Mechanisms of cell detachment and focal adhesion
disassembly: a role for calpain
There is evidence that the rate of detachment at the cell's rear is the rate-
limiting step of cell migration speed under many conditions. Release of
integrin-mediated adhesions involves the formation of a contractile force and
the contribution of signalling mechanisms that allows for adhesive release.
Contraction supplies the force required to break integrin-mediated adhesions,
while signalling mechanisms may determine whether the integrin-ligand or
integrin-cytoskeletal bond is preferentially broken at the cell's rear.
Mechanisms of cell detachment are distinct in different cell types and under
different extracellular matrix environments. Under conditions of low
adhesiveness between the cell and the extracellular matrix, preferential release
of the integrin-ligand bond is most likely. In contrast, under conditions of
higher adhesiveness, there is evidence that cleavage of the integrin-
cytoskeletal linkage occurs with higher frequency in fibroblasts. This results
in integrin being left behind on the substrate after cell detachment. Slower
moving cells, such as fibroblasts, favour breaking the integrin-cytoskeletal
bond, whereas rapidly moving cells may have mechanisms that release the
integrin-ligand bond instead. This latter mechanism retains integrins on the
cell surface and/or allows the integrins to be taken up into endocytic vesicles
with tracking to the cell centre.
Several lines of evidence support a role for actin-myosin generated
contraction in adhesion disassembly at the cell's rear during migration.
Amoebae deficient in myosin II exhibit decreased migration which is
exaggerated on more adhesive substrata (Eddy et al., 2000; Wessels et al.,
1988). Also, mitogen-activated protein kinase (MAPK) was found to enhance
the migration of fibroblast-like cells through stimulating myosin light chain
phosphorylation with an associated increase in contraction (Klemke et al.,
1997). Another signalling mechanism contributing to rear release involves the
small GTP-binding protein, Rho. Inhibition of Rho reduces rear release in
migrating leukocytes (Worthylake et al., 2001). It is possible that Rho
mediates rear release by stimulating contractility (Chrzanowska-Wodnicka
and Burridge, 1996), or alternatively, the effects of Rho on rear retraction may
be through modulation of the adhesive complex stability.
A role for calcium transients in adhesive release at the rear of migrating
neutrophils has been clearly demonstrated (Cox and Huttenlocher, 1998).
Calcium depletion or calcium buffering inhibits neutrophil migration on
fibronectin and vitronectin by specifically inhibiting rear release (Hendey and
Maxfield, 1993). Further evidence for calcium transients regulating adhesive
release is provided by the observation that in highly adhesive keratocytes,
stretch-activated calcium channels result in influx of calcium at the cell rear
immediately preceding adhesive release (Lee et al., 1997). Calcium-dependent