A Possible Role of Homeostasis Between Monomeric and Filamentous Actin in Filament Nucleation Revealed by Pharmacokinetic Modeling

The currently prevailing method to elucidate molecular functions in vivo is to knock out or knock down gene expression and to observe the resultant phenotypic changes. For example, fibroblasts extend pseudopods called lamellipodia toward the source of the platelet-derived growth factor (PDGF) gradient. We now know tens of molecules, both cell signaling intermediates and cytoskeleton regulators, involved in such cell polarization and migration. They interact with each other in a complicated manner both spatially and temporally. Our laboratory has recently carried out a screening to examine the effect of knockdown of Rho family GTPases on PDGF-induced chemotaxis. Many of these molecules have been implicated in the regulation of the cytoskeleton. Depletion of several Rho family GTPases had profound and distinct effects on cell morphology during migrating (Monypenny, J. et al., submitted). But given the complexity of the molecular interplay, it often remains unclear whether the induced phenotype is a direct or an indirect consequence of individual treatment. Even with the data showing clear morphological phenotypes, the question as to whether individual molecules function locally or globally, for example, is not easy to answer.

How, then, can we investigate such fast and complex mechanisms in detail?