A novel topology and redox regulation of the rat brain K+-dependent Na+/Ca2+ exchanger, NCKX2

Coordinated microRNA/mRNA Expression Profiles Reveal Unique Skin Color Regulatory Mechanisms in Chinese Giant Salamander (Andrias davidianus)

The Chinese giant salamander (Andrias davidianus) has been increasingly popular in the aquaculture market in China in recent years. In the breeding process of Andrias davidianus, we found that some albino individuals were extremely rare and could not be inherited stably, which severely limits their commercialization in the aquaculture market. In this study, we performed transcriptome and small RNA (sRNA) sequencing analyses in the skin samples of wild-type (WT) and albino (AL) Andrias davidianus. In total, among 5517 differentially expressed genes (DEGs), 2911 DEGs were down-regulated in AL, including almost all the key genes involved in melanin formation. A total of 25 miRNAs were differentially expressed in AL compared to WT, of which 17 were up-regulated. Through the integrated analysis, no intersection was found between the target genes of the differentially expressed miRNAs and the key genes for melanin formation. Gene Ontology (GO) and KEGG pathway analyses on DEGs showed that these genes involved multiple processes relevant to melanin synthesis and the key signal pathway MAPK. Interestingly, the transcription factors SOX10 and PAX3 and the Wnt signaling pathway that play a key role in other species were not included, while the other two transcription factors in the SOX family, SOX21 and SOX7, were included. After analyzing the key genes for melanin formation, it was interesting to note an alternative splicing form of the MITF in WT and a critical mutation of the SLC24A5 gene in AL, which might be the main reason for the skin color change of Andrias davidianus. The results contributed to understanding the molecular mechanism of skin pigmentation in Andrias davidianus and accelerating the acquisition process of individuals with specific body colors by genetic means.

Regulation of K+-Dependent Na+/Ca2+-Exchangers (NCKX)

Potassium-dependent sodium-calcium exchangers (NCKX) have emerged as key determinants of calcium (Ca²⁺) signaling and homeostasis, especially in environments where ion concentrations undergo large changes, such as excitatory cells and transport epithelia. The regulation of NCKX transporters enables them to respond to the changing cellular environment thereby helping to shape the extent and kinetics of Ca²⁺ signals. This review examines the current knowledge of the different ways in which NCKX activity can be modulated. These include (i) cellular and dynamic subcellular location (ii); changes in protein expression mediated at the gene, transcript, or protein level (iii); genetic changes resulting in altered protein structure or expression (iv); regulation via changes in substrate concentration (v); and post-translational modification, partner protein interactions, and allosteric regulation. Detailed mechanistic understanding of NCKX regulation is an emerging area of research with the potential to provide important new insights into transporter function, the control of Ca²⁺ signals, and possible interventions for dysregulated Ca²⁺ homeostasis.

Signal pathway analysis of selected obesity-associated melanocortin-4 receptor class V mutants

Mutations in the melanocortin-4 receptor (MC4R) in humans are the single most common cause of rare monogenic 1severe obesity, and polymorphisms in this gene are also associated with obesity in the general population. The MC4R is a G-protein coupled receptor, and in vitro analysis suggests that MC4R can signal through several different G-protein subtypes. In vivo studies show complex outcomes, with different G-proteins in different cells responsible for different physiological responses linked to obesity. There is an emerging consensus that Gαq-linked signals in the paraventricular nucleus of the hypothalamus are essential for normal satiety and the control of feeding behavior. Many MC4R mutations have been analyzed for the molecular defect underlying their association with obesity, which has revealed a group - referred to as class V mutants - with no measurable change in receptor function. However, Gαq-linked signaling leading to Ca²⁺ release has only been examined for a few MC4R mutations. In this study, we have examined seven MC4R class V mutants, as well as two other well-characterized signal-defective mutants as controls, with respect to G-protein signaling coupled to cAMP production, mitogen-activated protein kinase (MAPK) activation, and Ca²⁺ release. These data confirm, with one exception (E308K), the expected pattern of cAMP and MAPK signaling for wild type and mutant MC4R. Our results also demonstrate normal MSH-induced Ca²⁺ signals for wild type as well as all the class V mutants, but not the signal-defective controls. Thus, the means by which class V MC4R mutations lead to obesity remains an open question.

A SLC24A2 Gene Variant Uncovered in Pancreatic Ductal Adenocarcinoma by Whole Exome Sequencing

Pancreatic ductal adenocarcinoma (PDAC) presents as an aggressive malignancy caused by environmental and genetic factors. In order to identify causal genes for PDAC, we performed whole exome sequencing (WES) to detect gene mutations in seven pairs of PDAC tissue and adjacent non-tumor tissue samples. Finally, we found a new nonsynonymous single nucleotide variant (nsSNV) in solute carrier 24 family member 2 (SLC24A2) gene resulting in the substitution of native glutamic acid (E) into aspartic acid (D) at position of 287 amino acid (E287D) in SLC24A2 protein, and confirmed this variant by Sanger gene sequencing. SLC24A2 is a potassium-dependent sodium-calcium exchanger and can transport metal ion across cell membrane. Multiple in silico variants' effects analyses methods including SIFT, PolyPhen, PROVEAN, and PANTHER demonstrated this variant had probably damaging effects, which was consistent with the results obtained from Mutation Taster software analysis with a probability of 0.99999997 to be "disease causing." The three dimension (3D) structure analysis results suggested this variant had little effects on the solubility and hydrophobicity of the protein; but it could decrease the protein stability by increasing the total protein structure energy (-8874.33 kJ/mol for the mutant and -8963.54 kJ/mol for the native) and by causing the mutant protein decreasing three stabilizing residues. Less stability of the mutant 287D protein than the native E287 protein was also supported by I-Mutant and Western-blotting analysis results. Overall, a new mutation in SLC24A2 gene was identified to decrease the stability of SLC24A2, which may have potential clinical usages.

Insights into the early evolution of animal calcium signaling machinery: a unicellular point of view

The basic principles of Ca(2+) regulation emerged early in prokaryotes. Ca(2+) signaling acquired more extensive and varied functions when life evolved into multicellular eukaryotes with intracellular organelles. Animals, fungi and plants display differences in the mechanisms that control cytosolic Ca(2+) concentrations. The aim of this review is to examine recent findings from comparative genomics of Ca(2+) signaling molecules in close unicellular relatives of animals and in common unicellular ancestors of animals and fungi. Also discussed are the evolution and origins of the sperm-specific CatSper channel complex, cation/Ca(2+) exchangers and four-domain voltage-gated Ca(2+) channels. Newly identified evolutionary evidence suggests that the distinct Ca(2+) signaling machineries in animals, plants and fungi likely originated from an ancient Ca(2+) signaling machinery prior to early eukaryotic radiation.

The topology of the C-terminal sections of the NCX1 Na (+) /Ca ( 2+) exchanger and the NCKX2 Na (+) /Ca ( 2+) -K (+) exchanger

Mammalian Na (+) /Ca ( 2+) (NCX) and Na (+) /Ca ( 2+) -K (+) exchangers (NCKX) are polytopic membrane proteins that play critical roles in calcium homeostasis in many cells. Although hydropathy plots for NCX and NCKX are very similar, reported topological models for NCX1 and NCKX2 differ in the orientation of the three C-terminal transmembrane segments (TMS). NCX1 is thought to have 9 TMS and a re-entrant loop, whereas NCKX2 is thought to have 10 TMS. The current topological model of NCKX2 is very similar to the 10 membrane spanning helices seen in the recently reported crystal structure of NCX_MJ, a distantly related archaebacterial Na (+) /Ca ( 2+) exchanger. Here we reinvestigate the orientation of the three C-terminal TMS of NCX1 and NCKX2 using mass-tagging experiments of substituted cysteine residues. Our results suggest that NCX1, NCKX2 and NCX_MJ all share the same 10 TMS topology.

The topology of the cardiac Na⁺/Ca²⁺ exchanger, NCX1

The topology of the plasma membrane Na(+)/Ca(2+) exchanger of cardiac muscle, NCX1, is uncertain. Biochemical analyses have indicated the presence of 9 transmembrane segments (TMSs) whereas the recent crystal structure of a prokaryotic homologue has 10 TMSs. The discrepancy is towards the C-terminus of the proteins where the prokaryotic homologue has an additional TMS8. To resolve this apparent disagreement, we re-assessed the topology of the C-terminal TMSs of NCX1. We examined the ability of internal or external cysteine residues in the N-terminal portion of NCX1 to crosslink with cysteine residues, of uncertain orientation, in the C-terminal portion of the protein. The results strongly support a model of NCX1 with 10 TMSs as found in the prokaryotic homologue.

KIF21A-mediated axonal transport and selective endocytosis underlie the polarized targeting of NCKX2

We have previously shown that K(+)-dependent Na(+)/Ca(2+) exchanger (NCKX) is a major calcium clearance mechanism at the large axon terminals of central neurons, whereas their somata display little NCKX activity. We investigated mechanisms underlying the axonal polarization of NCKX2 in rat hippocampal neurons. We identified NCKX2 as the first neuron-specific cargo molecule of kinesin family member 21A (KIF21A). The intracellular loop of NCKX2 specifically interacted with the WD-40 repeats, a putative cargo-binding domain, of KIF21A. Dominant-negative mutant or depletion of KIF21A inhibited the transport of NCKX2-GFP to axon fibers. Knockdown of KIF21A caused calcium dysregulation at axonal boutons but not at somatodendritic regions. Despite the axonal polarization of the NCKX activity, both somatodendritic and axonal regions were immunoreactive to NCKX2. The surface expression of NCKX2 revealed by live-cell immunocytochemistry, however, displayed highly polarized distribution to the axon. Inhibition of endocytosis increased the somatodendritic surface NCKX2 and thus abolished the axonal polarization of surface NCKX2. These results indicate that KIF21A-mediated axonal transport and selective somatodendritic endocytosis underlie the axonal polarized surface expression of NCKX2.

Reciprocal regulation of the platelet-derived growth factor receptor-beta and G protein-coupled receptor kinase 5 by cross-phosphorylation: effects on catalysis

Signaling by the platelet-derived growth factor receptor-beta (PDGFRbeta) is diminished when the PDGFRbeta is phosphorylated on seryl residues by G protein-coupled receptor kinase-5 (GRK5), but mechanisms for GRK5 activation by the PDGFRbeta remain obscure. We therefore tested whether the PDGFRbeta is able to tyrosine-phosphorylate and thereby activate GRK5. Purified GRK5 was tyrosine-phosphorylated by the wild-type PDGFRbeta to a stoichiometry of 0.8 mol phosphate/mol GRK5, an extent approximately 5 times greater than observed with a Y857F PDGFRbeta mutant that fails to phosphorylate exogenous substrates but autophosphorylates and activates Src normally. The degree of PDGFRbeta-mediated phosphorylation of GRK5 correlated with GRK5 activity, as assessed by seryl phosphorylation of the PDGFRbeta in purified protein preparations, in intact cells expressing a tyrosine-to-phenylalanine GRK5 mutant, and in GRK5 peptide phosphorylation assays. However, tyrosyl phosphorylation of GRK5 was not necessary for GRK5-mediated phosphorylation of the beta(2)-adrenergic receptor, even though beta(2)-adrenergic receptor activation promoted tyrosyl phosphorylation of GRK5 in smooth muscle cells. Phosphorylation of the PDGFRbeta by GRK5 in smooth muscle cells or in purified protein preparations reduced PDGFRbeta-mediated peptide phosphorylation. In contrast, phosphorylation of GRK5 by the PDGFRbeta enhanced the V(max) of GRK5-mediated peptide phosphorylation, by 3.4-fold, without altering the GRK5 K(M) for peptide. We conclude that GRK5 tyrosyl phosphorylation is required for the activation of GRK5 by the PDGFRbeta, but not by the beta(2)-adrenergic receptor, and that by activating GRK5, the PDGFRbeta triggers its own desensitization.

Searching for a role of NCX/NCKX exchangers in neurodegeneration

Control of intracellular calcium signaling is essential for neuronal development and function. Maintenance of Ca2+ homeostasis depends on the functioning of specific transport systems that remove calcium from the cytosol. Na+/Ca2+ exchange is the main calcium export mechanism across the plasma membrane that restores resting levels of calcium in neurons after stimulation. Two families of Na+/Ca2+ exchangers exist, one of which requires the co-transport of K+ and Ca2+ in exchange for Na+ ions. The malfunctioning of Na+/Ca2+ exchangers has been related to the development of pathological conditions in the regulation of neuronal death after hypoxia-anoxia, brain trauma, and nerve injury. In addition, the Na+/Ca2+ exchanger function has been associated with impaired Ca2+ homeostasis during aging of the brain, as well as with a role in Alzheimer's disease by regulating beta-amyloid toxicity. In this review, we summarize the current knowledge about the Na+/Ca2+ exchanger families and their implications in neurodegenerative disorders.

Na+/Ca2+ exchangers: three mammalian gene families control Ca2+ transport

Mammalian Na+/Ca2+ exchangers are members of three branches of a much larger family of transport proteins [the CaCA (Ca2+/cation antiporter) superfamily] whose main role is to provide control of Ca2+ flux across the plasma membranes or intracellular compartments. Since cytosolic levels of Ca2+ are much lower than those found extracellularly or in sequestered stores, the major function of Na+/Ca2+ exchangers is to extrude Ca2+ from the cytoplasm. The exchangers are, however, fully reversible and thus, under special conditions of subcellular localization and compartmentalized ion gradients, Na+/Ca2+ exchangers may allow Ca2+ entry and may play more specialized roles in Ca2+ movement between compartments. The NCX (Na+/Ca2+ exchanger) [SLC (solute carrier) 8] branch of Na+/Ca2+ exchangers comprises three members: NCX1 has been most extensively studied, and is broadly expressed with particular abundance in heart, brain and kidney, NCX2 is expressed in brain, and NCX3 is expressed in brain and skeletal muscle. The NCX proteins subserve a variety of roles, depending upon the site of expression. These include cardiac excitation-contraction coupling, neuronal signalling and Ca2+ reabsorption in the kidney. The NCKX (Na2+/Ca2+-K+ exchanger) (SLC24) branch of Na+/Ca2+ exchangers transport K+ and Ca2+ in exchange for Na+, and comprises five members: NCKX1 is expressed in retinal rod photoreceptors, NCKX2 is expressed in cone photoreceptors and in neurons throughout the brain, NCKX3 and NCKX4 are abundant in brain, but have a broader tissue distribution, and NCKX5 is expressed in skin, retinal epithelium and brain. The NCKX proteins probably play a particularly prominent role in regulating Ca2+ flux in environments which experience wide and frequent fluctuations in Na+ concentration. Until recently, the range of functions that NCKX proteins play was generally underappreciated. This situation is now changing rapidly as evidence emerges for roles including photoreceptor adaptation, synaptic plasticity and skin pigmentation. The CCX (Ca2+/cation exchanger) branch has only one mammalian member, NCKX6 or NCLX (Na+/Ca2+-Li+ exchanger), whose physiological function remains unclear, despite a broad pattern of expression.

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.

Redox control of calcium channels: from mechanisms to therapeutic opportunities

Calcium plays an integral role in cellular function. It is a well-recognized second messenger necessary for signaling cellular responses, but in excessive amounts can be deleterious to function, causing cell death. The main route by which calcium enters the cytoplasm is either from the extracellular compartment or internal addistores via calcium channels. There is good evidence that calcium channels can respond to pharmacological compounds that reduce or oxidize thiol groups on the channel protein. In addition, reactive oxygen species such as hydrogen peroxide and superoxide that can mediate oxidative pathology also mediate changes in channel function via alterations of thiol groups. This review looks at the structure and function of calcium channels, the evidence that changes in cellular redox state mediate changes in channel function, and the role of redox modification of channels in disease processes. Understanding how redox modification of the channel protein alters channel structure and function is providing leads for the design of therapeutic interventions that target oxidative stress responses.

Ionic Selectivity of NCKX2, NCKX3, and NCKX4 for Monovalent Cations at K+-Binding Site

To determine the ionic selectivity of K+-sites in three members of Na+/Ca2++K+ exchanger (NCKX) family: NCKX2, NCKX3 and NCKX4, we compared the amplitudes of reverse mode NCKX current (I(NCKX)) activated by K+-substitutes (Rb+, NH4+, Cs+, or Li+) relative to that by K+ in an HEK293 cell overexpressing each of NCKX isoforms. In all three isoforms, monovalent cations activated I(NCKX) with similar order of potency: K+ > Rb+ > NH4+ > Cs+ >> Li+. However, the relative potency of Cs+ and NH4+ for activating NCKX2 was significantly higher than those for NCKX3 and NCKX4, indicating that the selectivity of NCKX2 for K+ is weaker than the other two isoforms.

Analysis of Ion Interactions with the K+ -dependent Na+/Ca+ Exchangers NCKX2, NCKX3, and NCKX4

K(+)-dependent Na(+)/Ca(2+) exchangers (NCKX) catalyze cytosolic Ca(2+) extrusion and are particularly important for neuronal Ca(2+) signaling. Of the five mammalian isoforms, the detailed functional characteristics have only been reported for NCKX1 and -2. In the current study, the functional characteristics of recombinant NCKX3 and -4 expressed in HEK293 cells were determined and compared with those of NCKX2. Although the apparent affinities of the three isoforms for Ca(2+) and Na(+) were similar, NCKX3 and -4 displayed approximately 40-fold higher affinities for K(+) ions than NCKX2. Functional analysis of various NCKX2 mutants revealed that mutation of Thr-551 to Ala, the corresponding residue in NCKX4, resulted in an apparent K(+) affinity shift to one similar to that of NCKX4 without a parallel shift in apparent Ca(2+) affinity. In the converse situation, when Gln-476 of NCKX4 was converted to Lys, the corresponding residue in NCKX2, both the K(+) and Ca(2+) affinities were reduced. These results indicate that the apparently low K(+) affinity of NCKX2 requires a Thr residue at position 551 that may reduce the conformational flexibility and/or K(+) liganding strength of side-chain moieties on critical neighboring residues. This interaction appears to be specific to the structural context of the NCKX2 K(+) binding pocket, because it was not possible to recreate the K(+)-specific low affinity phenotype with reciprocal mutations in NCKX4. The results of this study provide important information about the structure and function of NCKX proteins and will be critical to understanding their roles in neuronal Ca(2+) signaling.

Protein kinase C-dependent enhancement of activity of rat brain NCKX2 heterologously expressed in HEK293 cells

Different members of the Na+/Ca2++K+ exchanger (NCKX) family are present in distinct brain regions, suggesting that they may have cell-specific functions. Many neuronal channels and transporters are regulated via phosphorylation. Regulation of the rat brain NCKXs by protein kinases, however, has not been described. Here, we report an increase in NCKX2 activity in response to protein kinase C (PKC) activation. Outward current of NCKX2 heterologously expressed in HEK293 cells was enhanced by beta-phorbol dibutyrate (PDBu), whereas PDBu had little effect on activity of NCKX3 or NCKX4. The PDBu-induced enhancement (PIE) of NCKX2 activity was abolished by PKC inhibitors and significantly reduced when the dominant negative mutant of PKCepsilon (K437R) was overexpressed. Moreover, PDBu accelerated the decay rate of the Ca2+ transient at the calyx of Held, where NCKX is the major Ca2+-clearance mechanism. Intracellular perfusion with alkaline phosphatase completely inhibited PIE. Consistently, beta-phorbol myristate acetate (PMA), but not 4alpha-PMA, induced a 3-fold stimulation of 32P incorporation into NCKX2 expressed in HEK293 cells. To investigate the sites involved, PIE of wild-type NCKX2 was compared with mutant NCKX2 in which the three putative PKC consensus sites were replaced with alanine, either individually or in combination. Double-site mutation involving Thr-476 (T166A/T476A and T476A/S504A) disrupted PIE, whereas single mutation of Thr-166, Thr-476, or Ser-504 or the double mutant T166A/S504A failed to completely prevent PIE. These findings suggest that PKC-mediated activation of NCKX2 is sensitive to mutation of multiple PKC consensus sites via a mechanism that may involve several phosphorylation events.

Importance of K+-dependent Na+/Ca2+-exchanger 2, NCKX2, in motor learning and memory

Plasma membrane Na+/Ca2+-exchangers play a predominant role in Ca2+ extrusion in brain. Neurons express several different Na+/Ca2+-exchangers belonging to both the K+-independent NCX family and the K+-dependent NCKX family. The unique contributions of each of these proteins to neuronal Ca2+ homeostasis and/or physiology remain largely unexplored. To address this question, we generated mice in which the gene encoding the abundant neuronal K+ -dependent Na+/Ca2+-exchanger protein, NCKX2, was knocked out. Analysis of these animals revealed a significant reduction in Ca2+ flux in cortical neurons, a profound loss of long term potentiation and an increase in long term depression at hippocampal Schaffer/CA1 synapses, and clear deficits in specific tests of motor learning and spatial working memory. Surprisingly, there was no obvious loss of photoreceptor function in cones, where expression of the NCKX2 protein had been reported previously. These data emphasize the critical and non-redundant role of NCKX2 in the local control of neuronal [Ca2+] that is essential for the development of synaptic plasticity associated with learning and memory.

Different cation sensitivities and binding site domains of Na+-Ca2+-K+ and Na+-Ca2+ exchangers

We examined inhibitory effects of external multivalent cations Ni(2+), Co(2+), Cd(2+), La(3+), Mg(2+), and Mn(2+) on reverse-mode exchange of the K(+)-dependent Na(+)/Ca(2+) exchanger NCKX2 and the K(+)-independent exchanger NCX1 expressed in CCL-39 cells by measuring the rate of Ca(2+) uptake with radioisotope tracer and electrophysiological techniques. The apparent affinities for block of Ca(2+) uptake by multivalent cations was higher in NCKX2 than NCX1, and the rank order of inhibitory potencies among these cations was different. Additional experiments also showed that external Li(+) stimulated reverse-mode exchange by NCX1, but not NCKX2 in the presence of 5 mM K(+). Thus, both exchangers exhibited differential sensitivities to not only K(+) but also many other external cations. We attempted to locate the putative binding sites within the alpha motifs for multivalent cations by site-directed mutagenesis experiments. The cation affinities of NCKX2 were altered by mutations of amino acid residues in the alpha-1 motif, but not by mutations in the alpha-2 motif. These results contrast with those for NCX1 where mutations in both alpha-1 and alpha-2 motifs have been shown previously to affect cation affinities. Susceptibility tests with sulfhydryl alkylating agents suggested that the alpha-1 and alpha-2 motifs are situated extracellularly and intracellularly, respectively, in both exchangers. A topological model is proposed in which the extracellular-facing alpha-1 motif forms an external cation binding site that includes key residues N203, G207C, and I209 in NCKX2, while both alpha-1 and alpha-2 motifs together form the binding sites in NCX1.

The cation/Ca(2+) exchanger superfamily: phylogenetic analysis and structural implications

Cation/Ca(2+) exchangers are an essential component of Ca(2+) signaling pathways and function to transport cytosolic Ca(2+) across membranes against its electrochemical gradient by utilizing the downhill gradients of other cation species such as H(+), Na(+), or K(+). The cation/Ca(2+) exchanger superfamily is composed of H(+)/Ca(2+) exchangers and Na(+)/Ca(2+) exchangers, which have been investigated extensively in both plant cells and animal cells. Recently, information from completely sequenced genomes of bacteria, archaea, and eukaryotes has revealed the presence of genes that encode homologues of cation/Ca(2+) exchangers in many organisms in which the role of these exchangers has not been clearly demonstrated. In this study, we report a comprehensive sequence alignment and the first phylogenetic analysis of the cation/Ca(2+) exchanger superfamily of 147 sequences. The results present a framework for structure-function relationships of cation/Ca(2+) exchangers, suggesting unique signature motifs of conserved residues that may underlie divergent functional properties. Construction of a phylogenetic tree with inclusion of cation/Ca(2+) exchangers with known functional properties defines five protein families and the evolutionary relationships between the members. Based on this analysis, the cation/Ca(2+) exchanger superfamily is classified into the YRBG, CAX, NCX, and NCKX families and a newly recognized family, designated CCX. These findings will provide guides for future studies concerning structures, functions, and evolutionary origins of the cation/Ca(2+) 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.

The SLC24 Na+/Ca2+-K+ exchanger family: vision and beyond

Na(+)/Ca(2+)-K(+) exchange (NCKX) was first discovered in the outer segments of vertebrate rod photoreceptors (ROS), where it is the only mechanism for extruding the Ca(2+) that enters ROS via the light-sensitive and cGMP-gated channels. ROS NCKX1 is the only NCKX gene family member studied extensively in situ. ROS NCKX1 cDNAs have been cloned subsequently from a number of species including man and shown to be the first member of a new gene family ( SLCA24). Three further members of the human NCKX gene family have been cloned subsequently ( NCKX2- 4) by homology with NCKX1, while a partial sequence of a fifth human NCKX gene has appeared in the data base. NCKX-related genes have also been identified in lower animals including fruit flies, worms and sea urchins. NCKX2 is expressed in the brain, in retinal cone photoreceptors and in retinal ganglion cells, while NCKX3 and NCKX4 show a broader expression pattern. In situ NCKX1 and heterologously expressed NCKX2 operate at a 4Na(+):1Ca(2+)+1 K(+) stoichiometry; both NCKX1 and NCKX2 are bidirectional transporters normally extruding Ca(2+) from the cell (forward exchange), but also able to carry Ca(2+) into the cell (reverse exchange) when the transmembrane Na(+) gradient is reversed. Sequence changes have been observed for both NCKX1 and NCKX2 in patients with retinal diseases, but a definitive association with retinal disease has not been shown.

Molecular cloning of a fourth member of the potassium-dependent sodium-calcium exchanger gene family, NCKX4

We report here the identification and characterization of a fourth member of the potassium-dependent sodium-calcium exchanger gene family, NCKX4 (gene SLC24A4), which mapped to the chromosomal region 14q32. Human NCKX4 encoded a protein of 605 amino acids that displayed a high level of sequence identity to previously described family members, rod NCKX1 (gene SLC24A1), cone/neuronal NCKX2 (gene SLC24A2), and ubiquitous NCKX3 (gene SLC24A3), in the hydrophobic regions surrounding the alpha-repeat sequences thought to form the ion-binding pocket used for transport. The protein product of the NCKX4 gene shared the highest level of amino acid identity, as well as an almost identical arrangement of exon boundaries, with NCKX3, indicating that these two genes have arisen from a recent duplication event. NCKX4 transcripts were abundantly expressed in all brain regions, aorta, lung, and thymus, as well as at a lower level in many other tissues. The NCKX4 protein demonstrated potassium-dependent sodium calcium exchanger activity when assayed in transfected HEK293 cells using digital imaging of fura-2 fluorescence. The discovery of NCKX4, as far as can be ascertained from the current version of the human genome sequence, completes the mammalian potassium-dependent sodium-calcium exchanger gene family.