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Blood. 2016 Dec 29;128(26):3137-3145. doi: 10.1182/blood-2016-04-711630. Epub 2016 Oct 13.

Activity of nonmuscle myosin II isoforms determines localization at the cleavage furrow of megakaryocytes.

Roy A1,2,3, Lordier L1,2,3, Mazzi S1,2,3, Chang Y1,2,3, Lapierre V4, Larghero J5, Debili N1,2,3, Raslova H1,2,3, Vainchenker W1,2,3.

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Institut National de la Santé et de la Recherche Médicale, Unité mixte de recherche (UMR) 1170, Villejuif, France.
Université Paris-Saclay, UMR 1170, Villejuif, France.
Gustave Roussy, UMR 1170, Villejuif, France.
Gustave Roussy, Unité de Thérapie Cellulaire, Villejuif, France; and.
Assistance Publique-Hôpitaux de Paris, Unité de biotherapies cellulaires et tissulaires, Hôpital Saint-Louis, Paris, France.


Megakaryocyte polyploidy is characterized by cytokinesis failure resulting from defects in contractile forces at the cleavage furrow. Although immature megakaryocytes express 2 nonmuscle myosin II isoforms (MYH9 [NMIIA] and MYH10 [NMIIB]), only NMIIB localizes at the cleavage furrow, and its subsequent absence contributes to polyploidy. In this study, we tried to understand why the abundant NMIIA does not localize at the furrow by focusing on the RhoA/ROCK pathway that has a low activity in polyploid megakaryocytes. We observed that under low RhoA activity, NMII isoforms presented different activity that determined their localization. Inhibition of RhoA/ROCK signaling abolished the localization of NMIIB, whereas constitutively active RhoA induced NMIIA at the cleavage furrow. Thus, although high RhoA activity favored the localization of both the isoforms, only NMIIB could localize at the furrow at low RhoA activity. This was further confirmed in erythroblasts that have a higher basal RhoA activity than megakaryocytes and express both NMIIA and NMIIB at the cleavage furrow. Decreased RhoA activity in erythroblasts abolished localization of NMIIA but not of NMIIB from the furrow. This differential localization was related to differences in actin turnover. Megakaryocytes had a higher actin turnover compared with erythroblasts. Strikingly, inhibition of actin polymerization was found to be sufficient to recapitulate the effects of inhibition of RhoA/ROCK pathway on NMII isoform localization; thus, cytokinesis failure in megakaryocytes is the consequence of both the absence of NMIIB and a low RhoA activity that impairs NMIIA localization at the cleavage furrow through increased actin turnover.

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