Actin cytoskeleton: a signaling sensor in cell volume regulation

The actin microfilaments are well known dynamic structures that support and organize the cell membrane and functions associated with the membrane such as ion channels and transporters. In addition, many aspects of cellular physiology seem to be actively modulated by changes in actin cytoskeleton dynamics, which involve reorganization and restructuring of the filaments. For both of these reasons, the actin cytoskeleton has attracted special attention since the early days of cell volume regulation research. Mechanisms controlling the actin equilibrium in response to external stimuli were studied and the signaling cascades leading to the regulation of actin cytoskeleton dynamics have been partially elucidated. They include: a) activation of specific actin binding proteins that regulate actin polymerization dynamics, b) activation of protein kinases or phosphatases regulating phosphorylation of specific cytoskeletal proteins and c) activation of signal transduction pathways leading from membrane receptor activation to actin reorganization involving small GTPases of the Rho and Rac families. These intracellular signal transducers are activated by extracellular stimuli that include hormones, growth factors, cytokines, or ions, many of them in turn are partially known to participate in cell volume regulation. These findings provide strong evidence that the actin cytoskeleton is involved in cell volume regulation by sensing and mediating extracellular signals.