Project Summary/Abstract The prevalence of Alzheimer?s disease (AD) and other dementias has reached epidemic proportions. However, to date, not a single disease modifying therapeutic intervention has been developed. Furthermore, besides postmortem analysis of brain tissue, there are no consensus methods for AD diagnosis, especially in the early stages of the disease when preventing further cognitive decline is of utmost importance to patients and their families. It is highly likely that a more comprehensive understanding of the molecular underpinnings of AD will do much to remedy this unfortunate situation. Therefore, the research in this proposal aims to address this lack of understanding through the study of two intracellular events that have emerged as potential, major drivers of AD pathogenesis: dysregulated cyclin-dependent kinase 5 (Cdk5) activity and aberrant modification of intracellular proteins by O-linked ?-N-acetylglucosamine (O-GlcNAc). Cdk5 is known to play a role in regulating, among others, two central intracellular events in AD: hyperphosphorylation of microtubule-associated protein tau (tau) and amyloidogenic processing of amyloid precursor protein (APP). O-GlcNAc, on the other hand, is a dynamic, inducible post-translational modification of proteins thought to regulate key cellular signaling pathways in response to a wide variety of stimuli such as stress, nutrient availability, and neuronal activity. Both our lab and others have reported a striking decrease in the levels of O-GlcNAc transferase (OGT), the only known enzyme that catalyzes the addition of O-GlcNAc to proteins, and protein O-GlcNAc glycosylation (O- GlcNAcylation) in the brains of AD patients versus age-matched controls, implying that decreased levels of O- GlcNAcylation may also have an important role in the pathophysiology of AD in humans. Recently, we found that Cdk5 activity is increased in the absence of OGT and dynamically regulated by O-GlcNAcylation. Thus, there may also be a link between dysregulated Cdk5 activity and decreased O-GlcNAcylation in AD. On the basis of this work, we hypothesize that decreased Cdk5 O-GlcNAcylation increases Cdk5 activity and therefore has the potential to induce a neurodegenerative neuronal phenotype. Starting with Aim 1, we will explore how changes in Cdk5 O-GlcNAcylation affect cellular markers of neurodegeneration, such as tau and APP phosphorylation. In Aim 2, we will study the role of Cdk5 O-GlcNAcylation in synaptic plasticity with the goal of understanding how specific molecular events might lead to the memory deficits, disproportionate to neuronal loss, seen in AD. In summary, the studies proposed herein will provide critically needed insight into the molecular basis of AD, uncover a key link between two seemingly disparate cellular mediators of AD pathology (dysregulated Cdk5 activity and aberrant protein O-GlcNAcylation), and potentially inform the development of new diagnostic and treatment methods for AD.