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1 Department Geriatric and Respiratory Med, Tohoku Univ., Sendai, Miyagi, Japan; Center for Interdisciplinary Research, Tohoku Univ, Sendai, Miyagi, Japan
2 Department Geriatric and Respiratory Med, Tohoku Univ., Sendai, Miyagi, Japan; Harvard School, Pub. Health and Harvard Medical School, Boston, MA, USA
3 Department Geriatric and Respiratory Med, Tohoku Univ., Sendai, Miyagi, Japan
4 Center for Interdisciplinary Research, Tohoku Univ, Sendai, Miyagi, Japan; Department Metallurgy, Tohoku Univ, Sendai, Miyagi, Japan
* To whom correspondence should be addressed. E-mail: myan{at}mail.cc.tohoku.ac.jp.
Intracellular rheology is a useful probe of the mechanisms underlying spontaneous or chemotactic locomotion and transcellular migration of leukocytes. We characterized regional rheological differences between the leading, body, and trailing regions of isolated, adherent, and spontaneously locomoting human neutrophils. We optically trapped intracellular granules and measured their displacement for 500ms following a 100nm step change in the trap position. Results were analyzed in terms of simple viscoelasticity, and using structural damping (stress relaxation follows a power law in time). Structural damping fit the data better than viscoelasticity. Regional viscoelastic stiffness and viscosity or structural damping storage and loss moduli were all significantly lower in leading regions compared to pooled body/trailing regions (the latter were not significantly different). Structural damping showed similar levels of elastic and dissipative stresses in body/trailing regions; leading regions were significantly more fluid-like (increased power law exponent). Cytoskeletal disruption with cytochalasin-D or nocodazole made body/trailing regions ~50% less elastic and less viscous. Cytochalasin-D completely suppressed pseudopodial formation and locomotion; nocodazole had no effect on leading regions. Neither drug changed the dissipative/storage energy ratio. These results differ from studies on neutrophils and other cell types, probed at the cell membrane via 
2 integrin receptors; this suggests a distinct role for the cell cortex or focal adhesion complexes. We conclude that (a) structural damping well describes intracellular rheology, and (b) while not conclusive, the significantly more fluid-like behavior of the leading edge supports the idea that intracellular pressure may be the origin of motive force in neutrophil locomotion.
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