The clutch hypothesis revisited: Ascribing the roles of actin-associated proteins in filopodial protrusion in the nerve growth cone

We seek to understand how the nerve growth cone acts as a sensory motile ma chine to respond to chemical cues in the developing embryo. This review foc uses on filopodial protrusion and F-actin-based motility because there is g ood evidence that these processes are required for axon guidance. The clutc h hypothesis, which states that filopodial protrusion occurs by actin assem bly when an actin filament is fixed with respect to the substrate (i.e., a clutch is engaged), was postulated by Mitchison and Kirscher to link protru sion to actin dynamics. Protrusion would require functional modules for mov ement of material into filopodia, clutching the F-actin, F-actin assembly a t the tip, and retrograde flow, in this review, recent studies of actin-ass ociated proteins involved in filopodial protrusion will be summarized, and their roles will be assessed in the contest of the clutch hypothesis. The l arge number of proteins involved in filopodial motility and their complex i nteractions make it difficult to understand how these proteins act in protr usion. Recently, we have used microscale chromophore-assisted laser inactiv ation (micro-CALI) for the focal and acute inactivation of specific actin-a ssociated proteins during filopodial protrusion to address their in situ ro les. Our findings suggest that myosin V functions in moving membranes or ot her material forward in extending filopodia, that talin acts in the clutch module, and that zyxin acts in actin assembly at the tip during filopodial protrusion, perhaps by recruiting Ena/VASP family members to promote actin elongation at this site. 2000 John Wiley & Sons, Inc.