Intracellular elasticity and viscosity in the body, leading, and trailing regions of locomoting neutrophils

Toinvestigate the mechanisms underlying pseudopod protrusion inlocomoting neutrophils, we measured the intracellular stiffness andviscosity in the leading region, main body, and trailingregion from displacements of oscillating intracellulargranules driven with an optical trap. Experiments were done in controlconditions and after treatment with cytochalasin D or nocodazole. Wefound 1) in the body and trailingregion, the granules divided into a "fixed" population (too stiffto measure) and a "free" population (easily oscillated; fixedfraction 65%, free fraction 35%). By contrast, the fixed fraction inthe leading region was <5%. 2) Inthe body and trailing region, there was no difference in stiffness orviscosity, but both were sharply lower in the leading region (respectively, 20-fold and 5-fold).3) Neither cytochalasin D nornocodazole caused a decrease in stiffness, but both treatments markedlyreduced the fixed fraction in the body and trailing region to <20%and <40%, respectively. These observations suggest a discrete lattice structure in the body and trailing region and suggest that thedeveloping pseudopod has a core that is more fluidlike, in thesense of a much lower viscosity and an almost total loss of stiffness.This is consistent with the contraction/solation hypothesis ofpseudopodial formation.