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modifying the cytoskeletal linkage, or destabilizing the focal complex. As
a result, calpain-mediated proteolysis promotes complex translocation,
disassembly and rear retraction.
Calpain during adhesion formation and directional migration
There is also evidence supporting a role for calpain in adhesion formation and
cell spreading (Potter et al., 1998; Stewart et al., 1998). Inhibition of calpain
activity reduces lymphocyte (Stewart et al., 1998) and endothelial cell
spreading (Kulkarni et al., 1999) on extracellular matrix. In contrast,
fibroblast spreading is not significantly affected by calpain inhibition and
calpain inhibition can, in fact, enhance the adhesiveness and spreading of
primary neutrophils (Lokuta et al., 2003). These differences highlight the
importance of cellular context in calpain-mediated modulation of the actin
cytoskeleton and cell migration (Figure 14.2).
In addition to physically altering protein complexes such as focal adhesions,
calpain is also likely to participate in cell motility by affecting intracellular
signalling pathways. Studies by Kulkarni et al. (1999) suggest that calpain
may mediate integrin-mediated signalling and cell spreading upstream of Rho
family members by activating Rac and Rho activity (Kulkarni et al., 1999).
Specifically, they hypothesize that calpain inhibition reduces Rac and Rho
activity and the effects of calpain inhibition may be rescued by activating Rac
and Rho. In addition, in endothelial cells calpain is required for the formation
of focal contacts, small clusters of integrins that form early in the cell
spreading process. These early integrin clusters contain cleaved b3 integrin, a
known substrate of m-calpain and m-calpain (Kulkarni and Fox, 2000). Focal
complexes and focal adhesions, which form later in spreading, were not found
to contain either m-calpain or cleaved b integrin.
Recent studies show that the focal complexes formed in cells that are
deficient in calpain activity are different from those formed in control
cells (Bhatt et al., 2002). It is possible that these abnormal complexes
display distinct signalling properties that contribute to the changes in cell
protrusion observed after calpain inhibition. In support of this possibility,
calpain-deficient fibroblasts (Capn4
7/7
fibroblasts) not only exhibit reduced
migration rates, but also have increased cell protrusions (Dourdin et al.,
2001). Specifically, Capn4
7/7
embryonic fibroblasts demonstrate enhanced
formation of membrane projections, suggesting that calpain may normally
function to suppress protrusion and the formation of membrane projec-
tions. In accordance with this possibility we find that the activity of Cdc42
and Rac is increased after calpain inhibition in neutrophils (Lokuta et al.,
2003).
