Diverging signaling events control the pathway of GPVI down-regulation in vivo

Coronary artery thrombosis is often initiated by platelet activation on collagen-rich subendothelial layers in the disrupted atherosclerotic plaque. The activating platelet collagen receptor glycoprotein A (GPVI) noncovalently associates with the Fc receptor gamma-chain (FcR gamma), which signals through its immunoreceptor-tyrosine-based activation motif (ITAM) via the adaptor LAT leading to the activation of phospholipase C gamma 2 (PLC gamma 2). GPVI is a promising antithrombotic target as anti-GPVI antibodies induce the irreversible loss of the receptor from circulating platelets by yet undefined mechanisms in humans and mice and long-term antithrombotic protection in the latter. However, the treatment is associated with transient but severe thrombocytopenia and reduced platelet reactivity to thrombin questioning its clinical usefulness. Here we show that GPVI down-regulation occurs through 2 distinct pathways, namely ectodomain shedding or internalization/intracellular clearing, and that both processes are abrogated in mice carrying a point mutation in the FcR gamma-associated ITAM. In mice lacking LAT or PLC gamma 2, GPVI shedding is abolished, but the receptor is irreversibly down-regulated through internalization/ intracellular clearing. This route of GPVI loss is not associated with thrombocytopenia or altered thrombin responses. These results reveal the existence of 2 distinct signaling pathways downstream of the FcR gamma-ITAM and show that it is possible to uncouple GPVI down-regulation from undesired side effects with obvious therapeutic implications.