MEGAKARYOCYTE-RESTRICTED MYH9 INACTIVATION DRAMATICALLY AFFECTS HAEMOSTASIS WHILE PRESERVING PLATELET AGGREGATION AND SECRETION RESPONSES
Abstract number: O-S-020
Leon1 C., Eckly1 A., Hechler1 B., Aleil1 B., Freund1 M., Nonne1 C., Weber1 J., Tiedt2 R., Gratacap3 M., Séverin3 S., Cazenave1 J., Lanza1 F., Skoda2 R., Gachet1 C.
11INSERM U311, EFS-Alsace, Strasbourg, France 22Research experimental hematology, University hospital Basel, Basel, Switzerland 33INSERM U563, CHU Purpan, Toulouse, France
How-to-cite Leon C, Eckly A, Hechler B, Aleil B, Freund M, Nonne C, Weber J, Tiedt R, Gratacap M, Séverin S, Cazenave J, Lanza F, Skoda R, Gachet C. MEGAKARYOCYTE-RESTRICTED MYH9 INACTIVATION DRAMATICALLY AFFECTS HAEMOSTASIS WHILE PRESERVING PLATELET AGGREGATION AND SECRETION RESPONSES. J Thromb Haemost 2007; 5 Supplement 2: O-S-020
Introduction: Mutations in MYH9 encoding the non-muscle myosin heavy chain-IIA (NMHC-IIA) result in MYH9-related disorders, characterized by a macrothrombocytopenia and by haemorrhagic manifestations of variable severity. The aim of the study was to understand the role of myosin in platelet functions and haemostasis.
Methods: Mice were generated exhibiting a severe deficiency in platelet NMHC-IIA (15% of WT) following megakaryocyte-restricted MYH9 inactivation (MYH9D) with a PF4-Cre KO strategy.
Results: Myosin deficiency resulted in thrombocytopenia (platelet count representing 36% of wild-type) and heterogeneity in platelet morphology with a mean size twice that of controls and a mixed population of normal discoid and more ovoid morphology. The tail bleeding time was severely prolonged and clot retraction was abolished. Unexpectedly, near normal platelet aggregation and secretion responses were observed, despite absence of initial platelet contraction and shape change. Integrin outside-in signaling was abnormal, as evidenced by decreased integrin b3 phosphorylation and PtdIns(3,4)P2 accumulation following thrombin stimulation. MYH9D platelets were able to extend lamellipodia upon adhesion to a fibrinogen matrix but lacked stress fiber formation. As a consequence, thrombus growth investigated under flow by perfusing whole blood over collagen was altered. Thrombi that formed were loosely packed and composed of only a few layers of platelets, compared to the control condition where compact aggregates were composed of tightly packed platelets. Thrombus growth was also decreased in vivo in a model of laser-induced injury of mesenteric arterioles.
Conclusions: Overall, these results have established a key role of myosin in platelet functions, and remarkably in integrin outside-in signaling which, in addition to thrombocytopenia, account for the haemostatic defects in MYH9-related diseases.