Microdiversity of human-plasma-membrane calcium-pump isoform 2 generated by alternative RNA splicing in the N-terminal coding region

Differential expression of PMCA2 mRNA isoforms in a cohort of Spanish patients with breast tumor types

The present study examined the mRNA expression levels of different isoforms of the plasma membrane calcium ATPase 2 (PMCA2) gene generated by alternative splicing at the first intracellular loop (site A) and C-terminal region (site C) in 85 human breast cancer tumor and 69 adjacent non-tumor tissues. Associations were identified between the expression of PMCA2 splice isoforms and the following clinical variables: Estrogen receptor (ER), progesterone receptor (PR) and human epidermal growth factor receptor 2 (HER2) status, tumor size, staging and histological classification, and lymph node status. Transcripts including splice site A or splice site C were amplified by reverse transcription-quantitative polymerase chain reaction using PMCA2 isoform-specific primers. Tumor and adjacent tissues were determined to express the different PMCA2 splice isoforms 2w, 2× and 2z (site A), and 2b (site C). The mRNA levels for these variants indicated high biological variability, but increased expression was observed in breast tumor tissues, compared with in adjacent tissues. Significantly increased PMCA2×/b expression levels were detected in breast tumor tissues histologically classified as lobulillar, compared with in ductal-types breast tumor tissues (P<0.028). Furthermore, PMCA2z expression was significantly associated with PR status (P<0.024, compared with in PR-negative tumor tissues), and PMCA2w expression was significantly associated with ER status (P<0.048, increased in ER-positive tumor tissues, compared with ER-negative tumor tissues). Finally, PMCA2b was overexpressed in HER2-positive tumor tissues, compared with in HER2-negative tumor tissues (P<0.014). The data demonstrated the differential mRNA expression of a number of splice site A and C variants of PMCA2 in breast tumor and adjacent tissues, depending on tumor hormone receptor status and histological classification. In agreement with previous data, PMCA2b was overexpressed in HER2-positive tumor tissues, indicating that high mRNA levels of this variant could be a marker of poor prognosis.

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.

Protection by dietary restriction in the YAC128 mouse model of Huntington's disease: Relation to genes regulating histone acetylation and HTT

Huntington's disease (HD) is a fatal neurodegenerative disease characterized by metabolic, cognitive, and motor deficits. HD is caused by an expanded CAG repeat in the first exon of the HTT gene, resulting in an expanded polyglutamine section. Dietary restriction (DR) increases lifespan and ameliorates age-related pathologies, including in a model of HD, but the mechanisms mediating these protective effects are unknown. We report metabolic and behavioral effects of DR in the full-length YAC128 HD mouse model, and associated transcriptional changes in hypothalamus and striatum. DR corrected many effects of the transgene including increased body weight, decreased blood glucose, and impaired motor function. These changes were associated with reduced striatal human (but not mouse) HTT expression, as well as alteration in gene expression regulating histone acetylation modifications, particularly Hdac2. Other mRNAs related to Huntington's pathology in striatal tissue showed significant modulation by the transgene, dietary restriction or both. These results establish a protective role of DR in a transgenic model that contains the complete human HTT gene and for the first time suggest a role for DR in lowering HTT level, which correlates with severity of symptoms.

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.

Alternative splice variants of plasma membrane calcium-ATPases in human corneal epithelium

Plasma membrane calcium-ATPases (PMCAs) play a critical role in regulating intracellular calcium concentration. Four genes encode PMCA proteins with alternative splicing of transcripts at three sites (A, B and C) serving to increase isoform diversity. Our previous work shows that all four PMCAs are expressed and have specific locations in human corneal epithelium (hCE). The present work examined which splice variants of PMCAs are expressed in hCE. Total RNA was extracted from hCE scraped from cadaver corneas of five different donors (two females and three males, age range 55-76 years). RT-PCR was performed using PMCA isoform-specific primers designed to amplify transcripts that included either splice site A or splice sites B and C. PMCA cDNAs were sequenced or cloned, and then sequenced. There was uniformity in the PMCA1 and PMCA4 expression profile among the five donors. Specifically, every donor expressed PMCA4 transcripts (4x at site A and 4b at site B/C). Every donor also expressed PMCA1 transcripts at sites B/C, specifically PMCA1b and PMCA1kb. In contrast, PMCA2 and PMCA3 expression varied; PCR DNAs were detected in two of five donors. One donor expressed PMCA2a and a novel PMCA2 variant we termed PMCA2((i)). PMCA3a transcript was demonstrated in a different donor. Finally, for all the donors, bands encoding site A transcripts for PMCA4 were obtained but no PCR transcripts were detected at site A for PMCA1, PMCA2 and PMCA3. This investigation showed that hCE expressed multiple splice variants of PMCA isoforms. Furthermore, this study documented the expression of the PMCA1k variant (PMCA1kb) previously only described in intestine and pancreatic beta cells and describes a novel PMCA2((i)) variant. Finally, this study suggests that the molecular configuration of PMCA1, PMCA2 and PMCA3 in the region of splice site A in hCE must be different than in other tissues since the same primers that produced site A transcripts in several other tissues were ineffective in priming PCR in hCE.

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.

Deletions in the ALregion of the h4xb plasma membrane Ca2+pump

Mutants of the plasma membrane Ca(2+) pump (human isoform 4xb) with deletions in the linker between domain A and transmembrane segment M3 (A(L) region) were constructed and expressed in Chinese hamster ovary cells. The total or partial removal of the amino acid segment 300-349 did not change the maximal Ca(2+) transport activity, but mutants with deletions involving residues 300-338 exhibited a higher apparent affinity for Ca(2+) than the wild type h4xb enzyme. Deletion of the putative acidic lipid interacting sequence (residues 339-349) had no observable functional consequences. The removal of either residues 300-314 or 313-338 resulted in a similar increase in the apparent Ca(2+) affinity of the pump although the increase was somewhat lower than that obtained by the deletion 300-349 suggesting that both deletions affected the same structural determinant. The results show that alterations in the region of the alternative splicing site A change the sensitivity to Ca(2+) of the human isoform 4 of the PMCA.

Transcribed short tandem repeats occur in couples with strongly preferred registers

Short tandem repeats (STRs) have been widely observed, but most STRs have no recognized organization or function. Here we show that for diverse mRNAs, 84% of (GC)(n) repeats were found unexpectedly coupled with another STR, (GU)(n). These STR couples exhibited preferred polarity and register. In 3(') untranslated mRNA sequences (UTRs) 100% of (GC)(n>6) repeats were tightly coupled with (GU)(n). For (GC)(n), stem folding energy correlated with the length and number of neighboring, non-folding (GU)(n) partners (p=0.014). Approximately 20% of (AU)(n>/=14) repeats were coupled with (GU)(n). The STR couple (AC)(n)(AG)(n) also exhibited polarity and register preferences. The sequence arrangement at STR-couple joints was conserved rigorously, suggesting that these sequences were under selection pressure. Some STR couples may function as mRNA processing landmarks, based on alternative transcript comparisons. These observations suggest that some transcribed STRs may be functional UTR signals with predictable organization and usage patterns.

Alternative splicing of the first intracellular loop of plasma membrane Ca2+-ATPase isoform 2 alters its membrane targeting

Plasma membrane Ca(2+)-ATPases (PMCAs) are involved in local Ca(2+) signaling and in the spatial control of Ca(2+) extrusion, but how different PMCA isoforms are targeted to specific membrane domains is unknown. In polarized MDCK epithelial cells, a green fluorescent protein-tagged PMCA4b construct was targeted to the basolateral membrane, whereas a green fluorescent protein-tagged PMCA2b construct was localized to both the apical and basolateral domain. The PDZ protein-binding COOH-terminal tail of PMCA2b was not responsible for its apical membrane localization, as a chimeric pump made of an NH(2)-terminal portion from PMCA4 and a COOH-terminal tail from PMCA2b was targeted to the basolateral domain. Deletion of the last six residues of the COOH terminus of either PMCA2b or PMCA4b did not alter their membrane targeting, suggesting that PDZ protein interactions are not essential for proper membrane localization of the pumps. Instead, we found that alternative splicing affecting the first cytosolic loop determined apical membrane targeting of PMCA2. Only the "w" form, which contains a 45-amino acid residue insertion, showed prominent apical membrane localization. By contrast, the x and z splice variants containing insertions of 14 and 0 residues, respectively, localized to the basolateral membrane. The w splice insert was the crucial determinant of apical PMCA2 localization, and this was independent of the splice configuration at the COOH-terminal end of the pump; both PMCA2w/b and PMCA2w/a showed prominent apical targeting, whereas PMCA2x/b, PMCA2z/b, and PMCA2z/a were confined to the basolateral membrane. These data report the first differential effect of alternative splicing within the first cytosolic loop of PMCA2 and help explain the selective enrichment of specific PMCA2 isoforms in specialized membrane compartments such as stereocilia of auditory hair cells.

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

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

Deletions in the acidic lipid-binding region of the plasma membrane Ca2+ pump. A mutant with high affinity for Ca2+ resembling the acidic lipid-activated enzyme

The C-terminal segment of the loop between transmembrane helices 2 and 3 (A(L) region) of the plasma membrane Ca(2+) pump (PMCA) is not conserved in other P-ATPases. Part of this region, just upstream from the third transmembrane domain, has been associated with activation of the PMCA by acidic lipids. cDNAs coding for mutants of the Ca(2+) pump isoform h4xb with deletions in the A(L) region were constructed, and the proteins were successfully expressed in either COS or Chinese hamster ovary cells. Mutants with deletions in the segment 296-349 had full Ca(2+) transport activity, but deletions involving the segment of amino acids 350-356 were inactive suggesting that these residues are required for a functional PMCA. In the absence of calmodulin the V(max) of mutant d296-349 was similar to that of the recombinant wild type pump, but its K(0.5) for Ca(2+) was about 5-fold lower. The addition of calmodulin increased the V(max) and the apparent Ca(2+) affinity of both the wild type and d296-349 enzymes indicating that the activating effects of calmodulin were not affected by the deletion. At low concentrations of Ca(2+) and in the presence of saturating amounts of calmodulin, the addition of phosphatidic acid increased about 2-fold the activity of the recombinant wild type pump. In contrast, under these conditions phosphatidic acid did not significantly change the activity of mutant d296-349. Taken together these results suggest that (a) deletion of residues 296-349 recreates a form of PMCA similar to that resulting from the binding of acidic lipids at the A(L) region; (b) the A(L) region acts as an acidic lipid-binding inhibitory domain capable of adjusting the Ca(2+) affinity of the PMCA to the lipid composition of the membrane; and (c) the function of the A(L) region is independent of the autoinhibition by the C-terminal calmodulin-binding region.

Plasma membrane Ca2+-atpase isoforms 2b and 4b interact promiscuously and selectively with members of the membrane-associated guanylate kinase family of PDZ (PSD95/Dlg/ZO-1) domain-containing proteins

Spatial and temporal regulation of intracellular Ca(2+) signaling depends on localized Ca(2+) microdomains containing the requisite molecular components for Ca(2+) influx, efflux, and signal transmission. Plasma membrane Ca(2+)-ATPase (PMCA) isoforms of the "b" splice type contain predicted PDZ (PSD95/Dlg/ZO-1) interaction domains. The COOH-terminal tail of PMCA2b isolated the membrane-associated guanylate kinase (MAGUK) protein SAP97/hDlg as a binding partner in a yeast two-hybrid screen. The related MAGUKs SAP90/PSD95, PSD93/chapsyn-110, SAP97, and SAP102 all bound to the COOH-terminal tail of PMCA4b, whereas only the first three bound to the tail of PMCA2b. Coimmunoprecipitations confirmed the interaction selectivity between PMCA4b and SAP102 as opposed to the promiscuity of PMCA2b and 4b in interacting with other SAPs. Confocal immunofluorescence microscopy revealed the exclusive presence and colocalization of PMCA4b and SAP97 in the basolateral membrane of polarized Madin-Darby canine kidney epithelial cells. In hippocampal neurons, PMCA2b was abundant throughout the somatodendritic compartment and often extended into the neck and head of individual spines where it colocalized with SAP90/PSD95. These data show that PMCA "b" splice forms interact promiscuously but also with specificity with different members of the PSD95 family of SAPs. PMCA-SAP interactions may play a role in the recruitment and maintenance of the PMCA at specific membrane domains involved in local Ca(2+) regulation.

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.

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

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

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.

Ca2+-ATPases and their expression in the mammary gland of pregnant and lactating rats

The transcellular Ca2+ fluxes required for milk production must be rigorously regulated to maintain the low cytosolic Ca2+ concentrations critical to cell function. Ca2+-ATPases play a critical role in the maintenance of this cellular Ca2+ homeostasis. Using RT-PCR and sequencing, we identified six Ca2+ pumps in lactating mammary tissue. Three plasma membrane Ca2+-ATPases (PMCAs) were found (PMCA1b, PMCA2b, and PMCA4b). Two sarco (endo)plasmic reticulum Ca2+-ATPases (SERCAs) were identified (SERCA2 and SERCA3), and the rat homologue to the yeast Golgi Ca2+-ATPase RS-10 was also found. The pattern of mRNA expression of each of these pumps was examined in rat mammary tissue from the 7th day of pregnancy to the 21st day of lactation. Northern blots revealed increased mRNA expression for all Ca2+ pumps by the 14th day of lactation, and transcripts continued to increase through the 18th day of lactation. PMCA1b, PMCA4b, SERCA2, and SERCA3 showed the lowest levels of expression. RS-10 transcripts were more abundant than SERCA2, SERCA3, PMCA1b, and PMCA4b. RS-10 was the only pump to increase in expression before parturition. PMCA2b was the most abundant transcript found in lactating mammary tissue. At peak lactation, expression of PMCA2b approached that of actin. The high expression, high affinity for Ca2+, and high activity at low calmodulin concentrations exhibited by PMCA2b suggest that it is uniquely suited for maintenance of Ca2+ homeostasis in the lactating mammary gland. The pattern of expression and abundance of RS-10 suggest that it is a candidate for the Golgi Ca2+-ATPase shown to be important in maintaining the Golgi Ca2+ concentration required for casein synthesis and micelle formation.

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.

A novel calmodulin-regulated Ca2+-ATPase (ACA2) from Arabidopsis with an N-terminal autoinhibitory domain

To study transporters involved in regulating intracellular Ca2+, we isolated a full-length cDNA encoding a Ca2+-ATPase from a model plant, Arabidopsis, and named it ACA2 (Arabidopsis Ca2+-ATPase, isoform 2). ACA2p is most similar to a "plasma membrane-type" Ca2+-ATPase, but is smaller (110 kDa), contains a unique N-terminal domain, and is missing a long C-terminal calmodulin-binding regulatory domain. In addition, ACA2p is localized to an endomembrane system and not the plasma membrane, as shown by aqueous-two phase fractionation of microsomal membranes. ACA2p was expressed in yeast as both a full-length protein (ACA2-1p) and an N-terminal truncation mutant (ACA2-2p; Delta residues 2-80). Only the truncation mutant restored the growth on Ca2+-depleted medium of a yeast mutant defective in both endogenous Ca2+ pumps, PMR1 and PMC1. Although basal Ca2+-ATPase activity of the full-length protein was low, it was stimulated 5-fold by calmodulin (50% activation around 30 nM). In contrast, the truncated pump was fully active and insensitive to calmodulin. A calmodulin-binding sequence was identified within the first 36 residues of the N-terminal domain, as shown by calmodulin gel overlays on fusion proteins. Thus, ACA2 encodes a novel calmodulin-regulated Ca2+-ATPase distinguished by a unique N-terminal regulatory domain and a non-plasma membrane localization.

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.

Immunolocalization of the plasma membrane Ca2+ pump isoforms in the rat brain

Ca2+ homeostasis in nerve cells is dependent on at least three mechanisms: Ca2+ channels, calcium-binding proteins and Ca2+ exchangers/pumps. Only limited information is available on the regional/cellular distribution of these Ca2+-regulating systems in the brain. The distribution of three of the isoforms of one of the systems, plasma membrane Ca2+-ATPase (PMCA), was analyzed in this study. Using antibodies against epitopes specific for each isoform, a map of the distribution of the pump in the whole brain was produced. The pump was mainly expressed in neurons and was apparently absent from glia cells. Isoform 1 was ubiquitous and occurred in varying, but always significant, concentrations in almost all nerve cells. Isoform 2 was abundant in cerebellar Purkinje cells but less concentrated in other brain regions. Isoform 3 had a predominantly extra neuronal location, e.g. it was abundant in the choroid plexuses. The three isoforms were found to be distributed in a highly characteristic manner, suggesting that nerve cells have different requirements for the preservation of their intracellular calcium homeostasis.

Analysis of plasma membrane Ca(2+)-ATPase expression in control and SV40-transformed human fibroblasts

It has been long known that neoplastic transformation is accompanied by a lowered requirement for extracellular Ca2+ for growth. The studies presented here demonstrate that human fibroblastic cell lines produce the two commonly found 'housekeeping' isoforms of the plasma membrane Ca(2+)-ATPase (PMCA), PMCA1b and 4b, and at the expression of both is demonstrably lower in cell lines neoplastically transformed by SV40 than in the corresponding parental cell lines. Western blot analyses of lysates from control (GM00037) and SV40-transformed (GM00637) skin fibroblasts revealed a 138 kDa PMCA whose level was significantly lower in the SV40-transformed cells relative to either total cellular protein or alpha-tubulin. Similar analyses of plasma membrane preparations from control WI-38) and SV40-transformed (WI-38VA13) lung fibroblasts revealed 3-4-fold lower levels of PMCA in the SV40-transformed cells. Competitive ELISAs performed on detergent solubilized plasma membrane preparations indicated at least 3-4-fold lower levels of PMCA in the SV40-transformed cell lines compared to controls. Reverse transcriptase coupled-PCR analyses showed that PMCA1b and PMCA4b were the only isoforms expressed in all four cell lines. The PMCA4b mRNA level detected by Northern analysis also was substantially lower in SV40 transformed skin fibroblasts than in non-transformed fibroblasts. Quantitative RT-PCR analyses showed levels of PMCA1b and 4b mRNAs to be 5 and 10-fold lower, respectively, in GM00637 than in GM00037 when the levels of PCR products were normalized to glyceraldehyde-3-phosphate dehydrogenase (G3PDH) mRNA. These results demonstrate that the expression of these distinct PMCA genes is substantially lower in SV40 transformed human skin and lung fibroblasts and may be coordinately regulated in these cells.

Primary structure of human plasma membrane Ca(2+)-ATPase isoform 3

The complete coding sequence of the human plasma membrane calcium ATPase (PMCA) isoform 3 was determined from overlapping genomic and cDNA clones. The cDNAs for the two major alternative splice variants 3a (3CII) and 3b (3CI) code for proteins of 1173 and 1220 amino-acid residues, respectively, which show 98% identity with the corresponding rat isoforms. On a multiple human tissue Northern blot, a major PMCA3 transcript of about 7 kb was detected exclusively in the brain, demonstrating the highly restricted pattern of expression of this isoform to human neuronal tissues. With the elucidation of the human PMCA3 primary structure, complete sequence information is now available for the entire family of human PMCA isoforms.

Expression and functional characterization of isoforms 4 of the plasma membrane calcium pump

PMCA isoforms 4CII (generated by splicing at the C-terminus) and 4BICI (a pump version lacking the 10th transmembrane domain) were expressed in Sf9 cells using the baculovirus system. The purified PMCA4CII had a 20-fold lower affinity for calmodulin than the PMCA4CI, the PMCA4 isoform of the erythrocytes' membranes, but had a higher activity in the absence of calmodulin. The amount of phosphoenzyme intermediate formed by PMCA4CII in the presence of Ca2+ alone was almost 3 times higher than in PMCA4CI and was increased by La3+ less than in the PMCA4CI. The isoform lacking the 10th transmembrane domain (PMCA4BICI) had no Ca2+-dependent ATPase activity, but was still able to form the phosphoenzyme intermediate starting from phosphate. When expressed in COS cells, this isoform was retained in the endoplasmic reticulum; changes in membrane architecture apparently occurred during its expression; the C-terminal portion of the isoform was located in the cytosol, indicating that the deletion of the 10th transmembrane domain resulted in the loss of at least another transmembrane domain.

Analysis of the 5' end of the rat plasma membrane Ca(2+)-ATPase isoform 3 gene and identification of extensive trinucleotide repeat sequences in the 5' untranslated region

We have characterized the 5' end of the rat gene encoding isoform 3 of the plasma membrane Ca(2+)-ATPase using S1 nuclease protection and DNA sequence analysis. The 5'-untranslated region consists of over 900 nucleotides and includes a 217-nucleotide sequence composed of alternating tracts of TCC and ACC trinucleotides. Analysis of genomic sequences 5' to the transcription initiation site revealed potential binding sites for transcription factors that are active in muscle and brain.

Reverse transcriptase-polymerase chain reaction (RT-PCR): a sensitive method to examine basic fibroblast growth factor-induced expression of the early growth response gene-1 (egr-1) in human umbilical arterial endothelial cells

Immediate-early genes are expressed upon growth and differentiation in a large variety of cells and species. In the present study we investigated the effect of basic fibroblast growth factor (bFGF) on early growth response gene-1 (egr-1)-mRNA expression in human umbilical arterial endothelial cells (HUAEC). The detection of this gene in HUAEC was performed by Northern blotting and by reverse transcriptase-polymerase chain reaction (RT-PCR). For RT-PCR specific primers for egr-1 and glyceraldehyde-3-phosphate-dehydrogenase (GAPDH) were constructed and PCR conditions were optimized. bFGF induced a time- and concentration-dependent increase of egr-1 expression. Maximal expression occurred within 30 min of stimulation with bFGF at a concentration of 50-100 ng ml-1. RT-PCR gave highly reproducible and specific results. The comparison of both methods showed comparable results but a higher sensitivity for RT-PCR in detecting the egr-1 mRNA. RT-PCR is an excellent method for detecting the expression of egr-1 mRNA in HUAEC.

Characterisation of endoplasmic reticulum and plasma membrane Ca(2+)-ATPases in pancreatic beta-cells and in islets of Langerhans

We have investigated the plasma membrane (PMCA) and endoplasmic reticulum (SERCA) Ca(2+)-ATPases involved in active transport of Ca2+ in pancreatic beta-cell lines (MIN6, HIT T15, RINm5F) and in islets of Langerhans. Under selective membrane phosphorylation conditions (at low ATP concentration, in the presence of Ca2+ and La3+ and in the absence of Mg2+ at 4 degrees C) the only labelled proteins are the phosphoenzyme intermediates of the Ca(2+)-ATPases. Under these conditions, beta-cell membranes incorporated 32P from [gamma-32P]ATP into two proteins with molecular mass on acidic SDS-polyacrylamide gels of around 115 and 150 kDa. The 150 kDa band was identified as PMCA (i) by reaction with a monoclonal anti-human erythrocyte plasma membrane Ca(2+)-ATPase antibody; (ii) by its typical tryptic cleavage pattern which generated an 80 kDa band; (iii) by lack of inhibition of its autophosphorylation by SERCA-specific inhibitors. The 115 kDa band was identified as SERCA (i) by reaction with a polyclonal anti-rat fast skeletal muscle Ca(2+)-ATPase antibody; (ii) by the concentration-dependent inhibition of its autophosphorylation by thapsigargin and 2,5-di(t-butyl)-1,4-benzohydroquinone (tBHQ), which are specific inhibitors of SERCA. The 115 kDa band was further characterised as the SERCA-2b isoform by reaction with a polyclonal rabbit antibody against the 12 C-terminal amino acids of SERCA-2b.

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.

Transcript distribution of plasma membrane Ca2+ pump isoforms and splice variants in the human brain

Plasma membrane calcium pumps (PMCAs) play a major role in the maintenance and fine regulation of the intracellular Ca2+ concentration. Fourteen subregions of the normal human brain were carefully dissected and analyzed by reverse transcriptase-polymerase chain reaction for the distribution of mRNAs corresponding to the four known PMCA genes as well as their alternative splicing products at two previously defined 'hotspots' A and C. All PMCA genes were found to be expressed in every brain subregion; however, consistent differences were found in the distribution of alternative splice options. The four cortical regions and hippocampus were characterized by the relative preference of variants that include an entire exon at site C and lead to the expression of isoforms of the a-type. Inferior olive and olfactory bulb showed a relative preponderance of the b-form 'default' types of alternative splicing at site C, and a decrease or even the lack of 'differentiated' forms such as variants 1a and 1c. At the N-terminal splice site A, the default x-type variants were predominant in all brain regions for PMCA 1, 3, and 4. By contrast, the pattern of PMCA2 variants was the most variable, ranging from the presence of the entire set of 2x, 2w, and 2z forms in inferior olive to the almost exclusive presence of form 2z (excluding all alternatively spliced sequences) in the four cortical regions, caudate, and hippocampus. Regional differences in the PMCA splice type distribution in normal human brain may correlate with different demands on the regulation of the set-point resting Ca2+ levels in these areas. Changes in these patterns may correlate with altered physiological states of the affected regions and/or reflect an (early) sign of Ca2+ dyshomeostasis characteristic of many neurodegenerative diseases.

Plasma-membrane calcium-pump isoforms in human and rat liver

Plasma-membrane Ca(2+)-pumping ATPases (PMCAs) extrude Ca2+ from the cytoplasm of all cells. Some previous studies of ATP-dependent Ca2+ transport by liver membranes suggested there exist specific properties of the hepatic PMCA, including regulation by hormones which affect calcium signalling. Multiple PMCA isoforms are now known to result from expression of four different genes (known as PMCA 1-4) and alternative RNA splicing at three possible sites (A, B and C). We investigated which isoforms are expressed in adult human and rat liver RNA using reverse-transcription polymerase chain reaction with mixed primers designed to amplify parts of all the known PMCA transcripts. In human liver, products were identified by sequencing from PMCA1, PMCA2 and PMCA4, but not from PMCA3 or from any new gene. In rat liver, by contrast, only PMCA1 and PMCA2 were detectable, although we confirmed that the primers were able to amplify from rat lung a new sequence which is part of rat PMCA4. Of the alternatively spliced variants, at site A in the PMCA2 sequences, all the exons were included in both adult and fetal human liver. In human liver, the exon at site B was excluded in some products from PMCA1 and PMCA4, and at site C, only PMCA1b and one form of PMCA4 were found. Blots of human liver RNA showed PMCA1 and PMCA4 were abundantly expressed, unlike PMCA2. On blots of rat liver RNA, PMCA1 was more abundant than PMCA2, and purified rat parenchymal cell RNA gave similar findings. In summary, no new hepatic PMCA isoforms have been demonstrated, but differences between the predominant human and rat isoforms may have consequences for Ca2+ signalling or the response to liver cell injury.

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.

Molecular cloning of a plasma membrane calcium pump from human osteoblasts

The osteoblast plays a critical role in bone formation, bone remodelling, bone matrix formation, and matrix calcification. To better understand the process of osteoblast-controlled bone formation, we determined the structure and isoform types of the plasma membrane calcium pump from normal human osteoblasts. A complementary DNA library from normal human osteoblasts was screened for plasma membrane calcium pump clones. Sequencing and analysis of cDNA clones revealed the presence of a 3986 base pair cDNA that encoded a 1220 amino acid protein that was similar to the human plasma membrane calcium pump isoform 1. Polyadenylated RNA from human osteoblast cells contains bands of RNA approximately 5050 and 6750 bases long. Reverse transcription of polyadenylated RNA from human osteoblasts followed by amplification of the RNA-DNA duplex with calcium pump isoform-specific primers revealed the presence of isoforms 1 and 2 of the calcium pump. Isoform 4 was not detected. We conclude that normal adult human osteoblasts contain a plasma membrane calcium pump that is similar to the human plasma membrane calcium pump isoform 1. It is likely that this pump plays an important role in the cell biology of the human osteoblast.

Localization of the intronless gene coding for calmodulin-like protein CLP to human chromosome 10p13-ter

The functional intronless gene coding for a calmodulin-like protein (CLP) has been localized to human chromosome 10p13-ter. Chromosomal assignment was performed by Southern blot analysis of DNA from human-rodent somatic cell hybrids and amplification of a CLP gene-specific 1090-bp DNA fragment by the polymerase chain reaction (PCR) on DNA from human-hamster cell hybrids. Chromosomal sublocalization was carried out by in situ hybridization of human chromosome metaphase spreads. The CLP gene is the first member of the human calmodulin/calmodulin-like gene family to be chromosomally sublocalized. Its presence near the telomeric end of the short arm of chromosome 10 may be of significance with respect to its highly (epithelial) cell-type restricted expression in vivo and strong downregulation upon malignant transformation. The generation of a human CLP gene-specific sequence tag site specified by the two primers used for PCR should prove useful for future linkage studies.

Ca(2+)-transport ATPases and their regulation in muscle and brain

Eukaryotic cells express one or more isoforms of a sarco(endo)plasmic reticulum (SERCA) and of a plasma membrane (PMCA) Ca2+ pump. Both the SERCA and PMCA gene transcripts are subject to alternative processing in a differentiation stage-dependent and tissue-dependent manner. The Ca2+ pump isoforms thus generated may present different functional properties. This is exemplified by the SERCA2a and SERCA2b isoforms which differ in their Ca2+ sensitivity. Analysis of the cDNA structures for PMCA1 predicts protein isoforms with variant calmodulin- and phospholipid-binding domains. A comparative study of the tissue-specific mechanisms governing SERCA-PMCA transcript processing and a more detailed study of the functional implication of the PMCA pumps isoform diversity will be challenging subjects for future studies.

Expression of plasma membrane calcium-pumping ATPase mRNAs in developing rat brain and adult brain subregions: evidence for stage-specific expression

The plasma membrane calcium-pumping ATPases (Ca(2+)-ATPases) maintain resting free cytosolic calcium concentrations in cells at the submicromolar level. These Ca(2+)-ATPases are encoded by four genes that can be alternately spliced to produce nine different mRNAs, each of which has a unique tissue-specific distribution. Examination of the expression of these mRNAs in rat brain during development revealed that transcripts from three of the four known genes are expressed by the end of gestation. However, the stage of transcription induction varies among the isoforms. The mRNA encoding plasma membrane Ca(2+)-ATPase (PMCA) 1b, the isoform though to maintain a housekeeping function, was present from embryonic day 10. The other alternatively spliced PMCA1 mRNAs, PMCA1a and c, which are preferentially expressed in the brain, did not appear until embryonic day 14. PMCA2a mRNA and the alternatively spliced PMCA2b and c transcripts were coordinately induced on embryonic day 18. The PMCA3a transcript first appeared on embryonic day 18 but did not reach steady-state levels until postnatal day 3, whereas production of PMCA3b mRNA first occurred on embryonic day 10 and reached steady-state expression by embryonic day 18. Several PMCA mRNAs tested varied in expression in specific regions of the brain that were examined at three postnatal time points.