Mechanical Regulation of Cell Death Public
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Valvular disease is the cause of over 300,000 heart valve replacement surgeries each year worldwide. Calcific aortic valvular disease (CAVD), the most common valvular pathology, results in the stiffening and mineralization of the aortic valve leaflets, which hinders proper opening and closing of the leaflets. Previously thought to be a passive disease, CAVD is now known to be an active process mediated by valvular cells. Currently, there are no therapies available for the reversal or prevention of CAVD; treatments mostly comprise invasive surgeries and complete replacement of the valve. The mechanisms that initiate and regulate this disease remain largely unknown. However, studies have shown that the regulation of cell tension and programmed cell death (apoptosis) by the extracellular mechanical environment may be an underlying cause. It is generally hypothesized that cells maintain homeostatic levels of internal tension, and when mechanical forces such as external loading or cell-cell contact adversely affect that tension state, cells react in attempt to reestablish homeostasis. If internal tension remains too high or low, apoptosis can occur, triggering disease progression. Therefore, understanding the fundamental mechanisms that regulate cell tension and apoptosis is a critical step toward developing therapeutic treatments for CAVD. This dissertation examines cell behavior in response to different mechanical stress stimuli. We first investigated how mechanical stress affects cell survival and cytoskeletal remodeling in single cells that are dynamically stretched. We found that cyclically stretching cells in low-stress environments facilitates cellular spreading and decreases in apoptosis. While studies of single cells allow for the isolation of specific cellular behaviors independent of cell-cell contacts, multicellular systems are more characteristic of in vivo environments. Therefore, we next identified specific mechanical stress parameters and mechanotransduction pathways that mediate cell tension and apoptosis in two-dimensional multicellular aggregates. We determined that cells in aggregates display regional differences in stress-associated biomarkers, with low stress in central regions and high stress in peripheral regions. Additionally, the mechanosensitive transcriptional cofactors, YAP and MRTF-A, localize to the nucleus in regions of high stress and help promote cell survival. Overall, we found that low-stress conditions, such as those found on soft substrates or in the central region of aggregates, initiate apoptosis, and cells can be rescued if intracellular tensional levels are restored to homeostatic levels by external cyclic stretch or cell-to-cell stress transfer. This work offers new insight into how mechanical stress regulates cell fate through mechanotransduction pathways.
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