mRNAs for plasma membrane calcium pump isoforms differing in their regulatory domain are generated by alternative splicing that involves two internal donor sites in a single exon

Analysis of cholesterol-recognition motifs of the plasma membrane Ca2+-ATPase

The plasma membrane Ca2+-ATPase (PMCA) is crucial for the fine tuning of intracellular calcium levels in eukaryotic cells. In this study, we show the presence of CARC sequences in all human and rat PMCA isoforms and we performed further analysis by molecular dynamics simulations. This analysis focuses on PMCA1, containing three CARC motifs, and PMCA4, with four CARC domains. In PMCA1, two CARC motifs reside within transmembrane domains, while the third is situated at the intracellular interface. The simulations depict more stable RMSD values and lower RMSF fluctuations in the presence of cholesterol, emphasizing its potential stabilizing effect. In PMCA4, a distinct dynamic was found. Notably, the total energy differences between simulations with cholesterol and phospholipids are pronounced in PMCA4 compared to PMCA1. RMSD values for PMCA4 indicate a more energetically favorable conformation in the presence of cholesterol, suggesting a robust interaction between CARCs and this lipid in the membranes. Furthermore, RMSF analysis for CARCs in both PMCA isoforms exhibit lower values in the presence of cholesterol compared to POPC alone. The analysis of H-bond occupancy and total energy values strongly suggests the potential interaction of CARCs with cholesterol. Given the crucial role of PMCAs in physiological calcium regulation and their involvement in diverse pathological processes, this study underscores the significance of CARC motifs and their interaction with cholesterol in elucidating PMCA function. These insights into the energetic preferences associated with CARC-cholesterol interactions offer valuable implications for understanding PMCA function in maintaining calcium homeostasis and addressing potential associated pathologies.

A link between plasma membrane calcium ATPase 2 (PMCA2), estrogen and estrogen receptor α signaling in mechanical pain

Earlier studies on genetically modified mice indicated that plasma membrane calcium ATPase 2 (PMCA2), a calcium extrusion pump, plays a novel and sex-dependent role in mechanical pain responses: female, but not male, PMCA2^+/− mice manifest increased mechanical pain compared to female PMCA2^+/+ mice. The goal of the present studies was to determine the contribution of ovarian steroids to the genotype- and sex-dependent manifestation of mechanical pain in PMCA2^+/+ versus PMCA2^+/− mice. Ovariectomy increased mechanical pain sensitivity and 17β-estradiol (E2) replacement restored it to basal levels in PMCA2^+/+ mice, but not in PMCA2^+/− littermates. Intrathecal administration of an estrogen receptor alpha (ERα) agonist induced ERα signaling in the dorsal horn (DH) of female PMCA2^+/+ mice, but was ineffective in PMCA2^+/− mice. In male PMCA2^+/+ and PMCA2^+/− mice, E2 treatment following orchidectomy did not recapitulate the genotype-dependent differential pain responses observed in females and the agonist did not elicit ERα signaling. These findings establish a novel, female-specific link between PMCA2, ERα and mechanical pain. It is postulated that PMCA2 is essential for adequate ERα signaling in the female DH and that impaired ERα signaling in the female PMCA2^+/− mice hinders the analgesic effects of E2 leading to increased sensitivity to mechanical stimuli.

Regulation of the Plasma Membrane Calcium ATPases by the actin cytoskeleton

Associations between the cortical cytoskeleton and the components of the plasma membrane are no longer considered to be merely of structural and mechanical nature but are nowadays recognized as dynamic interactions that modulate a plethora of cellular responses. Reorganization of actin filaments upon diverse stimuli - among which is the rise in cytosolic Ca²⁺ - is involved in cell motility and adhesion, phagocytosis, cytokinesis, and secretion. Actin dynamics also participates in the regulation of ion transport across the membranes where it not only plays a key role in the delivery and stabilization of channels and transporters in the plasma membrane but also in the regulation of their activity. The recently described functional interaction between actin and the Plasma Membrane Ca²⁺-ATPase (PMCA) represents a novel regulatory mechanism of the pump at the time that unveils a new pathway by which the cortical actin cytoskeleton participates in the regulation of cytosolic Ca²⁺ homeostasis. In this review, we summarize the current knowledge on the interaction between the cortical actin cytoskeleton and the PMCA and discuss the possible mechanisms that may explain the pump's modulation.

Contribution of Plasma Membrane Calcium ATPases to neuronal maladaptive responses: Focus on spinal nociceptive mechanisms and neurodegeneration

Plasma membrane calcium ATPases (PMCAs) are ion pumps that expel Ca²⁺ from cells and maintain Ca²⁺ homeostasis. Four isoforms and multiple splice variants play important and non-overlapping roles in cellular function and integrity and have been implicated in diseases including disorders of the central nervous system (CNS). In particular, one of these isoforms, PMCA2, is critical for spinal cord (SC) neuronal function. PMCA2 expression is decreased in SC neurons at onset of symptoms in animal models of multiple sclerosis. Decreased PMCA2 expression affects the function and viability of SC neurons, with motor neurons being the most vulnerable population. Recent studies have also shown that PMCA2 could be an important contributor to pain processing in the dorsal horn (DH) of the SC. Pain sensitivity was altered in female, but not male, PMCA2^+/- mice compared to PMCA2^+/+ littermates in a modality-dependent manner. Changes in pain responsiveness in the female PMCA2^+/- mice were paralleled by female-specific alterations in the expression of effectors, which have been implicated in the excitability of DH neurons, in mechanisms governing nociception and in the transmission of pain signals. Other PMCA isoforms and in particular, PMCA4, also contribute to the excitability of neurons in the dorsal root ganglia (DRG), which contain the first-order sensory neurons that convey nociceptive information from the periphery to the DH. These findings suggest that specific PMCA isoforms play specialized functions in neurons that mediate pain processing. Further investigations are necessary to unravel the precise contribution of PMCAs to mechanisms governing pathological pain in models of injury and disease.

Implications of the thyroid hormone on neuronal development with special emphasis on the calmodulin-kinase IV pathway

Thyroid hormones influence brain development through regulation of gene expression. This is especially true for Ca²⁺-dependent regulation since a major pathway is controlled by the Ca²⁺/calmodulin-dependent protein kinase IV (CaMKIV) which in turn is induced by the thyroid hormone T₃. In addition, CaMKIV is involved in regulation of alternative splicing of a number of protein isoforms, among them PMCA1a, the neuronal specific isoform of the plasma membrane calcium pump. On the other hand, hypothyroidism or CaMKIV deficiency can have a severe influence on brain development. This article is part of a Special Issue entitled: ECS Meeting edited by Claus Heizmann, Joachim Krebs and Jacques Haiech.

Relevance of the plasma membrane calcium-ATPase in the homeostasis of calcium in the fetal liver

During the early stages of development, the embryo depends on the placenta as provider of oxygen and calcium, among other essential compounds. Although fetal liver accomplishes a well-known haematopoietic function, its contribution to calcium homeostasis upon development is poorly understood. The homeostasis of cell calcium contributes to diverse signaling pathways across developmental stages of most tissues and the calcium-ATPase located at the plasma membrane (PMCA) helps pumping excess calcium into the extracellular space. To date, the understanding of the equilibrium shift between PMCA isoforms during liver development is still missing. This review focuses on the characterization of the hepatic PMCA along the early stages of development, followed by a description of modern approaches to study calcium homeostasis involving several types of pluripotent cells. The application of interdisciplinary techniques to improve our understanding of liver development and the role calcium homeostasis plays in the definition of pathogenesis is also discussed.

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

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

Ca(2+) homeostasis and endoplasmic reticulum (ER) stress: An integrated view of calcium signaling

Cellular Ca(2+) homeostasis is maintained through the integrated and coordinated function of Ca(2+) transport molecules, Ca(2+) buffers and sensors. These molecules are associated with the plasma membrane and different cellular compartments, such as the cytoplasm, nucleus, mitochondria, and cellular reticular network, including the endoplasmic reticulum (ER) to control free and bound Ca(2+) levels in all parts of the cell. Loss of nutrients/energy leads to the loss of cellular homeostasis and disruption of Ca(2+) signaling in both the reticular network and cytoplasmic compartments. As an integral part of cellular physiology and pathology, this leads to activation of ER stress coping responses, such as the unfolded protein response (UPR), and mobilization of pathways to regain ER homeostasis.

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.

Emerging mechanisms and consequences of calcium regulation of alternative splicing in neurons and endocrine cells

Alternative splicing contributes greatly to proteomic complexity. How it is regulated by external stimuli to sculpt cellular properties, particularly the highly diverse and malleable neuronal properties, is an underdeveloped area of emerging interest. A number of recent studies in neurons and endocrine cells have begun to shed light on its regulation by calcium signals. Some mechanisms include changes in the trans-acting splicing factors by phosphorylation, protein level, alternative pre-mRNA splicing, and nucleocytoplasmic redistribution of proteins to alter protein-RNA or protein-protein interactions, as well as modulation of chromatin states. Importantly, functional analyses of the control of specific exons/splicing factors in the brain point to a crucial role of this regulation in synaptic maturation, maintenance, and transmission. Furthermore, its deregulation has been implicated in the pathogenesis of neurological disorders, particularly epilepsy/seizure. Together, these studies have not only provided mechanistic insights into the regulation of alternative splicing by calcium signaling but also demonstrated its impact on neuron differentiation, function, and disease. This may also help our understanding of similar regulations in other types of cells.

Emanuel Strehler's work on calcium pumps and calcium signaling

Cells are equipped with mechanisms to control tightly the influx, efflux and resting level of free calcium (Ca(2+)). Inappropriate Ca(2+) signaling and abnormal Ca(2+) levels are involved in many clinical disorders including heart disease, Alzheimer's disease and stroke. Ca(2+) also plays a major role in cell growth, differentiation and motility; disturbances in these processes underlie cell transformation and the progression of cancer. Accordingly, research in the Strehler laboratory is focused on a better understanding of the molecular "toolkit" needed to ensure proper Ca(2+) homeostasis in the cell, as well as on the mechanisms of localized Ca(2+) signaling. A long-term focus has been on the plasma membrane calcium pumps (PMCAs), which are linked to multiple disorders including hearing loss, neurodegeneration, and heart disease. Our work over the past 20 years or more has revealed a surprising complexity of PMCA isoforms with different functional characteristics, regulation, and cellular localization. Emerging evidence shows how specific PMCAs contribute not only to setting basal intracellular Ca(2+) levels, but also to local Ca(2+) signaling and vectorial Ca(2+) transport. A second major research area revolves around the calcium sensor protein calmodulin and an enigmatic calmodulin-like protein (CALML3) that is linked to epithelial differentiation. One of the cellular targets of CALML3 is the unconventional motor protein myosin-10, which raises new questions about the role of CALML3 and myosin-10 in cell adhesion and migration in normal cell differentiation and cancer.

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.

Plasma membrane calcium-ATPase isoform four distribution changes during corneal epithelial wound healing

PURPOSE

Plasma Membrane Calcium-ATPases (PMCAs) are integral membrane proteins essential to the control of intracellular Ca2+ concentration. In humans, four genes encode PMCA proteins termed PMCA1-PMCA4. PMCA4 is the major PMCA isoform expressed in human corneal epithelium (hCE); however, little is known about its role. The present study documented expression of PMCA4 in rabbit CE (rbCE) and followed the distribution of PMCA4 during CE wound healing in a rabbit (rb) model.

METHODS

Reverse transcriptase PCR using PMCA4 isoform gene-specific primers that flanked alternative splice site A was used to examine the presence of PMCA4 mRNA in rbCE. Protein expression was assessed by immunoblotting using panPMCA- and PMCA4-specific antibodies. Immunocytochemistry was employed to examine PMCA immunolocalization in frozen, formaldehyde-fixed sections of control and wounded rb corneas. In wound healing studies, circular, 6-mm diameter corneal wounds were produced in the central CE using the n-heptanol technique. The distribution of PMCA4 in CE was examined by immunohistochemical staining of frozen sections using PMCA4 isoform-specific antibody at 6-, 24-, 36-, and 48 h post-injury. siRNA(PMCA4) was used to transfect telomerase-immortalized human corneal epithelial (hTCEpi) cells. Cell cultures were wounded 48 h after transfection, and the wound area was measured at 0 h and at 3 h intervals post-wounding.

RESULTS

Direct sequencing of PCR DNAs documented the presence of PMCA4 transcripts in rbCE and showed that the splice variant at site A was PMCA4x. Immunoblot analysis for PMCA4 detected an intense band at approximately 160 kDa and a faint band at approximately 142 kDa. Immunohistochemistry with the panPMCA antibody demonstrated strong immunoreactivity (IR) in all layers of uninjured rbCE. Immunohistochemistry with a PMCA4-specific antibody demonstrated a similar pattern of intense IR along the plasma membrane of cells in all layers of CE, except for the notable absence of PMCA4 IR along the basal cell membranes adjacent to the stroma. The pattern of PMCA4 IR changed following wound healing. During the lag phase of corneal epithelial wound healing, PMCA4 IR was seen mostly on apical plasma membranes of basal cells near the wound margin, with little staining of basal plasma membranes. During the migration phase (24 h), PMCA4 IR was found mostly on basal cell membranes adjacent to the stroma. At 6 h and 24 h following wounding, PMCA4 IR of the cytoplasm was increased compared to control eyes. After closure of the denuded area and stratification, PMCA4 IR was again primarily found along the apical and lateral plasma membranes of basal cells and was again absent from basal cell membranes adjacent to the stroma; PMCA4 IR of the cytoplasm was also similar to that observed in control eyes. siRNA(PMCA4) transfected hTCEpi cells failed to seal the wound area, whereas wounds in control cultures transfected with a scrambled construct were completed healed.

CONCLUSIONS

PMCA4 is strongly expressed in rabbit CE and its immunolocalization exhibits marked changes in distribution during the wound healing process. Knockdown of PMCA4 expression in hTCEpi cells decreases wound healing. Present findings suggest that PMCA4 redistribution could function as one factor in mediating calcium-regulated events necessary for cell migration in regenerating CE.

The influence of calcium signaling on the regulation of alternative splicing

In this review the influence of calcium signaling on the regulation of alternative splicing is discussed with respect to its influence on cell- and developmental-specific expression of different isoforms of the plasma membrane calcium pump (PMCA). In a second part the possibility is discussed that due to the interaction of the calcium-binding protein ALG-2 with a spliceosomal regulator of alternative splicing, RBM22, Ca2+-signaling may thus influence its regulatory property.

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.

Is there a specific role for the plasma membrane Ca2+ -ATPase in the hepatocyte?

The plasma membrane Ca2+ -ATPase (PMCA) is responsible for the fine, long-term regulation of the cytoplasmic calcium concentration by extrusion of this cation from the cell. Although the general kinetic mechanisms for the action of both, well coordinated hydrolytic activity and calcium transport are reasonably understood in the majority of cell types, due to the complex physiologic and biochemical characteristics shown by the hepatocyte, the study of this enzyme in this cell type has become a real challenge. Here, we review the various molecular aspects known to date to be associated with liver PMCA activity, and outline the strategies to follow for establishing the role of this enzyme in the overall physiology of the hepatocyte. In this way, we first concentrate on the basic biochemical aspects of liver cell PMCA, and place an important emphasis on expression of its molecular forms to finally focus on the critical hormonal regulation of the enzyme. Although these complex aspects have been studied mainly under normal conditions, the significance of PMCA in the calcium homeostasis of an abnormal liver cell is also reviewed.

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.

Effect of Hailey-Hailey Disease mutations on the function of a new variant of human secretory pathway Ca2+/Mn2+-ATPase (hSPCA1)

ATP2C1, encoding the human secretory pathway Ca2+/Mn2+ ATPase (hSPCA1), was recently identified as the defective gene in Hailey-Hailey Disease (HHD), an autosomal dominant skin disorder characterized by persistent blisters and erosions. To investigate the underlying cause of HHD, we have analyzed the changes in expression level and function of hSPCA1 caused by mutations found in HHD patients. Mutations were introduced into hSPCA1d, a novel splice variant expressed in keratinocytes, described here for the first time. Encoded by the full-length of optional exons 27 and 28, hSPCA1d was longer than previously identified splice variants. The protein competitively transported Ca2+ and Mn2+ with equally high affinity into the Golgi of COS-1 cells. Ca2+- and Mn2+-dependent phosphoenzyme intermediate formation in forward (ATP-fuelled) and reverse (Pi-fuelled) directions was also demonstrated. HHD mutant proteins L341P, C344Y, C411R, T570I, and G789R showed low levels of expression, despite normal levels of mRNA and correct targeting to the Golgi, suggesting instability or abnormal folding of the mutated hSPCA1 polypeptides. P201L had little effect on the enzymatic cycle, whereas I580V caused a block in the E1 approximately P --> E2-P conformational transition. D742Y and G309C were devoid of Ca2+- and Mn2+-dependent phosphoenzyme formation from ATP. The capacity to phosphorylate from Pi was retained in these mutants but with a loss of sensitivity to both Ca2+ and Mn2+ in D742Y and a preferential loss of sensitivity to Mn2+ in G309C. These results highlight the crucial role played by Asp-742 in the architecture of the hSPCA1 ion-binding site and reveal a role for Gly-309 in Mn2+ transport selectivity.

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.

Urate transport via human PAH transporter hOAT1 and its gene structure

BACKGROUND

We recently cloned the human organic anion transporter 1 (hOAT1) as a p-aminohippurate (PAH) transporter. Whether urate is transported by the PAH transporter in humans remains unclear. Familial juvenile gouty nephropathy (FJGN) is thought to develop as a result of an abnormality in the urate transporter.

METHODS

To determine if hOAT1 transported urate, the cellular uptakes of PAH and urate were determined, as were the inhibition profiles of inorganic anions, and uricosuric and antiuricosuric agents using a mouse S2 cell line expressing hOAT1. The hOAT1 gene was cloned from a genomic library using full-length hOAT1-1 cDNA as a probe. The coding regions of the hOAT1 genes of two sisters with FJGN were sequenced. Also, immunohistochemical fluorescence analysis of hOAT1 in the kidney of the younger sister with FJGN was performed.

RESULTS

The Km and Vmax values of urate transport via hOAT1 were 943 +/- 84 micromol/L and 1286 +/- 162 pmol/mg protein/min, respectively. The order of the IC50 of urate transport via hOAT1 was benzbromarone < probenecid < salicylate or pyrazine carboxylic acid. The 10.9 kb hOAT1 gene was found to be interrupted by nine introns. Mutations in the coding region of the hOAT1 gene from the two sisters with FJGN were undetectable. Immunohistochemical fluorescent staining showed that hOAT1 in the kidney of the younger sister was similar to that of control individuals.

CONCLUSIONS

Our data show that hOAT1 transports urate, and the inhibition profiles of uricosuric and antiuricosuric agents are defined. hOAT1 is not responsible for FJGN in the two sisters examined in this study.

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.

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

When stimulated by glucose, the pancreatic beta-cell displays large oscillations of intracellular free Ca2+ concentration ([Ca2+]i). To control [Ca2+]i, the beta-cell must be equipped with potent mechanisms for Ca2+ extrusion. We studied the expression of the plasma membrane Ca(2+)-ATPases (PMCA) in three insulin secreting preparations (a pure beta-cell preparation, RINm5F cells and pancreatic islet cells), using reverse-transcribed PCR, RNase protection assay and Western blotting. The four main isoforms, PMCA1, PMCA2, PMCA3 and PMCA4 were expressed in the three preparations. Six alternative splice mRNA variants, characterized at splice sites A, B and C were detected in the three preparations (rPMCA1xb, 2yb, 2wb, 3za, 3zc, 4xb), plus two additional variants in pancreatic islet cells (PMCA4za, 1xkb). The latter variant corresponded to a novel variant of rat PMCA1 gene lacking the exon coding for the 10th transmembrane segment, at splice site B. At the mRNA and protein level, five variants predominated (1xb, 2wb, 3za, 3zc, 4xb), whilst one additional isoform (4za), predominated at the protein level only. This provides the first evidence for the presence of PMCA2 and PMCA3 isoforms at the protein level in non-neuronal tissue. Hence, the pancreatic beta-cell is equipped with multiple PMCA isoforms with possible differential regulation, providing a full range of PMCAs for [Ca2+]i regulation.

Alternative transcription initiation sites generate two LCA1 Ca2+-ATPase mRNA transcripts in tomato roots

The tomato LCA1 gene encodes a Ca2+-ATPase and gives rise to two major mRNA transcripts and two distinct protein products of different size in tomato roots. The basis of the transcript size difference was investigated to assess whether the mRNA transcripts encoded distinct protein products. Primer extension and S1 nuclease analysis identified two transcription initiation sites at -72 and -1392 from the start of translation. RNA gel blot analysis of poly(A)+ RNA isolated from phosphate-starved tomato roots using probes designed to domains of the 5'-untranslated region (UTR) or the full-length LCA1 cDNA identified mRNAs of 4.7 and 3.6 kb, corresponding to mRNA originating from transcription initiation sites -1392 and -72, respectively. Screening of a cDNA library derived from phosphate-starved tomato roots yielded three cDNA clones, LCA1A, LCA1B and LCA1C (3.6, 4.5 and 5.1 kb respectively). These cDNAs contain full-length LCA1 mRNA sequence derived from each transcription initiation site, with LCA1C additionally containing an intron of 0.6 kb. Sequence analysis indicated 100% identity between the three size classes of cDNA clones except for the differential 5'-UTR and the unspliced intron. Overall, the results indicate that the two major LCA1 mRNA transcripts are derived by differential transcription initiation and that two of the mRNAs may encode identical protein products, while a third mRNA may correspond to a non-functional truncated protein.

Identification and functional expression of the plasma membrane calcium ATPase gene family from Caenorhabditis elegans

Calcium-pumping ATPases are an essential component of the intracellular calcium homeostasis system and have been characterized in a large variety of species and cell types. In mammalian genomes, these proteins are encoded by gene families whose individual members feature complex tissue-specific expression and alternative splicing. In the search for a less complex system that is more amenable to genetic manipulation, we have identified a family of three genes (mca-1, mca-2, and mca-3) encoding putative calcium ATPases in the Caenorhabditis elegans Genome Project data and completed their transcript structure. In this work, we report the cloning and functional expression of the mca-1 gene, which encodes a calcium-stimulated ATPase whose features resemble those of the plasma membrane calcium adenosine triphosphatase family of mammalian cells and appears to be regulated by a multipartite promoter.

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

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

Balance and hearing deficits in mice with a null mutation in the gene encoding plasma membrane Ca2+-ATPase isoform 2

Plasma membrane Ca2+-ATPase isoform 2 (PMCA2) exhibits a highly restricted tissue distribution, suggesting that it serves more specialized physiological functions than some of the other isoforms. A unique role in hearing is indicated by the high levels of PMCA2 expression in cochlear outer hair cells and spiral ganglion cells. To analyze the physiological role of PMCA2 we used gene targeting to produce PMCA2-deficient mice. Breeding of heterozygous mice yielded live homozygous mutant offspring. PMCA2-null mice grow more slowly than heterozygous and wild-type mice and exhibit an unsteady gait and difficulties in maintaining balance. Histological analysis of the cerebellum and inner ear of mutant and wild-type mice revealed that null mutants had slightly increased numbers of Purkinje neurons (in which PMCA2 is highly expressed), a decreased thickness of the molecular layer, an absence of otoconia in the vestibular system, and a range of abnormalities of the organ of Corti. Analysis of auditory evoked brainstem responses revealed that homozygous mutants were deaf and that heterozygous mice had a significant hearing loss. These data demonstrate that PMCA2 is required for both balance and hearing and suggest that it may be a major source of the calcium used in the formation and maintenance of otoconia.

The alveolar macrophage as a model of calcium signaling in oxidative stress

Regulation of the free intracellular calcium concentration, [Ca2+]i, plays a major role in physiological signal transduction. Many of the essential enzymes in signaling cascades are Ca(2+)-dependent, as are numerous proteins that participate in the regulated function. Oxidative stress, which for many years was considered synonymous with cell and tissue injury, has more recently been demonstrated to alter signal transduction in both positive and negative directions. The realization that hydrogen peroxide and lipid hydroperoxides are produced as part of normal metabolism has led to the proposal that these oxidants function as second messengers. Exposure to environmental and other agents that produce hydroperoxides or the addition of exogenous hydroperoxides also causes elevation of [Ca2+]i in some cells. At sublethal exposure to hydroperoxides, the elevation in [Ca2+]i can either alter or mimic physiological stimulation. In addition to endoplasmic reticulum, mitochondria, and the extracellular space, the phospholipid- and Ca(2+)-binding proteins known as annexins constitute a Ca2+ pool from which this ion may be released under situations of oxidative stress. In this article, the source and consequences of Ca2+ elevation are reviewed with an emphasis on studies done with alveolar macrophages. These phagocytes, which modulate much of the physiological and immunological function of the lung, are susceptible targets for environmental oxidants.

Plasma membrane Ca2+ pump isoforms 2a and 2b are unusually responsive to calmodulin and Ca2+

The full-length a and b variants of the rat plasma membrane calcium pump, isoform 2 (rPMCA2a and rPMCA2b), were constructed and expressed in COS-7 cells. To characterize these isoforms, calcium transport was determined in a microsomal fraction. Both rPMCA2a and rPMCA2b had a much higher affinity for calmodulin than the corresponding forms of hPMCA4, and rPMCA2b had the highest affinity among the isoforms that have been tested so far. When analyzed at a relatively high calmodulin concentration, rPMCA2b and, to a lesser extent, rPMCA2a showed higher apparent calcium affinity; i.e. they were more active at lower Ca2+ concentrations than hPMCA4b. This indicates that these two variants of rat isoform 2 will tend to maintain a lower free cytosolic Ca2+ level in cells where they are expressed. Both variants also showed a higher level of basal activity (in the complete absence of calmodulin) than hPMCA4b, a property which would reinforce their ability to maintain a low free cytosolic Ca2+ concentration. Experiments designed to determine the source of the higher apparent Ca2+ affinity of rPMCA2b showed that it came from the properties of the carboxyl terminus, rather than from any difference in the catalytic core.

Cloning and functional characterization of a rat renal organic cation transporter isoform (rOCT1A)

Polyspecific organic cation transporters in the renal proximal tubule mediate the secretion of many clinically used drugs as well as endogenous metabolites. Recently, two organic cation transporters (rOCT1 and rOCT2) were cloned from rat kidney. In this study, we report the cloning and functional expression of an rOCT1 isoform, rOCT1A, from rat kidney. Genomic DNA cloning and sequencing demonstrated that rOCT1A is an alternatively spliced variant of rOCT1 with a deletion of 104 base pairs near the 5'-end. The uptake of [14C]tetraethylammonium (TEA) in oocytes injected with the cRNA-encoding rOCT1A was increased 16-fold over that in water-injected oocytes (29 +/- 2.8 pmol/oocyte/h versus 1.8 +/- 0.13 pmol/oocyte/h, mean +/- S.E., p < 0.05). [14C]TEA uptake in the cRNA-injected oocytes was saturable (Km = 42 +/- 11 microM) and was inhibited significantly by organic cations, including cimetidine and N1-methylnicotinamide. The amino acid sequence was deduced from the cDNA after examination of all three reading frames. Two overlapping open reading frames were found. Studies with synthetic constructs suggest that a functional organic cation transporter is encoded by the larger open reading frame. The larger open reading frame encodes a 430-amino acid protein (termed rOCT1A) that is 92% identical to rOCT1 and 57% identical to rOCT2. From hydropathy analysis, rOCT1A is predicted to have 10 transmembrane domains with both amino and carboxyl termini intracellular. RNase protection assays demonstrate the presence of rOCT1A mRNA transcripts in rat kidney cortex, medulla, and intestine. These studies demonstrate the presence of a functional, alternatively spliced organic cation transporter (rOCT1A) in rat kidney.

A serine-to-proline mutation in the copper-transporting P-type ATPase gene of the macular mouse

We have investigated the cDNA sequence of the copper-transporting P-type ATPase (Atp7a) gene of the macular mouse, a model for human Menkes disease. A point mutation (T to C) that results in substitution of proline for serine in a putative eighth transmembrane domain of the ATP7A was identified. This contrasts with abnormalities identified in the Atp7a of other mottled mouse strains: lack of expression of Atp7a mRNA in the dappled mouse, and a splicing mutation in the blotchy mouse.

Cloning and molecular analysis of the plasma membrane Ca(2+)-ATPase gene in Paramecium tetraurelia

We have determined the DNA sequence of the gene encoding the protein of the plasma membrane Ca(2+)-ATPase in Paramecium tetraurelia. The predicted amino acid sequence of the plasma membrane Ca(2+)-ATPase shows homology to conserved regions of known plasma membrane Ca(2+)-ATPases and contains the known binding sites for ATP (FITC), acylphosphate formation, and calmodulin, as well as the "hinge" region: all characteristics common to plasma membrane Ca(2+)-ATPases. The deduced molecular weight for this sequence is 131 kDa. The elucidation of this gene will assist in the studies of the mechanisms by which this excitable cell removes calcium entering through voltage gated calcium channels and the pump functions in chemosensory signal transduction.

Expression of the plasma membrane Ca2+-ATPase in myogenic cells

To study the physiological function of the plasma membrane calmodulin-dependent calcium ATPase (PMCA) in intact cells, L6 myogenic cell lines stably overexpressing the human PMCA isoform 4CI (= human PMCA isoform 4b) were generated. Several independent L6 clones and controls stably transfected with the empty expression vector were analyzed in detail. The resting cytosolic calcium level in hPMCA4CI-overexpressing muscle cells (measured by the Fura-2 method) was significantly reduced by 20-30% compared with controls. This was shown in a cytosolic window of 1322 single cells (p < 0.01). Furthermore, the differentiation process of these cells was remarkably accelerated compared with control myoblasts and parental nontransfected L6 cells as assessed by multinucleated myotube formation and creatine phosphokinase activity elevation. After 4 and 6 days of differentiation, PMCA-overexpressing L6 cells from four independent clones displayed a 3- and 4-fold higher creatine phosphokinase activity compared with controls (n = 5, p < 0.02). These results may extend the concept of the function of the PMCA from simple prevention of calcium overload to an active involvement in intracellular calcium regulation with potentially important consequences for cellular functions.

A unique combination of plasma membrane Ca2+-ATPase isoforms is expressed in islets of Langerhans and pancreatic beta-cell lines

Changes in free intracellular Ca2+ concentration regulate insulin secretion from pancreatic beta-cells. The existence of steep Ca2+ gradients within the beta-cell requires the presence of specialized Ca2+ exclusion systems. In this study we have characterized the plasma membrane Ca2+-ATPases (PMCAs) which extrude Ca2+ from the cytoplasm. PMCA isoform- and subtype-specific mRNA expression was investigated in rodent pancreatic alpha- and beta-cell lines, and in human and rat islets of Langerhans using reverse-transcription PCR with primers flanking the calmodulin-binding region of rat PMCA. The expression pattern of PMCA 1 and 2 was conserved in different species and islet-cell types since both rat and human islets of Langerhans and all cell lines tested contained the 1b and 2b forms. PMCA 4 isoform subtypes, however, were expressed in a cell-type-specific manner since beta-cells expressed PMCA 4b only, whereas in islets of Langerhans, which contain alpha, beta, delta and polypeptide-secreting cells, PMCA 4a and 4b were simultaneously present. No evidence was obtained for the expression of PMCA 3. Characterization of the beta-cell Ca2+-pump protein showed that it shared several similarities with the erythrocyte PMCA. It is a P-type ATPase; its phosphorylated intermediate was stabilized by La3+; it reacted with a PMCA-specific antibody; and it was not N-glycosylate. However, the beta-cell PMCA had a higher molecular mass than that of the erythrocyte; this difference could be explained by either predominant translation of the PMCA2 form, which has a molecular mass 3-8 kDa higher than the erythrocyte PMCA 1 and 4 proteins, or by a possible sequence insertion. Thus a unique combination of functionally distinct PMCA isoforms (1b, 2b, 4b) participates in Ca2+ homoeostasis in the beta-cell.

Analysis of mRNA expression and cloning of a novel plasma membrane Ca(2+)-ATPase splice variant in human heart

Four different plasma membrane Ca(2+)-ATPase (PMCA) genes and three sarco(endo)plasmic reticulum Ca(2+)-ATPase (SERCA) genes have been previously cloned and characterized. In this study we have investigated the expression of the mRNA encoding the various PMCA and SERCA proteins in fetal and adult human heart and placenta by the reverse-transcriptase-polymerase-chain-reaction (RT-PCR) and cDNA cloning. We have found that PMCA1 and PMCA4 genes were expressed in 8-, 12- and 20-week fetal heart and in adult heart. PMCA2 gene was expressed at low levels in adult heart but was not detected in fetal heart. PMCA3 mRNA was not detected in the heart nor placenta. In contrast, the mRNA encoding SERCA2a, SERCA2b and SERCA3 were expressed in all cardiac developmental stages. Multiple alternatively spliced mRNA transcripts which differ at splice site A and B/C of the PMCA1, PMCA2 and PMCA4 genes were detected in the human heart. Interestingly, a novel tissue specific variant of the PMCA4 gene was detected in both fetal and adult human heart but not in placenta that accounts for about 30% of the total PMCA4 mRNA variant expression. DNA sequence analysis of this novel variant revealed that it corresponds to the equivalent of the PMCA1d variant and accordingly we have named it PMCA4d. We cloned and sequenced eight cDNA inserts encoding for the PMCA1 and PMCA4 variants from a fetal human heart cDNA library confirming that these are the two main PMCA genes expressed in cardiac muscle.

Region-specific activity of the plasma membrane Ca2+ pump and delayed activation of Ca2+ entry characterize the polarized, agonist-evoked Ca2+ signals in exocrine cells

The initial release of Ca2+ from the intracellular Ca2+ stores is followed by a second phase during which the agonist-dependent Ca2+ response becomes sensitive to the extracellular Ca2+, indicating the involvement of the plasma membrane (PM) Ca2+ transport systems. The time course of activation of these transport systems, which consist of both Ca2+ extrusion and Ca2+ entry pathways, is not well established. To investigate the participation of these processes during the agonist-evoked Ca2+ response, isolated pancreatic acinar cells were exposed to maximal concentrations of an inositol 1,4,5-trisphosphate-mobilizing agonist (acetylcholine, 10 microM) in different experimental conditions. Following the increase of [Ca2+]i, there was an almost immediate activation of the PM Ca2+ extrusion system, and maximal activity was reached within less than 2s. The rate of Ca2+ extrusion was dependent on the level of [Ca2+]i, with a steep activation at values just above the resting [Ca2+]i and reached a plateau value at 700 nM Ca2+. In contrast, the PM Ca2+ entry pathway was activated with a much slower time course. There was also a delay of 3-4 s between the maximal effective depletion of the intracellular Ca2+ stores and the activation of this entry pathway. By use of digital imaging data, the PM Ca2+ transport systems were also analyzed independently in two regions of the cells, the lumenal and the basal poles. With respect to the activation of the Ca2+ entry pathways, no significant difference existed between these two regions. In contrast, the PM Ca2+ pump displayed a different pattern of activity in these regions. In the basal pole, the pump activity was more sensitive to changes of [Ca2+]i and had a higher maximal activity. Also, in the lumenal pole, the pump became saturated at values of [Ca2+]i around 700 nM, whereas at the basal pole [Ca2+]i had a biphasic effect on the pump activity, and higher [Ca2+]i inhibited the pump. It is argued that these differences in sensitivity to the levels of [Ca2+]i and the different relationship between [Ca2+]i and the rate of extrusion at the two functional poles of the pancreatic acinar cells indicate that the plasma membrane Ca2+ ATPase might play an important role in the polarization of the Ca2+ response.

Primary structure of rat plasma membrane Ca(2+)-ATPase isoform 4 and analysis of alternative splicing patterns at splice site A

We have determined the primary structure of the rat plasma membrane Ca(2+)-ATPase isoform 4 (PMCA4), and have analysed its mRNA tissue distribution and alternative splicing patterns at splice site A. Rat PMCA4 (rPMCA4) genomic clones were isolated and used to determine the coding sequences and intron/exon organization of the 5'-end of the gene, and the remaining coding sequence was determined from PCR-amplified cDNA fragments. Pairwise comparisons reveal that the amino acid sequence of rPMCA4 has diverged substantially from those of rPMCA isoforms 1, 2 and 3 (73-76% identity) and from that of human PMCA4 (87%). Despite the high degree of sequence divergence between the two species, comparisons of intron and untranslated mRNA sequences with the corresponding human sequences confirm the identity of this rat isoform as PMCA4. Northern blot studies demonstrate that the PMCA4 mRNA is expressed in all rat tissues examined except liver, with the highest levels in uterus and stomach. A combination of PCR analysis of alternative splicing patterns and sequence analysis of the gene demonstrate that a 36 nt exon at site A is included in PMCA4 mRNAs of most tissues but is largely excluded in heart and testis. Alternative splicing of both the 36 nt exon and a previously characterized 175 nt exon at splice site C, each of which can be either included or excluded in a highly tissue-specific manner, leads to the production of four different PMCA4 variants ranging in size from 1157 to 1203 amino acids.

The plasma membrane calcium pump: functional domains, regulation of the activity, and tissue specificity of isoform expression

The plasma membrane Ca2+ pump is responsible for the fine regulation of the intracellular Ca2+ level and is thus involved in the control of several cellular processes. The activity of the pump is regulated by a multiplicity of mechanisms, among which are calmodulin, acidic phospholipids, kinase-mediated phosphorylation, or an oligomerization process. The C-terminal part of the molecule interacts with the region of the pump close to the active site, leading to the decrease of the activity in the resting state. Four genes coding for different isoforms of the plasma membrane Ca2+ ATPase are known in humans. Isoform 1 and 4 represent housekeeping isoforms, whereas isoforms 2 and 3 are only present in specialized tissues. The variability of the protein is further increased by alternative RNA splicing at two sites (A, C). Alternative splicing occurs within (splice site C) or near (splice site A) regions coding for regulatory domains of the protein. In all isoforms a corresponding splice form exists at both splice sites. These common splice forms are present in all tissues, whereas isoform unique splice forms are normally only present in specialized tissues. In neuronal tissues all isoforms and almost the complete set of splice forms are found. The transcripts of the different isoforms are distributed in a region-specific manner in neuronal tissues.

Osteoblasts express the PMCA1b isoform of the plasma membrane Ca(2+)-ATPase

We report here that osteoblasts and osteoblast-like osteosarcoma cells express PMCA1b, an alternatively spliced transcript of plasma membrane Ca(2+)-ATPase. Synthetic oligonucleotide pairs were designed based upon unique regions of the cDNA encoding known PMCA isoforms (PMCA1-3) and used as primers in PCR-mediated amplification of cDNA synthesized from ROS 17/2.8 osteosarcoma cell RNA. A product was observed only when PMCA1-specific primers were present; no products were seen with PMCA2 or PMCA3 primers unless cDNA synthesized from rat brain RNA was present. Examination of the cDNA encoding the C terminus of PMCA1 from ROS 17/2.8 cells revealed that the mRNA is spliced to yield the PMCA1b isoform, a Ca(2+)-ATPase containing a consensus phosphorylation site for cAMP-dependent protein kinase A and a modified calmodulin binding domain. PMCA1b was also detected in UMR-106-01 osteosarcoma cells and unpassaged primary rat calvarial osteoblasts. These results suggest that the regulation of osteoblast function by agents that act via cAMP-mediated pathways may involve alterations in the activity of the plasma membrane Ca(2+)-ATPase.

Use of the polymerase chain reaction for the detection of alternatively spliced mRNAs of plasma membrane calcium pump

Polymerase chain reaction [PCR, reverse transcriptase-PCR (RT-PCR)] has been used to amplify the mRNA subspecies of the plasma membrane calcium pump isoform 1 (PMCA1) in total RNA extracted from hamster tissues. Two primers were synthesized that encompass the site at which a 154-bp exon is included totally (PMCA1a), partially (PMCA1c and d), or completely excluded (PMCA1b) in the carboxy-terminal regulatory region. PCR amplification revealed two bands (PMCA1b and 1c) that are more abundant in various tissues, while Southern hybridization of the samples after PCR amplification has detected two additional mRNA variants corresponding to PMCA1a and 1d. The distribution of these mRNA variants are tissue specific and correlate well with the pump protein distribution patterns on immunoblot. Since these multiple bands on the immunoblot are not derived from proteolysis, it is suggested that they represent the PMCA1 isozymes encoded by these alternatively spliced mRNAs. To our knowledge, this is the first report to show all four alternatively spliced mRNAs that are simultaneously detected in one single RNA sample using PCR technique. Since these isozymes are different in their regulatory domain, their tissue-specific expression may be physiologically important.