The actin cortex is the mechanical engine that drives diverse modes of cellular shape change and facilitates vital physiological processes, such as cell polarization, migration, cytokinesis, wound healing, and morphogenesis. Unlike the highly organized architecture found in muscle sarcomeres, the cell cortex is a disordered actomyosin network that endows both cellular rigidity and fluidity through the maintenance of a stiff, yet highly dynamic, random meshwork below the plasma membrane. Despite its lack of sarcomeric organization, this semi-flexible polymer network is capable of generating unstable contractile flows in response to mechanical perturbations and biochemical signals. Recent work has identified F-actin severing under compression as the dominant mechanism that allows disordered systems to initiate contraction. In fact, the cell cortex has the ability to switch between stable and contractile network modes to drive a broad range of mechanical processes and serve as a stimulus-response system in cells. Reconstitution of actomyosin stable states in vitro have been largely unsuccessful due to the spontaneous contractile interactions that destabilize mixtures of these proteins. Thus, it remains unknown how the cortical actin cytoskeleton, whose primary components are inherently unstable in vitro, remains stable in the presence of myosin activity in vivo. Further, it is unclear how the cell cortex modulates the transition from stable to contractile states, and the influence of network organization on the magnitude and dynamics of contraction in unstable network regimes.
This work aims to address fundamental questions about cortical stability, contractility, and the transition between these regimes by using both agent-based simulations and highly controlled in vitro experiments within a minimal model of the actin cytoskeleton. We reconstitute our biomimetic model of the cell cortex from purified lipids and cytoskeletal proteins, and we engineer the system to facilitate spatiotemporal control of myosin II motor activity using light. The network-level origins of stability are investigated within our reconstituted system by characterizing its active nematic liquid crystalline properties, which reflect the semi-flexible nature of its polymer constituents, thus classifying it as a soft active nematic. This engineered material exhibits fluctuations with unconventional scaling exponents that deviate from the active gel model predictions. We use our light activation assay to spatially and temporally control myosin activity and show that non-muscle myosin II (NMM) is sufficient to generate highly cooperative contractions in uncrosslinked networks above a critical myosin density. Further, we find that the contraction velocity in disordered actomyosin systems exhibits the same telescopic scaling (velocity increases linearly with size) observed in muscle sarcomeres.
Finally, we use coarse-grained molecular dynamics simulations to characterize the switch-like transition between stable and contractile network states observed for NMM in our light activation assay. We introduce passive crosslinking proteins and catch-bond myosin motors to identify how the network properties modulate the cooperativity of the contractile response. Our results suggest that disordered actomyosin networks transition from stable to contractile states upon reaching a threshold level of network rigidity, whereas the network connectivity modulates the sensitivity of the contractile response. Interestingly, we find that poorly connected networks exhibit the most cooperative contraction, and increasing levels of connectivity diminish this cooperative behavior.
|Advisor:||Murrell, Michael P.|
|School Location:||United States -- Connecticut|
|Source:||DAI-B 80/07(E), Dissertation Abstracts International|
|Subjects:||Bioengineering, Biomechanics, Biophysics|
|Keywords:||Actin, Active Nematics, Contraction, Cytoskeleton, Mechanics, Myosin|
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