Biology Reference
In-Depth Information
mechanisms that have previously been implicated in cell detachment include a
role for the phosphatase calcineurin in neutrophils on vitronectin (Hendey
et al., 1992).
More recent studies from our laboratory and other laboratories implicate a
central role for the protease calpain as a calcium-dependent molecule involved
in cell detachment. We have previously reported that inhibition of calpain
activity reduces the migration rate of CHO cells in vitro (Huttenlocher et al.,
1997). This decreased migration rate after calpain inhibition is the result of an
inhibition of rear retraction. Calpain inhibition also reduces the amount of
integrin that remains behind on the substratum after rear detachment (Palecek
et al., 1998) suggesting that calpain may weaken the integrin-cytoskeletal
bonds in this region, facilitating rear release. Interestingly, calpain inhibition
does not seem to affect cell detachment of neutrophils under most conditions
(Lokuta et al., 2003). This observation suggests that rapid (less adhesive) and
slow moving (more adhesive) cells may use different mechanisms to regulate
adhesive release at the cell's rear. In fact, at low cell-substratum
adhesiveness, calpain inhibitors do not significantly affect migration or
detachment rates of fibroblasts, suggesting that at lower adhesiveness,
contractile forces are su
cient to sever the integrin-ligand bond. However,
at higher cell-substratum adhesiveness, contractile forces may not be su
cient
to allow adhesive release and calpain-mediated proteolysis may also be
required.
The effects of calpain on rear detachment are likely to be mediated through
calpain-dependent remodelling of focal adhesions. Inhibiting calpain in
fibroblasts alters the morphology of focal adhesions in comparison to control
cells (Huttenlocher et al., 1997). Focal adhesions are larger, more stable and
located at the periphery of the cell after calpain inhibition instead of being
distributed throughout the cell interior. Actin stress fibres that normally cross
the cell and terminate in focal adhesions are also altered when calpain is
inhibited. After calpain inhibition the actin stress fibres are primarily cortical.
Similar changes are observed in calpain-deficient embryonic fibroblasts
(Capn4
7/7
fibroblasts), supporting a central role for calpain in the regulation
of focal adhesions and the actin cytosekeleton (Dourdin et al., 2001). Previous
studies have supported a role for calpain in focal adhesion disassembly. For
example, fragments of collagen promote focal adhesion disassembly by the
calpain-dependent cleavage of FAK, paxillin and talin (Carragher et al.,
1999). In live studies, we have recently shown that calpain inhibition
modulates focal complex composition and disassembly. Calpain inhibition
blocks localization of a-actinin to the focal adhesion and prevents subsequent
translocation of the focal adhesion to the cell interior as well as focal complex
disassembly (Bhatt et al., 2002). It has been proposed that cleavage of some
component of the focal adhesion, talin for example, may alter the composition
of the focal adhesion thus allowing the recruitment of different proteins,
