Factors involved in megakaryocyte development and platelet production (Homo sapiens)
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Megakaryocytes (MKs) give rise to circulating platelets (thrombocytes) through terminal differentiation of MKs which release cytoplasmic fragments as circulating platelets. As MKs mature they undergo endoreduplication (polyploidisation) and expansion of cytoplasmic mass to cell sizes larger than 50-100 microns, and ploidy ranges up to 128 N. As MKs mature, the polyploid nucleus becomes horseshoe-shaped, the cytoplasm expands, and platelet organelles and the demarcation membrane system are amplified. Proplatelet projections form which give rise to de novo circulating platelets (Deutsch & Tomer 2006).
The processes of megakaryocytopoiesis and platelet production occur within a complex microenvironment where chemokines, cytokines and adhesive interactions play major roles (Avecilla et al. 2004). Megakaryocytopoiesis is regulated at several levels including proliferation, differentiation and platelet release (Kaushansky 2003). Thrombopoietin (TPO/c-Mpl ligand) is the most potent cytokine stimulating proliferation and maturation of MK progenitors (Kaushansky 2005) but many other growth factors are involved. MK development is controlled by the action of multiple transcription factors. Many MK-specific genes are co-regulated by GATA and friend of GATA (FOG), RUNX1 and ETS proteins. Nuclear factor erythroid 2 (NF-E2), which has an MK-erythroid specific 45-kDa subunit, controls terminal MK maturation, proplatelet formation and platelet release (Schulze & Shivdasani 2004). NF-E2 deficient mice have profound thrombocytopenia (Shiraga et al. 1999). MYB (c-myb) functions with EP300 (p300) as a negative regulator of thrombopoiesis (Metcalf et al. 2005). During MK maturation, internal membrane systems, granules and organelles are assembled. Cytoplasmic fragmentation requires changes in the MK cytoskeleton and formation of organelles and channels. Individual organelles migrate from the cell body to the proplatelet ends, with approximately 30 percent of organelles/granules in motion at any given time (Richardson et al. 2005). View original pathway at:Reactome.
The processes of megakaryocytopoiesis and platelet production occur within a complex microenvironment where chemokines, cytokines and adhesive interactions play major roles (Avecilla et al. 2004). Megakaryocytopoiesis is regulated at several levels including proliferation, differentiation and platelet release (Kaushansky 2003). Thrombopoietin (TPO/c-Mpl ligand) is the most potent cytokine stimulating proliferation and maturation of MK progenitors (Kaushansky 2005) but many other growth factors are involved. MK development is controlled by the action of multiple transcription factors. Many MK-specific genes are co-regulated by GATA and friend of GATA (FOG), RUNX1 and ETS proteins. Nuclear factor erythroid 2 (NF-E2), which has an MK-erythroid specific 45-kDa subunit, controls terminal MK maturation, proplatelet formation and platelet release (Schulze & Shivdasani 2004). NF-E2 deficient mice have profound thrombocytopenia (Shiraga et al. 1999). MYB (c-myb) functions with EP300 (p300) as a negative regulator of thrombopoiesis (Metcalf et al. 2005). During MK maturation, internal membrane systems, granules and organelles are assembled. Cytoplasmic fragmentation requires changes in the MK cytoskeleton and formation of organelles and channels. Individual organelles migrate from the cell body to the proplatelet ends, with approximately 30 percent of organelles/granules in motion at any given time (Richardson et al. 2005). View original pathway at:Reactome.
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complex:REST:REST
DNA binding sitesEH domain containing
proteins:Rabenosyn-5protein alpha
subunitmethylated at
lysine-9region of
beta-globinAnnotated Interactions
complex:REST:REST
DNA binding sitesEH domain containing
proteins:Rabenosyn-5protein alpha
subunitmethylated at
lysine-9region of
beta-globinregion of
beta-globinNF-E2 binds to an extended AP-1-like element, TGCTGA(G/C)TCA, which is found in the locus control regions (LCRs) of the alpha- and beta-globin genes and in the promoters of several heme biosynthetic enzyme genes (see Motohashi et al. 1997). NF-E2 binding sites in the DNase I hypersensitive site 2 (HS2) of the beta-globin LCR are essential for its enhancer activity (Ney et al. 1990, Talbot & Grosveld 1991).
NFE2-p45 null mice have a mild defect in globin gene expression, suggesting that other members of the CNC protein family can substitute for function in vivo (Shivdasani & Orkin 1995).EH domain family proteins have regulatory roles in endocytic membrane transport events (Naslavsky & Caplan 2005); the EHD subfamily is believed to regulate endocytic recycling (George et al. 2007). All four human EHD proteins can rescue the vacuolated intestinal phenotype observed when the C. elegans orthologue rme-1 is mutated (George et al. 2007).
Over 20 interaction partners have been reported for the C-terminal EHD proteins including clathirin, syndaptins and Arp2/3 (see Naslavsky & Caplan 2005). EHD1-3 all interact with Rabenosyn-5 (Rab5), a Rab5 effector (Naslavsky et al. 2004).c-Myb (gene symbol MYB) is highly conserved in all vertebrates and some invertebrate species (Lipsick 1996). It plays an important role in the control of proliferation and differentiation of hematopoietic progenitor cells (Duprey & Boettiger 1985); Down-regulation of c-Myb is believed to be critical for the commitment of cells to terminal differentiation (Oh & Reddy, 1999). c-Myb interacts with many other transcription factors including CBP, several CCAAT binding protein (c/EBP) family members, and Ets family proteins such as Ets-2 (Oh & Reddy, 1999).
Loss of c-Myb function results in embryonic lethality due to failure of fetal hepatic hematopoiesis (Mucenski et al. 1991).
The HP1 amino-terminal chromodomain binds methylated lysine-9 of histone H3, causing transcriptional repression (Lachner et al. 2001). A crystal structure of human HP1 alpha in complex with H3K9(me)3 peptide is available (Amaya et al. 2008). The highly-conserved carboxy-terminal chromoshadow domain enables dimerization and also serves as a docking site for proteins involved in a wide variety of nuclear functions, from transcription to nuclear architecture.
REST is a modular transcriptional regulator that recruits CoREST and other regulatory cofactors to activate or repress transcription through dynamic epigenetic mechanisms (Ballas & Mandel 2005).