HYPERPHOSPHORYLATION OF TAU BY GSK-3β IN ALZHEIMER'S DISEASE: THE INTERACTION OF Aβ AND SPHINGOLIPID MEDIATORS AS A THERAPEUTIC TARGET

Alzheimer's disease (AD) is a progressive neurodegenerative disorder characterized by the extracellular deposits of beta amyloid peptides (A beta) in senile plaques, and intracellular aggregates of hyperphosphorylated tau in neurofibrillary tangles (NFT). Although accumulation of A beta has been long considered a leading hypothesis in the disease pathology, it is increasingly evident that the role hyperphosphorylation of tau in destabilization of microtubule assembly and disturbance of axonal transport is equally detrimental in the neurodegenerative process. The main kinase involved in phosphorylation of tau is glycogen-synthase kinase 3-beta (GSK-3 beta). Intracellular accumulation of A beta also likely induces increase in hyperphosphorylated tau by a mechanism dependent on GSK-3 beta. In addition, A beta affects production of ceramides, the major sphingolipids in mammalian cells, by acting on sphingomyelinases, enzymes responsible for the catabolic formation of ceramides from the sphingomyelin. Generated ceramides in turn increase production of A beta by acting on beta-secretase, a key enzyme in the proteolytic processing of the amyloid precursor protein (APP), altogether leading to a ceramide-A beta-hyperphosphorylated tau cascade that ends in neuronal death. Modulators and inhibitors acting on members of this devastating cascade are considered as potential targets for AD therapy. There is still no adequate treatment for AD patients. Novel therapeutic strategies increasingly consider the combination of multiple targets and interactions among the key members of implicated molecular pathways. This review summarizes recent findings and therapeutic perspectives in the pathology and treatment of AD, with the emphasis on the interplay between hyperphosphorylated tau, amyloid beta, and sphingolipid mediators.