Elevated transglutaminase-induced bonds in PHF tau in Alzheimer's disease.

Transglutaminase-induced epsilon-(gamma-glutamyl)lysine bonds covalently cross-link and polymerize peptides into insoluble high molecular weight protein aggregates resistant to degradation and proteolytic digestion. We investigated the hypothesis that excessive deposition of epsilon-(gamma-glutamyl)lysine bonds is a neuropathological mechanism which induces the polymerization of tau protein into stable aggregates leading to the formation of paired helical filaments (PHFs) which deposit into neurofibrillary tangles in Alzheimer's disease (AD) brain. We demonstrate a significant (45%) elevation in epsilon-(gamma-glutamyl)lysine cross-links in AD cortex as compared to control cortex. In vivo, PHF tau, and high and medium molecular weight neurofilament proteins have significantly greater cross-linking by epsilon-(gamma-glutamyl)lysine bonds in AD brains as compared to controls. The cross-linking of PHF tau occurs both intra-molecularly and inter-molecularly. The inter-molecular cross-linking of tau could account for the formation of high molecular weight tau polymers. These results suggest that transglutaminase-induced cross-linking of tau protein could play a role in the formation and stabilization of neurofibrillary tangles. Inhibition of transglutaminase-induced cross-linking may therefore, provide a novel strategy for the treatment of AD.

Tau in aluminum-induced neurofibrillary tangles.

Aluminum is a neurotoxin and in susceptible species induces a neurofibrillary pathology characterized by argentophilic masses in neuronal perikarya and in axonal spheroids. These inclusions are known to contain neurofilament proteins. Using immunocytochemistry and immunoblotting, we demonstrate that tau is a component of these aluminum-induced neurofibrillary tangles (Al-NFTs) in rabbits. Double-label immunocytochemistry experiments reveal co-localization of phosphorylated neurofilaments (using SMI31) and tau (using tau-1, tau-5, AT8, and PHF-1) in the perikaryal Al-NFTs. Non-phosphorylated tau (detected using tau-1) occupies a smaller area of the Al-NFT than the total pool of tau proteins (detected using tau-5). The area of total tau and non-phosphorylated tau immunolabeling in the Al-NFT increases as the size of the Al-NFT (i.e., the proportion of cell area occupied by the Al-NFT) increases. The proportion of cell area (outside of the Al-NFT) occupied by tau (as indicated by tau-5) decreases as the area of tau in the Al-NFT increases and as the size of the Al-NFT in the cell increases. Immunoblotting experiments demonstrate 1) the specificity of the tau antibody labeling and verify a lack of cross-reactivity of the tau-5 antibody to neurofilament proteins in rabbit tissue; and 2) no alterations in the levels of tau resulting from aluminum-treatment. These data suggest that as the size of the Al-NFT in a cell increases there is less tau in the neuronal perikarya. Therefore, there may be less tau in the perikarya available to perform normal functions such as microtubule polymerization and stabilization. Tau and neurofilament proteins are perturbed in a number of neurodegenerative disorders such as Alzheimer's disease, diffuse Lewy body disease, and Parkinson's disease. Aluminum-induced neurofibrillary pathology may provide a model to study perturbation in tau and neurofilaments, their phosphorylation and deposition into pathological inclusions.