In 2001, their extensive examinations of human lung cells led Ben Fabry
and Jeffrey Fredberg, two scientists at the Harvard School of Public
Health, to ask this provocative question. They were, of course, not
literally suggesting that humans are built out of window glass. Instead
they wanted to draw attention to a perplexing similarity between the
mechanical behavior of individual living cells and that of a large class
of inanimate materials comprising foams, emulsions, slurries and pastes.
All these materials exhibit apparent power-law rheology power-law
increase in strain over time following application of a step stress)
with very small power-law exponents, the fingerprint of ``soft glassy
rheology''. The ubiquity of power-law rheology and the corresponding
broad, scale-free spectrum of internal relaxation times in soft
condensed matter had been interpreted in terms of rough free energy
landscapes by the theoretical physicist Peter Sollich and his coworkers
a few years earlier. Viewed in this light, the observations of Fabry and
Fredberg suggested that many organisms live close to a glass transition
- a notion referring to an unusual state change of matter, where the
dynamics slows down dramatically without any noticeable structural
change. Could this property help cells to fulfill their multiple
mechanical tasks in scaffolding, force generation, division and
proliferation within a soft tissue, where they experience a wide range
of dynamical stimuli - from the sudden jerk of a muscle to the gradual
growth of a bone? Despite substantial research effort, the physical or
biological origin of the apparently very robust and universal glassy
dynamics of cells has remained obscure.
The mechanical properties of human and animal cells
and tissues are dominated by biopolymers like actin, microtubules,
etc. These have now been shown to undergo a glass transition upon
cooling.
As reported in the forthcoming issue of the
Proceedings of the National Academy of Sciences USA (PNAS), a
promising candidate for a universal mechanism underlying the
intriguing observations of soft glassy rheology in living cells has
now been discovered. Combining diverse physical measurement techniques
and theory, three German biophysics groups located in J�lich, Leipzig
and Munich have demonstrated that entangled solutions of pure
filamentous actin undergo a dramatic slowdown upon slight changes of
composition or ambient conditions.
Actin is the dominant polymeric constituent of the cytoskeleton (the "bones
and muscles" in the interior of a cell) and therefore forms a
promising target in the search for a unified physical theory of cell
mechanics. Using highprecision dynamic light scattering, the group at
the J�lich Research Center headed by Rudolf Merkel was able to detect
a curious timestretching in the Brownian dynamics of actin: upon
lowering the temperature only slightly, the spectrum of relaxation
rates of the actin filaments spread out enormously. This property is
also a key element of a new theoretical model of cell mechanics,
called the glassy wormlike chain. This theory not only fits the data
over many decades in time, it also explains the complicated nonlinear
response to an applied stress or strain that was measured by the group
of Andreas Bausch at the Technical University Munich. Unlike cells and
tissues, actin filament solutions flow like a liquid over long times.
However, if stretched sufficiently rapidly, they reveal a broad linear
response followed by pronounced stiffening as the stress is increased.
This property is strongly reminiscent of known results for crosslinked
actin networks, living cells, and tissues (which you can easily
confirm by pulling on your cheeks). For actin, the degree of
stiffening increases sharply with decreasing sample temperature, in
accord with the common experience that your body feels stiff when it
is cold. Remarkably, as the research team reports, not only
temperature but many different physiological parameters can trigger
exactly the same stiffening transition.
Further, what first appear to be distinct mechanical signatures in the
response to stress turn out to be equivalent after rescaling the
deformation speed (or time). If this "rheological redundancy'' also
applied to living cells, it would mean they could choose from a vast
arsenal of different molecular mechanisms to adjust their mechanical
properties to the same unique overall performance. A related
intriguing question concerns the role played by cell mechanics in
regulating the internal biological clock, which endows living
organisms with an emergent concept of time - or, as Klaus Kroy from
the Institute of Theoretical Physics in Leipzig puts it, "whether the
physical phenomenon of a glass transition could be the clue to
understanding the coherent slowdown of the physiological functions of
cold-blooded or hibernating animals or how bacteria manage to survive
millions of years in permafrost".
Further Information and Source:
-
Christine Semmrich, Tobias Storz, Jens Glaser, Rudolf Merkel,
Andreas R. Bausch and Klaus Kroy: Glass transition and rheological redundancy in F-actin solutions.
In: Proceedings of the National Academy of Sciences; PNAS;
December 18, 2007; vol. 104; no. 51; 20199-20203; doi
10.1073/pnas.0705513104.
