Cloning and characterization of a potassium-dependent sodium/calcium exchanger in Drosophila

Circadian programming of the ellipsoid body sleep homeostat in Drosophila

Homeostatic and circadian processes collaborate to appropriately time and consolidate sleep and wake. To understand how these processes are integrated, we scheduled brief sleep deprivation at different times of day in Drosophila and find elevated morning rebound compared to evening. These effects depend on discrete morning and evening clock neurons, independent of their roles in circadian locomotor activity. In the R5 ellipsoid body sleep homeostat, we identified elevated morning expression of activity dependent and presynaptic gene expression as well as the presynaptic protein BRUCHPILOT consistent with regulation by clock circuits. These neurons also display elevated calcium levels in response to sleep loss in the morning, but not the evening consistent with the observed time-dependent sleep rebound. These studies reveal the circuit and molecular mechanisms by which discrete circadian clock neurons program a homeostatic sleep center.

The CaCA superfamily genes in Saccharum: comparative analysis and their functional implications in response to biotic and abiotic stress

BACKGROUND

In plants, Calcium (Ca2+) acts as a universal messenger in various signal transduction pathways, including responses to biotic and abiotic stresses and regulation of cellular and developmental processes. The Ca2+/cation antiporter (CaCA) superfamily proteins play vital roles in the transport of Ca2+ and/or other cations. However, the characteristics of these superfamily members in Saccharum and their evolutionary and functional implications have remained unclear.

RESULTS

A total of 34 CaCA genes in Saccharum spontaneum, 5 CaCA genes in Saccharum spp. R570, and 14 CaCA genes in Sorghum bicolor were identified and characterized. These genes consisted of the H+/cation exchanger (CAX), cation/Ca2+ exchanger (CCX), EF-hand / CAX (EFCAX), and Mg2+/H+ exchanger (MHX) families, among which the CCX and EFCAX could be classified into three groups while the CAX could be divided into two groups. The exon/intron structures and motif compositions suggested that the members in the same group were highly conserved. Synteny analysis of CaCAs established their orthologous and paralogous relationships among the superfamily in S. spontaneum, R570, and S. bicolor. The results of protein-protein interactions indicated that these CaCA proteins had direct or indirect interactions. Quantitative reverse transcription polymerase chain reaction (qRT-PCR) analysis demonstrated that most members of Saccharum CaCA genes exhibited a similar expression pattern in response to hormonal (abscisic acid, ABA) treatment but played various roles in response to biotic (Sporisorium scitamineum) and abiotic (cold) stresses. Furthermore, ScCAX4, a gene encoding a cytoplasm, plasma membrane and nucleus positioning protein, was isolated from sugarcane. This gene was constitutively expressed in different sugarcane tissues and its expression was only induced at 3 and 6 h time points after ABA treatment, however was inhibited and indued in the whole process under cold and S. scitamineum stresses, respectively.

CONCLUSIONS

This study systematically conducted comparative analyses of CaCA superfamily genes among S. spontaneum, R570, and S. bicolor, delineating their sequence and structure characteristics, classification, evolutionary history, and putative functions. These results not only provided rich gene resources for exploring the molecular mechanism of the CaCA superfamily genes but also offered guidance and reference for research on other gene families in Saccharum.

CG4928 Is Vital for Renal Function in Fruit Flies and Membrane Potential in Cells: A First In-Depth Characterization of the Putative Solute Carrier UNC93A

The number of transporter proteins that are not fully characterized is immense. Here, we used Drosophila melanogaster and human cell lines to perform a first in-depth characterization of CG4928, an ortholog to the human UNC93A, of which little is known. Solute carriers regulate and maintain biochemical pathways important for the body, and malfunctioning transport is associated with multiple diseases. Based on phylogenetic analysis, CG4928 is closely related to human UNC93A and has a secondary and a tertiary protein structure and folding similar to major facilitator superfamily transporters. Ubiquitous knockdown of CG4928 causes flies to have a reduced secretion rate from the Malpighian tubules; altering potassium content in the body and in the Malpighian tubules, homologous to the renal system; and results in the development of edema. The edema could be rescued by using amiloride, a common diuretic, and by maintaining the flies on ion-free diets. CG4928-overexpressing cells did not facilitate the transport of sugars and amino acids; however, proximity ligation assay revealed that CG4928 co-localized with TASK¹ channels. Overexpression of CG4928 resulted in induced apoptosis and cytotoxicity, which could be restored when cells were kept in high-sodium media. Furthermore, the basal membrane potential was observed to be disrupted. Taken together, the results indicate that CG4928 is of importance for generating the cellular membrane potential by an unknown manner. However, we speculate that it most likely acts as a regulator or transporter of potassium flows over the membrane.

Structure-function relationships of K+-dependent Na+/Ca2+ exchangers (NCKX)

K⁺-dependent Na⁺/Ca²⁺ exchanger proteins (NCKX1-5) of the SLC24 gene family play important roles in a wide range of biological processes including but not limited to rod and cone photoreceptor vision, olfaction, enamel formation and skin pigmentation. NCKX proteins are also widely expressed throughout the brain and NCKX2 and NCKX4 knockouts in mice have specific phenotypes. Here we review our work on structure-function relationships of NCKX proteins. We discuss membrane topology, domains critical to transport function, and residues critical to cation binding and transport function, all in the context of crystal structures that were obtained for the archaeal Na⁺/Ca²⁺ exchanger NCX_Mj.

Evolution of Phototransduction Genes in Lepidoptera

Vision is underpinned by phototransduction, a signaling cascade that converts light energy into an electrical signal. Among insects, phototransduction is best understood in Drosophila melanogaster. Comparison of D. melanogaster against three insect species found several phototransduction gene gains and losses, however, lepidopterans were not examined. Diurnal butterflies and nocturnal moths occupy different light environments and have distinct eye morphologies, which might impact the expression of their phototransduction genes. Here we investigated: 1) how phototransduction genes vary in gene gain or loss between D. melanogaster and Lepidoptera, and 2) variations in phototransduction genes between moths and butterflies. To test our prediction of phototransduction differences due to distinct visual ecologies, we used insect reference genomes, phylogenetics, and moth and butterfly head RNA-Seq and transcriptome data. As expected, most phototransduction genes were conserved between D. melanogaster and Lepidoptera, with some exceptions. Notably, we found two lepidopteran opsins lacking a D. melanogaster ortholog. Using antibodies we found that one of these opsins, a candidate retinochrome, which we refer to as unclassified opsin (UnRh), is expressed in the crystalline cone cells and the pigment cells of the butterfly, Heliconius melpomene. Our results also show that butterflies express similar amounts of trp and trpl channel mRNAs, whereas moths express ∼50× less trp, a potential adaptation to darkness. Our findings suggest that while many single-copy D. melanogaster phototransduction genes are conserved in lepidopterans, phototransduction gene expression differences exist between moths and butterflies that may be linked to their visual light environment.

Calcium-Activated Calpain Specifically Cleaves Glutamate Receptor IIA But Not IIB at the Drosophila Neuromuscular Junction

Calpains are calcium-dependent, cytosolic proteinases active at neutral pH. They do not degrade but cleave substrates at limited sites. Calpains are implicated in various pathologies, such as ischemia, injuries, muscular dystrophy, and neurodegeneration. Despite so, the physiological function of calpains remains to be clearly defined. Using the neuromuscular junction of Drosophila of both sexes as a model, we performed RNAi screening and uncovered that calpains negatively regulated protein levels of the glutamate receptor GluRIIA but not GluRIIB. We then showed that calpains enrich at the postsynaptic area, and the calcium-dependent activation of calpains induced cleavage of GluRIIA at Q788 of its C terminus. Further genetic and biochemical experiments revealed that different calpains genetically and physically interact to form a protein complex. The protein complex was required for the proteinase activation to downregulate GluRIIA. Our data provide a novel insight into the mechanisms by which different calpains act together as a complex to specifically control GluRIIA levels and consequently synaptic function.SIGNIFICANCE STATEMENT Calpain has been implicated in neural insults and neurodegeneration. However, the physiological function of calpains in the nervous system remains to be defined. Here, we show that calpain enriches at the postsynaptic area and negatively and specifically regulates GluRIIA, but not IIB, level during development. Calcium-dependent activation of calpain cleaves GluRIIA at Q788 of its C terminus. Different calpains constitute an active protease complex to cleave its target. This study reveals a critical role of calpains during development to specifically cleave GluRIIA at synapses and consequently regulate synaptic function.

NCX-DB: a unified resource for integrative analysis of the sodium calcium exchanger super-family

Na⁺/Ca²⁺ exchangers are low-affinity high-capacity transporters that mediate Ca²⁺ extrusion by coupling Ca²⁺ efflux to the influx of Na⁺ ions. The Na⁺/Ca²⁺ exchangers form a super-family comprised of three branches each differing in ion-substrate selectivity: Na⁺/Ca²⁺ exchangers (NCX), Na⁺/Ca²⁺/K⁺ exchangers, and Ca²⁺/cation exchangers. Their primary function is to maintain Ca²⁺ homeostasis and play a particularly important role in excitable cells that experience transient Ca²⁺ fluxes. Research into the role and activity of Na⁺/Ca²⁺ exchangers has focused extensively on the cardio-vascular system, however, growing evidence suggests that Na⁺/Ca²⁺ exchangers play a key role in neuronal processes such as memory formation, learning, oligodendrocyte differentiation, neuroprotection during brain ischemia and axon guidance. They have also been implicated in pathologies such as Alzheimer’s disease, Parkinson’s disease, Multiple Sclerosis and Epilepsy, however, a clear understanding of their mechanism during disease is lacking. To date, there has never been a central resource or database for Na⁺/Ca²⁺ exchangers. With clear disease relevance and ever-increasing research on Na⁺/Ca²⁺ exchangers from both model and non-model species, a database that unifies the data on Na⁺/Ca²⁺ exchangers is needed for future research. NCX-DB is a publicly available database with a web interface that enables users to explore various Na⁺/Ca²⁺ exchangers, perform cross-species sequence comparison, identify new exchangers, and stay-up to date with recent literature. NCX-DB is available on the web via an interactive user interface with an intuitive design, which is applicable for the identification and comparison of Na⁺/Ca²⁺ exchanger proteins across diverse species.

Ca2+/Cation Antiporters (CaCA): Identification, Characterization and Expression Profiling in Bread Wheat (Triticum aestivum L.)

The Ca2+/cation antiporters (CaCA) superfamily proteins play vital function in Ca2+ ion homeostasis, which is an important event during development and defense response. Molecular characterization of these proteins has been performed in certain plants, but they are still not characterized in Triticum aestivum (bread wheat). Herein, we identified 34 TaCaCA superfamily proteins, which were classified into TaCAX, TaCCX, TaNCL, and TaMHX protein families based on their structural organization and evolutionary relation with earlier reported proteins. Since the T. aestivum comprises an allohexaploid genome, TaCaCA genes were derived from each A, B, and D subgenome and homeologous chromosome (HC), except chromosome-group 1. Majority of genes were derived from more than one HCs in each family that were considered as homeologous genes (HGs) due to their high similarity with each other. These HGs showed comparable gene and protein structures in terms of exon/intron organization and domain architecture. Majority of TaCaCA proteins comprised two Na_Ca_ex domains. However, TaNCLs consisted of an additional EF-hand domain with calcium binding motifs. Each TaCaCA protein family consisted of about 10 transmembrane and two α-repeat regions with specifically conserved signature motifs except TaNCL, which had single α-repeat. Variable expression of most of the TaCaCA genes during various developmental stages suggested their specified role in development. However, constitutively high expression of a few genes like TaCAX1-A and TaNCL1-B indicated their role throughout the plant growth and development. The modulated expression of certain genes during biotic (fungal infections) and abiotic stresses (heat, drought, salt) suggested their role in stress response. Majority of TaCCX and TaNCL family genes were found highly affected during various abiotic stresses. However, the role of individual gene needs to be established. The present study unfolded the opportunity for detail functional characterization of TaCaCA proteins and their utilization in future crop improvement programs.

Candidate genes for individual recognition in Polistes fuscatus paper wasps

Few animals are known to individually recognize conspecifics, i.e. learn and recall unique individuals during subsequent encounters, and nearly all are social vertebrates. Remarkably, the social paper wasp Polistes fuscatus has recently been discovered to possess this ability, which is useful for remembering identities during competitive social interactions. We analyzed brain gene expression in staged encounters between pairs of individuals to explore potential mechanisms underlying wasps' ability to recall familiar individuals using real-time qRT-PCR. We identified four candidate genes (IP3K, IP3R, Nckx30C and Su(var)2-10) that were down-regulated in the presence of familiar individuals compared to single wasps and pairs of wasps meeting for the first time. These candidate genes are related to calcium signaling, therefore, we treated wasps with lithium chloride, a pharmacological agent that inhibits calcium signaling in neurons. This treatment decreased aggression in paper wasps, but did not affect expression of genes related to calcium signaling. The results suggest calcium signaling differences may be related to individual memory recall in wasps, and we present four promising candidate genes for future study. These data suggest genes associated with dominance behavior may be co-opted for individual recognition, but further work is needed to establish a causal association with the behavior.

Genome-wide investigation and expression analysis of Sodium/Calcium exchanger gene family in rice and Arabidopsis

BACKGROUND

The Na(+)/Ca(2+) Exchanger (NCX) protein family is a member of the Cation/Ca(2+) exchanger superfamily and its members play important roles in cellular Ca(2+) homeostasis. While the functions of NCX family of proteins is well understood in humans, not much is known about the total complement of Na(+)/Ca(2+) exchangers in plants and their role in various physiological and developmental processes. In the present study, we have identified all the NCX proteins encoded in the genomes of rice and Arabidopsis and studied their phylogeny, domain architecture and expression profiles across different tissues, at various developmental stages and under stress conditions.

RESULTS

Through whole genome investigation, we identified twenty-two NCX proteins encoded by fifteen genes in rice and sixteen NCX proteins encoded by thirteen genes in Arabidopsis. Based on phylogenetic reconstruction, these could be classified into five clades, members of most of which were found to possess distinct domain architecture. Expression profiling of the identified NCX genes using publicly available MPSS and microarray data showed differential expression patterns under abiotic stresses, and at various development stages. In rice, OsNCX1, OsNCX8, OsNCX9 and OsNCX15 were found to be highly expressed in all the plant parts and various developmental stages. qRT-PCR based expression analysis revealed that OsNCX3, OsNCX10 and OsNCX15 were highly induced by salt and dehydration stress. Besides, expression profiling showed differential regulation of rice NCX genes in response to calcium and EGTA. Interestingly, expression of none of the NCX genes was found to be co-regulated by NaCl and calcium.

CONCLUSIONS

Together, our results present insights into the potential role of NCX family of proteins in abiotic stresses and development. Findings of the present investigation should serve as a starting point for future studies aiming functional characterization of plant NCX family proteins.

Dynamic dendritic compartmentalization underlies stimulus-specific adaptation in an insect neuron

In many neural systems, repeated stimulation leads to stimulus-specific adaptation (SSA), with responses to repeated signals being reduced while responses to novel stimuli remain unaffected. The underlying mechanisms of SSA remain mostly hypothetical. One hypothesis is that dendritic processes generate SSA. Evidence for such a mechanism was recently described in an insect auditory interneuron (TN-1 in Neoconocephalus triops). Afferents, tuned to different frequencies, connect with different parts of the TN-1 dendrite. The specific adaptation of these inputs relies on calcium and sodium accumulation within the dendrite, with calcium having a transient and sodium a tonic effect. Using imaging techniques, we tested here whether the accumulation of these ions remained limited to the stimulated parts of the dendrite. Stimulation with a fast pulse rate, which results in strong adaptation, elicited a transient dendritic calcium signal. In contrast, the sodium signal was tonic, remaining high during the fast pulse rate stimulus. These time courses followed the predictions from the previous pharmacological experiments. The peak positions of the calcium and sodium signals differed with the carrier frequency of the stimulus; at 15 kHz, peak locations were significantly more rostral than at 40 kHz. This matched the predictions made from neuroanatomical data. Our findings confirm that excitatory postsynaptic potentials rather than spiking cause the increase of dendritic calcium and sodium concentrations and that these increases remain limited to the stimulated parts of the dendrite. This supports the hypothesis of "dynamic dendritic compartmentalization" underlying SSA in this auditory interneuron.

Genome-wide screen for modifiers of Na (+) /K (+) ATPase alleles identifies critical genetic loci

Background

Mutations affecting the Na⁺/ K⁺ATPase (a.k.a. the sodium-potassium pump) genes cause conditional locomotor phenotypes in flies and three distinct complex neurological diseases in humans. More than 50 mutations have been identified affecting the human ATP1A2 and ATP1A3 genes that are known to cause rapid-onset Dystonia Parkinsonism, familial hemiplegic migraine, alternating hemiplegia of childhood, and variants of familial hemiplegic migraine with neurological complications including seizures and various mood disorders. In flies, mutations affecting the ATPalpha gene have dramatic phenotypes including altered longevity, neural dysfunction, neurodegeneration, myodegeneration, and striking locomotor impairment. Locomotor defects can manifest as conditional bang-sensitive (BS) or temperature-sensitive (TS) paralysis: phenotypes well-suited for genetic screening.

Results

We performed a genome-wide deficiency screen using three distinct missense alleles of ATPalpha and conditional locomotor function assays to identify novel modifier loci. A secondary screen confirmed allele-specificity of the interactions and many of the interactions were mapped to single genes and subsequently validated. We successfully identified 64 modifier loci and used classical mutations and RNAi to confirm 50 single gene interactions. The genes identified include those with known function, several with unknown function or that were otherwise uncharacterized, and many loci with no described association with locomotor or Na⁺/K⁺ ATPase function.

Conclusions

We used an unbiased genome-wide screen to find regions of the genome containing elements important for genetic modulation of ATPalpha dysfunction. We have identified many critical regions and narrowed several of these to single genes. These data demonstrate there are many loci capable of modifying ATPalpha dysfunction, which may provide the basis for modifying migraine, locomotor and seizure dysfunction in animals.

Electronic supplementary material

The online version of this article (doi:10.1186/s13041-014-0089-3) contains supplementary material, which is available to authorized users.

Insight into the family of Na+/Ca2+ exchangers of Caenorhabditis elegans

Here we provide the first genome-wide in vivo analysis of the Na+/Ca2+ exchanger family in the model system Caenorhabditis elegans. We source all members of this family within the Caenorhabditis genus and reconstruct their phylogeny across humans and Drosophila melanogaster. Next, we provide a description of the expression pattern for each exchanger gene in C. elegans, revealing a wide expression in a number of tissues and cell types including sensory neurons, interneurons, motor neurons, muscle cells, and intestinal tissue. Finally, we conduct a series of behavioral and functional analyses through mutant characterization in C. elegans. From these data we demonstrate that, similar to mammalian systems, the expression of Na+/Ca2+ exchangers in C. elegans is skewed toward excitable cells, and we propose that C. elegans may be an ideal model system for the study of Na+/Ca2+ exchangers.

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.

Glial solute carrier transporters in Drosophila and mice

Glia regulate brain physiology primarily by regulating the movement and concentration of substances in the extracellular fluid. Therefore, one approach to understanding the role of glia in brain physiology is to study what happens when glial transporters are removed or modified. The largest and most highly conserved class of transporter is solute carrier (SLC) proteins. SLC proteins are highly expressed in brain, and many are found in glia. The function of many SLC proteins in the brain--particularly in glia--is very poorly understood. SLC proteins can be relatively easily knocked out or modified in genetic model organisms to better understand glial function. Drosophila are popular genetic model organisms that offer a nice balance between genetic malleability and brain complexity. They are ideal for such an endeavor. This article lists and discusses SLC transporter family members that are expressed in both mouse and Drosophila glia, in an effort to provide a foundation for studies of glial SLC transporters using Drosophila as a model.

Novel stimulus-induced calcium efflux in Drosophila mushroom bodies

The mushroom body (MB) is an important part of the Drosophila brain, and is involved in many behaviors, including olfactory learning and memory and some visual cognition. However, the physiological properties of MB neurons remain elusive. Here we used a calcium-imaging technique to study calcium signals in Drosophila MB. We found that, rather than increasing calcium spread, electrical stimuli dramatically decreased calcium signals in the terminals of MB fibers. This novel calcium decrease occurred at all developmental stages from larvae to adults, but was specific for certain regions of the MB neurons. GABA receptor blockade promoted calcium propagation through the MB fibers, but did not disrupt the stimulus-induced decrease in calcium. Furthermore, this decrease in calcium was independent of extracellular calcium concentration and was not due to altered uptake by intracellular calcium stores and mitochondria. Rather, we found that inhibition of sodium-calcium exchangers significantly attenuated the stimulus-induced decrease in calcium, whereas the decrease persisted when membrane calcium pumps were blocked. Our findings indicate that MB neurons exhibit a novel stimulus-induced calcium efflux, which may be importantly regulated by sodium-calcium exchangers in the Drosophila MB.

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.

RNA editing in Drosophila melanogaster: New targets and functional consequences

Adenosine deaminases that act on RNA [adenosine deaminase, RNA specific (ADAR)] catalyze the site-specific conversion of adenosine to inosine in primary mRNA transcripts. These re-coding events affect coding potential, splice sites, and stability of mature mRNAs. ADAR is an essential gene, and studies in mouse, Caenorhabditis elegans, and Drosophila suggest that its primary function is to modify adult behavior by altering signaling components in the nervous system. By comparing the sequence of isogenic cDNAs to genomic DNA, we have identified and experimentally verified 27 new targets of Drosophila ADAR. Our analyses led us to identify new classes of genes whose transcripts are targets of ADAR, including components of the actin cytoskeleton and genes involved in ion homeostasis and signal transduction. Our results indicate that editing in Drosophila increases the diversity of the proteome, and does so in a manner that has direct functional consequences on protein function.

Calnexin is essential for rhodopsin maturation, Ca2+ regulation, and photoreceptor cell survival

In sensory neurons, successful maturation of signaling molecules and regulation of Ca2+ are essential for cell function and survival. Here, we demonstrate a multifunctional role for calnexin as both a molecular chaperone uniquely required for rhodopsin maturation and a regulator of Ca2+ that enters photoreceptor cells during light stimulation. Mutations in Drosophila calnexin lead to severe defects in rhodopsin (Rh1) expression, whereas other photoreceptor cell proteins are expressed normally. Mutations in calnexin also impair the ability of photoreceptor cells to control cytosolic Ca2+ levels following activation of the light-sensitive TRP channels. Finally, mutations in calnexin lead to retinal degeneration that is enhanced by light, suggesting that calnexin's function as a Ca2+ buffer is important for photoreceptor cell survival. Our results illustrate a critical role for calnexin in Rh1 maturation and Ca2+ regulation and provide genetic evidence that defects in calnexin lead to retinal degeneration.

Microdomains of intracellular Ca2+: molecular determinants and functional consequences

Calcium ions are ubiquitous and versatile signaling molecules, capable of decoding a variety of extracellular stimuli (hormones, neurotransmitters, growth factors, etc.) into markedly different intracellular actions, ranging from contraction to secretion, from proliferation to cell death. The key to this pleiotropic role is the complex spatiotemporal organization of the [Ca(2+)] rise evoked by extracellular agonists, which allows selected effectors to be recruited and specific actions to be initiated. In this review, we discuss the structural and functional bases that generate the subcellular heterogeneity in cellular Ca(2+) levels at rest and under stimulation. This complex choreography requires the concerted action of many different players; the central role is, of course, that of the calcium ion, with the main supporting characters being all the entities responsible for moving Ca(2+) between different compartments, while the cellular architecture provides a determining framework within which all the players have their exits and their entrances. In particular, we concentrate on the molecular mechanisms that lead to the generation of cytoplasmic Ca(2+) microdomains, focusing on their different subcellular location, mechanism of generation, and functional role.

Site-Directed Disulfide Mapping of Residues Contributing to the Ca2+ and K+ Binding Pocket of the NCKX2 Na+/Ca2+-K+ Exchanger

The Na+/Ca2+-K+ exchanger (NCKX) gene products are polytopic membrane proteins that utilize the existing cellular Na+ and K+ gradients to extrude cytoplasmic Ca2+. NCKX proteins are made up of two clusters of hydrophobic segments, both thought to consist of five putative membrane-spanning alpha-helices, and separated by a large cytoplasmic loop. The two most conserved regions within the NCKX sequence are known as the alpha1 and alpha2 repeats, and are found within the first and second set of transmembrane domains, respectively. The alpha repeats have previously been shown to contain residues critical for transport function. Here we used site-directed disulfide mapping to report that the alpha repeats are found in close proximity in three-dimensional space, bringing together key functional NCKX residues, e.g., the two critical acidic residues, Glu188 and Asp548. Glu188Cys in the alpha1 repeat could form a disulfide cross-link with Asp548Cys in the alpha2 repeat. Surprisingly, cysteine substitutions of Ser185 in the alpha1 repeat could form disulfide cross-links with cysteine substitutions of three residues in the alpha2 repeat (Ser545, Asp548, and Ser552), thought to cover close to two full turns of an alpha helix, implying an area of increased flexibility. Using the same method, Asp575, a residue critical for the K+ dependence of NCKX, was shown to be in the proximity of Ser185 and Glu188, consistent with its role in enabling K+ to bind to a single Ca2+ and K+ binding pocket.

Light activation, adaptation, and cell survival functions of the Na+/Ca2+ exchanger CalX

In sensory neurons, Ca(2+) entry is crucial for both activation and subsequent attenuation of signaling. Influx of Ca(2+) is counterbalanced by Ca(2+) extrusion, and Na(+)/Ca(2+) exchange is the primary mode for rapid Ca(2+) removal during and after sensory stimulation. However, the consequences on sensory signaling resulting from mutations in Na(+)/Ca(2+) exchangers have not been described. Here, we report that mutations in the Drosophila Na(+)/Ca(2+) exchanger calx have a profound effect on activity-dependent survival of photoreceptor cells. Loss of CalX activity resulted in a transient response to light, a dramatic decrease in signal amplification, and unusually rapid adaptation. Conversely, overexpression of CalX had reciprocal effects and greatly suppressed the retinal degeneration caused by constitutive activity of the TRP channel. These results illustrate the critical role of Ca(2+) for proper signaling and provide genetic evidence that Ca(2+) overload is responsible for a form of retinal degeneration resulting from defects in the TRP channel.

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+).

Role of cysteine residues in the NCKX2 Na+/Ca(2+)-K+ Exchanger: generation of a functional cysteine-free exchanger

Cysteine residues play an important role in many proteins, either in enzymatic activity or by mediating inter- or intramolecular interactions. The Na(+)/Ca(2+)-K(+) exchanger plays a critical role in Ca(2+) homeostasis in retinal rod (NCKX1) and cone (NCKX2) photoreceptors by extruding Ca(2+) that enters rod and cone cells via the cGMP-gated channels. NCKX1 and NCKX2 contain five highly conserved cysteine residues. The objectives of this study were threefold: (1) to examine the importance of cysteine residues in NCKX2 protein function; (2) to examine their role in the interaction between NCKX2 and the CNGA subunit of the cGMP-gated channel; and (3) to generate a functional cysteine-free NCKX2 protein. The latter will facilitate structural studies taking advantage of the unique chemistry of the thiol group following insertion of cysteine residues at specific positions in the cysteine-free background. We generated a cysteine-free NCKX2 mutant protein that showed normal protein synthesis and processing and approximately 50% wild-type cation transport function. Cysteine residues were also not critical for the formation of NCKX2 homo-oligmers or NCKX2 hetero-oligomers with the CNGA subunit of the cGMP-gated channel. Our results appear to rule out a critical importance of an intramolecular disulfide linkage in NCKX2 protein synthesis and folding as had been reported before.

Molecular characterization, functional expression and tissue distribution of a second NCKX Na+/Ca2+-K+ exchanger from Drosophila

The Na+/Ca2+ -K+ exchanger (NCKX) utilizes the inward Na+ gradient and the outward K+ gradient to promote Ca2+ extrusion from cells. Here, we have characterized a second NCKX from Drosophila. Based on its chromosomal location (X chromosome) we have named it Ncxk-x. Three splice variants were isolated with three distinct N-terminal sequences. NCKX-X differs from NCKX proteins described so far in other species by lacking an N-terminal signal peptide. Heterologous expression of the respective cDNA's resulted in NCKX-X protein expression and K+ -dependent Na+/Ca2+ exchange activity for two of the three splice variants. Transcript localization of Nckx-x was investigated and compared with that previously described by us for Nckx30C.

Calcium homeostasis in fly photoreceptor cells

In fly photoreceptor cells, two processes dominate the Ca2+ homeostasis: light-induced Ca2+ influx through members of the TRP family of ion channels, and Ca2+ extrusion by Na+/Ca2+ exchange. Ca2+ release from intracellular stores is quantitatively insignificant. Both, the light-activated channels and the Ca2+-extruding exchangers are located in or close to the rhabdomeric microvilli, small protrusions of the plasma membrane. The microvilli also contain the molecular machinery necessary for generating quantum bumps, short electrical responses caused by the absorption of a single photon. Due to this anatomical arrangement, the light-induced Ca2+ influx results in two separate Ca2+ signals that have different functions: a global, homogeneous increase of the Ca2+ concentration in the cell body, and rapid but large amplitude Ca2+ transients in the microvilli. The global rise of the Ca2+ concentration mediates light adaptation, via regulatory actions on the phototransduction cascade, the voltage-gated K+ channels and small pigment granules controlling the light intensity. The local Ca2+ transients in the microvilli are responsible for shaping the quantum bumps into fast, all-or-nothing events. They achieve this by facilitating strongly the phototransduction cascade at early stages ofthe light response and subsequently inhibiting it. Many molecular targets of these feedback mechanisms have been identified and characterized due to the availability of numerous Drosophila mutant showing defects in the phototransduction.

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

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

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

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

Topology of the retinal cone NCKX2 Na/Ca-K exchanger

The Na/Ca-K exchanger (NCKX) is a polytopic membrane protein that plays a critical role in Ca(2+) homeostasis in retinal rod and cone photoreceptors. The NCKX1 isoform is found in rods, while the NCKX2 isoform is found in cones, in retinal ganglion cells, and in various parts of the brain. The topology of the Na/Ca-K exchanger is thought to consist of two large hydrophilic loops and two sets of transmembrane spanning segments (TMs). The first large hydrophilic loop is located extracellularly at the N-terminus; the other is cytoplasmic and separates the two sets of TMs. The TMs consist of either five and five membrane spanning helices or five and six membrane spanning helices, depending upon the predictive algorithm used. Little specific information is yet available on the orientation of the various membrane spanning helices and the localization of the short loops connecting these helices. In this study, we have determined which of the connecting loops are exposed to the extracellular milieu using two different methods: accessibility of substituted cysteine residues and insertion of N-glycosylation sites. The two methods resulted in a consistent NCKX topology in which the two sets of TMs each contain five membrane spanning helices. Our new model places what was previously membrane spanning helix six in the cytoplasm, which places the C-terminus on the extracellular surface. Surprisingly, this NCKX topology model is different from the current NCX topology model with respect to the C-terminal three membrane helices.

Scanning mutagenesis of the alpha repeats and of the transmembrane acidic residues of the human retinal cone Na/Ca-K exchanger

The Na/Ca-K exchanger (NCKX) utilizes the inward sodium gradient and outward potassium gradient for Ca(2+) extrusion; two distinct NCKX isoforms are expressed in the outer segments of retinal rod (NCKX1) and cone (NCKX2) photoreceptors, respectively, where NCKX extrudes Ca(2+) that enters photoreceptors via the cGMP-gated channels. We carried out the first systematic NCKX mutagenesis study in which 96 residues were mutated in the human cone NCKX2 cDNA, and functional consequences of these mutations were measured; the residues selected for mutagenesis are conserved between rod and cone NCKX, the large majority are also conserved in NCKX paralogs found in lower organisms, and finally, they include the few residues conserved between members of the NCKX and members of the NCX (potassium-independent Na/Ca exchange) gene families. Twenty-five residues were identified for which mutagenesis reduced NCKX function to <20% of wild-type cone NCKX2 activity, while protein expression and plasma membrane targeting were not affected. Three classes of residues were found to be most sensitive toward mutagenesis: acidic (glutamate/aspartate) residues, polar (serines/threonine) residues, and glycine residues. These results are discussed with respect to residues that may contribute to the NCKX cation binding site(s).

The Na/Ca-K exchanger gene family

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

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

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

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

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

Generation of a semi-dominant mutation with temperature sensitive effects on both locomotion and phototransduction in Drosophila melanogaster

A novel Drosophila mutant named Tripped-and-fell (Taf) was isolated in a F1 screen for dominant temperature sensitive paralytics. Recombination mapping using multiply marked chromosomes and P elements have pinpointed the locus of Taf to polytene band 93 on the right arm of the third chromosome (3R). When exposed to restrictive temperatures, both Taf heterozygotes and homozygotes paralyzed; however, homozygotes paralyzed at lower temperatures and took longer to recover than heterozygotes. There are also positive correlations between recovery time from paralysis and both duration and temperature of exposure. Electroretinograms (ERGs) revealed that both homozygotes and heterozygotes have a grossly normal light response at 22 degrees C, but at 37 degrees C, the ERGs from both homozygotes and heterozygotes are unable to maintain a normal sustained depolarization and have a reduced off-transient potential. The severity of the ERG repolarization phenotype is greater in homozygotes than in heterozygotes.

Stoichiometry of the retinal cone Na/Ca-K exchanger heterologously expressed in insect cells: comparison with the bovine heart Na/Ca exchanger

The transport stoichiometry is an essential property of antiporter and symporter transport proteins. In this study, we determined the transport stoichiometry of the retinal cone potassium-dependent Na/Ca exchanger (NCKX) expressed in sodium-loaded cultured insect cells. The Na/Ca and Rb/Ca coupling ratios were obtained by direct measurements of the levels of (86)Rb and (45)Ca uptake and sodium release associated with reverse Na/Ca exchange. Rb/Ca coupling ratios of 0.98 [standard deviation (SD) of 0.12, 15 observations] and 0.92 (SD of 0.12, 13 observations) were obtained for the chicken and human retinal cone NCKX, respectively. Na/Ca coupling ratios of 4.11 (SD of 0.24, 10 observations) and 3.98 (SD of 0.34, 15 observations) were obtained for the chicken and human retinal cone NCKX, respectively, whereas a lower average coupling ratio of 3.11 (SD of 0.34, 10 observations) was obtained with cells expressing the bovine Na/Ca exchanger (NCX1). These results are consistent with a 4Na/1Ca + 1K stoichiometry for retinal cone NCKX. High Five cells expressing full-length dolphin rod NCKX, Caenorhabditis elegans NCKX, or bovine rod NCKX from which the two large hydrophilic loops were removed all showed a significant calcium-dependent (86)Rb uptake, whereas no calcium-dependent (86)Rb uptake was observed in cells expressing bovine NCX1. The calcium dependence of (45)Ca uptake yielded values between 1 and 2.5 microM for the external calcium dissociation constant of the different NCKX proteins studied here.

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(+)).

Sodium-calcium exchanger (NCX-1) and calcium modulation: NCX protein expression patterns and regulation of early heart development

Ouabain-induced inhibition of early heart development indicated that Na/K-ATPase plays an important role in maintaining normal ionic balances during differentiation of cardiomyocytes (Linask and Gui [1995] Dev Dyn 203:93-105). Inhibition of the sodium pump is generally accepted to affect the activity of the Na(+)-Ca(++) exchanger (NCX) to increase intracellular [Ca(++)]. These previous findings suggested that Ca(++) signaling may be an important modulator during differentiation of cardiomyocytes. In order to identify a connection between heart development and NCX-mediated Ca(++) regulation, we determined the embryonic spatiotemporal protein expression pattern of NCX-1 during early developmental stages. In both chick and mouse embryos, NCX-1 (the cardiac NCX isoform) is asymmetrically expressed during gastrulation; in the right side of the Hensen's node in the chick, in the right lateral mesoderm in the mouse. At slightly later stages, NCX-1 is expressed in the heart fields at comparable stages of heart development, in the chick at stage 7 and in the mouse at embryonic day (ED) 7.5. By ED 8 in the mouse, the exchanger protein displays a rostrocaudal difference in cardiac expression and an outer curvature-inner curvature ventricular difference. By ED 9.5, cardiac expression has increased from that seen at ED8 and NCX-1 is distributed throughout the myocardium consistent with the possibility that it is important in regulating initial cardiac contractile function. Only a low level of expression is detected in inflow and outflow regions. To substantiate a role for the involvement of calcium-mediated signaling, using pharmacologic approaches, ionomycin (a Ca(++) ionophore) was shown to perturb cardiac cell differentiation in a manner similar to ouabain as assayed by cNkx2.5 and sarcomeric myosin heavy chain expression. In addition, we show that an inhibitor of NCX, KB-R7943, can similarly and adversely affect early cardiac development at stage 4/5 and arrests cardiac cell contractility in 12-somite embryos. Thus, based upon NCX-1 protein expression patterns in the embryo, experimental Ca(++) modulation, and inhibition of NCX activity by KB-R7943, these results suggest an early and central role for calcium-mediated signaling in cardiac cell differentiation and NCX's regulation of the initial heartbeats in the embryo.

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

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

Na(+)-Ca(2+)-K(+) currents measured in insect cells transfected with the retinal cone or rod Na(+)-Ca(2+)-K(+) exchanger cDNA

The recently cloned retinal cone Na(+)-Ca(2+)-K(+) exchanger (NCKX) was expressed in cultured insect cells, and whole-cell patch clamp was used to measure transmembrane currents generated by this transcript and compare them with currents generated by retinal rod NCKX or by a deletion mutant rod NCKX from which the two large hydrophilic loops were removed. We have characterized the ionic currents generated by both the forward (Ca(2+) extrusion) and reverse (Ca(2+) influx) modes of all three NCKX proteins. Reverse NCKX exchange generated outward current that required the simultaneous presence of both external Ca(2+) and external K(+). Forward NCKX exchange carried inward current with Na(+), but not with Li(+) in the bath solution. The cation dependencies of the three NCKX tested (external K(+), external Na(+), internal Ca(2+)) were very similar to each other and to those reported previously for the in situ rod NCKX. These findings provide the first electrophysiological characterization of cone NCKX and the first electrophysiological characterization of potassium-dependent Na(+)-Ca(+) exchangers in heterologous systems. Our results demonstrate the feasibility of combining heterologous expression and biophysical measurements for detailed NCKX structure/function studies.

Calcium imaging demonstrates colocalization of calcium influx and extrusion in fly photoreceptors

During illumination, Ca(2+) enters fly photoreceptor cells through light-activated channels that are located in the rhabdomere, the compartment specialized for phototransduction. From the rhabdomere, Ca(2+) diffuses into the cell body. We visualize this process by rapidly imaging the fluorescence in a cross section of a photoreceptor cell injected with a fluorescent Ca(2+) indicator in vivo. The free Ca(2+) concentration in the rhabdomere shows a very fast and large transient shortly after light onset. The free Ca(2+) concentration in the cell body rises more slowly and displays a much smaller transient. After approximately 400 ms of light stimulation, the Ca(2+) concentration in both compartments reaches a steady state, indicating that thereafter an amount of Ca(2+), equivalent to the amount of Ca(2+) flowing into the cell, is extruded. Quantitative analysis demonstrates that during the steady state, the free Ca(2+) concentration in the rhabdomere and throughout the cell body is the same. This shows that Ca(2+) extrusion takes place very close to the location of Ca(2+) influx, the rhabdomere, because otherwise gradients in the steady-state distribution of Ca(2+) should be measured. The close colocalization of Ca(2+) influx and Ca(2+) extrusion ensures that, after turning off the light, Ca(2+) removal from the rhabdomere is faster than from the cell body. This is functionally significant because it ensures rapid dark adaptation.

The N-terminal portion of the main cytosolic loop mediates K+ sensitivity in the retinal rod Na+/Ca2+-K+-exchanger

Two types of Na+/Ca2+-exchangers have been characterized in the literature: The first is the cardiac, skeletal muscle and brain type, which exchanges 1 Ca2+ for 3 Na+, the second, found in retinal photosensor cells, transports 1 Ca2+ and 1 K+ in exchange for 4 Na+. The present work describes the properties of chimeric constructs of the two exchanger types. Ca2+ gel overlay experiments have identified a high affinity (Kd in the 1 microM range) Ca2+-binding domain between Glu601 and Asp733 in the main cytosolic loop of the retinal protein, just after transmembrane domain 5. Insertion of the retinal Ca2+-binding domain in the cytosolic loop of the cardiac exchanger conferred K+-dependence to the Ca2+ uptake activity of the chimeric constructs expressed in HeLa cells. The apparent Km of the K+ effect was about 1 mM. Experiments with C-terminally truncated versions of the retinal insert indicated that the sequence between Leu643 and Asp733 was critical in mediating K+ sensitivity of the recombinant chimeras. Thus, the high affinity Ca2+-binding domain in the main cytosolic loop of the retinal exchanger may regulate the activity of the retinal protein by binding Ca2+, and by conferring to it K+ sensitivity.