A regulatory node involving Gα_q, PLCβ, and RGS proteins modulates platelet reactivity to critical agonists

Background: Most platelet agonists work through G protein-coupled receptors, activating pathways that involve members of the G_q, G_i, and G₁₂/G₁₃ families of heterotrimeric G proteins. G_q signaling has been shown to be critical for efficient platelet activation. Growing evidence suggests that regulatory mechanisms converge on G protein-coupled receptors and G_q to prevent overly robust platelet reactivity.

Objectives: To identify and characterize mechanisms by which G_q signaling is regulated in platelets.

Methods: Based on our prior experience with a Gα_i2 variant that escapes regulation by regulator of G protein signaling (RGS) proteins, a Gα_q variant was designed with glycine 188 replaced with serine (G188S) and then incorporated into a mouse line so that its effects on platelet activation and thrombus formation could be studied in vitro and in vivo.

Results and conclusions: As predicted, the G188S substitution in Gα_q disrupted its interaction with RGS18. Unexpectedly, it also uncoupled PLCβ-3 from activation by platelet agonists as evidenced by a loss rather than a gain of platelet function in vitro and in vivo. Binding studies showed that in addition to preventing the binding of RGS18 to Gα_q, the G188S substitution also prevented the binding of PLCβ-3 to Gα_q. Structural analysis revealed that G188 resides in the region that is also important for Gα_q binding to PLCβ-3 in platelets. We conclude that the Gα_q signaling node is more complex than that has been previously understood, suggesting that there is cross-talk between RGS proteins and PLCβ-3 in the context of Gα_q signaling.