Expression of multiple plasma membrane Ca(2+)-ATPases in rat pancreatic islet cells

Characterization of the Molecular Mechanisms Underlying Glucose Stimulated Insulin Secretion from Isolated Pancreatic β-cells Using Post-translational Modification Specific Proteomics (PTMomics)

Normal pancreatic islet β-cells (PBCs) abundantly secrete insulin in response to elevated blood glucose levels, in order to maintain an adequate control of energy balance and glucose homeostasis. However, the molecular mechanisms underlying the insulin secretion are unclear. Improving our understanding of glucose-stimulated insulin secretion (GSIS) mechanisms under normal conditions is a prerequisite for developing better interventions against diabetes. Here, we aimed at identifying novel signaling pathways involved in the initial release of insulin from PBCs after glucose stimulation using quantitative strategies for the assessment of phosphorylated proteins and sialylated N-linked (SA) glycoproteins.Islets of Langerhans derived from newborn rats with a subsequent 9-10 days of maturation in vitro were stimulated with 20 mm glucose for 0 min (control), 5 min, 10 min, and 15 min. The isolated islets were subjected to time-resolved quantitative phosphoproteomics and sialiomics using iTRAQ-labeling combined with enrichment of phosphorylated peptides and formerly SA glycopeptides and high-accuracy LC-MS/MS. Using bioinformatics we analyzed the functional signaling pathways during GSIS, including well-known insulin secretion pathways. Furthermore, we identified six novel activated signaling pathways (e.g. agrin interactions and prolactin signaling) at 15 min GSIS, which may increase our understanding of the molecular mechanism underlying GSIS. Moreover, we validated some of the regulated phosphosites by parallel reaction monitoring, which resulted in the validation of eleven new phosphosites significantly regulated on GSIS. Besides protein phosphorylation, alteration in SA glycosylation was observed on several surface proteins on brief GSIS. Interestingly, proteins important for cell-cell interaction, cell movement, cell-ECM interaction and Focal Adhesion (e.g. integrins, semaphorins, and plexins) were found regulated at the level of sialylation, but not in protein expression. Collectively, we believe that this comprehensive Proteomics and PTMomics survey of signaling pathways taking place during brief GSIS of primary PBCs is contributing to understanding the complex signaling underlying GSIS.

The Plasma Membrane Calcium ATPases and Their Role as Major New Players in Human Disease

The Ca²⁺ extrusion function of the four mammalian isoforms of the plasma membrane calcium ATPases (PMCAs) is well established. There is also ever-increasing detail known of their roles in global and local Ca²⁺ homeostasis and intracellular Ca²⁺ signaling in a wide variety of cell types and tissues. It is becoming clear that the spatiotemporal patterns of expression of the PMCAs and the fact that their abundances and relative expression levels vary from cell type to cell type both reflect and impact on their specific functions in these cells. Over recent years it has become increasingly apparent that these genes have potentially significant roles in human health and disease, with PMCAs1-4 being associated with cardiovascular diseases, deafness, autism, ataxia, adenoma, and malarial resistance. This review will bring together evidence of the variety of tissue-specific functions of PMCAs and will highlight the roles these genes play in regulating normal physiological functions and the considerable impact the genes have on human disease.

The Na+/Ca2+ exchanger and the Plasma Membrane Ca2+-ATPase in β-cell function and diabetes

The rat pancreatic β-cell expresses 6 splice variants of the Plasma Membrane Ca²⁺-ATPase (PMCA) and two splice variants of the Na⁺/Ca²⁺ exchanger 1 (NCX1). In the β-cell Na⁺/Ca²⁺ exchange displays a high capacity, contributes to both Ca²⁺ outflow and influx and participates to the control of insulin release. Gain of function studies show that overexpression of PMCA2 or NCX1 leads to endoplasmic reticulum (ER) Ca²⁺ depletion with subsequent ER stress, decrease in β-cell proliferation and β-cell death by apoptosis. Loss of function studies show, on the contrary, that heterozygous inactivation of NCX1 (Ncx1^+/-) leads to an increase in β-cell function and a 5 fold increase in both β-cell mass and proliferation. The mutation also increases β-cell resistance to hypoxia, and Ncx1^+/- islets show a 2-4 times higher rate of diabetes cure than Ncx1^+/+ islets when transplanted in diabetic animals. Thus, down-regulation of the Na⁺/Ca²⁺ exchanger leads to various changes in β-cell function that are opposite to the major abnormalities seen in diabetes. In addition, the β-cell includes the mutually exclusive exon B in the alternative splicing region of NCX1, which confers a high sensitivity of its NCX splice variants (NCX1.3 & 1.7) to the inhibitory action of compounds like KBR-7943. Heterozygous inactivation of PMCA2 leads to apparented, though not completely similar results.These provide 2 unique models for the prevention and treatment of β-cell dysfunction in diabetes and following islet transplantation.

Heterozygous inactivation of plasma membrane Ca(2+)-ATPase in mice increases glucose-induced insulin release and beta cell proliferation, mass and viability

AIMS/HYPOTHESIS

Calcium plays an important role in the process of glucose-induced insulin release in pancreatic beta cells. These cells are equipped with a double system responsible for Ca(2+) extrusion--the Na/Ca exchanger (NCX) and the plasma membrane Ca(2+)-ATPase (PMCA). We have shown that heterozygous inactivation of NCX1 in mice increased glucose-induced insulin release and stimulated beta cell proliferation and mass. In the present study, we examined the effects of heterozygous inactivation of the PMCA on beta cell function.

METHODS

Biological and morphological methods (Ca(2+) imaging, Ca(2+) uptake, glucose metabolism, insulin release and immunohistochemistry) were used to assess beta cell function and proliferation in Pmca2 (also known as Atp2b2) heterozygous mice and control littermates ex vivo. Blood glucose and insulin levels were also measured to assess glucose metabolism in vivo.

RESULTS

Pmca (isoform 2) heterozygous inactivation increased intracellular Ca(2+) stores and glucose-induced insulin release. Moreover, increased beta cell proliferation, mass, viability and islet size were observed in Pmca2 heterozygous mice. However, no differences in beta cell glucose metabolism, proinsulin immunostaining and insulin content were observed.

CONCLUSIONS/INTERPRETATION

The present data indicates that inhibition of Ca(2+) extrusion from the beta cell and its subsequent intracellular accumulation stimulates beta cell function, proliferation and mass. This is in agreement with our previous results observed in mice displaying heterozygous inactivation of NCX, and indicates that inhibition of Ca(2+) extrusion mechanisms by small molecules in beta cells may represent a new approach in the treatment of type 1 and type 2 diabetes.

Plasma membrane calcium ATPases: From generic Ca(2+) sump pumps to versatile systems for fine-tuning cellular Ca(2.)

The plasma membrane calcium ATPases (PMCAs) are ATP-driven primary ion pumps found in all eukaryotic cells. They are the major high-affinity calcium extrusion system for expulsion of Ca(2+) ions from the cytosol and help restore the low resting levels of intracellular [Ca(2+)] following the temporary elevation of Ca(2+) generated during Ca(2+) signaling. Due to their essential role in the maintenance of cellular Ca(2+) homeostasis they were initially thought to be "sump pumps" for Ca(2+) removal needed by all cells to avoid eventual calcium overload. The discovery of multiple PMCA isoforms and alternatively spliced variants cast doubt on this simplistic assumption, and revealed instead that PMCAs are integral components of highly regulated multi-protein complexes fulfilling specific roles in calcium-dependent signaling originating at the plasma membrane. Biochemical, genetic, and physiological studies in gene-manipulated and mutant animals demonstrate the important role played by specific PMCAs in distinct diseases including those affecting the peripheral and central nervous system, cardiovascular disease, and osteoporosis. Human PMCA gene mutations and allelic variants associated with specific disorders continue to be discovered and underline the crucial role of different PMCAs in particular cells, tissues and organs.

NFAT1 and NFAT3 cooperate with HDAC4 during regulation of alternative splicing of PMCA isoforms in PC12 cells

Background

The bulk of human genes undergo alternative splicing (AS) upon response to physiological stimuli. AS is a great source of protein diversity and biological processes and is associated with the development of many diseases. Pheochromocytoma is a neuroendocrine tumor, characterized by an excessive Ca²⁺-dependent secretion of catecholamines. This underlines the importance of balanced control of calcium transport via regulation of gene expression pattern, including different calcium transport systems, such as plasma membrane Ca²⁺-ATPases (PMCAs), abundantly expressed in pheochromocytoma chromaffin cells (PC12 cells). PMCAs are encoded by four genes (Atp2b1, Atp2b2, Atp2b3, Atp2b4), whose transcript products undergo alternative splicing giving almost 30 variants.

Results

In this scientific report, we propose a novel mechanism of regulation of PMCA alternative splicing in PC12 cells through cooperation of the nuclear factor of activated T-cells (NFAT) and histone deacetylases (HDACs). Luciferase assays showed increased activity of NFAT in PC12 cells, which was associated with altered expression of PMCA. RT-PCR experiments suggested that inhibition of the transcriptional activity of NFAT might result in the rearrangement of PMCA splicing variants in PC12 cells. NFAT inhibition led to dominant expression of 2x/c, 3x/a and 4x/a PMCA variants, while in untreated cells the 2w,z/b, 3z,x/b,c,e,f, and 4x/b variants were found as well. Furthermore, chromatin immunoprecipitation experiments showed that NFAT1-HDAC4 or NFAT3-HDAC4 complexes might be involved in regulation of PMCA2x splicing variant generation.

Conclusions

We suggest that the influence of NFAT/HDAC on PMCA isoform composition might be important for altered dopamine secretion by PC12 cells.

Calcineurin/NFAT signaling represses genes Vamp1 and Vamp2 via PMCA-dependent mechanism during dopamine secretion by Pheochromocytoma cells

BACKGROUND

Plasma membrane Ca(2+)-ATPases (PMCA) extrude Ca(2+) ions out of the cell and contribute to generation of calcium oscillations. Calcium signaling is crucial for transcriptional regulation of dopamine secretion by neuroendocrine PC12 cells. Low resting [Ca(2+)]c in PC12 cells is maintained mainly by two Ca(2+)-ATPases, PMCA2 and PMCA3. Recently, we found that Ca(2+) dependent phosphatase calcineurin was excessively activated under conditions of experimental downregulation of PMCA2 or PMCA3. Thus, the aim of this study was to explain if, via modulation of the Ca(2+)/calcineurin-dependent nuclear factor of activated T cells (NFAT) pathway, PMCA2 and PMCA3 affect intracellular signaling in pheochromocytoma/neuronal cells/PC12 cells. Secondly, we tested whether this might influence dopamine secretion by PC12 cells.

RESULTS

PMCA2- and PMCA3-deficient cells displayed profound decrease in dopamine secretion accompanied by a permanent increase in [Ca(2+)]c. Reduction in secretion might result from changes in NFAT signaling, following altered PMCA pattern. Consequently, activation of NFAT1 and NFAT3 transcription factors was observed in PMCA2- or PMCA3-deficient cells. Furthermore, chromatin immunoprecipitation assay indicated that NFATs could be involved in repression of Vamp genes encoding vesicle associated membrane proteins (VAMP).

CONCLUSIONS

PMCA2 and PMCA3 are crucial for dopamine secretion in PC12 cells. Reduction in PMCA2 or PMCA3 led to calcium-dependent activation of calcineurin/NFAT signaling and, in consequence, to repression of the Vamp gene and deterioration of the SNARE complex formation in PC12 cells.

Na(+)/Ca (2+) exchange and the plasma membrane Ca(2+)-ATPase in β-cell function and diabetes

The rat pancreatic β-cell expresses two splice variants of the Na+/Ca(2+) exchanger 1 (NCX1) and six splice variants of the plasma membrane Ca(2+)-ATPase (PMCA). In the β-cell, Na(+)/Ca(2+) exchange displays a high capacity, contributes to both Ca(2+) outflow and influx and participates to the control of insulin release. Gain of function studies show that overexpression of NCX1 or PMCA2 leads to endoplasmic reticulum (ER) Ca(2+) depletion with subsequent ER stress, decrease in β-cell proliferation and β-cell death by apoptosis. Interestingly, chronic exposure to cytokines or high free fatty acids concentration also induces ER Ca(2+) depletion and β-cell death in diabetes. Loss of function studies shows, on the contrary, that heterozygous inactivation of NCX1 (Ncx1 ( +/- )) leads to an increase in β-cell function (insulin production and release) and a fivefold increase in both β-cell mass and proliferation. The mutation also increases β-cell resistance to hypoxia, and Ncx1 ( +/- ) islets show a four to seven times higher rate of diabetes cure than Ncx1 ( +/+ ) islets when transplanted in diabetic animals. Thus, downregulation of the Na(+)/Ca(2+) exchanger leads to various changes in β-cell function that are opposite to the major abnormalities seen in diabetes. In addition, the β-cell, which is an excitable cell, includes the mutually exclusive exon B in the alternative splicing region of NCX1, which confers a high sensitivity of its NCX splice variants (NCX1.3 & 1.7) to the inhibitory action of compounds like KB-R7943. This provides a unique model for the prevention and treatment of β-cell dysfunction in diabetes and following islet transplantation.

Plasma membrane calcium ATPases and related disorders

The plasma membrane Ca(2+) ATPases (PMCA pumps) cooperate with other transport systems in the plasma membrane and in the organelles in the regulation of cell Ca(2+). They have high Ca(2+) affinity and are thus the fine tuners of cytosolic Ca(2+). They belong to the superfamily of P-type ATPases: their four basic isoforms share the essential properties of the reaction cycle and the general membrane topography motif of 10 transmembrane domains and three large cytosolic units. However they also differ in other important properties, e.g., tissue distribution and regulatory mechanisms. Their chief regulator is calmodulin, that removes their C-terminal cytosolic tail from autoinhibitory binding sites next to the active site of the pump, restoring activity. The number of pump isoforms is increased to over 30 by alternative splicing of the transcripts at a N-terminal site (site A) and at site C within the C-terminal calmodulin binding domain: the splice variants are tissue specific and developmentally regulated. The importance of PMCAs in the maintenance of cellular Ca(2+) homeostasis is underlined by the disease phenotypes, genetic or acquired, caused by their malfunction. Non-genetic PMCA deficiencies have long been considered possible causative factors in disease conditions as important as cancer, hypertension, or neurodegeneration. Those of genetic origin are better characterized: some have now been discovered in humans as well. They concern all four PMCA isoforms, and range from cardiac dysfunctions, to deafness, to hypertension, to cerebellar ataxia.

β-Cell preservation and regeneration in diabetes by modulation of β-cell Ca²⁺ homeostasis

Ca(2+) extrusion from the β-cell is mediated by two processes the Na/Ca exchanger (NCX) and the plasma membrane Ca(2+) -ATPase (PMCA). Gain of function studies show that overexpression of NCX or PMCA leads to endoplasmic reticulum (ER) Ca(2+) depletion with subsequent ER stress, decrease in β-cell proliferation and β-cell death by apoptosis. Interestingly, chronic exposure to cytokines or high free fatty acid concentrations also induce ER Ca(2+) depletion and β-cell death in diabetes. Loss of function studies show, on the contrary, that heterozygous inactivation of NCX1 (Ncx1(+/-)) leads to an increase in β-cell function (insulin production and release), and a fivefold increase in both β-cell mass and proliferation. The mutation also increases β-cell resistance to hypoxia, and Ncx1(+/-) islets show a two to four times higher rate of diabetes cure than Ncx1(+/+) islets when transplanted in diabetic animals. Thus, down-regulation of the Na/Ca exchanger leads to various changes in β-cell function that are opposite to the major abnormalities seen in diabetes. This provides a unique model for the prevention and treatment of β-cell dysfunction in diabetes and following islet transplantation.

Calcium Pumps in Health and Disease

Ca2+-ATPases (pumps) are key actors in the regulation of Ca2+ in eukaryotic cells and are thus essential to the correct functioning of the cell machinery. They have high affinity for Ca2+ and can efficiently regulate it down to very low concentration levels. Two of the pumps have been known for decades (the SERCA and PMCA pumps); one (the SPCA pump) has only become known recently. Each pump is the product of a multigene family, the number of isoforms being further increased by alternative splicing of the primary transcripts. The three pumps share the basic features of the catalytic mechanism but differ in a number of properties related to tissue distribution, regulation, and role in the cellular homeostasis of Ca2+. The molecular understanding of the function of the pumps has received great impetus from the solution of the three-dimensional structure of one of them, the SERCA pump. These spectacular advances in the structure and molecular mechanism of the pumps have been accompanied by the emergence and rapid expansion of the topic of pump malfunction, which has paralleled the rapid expansion of knowledge in the topic of Ca2+-signaling dysfunction. Most of the pump defects described so far are genetic: when they are very severe, they produce gross and global disturbances of Ca2+ homeostasis that are incompatible with cell life. However, pump defects may also be of a type that produce subtler, often tissue-specific disturbances that affect individual components of the Ca2+-controlling and/or processing machinery. They do not bring cells to immediate death but seriously compromise their normal functioning.

Effects of plasma membrane Ca(2+)-ATPase overexpression upon D-glucose metabolism in insulin-producing BRIN-BD11 cells

In order to investigate the possible link between PMCA (plasma-membrane Ca(2+)-ATPase) activity and D-glucose catabolism in insulin-producing cells, BRIN-BD11 cells were transfected with two isoforms of PMCA2. Transfection of insulin-producing BRIN-BD11 cells with PMCA2yb and PMCA2wb was documented by RT-PCR (reverse transcription-PCR), Western blot analysis, indirect immunofluorescence microscopy and (45)Ca(2+) uptake by microsomes. In the transfected cells, the overexpression of PMCA coincided with three major anomalies of D-glucose metabolism, namely a lower rate of D-[5-(3)H]glucose utilization prevailing at a low extracellular concentration of D-glucose (1.1 mM), a low ratio between D-[U-(14)C]oxidation and D-[5-(3)H]glucose utilization prevailing at a high extracellular glucose concentration (16.7 mM), and a high ratio between the net generation of (14)C-labelled acidic metabolites and amino acids and that of (3)H(2)O from D-[5-(3)H]glucose. These anomalies resulted in a decreased estimated rate of ATP generation (linked to the catabolism of the hexose) and a lowered ATP cell content, whether at low or high extracellular D-glucose concentrations. The net uptake of (45)Ca(2+) by intact cells was also decreased in the transfected cells, but to a greater extent than can apparently be attributed to the change in the ATP-generation rate. These findings document the relevance of PMCA activity to both D-glucose metabolism and Ca(2+) handling in insulin-producing cells, with emphasis on the key role of both cytosolic and mitochondrial Ca(2+) concentrations in the regulation of D-glucose catabolism. They also reveal that overexpression of PMCA leads, in insulin-producing cells, to an imbalance between ATP generation and consumption.

Caloxins: a novel class of selective plasma membrane Ca2+ pump inhibitors obtained using biotechnology

Plasma membrane Ca2+ pumps (PMCA) extrude cellular Ca2+ with a high affinity and hence play a major role in Ca2+ homeostasis and signaling. Caloxins (selective extracellular PMCA inhibitors) would aid in elucidating the physiology of PMCA. PMCA proteins have five extracellular domains (exdoms). Our hypotheses are: 1) peptides that bind selectively to each exdom can be invented by screening a random peptide library, and 2) a peptide can modulate PMCA activity by binding to one of the exdoms. The first caloxin 2a1, selected for binding exdom 2 was selective for PMCA (Ki=529 microM). It has been used to examine the physiological role of PMCA. PMCA isoforms are encoded by four genes. PMCA isoform expression differs in various cell types, with PMCA1 and 4 being the most widely distributed. There are differences between PMCA1-4 exdom 1 sequences, which may be exploited for inventing isoform selective caloxins. Using exdom 1 of PMCA4 as a target, modified screening procedures and mutagenesis led to the high-affinity caloxin 1c2 (Ki=2.3 microM for PMCA4). It is selective for PMCA4 over PMCA1, 2, or 3. We hope that caloxins can be used to discern the roles of individual PMCA isoforms in Ca2+ homeostasis and signaling. Caloxins may also become clinically useful in cardiovascular diseases, neurological disorders, retinopathy, cancer, and contraception.

Alternative splice variants of plasma membrane calcium-ATPases in human corneal epithelium

Plasma membrane calcium-ATPases (PMCAs) play a critical role in regulating intracellular calcium concentration. Four genes encode PMCA proteins with alternative splicing of transcripts at three sites (A, B and C) serving to increase isoform diversity. Our previous work shows that all four PMCAs are expressed and have specific locations in human corneal epithelium (hCE). The present work examined which splice variants of PMCAs are expressed in hCE. Total RNA was extracted from hCE scraped from cadaver corneas of five different donors (two females and three males, age range 55-76 years). RT-PCR was performed using PMCA isoform-specific primers designed to amplify transcripts that included either splice site A or splice sites B and C. PMCA cDNAs were sequenced or cloned, and then sequenced. There was uniformity in the PMCA1 and PMCA4 expression profile among the five donors. Specifically, every donor expressed PMCA4 transcripts (4x at site A and 4b at site B/C). Every donor also expressed PMCA1 transcripts at sites B/C, specifically PMCA1b and PMCA1kb. In contrast, PMCA2 and PMCA3 expression varied; PCR DNAs were detected in two of five donors. One donor expressed PMCA2a and a novel PMCA2 variant we termed PMCA2((i)). PMCA3a transcript was demonstrated in a different donor. Finally, for all the donors, bands encoding site A transcripts for PMCA4 were obtained but no PCR transcripts were detected at site A for PMCA1, PMCA2 and PMCA3. This investigation showed that hCE expressed multiple splice variants of PMCA isoforms. Furthermore, this study documented the expression of the PMCA1k variant (PMCA1kb) previously only described in intestine and pancreatic beta cells and describes a novel PMCA2((i)) variant. Finally, this study suggests that the molecular configuration of PMCA1, PMCA2 and PMCA3 in the region of splice site A in hCE must be different than in other tissues since the same primers that produced site A transcripts in several other tissues were ineffective in priming PCR in hCE.

Role of Na/Ca exchange and the plasma membrane Ca2+-ATPase in beta cell function and death

Recent progresses concerning the Na/Ca exchanger (NCX) and the plasma membrane Ca2+-ATPase (PMCA) in the pancreatic beta cell are reviewed. The rat beta cell expresses two splice variants of NCX1 and six splice variants of the 4 PMCA isoforms. At the protein level, the most abundant forms are PMCA2 and PMCA3, providing the first evidence for the presence of these two isoforms in a non-neuronal tissue. Overexpression of NCX1 in an insulinoma cell line altered the initial rise in cytosolic-free Ca2+ concentration ([Ca2+]i) induced by membrane depolarization and the return of the [Ca2+]i to the baseline value on membrane repolarization, indicating that NCX contributes to both Ca2+ inflow and outflow in the beta cell. In contrast, overexpression of the PMCA markedly reduced the global rise in Ca2+ induced by membrane depolarization, indicating that the PMCA has a capacity higher than expected to extrude Ca2+. Glucose, the main physiological stimulus of insulin release from the beta cell, has opposite effect on NCX and PMCA transcription, expression and activity, inducing an increase in the case of NCX and a decrease in the case of the PMCA. This indicates that when exposed to glucose, the beta cell switches from a low-efficiency Ca2+ extruding mechanism, the PMCA, to a high-capacity system, the NCX, in order to better face the increase in Ca2+ inflow induced by the sugar. To our knowledge, this is the first demonstration of a reciprocal change in PMCA and NCX1 expression and activity in response to a given stimulus in any tissue.

Plasma membrane Ca2+ ATPases as dynamic regulators of cellular calcium handling

Plasma membrane Ca2+ ATPases (PMCAs) are essential components of the cellular toolkit to regulate and fine-tune cytosolic Ca2+ concentrations. Historically, the PMCAs have been assigned a housekeeping role in the maintenance of intracellular Ca2+ homeostasis. More recent work has revealed a perplexing multitude of PMCA isoforms and alternative splice variants, raising questions about their specific role in Ca2+ handling under conditions of varying Ca2+ loads. Studies on the kinetics of individual isoforms, combined with expression and localization studies suggest that PMCAs are optimized to function in Ca2+ regulation according to tissue- and cell-specific demands. Different PMCA isoforms help control slow, tonic Ca2+ signals in some cells and rapid, efficient Ca2+ extrusion in others. Localized Ca2+ handling requires targeting of the pumps to specialized cellular locales, such as the apical membrane of cochlear hair cells or the basolateral membrane of kidney epithelial cells. Recent studies suggest that alternatively spliced regions in the PMCAs are responsible for their unique targeting, membrane localization, and signaling cross-talk. The regulated deployment and retrieval of PMCAs from specific membranes provide a dynamic system for a cell to respond to changing needs of Ca2+ regulation.

Dynamics of calcium clearance in mouse pancreatic beta-cells

Pancreatic beta-cells maintain glucose homeostasis by their regulated Ca(2+)-dependent secretion of insulin. Several cellular mechanisms control intracellular Ca(2+) levels, but their relative significance in mouse beta-cells is not fully known. We used photometry to measure the dynamics of cytosolic Ca(2+) ([Ca(2+)](i)) clearance after brief, depolarization-induced Ca(2+) entry. Treatment with thapsigargin or cyclopiazonic acid, inhibitors of the sarco-endoplasmic reticulum Ca(2+)-ATPase (SERCA) pumps, nearly doubled the peak and slowed the decay of the depolarization-induced Ca(2+) transients. The remaining thapsigargin-insensitive decay was slowed further by inhibition of the plasma membrane Ca(2+)-ATPase (PMCA) and plasma membrane Na(+)/Ca(2+) exchanger (NCX) via alkalization of the bath solution, by adding lanthanum, or by substitution of Na(+) with Li(+). Mitochondrial Ca(2+) uptake contributed little to clearance in thapsigargin-pretreated cells. Together, the SERCA, PMCA, and NCX transport mechanisms accounted for 89 to 97% of clearance in normal solutions. We developed a quantitative model for the dynamic role of removal mechanisms over a wide range of [Ca(2+)](i). According to our model, 50 to 64% of initial Ca(2+) removal is via the SERCA pump, whereas the NCX contributes 21-30% of the extrusion at high [Ca(2+)](i), and the PMCA contributes 21-27% at low [Ca(2+)](i).

Opposite effects of glucose on plasma membrane Ca2+-ATPase and Na/Ca exchanger transcription, expression, and activity in rat pancreatic beta-cells

When stimulated by glucose the pancreatic beta-cell displays large oscillations of the intracellular free Ca2+concentration, resulting from intermittent Ca2+ entry from the outside and outflow from the inside, the latter process being mediated by the plasma membrane Ca2+-ATPase (PMCA) and the Na+/Ca2+ exchanger (NCX). To understand the respective role of these two mechanisms, we studied the effect of glucose on PMCA and NCX transcription, expression, and activity in rat pancreatic islet cells. Glucose (11.1 and 22.2 mm) induced a parallel decrease in PMCA transcription, expression, and activity. In contrast the sugar induced a parallel increase in NCX transcription, expression, and activity. The effects of the sugar were mimicked by the metabolizable insulin secretagogue alpha-ketoisocaproate and persisted in the presence of the Ca2+-channel blocker nifedipine. The above results are compatible with the view that, when stimulated, the beta-cell switches from a low efficiency Ca2+-extruding mechanism, the PMCA, to a high capacity system, the Na/Ca exchanger, to better face the increase in Ca2+ inflow. These effects of glucose do not result from a direct effect of the sugar itself and are not mediated by the increase in intracellular free Ca2+ concentration induced by the sugar.

Plasma membrane Ca(2+)-ATPase overexpression reduces Ca(2+) oscillations and increases insulin release induced by glucose in insulin-secreting BRIN-BD11 cells

In the mouse beta-cell, glucose generates large amplitude oscillations of the cytosolic-free Ca(2+) concentration ([Ca(2+)](i)) that are synchronous to insulin release oscillations. To examine the role played by [ Ca(2+)](i) oscillations in the process of insulin release, we examined the effect of plasma membrane Ca(2+)-ATPase (PMCA) overexpression on glucose-induced Ca(2+) oscillations and insulin release in BRIN-BD11 cells. BRIN-BD11 cells were stably transfected with PMCA2wb. Overexpression could be assessed at the mRNA and protein level, with appropriate targeting to the plasma membrane assessed by immunofluorescence and the increase in PMCA activity. In response to K(+), overexpressing cells showed a markedly reduced rise in [Ca(2+)](i). In response to glucose, control cells showed large amplitude [Ca(2+)](i) oscillations, whereas overexpressing cells showed markedly reduced increases in [Ca(2+)](i) without such large oscillations. Suppression of [Ca(2+)](i) oscillations was accompanied by an increase in glucose metabolism and insulin release that remained oscillatory despite having a lower periodicity. Hence, [Ca(2+)] (i) oscillations appear unnecessary for glucose-induced insulin release and may even be less favorable than a stable increase in [ Ca(2+)](i) for optimal hormone secretion. [Ca(2+)](i) oscillations do not directly drive insulin release oscillations but may nevertheless intervene in the fine regulation of such oscillations.

Na/Ca exchanger overexpression induces endoplasmic reticulum-related apoptosis and caspase-12 activation in insulin-releasing BRIN-BD11 cells

Ca(2+) may trigger programmed cell death (apoptosis) and regulate death-specific enzymes. Therefore, the development of strategies to control Ca(2+) homeostasis may represent a potential approach to prevent or enhance cell apoptosis. To test this hypothesis, the plasma membrane Na/Ca exchanger (NCX1.7 isoform) was stably overexpressed in insulin-secreting tumoral cells. NCX1.7 overexpression increased apoptosis induced by endoplasmic reticulum (ER) Ca(2+)-ATPase inhibitors, but not by agents increasing intracellular calcium concentration ([Ca(2+)](i)), through the opening of plasma membrane Ca(2+)-channels. NCX1.7 overexpression reduced the rise in [Ca(2+)](i) induced by all agents, depleted ER Ca(2+) stores, sensitized the cells to Ca(2+)-independent proapoptotic signaling pathways, and reduced cell proliferation by approximately 40%. ER Ca(2+) stores depletion was accompanied by the activation of the ER-specific caspase (caspase-12), and the activation was enhanced by ER Ca(2+)-ATPase inhibitors. Hence, Na/Ca exchanger overexpression, by depleting ER Ca(2+) stores, triggers the activation of caspase-12 and increases apoptotic cell death. By increasing apoptosis and decreasing cell proliferation, overexpression of Na/Ca exchanger may represent a new potential approach in cancer gene therapy. On the other hand, our results open the way to the development of new strategies to control cellular Ca(2+) homeostasis that could, on the contrary, prevent the process of apoptosis that mediates, in part, beta-cell autoimmune destruction in type 1 diabetes.