The pancreas-specific form of secretory pathway calcium ATPase 2 regulates multiple pathways involved in calcium homeostasis.

Acinar cell exocytosis requires spatiotemporal Ca2+ signals regulated through endoplasmic reticulum (ER) stores, Ca2+ATPases, and store-operated Ca2+ entry (SOCE). The secretory pathway Ca2+ATPase 2 (SPCA2) interacts with Orai1, which is involved in SOCE and store independent Ca2+ entry (SICE). However, in the pancreas, only a C-terminally truncated form of SPCA2 (termed SPAC2C) exists. The goal of this study was to determine if SPCA2C effects Ca2+ homeostasis in a similar fashion to the full-length SPCA2. Using epitope-tagged SPCA2C (SPCA2CFLAG) expressed in HEK293A cells and Fura2 imaging, cytosolic [Ca2+] was examined during SICE, SOCE and secretagogue-stimulated signaling. Exogenous SPCA2C expression increased resting cytosolic [Ca2+], Ca2+ release in response to carbachol, ER Ca2+ stores, and store-mediated and independent Ca2+ influx. Co-IP detected Orai1-SPCA2C interaction, which was altered by co-expression of STIM1. Importantly, SPCA2C's effects on store-mediated Ca2+ entry were independent of Orai1. These findings indicate SPCA2C influences Ca2+ homeostasis through multiple mechanisms, some of which are independent of Orai1, suggesting novel and possibly cell-specific Ca2+ regulation.

Atp2c2 Is Transcribed From a Unique Transcriptional Start Site in Mouse Pancreatic Acinar Cells.

Proper regulation of cytosolic Ca(2+) is critical for pancreatic acinar cell function. Disruptions in normal Ca(2+) concentrations affect numerous cellular functions and are associated with pancreatitis. Membrane pumps and channels regulate cytosolic Ca(2+) homeostasis by promoting rapid Ca(2+) movement. Determining how expression of Ca(2+) modulators is regulated and the cellular alterations that occur upon changes in expression can provide insight into initiating events of pancreatitis. The goal of this study was to delineate the gene structure and regulation of a novel pancreas-specific isoform for Secretory Pathway Ca(2+) ATPase 2 (termed SPCA2C), which is encoded from the Atp2c2 gene. Using Next Generation Sequencing of RNA (RNA-seq), chromatin immunoprecipitation for epigenetic modifications and promoter-reporter assays, a novel transcriptional start site was identified that promotes expression of a transcript containing the last four exons of the Atp2c2 gene (Atp2c2c). This region was enriched for epigenetic marks and pancreatic transcription factors that promote gene activation. Promoter activity for regions upstream of the ATG codon in Atp2c2's 24th exon was observed in vitro but not in in vivo. Translation from this ATG encodes a protein aligned with the carboxy terminal of SPCA2. Functional analysis in HEK 293A cells indicates a unique role for SPCA2C in increasing cytosolic Ca(2+) . RNA analysis indicates that the decreased Atp2c2c expression observed early in experimental pancreatitis reflects a global molecular response of acinar cells to reduce cytosolic Ca(2+) levels. Combined, these results suggest SPCA2C affects Ca(2+) homeostasis in pancreatic acinar cells in a unique fashion relative to other Ca(2+) ATPases. J. Cell. Physiol. 231: 2768-2778, 2016. © 2016 Wiley Periodicals, Inc.