CAPN1 (Calpain1)-Dependent Cleavage of STIM1 (Stromal Interaction Molecule 1) Results in an Enhanced SOCE (Store-Operated Calcium Entry) in Human Neonatal Platelets

Altered intracellular Ca homeostasis in neonatal platelets has been previously reported. This study aims to examine the changes in the Ca entry through the store-operated calcium entry (SOCE) mechanism in neonatal platelets. Human platelets from either control women, mothers, and neonates were isolated and, following, were fixed after being treated as required. Platelet samples were analyzed by Western blotting, qRT-PCR, and MALDITOF/TOF. Ca homeostasis was also determined. Culture cells were used as surrogated of platelets to overexpress the proteins of interest to reproduce the alterations observed in platelets. Altered TG (thapsigargin)-evoked SOCE, alternative molecular weight form of STIM1 (stromal interaction molecule 1; s-STIM1 [short STIM1 isoform (478 aa)], around 60 kDa) and overexpression of SARAF (SOCE-associated regulatory factor) were found in neonatal platelets as compared to maternal and control women platelets. s-STIM1 may result due to CAPN1 (calpain1)-dependent processing, as confirmed in platelets and MEG01 cells by using calpeptin and overexpressing CAPN1, respectively. In HEK293 (STIM1 and STIM2 [stromal interaction molecule 2] double knockout) cells transfected either with c-STIM1 (canonical STIM1 [685 aa]), s-STIM1 (478), STIM1B (540), and CAPN1 overexpression plasmids, we found s-STIM1 and c-STIM1, except in cells overexpressing s-STIM1 (478) that lacked CAPN1 target residues. These results and the in silico analysis, lead us to conclude that STIM1 is cleaved at Q496 by CAPN1. Ca imaging analysis and coimmunoprecipitation assay using MEG01 and HEK293 cells overexpressing SARAF together with s-STIM1 (478) reported a reduced slow Ca -dependent inactivation, so reproducing the Ca -homeostasis pattern observed in neonatal platelets. CAPN1 may cleave STIM1 in neonatal platelets, hence, impairing SARAF coupling after SOCE activation. s-STIM1 may avoid slow Ca -dependent inactivation and, subsequently, results in an enhanced TG-evoked SOCE as observed in neonatal platelets.