P3‐132: Transcriptional Dysregulation in Alzheimer's disease

Alzheimer's disease (AD) is the most prevalent form of dementia in the elderly and is characterized by the presence of two characteristic pathological hallmarks: senile plaques and neurofibrillary tangles (NFTs). Senile plaques are composed of b-amyloid (Aß) peptides derived from sequential cleavage of the amyloid precursor protein. NFTs are composed mostly of abnormally hyperphosphorylated microtubule-associated protein tau. Egr-1, a zinc finger transcription factor regulates the transcription of a number of genes involved in immune response, differentiation, growth and development and plays a central role in the induction and maintenance of various vascular pathologies. The basal level of Egr-1 protein in the brainis relatively low, but it is rapidly and transiently induced by a large number of growth factors, cytokines, neurotransmitters, brain injury, ischemia, hypoxia and other stressors. Once induced, Egr-1 activates the expression of a number of genes including those involved in cell death and injury. Recent studies have shown that Egr-1 is up regulated in AD brain. The pathophysiological significance of this up regulation is not known. We have used rats and primary neurons in culture over expressiong Egr-1, Egr-1 -/- mice, PC12 cells exposed to NGF and in vitro kinases assays. We show that lentivirus-mediated over expression of Egr-1 in rat brain hippocampus and primary neurons in culture activates proline-directed kinaseCdk5, inactivates PP1, promotes tau phosphorylation at both proline-directed S396/404 and non-proline-directed S262 sites, and destabilizes microtubules. Furthermore, in Egr-1-/- mouse brain, Cdk5 activity is decreased, PP1 activity is increased and tau phosphorylation is reduced at both proline-directed and non-proline-directed sites. By using NGF-exposed PC12 cells, we determined that Egr-1 activates Cdk5 to promote phosphorylation of tau and inactivates PP1via phosphorylation. When Cdk5 is inhibited, tau phosphorylation at both proline- and non-proline directed sites as well as PP1 phosphorylation are blocked, indicating that Egr-1 acts through Cdk5. By using an in vitro kinase assay and HEK-293 cells transfected with tau, PP1, and Cdk5, we show that Cdk5 phosphorylates S396/404directly. In addition, by phosphorylating and inactivating PP1, Cdk5 promotestau phosphorylation at S262 indirectly. Our results indicate that Egr-1 is an in vivo regulator of tauphosphorylation and suggest that in AD brain, increased levels of Egr-1aberrantly activates an Egr-1/Cdk5/PP1 pathway leading to accumulation of hyperphosphorylated tau, thus destabilizing the microtubule cytoskeleton.