Genetic and Pharmacologic Evidence Shows That Cdc42 GTPase Plays a Central Role in the Regulation of Both GPVI- and Non-GPVI-Dependent Activation of Platelets.

Abstract 2997

Poster Board II-975

Cdc42 and Rac1, members of the Rho family of small GTPases, play critical roles in reorganization of actin cytoskeleton in platelets. Previously we have shown that Rac1 GTPase is involved in regulation of platelet secretion and aggregation by diverse signaling pathways (J Thromb Haemost 2007; 5: 1747-55). Others have reported that Rac1 is essential for GPVI-, but not G protein-dependent platelet aggregation (Pflugers Arch. 2009; 457:1173-85). Cdc42 was recently reported to be involved in collagen, but not collagen related peptide (CRP), a GPVI specific agonist, induced platelet aggregation (Platelets 2008; 19: 199-210). In this study we have investigated the role of Cdc42 in regulation of platelet function by using complementary approaches of (a) mouse gene targeting of Cdc42, and (b) specific inhibition of Cdc42 activity by a newly identified chemical inhibitor of Cdc42, CASIN (Cdc42 activity-specific inhibitor). Platelets from Cdc42^−/− mice exhibited a complete lack of filopodia formation and spreading on collagen coated surfaces. Threshold concentrations of collagen, CRP or thrombin failed to induce shape change or aggregation in platelets from Cdc42^−/− mice compared with induction of shape change and maximal aggregation in platelets from Cdc42^+/+ mice. Platelets from Cdc42^−/− mice, as compared to Cdc42^+/+ mice, exhibited a significant inhibition of CRP- or thrombin-induced secretion of ATP and release of P-selectin from the dense- and alpha-granules respectively. Increasing concentrations of the agonists only partially corrected the defective aggregation and secretion responses in Cdc42^−/− platelets. These data provide the genetic evidence that Cdc42 is required for collagen, CRP and thrombin mediated platelet signaling and activation. Treatment of platelets with CASIN, but not a pharmacologically inactive analog, blocked collagen induced activation of Cdc42 without detectably affecting the Rac1 activity. Human platelets pre-incubated with CASIN (10 micro-M) exhibited a complete lack of filopodia formation and spreading on collagen coated surfaces. Further, treatment of platelets with CASIN (1-10 micro-M) inhibited: (a) aggregation induced by collagen, CRP, thrombin, ADP or U46619; (b) release of P-selectin and secretion of ATP induced by U46619; and (c) collagen induced phosphorylation of Akt. Addition of CASIN to platelets also blocked collagen or CRP induced aggregation in aspirinated platelets in the presence of apyrase. In other experiments, addition of CASIN to citrated platelet-rich plasma inhibited thrombin induced clot retraction. Significantly, removal of CASIN from the platelet samples by washing reversed inhibition of aggregation as well as clot retraction, reflecting a reversible suppression of Cdc42 activity by CASIN. Administration of CASIN into C57Bl/6 mice inhibited ex vivo platelet aggregation induced by collagen or ADP as well as significantly prolonged tail bleeding times. These data suggest that: (a) Cdc42 plays an essential, non-redundant role in platelet filopodia formation, spreading, secretion, aggregation and clot retraction; (b) Cdc42 is involved in GPVI, non-GPVI- and G protein-dependent signaling in platelets; (c) the pharmacologic inhibitor CASIN is capable of specifically and reversibly inhibiting Cdc42 activity in platelets, mimicking Cdc42 genetic knockout in mice. Altogether, our studies strongly implicate Cdc42 as a novel anti-platelet target, and present evidence that the Cdc42 specific small molecule inhibitor, CASIN, may have therapeutic potential.