Date of Award

2017

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Graduate Group

Bioengineering

First Advisor

Robert L. Mauck

Abstract

Our improved understanding of cell mechano-sensing has been enabled by developments in biomaterials platforms and in vitro culture systems. These emerging systems model key aspects of the cellular niche in a controlled fashion, enabling one to ask and answer questions that could not be addressed in vivo due to the complex interactions of these multiple discrete signals. These emerging materials include key features of the native microenvironment, including complex mechanical properties, degradable domains, and native-scale feature sizes. In this thesis work, we designed a 2D methacrylated hyaluronic acid hydrogel (MeHA) system to recapitulate the multiple adhesive interactions normally present in the developing mesenchymal progenitor cell niche, including an RGD adhesive motif (from fibronectin) and the HAVDI adhesive motif (from N-Cadherin). This system allowed for the decoupled presentation of cell-ECM interactions and cell-cell interactions while simultaneously modulating other variables present in the microenvironment, such as changes in substrate stiffness. Using this system, we elucidated a new signaling pathway through which HAVDI ligation altered MSC mechanosensing of ECM stiffness, which culminated in downstream differences in cellular proliferation and differentiation. Our findings suggest that HAVDI presentation may be harnessed towards novel biomaterial design to direct cellular behavior and allow for tuning of response to substrate stiffness in regenerative medicine applications. Building from this platform, we further investigated nuclear morphology of MSCs on these 2D MeHA substrates and in native tissue, and showed that nuclear morphology and mechanotransduction were markedly different on planar substrates compared to within the native tissue niche. These findings suggested that dimensionality and the manner by which nuclear pre-stress is established in 3D regulates strain transmission to the nucleus. To better recapitulate these interactions, we cultured cells in 3D MMP-degradable HA hydrogels, and showed that these remodelable systems more accurately recapitulated the in vivo cellular response in vitro. Moving forward, work with these defined synthetic 3D niches will play a critical role in understanding of complex processes governing stem cell development and differentiation, as well as disease pathologies that arise when these processes become disrupted.

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