Platelet Function, Ultrastructure, and Survival in the May-Hegglin Anomaly

Hamilton, Robert W.; Shaikh, Bahu S.; Ottie, Jeffrey N.; Storch, Allan E.; Saleem, Abdus; White, James G.

doi:10.1093/ajcp/74.5.663

Cited by 48 publications

(18 citation statements)

References 21 publications

Supporting

Mentioning

Contrasting

Unclassified

Order By: Relevance

“…42 Nevertheless, it has been hypothesized that the defect in platelet production is linked to an abnormal PPT formation because both MKs numbers in bone marrow and platelet clearance were normal. 43 Altogether, the present report shows for the first time that inhibition of MLC2 phosphorylation by either Tat-C3 or Y27632 or P18 increases PPF but these PPTs were abnormal and associated with an increased size in the produced platelets. These data suggest that the regulation of pMLC2 probably plays a critical role in the formation of normalsized platelets and may prevent disorganized cytoplasmic extension.…”

Section: Discussionsupporting

confidence: 59%

Proplatelet formation is regulated by the Rho/ROCK pathway

Chang

Aurade

Larbret

et al. 2007

Blood

159

184

View full text Add to dashboard Cite

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,...

show abstract

Section: Discussionsupporting

confidence: 59%

Proplatelet formation is regulated by the Rho/ROCK pathway

Chang

Aurade

Larbret

et al. 2007

Blood

159

184

View full text Add to dashboard Cite

show abstract

“…both megakaryocyte numbers and platelet clearance are normal (74). Myosin has been theorized to function in platelet formation, specifically during the bending and branching process, where an actomyosin interaction could possibly provide the forces required to contort the proplatelet shaft (27).…”

Section: Figurementioning

confidence: 99%

The biogenesis of platelets from megakaryocyte proplatelets

Patel¹

2005

Journal of Clinical Investigation

500

406

View full text Add to dashboard Cite

Platelets are formed and released into the bloodstream by precursor cells called megakaryocytes that reside within the bone marrow. The production of platelets by megakaryocytes requires an intricate series of remodeling events that result in the release of thousands of platelets from a single megakaryocyte. Abnormalities in this process can result in clinically significant disorders. Thrombocytopenia (platelet counts less than 150,000/µl) can lead to inadequate clot formation and increased risk of bleeding, while thrombocythemia (platelet counts greater than 600,000/µl) can heighten the risk for thrombotic events, including stroke, peripheral ischemia, and myocardial infarction. This Review will describe the process of platelet assembly in detail and discuss several disorders that affect platelet production. Platelet formationMegakaryocyte development. Megakaryocytes are rare myeloid cells (constituting less than 1% of these cells) that reside primarily in the bone marrow (1) but are also found in the lung and peripheral blood. In early development, before the marrow cavities have enlarged sufficiently to support blood cell development, megakaryopoiesis occurs within the fetal liver and yolk sac. Megakaryocytes arise from pluripotent HSCs that develop into 2 types of precursors, burst-forming cells and colony-forming cells, both of which express the CD34 antigen (2). Development of both cell types continues along an increasingly restricted lineage culminating in the formation of megakaryocyte precursors that develop into megakaryocytes (1). Thrombopoietin (TPO), the primary regulator of thrombopoiesis, is currently the only known cytokine required for megakaryocytes to maintain a constant platelet mass (3). TPO is thought to act in conjunction with other factors, including IL-3, IL-6, and IL-11, although these cytokines are not essential for megakaryocyte maturation (4).Megakaryocytes tailor their cytoplasm and membrane systems for platelet biogenesis. Before a megakaryocyte has the capacity to release platelets, it enlarges considerably to an approximate diameter of 100 µm and fills with high concentrations of ribosomes that facilitate the production of platelet-specific proteins (5). Cellular enlargement is mediated by multiple rounds of endomitosis, a process that amplifies the DNA by as much as 64-fold (6-9). TPO, which binds to the c-Mpl receptor, promotes megakaryocyte endomitosis. During endomitosis, chromosomes replicate and the nuclear envelope breaks down. Although interconnected mitotic spindles assemble, the normal mitotic cycle is arrested during anaphase B. The spindles fail to separate, and both telophase and cytokinesis are bypassed. Nuclear envelope reformation (10, 11) results in a polyploid, multilobed nucleus with DNA contents ranging from 4N up to 128N within each megakaryocyte (12).In addition to expansion of DNA, megakaryocytes experience significant maturation as internal membrane systems, granules, and organelles are assembled in bulk during their development. In particular, there is th...

show abstract

“…9,10 In the May-Hegglin anomaly, macrothrombocytopenia is due to a defect in platelet release by megakaryocytes (MKs) in the bone marrow, but platelets that do form seem to circulate and function normally. [9][10][11] The MYH9-associated syndromes are thus regarded as disorders of thrombopoiesis, 12 and defective myosin-IIA complexes are presumed to perturb some aspect of MK differentiation, likely late in the course of cell maturation.Polyploid MKs accumulate an enormous and complex cytoplasm before they assemble and release blood platelets. Two key processes are thought to govern the timing and execution of platelet release.…”

mentioning

confidence: 99%

The May-Hegglin anomaly gene MYH9 is a negative regulator of platelet biogenesis modulated by the Rho-ROCK pathway

Chen

Naveiras

Balduini

et al. 2007

Blood

150

161

View full text Add to dashboard Cite

IntroductionMYH9 encodes myosin-IIA, a nonmuscle myosin heavy chain that assembles within actomyosin complexes and facilitates shape changes in diverse cell types. [1][2][3] Patients with autosomal dominant inherited MYH9-related disorders, 4-6 including the May-Hegglin anomaly, 7,8 exhibit macrothrombocytopenia and variable degrees of hearing loss, nephritis, and cataracts. These individuals experience mild bleeding symptoms as a result of reduced numbers of misshapen blood platelets that can be 2 to 5 times larger than normal. 9,10 In the May-Hegglin anomaly, macrothrombocytopenia is due to a defect in platelet release by megakaryocytes (MKs) in the bone marrow, but platelets that do form seem to circulate and function normally. [9][10][11] The MYH9-associated syndromes are thus regarded as disorders of thrombopoiesis, 12 and defective myosin-IIA complexes are presumed to perturb some aspect of MK differentiation, likely late in the course of cell maturation.Polyploid MKs accumulate an enormous and complex cytoplasm before they assemble and release blood platelets. Two key processes are thought to govern the timing and execution of platelet release. Mature MKs travel within the bone marrow and come to lie close to sinusoidal vessels 13 ; this process responds to cellular and humoral interactions, mediated in part by the chemokine stromal cell-derived factor 1 (Sdf-1 or CXCL12). [14][15][16] In their final stages, MKs extend long cytoplasmic projections called proplatelets and actively assemble nascent platelets at the tips of these structures. [17][18][19] Proplatelet formation (PPF) is preceded and accompanied by characteristic changes in cell shape, reorganization of the actin and microtubule cytoskeletons, and generation of intracellular force. 20 As an abundant motor protein within MKs and the only representative of its family, 21 myosin-IIA is a good candidate mediator of proplatelet extension or platelet release. Biochemical studies suggest that certain N-terminal mutations found in patients with MYH9-related disorders reduce intrinsic ATPase activity, 22 whereas other common C-terminal mutations may compromise myosin filament formation. 23 Although both classes of mutations may interfere with myosin-IIA function, it is unclear whether monoallelic MYH9 mutations produce clinical phenotypes as a result of haploinsufficiency or dominant-negative effects. The role of myosin-IIA in platelet assembly and release is hence uncertain.Conventional myosin-II complexes contain a dimer of heavy chains, each associated with the myosin regulatory light chain (MLC). Phosphorylated MLC enhances actin-dependent myosin motor activity, 24 and 3 kinases can phosphorylate MLC: Rhoassociated kinase (ROCK), myosin light chain kinase (MLCK), and p21-activated kinase. 24 Both ROCK and MLCK participate in platelet activation, 25-27 but their roles in MK maturation or platelet Submitted February 1, 2007; accepted March 21, 2007. Prepublished online as Blood First Edition paper, March 28, 2007; DOI 10.1182 DOI 10. /blood-2007 An In...

show abstract

Platelet Function, Ultrastructure, and Survival in the May-Hegglin Anomaly

Cited by 48 publications

References 21 publications

Proplatelet formation is regulated by the Rho/ROCK pathway

Proplatelet formation is regulated by the Rho/ROCK pathway

The biogenesis of platelets from megakaryocyte proplatelets

The May-Hegglin anomaly gene MYH9 is a negative regulator of platelet biogenesis modulated by the Rho-ROCK pathway

Contact Info

Product

Resources

About