Regulation of platelet surface receptor expression
Platelet adhesion and aggregation at sites of vascular injury is essential for hemostasis but is also a central pathomechanism underlying ischemic cardiovascular diseases such as myocardial infarction or stroke. These processes are mediated by adhesive and activatory membrane glycoproteins that are tightly regulated in their activity and/or expression on the platelet surface. Besides glycoprotein (GP) Ib-V-IX, which mediates initial platelet adhesion, GPVI has been identified as a central receptor in the early phase of thrombus formation. GPVI mediates cellular activation, which is a prerequisite for firm adhesion and aggregation, making it a promising target for antithrombotic therapy. In previous studies we could show that anti-GPVI antibodies induce the down-regulation of the receptor in circulating platelets and, consequently, long-term antithrombotic protection in mice. We could show that GPVI down-regulation can occur either by ectodomain shedding or – in case of defective platelet signaling – via internalization of the receptor.
Moreover, we demonstrated that the C-type lectin like receptor (CLEC-2), which is the platelet receptor for podoplanin and a hitherto unknown intravascular ligand, can also be down-regulated from the platelet surface in mice via anti-CLEC-2 antibodies. The down-regulation of CLEC-2 appears to occur predominantly in megakaryocytes, the platelet progenitors. However, in case of defective CLEC-2 signaling CLEC-2 is down-regulated from the platelet surface via receptor internalization.
Currently, we are investigating a third mechanism of antibody-mediated platelet receptor regulation, the ´disintegration´ (uncoupling) of cytoskeleton-connected transmembrane proteins, most notably β3- and β1- integrins, but also GPVI and possibly many more. We postulate that this receptor ‘disintegration’ represents a basic mechanism to regulate platelet adhesiveness and thus provides an interesting target to modulate platelet function. We combine intravital imaging and transgene mouse models with super-resolution microscopy and proteomic approaches to dissect the spatio-temporal events and molecular players driving receptor uncoupling and membrane disintegration.