Ca²⁺-dependent myosin II activation is required for uropod retraction during neutrophil migration

Buffering of intracellular Ca²⁺ transients in human neutrophils leads to reduced motility due to defective uropod detachment on fibronectin and vitronectin-coated surfaces. Since one potential target of a rise in [Ca²⁺](i) is the activation of myosin II, we characterized the role of myosin II during motility. Treatment of neutrophils with a myosin inhibitor (2,3-butanedione monoxime), or myosin light chain kinase inhibitors (ML-7, ML-9, or KT5926) resulted in impaired uropod retraction and a dose-dependent decrease in chemokinesis following stimulation with N-formyl-Met-Leu-Phe (fMLP). Treatment with ML-9 resulted in a redistribution of F-actin and talin to the non-retracted uropods, mimicking the redistribution observed during [Ca²⁺](i) buffering. Impairment of uropod retraction and redistribution of F-actin and talin by myosin II inhibition was only observed on adhesive substrates such as fibronectin and not on poorly adhesive substrates such as human serum-coated glass. At higher concentrations of ML-9, cell polarization was inhibited and pseudopod extension occurred radially. Using an antibody specific for serine 19-phosphorylated regulatory light chain of myosin II, regions of activated myosin II were found at the leading edge as well as the uropod in motile fMLP-stimulated cells. [Ca²⁺](i) depletion caused a 50% decrease in the level of serine 19-phosphorylated myosin II suggesting that activation of myosin II by intracellular Ca²⁺ transients may be an essential step in establishing a polarized pseudopod and providing the force required for uropod retraction during PMN motility on adhesive surfaces.