Abstract/Summary

HSP47, a collagen specific chaperone protein, has been identified within the platelet and has been shown to influence the ability of platelets to respond to collagen and initiate haemostasis. Megakaryocytes, which produce platelets, mature in the bone marrow (BM) but also secrete extracellular matrix factors that support the BM environment. We aimed to understand the role HSP47 plays in megakaryocyte maturation, proplatelet formation and collagen synthesis. We also aimed to determine whether HSP47 acts as a platelet surface receptor. Using a megakaryocyte lineage-specific transgenic HSP47 gene-deficient mouse, we identified that megakaryocyte ploidy and proplatelet formation was not altered in MKs from HSP47 deficient mice (P>0.05) however collagen production from the MKs was reduced (P<0.05) suggesting that HSP47 plays no role in MK maturation but is involved in the maintenance of the BM environment. Using peptide sequences identified to enable HSP47-collagen interactions, we identified that human platelets are able to adhere and spread on these sequences, however they do not support granule release, aggregation or thrombus formation. We also identified, using the HSP47 binding sequences, that HSP47 is able to modulate platelet-collagen interactions that result in platelet aggregation and activation but not thrombus formation. Finally, we confirm the presence of additional chaperone proteins within the platelet, and identify using inhibitors, that Endoplasmin may be involved in collagen receptor-mediated aggregation and thrombus formation on collagen, and that BiP may be implicated in TRAP6 stimulated aggregation and thrombus formation on collagen. Deletion of HSP47 from MKs suggests that HSP47 may be a therapeutic target to prevent fibrosis of the BM. Investigations using HSP47 binding sequences identified that HSP47 has dual functionality within the platelet, both supporting platelet adhesion in addition to modulating platelet collagen receptor-collagen interactions. Confirmation of additional chaperone proteins add to the increasing understanding that ER proteins regulate platelet function and can be targeted using inhibitors.