A Regulatory Node Involving Gα, 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, G, and G/G families of heterotrimeric G proteins. G 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 to prevent overly robust platelet reactivity.

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

Methods: Based on our prior experience with a Gα variant that escapes regulation by regulator of G protein signaling (RGS) proteins, a Gα 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 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, the G188S substitution also prevented the binding of PLCβ-3 to Gα. Structural analysis revealed that G188 resides in the region that is also important for Gα binding to PLCβ-3 in platelets. We conclude that the Gα 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 signaling.