טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
Ph.D Thesis
Ph.D StudentMaya Malik Garbi
SubjectScaling Behaviors in Steady-State Contractile Actomyosin
Network Flow
DepartmentDepartment of Physics
Supervisor Professor Keren Kinneret
Full Thesis textFull thesis text - English Version


Abstract

Contractile actomyosin networks with turnover have important roles in living cells, especially in large cells such as oocytes and early embryonic cells, where these networks are essential for processes such as chromosome congression, intracellular transport and spindle positioning. Despite their importance, experimental investigation of such networks has been hampered by the lack of suitable model systems. Here we harness the unique benefits provided by a reconstituted system based on Xenopus egg extracts encapsulated in water-in-oil droplets, that incorporates both myosin activity and physiological actin turnover rates, to investigate the properties of contractile 3D actin networks with turnover. Within minutes, the system reaches a dynamic steady-state characterized by a spherically-symmetric inward contractile flow. We measure the spatial distribution of the local contraction rate and the net network turnover rate at steady-state, allowing us for the first time to relate these properties with network structural organization. We modulate the composition of the system by introducing soluble actin nucleators, and auxiliary factors such as crosslinkers and disassembly factors. We find that under many conditions the networks exhibit homogenous, density-independent contraction. Furthermore, we find that the contraction rate scales with the actin turnover rate, such that their ratio remains nearly constant. This relation breaks down in the presence of excessive crosslinkers or branches, which lead to global changes in network structure and flow. Our findings serve as a basis for a theoretical model for the large-scale behavior of contractile networks with turnover. Within this model, local network contraction is shown to depend on the ratio between the active stress driving contraction and the effective network viscosity. Our results indicate that for a broad range of physiological conditions this ratio is independent of network density, implying that the active stress and the network viscosity scale similarly with density. Our findings suggest that cells use diverse biochemical mechanisms to generate robust, yet tunable, actin flows by regulating two parameters: turnover rate and network geometry.