| Summary: | Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Biological Engineering, September 2009. === "August 2009." Cataloged from PDF version of thesis. === Includes bibliographical references. === The primary motivation for this work is the manipulation of EGFR family signaling to influence regenerative responses of mesenchymal stem cells (MSC). Underlying the potential of regenerative medicine is the need to understand and control cell behavior. A 'cue, signal, response' paradigm has emerged as a framework for building predictive models for manipulation of cells to achieve desired responses. The HER receptor tyrosine kinase (RTK) family is an attractive target for manipulation of cues and signals, as its four members - epidermal growth factor receptor (EGFR or HERI), HER2, HER3 and HER4 - influence processes as diverse as development, wound healing, migration, and tissue homeostasis and family members are expressed by almost every cell type. All HER receptors require either homodimerization or heterodimerization with other family members for activation of signaling pathways, and the various dimer pairs are not equivalent in their ability to activate all the downstream pathways. Hence, signaling (and phenotypic) outcomes may be dictated not only by the number (or fraction) of each type of receptor ligated, but by the quantitative distribution of these receptors into various possible dimer pairs. The canonical physiological ligands for the HER family receptors are monomeric, allowing occupied receptors to freely homodimerize or heterodimerize. The premise of this work is that engineered bivalent ligands can drive specific dimerization events to enhance or inhibit signaling by various HER family receptors in a quantitative fashion that might be predicted on the basis of receptor expression. This work focuses on the design and implementation of engineered protein systems that are targeted to control homo and heterodimerization of HERI and HER3. One broad consequence of using homodimer ligands is to quantitatively force ligand-occupied HERI or HER3 to homodimerize and thus inhibit heterodimerization. Homodimerization may reinforce preferred signaling pathways (e.g, HERI-HERI vs HER1-HER2) - with implications for tissue regeneration and inhibit undesirable pathways (e.g. HER2-HER3) - with implications in cancer. Preliminary results suggest that whereas the monomeric HER3 ligand activates canonical signaling pathways expected from HER3-HER2 interactions, dimeric ligands inhibit signaling, presumably by forcing homodimerization of the kinase-inactive HER3 receptors. This thesis focuses on developing the design principles to use bivalent ligand dimers to control signaling, experimental testing of the hypothesis that signaling pathways can be controlled by such ligands and are quantitatively different than those for monovalent ligands, and demonstration of how such ligands influence proliferation of human marrow stromal cells, a cell type important for bone regeneration. In addition, the issue of practical implementation in a tissue engineering setting is addressed by implementing approaches to tether bivalent ligands to scaffolds in a manner that preserves signaling function. === by Luis M. Alvarez. === Ph.D.
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