Molecular cloning and sequencing of the plasma-membrane Ca2+ pump of pig smooth muscle

Evolving mechanisms of vascular smooth muscle contraction highlight key targets in vascular disease

Vascular smooth muscle (VSM) plays an important role in the regulation of vascular function. Identifying the mechanisms of VSM contraction has been a major research goal in order to determine the causes of vascular dysfunction and exaggerated vasoconstriction in vascular disease. Major discoveries over several decades have helped to better understand the mechanisms of VSM contraction. Ca2+ has been established as a major regulator of VSM contraction, and its sources, cytosolic levels, homeostatic mechanisms and subcellular distribution have been defined. Biochemical studies have also suggested that stimulation of Gq protein-coupled membrane receptors activates phospholipase C and promotes the hydrolysis of membrane phospholipids into inositol 1,4,5-trisphosphate (IP3) and diacylglycerol (DAG). IP3 stimulates initial Ca2+ release from the sarcoplasmic reticulum, and is buttressed by Ca2+ influx through voltage-dependent, receptor-operated, transient receptor potential and store-operated channels. In order to prevent large increases in cytosolic Ca2+ concentration ([Ca2+]c), Ca2+ removal mechanisms promote Ca2+ extrusion via the plasmalemmal Ca2+ pump and Na+/Ca2+ exchanger, and Ca2+ uptake by the sarcoplasmic reticulum and mitochondria, and the coordinated activities of these Ca2+ handling mechanisms help to create subplasmalemmal Ca2+ domains. Threshold increases in [Ca2+]c form a Ca2+-calmodulin complex, which activates myosin light chain (MLC) kinase, and causes MLC phosphorylation, actin-myosin interaction, and VSM contraction. Dissociations in the relationships between [Ca2+]c, MLC phosphorylation, and force have suggested additional Ca2+ sensitization mechanisms. DAG activates protein kinase C (PKC) isoforms, which directly or indirectly via mitogen-activated protein kinase phosphorylate the actin-binding proteins calponin and caldesmon and thereby enhance the myofilaments force sensitivity to Ca2+. PKC-mediated phosphorylation of PKC-potentiated phosphatase inhibitor protein-17 (CPI-17), and RhoA-mediated activation of Rho-kinase (ROCK) inhibit MLC phosphatase and in turn increase MLC phosphorylation and VSM contraction. Abnormalities in the Ca2+ handling mechanisms and PKC and ROCK activity have been associated with vascular dysfunction in multiple vascular disorders. Modulators of [Ca2+]c, PKC and ROCK activity could be useful in mitigating the increased vasoconstriction associated with vascular disease.

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

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

Ion channels in smooth muscle: regulators of intracellular calcium and contractility

Smooth muscle (SM) is essential to all aspects of human physiology and, therefore, key to the maintenance of life. Ion channels expressed within SM cells regulate the membrane potential, intracellular Ca2+ concentration, and contractility of SM. Excitatory ion channels function to depolarize the membrane potential. These include nonselective cation channels that allow Na+ and Ca2+ to permeate into SM cells. The nonselective cation channel family includes tonically active channels (Icat), as well as channels activated by agonists, pressure-stretch, and intracellular Ca2+ store depletion. Cl--selective channels, activated by intracellular Ca2+ or stretch, also mediate SM depolarization. Plasma membrane depolarization in SM activates voltage-dependent Ca2+ channels that demonstrate a high Ca2+ selectivity and provide influx of contractile Ca2+. Ca2+ is also released from SM intracellular Ca2+ stores of the sarcoplasmic reticulum (SR) through ryanodine and inositol trisphosphate receptor Ca2+ channels. This is part of a negative feedback mechanism limiting contraction that occurs by the Ca2+-dependent activation of large-conductance K+ channels, which hyper polarize the plasma membrane. Unlike the well-defined contractile role of SR-released Ca2+ in skeletal and cardiac muscle, the literature suggests that in SM Ca2+ released from the SR functions to limit contractility. Depolarization-activated K+ chan nels, ATP-sensitive K+ channels, and inward rectifier K+ channels also hyperpolarize SM, favouring relaxation. The expression pattern, density, and biophysical properties of ion channels vary among SM types and are key determinants of electrical activity, contractility, and SM function.

Characterization and expression of plasma membrane Ca2+ ATPase (PMCA3) in the crayfish Procambarus clarkii antennal gland during molting

The discontinuous pattern of crustacean cuticular mineralization (the molting cycle) has emerged as a model system to study the spatial and temporal regulation of genes that code for Ca2+-transporting proteins including pumps, channels and exchangers. The plasma membrane Ca2+-ATPase (PMCA) is potentially of significant interest due to its role in the active transport of Ca2+ across the basolateral membrane, which is required for routine maintenance of intracellular Ca2+ as well as unidirectional Ca2+ influx. Prior research has suggested that PMCA expression is upregulated during periods of elevated Ca2+ influx associated with postmolt cuticular mineralization. This paper describes the cloning, sequencing and functional characterization of a novel PMCA3 gene from the antennal gland (kidney) of the crayfish Procambarus clarkii. The complete sequence, the first obtained from a non-genetic invertebrate species, was obtained through reverse transcription-polymerase chain reaction (RTPCR) and rapid amplification of cDNA ends (RACE) techniques. Crayfish PMCA3 consists of 4148 bp with a 3546 bp open reading frame coding for 1182 amino acid residues with a molecular mass of 130 kDa. It exhibits 77.5-80.9% identity at the mRNA level and 85.3-86.9% identity at the protein level with PMCA3 from human, mouse and rat. Membrane topography was typical of published mammalian PMCAs. Northern blot analysis of total RNA from crayfish gill, antennal gland, cardiac muscle and axial abdominal muscle revealed that a 7.5 kb species was ubiquitous. The level of PMCA3 mRNA expression in all tissues (transporting epithelia and muscle) increased significantly in pre/postmolt stages compared with relatively low abundance in intermolt. Western analysis confirmed corresponding changes in PMCA protein expression (130 kDa).

Interspecies differences in hepatic Ca(2+)-ATPase activity and the effect of cold preservation on porcine liver Ca(2+)-ATPase function

The accumulation of intracellular calcium ([Ca(2+)](i)) caused by ischemia-reperfusion during liver transplantation has been implicated as a factor leading to primary graft nonfunction. Plasma membrane (PM) and endoplasmic reticulum (ER) Ca(2+)-adenosinetriphosphatases (ATPases) are the primary transporters that maintain [Ca(2+)](i) homeostasis in the liver. We hypothesized that the porcine liver is better than the rat liver as a model for the study of human liver Ca(2+)-ATPase activity. We also hypothesized that cold preservation would depress Ca(2+)-ATPase activity in the porcine liver. Pig and rat livers were harvested, and human liver samples were obtained from surgical resection specimens. All were preserved with University of Wisconsin solution, and porcine livers were also preserved on ice for 2 to 18 hours. Ca(2+)-ATPase activity was measured after incubation with (45)Ca(2+) and adenosine triphosphate in the presence of specific Ca(2+)-ATPase inhibitors. Porcine PM and ER Ca(2+)-ATPase activities were 0.47 +/- 0.03 and 1.57 +/- 0.10 nmol of Ca(2+)/mg of protein/min, respectively. This was not significantly different from human liver, whereas rat liver was significantly greater at 2.60 +/- 0.03 and 9.2 +/- 0.9 nmol of Ca(2+)/mg of protein/min, respectively. We conclude that the Ca(2+)-ATPase activity in the pig liver is equivalent to that of human liver, and thus, the pig liver is a better model than the rat liver. Cold preservation studies showed a significant decrease in porcine hepatic PM Ca(2+)-ATPase activity after 4 hours of storage and near-total inhibition after 12 hours. Porcine hepatic ER Ca(2+)-ATPase activity showed a 45% decrease in activity by 12 hours and a 69% decrease by 18 hours. We conclude that cold ischemia at clinically relevant times depresses PM Ca(2+)-ATPase more than ER Ca(2+)-ATPase activity in pig liver homogenates.

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 plasma membrane calcium ATPases in phenotypically distinct canine vascular smooth muscle cells

Our laboratory has identified at least two types of vascular smooth muscle cells (VSMCs) that exist in canine arteries and veins: type 1 cells, located in the media express muscle specific proteins but do not proliferate in culture; and type 2 cells, located in both media and adventitia, do not express muscle specific protein but proliferate in culture. Plasma membrane Ca(2+)-ATPases (PMCAs) have been implicated in proliferation control. The present study examines the expression of PMCA isoforms and calmodulin-binding domain splice variants in these two types of canine VSMCs. PMCA protein was found in both type 1 and type 2 cells. Reverse transcriptase-polymerase chain reaction assays were developed for canine PMCA calmodulin-binding domain splice variants. We cloned and sequenced isolates corresponding to PMCA1b, 4a and 4b from canine VSMCs. PMCA 2 and 3 were not detected. Freshly isolated type 1 cells expressed PMCA 1b, 4a and 4b, while freshly isolated type 2 cells expressed PMCA1b and 4b. Upon placement in culture, type 2 cells originating from either carotid artery or saphenous vein demonstrated a time-dependent upregulation of PMCA4a mRNA. Treatment with the phosphoinositide 3-kinase inhibitor wortmannin produced concentration-dependent inhibition of both PMCA4a upregulation and [(3)H]thymidine incorporation. These findings suggest a role for phosphoinositide 3-kinase in regulating PMCA expression, which may be important in the control of Ca(2+)-sensitive VSMC functions.

Stimulation of plasma membrane Ca2+ -pump ATPase of vascular smooth muscle by cGMP-dependent protein kinase: functional reconstitution with purified proteins

A 240-kDa protein isolated from porcine aortic smooth muscle as a substrate for cGMP-dependent protein kinase (cGMP kinase) whose phosphorylation was in a close association with stimulation of partially purified plasma membrane Ca2+ -pump ATPase by the kinase was later shown to represent splicing variants of type 1 inositol 1,4,5-trisphosphate (IP3) receptor. To further clarify the role played by this protein in the stimulation of Ca2+ -pumpATPase, it was attempted in thepresent study to specifically remove the protein by immunoprecipitation with an antibody specific to type 1 IP3 receptor. Contrary to expectation, stimulation of the ATPase by cGMP kinase was still observed after removal of the IP3 receptor. Furthermore, cGMP kinase stimulated a highly purified preparation of Ca2+ -pump ATPase deprived of IP3 receptor when the concentrations of the ATPase were low enough (10-20 nM) to make it retain a monomeric form, while it did not produce stimulation when the concentration of the enzyme was increased to 40 nM at which the enzyme is known to take an oligomeric, fully activated form insensitive to activation by calmodulin. Heat-inactivated cGMP kinase and cGMP kinase without cGMP failed to stimulate the highly purified Ca2+ -pumpATPase. In addition, type Ialpha but not type Ibeta cGMP kinase was found to stimulate the ATPase. The stimulation of Ca2+ -pump ATPase by cGMP kinase occurs without any detectable phosphorylation of the ATPase. In conclusion, cGMP kinase can stimulate the plasma membrane Ca2+ -pump ATPase when it is in a monomeric form without phosphorylating the Ca2+ -pump ATPase and that of the two cGMP kinase isozymes found in the vascular smooth muscle, only type Ialpha cGMP kinase participates in the stimulation.

Cytokines induce airway smooth muscle cell hyperresponsiveness to contractile agonists

The important pathophysiological features of the airways in asthma include exaggerated narrowing to bronchoconstrictor agonists and attenuated relaxation to beta adrenoceptor stimulation. These physiological perturbations are associated with inflammation and remodelling of the airways, the latter including an increase in airway smooth muscle cell mass, disruption of the airway epithelium, and changes in the airway tissue extracellular matrix. Recent evidence suggests that cytokines, important molecules modulating airway inflammation, also directly decrease airway smooth muscle responsiveness to beta adrenergic agents, stimulate cytokine secretion, inhibit or promote airway smooth muscle proliferation, and "prime" airway smooth muscle to become hyperresponsive to bronchoconstrictors. Characterisation of the cellular and biochemical events that are involved in activation of airway smooth muscle is likely to be the major consideration in the design of future therapies for asthma. Because calcium is an essential regulatory element for cell growth and cell contraction, it is likely that alterations in calcium mobilisation may, in part, play a role in creating an airway smooth muscle phenotype that is hyperresponsive to contractile agonists. Further studies will be required to determine the precise mechanisms involved in cytokine modulation of calcium homeostasis in airway smooth muscle.

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.

Purification of the synaptosomal plasma membrane (Ca(2+) + Mg(2+))-ATPase from pig brain

The Ca(2+)-ATPase from the synaptosomal plasma membrane has been purified nearly to homogeneity from pig brain by a new procedure involving the calmodulin-affinity-chromatography technique. This is a convenient alternative to the standard methods for the purification of the plasma membrane Ca(2+)-ATPase from different sources that were unsuitable to purify the enzyme from pig brain. The main feature of this procedure is the use of 15% (v/v) glycerol as stabilizing agent, instead of acidic phospholipid. By using this protocol the enzyme was purified 36-fold with respect to the plasma membrane vesicle fraction, showing a specific activity of 2.3 i.u. in the presence of acidic phospholipid. In SDS/PAGE, it appears as a single protein band around Mr140 000 that can be phosphorylated by [gamma-(32)P]ATP in the presence of La(3+) and recognized by specific antibodies against the plasma membrane Ca(2+)-ATPase from pig antral smooth muscle. Calmodulin activates the enzyme 1.5-1.8-fold in the presence of phosphatidylcholine but not in the presence of phosphatidylserine.

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.

Cloning and characterization of a putative calcium-transporting ATPase gene from Schistosoma mansoni

Complementary DNA was isolated, encoding a putative Ca(2+)-transport ATPase (SMA1) of the human parasitic trematode Schistosoma mansoni. The cDNA was isolated by a nested polymerase chain reaction based strategy. The oligonucleotides used were designed on the basis of conserved amino-acid regions found in P-type ATPases. The complete nucleotide sequence was determined. The primary structure and topology of the enzyme were deduced. SMA1 has 1022 amino acids and a predicted molecular mass of 113 kDa. This protein is 67% identical and phylogenetically related to several sarco/endoplasmic reticulum Ca(2+)-ATPases but lacks the phospholamban-binding domain that exists in the SERCA isoforms 1 and 2. The membrane topology predicted for SMA1 is characteristic of the P-type ATPases, showing two major cytoplasmic loops and ten conserved hydrophobic segments. Sequences and residues that are important for the function of the SER Ca(2+)-ATPase, such as the high-affinity Ca(2+)-binding sites, the putative fluorescein isothiocyanate binding site, the 5'-(p-fluorosulfonyl)benzoyladenosine binding site and the aspartyl phosphorylation site, are conserved in SMA1, suggesting that the cloned gene is a Ca(2+)-transport ATPase of the SERCA family. In addition, three PCR products were cloned which share homology with another SER Ca(2+)-ATPase, with the yeast secretory pathway Ca(2+)-ATPase PMR1 and its mammalian homologue, and with the alpha subunit of a Na+,K(+)-ATPase.

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.

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.

Alternative processing of the gene transcripts encoding a plasma-membrane and a sarco/endoplasmic reticulum Ca2+ pump during differentiation of BC3H1 muscle cells

The effect of differentiation on the RNA processing of the PMCA1 gene encoding a plasma-membrane Ca2+ pump and of the SERCA2 gene encoding a sarco(endo)plasmic reticulum Ca2+ pump was studied in the myogenic BC3H1 cell line. A differentiation stage-dependent change in the RNA processing was observed for both genes. Proliferating myoblasts only expressed the non-muscle mRNA isoform whereas in differentiated cells muscle-specific processing became activated. The switch to muscle-specific RNA processing for both the PMCA1 and SERCA2 genes was found to be linked to the myogenic conversion of the BC3H1 cells. Our results furthermore indicated that the myogenic RNA processing could be reversed for both types of Ca2+ pumps since the expression of the PMCA1 and SERCA2 muscle-specific messengers was rapidly down-regulated by cycloheximide treatment.

In situ hybridization and immunocytochemical localization of SERCA2 encoded Ca2+ pump in rabbit heart and stomach

Heart tissue contains large amounts of the protein encoded by the Ca2+ pump gene SERCA2. The SERCA2 RNA can be spliced alternatively to produce mRNA encoding the proteins SERCA2a and SERCA2b which differ in their C-terminal sequences. In this study we report the tissue distribution of SERCA2a and SERCA2b isoforms by in situ hybridization to rabbit heart and stomach. The expression of SERCA2 mRNA was high in myocardial cells, being the highest in the atrial region. In contrast, there was more SERCA2 protein in Western blots in ventricles than in atria. Myocardial cells expressed predominantly the mRNA for the isoform SERCA2a. Whereas the stomach smooth muscle and the neuronal plexus expressed SERCA2 at levels much lower than myocardial cells, the expression was very high in the stomach mucosa. Mucosa contained mainly the mRNA for SERCA2b. From immunocytochemistry it was concluded that the anti-heart SR Ca2+ pump antibody IID8 reacted much better with heart and surface mucosal cells in the stomach than with the stomach smooth muscle, and that IID8 reactivity was intracellular. In contrast PM4A2B, an antibody against the plasma membrane Ca2+ pump, reacted well with heart and stomach smooth muscle, plexus and mucosa, and its localization appeared to be in the plasma membrane. Thus, stomach smooth muscle expressed SERCA2b mRNA and protein at low levels, mucosa expressed SERCA2b mRNA and protein at high levels, atria and ventricle expressed SERCA2a mRNA and protein at high levels, mRNA being more in atria, but protein being more in ventricles.

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.

The Ca(2+)-transport ATPases from the plasma membrane

The initial studies on the plasma membrane (PM) Ca(2+)-transport ATPases were made in the erythrocyte, a structure that can not be taken as representing a typical eukaryotic cell. In other cell types however, the study of the PM Ca(2+)-transport ATPase is complicated by the simultaneous expression of related Ca(2+)-pumps in intracellular stores. Whereas there are as yet no known specific inhibitors for the PM Ca(2+)-transport ATPase, a number of selective inhibitors for the endo(sarco)plasmic reticulum Ca2+ pumps have been described: thapsigargin, cyclopiazonic acid and 2,5-di-(tert-butyl)-1,4-benzohydroquinone. With the recent introduction of the molecular biological approach, it became quickly obvious that a family of at least 5 different PM Ca(2+)-transport ATPase genes govern the tissue-dependent expression of PM Ca2+ pumps. Moreover alternative splicing of the primary gene transcripts was found to further enhance the number of pump variants. The PM Ca(2+)-transport ATPase are subject to modulatory control by calmodulin, by acidic phospholipids, and by the known families of protein kinases. Each of the ensuing effects are mutually related and interdependent. The wide variety PM Ca2+ pump isoforms and their regulation by such an intricate modulatory network allows the distinct tissues to adapt most adequately to the prevailing tissue and stimulus specific requirements.

Evidence for a fourth rat isoform of the plasma membrane calcium pump in the kidney

This study was conducted to identify plasma membrane Ca(2+)-transporting ATPases present in rat kidney. Characterization of the cDNAs of the plasma membrane Ca(2+)-ATPases revealed a family of proteins with regions of highly conserved amino acid sequence. To examine the extent of the diversity of rat renal plasma membrane Ca(2+)-ATPases, we used the polymerase chain reaction to detect additional gene products in rat kidney mRNA that shared these conserved regions. Sequences corresponding to three previously known rat plasma membrane Ca(2+)-ATPases were obtained. In addition, we found sequence corresponding to a new putative plasma membrane Ca(2+)-ATPase. Our results demonstrate that the rat kidney contains at least four different plasma membrane Ca(2+)-ATPases and the complexity of this multigene family is greater than previously thought.

New Ca2+ pump isoforms generated by alternative splicing of rPMCA2 mRNA

Alternative splices capable of generating proteins with altered functions were found (by PCR) in isoform 2 of the rat plasma membrane Ca2+ pump. These splices were concentrated in two hypervariable regions. One of these regions, near the N-terminus and the lipid-binding region, could be altered by the insertion of either or both of inserts x and y. Insertion of both x and y would add 45 amino acids to the molecule. The y insert causes the appearance of a rather hydrophobic stretch of amino acids in the middle of a highly polar region. The second variable region begins in the middle of the calmodulin-binding domain. Insertion of 229 nucleotides at this point of the message converts the b form to the a form, which has an altered (and shorter) C-terminus. The calmodulin-binding domain of this shortened form has a less basic character, which would decrease the affinity for calmodulin. The b form of isoenzyme 2 contains relatively weak protein kinase A substrate sequences, such as KQNSS and KNNS. These sequences are eliminated in form a, and a strongly activated kinase substrate sequence, RRQSS, appears in a different place. Different tissues use different combinations of alternative splices, with heart and brain showing the greatest diversity.

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

cDNA species covering the entire coding sequence of the human homologue of the rat plasma membrane Ca(2+)-ATPase (PMCA) isoform 2 have been isolated and characterized. The deduced amino acid sequence shows 99% identity with that of the rat protein and can be aligned with the latter without gaps except for one 14-amino-acid-residue insert in the region immediately preceding the putative phospholipid-sensitive domain in the human pump. cDNA clones isolated by anchored polymerase-chain reaction revealed additional microheterogeneity in the same N-terminal PMCA2-coding region. Alternative RNA splicing involving a region of 135 nucleotides generates three types of cDNA. One does not contain any of the 135 bp, and the other two contain 42 bp or the entire 135 bp of the optional sequence. Analysis of genomic DNA indicates that this sequence is encoded by three separate exons of 33, 60 and 42 bp. Although each of these exons could be inserted into the mRNA without changing the reading frame, polymerase-chain amplifications using cDNA libraries from several human tissues show that the 33-bp and the 60-bp exons are never independently used during splicing. The unequal distribution of the splice variants suggests tissue-specific regulation of the alternative-splicing pathways and indicates a functional specialization of the encoded isoform subtypes.

Plasma membrane calcium pump expression in human placenta and small intestine

To identify the forms of the plasma membrane calcium pump present in tissues that transport calcium, cDNA from human placenta and proximal small intestine was amplified by the polymerase chain reaction using a pair of mixed primers based on all the known human and rat plasma membrane calcium pump sequences. Clones were identified from the two human forms HPMCA1 and HPMCA4, but no new sequences were found in either tissue. RNA blots probed with HPMCA1 showed two bands in both tissues; probing with HPMCA4 gave a single, larger species. In placenta, HPMCA4 was the more abundant form and similar expression was found in full-term and second-trimester placentas. In contrast, in the small intestine, HPMCA1 was more abundant, suggesting that calcium absorption is not associated with any one specific isoform in calcium transporting cells.

Vitamin D and mineral deficiencies increase the plasma membrane calcium pump of chicken intestine

The basolateral membrane of the enterocyte was previously shown to contain an adenosine triphosphate-dependent calcium pump. Using immunological procedures, the localization of the Ca2+ pump in chick intestine, and the effect of dietary variables on the concentration of the pump, were studied. A monoclonal antibody produced against the human erythrocyte calcium pump was shown to cross-react with a chick intestinal Ca2+ pump epitope. The most intense staining of intestinal tissue, as determined immunohistochemically, occurred at the basolateral membrane of the duodenum, jejunum, ileum, and colon, with minor staining elsewhere. By the Western blotting procedure, vitamin D repletion of vitamin D-deficient chicks was shown to significantly increase the concentration of the Ca2+ pump epitope of duodenal, jejunal, and ileal mucosa by a factor of 2-3. Chicks were also fed diets deficient in calcium or phosphorus, a situation known to result in the stimulation of the synthesis of calbindin-D28k and an enhancement of the efficiency of Ca2+ absorption. Adaptation of the chicks to these deficient diets was verified by an increase in intestinal levels of calbindin-D28k, and is now shown to increase the Ca2+ pump epitope. From these immunological studies, it seems apparent that dietary variables that enhance intestinal Ca2+ absorption also increase the amount of the intestinal basolateral Ca2+ pump.

The plasma membrane calcium pump: a multiregulated transporter

Activation of many cells, especially nonexcitable cells, results in a Ca(2+) transient that is influenced in part by the kinetics of active extrusion of Ca(2+) across the plasma membrane. The molecular cloning of the plasma membrane Ca(2+)-pump has helped to clarify the relationship between its structure and function. The Ca(2+)-pump is controlled by multiple regulators, including calmodulin, phospholipids and various kinases. Longer term control is achieved through regulation of its gene expression, and the presence of a number of Ca(2+)-pump isoforms that differ in their regulatory domains provides potential functional diversity. In this review, we focus on the mechanisms that regulate the function of the Ca(2+)-pump, and their physiological significance.

Calcium pump isoforms: diversity, selectivity and plasticity. Review article

Ca2+ pumps are essential for removing cytosolic Ca2+ either across the plasma membrane (PM) or into internal organelles such as the sarcoplasmic reticulum (SR). Four genes (PMCA1, PMCA2, PMCA3 and PMCA4) have been reported to encode the PM Ca2+ pumps and three (SERCA1, SERCA2 and SERCA3) to encode the SR Ca2+ pumps. The PM Ca2+ pumps are stimulated by calmodulin, the SR Ca2+ pumps encoded by SERCA1 and SERCA2 are stimulated by phospholamban while the product of SERCA3 may be regulated directly by cAMP-dependent protein kinase. Alternative splicing of the primary transcripts of several of these genes has been reported to occur in a tissue selective manner and for others to alter during ontogeny. For the PM Ca2+ pump, alternative RNA splicing may result in isoforms with altered cyclic nucleotide dependent protein kinase sensitivity. The diversity in distribution of Ca2+ pump isoforms and their regulatory factors when coupled with different Ca2+ entry mechanisms allows for tissue selectivity and plasticity in stimulus-response coupling. The roles of various Ca2+ pump isoforms, the rationale behind their tissue selective expression and the plasticity in this expression are among the new challenges to researchers in this field.

Immunohistochemical localization of a calcium pump and calbindin-D28k in the oviduct of the laying hen

The localization of a plasma membrane calcium pump in the oviduct of the laying hen was investigated by immunohistochemical techniques, utilizing a monoclonal antibody (5F10) produced against the human erythrocyte calcium pump. This antibody was shown to react with an epitope of the pump in oviductal tissue, and prominent staining was observed on the microvilli of the tubular gland cells of the hen shell gland (uterus) and the isthmus. The Ca2+ pump was not detectable in the infundibulum or the magnum. Calbindin-D28k, also localized by immunohistochemical means, was observed to be present in the tubular gland cells of the shell gland and the distal isthmus (adjacent to shell gland) but not in either the proximal isthmus (adjacent to the magnum), the magnum or the infundibulum. The localization of the Ca2+ pump in the oviduct corresponds to known sites of mineral deposition during egg shell formation. The distribution of calbindin-D28k differed, co-localizing with the Ca2+ pump in the shell gland and distal isthmus but not in the proximal isthmus. This might reflect a greater rate of active Ca2+ secretion in the distal isthmus and shell gland as compared to the proximal isthmus.

Expression of cyclic-nucleotide-sensitive and -insensitive isoforms of the plasma membrane Ca2+ pump in smooth muscle and other tissues

cDNA clones encoding the plasma membrane Ca2+ pump isoform PMCA1 were obtained from rabbit stomach smooth muscle. The PMCA1 gene has a 154 base exon which can be alternatively spliced. In splices containing 0, 87 or 114 bases of this exon, the mRNA downstream from this position encodes a protein containing the peptide sequence Lys-Arg-Asn-Ser-Ser (KRNSS), which can be phosphorylated by cyclic-nucleotide-sensitive protein kinase. However, in those splices containing 154 bases, the mRNA encodes a protein that does not contain this sequence. The cDNA clone obtained in this study did not contain the latter exon, and thus it coded for KRNSS. The presence of the various splices of PMCA1 was determined in stomach smooth muscle and other tissues by reverse transcription followed by a polymerase chain reaction. Percentage of transcripts encoding the potentially cyclic-nucleotide-sensitive isoform in various tissues were as follows: liver, 100%; stomach mucosa, 100%; heart, 100%; stomach smooth muscle, 86%; aorta, 83%; brain, 55%. Thus brain was the only tissue which expressed a very high proportion of the isoform of PMCA1 that is insensitive to cyclic-nucleotide-dependent protein kinases.

Ca2+ extrusion across plasma membrane and Ca2+ uptake by intracellular stores

The aim of this review is to summarize the various systems that remove Ca2+ from the cytoplasm. We will initially focus on the Ca2+ pump and the Na(+)-Ca2+ exchanger of the plasma membrane. We will review the functional regulation of these systems and the recent progress obtained with molecular-biology techniques, which pointed to the existence of different isoforms of the Ca2+ pump. The Ca2+ pumps of the sarco(endo)plasmic reticulum will be discussed next, by summarizing the discoveries obtained with molecular-biology techniques, and by reviewing the physiological regulation of these proteins. We will finally briefly review the mitochondrial Ca(2+)-uptake mechanism.