Corresponding oscillations in neutrophil shape and filamentous actin content

Human neutrophils pretreated with 17-hydroxywortmannin responded to the chemotactic agonist formyl-Met-Leu-Phe with a transient doubling in filamentous actin content which was characterized by prominent oscillations. These oscillations closely matched transient oscillations in suspension turbidity measured in parallel. The experimental data could be simulated using A----B----C stochastic series kinetic models with an oscillating intermediate species (B), allowing quantitative comparison of the frequencies of the oscillations (0.092 +/- 0.006 and 0.094 +/- 0.004 Hz) and the overall reaction rate constants for actin mobilization and turbidity changes (0.11 +/- 0.02 and 0.14 +/- 0.03 s-1, respectively). The total cell volume remained constant, indicating that stimulus-induced extension of lamellipods reduces the body volume by an amount proportional to the mass displaced outward. Light scattering theory predicts that a decrease in body size decreases the turbidity and that fluctuations in body size due to lamellipod extension and retraction cycles like those exhibited by crawling neutrophils result in turbidity oscillations (lamellipods scatter very little by comparison to the cell body, and both aggregation and degranulation were absent). The experiments thus suggest that the cyclic variations in F-actin content are correlated with periodic fluctuations in lamellipod size. The available evidence appears to be consistent with the hypothesis that actin polymerization provides the main driving force for lamellipod extension and that depolymerization causes lamellipod retraction.