Ca2+-dependent structural rearrangements within Na+-Ca2+ exchanger dimers

Structure-Based Function and Regulation of NCX Variants: Updates and Challenges

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.

The Cardiac Na+ -Ca2+ Exchanger: From Structure to Function

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.

Design of a Proteolytically Stable Sodium-Calcium Exchanger 1 Activator Peptide for In Vivo Studies

The cardiac sodium–calcium exchanger (NCX1) is important for normal Na⁺- and Ca²⁺-homeostasis and cardiomyocyte relaxation and contraction. It has been suggested that NCX1 activity is reduced by phosphorylated phospholemman (pSer68-PLM); however its direct interaction with PLM is debated. Disruption of the potentially inhibitory pSer68-PLM-NCX1 interaction might be a therapeutic strategy to increase NCX1 activity in cardiac disease. In the present study, we aimed to analyze the binding affinities and kinetics of the PLM-NCX1 and pSer68-PLM-NCX1 interactions by surface plasmon resonance (SPR) and to develop a proteolytically stable NCX1 activator peptide for future in vivo studies. The cytoplasmic parts of PLM (PLM_cyt) and pSer68-PLM (pSer68-PLM_cyt) were found to bind strongly to the intracellular loop of NCX1 (NCX1_cyt) with similar K_D values of 4.1 ± 1.0 nM and 4.3 ± 1.9 nM, but the PLM_cyt-NCX1_cyt interaction showed higher on/off rates. To develop a proteolytically stable NCX1 activator, we took advantage of a previously designed, high-affinity PLM binding peptide (OPT) that was derived from the PLM binding region in NCX1 and that reverses the inhibitory PLM (S68D)-NCX1 interaction in HEK293. We performed N- and C-terminal truncations of OPT and identified PYKEIEQLIELANYQV as the minimum sequence required for pSer68-PLM binding. To increase peptide stability in human serum, we replaced the proline with an N-methyl-proline (NOPT) after identification of N-terminus as substitution tolerant by two-dimensional peptide array analysis. Mass spectrometry analysis revealed that the half-life of NOPT was increased 17-fold from that of OPT. NOPT pulled down endogenous PLM from rat left ventricle lysate and exhibited direct pSer68-PLM binding in an ELISA-based assay and bound to pSer68-PLM_cyt with a K_D of 129 nM. Excess NOPT also reduced the PLM_cyt-NCX1_cyt interaction in an ELISA-based competition assay, but in line with that NCX1 and PLM form oligomers, NOPT was not able to outcompete the physical interaction between endogenous full length proteins. Importantly, cell-permeable NOPT-TAT increased NCX1 activity in cardiomyocytes isolated from both SHAM-operated and aorta banded heart failure (HF) mice, indicating that NOPT disrupted the inhibitory pSer68-PLM-NCX1 interaction. In conclusion, we have developed a proteolytically stable NCX1-derived PLM binding peptide that upregulates NCX1 activity in SHAM and HF cardiomyocytes.

Basolateral Na+/Ca2+ exchanger 1 and Na+/K+-ATPase, which display light-enhanced gene and protein expression levels, could be involved in the absorption of exogenous Ca2+ through the ctenidium of the giant clam, Tridacna squamosa

Giant clams perform light-enhanced shell formation (calcification) and therefore need to increase the uptake of exogenous Ca²⁺ during illumination. The ctenidium of the fluted giant clam, Tridacna squamosa, is involved in light-enhanced Ca²⁺ uptake. It expresses the pore-forming voltage-gated calcium channel (VGCC) subunit alpha 1 (CACNA1) in the apical membrane of the epithelial cells, and the protein expression level of CACNA1 is upregulated in the ctenidium during illumination. This study aimed to elucidate the mechanism involved in the transport of the absorbed Ca²⁺ across the basolateral membrane of the ctenidial epithelial cells into the hemolymph. We obtained a homolog of Na⁺/Ca²⁺exchanger 1 (NCX1-like) from the ctenidium of T. squamosa, which comprised 2418 bp, encoding a protein of 806 amino acids (88.9 kDa). NCX1-like had a basolateral localization in the epithelial cells of the ctenidial filaments and tertiary water channels. Illumination resulted in significant increases in the transcript and protein levels of NCX1-like/NCX1-like in the ctenidium. Hence, NCX1-like could operate in conjunction with VGCC to increase the transport of Ca²⁺ from the ambient seawater into the hemolymph during illumination. Illumination also resulted in the upregulation of the gene and protein expression levels of Na⁺/K⁺-ATPase (NKA) α-subunit (NKAα/NKAα) in the ctenidium of T. squamosa. As light-enhanced extrusion of Ca²⁺ into the hemolymph through NCX1-like would lead to a greater influx of extracellular Na⁺, the increased expression of the basolateral NKA was required to augment the capacity of intracellular Na⁺ homeostasis.

Dynamic Palmitoylation of the Sodium-Calcium Exchanger Modulates Its Structure, Affinity for Lipid-Ordered Domains, and Inhibition by XIP

The transmembrane sodium-calcium (Na-Ca) exchanger 1 (NCX1) regulates cytoplasmic Ca levels by facilitating electrogenic exchange of Ca for Na. Palmitoylation, the only reversible post-translational modification known to modulate NCX1 activity, controls NCX1 inactivation. Here, we show that palmitoylation of NCX1 modifies the structural arrangement of the NCX1 dimer and controls its affinity for lipid-ordered membrane domains. NCX1 palmitoylation occurs dynamically at the cell surface under the control of the enzymes zDHHC5 and APT1. We identify the position of the endogenous exchange inhibitory peptide (XIP) binding site within the NCX1 regulatory intracellular loop and demonstrate that palmitoylation controls the ability of XIP to bind this site. We also show that changes in NCX1 palmitoylation change cytosolic Ca. Our results thus demonstrate the broad molecular consequences of NCX1 palmitoylation and highlight a means to manipulate the inactivation of this ubiquitous ion transporter that could ameliorate pathologies linked to Ca overload via NCX1.

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NCX1 is dynamically palmitoylated at the cell surface by zDHHC5 and APT1

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Palmitoylation modifies the NCX1 dimer’s structure and affinity for lipid rafts

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We identify the binding site of the endogenous XIP domain in NCX1’s regulatory loop

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Palmitoylation modifies NCX1 XIP affinity and hence regulates intracellular Ca

Insights into the molecular basis of the palmitoylation and depalmitoylation of NCX1

Catalyzed by zDHHC-PAT enzymes and reversed by thioesterases, protein palmitoylation is the only post-translational modification recognized to regulate the sodium/calcium exchanger NCX1. NCX1 palmitoylation occurs at a single site at position 739 in its large regulatory intracellular loop. An amphipathic ɑ-helix between residues 740-756 is a critical for NCX1 palmitoylation. Given the rich background of the structural elements involving in NCX1 palmitoylation, the molecular basis of NCX1 palmitoylation is still relatively poorly understood. Here we found that (1) the identity of palmitoylation machinery of NCX1 controls its spatial organization within the cell, (2) the NCX1 amphipathic ɑ-helix directly interacts with zDHHC-PATs, (3) NCX1 is still palmitoylated when it is arrested in either Golgi or ER, indicating that NCX1 is a substrate for multiple zDHHC-PATs, (4) the thioesterase APT1 but not APT2 as a part of NCX1-depalmitoylation machinery governs subcellular organization of NCX1, (5) APT1 catalyzes NCX1 depalmitoylation in the Golgi but not in the ER. We also report that NCX2 and NCX3 are dually palmitoylated, with important implications for substrate recognition and enzyme catalysis by zDHHC-PATs. Our results could support new molecular or pharmacological strategies targeting the NCX1 palmitoylation and depalmitoylation machinery.

Cracking the code of sodium/calcium exchanger (NCX) gating: Old and new complexities surfacing from the deep web of secondary regulations

Cell membranes spatially define gradients that drive the complexity of biological signals. To guarantee movements and exchanges of solutes between compartments, membrane transporters negotiate the passages of ions and other important molecules through lipid bilayers. The Na⁺/Ca²⁺ exchangers (NCXs) in particular play central roles in balancing Na⁺ and Ca²⁺ fluxes across diverse proteolipid borders in all eukaryotic cells, influencing cellular functions and fate by multiple means. To prevent progression from balance to disease, redundant regulatory mechanisms cooperate at multiple levels (transcriptional, translational, and post-translational) and guarantee that the activities of NCXs are finely-tuned to cell homeostatic requirements. When this regulatory network is disturbed by pathological forces, cells may approach the end of life. In this review, we will discuss the main findings, controversies and open questions about regulatory mechanisms that control NCX functions in health and disease.

Regulation of NCX1 by palmitoylation

Palmitoylation (S-acylation) is the reversible conjugation of a fatty acid (usually C16 palmitate) to intracellular cysteine residues of proteins via a thioester linkage. Palmitoylation anchors intracellular regions of proteins to membranes because the palmitoylated cysteine is recruited to the lipid bilayer. NCX1 is palmitoylated at a single cysteine in its large regulatory intracellular loop. The presence of an amphipathic α-helix immediately adjacent to the NCX1 palmitoylation site is required for NCX1 palmitoylation. The NCX1 palmitoylation site is conserved through most metazoan phlya. Although palmitoylation does not regulate the normal forward or reverse ion transport modes of NCX1, NCX1 palmitoylation is required for its inactivation: sodium-dependent inactivation and inactivation by PIP2 depletion are significantly impaired for unpalmitoylatable NCX1. Here we review the role of palmitoylation in regulating NCX1 activity, and highlight future questions that must be addressed to fully understand the importance of this regulatory mechanism for sodium and calcium transport in cardiac muscle.

Modulation of the cardiac Na+-Ca2+ exchanger by cytoplasmic protons: Molecular mechanisms and physiological implications

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.

The Intracellular Loop of the Na+/Ca2+ Exchanger Contains an "Awareness Ribbon"-Shaped Two-Helix Bundle Domain

The Na⁺/Ca²⁺ exchanger (NCX) is a ubiquitous single-chain membrane protein that plays a major role in regulating the intracellular Ca²⁺ homeostasis by the counter transport of Na⁺ and Ca²⁺ across the cell membrane. Other than its prokaryotic counterpart, which contains only the transmembrane domain and is self-sufficient as an active ion transporter, the eukaryotic NCX protein possesses in addition a large intracellular loop that senses intracellular calcium signals and controls the activation of ion transport across the membrane. This provides a necessary layer of regulation for the more complex function of eukaryotic cells. The Ca²⁺ sensor in the intracellular loop is known as the Ca²⁺-binding domain (CBD12). However, how the signaling of the allosteric intracellular Ca²⁺ binding propagates and results in transmembrane ion transportation still lacks a detailed explanation. Further structural and dynamics characterization of the intracellular loop flanking both sides of CBD12 is therefore imperative. Here, we report the identification and characterization of another structured domain that is N-terminal to CBD12 in the intracellular loop using solution nuclear magnetic resonance (NMR) spectroscopy. The atomistic structure of this domain reveals that two tandem long α-helices, connected by a short linker, form a stable crossover two-helix bundle (THB), resembling an "awareness ribbon". Considering the highly conserved amino acid sequence of the THB domain, the detailed structural and dynamics properties of the THB domain will be common among NCXs from different species and will contribute toward the understanding of the regulatory mechanism of eukaryotic Na⁺/Ca²⁺ exchangers.

Mapping the in vitro interactome of cardiac sodium (Na+ )-calcium (Ca2+ ) exchanger 1 (NCX1)

The sodium (Na⁺ )-calcium (Ca²⁺ ) exchanger 1 (NCX1) is an antiporter membrane protein encoded by the SLC8A1 gene. In the heart, it maintains cytosolic Ca²⁺ homeostasis, serving as the primary mechanism for Ca²⁺ extrusion during relaxation. Dysregulation of NCX1 is observed in end-stage human heart failure. In this study, we used affinity purification coupled with MS in rat left ventricle lysates to identify novel NCX1 interacting proteins in the heart. Two screens were conducted using: (1) anti-NCX1 against endogenous NCX1 and (2) anti-His (where His is histidine) with His-trigger factor-NCX1_cyt recombinant protein as bait. The respective methods identified 112 and 350 protein partners, of which several were known NCX1 partners from the literature, and 29 occurred in both screens. Ten novel protein partners (DYRK1A, PPP2R2A, SNTB1, DMD, RABGGTA, DNAJB4, BAG3, PDE3A, POPDC2, STK39) were validated for binding to NCX1, and two partners (DYRK1A, SNTB1) increased NCX1 activity when expressed in HEK293 cells. A cardiac NCX1 protein-protein interaction map was constructed. The map was highly connected, containing distinct clusters of proteins with different biological functions, where "cell communication" and "signal transduction" formed the largest clusters. The NCX1 interactome was also significantly enriched with proteins/genes involved in "cardiovascular disease" which can be explored as novel drug targets in future research.

An amphipathic α-helix directs palmitoylation of the large intracellular loop of the sodium/calcium exchanger

The electrogenic sodium/calcium exchanger (NCX) mediates bidirectional calcium transport controlled by the transmembrane sodium gradient. NCX inactivation occurs in the absence of phosphatidylinositol 4,5-bisphosphate and is facilitated by palmitoylation of a single cysteine at position 739 within the large intracellular loop of NCX. The aim of this investigation was to identify the structural determinants of NCX1 palmitoylation. Full-length NCX1 (FL-NCX1) and a YFP fusion protein of the NCX1 large intracellular loop (YFP-NCX1) were expressed in HEK cells. Single amino acid changes around Cys-739 in FL-NCX1 and deletions on the N-terminal side of Cys-739 in YFP-NCX1 did not affect NCX1 palmitoylation, with the exception of the rare human polymorphism S738F, which enhanced FL-NCX1 palmitoylation, and D741A, which modestly reduced it. In contrast, deletion of a 21-amino acid segment enriched in aromatic amino acids on the C-terminal side of Cys-739 abolished YFP-NCX1 palmitoylation. We hypothesized that this segment forms an amphipathic α-helix whose properties facilitate Cys-739 palmitoylation. Introduction of negatively charged amino acids to the hydrophobic face or of helix-breaking prolines impaired palmitoylation of both YFP-NCX1 and FL-NCX1. Alanine mutations on the hydrophilic face of the helix significantly reduced FL-NCX1 palmitoylation. Of note, when the helix-containing segment was introduced adjacent to cysteines that are not normally palmitoylated, they became palmitoylation sites. In conclusion, we have identified an amphipathic α-helix in the NCX1 large intracellular loop that controls NCX1 palmitoylation. NCX1 palmitoylation is governed by a distal secondary structure element rather than by local primary sequence.

Structure-based dynamic arrays in regulatory domains of sodium-calcium exchanger (NCX) isoforms

Mammalian Na⁺/Ca²⁺ exchangers, NCX1 and NCX3, generate splice variants, whereas NCX2 does not. The CBD1 and CBD2 domains form a regulatory tandem (CBD12), where Ca²⁺ binding to CBD1 activates and Ca²⁺ binding to CBD2 (bearing the splicing segment) alleviates the Na⁺-induced inactivation. Here, the NCX2-CBD12, NCX3-CBD12-B, and NCX3-CBD12-AC proteins were analyzed by small-angle X-ray scattering (SAXS) and hydrogen-deuterium exchange mass-spectrometry (HDX-MS) to resolve regulatory variances in the NCX2 and NCX3 variants. SAXS revealed the unified model, according to which the Ca²⁺ binding to CBD12 shifts a dynamic equilibrium without generating new conformational states, and where more rigid conformational states become more populated without any global conformational changes. HDX-MS revealed the differential effects of the B and AC exons on the folding stability of apo CBD1 in NCX3-CBD12, where the dynamic differences become less noticeable in the Ca²⁺-bound state. Therefore, the apo forms predefine incremental changes in backbone dynamics upon Ca²⁺ binding. These observations may account for slower inactivation (caused by slower dissociation of occluded Ca²⁺ from CBD12) in the skeletal vs the brain-expressed NCX2 and NCX3 variants. This may have physiological relevance, since NCX must extrude much higher amounts of Ca²⁺ from the skeletal cell than from the neuron.

Profound regulation of Na/K pump activity by transient elevations of cytoplasmic calcium in murine cardiac myocytes

Small changes of Na/K pump activity regulate internal Ca release in cardiac myocytes via Na/Ca exchange. We now show conversely that transient elevations of cytoplasmic Ca strongly regulate cardiac Na/K pumps. When cytoplasmic Na is submaximal, Na/K pump currents decay rapidly during extracellular K application and multiple results suggest that an inactivation mechanism is involved. Brief activation of Ca influx by reverse Na/Ca exchange enhances pump currents and attenuates current decay, while repeated Ca elevations suppress pump currents. Pump current enhancement reverses over 3 min, and results are similar in myocytes lacking the regulatory protein, phospholemman. Classical signaling mechanisms, including Ca-activated protein kinases and reactive oxygen, are evidently not involved. Electrogenic signals mediated by intramembrane movement of hydrophobic ions, such as hexyltriphenylphosphonium (C6TPP), increase and decrease in parallel with pump currents. Thus, transient Ca elevation and Na/K pump inactivation cause opposing sarcolemma changes that may affect diverse membrane processes.

Modeling Na+-Ca2+ exchange in the heart: Allosteric activation, spatial localization, sparks and excitation-contraction coupling

The cardiac sodium (Na⁺)/calcium (Ca²⁺) exchanger (NCX1) is an electrogenic membrane transporter that regulates Ca²⁺ homeostasis in cardiomyocytes, serving mainly to extrude Ca²⁺ during diastole. The direction of Ca²⁺ transport reverses at membrane potentials near that of the action potential plateau, generating an influx of Ca²⁺ into the cell. Therefore, there has been great interest in the possible roles of NCX1 in cardiac Ca²⁺-induced Ca²⁺ release (CICR). Interest has been reinvigorated by a recent super-resolution optical imaging study suggesting that ~18% of NCX1 co-localize with ryanodine receptor (RyR2) clusters, and ~30% of additional NCX1 are localized to within ~120nm of the nearest RyR2. NCX1 may therefore occupy a privileged position in which to modulate CICR. To examine this question, we have developed a mechanistic biophysically-detailed model of NCX1 that describes both NCX1 transport kinetics and Ca²⁺-dependent allosteric regulation. This NCX1 model was incorporated into a previously developed super-resolution model of the Ca²⁺ spark as well as a computational model of the cardiac ventricular myocyte that includes a detailed description of CICR with stochastic gating of L-type Ca²⁺ channels and RyR2s, and that accounts for local Ca²⁺ gradients near the dyad via inclusion of a peri-dyadic (PD) compartment. Both models predict that increasing the fraction of NCX1 in the dyad and PD decreases spark frequency, fidelity, and diastolic Ca²⁺ levels. Spark amplitude and duration are less sensitive to NCX1 spatial redistribution. On the other hand, NCX1 plays an important role in promoting Ca²⁺ entry into the dyad, and hence contributing to the trigger for RyR2 release at depolarized membrane potentials and in the presence of elevated local Na⁺ concentration. Whole-cell simulation of NCX1 tail currents are consistent with the finding that a relatively high fraction of NCX1 (~45%) resides in the dyadic and PD spaces, with a dyad-to-PD ratio of roughly 1:2. Allosteric Ca²⁺ activation of NCX1 helps to "functionally localize" exchanger activity to the dyad and PD by reducing exchanger activity in the cytosol thereby protecting the cell from excessive loss of Ca²⁺ during diastole.

Development of a high-affinity peptide that prevents phospholemman (PLM) inhibition of the sodium/calcium exchanger 1 (NCX1)

NCX1 (Na⁺/Ca²⁺ exchanger 1) is an important regulator of intracellular Ca²⁺ and a potential therapeutic target for brain ischaemia and for diastolic heart failure with preserved ejection fraction. PLM (phospholemman), a substrate for protein kinases A and C, has been suggested to regulate NCX1 activity. However, although several studies have demonstrated that binding of phosphorylated PLM (pSer⁶⁸-PLM) leads to NCX1 inhibition, other studies have failed to demonstrate a functional interaction of these proteins. In the present study, we aimed to analyse the biological function of the pSer⁶⁸-PLM–NCX1 interaction by developing high-affinity blocking peptides. PLM was observed to co-fractionate and co-immunoprecipitate with NCX1 in rat left ventricle, and in co-transfected HEK (human embryonic kidney)-293 cells. For the first time, the NCX1–PLM interaction was also demonstrated in the brain. PLM binding sites on NCX1 were mapped to two regions by peptide array assays, containing the previously reported PASKT and QKHPD motifs. Conversely, the two NCX1 regions bound identical sequences in the cytoplasmic domain of PLM, suggesting that NCX1-PASKT and NCX1-QKHPD might bind to each PLM monomer. Using two-dimensional peptide arrays of the native NCX1 sequence KHPDKEIEQLIELANYQVLS revealed that double substitution of tyrosine for positions 1 and 4 (K1Y and D4Y) enhanced pSer⁶⁸-PLM binding 8-fold. The optimized peptide blocked binding of NCX1-PASKT and NCX1-QKHPD to PLM and reversed PLM(S68D) inhibition of NCX1 activity (both forward and reverse mode) in HEK-293 cells. Altogether our data indicate that PLM interacts directly with NCX1 and inhibits NCX1 activity when phosphorylated at Ser⁶⁸.

Na+/Ca2+ exchange and Na+/K+-ATPase in the heart

This paper is the third in a series of reviews published in this issue resulting from the University of California Davis Cardiovascular Symposium 2014: Systems approach to understanding cardiac excitation–contraction coupling and arrhythmias: Na⁺ channel and Na⁺ transport. The goal of the symposium was to bring together experts in the field to discuss points of consensus and controversy on the topic of sodium in the heart. The present review focuses on cardiac Na⁺/Ca²⁺ exchange (NCX) and Na⁺/K⁺-ATPase (NKA). While the relevance of Ca²⁺ homeostasis in cardiac function has been extensively investigated, the role of Na⁺ regulation in shaping heart function is often overlooked. Small changes in the cytoplasmic Na⁺ content have multiple effects on the heart by influencing intracellular Ca²⁺ and pH levels thereby modulating heart contractility. Therefore it is essential for heart cells to maintain Na⁺ homeostasis. Among the proteins that accomplish this task are the Na⁺/Ca²⁺ exchanger (NCX) and the Na⁺/K⁺ pump (NKA). By transporting three Na⁺ ions into the cytoplasm in exchange for one Ca²⁺ moved out, NCX is one of the main Na⁺ influx mechanisms in cardiomyocytes. Acting in the opposite direction, NKA moves Na⁺ ions from the cytoplasm to the extracellular space against their gradient by utilizing the energy released from ATP hydrolysis. A fine balance between these two processes controls the net amount of intracellular Na⁺ and aberrations in either of these two systems can have a large impact on cardiac contractility. Due to the relevant role of these two proteins in Na⁺ homeostasis, the emphasis of this review is on recent developments regarding the cardiac Na⁺/Ca²⁺ exchanger (NCX1) and Na⁺/K⁺ pump and the controversies that still persist in the field.

Calpain-3-mediated regulation of the Na⁺-Ca²⁺ exchanger isoform 3

Ca²⁺ disturbances are observed when Ca²⁺-dependent cysteine proteases malfunction, causing muscle weakness and wasting. For example, loss of calpain-3 (CAPN3) activity leads to limb-girdle muscular dystrophy 2A (LGMD2A). In neuronal excitotoxicity, the cleavage of the Na⁺-Ca²⁺ exchanger isoform 3 (NCX3) has been associated with an increase in activity and elevation of the Ca²⁺ content in the endoplasmic reticulum (ER). Since NCX3 is expressed in skeletal muscle, we evaluated the cleavage of different NCX3 splice variants by CAPN1 and CAPN3. Using Fura-2-based cellular Ca²⁺ imaging, we showed for the first time that CAPN3 increases NCX3 activity and that only NCX3-AC, the variant predominantly expressed in skeletal muscle, is sensitive to calpain. The silencing of the endogenous CAPN1 and the expression of the inactive form of CAPN3 (C129S CAPN3) confirmed the specificity for CAPN1 and CAPN3. Functional studies revealed that cellular Ca²⁺ uptake through the reverse mode of NCX3 was significantly increased independently of the mode of activation of the exchanger by either a rise in intracellular Ca²⁺ ([Ca²⁺]_i) or Na⁺ ([Na⁺]_i). Subsequently, the sensitivity to CAPN1 and CAPN3 could be abrogated by removal of the six residues coded in exon C of NCX3-AC. Additionally, mutation of the Leu-600 and Leu-601 suggested the presence of a cleavage site at Leu-602. The increased Ca²⁺ uptake of NCX3 might participate in the Ca²⁺ refilling of the sarcoplasmic reticulum (SR) after the excitation-contraction uncoupling following exercise and therefore be implicated in the impaired reticular Ca²⁺ storage observed in LGMD2A.

Palmitoylation of the Na/Ca exchanger cytoplasmic loop controls its inactivation and internalization during stress signaling

The electrogenic Na/Ca exchanger (NCX) mediates bidirectional Ca movements that are highly sensitive to changes of Na gradients in many cells. NCX1 is implicated in the pathogenesis of heart failure and a number of cardiac arrhythmias. We measured NCX1 palmitoylation using resin-assisted capture, the subcellular location of yellow fluorescent protein–NCX1 fusion proteins, and NCX1 currents using whole-cell voltage clamping. Rat NCX1 is substantially palmitoylated in all tissues examined. Cysteine 739 in the NCX1 large intracellular loop is necessary and sufficient for NCX1 palmitoylation. Palmitoylation of NCX1 occurs in the Golgi and anchors the NCX1 large regulatory intracellular loop to membranes. Surprisingly, palmitoylation does not influence trafficking or localization of NCX1 to surface membranes, nor does it strongly affect the normal forward or reverse transport modes of NCX1. However, exchangers that cannot be palmitoylated do not inactivate normally (leading to substantial activity in conditions when wild-type exchangers are inactive) and do not promote cargo-dependent endocytosis that internalizes 50% of the cell surface following strong G-protein activation or large Ca transients. The palmitoylated cysteine in NCX1 is found in all vertebrate and some invertebrate NCX homologs. Thus, NCX palmitoylation ubiquitously modulates Ca homeostasis and membrane domain function in cells that express NCX proteins.—Reilly, L., Howie, J., Wypijewski, K., Ashford, M. L. J., Hilgemann, D. W., Fuller, W. Palmitoylation of the Na/Ca exchanger cytoplasmic loop controls its inactivation and internalization during stress signaling.

Structure-dynamic determinants governing a mode of regulatory response and propagation of allosteric signal in splice variants of Na+/Ca2+ exchange (NCX) proteins

Ca2+ binding to CBD1 (calcium-binding domain 1) and CBD2 regulates Na+/Ca2+ exchangers (NCX). In the present study, we demonstrate that Ca2+ binding rigidifies the main chain of CBD2, but not of CBD1, in a splice variant-dependent manner. The dynamic differences account for variant-dependent responses to Ca2+.

Function and regulation of the Na+-Ca2+ exchanger NCX3 splice variants in brain and skeletal muscle

Isoform 3 of the Na(+)-Ca(2+) exchanger (NCX3) is crucial for maintaining intracellular calcium ([Ca(2+)]i) homeostasis in excitable tissues. In this sense NCX3 plays a key role in neuronal excitotoxicity and Ca(2+) extrusion during skeletal muscle relaxation. Alternative splicing generates two variants (NCX3-AC and NCX3-B). Here, we demonstrated that NCX3 variants display a tissue-specific distribution in mice, with NCX3-B as mostly expressed in brain and NCX-AC as predominant in skeletal muscle. Using Fura-2-based Ca(2+) imaging, we measured the capacity and regulation of the two variants during Ca(2+) extrusion and uptake in different conditions. Functional studies revealed that, although both variants are activated by intracellular sodium ([Na(+)]i), NCX3-AC has a higher [Na(+)]i sensitivity, as Ca(2+) influx is observed in the presence of extracellular Na(+). This effect could be partially mimicked for NCX3-B by mutating several glutamate residues in its cytoplasmic loop. In addition, NCX3-AC displayed a higher capacity of both Ca(2+) extrusion and uptake compared with NCX3-B, together with an increased sensitivity to intracellular Ca(2+). Strikingly, substitution of Glu(580) in NCX3-B with its NCX3-AC equivalent Lys(580) recapitulated the functional properties of NCX3-AC regarding Ca(2+) sensitivity, Lys(580) presumably acting through a structure stabilization of the Ca(2+) binding site. The higher Ca(2+) uptake capacity of NCX3-AC compared with NCX3-B is in line with the necessity to restore Ca(2+) levels in the sarcoplasmic reticulum during prolonged exercise. The latter result, consistent with the high expression in the slow-twitch muscle, suggests that this variant may contribute to the Ca(2+) handling beyond that of extruding Ca(2+).

Sodium-calcium exchangers (NCX): molecular hallmarks underlying the tissue-specific and systemic functions

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.

The SLC8 gene family of sodium-calcium exchangers (NCX) - structure, function, and regulation in health and disease

The SLC8 gene family encoding Na(+)/Ca(2+) exchangers (NCX) belongs to the CaCA (Ca(2+)/Cation Antiporter) superfamily. Three mammalian genes (SLC8A1, SLC8A2, and SLC8A3) and their splice variants are expressed in a tissue-specific manner to mediate Ca(2+)-fluxes across the cell-membrane and thus, significantly contribute to regulation of Ca(2+)-dependent events in many cell types. A long-wanted mitochondrial Na(+)/Ca(2+) exchanger has been recently identified as NCLX protein, representing a gene product of SLC8B1. Distinct NCX isoform/splice variants contribute to excitation-contraction coupling, long-term potentiation of the brain and learning, blood pressure regulation, immune response, neurotransmitter and insulin secretion, mitochondrial bioenergetics, etc. Altered expression and regulation of NCX proteins contribute to distorted Ca(2+)-homeostasis in heart failure, arrhythmia, cerebral ischemia, hypertension, diabetes, renal Ca(2+) reabsorption, muscle dystrophy, etc. Recently, high-resolution X-ray structures of Ca(2+)-binding regulatory domains of eukaryotic NCX and of full-size prokaryotic NCX have become available and the dynamic properties have been analyzed by advanced biophysical approaches. Molecular silencing/overexpression of NCX in cellular systems and organ-specific KO mouse models provided useful information on the contribution of distinct NCX variants to cellular and systemic functions under various pathophysiological conditions. Selective inhibition or activation of predefined NCX variants in specific diseases might have clinical relevance, although this breakthrough has not yet been realized. A better understanding of the underlying molecular mechanisms as well as the development of in vitro procedures for high-throughput screening of "drug-like" compounds may lead to selective pharmacological targeting of NCX variants.

Population shift underlies Ca2+-induced regulatory transitions in the sodium-calcium exchanger (NCX)

In eukaryotic Na(+)/Ca(2+) exchangers (NCX) the Ca(2+) binding CBD1 and CBD2 domains form a two-domain regulatory tandem (CBD12). An allosteric Ca(2+) sensor (Ca3-Ca4 sites) is located on CBD1, whereas CBD2 contains a splice-variant segment. Recently, a Ca(2+)-driven interdomain switch has been described, albeit how it couples Ca(2+) binding with signal propagation remains unclear. To resolve the dynamic features of Ca(2+)-induced conformational transitions we analyze here distinct splice variants and mutants of isolated CBD12 at varying temperatures by using small angle x-ray scattering (SAXS) and equilibrium (45)Ca(2+) binding assays. The ensemble optimization method SAXS analysis demonstrates that the apo and Mg(2+)-bound forms of CBD12 are highly flexible, whereas Ca(2+) binding to the Ca3-Ca4 sites results in a population shift of conformational landscape to more rigidified states. Population shift occurs even under conditions in which no effect of Ca(2+) is observed on the globally derived Dmax (maximal interatomic distance), although under comparable conditions a normal [Ca(2+)]-dependent allosteric regulation occurs. Low affinity sites (Ca1-Ca2) of CBD1 do not contribute to Ca(2+)-induced population shift, but the occupancy of these sites by 1 mM Mg(2+) shifts the Ca(2+) affinity (Kd) at the neighboring Ca3-Ca4 sites from ∼ 50 nM to ∼ 200 nM and thus, keeps the primary Ca(2+) sensor (Ca3-Ca4 sites) within a physiological range. Thus, Ca(2+) binding to the Ca3-Ca4 sites results in a population shift, where more constraint conformational states become highly populated at dynamic equilibrium in the absence of global conformational transitions in CBD alignment.

Na/Ca exchange and contraction of the heart

Sodium-calcium exchange (NCX) is the major calcium (Ca) efflux mechanism of ventricular cardiomyocytes. Consequently the exchanger plays a critical role in the regulation of cellular Ca content and hence contractility. Reductions in Ca efflux by the exchanger, such as those produced by elevated intracellular sodium (Na) in response to cardiac glycosides, raise sarcoplasmic reticulum (SR) Ca stores. The result is an increased Ca transient and cardiac contractility. Enhanced Ca efflux activity by the exchanger, for example during heart failure, may reduce diadic cleft Ca and excitation-contraction (EC) coupling gain. This aggravates the impaired contractility associated with SR Ca ATPase dysfunction and reduced SR Ca load in failing heart muscle. Recent data from our laboratories indicate that NCX can also impact the efficiency of EC coupling and contractility independent of SR Ca load through diadic cleft priming with Ca during the upstroke of the action potential. This article is part of a Special Issue entitled "Na(+) Regulation in Cardiac Myocytes".

Cardiac Na+-Ca2+ exchanger: dynamics of Ca2+-dependent activation and deactivation in intact myocytes

Cardiac Na(+)-Ca(2+) exchange (NCX) activity is regulated by [Ca(2+)]i. The physiological role and dynamics of this process in intact cardiomyocytes are largely unknown. We examined NCX Ca(2+) activation in intact rabbit and mouse cardiomyocytes at 37°C. Sarcoplasmic reticulum (SR) function was blocked, and cells were bathed in 2 mm Ca(2+). We probed Ca(2+) activation without voltage clamp by applying Na(+)-free (0 Na(+)) solution for 5 s bouts, repeated each 10 s, which should evoke [Ca(2+)]i transients due to Ca(2+) influx via NCX. In rested rabbit myocytes, Ca(2+) influx was undetectable even after 0 Na(+) applications were repeated for 2-5 min or more, suggesting that NCX was inactive. After external electric field stimulation pulses were applied, to admit Ca(2+) via L-type Ca(2+) channels, 0 Na(+) bouts activated Ca(2+) influx efficaciously, indicating that NCX had become active. Calcium activation increased with more field pulses, reaching a maximum typically after 15-20 pulses (1 Hz). At rest, NCX deactivated with a time constant typically of 20-40 s. An increase in [Na(+)]i, either in rabbit cardiomyocytes as a result of inhibition of Na(+)-K(+) pumping, or in mouse cardiomyocytes where normal [Na(+)]i is higher vs. rabbit, sensitized NCX to self-activation by 0 Na(+) bouts. In experiments with the SR functional but initially empty, the activation time course was slowed. It is possible that the SR initially accumulated Ca(2+) that would otherwise cause activation. We modelled Ca(2+) activation as a fourth-order highly co-operative process ([Ca]i required for half-activation K0.5act = 375 nm), with dynamics severalfold slower than the cardiac cycle. We incorporated this NCX model into an established ventricular myocyte model, which allowed us to predict responses to twitch stimulation in physiological conditions with the SR intact. Model NCX fractional activation increased from 0.1 to 1.0 as the frequency was increased from 0.2 to 2 Hz. By adjusting Ca(2+) activation on a multibeat time scale, NCX might better maintain a stable long-term Ca(2+) balance while contributing to the ability of myocytes to produce Ca(2+) transients over a wide range of intensity.

Influence of lipids on protein-mediated transmembrane transport

Transmembrane proteins are responsible for transporting ions and small molecules across the hydrophobic region of the cell membrane. We are reviewing the evidence for regulation of these transport processes by interactions with the lipids of the membrane. We focus on ion channels, including potassium channels, mechanosensitive and pentameric ligand gated ion channels, and active transporters, including pumps, sodium or proton driven secondary transporters and ABC transporters. For ion channels it has been convincingly shown that specific lipid-protein interactions can directly affect their function. In some cases, a combined approach of molecular and structural biology together with computer simulations has revealed the molecular mechanisms. There are also many transporters whose activity depends on lipids but understanding of the molecular mechanisms is only beginning.

Coordinated regulation of cardiac Na(+)/Ca (2+) exchanger and Na (+)-K (+)-ATPase by phospholemman (FXYD1)

Phospholemman (PLM) is the founding member of the FXYD family of regulators of ion transport. PLM is a 72-amino acid protein consisting of the signature PFXYD motif in the extracellular N terminus, a single transmembrane (TM) domain, and a C-terminal cytoplasmic tail containing three phosphorylation sites. In the heart, PLM co-localizes and co-immunoprecipitates with Na(+)-K(+)-ATPase, Na(+)/Ca(2+) exchanger, and L-type Ca(2+) channel. The TM domain of PLM interacts with TM9 of the α-subunit of Na(+)-K(+)-ATPase, while its cytoplasmic tail interacts with two small regions (spanning residues 248-252 and 300-304) of the proximal intracellular loop of Na(+)/Ca(2+) exchanger. Under stress, catecholamine stimulation phosphorylates PLM at serine(68), resulting in relief of inhibition of Na(+)-K(+)-ATPase by decreasing K(m) for Na(+) and increasing V(max), and simultaneous inhibition of Na(+)/Ca(2+) exchanger. Enhanced Na(+)-K(+)-ATPase activity lowers intracellular Na(+), thereby minimizing Ca(2+) overload and risks of arrhythmias. Inhibition of Na(+)/Ca(2+) exchanger reduces Ca(2+) efflux, thereby preserving contractility. Thus, the coordinated actions of PLM during stress serve to minimize arrhythmogenesis and maintain inotropy. In acute cardiac ischemia and chronic heart failure, either expression or phosphorylation of PLM or both are altered. PLM regulates important ion transporters in the heart and offers a tempting target for development of drugs to treat heart failure.

20 years from NCX purification and cloning: milestones

The Na(+)/Ca(2+) exchanger protein was first isolated from cardiac sarcolemma in 1988 and cloned in 1990. This allowed study of Na(+)/Ca(2+) exchange at the molecular level to begin. I will review the story leading to the cloning of NCX and the research that resulted from this event. This will include structure-function studies such as determination of the numbers of transmembrane segments and topological arrangement. Information on ion transport sites has been gathered from site-directed mutagenesis. The regions involved in Ca(2+) regulation have been identified, analyzed, and crystallized.We have also generated genetically altered mice to study the role of NCX in the myocardium. Of special interest are mice with atrial- or ventricular-specific KO of NCX that reveal new information on the role of NCX in excitation-contraction coupling and in cardiac pacemaker activity.

Cardiac sodium-calcium exchange and efficient excitation-contraction coupling: implications for heart disease

Cardiovascular disease is a leading cause of death worldwide, with ischemic heart disease alone accounting for >12% of all deaths, more than HIV/AIDS, tuberculosis, lung, and breast cancer combined. Heart disease has been the leading cause of death in the United States for the past 85 years and is a major cause of disability and health-care expenditures. The cardiac conditions most likely to result in death include heart failure and arrhythmias, both a consequence of ischemic coronary disease and myocardial infarction, though chronic hypertension and valvular diseases are also important causes of heart failure. Sodium-calcium exchange (NCX) is the dominant calcium (Ca2+) efflux mechanism in cardiac cells. Using ventricular-specific NCX knockout mice, we have found that NCX is also an essential regulator of cardiac contractility independent of sarcoplasmic reticulum Ca2+ load. During the upstroke of the action potential, sodium (Na+) ions enter the diadic cleft space between the sarcolemma and the sarcoplasmic reticulum. The rise in cleft Na+, in conjunction with depolarization, causes NCX to transiently reverse. Ca2+ entry by this mechanism then "primes" the diadic cleft so that subsequent Ca2+ entry through Ca2+ channels can more efficiently trigger Ca2+ release from the sarcoplasmic reticulum. In NCX knockout mice, this mechanism is inoperative (Na+ current has no effect on the Ca2+ transient), and excitation-contraction coupling relies upon the elevated diadic cleft Ca2+ that arises from the slow extrusion of cytoplasmic Ca2+ by the ATP-dependent sarcolemmal Ca2+ pump. Thus, our data support the conclusion that NCX is an important regulator of cardiac contractility. These findings suggest that manipulation of NCX may be beneficial in the treatment of heart failure.

Molecular determinants of allosteric regulation in NCX proteins

Allosteric activation of NCX involves the binding of cytosolic Ca(2+) to regulatory domains CBD1 and CBD2. Previous studies with isolated CBD12 and full-size NCX identified synergistic interactions between the two CBD domains that modify the affinity and kinetic properties of Ca(2+) sensing, although it remains unclear how the Ca(2+)-binding signal is decoded and propagates to transmembrane domains. Biophysical analyses (X-ray, SAXS, and stopped-flow techniques) of isolated preparations of CBD1, CBD2, and CBD12 have shown that Ca(2+) binding to Ca3-Ca4 sites of CBD1 results in interdomain tethering of CBDs through specific amino acids on CBD1 (Asp499 and Asp500) and CBD2 (Arg532 and Asp565). Mutant analyses of isolated CBDs suggest that the two-domain interface governs Ca(2+)-driven conformational alignment of CBDs, resulting in slow dissociation of Ca(2+) from CBD12, and thus, it mediates Ca(2+)-induced conformational transitions associated with allosteric signal transmission. Specifically, occupation of Ca3-Ca4 sites by Ca(2+) induces disorder-to-order transition owing to charge neutralization and coordination, thereby constraining CBD conformational freedom, rigidifying the NCX1 f-loop, and triggering allosteric signal transmission to the membrane domain. The newly found interdomain switch is highly conserved among NCX isoform/splice variants, although some additional structural motifs may shape the regulatory specificity of NCX variants.

Structural arrangement of the intracellular Ca2+ binding domains of the cardiac Na+/Ca2+ exchanger (NCX1.1): effects of Ca2+ binding

The cardiac Na(+)/Ca(2+) exchanger (NCX1.1) serves as the primary means of Ca(2+) extrusion across the plasma membrane of cardiomyocytes after the rise in intracellular Ca(2+) during contraction. The exchanger is regulated by binding of Ca(2+) to its intracellular domain, which contains two structurally homologous Ca(2+) binding domains denoted as CBD1 and CBD2. NMR and x-ray crystallographic studies have provided structures for the isolated CBD1 and CBD2 domains and have shown how Ca(2+) binding affects their structures and motional dynamics. However, structural information on the entire Ca(2+) binding domain, denoted CBD12, and how binding of Ca(2+) alters its structure and dynamics is more limited. Site-directed spin labeling has been employed in this work to address these questions. Electron paramagnetic resonance measurements on singly labeled constructs of CBD12 have identified the regions that undergo changes in dynamics as a result of Ca(2+) binding. Double electron-electron resonance (DEER) measurements on doubly labeled constructs of CBD12 have shown that the β-sandwich regions of the CBD1 and CBD2 domains are largely insensitive to Ca(2+) binding and that these two domains are widely separated at their N and C termini. Interdomain distances measured by DEER have been employed to construct structural models for CBD12 in the presence and absence of Ca(2+). These models show that there is not a major change in the relative orientation of the two Ca(2+) binding domains as a result of Ca(2+) binding in the NCX1.1 isoform. Additional measurements have shown that there are significant changes in the dynamics of the F-G loop region of CBD2 that merit further characterization with regard to their possible involvement in regulation of NCX1.1 activity.

Identification of the dimer interface of a bacterial Ca(2+)/H(+) antiporter

Members of the calcium/cation antiporter superfamily, including the cardiac sodium/calcium exchangers, are secondary active transporters that play an essential role in cellular Ca(2+) homeostasis. A notable feature of this group of transporters is the high levels of sequence similarity in relatively short sequences constituting the functionally important α-1 and α-2 regions in contrast to relatively lower degrees of similarity in the extended adjoining sequences. This suggests a similar structure and function of core transport machinery but possible differences in topology and/or oligomerization, a topic that has not been adequately addressed. Here we present the first example of purification of a bacterial member of this superfamily (CAX(CK31)) and analyze its quaternary structure. Purification of CAX(CK31) required the presence of a choline headgroup-containing detergent or lipid to yield stable preparations of the monomeric transporter. H(+)-driven Ca(2+) transport was demonstrated by reconstituting purified CAX(CK31) into liposomes. Dimeric CAX(CK31) could be isolated by manipulation of detergent micelles. Dimer formation was shown to be dependent on micelle composition as well as protein concentration. Furthermore, we establish that CAX(CK31) forms dimers in the membrane by analysis of cross-linked proteins. Using a dimeric homology model derived from the monomeric structure of the archaeal NCX homologue (Protein Data Bank entry 3V5U ), we introduced cysteine residues and through cross-linking experiments established the role of transmembrane helices 2 and 6 in the putative dimer interface.

G503 is obligatory for coupling of regulatory domains in NCX proteins

In multidomain proteins, interdomain linkers allow an efficient transfer of regulatory information, although it is unclear how the information encoded in the linker structure coins dynamic coupling. Allosteric regulation of NCX proteins involves Ca(2+)-driven tethering of regulatory CBD1 and CBD2 (through a salt bridge network) accompanied by alignment of CBDs and Ca(2+) occlusion at the interface of the two CBDs. Here we investigated "alanine-walk" substitutions in the CBD1-CBD2 linker (501-HAGIFT-506) and found that among all linker residues, only G503 is obligatory for Ca(2+)-induced reorientations of CBDs and slow dissociation of occluded Ca(2+). Moreover, swapping between positions A502 and G503 in the CBD1-CBD2 linker results in a complete loss of slow dissociation of occluded Ca(2+), meaning that dynamic coupling of CBDs requires an exact pose of glycine at position 503. Therefore, accumulating data revealed that position 503 occupied by glycine is absolutely required for Ca(2+)-driven tethering of CBDs, which in turn limits the linker's flexibility and, thus, restricts CBD movements. Because G503 is extremely well conserved in eukaryotic NCX proteins, the information encoded in G503 is essential for dynamic coupling of the two-domain CBD tandem and, thus, for propagation of the allosteric signal.

Ca2+ regulation of ion transport in the Na+/Ca2+ exchanger

The binding of Ca(2+) to two adjacent Ca(2+)-binding domains, CBD1 and CBD2, regulates ion transport in the Na(+)/Ca(2+) exchanger. As sensors for intracellular Ca(2+), the CBDs form electrostatic switches that induce the conformational changes required to initiate and sustain Na(+)/Ca(2+) exchange. Depending on the presence of a few key residues in the Ca(2+)-binding sites, zero to four Ca(2+) ions can bind with affinities between 0.1 to 20 μm. Importantly, variability in CBD2 as a consequence of alternative splicing modulates not only the number and affinities of the Ca(2+)-binding sites in CBD2 but also the Ca(2+) affinities in CBD1.

2-Color calcium pump reveals closure of the cytoplasmic headpiece with calcium binding

The sarco(endo)plasmic reticulum calcium ATPase (SERCA) undergoes conformational changes while transporting calcium, but the details of the domain motions are still unclear. The objective of the present study was to measure distances between the cytoplasmic domains of SERCA2a in order to reveal the magnitude and direction of conformational changes. Using fluorescence microscopy of live cells, we measured intramolecular fluorescence resonance energy transfer (FRET) from a donor fluorescent protein fused to the SERCA N-terminus to an acceptor fluorescent protein fused to either the N-, P-, or transmembrane domain. The "2-color" SERCA constructs were catalytically active as indicated by ATPase activity in vitro and Ca uptake in live cells. All constructs exhibited dynamic FRET changes in response to the pump ligands calcium and thapsigargin (Tg). These FRET changes were quantified as an index of SERCA conformational changes. Intramolecular FRET decreased with Tg for the two N-domain fusion sites (at residue 509 or 576), while the P- (residue 661) and TM-domain (C-terminus) fusions showed increased FRET with Tg. The magnitude of the Tg-dependent conformational change was not decreased by coexpression of phospholamban (PLB), nor did PLB slow the kinetics of Tg binding. FRET in ionophore-permeabilized cells was lower in EGTA than in saturating calcium for all constructs, indicating a decrease in domain separation distance with the structural transition from E2 (Ca-free) to E1 (Ca-bound). The data suggest closure of the cytoplasmic headpiece with Ca-binding. The present results provide insight into the structural dynamics of the Ca-ATPase. In addition, the 2-color SERCA constructs developed for this study may be useful for evaluating candidate small molecule regulators of Ca uptake activity.

A common Ca2+-driven interdomain module governs eukaryotic NCX regulation

Na(+)/Ca(2+) exchanger (NCX) proteins mediate Ca(2+)-fluxes across the cell membrane to maintain Ca(2+) homeostasis in many cell types. Eukaryotic NCX contains Ca(2+)-binding regulatory domains, CBD1 and CBD2. Ca(2+) binding to a primary sensor (Ca3-Ca4 sites) on CBD1 activates mammalian NCXs, whereas CALX, a Drosophila NCX ortholog, displays an inhibitory response to regulatory Ca(2+). To further elucidate the underlying regulatory mechanisms, we determined the 2.7 Å crystal structure of mammalian CBD12-E454K, a two-domain construct that retains wild-type properties. In conjunction with stopped-flow kinetics and SAXS (small-angle X-ray scattering) analyses of CBD12 mutants, we show that Ca(2+) binding to Ca3-Ca4 sites tethers the domains via a network of interdomain salt-bridges. This Ca(2+)-driven interdomain switch controls slow dissociation of "occluded" Ca(2+) from the primary sensor and thus dictates Ca(2+) sensing dynamics. In the Ca(2+)-bound conformation, the interdomain angle of CBD12 is very similar in NCX and CALX, meaning that the interdomain distances cannot account for regulatory diversity in NCX and CALX. Since the two-domain interface is nearly identical among eukaryotic NCXs, including CALX, we suggest that the Ca(2+)-driven interdomain switch described here represents a general mechanism for initial conduction of regulatory signals in NCX variants.

Dynamic features of allosteric Ca2+ sensor in tissue-specific NCX variants

The Na(+)-Ca(2+) exchanger (NCX) mediated Ca(2+) fluxes are essential for handling Ca(2+) homeostasis in many cell-types. Eukaryotic NCX variants contain regulatory CBD1 and CBD2 domains, whereas in distinct variants the Ca(2+) binding to Ca3-Ca4 sites of CBD1 results either in sustained activation, inhibition or no effect. CBD2 contains an alternatively spliced segment, which is expressed in a tissue-specific manner although its impact on allosteric regulation remains unclear. Recent studies revealed that the Ca(2+) binding to Ca3-Ca4 sites results in interdomain tethering of CBDs, which rigidifies CBDs movements with accompanied slow dissociation of "occluded" Ca(2+). Here we investigate the effects of CBD2 variants on Ca(2+) occlusion in the two-domain construct (CBD12). Mutational studies revealed that both sites (Ca3 and Ca4) contribute to Ca(2+) occlusion, whereas after dissociation of the first Ca(2+) ion the second Ca(2+) ion becomes occluded. This mechanism is common for the brain, kidney and cardiac splice variants of CBD12, although the occluded Ca(2+) exhibits 20-50-fold difference in off-rates among the tested variants. Therefore, the spliced exons on CBD2 affect the rate-limiting step of the occluded Ca(2+) dissociation at the primary regulatory sensor to shape dynamic features of allosteric regulation in NCX variants.

Structural and dynamic aspects of Ca2+ and Mg2+ binding of the regulatory domains of the Na+/Ca2+ exchanger

Intracellular Ca2+ regulates the activity of the NCX (Na+/Ca2+ exchanger) through binding to the cytosolic CBD (Ca2+-binding domain) 1 and CBD2. In vitro studies of the structure and dynamics of CBD1 and CBD2, as well as studies of their kinetics and thermodynamics of Ca2+ binding, greatly enhanced our understanding of NCX regulation. We describe the fold of the CBDs in relation to other known structures and review Ca2+ binding of the different CBD variants from a structural perspective. We also report on new findings concerning Mg2+ binding to the CBDs and finally we discuss recent results on CBD1–CBD2 interdomain interactions.

Structural basis of the Ca2+ inhibitory mechanism of Drosophila Na+/Ca2+ exchanger CALX and its modification by alternative splicing

The Na(+)/Ca(2+) exchanger CALX promotes Ca(2+) efflux in Drosophila sensory neuronal cells to facilitate light-mediated Ca(2+) homeostasis. CALX activity is negatively regulated by specific Ca(2+) interaction within its two intracellular Ca(2+) regulatory domains CBD1 and CBD2, yet how the Ca(2+) binding is converted to molecular motion to operate the exchanger is unknown. Here, we report crystal structures of the entire Ca(2+) regulatory domain CBD12 from two alternative splicing isoforms, CALX 1.1 and 1.2, exhibiting distinct regulatory Ca(2+) dependency. The structures show an open V-shaped conformation with four Ca(2+) ions bound on the CBD domain interface, confirmed by LRET analysis. The structures together with Ca(2+)-binding analysis support that the Ca(2+) inhibition of CALX is achieved by interdomain conformational changes induced by Ca(2+) binding at CBD1. The conformational difference between the two isoforms also indicates that alternative splicing adjusts the interdomain orientation angle to modify the Ca(2+) regulatory property of the exchangers.