RUNX1-induced silencing of non-muscle myosin heavy chain IIB contributes to megakaryocyte polyploidization

Abstract

megakaryocytes are unique mammalian cells that undergo polyploidization (endomitosis) during differentiation, leading to an increase in cell size and protein production that precedes platelet production. Recent evidence demonstrates that endomitosis is a consequence of a late failure in cytokinesis associated with a contractile ring defect. Here we show that the non-muscle myosin IIB heavy chain (mYH10) is expressed in immature megakaryocytes and specifically localizes in the contractile ring. mYH10 downmodulation by short hairpin RnA increases polyploidization by inhibiting the return of 4n cells to 2n, but other regulators, such as of the G1/s transition, might regulate further polyploidization of the 4n cells. Conversely, re-expression of mYH10 in the megakaryocytes prevents polyploidization and the transition of 2n to 4n cells. During polyploidization, mYH10 expression is repressed by the major megakaryocyte transcription factor RunX1. Thus, RunX1-mediated silencing of mYH10 is required for the switch from mitosis to endomitosis, linking polyploidization with megakaryocyte differentiation.

Myeloid neoplasms with germ line RUNX1 mutation

“…Recently, it has been shown that wild-type RUNX1 negatively regulates the expression of non-muscle myosin IIB heavy chain 10 (MYH10), resulting in the megakaryocyte maturation [30]. Thus, haploinsufficiency of RUNX1 might constitutively activate MYH10 expression and affect megakaryocyte polyploidization [30]. RUNX1 also directly regulates the expression of myosin light chain 9 (MYL9) in megakaryocytes.…”

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

“…1 An absence of nonmuscle myosin II (NMII) and hence a defect in the cleavage furrow was observed during endomitosis. 2 NMII is a constitutive component of the actomyosin ring formed at the cleavage furrow and provides the motor force required for cytokinesis and karyokinesis. 3 A single unit of NMII consists of 2 heavy chains of NMII linked to 2 regulatory light chains that regulate NMII activity and 2 essential light chains that stabilize the heavy chain structure.…”

“…Blebbistatin, an inhibitor of NMII, increased both ploidy and proplatelet formation. 2,5 Two isoforms of NMII-MYH9 (NMIIA) and MYH10 (NMIIB)-are expressed during megakaryocyte differentiation. The 2 isoforms are highly similar in their sequence, but vary considerably in their tail region; thus, although NMIIA and NMIIB can sometimes substitute for the absence of the other, they often have unique functions and subcellular localizations.…”

Pathogenesis and management of inherited thrombocytopenias: rationale for the use of thrombopoietin-receptor agonists

“…cells, as well as formation and maturation of proplatelets [30]. The defect in polyploidization in vitro paralleled the finding of hypolobulated Mks at examination of BM biopsies, and could be explained by absent downregulation of MYH10, encoding for non-muscle myosin heavy chain (NMMHC)-IIB, whose silencing by RUNX1 is essential for the switch from mitosis to endomitosis in Mks [38]. Reduced transcription of MYH9 and MYL9, encoding for NMMHC-IIA and myosin regulatory light chains, respectively, could instead contribute to defective PPF [30].…”