Proplatelet formation deficit and megakaryocyte death contribute to thrombocytopenia in Myh9 knockout mice.
BACKGROUND
Inactivation of the mouse Myh9 gene (Myh9Δ) or its mutation in MYH9-related diseases leads to macrothrombocytopenia. Paradoxically, previous studies using in vitro differentiated megakaryocytes showed an increased capacity for proplatelet formation when myosin was absent or inhibited.


METHODS
To explore the origin of the thrombocytopenia induced by myosin deficiency, we studied proplatelet formation using bone marrow explants of wild-type (WT) and Myh9Δ mouse where megakaryocytes have matured in their native environment.


CONCLUSIONS
A dramatic decrease in the number and complexity of proplatelets was observed in megakaryocytes from Myh9Δ mice, while inhibition of myosin activity by blebbistatin increased proplatelet formation from WT mature megakaryocytes. Moreover, Myh9Δ megakaryocytes had a smaller size than the WT cells. These data indicate that myosin deficiency acts negatively on proplatelet formation, probably by impairing in situ megakaryocyte maturation, while myosin activity is dispensable at the latest stage of proplatelet formation. In addition, ultrastructural examination of Myh9Δ bone marrow revealed an increased proportion of megakaryocytes exhibiting signs of non-apoptotic cell death as compared with the WT mice.


CONCLUSIONS
These data indicate that thrombocytopenia in Myh9Δ mice results from defective development of megakaryocyte size, impaired proplatelet formation and increased cell death.
Eckly
A
A
UMR_S949 INSERM-Université de Strasbourg, Etablissement Français du Sang-Alsace, Strasbourg, France.
Rinckel
J-Y
JY
Laeuffer
P
P
Cazenave
J-P
JP
Lanza
F
F
Gachet
C
C
Léon
C
C
D000818:Animals; D001792:Blood Platelets / Q000166:cytology*; D001853:Bone Marrow / Q000648:ultrastructure; D053148:Caspase 3 / Q000378:metabolism; D016923:Cell Death; D019070:Cell Lineage; D002470:Cell Survival; D005260:Female; D006576:Heterocyclic Compounds, 4 or More Rings / Q000378:metabolism; D020287:In Situ Nick-End Labeling; D008297:Male; D008533:Megakaryocytes / Q000166:cytology*; D051379:Mice; D046529:Microscopy, Electron, Transmission / Q000379:methods; D009154:Mutation; D018995:Myosin Heavy Chains; D024507:Nonmuscle Myosin Type IIA / Q000235:genetics*; D013921:Thrombocytopenia / Q000209:etiology / Q000235:genetics*
D006576:Heterocyclic Compounds, 4 or More Rings; C525088:Myh9 protein, mouse; C472645:blebbistatin; D053148:Caspase 3; D024507:Nonmuscle Myosin Type IIA; D018995:Myosin Heavy Chains