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Duchateau L, Wawrzyniak N, Sleegers K. The ABC's of Alzheimer risk gene ABCA7. Alzheimers Dement 2024; 20:3629-3648. [PMID: 38556850 PMCID: PMC11095487 DOI: 10.1002/alz.13805] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/03/2024] [Revised: 02/27/2024] [Accepted: 02/29/2024] [Indexed: 04/02/2024]
Abstract
Alzheimer's disease (AD) is a growing problem worldwide. Since ABCA7's identification as a risk gene, it has been extensively researched for its role in the disease. We review its recently characterized structure and what the mechanistic insights teach us about its function. We furthermore provide an overview of identified ABCA7 mutations, their presence in different ancestries and protein domains and how they might cause AD. For ABCA7 PTC variants and a VNTR expansion, haploinsufficiency is proposed as the most likely mode-of-action, although splice events could further influence disease risk. Overall, the need to better understand expression of canonical ABCA7 and its isoforms in disease is indicated. Finally, ABCA7's potential functions in lipid metabolism, phagocytosis, amyloid deposition, and the interplay between these three, is described. To conclude, in this review, we provide a comprehensive overview and discussion about the current knowledge on ABCA7 in AD, and what research questions remain. HIGHLIGHTS: Alzheimer's risk-increasing variants in ABCA7 can be found in up to 7% of AD patients. We review the recently characterized protein structure of ABCA7. We present latest insights in genetics, expression patterns, and functions of ABCA7.
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Affiliation(s)
- Lena Duchateau
- Complex Genetics of Alzheimer's Disease group, VIB‐UAntwerp Center for Molecular NeurologyWilrijkAntwerpBelgium
- Department of Biomedical SciencesUniversity of AntwerpWilrijkAntwerpBelgium
| | - Nicole Wawrzyniak
- Complex Genetics of Alzheimer's Disease group, VIB‐UAntwerp Center for Molecular NeurologyWilrijkAntwerpBelgium
- Chávez‐Gutiérrez Lab, VIB‐KU Leuven Center for Brain and Disease Research, VIBLeuvenBelgium
| | - Kristel Sleegers
- Complex Genetics of Alzheimer's Disease group, VIB‐UAntwerp Center for Molecular NeurologyWilrijkAntwerpBelgium
- Department of Biomedical SciencesUniversity of AntwerpWilrijkAntwerpBelgium
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2
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Cevik S, Wangtiraumnuay N, Van Schelvergem K, Tsukikawa M, Capasso J, Biswas SB, Bodt B, Levin AV, Biswas-Fiss E. Protein modeling and in silico analysis to assess pathogenicity of ABCA4 variants in patients with inherited retinal disease. Mol Vis 2023; 29:217-233. [PMID: 38222458 PMCID: PMC10784225] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/18/2022] [Accepted: 10/23/2023] [Indexed: 01/16/2024] Open
Abstract
Purpose The retina-specific ABCA transporter, ABCA4, plays an essential role in translocating retinoids required by the visual cycle. ABCA4 genetic variants are known to cause a wide range of inherited retinal disorders, including Stargardt disease and cone-rod dystrophy. More than 1,400 ABCA4 missense variants have been identified; however, more than half of these remain variants of uncertain significance (VUS). The purpose of this study was to employ a predictive strategy to assess the pathogenicity of ABCA4 variants in inherited retinal diseases using protein modeling and computational approaches. Methods We studied 13 clinically well-defined patients with ABCA4 retinopathies and identified the presence of 10 missense variants, including one novel variant in the ABCA4 gene, by next-generation sequencing (NGS). All variants were structurally analyzed using AlphaFold2 models and existing experimental structures of human ABCA4 protein. The results of these analyses were compared with patient clinical presentations to test the effectiveness of the methods employed in predicting variant pathogenicity. Results We conducted a phenotype-genotype comparison of 13 genetically and phenotypically well-defined retinal disease patients. The in silico protein structure analyses we employed successfully detected the deleterious effect of missense variants found in this affected patient cohort. Our study provides American College of Medical Genetics and Genomics (ACMG)-defined supporting evidence of the pathogenicity of nine missense ABCA4 variants, aligning with the observed clinical phenotypes in this cohort. Conclusions In this report, we describe a systematic approach to predicting the pathogenicity of ABCA4 variants by means of three-dimensional (3D) protein modeling and in silico structure analysis. Our results demonstrate concordance between disease severity and structural changes in protein models induced by genetic variations. Furthermore, the present study suggests that in silico protein structure analysis can be used as a predictor of pathogenicity and may facilitate the assessment of genetic VUS.
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Affiliation(s)
- Senem Cevik
- Department of Medical and Molecular Sciences, University of Delaware College of Health Sciences, Newark, DE
| | - Nutsuchar Wangtiraumnuay
- Department of Ophthalmology, Queen Sirikit National Institute of Child Health, Bangkok, Thailand
| | | | - Mai Tsukikawa
- Department of Ophthalmology, Duke University, Durham, NC
| | - Jenina Capasso
- Departments of Ophthalmology and Pediatrics, Flaum Eye Institute and Golisano Children's Hospital, University of Rochester, Rochester, NY
| | - Subhasis B Biswas
- Department of Medical and Molecular Sciences, University of Delaware College of Health Sciences, Newark, DE
| | - Barry Bodt
- College of Health Sciences Biostatistics Core Facility, University of Delaware, Newark, DE
| | - Alex V Levin
- Departments of Ophthalmology and Pediatrics, Flaum Eye Institute and Golisano Children's Hospital, University of Rochester, Rochester, NY
| | - Esther Biswas-Fiss
- Department of Medical and Molecular Sciences, University of Delaware College of Health Sciences, Newark, DE
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3
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Segrest JP, Davidson WS, Heinecke JW. Phospholipid transport by ABCA1: the extracellular translocase or alternating access model? Curr Opin Lipidol 2023; 34:208-213. [PMID: 37548415 PMCID: PMC10528508 DOI: 10.1097/mol.0000000000000895] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 08/08/2023]
Abstract
PURPOSE OF REVIEW ATP-binding cassette transporter A1 (ABCA1) plays a key role in high-density lipoprotein (HDL) biogenesis and cholesterol export from artery wall cells. Recent evidence challenges the generally accepted model for lipid transport by ABCA1, termed the alternating access mechanism, which proposes that phospholipid moves from the inner leaflet to the outer leaflet of the plasma membrane. RECENT FINDINGS In contrast to the standard model, our computer simulations of ABCA1 indicate that ABCA1 extracts phospholipid from the plasma membrane's outer leaflet. The lipid then diffuses into the interior of ABCA1 to contact a structure termed the 'gateway'. A conformational change opens the gateway and forces the lipid through a ring-shaped domain, the 'annulus orifice', into the base of an elongated hydrophobic tunnel in the transporter's extracellular domain. Engineered mutations in the gateway and annulus strongly inhibited lipid export by ABCA1 without affecting cell-surface expression levels of the transporter, strongly supporting the proposed model. SUMMARY Our demonstration that ABCA1 extracts lipid from the outer face of the plasma membrane and forces it into an elongated hydrophobic tunnel contrasts with the alternating access model, which flops phospholipid from the membrane's inner leaflet to its outer leaflet. These results suggest that ABCA1 is a phospholipid translocase that transports lipids by a mechanism distinct from that of other ABC transporters.
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Affiliation(s)
- Jere P. Segrest
- Department of Medicine, Vanderbilt University Medical Center, Nashville, TN 37232
| | - W. Sean Davidson
- Department of Pathology and Laboratory Medicine, University of Cincinnati, Cincinnati OH 45237
| | - Jay W. Heinecke
- Department of Medicine, University of Washington, Seattle, WA 98109
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4
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Borovsky D, Rougé P. Heliothis virescens chymotrypsin is translationally controlled by AeaTMOF binding ABC putative receptor. ARCHIVES OF INSECT BIOCHEMISTRY AND PHYSIOLOGY 2023; 114:1-24. [PMID: 37526204 DOI: 10.1002/arch.22042] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Received: 05/15/2023] [Revised: 07/19/2023] [Accepted: 07/22/2023] [Indexed: 08/02/2023]
Abstract
Heliothis virescens larval chymotrypsin (GenBank accession number AF43709) was cloned, sequenced and its three dimensional (3D) conformation modeled. The enzyme's transcript was first detected 6 days after larval emergence and the transcript level was shown to fall between larval ecdysis periods. Comparisons between the activities of larval gut chymotrypsin and trypsin shows that chymotrypsin activity is only 16% of the total trypsin activity and the pH optimum of the larval chymotrypsin is between pH 9-10, however the enzyme also exhibited a broad activity between pH 4-6. Injections of AeaTMOF and several shorter analogues into 3rd instar larvae followed by Northern blot analyses showed that although the chymotrypsins activities were inhibited by 60%-80% the transcript level of the sequenced chymotrypsin was not reduced and was similar to controls in which the chymotrypsin activity was not inhibited, indicating that AeaTMOF and its analogues exert a translational control. Based on these observations a putative AeaTMOF receptor (ABCC4) homologous to the Ae. aegypti ABC receptor sequence was found in the H. virescens genome. 3D molecular modeling and docking of the AeaTMOF and several of its analogues to the ABCC4 receptor showed that it can bind AeaTMOF and its analogues as was shown before for the Ae. aegypti receptor.
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Affiliation(s)
- Dov Borovsky
- Department of Biochemistry and Molecular Genetics, University of Colorado Anschutz School of Medicine, Aurora, Colorado, USA
| | - Pierre Rougé
- UMR 152 Pharma-Dev, Faculté des Sciences Pharmaceutiques, Institut de Recherche et Développement, Université Toulouse 3, Toulouse, France
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5
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Ying W, Liao L, Wei H, Gao Y, Liu X, Sun L. Structural basis for abscisic acid efflux mediated by ABCG25 in Arabidopsis thaliana. NATURE PLANTS 2023; 9:1697-1708. [PMID: 37666962 PMCID: PMC10581904 DOI: 10.1038/s41477-023-01510-0] [Citation(s) in RCA: 1] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Received: 02/13/2023] [Accepted: 08/02/2023] [Indexed: 09/06/2023]
Abstract
Abscisic acid (ABA) is a phytohormone essential to the regulation of numerous aspects of plant growth and development. The cellular level of ABA is critical to its signalling and is determined by its rate of biosynthesis, catabolism and the rates of ABA transport. ABCG25 in Arabidopsis thaliana has been identified to be an ABA exporter and play roles in regulating stomatal closure and seed germination. However, its ABA transport mechanism remains unknown. Here we report the structures of ABCG25 under different states using cryo-electron microscopy single particle analysis: the apo state and ABA-bound state of the wild-type ABCG25 and the ATP-bound state of the ATPase catalytic mutant. ABCG25 forms a homodimer. ABA binds to a cone-shaped, cytosolic-facing cavity formed in the middle of the transmembrane domains. Key residues in ABA binding are identified and verified by a cell-based ABA transport assay. ATP binding leads to closing of the nucleotide-binding domains of opposing monomers and conformational transitions of the transmembrane domains. Together, these results provide insights into the substrate recognition and transport mechanisms of ABCG25 in Arabidopsis, and facilitate our understanding of the ABA transport and signalling pathway in plants.
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Affiliation(s)
- Wei Ying
- The First Affiliated Hospital of USTC, MOE Key Laboratory for Membraneless Organelles and Cellular Dynamics, Hefei National Research Center for Interdisciplinary Sciences at the Microscale, Division of Life Sciences and Medicine, University of Science and Technology of China, Hefei, China
| | - Lianghuan Liao
- The First Affiliated Hospital of USTC, MOE Key Laboratory for Membraneless Organelles and Cellular Dynamics, Hefei National Research Center for Interdisciplinary Sciences at the Microscale, Division of Life Sciences and Medicine, University of Science and Technology of China, Hefei, China
| | - Hong Wei
- The First Affiliated Hospital of USTC, MOE Key Laboratory for Membraneless Organelles and Cellular Dynamics, Hefei National Research Center for Interdisciplinary Sciences at the Microscale, Division of Life Sciences and Medicine, University of Science and Technology of China, Hefei, China
| | - Yongxiang Gao
- The First Affiliated Hospital of USTC, MOE Key Laboratory for Membraneless Organelles and Cellular Dynamics, Hefei National Research Center for Interdisciplinary Sciences at the Microscale, Division of Life Sciences and Medicine, University of Science and Technology of China, Hefei, China
| | - Xin Liu
- The First Affiliated Hospital of USTC, MOE Key Laboratory for Membraneless Organelles and Cellular Dynamics, Hefei National Research Center for Interdisciplinary Sciences at the Microscale, Division of Life Sciences and Medicine, University of Science and Technology of China, Hefei, China.
- Biomedical Sciences and Health Laboratory of Anhui Province, University of Science and Technology of China, Hefei, China.
| | - Linfeng Sun
- The First Affiliated Hospital of USTC, MOE Key Laboratory for Membraneless Organelles and Cellular Dynamics, Hefei National Research Center for Interdisciplinary Sciences at the Microscale, Division of Life Sciences and Medicine, University of Science and Technology of China, Hefei, China.
- Biomedical Sciences and Health Laboratory of Anhui Province, University of Science and Technology of China, Hefei, China.
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Kaltak M, Blanco-Garavito R, Molday LL, Dhaenens CM, Souied EE, Platenburg G, Swildens J, Molday RS, Cremers FPM. Stargardt disease-associated in-frame ABCA4 exon 17 skipping results in significant ABCA4 function. J Transl Med 2023; 21:546. [PMID: 37587475 PMCID: PMC10428568 DOI: 10.1186/s12967-023-04406-x] [Citation(s) in RCA: 1] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/05/2023] [Accepted: 08/01/2023] [Indexed: 08/18/2023] Open
Abstract
BACKGROUND ABCA4, the gene implicated in Stargardt disease (STGD1), contains 50 exons, of which 17 contain multiples of three nucleotides. The impact of in-frame exon skipping is yet to be determined. Antisense oligonucleotides (AONs) have been investigated in Usher syndrome-associated genes to induce skipping of in-frame exons carrying severe variants and mitigate their disease-linked effect. Upon the identification of a STGD1 proband carrying a novel exon 17 canonical splice site variant, the activity of ABCA4 lacking 22 amino acids encoded by exon 17 was examined, followed by design of AONs able to induce exon 17 skipping. METHODS A STGD1 proband was compound heterozygous for the splice variant c.2653+1G>A, that was predicted to result in in-frame skipping of exon 17, and a null variant [c.735T>G, p.(Tyr245*)]. Clinical characteristics of this proband were studied using multi-modal imaging and complete ophthalmological examination. The aberrant splicing of c.2653+1G>A was investigated in vitro in HEK293T cells with wild-type and mutant midigenes. The residual activity of the mutant ABCA4 protein lacking Asp864-Gly885 encoded by exon 17 was analyzed with all-trans-retinal-activated ATPase activity assay, along with its subcellular localization. To induce exon 17 skipping, the effect of 40 AONs was examined in vitro in WT WERI-Rb-1 cells and 3D human retinal organoids. RESULTS Late onset STGD1 in the proband suggests that c.2653+1G>A does not have a fully deleterious effect. The in vitro splice assay confirmed that this variant leads to ABCA4 transcripts without exon 17. ABCA4 Asp864_Gly863del was stable and retained 58% all-trans-retinal-activated ATPase activity compared to WT ABCA4. This sequence is located in an unstructured linker region between transmembrane domain 6 and nucleotide-binding domain-1 of ABCA4. AONs were designed to possibly reduce pathogenicity of severe variants harbored in exon 17. The best AON achieved 59% of exon 17 skipping in retinal organoids. CONCLUSIONS Exon 17 deletion in ABCA4 does not result in the absence of protein activity and does not cause a severe STGD1 phenotype when in trans with a null allele. By applying AONs, the effect of severe variants in exon 17 can potentially be ameliorated by exon skipping, thus generating partial ABCA4 activity in STGD1 patients.
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Affiliation(s)
- Melita Kaltak
- Department of Human Genetics, Radboud University Medical Center, Nijmegen, The Netherlands
- ProQR Therapeutics, Leiden, The Netherlands
| | - Rocio Blanco-Garavito
- Department of Ophthalmology, Intercommunal Hospital Center and Henri Mondor Hospital, Paris-Est Créteil University, Creteil, France
| | - Laurie L Molday
- Department of Biochemistry and Molecular Biology, Department of Ophthalmology and Visual Sciences, Centre for Macular Research, University of British Columbia, Vancouver, BC, Canada
| | - Claire-Marie Dhaenens
- University of Lille, Inserm, CHU Lille, U1172-LilNCog-Lille Neuroscience & Cognition, Lille, France
| | - Eric E Souied
- Department of Ophthalmology, Intercommunal Hospital Center and Henri Mondor Hospital, Paris-Est Créteil University, Creteil, France
| | | | | | - Robert S Molday
- Department of Biochemistry and Molecular Biology, Department of Ophthalmology and Visual Sciences, Centre for Macular Research, University of British Columbia, Vancouver, BC, Canada
| | - Frans P M Cremers
- Department of Human Genetics, Radboud University Medical Center, Nijmegen, The Netherlands.
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7
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Abstract
ABC transporters are essential for cellular physiology. Humans have 48 ABC genes organized into seven distinct families. Of these genes, 44 (in five distinct families) encode for membrane transporters, of which several are involved in drug resistance and disease pathways resulting from transporter dysfunction. Over the last decade, advances in structural biology have vastly expanded our mechanistic understanding of human ABC transporter function, revealing details of their molecular arrangement, regulation, and interactions, facilitated in large part by advances in cryo-EM that have rendered hitherto inaccessible targets amenable to high-resolution structural analysis. As a result, experimentally determined structures of multiple members of each of the five families of ABC transporters in humans are now available. Here we review this recent progress, highlighting the physiological relevance of human ABC transporters and mechanistic insights gleaned from their direct structure determination. We also discuss the impact and limitations of model systems and structure prediction methods in understanding human ABC transporters and discuss current challenges and future research directions.
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Affiliation(s)
- Amer Alam
- The Hormel Institute, University of Minnesota, Austin, Minnesota, USA
| | - Kaspar P Locher
- Institute of Molecular Biology and Biophysics, ETH Zurich, Switzerland;
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8
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Prokai L, Zaman K, Prokai-Tatrai K. Mass spectrometry-based retina proteomics. MASS SPECTROMETRY REVIEWS 2023; 42:1032-1062. [PMID: 35670041 PMCID: PMC9730434 DOI: 10.1002/mas.21786] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Received: 02/28/2022] [Revised: 04/28/2022] [Accepted: 04/29/2022] [Indexed: 06/15/2023]
Abstract
A subfield of neuroproteomics, retina proteomics has experienced a transformative growth since its inception due to methodological advances in enabling chemical, biochemical, and molecular biology techniques. This review focuses on mass spectrometry's contributions to facilitate mammalian and avian retina proteomics to catalog and quantify retinal protein expressions, determine their posttranslational modifications, as well as its applications to study the proteome of the retina in the context of biology, health and diseases, and therapy developments.
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Affiliation(s)
- Laszlo Prokai
- Department of Pharmacology and Neuroscience, University of North Texas Health Science Center, Fort Worth, Texas, USA
| | - Khadiza Zaman
- Department of Pharmacology and Neuroscience, University of North Texas Health Science Center, Fort Worth, Texas, USA
| | - Katalin Prokai-Tatrai
- Department of Pharmacology and Neuroscience, University of North Texas Health Science Center, Fort Worth, Texas, USA
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9
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Cevik S, Biswas SB, Biswas-Fiss EE. Structural and Pathogenic Impacts of ABCA4 Variants in Retinal Degenerations-An In-Silico Study. Int J Mol Sci 2023; 24:ijms24087280. [PMID: 37108442 PMCID: PMC10138569 DOI: 10.3390/ijms24087280] [Citation(s) in RCA: 2] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/24/2023] [Revised: 04/11/2023] [Accepted: 04/12/2023] [Indexed: 04/29/2023] Open
Abstract
The retina-specific ATP-binding cassette transporter protein ABCA4 is responsible for properly continuing the visual cycle by removing toxic retinoid byproducts of phototransduction. Functional impairment caused by ABCA4 sequence variations is the leading cause of autosomal recessive inherited retinal disorders, including Stargardt disease, retinitis pigmentosa, and cone-rod dystrophy. To date, more than 3000 ABCA4 genetic variants have been identified, approximately 40 percent of which have not been able to be classified for pathogenicity assessments. This study examined 30 missense ABCA4 variants using AlphaFold2 protein modeling and computational structure analysis for pathogenicity prediction. All variants classified as pathogenic (n = 10) were found to have deleterious structural consequences. Eight of the ten benign variants were structurally neutral, while the remaining two resulted in mild structural changes. This study's results provided multiple lines of computational pathogenicity evidence for eight ABCA4 variants of uncertain clinical significance. Overall, in silico analyses of ABCA4 can provide a valuable tool for understanding the molecular mechanisms of retinal degeneration and their pathogenic impact.
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Affiliation(s)
- Senem Cevik
- Department of Medical and Molecular Sciences, College of Health Sciences, University of Delaware, 16 West Main Street, Suite 302 WHL, Newark, DE 19716, USA
- Ammon Pinizzotto Biopharmaceutical Innovation Center, 590 Avenue 1743, Newark, DE 19713, USA
| | - Subhasis B Biswas
- Department of Medical and Molecular Sciences, College of Health Sciences, University of Delaware, 16 West Main Street, Suite 302 WHL, Newark, DE 19716, USA
- Ammon Pinizzotto Biopharmaceutical Innovation Center, 590 Avenue 1743, Newark, DE 19713, USA
| | - Esther E Biswas-Fiss
- Department of Medical and Molecular Sciences, College of Health Sciences, University of Delaware, 16 West Main Street, Suite 302 WHL, Newark, DE 19716, USA
- Ammon Pinizzotto Biopharmaceutical Innovation Center, 590 Avenue 1743, Newark, DE 19713, USA
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10
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Choi HY, Choi S, Iatan I, Ruel I, Genest J. Biomedical Advances in ABCA1 Transporter: From Bench to Bedside. Biomedicines 2023; 11:biomedicines11020561. [PMID: 36831097 PMCID: PMC9953649 DOI: 10.3390/biomedicines11020561] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/15/2023] [Revised: 02/07/2023] [Accepted: 02/13/2023] [Indexed: 02/17/2023] Open
Abstract
ATP-binding cassette transporter A1 (ABCA1) has been identified as the molecular defect in Tangier disease. It is biochemically characterized by absence of high-density lipoprotein cholesterol (HDL-C) in the circulation, resulting in the accumulation of cholesterol in lymphoid tissues. Accumulation of cholesterol in arteries is an underlying cause of atherosclerosis, and HDL-C levels are inversely associated with the presence of atherosclerotic cardiovascular disease (ASCVD). ABCA1 increases HDL-C levels by driving the generation of new HDL particles in cells, and cellular cholesterol is removed in the process of HDL generation. Therefore, pharmacological strategies that promote the HDL biogenic process by increasing ABCA1 expression and activity have been intensively studied to reduce ASCVD. Many ABCA1-upregulating agents have been developed, and some have shown promising effects in pre-clinical studies, but no clinical trials have met success yet. ABCA1 has long been an attractive drug target, but the failed clinical trials have indicated the difficulty of therapeutic upregulation of ABCA1, as well as driving us to: improve our understanding of the ABCA1 regulatory system; to develop more specific and sophisticated strategies to upregulate ABCA1 expression; and to search for novel druggable targets in the ABCA1-dependent HDL biogenic process. In this review, we discuss the beginning, recent advances, challenges and future directions in ABCA1 research aimed at developing ABCA1-directed therapies for ASCVD.
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Affiliation(s)
- Hong Y. Choi
- Research Institute of the McGill University Health Centre, Montréal, QC H4A 3J1, Canada
- Correspondence: ; Tel.: +1-514-934-1934 (ext. 35796)
| | - Senna Choi
- Research Institute of the McGill University Health Centre, Montréal, QC H4A 3J1, Canada
| | - Iulia Iatan
- Centre for Heart Lung Innovation, Department of Medicine, St. Paul’s Hospital, University of British Columbia, Vancouver, BC V6Z 1Y6, Canada
| | - Isabelle Ruel
- Research Institute of the McGill University Health Centre, Montréal, QC H4A 3J1, Canada
| | - Jacques Genest
- Research Institute of the McGill University Health Centre, Montréal, QC H4A 3J1, Canada
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11
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Le LTM, Thompson JR, Dehghani‐Ghahnaviyeh S, Pant S, Dang PX, French JB, Kanikeyo T, Tajkhorshid E, Alam A. Cryo-EM structures of human ABCA7 provide insights into its phospholipid translocation mechanisms. EMBO J 2023; 42:e111065. [PMID: 36484366 PMCID: PMC9890230 DOI: 10.15252/embj.2022111065] [Citation(s) in RCA: 6] [Impact Index Per Article: 6.0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/02/2022] [Revised: 11/07/2022] [Accepted: 11/10/2022] [Indexed: 12/13/2022] Open
Abstract
Phospholipid extrusion by ABC subfamily A (ABCA) exporters is central to cellular physiology, although the specifics of the underlying substrate interactions and transport mechanisms remain poorly resolved at the molecular level. Here we report cryo-EM structures of lipid-embedded human ABCA7 in an open state and in a nucleotide-bound, closed state at resolutions between 3.6 and 4.0 Å. The former reveals an ordered patch of bilayer lipids traversing the transmembrane domain (TMD), while the latter reveals a lipid-free, closed TMD with a small extracellular opening. These structures offer a structural framework for both substrate entry and exit from the ABCA7 TMD and highlight conserved rigid-body motions that underlie the associated conformational transitions. Combined with functional analysis and molecular dynamics (MD) simulations, our data also shed light on lipid partitioning into the ABCA7 TMD and localized membrane perturbations that underlie ABCA7 function and have broader implications for other ABCA family transporters.
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Affiliation(s)
- Le Thi My Le
- The Hormel InstituteUniversity of MinnesotaAustinMNUSA
| | | | - Sepehr Dehghani‐Ghahnaviyeh
- Theoretical and Computational Biophysics Group, NIH Center for Macromolecular Modeling and Bioinformatics, Beckman Institute for Advanced Science and Technology, Department of Biochemistry, and Center for Biophysics and Quantitative BiologyUniversity of Illinois at Urbana‐ChampaignUrbanaILUSA
| | - Shashank Pant
- Theoretical and Computational Biophysics Group, NIH Center for Macromolecular Modeling and Bioinformatics, Beckman Institute for Advanced Science and Technology, Department of Biochemistry, and Center for Biophysics and Quantitative BiologyUniversity of Illinois at Urbana‐ChampaignUrbanaILUSA
- Present address:
Loxo Oncology at LillyLouisvilleCOUSA
| | | | | | | | - Emad Tajkhorshid
- Theoretical and Computational Biophysics Group, NIH Center for Macromolecular Modeling and Bioinformatics, Beckman Institute for Advanced Science and Technology, Department of Biochemistry, and Center for Biophysics and Quantitative BiologyUniversity of Illinois at Urbana‐ChampaignUrbanaILUSA
| | - Amer Alam
- The Hormel InstituteUniversity of MinnesotaAustinMNUSA
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12
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Hou W, Xu D, Wang L, Chen Y, Chen Z, Zhou C, Chen Y. Plastic structures for diverse substrates: A revisit of human
ABC
transporters. Proteins 2022; 90:1749-1765. [DOI: 10.1002/prot.26406] [Citation(s) in RCA: 2] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/26/2022] [Revised: 07/31/2022] [Accepted: 08/01/2022] [Indexed: 12/18/2022]
Affiliation(s)
- Wen‐Tao Hou
- School of Life Sciences University of Science and Technology of China Hefei People's Republic of China
| | - Da Xu
- School of Life Sciences University of Science and Technology of China Hefei People's Republic of China
| | - Liang Wang
- School of Life Sciences University of Science and Technology of China Hefei People's Republic of China
| | - Yu Chen
- School of Life Sciences University of Science and Technology of China Hefei People's Republic of China
| | - Zhi‐Peng Chen
- School of Life Sciences University of Science and Technology of China Hefei People's Republic of China
| | - Cong‐Zhao Zhou
- School of Life Sciences University of Science and Technology of China Hefei People's Republic of China
| | - Yuxing Chen
- School of Life Sciences University of Science and Technology of China Hefei People's Republic of China
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13
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Segrest JP, Tang C, Song HD, Jones MK, Davidson WS, Aller SG, Heinecke JW. ABCA1 is an extracellular phospholipid translocase. Nat Commun 2022; 13:4812. [PMID: 35974019 PMCID: PMC9381790 DOI: 10.1038/s41467-022-32437-3] [Citation(s) in RCA: 16] [Impact Index Per Article: 8.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/01/2021] [Accepted: 07/29/2022] [Indexed: 11/18/2022] Open
Abstract
Production of high density lipoprotein (HDL) requires ATP-binding cassette transporter A1 (ABCA1) to drive phospholipid (PL) from the plasma membrane into extracellular apolipoprotein A-I. Here, we use simulations to show that domains of ABCA1 within the plasma membrane remove PL from the membrane’s outer leaflet. In our simulations, after the lipid diffuses into the interior of ABCA1’s outward-open cavity, PL extracted by the gateway passes through a ring-shaped domain, the annulus orifice, which forms the base of an elongated hydrophobic tunnel in the transporter’s extracellular domain. Engineered mutations in the gateway and annulus strongly inhibit lipid export by ABCA1 without affecting cell-surface expression levels. Our finding that ABCA1 extracts lipid from the outer face of the plasma membrane and forces it through its gateway and annulus into an elongated hydrophobic tunnel contrasts with the alternating access model, which proposes that ABCA1 flops PL substrate from the inner leaflet to the outer leaflet of the membrane. Consistent with our model, ABCA1 lacks the charged amino acid residues in the transmembrane domain found in the floppase members of the ABC transporter family. ATP-binding cassette transporter A1 (ABCA1) drives phospholipid (PL) from the plasma membrane into extracellular apolipoprotein A-I, for the production of high density lipoprotein (HDL). Here, the authors use simulations to assess the mechanism of ABCA1 function and show that ABCA1 extracts lipid from the outer face of the plasma membrane.
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Affiliation(s)
- Jere P Segrest
- Department of Medicine, Vanderbilt University Medical Center, Nashville, TN, USA.
| | - Chongren Tang
- Department of Medicine, University of Washington, Seattle, WA, USA
| | - Hyun D Song
- Department of Medicine, Vanderbilt University Medical Center, Nashville, TN, USA
| | - Martin K Jones
- Department of Medicine, Vanderbilt University Medical Center, Nashville, TN, USA
| | - W Sean Davidson
- Department of Pathology and Laboratory Medicine, University of Cincinnati, Cincinnati, OH, USA
| | - Stephen G Aller
- Department of Pharmacology & Toxicology, University of Alabama at Birmingham, Birmingham, AL, USA
| | - Jay W Heinecke
- Department of Medicine, University of Washington, Seattle, WA, USA
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14
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Xie T, Zhang Z, Yue J, Fang Q, Gong X. Cryo-EM structures of the human surfactant lipid transporter ABCA3. SCIENCE ADVANCES 2022; 8:eabn3727. [PMID: 35394827 PMCID: PMC8993109 DOI: 10.1126/sciadv.abn3727] [Citation(s) in RCA: 2] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/02/2023]
Abstract
The adenosine 5'-triphosphate (ATP)-binding cassette (ABC) transporter ABCA3 plays a critical role in pulmonary surfactant biogenesis. Mutations in human ABCA3 have been recognized as the most frequent causes of inherited surfactant dysfunction disorders. Despite two decades of research, in vitro biochemical and structural studies of ABCA3 are still lacking. Here, we report the cryo-EM structures of human ABCA3 in two distinct conformations, both at resolution of 3.3 Å. In the absence of ATP, ABCA3 adopts a "lateral-opening" conformation with the lateral surfaces of transmembrane domains (TMDs) exposed to the membrane and features two positively charged cavities within the TMDs as potential substrate binding sites. ATP binding induces pronounced conformational changes, resulting in the collapse of the potential substrate binding cavities. Our results help to rationalize the disease-causing mutations in human ABCA3 and suggest a conserved "lateral access and extrusion" mechanism for both lipid export and import mediated by ABCA transporters.
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15
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Aller SG, Segrest JP. The regulatory domains of the lipid exporter ABCA1 form domain swapped latches. PLoS One 2022; 17:e0262746. [PMID: 35120130 PMCID: PMC8815970 DOI: 10.1371/journal.pone.0262746] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/08/2021] [Accepted: 01/04/2022] [Indexed: 12/19/2022] Open
Abstract
ABCA1 and ABCA4 are enigmatic because they transport substrates in opposite directions yet share >50% amino acid identity. ABCA4 imports lipid conjugates but ABCA1 exports lipids. Both hydrolyze ATP to drive transport, and both contain cytoplasmic regulatory domains (RDs) following nucleotide-binding domains (NBDs) in the primary structure. The tertiary structures of several ABC importers, including ABCA4, show that each RD forms a domain-swapped latch that locks onto the opposing RD and holds the NBDs close together. Crucially, sequences encoding the RDs and their bridges are among the most conserved in the entire ABC-A subfamily. In the original cryo-EM structure of ABCA1, the RDs were modeled without crossover. After close inspection of that cryo-EM density map and the recent structure of ABCA4, we propose that the RDs of ABCA1 also form a domain-swapped latch. A refined ABCA1 model containing latches exhibited significantly improved overall protein geometry. Critically, the conserved crossover sequence leading to the RD-domain swap is directly supported by the original cryo-EM density map of ABCA1 and appears to have been overlooked. Our refined ABCA1 model suggests the possibility that ABCA1, despite being an exporter, has highly restrained NBDs that suggest a transport mechanism that is distinct from 'alternating access'.
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Affiliation(s)
- Stephen G. Aller
- Department of Pharmacology and Toxicology, University of Alabama at Birmingham, Birmingham, AL, United States of America
| | - Jere P. Segrest
- Division of Cardiovascular Medicine, Department of Medicine, Vanderbilt University Medical Center, Nashville, TN, United States of America
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16
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Piotter E, McClements ME, MacLaren RE. The Scope of Pathogenic ABCA4 Mutations Targetable by CRISPR DNA Base Editing Systems—A Systematic Review. Front Genet 2022; 12:814131. [PMID: 35154257 PMCID: PMC8830518 DOI: 10.3389/fgene.2021.814131] [Citation(s) in RCA: 2] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/12/2021] [Accepted: 12/14/2021] [Indexed: 12/26/2022] Open
Abstract
Stargardt macular dystrophy (STGD1) is the most common form of inherited childhood blindness worldwide and for which no current treatments exist. It is an autosomal recessive disease caused by mutations in ABCA4. To date, a variety of gene supplementation approaches have been tested to create a therapy, with some reaching clinical trials. New technologies, such as CRISPR-Cas based editing systems, provide an exciting frontier for addressing genetic disease by allowing targeted DNA or RNA base editing of pathogenic mutations. ABCA4 has ∼1,200 known pathogenic mutations, of which ∼63% are transition mutations amenable to this editing technology. In this report, we screened the known “pathogenic” and “likely pathogenic” mutations in ABCA4 from available data in gnomAD, Leiden Open Variation Database (LOVD), and ClinVar for potential PAM sites of relevant base editors, including Streptococcus pyogenes Cas (SpCas), Staphylococcus aureus Cas (SaCas), and the KKH variant of SaCas (Sa-KKH). Overall, of the mutations screened, 53% (ClinVar), 71% (LOVD), and 71% (gnomAD), were editable, pathogenic transition mutations, of which 35–47% had “ideal” PAM sites. Of these mutations, 16–20% occur within a range of multiple PAM sites, enabling a variety of editing strategies. Further, in relevant patient data looking at three cohorts from Germany, Denmark, and China, we find that 44–76% of patients, depending on the presence of complex alleles, have at least one transition mutation with a nearby SaCas, SpCas, or Sa-KKH PAM site, which would allow for potential DNA base editing as a treatment strategy. Given the complexity of the genetic landscape of Stargardt, these findings provide a clearer understanding of the potential for DNA base editing approaches to be applied as ABCA4 gene therapy strategies.
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Affiliation(s)
- Elena Piotter
- Nuffield Laboratory of Ophthalmology, Department of Clinical Neurosciences, University of Oxford, Oxford, United Kingdom
- *Correspondence: Elena Piotter,
| | - Michelle E. McClements
- Nuffield Laboratory of Ophthalmology, Department of Clinical Neurosciences, University of Oxford, Oxford, United Kingdom
| | - Robert E. MacLaren
- Nuffield Laboratory of Ophthalmology, Department of Clinical Neurosciences, University of Oxford, Oxford, United Kingdom
- Oxford Eye Hospital, Oxford University Hospitals NHS Trust and NIHR Biomedical Research Centre, Oxford, United Kingdom
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17
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Sun Y, Li X. Cholesterol efflux mechanism revealed by structural analysis of human ABCA1 conformational states. NATURE CARDIOVASCULAR RESEARCH 2022; 1:238-245. [PMID: 37181814 PMCID: PMC10181854 DOI: 10.1038/s44161-022-00022-y] [Citation(s) in RCA: 13] [Impact Index Per Article: 6.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/16/2023]
Abstract
ATP-binding cassette transporter A1 (ABCA1) utilizes energy derived from ATP hydrolysis to export cholesterol and phospholipids from macrophages. ABCA1 plays a central role in the biosynthesis of high-density lipoprotein (HDL), which mediates reverse cholesterol transport and prevents detrimental lipid deposition. Mutations in ABCA1 cause Tangier disease characterized by a remarkable reduction in the amount of HDL in blood. Here we present cryo-electron microscopy structures of human ABCA1 in ATP-bound and nucleotide-free states. Structural comparison reveals that ATP molecules pull the nucleotide-binding domains together, inducing movements of transmembrane helices 1, 2, 7 and 8 through a series of salt-bridge interactions. Subsequently, extracellular domains (ECDs) undergo a rotation and introduce conformational changes in the ECD-transmembrane interface. In addition, while we observe a sterol-like molecule in ECDs, no such density was observed in the structure of an HDL-deficiency mutant ABCA1Y482C, demonstrating the physiological importance of ECDs and a putative interaction mode between ABCA1 and its lipid acceptors. Thus, these structures, along with cholesterol efflux assays, advance the understanding ABCA1-mediated reverse cholesterol transport.
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Affiliation(s)
- Yingyuan Sun
- Department of Molecular Genetics, University of Texas Southwestern Medical Center, Dallas, TX, USA
| | - Xiaochun Li
- Department of Molecular Genetics, University of Texas Southwestern Medical Center, Dallas, TX, USA
- Department of Biophysics, University of Texas Southwestern Medical Center, Dallas, TX, USA
- Correspondence and requests for materials should be addressed to Xiaochun Li.
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18
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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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19
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Abstract
Rapid flip-flop of phospholipids across the two leaflets of biological membranes is crucial for many aspects of cellular life. The transport proteins that facilitate this process are classified as pump-like flippases and floppases and channel-like scramblases. Unexpectedly, Class A G protein-coupled receptors (GPCRs), a large class of signaling proteins exemplified by the visual receptor rhodopsin and its apoprotein opsin, are constitutively active as scramblases in vitro. In liposomes, opsin scrambles lipids at a unitary rate of >100,000 per second. Atomistic molecular dynamics simulations of opsin in a lipid membrane reveal conformational transitions that expose a polar groove between transmembrane helices 6 and 7. This groove enables transbilayer lipid movement, conceptualized as the swiping of a credit card (lipid) through a card reader (GPCR). Conformational changes that facilitate scrambling are distinct from those associated with GPCR signaling. In this review, we discuss the physiological significance of GPCR scramblase activity and the modes of its regulation in cells. Expected final online publication date for the Annual Review of Biophysics, Volume 51 is May 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.
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Affiliation(s)
- George Khelashvili
- Department of Physiology and Biophysics, Weill Cornell Medical College, New York, New York, USA; .,Institute of Computational Biomedicine, Weill Cornell Medical College, New York, New York, USA
| | - Anant K Menon
- Department of Biochemistry, Weill Cornell Medical College, New York, New York, USA;
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20
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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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21
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Namasivayam V, Stefan K, Pahnke J, Stefan SM. Binding mode analysis of ABCA7 for the prediction of novel Alzheimer's disease therapeutics. Comput Struct Biotechnol J 2021; 19:6490-6504. [PMID: 34976306 PMCID: PMC8666613 DOI: 10.1016/j.csbj.2021.11.035] [Citation(s) in RCA: 3] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/23/2021] [Revised: 11/20/2021] [Accepted: 11/22/2021] [Indexed: 12/17/2022] Open
Abstract
The adenosine-triphosphate-(ATP)-binding cassette (ABC) transporter ABCA7 is a genetic risk factor for Alzheimer's disease (AD). Defective ABCA7 promotes AD development and/or progression. Unfortunately, ABCA7 belongs to the group of 'under-studied' ABC transporters that cannot be addressed by small-molecules. However, such small-molecules would allow for the exploration of ABCA7 as pharmacological target for the development of new AD diagnostics and therapeutics. Pan-ABC transporter modulators inherit the potential to explore under-studied ABC transporters as novel pharmacological targets by potentially binding to the proposed 'multitarget binding site'. Using the recently reported cryogenic-electron microscopy (cryo-EM) structures of ABCA1 and ABCA4, a homology model of ABCA7 has been generated. A set of novel, diverse, and potent pan-ABC transporter inhibitors has been docked to this ABCA7 homology model for the discovery of the multitarget binding site. Subsequently, application of pharmacophore modelling identified the essential pharmacophore features of these compounds that may support the rational drug design of innovative diagnostics and therapeutics against AD.
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Key Words
- ABC transporter (ABCA1, ABCA4, ABCA7)
- ABC, ATP-binding cassette
- AD, Alzheimer’s disease
- APP, amyloid precursor protein
- ATP, Adenosine-triphosphate
- Alzheimer’s disease (AD)
- BBB, blood-brain barrier
- BODIPY-cholesterol, 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene-cholesterol
- ECD, extracellular domain
- EH, extracellular helix
- GSH, reduced glutathione
- HTS, high-throughput screening
- IC, intracellular helix
- MOE, Molecular Operating Environment
- MSD, membrane spanning domain
- Multitarget modulation (PANABC)
- NBD, nucleotide binding domain
- NBD-cholesterol, 7-nitro-2-1,3-benzoxadiazol-4-yl-cholesterol
- PDB, protein data bank
- PET tracer (PETABC)
- PET, positron emission tomography
- PLIF, protein ligand interaction
- PSO, particle swarm optimization
- Polypharmacology
- R-domain/region, regulatory domain/region
- RMSD, root mean square distance
- Rational drug design and development
- SNP, single-nucleotide polymorphism
- TM, transmembrane helix
- cryo-EM, cryogenic-electron microscopy
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Affiliation(s)
- Vigneshwaran Namasivayam
- Department of Pharmaceutical and Cellbiological Chemistry, Pharmaceutical Institute, University of Bonn, An der Immenburg 4, 53121 Bonn, Germany
| | - Katja Stefan
- Department of Pathology, Section of Neuropathology, Translational Neurodegeneration Research and Neuropathology Lab (www.pahnkelab.eu), University of Oslo and Oslo University Hospital, Sognsvannsveien 20, 0372 Oslo, Norway
| | - Jens Pahnke
- Department of Pathology, Section of Neuropathology, Translational Neurodegeneration Research and Neuropathology Lab (www.pahnkelab.eu), University of Oslo and Oslo University Hospital, Sognsvannsveien 20, 0372 Oslo, Norway
- LIED, University of Lübeck, Ratzeburger Allee 160, 23538 Lübeck, Germany
- Department of Pharmacology, Faculty of Medicine, University of Latvia, Jelgavas iela 1, 1004 Rīga, Latvia
| | - Sven Marcel Stefan
- Department of Pathology, Section of Neuropathology, Translational Neurodegeneration Research and Neuropathology Lab (www.pahnkelab.eu), University of Oslo and Oslo University Hospital, Sognsvannsveien 20, 0372 Oslo, Norway
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22
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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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23
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Onnée M, Fanen P, Callebaut I, de Becdelièvre A. Structure-Based Understanding of ABCA3 Variants. Int J Mol Sci 2021; 22:ijms221910282. [PMID: 34638622 PMCID: PMC8508924 DOI: 10.3390/ijms221910282] [Citation(s) in RCA: 6] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/16/2021] [Revised: 09/17/2021] [Accepted: 09/18/2021] [Indexed: 11/16/2022] Open
Abstract
ABCA3 is a crucial protein of pulmonary surfactant biosynthesis, associated with recessive pulmonary disorders such as neonatal respiratory distress and interstitial lung disease. Mutations are mostly private, and accurate interpretation of variants is mandatory for genetic counseling and patient care. We used 3D structure information to complete the set of available bioinformatics tools dedicated to medical decision. Using the experimental structure of human ABCA4, we modeled at atomic resolution the human ABCA3 3D structure including transmembrane domains (TMDs), nucleotide-binding domains (NBDs), and regulatory domains (RDs) in an ATP-bound conformation. We focused and mapped known pathogenic missense variants on this model. We pinpointed amino-acids within the NBDs, the RDs and within the interfaces between the NBDs and TMDs intracellular helices (IHs), which are predicted to play key roles in the structure and/or the function of the ABCA3 transporter. This theoretical study also highlighted the possible impact of ABCA3 variants in the cytosolic part of the protein, such as the well-known p.Glu292Val and p.Arg288Lys variants.
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Affiliation(s)
- Marion Onnée
- Institut Mondor de Recherche Biomédicale, Université Paris Est Creteil, F-94010 Créteil, France; (M.O.); (P.F.)
| | - Pascale Fanen
- Institut Mondor de Recherche Biomédicale, Université Paris Est Creteil, F-94010 Créteil, France; (M.O.); (P.F.)
- AP-HP, Département de Biochimie-Biologie Moléculaire, Pharmacologie, Génétique Médicale, Hôpital Henri Mondor, F-94010 Créteil, France
| | - Isabelle Callebaut
- Institut de Minéralogie de Physique des Matériaux et de Cosmochimie (IMPMC), Muséum National d’Histoire Naturelle, UMR CNRS 7590, Sorbonne Université, F-75005 Paris, France
- Correspondence: (I.C.); (A.d.B.)
| | - Alix de Becdelièvre
- Institut Mondor de Recherche Biomédicale, Université Paris Est Creteil, F-94010 Créteil, France; (M.O.); (P.F.)
- AP-HP, Département de Biochimie-Biologie Moléculaire, Pharmacologie, Génétique Médicale, Hôpital Henri Mondor, F-94010 Créteil, France
- Correspondence: (I.C.); (A.d.B.)
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24
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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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Structural basis of substrate recognition and translocation by human ABCA4. Nat Commun 2021; 12:3853. [PMID: 34158497 PMCID: PMC8219669 DOI: 10.1038/s41467-021-24194-6] [Citation(s) in RCA: 29] [Impact Index Per Article: 9.7] [Reference Citation Analysis] [Abstract] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/04/2021] [Accepted: 06/04/2021] [Indexed: 12/29/2022] Open
Abstract
Human ATP-binding cassette (ABC) subfamily A (ABCA) transporters mediate the transport of various lipid compounds across the membrane. Mutations in human ABCA transporters have been described to cause severe hereditary disorders associated with impaired lipid transport. However, little is known about the mechanistic details of substrate recognition and translocation by ABCA transporters. Here, we present three cryo-EM structures of human ABCA4, a retina-specific ABCA transporter, in distinct functional states at resolutions of 3.3–3.4 Å. In the nucleotide-free state, the two transmembrane domains (TMDs) exhibit a lateral-opening conformation, allowing the lateral entry of substrate from the lipid bilayer. The N-retinylidene-phosphatidylethanolamine (NRPE), the physiological lipid substrate of ABCA4, is sandwiched between the two TMDs in the luminal leaflet and is further stabilized by an extended loop from extracellular domain 1. In the ATP-bound state, the two TMDs display a closed conformation, which precludes the substrate binding. Our study provides a molecular basis to understand the mechanism of ABCA4-mediated NRPE recognition and translocation, and suggests a common ‘lateral access and extrusion’ mechanism for ABCA-mediated lipid transport. Here, cryo-EM structures of human retinal ABCA4 transporter, either in apo state, in complex with ATP or with the physiological lipid substrate N-retinylidene-phosphatidylethanolamine (NRPE), reveal lateral opening, substrate recognition and suggest ‘lateral access and extrusion’ mechanism for ABCA-mediated lipid transport.
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