10 A niversary of IIMCB

Fig. 2: Bleb induced by laser ablation in a detached L929 fibroblast. 0s: offset of ablation. Bar, 10µm (author: Julia Roensch).
In the long term, this approach will help defining the
minimal ingredients necessary for cortex contractility, paving
the way for a biomimetic system of the cell cortex.
2. Mechanisms of formation of blebs
The growth of blebs depends on myosin activity and
is commonly believed to directly result from intracellular
pressure, however this hypothesis has not been tested and
the mechanisms of bleb growth remain elusive.
We have shown that laser ablation of the cell cortex
leads to the formation of a bleb, supporting the view that
bleb expansion is a direct, mechanical, result of intracellular
pressure (Fig. 2). Moreover, multiple ablations of the same
cell at different locations indicate that the growth of a bleb
considerably reduces pressure. We have then induced blebs
on cells with different tensions and shown that the size of
a bleb directly depends on tension. This dependence can
be fitted with a theoretical model of the actomyosin cortex
and allows us to estimate elastic parameters of the cortex
and of the cytoplasm, and to accurately predict bleb shape
(collaboration with the group of J.F. Joanny, Institut Curie,
Paris). We now plan to further analyze the dynamics of
bleb expansion in cells with different tensions. Coupled to
theoretical modeling, this will allow us to elucidate which
dissipation source is the major limiting factor for bleb growth
mechanics. This is particularly important because the type
of protrusion formed by a cell is likely to modify its migration
pattern (cf point 4) – bellow. Modifying the magnitude
of dissipation linked to bleb growth can be used by cells
to favor or reduce the formation of blebs versus other
protrusion types such as lamellipodia.
3. Role of cortex tension and blebs during cytokinesis
We have discovered that ablation of the actin cortex during
cytokinesis leads to oscillations of the cleavage furrow and
results in division failure (Fig. 3). Similar furrow oscillations
can be observed after depletion of different actin binding
70 Annual Report 2008
proteins. Strikingly, small oscillations of the furrow can also
sometimes be observed in control divisions, although their
amplitude remains limited, allowing for division to proceed.
Based on our observations, we have proposed that the cortex
controls its own contractility during furrow ingression and
prevents the built-up of an imbalance in contractile forces
by self-disassembling and forming blebs above a threshold
tension. When this control is removed, like for example
after laser ablation, the cleavage becomes unstable and
cytokinesis fails. We are currently testing this interpretation
using both biophysical and molecular techniques.
4. Protrusion formation during migration in
3D-environments
In 3D-environments, bleb-based migration is a
widespread alternative to lamellipodial migration, and is
commonly used by cancer cells and during development.
It is not known why cells form one or the other type of
protrusion, and how the cells can switch between protrusion
types is poorly understood. Strikingly, certain cell types, e.g.
mesendodermal cells in Danio rerio (zebrafish) embryos, are
able to form both lamellipodia and blebs at the same time.
We have initiated a study of the mechanisms of formation
of these protrusions and of their respective contributions to
cell migration in the zebrafish embryo (collaboration with
the lab of C.P. Heisenberg, MPI-CBG). We have characterized
wild type migration and have shown that the protrusions
formed by mesendodermal progenitors consist of blebs
(50%), lamellipodia (35%) and filopodia (15%). We have also
shown that the expression of dominant negative (resp.
constitutively active) ezrin (a protein linking the actin cortex
to the membrane) shifts this distribution and leads to the
formation of more (resp. fewer) blebs. We are currently
investigating the effects of these treatments on migration.
The long term perpective is to better understand the
mechanisms of protrusion formation and the role of blebs
during migration in vivo and in 3D environments.