Megakaryocyte-restricted MYH9 inactivation dramatically affects hemostasis while preserving platelet aggregation and secretion.
Mutations in the MYH9 gene encoding the nonmuscle myosin heavy chain IIA result in bleeding disorders characterized by a macrothrombocytopenia. To understand the role of myosin in normal platelet functions and in pathology, we generated mice with disruption of MYH9 in megakaryocytes. MYH9Delta mice displayed macrothrombocytopenia with a strong increase in bleeding time and absence of clot retraction. However, platelet aggregation and secretion in response to any agonist were near normal despite absence of initial platelet contraction. By contrast, integrin outside-in signaling was impaired, as observed by a decrease in integrin beta3 phosphorylation and PtdIns(3,4)P(2) accumulation following stimulation. Upon adhesion on a fibrinogen-coated surface, MYH9Delta platelets were still able to extend lamellipodia but without stress fiber-like formation. As a consequence, thrombus growth and organization, investigated under flow by perfusing whole blood over collagen, were strongly impaired. Thrombus stability was also decreased in vivo in a model of FeCl(3)-induced injury of carotid arteries. Overall, these results demonstrate that while myosin seems dispensable for aggregation and secretion in suspension, it plays a key role in platelet contractile phenomena and outside-in signaling. These roles of myosin in platelet functions, in addition to thrombocytopenia, account for the strong hemostatic defects observed in MYH9Delta mice.
Léon
Catherine
C
INSERM U311, Etablissement Français du Sang-Alsace, 10 rue Spielmann, 67065 Strasbourg Cedex, France. catherine.leon@efs-alsace.fr
Eckly
Anita
A
Hechler
Béatrice
B
Aleil
Boris
B
Freund
Monique
M
Ravanat
Catherine
C
Jourdain
Marie
M
Nonne
Christelle
C
Weber
Josiane
J
Tiedt
Ralph
R
Gratacap
Marie-Pierre
MP
Severin
Sonia
S
Cazenave
Jean-Pierre
JP
Lanza
François
F
Skoda
Radek
R
Gachet
Christian
C
D000818:Animals; D001760:Bleeding Time; D001792:Blood Platelets / Q000378:metabolism* / Q000473:pathology; D005260:Female; D006487:Hemostasis / Q000235:genetics*; D008297:Male; D008533:Megakaryocytes / Q000378:metabolism*; D051379:Mice; D008810:Mice, Inbred C57BL; D008822:Mice, Transgenic; D018995:Myosin Heavy Chains / Q000235:genetics / Q000378:metabolism / Q000502:physiology; D024507:Nonmuscle Myosin Type IIA / Q000172:deficiency / Q000235:genetics* / Q000378:metabolism / Q000502:physiology; D009928:Organ Specificity / Q000235:genetics; D010974:Platelet Aggregation / Q000235:genetics*; D013921:Thrombocytopenia / Q000097:blood / Q000235:genetics / Q000473:pathology
D024507:Nonmuscle Myosin Type IIA; D018995:Myosin Heavy Chains