The role of microtubules as mechanotransducers in smooth muscle cells.

The emerging fields of tissue engineering and regenerative medicine hold enormous promise to fulfill the increasing need to replace damaged or dysfunctional tissues and organs. Any successful effort to develop functional engineered tissues must consider the underlying fundamental cell biology in order to understand and manipulate the signals required for normal development and function. Mechanical signals from the extracellular matrix represent one critical class of signals that influence normal development and function for tissues that reside in a mechanically-dynamic environment in the body (e.g., blood vessels, muscle, cartilage, bone). The studies described in this thesis focus on one potential mechanism whereby changes in the mechanical microenvironment are transduced into changes in cell phenotype. Microtubules (MTs), one element of the cytoskeleton, may act as potential mechanotransducers that can alter their assembly and organization in response to externally-applied mechanical forces in smooth muscle cells. Experimental systems were developed in this thesis to test the hypothesis that mechanical forces influence MT assembly. Quantification of the cellular tubulin distribution between MTs and monomeric tubulin revealed that MT assembly is influenced by the duration, magnitude, and direction of externally-applied step-changes in strain. Further studies suggested that alteration of an intrinsic cell-ECM force balance can equivalently induce changes in MT assembly. These data are consistent with models in which MT assembly is, in part, controlled by forces imposed on these structures, suggesting that MTs may act as mechanosensors in SMCs by altering their assembly in response to an altered mechanical microenvironment. Finally, a connection between mechanical-strain induced alterations in MT assembly and a critical soluble signaling pathway mediated by the Rho-family GTP-binding proteins was investigated in this thesis. Alterations in the signaling activity of RhoA and Rac in response to single step-changes in applied strain were found to be dependent on the presence of a dynamic MT array. This last crucial finding in this thesis suggests one potential means by which mechanical signals that influence the state of MT assembly can be converted to a specific chemical signal known to regulate a wide variety of cellular functions, including cell adhesion, migration, spreading, growth, and differentiation.Ph.D.Applied SciencesBiological SciencesBiomedical engineeringCellular biologyChemical engineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/123880/2/3001035.pd