Platelet-VWF complexes get the chop

VWF is made up of subunits (consisting of domains D′-D3-A1-A2-A3-D4-B1-B2-B3-C1-C2) forming disulfide-linked mutltimers of 20 million daltons or more. Multimer size is critical to the prothrombotic function of VWF, and regulation of multimer size by the ADAM (A Disintegrin And a Metalloproteinase)–family metalloproteinase ADAMTS13 controls a clinically important class of VWF-based pathologies, including thrombotic thrombocytopenic purpura (TTP). The more high molecular weight the VWF multimers (including those arising from ADAMTS13 dysfunction), the greater the potential for thrombosis.²  Establishing the mechanisms for regulating ADAMTS13-mediated VWF cleavage is the focus of much recent research because of the pathologic importance of understanding how this cleavage is regulated in vivo.^1,–3 

One of VWF multimers' primary roles is to support formation of platelet adhesion and aggregation. VWF contains 2 major platelet-binding domains: the A1 domain, which binds GPIbα of the GPIb-IX-V complex, and an Arg-Gly-Asp (RGD) sequence in the C1 domain that binds the platelet integrin α_IIbβ₃ (GPIIb-IIIa). ADAMTS13 cleaves at Tyr842/Met843 within the A2 domain, between A1 (which binds GPIbα) and A3 (which binds collagen). This cleavage, however, does not occur efficiently in native solution-phase VWF, and conformational changes of A2 are required to enable cleavage.²  Unlike A1 and A3, the A2 domain lacks an intra-domain disulfide bond, presumably enabling mechanical distortion and exposure of the cryptic Tyr842/Met843 cleavage site.

Previous studies suggest tensile stress applied across A1-A2-A3 facilitates ADAMTS13-dependent cleavage of A2.³  In this article, Shim et al use purified components in a reconstituted system under uniform shear conditions in a cone-and-plate viscometer to show shear-dependent platelet- and GPIbα-dependent ADAMTS13-mediated cleavage of VWF (inhibitable by anti-GPIbα antibodies or soluble GPIbα fragments).Thus, platelet GPIbα acts in 2 ways to favor VWF cleavage: by binding A1 (to remove an inhibitory effect on A2 cleavage)¹  and by anchoring different A1 domains on an individual multimer to 2 or more platelets, interactions that under sufficient shear stress stretch the A2 domain and promote ADAMTS13-dependent proteolysis (see figure). Dong and colleagues have previously reported that ultralarge VWF strings expressed on activated endothelial cells (via P-selectin) can be decorated by multiple platelets under flow, and that in this form the VWF is susceptible to proteolysis by added plasma (ADAMTS13 dependent).²  These results suggest that a developing thrombus under arterial flow involving platelet attachment to VWF would be self-limiting in size, since shear applied to platelet-VWF-platelet complexes would promote ADAMTS13-mediated cleavage of VWF.