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Original title:
Mechanical Properties of Complex Cytoskeleton Networks 
Translated title:
Mechanische Eigenschaften von komplexen Zytoskelett Netzwerken 
Document type:
Fakultät für Physik 
Bausch, Andreas (Prof. Dr.) 
Bausch, Andreas (Prof. Dr.); Frey, Erwin (Prof. Dr.) 
Subject group:
PHY Physik 
actin; semiflexible polymers; molecular motors; cytoskeleton; deplition forces; HMM; actin-networks 
Translated keywords:
Aktin; semiflexible Polymere; Molekulare Motoren; Zytoskelett; Verarmungskräfte; HMM; Aktin-Netzwerke 
Controlled terms:
Zellskelett; Elastizität 
TUM classification:
PHY 824d 
The structure and mechanical response of all eukaryotic cells is essentially determined by cytoskeletal actin networks which are regulated by actin binding proteins (ABPs). The major biological relevance of understanding the mechanisms cells are using to control cytoskeleton is the reason for the huge interest to characterize the mechanical response of these composite network structures. Here, we show a study of the elasticity and microstructure of complex composite filamentous actin networks, using bulk rheology, microrheology, and imaging techniques. A study of in vitro active-HMM/actin networks is presented in chapter 8. At high HMM concentrations (ractin/HMM<40) the overall shape of the frequency dependence of the viscoelastic moduli G’ and G’’ changes and with increasing HMM concentration G’ and G’’ are also drastically decreasing. Active HMM molecules are able to reduce the plateau modulus G0 to around 70% of the value found for pure entangled actin. This intriguing effect is discussed with respect to recent theoretical predictions, which take into account that the activity of molecular motors increases the internal temperature. Additionally, another possible explanation is suggested: By means of the Euler buckling instability it is shown that active motors are able to suppress long transverse wave modes of fluctuating filaments when executing the powerstroke. As a result the resistance to thermal bending, which is described by the persistence length Lp, behaves stiffer. Therefore, the effective Lp is increased in active-HMM/actin networks and as a result the elastic response decreases. When ATP is completely consumed, all motor heads are strongly bound to actin and HMM molecules can act as crosslinks between actin filaments. Thus the network is in rigor state. This sol-gel transition is characterized in chapter 6. In chapter 7 we study rigor-HMM/actin networks which were found to form an isotropic, homogenous crosslinked network. The linear mechanical response of rigor-HMM/actin networks is described with a model that predicts an entropic origin of the network elasticity. Using this model it is implicitly assumed that an externally applied stress is transmitted in an affine way throughout the whole sample volume. This means that macroscopic elasticity can be ascribed to single filament properties. Besides these specific interactions, in cells where about 20%-40% of the volume consists of globular proteins depletion forces will be strong enough to severely alter the cytoskeletal structure and mechanics. In chapter 5 it is demonstrated that the addition of polyethylene glycol as a depletion agent results not only in severe structural changes, but also in alterations of mechanical properties of actin solutions. 
Translated abstract:
Die Struktur und die mechanischen Eigenschaften von allen eukaryontischen Zellen sind maßgeblich bestimmt durch das Zytoskelett. Wie diese Eigenschaften des Zytoskeletts in der Zelle kontrolliert werden ist von großem Interesse in der aktuellen Forschung. Die Arbeit umfasst eine Studie der Mikro-Struktur und der mechanischen Eigenschaften von Zell-Nachahmenden in vitro polymerisierten Aktin Netzwerken. Dabei wurde die Netzwerkstruktur mit Hilfe von verschiedenen mikroskopischen Techniken untersu...    »
Publication :
Universitätsbibliothek der Technischen Universität München 
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