Tau is modified by tissue transglutaminase in situ: Possible functional and metabolic effects of polyamination

Tissue transglutaminase (tTG) is up-regulated in Alzheimer's disease brain and localizes to neurofibrillary tangles with the tau protein. Tau is an in vitro tTG substrate, being cross-linked and/or polyaminated. Further, the Gin and Lys residues in tau that are modified by tTG in vitro are located p rimarily within or adjacent to the microtubule-binding domains. Considering these and other previous findings, this study was carried out to determine if tau is modified in situ by tTG in human neuroblastoma SH-SY5Y cells, an d whether tTG-catalyzed tau polyamination modulates the function and/or met abolism of tau in vitro. For these studies, SH-SY5Y cells stably overexpres sing tTG were used, tTG coimmunoprecipitated with tau, and elevating intrac ellular calcium levels with maitotoxin resulted in a 52 +/- 4% increase in the amount of tTG that coimmunoprecipitated with tau. The increase in assoc iation of tTG with tau after treatment with maitotoxin corresponded to a co immunolocalization of tTG, tTG activity, and tau in the cells. Further, tau was modified by tTG in situ in response to maitotoxin treatment. In vitro polyaminated tau was significantly less susceptible to mu-calpain proteolys is; however, tTG-mediated polyamination of tau did not significantly alter the microtubule-binding capacity of tau. Thus, tau interacts with and is mo dified by tTG in situ, and modification of tau by tTG alters its metabolism . These data indicate that tau is likely to be modified physiologically and pathophysiologically by tTG, and tTG may play a role in Alzheimer's diseas e.