NAADP/SERCA3-Dependent Ca 2+ Stores Pathway Specifically Controls Early Autocrine ADP Secretion Potentiating Platelet Activation

Rationale: Ca2+ signaling is a key and ubiquitous actor of cell organization and its modulation controls many cellular responses. Sarco-endoplasmic reticulum Ca2+-ATPases (SERCAs) pump Ca2+ into internal stores that play a major role in the cytosolic Ca2+ concentration rise upon cell activation. Platelets exhibit 2 types of SERCAs, SERCA2b and SERCA3, which may exert specific roles, yet ill-defined. We have recently shown that Ca2+ mobilization from SERCA3-dependent stores was required for full platelet activation in weak stimulation conditions. Objective: To uncover the signaling mechanisms associated with Ca2+ mobilization from SERCA3-dependent stores leading to ADP secretion. Methods and Results: Using platelets from wild-type or SERCA3-deficient mice, we demonstrated that an early (within 5 to 10 seconds following stimulation) secretion of ADP specifically dependent on SERCA3 stored Ca2+ is exclusively mobilized by nicotinic acid adenosine dinucleotide phosphate (NAADP): both Ca2+ mobilization from SERCA3-dependent stores and primary ADP secretion are blocked by the NAADP receptor antagonist Ned-19, and reciprocally both are stimulated by permeant NAADP. In contrast, Ca2+ mobilization from SERCA3-dependent stores and primary ADP secretion were unaffected by inhibition of the production of inositol-1,4,5-trisphosphate (IP3) by phospholipase-C, and accordingly were not stimulated by permeant IP3. Conclusions: Upon activation a NAADP/SERCA3 Ca2+ mobilization pathway initiates an early ADP secretion, potentiating platelet activation, and a secondary wave of ADP secretion driven by both an IP3/SERCA2b-dependent Ca2+ stores pathway and the NAADP/SERCA3 pathway. This does not exclude that Ca2+ mobilized from SERCA3 stores may also enhance platelet global reactivity to agonists. Because of its modulating effect on platelet activation, this NAADP-SERCA3 pathway may be a relevant target for anti-thrombotic therapy.