Investigating the Role of O-GlcNAc Glycosylation in Neurodegeneration

O-GlcNAc glycosylation of nuclear and cytosolic proteins is an essential post-translational modification implicated in many diseases, from cancer to diabetes. Importantly, many important neuronal proteins are also O-GlcNAc modified, and aberrant O-GlcNAcylation of these proteins may contribute to the pathology of neurodegenerative diseases although these mechanisms have not been well defined. Here we investigated the role of O-GlcNAc glycosylation in the brain, utilizing both chemistry and molecular biology to study O-GlcNAc transferase (OGT), the enzyme that adds the sugar modification. To evaluate the role of OGT in adult neurons, we generated a forebrain-specific conditional knockout of OGT (OGT cKO) in mice. Although indistinguishable from wild-type littermates at birth, after three weeks we observe progressive neurodegeneration in OGT cKO mice. Hallmarks of Alzheimer’s disease, including neuronal loss, neuroinflammation, behavioral deficits, hyperphosphorylated tau, and amyloid beta peptide accumulation, are observed. Furthermore, decreases in OGT protein levels were found in human AD brain tissue, suggesting that altered O-GlcNAcylation likely contributes to neurodegenerative diseases in humans. This model is one of a few mouse models that recapitulate AD phenotypes without mutating and overexpressing human tau, amyloid precursor protein, or presenilin, highlighting the essential role of OGT in neurodegenerative pathways.

Given the importance of OGT in the brain, we further investigated the regulation of the OGT enzyme by phosphorylation. We found that phosphorylation of OGT near its C-terminus reduces its activity in cancer cells, and have developed phosphorylation-specific antibodies to aid mechanistic studies. Furthermore, mutation of this phosphorylation site on OGT, followed by overexpression in neurons was shown to enhance neurite outgrowth, demonstrating a functional consequence for this site. Thus phosphorylation of OGT inhibits its activity and enhances neurite outgrowth, and current studies aim to characterize the signaling pathway that regulates OGT phosphorylation in neurons.