Louise P. Cramer

The Randall Institute, Kings College London, 26-29 Drury Lane, London WC2B 5RL, UK.

Received 5/21/97; Accepted 5/26/97

5. FUNCTION OF RETROGRADE ACTIN FLOW IN LAMELLIPODIA

Retrograde flow of actin filaments relative to the substratum in lamellipodia has often been proposed to play some role in cell motility, although retrograde movement is not immediately reconcilable with net forward displacement of either leading edge structures (protrusion), or the cell body (cell body motility). As with studies of mechanism, those for function of retrograde flow have yielded seemingly ambiguous results. In Aplysia growth cones, as the rate of retrograde actin flow relative to the substratum decreases, the rate of growth cone locomotion increases, both in terms of protrusion and growth cone body motility (26) (Fig 6). This is consistent with a model in which retrograde actin flow is attenuated by coupling to substrate, and in Aplasia, the myosin which drives retrograde actin flow, instead generates 'forward thrust' (as in a 'molecular clutch' proposed in (55)). There is tentative support for this idea for growth cone body motility, but not protrusion in Aplysia. When myosin force in Aplysia is killed with BDM, retrograde flow and growth cone body motility are inhibited, but protrusion is promoted (from (29)). Further studies are required to determine if myosin force from retrograde flow is directly harnessed for growth cone body motility in Aplysia. This is because BDM inhibits more than one myosin (30, 31) and so theoretically different myosins may independently generate force for growth cone body motility and retrograde actin flow respectively. For protrusion in Aplysia the implication is that force from a myosin is not directly harnessed for protrusion, but that the rate of actin flow limits net protrusion (Fig 6).

Fig 6. In Aplysia growth cones, retrograde actin flow is inversely correlated with growth cone body motility and protrusion (26). (A) Actin filaments (thick chevrons) flow retrograde (arrow). (B) Actin filaments attach to adhesion sites (vertical black bar) and retrograde actin flow stops. The gap at the front of the lamellipodium created by retrograde actin flow in A, is filled by actin filament assembly (thin chevrons). (B to C) Growth cone body motility occurs (upper arrow in B, to new position in C). Current data can not determine if force from attenuation of retrograde flow is directly harnessed to drive growth cone body motility (see text). The absence of retrograde flow no longer limits protrusion; there is no gap to fill at the front of the lamellipodium, and on going actin assembly (C, thin chevrons) is instead coupled to net protrusion (lower arrow in B, to new position in C).

In contrast to Aplysia, in stationary tissue culture fibroblasts (25) and locomoting heart fibroblasts (19) there is no correlation between retrograde actin flow relative to the substratum in lamellipodia and either protrusion or cell body motility. Also, in locomoting Ascaris sperm cells the rate of retrograde flow of major sperm protein filaments in lamellipodia is unrelated to cell speed, although flow is faster in stationary cells (33). For protrusion, the difference between Aplysia and fibroblasts may simply reflect a different geometry. Aplysia growth cones are in contact with the substratum, whereas fibroblast lamellipodia are often raised up off the substratum, thus preventing efficient coupling with the substratum. For cell body motility, the difference between Aplysia and fibroblasts may reflect the exact nature of an Aplysia growth cone. Perhaps the front of an Aplysia growth cone is simply one structure, rather than a distinct lamellipodium and lamella. In a single structure, a single type of force has the potential to drive retrograde actin flow across the entire region from the front margin of the growth cone to the front of the growth cone body. In this case, it is easy to imagine why in locomoting Aplysia growth cones, there is a relationship between retrograde actin flow, and both protrusion and growth cone body motility. In contrast, in locomoting fibroblasts the force that drives retrograde actin flow, in lamellipodia is spatially separated from the cell body by stationary actin filaments in the lamella (19). Instead in fibroblasts, retrograde actin flow may regulate the formation of certain actin bundles (56).

A different role for retrograde flow has been proposed in locomoting newt lung epithelial cells (Waterman, Storer and Salmon, submitted). In these cells, retrograde flow alters the spatial orientation of microtubules, which in turn influences microtubule plus-end assembly dynamics. This might be important for communication between actin and tubulin cytoskeletal systems in locomoting cells and neuronal growth cones.