Interaction of cardiac Na-Ca exchanger and exchange inhibitory peptide with membrane phospholipids

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.

Insulin effects on cardiac Na+/Ca2+ exchanger activity: role of the cytoplasmic regulatory loop

Insulin can alter myocardial contractility, in part through an effect on the cardiac sarcolemmal Na(+)/Ca(2+) exchanger (NCX), but little is known about its mechanism of action. The large cytoplasmic domain (f-loop) of NCX is required for regulation by various intracellular factors, and we have shown previously that residues 562-679 are determinants of NCX inhibition by exchanger inhibitory peptide (XIP). Here we show that the same f-loop deletion eliminates the enhancement of NCX current by insulin, and we examine the signal pathways involved in the insulin response. NCX current (I(NCX)) was measured in freshly isolated or cultured (up to 48 h) adult guinea pig myocytes and in myocytes expressing canine NCX1.1 with the 562-679 f-loop deletion (NCX-(Delta562-679)) via adenoviral gene transfer. I(NCX) was recorded by whole-cell patch clamp as the Ni(2+)-sensitive current at 37 degrees C with intracellular Ca(2+) buffered. Insulin (1 microm) increased I(NCX) (at +80 mV) by 110 and 83% in fresh and cultured myocytes, respectively, whereas in myocytes expressing NCX-(Delta562-679) the response was eliminated (with 100 microm XIP included to suppress any native guinea pig I(NCX)). The insulin effect on I(NCX) was not inhibited by wortmannin, a nitric-oxide synthase inhibitor, or disruption of caveolae but was blocked by chelerythrine, implicating protein kinase C, but not phosphatidylinositol-3-kinase, in the mechanism. The insulin effect was also not additive with phosphatidylinositol-4,5-bisphosphate-induced activation of I(NCX). The finding that the 562-670 f-loop domain is implicated in both XIP and receptor-mediated modulation of NCX highlights its important role in acute physiological or pathophysiological regulation of Ca(2+) balance in the heart.

Kinetics and ion specificity of Na+/Ca2+ exchange mediated by the reconstituted beef heart mitochondrial Na+/Ca2+ antiporter

The Na(+)/Ca(2+) antiporter was purified from beef heart mitochondria and reconstituted into liposomes containing fluorescent probes selective for Na(+) or Ca(2+). Na(+)/Ca(2+) exchange was strongly inhibited at alkaline pH, a property that is relevant to rapid Ca(2+) oscillations in mitochondria. The effect of pH was mediated entirely via an effect on the K(m) for Ca(2+). When present on the same side as Ca(2+), K(+) activated exchange by lowering the K(m) for Ca(2+) from 2 to 0.9 microM. The K(m) for Na(+) was 8 mM. In the absence of Ca(2+), the exchanger catalyzed high rates of Na(+)/Li(+) and Na(+)/K(+) exchange. Diltiazem and tetraphenylphosphonium cation inhibited both Na(+)/Ca(2+) and Na(+)/K(+) exchange with IC(50) values of 10 and 0.6 microM, respectively. The V(max) for Na(+)/Ca(2+) exchange was increased about fourfold by bovine serum albumin, an effect that may reflect unmasking of an autoregulatory domain in the carrier protein.

The cardiac sodium-calcium exchanger associates with caveolin-3

The cardiac Na/Ca exchanger's (NCX1) role in calcium homeostasis during myocardial contractility makes it a possible target of signaling factors regulating inotropy. Caveolae, structured invaginations of the plasmalemma, are known to concentrate a wide variety of signaling factors. The predominant coat proteins of caveolae, caveolins, dock to and regulate the activity of these signaling factors and other proteins through interaction with their scaffolding domain. In this study we investigated the interaction of NCX1 with caveolin proteins. Western blots of bovine cardiac sarcolemmal vesicles revealed the presence of caveolin-1, -2, and -3. Immunoprecipitation of detergent-solubilized vesicle proteins with either NCX1 or caveolin-3 antibodies indicated that NCX1 coprecipitates with caveolin-3, but not with caveolin-1 and -2. Functional disruption of caveolae, by beta-cyclodextrin treatment of vesicles, diminished coprecipitation of caveolin-3 and NCX1 activity. NCX1 has five potential caveolin-binding motifs, two of which are in the transporter's exchange inhibitory peptide (XIP) domain. The presence of 50 mM XIP peptide enhanced coprecipitation of caveolin-3 with NCX1 independent of calcium concentration. We conclude that NCX1 associates specifically with caveolin-3. Partitioning of NCX1 in caveolae has implications for temporal and spatial regulation of excitation-contraction and -relaxation coupling in cardiac myocytes.

The molecular determinants of ionic regulatory differences between brain and kidney Na+/Ca2+ exchanger (NCX1) isoforms

The Na(+)/Ca(2+) exchanger gene NCX1 undergoes alternative splicing leading to several isoforms that differ in a small portion of the large cytoplasmic loop. This loop is involved in many regulatory processes of NCX1, including ionic regulation by the transported substrates Na(+) and Ca(2+). High intracellular Ca(2+) can alleviate intracellular Na(+)-dependent inactivation in exon A (NCX1.4)-containing isoforms but not in those containing the mutually exclusive exon B (NCX1.3). Giant excised patches from Xenopus oocytes expressing various NCX1 constructs were used to examine the specific amino acids responsible for these observed regulatory differences. Using a chimeric approach, the region responsible was narrowed down to the small central part of exon A (IDDEEYEKNKTF). Replacing the second aspartic acid of this sequence with arginine (the corresponding amino acid in exon B) in an exon A background completely prevented the effect of Ca(2+) on intracellular Na(+)-dependent inactivation. Mutating the second lysine to cysteine (exon B) had a similar, but only partial, effect. The converse double mutant, but neither single mutation alone, introduced into an exon B background (arginine to aspartic acid and cysteine to lysine) was able to restore the NCX1.4 regulatory phenotype. These data demonstrate that aspartic acid 610 and lysine 617 (using the rat NCX1.4 numbering scheme) are critical molecular determinants of the unique Ca(2+) regulatory properties of NCX1.4.

Evidence for cardiac sodium-calcium exchanger association with caveolin-3

The interaction of cardiac Na+-Ca2+ exchange (NCX1) with caveolin proteins was investigated in sarcolemmal vesicles. Western blots of sarcolemmal vesicles revealed the presence of caveolin-1, -2, and -3. NCX1 co-fractionated more closely with caveolin-3 than caveolin-1 on sucrose density gradients. NCX1 has five possible caveolin-binding motifs and NCX1 co-precipitated specifically with caveolin-3. Molecular sieve column chromatography indicated that this co-precipitation was not due to incomplete solubilization of lipid raft microdomains. Cholesterol chelation in vesicles decreased NCX1 transport activity and caveolin-3 co-precipitation. NCX1 may play a role in caveolar transmembrane signaling in addition to its role in excitation-contraction coupling.

Expressing and purifying membrane transport proteins in high yield

Structural analysis of native or recombinant membrane transport proteins has been hampered by the lack of effective methodologies to purify sufficient quantities of active protein. We addressed this problem by expressing a polyhistidine tagged construct of the cardiac sodium-calcium exchanger (NCX1) in Trichoplusia ni larvae (caterpillars) from which membrane vesicles were prepared. Larvae vesicles containing recombinant NCX1-his protein supported NCX1 transport activity that was mechanistically not different from activity in native cardiac sarcolemmal vesicles although the specific activity was reduced. SDS-PAGE and Western blot analysis demonstrated the presence of both the 120 and 70 kDa forms of the NCX1 protein. Larvae vesicle proteins were solubilized in sodium cholate detergent and fractionated on a chelated Ni(2+) affinity chromatography column. After extensive washing, eluted fractions were mixed with soybean phospholipids and reconstituted. The resulting proteoliposomes contained NCX1 activity suggesting the protein retained native conformation. SDS-PAGE revealed two major bands at 120 and 70 kDa. Purification of large amounts of active NCX1 via this methodology should facilitate biophysical analysis of the protein. The larva expression system has broad-based application for membrane proteins where expression and purification of quantities required for physical analyses is problematic.

Intracellular pH regulation by Na(+)/H(+) exchange requires phosphatidylinositol 4,5-bisphosphate

The carrier-mediated, electroneutral exchange of Na(+) for H(+) across the plasma membrane does not directly consume metabolic energy. Nevertheless, acute depletion of cellular ATP markedly decreases transport. We analyzed the possible involvement of polyphosphoinositides in the metabolic regulation of NHE1, the ubiquitous isoform of the Na(+)/H(+) exchanger. Depletion of ATP was accompanied by a marked reduction of plasmalemmal phosphatidylinositol 4,5-bisphosphate (PIP(2)) content. Moreover, sequestration or hydrolysis of plasmalemmal PIP(2), in the absence of ATP depletion, was associated with profound inhibition of NHE1 activity. Examination of the primary structure of the COOH-terminal domain of NHE1 revealed two potential PIP(2)-binding motifs. Fusion proteins encoding these motifs bound PIP(2) in vitro. When transfected into antiport-deficient cells, mutant forms of NHE1 lacking the putative PIP(2)-binding domains had greatly reduced transport capability, implying that association with PIP(2) is required for optimal activity. These findings suggest that NHE1 activity is modulated by phosphoinositides and that the inhibitory effect of ATP depletion may be attributable, at least in part, to the accompanying net dephosphorylation of PIP(2).

Interaction of PIP(2) with the XIP region of the cardiac Na/Ca exchanger

The sarcolemmal Na/Ca exchanger undergoes an inactivation process in which exchange activity decays over several seconds following activation by the application of Na to the intracellular surface of the protein. Inactivation is eliminated by an increase in membrane phosphatidylinositol 4,5-bisphosphate (PIP(2)). Inactivation is also strongly affected by mutations to a basic 20-amino acid segment of the exchanger known as the endogenous XIP region. The hypothesis that PIP(2) directly interacts with the XIP region of the exchanger was tested. First, we investigated the ability of a peptide with the same sequence as the XIP region to bind to immobilized phospholipid vesicles. (125)I-labeled XIP bound avidly to vesicles containing only a low concentration (<3%) of PIP(2). The binding was specific, in that binding was not displaced by other basic peptides. The effects of altering the sequence of XIP peptides also indicated binding specificity. Second, we examined the functional response to PIP(2) of exchangers with mutated XIP regions. Outward Na/Ca exchange currents were measured using the giant excised patch technique. The mutated exchangers either had no inactivation or accelerated inactivation. In both cases, the exchangers no longer responded to PIP(2) or to PIP(2) antibodies. Overall, the data indicate that the affinity of the endogenous XIP region for PIP(2) is an important determinant of the inactivation process.

Large-scale expression of recombinant cardiac sodium-calcium exchange in insect larvae

Recombinant bovine cardiac sodium-calcium exchange (NCX1) in a baculovirus construct was used to infect cabbage looper larvae (Trichoplusia ni). Infected larvae were homogenized and larvae membrane vesicles were purified. Western blot analysis indicated the presence of recombinant NCX1 protein in vesicles from infected larvae but not in controls. Vesicles from infected larvae expressed high levels of NCX1 activity (1.7 nmol Ca2+/mg protein/s) while vesicles from control larvae had no activity. NCX1 in larvae vesicles was bidirectional. Kinetic analysis yielded a Vmax of 3.6 nmol Ca2+/mg protein/s and a Km for Ca of 4.2 microM. NCX1 activity was inhibited by the exchange inhibitory peptide with an IC50 of 4 microM. These data demonstrate a novel and efficient method for the expression of large amounts of active recombinant NCX1 protein that has general application for expression and analysis of recombinant membrane proteins.

Na+/Ca2+ exchange and cellular Ca2+ homeostasis

The Na+/Ca2+ exchange system is the primary Ca2+ efflux mechanism in cardiac myocytes, and plays an important role in controlling the force of cardiac contraction. The exchanger protein contains 11 transmembrane segments plus a large hydrophilic domain between the 5th and 6th transmembrane segments; the transmembrane regions are responsible for mediating ion translocation while the hydrophilic domain is responsible for regulation of activity. Exchange activity is regulated in vitro by interconversions between an active state and either of two inactive states. High concentrations of cytosolic Na+ or the absence of cytosolic Ca2+ promote the formation of the inactive states; phosphatidylinositol-(4,5)bisphosphate (or other negatively charged phospholipids) and cytosolic Ca2+ counteract the inactivation process. The importance of these mechanisms in regulating exchange activity under normal physiological conditions is uncertain. Exchanger function is also dependent upon cytoskeletal interactions, and the exchanger's location with respect to intracellular Ca2+-sequestering organelles. An understanding of the exchanger's function in normal cell physiology will require more detailed information on the proximity of the exchanger and other Ca2+-transporting proteins, their interactions with the cytoskeleton, and local concentrations of anionic phospholipids and transported ions.

Functional comparison of the three isoforms of the Na+/Ca2+ exchanger (NCX1, NCX2, NCX3)

Three distinct mammalian Na+/Ca2+ exchangers have been cloned: NCX1, NCX2, and NCX3. We have undertaken a detailed functional comparison of these three exchangers. Each exchanger was stably expressed at high levels in the plasma membranes of BHK cells. Na+/Ca2+ exchange activity was assessed using three different complementary techniques: Na+ gradient-dependent 45Ca2+ uptake into intact cells, Na+ gradient-dependent 45Ca2+ uptake into membrane vesicles isolated from the transfected cells, and exchange currents measured using giant patches of excised cell membrane. Apparent affinities for the transported ions Na+ and Ca2+ were markedly similar for the three exchangers at both membrane surfaces. Likewise, generally similar responses to changes in pH, chymotrypsin treatment, and application of various inhibitors were obtained. Depletion of cellular ATP inhibited NCX1 and NCX2 but did not affect the activity of NCX3. Exchange activities of NCX1 and NCX3 were modestly increased by agents that activate protein kinases A and C. All exchangers were regulated by intracellular Ca2+. NCX1-induced exchange currents were especially large in excised patches and, like the native myocardial exchanger, were stimulated by ATP. Results may be influenced by our choice of expression system and specific splice variants, but, overall, the three exchangers appear to have very similar properties.

The selective activation of the cardiac sarcolemmal sodium-calcium exchanger by plasmalogenic phosphatidic acid produced by phospholipase D

Since plasmalogens are the predominant phospholipid of cardiac sarcolemma, the activation of the sodium-calcium exchanger by either plasmenylethanolamine or plasmalogenic phosphatidic acid generated by phospholipase D was explored. Sodium-calcium exchange activity was 7-fold greater in proteoliposomes comprised of plasmenylethanolamine compared to proteoliposomes comprised of only plasmenylcholine. Phospholipase D treatment of proteoliposomes resulted in 1 mol % conversion of plasmenylcholine or phosphatidylcholine to their respective phosphatidic acid molecular species with a concomitant 8-fold or 2-fold activation of sodium-calcium exchange activity, respectfully. Thus, phospholipase D-mediated hydrolysis of plasmalogens to phosphatidic acid may be an important mechanism for the regulation of the sodium-calcium exchanger.

Localization of an exchange inhibitory peptide (XIP) binding site on the cardiac sodium-calcium exchanger

The exchange inhibitory peptide (XIP; RRLLFYKYVYKRYRAGKQRG) is a potent inhibitor of cardiac Na-Ca exchange activity. This study attempted to identify the XIP binding site on the Na-Ca exchange protein. Bovine cardiac sarcolemmal vesicles were proteolyzed and fractionated by XIP-affinity column chromatography. A 24 kDa fragment was purified and subjected to amino acid sequence analysis. A negatively charged region of intracellular loop f of the Na-Ca exchange protein (IDDDIFEEDEN; aa 445-455) was identified. The affinity and specificity of XIP interaction with peptides IDDDIFEEDEN and GEDDDDDECGEE (another negatively charged region of the Na-Ca exchange protein) were examined. XIP cross-linked to peptide IDDDIFEEDEN but not GEDDDDDECGEE in a pH-dependent manner. Fluorescence titration binding studies indicated that binding of IDDDIFEEDEN with XIP was saturable (Kd=5 microM) while binding with GEDDDDDECGEE was not specific. These data suggest that amino acids 445-455 on Na-Ca exchange loop f are involved in XIP binding.

Plasmalogen and anionic phospholipid dependence of the cardiac sarcolemmal sodium-calcium exchanger

Although plasmalogens are the predominant phospholipids of cardiac sarcolemma, their physiological role has not been forthcoming. Since the cardiac sarcolemmal sodium-calcium exchanger has been proposed to be regulated by anionic phospholipids, the roles of plasmalogens and anionic phospholipids as regulators of the sodium-calcium exchanger were explored. Reconstituted sodium-calcium exchange activity in plasmalogen-containing proteoliposomes was 10-fold higher than that in control proteoliposomes comprised of only diacyl phospholipids. Additionally, exchange activity in plasmalogen-containing proteoliposomes was regulated by anionic phospholipids. Thus, plasmalogens provide a critical lipid environment in which anionic phospholipids serve as boundary lipids for the regulation of the trans-sarcolemmal sodium-calcium exchanger.

Regulation of cardiac Na+,Ca2+ exchange and KATP potassium channels by PIP2

Cardiac Na+,Ca2+ exchange is activated by a mechanism that requires hydrolysis of adenosine triphosphate (ATP) but is not mediated by protein kinases. In giant cardiac membrane patches, ATP acted to generate phosphatidylinositol-4,5-bisphosphate (PIP2) from phosphatidylinositol (PI). The action of ATP was abolished by a PI-specific phospholipase C (PLC) and recovered after addition of exogenous PI; it was reversed by a PIP2-specific PLC; and it was mimicked by exogenous PIP2. High concentrations of free Ca2+ (5 to 20 microM) accelerated reversal of the ATP effect, and PLC activity in myocyte membranes was activated with a similar Ca2+ dependence. Aluminum reversed the ATP effect by binding with high affinity to PIP2. ATP-inhibited potassium channels (KATP) were also sensitive to PIP2, whereas Na+,K+ pumps and Na+ channels were not. Thus, PIP2 may be an important regulator of both ion transporters and channels.