#7. Protein Modification: Tissue transglutaminase (tTG), an atypical member of the transglutaminase protein family, is a calcium-activated transamidating enzyme that modifies proteins by catalyzing the incorporation of polyamines, deamination or crosslinking. In addition, tTG binds and hydrolyzes GTP and has been proposed to act as a signal transducing G-protein. tTG is upregulated in several neurodegenerative diseases, including Alzheimer's disease, and it has been proposed that this is detrimental to cell survival because tTG can be pro-apoptotic. However, tTG can also be anti-apoptotic in neuronal cells depending on the stressor and further, we have clearly demonstrated that tTG can facilitate neurite outgrowth. These data demonstrate that tTG likely has multiple functions in the cell, and has the capacity to facilitate neuronal survival and function. Therefore, it can be proposed that the initial increase in tTG levels in neurodegenerative conditions may be a compensatory response and be beneficial to neuronal survival. We have recently found that tTG greatly enhances cAMP production resulting in phosphorylation and activation of the transcription factor cAMP-responsive element binding protein (CREB); an event that can support neuronal survival and plasticity. Further, we have demonstrated that the ability of tTG to facilitate cAMP production is dependent in part on its transamidating activity. These are very exciting findings, as they provide a possible mechanism by which tTG facilitate both neurite outgrowth and survival. Considering these and other data our overall hypothesis is that tTG directly modulates the activity of adenylyl cyclase, resulting in enhanced cAMP production and thus supports neuronal survival and plasticity.
The specific aims of this application are to test the hypotheses that: (1) modulation of cAMP production by tTG requires both its transamidating and GTPase activity, and (2) tTG regulates cAMP production by interacting with and/or modifying specific components of the cAMP signaling pathway. The results of these studies will provide important new information on the function of tTG in neurons and provide the basis for an R01 application.
