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

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.

A functional approach to understanding the role of NCKX5 in Xenopus pigmentation

NCKX5 is an ion exchanger expressed mostly in pigment cells; however, the functional role for this protein in melanogenesis is not clear. A variant allele of SLC24A5, the gene encoding NCKX5, has been shown to correlate with lighter skin pigmentation in humans, indicating a key role for SLC24A5 in determining human skin colour. SLC24A5 expression has been found to be elevated in melanoma. Knockdown analyses have shown SLC24A5 to be important for pigmentation, but to date the function of this ion exchanger in melanogenesis has not been fully established. Our data suggest NCKX5 may have an alternative activity that is key to its role in the regulation of pigmentation. Here Xenopus laevis is employed as an in vivo model system to further investigate the function of NCKX5 in pigmentation. SLC24A5 is expressed in the melanophores as they differentiate from the neural crest and develop in the RPE of the eye. Morpholino knockdown and rescue experiments were designed to elucidate key residues and regions of the NCKX5 protein. Unilateral morpholino injection at the 2 cell stage resulted in a reduction of pigmentation in the eye and epidermis of one lateral side of the tadpole. Xenopus and human SLC24A5 can rescue the morpholino effects. Further rescue experiments including the use of ion exchange inactive SLC24A5 constructs raise the possibility that full ion exchanger function of NCKX5 may not be required for rescue of pigmentation.

The SLC24 gene family of Na⁺/Ca²⁺-K⁺ exchangers: from sight and smell to memory consolidation and skin pigmentation

Members of the SLC24 gene family encode K(+)-dependent Na(+)/Ca(2+) exchangers (NCKX) that utilize both the inward Na(+) and outward K(+) gradients to extrude Ca(2+) from cells. There are five human SLC24 genes that play a role in biological process as diverse as vision in retinal rod and cone photoreceptors, olfaction, skin pigmentation and at least three of the five genes are also widely expressed in the brain. Here I review the functional, physiological and structural features of NCKX proteins that have emerged in the past few years.

Sodium-calcium exchangers contribute to the regulation of cytosolic calcium levels in mouse taste cells

Taste cells use multiple signalling mechanisms to generate unique calcium responses to distinct taste stimuli. Some taste stimuli activate G-protein coupled receptors (GPCRs) that cause calcium release from intracellular stores while other stimuli depolarize taste cells to cause calcium influx through voltage-gated calcium channels (VGCCs). We recently demonstrated that a constitutive calcium influx exists in taste cells that is regulated by mitochondrial calcium transport and that the magnitude of this calcium influx correlates with the signalling mechanisms used by the taste cells. In this study, we used calcium imaging to determine that sodium-calcium exchangers (NCXs) also routinely contribute to the regulation of basal cytosolic calcium and that their relative role correlates with the signalling mechanisms used by the taste cells. RT-PCR analysis revealed that multiple NCXs and sodium-calcium-potassium exchangers (NCKXs) are expressed in taste cells. Thus, a dynamic relationship exists between calcium leak channels and calcium regulatory mechanisms in taste cells that functions to keep cytosolic calcium levels in the appropriate range for cell function.

The co-presence of Na+/Ca2+-K+ exchanger and Na+/Ca2+ exchanger in bovine adrenal chromaffin cells

We have previously shown that there is high Na(+)/Ca(2+) exchange (NCX) activity in bovine adrenal chromaffin cells. In this study, by monitoring the [Ca(2+)](i) change in single cells and in a population of chromaffin cells, when the reverse mode of exchanger activity has been initiated, we have shown that the NCX activity is enhanced by K(+). The K(+)-enhanced activity accounted for a significant proportion of the Na(+)-dependent Ca(2+) uptake activity in the chromaffin cells. The results support the hypothesis that both NCX and Na(+)/Ca(2+)-K(+) exchanger (NCKX) are co-present in chromaffin cells. The expression of NCKX in chromaffin cells was further confirmed using PCR and northern blotting. In addition to the plasma membrane, the exchanger activity, measured by Na(+)-dependent (45)Ca(2+) uptake, was also present in membrane isolated from the chromaffin granules enriched fraction and the mitochondria enriched fraction. The results support that both NCX and NCKX are present in bovine chromaffin cells and that the regulation of [Ca(2+)](i) is probably more efficient with the participation of NCKX.

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.

Sodium/calcium exchanger expression in the mouse and rat olfactory systems

Sodium/calcium (Na(+)/Ca(2+)) exchangers are membrane transport systems that regulate Ca(2+)-homeostasis in many eukaryotic cells. In olfactory and vomeronasal sensory neurons ligand-induced olfactory signal transduction is associated with influx and elevation of intracellular Ca(2+), [Ca(2+)](i). While much effort has been devoted to the characterization of Ca(2+)-related excitation and adaptation events of olfactory chemosensory neurons (OSNs), much less is known about mechanisms that return [Ca(2+)](i) to the resting state. To identify proteins participating in the poststimulus Ca(2+)-clearance of mouse OSNs, we analyzed the expression of three potassium (K(+))-independent (NCX1, 2, 3) and three K(+)-dependent (NCKX1, 2, 3) Na(+)/Ca(2+) exchangers. In situ hybridization showed that mRNAs of all six Na(+)/Ca(2+) exchangers coexist in neurons of the olfactory and vomeronasal systems, and that some are already detectable in the embryo. Of these, NCX1 and NCKX1 represent the most and least abundant mRNAs, respectively. Moreover, immunohistochemistry revealed that the NCX1, 2, and 3 proteins are expressed in nearly all neurons of the olfactory epithelium, the vomeronasal organ, the septal organ of Masera, and the Grueneberg ganglion. These three exchanger proteins display different expression profiles in dendrites, knobs, and plasma membranes of OSNs and in sustentacular cells. Furthermore, we show that NCX1 mRNA in rat olfactory mucosa is expressed as 8 alternative splice variants. This is the first comprehensive analysis of Na(+)/Ca(2+) exchanger expression in the mammalian olfactory system. Our results suggest that Ca(2+)-extrusion by OSNs utilizes multiple different Na(+)/Ca(2+) exchangers and that different subtypes are targeted to different subcellular compartments.

Na+ /Ca2+ exchanger contributes to asterosap-induced elevation of intracellular Ca2+ concentration in starfish spermatozoa

Asterosap, a group of equally active isoforms of sperm-activating peptides from the egg jelly of the starfish Asterias amurensis, functions as a chemotactic factor for sperm. It transiently increases the intracellular cGMP level of sperm, which in turn induces a transient elevation of intracellular Ca(2+) concentration ([Ca(2+)](i)). Using a fluorescent Ca(2+)-sensitive dye, Fluo-4 AM, we measured the changes in sperm [Ca(2+)](i) in response to asterosap. KB-R7943 (KB), a selective inhibitor of Na(+)/Ca(2+) exchanger (NCX), significantly inhibited the asterosap-induced transient elevation of [Ca(2+)](i), suggesting that asterosap influences [Ca(2+)](i) through activation of a K+-dependent NCX (NCKX). An NCKX activity of starfish sperm also shows K(+) dependency like other NCKXs. Therefore, we cloned an NCKX from the starfish testes and predicted that it codes for a 616 amino acid protein that is a member of the NCKX family. Pharmacological evidence suggests that this exchanger participates in the asterosap-induced Ca(2+) entry into sperm.

Functional characterization and molecular cloning of the K+-dependent Na+/Ca2+ exchanger in intact retinal cone photoreceptors

Light-dependent changes in cytoplasmic free Ca(2+) are much faster in the outer segment of cone than rod photoreceptors in the vertebrate retina. In the limit, this rate is determined by the activity of an electrogenic Na(+)/Ca(2+) exchanger located in the outer segment plasma membrane. We investigate the functional properties of the exchanger activity in intact, single cone photoreceptors isolated from striped bass retina. Exchanger function is characterized through analysis both of the electrogenic exchanger current and cytoplasmic free Ca(2+) measured with optical probes. The exchanger in cones is K(+) dependent and operates both in forward and reverse modes. In the reverse mode, the K(+) dependence of the exchanger is described by binding to a single site with K(1/2) about 3.6 mM. From the retina of the fish we cloned exchanger molecules bassNCKX1 and bassNCKX2. BassNCKX1 is a single class of molecules, homologous to exchangers previously cloned from mammalian rods. BassNCKX2 exists in four splice variants that differ from each other by small sequence differences in the single, large cytoplasmic loop characteristic of these molecules. We used RT-PCR (reverse transcriptase polymerase chain reaction) of individual cells to identify the exchanger molecule specifically expressed in bass single and twin cone photoreceptors. Each and every one of the four bassNCKX2 splice variants is expressed in both single and twin cones indistinguishably. BassNCKX1 is not expressed in cones and, by exclusion, it is likely to be an exchanger expressed in rods.

Differential contribution of plasmalemmal Na/Ca exchange isoforms to sodium-dependent calcium influx and NMDA excitotoxicity in depolarized neurons

Inhibition of Na(+),K(+)-ATPase during NMDA applications greatly increased NMDA-induced excitotoxicity in primary cultures of forebrain neurons (FNs), but not in cerebellar granule cells (CGCs). Because Na(+),K(+)-ATPase inhibition promotes reversal of plasmalemmal Na(+)/Ca(2+) exchangers, we compared the activities of reversed K(+)-independent (NCX) and K(+)-dependent (NCKX) Na(+)/Ca(2+) exchangers in these cultures. To this end, we measured gramicidin-induced and Na(+)-dependent elevation in cytosolic [Ca(2+)] ([Ca(2+)](c)) that represents Ca(2+) influx via reversed NCX and NCKX; NCX activity was dissected out by removing external K(+). The [Ca(2+)](c) elevations mediated by NCX alone, and NCX plus NCKX combined, were 17 and 6 times more rapid in FNs than in CGCs, respectively. Northern blot analysis showed that FNs preferentially express NCX1 whereas CGCs expressed NCX3. Differences in expression of other isoforms (NCX2, NCKX2, NCKX3 and NCKX4) were less pronounced. We tested whether the NCX or NCKX family of exchangers contributes most to the toxic NMDA-induced Ca(2+) influx in depolarized neurons. We found that in FNs, inhibition of NCX alone was sufficient to significantly limit NMDA excitotoxicity, whereas in CGCs, inhibition of both NCX and NCKX was required. The data suggest that the high activity of NCX isoforms expressed in FNs, possibly NCX1, sensitizes these neurons to NMDA excitotoxicity.

AMPA receptor properties and coexpression with sodium-calcium exchangers in rat hypothalamic neurons

The histaminergic tuberomamillary (TM) nucleus, a center for the regulation of wakefulness, is excited by glutamatergic, aminergic and peptidergic inputs. AMPA receptor properties in relation to their expression were investigated in acutely isolated TM neurons with the help of whole-cell patch-clamp recordings combined with single-cell RT-PCR. The mRNAs encoding for the AMPA receptor GluR2 (100% of the neurons) and GluR1 (75%) were the most frequently detected, followed by the mRNA for GluR4 (56%), whereas GluR3 cDNA amplification did not yield a PCR product in any neuron. Flip splice variants prevailed over flop, in keeping with a strong glutamate-response potentiation by cyclothiazide. The expression pattern of AMPA subunits in their two splice variants was correlated with the different subtypes of Na+/Ca2+ (NCX) and Na+/Ca2+/K+ (NCKX) exchangers: glutamate receptor subunits GluR1-4 displayed no coordinated pattern with NCX. However, NCKX2 mRNA occurred only in TM cells with a fast desensitizing glutamate response, where it was coexpressed with the GluR4 subunit in the flop splice variant. NCKX3 mRNA was detected in neurons with fast or slow desensitization of glutamate responses. AMPA receptors in TM neurons were Ca2+-impermeable. As reverse Na+/Ca2+ exchange contributes to the immediate rise in intracellular calcium resulting from glutamate receptor activation, we suggest that the coordinated expression of NCKX2 with the fast desensitizing AMPA receptor-type reflects either a receptor-exchanger coupling or separate mechanisms for maintaining calcium homeostasis in neurons with fast or slow glutamate responses.

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.

Assembly of retinal rod or cone Na(+)/Ca(2+)-K(+) exchanger oligomers with cGMP-gated channel subunits as probed with heterologously expressed cDNAs

Proper control of intracellular free Ca(2+) is thought to involve subsets of proteins that co-localize to mediate coordinated Ca(2+) entry and Ca(2+) extrusion. The outer segments of vertebrate rod and cone photoreceptors present one example: Ca(2+) influx is exclusively mediated via cGMP-gated channels (CNG), whereas the Na(+)/Ca(2+)-K(+) exchanger (NCKX) is the only Ca(2+) extrusion protein present. In situ, a rod NCKX homodimer and a CNG heterotetramer are thought to be part of a single protein complex. However, NCKX-NCKX and NCKX-CNG interactions have been described so far only in bovine rod outer segment membranes. We have used thiol-specific cross-linking and co-immunoprecipitation to examine NCKX self-assembly and CNG-NCKX co-assembly after heterologous expression of either the rod or cone NCKX/CNG isoforms. Co-immunoprecipitation clearly demonstrated both NCKX homooligomerization and interactions between NCKX and CNG. The NCKX-NCKX and NCKX-CNG interactions were observed for both the rod and the cone isoforms. Thiol-specific cross-linking led to rod NCKX1 dimers and to cone NCKX2 adducts of an apparent molecular weight higher than that predicted for a NCKX2 dimer. The mass of the cross-link product critically depended on the location of the particular cysteine residue used by the cross-linker, and we cannot exclude that NCKX forms a higher oligomer. The NCKX-NCKX and NCKX-CNG interactions were not isoform-specific (i.e., rod NCKX could interact with cone NCKX, rod NCKX could interact with cone CNGA, and vice versa). Deletion of the two large hydrophilic loops from the NCKX protein did not abolish the NCKX oligomerization, suggesting that it is mediated by the highly conserved transmembrane spanning segments.

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.

K+-dependent Na+/Ca2+ exchangers in the brain

Sodium-calcium exchange was first characterized in heart myocytes and squid axon more than 3 decades ago. Since then, it has been appreciated that functioning of the Na/Ca exchanger molecule plays a critical role in calcium homeostasis in neurons. Genome analysis indicates that Na/Ca exchangers are a superfamily encoded by 7 different genes divided into 2 groups: the Na/Ca exchangers (NCX; SLC8) and the Na/Ca+K exchangers (NCKX; SLC24). Two different NCX genes, NCX1 and NCX2, are highly expressed in brain. We recently described the widespread expression of 2 NCKX-type exchangers in brain, NCKX2 and NCKX3, and uncovered evidence for expression of another, NCKX4. The unique role that each different exchanger plays in neuronal calcium homeostasis, however, awaits further investigation. To begin exploring this central question, we examined both the expression pattern and the functional properties of the K-dependent Na/Ca exchanger isoforms expressed in brain and compared and contrasted these with NCX-type exchangers. Distinct patterns of transcript abundance, regional distribution, and developmental expression were noted for each isoform. Functional properties, including stoichiometry and the kinetic characteristics of ion binding, were determined for NCKX2 and are discussed in the context of cellular Ca2+ signaling.

The Na/Ca-K exchanger gene family

Ca(2+) extrusion driven by both the inward Na(+) gradient as well as the outward K(+) gradient is essential for visual transduction in retinal rod and cone photoreceptors because it removes Ca(2+) that enters photoreceptors via the cGMP-gated and light-sensitive channels. We have cloned rod and cone Na/Ca-K exchanger (NCKX) cDNAs from several species, and we have cloned NCKX cDNAs from lower organisms that lack vertebrate-type vision. Although in situ NCKX physiology has only been documented for vertebrate photoreceptors, it is now clear that NCKX gene products have a much broader distribution pattern. Here, we review some of the structural and functional features that have emerged from our studies on different members of the NCKX gene family.

Potassium-dependent sodium-calcium exchange through the eye of the fly

In this review, we describe the characterization of a Drosophila sodium/calcium-potassium exchanger, Nckx30C. Sodium/calcium (-potassium) exchangers (NCX and NCKX) are required for the rapid removal of calcium in excitable cells. The deduced protein topology for NCKX30C is similar to that of mammalian NCKX, with 5 hydrophobic domains in the amino terminus separated from 6 at the carboxy-terminal end by a large intracellular loop. NCKX30C functions as a potassium-dependent sodium-calcium exchanger and is expressed in adult neurons and during ventral nerve cord development in the embryo. Nckx30C is expressed in a dorsal/ventral pattern in the eye-antennal disc, suggesting that large fluxes of calcium may be occurring during imaginal disc development in the larvae. NCKX30C may play a critical role in modulating calcium during development as well as in the removal of calcium and maintenance of calcium homeostasis in adults.

Calcium regulation in photoreceptors

In this review we describe some of the remarkable and intricate mechanisms through which the calcium ion (Ca2+) contributes to detection, transduction and synaptic transfer of light stimuli in rod and cone photoreceptors. The function of Ca2+ is highly compartmentalized. In the outer segment, Ca2+ controls photoreceptor light adaptation by independently adjusting the gain of phototransduction at several stages in the transduction chain. In the inner segment and synaptic terminal, Ca2+ regulates cells' metabolism, glutamate release, cytoskeletal dynamics, gene expression and cell death. We discuss the mechanisms of Ca2+ entry, buffering, sequestration, release from internal stores and Ca2+ extrusion from both outer and inner segments, showing that these two compartments have little in common with respect to Ca2+ homeostasis. We also investigate the various roles played by Ca2+ as an integrator of intracellular signaling pathways, and emphasize the central role played by Ca2+ as a second messenger in neuromodulation of photoreceptor signaling by extracellular ligands such as dopamine, adenosine and somatostatin. Finally, we review the intimate link between dysfunction in photoreceptor Ca2+ homeostasis and pathologies leading to retinal dysfunction and blindness.

Preferential expression of plasmalemmal K-dependent Na+/Ca2+ exchangers in neurons versus astrocytes

Numerous isoforms of plasmalemmal K-dependent (NCKX) and K-independent (NCX) Na+/Ca2+ exchangers are expressed in the brain. The physiological functions of each isoform are presently unknown. Therefore, in this study, we compared expression of NCKX and NCX transcripts between primary cultures of cerebellar granule cells, and astrocytes. Northern blot analysis showed that granule cells expressed NCKX2, NCKX3, NCKX4 and NCX3, whereas astrocytes expressed primarily NCX1. Consistent with this molecular characterization, a significant fraction of 45Ca2+ accumulation in Na-loaded granule cells, but not in astrocytes, depended on external K+. This is the first demonstration of native NCKX activity in neurons derived from the central nervous system. Our data suggest that NCKX isoform expression may correspond to the unique Ca2+ homeostasis requirements of neurons.

A flagellar K(+)-dependent Na(+)/Ca(2+) exchanger keeps Ca(2+) low in sea urchin spermatozoa

The metabolism, flagellar beating, and acrosome reaction of spermatozoa are regulated by ion flux across the plasma membrane. As is true of most cells, swimming sperm maintain intracellular Ca(2+) concentrations at submicromolar levels. Here we describe a K(+)-dependent Na(+)/Ca(2+) exchanger (suNCKX) from sea urchin sperm. The suNCKX is phylogenetically related to other NCKXs, which use high relative intracellular K(+), and high relative extracellular Na(+), to couple the efflux of 1 Ca(2+) and 1 K(+) to the influx of 4 Na(+). The 652-aa suNCKX shares structural topology with other NCKX proteins, and has two protein kinase A sites and a His-rich region in its cytoplasmic loop. The suNCKX is encoded by a single gene, which is highly expressed in testes. The suNCKX activity of whole sperm shows Na(+) and K(+) dependence, and like other NCKXs can run in reverse exchange mode. An inhibitor blocks the suNCKX activity and sperm motility. suNCKX localizes to the plasma membrane over the sperm flagellum. The suNCKX may play a major role in keeping Ca(2+) low in swimming sperm.

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.