SARAF overexpression impairs thrombin-induced Ca2+ homeostasis in neonatal platelets.

Neonatal platelets present a reduced response to the platelet agonist, thrombin (Thr), thus resulting in a deficient Thr-induced aggregation. These alterations are more pronounced in premature newborns. Here, our aim was to uncover the causes underneath the impaired Ca2+ homeostasis described in neonatal platelets. Both Ca2+ mobilization and Ca2+ influx in response to Thr are decreased in neonatal platelets compared to maternal and control woman platelets. In neonatal platelets, we observed impaired Ca2+ mobilization in response to the PAR-1 agonist (SFLLRN) or by blocking SERCA3 function with tert-butylhydroquinone. Regarding SOCE, the STIM1 regulatory protein, SARAF, was found overexpressed in neonatal platelets, promoting an increase in STIM1/SARAF interaction even under resting conditions. Additionally, higher interaction between SARAF and PDCD61/ALG2 was also observed, reducing SARAF ubiquitination and prolonging its half-life. These results were reproduced by overexpressing SARAF in MEG01 and DAMI cells. Finally, we also observed that pannexin 1 permeability is enhanced in response to Thr in control woman and maternal platelets, but not in neonatal platelets, hence, leading to the deregulation of the Ca2+ entry found in neonatal platelets. Summarizing, we show that in neonatal platelets both Ca2+ accumulation in the intracellular stores and Thr-evoked Ca2+ entry through either capacitative channels or non-selective channels are altered in neonatal platelets, contributing to deregulated Ca2+ homeostasis in neonatal platelets and leading to the altered aggregation observed in these subjects.

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

BACKGROUND: Altered intracellular Ca2+ homeostasis in neonatal platelets has been previously reported. This study aims to examine the changes in the Ca2+ entry through the store-operated calcium entry (SOCE) mechanism in neonatal platelets. METHODS: 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. Ca2+ 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. RESULTS: 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. Ca2+ imaging analysis and coimmunoprecipitation assay using MEG01 and HEK293 cells overexpressing SARAF together with s-STIM1 (478) reported a reduced slow Ca2+-dependent inactivation, so reproducing the Ca2+-homeostasis pattern observed in neonatal platelets. CONCLUSIONS: CAPN1 may cleave STIM1 in neonatal platelets, hence, impairing SARAF coupling after SOCE activation. s-STIM1 may avoid slow Ca2+-dependent inactivation and, subsequently, results in an enhanced TG-evoked SOCE as observed in neonatal platelets.