Molecular Cloning of a Third Isoform of the Calmodulin-sensitive Plasma Membrane Ca2+-Transporting ATPase That Is Expressed Predominantly in Brain and Skeletal Muscle

The Sarcoplasmic Reticulum of Skeletal Muscle Cells: A Labyrinth of Membrane Contact Sites

The sarcoplasmic reticulum of skeletal muscle cells is a highly ordered structure consisting of an intricate network of tubules and cisternae specialized for regulating Ca2+ homeostasis in the context of muscle contraction. The sarcoplasmic reticulum contains several proteins, some of which support Ca2+ storage and release, while others regulate the formation and maintenance of this highly convoluted organelle and mediate the interaction with other components of the muscle fiber. In this review, some of the main issues concerning the biology of the sarcoplasmic reticulum will be described and discussed; particular attention will be addressed to the structure and function of the two domains of the sarcoplasmic reticulum supporting the excitation-contraction coupling and Ca2+-uptake mechanisms.

Ca2+ Transportome and the Interorganelle Communication in Hepatocellular Carcinoma

Hepatocellular carcinoma (HCC) is a type of liver cancer with a poor prognosis for survival given the complications it bears on the patient. Though damages to the liver are acknowledged prodromic factors, the precise molecular aetiology remains ill-defined. However, many genes coding for proteins involved in calcium (Ca²⁺) homeostasis emerge as either mutated or deregulated. Ca²⁺ is a versatile signalling messenger that regulates functions that prime and drive oncogenesis, favouring metabolic reprogramming and gene expression. Ca²⁺ is present in cell compartments, between which it is trafficked through a network of transporters and exchangers, known as the Ca²⁺ transportome. The latter regulates and controls Ca²⁺ dynamics and tonicity. In HCC, the deregulation of the Ca²⁺ transportome contributes to tumorigenesis, the formation of metastasizing cells, and evasion of cell death. In this review, we reflect on these aspects by summarizing the current knowledge of the Ca²⁺ transportome and overviewing its composition in the plasma membrane, endoplasmic reticulum, and the mitochondria.

Structure, Function and Regulation of the Plasma Membrane Calcium Pump in Health and Disease

In this review, I summarize the present knowledge of the structural and functional properties of the mammalian plasma membrane calcium pump (PMCA). It is outlined how the cellular expression of the different spliced isoforms of the four genes are regulated under normal and pathological conditions.

Dolutegravir derivative inhibits proliferation and induces apoptosis of non-small cell lung cancer cells via calcium signaling pathway

Non-small cell lung cancer (NSCLC) is the most prevalent type of lung cancer. However, there has been little improvement in its cure rate in the last 30 years, due to its intricate heterogeneity and drug resistance. Accumulating evidences have demonstrated that dysregulation of calcium (Ca²⁺) homeostasis contributes to oncogenesis and promotes tumor development. Inhibitors of Ca²⁺ channels/transporters to restore intracellular Ca²⁺ level were found to arrest tumor cell division, induce apoptosis, and suppress tumor growth both in vitro and in vivo. Dolutegravir (DTG), which is a first-line drug for Acquired Immune Deficiency Syndrome (AIDs) treatment, has been shown to increase intracellular Ca²⁺ levels and Reactive oxygen species (ROS) levels in human erythrocytes, leading to suicidal erythrocyte death or eryptosis. To explore the potential of DTG as an antitumor agent, we have designed and synthesized a panel of compounds based on the principle of biologically active substructure splicing of DTG. Our data demonstrated that 7-methoxy-4-methyl-6,8-dioxo-N-(3-(1-(2-(trifluoromethyl)phenyl)-1H-1,2,3-triazol-4-yl)phenyl)-3,4,6,8,12,12a-hexahydro-2H-pyrido[1',2':4,5]pyrazino[2,1-b][1,3]oxazine-9-carboxamide (DTHP), a novel derivative of DTG, strongly inhibited the colony-forming ability and proliferation of NSCLC cells, but displayed no cytotoxicity to normal lung cells. DTHP treatment also induced apoptosis and upregulate intracellular Ca²⁺ level in NSCLC cells significantly. Inhibiting Ca²⁺ signaling alleviated DTHP-induced apoptosis, suggesting the perturbation of intracellular Ca²⁺ is responsible for DTHP-induced apoptosis. We further discovered that DTHP activates AMPK signaling pathway through binding to SERCA, a Ca²⁺-ATPase. On the other hand, DTHP treatment promoted mitochondrial ROS production, causing mitochondrial dysfunction and cell death. Finally, DTHP effectively inhibited tumor growth in the mouse xenograft model of lung cancer with low toxicity to normal organs. Taken together, our work identified DTHP as a superior antitumor agent, which will provide a novel strategy for the treatment of NSCLC with potential clinical application.

Calcium electroporation for treatment of sarcoma in preclinical studies

Calcium electroporation (CaEP) describes the use of electric pulses (electroporation) to transiently permeabilize cells to allow supraphysiological doses of calcium to enter the cytosol. Calcium electroporation has successfully been investigated for treatment of cutaneous metastases in a clinical study. This preclinical study explores the possible use of calcium electroporation for treatment of sarcoma.

A normal murine muscle cell line (C2C12), and a human rhabdomyosarcoma cell line (RD) were used in the undifferentiated and differentiated state. Electroporation was performed using 8 pulses of 100 μs at 600–1000 V/cm; with calcium (0, 0.5, 1, and 5 mM). Viability was examined by MTS assay, intracellular calcium levels were measured, and expression of plasma membrane calcium ATPase (PMCA) was investigated using western blotting. Calcium/sodium exchanger (NCX1), ryanodine receptor (RyR1) expression and cytoskeleton structure (zyxin/actin) were assessed by immunofluorescence. CaEP efficiency on RD tumors was tested in vivo in immuno-deficient mice.

CaEP was significantly more efficient in RD than in normal cells. Intracellular Ca²⁺ levels after CaEP increased significantly in RD, whereas a lower increase was seen in normal cells. CaEP caused decreased expression of PMCA and NCX1 in malignant cells and RyR1 in both cell lines whereas normal cells exhibited increased expression of NCX1 after CaEP. Calcium electroporation also affected cytoskeleton structure in malignant cells.

This study showed that calcium electroporation is tolerated significantly better in normal muscle cells than sarcoma cells and as an inexpensive and simple cancer treatment this could potentially be used in connection with sarcoma surgery for local treatment.

Of channels and pumps: different ways to boost the aldosterone?

The mineralocorticoid aldosterone is a major factor controlling the salt and water balance and thereby also the arterial blood pressure. Accordingly, primary aldosteronism (PA) characterized by an inappropriately high aldosterone secretion is the most common form of secondary hypertension. The physiological stimulation of aldosterone synthesis in adrenocortical glomerulosa cells by angiotensin II and an increased plasma K⁺ concentration depends on a membrane depolarization and an increase in the cytosolic Ca²⁺ activity. Recurrent gain-of-function mutations of ion channels and transporters have been identified in a majority of cases of aldosterone-producing adenomas and in familial forms of PA. In this review, the physiological role of these genes in the regulation of aldosterone synthesis and the altered function of the mutant proteins as well are described. The specific changes of the membrane potential and the cellular ion homoeostasis in adrenal cells expressing the different mutants are compared, and their impact on autonomous aldosterone production and proliferation is discussed.

Metabolic regulation of the PMCA: Role in cell death and survival

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The PMCA is an ATP-driven Ca²⁺ pump critical for the maintenance of low cytosolic calcium.

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The PMCA has an important but paradoxical role in cell death and survival.

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The PMCA can be differentially regulated by caspase/calpain cleavage.

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Glycolytic ATP supply may be sufficient to fuel the PMCA during metabolic stress.

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The ATP sensitivity of the PMCA can be regulated by acidic phospholipids.

The plasma membrane Ca²⁺-ATPase (PMCA) is a ubiquitously expressed, ATP-driven Ca²⁺ pump that is critical for maintaining low resting cytosolic Ca²⁺ ([Ca²⁺]_i) in all eukaryotic cells. Since cytotoxic Ca²⁺ overload has such a central role in cell death, the PMCA represents an essential “linchpin” for the delicate balance between cell survival and cell death. In general, impaired PMCA activity and reduced PMCA expression leads to cytotoxic Ca²⁺ overload and Ca²⁺ dependent cell death, both apoptosis and necrosis, whereas maintenance of PMCA activity or PMCA overexpression is generally accepted as being cytoprotective. However, the PMCA has a paradoxical role in cell death depending on the cell type and cellular context. The PMCA can be differentially regulated by Ca²⁺-dependent proteolysis, can be maintained by a localised glycolytic ATP supply, even in the face of global ATP depletion, and can be profoundly affected by the specific phospholipid environment that it sits within the membrane. The major focus of this review is to highlight some of the controversies surrounding the paradoxical role of the PMCA in cell death and survival, challenging the conventional view of ATP-dependent regulation of the PMCA and how this might influence cell fate.

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.

The plethora of PMCA isoforms: Alternative splicing and differential expression

In this review the four different genes of the mammalian plasma membrane calcium ATPase (PMCA) and their spliced isoforms are discussed with respect to their tissue distribution, their differences during development and their importance for regulating Ca²⁺ homeostasis under different conditions. This article is part of a Special Issue entitled: 13th European Symposium on Calcium.

Interaction of PDC-109, the Major Secretory Protein From Bull Seminal Vesicles, With Bovine Sperm Membrane Ca2+-ATPase

PDC-109 is the prevalent secretory protein from bovine seminal vesicles that binds to the midpiece of sperm once they pass the ampulla of the vas deferens during emission. Thereby, the protein changes biophysical membrane properties, eventually resulting in increased sperm motility. To elucidate the underlying biochemical mechanism, we have studied the ion-pumping activity (Ca(2+)-ATPase) in membrane preparations of bovine spermatozoa following in vitro incubation with the protein and analyzed whether PDC-109 influences sperm motility. PDC-109 was purified to homogeneity from bull seminal vesicle extracts using a newly described method. The effect of PDC-109 on sperm motility was analyzed using the CASA-method. These experiments clearly demonstrated that PDC-109 significantly increases sperm motility. Calcium-pumping mechanisms were analyzed by monitoring the effect of PDC-109 on various parameters of enzyme activity of Ca(2+)-ATPase in epididymal sperm plasma membranes and were compared with Ca(2+)-ATPase activities from other organs and from epididymal sperm of different species, respectively. Specificity studies were performed using different Ca(2+)-antagonists. Enzyme activities of both Mg(2+)-dependent and Mg(2+)-independent Ca(2+)-ATPases increased in a dose-dependent manner following the addition of the PDC-109 (range 5-20 microg). Preincubation of PDC-109 at temperatures above 37 degrees C and pHs ranging from below 6.5 and above 8.5 led to the loss of the stimulatory effect. An analysis of enzyme kinetics pointed to irreversible, cooperative interaction of PDC-109 with the enzyme. The effect was organ-specific, that is, restricted to sperm ATPases, but it was not species-specific, as it could be elicited also in rat sperm.

Fiber Types in Mammalian Skeletal Muscles

Mammalian skeletal muscle comprises different fiber types, whose identity is first established during embryonic development by intrinsic myogenic control mechanisms and is later modulated by neural and hormonal factors. The relative proportion of the different fiber types varies strikingly between species, and in humans shows significant variability between individuals. Myosin heavy chain isoforms, whose complete inventory and expression pattern are now available, provide a useful marker for fiber types, both for the four major forms present in trunk and limb muscles and the minor forms present in head and neck muscles. However, muscle fiber diversity involves all functional muscle cell compartments, including membrane excitation, excitation-contraction coupling, contractile machinery, cytoskeleton scaffold, and energy supply systems. Variations within each compartment are limited by the need of matching fiber type properties between different compartments. Nerve activity is a major control mechanism of the fiber type profile, and multiple signaling pathways are implicated in activity-dependent changes of muscle fibers. The characterization of these pathways is raising increasing interest in clinical medicine, given the potentially beneficial effects of muscle fiber type switching in the prevention and treatment of metabolic diseases.

Cellular and subcellular localization of the neuron-specific plasma membrane calcium ATPase PMCA1a in the rat brain

Regulation of intracellular calcium is crucial both for proper neuronal function and survival. By coupling ATP hydrolysis with Ca(2+) extrusion from the cell, the plasma membrane calcium-dependent ATPases (PMCAs) play an essential role in controlling intracellular calcium levels in neurons. In contrast to PMCA2 and PMCA3, which are expressed in significant levels only in the brain and a few other tissues, PMCA1 is ubiquitously distributed, and is thus widely believed to play a "housekeeping" function in mammalian cells. Whereas the PMCA1b splice variant is predominant in most tissues, an alternative variant, PMCA1a, is the major form of PMCA1 in the adult brain. Here, we use immunohistochemistry to analyze the cellular and subcellular distribution of PMCA1a in the brain. We show that PMCA1a is not ubiquitously expressed, but rather is confined to neurons, where it concentrates in the plasma membrane of somata, dendrites, and spines. Thus, rather than serving a general housekeeping function, our data suggest that PMCA1a is a calcium pump specialized for neurons, where it may contribute to the modulation of somatic and dendritic Ca(2+) transients.

Plasma membrane calcium ATPase proteins as novel regulators of signal transduction pathways

Emerging evidence suggests that plasma membrane calcium ATPases (PMCAs) play a key role as regulators of calcium-triggered signal transduction pathways via interaction with partner proteins. PMCAs regulate these pathways by targeting specific proteins to cellular sub-domains where the levels of intracellular free calcium are kept low by the calcium ejection properties of PMCAs. According to this model, PMCAs have been shown to interact functionally with the calcium-sensitive proteins neuronal nitric oxide synthase, calmodulin-dependent serine protein kinase, calcineurin and endothelial nitric oxidase synthase. Transgenic animals with altered expression of PMCAs are being used to evaluate the physiological significance of these interactions. To date, PMCA interactions with calcium-dependent partner proteins have been demonstrated to play a crucial role in the pathophysiology of the cardiovascular system via regulation of the nitric oxide and calcineurin/nuclear factor of activated T cells pathways. This new evidence suggests that PMCAs play a more sophisticated role than the mere ejection of calcium from the cells, by acting as modulators of signaling transduction pathways.

The localization of PMCA1b in epithelial cells and aposomes of the rat coagulating gland is influenced by androgens

BACKGROUND

Rat coagulating gland epithelial cells export proteins by an apocrine secretion mode within membrane blebs arising from the apical plasma membrane. Using a pan-PMCA antibody, we have recently shown the plasma membrane Ca(2+)-ATPase (PMCA) being part of the apical plasma membrane of epithelial cells and incorporated into the aposomal membrane. The mRNA of PMCA isoforms 1 and 4 respectively, have been detected by RT-PCR in rat coagulating gland.

METHODS

In order to identify which PMCA isoform is integrated into aposomes during apocrine secretion and whether or not PMCA export is influenced by androgens RT-PCR, in situ hybridization, Western blotting, and immunofluorescence experiments were performed.

RESULTS

PMCA1b is the isoform which is expressed and located in the apical plasma membrane of coagulating gland epithelial cells and is integrated into the aposomal membrane. In contrast, PMCA4 mRNA and protein are restricted to the stroma. Androgen deprivation by castration within 14 days leads to an accumulation of PMCA1b in coagulating gland epithelium, while aposomes are not detected anymore.

CONCLUSIONS

We showed for the first time that PMCA isoform 1b is released via aposomes of the epithelial cells of the rat coagulating gland and that the localization of PMCA1b in the epithelial cells is influenced by androgens.

The plasma membrane Ca2+ ATPase of animal cells: structure, function and regulation

Most important processes in cell life are regulated by calcium (Ca2+). A number of mechanisms have thus been developed to maintain the concentration of free Ca2+ inside cells at the level (100-200nM) necessary for the optimal operation of the targets of its regulatory function. The systems that move Ca2+ back and forth across membranes are important actors in its control. The plasma membrane calcium ATPase (PMCA pump) which ejects Ca2+ from all eukaryotic cell types will be the topic of this contribution. The pump uses a molecule of ATP to transport one molecule of Ca2+ from the cytosol to the external environment. It is a P-type ATPase encoded by four genes (ATP2B1-4), the transcripts of which undergo different types of alternative splicing. Many pump variants thus exist. Their multiplicity is best explained by the specific Ca2+ demands in different cell types. In keeping with these demands, the isoforms are differently expressed in tissues and cell types and have differential Ca2+ extruding properties. At very low Ca2+ concentrations the PMCAs are nearly inactive. They must be activated by calmodulin, by acid phospholipids, by protein kinases, and by other means, e.g., a dimerization process. Other proteins interact with the PMCAs (i.e., MAGUK and NHERF at the PDZ domain and calcineurin A in the main intracellular domain) to sort them to specific regions of the cell membrane or to regulate their function. In some cases the interaction is isoform, or even splice variant specific. PMCAs knock out (KO) mice have been generated and have contributed information on the importance of PMCAs to cells and organisms. So far, only one human genetic disease, hearing loss, has been traced back to a PMCA defect.

Inhibitory interaction of the 14-3-3 proteins with ubiquitous (PMCA1) and tissue-specific (PMCA3) isoforms of the plasma membrane Ca2+ pump

A previous study has demonstrated that the ubiquitous plasma membrane Ca(2+) pump PMCA4 interacted with isoform epsilon of the 14-3-3 protein, whereas the nervous tissue-specific PMCA2 did not. The 14-3-3 proteins are widely expressed small acidic proteins, which modulate cell signaling, intracellular trafficking, transcription and apoptosis. The investigation has been extended to the other tissue-restricted pump (PMCA3) and to the other ubiquitous pump (PMCA1). At variance with PMCA2, PMCA3 interacted with the 14-3-3epsilon protein in a two-hybrid system assay, which could not be used for PMCA1. The 14-3-3epsilon protein immunoprecipitated with both PMCA3 and PMCA1 when expressed in HeLa cells. Pull-down experiments using GST-PMCA1 and GST-PMCA3 fusion products confirmed the interaction of both pumps with the 14-3-3epsilon protein. The binding was phosphorylation-independent with both PMCA3 and PMCA1. The 14-3-3zeta isoform also interacted with PMCA3; however, it did not interact with PMCA1. The effect of the interaction on the activity of the two pumps, and thus on the homeostasis of Ca(2+), was investigated by co-expressing the 14-3-3epsilon protein and PMCA3 or PMCA1 in CHO cells together with the recombinant Ca(2+) indicator aequorin: the ability of cells to re-establish the basal Ca(2+) concentration following a Ca(2+) transient induced by an InsP(3)-producing agonist was substantially decreased with both pumps, indicating that the interaction with the 14-3-3 protein inhibited the activity of both PMCA3 and PMCA1.

Differential regulation of the apical plasma membrane Ca(2+) -ATPase by protein kinase A in parotid acinar cells

Cross-talk between intracellular calcium ([Ca(2+)](i)) signaling and cAMP defines the specificity of stimulus-response coupling in a variety of cells. Previous studies showed that protein kinase A (PKA) potentiates and phosphorylates the plasma membrane Ca(2+)-ATPase (PMCA) in a Ca(2+)-dependent manner in parotid acinar cells (Bruce, J. I. E., Yule, D. I., and Shuttleworth, T. J. (2002) J. Biol. Chem. 277, 48172-48181). The aim of this study was to further investigate the spatial regulation of [Ca(2+)](i) clearance in parotid acinar cells. Par-C10 cells were used to functionally isolate the apical and basolateral PMCA activity by applying La(3+) to the opposite side to inhibit the PMCA. Activation of PKA (using forskolin) differentially potentiated apical [Ca(2+)](i) clearance in mouse parotid acinar cells and apical PMCA activity in Par-C10 cells. Immunofluorescence of parotid tissue slices revealed that PMCA1 was distributed throughout the plasma membrane, PMCA2 was localized to the basolateral membrane, and PMCA4 was localized to the apical membrane of parotid acinar cells. However, in situ phosphorylation assays demonstrated that PMCA1 was the only isoform phosphorylated by PKA following stimulation. Similarly, immunofluorescence of acutely isolated parotid acinar cells showed that the regulatory subunit of PKA (RIIbeta) translocated to the apical region following stimulation. These data suggest that PKA-mediated phosphorylation of PMCA1 differentially regulates [Ca(2+)](i) clearance in the apical region of parotid acinar cells because of a dynamic translocation of PKA. Such tight spatial regulation of Ca(2+) efflux is likely important for the fine-tuning of Ca(2+)-dependent effectors close to the apical membrane important for the regulation of fluid secretion and exocytosis.

Distinct phenotypes among plasma membrane Ca2+-ATPase knockout mice

Ca2+ gradients across the plasma membrane, required for Ca2+ homeostasis and signaling, are maintained in part by plasma membrane Ca2+-ATPase (PMCA) isoforms 1-4. Gene targeting has been used to analyze the functions of PMCA1, PMCA2, and PMCA4 in mice. PMCA1 null mutant embryos die during the preimplantation stage, and loss of a single copy of the PMCA1 gene contributes to apoptosis in vascular smooth muscle. PMCA2 deficiency in sensory hair cells of the inner ear causes deafness and balance defects, most likely by affecting both intracellular Ca2+ and extracellular Ca2+ in the endolymph. PMCA2 is required for viability of certain neurons, consistent with a major role in maintenance of intracellular Ca2+. Surprisingly, loss of PMCA2 in lactating mammary glands causes a sharp reduction in milk Ca2+, consistent with a macrocalcium secretory function. Although PMCA4 is widely expressed and is the most abundant isoform in some tissues, null mutants appear healthy. However, male PMCA4 null mutants are infertile due to a failure of hyperactivated sperm motility resulting from the absence of PMCA4 in the sperm tail, and Ca2+ signaling in B lymphocytes, involving interactions between PMCA4, CD22, and the tyrosine phosphatase SHP-1, is defective. Studies of bladder smooth muscle from PMCA4 null mutants and PMCA1 heterozygous mice suggest that PMCA1 and PMCA4 play different roles in smooth muscle contractility, with PMCA1 contributing to overall Ca2+ clearance and PMCA4 being required for carbachol-stimulated contraction. These phenotypes indicate that PMCA1 serves essential housekeeping functions, whereas PMCA4 and particularly PMCA2 serve more specialized physiological functions.

Identification and localisation of SERCA 2 isoforms in mammalian sperm

Upon binding to the egg's zona pellucida, capacitated spermatozoa will undergo a calcium-dependent exocytotic event called acrosome reaction. During this process, Ca2+ depletion from internal stores is followed by an important rise in [Ca2+]i due to a massive Ca2+ influx. Previous reports have shown that the acrosome can act as a Ca2+ store and that depletion of thapsigargin-sensitive stores induces acrosome exocytosis in capacitated spermatozoa from different mammalian species. The effect of thapsigargin, a specific inhibitor of sarcoplasmic/endoplasmic reticulum Ca2+-ATPases (SERCAs), suggests the presence and implication of SERCA in the active Ca2+ uptake during mammalian sperm capacitation. Although the presence of a thapsigargin-sensitive Ca2+-ATPase has been debated, the aim of this study was to clearly determine whether SERCAs are present in mammalian spermatozoa. Using three different anti-SERCA 2 antibodies, mono- and polyclonal, which recognised the same protein, we successfully identified and localised SERCA 2 in human, mouse and bovine sperm. Western blot analysis suggests that more than one SERCA 2 splice variant are present, one detected in the fraction containing the outer acrosomal membranes and another one present in the subcellular fraction containing the sperm midpiece. These results were confirmed by indirect immunofluorescence where SERCA 2 was observed in the acrosome and midpiece regions of human sperm. SERCA 2 immunohistochemical studies on human testis and PCR-amplification of mRNA encoding for each SERCA 2 splice variant in spermatogenic cells support the presence of this Ca2+-ATPase family in mature spermatozoa. In this paper, we clearly demonstrate, for the first time, the presence of SERCA 2 in mammalian sperm.

Distinct roles of PMCA isoforms in Ca2+homeostasis of bladder smooth muscle: evidence from PMCA gene-ablated mice

We previously showed that plasma membrane Ca2+-ATPase (PMCA) activity accounted for 25–30% of relaxation in bladder smooth muscle ( 8 ). Among the four PMCA isoforms only PMCA1 and PMCA4 are expressed in smooth muscle. To address the role of these isoforms, we measured cytosolic Ca2+([Ca2+]i) using fura-PE3 and simultaneously measured contractility in bladder smooth muscle from wild-type (WT), Pmca1+/−, Pmca4+/−, Pmca4−/−, and Pmca1+/−Pmca4−/−mice. There were no differences in basal [Ca2+]ivalues between bladder preparations. KCl (80 mM) elicited both larger forces (150–190%) and increases in [Ca2+]i(130–180%) in smooth muscle from Pmca1+/−and Pmca1+/−Pmca4−/−bladders than those in WT or Pmca4−/−. The responses to carbachol (CCh: 10 μM) were also greater in Pmca1+/−(120–150%) than in WT bladders. In contrast, the responses in Pmca4−/−and Pmca1+/−Pmca4−/−bladders to CCh were significantly smaller (40–50%) than WT. The rise in half-times of force and [Ca2+]iincreases in response to KCl and CCh, and the concomitant half-times of their decrease upon washout of agonist were prolonged in Pmca4−/−(130–190%) and Pmca1+/−Pmca4−/−(120–250%) bladders, but not in Pmca1+/−bladders with respect to WT. Our evidence indicates distinct isoform functions with the PMCA1 isoform involved in overall Ca2+clearance, while PMCA4 is essential for the [Ca2+]iincrease and contractile response to the CCh receptor-mediated signal transduction pathway.

Physiology of epithelial Ca2+ and Mg2+ transport

Ca2+ and Mg2+ are essential ions in a wide variety of cellular processes and form a major constituent of bone. It is, therefore, essential that the balance of these ions is strictly maintained. In the last decade, major breakthrough discoveries have vastly expanded our knowledge of the mechanisms underlying epithelial Ca2+ and Mg2+ transport. The genetic defects underlying various disorders with altered Ca2+ and/or Mg2+ handling have been determined. Recently, this yielded the molecular identification of TRPM6 as the gatekeeper of epithelial Mg2+ transport. Furthermore, expression cloning strategies have elucidated two novel members of the transient receptor potential family, TRPV5 and TRPV6, as pivotal ion channels determining transcellular Ca2+ transport. These two channels are regulated by a variety of factors, some historically strongly linked to Ca2+ homeostasis, others identified in a more serendipitous manner. Herein we review the processes of epithelial Ca2+ and Mg2+ transport, the molecular mechanisms involved, and the various forms of regulation.

The plasma membrane calcium ATPase and disease

The plasma membrane calcium ATPase (PMCA) uses energy to pump calcium (Ca2+) ions out of the cytosol into the extracellular milieu, usually against a strong chemical gradient. This energy expenditure is necessary to maintain a relatively low intracellular net Ca2+ load. Mammals have four genes (ATP2B1-ATP2B4), encoding the proteins PMCA1 through PMCA4. Transcripts from each of these genes are alternatively spliced to generate several variant proteins that are in turn post-translationally modified in a variety of ways. Expressed ubiquitously and with some level of functional redundancy in most vital tissues, only one of the four genes--Atp2b2--has been causally linked through naturally occuring mutations to disease in mammals: specifically to deafness and ataxia in spontaneous mouse mutants. In humans, a missense amino acid substitution in PMCA2 modifies the severity of hearing loss. Targeted null mutations of the Atp2b1 and Atp2b4 genes in mouse are embryonic lethal and cause a sperm motility defect, respectively. These phenotypes point to complex human diseases like hearing loss, cardiac function and infertility. Changes in PMCA expression are associated with other diseases including cataract formation, carciniogenesis, diabetes, and cardiac hypertension and hypertrophy. Severity of these diseases may be affected by subtle changes in expression of the PMCA isoforms expressed in those tissues.

Temporal pattern of plasma membrane calcium ATPase 2 expression in the spinal cord correlates with the course of clinical symptoms in two rodent models of autoimmune encephalomyelitis

Axonal/neuronal pathology is an important and early feature of multiple sclerosis and its animal model, experimental autoimmune encephalomyelitis (EAE). However, the underlying molecular mechanisms remain elusive. We have previously reported that the levels of an important neuronal calcium pump, plasma membrane calcium ATPase 2 and synaptic proteins, synapsin IIa and syntaxin 1B are decreased in the rat spinal cord at onset of acute EAE. Whether the expression of these genes is restored during neurological recovery and affected in other EAE models is currently unknown. The present study was undertaken to address these issues by use of validated multiplex quantitative real-time RT-PCR with fluoro-primers, western blot and immunocytochemistry. We report that plasma membrane calcium ATPase 2 (PMCA2) transcript and protein levels return to control values during recovery from acute disease in the Lewis rat, whereas they are reduced throughout the course of chronic, non-remitting EAE in the C57Bl/6 mouse. These results indicate a close correlation between PMCA2 levels and disease course as defined by clinical scores reflecting motor deficits. Decrease in synapsin IIa expression also correlated with the onset and progression of neurological symptoms, whereas the pattern of syntaxin 1B mRNA and protein expression suggested post-transcriptional regulation. The decrease in PMCA2 transcript and protein levels and the correlation between expression and disease course in two different EAE models further highlight the importance of this calcium pump in neuronal dysfunction during inflammation.

The Sarcolemmal Calcium Pump Inhibits the Calcineurin/Nuclear Factor of Activated T-cell Pathway via Interaction with the Calcineurin A Catalytic Subunit

The calcineurin/nuclear factor of activated T-cell (NFAT) pathway represents a crucial transducer of cellular function. There is increasing evidence placing the sarcolemmal calcium pump, or plasma membrane calcium/calmodulin ATPase pump (PMCA), as a potential modulator of signal transduction pathways. We demonstrate a novel interaction between PMCA and the calcium/calmodulin-dependent phosphatase, calcineurin, in mammalian cells. The interaction domains were located to the catalytic domain of PMCA4b and the catalytic domain of the calcineurin A subunit. Endogenous calcineurin activity, assessed by measuring the transcriptional activity of its best characterized substrate, NFAT, was significantly inhibited by 60% in the presence of ectopic PMCA4b. This inhibition was notably reversed by the co-expression of the PMCA4b interaction domain, demonstrating the functional significance of this interaction. PMCA4b was, however, unable to confer its inhibitory effect in the presence of a calcium/calmodulin-independent constitutively active mutant calcineurin A suggesting a calcium/calmodulin-dependent mechanism. The modulatory function of PMCA4b is further supported by the observation that endogenous calcineurin moves from the cytoplasm to the plasma membrane when PMCA4b is overexpressed. We suggest recruitment by PMCA4b of calcineurin to a low calcium environment as a possible explanation for these findings. In summary, our results offer strong evidence for a novel functional interaction between PMCA and calcineurin, suggesting a role for PMCA as a negative modulator of calcineurin-mediated signaling pathways in mammalian cells. This study reinforces the emerging role of PMCA as a molecular organizer and regulator of signaling transduction pathways.

The interaction of ethanol with reconstituted synaptosomal plasma membrane Ca2+ -ATPase

The primary effect of ethanol is on the central nervous system. However, the molecular mechanisms responsible for the physiological symptoms of ethanol intoxication are still unknown. Low concentrations of ethanol were observed to stimulate the activity of the calcium pump from reconstituted synaptosomal plasma membrane Ca2+ -ATPase (PMCA), and ethanol inhibited Ca2+ -ATPase activity at concentrations above 5%. The greatest stimulating effect was obtained with 5% (v/v) ethanol and was lipid-dependent, being 74% when the protein had been reconstituted in phosphatidylcholine (PC) and less when the reconstituted protein had previously been activated by calmodulin or after removal of a 9-kDa autoinhibitory site by controlled trypsinization. Stimulation of the pump by ethanol was lower for the native or trypsin-digested protein in the presence of phosphatidylserine than in PC. These results suggest a direct ethanol-protein interaction, because the activating effect depended on the state of Ca2+ -ATPase (native or truncated, or in presence of calmodulin). The activating mechanism of ethanol may involve opening an autoinhibitory domain located close to the calmodulin binding domain.

Calcium extrusion protein expression in the hippocampal formation of chronic epileptic rats after kainate-induced status epilepticus

PURPOSE

The plasma membrane Ca2+ -adenosine triphosphatase (ATPase) (PMCA) and (potassium-dependent) sodium-calcium exchange [NC(K)X] represent two main calcium-extrusion mechanisms that are important for the restoration of [Ca2+]i levels after electrical activity. We investigated whether the expression of these calcium-extrusion proteins is altered in the course of epileptogenesis.

METHODS

Hippocampal-parahippocampal protein expression of NCX1, 2, and 3, PMCA1-4, and NCKX2 at an early and late stage after kainate-induced status epilepticus (SE) was compared with that in control rats by using immunocytochemistry.

RESULTS

Several alterations were found in chronic epileptic rats: (a) NCX1 expression was permanently decreased in the inner molecular layer (IML) of the dentate gyrus (DG) and entorhinal cortex layer III (ECIII), related to neuronal loss in hilus and ECIII, respectively; (b) PMCA and NCKX2 expression was transiently upregulated in the IML, and decreased in several areas where cell loss had occurred, (c) NCX3 expression, which in control rats is abundant in presynaptic terminals of mossy fibers (MF), was extensively and permanently decreased in stratum lucidum and hilar region. In addition, newly formed MF sprouts that project to the DG iml did not noticeably express NCX3; (d) NCX2 and NCKX2 were (transiently) upregulated in astrocytes of epileptic rats throughout the hippocampal formation, including ECIII.

CONCLUSIONS

These region-specific changes in calcium-extrusion proteins reflect a change in calcium regulation. Whether these regional-specific changes of calcium-extrusion proteins are associated with an abnormal calcium homeostasis must be determined. Because some alterations of calcium-extrusion protein expression are already present at an early stage of epileptogenesis, they could be involved in this process.

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).

Phenotypes of SERCA and PMCA knockout mice

P-type Ca2+-ATPases of the sarco(endo)plasmic reticulum (SERCAs) and plasma membrane (PMCAs) are responsible for maintaining the Ca2+ gradients across cellular membranes that are required for regulation of Ca2+-mediated signaling and other biological processes. Gene-targeting studies of SERCA isoforms 1, 2, and 3 and PMCA isoforms 1, 2, and 4 have confirmed some of the general functions proposed for these pumps, such as a major role in excitation-contraction coupling for SERCA1 and SERCA2 and housekeeping functions for PMCA1 and SERCA2, but have also revealed some unexpected phenotypes. These include squamous cell cancer and plasticity in the regulation of Ca2+-mediated exocytosis in SERCA2 heterozygous mutant mice, modulation of Ca2+ signaling in SERCA3-deficient mice, deafness and balance disorders in PMCA2 null mice, and male infertility in PMCA4 null mice. These unique phenotypes provide new information about the cellular functions of these pumps, the requirement of their activities for higher order physiological processes, and the pathophysiological consequences of pump dysfunction.

Targeted Ablation of Plasma Membrane Ca2+-ATPase (PMCA) 1 and 4 Indicates a Major Housekeeping Function for PMCA1 and a Critical Role in Hyperactivated Sperm Motility and Male Fertility for PMCA4

The relative importance of plasma membrane Ca2+-ATPase (PMCA) 1 and PMCA4 was assessed in mice carrying null mutations in their genes (Atp2b1 and Atp2b4). Loss of both copies of the gene encoding PMCA1 caused embryolethality, whereas heterozygous mutants had no overt disease phenotype. Despite widespread and abundant expression of PMCA4, PMCA4 null (Pmca4-/-) mutants exhibited no embryolethality and appeared outwardly normal. Loss of PMCA4 impaired phasic contractions and caused apoptosis in portal vein smooth muscle in vitro; however, this phenotype was dependent on the mouse strain being employed. Pmca4-/- mice on a Black Swiss background did not exhibit the phenotype unless they also carried a null mutation in one copy of the Pmca1 gene. Pmca4-/- male mice were infertile but had normal spermatogenesis and mating behavior. Pmca4-/- sperm that had not undergone capacitation exhibited normal motility but could not achieve hyperactivated motility needed to traverse the female genital tract. Ultrastructure of the motility apparatus in Pmca4-/- sperm tails was normal, but an increased incidence of mitochondrial condensation indicated Ca2+ overload. Immunoblotting and immunohistochemistry showed that PMCA4 is the most abundant isoform in testis and sperm and that it is localized to the principle piece of the sperm tail, which is also the location of the major Ca2+ channel (CatSper) required for sperm motility. These results are consistent with an essential housekeeping or developmental function for PMCA1, but not PMCA4, and show that PMCA4 expression in the principle piece of the sperm tail is essential for hyperactivated motility and male fertility.

Neural system-enriched gene expression: relationship to biological pathways and neurological diseases

To understand the commitment of the genome to nervous system differentiation and function, we sought to compare nervous system gene expression to that of a wide variety of other tissues by gene expression database construction and mining. Gene expression profiles of 10 different adult nervous tissues were compared with that of 72 other tissues. Using ANOVA, we identified 1,361 genes whose expression was higher in the nervous system than other organs and, separately, 600 genes whose expression was at least threefold higher in one or more regions of the nervous system compared with their median expression across all organs. Of the 600 genes, 381 overlapped with the 1,361-gene list. Limited in situ gene expression analysis confirmed that identified genes did represent nervous system-enriched gene expression, and we therefore sought to evaluate the validity and significance of these top-ranked nervous system genes using known gene literature and gene ontology categorization criteria. Diverse functional categories were present in the 381 genes, including genes involved in intracellular signaling, cytoskeleton structure and function, enzymes, RNA metabolism and transcription, membrane proteins, as well as cell differentiation, death, proliferation, and division. We searched existing public sites and identified 110 known genes related to mental retardation, neurological disease, and neurodegeneration. Twenty-one of the 381 genes were within the 110-gene list, compared with a random expectation of 5. This suggests that the 381 genes provide a candidate set for further analyses in neurological and psychiatric disease studies and that as a field, we are as yet, far from a large-scale understanding of the genes that are critical for nervous system structure and function. Together, our data indicate the power of profiling an individual biologic system in a multisystem context to gain insight into the genomic basis of its structure and function.

Novel functional interaction between the plasma membrane Ca2+ pump 4b and the proapoptotic tumor suppressor Ras-associated factor 1 (RASSF1)

Plasma membrane calmodulin-dependent calcium ATPases (PMCAs) are enzymatic systems implicated in the extrusion of calcium from the cell. We and others have previously identified molecular interactions between the cytoplasmic COOH-terminal end of PMCA and PDZ domain-containing proteins. These interactions suggested a new role for PMCA as a modulator of signal transduction pathways. The existence of other intracellular regions in the PMCA molecule prompted us to investigate the possible participation of other domains in interactions with different partner proteins. A two-hybrid screen of a human fetal heart cDNA library, using the region 652-840 of human PMCA4b (located in the catalytic, second intracellular loop) as bait, revealed a novel interaction between PMCA4b and the tumor suppressor RASSF1, a Ras effector protein involved in H-Ras-mediated apoptosis. Immunofluorescence co-localization, immunoprecipitation, and glutathione S-transferase pull-down experiments performed in mammalian cells provided further confirmation of the physical interaction between the two proteins. The interaction domain has been narrowed down to region 74-123 of RASSF1C (144-193 in RASSF1A) and 652-748 of human PMCA4b. The functionality of this interaction was demonstrated by the inhibition of the epidermal growth factor-dependent activation of the Erk pathway when PMCA4b and RASSF1 were co-expressed. This inhibition was abolished by blocking PMCA/RASSSF1 association with an excess of a green fluorescent protein fusion protein containing the region 50-123 of RASSF1C. This work describes a novel protein-protein interaction involving a domain of PMCA other than the COOH terminus. It suggests a function for PMCA4b as an organizer of macromolecular protein complexes, where PMCA4b could recruit diverse proteins through interaction with different domains. Furthermore, the functional association with RASSF1 indicates a role for PMCA4b in the modulation of Ras-mediated signaling.

Short tandem repeats are associated with diverse mRNAs encoding membrane-targeted proteins

Within the genomes of multicellular organisms, short tandem repeating sequences (STRs) are ubiquitous, yet usage patterns remain obscure. The repeats (AC)n and (GU)n appear frequently in the untranslated regions (UTRs) of messenger RNAs (mRNAs). To investigate STR usage patterns, we used three approaches: (1) comparisons of individual mRNA database sequences including annotations and linked references, (2) statistical analysis of complete, UTR databases and (3) study of a large gene family, the aquaporins. Among 500 (AC)n- or (GU)n-containing mRNAs, 58 (12%) had known functions. Of these, 50 (86%) encoded proteins whose activities involved membranes or lipids, including integral membrane proteins, peripheral membrane proteins, ion channels, lipid enzymes, receptors and secreted proteins. A control sequence (AU)n also occurred in mRNAs, but only 5% encoded membrane-related functions. Investigation of all reported 3' UTR sequences, demonstrated that the STR (AC)n was 9 times more common in mRNAs encoding membrane functions than in the total UTR database (P < 0.001). Similarly, (GU)n was 8 times more common in membrane-function mRNAs than in the total database (P < 0.001). These observations suggest that (AC)n and (GU)n may be UTR signals for some mRNAs encoding membrane-targeted proteins.

Localization of endoplasmic reticulum and plasma membrane Ca2+-ATPases in subcellular fractions and sections of pig cerebellum

Subcellular fractions and sections of the cerebellum were analysed to evaluate the relative activity and distribution of organellar and plasma membrane Ca2+-ATPases (SERCA and PMCA). Western blot analysis of the fractions with IID8 or Y/1F4 SERCA-specific antibodies or else with 5F10 or pbPMCA antibodies, specific to PMCA pump, revealed a major content of SERCA protein in microsomes and of PMCA protein in plasma membrane vesicles. The Ca2+-ATPase activity of microsomes was more sensitive to thapsigargin, a SERCA-specific inhibitor, whereas the activity of the plasma membrane vesicle fraction was inhibited more by vanadate, a blocker of PMCA activity. The SERCA and PMCA distribution analysed in cerebellar sections revealed IID8 antibody reactions in Purkinje cell cytoplasm, granule cells and cerebellar glomeruli. Y/1F4 gave immunostaining in Purkinje cells, molecular layer interneurons (basket and stellate cells) and glomeruli, but granule cells were not labelled. The 5F10 antibody reacted with Purkinje cells, including their dendritic spines, as well as cerebellar glomeruli, whereas the pbPMCA antibody labelled several processes in all three layers and some synaptic interaction sites. The differential content and localization of the two types of Ca2+ pumps in specific neuronal areas of pig cerebellum indicate precise Ca2+ requirements of specific cellular regions.

Crosstalk between cAMP and Ca2+ signaling in non-excitable cells

An impressive array of cytosolic calcium ([Ca2+](i)) signals exert control over a broad range of physiological processes. The specificity and fidelity of these [Ca2+](i) signals is encoded by the frequency, amplitude, and sub-cellular localization of the response. It is believed that the distinct characteristics of [Ca2+](i) signals underlies the differential activation of effectors and ultimately cellular events. This "shaping" of [Ca2+](i) signals can be achieved by the influence of additional signaling pathways modulating the molecular machinery responsible for generating [Ca2+](i) signals. There is a particularly rich source of potential sites of crosstalk between the cAMP and the [Ca2+](i) signaling pathways. This review will focus on the predominant molecular loci at which these classical signaling systems interact to impact the spatio-temporal pattern of [Ca2+](i) signaling in non-excitable cells.

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

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

A comparative functional analysis of plasma membrane Ca2+ pump isoforms in intact cells

The four basic isoforms of the plasma membrane Ca2+ pump and the two C-terminally truncated spliced variants PMCA4CII(4a) and 3CII(3a) were transiently overexpressed in Chinese hamster ovary cells together with aequorin targeted to the cytosol, the endoplasmic reticulum, and the mitochondria. As PMCA3CII(3a) had not yet been cloned and studied, it was cloned for this study, partially purified, and characterized. At variance with the corresponding truncated variant of PMCA4, which had been studied previously, PMCA3CII(3a) had very high calmodulin affinity. All four basic pump variants influenced the homeostasis of Ca2+ in the native intracellular environment. The level of [Ca2+] in the endoplasmic reticulum and the height of the [Ca2+] transients generated in the cytosol and in the mitochondria by the emptying of the endoplasmic reticulum store by inositol 1,4,5-trisphosphate were all reduced by the overexpression of the pumps. The effects were much greater with the neuron-specific PMCA2 and PMCA3 than with the ubiquitously expressed isoforms 1 and 4. Unexpectedly, the truncated PMCA3 and PMCA4 were as effective as the full-length variants in influencing the homeostasis of Ca2+ in the cytosol and the organelles. In particular, PMCA4CII(4a) was as effective as PMCA4CI(4b), even if its affinity for calmodulin is much lower. The results indicate that the availability of calmodulin may not be critical for the modulation of PMCA pumps in vivo.

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

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

Effect of spermine on the activity of synaptosomal plasma membrane Ca(2+)-ATPase reconstituted in neutral or acidic phospholipids

The activity of purified plasma membrane Ca(2+)-ATPase (PMCA) from pig brain was inhibited by spermine (a naturally occurring and highly abundant polycation in brain). The level of inhibition was dependent on the phospholipid used for reconstitution as well as on the intact or truncated state of the enzyme. An IC(50) value of 12.5 mM spermine was obtained for both, the intact protein plus calmodulin and the trypsin-digested protein, reconstituted in phosphatidylcholine (PC). In the absence of calmodulin the intact Ca(2+)-ATPase gave an IC(50) of 27 mM. This form was more sensitive to spermine inhibition when it was reconstituted with phosphatidylserine (PS), showing an IC(50) value of 2.5 mM spermine. However, the truncated form was less responsive to spermine inhibition, having an IC(50) value of 12.5 mM. Spermine has no effect on the affinity of the PMCA for Ca(2+) or ATP, but its effect on the protein is pH-dependent. It is suggested that spermine could bind to negatively charged residues on the ATPase with different accessibility, depending on the structural rearrangement of the protein. Further, when the protein is reconstituted in PS, spermine also binds to the lipid.

Upregulation of cardiac cell plasma membrane calcium pump in a canine model of Chagas disease

We have previously demonstrated that cardiac myocytes isolated from the hearts of adult dogs develop rapid repetitive cytosolic Ca2+ transients, membrane depolarization, and cell contraction by mobilization of sarcoplasmic reticulum Ca2+ stores when exposed to a soluble factor from the trypomastigotes of Trypanosoma cruzi. These findings led us to investigate the regulatory mechanisms of cytosolic Ca2+ in cardiac tissues from dogs chronically infected with T. cruzi. Expression of the plasma membrane calcium pump (PMCA) RNA and protein was determined by Northern and Western blotting, respectively, followed by densitometric analyses. A 642-bp PMCA 1b complementary DNA probe derived from canine epicardial tissue hybridized to 2 major transcripts (7.3 and 5.3 kb) in canine epicardium. Expression of the dominant transcript (7.3 kb) was 77% greater in cardiac tissues obtained from dogs with chronic T. cruzi infection (140 days after inoculation) in comparison with constitutive expression levels in normal dogs. Monoclonal antibody 5F10, known to recognize all isoforms of the PMCA, was used to detect expression of the PMCA protein in epicardial tissue. Expression of a 142-kDa protein was increased by 58% in the cardiac tissues of infected dogs when compared with those from uninfected dogs. To establish a link between the upregulation of PMCA in dogs chronically infected with Chagas disease and the ventricular-based arrhythmias and myocardial failure that occur during this stage of disease both in dogs and humans, further study will be required.

Physiological functions of plasma membrane and intracellular Ca2+ pumps revealed by analysis of null mutants

It is known that plasma membrane Ca(2+)-transporting ATPases (PMCAs) extrude Ca(2+) from the cell and that sarco(endo)plasmic reticulum Ca(2+)-ATPases (SERCAs) and secretory pathway Ca(2+)-ATPases (SPCAs) sequester Ca(2+) in intracellular organelles; however, the specific physiological functions of individual isoforms are less well understood. This information is beginning to emerge from studies of mice and humans carrying null mutations in the corresponding genes. Mice with targeted or spontaneous mutations in plasma membrane Ca(2+)-ATPase isoform 2 (PMCA2) are profoundly deaf and have a balance defect due to the loss of PMCA2 in sensory hair cells of the inner ear. In humans, mutations in SERCA1 (ATP2A1) cause Brody disease, an impairment of skeletal muscle relaxation; loss of one copy of the SERCA2 (ATP2A2) gene causes Darier disease, a skin disorder; and loss of one copy of the SPCA1 (ATP2C1) gene causes Hailey-Hailey disease, another skin disorder. In the mouse, SERCA2 null mutants do not survive to birth, and heterozygous SERCA2 mutants have impaired cardiac performance and a high incidence of squamous cell cancers. SERCA3 null mutants survive and appear healthy, but endothelium-dependent relaxation of vascular smooth muscle is impaired and Ca(2+) signaling is altered in pancreatic beta cells. The diversity of phenotypes indicates that the various Ca(2+)-transporting ATPase isoforms serve very different physiological functions.

Interaction of the plasma membrane Ca2+ pump 4b/CI with the Ca2+/calmodulin-dependent membrane-associated kinase CASK

Spatial and temporal regulation of intracellular Ca(2+) is a key event in many signaling pathways. Plasma membrane Ca(2+)-ATPases (PMCAs) are major regulators of Ca(2+) homeostasis and bind to PDZ (PSD-95/Dlg/ZO-1) domains via their C termini. Various membrane-associated guanylate kinase family members have been identified as interaction partners of PMCAs. In particular, SAP90/PSD95, PSD93/chapsyn-110, SAP97, and SAP102 all bind to the C-terminal tails of PMCA "b" splice variants. Additionally, it has been demonstrated that PMCA4b interacts with neuronal nitric-oxide synthase and that isoform 2b interacts with Na(+)/H(+) exchanger regulatory factor 2, both via a PDZ domain. CASK (calcium/calmodulin-dependent serine protein kinase) contains a calmodulin-dependent protein kinase-like domain followed by PDZ, SH3, and guanylate kinase-like domains. In adult brain CASK is located at neuronal synapses and interacts with various proteins, e.g. neurexin and Veli/LIN-7. In kidney it is localized to renal epithelia. Surprisingly, interaction with the Tbr-1 transcription factor, nuclear transport, binding to DNA T-elements (in a complex with Tbr-1), and transcriptional competence has been shown. Here we show that the C terminus of PMCA4b binds to CASK and that both proteins co-precipitate from brain and kidney tissue lysates. Immunofluorescence staining revealed co-expression of PMCA, CASK, and calbindin-d-28K in distal tubuli of rat kidney sections. To test if physical interaction of both proteins results in functional consequences we constructed a T-element-dependent reporter vector and investigated luciferase activity in HEK293 lysates, previously co-transfected with PMCA4b expression and control vectors. Expression of wild-type PMCA resulted in an 80% decrease in T-element-dependent transcriptional activity, whereas co-expression of a point-mutated PMCA, with nearly eliminated Ca(2+) pumping activity, had only a small influence on regulation of transcriptional activity. These results provide evidence of a new direct Ca(2+)-dependent link from the plasma membrane to the nucleus.

Ca2+-dependent protein kinase--a modulation of the plasma membrane Ca2+-ATPase in parotid acinar cells

Cross-talk between cAMP and [Ca(2+)](i) signaling pathways represents a general feature that defines the specificity of stimulus-response coupling in a variety of cell types including parotid acinar cells. We have reported recently that cAMP potentiates Ca(2+) release from intracellular stores, primarily because of a protein kinase A-mediated phosphorylation of type II inositol 1,4,5-trisphosphate receptors (Bruce, J. I. E., Shuttleworth, T. J. S., Giovannucci, D. R., and Yule, D. I. (2002) J. Biol. Chem. 277, 1340-1348). The aim of the present study was to evaluate the functional and molecular mechanism whereby cAMP regulates Ca(2+) clearance pathways in parotid acinar cells. Following an agonist-induced increase in [Ca(2+)](i) the rate of Ca(2+) clearance, after the removal of the stimulus, was potentiated substantially ( approximately 2-fold) by treatment with forskolin. This effect was prevented completely by inhibition of the plasma membrane Ca(2+)-ATPase (PMCA) with La(3+). PMCA activity, when isolated pharmacologically, was also potentiated ( approximately 2-fold) by forskolin. Ca(2+) uptake into the endoplasmic reticulum of streptolysin-O-permeabilized cells by sarco/endoplasmic reticulum Ca(2+)-ATPase was largely unaffected by treatment with dibutyryl cAMP. Finally, in situ phosphorylation assays demonstrated that PMCA was phosphorylated by treatment with forskolin but only in the presence of carbamylcholine (carbachol). This effect of forskolin was Ca(2+)-dependent, and protein kinase C-independent, as potentiation of PMCA activity and phosphorylation of PMCA by forskolin also occurred when [Ca(2+)](i) was elevated by the sarco/endoplasmic reticulum Ca(2+)-ATPase inhibitor cyclopiazonic acid and was attenuated by pre-incubation with the Ca(2+) chelator, 1,2-bis(o-aminophenoxy) ethane-N,N,N',N'-tetraacetic acid (BAPTA). The present study demonstrates that elevated cAMP enhances the rate of Ca(2+) clearance because of a complex modulation of PMCA activity that involves a Ca(2+)-dependent step. Tight regulation of both Ca(2+) release and Ca(2+) efflux may represent a general feature of the mechanism whereby cAMP improves the fidelity and specificity of Ca(2+) signaling.

Cell-specific expression of plasma membrane calcium ATPase isoforms in retinal neurons

Ca(2+) extrusion by high-affinity plasma membrane calcium ATPases (PMCAs) is a principal mechanism for the clearance of Ca(2+) from the cytosol. The PMCA family consists of four isoforms (PMCA1-4). Little is known about the selective expression of these isoforms in brain tissues or about the physiological function conferred upon neurons by any given isoform. We investigated the cellular and subcellular distribution of PMCA isoforms in a mammalian retina. Mouse photoreceptors, cone bipolar cells and horizontal cells, which respond to light with a graded polarization, express isoform 1 (PMCA1) of the PMCA family. PMCA2 is localized to rod bipolar cells, horizontal cells, amacrine cells, and ganglion cells, and PMCA3 is predominantly expressed in spiking neurons, including both amacrine and ganglion cells but is also found in horizontal cells. PMCA4 was found to be selectively expressed in both synaptic layers. Optical measurements of Ca(2+) clearance showed that PMCAs mediate Ca(2+) extrusion in both rod and cone bipolar cells. In addition, we found that rod bipolar cells, but not cone bipolar cells possess a prominent Na(+)/Ca(2+) exchange mechanism. We conclude that PMCA isoforms are selectively expressed in retinal neurons and that processes of Ca(2+) clearance are different in rod and cone bipolar cells.

Identification of a plasma membrane Ca2+-ATPase in epithelial cells and aposomes of the rat coagulating gland

BACKGROUND

Epithelial cells of the rat coagulating gland secrete a minor fraction of proteins by means of an alternative export mode named apocrine secretion. Thereby, proteins are released by means of membrane bounded blebs or "aposomes" arising from the apical plasma membrane. Little is known about the composition of the aposomal membrane and whether or not proteins located in the apical plasma membrane are integrated into the aposomes.

METHODS

To show expression and localization of Ca(2+)-ATPase in rat coagulating gland, reverse transcriptase-polymerase chain reaction, Western blotting experiments, and Ca(2+)-ATPase activity assays, as well as immunofluorescence studies were performed.

RESULTS

Ca(2+)-ATPase is located in the apical plasma membrane of the epithelial cells of the rat coagulating gland and is also included in the aposomes. Mg(2+)-dependent and Mg(2+)-independent Ca(2+)-ATPase activity was observed in coagulating gland primary epithelial cells and tissue. Gene expression of plasma membrane Ca(2+)-ATPase isoforms 1 and 4 was detected in cultured primary epithelial cells of the rat coagulating gland and coagulating gland tissue.

CONCLUSIONS

We show for the first time that Ca(2+)-ATPase as an important, functionally active membrane protein of the apical plasma membrane is incorporated into the aposomal membranes and is released from the cells during apocrine secretion process.

A mouse with a point mutation in plasma membrane Ca2+-ATPase isoform 2 gene showed the reduced Ca2+ influx in cerebellar neurons

We analyzed mutant mice showing behavioral defects such as severe tremor, up-and-down and side-to-side wriggling of neck without coordination, and found that the gene causing the defects was located between 46 and 60.55 centimorgans (cM) on the mouse chromosome 6. In this region, nucleotide transition of the plasma membrane Ca2+-ATPase isoform 2 (PMCA2) gene was found, which caused a glutamic acid to change into lysine. Since PMCA2 is expressed in the cerebellum and plays an important role to maintain the homeostasis of the intracellular Ca2+ as a Ca2+ pump, the behavioral defect can be ascribed to the impairment of Ca2+ regulation in neurons of the cerebellum. To confirm the defect of Ca2+ homeostasis in the mutant mice, we measured high K+-induced changes in intracellular Ca2+ concentration ([Ca2+]i) in the cerebellar neurons. Contrary to our expectation, the extent of the [Ca2+]i increase in all the regions tested in the cerebellar slice was far smaller than that of the wild type mice, while the resting [Ca2+]i remained almost unaltered. The rate of rise in [Ca2+]i during high K+-induced depolarization was significantly reduced, and the extrusion rate of increased [Ca2+]i was also reduced. These results suggested that voltage-gated Ca2+ channels were down-regulated in the mutant mice in order to regulate [Ca2+]i toward the normal homeostasis. The behavioral defects may be ascribed to the down-regulated Ca2+ homeostasis since dynamic changes in [Ca2+]i are important for various neuronal functions.

Deficiency in plasma membrane calcium ATPase isoform 2 increases susceptibility to noise-induced hearing loss in mice

Susceptibility to noise-induced hearing loss (NIHL) is poorly understood at the genetic level. Mice homozygous for a null mutation in the plasma membrane Ca2+-ATPase isoform 2 (PMCA2) gene are deaf (Kozel et al., 1998). PMCA2 is expressed on outer hair cell stereocilia (Furuta et al., 1998). Fridberger et al. (1998) observed that the outer hair cell cytoplasmic Ca2+ concentration rises following acoustic overstimulation. We hypothesized that Pmca2+/- mice may be more susceptible to NIHL. Since the auditory brainstem response (ABR) thresholds of Pmca2+/- mice vary with the presence of a modifier locus (Noben-Trauth et al., 1997), Pmca2+/- mice were outcrossed to normal hearing CAST/Ei mice. The pre-exposure ABR thresholds of the resulting Pmca2+/+ and Pmca2+/- siblings were indistinguishable. Groups of these mice were exposed to varying intensities of broadband noise, and ABR threshold shifts were calculated. Fifteen days following an 8 h, 113 dB noise exposure, the Pmca2+/- mice displayed significant (P < or = 0.0007) permanent threshold shifts at 16 and 32 kHz that were 15 or 25 dB greater than those observed in Pmca2+/+ littermates. Pmca2 may be the first gene with a known mutated protein product that confers increased susceptibility to NIHL.