Calcium homeostasis in vertebrate retinal rod outer segments

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.

Calmodulin binds and modulates K+-dependent Na+/Ca2+-exchanger isoform 4, NCKX4

The family of K+-dependent Na+/Ca2+-exchangers, NCKX, are important mediators of cellular Ca2+ efflux, particularly in neurons associated with sensory transduction. The NCKX family comprises five proteins, NCKX1-5, each being the product of a different SLC24 gene. NCKX4 (SLC24A4) has been found to have a critical role in termination and adaptation of visual and olfactory signals, melanocortin-dependent satiety signaling, and the maturation of dental enamel. To explore mechanisms that might influence the temporal control of NCKX4 activity, a yeast two-hybrid system was used to search for protein interaction partners. We identified calmodulin as a partner for NCKX4 and confirmed the interaction using glutathione-S-transferase fusion pull-down. Calmodulin binding to NCKX4 was demonstrated in extracts from mouse brain and in transfected HEK293 cells. Calmodulin bound in a Ca2+-dependent manner to a motif present in the central cytosolic loop of NCKX4 and was abolished by the double-mutant I328D/F334D. When cotransfected in HEK293 cells, calmodulin bound to NCKX4 under basal conditions and induced a ∼2.5-fold increase in NCKX4 abundance, but did not influence either cellular location or basal activity. When purinergic stimulation of NCKX4 was examined in these cells, coexpression of wild-type calmodulin, but not a Ca2+ binding-deficient calmodulin mutant, suppressed NCKX4 activation in a time-dependent manner. We propose that Ca2+ binding to calmodulin prepositioned on NCKX4 induces a slow conformational rearrangement that interferes with purinergic stimulation of the exchanger, possibly by obscuring T331, a previously identified potential protein kinase C site.

The transcriptome of the rat subfornical organ is altered in response to early postnatal overnutrition

Early postnatal overnutrition in humans is associated with long-term negative outcomes including obesity, increased risk of type-II diabetes, and cardiovascular disease. Hypothalamic neurons from rodents exposed to early postnatal overnutrition show altered expression of satiety signals and receptors, and exhibit altered responses to many satiety signals, suggesting a hypothalamic link between early overnutrition and development of these sequelae. Importantly, several hypothalamic nuclei receive information regarding circulating hormones (such as insulin, leptin and ghrelin) from the subfornical organ (SFO), a forebrain sensory circumventricular organ which lacks a blood brain barrier. Previous transcriptomic studies indicate that challenges to energy balance and hydration status stimulate changes in gene expression within the SFO, including genes encoding ion channels and receptors. In order to determine if early postnatal overnutrition also causes changes in SFO gene expression which may be associated with homeostatic dysregulation, we performed whole transcriptome sequencing on SFO tissue from rats raised in small (4 pups), or control (large, 12 pups) litters. Illumina RNA sequencing was performed on SFO tissue from rats raised from small and large litters, and read sequences were aligned to the Rat Rnor_6.0 genome. Control data were further compared to previously published microarray data set for validation. We found statistically significant (p < 0.05) changes in expression of 12 transcripts, three of which have likely roles in neuronal excitability, neurite outgrowth and differentiation, and food intake (Manf, Slc24a4, Cracr2b). Additionally, gene ontology analysis identified a trend among significantly altered transcripts in roles for oxidative stress response. We conclude that the SFO transcriptome is subtly altered by early postnatal overnutrition, and recommend further investigation of the effect of early postnatal overnutrition on SFO physiology and morphology.

Structural and molecular bases of rod photoreceptor morphogenesis and disease

The rod cell has an extraordinarily specialized structure that allows it to carry out its unique function of detecting individual photons of light. Both the structural features of the rod and the metabolic processes required for highly amplified light detection seem to have rendered the rod especially sensitive to structural and metabolic defects, so that a large number of gene defects are primarily associated with rod cell death and give rise to blinding retinal dystrophies. The structures of the rod, especially those of the sensory cilium known as the outer segment, have been the subject of structural, biochemical, and genetic analysis for many years, but the molecular bases for rod morphogenesis and for cell death in rod dystrophies are still poorly understood. Recent developments in imaging technology, such as cryo-electron tomography and super-resolution fluorescence microscopy, in gene sequencing technology, and in gene editing technology are rapidly leading to new breakthroughs in our understanding of these questions. A summary is presented of our current understanding of selected aspects of these questions, highlighting areas of uncertainty and contention as well as recent discoveries that provide new insights. Examples of structural data from emerging imaging technologies are presented.

Cation dependencies and turnover rates of the human K⁺-dependent Na⁺-Ca²⁺ exchangers NCKX1, NCKX2, NCKX3 and NCKX4

The Solute Carrier Family 24 (SLC24) belongs to the CaCA super family of Ca(2+)/cation antiporters and codes for five different K(+)- dependent Na(+)- Ca(2+) exchangers (NCKX1-5). NCKX proteins play a critical role in Ca(2+) homeostasis in a wide variety of biological processes such as vision, olfaction, enamel formation, Melanocortin-4-receptor-dependent satiety and skin pigmentation. NCKX transcripts are widely found throughout the brain. In this study we examine the differences between NCKX1-4 in terms of cation dependencies. We measured changes to Ca(2+) influx via the reverse exchange mode while manipulating external Ca(2+) or K(+) or internal Na(+) concentrations (External Ca(2+) Dependence, External K(+) Dependence and Internal Na(+) Dependence respectively); we also looked at the effect of external Na(+)/Ca(2+) competition and 3' 4'-Dichlorobenzamil on the transport of ions in HEK 293 cell lines. A fluorescence based assay was used to determine differences in transport kinetics of the four membrane spanning exchangers using the Michaelis-Menten constant (Km). Our results show that there are no significant differences between the NCKX isoforms to explain the variation in the specific expression pattern of these exchangers.

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.

Change of genes in calcium transport channels caused by hypoxic stress in the placenta, duodenum, and kidney of pregnant rats

Preeclampsia is a pregnancy-specific disease characterized by concurrent development of hypertension, proteinuria, and oxidative stress in the placenta. In this study, we induced hypoxic stress in rats during pregnancy to reproduce physiological conditions associated with preeclampsia. The maternal weight of hypoxic pregnant rats was lower than that of normoxic animals. The level of calcium ions were also increased in urine collected from the hypoxic animals. In contrast, urinary concentrations of sodium, chloride, and potassium ions declined in hypoxic rats, and developed to proteinuria. The expression of genes known as two biomarkers, sFLT1 (for preeclampsia) and HIF-1alpha (for hypoxia), were highly induced in the placenta, duodenum, and kidney by hypoxic stress. The overexpression of sFLT1 and HIF-1alpha demonstrated that our experimental conditions closely mimicked ones that are associated with preeclampsia. In the present study, we measured the expression of calcium transporters (TRPV5, TRPV6, PMCA1, NCKX3, NCX1, and CaBP-9k) in the placenta, duodenum, and kidney under hypoxic conditions on Gestational Day 19.5 in rats. Placental TRPV5, TRPV6, and PMCA1 expression was up-regulated in the hypoxic rats, whereas the levels of NCX1 and CaBP-9k were unchanged. In addition, NCKX3 expression was increased in the placenta of hypoxic rats. Duodenal expression of CaBP-9k, TRPV5, TRPV 6, and PMCA1 was decreased in the hypoxic rats, whereas levels of NCXs were not altered. Renal expression of NCKX3 and TRPV6 was increased, whereas NCX1 was decreased in the hypoxic rats compared to the normoxic controls. Taken together, these results indicate that physiological changes observed in the hypoxic rats were similar to ones associated with preeclampsia. Expression of calcium transport genes in the placenta, duodenum, and kidney perturbed by hypoxic stress during pregnancy may cause calcium loss in the urine, and thereby induce calcium-deficient characteristics of preeclampsia.

Expression and regulation of sodium/calcium exchangers, NCX and NCKX, in reproductive tissues: do they play a critical role in calcium transport for reproduction and development?

Plasma membrane sodium/calcium (Na(+)/Ca(2+)) exchangers are an important component of intracellular calcium [Ca(2+)](i) homeostasis and electrical conduction. Na(+)/Ca(2+) exchangers, NCX and NCKX, play a critical role in the transport of one [Ca(2+)](i) and potassium ion across the cell membrane in exchange for four extracellular sodium ions [Na(+)](e). Mammalian plasma membrane Na(+)/Ca(2+) exchange proteins are divided into two families: one in which Ca(2+) flux is dependent only on sodium (NCX1-3) and another in which Ca(2+) flux is also dependent on potassium (NCKX1-4). Both molecules are capable of forward- and reverse-mode exchange. In cells and tissues, Na(+)/Ca(2+) (and K(+)) gradients localize to the cell membrane; thus, the exchangers transport ions across a membrane potential. Uterine NCKX3 has been shown to be involved in the regulation of endometrial receptivity by [Ca(2+)](i). In the uterus and placenta, NCKX3 expression is regulated by the sex steroid hormone estrogen (E2) and hypoxia stress, respectively. In this chapter, we described the expression and regulation of these proteins for reproductive functions in various tissues including uterus, placenta, and kidney of humans and rodents. Evidence to date suggests that NCKX3 and NCX1 may be regulated in a tissue-specific manner. In addition, we focused on the molecular mechanism involved in the regulation of NCKX3 and NCX1 in mammals, based upon our recent results and those of others.

Functional and structural properties of the NCKX2 Na(+)-Ca (2+)/K (+) exchanger: a comparison with the NCX1 Na (+)/Ca (2+) exchanger

Na(+)/Ca(2+)-K(+) exchangers (NCKX), alongside the more widely known Na(+)/Ca(2+) exchangers (NCX), are important players in the cellular Ca(2+) toolkit. But, unlike NCX, much less is known about the physiological roles of NCKX, while emergent evidence indicates that NCKX has highly specialized functions in cells and tissues where it is expressed. As their name implies, there are functional similarities in the properties of the two Ca(2+) exchanger families, but there are specific differences as well. Here, we compare and contrast their key functional properties of ionic dependence and affinities, as well as report on the effects of KB-R7943 - a compound that is widely used to differentiate the two exchangers. We also review structural similarities and differences between the two exchangers. The aim is to draw attention to key differences that will aid in differentiating the two exchangers in physiological contexts where both exist but perhaps play distinct roles.

Protein Phylogenetic Analysis of Ca(2+)/cation Antiporters and Insights into their Evolution in Plants

Cation transport is a critical process in all organisms and is essential for mineral nutrition, ion stress tolerance, and signal transduction. Transporters that are members of the Ca(2+)/cation antiporter (CaCA) superfamily are involved in the transport of Ca(2+) and/or other cations using the counter exchange of another ion such as H(+) or Na(+). The CaCA superfamily has been previously divided into five transporter families: the YRBG, Na(+)/Ca(2+) exchanger (NCX), Na(+)/Ca(2+), K(+) exchanger (NCKX), H(+)/cation exchanger (CAX), and cation/Ca(2+) exchanger (CCX) families, which include the well-characterized NCX and CAX transporters. To examine the evolution of CaCA transporters within higher plants and the green plant lineage, CaCA genes were identified from the genomes of sequenced flowering plants, a bryophyte, lycophyte, and freshwater and marine algae, and compared with those from non-plant species. We found evidence of the expansion and increased diversity of flowering plant genes within the CAX and CCX families. Genes related to the NCX family are present in land plant though they encode distinct MHX homologs which probably have an altered transport function. In contrast, the NCX and NCKX genes which are absent in land plants have been retained in many species of algae, especially the marine algae, indicating that these organisms may share "animal-like" characteristics of Ca(2+) homeostasis and signaling. A group of genes encoding novel CAX-like proteins containing an EF-hand domain were identified from plants and selected algae but appeared to be lacking in any other species. Lack of functional data for most of the CaCA proteins make it impossible to reliably predict substrate specificity and function for many of the groups or individual proteins. The abundance and diversity of CaCA genes throughout all branches of life indicates the importance of this class of cation transporter, and that many transporters with novel functions are waiting to be discovered.

The Na(+)/Ca(2+) exchanger NCKX4 governs termination and adaptation of the mammalian olfactory response

Sensory perception requires accurate encoding of stimulus information by sensory receptor cells. We identified NCKX4, a potassium-dependent Na(+)/Ca(2+) exchanger, as being necessary for rapid response termination and proper adaptation of vertebrate olfactory sensory neurons (OSNs). Nckx4(-/-) (also known as Slc24a4) mouse OSNs displayed substantially prolonged responses and stronger adaptation. Single-cell electrophysiological analyses revealed that the majority of Na(+)-dependent Ca(2+) exchange in OSNs relevant to sensory transduction is a result of NCKX4 and that Nckx4(-/-) mouse OSNs are deficient in encoding action potentials on repeated stimulation. Olfactory-specific Nckx4(-/-) mice had lower body weights and a reduced ability to locate an odorous source. These results establish the role of NCKX4 in shaping olfactory responses and suggest that rapid response termination and proper adaptation of peripheral sensory receptor cells tune the sensory system for optimal perception.

Distinct expression of the calcium exchangers, NCKX3 and NCX1, and their regulation by steroid in the human endometrium during the menstrual cycle

Plasma membrane sodium/calcium exchangers are an important component of intracellular calcium homeostasis and electrical conduction. The potassium-dependent sodium/calcium exchangers NCKX3 (gene SLC24A3) and NCX1 (gene SLC8A1) play a critical role in the transport of intracellular calcium across the cell membrane in exchange for extracellular sodium ions. NCKX3 and NCX1 transcripts are most abundant in the brain and smooth muscle, but many other tissues, particularly the uterus, aorta, and intestine, also express this gene at lower levels. However, the expression patterns and physiological roles of NCKX3 and NCX1 in the human endometrium during the menstrual cycle are unknown. Thus, we examined the endometrial expression of NCKX3 and NCX1 messenger RNA (mRNA) and protein throughout the different phases of the menstrual cycle. Endometrial expression of NCKX3 mRNA and protein was increased 1.5- to 2.5-fold during the early-proliferative, mid-proliferative, and early-secretory phases compared with the other phases; however, no significant alteration in NCX1 expression level was observed. The effects of the sex-steroid hormones, 17β-estradiol (E2) and progesterone (P4), on the expression of NCKX3 and NCX1 in Ishikawa cells was also investigated. NCKX3 expression was significantly increased by E2 (10(-8) mol/L). However, the expression of NCX1 was not affected by E2 and P4. Subsequent immunohistochemical analysis revealed that the uterine NCKX3 and NCX1 proteins were abundantly localized in the cytoplasm of luminal and glandular epithelial cells throughout the menstrual cycle. Taken together, these results indicate that NCKX3 is abundantly expressed within the human endometrium at the transcriptional and translational levels, and its level appears to be regulated by a steroid hormone, in particular, E2 during the human menstrual cycle.

Retina expresses a novel variant of the ryanodine receptor

Calcium released from intracellular stores via the ryanodine receptor (RyR) mediates a variety of signalling processes. We previously showed that retina expresses the three known types of RyR, but retinal membrane preparations exhibit unique characteristics such as Ca2+-independent [3H]ryanodine-binding and inhibition by caffeine. We have heretofore suggested that the major retinal RyR isoform is novel. The present study aimed to identify this receptor isoform and to localize RyR in mammalian retina. Immunoblotting with specific and pan-antibodies showed that the major retinal RyR has a mobility similar to that of RyR2 or RyR3. Real-time PCR revealed that the major type is RyR2, and RT-PCR followed by sequencing showed a transcript that encodes a protein with approximately 99% identity to RyR2, yet lacking two regions of seven and 12 amino acids and including an additional insertion of eight amino acids. An antibody against RyR2 localized this type to somas and primary dendrites of most retinal neurons. An antibody against RyR1 localized RyR to most somas but also revealed staining in photoreceptor outer segments, concentrated on the disk membranes at their rim. The ryanodine-binding properties and the electrophoretic mobility of RyR from the outer segments were similar to those of the whole retinal preparation. The results thus identify a novel variant of RyR2 which can contribute to regulating photoreceptor Ca2+ concentrations. The restricted localization of the outer segment RyR to the disk rim suggests that its activation mechanism involves a coupling between retinal RyR and the cGMP-gated channel.

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.

Electrogenic Na+/Ca2+-exchange of nerve and muscle cells

The plasma membrane Na(+)/Ca(2+)-exchanger is a bi-directional electrogenic (3Na(+):1Ca(2+)) and voltage-sensitive ion transport mechanism, which is mainly responsible for Ca(2+)-extrusion. The Na(+)-gradient, required for normal mode operation, is created by the Na(+)-pump, which is also electrogenic (3Na(+):2K(+)) and voltage-sensitive. The Na(+)/Ca(2+)-exchanger operational modes are very similar to those of the Na(+)-pump, except that the uncoupled flux (Na(+)-influx or -efflux?) is missing. The reversal potential of the exchanger is around -40 mV; therefore, during the upstroke of the AP it is probably transiently activated, leading to Ca(2+)-influx. The Na(+)/Ca(2+)-exchange is regulated by transported and non-transported external and internal cations, and shows ATP(i)-, pH- and temperature-dependence. The main problem in determining the role of Na(+)/Ca(2+)-exchange in excitation-secretion/contraction coupling is the lack of specific (mode-selective) blockers. During recent years, evidence has been accumulated for co-localisation of the Na(+)-pump, and the Na(+)/Ca(2+)-exchanger and their possible functional interaction in the "restricted" or "fuzzy space." In cardiac failure, the Na(+)-pump is down-regulated, while the exchanger is up-regulated. If the exchanger is working in normal mode (Ca(2+)-extrusion) during most of the cardiac cycle, upregulation of the exchanger may result in SR Ca(2+)-store depletion and further impairment in contractility. If so, a normal mode selective Na(+)/Ca(2+)-exchange inhibitor would be useful therapy for decompensation, and unlike CGs would not increase internal Na(+). In peripheral sympathetic nerves, pre-synaptic alpha(2)-receptors may regulate not only the VSCCs but possibly the reverse Na(+)/Ca(2+)-exchange as well.

Structure-function relationships of the NCKX2 Na+/Ca2+-K+ exchanger

K+-dependent Na+/Ca2+ exchangers (NCKX) have been shown to play important roles in physiological processes as diverse as phototransduction in rod photoreceptors, motor learning and memory in mice, and skin pigmentation in humans. Most structure-function studies on NCKX proteins have been carried out on the NCKX2 isoform, but sequence similarity suggests that the results obtained with the NCKX2 isoform are likely to apply to all NCKX1-5 members of the human SLC24 gene family. Here we review our recent work on the NCKX2 protein concerning the topological arrangement of transmembrane segments carrying out cation transport, and concerning residues important for transport function and cation binding.

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.

Na+-dependent inactivation of the retinal cone/brain Na+/Ca2+-K+ exchanger NCKX2

The SLC24 gene family Na+/Ca2+-K+ exchangers (NCKX) are bidirectional plasma membrane transporters whose main function is the extrusion of Ca2+ from the cytosol. In this study, we used human embryonic kidney 293 cells expressing human retinal cone/brain NCKX2 to examine its Na+ affinity and kinetic parameters of Ca2+ transport. With the use of the ionophore gramicidin to control alkali cation concentrations across the plasma membrane, application of high intracellular Na+ promoted large NCKX2-mediated increases in intracellular free Ca2+ in the 15-20 microm range; this also resulted in inactivation of NCKX2 transport, the first description of this novel kinetic state. The affinity of NCKX2 for internal Na+ was found to be sigmoidal, with a Hill coefficient of 2.6 and Kd = 50 mm. The time-dependent (t(1/2) approximately 40s) inactivation of NCKX2 required high intracellular Na+ levels (Kd > 50 mm) as well as high occupancy of the extracellular Ca2+-binding site. Also reported are two residues whose substitution resulted in an increase in internal Na+ affinity to values of approximately 19 mm; these mutants also displayed enhanced inactivation, suggesting that inactivation requires binding of Na+ to its intracellular transport sites. These findings are the first report of a regulatory kinetic state of Ca2+ transport via NCKX2 Na+/Ca2+-K+ exchangers that may play a prominent role in regulation of Ca2+ extrusion in cellular environments such as neuronal synapses that experience frequent and dynamic Ca2+ fluxes.

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.

Residues contributing to the Ca2+ and K+ binding pocket of the NCKX2 Na+/Ca2+-K+ exchanger

The Na(+)/Ca(2+)-K(+) exchanger (NCKX) extrudes Ca(2+) from cells utilizing both the inward Na(+) gradient and the outward K(+) gradient. NCKX is thought to operate by a consecutive mechanism in which a cation binding pocket accommodates both Ca(2+) and K(+) and alternates between inward and outward facing conformations. Here we developed a simple fluorometric method to analyze changes in K(+) and Ca(2+) dependences of mutant NCKX2 proteins in which candidate residues within membrane-spanning domains were substituted. The largest shifts in both K(+) and Ca(2+) dependences compared with wild-type NCKX2 were observed for the charge-conservative substitutions of Glu(188) and Asp(548), whereas the size-conservative substitutions resulted in nonfunctional proteins. Substitution of several other residues including two proline residues (Pro(187) and Pro(547)), three additional acidic residues (Asp(258), Glu(265), Glu(533)), and two hydroxyl-containing residues (Ser(185) and Ser(545)) showed smaller shifts, but shifts in Ca(2+) dependence were invariably accompanied by shifts in K(+) dependence. We conclude that Glu(188) and Asp(548) are the central residues of a single cation binding pocket that can accommodate both K(+) and Ca(2+). Furthermore, a single set of residues lines a transport pathway for both K(+) and Ca(2+).

Forefront of Na+/Ca2+ exchanger studies: physiology and molecular biology of monovalent cation sensitivities in Na+/Ca2+ exchangers

Sensitivities of the reverse-mode Na+/Ca2+ exchange activity measured as the Na+i-dependent Ca2+ uptake to extracellular monovalent cations K+, Li+, and Na+ were compared between the K+ -dependent (NCKX2) and the K+ -independent Na+/Ca2+ exchanger (NCX1) overexpressed in a fibroblast cell. Interestingly, the exchange activity of NCKX2 was not influenced by Li+ while it was increased by K+. On the contrary, the activity of NCX1 was increased by Li+. Thus, the cation sensitivities to K+ and Li+ markedly differed between NCKX2 and NCX1. In addition, Na+ exerted a significantly smaller inhibitory effect on the activity in NCKX2 than in NCX1. The Na+/Ca2+ exchange activities of NCKX2 and NCX1 are considered to be regulated differentially via the respective binding site domains that have distinct sensitivities to the external monovalent cations.

Calmodulin and Ca2+-binding proteins (CaBPs): variations on a theme

Ca2+ is a ubiquitous second messenger that frequently exerts its effects through Ca2+-binding proteins. In response to changes in the intracellular [Ca2+], Ca2+-binding proteins modulate the cellular activities of enzymes, channels and structural proteins. Multiple Ca2+-binding proteins are expressed in the retina and, in most cases, in a unique cellular and sub-cellular manner. CaBPs are retinal Ca2+-binding proteins displaying a high similarity to calmodulin (CaM). CaBPs are able to mimic some of the interactions of CaM with effector enzymes, although their physiological role has not yet been resolved. CaBPs could be cell-type specific proteins that play a key role in the Ca2+ signaling of specialized retinal neurons.

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.

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.

A null mutation in guanylate cyclase-1 alters the temporal dynamics and light entrainment properties of the iodopsin rhythm in cone photoreceptor cells

Guanylate cyclase-1 (GC1) plays a critical role in visual phototransduction and its absence severely compromises the ability of the photoreceptor cells to transduce light for vision. In this study we sought to determine if the absence of GC1 has any effect on light entrainment of the circadian oscillators located in these cells. We compared the rhythmic changes in transcript levels of iodopsin, a photoreceptor-specific gene whose expression is regulated by circadian oscillators, in retinas of normal chickens and GUCY1*B (*B) chickens that carry a null mutation in GC1. Our results show that iodopsin rhythms are present in *B retinas and that they can be entrained to light; however, the rise and fall of iodopsin transcript levels in *B retina under cyclic light conditions is significantly more rapid than that observed in normal retina, and under constant dark conditions, the phase of the iodopsin rhythm in *B retina is advanced by 6 h relative to that observed in normal retina. In addition, the rate of entrainment of the iodopsin rhythm in *B retina to a reversal of the light cycle is significantly slower than normal. The results of our study show that a functioning visual phototransduction cascade is not essential for light entrainment of the oscillators that drive the iodopsin rhythm in photoreceptor cells. We propose that the abnormal synthesis of cGMP in *B photoreceptors underlies the irregular iodopsin rhythms observed in post-hatch *B retina.

Electrophysiological characterization and ionic stoichiometry of the rat brain K(+)-dependent NA(+)/CA(2+) exchanger, NCKX2

We have recently described a novel K(+)-dependent Na(+)/Ca(2+) exchanger, NCKX2, that is abundantly expressed in brain neurons (Tsoi, M., Rhee, K.-H., Bungard, D., Li, X.-F., Lee, S.-L., Auer, R. N., and Lytton, J. (1998) J. Biol. Chem. 273, 4115--4162). The precise role for NCKX2 in neuronal Ca(2+) homeostasis is not yet clearly understood but will depend upon the functional properties of the molecule. Here, we have performed whole-cell patch clamp analysis to characterize cation dependences and ion stoichiometry for rat brain NCKX2, heterologously expressed in HEK293 cells. Outward currents generated by reverse NCKX2 exchange depended on external Ca(2+) with a K(12) of 1.4 or 101 microm without or with 1 mm Mg(2+), and on external K(+) with a K(1/2) of about 12 or 36 mm with choline or Li(+) as counter ion, respectively. Na(+) inhibited outward currents with a K(1/2) of about 60 mm. Inward currents generated by forward NCKX2 exchange depended upon external Na(+) with a K(1/2) of 30 mm and a Hill coefficient of 2.8. K(+) inhibited the inward currents by a maximum of 40%, with a K(1/2) of 2 mm or less, depending upon the conditions. The transport stoichiometry of NCKX2 was determined by observing the change in reversal potential as individual ion gradients were altered. Our data support a stoichiometry for rat brain NCKX2 of 4 Na(+):(1 Ca(2+) + 1 K(+)). These findings provide the first electrophysiological characterization of rat brain NCKX2, and the first evidence that a single recombinantly expressed NCKX polypeptide encodes a K(+)-transporting Na(+)/Ca(2+) exchanger with a transport stoichiometry of 4 Na(+):(1 Ca(2+) + 1 K(+)).

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.

Adaptation in vertebrate photoreceptors

When light is absorbed within the outer segment of a vertebrate photoreceptor, the conformation of the photopigment rhodopsin is altered to produce an activated photoproduct called metarhodopsin II or Rh(*). Rh(*) initiates a transduction cascade similar to that for metabotropic synaptic receptors and many hormones; the Rh(*) activates a heterotrimeric G protein, which in turn stimulates an effector enzyme, a cyclic nucleotide phosphodiesterase. The phosphodiesterase then hydrolyzes cGMP, and the decrease in the concentration of free cGMP reduces the probability of opening of channels in the outer segment plasma membrane, producing the electrical response of the cell. Photoreceptor transduction can be modulated by changes in the mean light level. This process, called light adaptation (or background adaptation), maintains the working range of the transduction cascade within a physiologically useful region of light intensities. There is increasing evidence that the second messenger responsible for the modulation of the transduction cascade during background adaptation is primarily, if not exclusively, Ca(2+), whose intracellular free concentration is decreased by illumination. The change in free Ca(2+) is believed to have a variety of effects on the transduction mechanism, including modulation of the rate of the guanylyl cyclase and rhodopsin kinase, alteration of the gain of the transduction cascade, and regulation of the affinity of the outer segment channels for cGMP. The sensitivity of the photoreceptor is also reduced by previous exposure to light bright enough to bleach a substantial fraction of the photopigment in the outer segment. This form of desensitization, called bleaching adaptation (the recovery from which is known as dark adaptation), seems largely to be due to an activation of the transduction cascade by some form of bleached pigment. The bleached pigment appears to activate the G protein transducin directly, although with a gain less than Rh(*). The resulting decrease in intracellular Ca(2+) then modulates the transduction cascade, by a mechanism very similar to the one responsible for altering sensitivity during background adaptation.

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.

Minimal domain requirement for cation transport by the potassium-dependent Na/Ca-K exchanger. Comparison with an NCKX paralog from Caenorhabditis elegans

The retinal rod Na/Ca-K exchanger (NCKX) is a unique calcium extrusion protein utilizing both inward sodium gradient and outward potassium gradient. Three mammalian rod NCKX cDNAs have been cloned to date, but quantitative analysis of NCKX function in heterologous systems has proven difficult. Here, we describe a simple system for quantitative analysis of NCKX function; stable transformation of cultured insect cells with the novel pEA1/153A vector containing NCKX cDNAs was combined with measurements of potassium-dependent (45)Ca uptake in sodium-loaded cells. We carried out structure-function studies on NCKX with the following results: 1) two-thirds of the full-length sequence of bovine NCKX could be deleted without affecting potassium-dependent calcium transport and without affecting key properties of the potassium binding site; 2) the affinity of NCKX for potassium was about 10-fold greater in choline medium when compared with lithium medium; this shift was observed in rod outer segments or in cells expressing full-length rod NCKX, the above deletion mutant, or a distantly related NCKX paralog cloned from Caenorhabditis elegans. We conclude that the potassium binding site is highly conserved among members of the NCKX family and is formed by residues located within the two sets of transmembrane spanning segments in the NCKX sequence.

Localization of type I inositol 1,4,5-triphosphate receptor in the outer segments of mammalian cones

Calcium enters the outer segment of a vertebrate photoreceptor through a cGMP-gated channel and is extruded via a Na/Ca, K exchanger. We have identified another element in mammalian cones that might help to control cytoplasmic calcium. Reverse transcription-PCR performed on isolated photoreceptors identified mRNA for the SII- splice variant of the type I receptor for inositol 1,4,5-triphosphate (IP3), and Western blots showed that the protein also is expressed in outer segments. Immunocytochemistry showed type I IP3 receptor to be abundant in red-sensitive and green-sensitive cones of the trichromatic monkey retina, but it was negative or weakly expressed in blue-sensitive cones and rods. Similarly, the green-sensitive cones expressed the receptor in dichromatic retina (cat, rabbit, and rat), but the blue-sensitive cones did not. Immunostain was localized to disk and plasma membranes on the cytoplasmic face. To restore sensitivity after a light flash, cytoplasmic cGMP must rise to its basal level, and this requires cytoplasmic calcium to fall. Cessation of calcium release via the IP3 receptor might accelerate this fall and thus explain why the cone recovers much faster than the rod. Furthermore, because its own activity of the IP3 receptor depends partly on cytoplasmic calcium, the receptor might control the set point of cytoplasmic calcium and thus affect cone sensitivity.

cDNA cloning and functional expression of the dolphin retinal rod Na-Ca+K exchanger NCKX1: comparison with the functionally silent bovine NCKX1

cDNAs of human and bovine retinal rod Na+-Ca2++K+ exchanger (NCKX1) have previously been cloned, but potassium-dependent Na-Ca exchange activity upon heterologous expression has not been demonstrated. We have cloned NCKX1 cDNA from dolphin, examined function upon transfection in HEK293 cells, and compared the dolphin sequence encoded by the cDNA with those of human and bovine. The dolphin NCKX1 cDNA encodes 1013 amino acid residues. Comparison to bovine and human NCKX1 revealed strong conservation in the transmembrane domains (>95%), but relatively low conservation in the large extracellular ( approximately 50%) and cytosolic (<50%) domains. The dolphin cytosolic domain differs from the bovine sequence by the absence of a stretch of 114 amino acids. HEK293 cells transfected with dolphin NCKX1 cDNA showed K+-dependent Na-Ca exchange in >95% of the experiments, whereas transfection with bovine NCKX1 yielded no function. The bovine NCKX1 phenotype was imparted on dolphin NCKX1 when the dolphin cytosolic loop was replaced by that from bovine. Conversely, deletion of 114 amino acids from the bovine sequence to match the dolphin sequence resulted in a mutant bovine NCKX1 which performed K+-dependent Na-Ca exchange. These results suggest that domains within the large cytosolic loop of NCKX1 control functional activity when expressed in heterologous systems.

Molecular cloning of a novel potassium-dependent sodium-calcium exchanger from rat brain

We have isolated a novel cDNA clone from rat cerebral cortex encoding a protein of 670 amino acids (NCKX2) that has significant similarity to the 1199-amino acid-long Na/Ca-K exchanger of bovine rod outer segment (NCKX1). NCKX2 transcripts are 10.5 kilobase pairs in length and are expressed abundantly in neurons throughout the brain and with much lower abundance in selected other tissues. The predicted topology of the rat NCKX2 protein is very similar to that of bovine NCKX1, beginning with a solitary transmembrane segment (M0), which is removed as a "signal peptide" in bovine NCKX1, an extracellular loop, a cluster of five transmembrane spanning segments (M1 to M5), a long cytoplasmic loop, and a final hydrophobic cluster (M6 to M11). Within the hydrophobic clusters, rat NCKX2 shares 80% identity and 91% similarity with bovine NCKX1. The two larger hydrophilic loops are much shorter in NCKX2 than in NCKX1, accounting largely for the difference in length between the two proteins, and are dissimilar in sequence except for a 32-amino acid stretch with 69% identity in the cytosolic loop. NCKX2 was epitope-tagged in the extracellular domain and was shown to be expressed at the surface of transfected HEK cells. Analysis of NCKX2 function by fluorescent imaging of fura-2-loaded transfected cells demonstrated that NCKX2 is a potassium-dependent sodium/calcium exchanger.

Functional expression of Na-Ca exchanger clones measured with the fluorescent Ca(2+)-indicating dye fluo-3

The process of Ca2+ homeostasis is of prime importance to all cells because of the ubiquitous role of cytoplasmic Ca2+ as an intracellular messenger and the cytotoxicity of sustained elevated cytosolic Ca2+ concentrations. Two classes of plasma membrane proteins are responsible for maintaining cytosolic free Ca2+ in the submicromolar range against a very large electrochemical Ca2+ gradient across the plasma membrane, the ATP-driven Ca2+ pump and Na-Ca exchangers. Two types of Na-Ca exchangers are known, the 3Na:1Ca exchangers found in heart, brain, kidney, and most other tissues and the 4Na:1Ca+ 1K exchanger found in retinal rod and cone photoreceptors. Functional expression of Na-Ca(/K) exchangers is most often measured as 45Ca uptake in Na(+)-loaded cells or as Na-Ca exchange currents with the giant excised patch technique. In this study, two functional assays used to detect expression of the bovine heart Na-Ca exchanger in CHO cells are described. Both assays are based on measurements of cytosolic free Ca2+ with the fluorescent Ca(2+)-indicating dye fluo-3 and should be equally applicable in the study of functional expression of both Na-Ca and Na-Ca/K exchanger clones.