Mechanophenotyping Actin Networks in Minimal Cell Models

Abstract

Actin, a highly conserved and abundant protein, constructs a fibrous matrix via actin binding proteins (ABPs) equipping the cell to sustain, exert, and sense forces. Actin assembly plays an important role in directed cell migration during wound healing, cancer metastasis, immune response, and embryonic development. Although actin is a well-studied protein, owing to inherent complexity of the cell, significant gaps remain in understanding how different actin architectures transform, interact, and behave to endow the mechanical behaviors of the cell. In my thesis, I investigate how cells mechanically respond to the absence of native physical forces, how ABPs cooperate and compete to construct actin networks, and how different actin architectures dictate cellular mechanophenotypes. The bulk of my thesis work leverages bottom-up construction of minimal cell models to decouple actin networks from the complex cytoplasm milieu. In Chapter 3, we study mechanical response of human osteoblasts to simulated microgravity. Using a home-built random positioning machine to generate simulated microgravity, we find that actin networks become highly disorganized leading to less spread and more rounded cells. Furthermore, cells subjected to microgravity become significantly softer. These findings reveal that microgravity influences osteoblast cell mechanics through actin disassembly. In Chapter 4, we introduce bottom-up reconstitution of actin networks in a minimal cell to decouple actin networks from the rest of the cytoplasm. Here, we reverse engineer a minimal cell model using giant unilamellar vesicle (GUV) encapsulating actin networks. We study architectural phenotypes assembled by fascin and Arp2/3 complex. While fascin-bundled actin forms membrane protrusive structures, membrane-associated Arp2/3 complex assembles a uniform cortical dendritic shell. When co-encapsulated, we hypothesize that fascin and Arp2/3 cooperates/competes in a concentration dependent manner. Under this condition, we find that fascin-bundled membrane protrusions are reduced due to the branching effect of Arp2/3 complex that shortens filaments. Our results provide support that ABPs compete to generate diverse actin structures to meet the needs of a cell. In Chapter 5, I electrically deform different actin network-encapsulating GUVs to study differential cell mechanics. I discover that increasing concentrations of filamentous actin dampens GUV deformability. Furthermore, GUVs with alpha-actinin crosslinked actin networks and actin cortex both exhibit even larger dampening of electrodeformability. Our results highlight the significance of actin network architecture in governing cellular mechanics. In Chapter 6, I explore how actin cortex and bundles rearrange in response to loading exerted by micropipette aspiration. Interestingly, we find that protrusive actin bundles that are otherwise randomly oriented in a GUV lumen collapse and align along the axis of the micropipette. When uniform cortex-GUVs are aspirated, bleb-like cortex-free membrane is aspirated in the micropipette. These results reveal distinct responses in the rearrangement of actin networks subjected to physical forces. In summary, my dissertation characterizes actin network mechanics in cells and in minimal cell models and addresses how different ABPs cooperate and compete to assemble actin networks with architectures that in turns influence their mechanical behaviors and their responses to load. I believe that these findings improve our understanding of how precisely actin networks endow the mechanics of the cell using a low complexity cell-like environment.