Expression of a functionally active human renal sodium-calcium exchanger lacking a signal sequence

Membrane topological structure of neutral system N/A amino acid transporter 4 (SNAT4) protein

Members of system N/A amino acid transporter (SNAT) family mediate transport of neutral amino acids, including l-alanine, l-glutamine, and l-histidine, across the plasma membrane and are involved in a variety of cellular functions. By using chemical labeling, glycosylation, immunofluorescence combined with molecular modeling approaches, we resolved the membrane topological structure of SNAT4, a transporter expressed predominantly in liver. To analyze the orientation using the chemical labeling and biotinylation approach, the "Cys-null" mutant of SNAT4 was first generated by mutating all five endogenous cysteine residues. Based on predicted topological structures, a single cysteine residue was introduced individually into all possible nontransmembrane domains of the Cys-null mutant. The cells expressing these mutants were labeled with N-biotinylaminoethyl methanethiosulfonate, a membrane-impermeable cysteine-directed reagent. We mapped the orientations of N- and C-terminal domains. There are three extracellular loop domains, and among them, the second loop domain is the largest that spans from amino acid residue ∼242 to ∼335. The orientation of this domain was further confirmed by the identification of two N-glycosylated residues, Asn-260 and Asn-264. Together, we showed that SNAT4 contains 10 transmembrane domains with extracellular N and C termini and a large N-glycosylated, extracellular loop domain. This is the first report concerning membrane topological structure of mammalian SNAT transporters, which will provide important implications for our understanding of structure-function of the members in this amino acid transporter family.

Role of the signal peptide in the synthesis and processing of the glucagon-like peptide-1 receptor

BACKGROUND AND PURPOSE

The glucagon-like peptide-1 receptor (GLP-1R) belongs to Family B of the G protein-coupled receptor superfamily and is a target for treatment of type 2 diabetes. Family B G protein-coupled receptors contain a putative N-terminal signal peptide, but its role in receptor synthesis and trafficking are unclear. Further, the signal peptide is not cleaved in at least one family member.

EXPERIMENTAL APPROACH

We examined receptor glycosylation and the role of the signal peptide in GLP-1R synthesis and trafficking using constructs containing epitope tags at the N- and/or C-terminus and in which the signal peptide sequence was either present or absent.

KEY RESULTS

The signal peptide was absolutely required for GLP-1R synthesis but could be substituted to some extent by increasing positive charge in the N-terminal region of the receptor flanking the signal peptide. The signal peptide is cleaved during synthesis and processing of the receptor. An enhanced GFP-epitope tag at the N-terminus of the receptor permitted synthesis of the receptor but blocked signal peptide cleavage and prevented trafficking to the plasma membrane. Cleavage site mutation allowed synthesis of a full-length receptor, blocked signal peptide cleavage and caused retention within the endoplasmic reticulum.

CONCLUSIONS AND IMPLICATIONS

Signal peptide cleavage was not essential for receptor synthesis but was obligatory for processing and trafficking of receptors to the plasma membrane. Further, the GLP-1R is subject to N-linked glycosylation and only the mature, fully glycosylated form of the receptor is present in the plasma membrane. Inhibition of glycosylation prevents processing and cell surface expression of the GLP-1R.

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.

The early stages of the intracellular transport of membrane proteins: clinical and pharmacological implications

Intracellular transport mechanisms ensure that integral membrane proteins are delivered to their correct subcellular compartments. Efficient intracellular transport is a prerequisite for the establishment of both cell architecture and function. In the past decade, transport processes of proteins have also drawn the attention of clinicians and pharmacologists since many diseases have been shown to be caused by transport-deficient proteins. Membrane proteins residing within the plasma membrane are transported via the secretory (exocytotic) pathway. The general transport routes of the secretory pathway are well established. The transport of membrane proteins starts with their integration into the ER membrane. The ribosomes synthesizing membrane proteins are targeted to the ER membrane, and the nascent chains are co-translationally integrated into the bilayer, i.e., they are inserted while their synthesis is in progress. During ER insertion, the orientation (topology) of the proteins in the membrane is determined. Proteins are folded, and their folding state is checked by a quality control system that allows only correctly folded forms to leave the ER. Misfolded or incompletely folded forms are retained, transported back to the cytosol and finally subjected to proteolysis. Correctly folded proteins are transported in the membranes of vesicles through the ER/Golgi intermediate compartment (ERGIC) and the individual compartments of the Golgi apparatus ( cis, medial, trans) to the plasma membrane. In this review, the current knowledge of the first stages of the intracellular trafficking of membrane proteins will be summarized. This "early secretory pathway" includes the processes of ER insertion, topology determination, folding, quality control and the transport to the Golgi apparatus. Mutations in the genes of membrane proteins frequently lead to misfolded forms that are recognized and retained by the quality control system. Such mutations may cause inherited diseases like cystic fibrosis or retinitis pigmentosa. In the second part of this review, the clinical implications of the early secretory pathway will be discussed. Finally, new pharmacological strategies to rescue misfolded and transport-defective membrane proteins will be outlined.

Association of Genetic Polymorphisms of Sodium-Calcium Exchanger 1 Gene, NCX1, with Hypertension in a Japanese General Population

The Na+/Ca2+ exchanger (NCX) is a membrane protein involved in calcium homeostasis, catalyzing the exchange of one Ca2+ ion for three Na+ ions across the cell membrane. The Na+/Ca2+ exchange has been suggested to play a role in the pathogenesis of hypertension. Therefore, we examined whether genetic variations in NCX1 were associated with hypertension. Among 15 polymorphisms identified in 96 hypertensive subjects by sequencing the entire exon and promoter regions of NCX1, 7 representative polymorphisms with a minor allele frequency of greater than 4% were genotyped in 1,865 individuals, of whom 787 were hypertensive and 1,072 were normotensive. These subjects were residents of Suita City and were randomly selected as a population for the Suita cohort study. Multivariate logistic regression analysis performed after adjusting for age, body mass index, hyperlipidemia, diabetes mellitus, smoking, and drinking revealed that the -23200T>C and -23181T>C polymorphisms in the 5' upstream region of exon 1c were significantly associated with hypertension in men (-23200T>C: CC vs. TC+TT: odds ratio=0.61; 95% confidence intervals: 0.39 to 0.97; p =0.04) and in women (-23181T>C: CC vs. TC+TT: odds ratio=1.45; 95% confidence intervals: 1.04 to 2.02; p =0.03), respectively. Thus, our study suggests that NCX1 is one of the genes related to susceptibility to essential hypertension in the Japanese general population.

Molecular cloning of a sixth member of the K+-dependent Na+/Ca2+ exchanger gene family, NCKX6

Bioinformatic and molecular cloning tools were used to identify and isolate cDNA clones from mouse and human tissues that encode the sixth member of the K(+)-dependent Na+/Ca2+ exchanger family, NCKX6. The mouse NCKX6 protein is 585 amino acids long and shares about 62% sequence similarity with previously identified exchangers in the alpha-repeat regions but has little primary sequence similarity outside these regions. NCKX6 transcripts of 4 kb are abundantly expressed in all tissues examined and are thus more broadly distributed than previously described NC(K)X family members. Two alternatively spliced products of this novel gene were identified that encode proteins of different length. The short isoform differs from the full-length isoform at the C-terminal hydrophobic domain as a result of a shift in the reading frame caused by the deletion of two exons. Both NCKX6 isoforms were expressed in HEK-293 cells. Functional analysis by digital imaging of fura-2 loaded transfected HEK-293 cells demonstrated that the short isoform exhibited K(+)-dependent Na+/Ca2+ exchange activity whereas the full-length isoform did not. The latter was retained within the endoplasmic reticulum, whereas the short isoform was present at the plasma membrane in transfected cells. Immunofluorescence studies examining NCKX6 expression in native tissue using an NCKX6-specific antibody showed intense labeling of the cardiac sarcolemmal membrane. The discovery of NCKX6 therefore reveals a novel member of the Na+/Ca2+ exchanger superfamily whose ubiquitous expression in all tissues suggests an important role for K(+)-dependent Na+/Ca2+ exchange in maintaining cellular Ca2+ homeostasis in diverse tissues and cell types.

The signal peptide of the G protein-coupled human endothelin B receptor is necessary for translocation of the N-terminal tail across the endoplasmic reticulum membrane

The initial step of the intracellular transport of G protein-coupled receptors, their insertion into the membrane of the endoplasmic reticulum, follows one of two different pathways. Whereas one group uses the first transmembrane domain of the mature receptor as an uncleaved signal anchor sequence for this process, a second group possesses additional cleavable signal peptides. The reason this second subset requires the additional signal peptide is not known. Here we have assessed the functional significance of the signal peptide of the endothelin B (ET(B)) receptor in transiently transfected COS.M6 cells. A green fluorescent protein-tagged ET(B) receptor mutant lacking the signal peptide was nonfunctional and retained in the endoplasmic reticulum, suggesting that it has a folding defect. To determine the defect in more detail, ET(B) receptor fragments containing the N-terminal tail, first transmembrane domain, and first cytoplasmic loop were constructed. We assessed N tail translocation across the endoplasmic reticulum membrane in the presence and absence of a signal peptide and show that the signal peptide is necessary for N tail translocation. We postulate that signal peptides are necessary for those G protein-coupled receptors for which post-translational translocation of the N terminus is impaired or blocked by the presence of stably folded domains.

Nuclear UDP-glucuronosyltransferases: identification of UGT2B7 and UGT1A6 in human liver nuclear membranes

We have demonstrated the subcellular localization of the human UDP-glucuronosyltransferases (UGTs), UGT2B7 and UGT1A6, in endoplasmic reticulum (ER) and nuclear membrane from human hepatocytes and cell lines, by in situ immunostaining and Western blot. Double immunostaining for UGT2B7 and calnexin, an ER resident protein, showed that UGT2B7 was equally present in ER and nuclear membrane whereas calnexin was present almost exclusively in ER. Immunogold labeling of HK293 cells expressing UGT2B7 established the presence of UGT2B7 in both nuclear membranes. Enzymatic assays with UGT2B7 substrates confirmed the presence of functional UGT2B7 protein in ER, whole nuclei, and both outer and inner nuclear membranes. This study has identified, for the first time, the presence of UGT2B7 and UGT1A6 in the nucleus and of UGT2B7 in the inner and outer nuclear membranes. This localization may play an important functional role within nuclei: protection from toxic compounds and/or control of steady-state concentrations of nuclear receptor ligands.

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.

Sodium-calcium exchange: a molecular perspective

Plasma membrane Na(+)-Ca2+ exchange is an essential component of Ca2+ signaling pathways in several tissues. Activity is especially high in the heart where the exchanger is an important regulator of contractility. An expanding exchanger superfamily includes three mammalian Na(+)-Ca2+ exchanger genes and a number of alternative splicing products. New information indicates that the exchanger protein has nine transmembrane segments. The exchanger, which transports Na+ and Ca2+, is also regulated by these substrates. Some molecular information is available on regulation by Na+ and Ca2+ and by PIP2 and phosphorylation. Altered expression of the exchanger in pathophysiological states may contribute to various cardiac phenotypes. Use of transgenic approaches is beginning to improve our knowledge of exchanger function.

Calcineurin controls the transcription of Na+/Ca2+ exchanger isoforms in developing cerebellar neurons

The Na(+)/Ca(2+) exchanger (NCX) and the plasma membrane Ca(2+)-ATPase export Ca(2+) from the cytosol to the extracellular space. Three NCX genes (NCX1, NCX2, and NCX3), encoding proteins with very similar properties, are expressed at different levels in tissues. Essentially, no information is available on the mechanisms that regulate their expression. Specific antibodies have been prepared and used to explore the expression of NCX1 and NCX2 in rat cerebellum. The expression of NCX2 became strongly up-regulated during development, whereas comparatively minor effects were seen for NCX1. This was also observed in cultured granule cells induced to mature in physiological concentrations of potassium. By contrast, higher K(+) concentrations, which induce partial depolarization of the plasma membrane and promote the influx of Ca(2+), caused the complete disappearance of NCX2. Reverse transcription-polymerase chain reaction analysis showed that the process occurred at the transcriptional level and depended on the activation of the Ca(2+) calmodulin-dependent protein phosphatase, calcineurin. The NCX1 and NCX3 genes were also affected by the depolarizing treatment: the transcription of the latter became up-regulated, and the pattern of expression of the splice variants of the former changed. The effects on the NCX1 and NCX3 genes were calcineurin-independent.

The retinal rod Na(+)/Ca(2+),K(+) exchanger contains a noncleaved signal sequence required for translocation of the N terminus

The retinal rod Na(+)/Ca(2+),K(+) exchanger (RodX) is a polytopic membrane protein found in photoreceptor outer segments where it is the principal extruder of Ca(2+) ions during light adaptation. We have examined the role of the N-terminal 65 amino acids in targeting, translocation, and integration of the RodX using an in vitro translation/translocation system. cDNAs encoding human RodX and bovine RodX through the first transmembrane domain were correctly targeted and integrated into microsomal membranes; deletion of the N-terminal 65 amino acids (aa) resulted in a translation product that was not targeted or integrated. Deletion of the first 65 aa had no effect on membrane targeting of full-length RodX, but the N-terminal hydrophilic domain no longer translocated. Chimeric constructs encoding the first 65 aa of bovine RodX fused to globin were translocated across microsomal membranes, demonstrating that the sequence could function heterologously. Studies of fresh bovine retinal extracts demonstrated that the first 65 aa are present in the native protein. These data demonstrate that the first 65 aa of RodX constitute an uncleaved signal sequence required for the efficient membrane targeting and proper membrane integration of RodX.

An internal signal sequence mediates the targeting and retention of the human UDP-glucuronosyltransferase 1A6 to the endoplasmic reticulum

The human UDP-glucuronosyltransferase isoform UGT1A6 is predicted to be a type I transmembrane protein anchored in the endoplasmic reticulum by a single C-terminal transmembrane domain, followed by a short cytoplasmic tail. This topology is thought to be established through the sequential action of a cleavable N-terminal signal peptide and of a C-terminal stop transfer/anchor sequence. We found that the deletion of the signal peptide did not prevent membrane targeting and insertion of this protein expressed in an in vitro transcription/translation system or in yeast Pichia pastoris. Interestingly, the same results were obtained when the protein was depleted of both the signal peptide and the C-terminal transmembrane domain/cytoplasmic tail sequences, suggesting the presence of an internal topogenic element able to translocate and retain UGT1A6 in the endoplasmic reticulum membrane in vitro and in yeast cells. To identify such a sequence, the insertion of several N-terminal deletion mutants of UGT1A6 into microsomal membranes was investigated in vitro. The data clearly showed that the deletion of the N-terminal end did not affect endoplasmic reticulum targeting and retention until residues 140-240 were deleted. The signal-like activity of the 140-240 region was demonstrated by the ability of this segment to confer endoplasmic reticulum residency to the cytosolic green fluorescent protein expressed in mammalian cells. Finally, we show that this novel topogenic sequence can posttranslationally mediate the translocation of UGT1A6. This study provides the first evidence that the membrane assembly of the human UGT1A6 involves an internal signal retention sequence.

Truncation of the C terminus of the rat brain Na(+)-Ca(2+) exchanger RBE-1 (NCX1.4) impairs surface expression of the protein

The C terminus of the rat brain Na(+)-Ca(2+) exchanger (RBE-1; NCX1. 4) (amino acids 875-903) is modeled to contain the last transmembrane alpha helix (amino acids 875-894) and an intracellular extramembraneous tail of 9 amino acids (895-903). Truncation of the last 9 C-terminal amino acids, Glu-895 to stop, did not significantly impair functional expression in HeLa or HEK 293 cells. Truncation, however, of 10 amino acids (Leu-894 to stop; mutant C10) reduced Na(+) gradient-dependent Ca(2+) uptake to 35-39% relative to the wild type parent exchanger, and further truncation of 13 or more amino acids resulted in expression of trace amounts of transport activity. Western analysis indicated that Na(+)-Ca(2+) exchanger protein was produced whether transfection was carried out with functional or non-functional mutants. Immunofluorescence studies of HEK 293 cells expressing N-Flag epitope-tagged wild type and mutant Na(+)-Ca(2+) exchangers revealed that transport activity in whole cells correlated with surface expression. All cells expressing the wild type exchanger or C9 exhibited surface expression of the protein. Only 39% of the cells expressing C10 exhibited surface expression, and none was detected in cells transfected with non-functional mutants C13 and C29. Since functional and non-functional mutants were glycosylated, the C terminus is not mandatory to translocation into the endoplasmic reticulum (ER). Endoglycosidase H digestion of [(35)S]methionine-labeled protein derived from wild type Na(+)-Ca(2+) exchanger and from C10 indicated that resistance to the digestion was acquired after 1 and 5 h of chase, respectively. C29 did not acquire detectable resistance to endoglycosidase H digestion even after 10 h of chase. Taken together, these results suggest that the "cellular quality control machinery" can tolerate the structural change introduced by truncation of the C terminus up to Ser-893 albeit with reduced rate of ER-->Golgi transfer and reduced surface expression of the truncated protein. Further truncation of C-terminal amino acids leads to retention of the truncated protein in the ER, no transfer to the Golgi, and no surface expression.

Expression of a functionally active human hepatic UDP-glucuronosyltransferase (UGT1A6) lacking the N-terminal signal sequence in the endoplasmic reticulum

UDP-glucuronosyltransferase 1A6 (UGT1A6) is a membrane glycoprotein of the endoplasmic reticulum playing a key role in drug metabolism. It is synthesized as a precursor with an N-terminal cleavable signal peptide. We demonstrate that deletion of the signal peptide sequence does not prevent membrane targeting and integration of this human isoform when expressed in an in vitro transcription-translation system, as shown by N-glycosylation, resistance to alkaline treatment and protease protection. Furthermore, UGT1A6 lacking the signal peptide (UGT1A6delta sp) was targeted to the endoplasmic reticulum in mammalian cells as shown by immunofluorescence microscopy and was catalytically active with kinetic constants for 4-methylumbelliferone glucuronidation similar to that of the wild-type. These results provide evidence that the signal peptide is not essential for the membrane assembly and activity of UGT1A6 suggesting that additional topogenic element(s) mediate(s) this process.

Sodium/calcium exchange: its physiological implications

The Na+/Ca2+ exchanger, an ion transport protein, is expressed in the plasma membrane (PM) of virtually all animal cells. It extrudes Ca2+ in parallel with the PM ATP-driven Ca2+ pump. As a reversible transporter, it also mediates Ca2+ entry in parallel with various ion channels. The energy for net Ca2+ transport by the Na+/Ca2+ exchanger and its direction depend on the Na+, Ca2+, and K+ gradients across the PM, the membrane potential, and the transport stoichiometry. In most cells, three Na+ are exchanged for one Ca2+. In vertebrate photoreceptors, some neurons, and certain other cells, K+ is transported in the same direction as Ca2+, with a coupling ratio of four Na+ to one Ca2+ plus one K+. The exchanger kinetics are affected by nontransported Ca2+, Na+, protons, ATP, and diverse other modulators. Five genes that code for the exchangers have been identified in mammals: three in the Na+/Ca2+ exchanger family (NCX1, NCX2, and NCX3) and two in the Na+/Ca2+ plus K+ family (NCKX1 and NCKX2). Genes homologous to NCX1 have been identified in frog, squid, lobster, and Drosophila. In mammals, alternatively spliced variants of NCX1 have been identified; dominant expression of these variants is cell type specific, which suggests that the variations are involved in targeting and/or functional differences. In cardiac myocytes, and probably other cell types, the exchanger serves a housekeeping role by maintaining a low intracellular Ca2+ concentration; its possible role in cardiac excitation-contraction coupling is controversial. Cellular increases in Na+ concentration lead to increases in Ca2+ concentration mediated by the Na+/Ca2+ exchanger; this is important in the therapeutic action of cardiotonic steroids like digitalis. Similarly, alterations of Na+ and Ca2+ apparently modulate basolateral K+ conductance in some epithelia, signaling in some special sense organs (e.g., photoreceptors and olfactory receptors) and Ca2+-dependent secretion in neurons and in many secretory cells. The juxtaposition of PM and sarco(endo)plasmic reticulum membranes may permit the PM Na+/Ca2+ exchanger to regulate sarco(endo)plasmic reticulum Ca2+ stores and influence cellular Ca2+ signaling.

Identification of a Cytoplasmic Region of CD20 Required for Its Redistribution to a Detergent-Insoluble Membrane Compartment

CD20 is a B lymphocyte integral membrane protein with signal-transducing properties. Abs directed toward extracellular CD20 epitopes activate nonreceptor tyrosine kinases and modulate cell cycle progression of B lymphocytes. Recently, we demonstrated that binding of CD20 Abs to B cells induces the rapid redistribution of up to 95% of CD20 molecules to low density, detergent-insoluble membrane microdomains and induces the appearance of an approximately 50-kDa tyrosine-phosphorylated protein in the same compartment. Active relocalization of CD20 may thus be critical to the initiation of signaling events by CD20. The CD20 cDNA sequence predicts a nonglycosylated protein with four transmembrane-spanning regions and intracellular amino and carboxyl termini. Here we provide verification of the location of both the intracellular and extracellular regions of the CD20 molecule and identify a membrane-proximal sequence in the cytoplasmic carboxyl tail that is required for CD20 to redistribute to detergent-insoluble membrane microdomains.

Identification, expression pattern and potential activity of Na/Ca exchanger isoforms in rat pancreatic B-cells

In the pancreatic B-cell, Na/Ca exchange displays a quite high capacity and participates in the control of cytosolic free Ca2+ concentration. The Na/Ca exchanger was recently cloned in various tissues. Two genes coding for two different exchangers (NCX1 and NCX2) have been identified and evidence for several isoforms for NCX1 shown. To characterize the isoform(s) expressed in pancreatic B-cells, a RT-PCR analysis was performed on mRNA from rat pancreatic islets, purified B-cells and insulinoma B-cells (RINm5F cells). PCR amplification did not yield the expected NCX2 DNA fragment but yielded 2 NCX1 bands, corresponding to NaCa3 and NaCa7, in the three preparations. NaCa3 and NaCa7 were equally expressed in pancreatic islets and purified B-cells. In RINm5F cells, NaCa3 expression did not differ from that in islet and purified B-cells but NaCa7 was 3 times less expressed. This lower expression was accompanied by a 3 times lower Na/Ca exchange activity in RINm5F cells compared to islet cells. Our data indicate the existence of 2 NCX1 isoforms but not of NCX2 in pancreatic B-cells. The difference in both the expression patterns of NCX1 isoforms and the activity of Na/Ca exchange in islet cells and RINm5F cells is compatible with a difference in activity between NaCa3 and NaCa7.

Sodium-calcium exchange: recent advances

Na-Ca exchange proteins are involved in Ca homeostasis in a wide variety of tissues. Unique Na-Ca exchangers have been identified by molecular biological approaches and it appears that these may represent a superfamily of ion transporters, similar to that identified for ion channels. Major advances in our understanding of these transporters have occurred in the past decade by combining molecular approaches with electrophysiological analyses. The regulatory and transport properties of Na-Ca exchangers are beginning to become understood in molecular detail. It also appears that the physiological roles of Na-Ca exchange may be quite complex. This brief review highlights some recent advances in Na-Ca exchange research obtained through the combination of molecular biological and electrophysiological approaches.