O-GlcNAcylation and Hippocampal Synaptic Plasticity

描述(申请人提供)：在基础条件下，大部分葡萄糖通过糖酵解途径代谢，但2-4%通过氨基己糖生物合成途径(HBP)代谢。HBP修饰葡萄糖以产生O-连接的N-乙酰氨基葡萄糖(O-GlcNAc)部分，该部分可以在高度动态和可逆的反应中添加到蛋白质的丝氨酸/苏氨酸残基上。当葡萄糖水平过高时，通过HBP的流量增加，这可能导致O-GlcNacylated蛋白质的病理性增加。O-GlcNAc转移酶(OGT)和O-GlcNAcase分别负责将O-GlcNAc部分添加和去除为蛋白质中的丝氨酸/苏氨酸残基。海马神经元中O-GlcNAc转移酶(OGT)和O-GlcNAcase在前脑中表达最高。CA3-CA1突触效能的长期变化是海马依赖学习的基础。OGT和O-GlcNAcase在海马区的高表达表明，该脑区正常的突触功能受突触蛋白O-GlcNAc周转的调节。然而，关于O-GlcN酰化如何调节突触功能，我们知之甚少甚至一无所知。此外，存在一种可能性，即在突触蛋白上异常添加O-GlcNAc可能会干扰突触表达记忆处理所需的长期可塑性的能力，并可以解释已知的糖尿病动物模型中发生的海马突触功能和学习障碍，在糖尿病动物模型中，O-GlcN酰化在病理上是升高的。到目前为止，还没有研究在生理或病理条件下O-GlcN酰化对记忆形成的影响。在最近的研究中，我们发现OGT和O-GlcNAcase在张力上是活跃的，并双向调节基础突触传递的强度，这表明通过HBP的自然通量设定了回路中的兴奋性水平。此外，我们发现O-GlcN酰化的增加限制了突触表达正常LTP的能力，而对LTD没有影响。在这项研究中，我们将研究O-GlcN酰化增加引起的突触抑制的细胞和分子机制，并确定O-GlcN酰化的慢性增加是否会导致突触功能障碍和学习障碍。因此，成功证明O-GlcN酰化在调节突触传递和可塑性中的生理作用可能是学习和记忆领域的下一个重大发现。所获得的结果将开启一个新的研究领域，旨在了解高压氧引起的葡萄糖代谢波动如何在生理和病理条件下直接影响突触功能。

DESCRIPTION (provided by applicant): Under basal conditions, the majority of glucose is metabolized through the glycolytic pathway, but 2-4 % is metabolized 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 of proteins in a highly dynamic and reversible reaction. Flux through the HBP is increased when glucose levels are in excess, which can lead to a pathological increase of O-GlcNAcylated proteins. Two enzymes, O-GlcNAc transferase (OGT) and O-GlcNAcase are responsible for adding and removing, respectively, the O-GlcNAc moiety to serine/threonine residues in proteins. Hippocampal neurons have the highest expression of O- GlcNAc transferase (OGT) and O-GlcNAcase in forebrain. Long-term changes in the efficacy of CA3-CA1 synapses underlie hippocampal dependent learning. The high expression of OGT and O-GlcNAcase in hippocampus suggests that normal synaptic function in this brain region is modulated by O-GlcNAc turnover of synaptic proteins. However little to nothing is known regarding how O-GlcNAcylation modulates synaptic function. Furthermore, the possibility exists that abnormal addition of O-GlcNAc on synaptic proteins could interfere with the ability of synapses to express long-term plasticity required for memory processing and could explain deficits in hippocampal synaptic function and learning known to occur in animal models of diabetes, where O-GlcNAcylation is pathologically elevated. No study to date has investigated the effects of O-GlcNAcylation on memory formation, either under physiological or pathological conditions. In recent studies, 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 an increase in O-GlcNAcylation limits the ability of synapses to express normal LTP, with no effect on LTD. In this proposal we will investigate the cellular and molecular mechanisms mediating the synaptic depression induced by increased O-GlcNAcylation and determine whether chronic increases in O-GlcNAcylation causes synaptic dysfunction and learning deficits. 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.