Differentiation induces dynamic alterations in mesenchymal stem cell nuclear architecture and mechanotransduction

Mesenchymal stem cells (MSCs) are a promising cell source and widely used in a variety of regenerative applications given their multipotent nature. MSCs are subjected to various types of mechanical forces during tissue development and repair, and it is clear that, along with soluble factors, physiological forces play an important role in determining their lineage specification. However, the molecular mechanisms by which external mechanical stimuli are converted to a biological response remain unclear, and few studies have been performed to probe alterations in cell and nuclear architecture in response to physiological loading. In this thesis, we investigated relationships between MSC cellular/nuclear biophysical properties and mechanosensitivity, and determined their importance in MSC mechanotransduction. Our findings demonstrate that MSC differentiation mediated by either a soluble factor, TGF-?3 or resulting from dynamic tensile loading (DL) is accompanied by reorganization of nuclear structural elements (i.e. lamin A/C and chromatin). These changes increased nuclear mechanical properties, resulting in changes tto he manner in which MSCs respond to external mechanical perturbation. In addition, through a series of micromechanical experiments, the molecular mechanisms by which nuclear structure was altered as a consequence of load-induced MSC differentiation were elucidated. DL resulted in a rapid increase in chromatin condensation in MSCs, which depended on the activity of the histone-lysine N-methyltransferase EZH2. The ATP/purinergic signaling was a key regulator of this load induced chromatin condensation, and was mediated by acto-myosin cellular contractility. In follow on studies, we demonstrated that chromatin condensation in MSCs was regulated by interplay between purinergic signaling and RhoA/Rock activity, and that baseline TGF superfamily signaling played a role in establishing cell contractility and mediating this load-induced chromatin remodeling response. Overall, this thesis identified novel signaling pathways and mechanisms that regulate the mechanical properties of the nucleus in progenitor cells as they transition towards a differentiated state, and elucidated how dynamic loading regulates chromatin condensation to increase mesenchymal stem cell (MSC) nuclear mechanics in the absence of exogenous differentiation factors. This work has broad implications in the field of mesenchymal stem cell biology and mechanobiology, and will inform the development of engineered tissues, medical devices, and biological materials for tissue repair and regeneration