Tissue transglutaminase (tTG) is a highly regulated, calcium-dependent enzyme that plays a key role in several critical processes in the nervous system. The primary function of tTG is to catalyze a transamidating reaction. This transamidation reaction results in the incorporation of polyamines into specific protein-bound glutamine residues, or if a polypeptide bound lysine is the amine donor, the tTG-catalyzed reaction results in the formation of protein crosslink between the polypeptide bound glutamine and lysine residues. tTG is predominantly a cytosolic protein, however we have shown that it also localizes to the nucleus. Further, we have data indicating that the activity state of tTG regulates its localization within the cell. We have shown that increased tTG expression induces neurite outgrowth, and our preliminary studies indicate that tTG plays a facilitory role in modulating transcriptional events. These and other data clearly indicate that tTG is of critical importance in neuronal development and plasticity. In addition, we have also demonstrated that tTG is elevated in Huntington's disease (HD) brain. Further, in a cell culture model system, tTG selectively polyaminates proteins that associate with mutant huntingtin (which contains an expanded polyglutamine domain), but not huntingtin itself. These and other findings suggest that tTG may play a role in HD. The focus of this proposal, which is a competing continuation, is on investigating the role of tTG in physiological (neurite outgrowth) and pathological (HD) conditions, and the interrelationships between these two roles. Our overall working hypothesis is that tTG plays a fundamental role in human neural systems by modulating transcriptional events and the function of specific structural proteins in a concerted manner. Further, we hypothesize that tTG may modulate the effects of mutant huntingtin on transcriptional processes in HD brain. The specific goals of this proposal are to test the hypotheses that: (1) upregulation and nuclear localization of active tTG are necessary to induce neurite outgrowth in human neuroblastoma cells, (2) tTG modulates the activity of specific transcription factors, and that specific polyglutamine-rich transcription factors or regulators are substrates of tTG, (3) the suppression of specific transcriptional processes by mutant huntingtin is either directly or indirectly, modulated by increased tTG expression and (4) specific transcriptional events in immortalized striatal cells from wild type and HdhQ111 mice are differentially modulated by tTG. These studies will increase significantly our understanding of the function of tTG, and are likely to provide insight into the role of tTG in HD.
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