Proteomic identification of altered protein O-GlcNAcylation in a triple transgenic mouse model of Alzheimer's disease

Abstract

PET scan analysis demonstrated the early reduction of cerebral glucose metabolism in Alzheimer disease (AD) patients that can make neurons vulnerable to damage via the alteration of the hexosamine biosynthetic pathway (HBP). Defective HBP leads to flawed protein O-GlcNAcylation coupled, by a mutual inverse relationship, with increased protein phosphorylation on Ser/Thr residues. Altered O-GlcNAcylation of Tau and APP have been reported in AD and is closely related with pathology onset and progression. In addition, type 2 diabetes patients show an altered O-GlcNAcylation/phosphorylation that might represent a link between metabolic defects and AD progression. Our study aimed to decipher the specific protein targets of altered O-GlcNAcylation in brain of 12-month-old 3×Tg-AD mice compared with age-matched non-Tg mice. Hence, we analysed the global O-GlcNAc levels, the levels and activity of OGT and OGA, the enzymes controlling its cycling and protein specific O-GlcNAc levels using a bi-dimensional electrophoresis (2DE) approach. Our data demonstrate the alteration of OGT and OGA activation coupled with the decrease of total O-GlcNAcylation levels. Data from proteomics analysis led to the identification of several proteins with reduced O-GlcNAcylation levels, which belong to key pathways involved in the progression of AD such as neuronal structure, protein degradation and glucose metabolism. In parallel, we analysed the O-GlcNAcylation/phosphorylation ratio of IRS1 and AKT, whose alterations may contribute to insulin resistance and reduced glucose uptake. Our findings may contribute to better understand the role of altered protein O-GlcNAcylation profile in AD, by possibly identifying novel mechanisms of disease progression related to glucose hypometabolism.

Therapeutic potential of rescuing protein O-GlcNAcylation in tau-related pathologies

“…Studies on human AD brain showed a marked decrease in tau-specific and global levels of O-GlcNAcylation, which are associated with the decline of (1) cerebral glucose metabolism, (2) brain insulin signaling and (3) neuronal GLUTs expression [5,6,9], suggesting that the O-GlcNAc modification of protein could represent the link between glucose hypometabolism and NFT formation. Furthermore, aberrant O-GlcNAc levels have been found on several proteins involved in glucose metabolism, strengthening this link [10,11]. Besides, aberrant levels of O-GlcNAc has been reported also on APP implying that this modification could be involved in the induction of its nonamyloidogenic processing by γ-secretase and to the formation of senile plaques [12].…”

“…A foster crucial restrain, in considering the use of OGA inhibitors as a valuable therapeutic strategy for tau-related pathologies, is represented by the variability of O-GlcNAc levels alteration in pathological brains, as recently observed. Data collected so far, on AD brains from human and mouse models of the disease, suggest that the cytoplasmic and mitochondrial proteome of the hippocampal region represents one of the primary target of intervention [6,9,10], indicating that a brain region-specific administration strategy focusing on these areas should be preferred to a systemic delivery (by oral or intraperitoneal injection distribution). Hence, a targeted administration could allow to maximize the effects, rescue cognition, and avoid the modification of unaffected brain regions or peripheral organs.…”

The Glucose Metabolic Pathway as A Potential Target for Therapeutics: Crucial Role of Glycosylation in Alzheimer’s Disease

“…Tau-O-GlcNAc is neuroprotective whilst its hyperphosphorylation leads to the formation of the toxic NFTs observed in AD human and mouse brain [ 106 ]. A triple transgenic murine model of AD (3 × Tg-AD), which develop age-dependent and region-specific Aβ and tau aggregations that closely mimic the disease progression seen in humans, represents a valuable model for the study of O-GlcNAcylation [ 23 , 108 , 109 , 110 , 111 , 112 , 113 ]. In this regard, 12-month-old 3 × Tg-AD mice exhibited reduced levels of tau O-GlcNAcylation, which is consistent with tau hyperphosphorylation at Ser396 and Thr205 in the hippocampus [ 22 ].…”

“…These data suggest that tau-O-GlcNAc and tau phosphorylation are inversely related and have opposite effects, which suggests that O-GlcNAc is a possible novel therapeutic target in AD. Tramutola and colleagues reported an increase of OGA enzyme activity in 3 × Tg-AD, coupled with the decrease of total O-GlcNAcylation levels, which could translate as an increased removal of GlcNAc residues from proteins [ 23 ]. The pharmacological inhibition of OGA with Thiamet-G in the JNPL3 tauopathy mouse model resulted in increased tau O-GlcNAc and decreased neuronal cell loss and tau aggregate formation [ 114 , 115 ].…”

“…More than 4000 proteins have been identified as potential O-GlcNAc candidates and these are involved in all aspects of cellular metabolism [ 18 ]. In particular, many proteins in the brain are subjected to glycosylation, including β-amyloid (Aβ) associated protein [ 22 , 23 ], tau [ 24 , 25 ], and neurofilament proteins [ 25 , 26 ]. Dysfunction in the regulation of phosphorylation/glycosylation and glycosylation–deglycosylation processes could contribute to the etiopathology of Alzheimer’s disease (AD) [ 2 ].…”