The formin DIAPH1 (mDia1) regulates megakaryocyte proplatelet formation by remodeling the actin and microtubule cytoskeletons. Blood. 2014;124:3967-3977. [PMID: 25298036 DOI: 10.1182/blood-2013-12-544924]

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

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.

Cdc42-dependent F-actin dynamics drive structuration of the demarcation membrane system in megakaryocytes

Essentials Information about the formation of the demarcation membrane system (DMS) is still lacking. We investigated the role of the cytoskeleton in DMS structuration in megakaryocytes. Cdc42/Pak-dependent F-actin remodeling regulates DMS organization for proper megakaryopoiesis. These data highlight the mandatory role of F-actin in platelet biogenesis.

SUMMARY

Background Blood platelet biogenesis results from the maturation of megakaryocytes (MKs), which involves the development of a complex demarcation membrane system (DMS). Therefore, MK differentiation is an attractive model for studying membrane remodeling. Objectives We sought to investigate the mechanism of DMS structuration in relationship to the cytoskeleton. Results Using three-dimensional (3D) confocal imaging, we have identified consecutive stages of DMS organization that rely on F-actin dynamics to polarize membranes and nuclei territories. Interestingly, microtubules are not involved in this process. We found that the mechanism underlying F-actin-dependent DMS formation required the activation of the guanosine triphosphate hydrolase Cdc42 and its p21-activated kinase effectors (Pak1/2/3). Förster resonance energy transfer demonstrated that active Cdc42 was associated with endomembrane dynamics throughout terminal maturation. Inhibition of Cdc42 or Pak1/2/3 severely destructured the DMS and blocked proplatelet formation. Even though this process does not require containment within the hematopoietic niche, because DMS structuration was observed upon thrombopoietin-treatment in suspension, integrin outside-in signaling was required for Pak activation and probably resulted from secretion of extracellular matrix by MKs. Conclusions These data indicate a functional link, mandatory for MK differentiation, between actin dynamics, regulated by Cdc42/Pak1/2/3, and DMS maturation.

Coordination of Actin- and Microtubule-Based Cytoskeletons Supports Transport of Spermatids and Residual Bodies/Phagosomes During Spermatogenesis in the Rat Testis

Germ cell transport across the seminiferous epithelium during spermatogenesis requires the intricate coordination of cell junctions, signaling proteins, and both actin- and microtubule (MT)-based cytoskeletons. Although the involvement of cytoskeletons in germ cell transport has been suggested, the precise mechanism(s) remains elusive. Based on growing evidence that actin and MT interactions underlie fundamental cellular processes, such as cell motility, it is unlikely that actin- and MT-based cytoskeletons work independently to regulate germ cell transport in the testis. Using rats treated with adjudin, a potential male contraceptive that disrupts spermatid adhesion and transport in the testis, as a study model, we show herein that actin- and MT-based cytoskeletons are both necessary for transport of spermatids and residual bodies/phagosomes across the seminiferous epithelium in adult rat testes. Analysis of intratubular expression of F-actin and tubulin revealed disruption of both actin and MT networks, concomitant with misdirected spermatids and phagosomes in rats treated with adjudin. Actin regulatory proteins, epidermal growth factor receptor pathway substrate 8 and actin-related protein 3, were mislocalized and down-regulated at the actin-rich anchoring junction between germ and Sertoli cells (apical ectoplasmic specialization) after adjudin treatment. Nonreceptor tyrosine kinase p-FAK-Tyr(407), known to regulate F-actin nucleation via actin-related protein 3, was also mislocalized and down-regulated at the apical ectoplasmic specialization, corroborating the observation of actin cytoskeleton disruption. Additionally, spatiotemporal expression of MT regulatory protein end-binding protein 1, shown to be involved in MT-actin cross talk herein, was also disrupted after adjudin treatment. In summary, spermatid/phagosome transport across the epithelium during spermatogenesis requires the coordination between actin- and MT-based cytoskeletons.

A gain-of-function variant in DIAPH1 causes dominant macrothrombocytopenia and hearing loss

Macrothrombocytopenia (MTP) is a heterogeneous group of disorders characterized by enlarged and reduced numbers of circulating platelets, sometimes resulting in abnormal bleeding. In most MTP, this phenotype arises because of altered regulation of platelet formation from megakaryocytes (MKs). We report the identification of DIAPH1, which encodes the Rho-effector diaphanous-related formin 1 (DIAPH1), as a candidate gene for MTP using exome sequencing, ontological phenotyping, and similarity regression. We describe 2 unrelated pedigrees with MTP and sensorineural hearing loss that segregate with a DIAPH1 R1213* variant predicting partial truncation of the DIAPH1 diaphanous autoregulatory domain. The R1213* variant was linked to reduced proplatelet formation from cultured MKs, cell clustering, and abnormal cortical filamentous actin. Similarly, in platelets, there was increased filamentous actin and stable microtubules, indicating constitutive activation of DIAPH1. Overexpression of DIAPH1 R1213* in cells reproduced the cytoskeletal alterations found in platelets. Our description of a novel disorder of platelet formation and hearing loss extends the repertoire of DIAPH1-related disease and provides new insight into the autoregulation of DIAPH1 activity.

Inhibition of Diaphanous Formin Signaling In Vivo Impairs Cardiovascular Development and Alters Smooth Muscle Cell Phenotype

OBJECTIVE

We and others have previously shown that RhoA-dependent stimulation of myocardin-related transcription factor nuclear localization promotes smooth muscle cell (SMC) marker gene expression. The goal of this study was to provide direct in vivo evidence that actin polymerization by the diaphanous-related formins contributes to the regulation of SMC differentiation and phenotype.

APPROACH AND RESULTS

Conditional Cre-based genetic approaches were used to overexpress a well-characterized dominant-negative variant of mDia1 (DNmDia) in SMC. DNmDia expression in SM22-expressing cells resulted in embryonic and perinatal lethality in ≈20% of mice because of defects in myocardial development and SMC investment of peripheral vessels. Although most DNmDia(+)/SM22Cre(+) mice exhibited no overt phenotype, the re-expression of SMC differentiation marker gene expression that occurs after carotid artery ligation was delayed, and this effect was accompanied by a significant decrease in myocardin-related transcription factor-A nuclear localization. Interestingly, neointima growth was inhibited by expression of DNmDia in SMC and this was likely because of a defect in directional SMC migration and not to defects in SMC proliferation or survival. Finally, by using the tamoxifen-inducible SM MHC-CreER(T2) line, we showed that SMC-specific induction of DNmDia in adult mice decreased SMC marker gene expression.

CONCLUSIONS

Our demonstration that diaphanous-related formin signaling plays a role in heart and vascular development and the maintenance of SMC phenotype provides important new evidence that Rho/actin/myocardin-related transcription factor signaling plays a critical role in cardiovascular function.

Cytoskeletal regulation of platelet formation: Coordination of F-actin and microtubules

Platelets are small, anucleate blood cells which play an important role in haemostasis. Thrombocytopenia is a condition where the platelet count falls below 150×10(9)/l and patients suffering from severe forms of this condition can experience life-threatening bleeds requiring platelet transfusions. Platelets are produced from large progenitor cells called megakaryocytes which are found in the bone marrow. The process of megakaryocyte maturation and the formation of proplatelets are essential steps in the production of mature platelets and both depend heavily on the actin and microtubule cytoskeletons. Understanding these processes is important for the development of in vitro platelet production which will help to treat thrombocytopenia as well as produce model systems for studying platelet-associated disorders. This review will highlight some of the recent advances in our understanding of the role of the cytoskeleton in platelet production, especially the key molecules and signalling pathways that regulate actin and microtubule crosstalk.

Regulation of actin polymerization by tropomodulin-3 controls megakaryocyte actin organization and platelet biogenesis

The actin cytoskeleton is important for platelet biogenesis. Tropomodulin-3 (Tmod3), the only Tmod isoform detected in platelets and megakaryocytes (MKs), caps actin filament (F-actin) pointed ends and binds tropomyosins (TMs), regulating actin polymerization and stability. To determine the function of Tmod3 in platelet biogenesis, we studied Tmod3(-/-) embryos, which are embryonic lethal by E18.5. Tmod3(-/-) embryos often show hemorrhaging at E14.5 with fewer and larger platelets, indicating impaired platelet biogenesis. MK numbers are moderately increased in Tmod3(-/-) fetal livers, with only a slight increase in the 8N population, suggesting that MK differentiation is not significantly affected. However, Tmod3(-/-) MKs fail to develop a normal demarcation membrane system (DMS), and cytoplasmic organelle distribution is abnormal. Moreover, cultured Tmod3(-/-) MKs exhibit impaired proplatelet formation with a wide range of proplatelet bud sizes, including abnormally large proplatelet buds containing incorrect numbers of von Willebrand factor-positive granules. Tmod3(-/-) MKs exhibit F-actin disturbances, and Tmod3(-/-) MKs spreading on collagen fail to polymerize F-actin into actomyosin contractile bundles. Tmod3 associates with TM4 and the F-actin cytoskeleton in wild-type MKs, and confocal microscopy reveals that Tmod3, TM4, and F-actin partially colocalize near the membrane of proplatelet buds. In contrast, the abnormally large proplatelets from Tmod3(-/-) MKs show increased F-actin and redistribution of F-actin and TM4 from the cortex to the cytoplasm, but normal microtubule coil organization. We conclude that F-actin capping by Tmod3 regulates F-actin organization in mouse fetal liver-derived MKs, thereby controlling MK cytoplasmic morphogenesis, including DMS formation and organelle distribution, as well as proplatelet formation and sizing.

Cytoskeletal proteins in cortical development and disease: actin associated proteins in periventricular heterotopia

The actin cytoskeleton regulates many important cellular processes in the brain, including cell division and proliferation, migration, and cytokinesis and differentiation. These developmental processes can be regulated through actin dependent vesicle and organelle movement, cell signaling, and the establishment and maintenance of cell junctions and cell shape. Many of these processes are mediated by extensive and intimate interactions of actin with cellular membranes and proteins. Disruption in the actin cytoskeleton in the brain gives rise to periventricular heterotopia (PH), a malformation of cortical development, characterized by abnormal neurons clustered deep in the brain along the lateral ventricles. This disorder can give rise to seizures, dyslexia and psychiatric disturbances. Anatomically, PH is characterized by a smaller brain (impaired proliferation), heterotopia (impaired initial migration) and disruption along the neuroependymal lining (impaired cell-cell adhesion). Genes causal for PH have also been implicated in actin-dependent processes. The current review provides mechanistic insight into actin cytoskeletal regulation of cortical development in the context of this malformation of cortical development.