how
cells move: molecular hardware of cell motility
Individual cells in multicellular
organisms are able to move. This ability is essential for
virtually every aspect of the whole body functioning. Cell motility
is
a complicated multi-step process, which begins from making
a decision to move, which occurs at the cell surface. When
surface receptors receive external signals, they initiate
a signaling cascade inside the cell and issue commands to the
cytoskeleton to generate motility. Cytoskeleton is a complex
of detergent-resistant cytoplasmic components, which may
be
considered as molecular hardware for motility. The major
question of my research is how this molecular hardware works.
Fig. 1: Main components of the fibroblast
cytoskeleton.
Actin filaments (yellow), microtubules (red), and
intermediate filaments (blue).
Platinum replica\electron microscopy.
The design of an unknown machine can be understood based on
how its elements are structurally arranged, how they move during
action, and what happens if some element is missing. My experimental
approach is based on the same idea applied to cells and molecules.
I use platinum replica electron microscopy (EM) to analyze
the
structural organization of the cytoskeleton at the nanometer
scale level. This EM technique reveals a fascinating world
of the cell interior and frequently gives simple answers to complicated
questions. Since EM is not applicable to living cells, to
see
the machinery in action I use correlative analysis, in which
the dynamic observation of a cell is followed by EM of the
same cell.
Fig. 2: Correlative light and electron
microscopy.
After phase-contrast microscopy of a living Xenopus fibroblast
(left),
the sample was processed for platinum replica electron microscopy,
and the cell is re-localized (right). This technique allows
to correlate
motility with the cytoskeleton organization.
This approach allows establishing functional connections
between cytoskeletal dynamics and supramolecular organization.
Functional perturbations of specific molecules give further
insight
into details of the molecular design of cellular motile machinery.
Cell
locomotion consists of repeated cycles of leading edge protrusion
followed by cell body translocation. The current
focus of my
research is to understand the mechanisms of leading edge
protrusion. Protrusion is driven by polymerization of actin,
the major
structural and functional element of the cytoskeleton. Actin
filaments through
interaction with different sets of accessory proteins are
able to form a remarkable variety of superstructures with
different
design and functions.
Lamellipodia and filopodia are the
two major protrusive organelles with strikingly different
structural
organization and different sets of molecular players. Different
cell types use these two organelles to a different extent.
Lamellipodia, which are broad, flat protrusions are filled
with a branched
network of actin filaments. The current model of actin dynamics
in lamellipodia (“array treadmilling model”)
describes it as a cycle of dendritic nucleation, elongation,
capping, and
depolymerization of actin filaments.
Fig. 3: Dendritic actin network in
fibroblast lamellipodium.
Individual families of branching filaments are highlighted
in different colors.
Filopodia, which are
thin cellular processes, contain a tight bundle of parallel
actin
filaments, which elongate at the tip and depolymerize from
the rear, as described by the filament treadmilling model.
Fig 4: Assembly of the filopodial
bundle occurs at the tip,
similar to how the tower is built. As a result, the history
of the construction is imprinted in the design of the
structure allowing to understand how the filopodium was built.
We recently
showed that filopodia are formed by reorganization of the
lamellipodial dendritic network in a process which we called “convergent
elongation”. A special structure at filopodial tips,
the filopodial tip complex, functions as an organizing center
for
filopodia formation.
Although basic models for the leading
edge protrusion have been formulated, many questions remain
about the molecular
design
of the protrusive machinery and specific roles of individual
molecules. Many proteins have been found to localize to “the
scene of crime”, but their roles in actin machinery
remain unknown. My strategy is to employ a combination of
powerful
structural, dynamic, and functional approaches to investigate
roles of potentially
important proteins. When these pieces of information come
together, they always produce something novel and exciting.
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