The critical role of myosin IIA in platelet internal contraction

Abstract

Summary. Background: Shape change and centralization of granules surrounded by a microtubular coil (internal contraction) are among the earliest morphologic changes observed following platelet activation. Myosin IIA contributes to initiation of platelet shape change, but its role in internal contraction has not been defined. Objective: To define the contribution of myosin IIA to platelet internal contraction. Methods: Aspirin-treated platelets suspended in calcium-free buffer were activated with a low concentration (25 nM) of the thromboxane A 2 analog U46619 which initiated shape change and internal contraction via a Rho kinase pathway. Shape change and internal contraction were assessed by aggregometry and transmission electron microscopy (TEM), and Rho activation and myosin regulatory light chain (MRLC) phosphorylation were studied concurrently. Results and Conclusions: Low-concentration blebbistatin (10 lM) inhibited internal contraction in the majority of platelets with minimal inhibition of shape change without significant suppression of MRLC phosphorylation. Higher blebbistatin concentrations (25-100 lM) produced concentration-dependent inhibition of aggregation, shape change, Rho activation, and MRLC phosphorylation. These data demonstrate: (i) direct platelet myosin IIA participation in internal contraction; and (ii) inhibition of Rho activation and MRLC phosphorylation by >10 lM blebbistatin.

Identification of a fibrin-independent platelet contractile mechanism regulating primary hemostasis and thrombus growth

“…The importance of the platelet contractile apparatus in supporting the hemostatic function of platelets is well established, 4,8,22,37,38 with strong evidence that contractility is essential for platelet shape change 7,8 and retraction of fibrin clots. 11,12 However, the role of platelet contractility, in particular the extracellular transmission of cytoskeletal contractile forces, in regulating primary hemostasis has been less clearly defined.…”

“…4,5 The membrane skeleton is essential for maintaining the structure and integrity of the surface membrane, whereas the cytoskeleton, through its attachment to myosin, principally generates contractile forces within the cell. 6 The internal generation of contractile force has a well-defined role in regulating platelet shape change 7,8 and in promoting granule secretion, 9,10 whereas the extracellular transmission of cytoskeletal contractile force is essential for fibrin clot retraction. 11,12 Central to the generation of contractile force is the molecular motor myosin, representing one of the major proteins found in the platelet cytosol.…”

Regulation of platelet myosin light chain (MYL9) by RUNX1: implications for thrombocytopenia and platelet dysfunction in RUNX1 haplodeficiency

“…Platelet shape change results from a rapid reorganization of the cytoskeleton, including formation of new actin filaments, actomyosin polymerization through MLC phosphorylation, and centralization of granules. [28][29][30][31] Inhibition of MLC phosphorylation results in decreased secretion. [32][33][34] Megakaryocytes release platelets by using cytoplasmic extensions called proplatelets; this process requires extensive reorganization in the microtubules and actin filaments.…”

Learning from errors: when a low platelet count in neonate excludes immune thrombocytopenic purpura in mother

“…In the patient and his mother, macrothrombocytopenia was present, the healthy girl supported an autosomal dominant inheritance. The presence of basophilic Döhle-like inclusions in leukocytes narrowed suspected myosin heavy chain 9 (MYH9)-related platelet disorders (15)(16)(17)(18)(19)(20)(21).…”

“…The MYH9 gene encodes the nonmuscle myosin heavy chain llA (NMMHC-llA), cytoskeletal contractile protein that has been identifi ed in the long arm of chromosome 22 (22q12.3-q13.2) (15,17). Several mutations in the MYH9 gene lead to premature release of platelets from the bone marrow, macrothrombocytopenia, and cytoplasmic inclusion bodies within leukocyte (19).…”