Modulation of SERCA2 activity: regulated splicing and interaction with phospholamban

Endoplasmic Reticulum-Plasma Membrane Contact Sites as an Organizing Principle for Compartmentalized Calcium and cAMP Signaling

In eukaryotic cells, ultimate specificity in activation and action-for example, by means of second messengers-of the myriad of signaling cascades is primordial. In fact, versatile and ubiquitous second messengers, such as calcium (Ca2+) and cyclic adenosine monophosphate (cAMP), regulate multiple-sometimes opposite-cellular functions in a specific spatiotemporal manner. Cells achieve this through segregation of the initiators and modulators to specific plasma membrane (PM) subdomains, such as lipid rafts and caveolae, as well as by dynamic close contacts between the endoplasmic reticulum (ER) membrane and other intracellular organelles, including the PM. Especially, these membrane contact sites (MCSs) are currently receiving a lot of attention as their large influence on cell signaling regulation and cell physiology is increasingly appreciated. Depletion of ER Ca2+ stores activates ER membrane STIM proteins, which activate PM-residing Orai and TRPC Ca2+ channels at ER-PM contact sites. Within the MCS, Ca2+ fluxes relay to cAMP signaling through highly interconnected networks. However, the precise mechanisms of MCS formation and the influence of their dynamic lipid environment on their functional maintenance are not completely understood. The current review aims to provide an overview of our current understanding and to identify open questions of the field.

Identification of a new SERCA2 splice variant regulated during monocytic differentiation

Sarco/endoplasmic reticulum-type calcium transport ATPases (SERCA enzymes) pump calcium ions from the cytosol into the endoplasmic reticulum. We report that in addition to the ubiquitously expressed SERCA2b isoform, a new splice variant of SERCA2 can be detected (SERCA2c) that arises from the inclusion of a short intronic sequence located between exons 20 and 21 of the SERCA2a isoform. Sequence analysis revealed classical splice donor and acceptor sites, as well as a branch-point site. Due to the presence in the new exon of an in-frame stop codon that is preceded by a 17 bp coding sequence, this mRNA potentially codes for a protein with a truncated C-terminus containing a short unique C-terminal peptide stretch. SERCA2c message was detected in epithelial, mesenchymal, and hematopoietic cell lines, as well as in primary human monocytes. Moreover, we found that during monocytic differentiation total SERCA2 ATPase expression is induced on the protein and mRNA level and that the novel SERCA2c messenger is also up-regulated during this process. These data indicate that the alternative splicing pattern of the 3(') region of the SERCA2 primary transcript is more complex than that previously thought and that this enzyme may be involved in the process of monocyte differentiation.

Sarco(endo)plasmic reticulum calcium pumps: recent advances in our understanding of structure/function and biology (review)

This review examines the structure and function of the sarco(endo)plasmic reticulum calcium pump (SERCA1a) in the light of the recent publication of the 2.6 A resolution structure of this protein, and looks at the increasing awareness of the key role played by SERCAs in calcium signalling. The roles played by the calcium pump isoforms, SERCA1a/b, SERCA2a/b and SERCA3a/b/c in cellular function are discussed, and the modulation of SERCA activity by phospholamban, sarcolipin and other modulatory influences is examined. The recent discoveries of human SERCA mutations leading to disease states is reviewed, and the insights into SERCA function using transgenic approaches are outlined.

Mobilizing store Ca(2+) in the presence of La(3+) evokes exocytosis in bovine chromaffin cells

The effect on exocytosis of La(3+), a known inhibitor of plasma membrane Ca(2+)-ATPases and Na(+)/Ca(2+) exchangers, was studied using cultured bovine adrenal chromaffin cells. At high concentrations (0.3-3 mM), La(3+) substantially increased histamine-induced catecholamine secretion. This action was mimicked by other lanthanide ions (Nd(3+), Eu(3+), Gd(3+), and Tb(3+)), but not several divalent cations. In the presence of La(3+), the secretory response to histamine became independent of extracellular Ca(2+). La(3+) enhanced secretion evoked by other agents that mobilize intracellular Ca(2+) stores (angiotensin II, bradykinin, caffeine, and thapsigargin), but not that due to passive depolarization with 20 mM K(+). La(3+) still enhanced histamine-induced secretion in the presence of the nonselective inhibitors of Ca(2+)-permeant channels SKF96365 and Cd(2+), but the enhancement was abolished by prior depletion of intracellular Ca(2+) stores with thapsigargin. La(3+) inhibited (45)Ca(2+) efflux from preloaded chromaffin cells in the presence or absence of Na(+). It also enhanced and prolonged the rise in cytosolic [Ca(2+)] measured with fura-2 during mobilization of intracellular Ca(2+) stores with histamine in Ca(2+)-free buffer. The results suggest that the efficacy of intracellular Ca(2+) stores in evoking exocytosis is enhanced dramatically by inhibiting Ca(2+) efflux from the cell.

Amino acid sequence of a Ca(2+)-transporting ATPase from the sarcoplasmic reticulum of the cross-striated part of the adductor muscle of the deep sea scallop: comparison to serca enzymes of other animals

The RT PCR approach was used to obtain the nucleotide sequence of the mRNA of a sarco/endoplasmic reticulum calcium transporting ATPase (SERCA) from the cross-striated (phasic) part of the adductor muscle of the deep sea scallop. Initially, degenerate primers based on consensus sequences among SERCAs and tryptic fragments of the scallop Ca-ATPase were used. The sequence was then extended using homologous primers and the 5' and 3' ends of the transcript determined by 5' and 3' RACE. The mRNA codes for a polypeptide chain 994 amino acid residues long (coded for by 2982 nucleotides) and has a 195 bp 5' untranslated region, with a 697 bp 3' untranslated region. The scallop enzyme shows strongest amino acid similarity to the SERCA enzyme of Loligo, followed by those of Drosophila and Artemia. It resembles the vertebrate SERCA3 in that it does not possess the phospholamban binding motif and so is unlikely to be regulated by protein kinase A mediated signals.

Sequence elements surrounding the acceptor site suppress alternative splicing of the sarco/endoplasmic reticulum Ca2+-ATPase 2 gene transcript

Expression of the muscle-specific 2a isoform of the sarco/endoplasmic reticulum Ca2+-ATPase (SERCA2) requires activation of an inefficient optional splice process at the 3' end of the primary gene transcript. The sequence elements required for this regulated splice event were studied by modifying a minigene containing the 3' end of the SERCA2 gene. An important requirement appears to be a strong muscle-specific acceptor site, as replacing it by a weak one prevented the induction of muscle-type splicing during myogenic differentiation. The induction of muscle-type splicing did not depend on positive cis-active sequences in the muscle-specific exon. On the other hand, replacement of a broad region around the acceptor site dramatically deregulated the expression pattern, as this modification strongly induced muscle-type splicing in undifferentiated muscle cells and in fibroblasts. This cis-active region is also involved in the suppression of the neuronal type of splicing. Furthermore selective replacement of the acceptor site as well as deletions or replacements in the muscle-specific exon induced muscle-type splicing to various extents in undifferentiated myogenic cells. Therefore sequence elements in the distal part of the optional intron and in the muscle-specific exon contribute to the suppression of muscle-specific SERCA2 splicing.

Alternative processing of the sarco/endoplasmic reticulum Ca(2+)-ATPase transcripts during muscle differentiation is a specifically regulated process

Expression of the muscle-specific 2a isoform of the sarco/endoplasmic reticulum Ca(2+)-ATPase (SERCA2) requires activation of an otherwise inefficient splice process at the 3'-end of the primary gene transcript. We provide evidence that SERCA2 splicing is a specifically regulated process, rather than the result of an increase in general splice efficiency or a decrease in polyadenylation efficiency at the 5'-most polyadenylation site. This is indicated by the fact that changes in general splice and polyadenylation efficiency, as observed during B-cell maturation, did not affect SERCA2 splicing. Furthermore, expression and overexpression studies did not support the hypothesis that changes in the level of the alternative splice factor ASF/SF2 or other arginine and serine rich proteins are sufficient to obtain the regulation of muscle- and neuronal-specific splicing.

Structure-function relationships in the Ca(2+)-ATPase of sarcoplasmic reticulum: facts, speculations and questions for the future

Structural data on the Ca(2+)-ATPase of sarcoplasmic reticulum are integrated with kinetic data on Ca2+ transport. The emphasis is upon ATPase-ATPase interactions, the requirement for phospholipids, and the mechanism of Ca2+ translocation. The possible role of cytoplasmic [Ca2+] in the regulation of the synthesis of Ca(2+)-ATPase is discussed.