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Khananshvili D. Neuronal and astrocyte NCX isoform/splice variants: How do they participate in Na + and Ca 2+ signalling? Cell Calcium 2023; 116:102818. [PMID: 37918135 DOI: 10.1016/j.ceca.2023.102818] [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: 10/16/2023] [Revised: 10/20/2023] [Accepted: 10/22/2023] [Indexed: 11/04/2023]
Abstract
NCX1, NCX2, and NCX3 gene isoforms and their splice variants are characteristically expressed in different regions of the brain. The tissue-specific splice variants of NCX1-3 isoforms show specific expression profiles in neurons and astrocytes, whereas the relevant NCX isoform/splice variants exhibit diverse allosteric modes of Na+- and Ca2+-dependent regulation. In general, overexpression of NCX1-3 genes leads to neuroprotective effects, whereas their ablation gains the opposite results. At this end, the partial contributions of NCX isoform/splice variants to neuroprotective effects remain unresolved. The glutamate-dependent Na+ entry generates Na+ transients (in response to neuronal cell activities), whereas the Na+-driven Ca2+ entry (through the reverse NCX mode) raises Ca2+ transients. This special mode of signal coupling translates Na+ transients into the Ca2+ signals while being a part of synaptic neurotransmission. This mechanism is of general interest since disease-related conditions (ischemia, metabolic stress, and stroke among many others) trigger Na+ and Ca2+ overload with deadly outcomes of downstream apoptosis and excitotoxicity. The recently discovered mechanisms of NCX allosteric regulation indicate that some NCX variants might play a critical role in the dynamic coupling of Na+-driven Ca2+ entry. In contrast, the others are less important or even could be dangerous under altered conditions (e.g., metabolic stress). This working hypothesis can be tested by applying advanced experimental approaches and highly focused computational simulations. This may allow the development of structure-based blockers/activators that can selectively modulate predefined NCX variants to lessen the life-threatening outcomes of excitotoxicity, ischemia, apoptosis, metabolic deprivation, brain injury, and stroke.
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Affiliation(s)
- Daniel Khananshvili
- Department of Physiology and Pharmacology, Faculty of Medicine, Tel-Aviv University, Tel Aviv 69978, Israel.
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2
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Structure-Based Function and Regulation of NCX Variants: Updates and Challenges. Int J Mol Sci 2022; 24:ijms24010061. [PMID: 36613523 PMCID: PMC9820601 DOI: 10.3390/ijms24010061] [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: 10/09/2022] [Revised: 12/14/2022] [Accepted: 12/15/2022] [Indexed: 12/24/2022] Open
Abstract
The plasma-membrane homeostasis Na+/Ca2+ exchangers (NCXs) mediate Ca2+ extrusion/entry to dynamically shape Ca2+ signaling/in biological systems ranging from bacteria to humans. The NCX gene orthologs, isoforms, and their splice variants are expressed in a tissue-specific manner and exhibit nearly 104-fold differences in the transport rates and regulatory specificities to match the cell-specific requirements. Selective pharmacological targeting of NCX variants could benefit many clinical applications, although this intervention remains challenging, mainly because a full-size structure of eukaryotic NCX is unavailable. The crystal structure of the archaeal NCX_Mj, in conjunction with biophysical, computational, and functional analyses, provided a breakthrough in resolving the ion transport mechanisms. However, NCX_Mj (whose size is nearly three times smaller than that of mammalian NCXs) cannot serve as a structure-dynamic model for imitating high transport rates and regulatory modules possessed by eukaryotic NCXs. The crystal structures of isolated regulatory domains (obtained from eukaryotic NCXs) and their biophysical analyses by SAXS, NMR, FRET, and HDX-MS approaches revealed structure-based variances of regulatory modules. Despite these achievements, it remains unclear how multi-domain interactions can decode and integrate diverse allosteric signals, thereby yielding distinct regulatory outcomes in a given ortholog/isoform/splice variant. This article summarizes the relevant issues from the perspective of future developments.
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Ottolia M, John S, Hazan A, Goldhaber JI. The Cardiac Na + -Ca 2+ Exchanger: From Structure to Function. Compr Physiol 2021; 12:2681-2717. [PMID: 34964124 DOI: 10.1002/cphy.c200031] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/07/2022]
Abstract
Ca2+ homeostasis is essential for cell function and survival. As such, the cytosolic Ca2+ concentration is tightly controlled by a wide number of specialized Ca2+ handling proteins. One among them is the Na+ -Ca2+ exchanger (NCX), a ubiquitous plasma membrane transporter that exploits the electrochemical gradient of Na+ to drive Ca2+ out of the cell, against its concentration gradient. In this critical role, this secondary transporter guides vital physiological processes such as Ca2+ homeostasis, muscle contraction, bone formation, and memory to name a few. Herein, we review the progress made in recent years about the structure of the mammalian NCX and how it relates to function. Particular emphasis will be given to the mammalian cardiac isoform, NCX1.1, due to the extensive studies conducted on this protein. Given the degree of conservation among the eukaryotic exchangers, the information highlighted herein will provide a foundation for our understanding of this transporter family. We will discuss gene structure, alternative splicing, topology, regulatory mechanisms, and NCX's functional role on cardiac physiology. Throughout this article, we will attempt to highlight important milestones in the field and controversial topics where future studies are required. © 2021 American Physiological Society. Compr Physiol 12:1-37, 2021.
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Affiliation(s)
- Michela Ottolia
- Department of Anesthesiology and Perioperative Medicine, Division of Molecular Medicine, David Geffen School of Medicine, University of California Los Angeles, Los Angeles, California, USA
| | - Scott John
- Department of Medicine (Cardiology), UCLA, Los Angeles, California, USA
| | - Adina Hazan
- Smidt Heart Institute, Cedars Sinai Medical Center, Los Angeles, California, USA
| | - Joshua I Goldhaber
- Smidt Heart Institute, Cedars Sinai Medical Center, Los Angeles, California, USA
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Iwaki M, Refaeli B, van Dijk L, Hiller R, Giladi M, Kandori H, Khananshvili D. Structure-affinity insights into the Na + and Ca 2+ interactions with multiple sites of a sodium-calcium exchanger. FEBS J 2020; 287:4678-4695. [PMID: 32056381 DOI: 10.1111/febs.15250] [Citation(s) in RCA: 5] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/26/2019] [Revised: 12/22/2019] [Accepted: 02/12/2020] [Indexed: 01/30/2023]
Abstract
Selective recognition and transport of Na+ and Ca2+ ions by sodium-calcium exchanger (NCX) proteins is a primary prerequisite for Ca2+ signaling and homeostasis. Twelve ion-coordinating residues are highly conserved among NCXs, and distinct NCX orthologs contain two or three carboxylates, while sharing a common ion-exchange stoichiometry (3Na+ :1Ca2+ ). How these structural differences affect the ion-binding affinity, selectivity, and transport rates remains unclear. Here, the mutational effects of three carboxylates (E54, E213, and D240) were analyzed on the ion-exchange rates in the archaeal NCX from Methanococcus jannaschii and ion-induced structure-affinity changes were monitored by attenuated total reflection-Fourier-transform infrared spectroscopy (ATR-FTIR). The D240N mutation elevated the ion-transport rates by twofold to threefold, meaning that the deprotonation of D240 is not essential for transport catalysis. In contrast, mutating E54 or E213 to A, D, N, or Q dramatically decreased the ion-transport rates. ATR-FTIR revealed high- and low-affinity binding of Na+ or Ca2+ with E54 and E213, but not with D240. These findings reveal distinct structure-affinity states at specific ion-binding sites in the inward-facing (IF) and outward-facing orientation. Collectively, two multidentate carboxylate counterparts (E54 and E213) play a critical role in determining the ion coordination/transport in prokaryotic and eukaryotic NCXs, whereas the ortholog substitutions in prokaryotes (aspartate) and eukaryotes (asparagine) at the 240 position affect the ion-transport rates differently (kcat ), probably due to the structural differences in the transition state.
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Affiliation(s)
- Masayo Iwaki
- Department of Life Science and Applied Chemistry and OptoBioTechnology Research Center, Nagoya Institute of Technology, Japan
| | - Bosmat Refaeli
- Department of Physiology and Pharmacology, Tel-Aviv University, Israel
| | - Liat van Dijk
- Department of Physiology and Pharmacology, Tel-Aviv University, Israel
| | - Reuben Hiller
- Department of Physiology and Pharmacology, Tel-Aviv University, Israel
| | - Moshe Giladi
- Department of Physiology and Pharmacology, Tel-Aviv University, Israel
| | - Hideki Kandori
- Department of Life Science and Applied Chemistry and OptoBioTechnology Research Center, Nagoya Institute of Technology, Japan
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5
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Basic and editing mechanisms underlying ion transport and regulation in NCX variants. Cell Calcium 2020; 85:102131. [DOI: 10.1016/j.ceca.2019.102131] [Citation(s) in RCA: 15] [Impact Index Per Article: 3.8] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/11/2019] [Revised: 11/20/2019] [Accepted: 11/20/2019] [Indexed: 12/28/2022]
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Scranton K, John S, Escobar A, Goldhaber JI, Ottolia M. Modulation of the cardiac Na +-Ca 2+ exchanger by cytoplasmic protons: Molecular mechanisms and physiological implications. Cell Calcium 2019; 87:102140. [PMID: 32070924 DOI: 10.1016/j.ceca.2019.102140] [Citation(s) in RCA: 5] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/27/2019] [Revised: 12/06/2019] [Accepted: 12/07/2019] [Indexed: 01/31/2023]
Abstract
A precise temporal and spatial control of intracellular Ca2+ concentration is essential for a coordinated contraction of the heart. Following contraction, cardiac cells need to rapidly remove intracellular Ca2+ to allow for relaxation. This task is performed by two transporters: the plasma membrane Na+-Ca2+ exchanger (NCX) and the sarcoplasmic reticulum (SR) Ca2+-ATPase (SERCA). NCX extrudes Ca2+ from the cell, balancing the Ca2+entering the cytoplasm during systole through L-type Ca2+ channels. In parallel, following SR Ca2+ release, SERCA activity replenishes the SR, reuptaking Ca2+ from the cytoplasm. The activity of the mammalian exchanger is fine-tuned by numerous ionic allosteric regulatory mechanisms. Micromolar concentrations of cytoplasmic Ca2+ potentiate NCX activity, while an increase in intracellular Na+ levels inhibits NCX via a mechanism known as Na+-dependent inactivation. Protons are also powerful inhibitors of NCX activity. By regulating NCX activity, Ca2+, Na+ and H+ couple cell metabolism to Ca2+ homeostasis and therefore cardiac contractility. This review summarizes the recent progress towards the understanding of the molecular mechanisms underlying the ionic regulation of the cardiac NCX with special emphasis on pH modulation and its physiological impact on the heart.
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Affiliation(s)
- Kyle Scranton
- Department of Anesthesiology and Perioperative Medicine, Division of Molecular Medicine, UCLA, Los Angeles, CA 90095, USA
| | - Scott John
- Department of Medicine (Cardiology), UCLA, Los Angeles, CA 90095, USA; Cardiovascular Research Laboratory, UCLA, Los Angeles, CA 90095, USA
| | - Ariel Escobar
- Department of Bioengineering, School of Engineering, UC Merced, Merced, CA 95343, USA
| | - Joshua I Goldhaber
- Smidt Heart Institute, Cedars Sinai Medical Center, Los Angeles, CA 90048, USA
| | - Michela Ottolia
- Department of Anesthesiology and Perioperative Medicine, Division of Molecular Medicine, UCLA, Los Angeles, CA 90095, USA; Cardiovascular Research Laboratory, UCLA, Los Angeles, CA 90095, USA.
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Allosteric Regulation of NCLX by Mitochondrial Membrane Potential Links the Metabolic State and Ca 2+ Signaling in Mitochondria. Cell Rep 2019; 25:3465-3475.e4. [PMID: 30566870 DOI: 10.1016/j.celrep.2018.11.084] [Citation(s) in RCA: 51] [Impact Index Per Article: 10.2] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/25/2018] [Revised: 10/24/2018] [Accepted: 11/21/2018] [Indexed: 12/19/2022] Open
Abstract
Calcium is a key regulator of mitochondrial function under both normal and pathological conditions. The mechanisms linking metabolic activity to mitochondrial Ca2+ signaling remain elusive, however. Here, by monitoring mitochondrial Ca2+ transients while manipulating mitochondrial membrane potential (ΔΨm), we found that mild fluctuations in ΔΨm, which do not affect Ca2+ influx, are sufficient to strongly regulate NCLX, the major efflux pathway of Ca2+ from the mitochondria. Phosphorylation of NCLX or expression of phosphomimicking mutant (S258D) rescued NCLX activity from ΔΨm-driven allosteric inhibition. By screening ΔΨm sensitivity of NCLX mutants, we also identified amino acid residues that, through functional interaction with Ser258, control NCLX regulation. Finally, we find that glucose-driven ΔΨm changes in pancreatic β-cells control mitochondrial Ca2+ signaling primarily via NCLX regulation. Our results identify a feedback control between metabolic activity and mitochondrial Ca2+ signaling and the "safety valve" NCLX phosphorylation that can rescue Ca2+ efflux in depolarized mitochondria.
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Functional properties and mode of regulation of the mitochondrial Na +/Ca 2+ exchanger, NCLX. Semin Cell Dev Biol 2019; 94:59-65. [PMID: 30658153 DOI: 10.1016/j.semcdb.2019.01.009] [Citation(s) in RCA: 35] [Impact Index Per Article: 7.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/11/2018] [Revised: 01/14/2019] [Accepted: 01/14/2019] [Indexed: 12/23/2022]
Abstract
Mitochondrial Ca2+ transient is the earliest discovered organellar Ca2+ signaling pathway. It consist of a Ca2+ influx, mediated by mitochondrial Ca2+ uniporter (MCU), and mitochondrial Ca2+ efflux mediated by a Na+/Ca2+ exchanger (NCLX). Mitochondrial Ca2+ signaling machinery plays a fundamental role in linking metabolic activity to cellular Ca2+ signaling, and in controlling local Ca2+ concertation in distinct cellular compartments. Impaired balance between mitochondrial Ca2+ influx and efflux leads to mitochondrial Ca2+ overload, an early and key event in ischemic or neurodegenerative syndromes. Molecular identification of NCLX and MCU happened only recently. Surprisingly, MCU knockout yielded a relatively mild phenotype while conditional knockout of NCLX led to a rapid fatal heart failure. Here we will focus on recent functional and molecular studies on NCLX structure and its mode of regulation. We will describe the unique crosstalk of this exchanger with Na+ and Ca2+ signaling pathways in the cell membrane and the endoplasmic reticulum, and with protein kinases that posttranslationally modulate NCLX activity. We will critically compare selectivity of pharmacological blockers versus molecular control of NCLX expression and activity. Finally we will discuss why this exchanger is essential for survival and can serve as an attractive therapeutic target.
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9
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Function, regulation and physiological role of the mitochondrial Na + /Ca 2+ exchanger, NCLX. CURRENT OPINION IN PHYSIOLOGY 2018. [DOI: 10.1016/j.cophys.2018.02.007] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/27/2023]
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Bode K, O'Halloran DM. NCX-DB: a unified resource for integrative analysis of the sodium calcium exchanger super-family. BMC Neurosci 2018; 19:19. [PMID: 29649983 PMCID: PMC5898058 DOI: 10.1186/s12868-018-0423-2] [Citation(s) in RCA: 6] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/03/2017] [Accepted: 03/28/2018] [Indexed: 12/20/2022] Open
Abstract
Na+/Ca2+ exchangers are low-affinity high-capacity transporters that mediate Ca2+ extrusion by coupling Ca2+ efflux to the influx of Na+ ions. The Na+/Ca2+ exchangers form a super-family comprised of three branches each differing in ion-substrate selectivity: Na+/Ca2+ exchangers (NCX), Na+/Ca2+/K+ exchangers, and Ca2+/cation exchangers. Their primary function is to maintain Ca2+ homeostasis and play a particularly important role in excitable cells that experience transient Ca2+ fluxes. Research into the role and activity of Na+/Ca2+ exchangers has focused extensively on the cardio-vascular system, however, growing evidence suggests that Na+/Ca2+ exchangers play a key role in neuronal processes such as memory formation, learning, oligodendrocyte differentiation, neuroprotection during brain ischemia and axon guidance. They have also been implicated in pathologies such as Alzheimer’s disease, Parkinson’s disease, Multiple Sclerosis and Epilepsy, however, a clear understanding of their mechanism during disease is lacking. To date, there has never been a central resource or database for Na+/Ca2+ exchangers. With clear disease relevance and ever-increasing research on Na+/Ca2+ exchangers from both model and non-model species, a database that unifies the data on Na+/Ca2+ exchangers is needed for future research. NCX-DB is a publicly available database with a web interface that enables users to explore various Na+/Ca2+ exchangers, perform cross-species sequence comparison, identify new exchangers, and stay-up to date with recent literature. NCX-DB is available on the web via an interactive user interface with an intuitive design, which is applicable for the identification and comparison of Na+/Ca2+ exchanger proteins across diverse species.
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Affiliation(s)
- Katrin Bode
- Department of Biological Sciences, The George Washington University, Science and Engineering Hall 6000, 800 22nd St. N.W., Washington, DC, 20052, USA.,Institute for Neuroscience, The George Washington University, 636A Ross Hall, 2300 I St. N.W., Washington, DC, 20052, USA
| | - Damien M O'Halloran
- Department of Biological Sciences, The George Washington University, Science and Engineering Hall 6000, 800 22nd St. N.W., Washington, DC, 20052, USA. .,Institute for Neuroscience, The George Washington University, 636A Ross Hall, 2300 I St. N.W., Washington, DC, 20052, USA.
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Liu T, Zhao J, Ibarra C, Garcia MU, Uhlén P, Nistér M. Glycosylation controls sodium-calcium exchanger 3 sub-cellular localization during cell cycle. Eur J Cell Biol 2018. [PMID: 29526322 DOI: 10.1016/j.ejcb.2018.02.004] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/17/2022] Open
Abstract
The Na+/Ca2+ exchanger (NCX) is a membrane antiporter that has been identified in the plasma membrane, the inner membrane of the nuclear envelope and in the membrane of the endoplasmic reticulum (ER). In humans, three genes have been identified, encoding unique NCX proteins. Although extensively studied, the NCX's sub-cellular localization and mechanisms regulating the activity of different subtypes are still ambiguous. Here we investigated the subcellular localization of the NCX subtype 3 (NCX3) and its impact on the cell cycle. Two phenotypes, switching from one to the other during the cell cycle, were detected. One phenotype was NCX3 in the plasma membrane during S and M phase, and the other was NCX3 in the ER membrane during resting and interphase. Glycosylation of NCX3 at the N45 site was required for targeting the protein to the plasma membrane, and the N45 site functioned as an on-off switch for the translocation of NCX3 to either the plasma membrane or the membrane of the ER. Introduction of an N-glycosylation deficient NCX3 mutant led to an arrest of cells in the G0/G1 phase of the cell cycle. This was accompanied by accumulation of de-glycosylated NCX3 in the cytosol (that is in the ER), where it transported calcium ions (Ca2+) from the cytosol to the ER. These results, obtained in transfected HEK293T and HeLa and confirmed endogenously in SH-SY5Y cells, suggest that cells can use a dynamic Ca2+ signaling toolkit in which the NCX3 sub-cellular localization changes in synchrony with the cell cycle.
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Affiliation(s)
- Tong Liu
- Department of Oncology-Pathology, Karolinska Institutet, CCK R8:05, Karolinska University Hospital Solna, SE-171 76 Stockholm, Sweden
| | - Jian Zhao
- Department of Oncology-Pathology, Karolinska Institutet, CCK R8:05, Karolinska University Hospital Solna, SE-171 76 Stockholm, Sweden.
| | - Cristian Ibarra
- Department of Medical Biochemistry and Biophysics, Karolinska Institutet, SE-171 77 Stockholm, Sweden
| | - Maxime U Garcia
- Department of Oncology-Pathology, Karolinska Institutet, CCK R8:05, Karolinska University Hospital Solna, SE-171 76 Stockholm, Sweden
| | - Per Uhlén
- Department of Medical Biochemistry and Biophysics, Karolinska Institutet, SE-171 77 Stockholm, Sweden
| | - Monica Nistér
- Department of Oncology-Pathology, Karolinska Institutet, CCK R8:05, Karolinska University Hospital Solna, SE-171 76 Stockholm, Sweden.
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Giladi M, Shor R, Lisnyansky M, Khananshvili D. Structure-Functional Basis of Ion Transport in Sodium-Calcium Exchanger (NCX) Proteins. Int J Mol Sci 2016; 17:E1949. [PMID: 27879668 PMCID: PMC5133943 DOI: 10.3390/ijms17111949] [Citation(s) in RCA: 31] [Impact Index Per Article: 3.9] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/29/2016] [Revised: 11/13/2016] [Accepted: 11/14/2016] [Indexed: 01/19/2023] Open
Abstract
The membrane-bound sodium-calcium exchanger (NCX) proteins shape Ca2+ homeostasis in many cell types, thus participating in a wide range of physiological and pathological processes. Determination of the crystal structure of an archaeal NCX (NCX_Mj) paved the way for a thorough and systematic investigation of ion transport mechanisms in NCX proteins. Here, we review the data gathered from the X-ray crystallography, molecular dynamics simulations, hydrogen-deuterium exchange mass-spectrometry (HDX-MS), and ion-flux analyses of mutants. Strikingly, the apo NCX_Mj protein exhibits characteristic patterns in the local backbone dynamics at particular helix segments, thereby possessing characteristic HDX profiles, suggesting structure-dynamic preorganization (geometric arrangements of catalytic residues before the transition state) of conserved α₁ and α₂ repeats at ion-coordinating residues involved in transport activities. Moreover, dynamic preorganization of local structural entities in the apo protein predefines the status of ion-occlusion and transition states, even though Na⁺ or Ca2+ binding modifies the preceding backbone dynamics nearby functionally important residues. Future challenges include resolving the structural-dynamic determinants governing the ion selectivity, functional asymmetry and ion-induced alternating access. Taking into account the structural similarities of NCX_Mj with the other proteins belonging to the Ca2+/cation exchanger superfamily, the recent findings can significantly improve our understanding of ion transport mechanisms in NCX and similar proteins.
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Affiliation(s)
- Moshe Giladi
- Department of Physiology and Pharmacology, Sackler Faculty of Medicine, Tel-Aviv University, Tel-Aviv 699780, Israel.
- Tel-Aviv Sourasky Medical Center, Tel-Aviv 6423906, Israel.
| | - Reut Shor
- Department of Physiology and Pharmacology, Sackler Faculty of Medicine, Tel-Aviv University, Tel-Aviv 699780, Israel.
| | - Michal Lisnyansky
- Department of Physiology and Pharmacology, Sackler Faculty of Medicine, Tel-Aviv University, Tel-Aviv 699780, Israel.
| | - Daniel Khananshvili
- Department of Physiology and Pharmacology, Sackler Faculty of Medicine, Tel-Aviv University, Tel-Aviv 699780, Israel.
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Towards Understanding the Role of the Na+-Ca2+ Exchanger Isoform 3. Rev Physiol Biochem Pharmacol 2015; 168:31-57. [DOI: 10.1007/112_2015_23] [Citation(s) in RCA: 12] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/05/2023]
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Sodium recognition by the Na+/Ca2+ exchanger in the outward-facing conformation. Proc Natl Acad Sci U S A 2014; 111:E5354-62. [PMID: 25468964 DOI: 10.1073/pnas.1415751111] [Citation(s) in RCA: 56] [Impact Index Per Article: 5.6] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/28/2022] Open
Abstract
Na(+)/Ca(2+) exchangers (NCXs) are ubiquitous membrane transporters with a key role in Ca(2+) homeostasis and signaling. NCXs mediate the bidirectional translocation of either Na(+) or Ca(2+), and thus can catalyze uphill Ca(2+) transport driven by a Na(+) gradient, or vice versa. In a major breakthrough, a prokaryotic NCX homolog (NCX_Mj) was recently isolated and its crystal structure determined at atomic resolution. The structure revealed an intriguing architecture consisting of two inverted-topology repeats, each comprising five transmembrane helices. These repeats adopt asymmetric conformations, yielding an outward-facing occluded state. The crystal structure also revealed four putative ion-binding sites, but the occupancy and specificity thereof could not be conclusively established. Here, we use molecular-dynamics simulations and free-energy calculations to identify the ion configuration that best corresponds to the crystallographic data and that is also thermodynamically optimal. In this most probable configuration, three Na(+) ions occupy the so-called Sext, SCa, and Sint sites, whereas the Smid site is occupied by one water molecule and one H(+), which protonates an adjacent aspartate side chain (D240). Experimental measurements of Na(+)/Ca(2+) and Ca(2+)/Ca(2+) exchange by wild-type and mutagenized NCX_Mj confirm that transport of both Na(+) and Ca(2+) requires protonation of D240, and that this side chain does not coordinate either ion at Smid. These results imply that the ion exchange stoichiometry of NCX_Mj is 3:1 and that translocation of Na(+) across the membrane is electrogenic, whereas transport of Ca(2+) is not. Altogether, these findings provide the basis for further experimental and computational studies of the conformational mechanism of this exchanger.
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Sharma V, O'Halloran DM. Recent structural and functional insights into the family of sodium calcium exchangers. Genesis 2013; 52:93-109. [PMID: 24376088 DOI: 10.1002/dvg.22735] [Citation(s) in RCA: 20] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/10/2013] [Revised: 12/04/2013] [Accepted: 12/08/2013] [Indexed: 01/08/2023]
Abstract
Maintenance of calcium homeostasis is necessary for the development and survival of all animals. Calcium ions modulate excitability and bind effectors capable of initiating many processes such as muscular contraction and neurotransmission. However, excessive amounts of calcium in the cytosol or within intracellular calcium stores can trigger apoptotic pathways in cells that have been implicated in cardiac and neuronal pathologies. Accordingly, it is critical for cells to rapidly and effectively regulate calcium levels. The Na(+) /Ca(2+) exchangers (NCX), Na(+) /Ca(2+) /K(+) exchangers (NCKX), and Ca(2+) /Cation exchangers (CCX) are the three classes of sodium calcium antiporters found in animals. These exchanger proteins utilize an electrochemical gradient to extrude calcium. Although they have been studied for decades, much is still unknown about these proteins. In this review, we examine current knowledge about the structure, function, and physiology and also discuss their implication in various developmental disorders. Finally, we highlight recent data characterizing the family of sodium calcium exchangers in the model system, Caenorhabditis elegans, and propose that C. elegans may be an ideal model to complement other systems and help fill gaps in our knowledge of sodium calcium exchange biology.
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Affiliation(s)
- Vishal Sharma
- Department of Biological Sciences, The George Washington University, Washington, DC; Institute for Neuroscience, The George Washington University, Washington, DC
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Khananshvili D. Sodium-calcium exchangers (NCX): molecular hallmarks underlying the tissue-specific and systemic functions. Pflugers Arch 2013; 466:43-60. [PMID: 24281864 DOI: 10.1007/s00424-013-1405-y] [Citation(s) in RCA: 88] [Impact Index Per Article: 8.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/24/2013] [Revised: 11/06/2013] [Accepted: 11/09/2013] [Indexed: 12/19/2022]
Abstract
NCX proteins explore the electrochemical gradient of Na(+) to mediate Ca(2+)-fluxes in exchange with Na(+) either in the Ca(2+)-efflux (forward) or Ca(2+)-influx (reverse) mode, whereas the directionality depends on ionic concentrations and membrane potential. Mammalian NCX variants (NCX1-3) and their splice variants are expressed in a tissue-specific manner to modulate the heartbeat rate and contractile force, the brain's long-term potentiation and learning, blood pressure, renal Ca(2+) reabsorption, the immune response, neurotransmitter and insulin secretion, apoptosis and proliferation, mitochondrial bioenergetics, etc. Although the forward mode of NCX represents a major physiological module, a transient reversal of NCX may contribute to EC-coupling, vascular constriction, and synaptic transmission. Notably, the reverse mode of NCX becomes predominant in pathological settings. Since the expression levels of NCX variants are disease-related, the selective pharmacological targeting of tissue-specific NCX variants could be beneficial, thereby representing a challenge. Recent structural and biophysical studies revealed a common module for decoding the Ca(2+)-induced allosteric signal in eukaryotic NCX variants, although the phenotype variances in response to regulatory Ca(2+) remain unclear. The breakthrough discovery of the archaebacterial NCX structure may serve as a template for eukaryotic NCX, although the turnover rates of the transport cycle may differ ~10(3)-fold among NCX variants to fulfill the physiological demands for the Ca(2+) flux rates. Further elucidation of ion-transport and regulatory mechanisms may lead to selective pharmacological targeting of NCX variants under disease conditions.
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Affiliation(s)
- Daniel Khananshvili
- Department of Physiology and Pharmacology, Sackler School of Medicine, Tel-Aviv University, Ramat-Aviv, Tel-Aviv, 69978, Israel,
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