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Model systems of the actin cortex [Elektronische Ressource] / Oliver Lieleg


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Published 01 January 2008
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Physik Department
Model Systems of the Actin Cortex
a thesis
presented by
Oliver Lieleg
Technische Universität
MünchenTechnische Universität München
Physik Department
Lehrstuhl für Zellbiophysik E27
Model Systems of the Actin Cortex
Oliver Lieleg
Vollständiger Abdruck der von der Fakultät für Physik der
Technischen Universität München
zur Erlangung des akademischen Grades eines
Doktors der Naturwissenschaften
genehmigten Dissertation.
Vorsitzender: Univ.-Prof. Dr. Ralf Metzler
Prüfer der Dissertation: 1. Univ.-Prof. Dr. Andreas Bausch
2. Univ.-Prof. Dr. Erwin Frey,
Ludwig-Maximilians-Universität München
Die Dissertation wurde am 16.09.08 bei der Technischen Universität München
eingereicht und durch die Fakultät für Physik am 05.11.08 angenommen.µ

Παντα χωρ ι κoι oυδν ν ι

Alles bewegt sich fort und nichts bleibt. (Platon)
Montes fluxerunt a facie Domini et Sinai a
facie Domini Dei Israhel.

Und die Berge zerflossen vor dem Angesicht des Herrn, dem Gott
Israels, und ebenso der Sinai. (Bibel, Richter 5,5)Summary
Cross-linked or bundled actin networks are the key components in cell mechanics.
By locally activating different actin binding proteins (ABPs) the cytoskeletal mi-
crostructure can be adjusted to meet the changing needs of the cell. Bundles of
actin filaments are used for structural fortification during locomotion. Here, the
cell relies on mechanical stability in order to endure the large forces created at
the leading edge. Besides this mechanical stability the cytoskeleton has to allow
for an ongoing remodeling of its microstructure to provide the cell with crucially
needed adaptibility. To shed light on the microscopic principles in cell mechanics,
reconstituted actin networks have been proven of utmost importance. Only in
such reconstituted systems physical and biochemical parameters can be controlled
In this thesis the microstructure and viscoelastic properties of reconstituted actin
networks are correlated. The first part of this thesis focuses on the structural or-
ganization of actin filaments by different actin cross-linking and/or bundling pro-
teins. A remarkable polymorphism of network microstructures is observed; the
detailed network configuration is set by both the type and concentration of the
ABP used. Furthermore, it is demonstrated that the different structural phases
of reconstituted actin networks are directly correlated with distinct regimes in the
macromechanical network response. It is shown that the transitions between these
structural and mechanical phases can be induced by varying the effective concen-
tration of a given cross-linking protein. This can be achieved by either varying
the total amount of cross-linking molecules or by adjusting their binding affinity
towards actin by temperature. The resulting network structures can be either ho-
mogeneous or heterogeneous. In the latter case the local viscoelastic properties are
observed to drastically differ from the macroscopic network mechanics. As an out-
standing example of structural heterogeneities the formation of bundle clusters is
described. These clusters exhibit a fractal dimension which also extends to larger
mesoscopic length scales of the cluster network.
Local heterogeneities are also observed in weakly cross-linked networks and seem
to be a generic prerequesite for the transition into a homogeneous phase which is
dominated by cross-links rather than by entanglements between filaments. This
cross-link transition is parameterized and the microscopic origin of this transi-
ition is identified. Similarly, the transition from a weakly cross-linked phase into
a purely bundled network is investigated. In contrast to the so far commonly
accepted belief, in stiff bundle networks the microscopic origin of the network
elasticity is completely different from that of isotropically cross-linked networks:
non-affine bending deformations have to be considered instead of affine stretching
deformations. By comparing theoretical predictions of affine stretching models and
non-affine bending models with the elastic response of different actin networks it is
shown that the network microstructure sets the local deformation mode and with
that the static viscoelastic response of actin networks.
In a second part the dynamic properties of cross-linked actin networks are
adressed. The transient character of the cross-links and its impact on the vis-
coelastic frequency spectrum of cross-linked actin networks is analyzed. Ther-
mally activated cross-linker unbinding dominates the viscoelastic response at low
frequencies. Consistently, the cross-linker off-rate is identified as the pivotal time
scale that sets the frequency regime at which transient binding effects dictate
the viscoelastic spectrum. Based on this finding a simple semi-phenomenological
model is introduced which is predicated on single cross-link unbinding events and
successfully reproduces the frequency spectrum of transiently cross-linked actin
networks. Further experiments show that not only the cross-linker off-rate but the
whole microscopic interaction potential of an actin/ABP bond dictates the fre-
quency dependent viscoelastic response of transiently cross-linked actin networks.
Such an interaction potential is expected to be sensitive towards forces which act
on the actin/ABP bond. Therefore, the elastic behavior of transiently cross-linked
actin networks under mechanical load is investigated. A highly complex and tun-
able non-linear response is observed that is in sharp contrast to existing theoretical
predictions. It is demonstrated that forced cross-linker unbinding limits the sta-
bility of cross-linked actin networks. Surprisingly, even in complex actin bundle
networks the loading rate dependence of the rupture forces of actin/ABP bonds
follows the Bell prediction for a single molecular bond. This suggests that collec-
tive phenomena can be neglected. Furthermore, the non-linear behavior of actin
bundle networks reveals an equivalence of cross-linker density and force loading
rate. This observation is employed to establish a novel superposition principle
of cross-linker density and time which is then used to describe the self-similar
actin/fascin system with a master curve. Interestingly, such actin/fascin bundle
networks as well as bundle networks formed by α-actinin can be trapped in a
meta-stable state. Yet, the transient nature of cross-linking molecules allows for
an (at least partially) retarded equilibration of these kinetically arrested network
configurations which gives rise to very slow dynamics in these networks.
The physical principles and effects described in this thesis establish a micro-
scopic understanding of the structural organization and viscoelastic properties of
iireconstituted actin networks. These reconstituted networks constitute valuable
model systems of the actin cortex of living cells. Thus, the conclusions drawn in
this thesis may also help to gain a better understanding of complex cytoskeletal
networks in vivo.