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Fig. 5. Proposed signaling pathway describing the mechanical response of cytoplasm in cells subjected to shear flow. (A) Color-coded (see gradient legend bar) cell for each condition indicates changes in intracellular viscoelasticity with respect to unsheared conditions. Upon application of mechanical shear flow, activation of Rho causes the downstream activation of ROCK (ROK/Rho-kinase), LIM kinases, and mDia. Such activation results in (1) increased F-actin formation; (2) actomyosin contractility; (3) focal adhesion formation, and ultimately, significant intracellular cytoplasmic stiffening of Swiss 3T3 fibroblasts. Drug treatment experiments are shown in green. (B) Comparison of chemical and mechanical stimulation on intracellular mechanics. Biomechanical stimulus (shear flow) causes a sustained dramatic stiffening response whereas a biochemical stimulus (LPA) causes a transient intermediate stiffening response. Such an intermediate response is recovered in mechanical stimulation when actomyosin interactions are inhibited. One-way ANOVA test of shear data and previous LPA treatment data (Kole et al., 2004a) yielded P<0.0001. Stars denote P values from two-tailed t-tests within conditions (see Materials and Methods). (C) Illustration of twitch phenomenon in muscle cells versus sustained tetanus that occurs when there is insufficient time for relaxation, which is analogous to the difference in intracellular mechanics measured in biochemical versus biomechanical-stimulated cells.
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