Date of Award

2014

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Graduate Group

Chemical and Biomolecular Engineering

First Advisor

Dennis E. Discher

Abstract

Whether a stem cell remains or egresses away from its physiological niche is a function of mechanical and soluble factors in a time-dependent manner, which implicates a `memory' of prior mechanochemical conditioning. Virtually every organ in the body contains resident stem or progenitor cells that contribute to organ homeostasis or repair. The wound healing process in higher vertebrate animals is spatiotemporally complex and usually leads to scarring. Limitations for the use of stem cells as regenerative therapy include the lack of expansion capabilities in vitro as well as materials issues that complicate traditional biochemical protocols. A minimal `scar in a dish' model is developed to clarify the kinetics of tension-sensitive proteins in mesenchymal stem cells (MSCs), which possess plasticity to mechanochemical changes of the microenvironment that are typical of scars. The organization and expression of such proteins implicates transcription factors that ultimately steer cell fate. In contrast to classic mechano-transducers of matrix mechanics such as actin assembly-dependent serum response factor (SRF) signaling, a novel mechano-repressive role of NKX2.5 is implicated in maintaining intracellular tension in long-term stem cell cultures on stiff matrices via nucleo-cytoplasmic shuttling — ultimately setting up a 'mechanical memory'. Core gene circuits with known roles in stem cell mechanobiology are modeled based on the 'use it or lose it' concept: tension inhibits turnover of structural proteins such as extracellular collagens, cytoskeletal myosins and nucleoskeletal lamins. This theoretical approach is tested in a variety of processes in vitro and in vivo that involve forces including cardiac development, osteogenic commitment of MSCs, and fibrosis therapy. With the sophistication of the science and technology of biomaterials relevant to stem cell biology and medicine, matrix mechanics can thus be rigorously combined with biochemical instructions in order to maximize therapeutic utility of stem cells.

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