Calcineurin controls the expression of isoform 4CII of the plasma membrane Ca(2+) pump in neurons

Altered functional connectivity and network excitability in a model of cortical dysplasia

Focal cortical dysplasias (FCDs) are malformations of cortical development that often result in medically refractory epilepsy, with a greater incidence in the pediatric population. The relationship between the disturbed cortical morphology and epileptogenic activity of FCDs remains unclear. We used the BCNU (carmustine 1-3-bis-chloroethyl-nitrosourea) animal model of cortical dysplasia to evaluate neuronal and laminar alterations and how these result in altered activity of intracortical networks in early life. We corroborated the previously reported morphological anomalies characteristic of the BCNU model, comprising slightly larger and rounder neurons and abnormal cortical lamination. Next, the neuronal activity of live cortical slices was evaluated through large field-of-view calcium imaging as well as the neuronal response to a stimulus that leads to cortical hyperexcitability (pilocarpine). Examination of the joint activity of neuronal calcium time series allowed us to identify intracortical communication patterns and their response to pilocarpine. The baseline power density distribution of neurons in the cortex of BCNU-treated animals was different from that of control animals, with the former showing no modulation after stimulus. Moreover, the intracortical communication pattern differed between the two groups, with cortexes from BCNU-treated animals displaying decreased inter-layer connectivity as compared to control animals. Our results indicate that the altered anatomical organization of the cortex of BCNU-treated rats translates into altered functional networks that respond abnormally to a hyperexcitable stimulus and highlight the role of network dysfunction in the pathophysiology of cortical dysplasia.

The PLEKHA7-PDZD11 complex regulates the localization of the calcium pump PMCA and calcium handling in cultured cells

The plasma membrane calcium ATPase (PMCA) extrudes calcium from the cytosol to the extracellular space to terminate calcium-dependent signaling. Although the distribution of PMCA is crucial for its function, the molecular mechanisms that regulate the localization of PMCA isoforms are not well understood. PLEKHA7 is implicated by genetic studies in hypertension and the regulation of calcium handling. PLEKHA7 recruits the small adapter protein PDZD11 to adherens junctions, and together they control the trafficking and localization of plasma membrane associated proteins, including the Menkes copper ATPase. Since PDZD11 binds to the C-terminal domain of b-isoforms of PMCA, PDZD11 and its interactor PLEKHA7 could control the localization and activity of PMCA. Here, we test this hypothesis using cultured cell model systems. We show using immunofluorescence microscopy and a surface biotinylation assay that KO of either PLEKHA7 or PDZD11 in mouse kidney collecting duct epithelial cells results in increased accumulation of endogenous PMCA at lateral cell–cell contacts and PDZ-dependent ectopic apical localization of exogenous PMCA4x/b isoform. In HeLa cells, coexpression of PDZD11 reduces membrane accumulation of overexpressed PMCA4x/b, and analysis of cytosolic calcium transients shows that PDZD11 counteracts calcium extrusion activity of overexpressed PMCA4x/b, but not PMCA4x/a, which lacks the PDZ-binding motif. Moreover, KO of PDZD11 in either endothelial (bEnd.3) or epithelial (mouse kidney collecting duct) cells increases the rate of calcium extrusion. Collectively, these results suggest that the PLEKHA7–PDZD11 complex modulates calcium homeostasis by regulating the localization of PMCA.

Early Developmental PMCA2b Expression Protects From Ketamine-Induced Apoptosis and GABA Impairments in Differentiating Hippocampal Progenitor Cells

PMCA2 is not expressed until the late embryonic state when the control of subtle Ca²⁺ fluxes becomes important for neuronal specialization. During this period, immature neurons are especially vulnerable to degenerative insults induced by the N-methyl-D-aspartate (NMDA) receptor blocker, ketamine. As H19-7 hippocampal progenitor cells isolated from E17 do not express the PMCA2 isoform, they constitute a valuable model for studying its role in neuronal development. In this study, we demonstrated that heterologous expression of PMCA2b enhanced the differentiation of H19-7 cells and protected from ketamine-induced death. PMCA2b did not affect resting [Ca²⁺]_c in the presence or absence of ketamine and had no effect on the rate of Ca²⁺ clearance following membrane depolarization in the presence of the drug. The upregulation of endogenous PMCA1 demonstrated in response to PMCA2b expression as well as ketamine-induced PMCA4 depletion were indifferent to the rate of Ca²⁺ clearance in the presence of ketamine. Yet, co-expression of PMCA4b and PMCA2b was able to partially restore Ca²⁺ extrusion diminished by ketamine. The profiling of NMDA receptor expression showed upregulation of the NMDAR1 subunit in PMCA2b-expressing cells and increased co-immunoprecipitation of both proteins following ketamine treatment. Further microarray screening demonstrated a significant influence of PMCA2b on GABA signaling in differentiating progenitor cells, manifested by the unique regulation of several genes key to the GABAergic transmission. The overall activity of glutamate decarboxylase remained unchanged, but Ca²⁺-induced GABA release was inhibited in the presence of ketamine. Interestingly, PMCA2b expression was able to reverse this effect. The mechanism of GABA secretion normalization in the presence of ketamine may involve PMCA2b-mediated inhibition of GABA transaminase, thus shifting GABA utilization from energetic purposes to neurosecretion. In this study, we show for the first time that developmentally controlled PMCA expression may dictate the pattern of differentiation of hippocampal progenitor cells. Moreover, the appearance of PMCA2 early in development has long-standing consequences for GABA metabolism with yet an unpredictable influence on GABAergic neurotransmission during later stages of brain maturation. In contrast, the presence of PMCA2b seems to be protective for differentiating progenitor cells from ketamine-induced apoptotic death.

Regulation of Cell Calcium and Role of Plasma Membrane Calcium ATPases

The plasma membrane Ca²⁺ ATPase (PMCA pump) is a member of the superfamily of P-type pumps. It has 10 transmembrane helices and 2 cytosolic loops, one of which contains the catalytic center. Its most distinctive feature is a C-terminal tail that contains most of the regulatory sites including that for calmodulin. The pump is also regulated by acidic phospholipids, kinases, a dimerization process, and numerous protein interactors. In mammals, four genes code for the four basic isoforms. Isoform complexity is increased by alternative splicing of primary transcripts. Pumps 2 and 3 are expressed preferentially in the nervous system. The pumps coexist with more powerful systems that clear Ca²⁺ from the bulk cytosol: their role is thus the regulation of Ca²⁺ in selected subplasma membrane microdomains, where a number of important Ca²⁺-dependent enzymes interact with them. Malfunctions of the pump lead to disease phenotypes that affect the nervous system preferentially.

Ultrastructural and immunohistochemical localization of plasma membrane Ca2+-ATPase 4 in Ca2+-transporting epithelia

Plasma membrane Ca(2+)-ATPases (PMCAs) participate in epithelial Ca(2+) transport and intracellular Ca(2+) signaling. The Pmca4 isoform is enriched in distal nephron isolates and decreased in mice lacking the epithelial transient receptor potential vanilloid 5 Ca(2+) channel. We therefore hypothesized that Pmca4 plays a significant role in transcellular Ca(2+) flux and investigated the localization and regulation of Pmca4 in Ca(2+)-transporting epithelia. Using antibodies directed specifically against Pmca4, we found it expressed only in the smooth muscle layer of mouse and human intestines, whereas pan-specific Pmca antibodies detected Pmca1 in lateral membranes of enterocytes. In the kidney, Pmca4 showed broad localization to the distal nephron. In the mouse, expression was most abundant in segments coexpressing the epithelial ransient receptor potential vanilloid 5 Ca(2+) channel. Significant, albeit lower, expression was also evident in the region encompassing the cortical thick ascending limbs, macula densa, and early distal tubules as well as smooth muscle layers surrounding renal vessels. In the human kidney, a similar pattern of distribution was observed, with the highest PMCA4 expression in Na(+)-Cl(-) cotransporter-positive tubules. Electron microscopy demonstrated Pmca4 localization in distal nephron cells at both the basolateral membrane and intracellular perinuclear compartments but not submembranous vesicles, suggesting rapid trafficking to the plasma membrane is unlikely to occur in vivo. Pmca4 expression was not altered by perturbations in Ca(2+) balance, pointing to a housekeeping function of the pump in Ca(2+)-transporting epithelia. In conclusion, Pmca4 shows a divergent expression pattern in Ca(2+)-transporting epithelia, inferring diverse roles for this isoform not limited to transepithelial Ca(2+) transport.

Regulation of GAP43/calmodulin complex formation via calcineurin-dependent mechanism in differentiated PC12 cells with altered PMCA isoforms composition

Several lines of evidence suggest the contribution of age-related decline in plasma membrane calcium pump (PMCA) to the onset of neurodegenerative diseases. From four PMCA isoforms, PMCA2, and PMCA3 respond to a rapid removal of Ca²⁺ and are expressed predominantly in excitable cells. We have previously shown that suppression of neuron-specific PMCAs in differentiated PC12 cells accelerated cell differentiation, but increased apoptosis in PMCA2-deficient line. We also demonstrated that altered expression of voltage-dependent calcium channels correlated with their higher contribution to Ca²⁺ influx, which varied between PMCA-reduced lines. Here, we propose a mechanism unique for differentiated PC12 cells by which PMCA2 and PMCA3 regulate pGAP43/GAP43 ratio and the interaction between GAP43 and calmodulin (CaM). Although down-regulation of PMCA2 or PMCA3 altered the content of GAP43/pGAP43, of paramount importance for the regulatory mechanism is a disruption of isoform-specific inhibitory PMCA/calcineurin interaction. In result, higher endogenous calcineurin (CaN) activity leads to hypophosphorylation of GAP43 in PMCA2- or PMCA3-deficient lines and intensification of GAP43/CaM complex formation, thus potentially limiting the availability of free CaM. In overall, our results indicate that both “fast” PMCA isoforms could actively regulate the local CaN function and CaN-downstream processes. In connection with our previous observations, we also suggest a negative feedback of cooperative action of CaM, GAP43, and CaN on P/Q and L-type channels activity. PMCAs- and CaN-dependent mechanism presented here, may signify a protective action against calcium overload in neuronal cells during aging, as well a potential way for decreasing neuronal cells vulnerability to neurodegenerative insults.

Long-term mTOR inhibitors administration evokes altered calcium homeostasis and platelet dysfunction in kidney transplant patients

The use of the mammal target of rapamycin (mTOR) inhibitors has been consolidated as the therapy of election for preventing graft rejection in kidney transplant patients, despite their immunosuppressive activity is less strong than anti-calcineurin agents like tacrolimus and cyclosporine A. Furthermore, as mTOR is widely expressed, rapamycin (a macrolide antibiotic produced by Streptomyces hygroscopicus) is recommended in patients presenting neoplasia due to its antiproliferative actions. Hence, we have investigated whether rapamycin presents side effects in the physiology of other cell types different from leucocytes, such as platelets. Blood samples were drawn from healthy volunteers and kidney transplant patients long-term medicated with rapamycin: sirolimus and everolimus. Platelets were either loaded with fura-2 or directly stimulated, and immunoassayed or fixed with Laemmli's buffer to perform the subsequent analysis of platelet physiology. Our results indicate that rapamycin evokes a biphasic time-dependent alteration in calcium homeostasis and function in platelets from kidney transplant patients under rapamycin regime, as demonstrated by the reduction in granule secretion observed and subsequent impairment of platelet aggregation in these patients compared with healthy volunteers. Platelet count was also reduced in these patients, thus 41% of patients presented thrombocytopenia. All together our results show that long-term administration of rapamycin to kidney transplant patients evokes alteration in platelet function.

Temporal regulation of nuclear factor one occupancy by calcineurin/NFAT governs a voltage-sensitive developmental switch in late maturing neurons

Dendrite and synapse development are critical for establishing appropriate neuronal circuits, and disrupted timing of these events can alter neural connectivity. Using microarrays, we have identified a nuclear factor I (NFI)-regulated temporal switch program linked to dendrite formation in developing mouse cerebellar granule neurons (CGNs). NFI function was required for upregulation of many synapse-related genes as well as downregulation of genes expressed in immature CGNs. Chromatin immunoprecipitation analysis revealed that a central feature of this program was temporally regulated NFI occupancy of late-expressed gene promoters. Developing CGNs undergo a hyperpolarizing shift in membrane potential, and depolarization inhibits their dendritic and synaptic maturation via activation of calcineurin (CaN) (Okazawa et al., 2009). Maintaining immature CGNs in a depolarized state blocked NFI temporal occupancy of late-expressed genes and the NFI switch program via activation of the CaN/nuclear factor of activated T-cells, cytoplasmic (NFATc) pathway and promotion of late-gene occupancy by NFATc4, and these mechanisms inhibited dendritogenesis. Conversely, inhibition of the CaN/NFATc pathway in CGNs maturing under physiological nondepolarizing conditions upregulated the NFI switch program, NFI temporal occupancy, and dendrite formation. NFATc4 occupied the promoters of late-expressed NFI program genes in immature mouse cerebellum, and its binding was temporally downregulated with development. Further, NFI temporal binding and switch gene expression were upregulated in the developing cerebellum of Nfatc4 (-/-) mice. These findings define a novel NFI switch and temporal occupancy program that forms a critical link between membrane potential/CaN and dendritic maturation in CGNs. CaN inhibits the program and NFI occupancy in immature CGNs by promoting NFATc4 binding to late-expressed genes. As maturing CGNs become more hyperpolarized, NFATc4 binding declines leading to onset of NFI temporal binding and the NFI switch program.

FKBP52 is involved in the regulation of SOCE channels in the human platelets and MEG 01 cells

Immunophilins are FK506-binding proteins that have been involved in the regulation of calcium homeostasis, either by modulating Ca(2+) channels located in the plasma membrane or in the rough endoplasmic reticulum (RE). We have investigated whether immunophilins would participate in the regulation of stored-operated Ca(2+) entry (SOCE) in human platelets and MEG 01. Both cell types were loaded with fura-2 for determining cytosolic calcium concentration changes ([Ca(2+)](c)), or stimulated and fixed to evaluate the protein interaction profile by performing immunoprecipitation and western blotting. We have found that incubation of platelets with FK506 increases Ca(2+) mobilization. Thapsigargin (TG)-evoked, Thr-evoked SOCE and TG-evoked Mn(2+) entry resulted in significant reduction by treatment of platelets with immunophilin antagonists. We confirmed by immunoprecipitation that immunophilins interact with transient receptor potential channel 1 (TRPC1) and Orai1 in human platelets. FK506 and rapamycin reduced the association between TRPC1 and Orai1 with FK506 binding protein (52) (FKBP52) in human platelets, and between TRPC1 and the type II IP(3)R, which association is known to be crucial for the maintenance of SOCE in human platelets. FKBP52 role in SOCE activation was confirmed by silencing FKBP52 using SiRNA FKBP52 in MEG 01 as demonstrated by single cell configuration imaging technique. TRPC1 silencing and depletion of cell of TRPC1 and FKBP52 simultaneously, impair activation of SOCE evoked by TG in MEG 01. Finally, in MEG 01 incubated with FK506 we observed a reduction in TRPC1/FKBP52 coupling, and similarly, FKBP52 silencing reduced the association between IP3R type II and TRPC1 during SOCE. All together, these results demonstrate that immunophilins participate in the regulation of SOCE in human platelets.

Plasma membrane calcium ATPases and related disorders

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

Ca2+-dependent transcriptional control of Ca2+ homeostasis

Intracellular free Ca(2+) ions regulate many cellular functions, and in turn, the cell devotes many genes/proteins to keep tight control of the level of intracellular free Ca(2+). Here, we review recent work on Ca(2+)-dependent mechanisms and effectors that regulate the transcription of genes encoding proteins involved in the maintenance of the homeostasis of Ca(2+) in the cell.

Gene expression pattern in PC12 cells with reduced PMCA2 or PMCA3 isoform: selective up-regulation of calmodulin and neuromodulin

Cellular calcium homeostasis is controlled predominantly by the plasma membrane calcium pump (PMCA). From four PMCA isoforms, PMCA1 and PMCA4 are ubiquitous, while PMCA2 and PMCA3 are found in excitable cells. We have previously shown that suppression of neuron-specific PMCAs in non-differentiated PC12 cells changed the cell morphology and triggered neuritogenesis. Using the microarrays, real-time PCR and immunodetection, we analyzed the effect of PMCA2 or PMCA3 reduction in PC12 cells on gene expression, with emphasis on calmodulin (CaM), neuromodulin (GAP43) and MAP kinases. In PMCA-suppressed lines total CaM increased, and the calm I and calm II genes appeared to be responsible for this effect. mRNA and protein levels of GAP43 were increased, however, the amount of phosphorylated form was lower than in control cells. Localization of CaM/GAP43 and CaM/pGAP43 differed between control and PMCA-reduced cells. In both PMCA-modified lines, amounts of ERK1/2 increased. While pERK1 decreased, the pERK2 level was similar in all examined lines. PMCA suppression did not change the p38 amount, but the p-p38 diminished. JNK2 protein decreased in both PMCA-reduced cells without changes in pJNK level. Microarray analysis revealed distinct expression patterns of certain genes involved in the regulation of cell cycle, proliferation, migration, differentiation, apoptosis and cell signaling. Suppression of neuron-specific PMCA isoforms affected the phenotype of PC12 cells enabling adaptation to the sustained increase in cytosolic Ca(2+) concentration. This is the first report showing function of PMCA2 and PMCA3 isoforms in the regulation of signaling pathways in PC12 cells.

Ca2+-signaling, alternative splicing and endoplasmic reticulum stress responses

Ca(2+)-signaling, alternative splicing, and stress responses by the endoplasmic reticulum are three important cellular activities which can be strongly interconnected to alter the expression of protein isoforms in a tissue dependent manner or during development depending on the environmental conditions. This integrated network of signaling pathways permits a high degree of versatility and adaptation to metabolic, developmental and stress processes. Defects in its regulation may lead to cellular malfunction.

The plasma membrane Ca²+ ATPase and the plasma membrane sodium calcium exchanger cooperate in the regulation of cell calcium

Calcium is an ambivalent signal: it is essential for the correct functioning of cell life, but may also become dangerous to it. The plasma membrane Ca(2+) ATPase (PMCA) and the plasma membrane Na(+)/Ca(2+) exchanger (NCX) are the two mechanisms responsible for Ca(2+) extrusion. The NCX has low Ca(2+) affinity but high capacity for Ca(2+) transport, whereas the PMCA has a high Ca(2+) affinity but low transport capacity for it. Thus, traditionally, the PMCA pump has been attributed a housekeeping role in maintaining cytosolic Ca(2+), and the NCX the dynamic role of counteracting large cytosolic Ca(2+) variations (especially in excitable cells). This view of the roles of the two Ca(2+) extrusion systems has been recently revised, as the specific functional properties of the numerous PMCA isoforms and splicing variants suggests that they may have evolved to cover both the basal Ca(2+) regulation (in the 100 nM range) and the Ca(2+) transients generated by cell stimulation (in the μM range).

Role of calcineurin signaling in membrane potential-regulated maturation of cerebellar granule cells

At the early postnatal period, cerebellar granule cells proliferate, differentiate, migrate, and finally form refined synaptic connections with mossy fibers. During this period, the resting membrane potential of immature granule cells is relatively depolarized, but it becomes hyperpolarized in mature cells. This investigation was conducted to examine the role of this alteration in membrane potential and its downstream signaling mechanism in development and maturation of granule cells. Experiments were designed to precisely characterize the ontogenic processes of developing granule cells by combining organotypic cerebellar cultures with the specific expression of EGFP (enhanced green fluorescent protein) in granule cells by use of DNA transfection. Multiple approaches using morphology, electrophysiology, and immunohistochemistry demonstrated that granule cells developed and matured at the physiological KCl concentration in organotypic cultures in a temporally regulated manner. We addressed how persistent membrane depolarization influences the developmental and maturation processes of granule cells by depolarizing organotypic cultures with high KCl. Depolarization preserved the developmental processes of granule cells up to the stage of formation of immature dendrites but prevented the maturation processes for synaptic formation by granule cells. Importantly, this blockade of the terminal maturation of granule cells was reversed by inactivation of calcineurin with its specific inhibitor. This investigation has demonstrated that alteration of the membrane potential and its downstream calcineurin signaling play a pivotal role in triggering the maturation program for the synaptic organization of postnatally developing granule cells.

Inverse regulation of plasticity-related immediate early genes by calcineurin in hippocampal neurons

In the mammalian hippocampus, changes in the expression of immediate early genes (IEGs) is thought to contribute to long term plastic changes in neurons brought about by learning tasks and high frequency stimulation of synapses. The phosphatase calcineurin has emerged as an important negative regulator of hippocampus-dependent learning and long term potentiation. Here we investigated the possibility that the constraining action of calcineurin on hippocampal plasticity is mediated in part by regulation of gene expression through negative control of transcription factors, such as cAMP-response element (CRE)-binding protein (CREB). We assessed the effect of calcineurin inhibitors on CREB activation by neuronal activity and show that calcineurin activity is in fact required for CREB-mediated gene expression. However, inhibition of calcineurin had disparate effects on the transcriptional induction of CREB-dependent IEGs. We find that the IEG c-fos is unaffected by suppression of calcineurin activity, the plasticity-related genes Egr1/Zif268 and Egr2/Krox-20 are up-regulated, and genes encoding the orphan nuclear hormone receptors Nor1 and Nur77 are down-regulated. We further show that the up-regulation of particular IEGs is probably due to the presence of serum response elements (SREs) in their promoters, because SRE-mediated gene expression is enhanced by calcineurin blockers. Moreover, expression of the c-fos gene, which is unaffected by calcineurin inhibitors, could be down-regulated by mutating the SRE. Conversely, SRE-mediated c-fos induction in the absence of a functional CRE was enhanced by calcineurin inhibitors. Our experiments thus implicate calcineurin as a negative regulator of SRE-dependent neuronal genes.

Control of alternative pre-mRNA splicing by Ca(++) signals

Alternative pre-mRNA splicing is a common way of gene expression regulation in metazoans. The selective use of specific exons can be modulated in response to various manipulations that alter Ca(++) signals, particularly in neurons. A number of splicing factors have also been found to be controlled by Ca(++) signals. Moreover, pre-mRNA elements have been identified that are essential and sufficient to mediate Ca(++)-regulated splicing, providing model systems for dissecting the involved molecular components. In neurons, this regulation likely contributes to the fine-tuning of neuronal properties.

Plasma membrane Ca(2+)-ATPase: from a housekeeping function to a versatile signaling role

Plasma membrane Ca(2+)-ATPases (PMCAs) are high-affinity calcium pumps that contribute to the maintenance of intracellular Ca(2+) homeostasis by exporting Ca(2+) from the cytosol to the extracellular environment. Mammals have four genes encoding the proteins PMCA1 through PMCA4. Each gene transcript is alternatively spliced to generate several variants. Their distribution is tissue- and cell-specific and undergoes regulation during cell development and differentiation. Traditionally, these pumps have been considered to play a housekeeping role in controlling basal Ca(2+) levels, but more recently, it became clear that the presence (and the co-expression) of different isoforms must be related to a more specialized function. Only one of the four genes (encoding PMCA2) has been causally linked to disease in mammals: Several spontaneous mutations are responsible for deafness and ataxia. Other complex human disease phenotype like hearing loss, cardiac function, and infertility are likely to be associated with PMCA function, but no spontaneous mutations in other PMCA genes than PMCA2 are so far identified. The evidence of their involvement in disease phenotypes comes from studies on isoform-specific knockout mice. In this review, I will discuss briefly the general role of PMCA as essential component of Ca(2+) homeostasis machinery and focus on its emerging role as signaling molecule with particular attention on the diseases caused by PMCA dysfunction.

Exporting calcium from cells

All eukaryotic cells import Ca2+ through a number of variously gated plasma membrane channels. Once inside cells, Ca2+ transmits information to a large number of (enzyme) targets. Eventually, it must be exported again, to prevent the overloading of the cytosol with Ca2+. Two systems export Ca2+ from cells: a high affinity, low capacity Ca2+-ATPase, and a lower affinity, but much larger capacity, Na+/Ca2+ exchanger. The ATPase (commonly called the Ca2+ pump) is the fine-tuner of cell Ca2+, as it functions well even if the concentration of the ion drops below the microM level. It is a large enzyme, with 10 transmembrane domains and a C-terminal cytosolic tail that contains regulatory sites, including a calmodulin-binding domain. Four distinct gene products plus a large number of splice variants have been described. Some are tissue specific, the isoform 2 being specifically expressed in the sensorial cells of the Corti organ in the inner-ear. Its genetic absence causes deafness in mice. Two different families of the Na+/Ca2+ exchanger exist, one of which, originally described in photoreceptors, transports K+ and Ca2+ in exchange for Na+. The exchanger is particularly active in excitable cells, e.g., heart, where the necessity cyclically arises to rapidly eject large amounts of Ca2+. In addition to heart, the exchanger is particularly important to neurons: the cleavage of the most important neuronal isoform (NCX3) by calpains activated by excitotoxic treatments generates Ca2+ overload and eventually cell death.

p66SHC promotes T cell apoptosis by inducing mitochondrial dysfunction and impaired Ca2+ homeostasis

p66Shc, a redox enzyme that enhances reactive oxygen species (ROS) production by mitochondria, promotes T cell apoptosis. We have addressed the mechanisms regulating p66Shc-dependent apoptosis in T cells exposed to supraphysiological increases in [Ca2+]c. p66Shc expression resulted in profound mitochondrial dysfunction in response to the Ca2+ ionophore A23187, as revealed by dissipation of mitochondrial transmembrane potential, cytochrome c release and decreased ATP levels. p66Shc expression also caused a dramatic alteration in the cells' Ca2+-handling ability, which resulted in Ca2+ overload after A23187 treatment. The impairment in Ca2+ homeostasis was ROS dependent and caused by defective Ca2+ extrusion due at least in part to decreased plasma membrane ATPase (PMCA) expression. Both effects of p66Shc required Ca2+-dependent serine-36 phosphorylation. The mitochondrial effects of p66Shc were potentiated by but not strictly dependent on the rise in [Ca2+]c. Thus, Ca2+-dependent p66Shc phosphorylation causes both mitochondrial dysfunction and impaired Ca2+ homeostasis, which synergize in promoting T cell apoptosis.

Downstream regulatory element antagonist modulator regulates Ca2+ homeostasis and viability in cerebellar neurons

The Na+/Ca2+ exchangers NCX1, NCX2, and NCX3 are vital for the control of cellular Ca2+ homeostasis. Here, we show that a doublet of downstream regulatory element sites in the promoter of the NCX3 gene mediates transcriptional repression of NCX3 by the Ca2+-modulated transcriptional repressor downstream regulatory element antagonist modulator (DREAM). Overexpression of a DREAM EF-hand mutant insensitive to Ca2+ (EFmDREAM) in hippocampus and cerebellum of transgenic mice significantly reduced NCX3 mRNA and protein levels without modifying NCX1 and NCX2 expression. Cerebellar granules from EFmDREAM transgenic mice showed increased levels of cytosolic Ca2+ and were more vulnerable to increased Ca2+ influx after partial opening of voltage-gated plasma membrane Ca2+ channels induced by increasing K+ in the culture medium but survived better in the conditions of reduced Ca2+ influx prevailing in low extracellular K+. Overexpression of NCX3 in EFmDREAM transgenic granules using a lentiviral vector restored the normal survival response to high K+ observed in wild-type granules. Thus, the downregulation of the regulator of Ca2+ homeostasis NCX3 by Ca2+-regulated DREAM is a striking example of the autoregulatory property of the Ca2+ signal in neurons.

Plasma membrane calcium-ATPase 2 and 4 in human breast cancer cell lines

There is evidence to suggest that plasma membrane Ca2+-ATPase (PMCA) isoforms are important mediators of mammary gland physiology. PMCA2 in particular is upregulated extensively during lactation. Expression of other isoforms such as PMCA4 may influence mammary gland epithelial cell proliferation and aberrant regulation of PMCA isoform expression may lead or contribute to mammary gland pathophysiology in the form of breast cancers. To explore whether PMCA2 and PMCA4 expression may be deregulated in breast cancer, we compared mRNA expression of these PMCA isoforms in tumorigenic and non-tumorigenic human breast epithelial cell lines using real time RT-PCR. PMCA2 mRNA has a higher level of expression in some breast cancer cell lines and is overexpressed more than 100-fold in ZR-75-1 cells, compared to non-tumorigenic 184B5 cells. Although differences in PMCA4 mRNA levels were observed between breast cell lines, they were not of the magnitude observed for PMCA2. We conclude that PMCA2 mRNA can be highly overexpressed in some breast cancer cells. The significance of PMCA2 overexpression on tumorigenicity and its possible correlation with other properties such as invasiveness requires further study.

Somatostatin and the somatostatin receptor 2 are reciprocally controlled by calcineurin during cerebellar granule cell maturation

Activity-dependent Ca2+ influx into neurones and the subsequent changes in gene expression are thought to be important in shaping neuronal development. In this study, we investigated whether an important mediator of neuronal migration, somatostatin (Srif), alongside its receptors, is controlled in this manner in cerebellar granule cells. We show that Ca2+ influx increases the expression of somatostatin mRNA (srif), while somatostatin receptor 2 (sst2) mRNA expression is decreased. Both genes appear to be regulated independently of each other and in a calcineurin-dependent manner that does not depend on either the ERK1/2 MAP kinase or the cAMP/CREB pathway. Nonetheless, a second pathway is required to induce changes in srif and sst2 expression, since constitutively active calcineurin alone is not sufficient to induce these changes. Furthermore, calcineurin activation reciprocally regulates the expression of brain-derived neurotrophic factor, bdnf, and its receptor trkb, which have also been shown to play a role in neuronal migration. Finally, calcineurin appears to control the expression of the neuronal marker transient axonal glycoprotein 1, tag-1, thereby strongly suggesting that calcineurin activation in vivo occurs during the late stages of neuronal migration, possibly during synaptogenesis with mossy fibres. We therefore propose that calcineurin might play an important role as a switch between transcriptional programs during neuronal development.

Calcium - a universal carrier of biological signals

Calcium is the most universal carrier of signals to cells. Chosen by evolution because of its peculiar flexibility as a ligand, it now regulates all important aspects of cell activity, beginning with the creation of new life at fertilization and ending with the dramatic event of apoptotic suicide at the end of the life cycle. The process of signal transduction by Ca2+ displays a number of properties that make it unique among all other carriers of signals: for instance, the ability to perform both a first messenger and a second messenger function, or the frequent activation of autoregulatory mechanisms. The aspect that distinguishes the Ca2+ signaling function most dramatically is ambivalence. Cells have an absolute dependence on the messenger function of Ca2+ in order to function properly and must control its homeostasis with precision to maintain its free concentration in their interior at the appropriate low level. Catastrophy, however, invariably follows whenever protracted failures of the control mechanisms lead to sustained Ca2+ overload.

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).

Calcineurin in memory and bidirectional plasticity

The molecular mechanisms of learning and memory, and the underlying bidirectional changes in synaptic plasticity that sustain them largely implicate protein kinases and phosphatases. Specifically, Ca(2+)-dependent kinases and phosphatases actively control neuronal processing by forming a tightly regulated balance in which they oppose each other. In this balance, calcineurin (PP2B) is a critical protein phosphatase whose main function is to negatively modulate learning, memory, and plasticity. It acts by dephosphorylating numerous substrates in different neuronal compartments. This review outlines some of CN neuronal targets and their implication in synaptic functions, and describes the role of CN in the acquisition, storage, retrieval, and extinction of memory, as well as in bidirectional plasticity.

Calcineurin regulation of neuronal plasticity

From the most basic of nervous systems to the intricate circuits found within the human brain, a fundamental requirement of neuronal function is that it be malleable, altering its output based upon experience. A host of cellular proteins are recruited for this purpose, which themselves are regulated by protein phosphorylation. Over the past several decades, research has demonstrated that the Ca(2+) and calmodulin-dependent protein phosphatase calcineurin (protein phosphatase 2B) is a critical regulator of a diverse array of proteins, leading to both short- and long-term effects on neuronal excitability and function. This review describes many of the influences of calcineurin on a variety of proteins, including ion channels, neurotransmitter receptors, enzymes, and transcription factors. Intriguingly, due to the bi-directional influences of Ca(2+) and calmodulin on calcineurin activity, the strength and duration of particular stimulations may cause apparently antagonistic functions of calcineurin to work in concert.

Calcineurin-independent regulation of plasma membrane Ca2+ ATPase-4 in the vascular smooth muscle cell cycle

Calcineurin mediates repression of plasma membrane Ca2+-ATPase-4 (PMCA4) expression in neurons, whereas c-Myb is known to repress PMCA1 expression in vascular smooth muscle cells (VSMC). Here, we describe a novel mouse VSMC line (MOVAS) in which 45Ca efflux rates decreased 50%, fura 2-AM-based intracellular Ca2+ concentrations ([Ca2+]i) increased twofold, and real-time RT-PCR and Western blot revealed a approximately 40% decrease in PMCA4 expression levels from G0 to G1/S in the cell cycle, where PMCA4 constituted approximately 20% of total PMCA protein. Although calcineurin activity increased fivefold as MOVAS progressed from G0 to G1/S, inhibition of this increase with either BAPTA or retroviral transduction with peptide inhibitors of calcineurin (CAIN), or its downstream target nuclear factor of activated T cells (NFAT) (VIVIT), had no effect on the repression of PMCA4 mRNA expression at G1/S. By contrast, Ca2+-independent activity of the calmodulin-dependent protein kinase-II (CaMK-II) increased eightfold as MOVAS progressed from G0 to G1/S, and treatment with an inhibitor of CaMK-II (KN-93) or transduction of a c-Myb-neutralizing antibody significantly alleviated the G1/S-associated repression of PMCA4. These data show that G1/S-specific PMCA4 repression in proliferating VSMC is brought about by c-Myb and CaMK-II and that calcineurin may regulate cell cycle-associated [Ca2+]i through alternate targets.

Calcium signalling: dynamics, homeostasis and remodelling

Ca2+ is a highly versatile intracellular signal that operates over a wide temporal range to regulate many different cellular processes. An extensive Ca2+-signalling toolkit is used to assemble signalling systems with very different spatial and temporal dynamics. Rapid highly localized Ca2+ spikes regulate fast responses, whereas slower responses are controlled by repetitive global Ca2+ transients or intracellular Ca2+ waves. Ca2+ has a direct role in controlling the expression patterns of its signalling systems that are constantly being remodelled in both health and disease.

Calcineurin controls the expression of numerous genes in cerebellar granule cells

The Ca(2+)/calmodulin-dependent phosphatase calcineurin plays a crucial role in gene expression in different cell types such as T-lymphocytes, cardiac myocytes, and smooth muscle cells. A possible role for calcineurin in gene expression was recently found in neurons, where calcineurin regulates the expression of several genes involved in Ca(2+) homeostasis. To detect additional genes regulated in a calcineurin-dependent way in neurons we analysed gene expression profiles of cerebellar granule cells cultured in depolarising conditions in the presence or absence of the calcineurin inhibitory agents FK506 and CsA. Using oligonucleotide arrays we identified 34 genes that are differentially expressed between the samples and confirmed the calcineurin-dependent regulation of some of these genes by RT-PCR. Therefore, our results, which are likely not to be comprehensive, suggest that calcineurin plays a fundamental role in neuronal gene expression by either activating or repressing the expression of genes such as receptors, transcription factors, and signalling molecules.

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.

Control of the Na+/Ca2+ exchanger 3 promoter by cyclic adenosine monophosphate and Ca2+ in differentiating neurons

The human gene for member 3 of solute carrier family 8 (SLC8A3), encoding the Na+/Ca2+ exchanger isoform 3 (NCX3), was identified on chromosome 14q24.2. The minimal promoter region was predicted 250 bp upstream of exon 1. This was confirmed by luciferase reporter assays of pGL3-promoter constructs in transfected SH-SY5Y cells. The promoter activity was monitored during the differentiation of this cell line elicited by the sequential treatment with retinoic acid and brain-derived neurotrophic factor (BDNF). The activity was induced by cyclic AMP (cAMP) via the CRE (cAMP response element) and was stimulated by retinoic acid. The increase of intracellular Ca2+ induced by the partial depolarization of the plasma membrane with KCl down-regulated both the basal and the cAMP-stimulated transcription. The down-regulation of the latter may be mediated by the phosphorylation of the CRE-binding protein by a calmodulin-dependent kinase (CaMKII). The exposure of cells to BDNF after treatment with retinoic acid rapidly induced promoter activity during the initial five hours and phosphorylation of CRE-binding protein during the first two hours. The promoter activity was further enhanced by cAMP, but became insensitive to Ca2+. In BDNF-stimulated cells cAMP elevation caused the preferential phosphorylation of ATF1 instead of that of CRE-binding protein.

The expression of the PDZ protein MALS-1/velis is regulated by calcium and calcineurin in cerebellar granule cells

Activity-dependent gene expression is thought to be important in shaping neuronal development and in modifying the protein content of neurons. Ca(2+) entry into neurons appears to be one of the key effectors of activity-dependent gene expression. Among the possible downstream targets of calcium, the protein phosphatase calcineurin represents a prime candidate. We hereby report that in cultured cerebellar granule cells the activation of the Ca(2+)/calcineurin pathway via either voltage- or ligand- operated Ca(2+) channels regulates MALS-1 and MALS-2 expression at the transcriptional level. These proteins are integral parts of the post-synaptic density and are also involved in receptor trafficking. MALS regulation is not at the level of mRNA stability and does not require de novo protein synthesis, thereby suggesting a direct pathway. These data suggest that Ca(2+) entry by means of calcineurin is capable of controlling the structure of the post-synaptic density by controlling the expression of key components at the transcriptional level.

The role of calmodulin for inositol 1,4,5-trisphosphate receptor function

Intracellular calcium release is a fundamental signaling mechanism in all eukaryotic cells. The ryanodine receptor (RyR) and inositol 1,4,5-trisphosphate receptor (IP(3)R) are intracellular calcium release channels. Both channels can be regulated by calcium and calmodulin (CaM). In this review we will first discuss the role of calcium as an activator and inactivator of the IP(3)R, concluding that calcium is the most important regulator of the IP(3)R. In the second part we will further focus on the role of CaM as modulator of the IP(3)R, using results of the voltage-dependent Ca(2+) channels and the RyR as reference material. Here we conclude that despite the fact that different CaM-binding sites have been characterized, their function for the IP(3)R remains elusive. In the third part we will discuss the possible functional role of CaM in IP(3)-induced Ca(2+) release (IICR) by direct and indirect mechanisms. Special attention will be given to the Ca(2+)-binding proteins (CaBPs) that were shown to activate the IP(3)R in the absence of IP(3).

A study of the activity of the plasma membrane Na/Ca exchanger in the cellular environment

The Na/Ca exchanger is a large-capacity system that is supposed to be predominant in the ejection of calcium in excitable tissues. Its activity is normally evaluated in electro-physiological experiments or using Na-loaded cells or plasma membrane vesicles in experiments in which the activity is estimated by measuring the uptake of isotopic calcium. Here the activity of the exchanger has been evaluated in model cells that coexpress the exchanger together with the (protein) Ca indicator aequorin targeted to the different subcellular compartments. Exchanger isoform 1 (NCX1) has been used for experiments in which the effect of its overexpression has been evaluated on the Ca homeostasis in the cytoplasm at large, in the endoplasmic reticulum, in mitochondria, and in the layer of cytoplasm underneath the plasma membrane. The experiments have shown that, despite its very low calcium affinity, the exchanger was active in cells at rest in which calcium is around 100 nM. They have also shown that expressed NCX1 significantly reduced the cytosolic and mitochondrial calcium transients produced by the emptying of the ER calcium stores with InsP3-linked agonists. The effect of the expressed NCX1 on the calcium homeostasis in the domain underneath the plasma membrane was more complex: The exchanger apparently catalyzed a reverse operation, leading to increased penetration of calcium.

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.

Calcium signaling: a tale for all seasons

An experiment performed in London nearly 120 years ago, which by today's standards would be considered unacceptably sloppy, marked the beginning of the calcium (Ca(2+)) signaling saga. Sidney Ringer [Ringer, S. (1883) J. Physiol. 4, 29-43] was studying the contraction of isolated rat hearts. In earlier experiments, Ringer had suspended them in a saline medium for which he admitted to having used London tap water, which is hard: The hearts contracted beautifully. When he proceeded to replace the tap water with distilled water, he made a startling finding: The beating of the hearts became progressively weaker, and stopped altogether after about 20 min. To maintain contraction, he found it necessary to add Ca(2+) salts to the suspension medium. Thus, Ringer had serendipitously discovered that Ca(2+), hitherto exclusively considered as a structural element, was active in a tissue that has nothing to do with bone or teeth, and performed there a completely novel function: It carried the signal that initiated heart contraction. It was a landmark observation, which should have immediately aroused wide interest. Unexpectedly, however, for decades it attracted no particular attention. Occasionally, farsighted pioneers argued forcefully for a messenger role of Ca(2+), offering compelling experimental evidence. Among them, one could quote L. V. Heilbrunn [Heilbrunn, L. V. (1940) Physiol. Zool. 13, 88-94], who contracted frog muscle fibers by applying Ca(2+) salts to their cut ends, but not to their surfaces. Heilbrunn correctly concluded that Ca(2+) had diffused from the cut ends to the internal contractile elements to elicit their contraction. One could also quote K. Bailey [Bailey, K. (1942) Biochem. J. 36, 121-139], who showed that the ATPase activity of myosin was strongly activated by Ca(2+) (but not by Mg(2+)), and concluded that the liberation of Ca(2+) in the neighborhood of the myosin controlled muscle contraction. Clearly, enough evidence was there, but only a handful of people had the vision to see it and to foresee its far-reaching implications. Perhaps no better example of clairvoyance can be offered than the quip by O. Loewy in 1959: "Ja Kalzium, das ist alles!"

The neuroprotective actions of FK506 binding protein ligands: neuronal survival is triggered by de novo RNA synthesis, but is independent of inhibition of JNK and calcineurin

The immunosuppressant FK506 displays substantial neuroprotective and neuroregenerative effects. It is not fully understood to which extent these effects depend on the inhibition of the calcineurin phosphatase (PP2B). The present study has re-addressed this issue using Lie120, a novel highly specific inhibitor of calcineurin, which does not block the enzymatic activity of FKBPs or cyclophilins, respectively. We have determined the effect of FK506 (10-500 nM), V-10,367 (a FK506 derivative which does not block calcineurin; 1-5 microM) and Lie120 (a novel specific inhibitor of calcineurin, 0.1-5 microM) on the cellular survival and the pro-degenerative JNK activity of PC12 and Neuro2A cells following application of 200 microM H(2)O(2). FK506 and V-10,367, but not Lie120, protected both cell lines against H(2)O(2)-mediated death, whereas an increase in JNK1 activity was blocked by FK506 and Lie120, but not by V-10,367. Co-incubation of FK506 and V-10,367 with the mRNA synthesis inhibitor actinomycin D abolished the protective effect of FK506 and V-10,367. This antagonization was effective when actinomycin D was applied 30 min or 1 h, but not 2 or 4 h, after H(2)O(2) suggesting that FKBP-ligands confer their neuroprotection by rapid de novo synthesis of (functionally) anti-apoptotic proteins. The search for the corresponding effector genes revealed that the expression of FKBP25, FKBP38 and FKBP52 (analysis by reverse transcription-polymerase chain reaction (RT-PCR) did not change following H(2)O(2) or FK506, and this was also true for the expression of apoptosis-related genes caspase 3, bax, bcl-2 and bcl-xL (analysis by Multiplex-PCR). Summarizing, neuronal protection by FKBP-ligands is not mediated either by calcineurin or by JNK1 in this experimental set-up, whereas the FK506 mediated inhibition of JNK1 is realized by the inhibition of calcineurin, an effective activator of JNK1 in neurons.

Regulation of SERCA Ca2+ pump expression by cytoplasmic Ca2+ in vascular smooth muscle cells

Vascular smooth muscle cells (VSMC) express three isoforms of the sarcoplasmic or endoplasmic reticulum Ca2+-ATPase (SERCA) pump; SERCA2b predominates (91%), whereas SERCA2a (6%) and SERCA3 (3%) are present in much smaller amounts. Treatment with thapsigargin (Tg) or A-23187 increased the level of mRNA encoding SERCA2b four- to fivefold; SERCA3 increased about 10-fold, whereas SERCA2a was unchanged. Ca2+ chelation prevented the Tg-induced SERCA2b increase, whereas Ca2+ elevation itself increased SERCA2b expression. These responses were discordant with those of 78-kDa glucose-regulated protein/immunoglobulin-binding protein (grp78/BiP), an endoplasmic reticulum stress-response protein. SERCA2b mRNA elevation was much larger than could be accounted for by the observed increase in message stability. The induction of SERCA2b by Tg did not require protein synthesis, nor was it affected by inhibitors of calcineurin, protein kinase C, Ca2+/calmodulin-dependent protein kinase, or tyrosine protein kinases. Treatment with the nonselective protein kinase inhibitor H-7 prevented Tg-induced SERCA2b expression from occurring, whereas another nonselective inhibitor, staurosporine, was without effect. We conclude that changes in cytosolic Ca2+ control the expression of SERCA2b in VSMC via a mechanism involving a currently uncharacterized, H-7-sensitive but staurosporine-insensitive, protein kinase.

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.

Development of a real-time RT-PCR assay for plasma membrane calcium ATPase isoform 1 (PMCA1) mRNA levels in a human breast epithelial cell line

The plasma membrane Ca(2+) pump is a key regulator of cytosolic free Ca(2+). Recent studies have demonstrated the dynamic expression of the plasma membrane Ca(2+) pump in a variety of cell types. Furthermore, alterations in plasma membrane calcium pump activity have now been implicated in human disease. In this study, the development of a technique to quantitatively assess mRNA expression of the human plasma membrane Ca(2+) ATPase (PMCA1) isoform of the plasma membrane Ca(2+) pump, using a real-time reverse transcriptase-polymerase chain reaction (real-time RT-PCR) assay in a human breast epithelial cell line (MCF-7) is described. The sequences of the PMCA1 primers and probe for real-time RT-PCR are presented. The results also indicate that PMCA1 mRNA can be normalized to both 18S ribosomal RNA (18S rRNA) and human glyceraldehyde-3-phosphate dehydrogenase (hGAPDH) in MCF-7 cells. Real-time RT-PCR will be most useful in assessing PMCA1 mRNA expression in cases where only low amounts of RNA are available and/or when numerous samples must be assessed simultaneously.

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.

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

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