Size sensitive rheology of reconstituted actin and vimentin filament networks
by Kaan Ürgüp
Date of Examination:2025-07-03
Date of issue:2025-08-29
Advisor:Prof. Dr. Sarah Köster
Referee:Prof. Dr. Sarah Köster
Referee:Prof. Dr. Andreas Janshoff
Persistent Address:
http://dx.doi.org/10.53846/goediss-11476
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Abstract
English
Intermediate filament (IF) networks provide mechanical resilience to cells. Understanding their viscoelastic properties—and crucially, their dependence on the length scale of deformation—is vital for deciphering physiological function. This study investigates the scale-dependent mechanics of vimentin IF networks across linear and nonlinear regimes, systematically contrasting them with actin networks, which are primarily responsible for facilitating cell movement. We integrate single-point (1P) and two-point (2P) passive microrheology with optical-tweezers-based microrheology (both passive and active) to systematically dissect size-dependent mechanics in in vitro protein networks. A new automated approach is developed to characterize probes on different length scales simultaneously and quantify the magnitude of these changes with regard to their statistical significance. A significant resulting contribution is that while employing polydisperse probes within identical samples, eliminating artifacts from sample preparation and enabling direct comparison of local versus bulk dynamics also using different-sized particle pair correlations. The shear modulus plateaus were shown to be dependent when probe size exceeds the network mesh size, as demonstrated in vimentin and confirmed in actin. This confirms that observed size effects reflect intrinsic network physics. Mesh sizes were also quantified for both systems. The work further establishes the scope of strain-relaxation dynamics measurements in pure vimentin. Due to its complex viscoelasticity, active-passive calibration was implemented prior. To probe network properties by separate means, we also introduced fluorescently labeled filaments and extracted key filament characteristics via imaging for quantitative model comparison. Actin networks was shown to be conforming to semiflexible polymer theory. Vimentin, however, displays unique complexity—its linear mechanics fit semiflexible predictions, but its nonlinear viscoelasticity necessitates empirical models (such as fractional Kelvin-Voigt). This reflects vimentin’s broader relaxation spectrum and molecular intricacies, potentially involving load-induced structural transitions. Critically, a strain-rate-dependent crossover in force relaxation was observed: deformations exceeding characteristic network timescales shift responses from exponential to power-law decay, possibly linking filament reorganization to emergent nonlinearity.
Keywords: microrheology; intermediate filaments; IF; vimentin; actin; optical tweezers; polydisperse 2P-correlations; active-passive calibration; multiple particle tracking
