RhoA/Cdc42 signaling drives cytoplasmic maturation but not endomitosis in megakaryocytes

A circle of life: platelet and megakaryocyte cytoskeleton dynamics in health and disease

Platelets are blood cells derived from megakaryocytes that play a central role in regulating haemostasis and vascular integrity. The microtubule cytoskeleton of megakaryocytes undergoes a critical dynamic reorganization during cycles of endomitosis and platelet biogenesis. Quiescent platelets have a discoid shape maintained by a marginal band composed of microtubule bundles, which undergoes remarkable remodelling during platelet activation, driving shape change and platelet function. Disrupting or enhancing this process can cause platelet dysfunction such as bleeding disorders or thrombosis. However, little is known about the molecular mechanisms underlying the reorganization of the cytoskeleton in the platelet lineage. Recent studies indicate that the emergence of a unique platelet tubulin code and specific pathogenic tubulin mutations cause platelet defects and bleeding disorders. Frequently, these mutations exhibit dominant negative effects, offering valuable insights into both platelet disease mechanisms and the functioning of tubulins. This review will highlight our current understanding of the role of the microtubule cytoskeleton in the life and death of platelets, along with its relevance to platelet disorders.

Modelling Takenouchi-Kosaki syndrome using disease-specific iPSCs

Takenouchi-Kosaki Syndrome (TKS) is a congenital multi-organ disorder caused by the de novo missense mutation c.191A > G p. Tyr64Cys (Y64C) in the CDC42 gene. We previously elucidated the functional abnormalities and thrombopoietic effects of Y64C using HEK293 and MEG01 cells. In the present study, we used iPSCs derived from TKS patients to model the disease and successfully recapitulated macrothrombocytopenia, a prominent TKS phenotype. The megakaryopoietic differentiation potential of TKS-iPSCs and platelet production capacity were examined using an efficient platelet production method redesigned from existing protocols. The results obtained showed that TKS-iPSCs produced fewer hematopoietic progenitor cells, exhibited defective megakaryopoiesis, and released platelets with an abnormally low count and giant morphology. We herein report the first analysis of TKS-iPSC-derived megakaryocytes and platelets, and currently utilize this model to perform drug evaluations for TKS. Therefore, our simple yet effective differentiation method, which mimics the disease in a dish, is a feasible strategy for studying hematopoiesis and related diseases.

Loss of zinc transporters ZIP1 and ZIP3 augments platelet reactivity in response to thrombin and accelerates thrombus formation in vivo

Zinc (Zn2+) is considered as important mediator of immune cell function, thrombosis and haemostasis. However, our understanding of the transport mechanisms that regulate Zn2+ homeostasis in platelets is limited. Zn2+ transporters, ZIPs and ZnTs, are widely expressed in eukaryotic cells. Using mice globally lacking ZIP1 and ZIP3 (ZIP1/3 DKO), our aim was to explore the potential role of these Zn2+ transporters in maintaining platelet Zn2+ homeostasis and in the regulation of platelet function. While ICP-MS measurements indicated unaltered overall Zn2+ concentrations in platelets of ZIP1/3 DKO mice, we observed a significantly increased content of FluoZin3-stainable free Zn2+, which, however, appears to be released less efficiently upon thrombin-stimulated platelet activation. On the functional level, ZIP1/3 DKO platelets exhibited a hyperactive response towards threshold concentrations of G protein-coupled receptor (GPCR) agonists, while immunoreceptor tyrosine-based activation motif (ITAM)-coupled receptor agonist signalling was unaffected. This resulted in enhanced platelet aggregation towards thrombin, bigger thrombus volume under flow ex vivo and faster in vivo thrombus formation in ZIP1/3 DKO mice. Molecularly, augmented GPCR responses were accompanied by enhanced Ca2+ and PKC, CamKII and ERK1/2 signalling. The current study thereby identifies ZIP1 and ZIP3 as important regulators for the maintenance of platelet Zn2+ homeostasis and function.

A novel transcription factor, BmZFP67, regulates endomitosis switch by controlling the expression of cyclin B in silk glands

Endomitosis is involved in developmental processes associated with an increase in metabolic cell activity, which is characterized by repeated rounds of DNA replication without cytokinesis. Endomitosis cells are widespread in protozoa, plants, animals and humans. Endomitosis cell cycle is currently viewed as a variation of the canonical cell cycle and transformed from mitotic cell cycle. However, the meaningful question about how endomitosis transformed from mitosis is still unclear. Herein, we identified a novel transcription factor in silk glands, ZFP67, which is gradually reduced in silk glands during the transition of mitosis to endomitosis. In addition, over-expressed ZFP67 in silk glands led to the transition delayed. And, knock-out of ZFP67 led to abnormal chromatin division and unsuccessful cell division. These data reveled that ZFP67 played an important role in transition of mitosis to endomitosis. Furthermore, ZFP67 can regulate the transcription of cyclin B, a key cyclin related to cell division and G2/M phase, which is demonstrated by chromatin immunoprecipitation and dual luciferase reporter system in this article. In conclusion, it can be speculated that the decreasing expression of ZFP67 in silk glands during the transition stage of mitosis-to-endomitosis resulted in the lack of cyclin B, which further led to unsuccessful cytokinesis and then promoted the transition from mitosis to endomitosis of silk gland cells.

Targeting of a Conserved Epitope in Mouse and Human GPVI Differently Affects Receptor Function

Glycoprotein (GP) VI is the major platelet collagen receptor and a promising anti-thrombotic target. This was first demonstrated in mice using the rat monoclonal antibody JAQ1, which completely blocks the Collagen-Related Peptide (CRP)-binding site on mouse GPVI and efficiently inhibits mouse platelet adhesion, activation and aggregation on collagen. Here, we show for the first time that JAQ1 cross-reacts with human GPVI (huGPVI), but not with GPVI in other tested species, including rat, rabbit, guinea pig, swine, and dog. We further demonstrate that JAQ1 differently modulates mouse and human GPVI function. Similar to its effects on mouse GPVI (mGPVI), JAQ1 inhibits CRP-induced activation in human platelets, whereas, in stark contrast to mouse GPVI, it does not inhibit the adhesion, activation or aggregate formation of human platelets on collagen, but causes instead an increased response. This effect was also seen with platelets from newly generated human GPVI knockin mice (hGP6^tg/tg). These results indicate that the binding of JAQ1 to a structurally conserved epitope in GPVI differently affects its function in human and mouse platelets.

Impaired microtubule dynamics contribute to microthrombocytopenia in RhoB-deficient mice

RhoB-deficient mice display microthrombocytopenia.

RhoB exerts nonredundant functions in the megakaryocyte lineage compared with RhoA and regulates microtubule dynamics.

Megakaryocytes are large cells in the bone marrow that give rise to blood platelets. Platelet biogenesis involves megakaryocyte maturation, the localization of the mature cells in close proximity to bone marrow sinusoids, and the formation of protrusions, which are elongated and shed within the circulation. Rho GTPases play important roles in platelet biogenesis and function. RhoA-deficient mice display macrothrombocytopenia and a striking mislocalization of megakaryocytes into bone marrow sinusoids and a specific defect in G-protein signaling in platelets. However, the role of the closely related protein RhoB in megakaryocytes or platelets remains unknown. In this study, we show that, in contrast to RhoA deficiency, genetic ablation of RhoB in mice results in microthrombocytopenia (decreased platelet count and size). RhoB-deficient platelets displayed mild functional defects predominantly upon induction of the collagen/glycoprotein VI pathway. Megakaryocyte maturation and localization within the bone marrow, as well as actin dynamics, were not affected in the absence of RhoB. However, in vitro–generated proplatelets revealed pronouncedly impaired microtubule organization. Furthermore, RhoB-deficient platelets and megakaryocytes displayed selective defects in microtubule dynamics/stability, correlating with reduced levels of acetylated α-tubulin. Our findings imply that the reduction of this tubulin posttranslational modification results in impaired microtubule dynamics, which might contribute to microthrombocytopenia in RhoB-deficient mice. Importantly, we demonstrate that RhoA and RhoB are localized differently and have selective, nonredundant functions in the megakaryocyte lineage.

Don't you forget about me(gakaryocytes)

Platelets (small, anucleate cell fragments) derive from large precursor cells, megakaryocytes (MKs), that reside in the bone marrow. MKs emerge from hematopoietic stem cells in a complex differentiation process that involves cytoplasmic maturation, including the formation of the demarcation membrane system, and polyploidization. The main function of MKs is the generation of platelets, which predominantly occurs through the release of long, microtubule-rich proplatelets into vessel sinusoids. However, the idea of a 1-dimensional role of MKs as platelet precursors is currently being questioned because of advances in high-resolution microscopy and single-cell omics. On the one hand, recent findings suggest that proplatelet formation from bone marrow-derived MKs is not the only mechanism of platelet production, but that it may also occur through budding of the plasma membrane and in distant organs such as lung or liver. On the other hand, novel evidence suggests that MKs not only maintain physiological platelet levels but further contribute to bone marrow homeostasis through the release of extracellular vesicles or cytokines, such as transforming growth factor β1 or platelet factor 4. The notion of multitasking MKs was reinforced in recent studies by using single-cell RNA sequencing approaches on MKs derived from adult and fetal bone marrow and lungs, leading to the identification of different MK subsets that appeared to exhibit immunomodulatory or secretory roles. In the following article, novel insights into the mechanisms leading to proplatelet formation in vitro and in vivo will be reviewed and the hypothesis of MKs as immunoregulatory cells will be critically discussed.

Reduced platelet forces underlie impaired hemostasis in mouse models of MYH9-related disease

MYH9-related disease patients with mutations in the contractile protein nonmuscle myosin heavy chain IIA display, among others, macrothrombocytopenia and a mild-to-moderate bleeding tendency. In this study, we used three mouse lines, each with one point mutation in the Myh9 gene at positions 702, 1424, or 1841, to investigate mechanisms underlying the increased bleeding risk. Agonist-induced activation of Myh9 mutant platelets was comparable to controls. However, myosin light chain phosphorylation after activation was reduced in mutant platelets, which displayed altered biophysical characteristics and generated lower adhesion, interaction, and traction forces. Treatment with tranexamic acid restored clot retraction in the presence of tPA and reduced bleeding. We verified our findings from the mutant mice with platelets from patients with the respective mutation. These data suggest that reduced platelet forces lead to an increased bleeding tendency in patients with MYH9-related disease, and treatment with tranexamic acid can improve the hemostatic function.

G6b-B regulates an essential step in megakaryocyte maturation

Loss of G6b-B leads to an unexpected megakaryocyte development defect resulting in severe macrothrombocytopenia.

G6b-B–deficient mice display reduced levels of MK-specific transcripts, surface receptors, GATA-1, and thrombopoietin signaling.

G6b-B is a megakaryocyte lineage-specific immunoreceptor tyrosine-based inhibition motif–containing receptor, essential for platelet homeostasis. Mice with a genomic deletion of the entire Mpig6b locus develop severe macrothrombocytopenia and myelofibrosis, which is reflected in humans with null mutations in MPIG6B. The current model proposes that megakaryocytes lacking G6b-B develop normally, whereas proplatelet release is hampered, but the underlying molecular mechanism remains unclear. We report on a spontaneous recessive single nucleotide mutation in C57BL/6 mice, localized within the intronic region of the Mpig6b locus that abolishes G6b-B expression and reproduces macrothrombocytopenia, myelofibrosis, and osteosclerosis. As the mutation is based on a single-nucleotide exchange, Mpig6b^mut mice represent an ideal model to study the role of G6b-B. Megakaryocytes from these mice were smaller, displayed a less-developed demarcation membrane system, and had a reduced expression of receptors. RNA sequencing revealed a striking global reduction in the level of megakaryocyte-specific transcripts, in conjunction with decreased protein levels of the transcription factor GATA-1 and impaired thrombopoietin signaling. The reduced number of mature MKs in the bone marrow was corroborated on a newly developed Mpig6b-null mouse strain. Our findings highlight an unexpected essential role of G6b-B in the early differentiation within the megakaryocytic lineage.

The Role of LIM Kinases during Development: A Lens to Get a Glimpse of Their Implication in Pathologies

The organization of cell populations within animal tissues is essential for the morphogenesis of organs during development. Cells recognize three-dimensional positions with respect to the whole organism and regulate their cell shape, motility, migration, polarization, growth, differentiation, gene expression and cell death according to extracellular signals. Remodeling of the actin filaments is essential to achieve these cell morphological changes. Cofilin is an important binding protein for these filaments; it increases their elasticity in terms of flexion and torsion and also severs them. The activity of cofilin is spatiotemporally inhibited via phosphorylation by the LIM domain kinases 1 and 2 (LIMK1 and LIMK2). Phylogenetic analysis indicates that the phospho-regulation of cofilin has evolved as a mechanism controlling the reorganization of the actin cytoskeleton during complex multicellular processes, such as those that occur during embryogenesis. In this context, the main objective of this review is to provide an update of the respective role of each of the LIM kinases during embryonic development.