Abstract/Summary

Background: Heat shock protein 47 (HSP47) is an intracellular chaperone protein with an indispensable role in collagen biosynthesis in collagen-secreting cells. This chaperone has also been shown to be present on the surface of platelets. The inhibition of HSP47 in human platelets or its ablation in mouse platelets reduced their function in response to collagen and the GPVI agonist (CRP-XL), however, responses to thrombin were unaltered. Aims: Given the importance of collagen and its interactions with platelets in triggering hemostasis and thrombosis, in this study we sought to understand the mechanisms by which HSP47 modulates platelet-collagen adhesion. We have explored the location, levels, intracellular associations of HSP47 with different platelet organelles and identified cellular events that mediate HSP47 mobilisation to platelet surface on stimulation. We examined the impact of HSP47 inhibition on the binding of platelets to various collagen receptor-specific ligands and its implication in the modulation of collagen receptors GPVI and integrin α2β1. Signalling events downstream of these receptors were also explored in presence of HSP47 inhibitor. Results: HSP47 colocalisation with the dense tubular system was established using immunofluorescence microscopy imaging and subcellular fractionation analysis. Following platelet stimulation, HSP47 peripheral mobilisation was shown to be dependent on actin polymerisation. Inhibition of HSP47 attenuated platelet adhesion to collagen and CRP-XL, whereas, adhesion to GFOGER was unaltered. GPVI dimerisation was reduced in HSP47 inhibited platelets. Co-immunoprecipitation and microscopic studies revealed the association of HSP47 with GPVI. HSP47 has shown to support collagen and GPVI-mediated signalling. However, outside-in signalling events in response to GFOGER stimulation were not affected by HSP47 inhibition. Conclusions: The present study identifies a possible mechanism by which HSP47 supports platelet collagen responses and thereby contributes to haemostasis and thrombosis via GPVI dimerisation.