Increased O-GlcNAcylation rapidly decreases GABAAR currents in hippocampus but depresses neuronal output

Abstract

O-GlcNAcylation, a post-translational modification involving O-linkage of β-N-acetylglucosamine to Ser/Thr residues on target proteins, is increasingly recognized as a critical regulator of synaptic function. Enzymes that catalyze O-GlcNAcylation are found at both presynaptic and postsynaptic sites, and O-GlcNAcylated proteins localize to synaptosomes. An acute increase in O-GlcNAcylation can affect neuronal communication by inducing long-term depression (LTD) of excitatory transmission at hippocampal CA3-CA1 synapses, as well as suppressing hyperexcitable circuits in vitro and in vivo. Despite these findings, to date, no studies have directly examined how O-GlcNAcylation modulates the efficacy of inhibitory neurotransmission. Here we show an acute increase in O-GlcNAc dampens GABAergic currents onto principal cells in rodent hippocampus likely through a postsynaptic mechanism, and has a variable effect on the excitation/inhibition balance. The overall effect of increased O-GlcNAc is reduced synaptically-driven spike probability via synaptic depression and decreased intrinsic excitability. Our results position O-GlcNAcylation as a novel regulator of the overall excitation/inhibition balance and neuronal output. Synaptic integration and spike initiation in neurons is controlled by synaptic inhibition, which strongly influences neuronal output and information processing 1. Importantly, the balance of excitation to inhibition (E/I) is crucial to the proper functioning of circuits, and E/I imbalances have been implicated in a number of neurodevelopmental disorders and neurodegenerative diseases including schizophrenia, autism spectrum disorders, and Alzheimer's disease 2-5. Thus, understanding the mechanisms that modulate the strength of inhibitory transmission is fundamental to unraveling how neuronal circuits function in normal and disease states. Fast inhibitory transmission in the central nervous system is mediated by GABA A receptors (GABA A Rs), which are pentameric ligand-gated ion channels. The strength of this inhibition can be rapidly up-or down-regulated by post-translational modifications including ubiquitination 6 , palmitoylation 7,8 , and phosphorylation 9 of GABA A R subunits and/or associated proteins, which can alter channel function, trafficking, or stability at the membrane. While there is a vast body of literature on these post-translational modifications, O-GlcNAcylation, involving the O-linkage of β-N-acetylglucosamine (O-GlcNAc) to Ser/Thr residues on target proteins, remains severely understudied, with no examination to date of the effect of protein O-GlcNAcylation on inhibitory synapse physiology. Addition and removal of O-GlcNAc are catalyzed by the single pair of enzymes O-GlcNAc transferase (OGT) and O-GlcNAcase (OGA). O-GlcNAcylation is essential, as genetic deletion of OGT and OGA are lethal 10,11. O-GlcNAcylation is metabolically-regulated and highly dynamic; global changes reversibly occur within minutes, and are dictated by availability of UDP-GlcNAc, which is synthesized from glucos...

Daily Vinegar Ingestion Improves Depression Scores and Alters the Metabolome in Healthy Adults: A Randomized Controlled Trial

“…An acute increase in N-acetylglucosamine glycosylation can affect neuronal communication by inducing long-term depression of excitatory transmission at hippocampal synapses, as well as suppressing hyperexcitable circuits in vivo [36,37]. In addition, N-acetylglucosamine glycosylation reduces action potential probability in pyramidal cells [36]. These findings from previous trials may help explain the improvements in depressive symptoms noted in the present trial.…”

“…Although unexplored, this is a possible mechanism for the favorable effects of liquid vinegar ingestion on insulin sensitivity noted in human trials. Moreover, and pertinent to the present trial, the brain contains some of the highest levels of N-acetylglucosamine glycated proteins in the body, with the hippocampus expressing high levels of N-acetylglucosamine transferase and N-acetylglucosaminase [36]. An acute increase in N-acetylglucosamine glycosylation can affect neuronal communication by inducing long-term depression of excitatory transmission at hippocampal synapses, as well as suppressing hyperexcitable circuits in vivo [36,37].…”

“…Moreover, and pertinent to the present trial, the brain contains some of the highest levels of N-acetylglucosamine glycated proteins in the body, with the hippocampus expressing high levels of N-acetylglucosamine transferase and N-acetylglucosaminase [36]. An acute increase in N-acetylglucosamine glycosylation can affect neuronal communication by inducing long-term depression of excitatory transmission at hippocampal synapses, as well as suppressing hyperexcitable circuits in vivo [36,37]. In addition, N-acetylglucosamine glycosylation reduces action potential probability in pyramidal cells [36].…”

Acute inhibition of OGA sex-dependently alters the networks associated with bioenergetics, autophagy, and neurodegeneration

“…Increased autophagic flux has also been reported with TG administration in drinking water at 500 mg/kg/d for 2 weeks to RFP-GFP-LC3 mice [24]. In addition to apparently divergent effects on neuronal autophagy, an acute increase in O-GlcNAcylation levels in vivo impairs learning and memory [25] and it has been reported that protein O-GlcNAcylation is a regulator of neuronal excitability [25][26][27]. In other excitable cells, such as cardiomyocytes, increases in mitochondrial O-GlcNAc levels have been reported to increase oxidative phosphorylation as well as lead to impaired mitochondrial function [28][29][30].…”

“…To date, the majority of the studies related to the effects of increased O-GlcNAc in the brain have focused on chronic disease models or prolonged treatment with OGA inhibition, despite the fact that as discussed above short-term treatment with OGA inhibitors can have acute neurological effects [25][26][27]. We have recently shown that only 3 h after a single treatment with the OGA inhibitor thiamet G there was marked increases in O-GlcNAcylation (> 1.5-fold) of 65 proteins [35].…”