Gijsje Koenderink's research page
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The cytoskeleton is a complex and highly dynamic network of
protein filaments, motors, regulatory agents, and membranes that gives
eukaryotic cells their shape and mechanical strength and drives
dynamic cell functions such as cell locomotion, division, and growth.
I study physical properties, in particular mechanics, of cytoskeletal protein networks
in vivo and in vitro, aiming towards a quantitative understanding of cell mechanics that
links the molecular to the cellular level. I work jointly with the Weitz
lab and the Physics of
Complex Systems lab of Prof. C.F. Schmidt in Amsterdam, the
Netherlands.
The mechanical response of cells is controlled by an elastic
network of interconnected semiflexible protein filaments and
membranes known as the cytoskeleton. The cytoskeleton has
a highly organized yet very dynamic structure, carefully
regulated by a myriad of accessory proteins that control the
lengths and spatial organization of the filaments. Mechanical properties
of cells are essential for many cell functions, including cell
crawling, division, and mechanosensing. Of crucial importance to our understanding of the force
generation and the mechanical response of cells is a quantitative
model of the cytoskeleton and associated proteins that
cross-link, bundle, and/or act as molecular motors. This requires
a fundamental understanding of
- the network properties of cytoskeletal filaments and
their composite structures
- the effects of contractile elements such as motors
- the nature of stress propagation in the cytoskeleton
Why are physicists interested in cell mechanics? A soft condensed
matter physicist likes to view the cell as a complex and unique
soft condensed matter material with material properties that are very distinct from
materials made of common, flexible polymers:
- The semiflexible nature of its components leads to a
shear rigidity orders of magnitude higher than that of a
comparable flexible polymer network and a strongly
non-linear behaviour under strain (strain hardening).
- The cytoskeleton is a dissipative, non-equilibrium
system, since the polymerization of the filaments as well
as the activity of motor proteins require nucleotide
hydrolysis.
- The length and time scales associated with the structure
and dynamics of cytoskeletal protein networks are several
orders of magnitude larger than those of typical
synthetic polymers. Therefore we can study the connection
between the macroscopic material properties and the
structure and dynamics of the individual filaments, which
are observable with light microscopy.
Viscoelastic properties of polymer solutions are traditionally
probed with macroscopic rheometers. However, rheometers use large
amounts of material and are insensitive to local variations of
mechanical properties, expected in the intrinsically
heterogeneous cytoskeleton of cells. To overcome these
limitations, local probes of the viscoelastic properties ("microrheology")
have been developed in recent years.
Microrheology uses the motions of thermally excited
micron-sized probe particles embedded in a complex fluid to make local measurements
of linear viscoelastic properties of
the material at the length scale of the probe particle. Since no
single technique has a sufficiently wide frequency range and sensitivity,
we use a combination of optical microrheology techniques:
- Multiple-Particle video Tracking (MPT): a large
number of probe beads are imaged simultaneously and a
statistical analysis of the variations in the bead
motions yields spatially resolved information about
structural and dynamic heterogeneities
- Diffusing Wave Spectroscopy (DWS): a high
frequency (up to 1 MHz) technique especially suitable for
highly crosslinked, rigid networks
- Laser interferometry (in Amsterdam): microrheology based on laser
tweezers and interferometric detection of bead motions; 1
nm displacements can be tracked with 100 kHz time
resolution and the beads can optionally be actively
manipulated to study non-linear viscoelastic properties
In addition to the microrheological techniques we use
conventional rheometry with commercial cone-and-plate rheometers
to study rheology at low frequencies (10-4 to 10 Hz) and large
strains.
Microrheology of actin, microtubule, and neurofilament networks
We use two-particle microrheology based on video tracking as well as
laser interferometry to measure the linear viscoelastic moduli of networks of actin,
microtubules, and neurofilaments (a type of intermediate filament present in neurons).
We study composite networks of purified proteins and add physiological regulatory proteins
(cross-linkers and bundlers) to mimick the cytoskeleton.
Mechanics of cytoskeletal protein networks with active
contractile elements
It is known that motor proteins generate tension and mediate
sliding of filaments, but it is largely unknown how this affects
the viscoelastic properties of cytoskeletal polymer networks. We
study simplified model systems of semiflexible actin filaments
with added motor proteins (skeletal muscle myosin II), and measure the mechanics with
(mostly) microrheology techniques.
Thermal bending of semiflexible polymers in entangled and
crosslinked networks
We image fluorescently labeled actin filaments and
microtubules in a background network of unlabeled actin and
determine the confinement of the thermal bending modes by the
entangled/cross-linked network.
Other people involved in these
projects: Cliff Brangwynne, Karen Kasza, Jiayu Liu, Yi-Chia Lin, Maryam
Atakhorrami
- Zvonimir Dogic, Complex
Fluids, Rowland Institute at Harvard, Cambridge MA,
US
- Christoph Schmidt, Physics of Complex
Systems, Vrije Universiteit, Amsterdam, the
Netherlands
- Frederick MacKintosh, Theoretical
Physics, Vrije Universiteit, Amsterdam, the
Netherlands
- Alex Levine, Dept. of
Physics, University of Massachusetts, Amherst MA, US
To be added.
Research Page
Last Updated January 24, 2005
Web Page by Gijsje Koenderink