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Revealing Mechanisms of Morphological and Dynamical Transformation of the Actin Cytoskeleton
thesisposted on 24.06.2021, 03:58 by Wonyeong JungWonyeong Jung
Mechanical forces play a crucial role in cell functions. Cells can generate force, change their shape, and sense external mechanical stimuli, which allows diverse cell functions such as cell migration, cytokinesis, and morphogenesis. Mechanics of cells mainly come from the molecular interactions between actin filaments and diverse actin-binding proteins in the actin cytoskeleton. Actin-crosslinking proteins connect actin filaments to form actin structures; myosin, a molecular motor, generates force on the actin filaments. Due to the complex geometry of the actin structures in non-muscle cells, it has not been well understood how the actin cytoskeleton generates force and remodels itself. To better understand the molecular interactions, many in vitro studies employed a minimal system composed of actin filaments, actin-crosslinking proteins, and myosin motors. For example, myosin motility assays have been used to understand the self-organization and collective behavior of actin filaments, which enable the formation of diverse actin structures in cells. Reconstituted actomyosin networks have been used to understand myosin-induced contraction of the cell cortex, which allow cell shape change. A computational model can give additional information that is critical for understanding the mechanics of the cytoskeleton which in vitro assay cannot offer, such asthelocation or force of each molecule. Most of the previous models lack some mechanical details that could potentially be critical in the mechanics of the actin cytoskeleton. In this study, we used an agent-based computational model based on Brownian dynamics for simulating the motility assay and actomyosin network. The model describes the detailed mechanics and dynamics, thus enabling the investigation of previously unexplored aspects of cytoskeleton mechanics.
In the first study, we investigated how the properties of actin filaments and motors affect gliding motions and the self-organization of actin filaments on the motility assay. We found that the length of actin filaments, the average spacing between neighboring motors, and the processivity of motors regulate the gliding speed of actin filaments. We also demonstrated that cross-linking proteins could lead to contractile behaviors of actin networks on the motility assay.
In the second study, we showed that volume-exclusion effects between actin filaments can induce self-organization and collective motion of actin filaments. Bands and ring-like patterns were formed through self-organization; the patterns could be regulated by bending stiffness of filaments, actin concentration, and actin filament length.
In the third study, we sought to understand an alternative mechanism of contraction of actin networks by myosin motors. Previously, unbinding of cross-linkers and severing of actin-filaments by buckling have been identified as important regulators of actin network contraction. We investigated how F-actin fragmentation by stretching, which has not been studied but could potentially regulate contraction dynamics, contributes to the contraction of actin networks. In in vitro experiments, we observed that some actin filaments are indeed fragmented due to tensile forces. Using the computational model, we demonstrated that F-actin fragmentation is particularly important for the contraction of networks composed of long actin filaments with numerous motors, whereas cross-linker unbinding is more important for the contraction of networks with short actin filaments.
In the fourth study, we investigated how actin network remodeling by myosin activity regulates the motions of motors in turn. We demonstrated that myosin motions can be confined due to force generation or force transmission; we identified conditions where each of the two mechanisms is dominant. We also found that turnover of cross-linking proteins can trap motors and verified it in an in vitro experiment. On the other hand, turnover of actin filaments was shown to promote motor movement and inhibit confinement.
This study gives new biophysical insights into the self-organization, contraction, and transport in actin networks, which enable a more complete understanding of cellular processes regulated by the dynamics and mechanics of actin networks.