Multiple transport modes of the cardiac Na+/Ca2+ exchanger

Kang, Tong Mook; Hilgemann, Donald W.

doi:10.1038/nature02271

Cited by 139 publications

(128 citation statements)

References 27 publications

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“…Thus, the detection of concurrent electron density signals for three Na + and one Ca 2+ must reflect the coexistence of protein molecules with either of these two ion configurations in the crystal lattice. On the other hand, configurations in which Na + and Ca 2+ are simultaneously bound cannot be entirely ruled out; indeed, a minor transport mode has been reportedly observed for cardiac NCX1 whereby one Na + and one Ca 2+ are cotranslocated (9). Either way, it is puzzling that the local structure of the binding sites revealed by the electron density is uniquely defined, as it seems unlikely that this structure is identical irrespective of which ion type is bound.…”

mentioning

confidence: 86%

“…E213 and E54 are, however, always ionized, because in all considered cases they coordinate a cation directly. In addition, we explore the configurations that might be expected for a K + -dependent exchanger, whether or not K + is transported, as well as mixed Na + /Ca 2+ configurations that might correspond to atypical stoichiometries, as noted for cardiac NCX1 (9).…”

Section: Simulations Identify Three Ion Configurations Similarly Consmentioning

confidence: 99%

“…It is well established that NCXs are reversible and electrogenic, but it has been debated whether the Na + /Ca 2+ exchange stoichiometry is 3:1 or 4:1 (3,5,(7)(8)(9)(10)(11)(12). In any case, NCXs can also facilitate Na + /Na + and Ca 2+ /Ca 2+ exchange, implying that the translocation of Na + and Ca 2+ are two distinct reactions (3,5,6,13).…”

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confidence: 99%

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Sodium recognition by the Na ⁺ /Ca ²⁺ exchanger in the outward-facing conformation

Marinelli

Almagor

Hiller

et al. 2014

Proc. Natl. Acad. Sci. U.S.A.

124

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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 S ext , S Ca , and S int sites, whereas the S mid 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 S mid . 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.secondary transporters | membrane antiporters | ion specificity | CaCA superfamily | molecular-dynamics simulations C a 2+ signals control a variety of cellular processes essential for the basic function of multiple organs. In cardiac cells, for example, Ca 2+ release from the sarcoplasmic reticulum is a necessary step for heart contraction, whereas Ca 2+ extrusion from the cell is required for cardiac relaxation. These fluctuations in the cytosolic Ca 2+ concentration underlie the initiation of the heartbeat (1, 2). Na + /Ca 2+ exchangers (NCXs) play a central role in the homeostasis of cellular Ca 2+ (3-5). These integral membrane proteins are ubiquitous in many types of tissues including the heart, brain, and kidney (4), and consequently their dysfunction is associated with numerous human pathologies such as cardiac arrhythmia, hypertension, skeletal muscle dystrophy, and postischemic brain damage (5). NCXs facilitate the translocation of either Ca 2+ or Na + across the membrane; thus, they can harness a transmembrane sodium motive force to energize Ca 2+ transport against a concentration gradient. For example, the cardiac exchanger NCX1 mediates the extrusion of intracellular Ca 2+ driven by a Na + transmembrane gradient maintained by the Na ...

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confidence: 86%

Section: Simulations Identify Three Ion Configurations Similarly Consmentioning

confidence: 99%

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Sodium recognition by the Na ⁺ /Ca ²⁺ exchanger in the outward-facing conformation

Marinelli

Almagor

Hiller

et al. 2014

Proc. Natl. Acad. Sci. U.S.A.

124

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“…Uniquely, under physiological conditions, the sarcolemmal Na + /Ca 2+ exchanger (NCX) operates in both the forward-mode (Ca 2+ extrusion/Na + entry) and reverse-mode (Ca 2+ entry/Na + extrusion) during different phases of the cardiac cycle, depending on the dynamically changing membrane potential and gradients of Na + and Ca 2+ across the sarcolemma. Its sensitivity to membrane voltage is conferred by its electrogenic transport properties (predominantly 3Na + :1Ca 2+ stoichiometry, but see [2] for evidence of other transport modes). The complex dependence of NCX on the transmembrane gradients of Na + and Ca 2+ , as well as membrane voltage, makes it difficult to make simple assumptions about its role in cardiac function in healthy and diseased states.…”

Section: Na I + Balance: the Playersmentioning

confidence: 99%

The role of Na dysregulation in cardiac disease and how it impacts electrophysiology

O’Rourke

Maack

2007

Drug Discovery Today: Disease Models

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Ca 2+ is well known as the central player in cardiac cell physiology, mediating Ca 2+ activation of myosin ATPase and contraction, the stimulation of Ca 2+ -activated signaling pathways and modulation of mitochondrial energy production. Abnormalities of Ca 2+ handling are a well-studied mechanism of decompensation in heart failure. Less appreciated is the role of cytosolic Na + (Na i + ), which can dramatically influence the transfer rates and distribution of Ca 2+ among the intracellular compartments of the myocyte. Since Na i + can vary widely under different physiological and pathological conditions, and its effects depend on multiple ion gradients and membrane electrical potentials, unraveling the global influence of Na i + on cell function is complex, requiring an integrative view of cardiomyocyte physiology. Here, we discuss how abnormal Na i + regulation not only influences the cytosolic Ca 2+ transient and the cellular action potential but also alters mitochondrial Ca 2+ uptake and the balance of energy supply and demand of the cardiomyocyte, which may contribute to oxidative stress and cardiac decompensation. The implications for sudden cardiac death and the potential for novel therapeutic interventions are discussed. Na i + balance: the playersNa i + balance in the heart cell is maintained by a variety of ion channels, pumps and exchangers whose function is strongly influenced by the transmembrane gradients of various ions, and in some cases, by the electrical potential across the sarcolemmal or organelle membranes ( Fig. 1; see [1] for a review). At the sarcolemma, two well-known processes contribute to unidirectional Na + fluxes during the cardiac cycle; voltage-dependent Na + channels mediate the rapid inward Na + current (I Na ) during the upstroke of the cellular action potential and may also contribute to persistent Na + influx, while the Na + /K + ATPase (NKA) utilizes the energy released by ATP hydrolysis to pump Na + out of the cell against a concentration gradient in exchange for K + in an electrogenic process (3Na + :2K + ). A variety of electroneutral exchangers also influence Na i + when ion homeostasis is disturbed under pathological conditions; these include the Na + /H + exchanger (NHE), the Na + /HCO 3 -cotransporter (NBC), the Na + /K + / 2Cl -cotransporter (NKCC) and the Na + /Mg 2+ exchanger (NMgX). Influx of Na + through these pathways is generally well compensated by NKA action. Uniquely, under physiological conditions, the sarcolemmal Na + /Ca 2+ exchanger (NCX) operates in both the forward-mode (Ca 2+ extrusion/Na + entry) and reverse-mode (Ca 2+ entry/Na + extrusion) during different phases of the cardiac cycle, depending on the dynamically changing membrane potential and gradients of Na + and Ca 2+ across the sarcolemma. Its sensitivity to membrane voltage is

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“…Considering that mast cells have a resting potential of about Ϫ70 mV and Fc⑀RI receptor stimulation depolarizes the cells by Ͻ20 mV (26,27), an inversion of the transport is very unlikely and, therefore, NCKX exchangers might in the main support Ca 2ϩ efflux from mast cells. Conversely, the K ϩ -independent exchanger NCX1 can mediate Ca 2ϩ efflux, electroneutral Ca 2ϩ influx as well as Na ϩ influx (28). Based on the stoichiometry of 3 Na ϩ :1 Ca 2ϩ for the major transport mode, it can be predicted that the reversal potential of K ϩ -independent Na ϩ /Ca 2ϩ exchangers is close to Ϫ60 mV under physiological conditions (7).…”

Section: Role Of Na ϩ /Ca 2ϩ Exchangers In Mast Cellsmentioning

confidence: 99%

Modulation of Ca2+ Signaling by Na+/Ca2+ Exchangers in Mast Cells

Aneiros

Philipp

Lis

et al. 2005

The Journal of Immunology

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Mast cells rely on Ca2+ signaling to initiate activation programs leading to release of proinflammatory mediators. The interplay between Ca2+ release from internal stores and Ca2+ entry through store-operated Ca2+ channels has been extensively studied. Using rat basophilic leukemia (RBL) mast cells and murine bone marrow-derived mast cells, we examine the role of Na+/Ca2+ exchangers. Calcium imaging experiments and patch clamp current recordings revealed both K+-independent and K+-dependent components of Na+/Ca2+ exchange. Northern blot analysis indicated the predominant expression of the K+-dependent sodium-calcium exchanger NCKX3. Transcripts of the exchangers NCX3 and NCKX1 were additionally detected in RBL cells with RT-PCR. The Ca2+ clearance via Na+/Ca2+ exchange represented ∼50% of the total clearance when Ca2+ signals reached levels ≥200 nM. Ca2+ signaling and store-operated Ca2+ entry were strongly reduced by inverting the direction of Na+/Ca2+ exchange, indicating that Na+/Ca2+ exchangers normally extrude Ca2+ ions from cytosol and prevent the Ca2+-dependent inactivation of store-operated Ca2+ channels. Working in the Ca2+ efflux mode, Na+/Ca2+ exchangers such as NCKX3 and NCX3 might, therefore, play a role in the Ag-induced mast cell activation by controlling the sustained phase of Ca2+ mobilization.

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Multiple transport modes of the cardiac Na+/Ca2+ exchanger

Cited by 139 publications

References 27 publications

Sodium recognition by the Na ⁺ /Ca ²⁺ exchanger in the outward-facing conformation

Sodium recognition by the Na ⁺ /Ca ²⁺ exchanger in the outward-facing conformation

The role of Na dysregulation in cardiac disease and how it impacts electrophysiology

Modulation of Ca2+ Signaling by Na+/Ca2+ Exchangers in Mast Cells

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Multiple transport modes of the cardiac Na+/Ca2+ exchanger

Cited by 139 publications

References 27 publications

Sodium recognition by the Na + /Ca 2+ exchanger in the outward-facing conformation

Sodium recognition by the Na + /Ca 2+ exchanger in the outward-facing conformation

The role of Na dysregulation in cardiac disease and how it impacts electrophysiology

Modulation of Ca2+ Signaling by Na+/Ca2+ Exchangers in Mast Cells

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Sodium recognition by the Na ⁺ /Ca ²⁺ exchanger in the outward-facing conformation

Sodium recognition by the Na ⁺ /Ca ²⁺ exchanger in the outward-facing conformation