Overview of Cell Biology/Regulation of Actin Dynamics in Cells

The Fibroblast
Dynamic structures containing actin are involved in cell migration. One model cell type used on the study of cell locomotion is the fibroblast, a connective tissue cell. The front of the fibroblast regularly extends thin sheet-like processes called lamellipodia, which is organized as a meshwork, and thin needle-like projections called microspikes or filopodia, both of which are filled with actin filaments and organized in parallel bundles. Fibroblasts also contain cables of actin called stress-fibers which help facilitate cell adhesion and contraction.

Actin filaments in the lamellipodia, filopodia, and stress fibers are oriented with their barbed (+) ends facing the membrane.

The Leading Edge of the Cell Nucleates Actin Polymerization
The behavior of actin filaments in the lamellipodia of fibroblasts can be traced by microinjecting the cells with fluorescently labeled actin. Actin filaments are thought to be nucleated by the Arp2/3 complex in conjunction with activated WASP family proteins.

Retrograde flux and Depolymerization
After actin filaments are nucleated at the leading edge of the fibroblast, the entire network of filaments flux backwards towards the interior of the cell in a process called retrograde flow. Cofilin is thought to be the major actin depolymerizing activity at the leading edge. The action of ADF/cofilin can be inhibited by LIM-kinase.

Listeria monocytogenes
The comet tails of Listeria monocytogenes drive motility. Listeria has a protein called ActA which functions like WASP family proteins and recruits the Arp2/3 complexes of the host. Listeria comet tail formation and motility requires actin, the Arp 2/3 complex, profilin, capping proteins, and cofilin. Listeria motility can be reconstituted in cell cytoplasmic extracts containing these purified proteins.

Rho GTPase Proteins
Rho proteins are part of the Ras superfamily' of proteins which bind and hydrolyze GTP.

Active GTP in its bound state can interact with effector molecules. GDI (guanine nucleotide dissociation inhibitor) stabilizes the Rho-GDP form.

Rho, Rac, and Cdc42 Regulation
Microinjection of Rho-GTP causes the formation of stress fibers.

Microinjection of Rac-GTP causes the formation of lamellipodia.

Microinjection of Cdc42-GTP causes the formation of filopodia.

WASP
Was is one of the effectors of the Rho family that is most studied. WASP is activated by Cdc42. WASP is always present in an inactive, autoinhibited form in the absence of Cdc42-GTP. The GTPase binding domain (GBD) of WASP binds to Cdc42-GTP and this induces a comformational change which opens the molecule, exposing the WCA region (on the C-terminus). The WCA region interacts with Arp2/3 and action and begins the process of nucleation.