Chapter 11 Regulation of Actin and Myosin II Dynamics in Living Cells

This chapter discusses the regulation of the structure and dynamics of actin-based cytoskeleton. Actin, myosins, and associated proteins exhibit complex and varied behavior in muscle and nonmuscle cells alike: assembling and disassembling and forming fibers, bundles, and isotropic gel networks. In turn, the resulting actin-based structures display yet another level of behavior, as they move about the cell. The chapter discusses quiescent fibroblasts, as a model system, the multimode approach, actomyosin dynamics in serum-starved 3T3 fibroblasts, regulation of stress fiber contraction, stress fiber movement and cell motility, and some future prospects. Recent advances allow viewing the dynamic behavior of multiple intracellular variables simultaneously or in very rapid succession. Multimode microscopy is capable of revealing not only the precise dynamics of important structural elements, but also the order of these movements relative to that of other dynamic macromolecules. Photodetectors, radio imaging, and automated or semi-automated image analysis make precise, quantitative answers possible. Such quantitation seems crucial, given the complex and usually subtle interconnections among regulatory pathways. When viewed over long time periods, stress fibers can be very dynamic. In serum-starved Swiss 3T3 fibro-blasts, a large proportion of the stress fibers, particularly those lying parallel to the cells distal margins, are in continuous motion. They move centripetally, that is, from the cell periphery toward the nucleus, until they reach the perinuclear cytoplasm, where they disappear. Microinjection studies offer the advantage that they directly and immediately alter cellular concentration of a given component, albeit at the expense of introducing it in a “less natural” state.