Ionic regulatory properties of brain and kidney splice variants of the NCX1 Na(+)-Ca(2+) exchanger

K+ -dependent Na+/Ca2+ exchangers: key contributors to Ca2+ signaling

An elevation in cytosolic Ca2+ is a universal signaling mechanism that controls a vast array of physiological processes. K+ -dependent Na+/Ca2+ exchangers are a newly identified family of Ca2+ efflux transporters that play important and diverse roles in cellular Ca2+ homeostasis.

A comparison of adenovirally delivered molecular methods to inhibit Na+/Ca2+ exchange

The ability to use molecular biology tools to down-regulate Na+/Ca2+ exchanger (NCX) expression will allow us to better understand the regulation of Ca(i)2+ and contractility in heart. Three different techniques to deplete NCX expression were compared: short hairpin RNA (shRNA), antisense RNA and exchanger inhibitory peptide expression via adenoviral transfection. Our results demonstrate that the most efficient method to deplete NCX expression and activity from cardiomyocytes is shRNA. It is also possible to replace the endogenous NCX with alternative isoforms or mutant forms of the NCX. Adenovirally delivered shRNA is an efficient tool for the study of the NCX and could be adapted for many other cardiac proteins.

Ca2+ regulation in the Na+/Ca2+ exchanger involves two markedly different Ca2+ sensors

The plasma membrane Na+/Ca2+ exchanger (NCX) is almost certainly the major Ca2+ extrusion mechanism in cardiac myocytes. Binding of Na+ and Ca2+ ions to its large cytosolic loop regulates ion transport of the exchanger. We determined the solution structures of two Ca2+ binding domains (CBD1 and CBD2) that, together with an alpha-catenin-like domain (CLD), form the regulatory exchanger loop. CBD1 and CBD2 are very similar in the Ca2+ bound state and describe the Calx-beta motif. Strikingly, in the absence of Ca2+, the upper half of CBD1 unfolds while CBD2 maintains its structural integrity. Together with a 7-fold higher affinity for Ca2+, this suggests that CBD1 is the primary Ca2+ sensor. Specific point mutations in either domain largely allow the interchange of their functionality and uncover the mechanism underlying Ca2+ sensing in NCX.

Cells expressing unique Na+/Ca2+exchange (NCX1) splice variants exhibit different susceptibilities to Ca2+overload

The Na+/Ca2+exchanger (NCX) NCX1 exhibits tissue-specific alternative splicing. Such NCX splice variants as NCX1.1 and NCX1.3 are also differentially regulated by Na+and Ca2+, although the physiological implications of these regulatory characteristics are unclear. On the basis of their distinct regulatory profiles, we hypothesized that cells expressing these different splice variants might exhibit unique responses to conditions promoting Ca2+overload, such as during exposure to cardiac glycosides or simulated ischemia. NCX1.1 or NCX1.3 was expressed in human embryonic kidney (HEK)-293 cells or rat neonatal ventricular cardiomyocytes (NVC), and expression was confirmed by Western blotting and immunocytochemical analyses. HEK-293 cells lacked NCX1 protein before transfection. With use of adenoviral vectors, neonatal cardiomyocytes were induced to overexpress the NCX1.1 splice variant by nearly twofold, whereas the NCX1.3 isoform was expressed on the endogenous NCX1.1 background. Total expression was comparable for NCX1.1 and NCX1.3. Exposure of NVC to ouabain induced a significant increase in cellular Ca2+, an effect that was exaggerated in cells overexpressing NCX1.1, but not NCX1.3. The increase in intracellular Ca2+was inhibited by 5 μM KB-R7943. Cardiomyocytes overexpressing NCX1.1 also exhibited a greater accumulation of intracellular Ca2+in response to simulated ischemia than did cells expressing NCX1.3. Similar responses were observed in HEK-293 cells where NCX1.1 was expressed. We conclude that expression of the NCX1.3 splice variant protects against severe Ca2+overload, whereas NCX1.1 promotes Ca2+overload in response to cardiac glycosides and ischemic challenges. These results highlight the importance of ionic regulation in controlling NCX1 activity under conditions that promote Ca2+overload.

Disturbed Ca2+-signaling by chloroacetaldehyde: a possible cause for chronic ifosfamide nephrotoxicity

BACKGROUND

Renal damage following chemotherapy with ifosfamide is attributed to the metabolic activation of the drug and the generation of chloroacetaldehyde (CAA). Little is known about the mechanism by which CAA impairs renal function. In this study the effect of CAA on intracellular Ca(2+) homeostasis in human renal proximal tubule cells (RPTEC) in primary culture was investigated.

METHODS

Intracellular Ca(2+) was measured using the Ca(2+)-sensitive dye fura-2. Cell viability was determined by protein content and cell number. Oncotic and apoptotic cell death was assayed using trypan blue exclusion, caspase-3 activity, and 4',6-diamino-2-phenylindole (DAPI) staining.

RESULTS

CAA (1.5 to 150 micromol/L) induced sustained elevations of intracellular free calcium ([Ca(2+)](i)) from 75 +/- 3 nmol/L to maximal 151 +/- 6 nmol/L. This effect was dependent on extracellular Ca(2+), but not Ca(2+) entry. The rise in [Ca(2+)](i) mediated by CAA could be attributed to inhibition of Na(+)-dependent extrusion of intracellular Ca(2+), indicating an inhibitory action of CAA on Na(+)/Ca(2+) exchange. Modulation of protein kinase A (PKA), but not protein kinase C (PKC) blunted the effect of CAA. Thus, CAA seems to inhibit Na(+)/Ca(2+) exchange by interaction with cyclic adenosine monophosphate (cAMP)-PKA-signaling. A 48-hour exposure to 15 micromol/L CAA significantly reduced cell number and protein content of RPTEC by induction of necrosis. This effect of 15 micromol/L CAA could be overcome by coadministration of the intracellular Ca(2+) chelator 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid acetoxymethyl ester (BAPTA-AM).

CONCLUSION

First, CAA inhibits the Na+/Ca2+-exchanger. Second, this effect is dependent on PKA. Third, CAA induces necrotic rather than apoptotic cell death. Finally, disturbed Ca(2+) homeostasis via Na(+)/Ca(2+) exchange contributes to the nephrotoxic action of CAA in RPTEC.

Frequency-dependent regulation of cardiac Na(+)/Ca(2+) exchanger

The activity of the cardiac Na(+)/Ca(2+) exchanger (NCX1.1) undergoes continuous modulation during the contraction-relaxation cycle because of the accompanying changes in the electrochemical gradients for Na(+) and Ca(2+). In addition, NCX1.1 activity is also modulated via secondary, ionic regulatory mechanisms mediated by Na(+) and Ca(2+). In an effort to evaluate how ionic regulation influences exchange activity under pulsatile conditions, we studied the behavior of the cloned NCX1.1 during frequency-controlled changes in intracellular Na(+) and Ca(+) (Na(i)(+) and Ca(i)(2+)). Na(+)/Ca(2+) exchange activity was measured by the giant excised patch-clamp technique with conditions chosen to maximize the extent of Na(+)- and Ca(2+)-dependent ionic regulation so that the effects of variables such as pulse frequency and duration could be optimally discerned. We demonstrate that increasing the frequency or duration of solution pulses leads to a progressive decline in pure outward, but not pure inward, Na(+)/Ca(2+) exchange current. However, when the exchanger is permitted to alternate between inward and outward transport modes, both current modes exhibit substantial levels of inactivation. Changes in regulatory Ca(2+), or exposure of patches to limited proteolysis by alpha-chymotrypsin, reveal that this "coupling" is due to Na(+)-dependent inactivation originating from the outward current mode. Under physiological ionic conditions, however, evidence for modulation of exchange currents by Na(i)(+)-dependent inactivation was not apparent. The current approach provides a novel means for assessment of Na(+)/Ca(2+) exchange ionic regulation that may ultimately prove useful in understanding its role under physiological and pathophysiological conditions.

cDNA cloning and expression of the cardiac Na+/Ca2+ exchanger from Mozambique tilapia (Oreochromis mossambicus) reveal a teleost membrane transporter with mammalian temperature dependence

The complete cDNA sequence of the tilapia cardiac Na(+)/Ca2+ exchanger (NCX-TL1.0) was determined. The 3.1-kb transcript encodes a protein 957 amino acids in length, with a predicted signal peptide cleaved at residue 31 and two potential N-glycosylation sites in the extracellular N terminus. Hydropathy analysis and sequence comparison predicted a mature protein with nine transmembrane-spanning segments, consistent with the structural topologies of other known mammalian and teleost NCX isoforms. Overall sequence comparison shows high identity to both trout NCX-TR1.0 ( approximately 81%) and mammalian NCX1.1 ( approximately 73%), and phylogenetic analyses confirmed its identity as a member of the NCX1 gene family, expressing exons A, C, D, and F in the alternative splice site. Sequence identity is even higher in the alpha-repeats, the exchanger inhibitory peptide (XIP) site, and Ca(2+)-binding domains, which is reflected in the functional and regulatory properties of tilapia NCX-TL1.0. When NCX-TL1.0 was expressed in Xenopus oocytes and the currents were measured in giant excised patches, they displayed both positive regulation by Ca2+ and Na(+)-dependent inactivation in a manner similar to trout NCX-TR1.0. However, tilapia NCX-TL1.0 exhibited a relatively high sensitivity to temperature compared with trout NCX-TR1.0. Whereas trout NCX-TR1.0 currents displayed activation energies of approximately 7 kJ/mol, tilapia NCX-TL1.0 currents showed mammal-like temperature dependence, with peak and steady-state current activation energies of 53 +/- 9 and 67 +/- 21 kJ/mol, respectively. Using comparative sequence analysis, we highlighted 10 residue positions in the N-terminal domain of the NCX that, in combination, may confer exchanger temperature dependence through subtle changes in protein flexibility. Tilapia NCX-TL1.0 represents the first non-mammalian NCX to exhibit a mammalian temperature dependence phenotype and will prove to be a useful model in defining the interplay between molecular flexibility and stability in NCX function.

Normal expression and inflammation-induced changes of Na and Na/K ATPase activity in spinal dorsal horn of the rat

We tested whether that peripheral inflammation induces changes in the spinal dorsal horn ATPase activity. Adult Sprague-Dawley rats were anesthetized (thiobarbital), the left hind paw (inflammation group; n = 15) was immersed in water at 60 degrees C for 60s, which induced a local inflammation. A control group (n = 12) was tested with water at room temperature. After 60 min of peripheral inflammation left (LDH) or right lumbar dorsal horn (RDH) were processed for total, Na/K, Na and remanent ATPase activities (nM P(i) (mgprotein)(-1) min(-1)). In control animals isoenzymatic activities were: Na (31.2%); Na/K (20.6%) and remanent (48.2%) from total ATPase activity. No LDH-RDH asymmetry was found. The inflammation group presented an ipsilateral increase of total ATPase activity in LDH (X+/-S.E.M.; 4798.9+/-601) over the RDH (3982.2+/-451; Delta+817; P<0.05). This is due to an increase in Na ATPase activity (1609.3+/-297) over RDH (1164.2+/-166; Delta+445; P<0.05). ATPase activities were increased in LDH from inflamed over the control group as follows: total (4798.9+/-601; Delta+840; P<0.05), Na/K (1298.1+/-301; Delta+483; P<0.05) and Na (1609.3+/-297; Delta+373; P<0.05). These increased ATPase activities, induced in a short time, can be considered a functional marker of nociceptive neuronal activity.

The sodium/calcium exchanger family-SLC8

The Na(+)/Ca(2+) exchanger gene family encompasses three distinct proteins, NCX1, NCX2, and NCX3, which mediate cellular Ca(2+) efflux and thus contribute to intracellular Ca(2+) homeostasis. NCX1 is expressed ubiquitously while NCX2 and NCX3 are limited to brain and skeletal muscle. NCX1 exchanges 3 extracellular Na(+) for 1 intracellular Ca(2+). In addition to transporting Na(+) and Ca(2+), NCX1 activity is also regulated by these cations. NCX1 is especially important in regulating cardiac contractility.

Forefront of Na+/Ca2+ Exchanger Studies: Regulation Kinetics of Na+/Ca2+ Exchangers

Five isoforms of Na+/Ca2+ exchanger have been identified: NCX1, NCX2, NCX3, NCX-SQ1, and CALX. In all of the exchangers, the Na+/Ca2+ exchange current, which was recorded in inside-out membrane patches, was regulated by cytoplasmic Ca2+. However, the mode of regulation is different among the exchangers. NCX1, NCX2, and NCX-SQ1 are positively regulated by cytoplasmic Ca2+, but CALX is negatively regulated. NCX3 apparently has both positive and negative regulation mechanisms. In this review, I briefly summarize the Ca2+ -dependent regulation mechanisms of the exchangers.

Molecular cloning of a sixth member of the K+-dependent Na+/Ca2+ exchanger gene family, NCKX6

Bioinformatic and molecular cloning tools were used to identify and isolate cDNA clones from mouse and human tissues that encode the sixth member of the K(+)-dependent Na+/Ca2+ exchanger family, NCKX6. The mouse NCKX6 protein is 585 amino acids long and shares about 62% sequence similarity with previously identified exchangers in the alpha-repeat regions but has little primary sequence similarity outside these regions. NCKX6 transcripts of 4 kb are abundantly expressed in all tissues examined and are thus more broadly distributed than previously described NC(K)X family members. Two alternatively spliced products of this novel gene were identified that encode proteins of different length. The short isoform differs from the full-length isoform at the C-terminal hydrophobic domain as a result of a shift in the reading frame caused by the deletion of two exons. Both NCKX6 isoforms were expressed in HEK-293 cells. Functional analysis by digital imaging of fura-2 loaded transfected HEK-293 cells demonstrated that the short isoform exhibited K(+)-dependent Na+/Ca2+ exchange activity whereas the full-length isoform did not. The latter was retained within the endoplasmic reticulum, whereas the short isoform was present at the plasma membrane in transfected cells. Immunofluorescence studies examining NCKX6 expression in native tissue using an NCKX6-specific antibody showed intense labeling of the cardiac sarcolemmal membrane. The discovery of NCKX6 therefore reveals a novel member of the Na+/Ca2+ exchanger superfamily whose ubiquitous expression in all tissues suggests an important role for K(+)-dependent Na+/Ca2+ exchange in maintaining cellular Ca2+ homeostasis in diverse tissues and cell types.

Inhibition of canine (NCX1.1) and Drosophila (CALX1.1) Na(+)-Ca(2+) exchangers by 7-chloro-3,5-dihydro-5-phenyl-1H-4,1-benzothiazepine-2-one (CGP-37157)

The electrophysiological effects of the benzothiazepine 7-chloro-3,5-dihydro-5-phenyl-1H-4,1-benzothiazepine-2-one (CGP-37157) (CGP) were investigated on the canine (NCX1.1) and Drosophila (CALX1.1) plasmalemmal Na+-Ca2+ exchangers. These exchangers were selected for study because they show opposite responses to cytoplasmic regulatory Ca2+, thereby allowing us to examine the role of this regulatory mechanism in the inhibitory effects of CGP. CGP blocked Na+-Ca2+ exchange current mediated by both transporters with moderate potency (IC50 values = approximately 3-17 microM) compared with other recently reported blockers of Na+-Ca2+ exchange [e.g., 2-[4-[2,5-difluorophenyl) methoxy]phenoxy]phenoxy]-5-ethoxyaniline (KB-R7943) and 2-[2-[4-(4-nitrobenzyloxy)phenyl]ethyl]isothiourea (SEA0400)]. Experiments using alpha-chymotrypsin to remove autoregulation of Na+-Ca2+ exchange showed that block by CGP was reduced, suggesting that part of the effects of this drug may require intact ionic regulatory mechanisms. For NCX1.1, the inhibition produced by CGP was greater for outward Na+-Ca2+ exchange currents compared with inward currents. When CALX1.1 was examined, the extent of inhibition was similar for both inward and outward exchange currents. Although the extent and potency of CGP-mediated inhibition of Na+-Ca2+ exchange are less than those observed with SEA0400 and KB-R7943, our data demonstrate that CGP constitutes a novel class of plasmalemmal Na+-Ca2+ exchange inhibitors. Moreover, the widespread use of CGP as a selective mitochondrial Na+-Ca2+ exchange inhibitor should be reconsidered in light of these additional inhibitory effects.

Sodium/calcium exchanger subtypes NCX1, NCX2 and NCX3 show cell-specific expression in rat hippocampus cultures

Na(+)/Ca(2+) exchange activity is known to be expressed throughout the brain in both glial and neuronal tissue. mRNA of all three major subtypes of the mammalian Na(+)/Ca(2+) exchanger protein (NCX1, NCX2, NCX3) has been detected in most brain areas, albeit at varying densities. [The term 'subtype' is used for exchangers that are products of different genes (NCX1, NCX2, NCX3); 'isoform' is used for splice variants of a single gene product]. However, for lack of subtype specific labels, the cellular expression pattern of this transport protein has remained largely unknown. We have now used three subtype-specific antibodies, two monoclonal and one polyclonal, to identify the cellular distribution of the exchanger subtypes in rat hippocampus cell cultures. Surprisingly, we found little overlap for the expression of this membrane protein in different cell types. NCX1 labeled mainly the membranes of neuronal cells and their associated dendritic network. It was found in nearly all neuronal cells of the population growing in culture. In cultures maintained for more than 3 weeks, NCX1 was increasingly detected in the membrane of glia cells. NCX2 immunoreactivity was predominantly localized in various types of glia cells. It was also detected in the membranes of a few neuronal cell bodies but never in the dendritic network. In addition to labeling membranes, the NCX2 antibody strongly cross-reacted with an unidentified glial fibrillar protein. NCX3 expression appeared very low in hippocampus cultures and was restricted to a small subpopulation of neuronal cells. It was never detected in glia cells. Our results provide novel information on the cell-specific expression of the three Na(+)/Ca(2+) exchanger subtypes (NCX1, NCX2 and NCX3) in mammalian brain. These data may reflect functional differences among the subtypes that are not obvious from studies in recombinant cell lines and hence, may help to understand the functional role of specific glia- or neuron-associated Ca(2+) transport systems.

Tissue-specific modes of Na/Ca exchanger regulation

Ionic regulation of Na(+)/Ca(2+) exchange describes the secondary modulating effects exerted on exchange activity by the transport substrates Na(+) and Ca(2+). These effects have been extensively characterized for the cardiac Na(+)/Ca(2+) exchanger, NCX1.1, primarily by the giant excised patch-clamp technique. Moreover, several studies have provided functional evidence for ionic regulation of Na(+)/Ca(2+) exchange activity in intact cellular systems. Through structure-function analyses, important protein domains involved in these regulatory processes have been identified. However, despite major progress in characterizing ionic regulation at the functional and molecular levels, the physiological importance of these processes remains unknown. In this study, we have examined Na(+)/Ca(2+) exchange activity for three members of the NCX1 family, namely NCX1.1, NCX1.3, and NCX1.4. These exchangers were expressed in Xenopus laevis oocytes and were characterized using the giant excised patch-clamp technique. We show that these three splice variants exhibit considerable differences in the kinetic features of their ionic regulatory profiles. Information of this type is beginning to provide insight into the physiological basis for tissue-specific expression of alternatively spliced Na(+)/Ca(2+) exchangers.

The Na+/Ca2+ exchange molecule: an overview

An overview of the molecular physiology of the Na(+)/Ca(2+) exchanger is presented. This includes information on the variety of exchangers that have been described and their regulatory properties. Molecular insight is most detailed for the cardiac Na(+)/Ca(2+) exchanger (NCX1). Parts of the NCS1 molecule involved in regulation and ion transport have been elucidated, and initial information on the topology and structure is available.

Functional regulation of alternatively spliced Na+/Ca2+ exchanger (NCX1) isoforms

Alternative splicing of RNA transcripts is a general characteristic for NCX genes in mammals, mollusks, and arthropods. Among the family of three NCX genes in mammals, the NCX1 gene contains six exons, namely, A, B, C, D, E, and F, that make up the alternatively spliced region. Studies of the NCX1 gene transcripts suggested that 16 distinct gene products can be produced from the NCX1 gene. The exons A and B are mutually exclusive when expressed. Generally, exon A-containing transcripts are predominantly found in excitable cells like cardiomyoctes and neurons, whereas exon B-containing transcripts are mostly found in nonexcitable cells like astrocytes and kidney cells. Other alternatively spliced exons (C-F) appear to be cassette-type exons and are found in various combinations. Interestingly, exon D is present in all characterized transcripts. The alternatively spliced isoforms of NCX1 show tissue-specific expression patterns, suggesting functional adaptation to tissues. To investigate functional differences among alternatively spliced isoforms of NCX1, we expressed an exon A-containing transcript present in cardiac tissue (NCX1.1) and an exon B-containing transcript found in the kidney (NCX1.3) in Xenopus oocytes. We demonstrated that the Na(+)/Ca(2+) exchangers expressed by exon A- and exon B-containing transcripts display differences in activation by PKA and by [Ca(2+)](i). We also observed that these two isoforms show differences in voltage dependence. Surprisingly, the alternatively spliced isoforms of NCX1 display greater functional differences among themselves than the products of different gene loci, NCX1, NCX2, and NCX3.

Immunohistochemical detection of the sodium-calcium exchanger in rat hippocampus cultures using subtype-specific antibodies

All of the known Na+/Ca2+ exchanger subtypes, NCX1-3, are expressed in the brain, albeit with marked regional differences. On the mRNA level, overall expression seems most prominent for NCX2, intermediate for NCX1, and, except for a few regions, low for NCX3. Using three subtype-specific antibodies, we have now studied the cellular expression of the NCX subtypes in rat hippocampus cultures by immunohistochemical techniques. Our results provide evidence for a highly cell-specific expression pattern of NCX subtypes and show surprisingly little colocalization. NCX1 and NCX3 are both primarily expressed in neuronal cells. While NCX1 is found in the large majority of neurons, NCX3 expression was restricted to a small minority of cells. By contrast, NCX2 was almost exclusively present in glial cells. The NCX2 antibody, a IgM, stained glial cell membranes as well as an intermediate fibrillar system. In spite of extensive screening, the nature of this fiber system has not yet been identified.

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.

Na(+)/Ca(2+) exchanger expression in the developing rat cortex

The Na(+)/Ca(2+) exchanger (NCX) participates in the regulation of neuronal Ca(2+) homeostasis and is also believed to be involved in the neuronal responses to hypoxia. However, there are very limited data on how NCX mRNA and protein expression are regulated during brain development. In the present study, we sought to elucidate the developmental expression of NCX1 and NCX2 in the rat cortex from late fetal to adult stages using reverse transcription-polymerase chain reaction and western blot assays. The primers for NCX1 mRNA targeted the alternative splicing domain to allow differentiation between NCX1 splice variants. Our results show that: (1) only two NCX1 mRNA splice variants (NCX1.5 and NCX1.4) are present in the cortex and their expression is age-dependent; (2) total NCX1 mRNA levels are low in fetal tissue, reach maximum density at postnatal day 8 and substantially decline with further maturation; (3) NCX2 mRNA density is significantly greater than total NCX1 mRNA for all ages and increases markedly during maturation from fetus/neonate to adult; and (4) NCX1 protein expression is lowest in late fetal cortex and reaches maximum levels after 2 weeks postnatally, even though expression levels are not significantly different between newborn and adult animals. Also, we found a similar NCX1 protein trend in the subcortical and cerebellar regions during development. From these data we suggest that NCX1 and NCX2 are differentially expressed in the cortex with a predominance of NCX2 levels during postnatal development. We speculate that the developmental increase in NCX2 expression is responsible for the overall increase in Na(+)/Ca(2+) exchange capacity during maturation.

Expression of Na+/Ca(2+) exchanger (NCX1) gene in the developmental mouse embryo and adult mouse brain

The Na(+)/Ca(2+) exchanger gene, NCX1, is widely expressed in many tissues, encoding several isoforms through alternative RNA splicing. NCX1 deficient mice are known to be lethal at embryonic day 9-10 (E9-10). However, its expression pattern during embryogenesis is largely unknown. Therefore, to identify and compare the localization and alternatively spliced isoforms of NCX1 mRNA expressed in the developmental stages, we analyzed the mouse embryo. Northern blot analysis demonstrated that NCX1 mRNA was expressed from the earliest stage examined, E7. In situ hybridization analysis revealed that NCX1 mRNA was expressed in the heart alone until E10.5. However, at E14.5 and 16.5, NCX1 mRNA was expressed not only in the heart, but also in neuronal cells. In addition, the expression of NCX1 mRNA in the adult brain was most abundant in the hippocampus. Using reverse transcription-polymerase chain reaction (RT-PCR), we also identified the alternatively spliced isoforms expressed during each developmental stage. The restricted expression of the NCX1 gene suggested that NCX1 may play an important role in the developing mouse embryo.

Molecular cloning of a third member of the potassium-dependent sodium-calcium exchanger gene family, NCKX3

We describe here the identification and characterization of a novel member of the family of K(+)-dependent Na(+)/Ca(2+) exchangers, NCKX3 (gene SLC24A3). Human NCKX3 encodes a protein of 644 amino acids that displayed a high level of sequence identity to the other family members, rod NCKX1 and cone/neuronal NCKX2, in the hydrophobic regions surrounding the "alpha -repeat" sequences thought to form the ion-binding pocket for transport. Outside of these regions NCKX3 showed no significant identity to other known proteins. As anticipated from this sequence similarity, NCKX3 displayed K(+)-dependent Na(+)/Ca(2+) exchanger activity when assayed in heterologous expression systems, using digital imaging of fura-2 fluorescence, electrophysiology, or radioactive (45)Ca(2+) uptake. The N-terminal region of NCKX3, although not essential for expression, increased functional activity at least 10-fold and may represent a cleavable signal sequence. NCKX3 transcripts were most abundant in brain, with highest levels found in selected thalamic nuclei, in hippocampal CA1 neurons, and in layer IV of the cerebral cortex. Many other tissues also expressed NCKX3 at lower levels, especially aorta, uterus, and intestine, which are rich in smooth muscle. The discovery of NCKX3 thus expands the K(+)-dependent Na(+)/Ca(2+) exchanger family and suggests this class of transporter has a more widespread role in cellular Ca(2+) handling than previously appreciated.

Cardiac Na + -Ca 2+ Exchange

Abstract —The Na + -Ca 2+ exchanger (NCX) is one of the essential regulators of Ca 2+ homeostasis in cardiomyocytes and thus an important modulator of the cardiac contractile function. The purpose of this review is to survey recent advances in cardiac NCX research, with particular emphasis on molecular and pharmacological aspects. The NCX function is thought to be regulated by a variety of cellular factors. However, data obtained by use of different experimental systems often appear to be in conflict. Where possible, we endeavor to provide a rational interpretation of such data. We also provide a summary of current work relating to the structure and function of the cardiac NCX. Recent molecular studies of the NCX protein are beginning to shed light on structural features of the ion translocation pathway in the NCX membrane domain, which seems likely to be formed, at least partly, by the phylogenetically conserved α-1 and α-2 repeat structures and their neighboring membrane-spanning segments. Finally, we discuss new classes of NCX inhibitors with improved selectivity. One of these, 2-[2-[4-(4-nitrobenzyloxy)phenyl]ethyl]isothiourea methanesulfonate (KB-R7943), appears to exhibit unique selectivity for Ca 2+ -influx–mode NCX activity. Data obtained with these inhibitors should provide a basis for designing more selective and clinically useful drugs targeting NCX.

Functional differences between cardiac and renal isoforms of the rat Na+-Ca2+ exchanger NCX1 expressed in Xenopus oocytes

The transcript of the Na+-Ca2+ exchanger gene NCX1 undergoes alternative splicing to produce tissue-specific isoforms. The cloned NCX1 isoforms were expressed in Xenopus oocytes and studied using a two-electrode voltage clamp method to measure Na+-Ca2+ exchanger activity. The cardiac isoform (NCX1.1) expressed in oocytes was less sensitive to depolarizing voltages and to activation by [Ca2+]i than the renal isoform (NCX1.3). The cardiac isoform of NCX1 is more sensitive to activation by protein kinase A (PKA) than the renal isoform which may be explained by preferential phosphorylation. The cardiac isoform of NCX1 is phosphorylated to a greater extent than the renal isoform. The action of PKA phosphorylation which increases the activity of the cardiac isoform of the Na+-Ca2+ exchanger in oocytes was confirmed in adult rat ventricular cardiomyocytes by measuring Na+-dependent Ca2+ flux. We conclude that alternative splicing of NCX1 confers distinct functional characteristics to tissue-specific isoforms of the Na+-Ca2+ exchanger.

Alternatively spliced isoforms of the rat eye sodium/calcium+potassium exchanger NCKX1

We have investigated the structure, function, and expression of the rat eye sodium/calcium+potassium exchanger NCKX1. The sequence of independent rat NCKX1 clones and the analysis of rat eye mRNA by RT-PCR revealed a region of alternative splicing that comprised four exons and encoded a stretch of 113 amino acids near the beginning of the large cytosolic loop. In comparison with other NCKX1 molecules and the rat NCKX2 protein, rat NCKX1 was highly conserved within the hydrophobic regions but was quite divergent in the two large hydrophilic loops. The only exception was the region of the cytosolic loop encoded by the second alternatively spliced exon, which was approximately 60% identical. Similar to bovine, but different from human, rat NCKX1 possessed an acidic motif that was repeated 14 times in the cytoplasmic loop. Analysis of NCKX1 expression in different rat tissues by Northern blot revealed a very high level of expression of a 7-kb transcript in the eye but also lower levels of transcripts of various lengths in other tissues. The recombinant rat NCKX1 protein was tagged in the extracellular loop with the FLAG epitope and expressed in HEK-293 cells. Surface delivery and potassium-dependent sodium/calcium exchange activity were observed for each spliced variant.