cDNA cloning, expression and chromosomal localization of the human sarco/endoplasmic reticulum Ca(2+)-ATPase 3 gene

ADP receptor P2Y12 is the capstone of the cross-talk between Ca2+ mobilization pathways dependent on Ca2+ ATPases sarcoplasmic/endoplasmic reticulum type 3 and type 2b in platelets

Background

Blood platelet Ca²⁺ stores are regulated by 2 Ca²⁺-ATPases (SERCA2b and SERCA3). On thrombin stimulation, nicotinic acid adenosine dinucleotide phosphate mobilizes SERCA3-dependent stores, inducing early adenosine 5'-diphosphate (ADP) secretion, potentiating later SERCA2b-dependent secretion.

Objectives

The aim of this study was to identify which ADP P2 purinergic receptor (P2Y1 and/or P2Y12) is(are) involved in the amplification of platelet secretion dependent on the SERCA3-dependent Ca²⁺ mobilization pathway (SERCA3 stores mobilization) as triggered by low concentration of thrombin.

Methods

The study used the pharmacologic antagonists MRS2719 and AR-C69931MX, of the P2Y1 and P2Y12, respectively, as well as Serca3 ^-/- mice and mice exhibiting platelet lineage-specific inactivation of the P2Y1 or P2Y12 genes.

Results

We found that in mouse platelets, pharmacological blockade or gene inactivation of P2Y12 but not of P2Y1 led to a marked inhibition of ADP secretion after platelet stimulation with low concentration of thrombin. Likewise, in human platelets, pharmacological inhibition of P2Y12 but not of P2Y1 alters amplification of thrombin-elicited secretion through SERCA2b stores mobilization. Finally, we show that early SERCA3 stores secretion of ADP is a dense granule secretion, based on parallel adenosine triphosphate and serotonin early secretion. Furthermore, early secretion involves a single granule, based on the amount of adenosine triphosphate released.

Conclusion

Altogether, these results show that at low concentrations of thrombin, SERCA3- and SERCA2b-dependent Ca²⁺ mobilization pathways cross-talk via ADP and activation of the P2Y12, and not the P2Y1 ADP receptor. The relevance in hemostasis of the coupling of the SERCA3 and the SERCA2b pathways is reviewed.

A Role for SERCA Pumps in the Neurobiology of Neuropsychiatric and Neurodegenerative Disorders

Calcium (Ca²⁺) is a fundamental regulator of cell fate and intracellular Ca²⁺ homeostasis is crucial for proper function of the nerve cells. Given the complexity of neurons, a constellation of mechanisms finely tunes the intracellular Ca²⁺ signaling. We are focusing on the sarco/endoplasmic reticulum (SR/ER) calcium (Ca²⁺)-ATPase (SERCA) pump, an integral ER protein. SERCA's well established role is to preserve low cytosolic Ca²⁺ levels ([Ca²⁺]_cyt), by pumping free Ca²⁺ ions into the ER lumen, utilizing ATP hydrolysis. The SERCA pumps are encoded by three distinct genes, SERCA1-3, resulting in 12 known protein isoforms, with tissue-dependent expression patterns. Despite the well-established structure and function of the SERCA pumps, their role in the central nervous system is not clear yet. Interestingly, SERCA-mediated Ca²⁺ dyshomeostasis has been associated with neuropathological conditions, such as bipolar disorder, schizophrenia, Parkinson's disease and Alzheimer's disease. We summarize here current evidence suggesting a role for SERCA in the neurobiology of neuropsychiatric and neurodegenerative disorders, thus highlighting the importance of this pump in brain physiology and pathophysiology.

Calcium handling proteins: structure, function, and modulation by exercise

Heart failure is a serious public health issue with a growing prevalence, and it is related with the aging of the population. Hypertension is identified as the main precursor of left ventricular hypertrophy and therefore can lead to diastolic dysfunction and heart failure. Scientific studies have confirmed the beneficial effects of the physical exercise by reducing the blood pressure and improving the functional status of the heart in hypertension. Several proteins are involved in the mobilization of calcium during the coupling excitation-contraction process in the heart among those are sarcoplasmic reticulum Ca(2+)-ATPase, phospholamban, calsequestrin, sodium-calcium exchanger, L-type calcium's channel, and ryanodine receptors. Our goal is to address the beneficial effects of exercise on the calcium handling proteins in a heart with hypertension.

Role of cis-acting elements in the control of SERCA2b Ca2+ pump mRNA decay by nuclear proteins

Alternative splicing at position 3495 b yields SERCA2 (sarco/endoplasmic reticulum Ca2+ pump 2) RNA species, namely SERCA2a and SERCA2b which differ in 3'-end regions. This results in SERCA2b RNA being less stable. In vitro decay experiments show that, in the presence of protein extracts from nuclei of LVMs (left ventricular myocytes), the rate of decay of both SERCA2b RNA and synthetic RNA from its 3'-region is greater than that of the corresponding SERCA2a RNA. To search for cis-acting instability elements in the 3'-region of SERCA2b, we examined the effects of LVM nuclear protein extracts on the in vitro decay of six short overlapping capped [m7G(5')ppp(5')Gm] and polyadenylated (A40) RNA fragments from the 3'-end region (3444-4472) of SERCA2b. The proximal fragment 2B1 (3444-3753) was the most unstable. 2B1 RNA without a cap or a polyadenylated tail was analysed further in electrophoretic mobility-shift assays, and was observed to bind to protein(s) in the nuclear extracts. Based on competition for binding to nuclear proteins between radiolabelled 2B1 RNA and short unlabelled RNA fragments, the cis-acting element involved in this binding was the sequence 2B1-4. 2B1-4 is a 35-base (3521-3555, CCAGUCCUGCUCGUUGUGGGCGUGCACCGAGGGGG) GC-rich region just past the splice site (3495). Nuclear extracts decreased the electrophoretic mobility of the radiolabelled 2B1-4 RNA which bound to two proteins (19 and 21 kDa) in cross-linking experiments. Excess 2B1-4 RNA decreased the decay of the 2B1 RNA by the nuclear protein extract. 2B1-del 4 RNA (2B1 with the 2B1-4 domain deleted) also decayed more slowly than the control 2B1 RNA. Thus SERCA2b contains a novel GC-rich cis-acting element involved in its decay by nuclear proteins.

How many Ca2+ATPase isoforms are expressed in a cell type? A growing family of membrane proteins illustrated by studies in platelets

Ca(2+) signaling plays a key role in normal and abnormal platelet functions. Understanding platelet Ca(2+) signaling requires the knowledge of proteins involved in this process. Among these proteins are Ca(2+)ATPases or Ca(2+) pumps that deplete the cytosol of Ca(2+) ions. Here, we will particularly focus on two Ca(2+) pump families: the plasma membrane Ca(2+)ATPases (PMCAs) that extrude cytosolic Ca(2+) towards the extracellular medium and the sarco/endoplasmic reticulum Ca(2+)ATPases (SERCAs) that pump Ca(2+) into the endoplasmic reticulum (ER). In the present review, we will summarize data on platelet Ca(2+)ATPases including their identification and biogenesis. First of all, we will present the Ca(2+)ATPase genes and their isoforms expressed in platelets. We will especially focus on a member of the SERCA family, SERCA3, recently found to give rise to a number of species-specific isoforms. Next, we will describe the differences in Ca(2+)ATPase patterns observed in human and rat platelets. Last, we will analyze how the expression of Ca(2+)ATPase isoforms changes during megakaryocytic maturation and show that megakaryocytopoiesis is associated with a profound reorganization of the expression and/or activity of Ca(2+)ATPases. Taken together, these data provide new aspects of investigations to better understand normal and abnormal platelet Ca(2+) signaling.

New perspectives on the role of SERCA2's Ca2+ affinity in cardiac function

Cardiomyocyte relaxation and contraction are tightly controlled by the activity of the cardiac sarco(endo)plasmic reticulum (SR) Ca2+ transport ATPase (SERCA2a). The SR Ca2+ -uptake activity not only determines the speed of Ca(2+) removal during relaxation, but also the SR Ca2+ content and therefore the amount of Ca2+ released for cardiomyocyte contraction. The Ca2+ affinity is the major determinant of the pump's activity in the physiological Ca2+ concentration range. In the heart, the affinity of the pump for Ca2+ needs to be controlled between narrow borders, since an imbalanced affinity may evoke hypertrophic cardiomyopathy. Several small proteins (phospholamban, sarcolipin) adjust the Ca2+ affinity of the pump to the physiological needs of the cardiomyocyte. It is generally accepted that a chronically reduced Ca2+ affinity of the pump contributes to depressed SR Ca2+ handling in heart failure. Moreover, a persistently lower Ca2+ affinity is sufficient to impair cardiomyocyte SR Ca2+ handling and contractility inducing dilated cardiomyopathy in mice and humans. Conversely, the expression of SERCA2a, a pump with a lower Ca2+ affinity than the housekeeping isoform SERCA2b, is crucial to maintain normal cardiac function and growth. Novel findings demonstrated that a chronically increased Ca2+ affinity also may trigger cardiac hypertrophy in mice and humans. In addition, recent studies suggest that some models of heart failure are marked by a higher affinity of the pump for Ca2+, and hence by improved cardiomyocyte relaxation and contraction. Depressed cardiomyocyte SR Ca2+ uptake activity may therefore not be a universal hallmark of heart failure.

SERCA2a in heart failure: role and therapeutic prospects

Ca(2+) is a key molecule controlling several cellular processes, from fertilization to cell death, in all cell types. In excitable and contracting cells, such as cardiac myocytes, Ca(2+) controls muscle contractility. The spatial and temporal segregation of Ca(2+) concentrations are central to maintain its concentration gradients across the cells and the cellular compartments for proper function. SERCA2a is a cornerstone molecule for maintaining a balanced concentration of Ca(2+) during the cardiac cycle, since it controls the transport of Ca(2+) to the sarcoplasmic reticulum (SR) during relaxation. Alterations of the activity of this pump have been widely investigated, emphasizing its central role in the control of Ca(2+) homeostasis and consequently in the pathogenesis of the contractile defect seen with heart failure. This review focuses on the molecular characteristics of the pump, its role during the cardiac cycle and the prospects derived from the manipulation of SERCA2a for heart failure treatment.

Expression des pompes calciques de type SERCA au cours de la différenciation cellulaire et de la tumorigenèse: application à la carcinogenèse colique

Calcium homeostasis of the endoplasmic reticulum (ER) is involved in intracellular signaling pathways and is implicated in major cell functions such as cell growth, differentiation, protein synthesis and apoptosis. The accumulation of calcium in the ER is performed by specific sarco/endoplasmic reticulum calcium transport ATPases (SERCA iso-enzymes). The expression of biochemically distinct SERCA isoforms is cell type dependent and developmentally regulated. This review summarizes pertinent data about the modulation of the expression of SERCA enzymes during the differentiation of normal and tumor cells. These data support the implication of SERCA pumps and especially SERCA3 in the differentiation program of cancer and leukemia cells. During the multi-step process of colon carcinogenesis, the decrease of SERCA3 expression seems to be linked to enhanced APC/ss-catenin/TCF4 signaling and deficient Sp1-like factor-dependent transcription.

Congenital and acquired platelet disorders: current dilemmas and treatment strategies

Platelets are important for primary hemostasis. When a blood vessel is damaged, platelets adhere to exposed subendothelial connective tissue and form a hemostatic plug. Formation of the plug is contingent upon a series of processes, with adhesion, activation, and aggregation all being involved. Patients with quantitative platelet disorders have reduced numbers of platelets. Patients with qualitative disorders have platelets that exhibit abnormal functioning. Defects that impair function can affect platelet receptors, secretory responses, or intracellular signaling pathways. Examples of qualitative platelet disorders include Glanzmann's thrombasthenia (GT) and Bernard-Soulier syndrome (BSS). The treatment of platelet disorders is primarily with platelet concentrates. However, in patients with abnormalities of their platelet surface receptors, platelet transfusion may provoke an immune response. Recombinant factor VIIa (rFVIIa) may provide hemostatically effective therapy in such patients.

Effects of peroxynitrite on sarco/endoplasmic reticulum Ca2+ pump isoforms SERCA2b and SERCA3a

Sarco(endo)plasmic reticulum Ca2+ (SERCA) pumps are important for cell signaling. Three different genes, SERCA1, 2, and 3, encode these pumps. Most tissues, including vascular smooth muscle, express a splice variant of SERCA2 (SERCA2b), whereas SERCA3a is widely distributed in tissues such as vascular endothelium, tracheal epithelium, mast cells, and lymphoid cells. SERCA2b protein is readily inactivated by peroxynitrite that may be formed during cardiac ischemia reperfusion or during immune response after infection. Here, we compared the peroxynitrite sensitivity of SERCA2b and SERCA3a by using microsomes prepared from HEK-293T cells overexpressing the pumps. We incubated the microsomes with different concentrations of peroxynitrite and determined Ca2+ uptake, Ca2+-Mg2+-ATPase, Ca2+-dependent formation of acylphosphate intermediate, and protein mobility in Western blots. Ca2+ uptake, Ca2+-Mg2+-ATPase, and Ca2+-dependent formation of acylphosphate intermediate were inactivated for both SERCA2b and SERCA3a, but the latter was more resistant to the inactivation. Western blots showed that SERCA2b and SERCA3a proteins oligomerized after treatment with peroxynitrite, but each with a slightly different pattern. Compared with monomers, the oligomers may be less efficient in forming the acylphosphate intermediate and in conducting the remainder of the steps in the reaction cycle. We conclude that the resistance of SERCA3a to peroxynitrite may aid the cells expressing them in functioning during exposure to oxidative stress.

Sarcoendoplasmic reticulum Ca2+ ATPase type 2 downregulated in human oral squamous cell carcinoma

Mice with a heterozygous deletion of the Atp2a2 gene (Atp2a2(+/-)) encoding SERCA2 spontaneously develop SCCs of the skin and upper digestive tract, including the oral cavity. To elucidate the contribution of ATP2A2 to human oral carcinogenesis, we analyzed genetic and epigenetic changes as well as mRNA and protein expression in primary OSCCs and OPLs. With the exception of one OSCC-derived cell line showing a 12 bp deletion of ATP2A2, we found no mutations in the coding sequence of the gene in primary OSCCs (n = 52), OPLs (n = 32) and cell lines (n = 8). In immunohistochemistry, however, high frequencies of ATP2A2 downregulation were evident not only in primary OSCCs (42%, 42/100) but also in OPLs (31%, 10/32). Real-time quantitative RT-PCR data were consistent with the protein expression status. Aberrant DNA methylation within ATP2A2 also was detected in 9 of 30 ATP2A2-downregulated OSCCs. Moreover, restoration or elevated expression of the ATP2A2 protein was induced in most of the cell lines showing ATP2A2 methylation after treatment with 5-aza-2'-dC, a DNA demethylating agent. These results suggest that inactivation of the ATP2A2 gene is a frequent and early event during oral carcinogenesis and that loss of expression may be regulated partly by an epigenetic mechanism.

Sarco/endoplasmic-reticulum calcium ATPase SERCA1 is maintained in the endoplasmic reticulum by a retrieval signal located between residues 1 and 211

The location of sarco/endoplasmic-reticulum calcium ATPase (SERCA) retention/retrieval motifs in the sequence of the SERCA1 has been investigated by examining the subcellular location in COS-7 cells of enhanced-green-fluorescent-protein-tagged calcium-pump chimaeras. These chimaeras have been constructed from the fast-twitch SERCA1 and the plasma-membrane calcium ATPase PMCA3. The N-terminal, central and C-terminal segments of these calcium pumps were exchanged between SERCA1 and PMCA3. The segments exchanged correspond to residues 1-211, 212-711 and 712-994 of SERCA1, and residues 1-264, 265-788 and 789-1159 of PMCA3 respectively. Only chimaeras containing the N-terminal segment of SERCA1 were located in the endoplasmic reticulum (ER), whereas chimaeras containing the N-terminal segment from PMCA3 were able to escape from the ER and enter the endomembrane pathway en route for the plasma membrane. Co-localization of SERCA1 in COS-7 cells with the ER/Golgi-intermediate compartment marker ERGIC53 indicates that SERCA1 is maintained in the ER by a process of retrieval. These results indicate that the N-terminal region of SERCA1, containing transmembrane helices M1 and M2, contains an ER-retrieval signal.

Acrokeratosis verruciformis of Hopf is caused by mutation in ATP2A2: evidence that it is allelic to Darier's disease

Acrokeratosis verruciformis of Hopf is a localized disorder of keratinization affecting the distal extremities. Onset is early in life and the disease is inherited in an autosomal dominant fashion. Although histology of acrokeratosis verruciformis lesions shows no evidence of dyskeratosis, a possible relationship with Darier's disease has long been postulated on the basis of clinical similarity. ATP2A2 encoding the sarco(endo)plasmic reticulum Ca2+ ATPase2 pump has been identified as the defective gene in Darier's disease. In this report, we studied a family affected with acrokeratosis verruciformis in six generations and identified a heterozygous P602L mutation in ATP2A2. This mutation predicts a nonconservative amino acid substitution in the ATP binding domain of the molecule. The mutation segregates with the disease phenotype in the family and was not found in 50 controls. Moreover, functional analysis of the P602L mutant showed that it has lost its ability to transport Ca2+. This result demonstrates loss of function of the sarco(endo)plasmic reticulum Ca2+ ATPase2 mutant in acrokeratosis verruciformis, thus providing evidence that acrokeratosis verruciformis and Darier's disease are allelic disorders.

Dissection of the functional differences between sarco(endo)plasmic reticulum Ca2+-ATPase (SERCA) 1 and 3 isoforms by steady-state and transient kinetic analyses

Steady-state and transient-kinetic studies were conducted to characterize the overall and partial reactions of the Ca(2+)-transport cycle mediated by the human sarco(endo)plasmic reticulum Ca(2+)-ATPase 3 (SERCA3) isoforms: SERCA3a, SERCA3b, and SERCA3c. Relative to SERCA1a, all three human SERCA3 enzymes displayed a reduced apparent affinity for cytosolic Ca(2+) in activation of the overall reaction due to a decreased E(2) to E(1)Ca(2) transition rate and an increased rate of Ca(2+) dissociation from E(1)Ca(2). At neutral pH, the ATPase activity of the SERCA3 enzymes was not significantly enhanced upon permeabilization of the microsomal vesicles with calcium ionophore, indicating a difference from SERCA1a with respect to regulation of the lumenal Ca(2+) level (either an enhanced efflux of lumenal Ca(2+) through the pump in E(2) form or insensitivity to inhibition by lumenal Ca(2+)). Other differences from SERCA1a with respect to the overall ATPase reaction were an alkaline shift of the pH optimum, increased catalytic turnover rate at pH optimum (highest for SERCA3b, the isoform with the longest C terminus), and an increased sensitivity to inhibition by vanadate that disappeared under equilibrium conditions in the absence of Ca(2+) and ATP. The transient-kinetic analysis traced several of the differences from SERCA1a to an enhancement of the rate of dephosphorylation of the E(2)P phosphoenzyme intermediate, which was most pronounced at alkaline pH and increased with the length of the alternatively spliced C terminus.

Molecular physiology of the SERCA and SPCA pumps

Intracellular Ca(2+)-transport ATPases exert a pivotal role in the endoplasmic reticulum and in the compartments of the cellular secretory pathway by maintaining a sufficiently high lumenal Ca(2+) (and Mn(2+)) concentration in these compartments required for an impressive number of vastly different cell functions. At the same time this lumenal Ca(2+) represents a store of releasable activator Ca(2+) controlling an equally impressive number of cytosolic functions. This review mainly focuses on the different Ca(2+)-transport ATPases found in the intracellular compartments of mainly animal non-muscle cells: the sarco(endo)plasmic reticulum Ca(2+)-ATPase (SERCA) pumps. Although it is not our intention to treat the ATPases of the specialized sarcoplasmic reticulum in depth, we can hardly ignore the SERCA1 pump of fast-twitch skeletal muscle since its structure and function is by far the best understood and it can serve as a guide to understand the other members of the family. In a second part of this review we describe the relatively novel family of secretory pathway Ca(2+)/Mn(2+) ATPases (SPCA), which in eukaryotic cells are primarily found in the Golgi compartment.

All three splice variants of the human sarco/endoplasmic reticulum Ca2+-ATPase 3 gene are translated to proteins: a study of their co-expression in platelets and lymphoid cells

The molecular cloning of two previously unknown human sarco/endoplasmic reticulum Ca(2+)-ATPase 3 (SERCA3) 3'-end transcripts, 3b and 3c, has been recently published. Data were lacking, however, for the presence of these SERCA3 variants in different tissue or cell types at the protein level. Here we report the co-expression of three human SERCA3 protein isoforms in platelets and T lymphoid Jurkat cells. Isoform-specific polyclonal anti-peptide antibodies have been generated that recognize specifically the SERCA3a, 3b or 3c splice variants at their C-termini, and this has been confirmed by peptide-competition experiments as well. None of these antibodies cross-reacted with the housekeeping SERCA2b isoform co-expressed endogenously with SERCA3 proteins in non-muscle cells. Although all three SERCA3 isoforms could be detected in platelets, the 3a form was the most abundantly expressed species. Its size matched the apparent size of SERCA3a over-expressed in HEK-293 cells. Immunoprecipitation of the SERCA3 variants from platelet membranes using a PL/IM 430-affinity matrix provided evidence that the putative pan-anti-SERCA3 antibody, PL/IM 430, recognizes all SERCA3 protein isoforms. The epitope for the PL/IM 430 antibody could be localized in a 40 kDa N-terminal tryptic fragment common to all three SERCA3 variants. Comparative Western-blot analysis showed that the expression level of the SERCA3a, 3b and 3c isoforms was more than 10 times lower in Jurkat cells than in platelets, whereas expression of the ubiquitous SERCA2b was nearly identical. This work highlights new Ca(2+)-transporting proteins of haematopoietic cells and provides specific antibodies for their detection.

Sarco(endo)plasmic reticulum calcium pumps: recent advances in our understanding of structure/function and biology (review)

This review examines the structure and function of the sarco(endo)plasmic reticulum calcium pump (SERCA1a) in the light of the recent publication of the 2.6 A resolution structure of this protein, and looks at the increasing awareness of the key role played by SERCAs in calcium signalling. The roles played by the calcium pump isoforms, SERCA1a/b, SERCA2a/b and SERCA3a/b/c in cellular function are discussed, and the modulation of SERCA activity by phospholamban, sarcolipin and other modulatory influences is examined. The recent discoveries of human SERCA mutations leading to disease states is reviewed, and the insights into SERCA function using transgenic approaches are outlined.

Biogenesis of endoplasmic reticulum proteins involved in Ca2+ signalling during megakaryocytic differentiation: an in vitro study

The endoplasmic reticulum (ER) plays a key role in Ca(2+) signalling through Ca(2+) release via inositol 1,4,5-trisphosphate receptors (InsP(3)-Rs) and Ca(2+) uptake by sarco/endoplasmic reticulum Ca(2+)-ATPases (SERCAs). Here, we investigated the organization of platelet ER and its biogenesis during megakaryocytopoiesis. First, erythro/megakaryoblastic MEG 01, UT7, M-O7e and CHRF 288-11 cell lines, platelets and thrombopoietin-induced UT7-Mpl cells were selected for the study of SERCA2b and SERCA3 proteins by Western blotting using the antibodies IID8 and PL/IM430, respectively. As judged by platelet glycoprotein IIIa (GPIIIa) expression, an increase in SERCA3 proteins was observed while that of SERCA2b remained unchanged throughout maturation. Second, these studies were extended to the newly described alternatively spliced SERCA3a-c RNAs and InsP(3)-Rs using the in vitro model of PMA-induced differentiation of MEG 01 cells. Time-course and dose-response studies showed a maximal approx. 4-fold up-regulation of SERCA3 proteins using 10(-8) M PMA for 3 days, which paralleled induction of GPIIIa expression. SERCA3 induction was found to occur at the level of mRNA. The modulation of the different SERCA3 species (i.e. 3a, 3b and 3c) was isoform-specific: while SERCA3a was slightly increased, an approx. 3-fold induction of SERCA3b, and a 4-fold induction of SERCA3c, was observed after 24 h of PMA treatment. Isoform-specific Western blotting and/or reverse transcriptase PCR studies showed that InsP(3)-R types I, II and III are expressed in MEG 01 cells, as well as in platelets. Study of the expression of these InsP(3)-R types in PMA-induced MEG 01 cells revealed that: (i) InsP(3)-RI protein and mRNA showed no changes; (ii) InsP(3)-RII mRNA was up-regulated and peaked at hour 48 and (iii) InsP(3)-RIII mRNA and protein showed a transitory maximal 3- and 2.3-fold increase at hours 6 and 30, respectively. Upon PMA treatment of CHRF 288-11 cells, in which GPIIIa is not induced upon treatment, a similar pattern of regulation of InsP(3)-R types II and III was seen, but a distinct pattern of SERCA3 regulation was observed. These results suggest a profound reorganization of ER-protein patterns during megakaryocytopoiesis and underline the role of SERCA3 gene regulation in the control of Ca(2+)-dependent platelet functions.

Calcium homeostatic pathways change with gestation in human myometrium

A rise in intracellular calcium is the primary trigger for contractile activity in pregnant human myometrium. It is hypothesized that key proteins involved in myometrial calcium homeostasis are gestationally regulated and play an important role in the preparation for labor. The aims of the study were to investigate the role of sarcoplasmic reticulum Ca ATPases (SERCAs) in regulating spontaneous contractile activity in myometrium, and to determine the expression of SERCA isoforms 2a and 2b, and the plasma membrane Ca ATPase (PMCA), at term and during labor. Western blot analysis demonstrated that the expression of SERCA 2a and 2b significantly increased in myometrium of women in labor compared with those not in labor. The augmentation of contractile activity in laboring myometrium in the presence of a SERCA 2 inhibitor, cyclopiazonic acid (CPA), demonstrated the functional significance of this observation. It is interesting that the application of CPA in the presence of a calcium-activated potassium channel inhibitor to term nonlabor myometrium mimicked the response of myometrium from women in active labor to CPA alone. We conclude that the activity of SERCA isoforms becomes increasingly important in the maintenance of regular contractile activity during labor and may compensate for the functional loss of other calcium control pathways at term.

Function and regulation of sarcoplasmic reticulum Ca2+-ATPase: advances during the past decade and prospects for the coming decade

In cardiac muscle, the contraction-relaxation cycle is tightly controlled by the regulated release and uptake of intracellular Ca2+ between sarcoplasmic reticulum and cytoplasm. A major protein controlling Ca2+ cycling is Ca2+-ATPase (SERCA2a) located in the sarcoplasmic reticulum membrane. The function of SERCA2a protein is regulated by the phosphorylatable protein, phospholamban. Phosphorylation of phospholamban releases its inhibitory effect on SERCA2a through direct molecular interaction. Recently, mice whose SERCA2a function is increased (overexpression of the gene) or lost (knock out) were developed. These mice demonstrated that SERCA2a pump levels are a major determinant of cardiac muscle contractility and relaxation. These studies open the prospect that the overexpression of SERCA2a can correct cardiac dysfunction seen in heart failure. Advances in knowledge concerning the function and gene regulation of SERCA2a are discussed in this review.

Sarco/endoplasmic reticulum Ca2+-pump isoform SERCA3a is more resistant to superoxide damage than SERCA2b

Endo/sarcoplasmic reticulum (ER) Ca2+-pumps are important for cell survival and communication but they are inactivated by reactive oxygen species (ROS). We have previously reported that the Ca2+-pump isoform SERCA3a is more resistant than SERCA2b to damage by peroxide. Since peroxide and superoxide differ in their redox potentials, we now report the effects of superoxide on the two Ca2+-pump isoforms. We isolated microsomes from HEK293 cells transiently transfected with SERCA2b or SERCA3a cDNA. We exposed these microsomes to superoxide which was generated using xanthine plus xanthine oxidase and catalase to prevent accumulation of peroxide due to superoxide dismutation. Superoxide damaged the Ca2+-transport activity of both isoforms but SERCA3a was damaged at higher concentrations of superoxide and upon longer periods of exposures than was SERCA2b. Thus the SERCA3a isoform is more resistant than SERCA2b to inactivation by both superoxide and peroxide.

Identification of avian sarcoplasmic reticulum Ca(2+)-ATPase (SERCA3) as a novel 1,25(OH)(2)D(3) target gene in the monocytic lineage

Osteoclasts are postmitotic, multinucleated giant cells generated by the fusion of hematopoietic mononuclear precursors from the monocyte-macrophage lineage. In culture, adherent macrophages from blood-derived monocytes grow, gather, and fuse together to form multinucleated osteoclast-like cells. These events are controlled by 1,25(OH)(2)D(3). To sort out new 1,25(OH)(2)D(3) target genes involved in osteoclast differentiation, we have performed an RT-PCR differential display using mRNA from macrophages induced for 10 h by 1,25(OH)(2)D(3) compared to nontreated cells. We have identified a new target gene, a chick ATP-dependent Ca(2+) pump, ChkSERCA3. Although the level of the corresponding transcript increases during the differentiation process from macrophages to osteoclast-like cells, its steady-state level is downregulated by hormone treatment. The action of 1,25(OH)(2)D(3) on the Ca(2+)-ATPase gene expression is independent of de novo protein synthesis and is hormone dose dependent. This expression in adult chick was restricted to the hematopoietic cell lineage, spleen, lung, intestine, and brain, whereas no expression was detected in embryos. The function of the protein can be predicted from its high homology with the other members of the SR ATP-dependent Ca(2+) pump family, i.e., storage and control of cytosolic Ca(2+) directly regulated by 1, 25(OH)(2)D(3), further supporting the critical role for intracellular calcium in highly specialized cells such as osteoclasts.

Mechanisms involved in the cellular calcium homeostasis in vascular smooth muscle: calcium pumps

The regulation of cytosolic Ca2+ homeostasis is essential for cells, and particularly for vascular smooth muscle cells. In this regulation, there is a participation of different factors and mechanisms situated at different levels in the cell, among them Ca2+ pumps play an important role. Thus, Ca2+ pump, to extrude Ca2+; Na+/Ca2+ exchanger; and different Ca2+ channels for Ca2+ entry are placed in the plasma membrane. In addition, the inner and outer surfaces of the plasmalemma possess the ability to bind Ca2+ that can be released by different agonists. The sarcoplasmic reticulum has an active role in this Ca2+ regulation; its membrane has a Ca2+ pump that facilitates luminal Ca2+ accumulation, thus reducing the cytosolic free Ca2+ concentration. This pump can be inhibited by different agents. Physiologically, its activity is regulated by the protein phospholamban; thus, when it is in its unphosphorylated state such a Ca2+ pump is inhibited. The sarcoplasmic reticulum membrane also possesses receptors for 1,4,5-inositol trisphosphate and ryanodine, which upon activation facilitates Ca2+ release from this store. The sarcoplasmic reticulum and the plasmalemma form the superficial buffer barrier that is considered as an effective barrier for Ca2+ influx. The cytosol possesses different proteins and several inorganic compounds with a Ca2+ buffering capacity. The hypothesis of capacitative Ca2+ entry into smooth muscle across the plasma membrane after intracellular store depletion and its mechanisms of inhibition and activation is also commented.

Functional characterization of alternatively spliced human SERCA3 transcripts

The sarcoplasmic (or endoplasmic) reticulum Ca2+-ATPase (SERCA)-3 has been implicated in the possible dysregulation of Ca2+ homeostasis that accompanies the pathology of hypertension and diabetes. We report the molecular cloning of two alternatively spliced transcripts from the human SERCA3 gene, ATP2A3, that encode proteins that differ at their carboxy termini by 36 amino acids. SERCA3 transcripts were most abundantly expressed in lymphoid tissues, intestine, pancreas, and prostate. The two human SERCA3 proteins encoded by alternatively spliced transcripts were recognized by the monoclonal antibody PL/IM430 and demonstrated Ca2+ uptake and ATPase activity with an apparent Ca2+ affinity 0.5 pCa unit lower than that of other SERCA gene products. The subcellular distribution of SERCA3 protein was indistinguishable from that of SERCA2b, with expression in the nuclear envelope and in the endoplasmic reticulum throughout the cell. Two variant SERCA3 constructs, huS3-I and huS3-II, were isolated that encode proteins with three amino acid differences: Ala-673 (in huS3-I) substituted for Thr (in huS3-II), Ile-817 substituted for Met, and an insertion of Glu-994. huS3-I displayed a 10-fold lower capacity to transport Ca2+ than huS3-II.

Schistosoma mansoni Ca2+-ATPase SMA2 restores viability to yeast Ca2+-ATPase-deficient strains and functions in calcineurin-mediated Ca2+ tolerance

The sarco(endo)plasmic reticulum of animal cells contains an ATP-powered Ca2+ pump that belongs to the P-type family of membrane-bound cation-translocating enzymes. In Schistosoma mansoni, the sarco(endo)plasmic reticulum Ca2+-ATPase (SERCA) is encoded by the SMA1 and SMA2 genes. A full-length SMA2 cDNA clone was isolated, sequenced, and expressed into a yeast Ca2+-ATPase-deficient strain requiring plasmid-borne rabbit SERCA1a for viability. The S. mansoni Ca2+-ATPase supports growth of mutant cells lacking SERCA1a, indicating functional expression in yeast and a role in calcium sequestration. Subcellular fractionation showed that the SMA2 ATPase is localized in yeast internal membranes. SMA2 expression was found to be associated with thapsigargin-sensitive, Ca2+-dependent ATPase activity. The activity increased 2-fold upon calcineurin inactivation, which correlates with in vivo stimulated contribution of SMA2 in calcium tolerance. These results suggest that calcineurin controls calcium homeostasis by inhibiting Ca2+-ATPase activity in an internal compartment.

Expression of sarcoplasmic/endoplasmic reticulum Ca2+ ATPases in the rat extensor digitorum longus (EDL) muscle regenerating from notexin-induced necrosis

The level of sarcoplasmic/endoplasmic reticulum Ca2+ ATPase (SERCA) mRNAs and proteins have been assessed by RT-PCR, immunoblotting and immunocytochemistry in the rat extensor digitorum longus (EDL) muscles during regeneration from notexin-induced necrosis. As a result of the necrosis, SERCA1 and SERCA2 declined on days 1 and 3 after administration of the toxin. Thereupon the mRNA of the fast isoform SERCA1 rapidly increased between days 5 and 10 to the normal level. The mRNA level of the "housekeeping" SERCA2b isoform increased markedly during the actual necrosis at days 1 and 5, probably due to invading cells. Then the mRNA level of the neonatal SERCA1b splice variant increased first, and exceeded the level of the adult SERCA1a transcript on day 5. At later stages of regeneration the neonatal form was gradually replaced by the adult SERCA1a form, thus recapitulating similar changes known to occur during normal ontogenesis. Along with SERCA1, the levels of the slow isoform (SERCA2a) mRNA and protein increased on day 5, but the SERCA2a mRNA levels never rose above 10% of SERCA1 and after 10 days gradually declined again. In the normal and regenerated muscles, SERCA1 was expressed in 97% of the fibres which accounted for the population of fast-twitch fibres (type IIa, type IIb and probably type IIx/d). SERCA2a was present in 6% of the fibres of normal muscle (mostly in the slow-twitch type I fibres). At the end of regeneration the number of fibres expressing SERCA2a was twice as high and were found in small groups, unlike in normal EDL, but about 50% of these clustered fibres also expressed SERCA1.

Molecular characterization of a sarco(endo)plasmic reticulum Ca2+-ATPase gene from Paramecium tetraurelia and localization of its gene product to sub-plasmalemmal calcium stores

A cDNA encoding the gene for a sarco(endo)plasmic reticulum-type Ca2+-ATPase (SERCA) was isolated from a cDNA library of Paramecium tetraurelia by using degenerated primers according to conserved domains of SERCA-type ATPases. The identified nucleotide sequence (PtSERCA) is 3114 nucleotides in length with an open reading frame of 1037 amino acids. An intron of only 22 nucleotides occurs. Homology searches for the deduced amino acid sequence revealed 38-49% similarity to SERCA-type ATPases from organisms ranging from protozoans to mammals, with no more similarity to some parasitic protozoa of the same phylum. The calculated molecular mass of the encoded protein is 114.7 kDa. It contains the typical 10 transmembrane domains of SERCA-type ATPases and other conserved domains, such as the phosphorylation site and the ATP binding site. However, there are no binding sites for phospholamban and thapsigargin present in the PtSERCA. Antibodies raised against a cytoplasmic loop peptide between the phosphorylation site and the ATP binding site recognize on Western blots a protein of 106 kDa, exclusively in the fraction of sub-plasmalemmal calcium stores ('alveolar sacs'). In immunofluorescence studies the antibodies show labelling exclusively in the cell cortex of permeabilized cells in a pattern characteristic of the arrangement of alveolar sacs. When alveolar sacs where tested for phosphoenzyme-intermediate formation a phosphoprotein of the same molecular mass (106 kDa) could be identified.

Structure of the human sarco/endoplasmic reticulum Ca2+-ATPase 3 gene. Promoter analysis and alternative splicing of the SERCA3 pre-mRNA

Human chromosome 17-specific genomic clones extending over 90 kilobases (kb) of DNA and coding for sarco/endoplasmic reticulum Ca2+-ATPase 3 (SERCA3) were isolated. The presence of the D17S1828 genetic marker in the cosmid contig enabled us to map the SERCA3 gene (ATP2A3) 11 centimorgans from the top of the short arm p of chromosome 17, in the vicinity of the cystinosis gene locus. The SERCA3 gene contains 22 exons spread over 50 kb of genomic DNA. The exon/intron boundaries are well conserved between human SERCA3 and SERCA1 genes, except for the junction between exons 8 and 9 which is found in the SERCA1 gene but not in SERCA3 and SERCA2 genes. The transcription start site (+1) is located 152 nucleotides (nt) upstream of the AUG codon. The 5'-flanking region, including exon 1, is embedded in a 1.5-kb CpG island and is characterized by the absence of a TATA box and by the presence of 14 putative Sp1 sites, 11 CACCC boxes, 5 AP-2-binding motifs, 3 GGCTGGGG motifs, 3 CANNTG boxes, a GATA motif, as well as single sites for Ets-1, c-Myc, and TFIIIc. Functional promoter analysis indicated that the GC-rich region (87% G + C) from -135 to -31 is of critical importance in initiating SERCA3 gene transcription in Jurkat cells. Exon 21 (human, 101 base pairs; mouse, 86 base pairs) can be alternatively excluded, partially included, or totally included, thus generating, respectively, SERCA3a (human and mouse, 999 amino acids (aa)), SERCA3b (human, 1043 aa; mouse, 1038 aa), or SERCA3c (human, 1024 aa; mouse, 1021 aa) isoforms with different C termini. Expression of the mouse SERCA3 isoforms in COS-1 cells demonstrated their ability to function as active pumps, although with different apparent affinities for Ca2+.

Characterization of the 3' end of the mouse SERCA 3 gene and tissue distribution of mRNA spliced variants

The sarco(endo)plasmic reticulum Ca2+ ATPase (SERCA) type 1 and 2 genes are alternatively spliced at their 3' end. We hypothesized that similar mechanism may occur for SERCA 3. Two spliced variants were identified by RNase protection analysis. We then isolated and sequenced the 3' end portion of the mouse SERCA 3 gene, and confirmed the presence of an alternative mRNA transcript by sequencing a cDNA fragment obtained by RT-PCR. Tissue distribution of the alternatively spliced mRNAs was studied by RT-PCR: SERCA 3b was the only isoform expressed in endothelial cells from aorta and heart and also was the major isoform in lung and kidney whereas SERCA 3a and 3b were coexpressed in trachea, intestine, thymus, spleen, and fetal liver.

The exon-intron architecture of human chloride channel genes is not conserved

The human CLCN6 gene contains a 167 bp exon that is optionally included or excluded in ClC-6 mRNAs. The corresponding region (3.4 kbp) of the human CLCN7 gene has now been cloned and sequenced. A comparison of the human CLCN1, CLCN5, CLCN6 and CLCN7 genes indicates that there is no homologue of the optional CLCN6 exon in the CLCN1, CLCN5 or CLCN7 genes. Thus, the CLCN6 type of alternative splicing and the ensuing structural diversity is not conserved within the CLC gene family.

cDNA cloning and predicted primary structure of scallop sarcoplasmic reticulum Ca(2+)-ATPase

Sarcoplasmic reticulum (SR) Ca(2+)-ATPase of the scallop cross-striated adductor muscle was purified with deoxycholate and digested with lysyl endopeptidase for sequencing of the digested fragments. Overlapping cDNA clones of the ATPase were isolated by screening the cDNA library with an RT-PCR product as a hybridization probe, which encodes the partial amino acid sequence of the ATPase. The predicted amino acid sequence of the ATPase contained all the partial sequences determined with the proteolytic fragments and consisted of the 993 residues with approximately 70% overall sequence similarity to those of the SR ATPases from rabbit fast-twitch and slow-twitch muscles. An outline of the structure of the scallop ATPase molecule is predicted to mainly consist of ten transmembrane and five 'stalk' domains with two large cytoplasmic regions as observed with the rabbit ATPase molecules. The sequence relationship between scallop and other sarco/endoplasmic reticulum-type Ca(2+)-ATPases is discussed.

Distribution and isoform diversity of the organellar Ca2+ pumps in the brain

The gene family of organellar-type Ca2+ transport ATPases consists of three members. SERCA1 is expressed exclusively in fast skeletal muscle; SERCA2 is ubiquitously expressed, whereas SERCA3 is considered to be mainly expressed in cells of the hematopoietic lineage and in some epithelial cells. In the brain, the organellar-type Ca2+ transport ATPases are almost exclusively transcribed from the SERCA2 gene. Four different SERCA2 mRNAs have been described (classes 1-4). However, unlike in nonneuronal cells, which express the class 1, 2, and 3 splice variants, the main SERCA2 mRNA in the brain is the class 4 messenger. Similar to classes 2 and 3, the class 4 codes for the ubiquitously expressed SERCA2b protein. Recently, we have reported the distribution of the SERCA isoforms in the brain (Baba-Aissa et al., 1996a,b). SERCA2b was present in most neurons of all investigated brain regions. The highest levels were found in the Purkinje neurons of the cerebellum and in the pyramidal cells of the hippocampus. Interestingly, SERCA3 and SERCA2a are coexpressed along with SERCA2b in the Purkinje neurons, but are weakly expressed in the other brain regions if present at all. Since these three protein isoforms have a different affinity for Ca2+, their possible roles in relation to Ca2+ stores in neurons are discussed.

Expression of two isoforms of the third sarco/endoplasmic reticulum Ca2+ ATPase (SERCA3) in platelets. Possible recognition of the SERCA3b isoform by the PL/IM430 monoclonal antibody

Human platelets express several sarco/endoplasmic reticulum Ca2+ ATPase (SERCA) isoenzymes: SERCA2b of 100 kDa apparent molecular mass and two distinct enzymes of 97 kDa, one of them identified as being the SERCA3a isoform. The molecular identity of the third enzyme specifically recognized by the PL/IM430 monoclonal antibody has remained elusive. First, the study of the 3'-end part of platelet SERCA3 mRNA, by means of RT-PCR amplification using sets of primers covering the N-3 to N (ultimate) exons of the human SERCA3 sequence, revealed the presence of two distinct mRNA sequences, SERCA3a and a longer variant. Second, this additional sequence was identified as SERCA3b and found to refer to the insertion of a new exon of 73 bp, located at bp 349 from the beginning of the intronic sequence, linking the penultimate (N-1) exon to the last exon (N) of the human SERCA3 gene. Third, a relationship between the expression of this SERCA3b mRNA and the PL/ IM430 recognizable SERCA protein was observed. SERCA3b mRNA was found to be absent in epithelial HeLa cells not recognized by the PL/IM430 antibody and the expression of this SERCA3b RNA species correlated with that of the SERCA protein recognized by PL/IM430 which was down-modulated in the platelet precursor megakaryocytic CHRF 288-11 cell line as well as upon in vitro lymphocyte activation. Taken together, these results strongly support the notion of the presence of the SERCA3b protein in human cells by showing SERCA3b mRNA in platelets and the fact that the protein corresponding to this mRNA species is very likely the 97 kDa protein recognized by the PL/IM430 antibody.

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.

Defective endothelium-dependent relaxation of vascular smooth muscle and endothelial cell Ca2+ signaling in mice lacking sarco(endo)plasmic reticulum Ca2+-ATPase isoform 3

Sarco(endo)plasmic reticulum Ca2+ ATPase isoform 3 (SERCA3) is one of two Ca2+ pumps serving intracellular Ca2+ signaling pools in non-muscle tissues; however, unlike the ubiquitous SERCA2b, it exhibits a restricted cell-type distribution. Gene targeting was used to generate a mouse with a null mutation in the SERCA3 gene. Homozygous mutant mice were viable, fertile, and did not exhibit an overt disease phenotype. Because SERCA3 is expressed in arterial endothelial cells, aortic ring preparations were analyzed to determine whether it is involved in the regulation of vascular tone. Contraction-isometric force relations in response to phenylephrine or KCl, as well as relaxation produced by exposure to a nitric oxide donor, were similar in wild-type and null mutant aortas. Acetylcholine-induced endothelium-dependent relaxation of aortas after precontraction with phenylephrine was significantly reduced in homozygous mutants (61.3 +/- 5.6% in wild type, 35.4 +/- 7.3% in mutants). Ca2+ imaging of cultured aortic endothelial cells demonstrated that the acetylcholine-induced intracellular Ca2+ signal is sharply diminished in SERCA3-deficient cells and also indicated that replenishment of the acetylcholine-responsive Ca2+ stores is severely impaired. These results indicate that SERCA3 plays a critical role in endothelial cell Ca2+ signaling events involved in nitric oxide-mediated relaxation of vascular smooth muscle.

Mutation Lys758 --> Ile of the sarcoplasmic reticulum Ca2+-ATPase enhances dephosphorylation of E2P and inhibits the E2 to E1Ca2 transition

The highly conserved lysine residue Lys758 in the fifth stalk segment of the sarcoplasmic reticulum Ca2+-ATPase was substituted with either isoleucine or arginine by site-directed mutagenesis. The substitution with arginine was without significant effects on Ca2+-ATPase function, whereas multiple changes of functional characteristics were observed with the Lys758 --> Ile mutant. These included insensitivity of ATPase activity to the calcium ionophore A23187, an alkaline shift of the pH dependence of ATPase activity, reduced maximum molecular turnover rate and steady-state phosphorylation level, reduced apparent affinities for Ca2+ and inorganic phosphate, as well as increased sensitivity to inhibition by vanadate. Analysis of the partial reaction steps of the enzyme cycle traced these changes to two steps. The rate of dephosphorylation of the ADP-insensitive phosphoenzyme intermediate (E2P) was increased, irrespective of variations of pH, K+, Ca2+, and dimethyl sulfoxide concentration. In addition, the rate of conversion of the dephosphoenzyme with low Ca2+ affinity (E2) to the Ca2+-bound form activated for phosphorylation (E1Ca2) was reduced in the mutant, and the ATP-induced rate enhancement of this step required higher ATP concentrations in the mutant compared with the wild type.

Modulation of endoplasmic reticulum calcium pump expression during T lymphocyte activation

Calcium mobilization from intracellular storage organelles is a key component of the second messenger system inducing cell activation. Calcium transport ATPases associated with intracellular calcium storage organelles play a major role in controlling this process by accumulating calcium from the cytosol into intracellular calcium pools. In this study the modulation of the expression of the sarco-endoplasmic reticulum calcium transport ATPase (SERCA) isoenzymes has been studied in lymphocytes undergoing phorbol myristate acetate and ionomycin-induced activation. In several T lymphocyte cell lines a combined treatment by the two drugs resulted in an approximately 90% decrease of the expression of the calcium pump isoform recognized by the PLIM430 isoform-specific antibody, whereas the expression of the SERCA 2b isoform was increased approximately 2-fold. Phorbol ester or ionomycin applied separately was ineffective. In Jurkat T cells the down-modulation of expression of the SERCA isoform recognized by the PLIM430 antibody appeared concomitantly with the induction of interleukin-2 expression and could be inhibited by the immunosuppressant drug cyclosporine-A. These data indicate that T cell activation induces a selective and cyclosporine-A-sensitive modulation of the expression of the SERCA calcium pump isoforms. This reflects a profound reorganization of the calcium homeostasis of T cells undergoing activation and may open new avenues in the understanding of the plasticity of the calcium homeostasis of differentiating cells and in the pharmacological modulation of lymphocyte function.