Alzheimer's disease (AD) manifests both by the development of extracellular amyloid deposits (plaques) and intracellular aggregates known as neurofibrillary tangles (NFTs). The plaques result from accumulation of the insoluble peptide amyloid b (A2) that is proteolytically released from the amyloid b precursor protein (APP). The NFTs arise from an abnormally hyperphosphorylated form of the cytoskeletal protein tau that assembles into aggregated, paired helical filaments (PHFs). Accumulating evidence suggests that a synergistic interplay between Ab and hyperphosphorylated tau results in the full development of AD symptoms. It has emerged over the past number of years that the addition of O-linked N-acetylglucosamine units (O-GlcNAc) on intracellular proteins can regulate their activities and stabilities. Recently, one mechanism for regulating tau phosphorylation has been shown to involve addition of O-GlcNAc units. Tau O-GlcNAc levels in human AD brains are markedly lower than in normal brains and hyperphosphorylated tau bears little O- GlcNAc. These results, and others, demonstrate that tau O-GlcNAc levels and tau hyperphosphorylation are linked in a reciprocal manner. Maintenance of an appropriate balance between these two modifications may be critical to avoid the development of the pathogenic tau species that are associated with disease progression in AD. These observations suggest that blocking the removal of O-GlcNAc from tau will prevent hyperphosphorylation and, in turn, arrest the formation of tau aggregates. APP has also been shown to be O-GlcNAc modified, and preliminary reports indicate that O-GlcNAc levels affect APP processing. Accordingly, there is clear potential for pharmacological enhancement of O-GlcNAc levels of tau and, to a lesser extent, APP as a method to prevent the pathological processes of tau hyperphosphorylation and plaque formation. We have recently devised several potent and selective small-molecule inhibitors of O-GlcNAcase, the enzyme responsible for removal of O-GlcNAc from proteins. We have also demonstrated that oral dosing with several of these inhibitors dramatically lowers tau phosphorylation levels in rats.
We aim to establish proof-of- principle that pharmacologic blockade of O-GlcNAcase prevents tau hyperphosphorylation and thereby blocks the development of AD-like symptoms. This experiment will be carried out in vivo using an animal model of AD. Transgenic TAPP mice (containing mutant forms of human tau and APP) will be dosed orally with inhibitor for 36 weeks and the effects of treatment will be assessed using Western blot analyses, immunocytochemistry, and histology. Motor skills and cognitive performance will be assessed using behavioral tests from week 12 onward. Mice will be sacrificed at weeks 14, 24, and 36, and tested for levels of tau phosphorylation, NFT formation, and Ab deposition. Results for the dosed groups will be compared to those for control groups, receiving vehicle alone, to assess the effects of inhibitor on disease progression.
The defining features of Alzheimer's disease are the formation of pathogenic forms of tau protein and deposition of amyloid peptide in the brain. The proposed work will test an inhibitor of an enzyme involved in the regulation of these proteins in transgenic mice;mice that develop symptoms similar to Alzheimer's disease. If the mice receiving inhibitor show delayed disease progression, this will validate this enzyme as a drug target and may lead to a new and more effective class of drugs for treatment of Alzheimer's disease.
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