10 A niversary of IIMCB

Research
The main goal of the group’s research is to understand
how the mechanical properties of the cell are regulated
at the protein-level in order to achieve controlled cellular
deformations. We particularly focus on the cell cortex, a
network of actin, myosin and associated proteins that lies
beneath the plasma membrane and determines the shape of
the cell body. The cortex enables the cell to resist externally
applied forces and to exert mechanical work. As such, it
plays a role in normal physiology during events involving
cell deformation such as mitosis, cytokinesis and cell
locomotion, and in the patho-physiology of diseases such
as cancer where cortical contractility is often upregulated.
Despite its importance, very little is known about how the
cortex is assembled and regulated.
The biological function of the cortex relies on its ability
to contract and to exert forces. As such, the cortex is an
intrinsically mechanical structure and its biological properties
cannot be understood in isolation from its mechanics.
Our main focus is to investigate how cortical mechanical
properties are determined by the molecular components of
the cortex and how these properties are regulated, locally
and globally, to allow the cell to undergo deformations
during cell division and migration. We particularly focus
on blebs, spherical membrane protrusions driven by
contractions of the actomyosin cortex, which commonly
occur during apoptosis, cell spreading, cytokinesis and
migration.
The staff composed of biologists and physicists combine
biophysical and molecular approaches. Our main lines of
research are:
1. Regulation of the mechanical properties of the cortex
We aim to characterize the role of the various cortical
components in cortex mechanics. For this we have chosen
two readouts: cortical tension, which characterizes the cell
mechanical state and cortex flows, which reveal cortex
dynamics.
We have measured cortical tension in various cell lines and
have shown that it has a well-defined quantity for a given cell
line but that it can also considerably vary between lines. This
suggests the existence of feedback loops allowing the cell to
adjust its own tension. We are currently investigating how such
mechanosensing feedbacks are achieved. Moreover, we have
started assessing the influence of various cortical proteins on
cortical tension. We have shown that tension depends not
only on the activity of myosin motors, but also on the level
of proteins involved in actin turnover (Fig. 1). We are currently
extending this analysis to a larger set of cortical components.
Concomitantly, we will check the effect of depletion of
cortical proteins on cortex dynamics. To that aim, we are
analyzing cortical flows during cortical oscillations, which can
be triggered by depolymerization of microtubules (Paluch
et al., Biophys. J. 2005). In an alternative approach, we plan to
monitor cortical flows during cleavage furrow establishment
during cytokinesis and analyze the effect of target protein
depletion on the dynamics of these flows.
Fig. 1: Cortex tension depends on the level of myosin activity and on
actin turnover. A. Schematic of the aspiration setup: a cell is gradually
aspirated into a micropipette until it forms a hemispherical bulge inside
the pipette. At this critical pressure, cortical tension is given by the
Laplace law (formula indicated). The image displays a L929 detached
fibroblast aspirated into a micropipette close to the critical pressure.
B. Cortex tension after various treatments affecting myosin activity.
Y27632: ROCK inhibitor. Blebbistatin: myosin II inhibitor. RhoAQ63L:
constitutively active RhoA. C. Cortex tension after various treatments
affecting actin and actin binding proteins. CD: Cytochalasin D (authors:
Jean-Yves Tineves and Ulrike Schulze).
Laboratory of Cell Cortex Mechanics 69