Role of RhoA-specific guanine exchange factors in regulation of endomitosis in megakaryocytes

Rho GTPases as regulators of mitosis and cytokinesis in mammalian cells

Rho GTPases regulate a diverse range of cellular functions primarily through their ability to modulate microtubule dynamics and the actin-myosin cytoskeleton. Both of these cytoskeletal structures are crucial for a mitotic cell division. Specifically, their assembly and disassembly is tightly regulated in a temporal manner to ensure that each mitotic stage occurs in the correct sequential order and not prematurely until the previous stage is completed. Thus, it is not surprising that the Rho GTPases, RhoA, and Cdc42, have reported roles in several stages of mitosis: cell cortex stiffening during cell rounding, mitotic spindle formation, and bi-orient attachment of the spindle microtubules to the kinetochore and during cytokinesis play multiple roles in establishing the division plane, assembly, and activation of the contractile ring, membrane ingression, and abscission. Here, I review the molecular mechanisms regulating the spatial and temporal activation of RhoA and Cdc42 during mitosis, and how this is critical for mitotic progression and completion.

Biogenesis of the demarcation membrane system (DMS) in megakaryocytes

Using state-of-the-art three-dimensional electron microscopy approaches, we show that the onset of the DMS formation is at the megakaryocyte plasma membrane. A pre-DMS structure is formed in the perinuclear region, through a PM invagination process that resembles cleavage furrow formation.

Visualizing developmentally programmed endoreplication in mammals using ubiquitin oscillators

The majority of mammalian somatic cells maintain a diploid genome. However, some mammalian cell types undergo multiple rounds of genome replication (endoreplication) as part of normal development and differentiation. For example, trophoblast giant cells (TGCs) in the placenta become polyploid through endoreduplication (bypassed mitosis), and megakaryocytes (MKCs) in the bone marrow become polyploid through endomitosis (abortive mitosis). During the normal mitotic cell cycle, geminin and Cdt1 are involved in ‘licensing’ of replication origins, which ensures that replication occurs only once in a cell cycle. Their protein accumulation is directly regulated by two E3 ubiquitin ligase activities, APCCdh1 and SCFSkp2, which oscillate reciprocally during the cell cycle. Although proteolysis-mediated, oscillatory accumulation of proteins has been documented in endoreplicating Drosophila cells, it is not known whether the ubiquitin oscillators that control normal cell cycle transitions also function during mammalian endoreplication. In this study, we used transgenic mice expressing Fucci fluorescent cell-cycle probes that report the activity of APCCdh1 and SCFSkp2. By performing long-term, high temporal-resolution Fucci imaging, we were able to visualize reciprocal activation of APCCdh1 and SCFSkp2 in differentiating TGCs and MKCs grown in our custom-designed culture wells. We found that TGCs and MKCs both skip cytokinesis, but in different ways, and that the reciprocal activation of the ubiquitin oscillators in MKCs varies with the polyploidy level. We also obtained three-dimensional reconstructions of highly polyploid TGCs in whole, fixed mouse placentas. Thus, the Fucci technique is able to reveal the spatiotemporal regulation of the endoreplicative cell cycle during differentiation.

Polyploidization in liver tissue

Polyploidy (alias whole genome amplification) refers to organisms containing more than two basic sets of chromosomes. Polyploidy was first observed in plants more than a century ago, and it is known that such processes occur in many eukaryotes under a variety of circumstances. In mammals, the development of polyploid cells can contribute to tissue differentiation and, therefore, possibly a gain of function; alternately, it can be associated with development of disease, such as cancer. Polyploidy can occur because of cell fusion or abnormal cell division (endoreplication, mitotic slippage, or cytokinesis failure). Polyploidy is a common characteristic of the mammalian liver. Polyploidization occurs mainly during liver development, but also in adults with increasing age or because of cellular stress (eg, surgical resection, toxic exposure, or viral infections). This review will explore the mechanisms that lead to the development of polyploid cells, our current state of understanding of how polyploidization is regulated during liver growth, and its consequence on liver function.

The incredible journey: From megakaryocyte development to platelet formation

Circulating blood platelets are specialized cells that prevent bleeding and minimize blood vessel injury. Large progenitor cells in the bone marrow called megakaryocytes (MKs) are the source of platelets. MKs release platelets through a series of fascinating cell biological events. During maturation, they become polyploid and accumulate massive amounts of protein and membrane. Then, in a cytoskeletal-driven process, they extend long branching processes, designated proplatelets, into sinusoidal blood vessels where they undergo fission to release platelets. Given the need for platelets in many pathological situations, understanding how this process occurs is an active area of research with important clinical applications.

RhoA is essential for maintaining normal megakaryocyte ploidy and platelet generation

RhoA plays a multifaceted role in platelet biology. During platelet development, RhoA has been proposed to regulate endomitosis, proplatelet formation, and platelet release, in addition to having a role in platelet activation. These processes were previously studied using pharmacological inhibitors in vitro, which have potential drawbacks, such as non-specific inhibition or incomplete disruption of the intended target proteins. Therefore, we developed a conditional knockout mouse model utilizing the CRE-LOX strategy to ablate RhoA, specifically in megakaryocytes and in platelets to determine its role in platelet development. We demonstrated that deleting RhoA in megakaryocytes in vivo resulted in significant macrothrombocytopenia. RhoA-null megakaryocytes were larger, had higher mean ploidy, and exhibited stiff membranes with micropipette aspiration. However, in contrast to the results observed in experiments relying upon pharmacologic inhibitors, we did not observe any defects in proplatelet formation in megakaryocytes lacking RhoA. Infused RhoA-null megakaryocytes rapidly released platelets, but platelet levels rapidly plummeted within several hours. Our evidence supports the hypothesis that changes in membrane rheology caused infused RhoA-null megakaryocytes to prematurely release aberrant platelets that were unstable. These platelets were cleared quickly from circulation, which led to the macrothrombocytopenia. These observations demonstrate that RhoA is critical for maintaining normal megakaryocyte development and the production of normal platelets.

Proplatelet generation in the mouse requires PKCε-dependent RhoA inhibition

During thrombopoiesis, megakaroycytes undergo extensive cytoskeletal remodeling to form proplatelet extensions that eventually produce mature platelets. Proplatelet formation is a tightly orchestrated process that depends on dynamic regulation of both tubulin reorganization and Rho-associated, coiled-coil containing protein kinase/RhoA activity. A disruption in tubulin dynamics or RhoA activity impairs proplatelet formation and alters platelet morphology. We previously observed that protein kinase Cepsilon (PKCε), a member of the protein kinase C family of serine/threonine-kinases, expression varies during human megakaryocyte differentiation and modulates megakaryocyte maturation and platelet release. Here we used an in vitro model of murine platelet production to investigate a potential role for PKCε in proplatelet formation. By immunofluorescence we observed that PKCε colocalizes with α/β-tubulin in specific areas of the marginal tubular-coil in proplatelets. Moreover, we found that PKCε expression escalates during megakarocyte differentiation and remains elevated in proplatelets, whereas the active form of RhoA is substantially downregulated in proplatelets. PKCε inhibition resulted in lower proplatelet numbers and larger diameter platelets in culture as well as persistent RhoA activation. Finally, we demonstrate that pharmacological inhibition of RhoA is capable of reversing the proplatelet defects mediated by PKCε inhibition. Collectively, these data indicate that by regulating RhoA activity, PKCε is a critical mediator of mouse proplatelet formation in vitro.

Endoreplication and polyploidy: insights into development and disease

Polyploid cells have genomes that contain multiples of the typical diploid chromosome number and are found in many different organisms. Studies in a variety of animal and plant developmental systems have revealed evolutionarily conserved mechanisms that control the generation of polyploidy and have recently begun to provide clues to its physiological function. These studies demonstrate that cellular polyploidy plays important roles during normal development and also contributes to human disease, particularly cancer.

New insights into the dynamics of plant cell nuclei and chromosomes

The plant lamin-like protein NMCP/AtLINC and orthologues of the SUN-KASH complex across the nuclear envelope (NE) show the universality of nuclear structure in eukaryotes. However, depletion of components in the connection complex of the NE in plants does not induce severe defects, unlike in animals. Appearance of the Rabl configuration is not dependent on genome size in plant species. Topoisomerase II and condensin II are not essential for plant chromosome condensation. Plant endoreduplication shares several common characteristics with animals, including involvement of cyclin-dependent kinases and E2F transcription factors. Recent finding regarding endomitosis regulator GIG1 shed light on the suppression mechanism of endomitosis in plants. The robustness of plants, compared with animals, is reflected in their genome redundancy. Spatiotemporal functional analyses using chromophore-assisted light inactivation, super-resolution microscopy, and 4D (3D plus time) imaging will reveal new insights into plant nuclear and chromosomal dynamics.

Induction of megakaryocyte differentiation drives nuclear accumulation and transcriptional function of MKL1 via actin polymerization and RhoA activation

How components of the cytoskeleton regulate complex cellular responses is fundamental to understanding cellular function. Megakaryoblast leukemia 1 (MKL1), an activator of serum response factor (SRF) transcriptional activity, promotes muscle, neuron, and megakaryocyte differentiation. In muscle cells, where MKL1 subcellular localization is one mechanism by which cells control SRF activity, MKL1 translocation from the cytoplasm to the nucleus in response to actin polymerization is critical for its function as a transcriptional regulator. MKL1 localization is cell-type specific; it is predominantly cytoplasmic in unstimulated fibroblasts and some muscle cell types and is constitutively nuclear in neuronal cells. In the present study, we report that in megakaryocytes, subcellular localization and regulation of MKL1 is dependent on RhoA activity and actin organization. Induction of megakaryocytic differentiation of human erythroleukemia cells by 12-O-tetradecanoylphorbol-13-acetate and primary megakaryocytes by thrombopoietin promotes MKL1 nuclear localization. This MKL1 localization is blocked by drugs inhibiting RhoA activity or actin polymerization.We also show that nuclear-localized MKL1 activates the transcription of SRF target genes. This report broadens our knowledge of the molecular mechanisms regulating megakaryocyte differentiation.

Presence of a defect in karyokinesis during megakaryocyte endomitosis

Megakaryocyte is the naturally polyploid cell that gives rise to platelets. Polyploidization occurs by endomitosis, a process corresponding to a late failure of cytokinesis with a backward movement of the daughter cells. Generally, a pure defect in cytokinesis produces a multinucleated cell, but megakaryocytes are characterized by a single polylobulated nucleus with a 2 (N) ploidy. Here, we show the existence of a defect in karyokinesis during the endomitotic process. From late telophase until the reversal of cytokinesis, some dipolar mitosis/endomitosis and most multipolar endomitosis present a thin DNA link between the segregated chromosomes surrounded by an incomplete nuclear membrane formation, which implies that sister chromatid separation is not complete. This observation may explain why polyploid megakaryocytes display a single polylobulated nucleus along with an increase in ploidy.

Cell cycle regulation of Rho signaling pathways

The dynamics of the actin cytoskeleton and its regulation by Rho GTPases are essential to maintain cell shape, to allow cell motility and are also critical during cell cycle progression and mitosis. Rho GTPases and their effectors are involved in cell rounding at mitosis onset, in chromosomes alignment and are required for contraction of the actomyosin ring that separates daughter cells at the end of mitosis. Recent studies have revealed how a number of nucleotide exchange factors and GTPase-activating proteins regulate the activity of Rho GTPases during these processes. This review will focus on how the cell cycle machinery, in turn, regulates expression of proteins in the Rho signaling pathways through transcriptional activation, ubiquitylation and proteasomal degradation and modulates their activity through phosphorylation by mitotic kinases.

MKL1 and MKL2 play redundant and crucial roles in megakaryocyte maturation and platelet formation

Serum response factor and its transcriptional cofactor MKL1 are critical for megakaryocyte maturation and platelet formation. We show that MKL2, a homologue of MKL1, is expressed in megakaryocytes and plays a role in megakaryocyte maturation. Using a megakaryocyte-specific Mkl2 knockout (KO) mouse on the conventional Mkl1 KO background to produce double KO (DKO) megakaryocytes and platelets, a critical role for MKL2 is revealed. The decrease in megakaryocyte ploidy and platelet counts of DKO mice is more severe than in Mkl1 KO mice. Platelet dysfunction in DKO mice is revealed by prolonged bleeding times and ineffective platelet activation in vitro in response to adenosine 5'-diphosphate. Electron microscopy and immunofluorescence of DKO megakaryocytes and platelets indicate abnormal cytoskeletal and membrane organization with decreased granule complexity. Surprisingly, the DKO mice have a more extreme thrombocytopenia than mice lacking serum response factor (SRF) expression in the megakaryocyte compartment. Comparison of gene expression reveals approximately 4400 genes whose expression is differentially affected in DKO compared with megakaryocytes deficient in SRF, strongly suggesting that MKL1 and MKL2 have both SRF-dependent and SRF-independent activity in megakaryocytopoiesis.

GEFs on the RhoAd to a colossal nucleus

Cytokinesis in normal cell division requires RhoA-regulated actomyosin contraction of the cleavage furrow; this process is aborted in megakaryocyte endomitosis, leading to polyploidy. In this issue of Developmental Cell, Gao et al. (2012) trace the basis of endomitosis to sequential downregulation of guanine nucleotide exchange factors GEF-H1 and ECT2.

Hepatocytes polyploidization and cell cycle control in liver physiopathology

Most cells in mammalian tissues usually contain a diploid complement of chromosomes. However, numerous studies have demonstrated a major role of "diploid-polyploid conversion" during physiopathological processes in several tissues. In the liver parenchyma, progressive polyploidization of hepatocytes takes place during postnatal growth. Indeed, at the suckling-weaning transition, cytokinesis failure events induce the genesis of binucleated tetraploid liver cells. Insulin signalling, through regulation of the PI3K/Akt signalling pathway, is essential in the establishment of liver tetraploidization by controlling cytoskeletal organisation and consequently mitosis progression. Liver cell polyploidy is generally considered to indicate terminal differentiation and senescence, and both lead to a progressive loss of cell pluripotency associated to a markedly decreased replication capacity. Although adult liver is a quiescent organ, it retains a capacity to proliferate and to modulate its ploidy in response to various stimuli or aggression (partial hepatectomy, metabolic overload (i.e., high copper and iron hepatic levels), oxidative stress, toxic insult, and chronic hepatitis etc.). Here we review the mechanisms and functional consequences of hepatocytes polyploidization during normal and pathological liver growth.