Diabetes affects over 20 million people in the US alone. Previous studies show that abnormal insulin and plasma glucose levels in diabetes leads to synaptic dysfunction and cognitive impairment;as such diabetics have an increased risk of Alzheimer's disease and vascular dementia. Under basal conditions, glucose is processed through the glycolytic pathway;while 2-4% is process via the hexosamine biosynthetic pathway (HBP). The HBP modifies glucose to produce an O-linked N-acetylglucosamine (O-GlcNAc) moiety that can be added to serine/threonine residues. Flux through the HBP is increased when glucose levels are in excess, which leading to a pathologically increase amount of proteins with an O-linked glycosylation tag. Hippocampal neurons have the highest expression of O-GlcNAc transferase (OGT) and O-GlcNAcase in forebrain. These two enzymes are responsible for adding and removing, respectively, the O-GlcNAc moiety to serine/threonine residues in proteins. Thus, the high expression of OGT and O-GlcNAcase in hippocampus suggests that normal synaptic function in this brain region is modulated by GlcNAc turnover of synaptic proteins. Despite this biochemical information and known cognitive deficits in diabetes where O-GlcNAcylation is increased, no study to date has investigated how O-GlcNAcylation impacts synaptic function required for normal learning and memory. Long-term changes in function of CA3-CA1 synapses underlie hippocampal dependent learning. The possibility exists that abnormal addition of O-GlcNAc on synaptic proteins could interfere with the ability of synapses to express LTP and LTD required for memory processing. In fact, this mechanism could explain deficits in synaptic function in animal models of diabetes. In preliminary experiments, we find that OGT and O-GlcNAcase are tonically active and bidirectionally modulate the strength of basal synaptic transmission, suggesting the natural flux through the HBP sets the level of excitability in the circuit. Furthermore, we find that strongly stimulation of the HBP prevents expression of LTP. Finally, we find that in two diabetic animal models, O-GlcNAcylation is significantly increased, consistent with the notion that abnormal addition of O-GlcNAc on synaptic proteins is causal to deficits in LTP and learning in animal models of diabetes. In this proposal, we will test the hypothesis that O-GlcNAcylation modulates synaptic function at hippocampal CA3-CA1 synapses normally and 2) that chronic increases in O-GlcNAc modification of synaptic proteins in animal models of diabetes interferes with normal synaptic function and learning. Thus, the successful demonstration of a physiological role of O-GlcNAcylation in modulating synaptic transmission and plasticity could be the next major discovery in the field of learning and memory. The results obtained will launch a new area of investigation aimed at understanding how fluctuations in glucose metabolism by the HBP can directly affect synaptic function in physiological and pathological conditions.
These proposed studies are a new area of investigation aimed at understanding how alterations in glucose metabolism can directly affect synaptic function via O-linked glycosylation in physiological and pathological conditions. The results of these studies could provide a mechanistic understanding of deficits in synaptic plasticity and learning, consequences of diabetes and Alzheimer's disease, where O-linked glycosylation is pathologically increased and decreased, respectively.
| Taylor, Erica W; Wang, Kai; Nelson, Amy R et al. (2014) O-GlcNAcylation of AMPA receptor GluA2 is associated with a novel form of long-term depression at hippocampal synapses. J Neurosci 34:10-21 |
