Fibroblasts on the move [published erratum appears in J Cell Biol 1988 Apr;106(4):following 1403]

T r~ERE are widely divergent views about how animal cells, as exemplified by fibroblasts, move over a substrate. All models assume that fibroblasts are anchored to the substratum by plasma membrane receptors which bind transiently to specific components of the extracellular matrix. It is through these individual molecular feet (not to be confused with focal contacts) that force has to be generated: the cell is moved forward over the substratum as it pushes against those feet that are, at that moment, attached to the substratum. This, of course, will lead to the accumulation of feet towards the rear of the cell: some mechanism must exist to return the feet to the leading front of the cell to attach it to the substratum. But what provides the driving force to push the cell forward? This is where there is most disagreement. Cytoskeletal models (see, for example, Darnell et al., 1986) presume that the cell's feet span the plasma membrane and are attached (perhaps via other proteins) to the actin network in the cell cytoplasm. Force is somehow applied to those feet, by perhaps actin-myosin interactions, which pulls the cell forward. This could be analogous to a ship maneuvering by pulling on its anchor. An alternative membrane flow model (Bretscher, t984) is completely different. In this there exists a continual flow of plasma membrane from the front of the cell towards the rear, both on the dorsal and ventral cell surfaces. It is this flow which pushes on those feet attached to the substrate and thereby produces the force to move the cell forwards. This flow arises from the cell's endocytic cycle: specific receptors and lipids are endocytosed by randomly distributed coated pits on the cell surface, and these molecules are later returned by exocytosis to the cell surface at the cell's leading edge. It is this separation between the sites of endocytosis and exocytosis which generates the flow. Furthermore, exocytosis of membrane at the leading front enables the cell to extend itself forward, with respect to the substratum. A crude mechanical analogy compares the cell with a tank, the lower tread is the plasma membrane with feet attached to the substratum, the upper tread being membrane cycled through the cell to the leading front. A curious feature of this model is that the cyclic flow of membrane from the front of the cell to its back and through the cell again is restricted to just those molecules which are endocytosed by coated pits. Other plasma membrane proteins, which are excluded from coated pits and therefore do not recycle (Bretscher et al., 1980), may therefore find themselves being continually swept backwards to the rear of the cell. Whether such a noncirculating protein does get swept backwards or not depends on how fast it can diffuse by Brownian motion, and how fast the flow is. Calculation shows that if such a protein has a high diffusion coefficient (D • 10 -g cm2/s), the flow on a fibroblast would be too slow to sweep it effectively towards the cell's r ea r Brownian motion would win ou t -and the distribution of the protein over the cell's surface would be roughly uniform. By contrast, a slowly diffusing protein would be swept towards the rear of the cell. In this way, the molecular feet of fibroblasts, the fibronectin receptors, whilst tethered to the substrate fibronectin and therefore unable to diffuse at all, would be pushed backwards with respect to the advancing front of the cell. On stationary cells, this model presumes that there is no net membrane flow because, although the endocytic cycle remains, both endocytosis and exocytosis occur randomly over the cell's surface. A most interesting paper by Ishihara, Holifield, and Jacobson (1988) occurs elsewhere in this issue. In this the authors study a protein, GP80, on the surface of fibroblasts. The interest in this protein is that it has an unusually low diffusion coefficient (Jacobson'et al., 1984) and is known not to participate in the endocytic cycle (Bretscher et al., 1980). They find that, when the fibroblasts move, the GP80 protein is concentrated towards the rear of the cell. When the cells stop moving, the protein randomizes itself by diffusion. The observation that GP80, which is noncirculating, is swept backwards on motile cells, but not stationary ones, fits nicely into the membrane flow scheme outlined above. However, does the magnitude of the gradient observed match that estimated by calculation? Ishihara et al. (1988) find that on a typical cell (50-1xm long) moving at 0,5 Ixm/min, GP80 is '~20-fold more concentrated at the rear of the cell than at its front. Calculation of the effective diffusion coefficient for GP80 which would give this gradient gives D '~ 7 x 10 -1~ cm2/s which is in reasonably good agreement with that previously measured (3.2 x 10 -t~ cm2/s, Jacobson et al., 1984). Further observations of Ishihara et al. (1988) are concerned with what happens when a fibroblast held at 0~ where GP80 is uniformly distributed, is warmed up to 37~ They find that GP80 is absent from any newly formed lamellae, that the boundary of GP80 between these bare regions and the rest of the cell is initially rather sharp, and that, at short times after the warm up, the new boundary does not move appreciably backwards with respect to the substrate, although it obviously does with respect to the new and advancing leading edge. Ishihara et al. (1988) suggest that the lack of rearward movement of the boundary (with respect to