Megakaryocytes (MKs) are the giant polyploid cells in bone marrow that produce platelets. Patients with mutations in MYH9, the gene for the non-muscle myosin IIA (NMIIA) heavy chain, have a range of syndromes referred to as MYH9-related diseases (MYH9-RD). These patients all have elevated bleeding tendency with macrothrombocytopenia (reduced numbers of enlarged platelets), and some may acquire progressive kidney disease, deafness, or cataracts. Mutations in MYH9 are known to impair MK morphogenesis (without affecting ploidy), resulting in reduced proplatelet formation and fewer and larger circulating platelets (macrothrombocytopenia).


While the MYH9-RD phenotype has been described, the cellular and molecular basis for impaired proplatelet formation remains unclear. Human patients with mutations in different domains of MYH9 exhibit variable severity of the disease. We are using Myh9 transgenic mouse models that model the human disease phenotype to test the hypothesis that NMIIA regulates MK migration in the bone marrow to the vascular niche for platelet release, and/or regulates the membrane trafficking and organelle biogenesis pathways required for platelet formation. In parallel, we are examining tropomodulin (Tmod) or tropomyosin (TM) knockout mice to determine whether NMIIA, Tmods and TMs function via common or diverse F-actin contractility pathways in MKs to regulate MK maturation, membrane trafficking and platelet formation. We study MKs in situ in bone marrow using fluorescence confocal microscopy, and MK migration, membrane dynamics and proplatelet formation in living cells using time-lapse microscopy. We identify NMIIA binding partners using proteomics, evaluate associations in MKs using proximity ligation assays (PLA) and super-resolution microscopy, and test functions by viral transduction of candidates into cultured MKs. These studies will elucidate the basis for MK and platelet defects in MYH9-RD, and will be applicable to inherited thrombocytopenias due to mutations in other actin filament (F-actin) binding proteins (e.g., TM4, α-actinin1). In addition, our studies may also provide new strategies to improve ex-vivo PLT generation with high yields for therapeutic purposes.


Relevant publications:

Sui Z, Nowak RB, Sanada C, Halene S, Krause DS, Fowler VM. Regulation of actin polymerization by tropomodulin3 controls megakaryocyte actin organization and platelet biogenesis. Blood. 2015 Jul 23;126(4):520-30.



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