Aspects of platelet formation and release

Scurfield, G.; Radley, J.M.

doi:10.1002/ajh.2830100308

Cited by 68 publications

(39 citation statements)

References 18 publications

Supporting

Mentioning

Contrasting

Order By: Relevance

“…Megakaryocytes cultured in the presence of TPO extend numerous proplatelets, consistent with the flow model. The proplatelets generated in the in vitro systems are structurally similar to those seen in vivo extending into bone marrow sinusoids and within the bloodstream (30)(31)(32). Significantly, platelets released from megakaryocytes in vitro are structurally and functionally similar to those found in vivo (28,29).…”

supporting

confidence: 49%

The biogenesis of platelets from megakaryocyte proplatelets

Patel¹

2005

Journal of Clinical Investigation

490

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

supporting

confidence: 49%

The biogenesis of platelets from megakaryocyte proplatelets

Patel¹

2005

Journal of Clinical Investigation

490

406

View full text Add to dashboard Cite

show abstract

“…[1][2][3] Nascent platelets are assembled within these structures. 4 The need for dramatic cytoplasmic and cytoskeletal reorganization and concomitant assembly of anucleate platelets presents unusual challenges to differentiated MKs.…”

Section: Introductionmentioning

confidence: 99%

A role for Rab27b in NF-E2-dependent pathways of platelet formation

Tiwari

Italiano

Barral

et al. 2003

Blood

View full text Add to dashboard Cite

Megakaryocytes release platelets by reorganizing the cytoplasm into proplatelet extensions. Fundamental to this process is the need to coordinate transport of products and organelles in the appropriate abundance to nascent platelets. The importance of the Rab family of small GTPases (guanosine 5-triphosphatases) in platelet biogenesis is revealed in gunmetal (gm/gm) mice, which show deficient Rab isoprenylation and macrothrombocytopenia with few granules and abnormal megakaryocyte morphology. Although some Rab proteins are implicated in vesicle and organelle transport along microtubules or actin, the role of any Rab protein in platelet biogenesis is unknown. The limited number of Rab proteins with defective membrane association in gm/gm megakaryocytes prominently includes Rab27a and Rab27b. Normal expression of Rab27b is especially increased with terminal megakaryocyte differentiation and dependent on nuclear factor-erythroid 2 (NF-E2), a transcription factor required for thrombopoiesis. Chromatin immunoprecipitation demonstrates recruitment of NF-E2 to the putative Rab27B promoter. Inhibition of endogenous IntroductionMammalian blood platelets are released from bone marrow megakaryocytes (MKs) in a process that transforms the entire MK cytoplasm into long pseudopodia known as proplatelets. 1-3 Nascent platelets are assembled within these structures. 4 The need for dramatic cytoplasmic and cytoskeletal reorganization and concomitant assembly of anucleate platelets presents unusual challenges to differentiated MKs. The cellular and molecular response to these challenges is poorly understood.Selected mouse models of thrombocytopenia and qualitative platelet defects have proved invaluable in probing the molecular basis of platelet biogenesis. Mice lacking either of 2 erythromegakaryocytic transcription factors, GATA1, and nuclear factorerythroid 2 (NF-E2), show dramatically arrested MK maturation 5,6 and thus implicate their target genes in aspects of platelet assembly. However, the transcriptional targets of GATA1 and NF-E2 that are immediately relevant to MK fragmentation and platelet release are not known. Other insights derive from findings in animal models of the human Hermansky-Pudlak syndrome (HPS), a multigenic group of recessively inherited disorders that result from abnormal synthesis of 3 related organelles: melanosomes, platelet-dense granules, and lysosomes. 7 HPS proteins regulate intracellular vesicle traffic, including the ␦ and ␤3A subunits of the AP-3 adaptor complex, which captures organelle membrane proteins at the trans-Golgi apparatus and/or endosomes, and the pallid protein, which interacts with syntaxin-13 to mediate vesicle fusion. 8,9 These HPS proteins highlight molecular pathways that serve to assemble and transport organelles but they do not overtly influence the efficiency with which MKs release platelets. In contrast, thrombocytopenia is a cardinal feature of the gunmetal mouse. 10 The cellular morphology of gm/gm MKs closely resembles that seen in the absence of NF-E2, with reduced ...

show abstract

“…A role for these proplatelet promoting factors is particularly attractive because proplatelet formation occurs in the perivascular areas of the marrow, precisely where these substances reside. 7,22 Despite these morphologic and cell culture clues, however, the specific signal transduction pathways governing proplatelet formation are still poorly defined. In the present series of experiments, we used a murine model of MK proplatelet formation and began to explore the molecular mechanisms of this process.…”

mentioning

confidence: 99%

Actin reorganization and proplatelet formation in murine megakaryocytes: the role of protein kinase Cα

Rojnuckarin

Kaushansky

2001

Blood

View full text Add to dashboard Cite

With the recent cloning and characterization of thrombopoietin, appreciation of the molecular events surrounding megakaryocyte (MK) development is growing. However, the final stages of platelet formation are less well understood. Platelet production occurs after the formation of MK proplatelet processes. In a study to explore the molecular mechanisms underlying this process, mature MKs isolated from suspension murine bone marrow cell cultures were induced to form proplatelets by exposure to plasma, and the role of various cellsignaling pathways was assessed. The results showed that (1) bis-indolylmaleimide I, which blocks protein kinase C (PKC) activation; (2) down-modulation of conventional or novel classes of PKC by phorbol myristate acetate; and (3) ribozymes specific for PKC␣ each inhibited proplatelet formation. Inhibition of several MAP kinases, PI3 kinase, or protein kinase A failed to affect MK proplatelet formation. To gain further insights into the function of PKC␣ in proplatelet formation, its subcellular localization was investigated. In cultures containing active proplatelet formation, cytoplasmic polymerized actin was highly aggregated, its subcellular distribution was reorganized, and PKC␣ colocalized with the cellular actin aggregates. A number of MK manipulations, including blockade of integrin signaling with a disintegrin or inhibition of actin polymerization with cytochalasin D, interrupted actin reorganization, PKC relocalization, and proplatelet formation. These findings suggest an important role for PKC␣ in proplatelet development and suggest that it acts by altering actin dynamics in proplatelet-forming MKs. Identification of the upstream and downstream pathways involved in proplatelet formation should provide greater insights into thrombopoiesis, potentially allowing pharmacologic manipulation of the process. IntroductionPlatelet formation represents the terminal stage of megakaryocyte (MK) development. This process, during which a large polyploid cell fragments into thousands of anucleate progeny, is virtually unique in mammalian cell biology. In addition to its intrinsic interest to cell biologists, the generation of platelets holds much medical interest. Several clinical states characterized by inadequate platelet production, such as myelodysplasia, may be due, at least in part, to failure of platelet formation from seemingly normal numbers of MKs. Moreover, although thrombopoietin (TPO), the major regulator of MK development, has been cloned and is undergoing clinical testing, the therapeutic response is slow because of the long time required for MK progenitor cells to mature into platelets. Theoretically, direct stimulation of MK platelet formation would be more rapid. It is clear that a better understanding of the mechanisms of platelet formation may lead to improved therapies for thrombocytopenia or pharmacologic inhibition of this process in the treatment of thrombocytosis.It has long been observed that fully mature MKs form proplatelets in culture. Proplatelets are long cytoplas...

show abstract

Aspects of platelet formation and release

Cited by 68 publications

References 18 publications

The biogenesis of platelets from megakaryocyte proplatelets

The biogenesis of platelets from megakaryocyte proplatelets

A role for Rab27b in NF-E2-dependent pathways of platelet formation

Actin reorganization and proplatelet formation in murine megakaryocytes: the role of protein kinase Cα

Contact Info

Product

Resources

About