Munc13-4 is a limiting factor in the pathway required for platelet granule release and hemostasis

Activation-dependent platelet granule release is mediated by integral membrane proteins called soluble N-ethylmaleimide-sensitive fusion protein attachment protein receptors (SNAREs) and their regulators; however, the mechanisms for this process are ill-defined. To further characterize platelet secretion, we analyzed the function of platelets from Unc13d^Jinx mice. Platelets from these animals lack the putative vesicle priming factor, Munc13-4, and have a severe secretion defect. Release from dense granules was completely ablated and that from α-granules and lysosomes was severely compromised. Unc13d^Jinx platelets showed attenuated aggregation and, consequently, Unc13d^Jinx mice had prolonged tail-bleeding times. The secretion defect was not due to altered expression of SNAREs or SNARE regulators, defective granule biogenesis, or faulty platelet activation. The defective release could be rescued by adding recombinant Munc13-4 to permeabilized Unc13d^Jinx platelets. In wild-type mouse platelets, Munc13-4 levels were lower than those of SNAREs suggesting that Munc13-4 could be a limiting component of the platelets' secretory machinery. Consistently, Munc13-4 levels directly correlated with the extent of granule release from permeabilized platelets and from intact, heterozygous Unc13d^Jinx platelets. These data highlight the importance of Munc13-4 in platelets and indicate that it is a limiting factor required for platelet secretion andhemostasis.