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Carder J, Barile B, Shisler KA, Pisani F, Frigeri A, Hipps KW, Nicchia GP, Brozik JA. Thermodynamics and S-Palmitoylation Dependence of Interactions between Human Aquaporin-4 M1 Tetramers in Model Membranes. J Phys Chem B 2024; 128:603-621. [PMID: 38212942 PMCID: PMC10824246 DOI: 10.1021/acs.jpcb.3c04529] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/05/2023] [Revised: 12/09/2023] [Accepted: 12/21/2023] [Indexed: 01/13/2024]
Abstract
Aquaporin-4 (AQP4) is a water channel protein found primarily in the central nervous system (CNS) that helps to regulate water-ion homeostasis. AQP4 exists in two major isoforms: M1 and M23. While both isoforms have a homotetrameric quaternary structure and are functionally identical when transporting water, the M23 isoform forms large protein aggregates known as orthogonal arrays of particles (OAPs). In contrast, the M1 isoform creates a peripheral layer around the outside of these OAPs, suggesting a thermodynamically stable interaction between the two. Structurally, the M1 isoform has an N-terminal tail that is 22 amino acids longer than the M23 isoform and contains two solvent-accessible cysteines available for S-palmitoylation at cysteine-13 (Cys-13) and cysteine-17 (Cys-17) in the amino acid sequence. Earlier work suggests that the palmitoylation of these cysteines might aid in regulating AQP4 assemblies. This work discusses the thermodynamic driving forces for M1 protein-protein interactions and how the palmitoylation state of M1 affects them. Using temperature-dependent single-particle tracking, the standard state free energies, enthalpies, and entropies were measured for these interactions. Furthermore, we present a binding model based on measured thermodynamics and a structural modeling study. The results of this study demonstrate that the M1 isoform will associate with itself according to the following expressions: 2[AQP4-M1]4 ↔ [[AQP4-M1]4]2 when palmitoylated and 3[AQP4-M1]4 ↔ [AQP4-M1]4 + [[AQP4-M1]4]2 ↔ [[AQP4-M1]4]3 when depalmitoylated. This is primarily due to a conformational change induced by adding the palmitic acid groups at Cys-13 and Cys-17 in the N-terminal tails of the homotetramers. In addition, a statistical mechanical model was developed to estimate the Gibbs free energy, enthalpy, and entropy for forming dimers and trimers. These results were in good agreement with experimental values.
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Affiliation(s)
- Jessica
D. Carder
- Department
of Chemistry, Washington State University, PO Box 644630, Pullman, Washington 99164-4630, United States
| | - Barbara Barile
- Department
of Bioscience, Biotechnologies and Environment, University of Bari Aldo Moro, Bari 70124, Italy
| | - Krista A. Shisler
- Department
of Chemistry, Washington State University, PO Box 644630, Pullman, Washington 99164-4630, United States
| | - Francesco Pisani
- Department
of Bioscience, Biotechnologies and Environment, University of Bari Aldo Moro, Bari 70124, Italy
| | - Antonio Frigeri
- Department
of Translational Medicine and Neuroscience, University of Bari Aldo Moro, Bari 70124, Italy
- Dominick
P. Purpura Department of Neuroscience, Albert
Einstein College of Medicine, 840 Kennedy Center, Bronx, New York 10461, United States
| | - K. W. Hipps
- Department
of Chemistry, Washington State University, PO Box 644630, Pullman, Washington 99164-4630, United States
- Materials
Science & Engineering Program, Washington
State University, Pullman, Washington 99163-2711, United States
| | - Grazia Paola Nicchia
- Department
of Bioscience, Biotechnologies and Environment, University of Bari Aldo Moro, Bari 70124, Italy
- Dominick
P. Purpura Department of Neuroscience, Albert
Einstein College of Medicine, 840 Kennedy Center, Bronx, New York 10461, United States
| | - James A. Brozik
- Department
of Chemistry, Washington State University, PO Box 644630, Pullman, Washington 99164-4630, United States
- Materials
Science & Engineering Program, Washington
State University, Pullman, Washington 99163-2711, United States
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2
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Cao Y, Wei H, Jiang S, Lu T, Nie P, Yang C, Liu N, Lee I, Meng X, Wang W, Yuan Z. Effect of AQP4 and its palmitoylation on the permeability of exogenous reactive oxygen species: Insights from computational study. Int J Biol Macromol 2023; 253:127568. [PMID: 37866582 DOI: 10.1016/j.ijbiomac.2023.127568] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/02/2023] [Revised: 10/16/2023] [Accepted: 10/18/2023] [Indexed: 10/24/2023]
Abstract
Aquaporin 4 (AQP4) facilitates the transport of reactive oxygen species (ROS). Both cancer cells and the ionizing radiation microenvironment can induce posttranslational modifications (PTMs) in AQP4, which may affect its permeability to ROS. Because this ROS diffusion process is rapid, microscopic, and instantaneous within and outside cells, conventional experimental methods are inadequate for elucidating the molecular mechanisms involved. In this study, computational methods were employed to investigate the permeability of exogenous ROS mediated by radiation in AQP4 at a molecular scale. We constructed a simulation system incorporating AQP4 and AQP4-Cysp13 in a complex lipid environment with ROS. Long-timescale molecular dynamics simulations were conducted to assess the structural stability of both AQP4 and AQP4-Cysp13. Free energy calculations were utilized to determine the ROS transport capability of the two AQP4 proteins. Computational electrophysiology and channel structural analysis quantitatively evaluated changes in ROS transport capacity under various radiation-induced transmembrane voltage microenvironments. Our findings demonstrate the distinct transport capabilities of AQP4 channels for water molecules and various types of ROS and reveal a decrease in transport efficiency when AQP4 undergoes palmitoylation modification. In addition, we have simulated the radiation-induced alteration of cell membrane voltage, which significantly affected the ROS transport capacity. We propose that this research will enhance the understanding of the molecular mechanisms governing the transport of exogenous ROS by AQP4 and elucidate the influence of palmitoylation on ROS transport. This study will also help clarify how different structural features of AQP4 affect the transport of exogenous ROS mediated by radiotherapy, thereby providing a theoretical molecular basis for the development of new treatment strategies that combine with radiotherapy.
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Affiliation(s)
- Yipeng Cao
- Key Laboratory of Cancer Prevention and Therapy, Tianjin's Clinical Research Center for Cancer, National Clinical Research Center for Cancer, Tianjin Medical University Cancer Institute and Hospital, 300060, PR China; National Supercomputer Center in Tianjin, 300457, PR China.
| | - Hui Wei
- Key Laboratory of Cancer Prevention and Therapy, Tianjin's Clinical Research Center for Cancer, National Clinical Research Center for Cancer, Tianjin Medical University Cancer Institute and Hospital, 300060, PR China
| | - Shengpeng Jiang
- Key Laboratory of Cancer Prevention and Therapy, Tianjin's Clinical Research Center for Cancer, National Clinical Research Center for Cancer, Tianjin Medical University Cancer Institute and Hospital, 300060, PR China
| | - Tong Lu
- Key Laboratory of Cancer Prevention and Therapy, Tianjin's Clinical Research Center for Cancer, National Clinical Research Center for Cancer, Tianjin Medical University Cancer Institute and Hospital, 300060, PR China
| | - Pengfei Nie
- National Supercomputer Center in Tianjin, 300457, PR China
| | - Chengwen Yang
- Key Laboratory of Cancer Prevention and Therapy, Tianjin's Clinical Research Center for Cancer, National Clinical Research Center for Cancer, Tianjin Medical University Cancer Institute and Hospital, 300060, PR China
| | - Ningbo Liu
- Key Laboratory of Cancer Prevention and Therapy, Tianjin's Clinical Research Center for Cancer, National Clinical Research Center for Cancer, Tianjin Medical University Cancer Institute and Hospital, 300060, PR China
| | - Imshik Lee
- College of Physics, Nankai University, Tianjin 300071, PR China
| | - Xiangfei Meng
- National Supercomputer Center in Tianjin, 300457, PR China.
| | - Wei Wang
- Key Laboratory of Cancer Prevention and Therapy, Tianjin's Clinical Research Center for Cancer, National Clinical Research Center for Cancer, Tianjin Medical University Cancer Institute and Hospital, 300060, PR China.
| | - Zhiyong Yuan
- Key Laboratory of Cancer Prevention and Therapy, Tianjin's Clinical Research Center for Cancer, National Clinical Research Center for Cancer, Tianjin Medical University Cancer Institute and Hospital, 300060, PR China.
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Sisto M, Ribatti D, Lisi S. Aquaporin water channels: New perspectives on the potential role in inflammation. ADVANCES IN PROTEIN CHEMISTRY AND STRUCTURAL BIOLOGY 2019; 116:311-345. [PMID: 31036295 DOI: 10.1016/bs.apcsb.2018.11.010] [Citation(s) in RCA: 16] [Impact Index Per Article: 3.2] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 12/13/2022]
Abstract
Aquaporins (AQPs) are a family of membrane water channel proteins that osmotically modulate water fluid homeostasis in several tissues; some of them also transport small solutes such as glycerol. At the cellular level, the AQPs regulate not only cell migration and transepithelial fluid transport across membranes, but also common events that are crucial for the inflammatory response. Emerging data reveal a new function of AQPs in the inflammatory process, as demonstrated by their dysregulation in a wide range of inflammatory diseases including edematous states, cancer, obesity, wound healing and several autoimmune diseases. This chapter summarizes the discoveries made so far about the structure and functions of the AQPs and provides updated information on the underlying mechanisms of AQPs in several human inflammatory diseases. The discovery of new functions for AQPs opens new vistas offering promise for the discovery of mechanisms and therapeutic opportunities in inflammatory disorders.
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Affiliation(s)
- Margherita Sisto
- Department of Basic Medical Sciences, Neurosciences and Sensory Organs (SMBNOS), Section of Human Anatomy and Histology, University of Bari "Aldo Moro", Bari, Italy.
| | - Domenico Ribatti
- Department of Basic Medical Sciences, Neurosciences and Sensory Organs (SMBNOS), Section of Human Anatomy and Histology, University of Bari "Aldo Moro", Bari, Italy
| | - Sabrina Lisi
- Department of Basic Medical Sciences, Neurosciences and Sensory Organs (SMBNOS), Section of Human Anatomy and Histology, University of Bari "Aldo Moro", Bari, Italy
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Mukherjee A, Wang Z, Kinlough CL, Poland PA, Marciszyn AL, Montalbetti N, Carattino MD, Butterworth MB, Kleyman TR, Hughey RP. Specific Palmitoyltransferases Associate with and Activate the Epithelial Sodium Channel. J Biol Chem 2017; 292:4152-4163. [PMID: 28154191 DOI: 10.1074/jbc.m117.776146] [Citation(s) in RCA: 16] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/10/2017] [Indexed: 11/06/2022] Open
Abstract
The epithelial sodium channel (ENaC) has an important role in regulating extracellular fluid volume and blood pressure, as well as airway surface liquid volume and mucociliary clearance. ENaC is a trimer of three homologous subunits (α, β, and γ). We previously reported that cytoplasmic residues on the β (βCys-43 and βCys-557) and γ (γCys-33 and γCys-41) subunits are palmitoylated. Mutation of Cys that blocked ENaC palmitoylation also reduced channel open probability. Furthermore, γ subunit palmitoylation had a dominant role over β subunit palmitoylation in regulating ENaC. To determine which palmitoyltransferases (termed DHHCs) regulate the channel, mouse ENaCs were co-expressed in Xenopus oocytes with each of the 23 mouse DHHCs. ENaC activity was significantly increased by DHHCs 1, 2, 3, 7, and 14. ENaC activation by DHHCs was lost when γ subunit palmitoylation sites were mutated, whereas DHHCs 1, 2, and 14 still activated ENaC lacking β subunit palmitoylation sites. β subunit palmitoylation was increased by ENaC co-expression with DHHC 7. Both wild type ENaC and channels lacking β and γ palmitoylation sites co-immunoprecipitated with the five activating DHHCs, suggesting that ENaC forms a complex with multiple DHHCs. RT-PCR revealed that transcripts for the five activating DHHCs were present in cultured mCCDcl1 cells, and DHHC 3 was expressed in aquaporin 2-positive principal cells of mouse aldosterone-sensitive distal nephron where ENaC is localized. Treatment of polarized mCCDcl1 cells with a general inhibitor of palmitoylation reduced ENaC-mediated Na+ currents within minutes. Our results indicate that specific DHHCs have a role in regulating ENaC.
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Affiliation(s)
| | | | | | | | | | | | | | | | - Thomas R Kleyman
- From the Departments of Medicine, .,Cell Biology, and.,Pharmacology and Chemical Biology, University of Pittsburgh School of Medicine, Pittsburgh, Pennsylvania 15261
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Mangiatordi GF, Alberga D, Trisciuzzi D, Lattanzi G, Nicolotti O. Human Aquaporin-4 and Molecular Modeling: Historical Perspective and View to the Future. Int J Mol Sci 2016; 17:ijms17071119. [PMID: 27420052 PMCID: PMC4964494 DOI: 10.3390/ijms17071119] [Citation(s) in RCA: 10] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/27/2016] [Revised: 06/30/2016] [Accepted: 07/02/2016] [Indexed: 12/26/2022] Open
Abstract
Among the different aquaporins (AQPs), human aquaporin-4 (hAQP4) has attracted the greatest interest in recent years as a new promising therapeutic target. Such a membrane protein is, in fact, involved in a multiple sclerosis-like immunopathology called Neuromyelitis Optica (NMO) and in several disorders resulting from imbalanced water homeostasis such as deafness and cerebral edema. The gap of knowledge in its functioning and dynamics at the atomistic level of detail has hindered the development of rational strategies for designing hAQP4 modulators. The application, lately, of molecular modeling has proved able to fill this gap providing a breeding ground to rationally address compounds targeting hAQP4. In this review, we give an overview of the important advances obtained in this field through the application of Molecular Dynamics (MD) and other complementary modeling techniques. The case studies presented herein are discussed with the aim of providing important clues for computational chemists and biophysicists interested in this field and looking for new challenges.
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Affiliation(s)
- Giuseppe Felice Mangiatordi
- Dipartimento di Farmacia-Scienze del Farmaco, Via Orabona, 4, University of Bari "Aldo Moro", 70126 Bari, Italy.
| | - Domenico Alberga
- Institut de Recherche de Chimie Paris CNRS Chimie ParisTech, PSL Research University, 11 rue P. et M. Curie, F-75005 Paris, France.
| | - Daniela Trisciuzzi
- Dipartimento di Farmacia-Scienze del Farmaco, Via Orabona, 4, University of Bari "Aldo Moro", 70126 Bari, Italy.
| | - Gianluca Lattanzi
- INFN-Sez. di Bari and Dipartimento di Medicina Clinica e Sperimentale, University of Foggia, Viale Pinto, 71122 Foggia, Italy.
| | - Orazio Nicolotti
- Dipartimento di Farmacia-Scienze del Farmaco, Via Orabona, 4, University of Bari "Aldo Moro", 70126 Bari, Italy.
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Verkman AS, Phuan PW, Asavapanumas N, Tradtrantip L. Biology of AQP4 and anti-AQP4 antibody: therapeutic implications for NMO. Brain Pathol 2014; 23:684-95. [PMID: 24118484 DOI: 10.1111/bpa.12085] [Citation(s) in RCA: 84] [Impact Index Per Article: 8.4] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/12/2013] [Accepted: 08/14/2013] [Indexed: 12/18/2022] Open
Abstract
The water channel aquaporin-4 (AQP4) is the target of the immunoglobulin G autoantibody (AQP4-IgG) in neuromyelitis optica (NMO). AQP4 is expressed in foot processes of astrocytes throughout the central nervous system, as well as in skeletal muscle and epithelial cells in kidney, lung and gastrointestinal organs. Phenotype analysis of AQP4 knockout mice indicates the involvement of AQP4 in water movement into and out of the brain, astrocyte migration, glial scar formation and neuroexcitatory phenomena. AQP4 monomers form tetramers in membranes, which further aggregate to form supramolecular assemblies called orthogonal arrays of particles. AQP4-IgG is pathogenic in NMO by a mechanism involving complement- and cell-mediated astrocyte cytotoxicity, which produces an inflammatory response with oligodendrocyte injury and demyelination. AQP4 orthogonal arrays are crucial in NMO pathogenesis, as they increase AQP4-IgG binding to AQP4 and greatly enhance complement-dependent cytotoxicity. Novel NMO therapeutics are under development that target AQP4-IgG or AQP4, including aquaporumab monoclonal antibodies and small molecules that block AQP4-IgG binding to AQP4, and enzymatic inactivation strategies to neutralize AQP4-IgG pathogenicity.
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Affiliation(s)
- A S Verkman
- Department of Medicine, University of California, San Francisco, CA; Department of Physiology, University of California, San Francisco, CA
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7
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Shipston MJ. Ion channel regulation by protein S-acylation. J Gen Physiol 2014; 143:659-78. [PMID: 24821965 PMCID: PMC4035745 DOI: 10.1085/jgp.201411176] [Citation(s) in RCA: 52] [Impact Index Per Article: 5.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/30/2014] [Accepted: 04/18/2014] [Indexed: 01/09/2023] Open
Abstract
Protein S-acylation, the reversible covalent fatty-acid modification of cysteine residues, has emerged as a dynamic posttranslational modification (PTM) that controls the diversity, life cycle, and physiological function of numerous ligand- and voltage-gated ion channels. S-acylation is enzymatically mediated by a diverse family of acyltransferases (zDHHCs) and is reversed by acylthioesterases. However, for most ion channels, the dynamics and subcellular localization at which S-acylation and deacylation cycles occur are not known. S-acylation can control the two fundamental determinants of ion channel function: (1) the number of channels resident in a membrane and (2) the activity of the channel at the membrane. It controls the former by regulating channel trafficking and the latter by controlling channel kinetics and modulation by other PTMs. Ion channel function may be modulated by S-acylation of both pore-forming and regulatory subunits as well as through control of adapter, signaling, and scaffolding proteins in ion channel complexes. Importantly, cross-talk of S-acylation with other PTMs of both cysteine residues by themselves and neighboring sites of phosphorylation is an emerging concept in the control of ion channel physiology. In this review, I discuss the fundamentals of protein S-acylation and the tools available to investigate ion channel S-acylation. The mechanisms and role of S-acylation in controlling diverse stages of the ion channel life cycle and its effect on ion channel function are highlighted. Finally, I discuss future goals and challenges for the field to understand both the mechanistic basis for S-acylation control of ion channels and the functional consequence and implications for understanding the physiological function of ion channel S-acylation in health and disease.
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Affiliation(s)
- Michael J Shipston
- Centre for Integrative Physiology, College of Medicine and Veterinary Medicine, University of Edinburgh, Edinburgh EH8 9XD Scotland, UK
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8
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Abstract
AQP4 (aquaporin-4), a water channel protein that is predominantly expressed in astrocyte end-feet, plays an important role in the brain oedema formation, and is thereby considered to be a potential therapeutic target. Using a stopped-flow analysis, we showed that propofol (2,6-diisopropylphenol), a general anaesthetic drug, profoundly inhibited the osmotic water permeability of AQP4 proteoliposomes in the presence of Zn²⁺. This propofol inhibition was not observed in AQP1, suggesting the specificity for AQP4. In addition, the inhibitory effects of propofol could be reversed by the removal of Zn²⁺. Other lipid membrane fluidizers also similarly inhibited AQP4, suggesting that the modulation of protein-lipid interactions plays an essential role in the propofol-induced inhibition of AQP4. Accordingly, we used Blue native PAGE and showed that the profound inhibition caused by propofol in the presence of Zn²⁺ is coupled with the reversible clustering of AQP4 tetramers. Site-directed mutagenesis identified that Cys²⁵³, located at the membrane interface connecting to the C-terminal tail, is responsible for Zn²⁺-mediated propofol inhibition. Overall, we discovered that propofol specifically and reversibly inhibits AQP4 through the interaction between Zn²⁺ and Cys²⁵³. The findings provide new insight into the functional regulation of AQP4 and may facilitate the identification of novel AQP4-specific inhibitors.
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Tong J, Briggs MM, McIntosh TJ. Water permeability of aquaporin-4 channel depends on bilayer composition, thickness, and elasticity. Biophys J 2013. [PMID: 23199918 DOI: 10.1016/j.bpj.2012.09.025] [Citation(s) in RCA: 62] [Impact Index Per Article: 5.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022] Open
Abstract
Aquaporin-4 (AQP4) is the primary water channel in the mammalian brain, particularly abundant in astrocytes, whose plasma membranes normally contain high concentrations of cholesterol. Here we test the hypothesis that the water permeabilities of two naturally occurring isoforms (AQP4-M1 and AQP4-M23) depend on bilayer mechanical/structural properties modulated by cholesterol and phospholipid composition. Osmotic stress measurements were performed with proteoliposomes containing AQP4 and three different lipid mixtures: 1), phosphatidylcholine (PC) and phosphatidylglycerol (PG); 2), PC, PG, with 40 mol % cholesterol; and 3), sphingomyelin (SM), PG, with 40 mol % cholesterol. The unit permeabilities of AQP4-M1 were 3.3 ± 0.4 × 10(-13) cm(3)/s (mean ± SE), 1.2 ± 0.1 × 10(-13) cm(3)/s, and 0.4 ± 0.1 × 10(-13) cm(3)/s in PC:PG, PC:PG:cholesterol, and SM:PG:cholesterol, respectively. The unit permeabilities of AQP4-M23 were 2.1 ± 0.2 × 10(-13) cm(3)/s, 0.8 ± 0.1 × 10(-13) cm(3)/s, and 0.3 ± 0.1 × 10(-13) cm(3)/s in PC:PG, PC:PG:cholesterol, and SM:PG:cholesterol, respectively. Thus, for each isoform the unit permeabilities strongly depended on bilayer composition and systematically decreased with increasing bilayer compressibility modulus and bilayer thickness. These observations suggest that altering lipid environment provides a means of regulating water channel permeability. Such permeability changes could have physiological consequences, because AQP4 water permeability would be reduced by its sequestration into SM:cholesterol-enriched raft microdomains. Conversely, under ischemic conditions astrocyte membrane cholesterol content decreases, which could increase AQP4 permeability.
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Affiliation(s)
- Jihong Tong
- Department of Cell Biology, Duke University Medical Center, Durham, North Carolina, USA
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10
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Jones ML, Tay CL, Rayner JC. Getting stuck in: protein palmitoylation in Plasmodium. Trends Parasitol 2012; 28:496-503. [DOI: 10.1016/j.pt.2012.08.009] [Citation(s) in RCA: 16] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/26/2012] [Revised: 08/28/2012] [Accepted: 08/28/2012] [Indexed: 11/26/2022]
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Neuromyelitis optica: aquaporin-4 based pathogenesis mechanisms and new therapies. Int J Biochem Cell Biol 2012; 44:1519-30. [PMID: 22713791 DOI: 10.1016/j.biocel.2012.06.013] [Citation(s) in RCA: 77] [Impact Index Per Article: 6.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/06/2012] [Revised: 05/29/2012] [Accepted: 06/10/2012] [Indexed: 12/21/2022]
Abstract
Neuromyelitis optica (NMO) is an autoimmune 'aquaporinopathy' of the central nervous system that causes inflammatory demyelinating lesions primarily in spinal cord and optic nerve, leading to paralysis and blindness. NMO lesions show loss of aquaporin-4 (AQP4), GFAP and myelin, infiltration of granulocytes and macrophages, and perivascular deposition of activated complement. Most patients with NMO are seropositive for immunoglobulin autoantibodies (AQP4-IgG) against AQP4, the principal water channel of astrocytes. There is strong evidence that AQP4-IgG is pathogenic in NMO, probably by a mechanism involving complement-dependent astrocyte cytotoxicity, causing leukocyte infiltration, cytokine release and blood-brain barrier disruption, which leads to oligodendrocyte death, myelin loss and neuron death. Here, we review the evidence for this and alternative proposed NMO pathogenesis mechanisms, such as AQP4-IgG-induced internalization of AQP4 and glutamate transporters, complement-independent cell-mediated cytotoxicity, and AQP4-IgG inhibition of AQP4 water transport function. Based on the initiating pathogenic role of AQP4-IgG binding to astrocyte AQP4 in NMO, selective blocker therapies are under development in which AQP4-targeted monoclonal antibodies or small molecules block binding of AQP4-IgG to astrocytes and consequent downstream pathology.
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12
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Verkman AS, Rossi A, Crane JM. Live-cell imaging of aquaporin-4 supramolecular assembly and diffusion. Methods Enzymol 2012; 504:341-54. [PMID: 22264543 DOI: 10.1016/b978-0-12-391857-4.00017-3] [Citation(s) in RCA: 10] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/23/2022]
Abstract
Aquaporin-4 (AQP4) is a water channel expressed in astrocytes throughout the central nervous system, as well as in epithelial cells in various peripheral organs. AQP4 is involved in brain water balance, neuroexcitation, astrocyte migration, and neuroinflammation and is the target of pathogenic autoantibodies in neuromyelitis optica. Two AQP4 isoforms produced by alternative splicing, M1 and M23 AQP4, form heterotetramers that assemble in cell plasma membranes in supramolecular aggregates called orthogonal arrays of particles (OAPs). OAPs have been studied morphologically, by freeze-fracture electron microscopy, and biochemically, by native gel electrophoresis. We have applied single-molecule and high-resolution fluorescence microscopy methods to visualize AQP4 and OAPs in live cells. Quantum dot single particle tracking of fluorescently labeled AQP4 has quantified AQP4 diffusion in membranes, and has elucidated the molecular determinants and regulation of OAP formation. The composition, structure, and kinetics of OAPs containing fluorescent protein-AQP4 chimeras have been studied utilizing total internal reflection fluorescence microscopy, single-molecule photobleaching, and super-resolution imaging methods. The biophysical data afforded by live-cell imaging of AQP4 and OAPs has provided new insights in the roles of AQP4 in organ physiology and neurological disease.
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Affiliation(s)
- A S Verkman
- Departments of Medicine and Physiology, University of California, San Francisco, San Francisco, California, USA
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13
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Rossi A, Baumgart F, van Hoek AN, Verkman AS. Post-Golgi supramolecular assembly of aquaporin-4 in orthogonal arrays. Traffic 2011; 13:43-53. [PMID: 21981006 DOI: 10.1111/j.1600-0854.2011.01299.x] [Citation(s) in RCA: 25] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
Abstract
The supramolecular assembly of aquaporin-4 (AQP4) in orthogonal arrays of particles (OAPs) involves N-terminus interactions of the M23-AQP4 isoform. We found AQP4 OAPs in cell plasma membranes but not in endoplasmic reticulum (ER) or Golgi, as shown by: (i) native gel electrophoresis of brain and AQP4-transfected cells, (ii) photobleaching recovery of green fluorescent protein-AQP4 chimeras in live cells and (iii) freeze-fracture electron microscopy (FFEM). We found that AQP4 OAP formation in plasma membranes, but not in the Golgi, was not related to AQP4 density, pH, membrane lipid composition, C-terminal PDZ domain interactions or α-syntrophin expression. Remarkably, however, fusion of AQP4-containing Golgi vesicles with (AQP4-free) plasma membrane vesicles produced OAPs, suggesting the involvement of plasma membrane factor(s) in AQP4 OAP formation. In investigating additional possible determinants of OAP assembly we discovered membrane curvature-dependent OAP assembly, in which OAPs were disrupted by extrusion of plasma membrane vesicles to ∼110 nm diameter, but not to ∼220 nm diameter. We conclude that AQP4 supramolecular assembly in OAPs is a post-Golgi phenomenon involving plasma membrane-specific factor(s). Post-Golgi and membrane curvature-dependent OAP assembly may be important for vesicle transport of AQP4 in the secretory pathway and AQP4-facilitated astrocyte migration, and suggests a novel therapeutic approach for neuromyelitis optica.
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Affiliation(s)
- Andrea Rossi
- Departments of Medicine and Physiology, University of California-San Francisco, CA 94143, USA
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14
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Aquaporin-4: orthogonal array assembly, CNS functions, and role in neuromyelitis optica. Acta Pharmacol Sin 2011; 32:702-10. [PMID: 21552296 DOI: 10.1038/aps.2011.27] [Citation(s) in RCA: 58] [Impact Index Per Article: 4.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/18/2022] Open
Abstract
Aquaporin-4 (AQP4) is a water-selective transporter expressed in astrocytes throughout the central nervous system, as well as in kidney, lung, stomach and skeletal muscle. The two AQP4 isoforms produced by alternative spicing, M1 and M23 AQP4, form heterotetramers that assemble in cell plasma membranes in supramolecular structures called orthogonal arrays of particles (OAPs). Phenotype analysis of AQP4-null mice indicates the involvement of AQP4 in brain and spinal cord water balance, astrocyte migration, neural signal transduction and neuroinflammation. AQP4-null mice manifest reduced brain swelling in cytotoxic cerebral edema, but increased brain swelling in vasogenic edema and hydrocephalus. AQP4 deficiency also increases seizure duration, impairs glial scarring, and reduces the severity of autoimmune neuroinflammation. Each of these phenotypes is likely explicable on the basis of reduced astrocyte water permeability in AQP4 deficiency. AQP4 is also involved in the neuroinflammatory demyelinating disease neuromyelitis optica (NMO), where autoantibodies (NMO-IgG) targeting AQP4 produce astrocyte damage and inflammation. Mice administered NMO-IgG and human complement by intracerebral injection develop characteristic NMO lesions with neuroinflammation, demyelination, perivascular complement deposition and loss of glial fibrillary acidic protein and AQP4 immunoreactivity. Our findings suggest the potential utility of AQP4-based therapeutics, including small-molecule modulators of AQP4 water transport function for therapy of brain swelling, injury and epilepsy, as well as small-molecule or monoclonal antibody blockers of NMO-IgG binding to AQP4 for therapy of NMO.
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Abstract
Protein S-palmitoylation, the reversible thioester linkage of a 16-carbon palmitate lipid to an intracellular cysteine residue, is rapidly emerging as a fundamental, dynamic, and widespread post-translational mechanism to control the properties and function of ligand- and voltage-gated ion channels. Palmitoylation controls multiple stages in the ion channel life cycle, from maturation to trafficking and regulation. An emerging concept is that palmitoylation is an important determinant of channel regulation by other signaling pathways. The elucidation of enzymes controlling palmitoylation and developments in proteomics tools now promise to revolutionize our understanding of this fundamental post-translational mechanism in regulating ion channel physiology.
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Affiliation(s)
- Michael J Shipston
- Centre for Integrative Physiology, College of Medicine and Veterinary Medicine, University of Edinburgh, Edinburgh EH8 9XD, Scotland, United Kingdom.
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Wolburg H, Wolburg-Buchholz K, Fallier-Becker P, Noell S, Mack AF. Structure and functions of aquaporin-4-based orthogonal arrays of particles. INTERNATIONAL REVIEW OF CELL AND MOLECULAR BIOLOGY 2011; 287:1-41. [PMID: 21414585 DOI: 10.1016/b978-0-12-386043-9.00001-3] [Citation(s) in RCA: 99] [Impact Index Per Article: 7.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 02/07/2023]
Abstract
Orthogonal arrays or assemblies of intramembranous particles (OAPs) are structures in the membrane of diverse cells which were initially discovered by means of the freeze-fracturing technique. This technique, developed in the 1960s, was important for the acceptance of the fluid mosaic model of the biological membrane. OAPs were first described in liver cells, and then in parietal cells of the stomach, and most importantly, in the astrocytes of the brain. Since the discovery of the structure of OAPs and the identification of OAPs as the morphological equivalent of the water channel protein aquaporin-4 (AQP4) in the 1990s, a plethora of morphological work on OAPs in different cells was published. Now, we feel a need to balance new and old data on OAPs and AQP4 to elucidate the interrelationship of both structures and molecules. In this review, the identity of OAPs as AQP4-based structures in a diversity of cells will be described. At the same time, arguments are offered that under pathological or experimental circumstances, AQP4 can also be expressed in a non-OAP form. Thus, we attempt to project classical work on OAPs onto the molecular biology of AQP4. In particular, astrocytes and glioma cells will play the major part in this review, not only due to our own work but also due to the fact that most studies on structure and function of AQP4 were done in the nervous system.
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Affiliation(s)
- Hartwig Wolburg
- Institute of Pathology, University of Tübingen, Tübingen, Germany
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Mitsuma T, Tani K, Hiroaki Y, Kamegawa A, Suzuki H, Hibino H, Kurachi Y, Fujiyoshi Y. Influence of the cytoplasmic domains of aquaporin-4 on water conduction and array formation. J Mol Biol 2010; 402:669-81. [PMID: 20709083 DOI: 10.1016/j.jmb.2010.07.060] [Citation(s) in RCA: 17] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/09/2010] [Revised: 07/24/2010] [Accepted: 07/28/2010] [Indexed: 01/15/2023]
Abstract
Phosphorylation of Ser180 in cytoplasmic loop D has been shown to reduce the water permeability of aquaporin (AQP) 4, the predominant water channel in the brain. However, when the structure of the S180D mutant (AQP4M23S180D), which was generated to mimic phosphorylated Ser180, was determined to 2.8 Å resolution using electron diffraction patterns, it showed no significant differences from the structure of the wild-type channel. High-resolution density maps usually do not resolve protein regions that are only partially ordered, but these can sometimes be seen in lower-resolution density maps calculated from electron micrographs. We therefore used images of two-dimensional crystals and determined the structure of AQP4M23S180D at 10 A resolution. The features of the 10-A density map are consistent with those of the previously determined atomic model; in particular, there were no indications of any obstruction near the cytoplasmic pore entrance. In addition, water conductance measurements, both in vitro and in vivo, show the same water permeability for wild-type and mutant AQP4M23, suggesting that the S180D mutation neither reduces water conduction through a conformational change nor reduces water conduction by interacting with a protein that would obstruct the cytoplasmic channel entrance. Finally, the 10-A map shows a cytoplasmic density in between four adjacent tetramers that most likely represents the association of four N termini. This finding supports the critical role of the N terminus of AQP4 in the stabilization of orthogonal arrays, as well as their interference through lipid modification of cysteine residues in the longer N-terminal isoform.
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Affiliation(s)
- Tadanori Mitsuma
- Department of Biophysics, Faculty of Science, Kyoto University, Oiwake, Kitashirakawa, Sakyo-ku, Kyoto 606-8502, Japan
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Mueller GM, Maarouf AB, Kinlough CL, Sheng N, Kashlan OB, Okumura S, Luthy S, Kleyman TR, Hughey RP. Cys palmitoylation of the beta subunit modulates gating of the epithelial sodium channel. J Biol Chem 2010; 285:30453-62. [PMID: 20663869 DOI: 10.1074/jbc.m110.151845] [Citation(s) in RCA: 43] [Impact Index Per Article: 3.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/13/2022] Open
Abstract
The epithelial Na(+) channel (ENaC) is comprised of three homologous subunits (α, β, and γ) that have a similar topology with two transmembrane domains, a large extracellular region, and cytoplasmic N and C termini. Although ENaC activity is regulated by a number of factors, palmitoylation of its cytoplasmic Cys residues has not been previously described. Fatty acid-exchange chemistry was used to determine whether channel subunits were Cys-palmitoylated. We observed that only the β and γ subunits were modified by Cys palmitoylation. Analyses of ENaCs with mutant β subunits revealed that Cys-43 and Cys-557 were palmitoylated. Xenopus oocytes expressing ENaC with a β C43A,C557A mutant had significantly reduced amiloride-sensitive whole cell currents, enhanced Na(+) self-inhibition, and reduced single channel P(o) when compared with wild-type ENaC, while membrane trafficking and levels of surface expression were unchanged. Computer modeling of cytoplasmic domains indicated that β Cys-43 is in proximity to the first transmembrane α helix, whereas β Cys-557 is within an amphipathic α-helix contiguous with the second transmembrane domain. We propose that β subunit palmitoylation modulates channel gating by facilitating interactions between cytoplasmic domains and the plasma membrane.
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Affiliation(s)
- Gunhild M Mueller
- Renal-Electrolyte Division, Department of Medicine, University of Pittsburgh, Pittsburgh, Pennsylvania 15261, USA
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Tajima M, Crane JM, Verkman AS. Aquaporin-4 (AQP4) associations and array dynamics probed by photobleaching and single-molecule analysis of green fluorescent protein-AQP4 chimeras. J Biol Chem 2010; 285:8163-70. [PMID: 20071343 DOI: 10.1074/jbc.m109.093948] [Citation(s) in RCA: 34] [Impact Index Per Article: 2.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
The plasma membrane assembly of aquaporin-4 (AQP4) water channels into orthogonal arrays of particles (OAPs) involves interactions of AQP4 N-terminal domains. To study in live cells the site of OAP assembly, the size and dynamics of plasma membrane OAPs, and the heterotetrameric associations of AQP4, we constructed green fluorescent protein (GFP)-labeled AQP4 "long" (M1) and "short" (M23) isoforms in which GFP was inserted at the cytoplasm-facing N or C terminus or between Val-141 and Val-142 in the second extracellular loop of AQP4. The C-terminal and extracellular loop GFP insertions did not interfere with the rapid unrestricted membrane diffusion of GFP-labeled M1 or the restricted diffusion and OAP assembly of GFP-labeled M23. Photobleaching of brefeldin A-treated cells showed comparable and minimally restricted diffusion of M1 and M23, indicating that OAP assembly occurs post-endoplasmic reticulum. Single-molecule step photobleaching and intensity analysis of GFP-labeled M1 in the absence versus presence of excess unlabeled M1 or M23 with an OAP-disrupting mutation indicated heterotetrameric AQP4 association. Time-lapse total internal reflection fluorescence imaging of M23 in live cells at 37 degrees C indicated that OAPs diffuse slowly (D approximately 10(-12) cm(2)/s) and rearrange over tens of minutes. Our biophysical measurements in live cells thus reveal extensive AQP4 monomer-monomer and tetramer-tetramer interactions.
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Affiliation(s)
- Masato Tajima
- Departments of Medicine and Physiology, University of California, San Francisco, California 94143, USA
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