Tau Proteolysis: A Piece to the Alzheimer's Puzzle

          Alzheimer’s disease is a progressive, neurodegenerative disorder affecting millions of older Americans. The disease is characterized by continued mental deterioration, finally resulting in profound dementia. At present, there is no defined cause or cure of Alzheimer’s disease. One of the characteristic changes in the brains of Alzheimer’s disease patients is the development of abnormal structures within nerve cells, called neurofibrillary tangles (NFfs). These NFTs appear to be composed of proteins that occur normally within nerve cells, however there is evidence to sugges t that the proteins which form the NFTs have been abnormally modified, and this contributes to their pathological accumulation. This aberrant accumulation of proteins and the development of NFTs appears to be a contributing factor in the eventual death of the nerve cell.                     One of the proteins that is a major component of NFTs is called tau. Tau is a group of closely related , structural proteins that play a role in the normal form and function of nerve cells. In Alzheimer’s disease tau is present, however it appears to be modified by being abnormally phosphorylated. It has been suggested that this abnormally phosphorylated tau may be more resistant to degradation by enzymes that breakdown proteins (proteases) and that this may be one of the reasons that tau accumulates and forms NFTs. Previous work from our laboratory has indicated that if “normal” tau is phosphorylated in a specific way, it becomes more resistant to a certain brain protease, calpain, indicating that the abnormal phosphorylation of tau may contribute to the formation of NFTs.                 The first hypothesis to be tested is that the tau isoforms are degraded differently by the protease, cal pain. This goal will be accomplished by studying the degradation of the individual human tau isoforms isolated from clones.                   The second hypothesis to be tested i s that site-specific phosphorylation of tau alters its susceptibility to degradation by calpain. This will be accomplished by completing three specific aims using both tau and the individual human tau isoforms.                      1. Phosphorylating tau b y three different enzymes that put phosphate groups on proteins (kinases) and then determining if the phosphorylated tau is hydrolyzed differently by calpain when compared with control tau.                      2. Identification of the specific sites on tau that are phosphorylated by the different kinases.                      3. Determining if removing phosphates from tau alters hydrolysis of tau.                      The third hypothesis to be tested is that calmodulin-binding (a calcium-dependent modulatory protein) alters the degradation of tau and the individual isoforms by calpain. This will be accomplished by completing the following specific aims.                      1 . Determining if tau and the individual isoforms bind calmodulin.                      2. Determining if the binding of calmodulin to tau and the recombinant isoforms alters calpain-induced hydrolysis.                      3. Identifying the calmodulin binding domains on tau responsible for alterations in calpain-mediated proteolysis.                  The long term goals of the project are to determine the factors involved in altering the hydrolysis of tau, and if differences exist between the tau isoforms in the way they are processed and metabolized. These goals are important because they are likely to reveal the functional significance of the tau isoforms and provide insight into the mechanisms underlying the formation of NFTs and other neuropathological structures in Alzheimer’s disease.