Platelets are released by megakaryocytes (MKs) via cytoplasmic extensions called proplatelets, which require profound changes in the microtubule and actin organization. Here, we provide evidence that the Rho/ROCK pathway, a well-known regulator of actin cytoskeleton, acts as a negative regulator of proplatelet formation (PPF). Rho is expressed at a high level during the entire MK differentiation including human CD34 ؉ cells. Thrombopoietin stimulates its activity but at a higher extent in immature than in mature MKs. Overexpression of a dominantnegative or a spontaneously active RhoA leads to an increase or a decrease in PPF indicating that Rho activation inhibits PPF. This inhibitory effect is mediated through the main Rho effector, Rho kinase (ROCK), the inhibition of which also increases PPF. Furthermore, inhibition of Rho or ROCK in MKs leads to a decrease in myosin light chain 2 (MLC2) phosphorylation, which is required for myosin contractility. Interestingly, inhibition of the MLC kinase also decreases MLC2 phosphorylation while increasing PPF. Taken together, our results suggest that MLC2 phosphorylation is regulated by both ROCK and MLC kinase and plays an important role in platelet biogenesis by controlling PPF and fragmentation. IntroductionMegakaryocytes (MKs) are the highly specialized precursor cells that lead to platelet production. MK differentiation is a continuous process characterized by sequential steps. 1 First, MKs increase their ploidy via endomitosis and begin to increase their size. 2 Then, the synthesis of storage organelles is enhanced, as well as the synthesis of plasma membrane to form the demarcation membranes. This cytoplasmic maturation is associated with a marked increase in the MK size. Finally, mature MKs release platelets probably through cytoplasmic fragmentation at the tips of long and thin extensions called proplatelets (PPTs) that contain all the platelet organelles. 3,4 The mechanisms controlling proplatelet formation (PPF) are still incompletely understood. However, PPF is associated with remarkable morphologic changes that require a profound reorganization of the cytoskeleton. 5 Increasing evidence indicates that PPTs arise from the unfolding of demarcation membranes. The microtubule cytoskeleton provides the sliding power to unfold demarcation membranes and thus to induce the pseudopodial elongations corresponding to PPTs. 6 In addition, microtubules permit the organelle transport in the PPTs and maintain the platelet discoid shape. [7][8][9][10] Although not studied in detail, the actin cytoskeleton may also participate in PPF because cytoplasmic polymerized actin is associated with demarcation membranes and actin is highly aggregated in cultured MKs when PPF occurs. 11 In addition, a crucial role of the actin cytoskeleton has been reported in platelet functions since it regulates platelet shape in unstimulated and activated platelets. 12 Evidence suggests that actin cytoskeleton may play important roles during PPT formation at 2 different stages: (1) at early stages,...

Megakaryocyte (MK) is the naturally polyploid cell that gives rise to platelets. Polyploidization occurs by endomitosis, which was a process considered to be an incomplete mitosis aborted in anaphase. Here, we used time-lapse confocal video microscopy to visualize the endomitotic process of primary human megakaryocytes. Our results show that the switch from mitosis to endomitosis corresponds to a late failure of cytokinesis accompa-

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.

Heat-shock factors (HSFs) are associated with multiple developmental processes, but their mechanisms of action in these processes remain largely enigmatic. Hsf2-null mice display gametogenesis defects and brain abnormalities characterized by enlarged ventricles. Here, we show that Hsf2 −/− cerebral cortex displays mispositioning of neurons of superficial layers. HSF2 deficiency resulted in a reduced number of radial glia fibers, the architectural guides for migrating neurons, and of Cajal-Retzius cells, which secrete the positioning signal Reelin. Therefore, we focused on the radial migration signaling pathways. The levels of Reelin and Dab1 tyrosine phosphorylation were reduced, suggesting that the Reelin cascade is affected in Hsf2 −/− cortices. The expression of p35, an activator of cyclin-dependent kinase 5 (Cdk5), essential for radial migration, was dependent on the amount of HSF2 in gain-and loss-of-function systems. p39, another Cdk5 activator, displayed reduced mRNA levels in Hsf2 −/− cortices, which, together with the lowered p35 levels, decreased Cdk5 activity. We demonstrate in vivo binding of HSF2 to the p35 promoter and thereby identify p35 as the first target gene for HSF2 in cortical development. In conclusion, HSF2 affects cellular populations that assist in radial migration and directly regulates the expression of p35, a crucial actor of radial neuronal migration.[Keywords: Corticogenesis; heat-shock factor; p35-Cdk5; radial cortical migration] Supplemental material is available at http://www.genesdev.org. Heat-shock factors (HSFs) were initially discovered to regulate heat-shock genes and the heat-shock response. The heat-shock response, conserved from yeast to man, is characterized by the induction of heat-shock genes encoding molecular chaperones (for review, see Pirkkala et al. 2001). A unique gene constitutes HSF in yeast, nematode, and fruit fly, whereas a family of four members is present in vertebrates. HSF1 and HSF2 are found in all vertebrate species, while HSF3 is specific for avian species and HSF4 is specific for mammals (Rabindran et al. 1991;Sarge et al. 1991;Schuetz et al. 1991;Nakai and Morimoto 1993;Nakai et al. 1997;Råbergh et al. 2000;Hilgarth et al. 2004;Le Goff et al. 2004). In vertebrates, HSF1 is the stress-responsive prototype, which cannot be substituted by any other HSF in stress-inducible hsp gene expression or in acquired thermotolerance (McMillan et al. 1998;Xiao et al. 1999;Zhang et al. 2002).A developmental role for the HSFs began to emerge when the Drosophila HSF was found to be required for oogenesis and early larval development (Jedlicka et al. 1997). Strikingly, these developmental effects of Drosophila HSF are not mediated by hsp gene induction. The basal expression levels of hsps during embryonic development in mouse are not affected by the lack of HSF1 (Xiao et al. 1999). Therefore, other target genes are likely to be controlled by HSF1 in development. Recently, binding of HSF1 and HSF4 to the FGF-7 promoter with opposing effects on FGF-7 gene expression su...

Summary. Megakaryocytopoiesis is the process that leads to the production of platelets. This process involves the commitment of multipotent hematopoietic stem cells toward megakaryocyte (MK) progenitors, the proliferation and differentiation of MK progenitors, the polyploidization of MK precursors and the maturation of MK. Mature MK produce platelets by cytoplasmic fragmentation occurring through a dynamic and regulated process, called proplatelet formation, and consisting of long pseudopodial elongations that break in the blood flow. Recent insights have demonstrated that the MK and erythroid lineages are tightly associated at both the cellular and molecular levels, especially in the transcription factors that regulate their differentiation programs. Megakaryocytopoiesis is regulated by two types of transcription factors, those regulating the differentiation process, such as GATA-1, and those regulating proplatelet formation, such as NF-E2. The humoral factor thrombopoietin (TPO) is the primary regulator of MK differentiation and platelet production through the stimulation of its receptor MPL. Numerous acquired or congenital pathologies of the MK lineage are now explained by molecular abnormalities in the activity of the transcription factors involved in megakaryocytopoiesis, in the Tpo or c-mpl genes, as well as in signaling molecules associated with MPL. The recent development of MPL agonists may provide efficient agents for the treatment of some thrombocytopenias.

Summary. Each day in every human, approximately 1 · 10 11 platelets are produced by the cytoplasmic fragmentation of megakaryocytes (MK), their marrow precursor cells. Platelets are the predominating factor in the process of hemostasis and thrombosis. Recent studies have shown that platelets also play a hitherto unsuspected role in several other processes such as inflammation, innate immunity, neoangiogenesis and tumor metastasis. The late phases of MK differentiation identified by polyploidization, maturation and organized fragmentation of the cytoplasm leading to the release of platelets in the blood stream represent a unique model of differentiation. The molecular and cellular mechanisms regulating platelet biogenesis are better understood and may explain several platelet disorders. This review focuses on MK polyploidization, and platelet formation, and discusses their alteration in some platelet disorders.

Megakaryoblastic leukemia 1 (MAL) is a transcriptional coactivator of serum response factor (SRF). In acute megakaryoblastic leukemia, the MAL gene is translocated and fused with the gene encoding one twenty-two (OTT). Herein, we show that MAL expression increases during the late differentiation steps of neonate and adult human megakaryopoiesis and localized into the nucleus after Rho GTPase activation by adhesion on collagen I or convulxin. MAL knockdown in megakaryocyte progenitors reduced the percentage of cells forming filopodia, lamellipodia, and stress fibers after adhesion on the same substrates, and reduced proplatelet formation. MAL repression led to dysmorphic megakaryocytes with disorganized demarcation membranes and ␣ granules heterogeneously scattered in the cytoplasm. Gene expression profiling revealed a marked decrease in metalloproteinase 9 (MMP-9) and MYL9 expression after MAL inhibition. Luciferase assays in HEK293T cells and chromatin immunoprecipitation in primary megakaryocytes showed that the MAL/SRF complex directly regulates MYL9 and MMP9 in vitro. Megakaryocyte migration in response to stromal cell-derived factor 1, through Matrigel was considerably decreased after MAL knockdown, implicating MMP9 in migration. Finally, the use of a shRNA to decrease MYL9 expression showed that MYL9 was involved in proplatelet formation. MAL/SRF complex is thus involved in platelet formation and megakaryocyte migration by regulating MYL9 and MMP9. IntroductionSerum response factor (SRF) is a widely expressed transcription factor required for the expression of immediate early, musclespecific, and cytoskeletal genes. 1-4 SRF contains a MADS domain that mediates homodimerization and DNA binding, and that allows recruitment of transcriptional cofactors. SRF binds to a CArG box present in promoter/enhancer regions of SRF-regulated genes. 5 Depending on cell lines, different extracellular stimuli activate SRF through 2 main signaling pathways: the MAP-kinase pathway through members of the ternary complex factor (TCF) 6,7 and the small GTPases pathway through the Rho family 8 members regulating the myocardin-related transcription factors (MRTFs). The Rho-actin signaling pathway 9-12 stimulates SRF by 2 ubiquitous MRTFs, megakaryoblastic leukemia 1 (MAL; MKL1, MRTF-A, BSAC) and MAL16 (MKL2, MRTF-B).MAL was initially identified in acute megakaryoblastic leukemia (AMKL, M7) as a chromosome 22 encoded protein fused in 3Ј with RNA-binding motif protein 15 (RBM15; OTT) located on chromosome 1. [13][14][15] The translocation t(1,22)(p13;q13) leads to the in-frame fusion of the quasi-totality of OTT/RBM15 to the MAL gene. The OTT-MAL fusion protein is restricted to AMKL occurring de novo in infancy, 16,17 in children older than 1 year or, occasionally, in Down syndrome patients. 18,19 The subcellular localization of MAL is regulated through its association with globular actin by its RPEL motifs in the Nterminal region. The modification of the actin treadmilling by the Rho pathway results in the nuclear accumulation of ...

Key Points• DIAPH1 (mDia1) is involved in both Rho-mediated actin polymerization and microtubule assembly and stability during proplatelet formation.Megakaryocytes are highly specialized precursor cells that produce platelets via cytoplasmic extensions called proplatelets. Proplatelet formation (PPF) requires profound changes in microtubule and actin organization. In this work, we demonstrated that DIAPH1 (mDia1), a mammalian homolog of Drosophila diaphanous that works as an effector of the small GTPase Rho, negatively regulates PPF by controlling the dynamics of the actin and microtubule cytoskeletons. Moreover, we showed that inhibition of both DIAPH1 and the Rho-associated protein kinase (Rock)/myosin pathway increased PPF via coordination of both cytoskeletons. We provide evidence that 2 major effectors of the Rho GTPase pathway (DIAPH1 and Rock/myosin II) are involved not only in Rho-mediated stress fibers assembly, but also in the regulation of microtubule stability and dynamics during PPF. (Blood. 2014;124(26):3967-3977)