Consequences of functional expression of the plasma membrane Ca2+ pump isoform 1a

Multifaceted plasma membrane Ca(2+) pumps: From structure to intracellular Ca(2+) handling and cancer

Plasma membrane Ca(2+) ATPases (PMCAs) are intimately involved in the control of intracellular Ca(2+) concentration. They reduce Ca(2+) in the cytosol not only by direct ejection, but also by controlling the formation of inositol-1,4,5-trisphosphate and decreasing Ca(2+) release from the endoplasmic reticulum Ca(2+) pool. In mammals four genes (PMCA1-4) are expressed, and alternative RNA splicing generates more than twenty variants. The variants differ in their regulatory characteristics. They localize into highly specialized membrane compartments and respond to the incoming Ca(2+) with distinct temporal resolution. The expression pattern of variants depends on cell type; a change in this pattern can result in perturbed Ca(2+) homeostasis and thus altered cell function. Indeed, PMCAs undergo remarkable changes in their expression pattern during tumorigenesis that might significantly contribute to the unbalanced Ca(2+) homeostasis of cancer cells. This article is part of a Special Issue entitled: Calcium and Cell Fate. Guest Editors: Jacques Haiech, Claus Heizmann, Joachim Krebs, Thierry Capiod and Olivier Mignen.

Altered vascular smooth muscle function in the ApoE knockout mouse during the progression of atherosclerosis

Objectives

Relaxation of vascular smooth muscle (VSM) requires re-uptake of cytosolic Ca²⁺ into the sarcoplasmic reticulum (SR) via the Sarco/Endoplasmic Reticulum Ca²⁺ ATPase (SERCA), or extrusion via the Plasma Membrane Ca²⁺ ATPase (PMCA) or sodium Ca²⁺ exchanger (NCX). Peroxynitrite, a reactive species formed in vascular inflammatory diseases, upregulates SERCA activity to induce relaxation but, chronically, can contribute to atherogenesis and altered vascular function by escalating endoplasmic reticulum stress. Our objectives were to determine if peroxynitrite-induced relaxation and Ca²⁺ handling processes within vascular smooth muscle cells were altered as atherosclerosis develops.

Methods

Aortae from control and ApoE^−/− mice were studied histologically, functionally and for protein expression levels of SERCA and PMCA. Ca²⁺ responses were assessed in dissociated aortic smooth muscle cells in the presence and absence of extracellular Ca²⁺.

Results

Relaxation to peroxynitrite was concentration-dependent and endothelium-independent. The abilities of the SERCA blocker thapsigargin and the PMCA inhibitor carboxyeosin to block this relaxation were altered during fat feeding and plaque progression. SERCA levels were progressively reduced, while PMCA expression was upregulated. In ApoE^−/− VSM cells, increases in cytosolic Ca²⁺ [Ca²⁺]_c in response to SERCA blockade were reduced, while SERCA-independent Ca²⁺ clearance was faster compared to control.

Conclusion

As atherosclerosis develops in the ApoE^−/− mouse, expression and function of Ca²⁺ handling proteins are altered. Up-regulation of Ca²⁺ removal via PMCA may offer a potential compensatory mechanism to help normalise the dysfunctional relaxation observed during disease progression.

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Expression and function of SERCA and PMCA are temporally altered in ApoE^−/− VSM.

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TG-induced increases in [Ca²⁺]_c were reduced in ApoE^−/− aortic SM cells.

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Ca²⁺ extrusion is upregulated in isolated ApoE^−/− aortic SM cells.

Silencing of Atp2b1 increases blood pressure through vasoconstriction

BACKGROUND

Recent genome-wide association studies (GWASs) have identified 30 genetic loci that regulate blood pressure, increasing our understanding of the cause of hypertension. However, it has been difficult to define the causative genes at these loci due to a lack of functional analyses.

METHOD

In this study, we aimed to validate the candidate gene ATP2B1 in 12q21, variants near which have the strongest association with blood pressure in Asians and Europeans. ATP2B1 functions as a calcium pump to fine-tune calcium concentrations - necessary for repolarization following muscular contractions. We silenced Atp2b1 using an siRNA complex, injected into mouse tail veins.

RESULTS

In treated mice, blood pressure rose and the mesenteric arteries increased in wall : lumen ratio. Moreover, the arteries showed enhanced myogenic responses to pressure, and contractile responses to phenylephrine increased compared with the control, suggesting that blood pressure is regulated by ATP2B1 through the contraction and dilation of the vessel, likely by controlling calcium concentrations in the resting state.

CONCLUSION

These results support that ATP2B1 is the causative gene in the blood pressure-associated 12q21 locus and demonstrate that ATP2B1 expression in the vessel influences blood pressure.

Caloxin 1b3: a novel plasma membrane Ca(2+)-pump isoform 1 selective inhibitor that increases cytosolic Ca(2+) in endothelial cells

The purpose of this study was to invent an extracellular inhibitor selective for the plasma membrane Ca(2+) pump(s) (PMCA) isoform 1. PMCA extrude Ca(2+) from cells during signalling and homeostasis. PMCA isoforms are encoded by 4 genes (PMCA1-4). Pig coronary artery endothelium and smooth muscle express the genes PMCA1 and 4. We showed that the endothelial cells contained mostly PMCA1 protein while smooth muscle cells had mostly PMCA4. A random peptide phage display library was screened for binding to synthetic extracellular domain 1 of PMCA1. The selected phage population was screened further by affinity chromatography using PMCA from rabbit duodenal mucosa which expressed mostly PMCA1. The peptide displayed by the selected phage was termed caloxin 1b3. Caloxin 1b3 inhibited PMCA Ca(2+)-Mg(2+)-ATPase in the rabbit duodenal mucosa (PMCA1) with a greater affinity (inhibition constant=17±2 μM) than the PMCA in the human erythrocyte ghosts (PMCA4, inhibition constant=45±4 μM). The affinity of caloxin 1b3 was also higher for PMCA1 than for PMCA2 and 3 indicating its selectivity for PMCA1. Consistent with an inhibition of PMCA1, caloxin 1b3 addition to the medium increased cytosolic Ca(2+) concentration in endothelial cells. Caloxin 1b3 is the first known PMCA1 selective inhibitor. We anticipate caloxin 1b3 to aid in understanding PMCA physiology in endothelium and other tissues.

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.

Aromatase deficiency causes altered expression of molecules critical for calcium reabsorption in the kidneys of female mice *

Kidney stones increase after menopause, suggesting a role for estrogen deficiency. ArKO mice have hypercalciuria and lower levels of calcium transport proteins, whereas levels of the klotho protein are elevated. Thus, estrogen deficiency is sufficient to cause altered renal calcium handling.

INTRODUCTION

The incidence of renal stones increases in women after menopause, implicating a possible role for estrogen deficiency. We used the aromatase deficient (ArKO) mouse, a model of estrogen deficiency, to test the hypothesis that estrogen deficiency would increase urinary calcium excretion and alter the expression of molecular regulators of renal calcium reabsorption.

MATERIALS AND METHODS

Adult female wildtype (WT), ArKO, and estradiol-treated ArKO mice (n = 5-12/group) were used to measure urinary calcium in the fed and fasting states, relative expression level of some genes involved in calcium reabsorption in the distal convoluted tubule by real-time PCR, and protein expression by Western blotting or immunohistochemistry. Plasma membrane calcium ATPase (PMCA) activity was measured in kidney membrane preparations. ANOVA was used to test for differences between groups followed by posthoc analysis with Dunnett's test.

RESULTS

Compared with WT, urinary Ca:Cr ratios were elevated in ArKO mice, renal mRNA levels of transient receptor potential cation channel vallinoid subfamily member 5 (TRPV5), TRPV6, calbindin-D28k, the Na+/Ca+ exchanger (NCX1), and the PMCA1b were significantly decreased, and klotho mRNA and protein levels were elevated. Estradiol treatment of ArKO mice normalized urinary calcium excretion, renal mRNA levels of TRPV5, calbindin-D(28k), PMCA1b, and klotho, as well as protein levels of calbindin-D28k and Klotho. ArKO mice treated with estradiol had significantly greater PMCA activity than either untreated ArKO mice or WT mice.

CONCLUSIONS

Estrogen deficiency caused by aromatase inactivation is sufficient for renal calcium loss. Changes in estradiol levels are associated with coordinated changes in expression of many proteins involved in distal tubule calcium reabsorption. Estradiol seems to act at the genomic level by increasing or decreasing (klotho) protein expression and nongenomically by increasing PMCA activity. PMCA, not NCX1, is likely responsible for extruding calcium in response to in vivo estradiol hormonal challenge. These data provide potential mechanisms for regulation of renal calcium handling in response to changes in serum estrogen levels.

Isoform-specific up-regulation of plasma membrane Ca2+ATPase expression during colon and gastric cancer cell differentiation

In this work we demonstrate a differentiation-induced up-regulation of the expression of plasma membrane Ca2+ATPase (PMCA) isoforms being present in various gastric/colon cancer cell types. We found PMCA1b as the major isoform in non-differentiated cancer cell lines, whereas the expression level of PMCA4b was significantly lower. Cell differentiation initiated with short chain fatty acids (SCFAs) and trichostatin A, or spontaneous differentiation of post-confluent cell cultures resulted in a marked induction of PMCA4b expression, while only moderately increased PMCA1b levels. Up-regulation of PMCA4b expression was demonstrated both at the protein and mRNA levels, and closely correlated with the induction of established differentiation markers. In contrast, the expression level of the Na+/K+-ATPase or that of the sarco/endoplasmic reticulum Ca2+ATPase 2 protein did not change significantly under these conditions. In membrane vesicles obtained from SCFA-treated gastric/colon cancer cells a marked increase in the PMCA-dependent Ca2+ transport activity was observed, indicating a general increase of PMCA function during the differentiation of these cancer cells. Because various PMCA isoforms display distinct functional characteristics, we suggest that up-regulated PMCA expression, together with a major switch in PMCA isoform pattern may significantly contribute to the differentiation of gastric/colon cancer cells. The analysis of PMCA expression may provide a new diagnostic tool for monitoring the tumor phenotype.

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.

The plasma membrane Ca2+ pump isoform 4a differs from isoform 4b in the mechanism of calmodulin binding and activation kinetics: implications for Ca2+ signaling

The inhibition by the regulatory domain and the interaction with calmodulin (CaM) vary among plasma membrane calcium pump (PMCA) isoforms. To explore these differences, the kinetics of CaM effects on PMCA4a were investigated and compared with those of PMCA4b. The maximal apparent rate constant for CaM activation of PMCA4a was almost twice that for PMCA4b, whereas the rates of activation for both isoforms showed similar dependence on Ca2+. The inactivation of PMCA4a by CaM removal was also faster than for PMCA4b, and Ca2+ showed a much smaller effect (2- versus 30-fold modification). The rate constants of the individual steps that determine the overall rates were obtained from stopped-flow experiments in which binding of TA-CaM was observed by changes in its fluorescence. TA-CaM binds to two conformations of PMCA4a, an "open" conformation with high activity, and a "closed" one with lower activity. Compared with PMCA4b (Penheiter, A. R., Bajzer, Z., Filoteo, A. G., Thorogate, R., Török, K., and Caride, A. J. (2003) Biochemistry 41, 12115-12124), the model for PMCA4a predicts less inhibition in the closed form and a much faster equilibrium between the open and closed forms. Based on the available kinetic parameters, we determined the constants to fit the shape of a Ca2+ signal in PMCA4b-overexpressing Chinese hamster ovary cells. Using the constants for PMCA4a, and allowing small variations in parameters of other systems contributing to a Ca2+ signal, we then simulated the effect of PMCA4a on the shape of a Ca2+ signal in Chinese hamster ovary cells. The results reproduce the published data (Brini, M., Coletto, L., Pierobon, N., Kraev, N., Guerini, D., and Carafoli, E. (2003) J. Biol. Chem. 278, 24500-24508), and thereby demonstrate the importance of altered regulatory kinetics for the different functional properties of PMCA isoforms.

Involvement of TR3/Nur77 translocation to the endoplasmic reticulum in ER stress-induced apoptosis

Nuclear orphan receptor TR3/Nur77/NGFI-B is a novel apoptotic effector protein that initiates apoptosis largely by translocating from the nucleus to the mitochondria, causing the release of cytochrome c. However, it is possible that TR3 translocates to other organelles. The present study was designed to determine the intracellular localization of TR3 following CD437-induced nucleocytoplasmic translocation and the mechanisms involved in TR3-induced apoptosis. In human neuroblastoma SK-N-SH cells and human esophageal squamous carcinoma EC109 and EC9706 cells, 5 microM CD437 induced translocation of TR3 to the endoplasmic reticulum (ER). This distribution was confirmed by immunofluorescence analysis, subcellular fractionation analysis and coimmunoprecipitation analysis. The translocated TR3 interacted with ER-targeting Bcl-2; initiated an early release of Ca(2+) from ER; resulted in ER stress and induced apoptosis through ER-specific caspase-4 activation, together with induction of mitochondrial stress and subsequent activation of caspase-9. Our results identified a novel distribution of TR3 in the ER and defined two parallel mitochondrial- and ER-based pathways that ultimately result in apoptotic cell death.

Mechanism of calmodulin recognition of the binding domain of isoform 1b of the plasma membrane Ca(2+)-ATPase: kinetic pathway and effects of methionine oxidation

Calmodulin (CaM) binds to a domain near the C-terminus of the plasma membrane Ca2+-ATPase (PMCA), causing the release of this domain and relief of its autoinhibitory function. We investigated the kinetics of dissociation and binding of Ca2+-CaM with a 28-residue peptide [C28W(1b)] corresponding to the CaM-binding domain of isoform 1b of PMCA. CaM was labeled with a fluorescent probe on either the N-terminal domain at residue 34 or the C-terminal domain at residue 110. Formation of complexes of CaM with C28W(1b) results in a decrease in the fluorescence yield of the fluorophore, allowing the kinetics of dissociation or binding to be detected. Using a maximum entropy method, we determined the minimum number and magnitudes of rate constants required to fit the data. Comparison of the fluorescence changes for CaM labeled on the C-terminal or N-terminal domain suggests sequential and ordered binding of the C-terminal and N-terminal domains of CaM with C28W(1b). For dissociation of C28W(1b) from CaM labeled on the N-terminal domain, we observed three time constants, indicating the presence of two intermediate states in the dissociation pathway. However, for CaM labeled on the C-terminal domain, we observed only two time constants, suggesting that the fluorescence label on the C-terminal domain was not sensitive to one of the kinetic steps. The results were modeled by a kinetic mechanism in which an initial complex forms upon binding of the C-terminal domain of CaM to C28W(1b), followed by binding of the N-terminal domain, and then formation of a tight binding complex. Oxidation of methionine residues in CaM resulted in significant perturbations to the binding kinetics. The rate of formation of a tight binding complex was reduced, consistent with the poorer effectiveness of oxidized CaM in activating the Ca2+ pump.

Activity of the hSPCA1 Golgi Ca2+ pump is essential for Ca2+-mediated Ca2+ response and cell viability in Darier disease

Keratinocyte differentiation, adhesion and motility are directed by extracellular Ca2+ concentration increases, which in turn increase intracellular Ca2+ levels. Normal keratinocytes, in contrast to most non-excitable cells, require Ca2+ release from both Golgi and endoplasmic reticulum Ca2+ stores for efficient Ca2+ signaling. Dysfunction of the Golgi human secretory pathway Ca2+-ATPase hSPCA1, encoded by ATP2C1, abrogates Ca2+ signaling and causes the acantholytic genodermatosis, Hailey-Hailey disease. We have examined the role of the endoplasmic reticulum Ca2+ store, established and maintained by the sarcoplasmic and endoplasmic reticulum Ca2+-ATPase SERCA2 encoded by ATP2A2, in Ca2+ signaling. Although previous studies have shown acute SERCA2 inactivation to abrogate Ca2+ signaling, we find that chronic inactivation of ATP2A2 in keratinocytes from patients with the similar acantholytic genodermatosis, Darier disease, does not impair the response to raised extracellular Ca2+ levels. This normal response is due to a compensatory upregulation of hSPCA1, as inactivating ATP2C1 expression with siRNA blocks the response to raised extracellular Ca2+ concentrations in both normal and Darier keratinocytes. ATP2C1 inactivation also diminishes Darier disease keratinocyte viability, suggesting that compensatory ATP2C1 upregulation maintains viability and partially compensates for defective endoplasmic reticulum Ca2+-ATPase in Darier disease keratinocytes. Keratinocytes thus are unique among mammalian cells in their ability to use the Golgi Ca2+ store to mediate Ca2+ signaling.

Role of plasma membrane Ca2+-ATPase in contraction-relaxation processes of the bladder: evidence from PMCA gene-ablated mice

We investigated the roles and relationships of plasma membrane Ca2+-ATPase (PMCA), sarco(endo)plasmic reticulum Ca2+-ATPase (SERCA)2, and Na+/Ca2+ exchanger (NCX) in bladder smooth muscle contractility in Pmca-ablated mice: Pmca4-null mutant ( Pmca4 −/−) and heterozygous Pmca1 and homozygous Pmca4 double gene-targeted ( Pmca1 +/− Pmca4 −/−) mice. Gene manipulation did not alter the amounts of PMCA1, SERCA2, and NCX. To study the role of each Ca2+ transport system, contraction of circular ring preparations was elicited with KCl (80 mM) plus atropine, and then the muscle was relaxed with Ca2+-free physiological salt solution containing EGTA. We measured the contributions of Ca2+ clearance components by inhibiting SERCA2 (with 10 μM cyclopiazonic acid) and/or NCX (by replacing NaCl with N-methyl-d-glucamine/HCl plus 10 μM KB-R7943). Contraction half-time (time to 50% of maximum tension) was prolonged in the gene-targeted muscles but marginally shortened when SERCA2 or NCX was inhibited. The inhibition of NCX significantly inhibited this prolongation, suggesting that NCX activity might be augmented to compensate for PMCA4 function in the gene-targeted muscles under nonstimulated conditions. Inhibition of SERCA2 and NCX as well as gene targeting all prolonged the relaxation half-time. The contribution of PMCA to relaxation was calculated to be ∼25–30%, with that of SERCA2 being 20% and that of NCX being 70%. PMCA and SERCA2 appeared to function additively, but the function of NCX might overlap with those of other components. In summary, gene manipulation of PMCA indicates that PMCA, in addition to SERCA2 and NCX, plays a significant role in both excitation-contraction coupling and the Ca2+ extrusion-relaxation relationship, i.e., Ca2+ homeostasis, of bladder smooth muscle.

Regulation of Ca2+-induced permeability transition by Bcl-2 is antagonized by Drp1 and hFis1

The regulation of mitochondrial permeability transition (MPT) is essential for cell survival. Un-controlled opening of the MPT pore is often associated with cell death. Anti-death protein Bcl-2 can block MPT as assessed by the enhanced capacity of mitochondria to accumulate and retain Ca2+. We report here that two proteins of the mitochondrial fission machinery, dynamin-related protein (Drp1) and human mitochondrial fission protein (hFis1), have an antagonistic effect on Bcl-2. Drp1, with the assistance of hFis1, sensitizes cells to MPT by reducing the mitochondrial Ca2+ retention capacity (CRC). While the reduction of CRC by Drp1/hFis1 is linked to mitochondrial fission, the antagonism between Bcl-2 and Drp1 appears to be mediated by mutually exclusive interactions of the two proteins with hFis1 . The complexity of protein-protein interactions demonstrated in the present study suggests that in addition to the previously described role of Bcl-2 in the control of apoptosis, Bcl-2 may also participate directly or indirectly in the regulation of mitochondrial fission.

Molecular biology of ion motive proteins in comparative models

This article will review the utility of comparative animal models in understanding the molecular biology of ion transport. Due to the breadth of this field some 'disclaimers' need to be established up front. 'Comparative' will be defined as non-mammalian. 'Genetic species' will be defined as organisms that have been selected as models for genetic studies and for which the genome has been largely sequenced. 'Non-genetic species' will include other non-mammalian organisms. The review will be limited to ions that play a major role in extracellular (EC) ionoregulation (Na/K/Ca/Cl) and not to micronutrients (Fe) or heavy metals (Cd, Zn). The review will focus only on ion motive proteins that have been associated with vectorial transfer at epithelial tissues. The review is therefore intended as a guidepost to researchers new to the field as well as to inform biologists of the power of comparative genomics.

Characterization and expression of plasma membrane Ca2+ ATPase (PMCA3) in the crayfish Procambarus clarkii antennal gland during molting

The discontinuous pattern of crustacean cuticular mineralization (the molting cycle) has emerged as a model system to study the spatial and temporal regulation of genes that code for Ca2+-transporting proteins including pumps, channels and exchangers. The plasma membrane Ca2+-ATPase (PMCA) is potentially of significant interest due to its role in the active transport of Ca2+ across the basolateral membrane, which is required for routine maintenance of intracellular Ca2+ as well as unidirectional Ca2+ influx. Prior research has suggested that PMCA expression is upregulated during periods of elevated Ca2+ influx associated with postmolt cuticular mineralization. This paper describes the cloning, sequencing and functional characterization of a novel PMCA3 gene from the antennal gland (kidney) of the crayfish Procambarus clarkii. The complete sequence, the first obtained from a non-genetic invertebrate species, was obtained through reverse transcription-polymerase chain reaction (RTPCR) and rapid amplification of cDNA ends (RACE) techniques. Crayfish PMCA3 consists of 4148 bp with a 3546 bp open reading frame coding for 1182 amino acid residues with a molecular mass of 130 kDa. It exhibits 77.5-80.9% identity at the mRNA level and 85.3-86.9% identity at the protein level with PMCA3 from human, mouse and rat. Membrane topography was typical of published mammalian PMCAs. Northern blot analysis of total RNA from crayfish gill, antennal gland, cardiac muscle and axial abdominal muscle revealed that a 7.5 kb species was ubiquitous. The level of PMCA3 mRNA expression in all tissues (transporting epithelia and muscle) increased significantly in pre/postmolt stages compared with relatively low abundance in intermolt. Western analysis confirmed corresponding changes in PMCA protein expression (130 kDa).

Role of Regulator of G Protein Signaling 2 (RGS2) in Ca2+ Oscillations and Adaptation of Ca2+ Signaling to Reduce Excitability of RGS2–/– Cells

Regulators of G protein signaling (RGS) proteins accelerate the GTPase activity of Galpha subunits to determine the duration of the stimulated state and control G protein-coupled receptor-mediated cell signaling. RGS2 is an RGS protein that shows preference toward Galpha(q). To better understand the role of RGS2 in Ca(2+) signaling and Ca(2+) oscillations, we characterized Ca(2+) signaling in cells derived from RGS2(-/-) mice. Deletion of RGS2 modified the kinetic of inositol 1,4,5-trisphosphate (IP(3)) production without affecting the peak level of IP(3), but rather increased the steady-state level of IP(3) at all agonist concentrations. The increased steady-state level of IP(3) led to an increased frequency of [Ca(2+)](i) oscillations. The cells were adapted to deletion of RGS2 by reducing Ca(2+) signaling excitability. Reduced excitability was achieved by adaptation of all transporters to reduce Ca(2+) influx into the cytosol. Thus, IP(3) receptor 1 was down-regulated and IP(3) receptor 3 was up-regulated in RGS2(-/-) cells to reduce the sensitivity for IP(3) to release Ca(2+) from the endoplasmic reticulum to the cytosol. Sarco/endoplasmic reticulum Ca(2+) ATPase 2b was up-regulated to more rapidly remove Ca(2+) from the cytosol of RGS2(-/-) cells. Agonist-stimulated Ca(2+) influx was reduced, and Ca(2+) efflux by plasma membrane Ca(2+) was up-regulated in RGS2(-/-) cells. The result of these adaptive mechanisms was the reduced excitability of Ca(2+) signaling, as reflected by the markedly reduced response of RGS2(-/-) cells to changes in the endoplasmic reticulum Ca(2+) load and to an increase in extracellular Ca(2+). These findings highlight the central role of RGS proteins in [Ca(2+)](i) oscillations and reveal a prominent plasticity and adaptability of the Ca(2+) signaling apparatus.

Identification, Expression, Function, and Localization of a Novel (Sixth) Isoform of the Human Sarco/Endoplasmic Reticulum Ca2+ATPase 3 Gene

Understanding of Ca(2+) signaling requires the knowledge of proteins involved in this process. Among these proteins are sarco/endoplasmic reticulum Ca(2+)-ATPases (SERCAs) that pump Ca(2+) into the endoplasmic reticulum (ER). Recently, the human SERCA3 gene was shown to give rise to five isoforms (SERCA3a-e (h3a-h3e)). Here we demonstrate the existence of an additional new member, termed SERCA3f (h3f). By reverse transcriptase-PCR using monocytic U937 cell RNA, h3f mRNA was found to exclude the antepenultimate exon 21. h3f mRNA expression appeared as a human-specific splice variant. It was not found in rats or mice. h3f mRNA gave rise to an h3f protein differing in its C terminus from h3a-h3e. Of particular interest, h3f diverged in the first amino acids after the first splice site but presented the same last 21 amino acids as h3b. Consequently, we further investigated the structure-function-location relationships of the h3b and h3f isoforms. Comparative functional study of h3b and h3f recombinant proteins in intact HEK-293 cells and in fractionated membranes showed the following distinct characteristics: (i) resting cytosolic Ca(2+) concentration ([Ca(2+)](c)) and (ii) ER Ca(2+) content ([Ca(2+)](er)); similar characteristics were shown for the following: (i) the effects of the SERCA inhibitor, thapsigargin, on Ca(2+) release ([Ca(2+)](Tg)) and subsequent Ca(2+) entry ([Ca(2+)](e)) and (ii) the low apparent Ca(2+) affinity and the enhanced rate of dephosphorylation of the E(2)P phosphoenzyme intermediate. Subcellular location of h3b and h3f by immunofluorescence and/or confocal microscopy using the h3b- and h3f-specific polyclonal and the pan-h3 monoclonal (PL/IM430) antibodies suggested overlapping but distinct ER location. The endogenous expression of h3f protein was also proved in U937 cells. Altogether these data suggest that the SERCA3 isoforms have a more widespread role in cellular Ca(2+) signaling than previously appreciated.

CD22 attenuates calcium signaling by potentiating plasma membrane calcium-ATPase activity

Binding of antigen to the B cell receptor induces a calcium response, which is required for proliferation and antibody production. CD22, a B cell surface protein, inhibits this signal through mechanisms that have been obscure. We report here that CD22 augments calcium efflux after B cell receptor crosslinking. Inhibition of plasma membrane calcium-ATPase (PMCA) attenuated these effects, as did disruption by homologous recombination of the gene encoding PMCA4a and PMCA4b. PMCA coimmunoprecipitated with CD22 in an activation-dependent way. CD22 cytoplasmic tyrosine residues were required for association with PMCA and enhancement of calcium efflux. Moreover, CD22 regulation of efflux and the calcium response required the tyrosine phosphatase SHP-1. Thus, SHP-1 and PMCA provide a mechanism by which CD22, a tissue-specific negative regulator, can affect calcium responses.

Characterization of Cos-7 cells overexpressing the rat secretory pathway Ca2+-ATPase

On the basis of sequence similarities to the yeast PMR1 and hSPCA gene, the rat alternatively spliced mRNA has been suggested to be a Golgi secretory pathway Ca2+-ATPase (SPCA). Data in this report lend further support for this hypothesis in that sucrose gradient fractionation of rat liver microsomes resulted in SPCA comigrating with the Golgi calcium binding protein CALNUC, which was well resolved from the endoplasmic reticulum marker calreticulin. Also, in PC-12 cells, antibody to SPCA colocalized with an antibody to the Golgi marker alpha-mannosidase II. To study the biological effects of SPCA expression, we performed stable overexpression of SPCA in COS-7 cells. Seven clones were selected for further comparison with COS-7 cells containing an empty expression vector. Overexpression of SPCA resulted in a significant reduction of plasma membrane Ca2+-ATPase, sarco(endo)plasmic reticulum Ca2+-ATPase, and calreticulin expression in these clones. In contrast, the expression of the Golgi calcium-binding protein CALNUC increased significantly. The phosphoenzyme intermediate formed using membranes from clone G11/5 was calcium dependent, significantly more intense than in COS-7 cells, and not affected by La3+ treatment. Calcium uptake by G11/5 microsomes was ATP dependent and significantly greater than in microsomes from parent COS-7 cells. The overexpression of SPCA significantly increased the growth rate of these cells compared with COS-7 cells containing only the empty vector. These data demonstrate that overexpression of the rat SPCA results in significant changes in the expression of calcium transport and storage proteins in COS-7 cells.

Expression, purification, and characterization of isoform 1 of the plasma membrane Ca2+ pump: focus on calpain sensitivity

The plasma membrane Ca2+ ATPase isoform 1(PMCA1) is ubiquitously distributed in tissues and cells, but only scarce information is available on its properties. The isoform was overexpressed in Sf9 cells, purified on calmodulin columns, and characterized functionally. The level of expression was very low, but sufficient amounts of the protein could be isolated for biochemical characterization. The affinity of PMCA1 for calmodulin was similar to that of PMCA4, the other ubiquitous PMCA isoform. The affinity of PMCA1 for ATP, evaluated by the formation of the phosphorylated intermediate, was higher than that of the PMCA4 pump. The recombinant PMCA1 pump was a much better substrate for the cAMP-dependent protein kinase than the PMCA2 and PMCA4 isoforms. Pulse and chase experiments on Sf9 cells overexpressing the PMCA pumps showed that PMCA1 was much less stable than the PMCA4 and PMCA2 isoforms, i.e. PMCA1 had a much higher sensitivity to degradation by calpain. The effect of calpain was not the result of a general higher susceptibility of the PMCA1 to proteolytic degradation, because the pattern of degradation by trypsin was the same in the three isoforms.

Plasma membrane calcium ATPase overexpression in arterial smooth muscle increases vasomotor responsiveness and blood pressure

In vascular smooth muscle cells (SMCs), several mechanisms act in concert to regulate the intracellular calcium concentration [Ca2+]i, which may in turn affect vascular tone. One such mechanism is the extrusion of Ca2+ by the plasma membrane calcium ATPase (PMCA). To address, in particular, the role of the neuronal nitric oxide synthase (nNOS)-associating isoform PMCA4b in regulating vascular tone, a doxycycline-responsive transgene for human PMCA4b was overexpressed in arterial SMCs of mice. Overexpression of hPMCA4b resulted in a 2-fold increase in total aortic PMCA4 protein expression and significant real-time RT-PCR-documented differences in the levels of endogenous mouse PMCA1, PMCA4, SERCA2, and IP3R1 gene expression in arterial SMCs. Whereas no significant difference in basal [Ca2+]i or Ca2+ sensitivity was observed in vascular SMCs or mesenteric arteries, respectively, from hPMCA4b-overexpressing versus control mice, hPMCA4b-overexpressing mice revealed a reduced set-point and increased extent of myogenic response and heightened sensitivity to vasoconstrictors. Treatment of arteries with an nNOS inhibitor resulted in a reduced set-point and increased extent of the myogenic response in control but not hPMCA4b-overexpressing mice. Moreover, aortic SMCs from hPMCA4b-overexpressing mice exhibited reduced levels of cGMP under both basal and phenylephrine-stimulated conditions. These changes were associated with significant doxycycline-reversible elevations in blood pressure. Taken together, these data show that overexpression of hPMCA4b in arterial SMCs increases vascular reactivity and blood pressure, an effect that may be mediated in part by negative regulation of nNOS.

Homer 2 tunes G protein-coupled receptors stimulus intensity by regulating RGS proteins and PLCbeta GAP activities

Homers are scaffolding proteins that bind G protein-coupled receptors (GPCRs), inositol 1,4,5-triphosphate (IP3) receptors (IP3Rs), ryanodine receptors, and TRP channels. However, their role in Ca2+ signaling in vivo is not known. Characterization of Ca2+ signaling in pancreatic acinar cells from Homer2-/- and Homer3-/- mice showed that Homer 3 has no discernible role in Ca2+ signaling in these cells. In contrast, we found that Homer 2 tunes intensity of Ca2+ signaling by GPCRs to regulate the frequency of [Ca2+]i oscillations. Thus, deletion of Homer 2 increased stimulus intensity by increasing the potency for agonists acting on various GPCRs to activate PLCbeta and evoke Ca2+ release and oscillations. This was not due to aberrant localization of IP3Rs in cellular microdomains or IP3R channel activity. Rather, deletion of Homer 2 reduced the effectiveness of exogenous regulators of G proteins signaling proteins (RGS) to inhibit Ca2+ signaling in vivo. Moreover, Homer 2 preferentially bound to PLCbeta in pancreatic acini and brain extracts and stimulated GAP activity of RGS4 and of PLCbeta in an in vitro reconstitution system, with minimal effect on PLCbeta-mediated PIP2 hydrolysis. These findings describe a novel, unexpected function of Homer proteins, demonstrate that RGS proteins and PLCbeta GAP activities are regulated functions, and provide a molecular mechanism for tuning signal intensity generated by GPCRs and, thus, the characteristics of [Ca2+]i oscillations.

From nitric oxide to endothelial cytosolic Ca2+: a negative feedback control

Nitric oxide production is stimulated by an increase of the concentration of cytosolic Ca(2+) in vascular endothelial cells. Recent evidence suggests that nitric oxide and cGMP might attenuate Ca(2+) influx and, at the same time, initiate a Ca(2+) removal mechanism, thereby decreasing the intracellular concentration of endothelial Ca(2+) in a negative feedback fashion. Such a negative feedback mechanism could serve to protect the endothelial cells from the detrimental effects of excessive nitric oxide and Ca(2+).

Plasma membrane Ca2+-ATPase mRNA expression in murine hepatocarcinoma and regenerating liver cells

The plasma membrane calcium ATPase (PMCA) is an ubiquitous enzyme that extrudes calcium from the cytoplasm to the extracellular space. Four PMCA genes through alternative splicing produce a large diversity of isoforms of this enzyme. We reported previously that the PMCA contained in AS-30D hepatocarcinoma cells showed significant differences in activity in comparison to normal and regenerating liver. In the present study we investigate if the difference in PMCA activity could be related to differential expression of mRNAs encoding different isoforms of PMCA. Using RT-PCR we found that variants 1b, 1x, and 4b are expressed in all liver samples. The hepatoma AS-30 and liver at 2 days of regeneration express low amounts of isoforms 2w, 4b and 4x, and do not express isoforms 4a, 4d and 4z. Fetal and neonatal liver do not express variants 4a and 4d, but they do express variants 4x and 4z. Immunoblot analysis showed a higher ratio ATPase/total protein in the hepatoma AS-30D in comparison to normal liver. Our results suggest that the Ca2+-ATPase kinetic pattern previously observed by us in the AS-30D cells, could be at least partially explained by changes in the mRNA expression of several of the PMCA isoforms expressed in the liver.

Antisense-inhibition of plasma membrane Ca2+ pump induces apoptosis in vascular smooth muscle cells

The effect of antisense oligodeoxynucleotides (ODNs) of plasma membrane Ca(2+)-pumping ATPase (PMCA) on rat aortic vascular smooth muscle cells (VSMCs) in primary culture was examined. More than 80% of the PMCA expressed in cultured VSMCs was the PMCA-1B subtype. Exposed to antisense ODNs against PMCA-1, not only the expression of the PMCA protein but also mRNA of PMCA-1B was diminished in a concentration-dependent manner. Extracellular Na(+)-independent (45)Ca(2+) efflux catalyzed via PMCA was inhibited with antisense ODNs. Both the resting and ionomycin- or ATP-stimulated levels of intracellular Ca(2+) were increased by antisense ODNs. Furthermore, prolonged treatment with antisense ODNs caused apoptosis in VSMCs. The occurrence of apoptosis was inhibited by FK506, a potent immunosuppressant. These results demonstrate that the PMCA was specifically inhibited by antisense ODNs and suggest that PMCA plays an important role in regulation of intracellular Ca(2+) concentrations, especially at the resting condition to prevent an occurrence of apoptosis that may be induced through the activation of calcineurin.

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.

Three novel sarco/endoplasmic reticulum Ca2+-ATPase (SERCA) 3 isoforms. Expression, regulation, and function of the membranes of the SERCA3 family

Sarco/endoplasmic reticulum Ca2+-ATPases (SERCAs) pump Ca2+ into the endoplasmic reticulum. Recently, three human SERCA3 (h3a-c) proteins and a previously unknown rat SERCA3 (r3b/c) mRNA have been described. Here, we (i) document two novel human SERCA3 splice variants h3d and h3e, (ii) provide data for the expression and mechanisms regulating the expression of all known SERCA3 variants (r3a, r3b/c, and h3a-e), and (iii) show functional characteristics of the SERCA3 isoforms. h3d and h3e are issued from the insertion of an additional penultimate exon 22 resulting in different carboxyl termini for these variants. Distinct distribution patterns of the SERCA3 gene products were observed in a series of cell lines of hematopoietic, epithelial, embryonic origin, and several cancerous types, as well as in panels of rat and human tissues. Hypertension and protein kinase C, calcineurin, or retinoic acid receptor signaling pathways were found to differently control rat and human splice variant expression, respectively. Stable overexpression of each variant was performed in human embryonic kidney 293 cells, and the SERCA3 isoforms were fully characterized. All SERCA3 isoforms were found to pump Ca2+ with similar affinities. However, they modulated the cytosolic Ca2+ concentration ([Ca2+]c) and the endoplasmic reticulum Ca2+ content ([Ca2+]er) in different manners. A newly generated polyclonal antibody and a pan-SERCA3 antibody proved the endogenous expression of the three novel SERCA3 proteins, h3d, h3e, and r3b/c. All these data suggest that the SERCA3 gene products have a more widespread role in cellular Ca2+ signaling than previously appreciated.

Regulation of sodium-calcium exchange and mitochondrial energetics by Bcl-2 in the heart of transgenic mice

Our previous work in cultured cells has shown that the maintenance of mitochondrial Ca(2+) homeostasis is essential for cell survival, and that the anti-apoptotic protein Bcl-2 is able to maintain a threshold level of mitochondrial Ca(2+) by the inhibition of permeability transition. To test whether Bcl-2 also affects the mitochondrial Na(+)-Ca(2+) exchange (NCE), a major efflux pathway for mitochondrial Ca(2+), studies using transgenic mice that overexpress Bcl-2 in the heart have been performed. NCE activity was determined as the Na(+)-dependent Ca(2+) efflux in the isolated mitochondria. Overexpression of Bcl-2 led to a significant reduction of NCE activity as well as increased resistance to permeability transition in the mitochondria of transgenic heart. This was accompanied by increased matrix Ca(2+) level, enhanced formation of NADH and enhanced oxidation of pyruvate, an NAD(+)-linked substrate. Furthermore, there was induction of cellular Ca(2+) transport proteins including the Na(+)-Ca(2+) exchanger of the sarcolemma (NCX). Bcl-2 not only stimulates NCX expression in the sarcolemma but also attenuates the Na(+)-Ca(2+) exchange in the mitochondria. These results are consistent with the protection by Bcl-2 against apoptosis in heart following ischemia/reperfusion.

Function of SERCA mediated calcium uptake and expression of SERCA3 in cerebral cortex from young and old rats

Previous work on peripheral sympathetic neurons indicated that a decline in sarco/endoplasmic reticulum calcium ATPase (SERCA) function occurs with advancing age. Therefore, an age-related decline in mechanisms controlling intracellular calcium homeostasis could contribute to altered neuronal function and/or degeneration. In this study we sought to extend the findings on peripheral neurons and to detect possible age-related declines in SERCA function and expression of SERCA3 in central neurons from cerebral cortex from young (6-month) and old (20-month) rats. Functional studies compared ATP-dependent 45Ca(2+)-uptake into microsomes and plasma membrane vesicles (PMVs). We and found no significant difference in 45Ca(2+)-uptake between microsomes or PMVs between young and old animals. On the other hand expression of SERCA3 mRNA in rat cerebral cortex showed a significant decline with advancing age. However, comparison of SERCA3 protein content did not reveal a corresponding decline; implying that SERCA mRNA turnover rates may be greater in the younger group. Although the present work with rat cerebral cortex does not indicate an age-related decline in SERCA function, previous work from our laboratory on sympathetic nerves and by others on the hippocampus indicate such a decline. In light of our previous and current studies, aging may affect calcium homeostatic mechanisms in central and peripheral autonomic neurons differently.

Plasma membrane calcium ATPase plays a role in reducing Ca(2+)-mediated cytotoxicity in PC12 cells

In many cell types, cell death induced by a variety of insults is accompanied by an increase in intracellular calcium. The Ca(2+) homeostatic mechanisms affected by such insults, however, have not been fully determined. Recent evidence indicates that kainic acid-induced seizures alter plasma membrane calcium ATPase mRNA expression within vulnerable hippocampal cell populations before the onset of cell death. We examined the effects of altering plasma membrane calcium ATPase expression on cell vulnerability in rat pheochromocytoma 12 cells. Pheochromocytoma 12 cells are vulnerable to Ca(2+) overload induced by the Ca(2+) ionophore A23187. Reverse transcriptase-PCR and Western blot data indicated that plasma membrane calcium ATPase isoform 4b constitutes a major calcium pump isoform in the pheochromocytoma 12 cells. Therefore, permanently transfected pheochromocytoma 12-derived cell lines were established that either over-expressed plasma membrane calcium ATPase isoform 4b, or suppressed the expression of the endogenous plasma membrane calcium ATPase isoform 4. Over-expressing clones were less vulnerable to Ca(2+)-mediated cell death induced by A23187 whereas "antisense" clones were considerably more susceptible. These data indicate that regulation of plasma membrane calcium ATPase expression may be critical to cellular survival when cells are exposed to pathological increases in intracellular calcium.

Plasticity and adaptation of Ca2+ signaling and Ca2+-dependent exocytosis in SERCA2(+/-) mice

Darier's disease (DD) is a high penetrance, autosomal dominant mutation in the ATP2A2 gene, which encodes the SERCA2 Ca2+ pump. Here we have used a mouse model of DD, a SERCA2(+/-) mouse, to define the adaptation of Ca2+ signaling and Ca2+-dependent exocytosis to a deletion of one copy of the SERCA2 gene. The [Ca2+]i transient evoked by maximal agonist stimulation was shorter in cells from SERCA2(+/-) mice, due to an up-regulation of specific plasma membrane Ca2+ pump isoforms. The change in cellular Ca2+ handling caused approximately 50% reduction in [Ca2+]i oscillation frequency. Nonetheless, agonist-stimulated exocytosis was identical in cells from wild-type and SERCA2(+/-) mice. This was due to adaptation in the levels of the Ca2+ sensors for exocytosis synaptotagmins I and III in cells from SERCA2(+/-) mice. Accordingly, exocytosis was approximately 10-fold more sensitive to Ca2+ in cells from SERCA2(+/-) mice. These findings reveal a remarkable plasticity and adaptability of Ca2+ signaling and Ca2+-dependent cellular functions in vivo, and can explain the normal function of most physiological systems in DD patients.

Role of alternative splicing in generating isoform diversity among plasma membrane calcium pumps

Calcium pumps of the plasma membrane (also known as plasma membrane Ca(2+)-ATPases or PMCAs) are responsible for the expulsion of Ca(2+) from the cytosol of all eukaryotic cells. Together with Na(+)/Ca(2+) exchangers, they are the major plasma membrane transport system responsible for the long-term regulation of the resting intracellular Ca(2+) concentration. Like the Ca(2+) pumps of the sarco/endoplasmic reticulum (SERCAs), which pump Ca(2+) from the cytosol into the endoplasmic reticulum, the PMCAs belong to the family of P-type primary ion transport ATPases characterized by the formation of an aspartyl phosphate intermediate during the reaction cycle. Mammalian PMCAs are encoded by four separate genes, and additional isoform variants are generated via alternative RNA splicing of the primary gene transcripts. The expression of different PMCA isoforms and splice variants is regulated in a developmental, tissue- and cell type-specific manner, suggesting that these pumps are functionally adapted to the physiological needs of particular cells and tissues. PMCAs 1 and 4 are found in virtually all tissues in the adult, whereas PMCAs 2 and 3 are primarily expressed in excitable cells of the nervous system and muscles. During mouse embryonic development, PMCA1 is ubiquitously detected from the earliest time points, and all isoforms show spatially overlapping but distinct expression patterns with dynamic temporal changes occurring during late fetal development. Alternative splicing affects two major locations in the plasma membrane Ca(2+) pump protein: the first intracellular loop and the COOH-terminal tail. These two regions correspond to major regulatory domains of the pumps. In the first cytosolic loop, the affected region is embedded between a putative G protein binding sequence and the site of phospholipid sensitivity, and in the COOH-terminal tail, splicing affects pump regulation by calmodulin, phosphorylation, and differential interaction with PDZ domain-containing anchoring and signaling proteins. Recent evidence demonstrating differential distribution, dynamic regulation of expression, and major functional differences between alternative splice variants suggests that these transporters play a more dynamic role than hitherto assumed in the spatial and temporal control of Ca(2+) signaling. The identification of mice carrying PMCA mutations that lead to diseases such as hearing loss and ataxia, as well as the corresponding phenotypes of genetically engineered PMCA "knockout" mice further support the concept of specific, nonredundant roles for each Ca(2+) pump isoform in cellular Ca(2+) regulation.

Cloning and characterization of sarco/endoplasmic reticulum Ca(2+)-ATPase (SERCA) from crayfish axial muscle. Sarco/Endoplasmic Reticulum Ca(2+)-ATPase

The discontinuous pattern of muscle growth during the moulting cycle of a freshwater crustacean (the crayfish Procambarus clarkii) was used as a model system to examine the regulation of the expression of Sarco/Endoplasmic Reticulum Ca(2+)-ATPase (SERCA). We describe the cloning, sequencing and characterization of a novel SERCA cDNA (3856 bp) obtained from crayfish axial abdominal muscle by reverse transcription/polymerase chain reaction (RT-PCR) followed by rapid amplification of cDNA ends (RACE). This complete sequence contains a 145 base pair (bp) noncoding region at the 5′ end, a 3006 bp open reading frame coding for 1002 amino acid residues with a molecular mass of 110 kDa and 705 bp of untranslated region at the 3′ end. This enzyme contains all the conserved domains found in ‘P’-type ATPases, and the hydropathy profile suggests a transmembrane organization typical of other SERCAs. It exhibits 80% amino acid identity with Drosophila melanogaster SERCA, 79% identity with Artemia franciscana SERCA, 72% identity with rabbit fast-twitch muscle neonatal isoform SERCA1b, 71% identity with slow-twitch muscle isoform SERCA2 and 67% identity with SERCA3. Sequence alignment revealed that regions anchoring the cytoplasmic domain in the membrane were highly conserved and that most differences were in the NH(2) terminus, the central loop region and the COOH terminus. Northern analysis of total RNA from crayfish tissues probed with the 460 bp fragment initially isolated showed four bands (7.6, 7.0, 5.8 and 4.5 kilobases) displaying tissue-specific expression. SERCA was most abundant in muscle (axial abdominal, cardiac and stomach), where it is involved in Ca(2+) resequestration during relaxation, and in eggs, where it may be implicated in early embryogenesis. The level of SERCA mRNA expression in axial abdominal muscle varied during the moulting cycle as determined by slot-blot analysis. SERCA expression was greatest during intermoult and decreased to approximately 50% of this level during pre- and postmoult. Patterns of gene expression for SERCA and other sarcomeric proteins during the crustacean moulting cycle may be regulated by ecdysteroids and/or mechanical stimulation.

Ca(2+)-ATPase protein expression in mammary tissue

Protein expression of plasma membrane Ca(2+)-ATPases (PMCAs) and the putative Golgi secretory pathway Ca(2+)-ATPase (SPCA) was examined in rat mammary tissue. As lactation started, PMCA protein expression increased dramatically, and this increased expression paralleled milk production. Mammary PMCA was primarily PMCA2b but was approximately 4,000 daltons larger than expected. RT-PCR showed that the primary mammary PMCA2b transcript was alternatively spliced, at splice site A, to include an additional 135 bp, resulting in the insertion of 45 amino acids. This splice form is designated 2bw. PMCA2bw is secreted into milk, associated with the milk fat globule membrane. Therefore, PMCA2bw is located on the apical membrane of the secretory cell. Smaller amounts of PMCA1b and 4b protein were found in mammary tissue. PMCA4b was the major PMCA expressed in developing tissue, and its level declined as lactation started. PMCA1b expression increased moderately during lactation. SPCA protein expression increased 1 wk before parturition and increased further as lactation proceeded. The abundance and cell location of PMCA2b suggest that it is important for macro-Ca(2+) homeostasis in lactating tissue. The pattern of expression and abundance of SPCA suggest that it is a candidate for the Golgi Ca(2+)-ATPase.

Hydrogen peroxide attenuates store-operated calcium entry and enhances calcium extrusion in thyroid FRTL-5 cells

Redox modulation participates in the regulation of intracellular free calcium concentration ([Ca(2+)](i)) in several cell types. In thyroid cells, including FRTL-5 cells, changes in [Ca(2+)](i) regulate several important functions, including the production of H(2)O(2) (hydrogen peroxide). As H(2)O(2) is of crucial importance for the production of thyroid hormones, we investigated the effects of H(2)O(2) on [Ca(2+)](i) in thyroid FRTL-5 cells. H(2)O(2) itself did not modulate basal [Ca(2+)](i). However, H(2)O(2) attenuated store-operated calcium entry evoked by thapsigargin, both in a sodium-containing buffer and in a sodium-free buffer. The effect of H(2)O(2) was abrogated by the reducing agent beta-mercaptoethanol. H(2)O(2) also attenuated the thapsigargin-evoked entry of barium and manganese. The effect of H(2)O(2) was, at least in part, mediated by activation of protein kinase C (PKC), as H(2)O(2) enhanced the binding of [(3)H]phorbol 12,13-dibutyrate. H(2)O(2) also stimulated the translocation of the isoenzyme PKCepsilon from the cytosolic fraction to the particulate fraction. Furthermore, H(2)O(2) did not attenuate store-operated calcium entry in cells treated with staurosporine or calphostin C, or in cells with down-regulated PKC. H(2)O(2) depolarized the membrane potential in bisoxonol-loaded cells and when patch-clamp in the whole-cell mode was used. The depolarization was attenuated in cells with down-regulated PKC. This depolarization, at least in part, explained the H(2)O(2)-evoked inhibition of calcium entry. In addition, H(2)O(2) enhanced the extrusion of calcium from cells stimulated with thapsigargin and this effect was abolished in cells with down-regulated PKC and after treatment of the cells with the reducing agent beta-mercaptoethanol. In conclusion H(2)O(2) attenuates an increase in [Ca(2+)](i). As H(2)O(2) is produced in thyroid cells in a calcium-dependent manner, our results suggest that H(2)O(2) may participate in the regulation of [Ca(2+)](i) in these cells via a negative-feedback mechanism involving activation of PKC.

Effects of PMCA and SERCA pump overexpression on the kinetics of cell Ca(2+) signalling

The dynamic interactions of the main pathways for active Ca(2+) transport have been analysed in living cells by altering the expression of their components. The plasma membrane (PMCA) and the endoplasmic reticulum (ER) (SERCA) Ca(2+) pumps were transiently overexpressed in CHO cells, and the Ca(2+) homeostasis in the subcellular compartments was investigated using specifically targeted chimaeras of the Ca(2+)- sensitive photoprotein aequorin. In resting cells, overexpression of the PMCA and SERCA pumps caused a reduction and an increase in ER [Ca(2+)] levels, respectively, while no significant differences were detected in cytosolic and mitochondrial [Ca(2+)]. Upon stimulation with an inositol 1,4, 5-trisphosphate (IP(3))-generating agonist, the amplitude of the mitochondrial and cytosolic Ca(2+) rises correlated with the ER [Ca(2+)] only up to a threshold value, above which the feedback inhibition of the IP(3) channel by Ca(2+) appeared to be limiting.

Polyglutamine expansion down-regulates specific neuronal genes before pathologic changes in SCA1

The expansion of an unstable CAG repeat causes spinocerebellar ataxia type 1 (SCA1) and several other neurodegenerative diseases. How polyglutamine expansions render the resulting proteins toxic to neurons, however, remains elusive. Hypothesizing that long polyglutamine tracts alter gene expression, we found certain neuronal genes involved in signal transduction and calcium homeostasis sequentially downregulated in SCA1 mice. These genes were abundant in Purkinje cells, the primary site of SCA1 pathogenesis; moreover, their downregulation was mediated by expanded ataxin-1 and occurred before detectable pathology. Similar downregulation occurred in SCA1 human tissues. Altered gene expression may be the earliest mediator of polyglutamine toxicity.

Modulation of mitochondrial Ca(2+) homeostasis by Bcl-2

We have investigated the role of mitochondrial Ca(2+) (Ca(m)) homeostasis in cell survival. Disruption of Ca(m) homeostasis via depletion of the mitochondrial Ca(2+) store was the earliest event that occurred during staurosporine-induced apoptosis in neuroblastoma cells (SH-SY5Y). The decrease of Ca(m) preceded activation of the caspase cascade and DNA fragmentation. Overexpression of the anti-apoptosis protein Bcl-2 led to increased Ca(m) load, increased mitochondrial membrane potential (DeltaPsi(m)), and inhibition of staurosporine-induced apoptosis. On the other hand, ectopic expression of the pro-apoptotic protein Bik led to decreased Ca(m) load and decreased DeltaPsi(m). Inhibition of calcium uptake into mitochondria by ruthenium red induced a dose-dependent apoptosis as determined by nuclear staining and DNA ladder assay. Similarly, reducing the Ca(m) load by lowering the extracellular calcium concentration also led to apoptosis. We suggest that the anti-apoptotic effect of Bcl-2 is related to its ability to maintain a threshold level of Ca(m) and DeltaPsi(m) while the pro-apoptotic protein Bik has the opposite effect. Furthermore, both ER and mitochondrial Ca(2+) stores are important, and the depletion of either one will result in apoptosis. Thus, our results, for the first time, provide evidence that the maintenance of Ca(m) homeostasis is essential for cell survival.

Selective decrease of mRNAs encoding plasma membrane calcium pump isoforms 2 and 3 in rat kidney

BACKGROUND

Although the existence of multiple isoforms of plasma membrane calcium ATPase (PMCA) is now well documented, their biological functions are not yet known. In this study, we set out to investigate the potential role of PMCA isoforms, previously identified in renal cortical tissue, in tubular reabsorption of calcium (Ca2+).

METHODS

With use of reverse transcription-polymerase chain reaction analysis, we determined levels of mRNAs encoding isoforms of PMCA1 through PMCA4 in renal cortex, liver, and brain of rats with hypercalciuria induced by feeding with a low-phosphate diet (LPD) as compared with Ca2+-retaining rats that were fed a high-phosphate diet (HPD).

RESULTS

We observed that in hypercalciuric LPD-fed rats, the mRNAs encoding isoforms PMCA2b and PMCA3(a + c) are significantly lower (Delta approximately-50%) than in HPD-fed hypocalciuric rats, whereas no changes in mRNAs encoding isoforms PMCA1b and PMCA4 were observed, and mRNA encoding calbindin 28 kDa was increased. On the other hand, the content of mRNAs encoding PMCA2b and PMCA3(a + c) in liver and brain, respectively, was not changed.

CONCLUSION

These findings are evidence that expression of PMCA isoforms in the kidney can be selectively modulated in response to pathophysiologic stimuli. The association of a decrease in mRNA encoding PMCA2b and PMCA3(a + c) with hypercalciuria suggests that the two PMCA isoforms may be operant in tubular reabsorption of Ca2+ and its regulation.

Glucocorticoid modulation of Ca2+ homeostasis in human B lymphoblasts

1. We determined the effect of cortisol (200 nM for 48 h) on the intracellular Ca2+ concentration ([Ca2+]i) and parameters of Ca2+i signalling in 19 lymphoblastoid cell lines (LCLs). 2. Using the fluorescent dye fura-2, the basal [Ca2+]i in Ca2+-containing medium was 63.5 +/- 2.4 nM in vehicle (ethanol)-treated LCLs and 55.7 +/- 2. 6 nM (mean +/- s.e.m.) in cortisol-treated LCLs. 3. Ca2+i signalling following platelet-activating factor (PAF, 100 nM) addition was enhanced by cortisol treatment, with LCLs having small PAF responses showing the largest percentage increase after cortisol treatment. Mean peak [Ca2+]i responses to PAF were enhanced 67.0% and 55.7% in Ca2+-free and Ca2+-containing medium, respectively. 4. The endoplasmic reticulum Ca2+-ATPase inhibitor thapsigargin (100 nM) caused a transient increase in [Ca2+]i in Ca2+-free medium in which the peak change was increased in cortisol-treated cells (98.5 +/- 5.8 vs. 79.8 +/- 4.5 nM). Peak changes in the freely exchangeable Ca2+ in response to 5 microM ionomycin were also enhanced in cortisol-treated cells (923.7 +/- 113.9 vs. 652.2 +/- 64.5 nM) and correlated to the PAF-evoked [Ca2+]i response. 5. Cortisol-treated LCLs exposed to thapsigargin to empty intracellular Ca2+ stores (10 min treatment in Ca2+-free medium) and exposed to CaCl2 or MnCl2 had a greater rate of Ca2+ entry (18.6 +/- 1.8 vs. 13.8 +/- 1.5 nM s-1) and higher rate constant for Mn2+ entry (0.0345 +/- 0.0029 vs. 0. 0217 +/- 0.0020) than vehicle-treated cells. Peak [Ca2+]i in cells exposed to CaCl2 was also enhanced (869.4 +/- 114.7 vs. 562.6 +/- 61.7 nM). Parameters of divalent cation influx were highly correlated to the peak [Ca2+]i elicited by thapsigargin or ionomycin. 6. Inclusion of RU 486 (a glucocorticoid antagonist) with cortisol prevented the decrease in basal [Ca2+]i and stimulatory actions of cortisol on all Ca2+i parameters. RU 486 alone had no apparent effects on basal [Ca2+]i or Ca2+i signalling. 7. Based on data obtained over a wide range of responses (in the presence and/or absence of cortisol or RU 486), the results show that cortisol stimulation of glucocorticoid receptors decreases basal [Ca2+]i and enhances PAF-evoked [Ca2+]i signalling, most probably through its effects on intracellular Ca2+ stores. In turn, the extent of Ca2+ entry via store-operated plasma membrane Ca2+ channels is closely linked to the size of the Ca2+ stores.

Overexpression of the sarcolemmal calcium pump in the myocardium of transgenic rats

The plasma membrane calmodulin-dependent calcium ATPase (PMCA) is a calcium-extruding enzyme controlling Ca2+ homeostasis in nonexcitable cells. However, its function in the myocardium is unclear because of the presence of the Na+/Ca2+ exchanger. We approached the question of the physiological function of the calcium pump using a transgenic "gain of function" model. Transgenic rat lines carrying the human PMCA 4 cDNA under control of the ventricle-specific myosin light chain-2 promoter were established, and expression in the myocardium was ascertained at the mRNA, protein, and functional levels. In vivo hemodynamic measurements in adult homozygous animals showed no differences in baseline and increased cardiac performance recruited by volume overload compared with controls. No differences between transgenic and control cardiomyocytes were found in patch clamp voltage dependence, activation/inactivation behavior of the L-type Ca2+ current, or fast [Ca2+]i transients (assessed by the Fura-2 method). To test whether the PMCA might be involved in processes other than beat-to-beat regulation of contraction/relaxation, we compared growth processes of neonatal transgenic and control cardiomyocytes. A 1.6- and 2.3-fold higher synthesis rate of total protein was seen in cells from transgenic animals compared with controls on incubation with 2% FCS for 24 hours and 36 hours, respectively. An effect of similar magnitude was observed using 20 micromol/L phenylephrine. A 1.4-fold- and 2.0-fold-higher protein synthesis peak was seen in PMCA-overexpressing cardiomyocytes after stimulation with isoproterenol for 12 hours and 24 hours, respectively. Because pivotal parts of the alpha- and beta-adrenergic signal transduction pathways recently have been localized to caveolae, we tested the hypothesis that the PMCA might alter the amplitude of alpha- and beta-adrenergic growth signals by virtue of its localization in caveolae. Biochemical as well as immunocytochemical studies suggested that the PMCA in large part was colocalized with caveolin 3 in caveolae of cardiomyocytes. These results indicate that the sarcolemmal Ca2+-pump has little relevance for beat-to-beat regulation of contraction/relaxation in adult animals but likely plays a role in regulating myocardial growth, possibly through modulation of caveolar signal transduction.

The plasma membrane calcium pump, its role and regulation: new complexities and possibilities

Significant progress has been achieved in elucidating the role of the plasma membrane Ca2(+)-ATPase in cellular Ca2+ homeostasis and physiology since the enzyme was first purified and physiology since the enzyme was first purified and cloned a number of years ago. The simple notion that the PM Ca2(+)-ATPase controls resting levels of [Ca2+]CYT has been challenged by the complexity arising from the finding of four major isoforms and splice variants of the Ca2+ pump, and the finding that these are differentially localized in various organs and subcellular regions. Furthermore, the isoforms exhibit differential sensitivities to Ca2+, calmodulin, ATP, and kinase-mediated phosphorylation. The latter pathways of regulation can give rise to activation or inhibition of the Ca2+ pump activity, depending on the kinase and the particular Ca2+ pump isoform. Significant progress is being made in elucidating subtle and more profound roles of the PM Ca2(+)-ATPase in the control of cellular function. Further understanding of these roles awaits new studies in both transfected cells and intact organelles, a process that will be greatly aided by the development of new and selective Ca2+ pump inhibitors.

Ca2+ in the dense tubules: a model of platelet Ca2+ load

In this work, we explored the relationship between the freely exchangeable Ca2+ (FECa2+) in the dense tubules (DT) and the sarco(endo)plasmic reticulum (SER) Ca2+-ATPase (SERCA) in circulating human platelets and examined the relationship between blood pressure (BP) and these platelet parameters. Studying platelets from 32 healthy men, we showed that the maximal reaction velocity (Vmax) of the SERCA significantly correlated with FECa2+ in the DT and with the protein expressions of SERCA 2 and 3. BP positively correlated with both the Vmax of the SERCA (r=.462, P=.010) and the FECa2+ sequestered in the DT (r=.492, P=.005). The relationships between these platelet Ca2+ parameters and BP were in part confounded by increased levels of serum triglycerides and diminished HDL cholesterol with a higher BP. No correlation was observed between the resting cytosolic Ca2+ and BP. Collectively, these findings indicate that (1) an increase in the cellular Ca2+ load in platelets is expressed by a higher activity of the SERCA and an increase in the expressions of SERCA 2 and 3 proteins, coupled with an increase in the FECa2+ in the DT, and (2) a higher BP is associated with an increase in platelet Ca2+ load in human beings, expressed by a rise in the FECa2+ in the DT and the upregulation of SERCA activity.

Interferon-gamma induces a decrease in the intracellular calcium pump in a human salivary gland cell line

Interferon-gamma (IFN-gamma) +/- tumor necrosis factor-alpha (TNF-alpha) induces antiproliferation and intracellular Ca2+ store depletion in a human submandibular ductal cell line (HSG), which can be reversed on cytokine removal [A. J. Wu, G. C. Chen, B. J. Baum, and I. S. Ambudkar. Am. J. Physiol. 270 (Cell Physiol. 39): C514-C521, 1996]. Here we have examined a possible mechanism for the IFN-gamma-induced intracellular Ca2+ store depletion. There was a time-dependent decrease in thapsigargin-dependent internal Ca2+ release after exposure of the cells to the cytokines. The intracellular Ca2+ pump [sarco(endo)plasmic reticulum Ca(2+)-ATPase (SERCA)] protein in lysates and membranes of cells treated with IFN-gamma +/- TNF-alpha, but not with TNF-alpha alone, showed a similar time-dependent decrease (examined using a SERCA2 antibody). Removal of the cytokines, which resulted in recovery of cell growth and refill of internal Ca2+ stores, also increased the level of SERCA protein. The decrease in SERCA is not a result of decreased cell proliferation, since thapsigargin, 2,5-di-(t-butyl)-1,4-hydroquinone, or serum-free growth conditions induced antiproliferative effects on HSG cells without any corresponding decrease in SERCA. We suggest that the IFN-gamma-induced decrease in the level of SERCA accounts for the depleted state of internal Ca2+ stores in cytokine-treated HSG cells. These data suggest a novel mechanism for the inhibition of HSG cell growth by IFN-gamma.