Heat Shock Protein 47: A New Platelet Collagen Receptor

Abstract 158

We recently reported that Heat shock protein 47 (Hsp47), a well established collagen binding protein, was present on the surface of human platelets. In the current study we demonstrate that anti-Hsp47 antibodies and a small molecule inhibitor of Hsp47 selectively prevent aggregation of platelets induced with collagen fibrils. Inhibition of Hsp47 was also found to reduce the size of thrombi produced upon perfusion of blood under arterial flow conditions over collagen in vitro. Inclusion of Integrilin, to prevent platelet-platelet interactions via fibrinogen, indicated that the reduced size of thrombi was due to fewer direct platelet-collagen interactions. Collagen-induced tyrosine phosphorylation of Syk, an early marker of signalling from the collagen receptor GPVI was unaffected by the Hsp47 inhibitor, suggesting that any contribution to platelet function by Hsp47 may be independent of GPVI-proximal signalling. Surface plasmon resonance analysis of recombinant his-tagged human Hsp47 revealed its ability to interact directly with collagen fibrils, an interaction that was disrupted by the small molecule inhibitor of Hsp47.

Mouse platelets were also found to contain Hsp47, and aggregation induced with collagen was reduced by inhibitors of Hsp47. Infusion of Hsp47 inhibitor into mice increased tail bleeding times and volumes of blood lost, even in the absence of anticoagulant. This effect was likely due to inhibition of platelet function, since the thrombi formed in cremaster muscle arterioles using an intravital laser-injury model of thrombosis were smaller following administration of the Hsp47 inhibitor.

Together these data indicate Hsp47 to function on the platelet surface as a collagen receptor that potentially represents a new target for antithrombotic therapy.