Actin polymerization in neutrophils. Cell density, molecular redistribution, and mass transfer rate-factors in response dynamics.

Abstract

Regulation of cellular responses to stimulation is one of the major areas in contemporary biology. I studied regulation of the actin polymerization response in neutrophils stimulated by chemotactic peptides. Actin polymerization in neutrophils, as well as in many other cell types, is essential for polarization, motility, chemotaxis, and phagocytosis. I investigated the relationship between the actin response dynamics and distribution of the eliciting receptors and actin within the cell. I developed a method to visualize both receptors and F-actin in the same cell and showed that in polarized neutrophils the receptor and F-actin distributions do not coincide. Based on this finding, I proposed a novel mechanism of response regulation by translocation of signalling molecules within the cell. I also demonstrated the importance of extracellular chemical factors in response termination; I showed that a rapid termination of the response to high concentrations of chemoattractants is a result of ligand loss and inhibition by endogenously produced factors in the medium. By keeping cells in a diluted suspension or by exchanging the medium, I was able to stimulate sustained actin polymerization. The binding rate of the agonist is another factor that can influence the response kinetics. With sufficiently numerous and high-affinity receptors on the cell surface, binding can be diffusion-limited. However, the conditions determining whether the reaction is diffusion-limited have been formulated for stationary liquid only, whereas experiments with cell suspensions are usually done with stirring. Therefore I did a theoretical analysis of the effect of stirring (it can be a different kind of convective transport as well) on the kinetics of ligand-receptor interactions. I also considered another practically important case: binding kinetics to cells forming a monolayer on a surface, with or without perfusion. My theoretical results may help experimentalists correctly perform measurements of the rates of ligand-receptor interactions.