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UBC Theses and Dissertations

The role of the actin cytoskeleton in regulating platelet lifespan and function De Silva, Enoli Shannon


Platelets are small discoid blood cells that regulate hemostasis and wound healing. Normal platelet function is largely contingent on the shape change reaction that occurs following the cells’ exposure to external stimuli and/or soluble agonists. Agonist-driven platelet shape change is mediated by reorganization of the actin cytoskeleton (the cell’s structural framework) although the underlying molecular mechanisms are not precisely defined. Moreover, platelet lifespan is partly determined by apoptosis (programmed cell death); how the actin cytoskeleton regulates platelet apoptosis is also undefined. In response to ABT-737 (an apoptosis-inducing drug that targets Bcl-xL pro-survival protein), the 3 apoptotic markers of mitochondrial depolarization, surface phosphatidylserine (PS) exposure and caspase-3 activation were all unaffected by the actin-depolymerizing drug cytochalasin D or the actin polymerizing drug jasplakinolide. Similar results were observed in platelets isolated from mice deficient in gelsolin, an actin-binding protein that is essential for normal cytoskeletal remodeling, suggesting that ABT-737 induces established markers of platelet apoptosis in an actin-independent manner. I then studied another actin-binding protein, filamin A (FLNA), which crosslinks actin filaments and also serves as an intracellular signalling scaffold. ABT-737 induced caspase-3 cleavage and PS exposure in control mouse platelets but these effects were essentially abrogated in platelets obtained from FLNA-null mice. These data point to a novel role for FLNA in the regulation of platelet apoptosis that is independent of the actin cytoskeleton. Furthermore, I studied the role of FLNA in regulating thrombin-driven platelet shape change. Preliminary data suggest that FLNA-null platelets show impaired early phosphorylation of myosin light chain (MLC), which is a key determinant of the shape change reaction. Collectively, my findings serve to delineate the respective roles for the actin cytoskeleton, and of FLNA, with respect to platelet shape change and apoptosis.

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