Regulation of Actin Network Dynamics by Actomyosin Contractility - Innovative Approaches to Cell Biomechanics - page 59

Fig. 5.1b . Moreover, Fig. 5.1a, b show that in the case of calyculin treatment, myo-
sin II motors are widely distributed over the entire lamellipodia, even at the leading
edge, which is believed to be less accessible to myosin II filaments.
In fact, in the case of calyculin treatment, the density of myosin II appears higher
compared with control, and visibly, most myosin II motors are localized along the
SFs at the back of the lamellipodia. A similar myosin II distribution was previously
reported (Kolega 2006 ; Svitkina et al. 1997 ). Since SF development is enhanced by
actomyosin contractility, together, these results demonstrate that calyculin treat-
ment enhances actomyosin activity, resulting in the formation of more SFs.

5.3.2 Inhibiting Actomyosin Contractility by Blebbistatin

Contrastingly, F-actin structure organization especially at the back of the lamellipodia
appears disrupted in cells treated with blebbistatin (Fig. 5.1e ). In fact, SFs are
barely visible (compare Fig. 5.1a with Fig. 5.1e ), further supporting the argument
that tension is necessary for the integrity of SFs. Moreover, low density of myosin
II in the lamellipodia of blebbistatin-treated cell in Fig. 5.1e also demonstrates the
effectiveness of blebbistatin treatment in inhibiting actomyosin contractility, which
is the main cause for loss of SFs.
Nevertheless, sparsely distributed myosin II spots are still visible as in Fig. 5.1e ,
and this implies partial inhibition. Remarkably, in all the three cases, even the cell
nucleus stains positive for myosin II, consistent with the high density of myosin II
motors around the nucleus (Li and Sarna 2009 ). A similar distribution of myosin II
has been reported (Svitkina et al. 1997 ).
Taken together, the actomyosin distribution results demonstrate that the pertur-
bation drugs can modulate actomyosin contractility by influencing the activity of
the myosin II motors. Moreover, these results highlight the importance of myosin
II-dependent contractility for the structural organization of the actin cytoskeleton
and the stability of SFs in particular, in agreement with the previous reports (Schaub
et al. 2007 ; Henson et al. 2003 ; Lin et al. 1996 ).

5.4 Actomyosin Perturbation Alters Network Flow

and Migration Speed

This section explores the effect of actomyosin perturbation on the dynamics of
F-actin dynamics as exhibited by actively migrating keratocytes. The case consid-
ered here is that of keratocytes microinjected with Qdot-phalloidin and then imaged
using fluorescence microscopy. As mentioned already in Chap. 4 , phallodin has
been reported to induce changes in the mechanical properties of F-actin by stabiliz-
ing F-actin against depolymerization, and also to promote polymerization of G-actin
to F-actin (Wehland et al. 1977 ; Dancker et al. 1975 ).