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Jia XM, Long YR, Yu XL, Chen RQ, Gong LK, Geng Y. Construction of stable membranal CMTM6-PD-L1 full-length complex to evaluate the PD-1/PD-L1-CMTM6 interaction and develop anti-tumor anti-CMTM6 nanobody. Acta Pharmacol Sin 2023; 44:1095-1104. [PMID: 36418428 PMCID: PMC10104848 DOI: 10.1038/s41401-022-01020-3] [Citation(s) in RCA: 1] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/12/2022] [Accepted: 10/23/2022] [Indexed: 11/24/2022] Open
Abstract
CKLF (chemokine-like factor)-MARVEL transmembrane domain containing protein 6 (CMTM6) is a novel regulator to maintain the stability of PD-L1. CMTM6 can colocalize and interact with PD-L1 on the recycling endosomes and cell membrane, preventing PD-L1 from lysosome-mediated degradation and proteasome-mediated degradation thus increasing the half-life of PD-L1 on the cell membrane. The difficulties in obtaining stable full-length PD-L1 and CMTM6 proteins hinder the research on their structures, function as well as related drug development. Using lauryl maltose neopentyl glycol (LMNG) as the optimized detergent and a cell membrane mimetic strategy, we assembled a stable membrane-bound full-length CMTM6-PD-L1 complex with amphipol A8-35. When the PD-1/PD-L1-CMTM6 interactions were analyzed, we found that CMTM6 greatly enhanced the binding and delayed the dissociation of PD-1/PD-L1, thus affecting immunosuppressive signaling and anti-apoptotic signaling. We then used the CMTM6-PD-L1 complex as immunogens to generate immune repertoires in camels, and identified a functional anti-CMTM6 nanobody, called 1A5. We demonstrated that the anti-CMTM6 nanobody greatly decreased T-cell immunosuppression and promoted apoptotic susceptibility of tumor cells in vitro, and mainly relied on the cytotoxic effect of CD8+ T-cells to exert tumor growth inhibitory effects in CT26 tumor-bearing mice. In conclusion, the stable membrane-bound full-length CMTM6-PD-L1 complex has been successfully used in studying PD-1/PD-L1-CMTM6 interactions and CMTM6-targeting drug development, suggesting CMTM6 as a novel tumor immunotherapy target.
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Affiliation(s)
- Xiao-Min Jia
- The CAS Key Laboratory of Receptor Research, Shanghai Institute of Materia Medica, Chinese Academy of Sciences, Shanghai, 201203, China
- University of Chinese Academy of Sciences, Beijing, 100049, China
| | - Yi-Ru Long
- University of Chinese Academy of Sciences, Beijing, 100049, China
- State Key Laboratory of Drug Research, Shanghai Institute of Materia Medica, Chinese Academy of Sciences, Shanghai, 201203, China
| | - Xiao-Lu Yu
- University of Chinese Academy of Sciences, Beijing, 100049, China
- State Key Laboratory of Drug Research, Shanghai Institute of Materia Medica, Chinese Academy of Sciences, Shanghai, 201203, China
| | - Run-Qiu Chen
- State Key Laboratory of Drug Research, Shanghai Institute of Materia Medica, Chinese Academy of Sciences, Shanghai, 201203, China
| | - Li-Kun Gong
- University of Chinese Academy of Sciences, Beijing, 100049, China.
- State Key Laboratory of Drug Research, Shanghai Institute of Materia Medica, Chinese Academy of Sciences, Shanghai, 201203, China.
- Zhongshan Institute for Drug Discovery, Shanghai Institute of Materia Medica, Chinese Academy of Sciences, Zhongshan, 528400, China.
| | - Yong Geng
- The CAS Key Laboratory of Receptor Research, Shanghai Institute of Materia Medica, Chinese Academy of Sciences, Shanghai, 201203, China.
- University of Chinese Academy of Sciences, Beijing, 100049, China.
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Molday RS, Garces FA, Scortecci JF, Molday LL. Structure and function of ABCA4 and its role in the visual cycle and Stargardt macular degeneration. Prog Retin Eye Res 2021; 89:101036. [PMID: 34954332 DOI: 10.1016/j.preteyeres.2021.101036] [Citation(s) in RCA: 20] [Impact Index Per Article: 6.7] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/24/2021] [Revised: 12/07/2021] [Accepted: 12/13/2021] [Indexed: 12/17/2022]
Abstract
ABCA4 is a member of the superfamily of ATP-binding cassette (ABC) transporters that is preferentially localized along the rim region of rod and cone photoreceptor outer segment disc membranes. It uses the energy from ATP binding and hydrolysis to transport N-retinylidene-phosphatidylethanolamine (N-Ret-PE), the Schiff base adduct of retinal and phosphatidylethanolamine, from the lumen to the cytoplasmic leaflet of disc membranes. This ensures that all-trans-retinal and excess 11-cis-retinal are efficiently cleared from photoreceptor cells thereby preventing the accumulation of toxic retinoid compounds. Loss-of-function mutations in the gene encoding ABCA4 cause autosomal recessive Stargardt macular degeneration, also known as Stargardt disease (STGD1), and related autosomal recessive retinopathies characterized by impaired central vision and an accumulation of lipofuscin and bis-retinoid compounds. High resolution structures of ABCA4 in its substrate and nucleotide free state and containing bound N-Ret-PE or ATP have been determined by cryo-electron microscopy providing insight into the molecular architecture of ABCA4 and mechanisms underlying substrate recognition and conformational changes induced by ATP binding. The expression and functional characterization of a large number of disease-causing missense ABCA4 variants have been determined. These studies have shed light into the molecular mechanisms underlying Stargardt disease and a classification that reliably predicts the effect of a specific missense mutation on the severity of the disease. They also provide a framework for developing rational therapeutic treatments for ABCA4-associated diseases.
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Affiliation(s)
- Robert S Molday
- Department of Biochemistry & Molecular Biology, University of British Columbia, Vancouver, B.C., Canada; Department of Ophthalmology & Visual Sciences, University of British Columbia, Vancouver, B.C., Canada.
| | - Fabian A Garces
- Department of Biochemistry & Molecular Biology, University of British Columbia, Vancouver, B.C., Canada
| | | | - Laurie L Molday
- Department of Biochemistry & Molecular Biology, University of British Columbia, Vancouver, B.C., Canada
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3
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Pöge M, Mahamid J, Imanishi SS, Plitzko JM, Palczewski K, Baumeister W. Determinants shaping the nanoscale architecture of the mouse rod outer segment. eLife 2021; 10:e72817. [PMID: 34931611 PMCID: PMC8758146 DOI: 10.7554/elife.72817] [Citation(s) in RCA: 21] [Impact Index Per Article: 7.0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/05/2021] [Accepted: 12/09/2021] [Indexed: 12/02/2022] Open
Abstract
The unique membrane organization of the rod outer segment (ROS), the specialized sensory cilium of rod photoreceptor cells, provides the foundation for phototransduction, the initial step in vision. ROS architecture is characterized by a stack of identically shaped and tightly packed membrane disks loaded with the visual receptor rhodopsin. A wide range of genetic aberrations have been reported to compromise ROS ultrastructure, impairing photoreceptor viability and function. Yet, the structural basis giving rise to the remarkably precise arrangement of ROS membrane stacks and the molecular mechanisms underlying genetically inherited diseases remain elusive. Here, cryo-electron tomography (cryo-ET) performed on native ROS at molecular resolution provides insights into key structural determinants of ROS membrane architecture. Our data confirm the existence of two previously observed molecular connectors/spacers which likely contribute to the nanometer-scale precise stacking of the ROS disks. We further provide evidence that the extreme radius of curvature at the disk rims is enforced by a continuous supramolecular assembly composed of peripherin-2 (PRPH2) and rod outer segment membrane protein 1 (ROM1) oligomers. We suggest that together these molecular assemblies constitute the structural basis of the highly specialized ROS functional architecture. Our Cryo-ET data provide novel quantitative and structural information on the molecular architecture in ROS and substantiate previous results on proposed mechanisms underlying pathologies of certain PRPH2 mutations leading to blindness.
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Affiliation(s)
- Matthias Pöge
- Max Planck Institute of Biochemistry, Department of Molecular Structural BiologyMartinsriedGermany
| | - Julia Mahamid
- Max Planck Institute of Biochemistry, Department of Molecular Structural BiologyMartinsriedGermany
| | - Sanae S Imanishi
- Eugene and Marilyn Glick Eye Institute and the Department of Ophthalmology, Indiana University School of MedicineyIndianapolisUnited States
| | - Jürgen M Plitzko
- Max Planck Institute of Biochemistry, Department of Molecular Structural BiologyMartinsriedGermany
| | - Krzysztof Palczewski
- Gavin Herbert Eye Institute and the Department of Ophthalmology, Center for Translational Vision Research, Department of Physiology & Biophysics, Department of Chemistry, Department of Molecular Biology and BiochemistryIrvineUnited States
| | - Wolfgang Baumeister
- Max Planck Institute of Biochemistry, Department of Molecular Structural BiologyMartinsriedGermany
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4
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Scortecci JF, Molday LL, Curtis SB, Garces FA, Panwar P, Van Petegem F, Molday RS. Cryo-EM structures of the ABCA4 importer reveal mechanisms underlying substrate binding and Stargardt disease. Nat Commun 2021; 12:5902. [PMID: 34625547 PMCID: PMC8501128 DOI: 10.1038/s41467-021-26161-7] [Citation(s) in RCA: 20] [Impact Index Per Article: 6.7] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/24/2021] [Accepted: 09/20/2021] [Indexed: 12/04/2022] Open
Abstract
ABCA4 is an ATP-binding cassette (ABC) transporter that flips N-retinylidene-phosphatidylethanolamine (N-Ret-PE) from the lumen to the cytoplasmic leaflet of photoreceptor membranes. Loss-of-function mutations cause Stargardt disease (STGD1), a macular dystrophy associated with severe vision loss. To define the mechanisms underlying substrate binding and STGD1, we determine the cryo-EM structure of ABCA4 in its substrate-free and bound states. The two structures are similar and delineate an elongated protein with the two transmembrane domains (TMD) forming an outward facing conformation, extended and twisted exocytoplasmic domains (ECD), and closely opposed nucleotide binding domains. N-Ret-PE is wedged between the two TMDs and a loop from ECD1 within the lumen leaflet consistent with a lateral access mechanism and is stabilized through hydrophobic and ionic interactions with residues from the TMDs and ECDs. Our studies provide a framework for further elucidating the molecular mechanism associated with lipid transport and disease and developing promising disease interventions.
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Affiliation(s)
| | - Laurie L Molday
- Department of Biochemistry and Molecular Biology, University of British Columbia, Vancouver, BC, Canada
| | - Susan B Curtis
- Department of Biochemistry and Molecular Biology, University of British Columbia, Vancouver, BC, Canada
| | - Fabian A Garces
- Department of Biochemistry and Molecular Biology, University of British Columbia, Vancouver, BC, Canada
| | - Pankaj Panwar
- Department of Biochemistry and Molecular Biology, University of British Columbia, Vancouver, BC, Canada
| | - Filip Van Petegem
- Department of Biochemistry and Molecular Biology, University of British Columbia, Vancouver, BC, Canada
| | - Robert S Molday
- Department of Biochemistry and Molecular Biology, University of British Columbia, Vancouver, BC, Canada.
- Department of Ophthalmology and Visual Sciences, University of British Columbia, Vancouver, BC, Canada.
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5
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Huang D, Heath Jeffery RC, Aung-Htut MT, McLenachan S, Fletcher S, Wilton SD, Chen FK. Stargardt disease and progress in therapeutic strategies. Ophthalmic Genet 2021; 43:1-26. [PMID: 34455905 DOI: 10.1080/13816810.2021.1966053] [Citation(s) in RCA: 15] [Impact Index Per Article: 5.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/15/2022]
Abstract
Background: Stargardt disease (STGD1) is an autosomal recessive retinal dystrophy due to mutations in ABCA4, characterized by subretinal deposition of lipofuscin-like substances and bilateral centrifugal vision loss. Despite the tremendous progress made in the understanding of STGD1, there are no approved treatments to date. This review examines the challenges in the development of an effective STGD1 therapy.Materials and Methods: A literature review was performed through to June 2021 summarizing the spectrum of retinal phenotypes in STGD1, the molecular biology of ABCA4 protein, the in vivo and in vitro models used to investigate the mechanisms of ABCA4 mutations and current clinical trials.Results: STGD1 phenotypic variability remains an challenge for clinical trial design and patient selection. Pre-clinical development of therapeutic options has been limited by the lack of animal models reflecting the diverse phenotypic spectrum of STDG1. Patient-derived cell lines have facilitated the characterization of splice mutations but the clinical presentation is not always predicted by the effect of specific mutations on retinoid metabolism in cellular models. Current therapies primarily aim to delay vision loss whilst strategies to restore vision are less well developed.Conclusions: STGD1 therapy development can be accelerated by a deeper understanding of genotype-phenotype correlations.
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Affiliation(s)
- Di Huang
- Centre for Molecular Medicine and Innovative Therapeutics, Murdoch University, Western Australia, Australia.,Centre for Ophthalmology and Visual Science (Incorporating Lions Eye Institute), the University of Western Australia, Nedlands, Western Australia, Australia.,Perron Institute for Neurological and Translational Science & the University of Western Australia, Nedlands, Western Australia, Australia
| | - Rachael C Heath Jeffery
- Centre for Ophthalmology and Visual Science (Incorporating Lions Eye Institute), the University of Western Australia, Nedlands, Western Australia, Australia
| | - May Thandar Aung-Htut
- Centre for Molecular Medicine and Innovative Therapeutics, Murdoch University, Western Australia, Australia.,Perron Institute for Neurological and Translational Science & the University of Western Australia, Nedlands, Western Australia, Australia
| | - Samuel McLenachan
- Centre for Ophthalmology and Visual Science (Incorporating Lions Eye Institute), the University of Western Australia, Nedlands, Western Australia, Australia
| | - Sue Fletcher
- Centre for Molecular Medicine and Innovative Therapeutics, Murdoch University, Western Australia, Australia.,Perron Institute for Neurological and Translational Science & the University of Western Australia, Nedlands, Western Australia, Australia
| | - Steve D Wilton
- Centre for Molecular Medicine and Innovative Therapeutics, Murdoch University, Western Australia, Australia.,Perron Institute for Neurological and Translational Science & the University of Western Australia, Nedlands, Western Australia, Australia
| | - Fred K Chen
- Centre for Ophthalmology and Visual Science (Incorporating Lions Eye Institute), the University of Western Australia, Nedlands, Western Australia, Australia.,Australian Inherited Retinal Disease Registry and DNA Bank, Department of Medical Technology and Physics, Sir Charles Gairdner Hospital, Nedlands, Western Australia, Australia.,Department of Ophthalmology, Royal Perth Hospital, Perth, Western Australia, Australia.,Department of Ophthalmology, Perth Children's Hospital, Nedlands, Western Australia, Australia
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6
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Al-Khuzaei S, Broadgate S, Foster CR, Shah M, Yu J, Downes SM, Halford S. An Overview of the Genetics of ABCA4 Retinopathies, an Evolving Story. Genes (Basel) 2021; 12:1241. [PMID: 34440414 PMCID: PMC8392661 DOI: 10.3390/genes12081241] [Citation(s) in RCA: 19] [Impact Index Per Article: 6.3] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/01/2021] [Revised: 08/09/2021] [Accepted: 08/10/2021] [Indexed: 11/16/2022] Open
Abstract
Stargardt disease (STGD1) and ABCA4 retinopathies (ABCA4R) are caused by pathogenic variants in the ABCA4 gene inherited in an autosomal recessive manner. The gene encodes an importer flippase protein that prevents the build-up of vitamin A derivatives that are toxic to the RPE. Diagnosing ABCA4R is complex due to its phenotypic variability and the presence of other inherited retinal dystrophy phenocopies. ABCA4 is a large gene, comprising 50 exons; to date > 2000 variants have been described. These include missense, nonsense, splicing, structural, and deep intronic variants. Missense variants account for the majority of variants in ABCA4. However, in a significant proportion of patients with an ABCA4R phenotype, a second variant in ABCA4 is not identified. This could be due to the presence of yet unknown variants, or hypomorphic alleles being incorrectly classified as benign, or the possibility that the disease is caused by a variant in another gene. This underlines the importance of accurate genetic testing. The pathogenicity of novel variants can be predicted using in silico programs, but these rely on databases that are not ethnically diverse, thus highlighting the need for studies in differing populations. Functional studies in vitro are useful towards assessing protein function but do not directly measure the flippase activity. Obtaining an accurate molecular diagnosis is becoming increasingly more important as targeted therapeutic options become available; these include pharmacological, gene-based, and cell replacement-based therapies. The aim of this review is to provide an update on the current status of genotyping in ABCA4 and the status of the therapeutic approaches being investigated.
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Affiliation(s)
- Saoud Al-Khuzaei
- Oxford Eye Hospital, John Radcliffe Hospital, Oxford University Hospitals NHS Foundation Trust, Oxford OX3 9DU, UK; (S.A.-K.); (M.S.)
- Nuffield Laboratory of Ophthalmology, Nuffield Department of Clinical Neuroscience, University of Oxford, Level 6 John Radcliffe Hospital, Headley Way, Oxford OX3 9DU, UK; (S.B.); (J.Y.)
| | - Suzanne Broadgate
- Nuffield Laboratory of Ophthalmology, Nuffield Department of Clinical Neuroscience, University of Oxford, Level 6 John Radcliffe Hospital, Headley Way, Oxford OX3 9DU, UK; (S.B.); (J.Y.)
| | | | - Mital Shah
- Oxford Eye Hospital, John Radcliffe Hospital, Oxford University Hospitals NHS Foundation Trust, Oxford OX3 9DU, UK; (S.A.-K.); (M.S.)
| | - Jing Yu
- Nuffield Laboratory of Ophthalmology, Nuffield Department of Clinical Neuroscience, University of Oxford, Level 6 John Radcliffe Hospital, Headley Way, Oxford OX3 9DU, UK; (S.B.); (J.Y.)
| | - Susan M. Downes
- Oxford Eye Hospital, John Radcliffe Hospital, Oxford University Hospitals NHS Foundation Trust, Oxford OX3 9DU, UK; (S.A.-K.); (M.S.)
- Nuffield Laboratory of Ophthalmology, Nuffield Department of Clinical Neuroscience, University of Oxford, Level 6 John Radcliffe Hospital, Headley Way, Oxford OX3 9DU, UK; (S.B.); (J.Y.)
| | - Stephanie Halford
- Nuffield Laboratory of Ophthalmology, Nuffield Department of Clinical Neuroscience, University of Oxford, Level 6 John Radcliffe Hospital, Headley Way, Oxford OX3 9DU, UK; (S.B.); (J.Y.)
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7
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Sander CL, Sears AE, Pinto AF, Choi EH, Kahremany S, Gao F, Salom D, Jin H, Pardon E, Suh S, Dong Z, Steyaert J, Saghatelian A, Skowronska-Krawczyk D, Kiser PD, Palczewski K. Nano-scale resolution of native retinal rod disk membranes reveals differences in lipid composition. J Cell Biol 2021; 220:e202101063. [PMID: 34132745 PMCID: PMC8240855 DOI: 10.1083/jcb.202101063] [Citation(s) in RCA: 19] [Impact Index Per Article: 6.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/11/2021] [Revised: 04/26/2021] [Accepted: 05/18/2021] [Indexed: 02/07/2023] Open
Abstract
Photoreceptors rely on distinct membrane compartments to support their specialized function. Unlike protein localization, identification of critical differences in membrane content has not yet been expanded to lipids, due to the difficulty of isolating domain-specific samples. We have overcome this by using SMA to coimmunopurify membrane proteins and their native lipids from two regions of photoreceptor ROS disks. Each sample's copurified lipids were subjected to untargeted lipidomic and fatty acid analysis. Extensive differences between center (rhodopsin) and rim (ABCA4 and PRPH2/ROM1) samples included a lower PC to PE ratio and increased LC- and VLC-PUFAs in the center relative to the rim region, which was enriched in shorter, saturated FAs. The comparatively few differences between the two rim samples likely reflect specific protein-lipid interactions. High-resolution profiling of the ROS disk lipid composition gives new insights into how intricate membrane structure and protein activity are balanced within the ROS, and provides a model for future studies of other complex cellular structures.
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Affiliation(s)
- Christopher L. Sander
- Department of Pharmacology, Case Western Reserve University, Cleveland, OH
- Department of Ophthalmology, Gavin Herbert Eye Institute, University of California, Irvine, Irvine, CA
| | - Avery E. Sears
- Department of Pharmacology, Case Western Reserve University, Cleveland, OH
- Department of Ophthalmology, Gavin Herbert Eye Institute, University of California, Irvine, Irvine, CA
| | - Antonio F.M. Pinto
- Clayton Foundation Laboratories for Peptide Biology, Salk Institute for Biological Studies, La Jolla, CA
| | - Elliot H. Choi
- Department of Pharmacology, Case Western Reserve University, Cleveland, OH
- Department of Ophthalmology, Gavin Herbert Eye Institute, University of California, Irvine, Irvine, CA
| | - Shirin Kahremany
- Department of Ophthalmology, Gavin Herbert Eye Institute, University of California, Irvine, Irvine, CA
| | - Fangyuan Gao
- Department of Ophthalmology, Gavin Herbert Eye Institute, University of California, Irvine, Irvine, CA
| | - David Salom
- Department of Ophthalmology, Gavin Herbert Eye Institute, University of California, Irvine, Irvine, CA
| | - Hui Jin
- Department of Pharmacology, Case Western Reserve University, Cleveland, OH
| | - Els Pardon
- Vlaams Instituut voor Biotechnologie–Vrije Universiteit Brussel Center for Structural Biology, Vlaams Instituut voor Biotechnologie, Brussels, Belgium
- Structural Biology Brussels, Vrije Universiteit Brussel, Brussels, Belgium
| | - Susie Suh
- Department of Pharmacology, Case Western Reserve University, Cleveland, OH
- Department of Ophthalmology, Gavin Herbert Eye Institute, University of California, Irvine, Irvine, CA
| | - Zhiqian Dong
- Department of Ophthalmology, Gavin Herbert Eye Institute, University of California, Irvine, Irvine, CA
| | - Jan Steyaert
- Vlaams Instituut voor Biotechnologie–Vrije Universiteit Brussel Center for Structural Biology, Vlaams Instituut voor Biotechnologie, Brussels, Belgium
- Structural Biology Brussels, Vrije Universiteit Brussel, Brussels, Belgium
| | - Alan Saghatelian
- Clayton Foundation Laboratories for Peptide Biology, Salk Institute for Biological Studies, La Jolla, CA
| | - Dorota Skowronska-Krawczyk
- Department of Ophthalmology, Gavin Herbert Eye Institute, University of California, Irvine, Irvine, CA
- Department of Physiology and Biophysics, University of California, Irvine, Irvine, CA
| | - Philip D. Kiser
- Department of Ophthalmology, Gavin Herbert Eye Institute, University of California, Irvine, Irvine, CA
- Department of Physiology and Biophysics, University of California, Irvine, Irvine, CA
- Research Service, VA Long Beach Healthcare System, Long Beach, CA
| | - Krzysztof Palczewski
- Department of Ophthalmology, Gavin Herbert Eye Institute, University of California, Irvine, Irvine, CA
- Department of Physiology and Biophysics, University of California, Irvine, Irvine, CA
- Department of Chemistry, University of California, Irvine, Irvine, CA
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8
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Zhu Q, Rui X, Li Y, You Y, Sheng XL, Lei B. Identification of Four Novel Variants and Determination of Genotype-Phenotype Correlations for ABCA4 Variants Associated With Inherited Retinal Degenerations. Front Cell Dev Biol 2021; 9:634843. [PMID: 33732702 PMCID: PMC7957020 DOI: 10.3389/fcell.2021.634843] [Citation(s) in RCA: 6] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/29/2020] [Accepted: 01/26/2021] [Indexed: 12/21/2022] Open
Abstract
Purpose The purpose of the study is to describe the genetic and clinical features of 17 patients with ABCA4-related inherited retinal degenerations (IRDs) and define the phenotype–genotype correlations. Methods In this multicenter retrospective study, 17 patients from 16 families were enrolled, and ABCA4 gene variants were detected using targeted next-generation sequencing using a custom designed panel for IRDs. Sanger sequencing and co-segregation analysis of the suspected pathogenic variants were performed with the family members. The pathogenicities of variants were evaluated according to the American College of Medical Genetics and Genomics guidelines (ACMG). Protein structure modifications mediated by the variants were studied using bioinformatic analyses. Results The probands were diagnosed with Stargardt disease 1 (7), cone-rod dystrophy type 3 (8), cone dystrophy (1), and retinitis pigmentosa 19 (1). Onset of symptoms occurred between 5 and 27 years of age (median age = 12.4 years). A total of 30 unique ABCA4 suspicious pathogenic variations were observed, including 18 missense mutations, seven frameshift mutations, two nonsense mutations, one canonical splice site mutation, one small in-frame deletion, and one insertion. Four novel ABCA4 variants were identified. Two novel frameshift variants, c.1290dupC (p.W431fs), and c.2967dupT (G990fs), were determined to be pathogenic. A novel missense variant c.G5761T (p.V1921L) was likely pathogenic, and another novel missense c.C170G (p.P57R) variant was of undetermined significance. All ABCA4 variants tested in this study inordinately changed the physico-chemical parameters and structure of protein based on in silico analysis. Conclusion ABCA4-related IRD is genetically and clinically highly heterogeneous. Four novel ABCA4 variants were identified. This study will expand the spectrum of disease-causing variants in ABCA4, which will further facilitate genetic counseling.
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Affiliation(s)
- Qing Zhu
- Zhengzhou University People's Hospital, Henan Provincial People's Hospital, Zhengzhou, China
| | - Xue Rui
- Ningxia Clinical Research Center of Blinding Eye Disease, Ningxia Eye Hospital, People's Hospital of Ningxia Hui Autonomous Region, First Affiliated Hospital of Northwest University for Nationalities, Yinchuan, China
| | - Ya Li
- Zhengzhou University People's Hospital, Henan Provincial People's Hospital, Zhengzhou, China.,Henan Branch of National Clinical Research Center for Ocular Diseases, Henan Eye Institute and Henan Eye Hospital, Henan Provincial People's Hospital, Zhengzhou, China
| | - Ya You
- Zhengzhou University People's Hospital, Henan Provincial People's Hospital, Zhengzhou, China.,Henan Branch of National Clinical Research Center for Ocular Diseases, Henan Eye Institute and Henan Eye Hospital, Henan Provincial People's Hospital, Zhengzhou, China
| | - Xun-Lun Sheng
- Ningxia Clinical Research Center of Blinding Eye Disease, Ningxia Eye Hospital, People's Hospital of Ningxia Hui Autonomous Region, First Affiliated Hospital of Northwest University for Nationalities, Yinchuan, China
| | - Bo Lei
- Zhengzhou University People's Hospital, Henan Provincial People's Hospital, Zhengzhou, China.,Henan Branch of National Clinical Research Center for Ocular Diseases, Henan Eye Institute and Henan Eye Hospital, Henan Provincial People's Hospital, Zhengzhou, China
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9
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Liu F, Lee J, Chen J. Molecular structures of the eukaryotic retinal importer ABCA4. eLife 2021; 10:63524. [PMID: 33605212 PMCID: PMC7932691 DOI: 10.7554/elife.63524] [Citation(s) in RCA: 24] [Impact Index Per Article: 8.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/28/2020] [Accepted: 02/18/2021] [Indexed: 12/20/2022] Open
Abstract
The ATP-binding cassette (ABC) transporter family contains thousands of members with diverse functions. Movement of the substrate, powered by ATP hydrolysis, can be outward (export) or inward (import). ABCA4 is a eukaryotic importer transporting retinal to the cytosol to enter the visual cycle. It also removes toxic retinoids from the disc lumen. Mutations in ABCA4 cause impaired vision or blindness. Despite decades of clinical, biochemical, and animal model studies, the molecular mechanism of ABCA4 is unknown. Here, we report the structures of human ABCA4 in two conformations. In the absence of ATP, ABCA4 adopts an outward-facing conformation, poised to recruit substrate. The presence of ATP induces large conformational changes that could lead to substrate release. These structures provide a molecular basis to understand many disease-causing mutations and a rational guide for new experiments to uncover how ABCA4 recruits, flips, and releases retinoids.
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Affiliation(s)
- Fangyu Liu
- Laboratory of Membrane Biology and Biophysics, The Rockefeller University, New York, United States.,Tri-Institutional Training Program in Chemical Biology, New York, United States
| | - James Lee
- Laboratory of Membrane Biology and Biophysics, The Rockefeller University, New York, United States
| | - Jue Chen
- Laboratory of Membrane Biology and Biophysics, The Rockefeller University, New York, United States.,Howard Hughes Medical Institute, Chevy Chase, United States
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10
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Stanchev LD, Marek M, Xian F, Klöhn M, Silvestro D, Dittmar G, López-Marqués RL, Günther Pomorski T. Functional Significance of Conserved Cysteines in the Extracellular Loops of the ATP Binding Cassette Transporter Pdr11p. J Fungi (Basel) 2020; 7:jof7010002. [PMID: 33375075 PMCID: PMC7822014 DOI: 10.3390/jof7010002] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/01/2020] [Accepted: 12/18/2020] [Indexed: 12/16/2022] Open
Abstract
The pleiotropic drug resistance (PDR) transporter Pdr11p is expressed under anaerobic growth conditions at the plasma membrane of the yeast Saccharomyces cerevisiae, where it facilitates the uptake of exogenous sterols. Members of the fungal PDR family contain six conserved cysteines in their extracellular loops (ECL). For the functional analysis of these cysteine residues in Pdr11p, we generated a series of single cysteine-to-serine mutants. All mutant proteins expressed well and displayed robust ATPase activity upon purification. Mass-spectrometry analysis identified two cysteine residues (C582 and C603) in ECL3 forming a disulfide bond. Further characterization by cell-based assays showed that all mutants are compromised in facilitating sterol uptake, protein stability, and trafficking to the plasma membrane. Our data highlight the fundamental importance of all six extracellular cysteine residues for the functional integrity of Pdr11p and provide new structural insights into the PDR family of transporters.
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Affiliation(s)
- Lyubomir Dimitrov Stanchev
- Department of Molecular Biochemistry, Faculty of Chemistry and Biochemistry, Ruhr University Bochum, 44801 Bochum, Germany; (L.D.S.); (M.K.)
- Department of Plant and Environmental Sciences, University of Copenhagen, Thorvaldsensvej 40, DK-1871 Frederiksberg, Denmark; (M.M.); (D.S.); (R.L.L.-M.)
| | - Magdalena Marek
- Department of Plant and Environmental Sciences, University of Copenhagen, Thorvaldsensvej 40, DK-1871 Frederiksberg, Denmark; (M.M.); (D.S.); (R.L.L.-M.)
| | - Feng Xian
- Proteomics of Cellular Signaling, Luxembourg Institute of Health, Rue Thomas Edison 1A-B, L-1445 Strassen, Luxembourg; (F.X.); (G.D.)
| | - Mara Klöhn
- Department of Molecular Biochemistry, Faculty of Chemistry and Biochemistry, Ruhr University Bochum, 44801 Bochum, Germany; (L.D.S.); (M.K.)
| | - Daniele Silvestro
- Department of Plant and Environmental Sciences, University of Copenhagen, Thorvaldsensvej 40, DK-1871 Frederiksberg, Denmark; (M.M.); (D.S.); (R.L.L.-M.)
| | - Gunnar Dittmar
- Proteomics of Cellular Signaling, Luxembourg Institute of Health, Rue Thomas Edison 1A-B, L-1445 Strassen, Luxembourg; (F.X.); (G.D.)
| | - Rosa Laura López-Marqués
- Department of Plant and Environmental Sciences, University of Copenhagen, Thorvaldsensvej 40, DK-1871 Frederiksberg, Denmark; (M.M.); (D.S.); (R.L.L.-M.)
| | - Thomas Günther Pomorski
- Department of Molecular Biochemistry, Faculty of Chemistry and Biochemistry, Ruhr University Bochum, 44801 Bochum, Germany; (L.D.S.); (M.K.)
- Department of Plant and Environmental Sciences, University of Copenhagen, Thorvaldsensvej 40, DK-1871 Frederiksberg, Denmark; (M.M.); (D.S.); (R.L.L.-M.)
- Correspondence: ; Tel.: +49-234-322-4430
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11
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Curtis SB, Molday LL, Garces FA, Molday RS. Functional analysis and classification of homozygous and hypomorphic ABCA4 variants associated with Stargardt macular degeneration. Hum Mutat 2020; 41:1944-1956. [PMID: 32845050 DOI: 10.1002/humu.24100] [Citation(s) in RCA: 19] [Impact Index Per Article: 4.8] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/29/2020] [Revised: 08/06/2020] [Accepted: 08/24/2020] [Indexed: 12/14/2022]
Abstract
Stargardt macular degeneration (Stargardt disease 1 [STGD1]) is caused by mutations in the gene encoding ABCA4, an ATP-binding cassette protein that transports N-retinylidene-phosphatidylethanolamine (N-Ret-PE) across photoreceptor membranes. Reduced ABCA4 activity results in retinoid accumulation leading to photoreceptor degeneration. The disease onset and severity vary from severe loss in visual acuity in the first decade to mild visual impairment late in life. We determined the effect of 22 disease-causing missense mutations on the expression and ATPase activity of ABCA4 in the absence and presence of N-Ret-PE. Three classes were identified that correlated with the disease onset in homozygous STGD1 individuals: Class 1 exhibited reduced ABCA4 expression and ATPase activity that was not stimulated by N-Ret-PE; individuals homozygous for these variants had an early disease onset (≤13 years); Class 2 showed reduced ATPase activity with limited stimulation by N-Ret-PE; these correlated with moderate disease onset (14-40 years); and Class 3 displayed high expression and ATPase activity that was strongly activated by N-Ret-PE; these were associated with late disease onset (>40 years). On the basis of our results, we introduce a functionality index for gauging the effect of missense mutations on STGD1 severity. Our studies support the mild phenotype exhibited by the p.Gly863Ala, p.Asn1868Ile, and p.Gly863Ala/p.Asn1868Ile variants.
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Affiliation(s)
- Susan B Curtis
- Department of Biochemistry and Molecular Biology, University of British Columbia, Vancouver, British Columbia, Canada
| | - Laurie L Molday
- Department of Biochemistry and Molecular Biology, University of British Columbia, Vancouver, British Columbia, Canada
| | - Fabian A Garces
- Department of Biochemistry and Molecular Biology, University of British Columbia, Vancouver, British Columbia, Canada
| | - Robert S Molday
- Department of Biochemistry and Molecular Biology, University of British Columbia, Vancouver, British Columbia, Canada.,Department of Ophthalmology and Visual Sciences, University of British Columbia, Vancouver, British Columbia, Canada
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12
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von Lintig J, Moon J, Babino D. Molecular components affecting ocular carotenoid and retinoid homeostasis. Prog Retin Eye Res 2020; 80:100864. [PMID: 32339666 DOI: 10.1016/j.preteyeres.2020.100864] [Citation(s) in RCA: 30] [Impact Index Per Article: 7.5] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/25/2020] [Revised: 04/13/2020] [Accepted: 04/17/2020] [Indexed: 12/15/2022]
Abstract
The photochemistry of vision employs opsins and geometric isomerization of their covalently bound retinylidine chromophores. In different animal classes, these light receptors associate with distinct G proteins that either hyperpolarize or depolarize photoreceptor membranes. Vertebrates also use the acidic form of chromophore, retinoic acid, as the ligand of nuclear hormone receptors that orchestrate eye development. To establish and sustain these processes, animals must acquire carotenoids from the diet, transport them, and metabolize them to chromophore and retinoic acid. The understanding of carotenoid metabolism, however, lagged behind our knowledge about the biology of their receptor molecules. In the past decades, much progress has been made in identifying the genes encoding proteins that mediate the transport and enzymatic transformations of carotenoids and their retinoid metabolites. Comparative analysis in different animal classes revealed how evolutionary tinkering with a limited number of genes evolved different biochemical strategies to supply photoreceptors with chromophore. Mutations in these genes impair carotenoid metabolism and induce various ocular pathologies. This review summarizes this advancement and introduces the involved proteins, including the homeostatic regulation of their activities.
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Affiliation(s)
- Johannes von Lintig
- Department of Pharmacology, School of Medicine, Case Western Reserve University, Cleveland, OH, USA.
| | - Jean Moon
- Department of Pharmacology, School of Medicine, Case Western Reserve University, Cleveland, OH, USA
| | - Darwin Babino
- Department of Ophthalmology, School of Medicine, University of Washington, Seattle, WA, USA
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13
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Cremers FPM, Lee W, Collin RWJ, Allikmets R. Clinical spectrum, genetic complexity and therapeutic approaches for retinal disease caused by ABCA4 mutations. Prog Retin Eye Res 2020; 79:100861. [PMID: 32278709 PMCID: PMC7544654 DOI: 10.1016/j.preteyeres.2020.100861] [Citation(s) in RCA: 156] [Impact Index Per Article: 39.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/06/2019] [Revised: 03/13/2020] [Accepted: 03/18/2020] [Indexed: 12/18/2022]
Abstract
The ABCA4 protein (then called a “rim protein”) was first
identified in 1978 in the rims and incisures of rod photoreceptors. The
corresponding gene, ABCA4, was cloned in 1997, and variants
were identified as the cause of autosomal recessive Stargardt disease (STGD1).
Over the next two decades, variation in ABCA4 has been
attributed to phenotypes other than the classically defined STGD1 or fundus
flavimaculatus, ranging from early onset and fast progressing cone-rod dystrophy
and retinitis pigmentosa-like phenotypes to very late onset cases of mostly mild
disease sometimes resembling, and confused with, age-related macular
degeneration. Similarly, analysis of the ABCA4 locus uncovered
a trove of genetic information, including >1200 disease-causing mutations
of varying severity, and of all types – missense, nonsense, small
deletions/insertions, and splicing affecting variants, of which many are located
deep-intronic. Altogether, this has greatly expanded our understanding of
complexity not only of the diseases caused by ABCA4 mutations,
but of all Mendelian diseases in general. This review provides an in depth
assessment of the cumulative knowledge of ABCA4-associated retinopathy –
clinical manifestations, genetic complexity, pathophysiology as well as current
and proposed therapeutic approaches.
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Affiliation(s)
- Frans P M Cremers
- Department of Human Genetics, Radboud University Medical Center, PO Box 9101, 6500 HB, Nijmegen, the Netherlands; Donders Institute for Brain, Cognition and Behaviour, Radboud University Medical Center, PO Box 9104, 6500 HE, Nijmegen, the Netherlands.
| | - Winston Lee
- Department of Ophthalmology, Columbia University, New York, NY, 10032, USA; Department of Genetics & Development, Columbia University, New York, NY, 10032, USA
| | - Rob W J Collin
- Department of Human Genetics, Radboud University Medical Center, PO Box 9101, 6500 HB, Nijmegen, the Netherlands; Donders Institute for Brain, Cognition and Behaviour, Radboud University Medical Center, PO Box 9104, 6500 HE, Nijmegen, the Netherlands
| | - Rando Allikmets
- Department of Ophthalmology, Columbia University, New York, NY, 10032, USA; Department of Pathology & Cell Biology, Columbia University, New York, NY, 10032, USA.
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14
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Raj RK, Dhoble P, Anjanamurthy R, Chermakani P, Kumaran M, Devarajan B, Sundaresan P. Genetic characterization of Stargardt clinical phenotype in South Indian patients using sanger and targeted sequencing. EYE AND VISION 2020; 7:3. [PMID: 31934596 PMCID: PMC6950877 DOI: 10.1186/s40662-019-0168-8] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Subscribe] [Scholar Register] [Received: 06/04/2019] [Accepted: 12/07/2019] [Indexed: 11/10/2022]
Abstract
Background Stargardt disease 1 (STGD1; MIM 248200) is a monogenic form of autosomal recessive genetic disease caused by mutation in ABCA4. This gene has a major role in hydrolyzing N-retinylidene-phosphatidylethanolamine to all-trans-retinal and phosphatidylethanolamine. The purpose of this study is to identify the frequency of putative disease-causing mutations associated with Stargardt disease in a South Indian population. Methods A total of 28 clinically diagnosed Stargardt-like phenotype patients were recruited from south India. Ophthalmic examination of all patients was carefully carried out by a retina specialist based on the stages of fundus imaging and ERG grouping. Genetic analysis of ABCA4 was performed for all patients using Sanger sequencing and clinical exome sequencing. Results This study identified disease-causing mutations in ABCA4 in 75% (21/28) of patients, 7% (2/28) exhibited benign variants and 18% (5/28) were negative for the disease-causing mutation. Conclusion This is the first study describing the genetic association of ABCA4 disease-causing mutation in South Indian Stargardt 1 patients (STGD1). Our findings highlighted the presence of two novel missense mutations and an (in/del, single base pair deletion & splice variant) in ABCA4. However, genetic heterogeneity in ABCA4 mutants requires a larger sample size to establish a true correlation with clinical phenotype.
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Affiliation(s)
- Rajendran Kadarkarai Raj
- 1Department of Genetics, Aravind Medical Research Foundation-Madurai, No.1 Anna Nagar, Madurai, Tamil Nadu 625 020 India
| | - Pankaja Dhoble
- 2Retina Consultant, Department of Vitreo Retinal services, Aravind Eye Hospital-Pondicherry, Puducherry, India
| | - Rupa Anjanamurthy
- 3Department of Paediatrics and Adult strabismus, Aravind Eye Hospital-Madurai, Madurai, Tamil Nadu India
| | - Prakash Chermakani
- 1Department of Genetics, Aravind Medical Research Foundation-Madurai, No.1 Anna Nagar, Madurai, Tamil Nadu 625 020 India
| | - Manojkumar Kumaran
- 4Department of Bioinformatics, Aravind Medical Research Foundation-Madurai, Madurai, Tamil Nadu India
| | - Bharanidharan Devarajan
- 4Department of Bioinformatics, Aravind Medical Research Foundation-Madurai, Madurai, Tamil Nadu India
| | - Periasamy Sundaresan
- 1Department of Genetics, Aravind Medical Research Foundation-Madurai, No.1 Anna Nagar, Madurai, Tamil Nadu 625 020 India
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15
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Joo K, Seong MW, Park KH, Park SS, Woo SJ. Genotypic profile and phenotype correlations of ABCA4-associated retinopathy in Koreans. Mol Vis 2019; 25:679-690. [PMID: 31814693 PMCID: PMC6857773] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/08/2019] [Accepted: 11/10/2019] [Indexed: 11/29/2022] Open
Abstract
PURPOSE This study was conducted to analyze the clinical features associated with the pathogenic variants of ABCA4 in Korean patients with inherited retinal dystrophies (IRDs). METHODS We enrolled patients with IRDs who visited a tertiary referral hospital and identified the pathogenic variants of ABCA4 through targeted gene panel sequencing and whole exome sequencing. We analyzed the clinical characteristics and phenotypic spectrum according to genotype. RESULTS Eleven patients (from nine families) with IRDs and pathogenic variants in ABCA4 were included. Eight patients (from seven families) with Stargardt disease (STGD), two (from one family) with cone-rod dystrophy (CRD), and one with early-onset retinitis pigmentosa (RP) were included. Two heterozygous mutations were identified in eight families, and one variant was found in a patient with fundus flavimaculatus. Two variants, p.Gln294Ter and p.Gln636Lys, were associated with severe phenotypes, such as early-onset RP and CRD. Four novel pathogenic variants, p.Gln636Lys, p.Ile1114del, p.Thr1117Ala, and p.Asn1588Tyr, were identified. p.Gln294Ter, p.Leu1157Ter, and p.Lys2049ArgfsTer12 were repeatedly detected in Koreans with ABCA4-associated retinal diseases (ABCA4-RD). CONCLUSIONS Various pathogenic variants of ABCA4, including four novel variants, were identified, and ABCA4-RD exhibited various phenotypes and disease severities in a Korean IRD cohort. These findings will be useful for understanding the clinical features of ABCA4-RD and ethnicity-specific variants in East Asians.
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Affiliation(s)
- Kwangsic Joo
- Department of Ophthalmology, Seoul National University Bundang Hospital, Seoul National University College of Medicine, Seongnam, Republic of Korea
| | - Moon-Woo Seong
- Department of Laboratory Medicine, Seoul National University Hospital, Seoul National University College of Medicine, Seoul, Republic of Korea
| | - Kyu Hyung Park
- Department of Ophthalmology, Seoul National University Bundang Hospital, Seoul National University College of Medicine, Seongnam, Republic of Korea
| | - Sung Sup Park
- Department of Laboratory Medicine, Seoul National University Hospital, Seoul National University College of Medicine, Seoul, Republic of Korea
| | - Se Joon Woo
- Department of Ophthalmology, Seoul National University Bundang Hospital, Seoul National University College of Medicine, Seongnam, Republic of Korea
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16
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Jha NK, Kar R, Niranjan R. ABC Transporters in Neurological Disorders: An Important Gateway for Botanical Compounds Mediated Neuro-Therapeutics. Curr Top Med Chem 2019; 19:795-811. [PMID: 30977450 DOI: 10.2174/1568026619666190412121811] [Citation(s) in RCA: 13] [Impact Index Per Article: 2.6] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/21/2019] [Revised: 02/27/2019] [Accepted: 03/07/2019] [Indexed: 12/13/2022]
Abstract
Neurodegeneration is a distinguishing feature of many age related disorders and other vector borne neuroinflammatory diseases. There are a number of factors that can modulate the pathology of these disorders. ATP-binding cassette (ABC) transporters are primarily involved in the maintenance of normal brain homeostasis by eliminating toxic peptides and compounds from the brain. Also, ABC transporters protect the brain from the unwanted effects of endogenous and exogenous toxins that can enter the brain parenchyma. Therefore, these transporters have the ability to determine the pathological outcomes of several neurological disorders. For instance, ABC transporters like P-glycoprotein (ABCB1), and BCRP (ABCG2) have been reported to facilitate the clearance of peptides such as amyloid-β (Aβ) that accumulate in the brain during Alzheimer's disease (AD) progression. Other members such as ABCA1, ABCA2, ABCC8, ABCC9, ABCG1 and ABCG4 also have been reported to be involved in the progression of various brain disorders such as HIV-associated dementia, Multiple sclerosis (MS), Ischemic stroke, Japanese encephalitis (JE) and Epilepsy. However, these defective transporters can be targeted by numerous botanical compounds such as Verapamil, Berberine and Fascalpsyn as a therapeutic target to treat these neurological outcomes. These compounds are already reported to modulate ABC transporter activity in the CNS. Nonetheless, the exact mechanisms involving the ABC transporters role in normal brain functioning, their role in neuronal dysfunction and how these botanical compounds ensure and facilitate their therapeutic action in association with defective transporters still remain elusive. This review therefore, summarizes the role of ABC transporters in neurological disorders, with a special emphasis on its role in AD brains. The prospect of using botanical/natural compounds as modulators of ABC transporters in neurological disorders is discussed in the latter half of the article.
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Affiliation(s)
- Niraj Kumar Jha
- Department of Biotechnology, Noida Institute of Engineering & Technology (NIET), Greater Noida, India
| | - Rohan Kar
- Department of Biotechnology, Noida Institute of Engineering & Technology (NIET), Greater Noida, India
| | - Rituraj Niranjan
- Unit of Microbiology and Molecular Biology, ICMR-Vector Control Research Center, Puducherry-605006, India
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17
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Abstract
Rods and cones are retinal photoreceptor neurons required for our visual sensation. Because of their highly polarized structures and well-characterized processes of G protein-coupled receptor-mediated phototransduction signaling, these photoreceptors have been excellent models for studying the compartmentalization and sorting of proteins. Rods and cones have a modified ciliary compartment called the outer segment (OS) as well as non-OS compartments. The distinct membrane protein compositions between OS and non-OS compartments suggest that the OS is separated from the rest of the cellular compartments by multiple barriers or gates that are selectively permissive to specific cargoes. This review discusses the mechanisms of protein sorting and compartmentalization in photoreceptor neurons. Proper sorting and compartmentalization of membrane proteins are required for signal transduction and transmission. This review also discusses the roles of compartmentalized signaling, which is compromised in various retinal ciliopathies.
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Affiliation(s)
- Yoshikazu Imanishi
- Department of Pharmacology, School of Medicine, Case Western Reserve University, Cleveland, Ohio 44106, USA;
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18
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Molday LL, Wahl D, Sarunic MV, Molday RS. Localization and functional characterization of the p.Asn965Ser (N965S) ABCA4 variant in mice reveal pathogenic mechanisms underlying Stargardt macular degeneration. Hum Mol Genet 2019; 27:295-306. [PMID: 29145636 DOI: 10.1093/hmg/ddx400] [Citation(s) in RCA: 22] [Impact Index Per Article: 4.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/31/2017] [Accepted: 11/08/2017] [Indexed: 11/12/2022] Open
Abstract
ABCA4 is a member of the superfamily of ATP-binding cassette (ABC) proteins that transports N-retinylidene-phosphatidylethanolamine (N-Ret-PE) across outer segment disc membranes thereby facilitating the removal of potentially toxic retinoid compounds from photoreceptor cells. Mutations in the gene encoding ABCA4 are responsible for Stargardt disease (STGD1), an autosomal recessive retinal degenerative disease that causes severe vision loss. To define the molecular basis for STGD1 associated with the p.Asn965Ser (N965S) mutation in the Walker A motif of nucleotide binding domain 1 (NBD1), we generated a p.Asn965Ser knockin mouse and compared the subcellular localization and molecular properties of the disease variant with wild-type (WT) ABCA4. Here, we show that the p.Asn965Ser ABCA4 variant expresses at half the level of WT ABCA4, partially mislocalizes to the endoplasmic reticulum (ER) of photoreceptors, is devoid of N-Ret-PE activated ATPase activity, and causes an increase in autofluorescence and the bisretinoid A2E associated with lipofuscin deposits in retinal pigment epithelial cells as found in Stargardt patients and Abca4 knockout mice. We also show for the first time that a significant fraction of WT ABCA4 is retained in the inner segment of photoreceptors. On the basis of these studies we conclude that loss in substrate-dependent ATPase activity and protein misfolding are mechanisms underlying STGD1 associated with the p.Asn965Ser mutation in ABCA4. Functional and molecular modeling studies further suggest that similar pathogenic mechanisms are responsible for Tangiers disease associated with the p.Asn935Ser (N935S) mutation in the NBD1 Walker A motif of ABCA1.
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Affiliation(s)
- Laurie L Molday
- Department of Biochemistry and Molecular Biology, University of British Columbia, Vancouver, BC, Canada V6T 1Z3
| | - Daniel Wahl
- School of Engineering Science, Simon Fraser University, Burnaby, BC, Canada V5A 1S6
| | - Marinko V Sarunic
- School of Engineering Science, Simon Fraser University, Burnaby, BC, Canada V5A 1S6
| | - Robert S Molday
- Department of Biochemistry and Molecular Biology, University of British Columbia, Vancouver, BC, Canada V6T 1Z3.,Department of Ophthalmology and Visual Sciences, University of British Columbia, Vancouver, BC, Canada V5Z 3N9
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19
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Qi YH, Gao FJ, Hu FY, Zhang SH, Chen JY, Huang WJ, Tian GH, Wang M, Gan DK, Wu JH, Xu GZ. Next-Generation Sequencing-Aided Rapid Molecular Diagnosis of Occult Macular Dystrophy in a Chinese Family. Front Genet 2017; 8:107. [PMID: 28890726 PMCID: PMC5574873 DOI: 10.3389/fgene.2017.00107] [Citation(s) in RCA: 12] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/02/2017] [Accepted: 08/02/2017] [Indexed: 11/15/2022] Open
Abstract
Purpose: To show early, rapid and accurate molecular diagnosis of occult macular dystrophy (OMD) in a four-generation Chinese family with inherited macular dystrophy. Methods: In the current study, we comprehensively screened 130 genes involved in common inherited non-syndromic eye diseases with next-generation sequencing-based target capture sequencing of the proband of a four-generation Chinese family that has suffered from maculopathy without a definitive diagnosis for over 10 years. Variants were filtered and analyzed to identify possible disease-causing variants before validation by Sanger sequencing. Results: Two heterozygous mutations—RP1L1 c.133 C > T (p.Arg45Trp), which is a hot spot for OMD, and ABCA4 c.6119 G > A (p.Arg2040Gln), which was identified in Stargardt’s disease were found in three patients, but neither of the mutations was found in the unaffected individuals in the same family, who are phenotypically normal or in the normal control volunteers. Conclusion: These results cannot only confirm the diagnosis of OMD in the proband, but also provide presymptomatic diagnosis of the proband’s children before the onset of visual acuity impairment and guidance regarding the prognosis and management of these patients. Heterozygous mutations of RP1L1 c.133 C > T (p.Arg45Trp) and ABCA4 c.6119 G > A (p.Arg2040Gln) are likely responsible for OMD. Our results further extend our current understanding of the genetic basis of OMD, and emphasize the importance of molecular diagnosis and genetic counseling for OMD.
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Affiliation(s)
- Yu-He Qi
- Eye Institute, Eye and ENT Hospital, College of Medicine, Fudan UniversityShanghai, China
| | - Feng-Juan Gao
- Eye Institute, Eye and ENT Hospital, College of Medicine, Fudan UniversityShanghai, China
| | - Fang-Yuan Hu
- Eye Institute, Eye and ENT Hospital, College of Medicine, Fudan UniversityShanghai, China
| | - Sheng-Hai Zhang
- Eye Institute, Eye and ENT Hospital, College of Medicine, Fudan UniversityShanghai, China.,Shanghai Key Laboratory of Visual Impairment and Restoration, Science and Technology Commission of Shanghai MunicipalityShanghai, China.,Key Laboratory of Myopia, Ministry of HealthShanghai, China
| | - Jun-Yi Chen
- Eye Institute, Eye and ENT Hospital, College of Medicine, Fudan UniversityShanghai, China
| | - Wan-Jing Huang
- Eye Institute, Eye and ENT Hospital, College of Medicine, Fudan UniversityShanghai, China
| | - Guo-Hong Tian
- Eye Institute, Eye and ENT Hospital, College of Medicine, Fudan UniversityShanghai, China
| | - Min Wang
- Eye Institute, Eye and ENT Hospital, College of Medicine, Fudan UniversityShanghai, China
| | - De-Kang Gan
- Eye Institute, Eye and ENT Hospital, College of Medicine, Fudan UniversityShanghai, China
| | - Ji-Hong Wu
- Eye Institute, Eye and ENT Hospital, College of Medicine, Fudan UniversityShanghai, China.,Shanghai Key Laboratory of Visual Impairment and Restoration, Science and Technology Commission of Shanghai MunicipalityShanghai, China.,Key Laboratory of Myopia, Ministry of HealthShanghai, China
| | - Ge-Zhi Xu
- Eye Institute, Eye and ENT Hospital, College of Medicine, Fudan UniversityShanghai, China.,Shanghai Key Laboratory of Visual Impairment and Restoration, Science and Technology Commission of Shanghai MunicipalityShanghai, China.,Key Laboratory of Myopia, Ministry of HealthShanghai, China
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20
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Qian H, Zhao X, Cao P, Lei J, Yan N, Gong X. Structure of the Human Lipid Exporter ABCA1. Cell 2017; 169:1228-1239.e10. [DOI: 10.1016/j.cell.2017.05.020] [Citation(s) in RCA: 179] [Impact Index Per Article: 25.6] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/24/2017] [Revised: 05/01/2017] [Accepted: 05/12/2017] [Indexed: 01/10/2023]
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21
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Hofrnann L, Alexander NS, Sun W, Zhang J, Orban T, Palczewski K. Hydrogen/Deuterium Exchange Mass Spectrometry of Human Green Opsin Reveals a Conserved Pro-Pro Motif in Extracellular Loop 2 of Monostable Visual G Protein-Coupled Receptors. Biochemistry 2017; 56:2338-2348. [PMID: 28402104 PMCID: PMC5501310 DOI: 10.1021/acs.biochem.7b00165] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/01/2023]
Abstract
Opsins comprise the protein component of light sensitive G protein-coupled receptors (GPCRs) in the retina of the eye that are responsible for the transduction of light into a biochemical signal. Here, we used hydrogen/deuterium (H/D) exchange coupled with mass spectrometry to map conformational changes in green cone opsin upon light activation. We then compared these findings with those reported for rhodopsin. The extent of H/D exchange in green cone opsin was greater than in rhodopsin in the dark and bleached states, suggesting a higher structural heterogeneity for green cone opsin. Further analysis revealed that green cone opsin exists as a dimer in both dark (inactive) and bleached (active) states, and that the predicted glycosylation sites at N32 and N34 are indeed glycosylated. Comparison of deuterium uptake between inactive and active states of green cone opsin also disclosed a reduced solvent accessibility of the extracellular N-terminal region and an increased accessibility of the chromophore binding site. Increased H/D exchange at the extracellular side of transmembrane helix four (TM4) combined with an analysis of sequence alignments revealed a conserved Pro-Pro motif in extracellular loop 2 (EL2) of monostable visual GPCRs. These data present new insights into the locus of chromophore release at the extracellular side of TM4 and TM5 and provide a foundation for future functional evaluation.
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Affiliation(s)
- Lukas Hofrnann
- Department of Pharmacology and Cleveland Center for Membrane and Structural Biology, School of Medicine, Case Western Reserve University, 10900 Euclid Avenue, Cleveland, Ohio 44106, United States
| | - Nathan S. Alexander
- Department of Pharmacology and Cleveland Center for Membrane and Structural Biology, School of Medicine, Case Western Reserve University, 10900 Euclid Avenue, Cleveland, Ohio 44106, United States
| | - Wenyu Sun
- Department of Pharmacology and Cleveland Center for Membrane and Structural Biology, School of Medicine, Case Western Reserve University, 10900 Euclid Avenue, Cleveland, Ohio 44106, United States
| | - Jianye Zhang
- Department of Pharmacology and Cleveland Center for Membrane and Structural Biology, School of Medicine, Case Western Reserve University, 10900 Euclid Avenue, Cleveland, Ohio 44106, United States
| | - Tivadar Orban
- Department of Pharmacology and Cleveland Center for Membrane and Structural Biology, School of Medicine, Case Western Reserve University, 10900 Euclid Avenue, Cleveland, Ohio 44106, United States
| | - Krzysztof Palczewski
- Department of Pharmacology and Cleveland Center for Membrane and Structural Biology, School of Medicine, Case Western Reserve University, 10900 Euclid Avenue, Cleveland, Ohio 44106, United States
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Rajavel M, Orban T, Xu M, Hernandez-Sanchez W, de la Fuente M, Palczewski K, Taylor DJ. Dynamic peptides of human TPP1 fulfill diverse functions in telomere maintenance. Nucleic Acids Res 2016; 44:10467-10479. [PMID: 27655633 PMCID: PMC5137443 DOI: 10.1093/nar/gkw846] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/04/2016] [Revised: 09/12/2016] [Accepted: 09/14/2016] [Indexed: 12/12/2022] Open
Abstract
Telomeres are specialized nucleoprotein complexes that comprise the ends of linear chromosomes. Human telomeres end in a short, single-stranded DNA (ssDNA) overhang that is recognized and bound by two telomere proteins, POT1 and TPP1. Whereas POT1 binds directly to telomere ssDNA, its interaction with TPP1 is essential for localization of POT1 to the telomere. TPP1 also provides enhanced binding and sequence discrimination that regulates POT1-TPP1 interactions exclusively with telomere ssDNA. Finally, TPP1 recruits telomerase, the enzyme responsible for synthesis of telomere DNA, to the telomere. While the oligosaccharide-oligonucleotide-binding (OB)-fold domain of TPP1 has been solved by X-ray crystallography, the molecular interactions within the POT1-TPP1-ssDNA ternary complex and the conformational changes that contribute to its diverse functions remain ambiguous. We employed hydrogen/deuterium exchange combined with mass spectrometry to identify three peptides, all residing within the OB-fold of TPP1, that exhibit altered exchange rates upon complex formation or ssDNA binding. Mutation of these regions combined with functional assays revealed the diverse contributions of each moiety in protein-protein interactions, regulating telomerase activity or DNA-binding. Together, these functional data combined with biophysical analyses and homology modeling provide a molecular understanding of the diverse contributions of TPP1 in telomere maintenance.
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Affiliation(s)
- Malligarjunan Rajavel
- Department of Pharmacology, Case Western Reserve University, School of Medicine, Cleveland, OH 44106, USA
| | - Tivadar Orban
- Department of Pharmacology, Case Western Reserve University, School of Medicine, Cleveland, OH 44106, USA
| | - Mengyuan Xu
- Department of Pharmacology, Case Western Reserve University, School of Medicine, Cleveland, OH 44106, USA
| | - Wilnelly Hernandez-Sanchez
- Department of Pharmacology, Case Western Reserve University, School of Medicine, Cleveland, OH 44106, USA
| | - Maria de la Fuente
- Department of Pharmacology, Case Western Reserve University, School of Medicine, Cleveland, OH 44106, USA
| | - Krzysztof Palczewski
- Department of Pharmacology, Case Western Reserve University, School of Medicine, Cleveland, OH 44106, USA
| | - Derek J Taylor
- Department of Pharmacology, Case Western Reserve University, School of Medicine, Cleveland, OH 44106, USA .,Department of Biochemistry, Case Western Reserve University, School of Medicine, Cleveland, OH 44106, USA
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Abstract
Recent progress in molecular understanding of the retinoid cycle in mammalian retina stems from painstaking biochemical reconstitution studies supported by natural or engineered animal models with known genetic lesions and studies of humans with specific genetic blinding diseases. Structural and membrane biology have been used to detect critical retinal enzymes and proteins and their substrates and ligands, placing them in a cellular context. These studies have been supplemented by analytical chemistry methods that have identified small molecules by their spectral characteristics, often in conjunction with the evaluation of models of animal retinal disease. It is from this background that rational therapeutic interventions to correct genetic defects or environmental insults are identified. Thus, most presently accepted modulators of the retinoid cycle already have demonstrated promising results in animal models of retinal degeneration. These encouraging signs indicate that some human blinding diseases can be alleviated by pharmacological interventions.
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Affiliation(s)
- Philip D Kiser
- Department of Pharmacology, Cleveland Center for Membrane and Structural Biology, School of Medicine, Case Western Reserve University, Cleveland, Ohio 44106 ; Louis Stokes Cleveland VA Medical Center, Cleveland, Ohio 44106
| | - Krzysztof Palczewski
- Department of Pharmacology, Cleveland Center for Membrane and Structural Biology, School of Medicine, Case Western Reserve University, Cleveland, Ohio 44106
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Goldberg AFX, Moritz OL, Williams DS. Molecular basis for photoreceptor outer segment architecture. Prog Retin Eye Res 2016; 55:52-81. [PMID: 27260426 DOI: 10.1016/j.preteyeres.2016.05.003] [Citation(s) in RCA: 122] [Impact Index Per Article: 15.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/23/2016] [Revised: 05/27/2016] [Accepted: 05/29/2016] [Indexed: 01/11/2023]
Abstract
To serve vision, vertebrate rod and cone photoreceptors must detect photons, convert the light stimuli into cellular signals, and then convey the encoded information to downstream neurons. Rods and cones are sensory neurons that each rely on specialized ciliary organelles to detect light. These organelles, called outer segments, possess elaborate architectures that include many hundreds of light-sensitive membranous disks arrayed one atop another in precise register. These stacked disks capture light and initiate the chain of molecular and cellular events that underlie normal vision. Outer segment organization is challenged by an inherently dynamic nature; these organelles are subject to a renewal process that replaces a significant fraction of their disks (up to ∼10%) on a daily basis. In addition, a broad range of environmental and genetic insults can disrupt outer segment morphology to impair photoreceptor function and viability. In this chapter, we survey the major progress that has been made for understanding the molecular basis of outer segment architecture. We also discuss key aspects of organelle lipid and protein composition, and highlight distributions, interactions, and potential structural functions of key OS-resident molecules, including: kinesin-2, actin, RP1, prominin-1, protocadherin 21, peripherin-2/rds, rom-1, glutamic acid-rich proteins, and rhodopsin. Finally, we identify key knowledge gaps and challenges that remain for understanding how normal outer segment architecture is established and maintained.
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Affiliation(s)
- Andrew F X Goldberg
- Eye Research Institute, Oakland University, 417 Dodge Hall, Rochester, MI, 48309, USA.
| | - Orson L Moritz
- Department of Ophthalmology & Visual Sciences, University of British Columbia, Vancouver, BC, Canada
| | - David S Williams
- Department of Ophthalmology and Jules Stein Eye Institute, Department of Neurobiology, David Geffen School of Medicine at UCLA, University of California, Los Angeles, CA, USA
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25
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Cideciyan AV, Swider M, Schwartz SB, Stone EM, Jacobson SG. Predicting Progression of ABCA4-Associated Retinal Degenerations Based on Longitudinal Measurements of the Leading Disease Front. Invest Ophthalmol Vis Sci 2015; 56:5946-55. [PMID: 26377081 DOI: 10.1167/iovs.15-17698] [Citation(s) in RCA: 30] [Impact Index Per Article: 3.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/08/2023] Open
Abstract
PURPOSE To evaluate the progression of the earliest stage of disease in ABCA4-associated retinal degenerations (RDs). METHODS Near-infrared excited reduced-illuminance autofluorescence imaging was acquired across the retina up to 80 degrees eccentricity in 44 patients with two ABCA4 alleles. The eccentricity of the leading disease front (LDF) corresponding to the earliest stage of disease was measured along the four meridians. A mathematical model describing the expansion of the LDF was developed based on 6 years of longitudinal follow-up. RESULTS The extent of LDF along the superior, inferior, and temporal meridians showed a wide spectrum from 3.5 to 70 degrees. In patients with longitudinal data, the average centrifugal expansion rate was 2 degrees per year. The nasal extent of LDF between the fovea and ONH ranged from 4.3 to 16.5 degrees and expanded at 0.35 degrees per year. The extent of LDF beyond ONH ranged from 19 to 75 degrees and expanded on average at 2 degrees per year. A mathematical model fit well to the longitudinal data describing the expansion of the LDF. CONCLUSIONS The eccentricity of the LDF in ABCA4-RD shows a continuum from parafovea to far periphery along all four meridians consistent with a wide spectrum of severity observed clinically. The model of progression may provide a quantitative prediction of the LDF expansion based on the age and eccentricity of the LDF at a baseline visit, and thus contribute significantly to the enrollment of candidates appropriate for clinical trials planning specific interventions, efficacy outcomes, and durations.
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Affiliation(s)
- Artur V Cideciyan
- Scheie Eye Institute Department of Ophthalmology, Perelman School of Medicine, University of Pennsylvania, Philadelphia, Pennsylvania, United States
| | - Malgorzata Swider
- Scheie Eye Institute Department of Ophthalmology, Perelman School of Medicine, University of Pennsylvania, Philadelphia, Pennsylvania, United States
| | - Sharon B Schwartz
- Scheie Eye Institute Department of Ophthalmology, Perelman School of Medicine, University of Pennsylvania, Philadelphia, Pennsylvania, United States
| | - Edwin M Stone
- Stephen A. Wynn Institute for Vision Research, University of Iowa, Iowa City, Iowa, United States 3Howard Hughes Medical Institute, Department of Ophthalmology and Visual Sciences, University of Iowa Carver College of Medicine, Iowa City, Iowa, United Sta
| | - Samuel G Jacobson
- Scheie Eye Institute Department of Ophthalmology, Perelman School of Medicine, University of Pennsylvania, Philadelphia, Pennsylvania, United States
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Identification of Novel Mutations in ABCA4 Gene: Clinical and Genetic Analysis of Indian Patients with Stargardt Disease. BIOMED RESEARCH INTERNATIONAL 2015; 2015:940864. [PMID: 25922843 PMCID: PMC4398921 DOI: 10.1155/2015/940864] [Citation(s) in RCA: 18] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Subscribe] [Scholar Register] [Received: 12/26/2014] [Revised: 02/13/2015] [Accepted: 03/09/2015] [Indexed: 11/17/2022]
Abstract
Stargardt disease (STGD) is the leading cause of juvenile macular degeneration associated with progressive central vision loss, photophobia, and colour vision abnormalities. In this study, we have described the clinical and genetic features of Stargardt patients from an Indian cohort. The next generation sequencing was carried out in five clinically confirmed unrelated patients and their family members using a gene panel comprising 184 retinal specific genes. Sequencing results were analyzed by read mapping and variant calling in genes of interest, followed by their verification and interpretation. Genetic analysis revealed ABCA4 mutations in all of the five unrelated patients. Among these, four patients were found with compound heterozygous mutations and another one had homozygous mutation. All the affected individuals showed signs and symptoms consistent with the disease phenotype. We report two novel ABCA4 mutations in Indian patients with STGD disease, which expands the existing spectrum of disease-causing variants and the understanding of phenotypic and genotypic correlations. Screening for causative mutations in patients with STGD using panel of targeted gene sequencing by NGS would be a cost effective tool, might be helpful in confirming the precise diagnosis, and contributes towards the genetic counselling of asymptomatic carriers and isolated patients.
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Zhang N, Tsybovsky Y, Kolesnikov AV, Rozanowska M, Swider M, Schwartz SB, Stone EM, Palczewska G, Maeda A, Kefalov VJ, Jacobson SG, Cideciyan AV, Palczewski K. Protein misfolding and the pathogenesis of ABCA4-associated retinal degenerations. Hum Mol Genet 2015; 24:3220-37. [PMID: 25712131 DOI: 10.1093/hmg/ddv073] [Citation(s) in RCA: 59] [Impact Index Per Article: 6.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/05/2015] [Accepted: 02/22/2015] [Indexed: 12/13/2022] Open
Abstract
Mutations in the ABCA4 gene are a common cause of autosomal recessive retinal degeneration. All mouse models to date are based on knockouts of Abca4, even though the disease is often caused by missense mutations such as the complex allele L541P;A1038V (PV). We now show that the PV mutation causes severe human disease whereas the V mutation alone causes mild disease. Mutant ABCA4 proteins expressed heterologously in mammalian cells retained normal cellular localization. However, basal and all-trans-retinal-stimulated ATPase activities were reduced substantially for P and PV but only mildly for V. Electron microscopy revealed marked structural changes and misfolding for the P and PV mutants but few changes for the V mutant, consistent with the disease severity difference in patients. We generated Abca4(PV/PV) knock-in mice homozygous for the complex PV allele to investigate the effects of this misfolding mutation in vivo. Mutant ABCA4 RNA levels approximated WT ABCA4 RNA levels but, surprisingly, only trace amounts of mutant ABCA4 protein were noted in the retina. RNA sequencing of WT, Abca4(-/-) and Abca4(PV/PV) mice revealed mild gene expression alterations in the retina and RPE. Similar to Abca4(-/-) mice, Abca4(PV/PV) mice showed substantial A2E and lipofuscin accumulation in their RPE cells but no retinal degeneration up to 12 months of age. Thus, rapid degradation of this large misfolded mutant protein in mouse retina caused little detectable photoreceptor degeneration. These findings suggest likely differences in the unfolded protein response between murine and human photoreceptors and support development of therapies directed at increasing this capability in patients.
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Affiliation(s)
- Ning Zhang
- Department of Pharmacology and Cleveland Center for Membrane and Structural Biology and
| | - Yaroslav Tsybovsky
- Department of Pharmacology and Cleveland Center for Membrane and Structural Biology and
| | - Alexander V Kolesnikov
- Department of Ophthalmology and Visual Sciences, Washington University School of Medicine, 660 S. Euclid Avenue, Campus Box 8096, Saint Louis, MO 63110, USA
| | - Malgorzata Rozanowska
- Department of Ophthalmology, School of Medicine, Case Western Reserve University, 10900 Euclid Avenue, Cleveland, OH 44106, USA
| | - Malgorzata Swider
- Department of Ophthalmology, Scheie Eye Institute, Perelman School of Medicine, University of Pennsylvania, Philadelphia, PA, USA
| | - Sharon B Schwartz
- Department of Ophthalmology, Scheie Eye Institute, Perelman School of Medicine, University of Pennsylvania, Philadelphia, PA, USA
| | - Edwin M Stone
- Department of Ophthalmology, University of Iowa Carver College of Medicine, Iowa City, IA, USA Howard Hughes Medical Institute, Iowa City, IA, USA and
| | | | - Akiko Maeda
- Department of Ophthalmology, School of Medicine, Case Western Reserve University, 10900 Euclid Avenue, Cleveland, OH 44106, USA
| | - Vladimir J Kefalov
- Department of Ophthalmology and Visual Sciences, Washington University School of Medicine, 660 S. Euclid Avenue, Campus Box 8096, Saint Louis, MO 63110, USA
| | - Samuel G Jacobson
- Department of Ophthalmology, Scheie Eye Institute, Perelman School of Medicine, University of Pennsylvania, Philadelphia, PA, USA
| | - Artur V Cideciyan
- Department of Ophthalmology, Scheie Eye Institute, Perelman School of Medicine, University of Pennsylvania, Philadelphia, PA, USA
| | - Krzysztof Palczewski
- Department of Pharmacology and Cleveland Center for Membrane and Structural Biology and
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Molday RS. Insights into the Molecular Properties of ABCA4 and Its Role in the Visual Cycle and Stargardt Disease. PROGRESS IN MOLECULAR BIOLOGY AND TRANSLATIONAL SCIENCE 2015; 134:415-31. [DOI: 10.1016/bs.pmbts.2015.06.008] [Citation(s) in RCA: 50] [Impact Index Per Article: 5.6] [Reference Citation Analysis] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 12/26/2022]
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29
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Folding and stability of integral membrane proteins in amphipols. Arch Biochem Biophys 2014; 564:327-43. [PMID: 25449655 DOI: 10.1016/j.abb.2014.10.013] [Citation(s) in RCA: 44] [Impact Index Per Article: 4.4] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/30/2014] [Revised: 10/11/2014] [Accepted: 10/22/2014] [Indexed: 11/23/2022]
Abstract
Amphipols (APols) are a family of amphipathic polymers designed to keep transmembrane proteins (TMPs) soluble in aqueous solutions in the absence of detergent. APols have proven remarkably efficient at (i) stabilizing TMPs, as compared to detergent solutions, and (ii) folding them from a denatured state to a native, functional one. The underlying physical-chemical mechanisms are discussed.
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30
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Palczewski K. Chemistry and biology of the initial steps in vision: the Friedenwald lecture. Invest Ophthalmol Vis Sci 2014; 55:6651-72. [PMID: 25338686 DOI: 10.1167/iovs.14-15502] [Citation(s) in RCA: 38] [Impact Index Per Article: 3.8] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/01/2023] Open
Abstract
Visual transduction is the process in the eye whereby absorption of light in the retina is translated into electrical signals that ultimately reach the brain. The first challenge presented by visual transduction is to understand its molecular basis. We know that maintenance of vision is a continuous process requiring the activation and subsequent restoration of a vitamin A-derived chromophore through a series of chemical reactions catalyzed by enzymes in the retina and retinal pigment epithelium (RPE). Diverse biochemical approaches that identified key proteins and reactions were essential to achieve a mechanistic understanding of these visual processes. The three-dimensional arrangements of these enzymes' polypeptide chains provide invaluable insights into their mechanisms of action. A wealth of information has already been obtained by solving high-resolution crystal structures of both rhodopsin and the retinoid isomerase from pigment RPE (RPE65). Rhodopsin, which is activated by photoisomerization of its 11-cis-retinylidene chromophore, is a prototypical member of a large family of membrane-bound proteins called G protein-coupled receptors (GPCRs). RPE65 is a retinoid isomerase critical for regeneration of the chromophore. Electron microscopy (EM) and atomic force microscopy have provided insights into how certain proteins are assembled to form much larger structures such as rod photoreceptor cell outer segment membranes. A second challenge of visual transduction is to use this knowledge to devise therapeutic approaches that can prevent or reverse conditions leading to blindness. Imaging modalities like optical coherence tomography (OCT) and scanning laser ophthalmoscopy (SLO) applied to appropriate animal models as well as human retinal imaging have been employed to characterize blinding diseases, monitor their progression, and evaluate the success of therapeutic agents. Lately two-photon (2-PO) imaging, together with biochemical assays, are revealing functional aspects of vision at a new molecular level. These multidisciplinary approaches combined with suitable animal models and inbred mutant species can be especially helpful in translating provocative cell and tissue culture findings into therapeutic options for further development in animals and eventually in humans. A host of different approaches and techniques is required for substantial progress in understanding fundamental properties of the visual system.
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Affiliation(s)
- Krzysztof Palczewski
- Department of Pharmacology, Cleveland Center for Membrane and Structural Biology, School of Medicine, Case Western Reserve University, Cleveland, Ohio, United States
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31
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Abstract
Amphipols (APols) are short amphipathic polymers that can substitute for detergents at the transmembrane surface of membrane proteins (MPs) and, thereby, keep them soluble in detergent free aqueous solutions. APol-trapped MPs are, as a rule, more stable biochemically than their detergent-solubilized counterparts. APols have proven useful to produce MPs, most noticeably by assisting their folding from the denatured state obtained after solubilizing MP inclusion bodies in either SDS or urea. They facilitate the handling in aqueous solution of fragile MPs for the purpose of proteomics, structural and functional studies, and therapeutics. Because APols can be chemically labeled or functionalized, and they form very stable complexes with MPs, they can also be used to functionalize those indirectly, which opens onto many novel applications. Following a brief recall of the properties of APols and MP/APol complexes, an update is provided of recent progress in these various fields.
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Affiliation(s)
- Manuela Zoonens
- Laboratoire de Physico-Chimie Moléculaire des Protéines Membranaires, UMR 7099, Institut de Biologie Physico-Chimique (FRC 550), Centre National de la Recherche Scientifique/Université Paris-7, 13, rue Pierre-et-Marie-Curie, 75005 Paris, France
| | - Jean-Luc Popot
- Laboratoire de Physico-Chimie Moléculaire des Protéines Membranaires, UMR 7099, Institut de Biologie Physico-Chimique (FRC 550), Centre National de la Recherche Scientifique/Université Paris-7, 13, rue Pierre-et-Marie-Curie, 75005 Paris, France
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32
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Opačić M, Giusti F, Popot JL, Broos J. Isolation of Escherichia coli mannitol permease, EIImtl, trapped in amphipol A8-35 and fluorescein-labeled A8-35. J Membr Biol 2014; 247:1019-30. [PMID: 24952466 DOI: 10.1007/s00232-014-9691-7] [Citation(s) in RCA: 8] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/28/2014] [Accepted: 05/22/2014] [Indexed: 12/15/2022]
Abstract
Amphipols (APols) are short amphipathic polymers that keep integral membrane proteins water-soluble while stabilizing them as compared to detergent solutions. In the present work, we have carried out functional and structural studies of a membrane transporter that had not been characterized in APol-trapped form yet, namely EII(mtl), a dimeric mannitol permease from the inner membrane of Escherichia coli. A tryptophan-less and dozens of single-tryptophan (Trp) mutants of this transporter are available, making it possible to study the environment of specific locations in the protein. With few exceptions, the single-Trp mutants show a high mannitol-phosphorylation activity when in membranes, but, as variance with wild-type EII(mtl), some of them lose most of their activity upon solubilization by neutral (PEG- or maltoside-based) detergents. Here, we present a protocol to isolate these detergent-sensitive mutants in active form using APol A8-35. Trapping with A8-35 keeps EII(mtl) soluble and functional in the absence of detergent. The specific phosphorylation activity of an APol-trapped Trp-less EII(mtl) mutant was found to be ~3× higher than the activity of the same protein in dodecylmaltoside. The preparations are suitable both for functional and for fluorescence spectroscopy studies. A fluorescein-labeled version of A8-35 has been synthesized and characterized. Exploratory studies were conducted to examine the environment of specific Trp locations in the transmembrane domain of EII(mtl) using Trp fluorescence quenching by water-soluble quenchers and by the fluorescein-labeled APol. This approach has the potential to provide information on the transmembrane topology of MPs.
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Affiliation(s)
- Milena Opačić
- Unité Mixte de Recherche 7099, Centre National de la Recherche Scientifique and Université Paris 7, Institut de Biologie Physico-Chimique, CNRS FRC 550, 13 rue Pierre-et-Marie Curie, 75005, Paris, France
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Huynh KW, Cohen MR, Moiseenkova-Bell VY. Application of amphipols for structure-functional analysis of TRP channels. J Membr Biol 2014; 247:843-51. [PMID: 24894720 DOI: 10.1007/s00232-014-9684-6] [Citation(s) in RCA: 26] [Impact Index Per Article: 2.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/16/2014] [Accepted: 05/13/2014] [Indexed: 11/25/2022]
Abstract
Amphipathic polymers (amphipols), such as A8-35 and SApol, are a new tool for stabilizing integral membrane proteins in detergent-free conditions for structural and functional studies. Transient receptor potential (TRP) ion channels function as tetrameric protein complexes in a diverse range of cellular processes including sensory transduction. Mammalian TRP channels share ~20 % sequence similarity and are categorized into six subfamilies: TRPC (canonical), TRPV (vanilloid), TRPA (ankyrin), TRPM (melastatin), TRPP (polycystin), and TRPML (mucolipin). Due to the inherent difficulties in purifying eukaryotic membrane proteins, structural studies of TRP channels have been limited. Recently, A8-35 was essential in resolving the molecular architecture of the nociceptor TRPA1 and led to the determination of a high-resolution structure of the thermosensitive TRPV1 channel by cryo-EM. Newly developed maltose-neopentyl glycol (MNG) detergents have also proven to be useful in stabilizing TRP channels for structural analysis. In this review, we will discuss the impacts of amphipols and MNG detergents on structural studies of TRP channels by cryo-EM. We will compare how A8-35 and MNG detergents interact with the hydrophobic transmembrane domains of TRP channels. In addition, we will discuss what these cryo-EM studies reveal on the importance of screening different types of surfactants toward determining high-resolution structures of TRP channels.
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Affiliation(s)
- Kevin W Huynh
- Department of Pharmacology, School of Medicine, Case Western Reserve University, Cleveland, OH, 44106, USA
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Strauss AS, Wang D, Stock M, Gretscher RR, Groth M, Boland W, Burse A. Tissue-specific transcript profiling for ABC transporters in the sequestering larvae of the phytophagous leaf beetle Chrysomela populi. PLoS One 2014; 9:e98637. [PMID: 24887102 PMCID: PMC4041752 DOI: 10.1371/journal.pone.0098637] [Citation(s) in RCA: 33] [Impact Index Per Article: 3.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/31/2014] [Accepted: 05/05/2014] [Indexed: 11/18/2022] Open
Abstract
BACKGROUND Insects evolved ingenious adaptations to use extraordinary food sources. Particularly, the diet of herbivores enriched with noxious plant secondary metabolites requires detoxification mechanisms. Sequestration, which involves the uptake, transfer, and concentration of occasionally modified phytochemicals into specialized tissues or hemolymph, is one of the most successful detoxification strategies found in most insect orders. Due to the ability of ATP-binding cassette (ABC) carriers to transport a wide range of molecules including phytochemicals and xenobiotics, it is highly likely that they play a role in this sequestration process. To shed light on the role of ABC proteins in sequestration, we describe an inventory of putative ABC transporters in various tissues in the sequestering juvenile poplar leaf beetle, Chrysomela populi. RESULTS In the transcriptome of C. populi, we predicted 65 ABC transporters. To link the proteins with a possible function, we performed comparative phylogenetic analyses with ABC transporters of other insects and of humans. While tissue-specific profiling of each ABC transporter subfamily suggests that ABCB, C and G influence the plant metabolite absorption in the gut, ABCC with 14 members is the preferred subfamily responsible for the excretion of these metabolites via Malpighian tubules. Moreover, salicin, which is sequestered from poplar plants, is translocated into the defensive glands for further deterrent production. In these glands and among all identified ABC transporters, an exceptionally high transcript level was observed only for Cpabc35 (Cpmrp). RNAi revealed the deficiency of other ABC pumps to compensate the function of CpABC35, demonstrating its key role during sequestration. CONCLUSION We provide the first comprehensive phylogenetic study of the ABC family in a phytophagous beetle species. RNA-seq data from different larval tissues propose the importance of ABC pumps to achieve a homeostasis of plant-derived compounds and offer a basis for future analyses of their physiological function in sequestration processes.
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Affiliation(s)
- Anja S. Strauss
- Max Planck Institute for Chemical Ecology, Beutenberg Campus, Hans-Knoell-Str. 8, D-07745 Jena, Thuringia, Germany
| | - Ding Wang
- Max Planck Institute for Chemical Ecology, Beutenberg Campus, Hans-Knoell-Str. 8, D-07745 Jena, Thuringia, Germany
| | - Magdalena Stock
- Max Planck Institute for Chemical Ecology, Beutenberg Campus, Hans-Knoell-Str. 8, D-07745 Jena, Thuringia, Germany
| | - René R. Gretscher
- Max Planck Institute for Chemical Ecology, Beutenberg Campus, Hans-Knoell-Str. 8, D-07745 Jena, Thuringia, Germany
| | - Marco Groth
- Leibniz Institute for Age Research – Fritz Lipmann Institute, Beutenbergstr. 11, D-07745 Jena, Thuringia, Germany
| | - Wilhelm Boland
- Max Planck Institute for Chemical Ecology, Beutenberg Campus, Hans-Knoell-Str. 8, D-07745 Jena, Thuringia, Germany
| | - Antje Burse
- Max Planck Institute for Chemical Ecology, Beutenberg Campus, Hans-Knoell-Str. 8, D-07745 Jena, Thuringia, Germany
- * E-mail:
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Abstract
AbstractABC transporters comprise a large, diverse, and ubiquitous superfamily of membrane active transporters. Their core architecture is a dimer of dimers, comprising two transmembrane (TM) domains that bind substrate, and two ATP-binding cassettes, which use the cell's energy currency to couple substrate translocation to ATP hydrolysis. Despite the availability of over a dozen resolved structures and a wealth of biochemical and biophysical data, this field is bedeviled by controversy and long-standing mechanistic questions remain unresolved. The prevailing paradigm for the ABC transport mechanism is the Switch Model, in which the ATP-binding cassettes dimerize upon binding two ATP molecules, and thence dissociate upon sequential ATP hydrolysis. This cycle of nucleotide-binding domain (NBD) dimerization and dissociation is coupled to a switch between inward- or outward facing conformations of a single TM channel; this alternating access enables substrate binding on one face of the membrane and its release at the other. Notwithstanding widespread acceptance of the Switch Model, there is substantial evidence that the NBDs do not separate very much, if at all, and thus physical separation of the ATP cassettes observed in crystallographic structures may be an artefact. An alternative Constant Contact Model has been proposed, in which ATP hydrolysis occurs alternately at the two ATP-binding sites, with one of the sites remaining closed and containing occluded nucleotide at all times. In this model, the cassettes remain in contact and the active sites swing open in an alternately seesawing motion. Whilst the concept of NBD association/dissociation in the Switch Model is naturally compatible with a single alternating-access channel, the asymmetric functioning proposed by the Constant Contact model suggests an alternating or reciprocating function in the TMDs. Here, a new model for the function of ABC transporters is proposed in which the sequence of ATP binding, hydrolysis, and product release in each active site is directly coupled to the analogous sequence of substrate binding, translocation and release in one of two functionally separate substrate translocation pathways. Each translocation pathway functions 180° out of phase. A wide and diverse selection of data for both ABC importers and exporters is examined, and the ability of the Switch and Reciprocating Models to explain the data is compared and contrasted. This analysis shows that not only can the Reciprocating Model readily explain the data; it also suggests straightforward explanations for the function of a number of atypical ABC transporters. This study represents the most coherent and complete attempt at an all-encompassing scheme to explain how these important proteins work, one that is consistent with sound biochemical and biophysical evidence.
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Two-photon microscopy reveals early rod photoreceptor cell damage in light-exposed mutant mice. Proc Natl Acad Sci U S A 2014; 111:E1428-37. [PMID: 24706832 DOI: 10.1073/pnas.1317986111] [Citation(s) in RCA: 46] [Impact Index Per Article: 4.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/20/2022] Open
Abstract
Atrophic age-related and juvenile macular degeneration are especially devastating due to lack of an effective cure. Two retinal cell types, photoreceptor cells and the adjacent retinal pigmented epithelium (RPE), reportedly display the earliest pathological changes. Abca4(-/-)Rdh8(-/-) mice, which mimic many features of human retinal degeneration, allowed us to determine the sequence of light-induced events leading to retinal degeneration. Using two-photon microscopy with 3D reconstruction methodology, we observed an initial strong retinoid-derived fluorescence and expansion of Abca4(-/-)Rdh8(-/-) mouse rod cell outer segments accompanied by macrophage infiltration after brief exposure of the retina to bright light. Additionally, light-dependent fluorescent compounds produced in rod outer segments were not transferred to the RPE of mice genetically defective in RPE phagocytosis. Collectively, these findings suggest that for light-induced retinopathies in mice, rod photoreceptors are the primary site of toxic retinoid accumulation and degeneration, followed by secondary changes in the RPE.
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ATP-binding cassette transporter ABCA4 and chemical isomerization protect photoreceptor cells from the toxic accumulation of excess 11-cis-retinal. Proc Natl Acad Sci U S A 2014; 111:5024-9. [PMID: 24707049 DOI: 10.1073/pnas.1400780111] [Citation(s) in RCA: 96] [Impact Index Per Article: 9.6] [Reference Citation Analysis] [Abstract] [Key Words] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022] Open
Abstract
The visual cycle is a series of enzyme-catalyzed reactions which converts all-trans-retinal to 11-cis-retinal for the regeneration of visual pigments in rod and cone photoreceptor cells. Although essential for vision, 11-cis-retinal like all-trans-retinal is highly toxic due to its highly reactive aldehyde group and has to be detoxified by either reduction to retinol or sequestration within retinal-binding proteins. Previous studies have focused on the role of the ATP-binding cassette transporter ABCA4 associated with Stargardt macular degeneration and retinol dehydrogenases (RDH) in the clearance of all-trans-retinal from photoreceptors following photoexcitation. How rod and cone cells prevent the accumulation of 11-cis-retinal in photoreceptor disk membranes in excess of what is required for visual pigment regeneration is not known. Here we show that ABCA4 can transport N-11-cis-retinylidene-phosphatidylethanolamine (PE), the Schiff-base conjugate of 11-cis-retinal and PE, from the lumen to the cytoplasmic leaflet of disk membranes. This transport function together with chemical isomerization to its all-trans isomer and reduction to all-trans-retinol by RDH can prevent the accumulation of excess 11-cis-retinal and its Schiff-base conjugate and the formation of toxic bisretinoid compounds as found in ABCA4-deficient mice and individuals with Stargardt macular degeneration. This segment of the visual cycle in which excess 11-cis-retinal is converted to all-trans-retinol provides a rationale for the unusually high content of PE and its long-chain unsaturated docosahexaenoyl group in photoreceptor membranes and adds insight into the molecular mechanisms responsible for Stargardt macular degeneration.
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Tsybovsky Y, Palczewski K. Expression, purification and structural properties of ABC transporter ABCA4 and its individual domains. Protein Expr Purif 2014; 97:50-60. [PMID: 24583180 DOI: 10.1016/j.pep.2014.02.010] [Citation(s) in RCA: 10] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/29/2014] [Revised: 02/18/2014] [Accepted: 02/19/2014] [Indexed: 12/13/2022]
Abstract
ABCA4 is a member of the A subfamily of ATP-binding cassette transporters that consists of large integral membrane proteins implicated in inherited human diseases. ABCA4 assists in the clearance of N-retinylidene-phosphatidylethanolamine, a potentially toxic by-product of the visual cycle formed in photoreceptor cells during light perception. Structural and functional studies of this protein have been hindered by its large size, membrane association, and domain complexity. Although mammalian, insect and bacterial systems have been used for expression of ABCA4 and its individual domains, the structural relevance of resulting proteins to the native transporter has yet to be established. We produced soluble domains of ABCA4 in Escherichia coli and Saccharomyces cerevisiae and the full-length transporter in HEK293 cells. Electron microscopy and size exclusion chromatography were used to assess the conformational homogeneity and structure of these proteins. We found that isolated ABCA4 domains formed large, heterogeneous oligomers cross-linked with non-specific disulphide bonds. Incomplete folding of cytoplasmic domain 2 was proposed based on fluorescence spectroscopy results. In contrast, full-length human ABCA4 produced in mammalian cells was found structurally equivalent to the native protein obtained from bovine photoreceptors. These findings offer recombinantly expressed full-length ABCA4 as an appropriate object for future detailed structural and functional characterization.
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Affiliation(s)
- Yaroslav Tsybovsky
- Department of Pharmacology, School of Medicine, Case Western Reserve University, 10900 Euclid Ave, Cleveland, OH 44106, USA
| | - Krzysztof Palczewski
- Department of Pharmacology, School of Medicine, Case Western Reserve University, 10900 Euclid Ave, Cleveland, OH 44106, USA.
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Affiliation(s)
| | | | - Krzysztof Palczewski
- Department of Pharmacology, School of Medicine, Case
Western Reserve University, 2109 Adelbert Road, Cleveland, Ohio 44106-4965,
United States
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Orban T, Jastrzebska B, Palczewski K. Structural approaches to understanding retinal proteins needed for vision. Curr Opin Cell Biol 2013; 27:32-43. [PMID: 24680428 DOI: 10.1016/j.ceb.2013.11.001] [Citation(s) in RCA: 10] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/29/2013] [Revised: 10/31/2013] [Accepted: 11/01/2013] [Indexed: 12/21/2022]
Abstract
The past decade has witnessed an impressive expansion of our knowledge of retinal photoreceptor signal transduction and the regulation of the visual cycle required for normal eyesight. Progress in human genetics and next generation sequencing technologies have revealed the complexity behind many inherited retinal diseases. Structural studies have markedly increased our understanding of the visual process. Moreover, technical innovations and improved methodologies in proteomics, macromolecular crystallization and high resolution imaging at different levels set the scene for even greater advances. Pharmacology combined with structural biology of membrane proteins holds great promise for developing innovative accessible therapies for millions robbed of their sight or progressing toward blindness.
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Affiliation(s)
- Tivadar Orban
- Department of Pharmacology, Case Western Reserve University, Cleveland, OH 44106, USA
| | - Beata Jastrzebska
- Department of Pharmacology, Case Western Reserve University, Cleveland, OH 44106, USA
| | - Krzysztof Palczewski
- Department of Pharmacology, Case Western Reserve University, Cleveland, OH 44106, USA.
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Kevany BM, Tsybovsky Y, Campuzano IDG, Schnier PD, Engel A, Palczewski K. Structural and functional analysis of the native peripherin-ROM1 complex isolated from photoreceptor cells. J Biol Chem 2013; 288:36272-84. [PMID: 24196967 DOI: 10.1074/jbc.m113.520700] [Citation(s) in RCA: 34] [Impact Index Per Article: 3.1] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/19/2023] Open
Abstract
Peripherin and its homologue ROM1 are retina-specific members of the tetraspanin family of integral membrane proteins required for morphogenesis and maintenance of photoreceptor outer segments, regions that collect light stimuli. Over 100 pathogenic mutations in peripherin cause inherited rod- and cone-related dystrophies in humans. Peripherin and ROM1 interact in vivo and are predicted to form a core heterotetrameric complex capable of creating higher order oligomers. However, structural analysis of tetraspanin proteins has been hampered by their resistance to crystallization. Here we present a simplified methodology for high yield purification of peripherin-ROM1 from bovine retinas that permitted its biochemical and biophysical characterization. Using size exclusion chromatography and blue native gel electrophoresis, we confirmed that the core native peripherin-ROM1 complex exists as a tetramer. Peripherin, but not ROM1, is glycosylated and we examined the glycosylation site and glycan composition of ROM1 by liquid chromatographic tandem mass spectrometry. Mass spectrometry was used to analyze the native complex in detergent micelles, demonstrating its tetrameric state. Our electron microscopy-generated structure solved to 18 Å displayed the tetramer as an elongated structure with an apparent 2-fold symmetry. Finally, we demonstrated that peripherin-ROM1 tetramers induce membrane curvature when reconstituted in lipid vesicles. These results provide critical insights into this key retinal component with a poorly defined function.
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Affiliation(s)
- Brian M Kevany
- From the Department of Pharmacology, School of Medicine, Case Western Reserve University, Cleveland, Ohio 44106-4965 and
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Quazi F, Molday RS. Differential phospholipid substrates and directional transport by ATP-binding cassette proteins ABCA1, ABCA7, and ABCA4 and disease-causing mutants. J Biol Chem 2013; 288:34414-26. [PMID: 24097981 DOI: 10.1074/jbc.m113.508812] [Citation(s) in RCA: 156] [Impact Index Per Article: 14.2] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/25/2022] Open
Abstract
ABCA1, ABCA7, and ABCA4 are members of the ABCA subfamily of ATP-binding cassette transporters that share extensive sequence and structural similarity. Mutations in ABCA1 cause Tangier disease characterized by defective cholesterol homeostasis and high density lipoprotein (HDL) deficiency. Mutations in ABCA4 are responsible for Stargardt disease, a degenerative disorder associated with severe loss in central vision. Although cell-based studies have implicated ABCA proteins in lipid transport, the substrates and direction of transport have not been firmly established. We have purified and reconstituted ABCA1, ABCA7, and ABCA4 into liposomes for fluorescent-lipid transport studies. ABCA1 actively exported or flipped phosphatidylcholine, phosphatidylserine, and sphingomyelin from the cytoplasmic to the exocytoplasmic leaflet of membranes, whereas ABCA7 preferentially exported phosphatidylserine. In contrast, ABCA4 transported phosphatidylethanolamine in the reverse direction. The same phospholipids stimulated the ATPase activity of these ABCA transporters. The transport and ATPase activities of ABCA1 and ABCA4 were reduced by 25% in the presence of 20% cholesterol. Nine ABCA1 Tangier mutants and the corresponding ABCA4 Stargardt mutants showed significantly reduced phospholipid transport activity and subcellular mislocalization. These studies provide the first direct evidence for ABCA1 and ABCA7 functioning as phospholipid transporters and suggest that this activity is an essential step in the loading of apoA-1 with phospholipids for HDL formation.
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Affiliation(s)
- Faraz Quazi
- From the Department of Biochemistry and Molecular Biology, Centre for Macular Research, University of British Columbia, Vancouver, British Columbia V6T 1Z3, Canada
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Pollock NL, McDevitt CA, Collins R, Niesten PHM, Prince S, Kerr ID, Ford RC, Callaghan R. Improving the stability and function of purified ABCB1 and ABCA4: the influence of membrane lipids. BIOCHIMICA ET BIOPHYSICA ACTA-BIOMEMBRANES 2013; 1838:134-47. [PMID: 24036079 DOI: 10.1016/j.bbamem.2013.09.001] [Citation(s) in RCA: 25] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Received: 05/06/2013] [Revised: 08/27/2013] [Accepted: 09/03/2013] [Indexed: 12/17/2022]
Abstract
ATP Binding Cassette (ABC) transporters play prominent roles in numerous cellular processes and many have been implicated in human diseases. Unfortunately, detailed mechanistic information on the majority of ABC transporters has not yet been elucidated. The slow rate of progress of molecular and high resolution structural studies may be attributed to the difficulty in the investigation of integral membrane proteins. These difficulties include the expression of functional, non-aggregated protein in heterologous systems. Furthermore, the extraction of membrane proteins from source material remains a major bottle-neck in the process since there are relatively few guidelines for selection of an appropriate detergent to achieve optimal extraction. Whilst affinity tag strategies have simplified the purification of membrane proteins; many challenges remain. For example, the chromatographic process and associated steps can rapidly lead to functional inactivation, random aggregation, or even precipitation of the target protein. Furthermore, optimisation of high yield and purity, does not guarantee successful structure determination. Based on this series of potential issues, any investigation into structure-function of membrane proteins requires a systematic evaluation of preparation quality. In particular, the evaluation should focus on function, homogeneity and mono-dispersity. The present investigation provides a detailed assessment of the quality of purified ATP Binding Cassette (ABC) transporters; namely ABCB1 (P-gp) and ABCA4 (ABCR). A number of suggestions are provided to facilitate the production of functional, homogeneous and mono-disperse preparations using the insect cell expression system. Finally, the ABCA4 samples have been used to provide structural insights into this essential photo-receptor cell protein.
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Affiliation(s)
- Naomi L Pollock
- Nuffield Department of Clinical Laboratory Sciences, John Radcliffe Hospital, University of Oxford, Oxford OX3 9DU, UK
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