Na+-Ca2+ exchange in bovine rod outer segments requires and transports K+

Decreased eggshell strength caused by impairment of uterine calcium transport coincide with higher bone minerals and quality in aged laying hens

BACKGROUND

Deteriorations in eggshell and bone quality are major challenges in aged laying hens. This study compared the differences of eggshell quality, bone parameters and their correlations as well as uterine physiological characteristics and the bone remodeling processes of hens laying eggs of different eggshell breaking strength to explore the mechanism of eggshell and bone quality reduction and their interaction. A total of 240 74-week-old Hy-line Brown laying hens were selected and allocated to a high (HBS, 44.83 ± 1.31 N) or low (LBS, 24.43 ± 0.57 N) eggshell breaking strength group.

RESULTS

A decreased thickness, weight and weight ratio of eggshells were observed in the LBS, accompanied with ultrastructural deterioration and total Ca reduction. Bone quality was negatively correlated with eggshell quality, marked with enhanced structures and increased components in the LBS. In the LBS, the mammillary knobs and effective layer grew slowly. At the initiation stage of eggshell calcification, a total of 130 differentially expressed genes (DEGs, 122 upregulated and 8 downregulated) were identified in the uterus of hens in the LBS relative to those in the HBS. These DEGs were relevant to apoptosis due to the cellular Ca overload. Higher values of p62 protein level, caspase-8 activity, Bax protein expression and lower values of Bcl protein expression and Bcl/Bax ratio were seen in the LBS. TUNEL assay and hematoxylin-eosin staining showed a significant increase in TUNEL-positive cells and tissue damages in the uterus of the LBS. Although few DEGs were identified at the growth stage, similar uterine tissue damages were also observed in the LBS. The expressions of runt-related transcription factor 2 and osteocalcin were upregulated in humeri of the LBS. Enlarged diameter and more structural damages of endocortical bones and decreased ash were observed in femurs of the HBS.

CONCLUSION

The lower eggshell breaking strength may be attributed to a declined Ca transport due to uterine tissue damages, which could affect eggshell calcification and lead to a weak ultrastructure. Impaired uterine Ca transport may result in reduced femoral bone resorption and increased humeral bone formation to maintain a higher mineral and bone quality in the LBS.

Na+/Ca2+ exchange in enamel cells is dominated by the K+-dependent NCKX exchanger

Calcium (Ca2+) extrusion is an essential function of the enamel-forming ameloblasts, providing Ca2+ for extracellular mineralization. The plasma membrane Ca2+ ATPases (PMCAs) remove cytosolic Ca2+ (cCa2+) and were recently shown to be efficient when ameloblasts experienced low cCa2+ elevation. Sodium-calcium (Na+/Ca2+) exchange has higher capacity to extrude cCa2+, but there is limited evidence on the function of the two main families of Na+/Ca2+ exchangers in enamel formation. The purpose of this study was to analyze the function of the NCX (coded by SLC8) and the K+-dependent NCKX (coded by SLC24) exchangers in rat ameloblasts and to compare their efficacy in the two main stages of enamel formation: the enamel forming secretory stage and the mineralizing or maturation stage. mRNA expression profiling confirmed the expression of Slc8 and Slc24 genes in enamel cells, Slc24a4 being the most highly upregulated transcript during the maturation stage, when Ca2+ transport increases. Na+/Ca2+ exchange was analyzed in the Ca2+ influx mode in Fura-2 AM-loaded ameloblasts. We show that maturation-stage ameloblasts have a higher Na+/Ca2+ exchange capacity than secretory-stage cells. We also show that Na+/Ca2+ exchange in both stages is dominated by NCKX over NCX. The importance of NCKX function in ameloblasts may partly explain why mutations in the SLC24A4 gene, but not in SLC8 genes, result in enamel disease. Our results demonstrate that Na+/Ca2+ exchangers are fully operational in ameloblasts and that their contribution to Ca2+ homeostasis increases in the maturation stage, when Ca2+ transport need is higher.

Molecular basis of rod and cone differences

In the vertebrate retina, rods and cones both detect light, but they are different in functional aspects such as light sensitivity and time resolution, for example, and in some of cell biological aspects. For functional aspects, both photoreceptors are known to share a common mechanism, phototransduction cascade, consisting of a series of enzyme reactions to convert a photon-capture signal to an electrical signal. To understand the mechanisms of the functional differences between rods and cones at the molecular level, we compared biochemically each of the reactions in the phototransduction cascade between rods and cones using the cells isolated and purified from carp retina. Although proteins in the cascade are functionally similar between rods and cones, their activities together with their expression levels are mostly different between these photoreceptors. In general, reactions to generate a response are slightly less effective, as a total, in cones than in rods, but each of the reactions for termination and recovery of a response are much more effective in cones. These findings explain lower light sensitivity and briefer light responses in cones than in rods. In addition, our considerations suggest that a Ca²⁺-binding protein, S-modulin or recoverin, has a currently unnoticed role in shaping light responses. With comparison of the expression levels of proteins and/or mRNAs using purified cells, several proteins were found to be specifically or predominantly expressed in cones. These proteins would be of interest for future studies on the difference between rods and cones.

Intestinal Calcium Absorption

In this article, we focus on mammalian calcium absorption across the intestinal epithelium in normal physiology. Intestinal calcium transport is essential for supplying calcium for metabolism and bone mineralization. Dietary calcium is transported across the mucosal epithelia via saturable transcellular and nonsaturable paracellular pathways, both of which are under the regulation of 1,25-dihydroxyvitamin D₃ and several other endocrine and paracrine factors, such as parathyroid hormone, prolactin, 17β-estradiol, calcitonin, and fibroblast growth factor-23. Calcium absorption occurs in several segments of the small and large intestine with varying rates and capacities. Segmental heterogeneity also includes differential expression of calcium transporters/carriers (e.g., transient receptor potential cation channel and calbindin-D_9k ) and the presence of favorable factors (e.g., pH, luminal contents, and gut motility). Other proteins and transporters (e.g., plasma membrane vitamin D receptor and voltage-dependent calcium channels), as well as vesicular calcium transport that probably contributes to intestinal calcium absorption, are also discussed. © 2021 American Physiological Society. Compr Physiol 11:1-27, 2021.

Electrophysiological characterization of photoreceptor-like cells in human inducible pluripotent stem cell-derived retinal organoids during in vitro maturation

Retinal organoids (ROs) derived from human inducible pluripotent stem cells (hiPSCs) exhibit considerable therapeutic potential. However, current quality control of ROs during in vitro differentiation is largely limited to the detection of molecular markers, often by immunostaining, polymerase chain reaction (PCR) assays and sequencing, often without proper functional assessments. As such, in the current study, we systemically characterized the physiological maturation of photoreceptor-like cells in hiPSC-derived ROs. By performing patch-clamp recordings from photoreceptor-like cells in ROs at distinct differentiation stages (ie, Differentiation Day [D]90, D150, and D200), we determined the electrophysiological properties of the plasma membrane and several characteristic ion channels closely associated with the physiological functions of the photoreceptors. Ionic hallmarks, such as hyperpolarization-activated cyclic nucleotide-gated (HCN) channels and cyclic nucleotide-gated (CNG) channels, matured progressively during differentiation. After D200 in culture, these characteristic currents closely resembled those in macaque or human native photoreceptors. Furthermore, we demonstrated that the hyperpolarization-activated inward current/depolarization-activated outward current ratio (I_-120 /I₊₄₀ ), termed as the inward-outward current (IOC) ratio hereon, accurately represented the maturity of photoreceptors and could serve as a sensitive indicator of pathological state. Thus, this study provides a comprehensive dataset describing the electrophysiological maturation of photoreceptor-like cells in hiPSC-derived ROs for precise and sensitive quality control during RO differentiation.

Evolution of the genes mediating phototransduction in rod and cone photoreceptors

This paper reviews current knowledge of the evolution of the multiple genes encoding proteins that mediate the process of phototransduction in rod and cone photoreceptors of vertebrates. The approach primarily involves molecular phylogenetic analysis of phototransduction protein sequences, combined with analysis of the syntenic arrangement of the genes. At least 35 of these phototransduction genes appear to reside on no more than five paralogons - paralogous regions that each arose from a common ancestral region. Furthermore, it appears that such paralogs arose through quadruplication during the two rounds of genome duplication (2R WGD) that occurred in a chordate ancestor prior to the vertebrate radiation, probably around 600 millions years ago. For several components of the phototransduction cascade, it is shown that distinct isoforms already existed prior to WGD, with the likely implication that separate classes of scotopic and photopic photoreceptor cells had already evolved by that stage. The subsequent quadruplication of the entire genome then permitted the refinement of multiple distinct protein isoforms in rods and cones. A unified picture of the likely pattern and approximate timing of all the important gene duplications is synthesised, and the implications for our understanding of the evolution of rod and cone phototransduction are presented.

Transcriptome analysis of the endometrium from Chinese Erhualian sows that differ in calcium ion concentration and litter size

Embryonic survival rate, an important factor in the fecundity of sows, is affected by endometrium-secreting histotroph. A higher concentration of calcium ion has been observed in the uterus of highly prolific Erhualian sows (EH) compared with those of less prolific (EL) sows. This suggests that EH sows have better establishment and maintenance of pregnancies, thus increasing embryonic survival rate during the peri-implantation period. To understand the mechanisms of how the endometrium-secreting histotroph affects embryonic survival rate during the Erhualian peri-implantation period, the expression patterns of endometrial mRNA in the EH and EL sows on day 12 of gestation were analyzed using RNA sequencing technology. A total of 164 differentially expressed genes (DEGs) were identified (P_adj  < 0.05, |log₂ (FC)| ≥ 1), including 46 upregulated and 118 downregulated genes in EH compared to EL. Gene Ontology enrichment indicated that a subset of DEGs was involved in calcium ion binding and cell adhesion. Solute carrier family 8 member A3 and solute carrier family 24 member 4, identified as upregulated genes (P_adj  < 0.05) in EH, were considered key candidate genes expressed in the endometrium affecting embryonic survival rate during the peri-implantation period. The results improve understanding of the genetic mechanism underlying the variation in litter size of Erhualian pigs during the peri-implantation period.

Evolution of the calcium feedback steps of vertebrate phototransduction

We examined the genes encoding the proteins that mediate the Ca-feedback regulatory system in vertebrate rod and cone phototransduction. These proteins comprise four families: recoverin/visinin, the guanylyl cyclase activating proteins (GCAPs), the guanylyl cyclases (GCs) and the sodium/calcium-potassium exchangers (NCKXs). We identified a paralogon containing at least 36 phototransduction genes from at least fourteen families, including all four of the families involved in the Ca-feedback loop (recoverin/visinin, GCAPs, GCs and NCKXs). By combining analyses of gene synteny with analyses of the molecular phylogeny for each of these four families of genes for Ca-feedback regulation, we have established the likely pattern of gene duplications and losses underlying the expansion of isoforms, both before and during the two rounds of whole-genome duplication (2R WGD) that occurred in early vertebrate evolution. Furthermore, by combining our results with earlier evidence on the timing of duplication of the visual G-protein receptor kinase genes, we propose that specialization of proto-vertebrate photoreceptor cells for operation at high and low light intensities preceded the emergence of rhodopsin, which occurred during 2R WGD.

Characterization of the Cation Binding Sites in the NCKX2 Na+/Ca2+-K+ Exchanger

NCKX1-5 are proteins involved in K⁺-dependent Na⁺/Ca²⁺ exchange in various signal tissues. Here we present a homology model of NCKX2 based on the crystal structure of the NCX_Mj transporter found in Methanoccocus jannaschii. Molecular dynamics simulations were performed on the resultant wild-type NCKX2 model and two mutants (D548N and D575N) loaded with either four Na⁺ ions or one Ca²⁺ ion and one K⁺ ion, in line with the experimentally observed transport stoichiometry. The selectivity of the active site in wild-type NCKX2 for Na⁺, K⁺, and Li⁺ and the electrostatic interactions of the positive Na⁺ ions in the negatively charged active site of wild-type NCKX2 and the two mutants were evaluated from free energy perturbation calculations. For validation of the homology model, our computational results were compared to available experimental data obtained from numerous prior functional studies. The NCKX2 homology model is in good agreement with the discussed experimental data and provides valuable insights into the structure of the active site, which is lined with acidic and polar residues. The binding of the potassium and calcium ions is accomplished via Asp 575 and 548, respectively. Mutation of these residues to Asn alters the functionality of NCKX2 because of the elimination of the favorable carboxylate-cation interactions. The knowledge obtained from the NCKX2 model can be transferred to other isoforms of the NCKX family: newly discovered pathological mutations in NCKX4 and NCKX5 affect residues that are involved in ion binding and/or transport according to our homology model.

A Functional Study of Mutations in K+-dependent Na+-Ca2+ Exchangers Associated with Amelogenesis Imperfecta and Non-syndromic Oculocutaneous Albinism

K(+)-dependent Na(+)/Ca(2+) exchangers belong to the solute carrier 24 (SLC24A1-5) gene family of membrane transporters. Five different gene products (NCKX1-5) have been identified in humans, which play key roles in biological processes including vision, olfaction, and skin pigmentation. NCKXs are bi-directional membrane transporters that transport 1 Ca(2+)+K(+) ions in exchange for 4 Na(+) ions. Recent studies have linked mutations in the SLC24A4 (NCKX4) and SLC24A5 (NCKX5) genes to amylogenesis imperfecta (AI) and non-syndromic oculocutaneous albinism (OCA6), respectively. Here, we introduced mutations found in patients with AI and OCA6 into human SLC24A4 (NCKX4) cDNA leading to single residue substitutions in the mutant NCKX4 proteins. We measured NCKX-mediated Ca(2+) transport activity of WT and mutant NCKX4 proteins expressed in HEK293 cells. Three mutant NCKX4 cDNAs represent mutations found in the SCL24A4 gene and three represent mutations found in the SCL24A5 gene involving residues conserved between NCKX4 and NCKX5. Five mutant proteins had no observable NCKX activity, whereas one mutation resulted in a 78% reduction in transport activity. Total protein expression and trafficking to the plasma membrane (the latter with one exception) were not affected in the HEK293 cell expression system. We also analyzed two mutations in a Drosophila NCKX gene that have been reported to result in an increased susceptibility for seizures, and found that both resulted in mutant proteins with significantly reduced but observable NCKX activity. The data presented here support the genetic analyses that mutations in SLC24A4 and SLC24A5 are responsible for the phenotypic defects observed in human patients.

TRP-Na(+)/Ca(2+) Exchanger Coupling

Na(+)/Ca(2+) exchangers (NCXs) have traditionally been viewed principally as a means of Ca(2+) removal from non-excitable cells. However there has recently been increasing interest in the operation of NCXs in reverse mode acting as a means of eliciting Ca(2+) entry into these cells. Reverse mode exchange requires a significant change in the normal resting transmembrane ion gradients and membrane potential, which has been suggested to occur principally via the coupling of NCXs to localised Na(+) entry through non-selective cation channels such as canonical transient receptor potential (TRPC) channels. Here we review evidence for functional or physical coupling of NCXs to non-selective cation channels, and how this affects NCX activity in non-excitable cells. In particular we focus on the potential role of nanojunctions, where the close apposition of plasma and intracellular membranes may help create the conditions needed for the generation of localised rises in Na(+) concentration that would be required to trigger reverse mode exchange.

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.

Calcium extrusion regulatory molecules: differential expression during pregnancy in the porcine uterus

Calcium ions in the uterine endometrium are essential for the establishment and maintenance of pregnancy, but the cellular and molecular mechanisms of calcium ion regulation in the endometrium are not fully understood. Our previous study in pigs found that calcium regulatory molecules, transient receptor potential, vanilloid type 6 and calbindin-D9K, are expressed in the uterine endometrium during the estrous cycle and pregnancy. However, we did not determine the expression of calcium extrusion regulatory molecules, plasma membrane calcium ATPases (ATP2Bs), sodium/calcium exchangers (SLC8As), or potassium-dependent sodium/calcium exchangers (SLC24As), in the uterine endometrium and conceptuses. Thus, in this study we determine whether ATP2Bs, SCL8As, and SLC24As are expressed in the uterine endometrium during the estrous cycle and pregnancy and in conceptuses during early pregnancy. Real-time RT-PCR analysis showed that ATP2Bs, SLC8As, and SLC24As were expressed in the uterine endometrium in a pregnancy status- and stage-specific manner. Conceptuses during early pregnancy also expressed these molecules. In situ hybridization analysis showed that ATP2B1, SLC8A1, and SLC24A4 were localized mainly to luminal and glandular epithelium and stromal cells in the endometrium during pregnancy. These results indicate that calcium extrusion regulatory molecules are expressed in the uterine endometrium during the estrous cycle and pregnancy and in conceptuses during early pregnancy, indicating that calcium extrusion regulatory molecules may play important roles in the establishment and maintenance of pregnancy by regulating calcium ion concentration in the uterine endometrium in pigs.

A comprehensive model of the phototransduction cascade in mouse rod cells

Vertebrate visual phototransduction is perhaps the most well-studied G-protein signaling pathway. A wealth of available biochemical and electrophysiological data has resulted in a rich history of mathematical modeling of the system. However, while the most comprehensive models have relied upon amphibian biochemical and electrophysiological data, modern research typically employs mammalian species, particularly mice, which exhibit significantly faster signaling dynamics. In this work, we present an adaptation of a previously published, comprehensive model of amphibian phototransduction that can produce quantitatively accurate simulations of the murine photoresponse. We demonstrate the ability of the model to predict responses to a wide range of stimuli and under a variety of mutant conditions. Finally, we employ the model to highlight a likely unknown mechanism related to the interaction between rhodopsin and rhodopsin kinase.

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.

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.

Expression of the sodium/calcium/potassium exchanger, NCKX4, in ameloblasts

Transcellular calcium transport is an essential activity in mineralized tissue formation, including dental hard tissues. In many organ systems, this activity is regulated by membrane-bound sodium/calcium (Na(+)/Ca(2+)) exchangers, which include the NCX and NCKX [sodium/calcium-potassium (Na(+)/Ca(2+)-K(+)) exchanger] proteins. During enamel maturation, when crystals expand in thickness, Ca(2+) requirements vastly increase but exactly how Ca(2+) traffics through ameloblasts remains uncertain. Previous studies have shown that several NCX proteins are expressed in ameloblasts, although no significant shifts in expression were observed during maturation which pointed to the possible identification of other Ca(2+) membrane transporters. NCKX proteins are encoded by members of the solute carrier gene family, Slc24a, which include 6 different proteins (NCKX1-6). NCKX are bidirectional electrogenic transporters regulating Ca(2+) transport in and out of cells dependent on the transmembrane ion gradient. In this study we show that all NCKX mRNAs are expressed in dental tissues. Real-time PCR indicates that of all the members of the NCKX group, NCKX4 is the most highly expressed gene transcript during the late stages of amelogenesis. In situ hybridization and immunolocalization analyses clearly establish that in the enamel organ, NCKX4 is expressed primarily by ameloblasts during the maturation stage. Further, during the mid-late maturation stages of amelogenesis, the expression of NCKX4 in ameloblasts is most prominent at the apical poles and at the lateral membranes proximal to the apical ends. These data suggest that NCKX4 might be an important regulator of Ca(2+) transport during amelogenesis.

Molecular and functional characterization of the human platelet Na(+) /Ca(2+) exchangers

BACKGROUND AND PURPOSE The Na(+) /Ca(2+) exchanger is a bi-directional transporter that plays an important role in maintaining the concentration of cytosolic Ca(2+) ([Ca(2+) ](i) ) of quiescent platelets and increasing it during activation with some, but not all, agonists. There are two classes of Na(+) /Ca(2+) exchangers: K(+) -independent Na(+) /Ca(2+) exchanger (NCX) and K(+) -dependent Na(+) /Ca(2+) exchanger (NCKX). Platelets have previously been shown to express NCKX1. However, initial studies from our laboratory suggest that NCX may also play a role in platelet activation. The objective of this study was to determine if the human platelet expresses functional NCXs. EXPERIMENTAL APPROACH RT-PCR, DNA sequencing and Western blot analysis were utilized to characterize the human platelet Na(+) /Ca(2+) exchangers. Their function during quiescence and collagen-induced activation was determined by measuring [Ca(2+) ](i) with calcium-green/fura-red in response to: changes in the Na(+) and K(+) gradient, NCX pharmacological inhibitors (CBDMB, KB-R7943 and SEA0400) and antibodies specific to extracellular epitopes of the exchangers. KEY RESULTS Human platelets express NCX1.3, NCX3.2 and NCX3.4. The NCXs operate in the Ca(2+) efflux mode in resting platelets and also during their activation with thrombin but not collagen. Collagen-induced increase in [Ca(2+) ](i) was reduced with the pharmacological inhibitors of NCX (CBDMB, KB-R7943 or SEA0400), anti-NCX1 and anti-NCX3. In contrast, anti-NCKX1 enhanced the collagen-induced increase in [Ca(2+) ](i) . CONCLUSIONS AND IMPLICATIONS Human platelets express K(+) -independent Na(+) /Ca(2+) exchangers NCX1.3, NCX3.2 and NCX3.4. During collagen activation, NCX1 and NCX3 transiently reverse to promote Ca(2+) influx, whereas NCKX1 continues to operate in the Ca(2+) efflux mode to reduce [Ca(2+) ](i) .

Calcium stores in vertebrate photoreceptors

This review lays out the emerging evidence for the fundamental role of Ca(2+) stores and store-operated channels in the Ca(2+) homeostasis of rods and cones. Calcium-induced calcium release (CICR) is a major contributor to steady-state and light-evoked photoreceptor Ca(2+) homeostasis in the darkness whereas store-operated Ca(2+) channels play a more significant role under sustained illumination conditions. The homeostatic response includes dynamic interactions between the plasma membrane, endoplasmic reticulum (ER), mitochondria and/or outer segment disk organelles which dynamically sequester, accumulate and release Ca(2+). Coordinated activation of SERCA transporters, ryanodine receptors (RyR), inositol triphosphate receptors (IP3Rs) and TRPC channels amplifies cytosolic voltage-operated signals but also provides a memory trace of previous exposures to light. Store-operated channels, activated by the STIM1 sensor, prevent pathological decrease in [Ca(2+)]i mediated by excessive activation of PMCA transporters in saturating light. CICR and SOCE may also modulate the transmission of afferent and efferent signals in the outer retina. Thus, Ca(2+) stores provide additional complexity, adaptability, tuneability and speed to photoreceptor signaling.

The genetics of calcium signaling in Drosophila melanogaster

BACKGROUND

Genetic screens for behavioral and physiological defects in Drosophila melanogaster, helped identify several components of calcium signaling of which some, like the Trps, were novel. For genes initially identified in vertebrates, reverse genetic methods have allowed functional studies at the cellular and systemic levels.

SCOPE OF REVIEW

The aim of this review is to explain how various genetic methods available in Drosophila have been used to place different arms of Ca2+ signaling in the context of organismal development, physiology and behavior.

MAJOR CONCLUSION

Mutants generated in genes encoding a range of Ca2+ transport systems, binding proteins and enzymes affect multiple aspects of neuronal and muscle physiology. Some also affect the maintenance of ionic balance and excretion from malpighian tubules and innate immune responses in macrophages. Aspects of neuronal physiology affected include synaptic growth and plasticity, sensory transduction, flight circuit development and function. Genetic interaction screens have shown that mechanisms of maintaining Ca2+ homeostasis in Drosophila are cell specific and require a synergistic interplay between different intracellular and plasma membrane Ca2+ signaling molecules.

GENERAL SIGNIFICANCE

Insights gained through genetic studies of conserved Ca2+ signaling pathways have helped understand multiple aspects of fly physiology. The similarities between mutant phenotypes of Ca2+ signaling genes in Drosophila with certain human disease conditions, especially where homologous genes are causative factors, are likely to aid in the discovery of underlying disease mechanisms and help develop novel therapeutic strategies. This article is part of a Special Issue entitled Biochemical, biophysical and genetic approaches to intracellular calcium signalling.

Residues contributing to the Na(+)-binding pocket of the SLC24 Na(+)/Ca(2+)-K(+) Exchanger NCKX2

Na(+)/Ca(2+)-K(+) exchangers (NCKX; gene family SLC24) are plasma membrane Ca(2+) transporters that mediate the extrusion of one Ca(2+) ion and one K(+) ion in exchange for four Na(+) ions. NCKX is modeled to have two sets of five transmembrane segments separated by a large cytosolic loop; within each set of transmembrane segments are regions of internal symmetry termed alpha(1) and alpha(2) repeats. The central residues that are important for Ca(2+) and K(+) liganding and transport have been identified in NCKX2, and they comprise three central acidic residues, Glu(188) in alpha(1) and Asp(548) and Asp(575) in alpha(2), as well as Ser/Thr residues one-helical turn away from these residues. In this study, we have scanned through more than 100 single-residue substitutions of NCKX2 for shifts in Na(+) affinity using a fluorescence assay to monitor changes in free Ca(2+) in HEK293 cells treated with gramicidin to control intracellular Na(+). We have identified 31 residues that, when substituted, result in shifts in Na(+) affinity, either toward higher or lower K(m) values when compared with wild type NCKX2 (K(m) for Na(+) 58 mm). These residues include the central acidic residues Glu(188), Asp(548), and Asp(575), and their neighboring residues in alpha(1) and alpha(2), in addition to a number of newly investigated residues in transmembrane segment 3. Our results relate the identification of residues important for Na(+) transport in this study to those previously identified as important in the counter-transport of Ca(2+) and K(+), lending support to the alternating access model of transmembrane transport.

How vision begins: an odyssey

Retinal rods and cones, which are the front-end light detectors in the eye, achieve wonders together by being able to signal single-photon absorption and yet also able to adjust their function to brightness changes spanning 10(9)-fold. How these cells detect light is now quite well understood. Not surprising for almost any biological process, the intial step of seeing reveals a rich complexity as the probing goes deeper. The odyssey continues, but the knowledge gained so far is already nothing short of remarkable in qualitative and quantitative detail. It has also indirectly opened up the mystery of odorant sensing. Basic science aside, clinical ophthalmology has benefited tremendously from this endeavor as well. This article begins by recapitulating the key developments in this understanding from the mid-1960s to the late 1980s, during which period the advances were particularly rapid and fit for an intricate detective story. It then highlights some details discovered more recently, followed by a comparison between rods and cones.

Exporting calcium from cells

All eukaryotic cells import Ca2+ through a number of variously gated plasma membrane channels. Once inside cells, Ca2+ transmits information to a large number of (enzyme) targets. Eventually, it must be exported again, to prevent the overloading of the cytosol with Ca2+. Two systems export Ca2+ from cells: a high affinity, low capacity Ca2+-ATPase, and a lower affinity, but much larger capacity, Na+/Ca2+ exchanger. The ATPase (commonly called the Ca2+ pump) is the fine-tuner of cell Ca2+, as it functions well even if the concentration of the ion drops below the microM level. It is a large enzyme, with 10 transmembrane domains and a C-terminal cytosolic tail that contains regulatory sites, including a calmodulin-binding domain. Four distinct gene products plus a large number of splice variants have been described. Some are tissue specific, the isoform 2 being specifically expressed in the sensorial cells of the Corti organ in the inner-ear. Its genetic absence causes deafness in mice. Two different families of the Na+/Ca2+ exchanger exist, one of which, originally described in photoreceptors, transports K+ and Ca2+ in exchange for Na+. The exchanger is particularly active in excitable cells, e.g., heart, where the necessity cyclically arises to rapidly eject large amounts of Ca2+. In addition to heart, the exchanger is particularly important to neurons: the cleavage of the most important neuronal isoform (NCX3) by calpains activated by excitotoxic treatments generates Ca2+ overload and eventually cell death.

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.

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.

Modulation of the reaction cycle of the Na+:Ca2+, K+ exchanger

Ca(2+) concentration in retinal photoreceptor rod outer segment (OS) strongly affects the generator potential kinetics and the receptor light adaptation. The response to intense light stimuli delivered in the dark produce potential changes exceeding 40 mV: since the Ca(2+) extrusion in the OS is entirely controlled by the Na(+):Ca(2+), K(+) exchanger, it is important to assess how the exchanger ion transport rate is affected by the voltage and, in general, by intracellular factors. It is indeed known that the cardiac Na(+):Ca(2+) exchanger is regulated by Mg-ATP via a still unknown metabolic pathway. In the present work, the Na(+):Ca(2+), K(+) exchanger regulation was investigated in isolated OS, recorded in whole-cell configuration, using ionic conditions that activated maximally the exchanger in both forward and reverse mode. In all species examined (amphibia: Rana esculenta and Ambystoma mexicanum; reptilia: Gecko gecko), the forward (reverse) exchange current increased about linearly for negative (positive) voltages and exhibited outward (inward) rectification for positive (negative) voltages. Since hyperpolarisation increases Ca(2+) extrusion rate, the recovery of the dark level of Ca(2+) (and, in turn, of the generator potential) after intense light stimuli results accelerated. Mg-ATP increased the size of forward and reverse exchange current by a factor of approximately 2.3 and approximately 2.6, respectively, without modifying their voltage dependence. This indicates that Mg-ATP regulates the number of active exchanger sites and/or the exchanger turnover number, although via an unknown mechanism.

Examining Ca2+ extrusion of Na+/Ca2+-K+ exchangers

Na+/Ca2+-K+ exchangers (NCKX) are plasma membrane transporters that are thought to mainly mediate Ca2+ extrusion (along with K+) at the expense of the Na+ electrochemical gradient. However, because they are bidirectional, most assays have relied on measuring their activity in the reverse (Ca2+ import) mode. Herein we describe a method to control intracellular ionic conditions, and examine the forward (Ca2+ extrusion) mode of exchange of NCKX2.

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+-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.

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.

Sodium/calcium exchanger: influence of metabolic regulation on ion carrier interactions

The Na(+)/Ca(2+) exchanger's family of membrane transporters is widely distributed in cells and tissues of the animal kingdom and constitutes one of the most important mechanisms for extruding Ca(2+) from the cell. Two basic properties characterize them. 1) Their activity is not predicted by thermodynamic parameters of classical electrogenic countertransporters (dependence on ionic gradients and membrane potential), but is markedly regulated by transported (Na(+) and Ca(2+)) and nontransported ionic species (protons and other monovalent cations). These modulations take place at specific sites in the exchanger protein located at extra-, intra-, and transmembrane protein domains. 2) Exchange activity is also regulated by the metabolic state of the cell. The mammalian and invertebrate preparations share MgATP in that role; the squid has an additional compound, phosphoarginine. This review emphasizes the interrelationships between ionic and metabolic modulations of Na(+)/Ca(2+) exchange, focusing mainly in two preparations where most of the studies have been carried out: the mammalian heart and the squid giant axon. A surprising fact that emerges when comparing the MgATP-related pathways in these two systems is that although they are different (phosphatidylinositol bisphosphate in the cardiac and a soluble cytosolic regulatory protein in the squid), their final target effects are essentially similar: Na(+)-Ca(2+)-H(+) interactions with the exchanger. A model integrating both ionic and metabolic interactions in the regulation of the exchanger is discussed in detail as well as its relevance in cellular Ca(i)(2+) homeostasis.

Synaptic transmission at retinal ribbon synapses

The molecular organization of ribbon synapses in photoreceptors and ON bipolar cells is reviewed in relation to the process of neurotransmitter release. The interactions between ribbon synapse-associated proteins, synaptic vesicle fusion machinery and the voltage-gated calcium channels that gate transmitter release at ribbon synapses are discussed in relation to the process of synaptic vesicle exocytosis. We describe structural and mechanistic specializations that permit the ON bipolar cell to release transmitter at a much higher rate than the photoreceptor does, under in vivo conditions. We also consider the modulation of exocytosis at photoreceptor synapses, with an emphasis on the regulation of calcium channels.

Pharmacology of Brain Na+/Ca2+Exchanger: From Molecular Biology to Therapeutic Perspectives

In the last two decades, there has been a growing interest in unraveling the role that the Na+/Ca2+ exchanger (NCX) plays in the function and regulation of several cellular activities. Molecular biology, electrophysiology, genetically modified mice, and molecular pharmacology have helped to delve deeper and more successfully into the physiological and pathophysiological role of this exchanger. In fact, this nine-transmembrane protein, widely distributed in the brain and in the heart, works in a bidirectional way. Specifically, when it operates in the forward mode of operation, it couples the extrusion of one Ca2+ ion with the influx of three Na+ ions. In contrast, when it operates in the reverse mode of operation, while three Na+ ions are extruded, one Ca2+ enters into the cells. Different isoforms of NCX, named NCX1, NCX2, and NCX3, have been described in the brain, whereas only one, NCX1, has been found in the heart. The hypothesis that NCX can play a relevant role in several pathophysiological conditions, including hypoxia-anoxia, white matter degeneration after spinal cord injury, brain trauma and optical nerve injury, neuronal apoptosis, brain aging, and Alzheimer's disease, stems from the observation that NCX, in parallel with selective ion channels and ATP-dependent pumps, is efficient at maintaining intracellular Ca2+ and Na+ homeostasis. In conclusion, although studies concerning the involvement of NCX in the pathological mechanisms underlying brain injury during neurodegenerative diseases started later than those related to heart disease, the availability of pharmacological agents able to selectively modulate each NCX subtype activity and antiporter mode of operation will provide a better understanding of its pathophysiological role and, consequently, more promising approaches to treat these neurological disorders.

Substitution of a single residue, Asp575, renders the NCKX2 K+-dependent Na+/Ca2+ exchanger independent of K+

The Na(+)/Ca(2+)-K(+) exchanger (NCKX) is a polytopic membrane protein that uses both the inward Na(+) gradient and the outward K(+) gradient to drive Ca(2+) extrusion across the plasma membrane. NCKX1 is found in retinal rod photoreceptors, while NCKX2 is found in retinal cone photoreceptors and is also widely expressed in the brain. Here, we have identified a single residue (out of >100 tested) for which substitution removed the K(+) dependence of NCKX-mediated Ca(2+) transport. Charge-removing replacement of Asp(575) by either asparagine or cysteine rendered the mutant NCKX2 proteins independent of K(+), whereas the charge-conservative substitution of Asp(575) to glutamate resulted in a nonfunctional mutant NCKX2 protein, accentuating the critical nature of this residue. Asp(575) is conserved in the NCKX1-5 genes, while an asparagine is found in this position in the three NCX genes, coding for the K(+)-independent Na(+)/Ca(2+) exchanger.

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+).

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.

Inhibition of plasmalemmal Na(+)/Ca(2+) exchange by mitochondrial Na(+)/Ca(2+) exchange inhibitor 7-chloro-5-(2-chlorophenyl)-1,5-dihydro-4,1-benzothiazepin-2(3H)-one (CGP-37157) in cerebellar granule cells

In the heart, 7-chloro-5-(2-chlorophenyl)-1,5-dihydro-4,1-benzothiazepin-2(3H)-one (CGP-37157) inhibits mitochondrial but not sarcolemmal Na(+)/Ca(2+) exchange. Therefore, CGP-37157 is often used as an experimental tool to study the role of mitochondrial Na(+)/Ca(2+) exchange in Ca(2+) homeostasis in various cells, including neurons. However, neurons express several K(+)-dependent (NCKX) and/or K(+)-independent (NCX) isoforms of plasmalemmal Na(+)/Ca(2+) exchange not expressed in the sarcolemma. Because it has never been determined whether CGP-37157 inhibits plasmalemmal NCKX and/or NCX isoforms in neurons, we tested this possibility. As an index of NCKX and/or NCX activity, we studied Na-dependent and gramicidin-induced 45Ca(2+) accumulation in the presence and absence of K(+), respectively. In primary cultures of cerebellar granule cells, CGP-37157 with IC(50) of 13 microM inhibited over 70% of plasmalemmal NCX activity (P<0.01) but not NCKX activity. Our data suggest that the effects of CGP-37157 on neuronal Ca(2+) homeostasis include inhibition of certain plasmalemmal NCX isoform(s). Because cerebellar granule cells robustly express NCX3 transcripts, which are not expressed in the heart, it appears that this isoform may be CGP-37157 sensitive.

Regulation of the rod photoreceptor cyclic nucleotide-gated channel

The cGMP-gated channel of rod photoreceptors plays a key role in phototransduction by controlling the flow of cations into the outer segment in response to light-induced changes in cGMP. The channel is a heterotetramer composed of alpha-subunits required for channel activity and beta-subunits that are important in modulating the activity of channel. Earlier studies have shown that exogenous calmodulin binds to the beta-subunit of the channel and modulates the sensitivity of the channel for cGMP in a calcium dependent manner. In addition unidentified Ca2+-dependent endogenous proteins have been reported to modulate the activity of the frog rod channel. In this paper, we investigated whether endogenous calmodulin and other Ca2+ binding proteins interact with and modulate the cGMP-gated channel in bovine rod outer segments. Using immunoaffinity techniques in conjunction with ion flux assays, we show that endogenous calmodulin, but not other Ca2+ dependent proteins, binds and modulates the rod cGMP-gated channel in bovine rod outer segments. We also show that the beta-subunit of the channel is phosphorylated by endogenous and exogenous casein kinase 2. This posttranslational modification, however, does not alter the sensitivity of the channel for cGMP.

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.

In depolarized and glucose-deprived neurons, Na+ influx reverses plasmalemmal K+-dependent and K+-independent Na+/Ca2+ exchangers and contributes to NMDA excitotoxicity

Cerebellar granule cells (CGCs) express K+-dependent (NCKX) and K+-independent (NCX) plasmalemmal Na+/Ca2+ exchangers which, under plasma membrane-depolarizing conditions and high cytosolic [Na+], may reverse and mediate potentially toxic Ca2+ influx. To examine this possibility, we inhibited NCX or NCKX with KB-R7943 or K+-free medium, respectively, and studied how gramicidin affects cytosolic [Ca2+] and 45Ca2+ accumulation. Gramicidin forms pores permeable to alkali cations but not Ca2+. Therefore, gramicidin-induced Ca2+ influx is indirect; it results from fluxes of monovalent cations. In the presence of Na+, but not Li+ or Cs+, gramicidin induced Ca2+ influx that was inhibited by simultaneous application of KB-R7943 and K+-free medium. The data indicate that gramicidin-induced Na+ influx reverses NCX and NCKX. To test the role of NCX and/or NCKX in excitotoxicity, we studied how NMDA affects the viability of glucose-deprived and depolarized CGCs. To assure depolarization of the plasma membrane, we inhibited Na+,K+-ATPase with ouabain. Although inhibition of NCX or NCKX reversal failed to significantly limit 45Ca2+ accumulation and excitotoxicity, simultaneously inhibiting NCX and NCKX reversal was neuroprotective and significantly decreased NMDA-induced 45Ca2+ accumulation. Our data suggest that NMDA-induced Na+ influx reverses NCX and NCKX and leads to the death of depolarized and glucose-deprived neurons.

Binding of the retinal rod Na+/Ca2+-K+ exchanger to the cGMP-gated channel indicates local Ca(2+)-signaling in vertebrate photoreceptors

Ca(2+) ions enter the outer segment of rod or cone photoreceptors exclusively through the cGMP-gated channel and are extruded by the Na(+)/Ca(2+)-K(+) exchanger. Recent evidence indicates that in the plasma membrane, the Na(+)/Ca(2+)-K(+) exchanger is associated with the cGMP-gated channel. In this contribution, the possible physiologic significance of this protein complex is considered. Based on recent experimental evidence, the possibility of a direct functional interaction between the cGMP-gated channel and the Na(+)/Ca(2+)-K(+) exchanger is discussed. Furthermore, a quantitative estimation of the cytoplasmic Ca(2+) diffusion at the cGMP-gated channel indicates that Ca(2+) diffusion is largely confined to the complex of the cGMP-gated channel and the associated Na(+)/Ca(2+)-K(+) exchanger molecules.

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.