Biology Reference
In-Depth Information
Disruption of microtubules was shown to activate contractility in many
different cell types (Elbaum et al., 1999; Small et al., 2002). The mechanism of
microtubule-mediated suppression of contractility is not yet clear, and could
be based both on delivery and removal of contractility regulating signalling
molecules by the microtubule system, and on mechanical resistance of
microtubule arrays (with associated motors and crosslinking proteins) to
actomyosin contractility (see, for more detailed discussion Elbaum et al., 1999;
Ingber, 2002; Small et al., 2002; Wittmann and Waterman-Storer, 2001).
Thus, to understand possible role of microtubules in the regulation of cell
migration, it is necessary first to explain how formation of focal adhesions is
related to the regulation of cell contractility.
Mechanosensory function of focal adhesions and its
modulation by microtubules
Adherent cells apply physical forces to the extracellular matrix, generated
through the contractile activity of the actin cytoskeleton (Balaban et al., 2001;
Dembo and Wang, 1999; Galbraith and Sheetz, 1997; Harris et al., 1980; Tien
et al., 2002) and reviewed in Beningo and Wang (2002). The forces are
produced by myosin II and are transmitted via the focal adhesions. The
average magnitude of the force for fibroblast-like cells is about 5 nN per
square micrometre of focal adhesion plaque. In the course of these studies, a
surprising feature of focal adhesions was discovered. These structures appear
to assemble and maintain their integrity only under conditions in which forces
of this magnitude are applied to them. Any treatment that inhibits myosin
II-driven contractility - including incubation with chemical inhibitors of
myosin light chain kinase (ML-7 and KT5926), myosin ATPase (BDM), or
transfection with caldesmon, a protein that also inhibits actin-dependent
myosin II ATPase activity, prevents formation of new focal adhesions, and,
moreover, leads to rapid disassembly of the existing ones (Chrzanowska-
Wodnicka and Burridge, 1996; Helfman et al., 1999). In cells attached to a
flexible substrate, which can be deformed by the cell, the tension force acting
at the adhesion plaques are smaller than in an identical cell attached to the
solid substrate. Thus, the average size of the focal adhesions that cells form
upon attachment to flexible substrates is smaller than those formed in cells
attached to solid substrate (Pelham and Wang, 1997). Finally, the assembly of
the focal adhesions can be induced by externally applied tension forces, using
either a micropipette attached to the cell surface, or the local stretching of a
flexible substrate (Kaverina, 2002; Riveline et al., 2001). External force
applied to the focal adhesion can completely substitute the cell-generated
force, so that micropipette manipulation leads to focal adhesion growth even
