A Rac-PI3Kβ feedback loop drives GPVI-dependent platelet activation

Class I PI3Kβ is critical in controlling platelet activation and preserving thrombus stability under high shear. PI3Kβ is unique among class I phosphoinositide 3-kinases (PI3Ks) in being activated by Rho-family GTPases Rac and Cdc42, positioning it as a potential key link between Rho-family GTPases and platelet signaling. Here, we combined pharmacological inhibition with genetic approaches to define a direct Rho-PI3Kβ signaling axis in mouse platelets. Platelets from knockin mice carrying two point mutations within the Rho-binding domain (RBD) of the PI3Kβ catalytic subunit p110β exhibited impaired GPVI-mediated platelet signaling, functional responses, and spreading on fibrinogen. Pharmacological inhibition with the PI3Kβ-selective inhibitor AZD6482 demonstrated that these responses largely depend on the p110β RBD. Inhibition of Rac/Cdc42 in wild-type platelets phenocopied the knockin defects, supporting a direct role for Rho GTPases in PI3Kβ activation. Conversely, Rac activation was impaired in RBD-mutant and AZD6482-treated platelets, revealing a Rac-PI3Kβ feedback loop that amplifies GPVI-dependent platelet activation.