Cheng-Xin Gong

Microtubule-associated protein tau is abnormally hyperphosphorylated and aggregated into neurofibrillary tangles in brains of individuals with Alzheimer's disease (AD) and other tauopathies. Tau pathology is critical to pathogenesis and correlates to the severity of dementia. However, the mechanisms leading to abnormal hyperphosphorylation are unknown. Here, we demonstrate that human brain tau was modified by O-GlcNAcylation, a type of protein O-glycosylation by which the monosaccharide beta-N-acetylglucosamine (GlcNAc) attaches to serine/threonine residues via an O-linked glycosidic bond. O-GlcNAcylation regulated phosphorylation of tau in a site-specific manner both in vitro and in vivo. At most of the phosphorylation sites, O-GlcNAcylation negatively regulated tau phosphorylation. In an animal model of starved mice, low glucose uptake/metabolism that mimicked those observed in AD brain produced a decrease in O-GlcNAcylation and consequent hyperphosphorylation of tau at the majority of the phosphorylation sites. The O-GlcNAcylation level in AD brain extracts was decreased as compared to that in controls. These results reveal a mechanism of regulation of tau phosphorylation and suggest that abnormal hyperphosphorylation of tau could result from decreased tau O-GlcNAcylation, which probably is induced by deficient brain glucose uptake/metabolism in AD and other tauopathies.

O -linked N -acetylglucosamine ( O -GlcNAc) is a reversible posttranslational modification of Ser and Thr residues on cytosolic and nuclear proteins of higher eukaryotes catalyzed by O -GlcNAc transferase (OGT). O -GlcNAc has recently been found on Notch1 extracellular domain catalyzed by EGF domain-specific OGT. Aberrant O -GlcNAc modification of brain proteins has been linked to Alzheimer's disease (AD). However, understanding specific functions of O -GlcNAcylation in AD has been impeded by the difficulty in characterization of O -GlcNAc sites on proteins. In this study, we modified a chemical/enzymatic photochemical cleavage approach for enriching O -GlcNAcylated peptides in samples containing ∼100 μg of tryptic peptides from mouse cerebrocortical brain tissue. A total of 274 O -GlcNAcylated proteins were identified. Of these, 168 were not previously known to be modified by O -GlcNAc. Overall, 458 O -GlcNAc sites in 195 proteins were identified. Many of the modified residues are either known phosphorylation sites or located proximal to known phosphorylation sites. These findings support the proposed regulatory cross-talk between O -GlcNAcylation and phosphorylation. This study produced the most comprehensive O -GlcNAc proteome of mammalian brain tissue with both protein identification and O -GlcNAc site assignment. Interestingly, we observed O -β-GlcNAc on EGF-like repeats in the extracellular domains of five membrane proteins, expanding the evidence for extracellular O -GlcNAcylation by the EGF domain-specific OGT. We also report a GlcNAc-β-1,3-Fuc-α-1- O -Thr modification on the EGF-like repeat of the versican core protein, a proposed substrate of Fringe β-1,3- N -acetylglucosaminyltransferases.

In Alzheimer's disease (AD) brain, microtubule-associated protein tau is abnormally modified by hyperphosphorylation and glycosylation, and is aggregated as neurofibrillary tangles of paired helical filaments. To investigate the role of tau glycosylation in neurofibrillary pathology, we isolated various pools of tau protein from AD brain which represent different stages of tau pathology. We found that the non-hyperphosphorylated tau from AD brain but not normal brain tau was glycosylated. Monosaccharide composition analyses and specific lectin blots suggested that the tau in AD brain was glycosylated mainly through N-linkage. In vitro phosphorylation indicated that the glycosylated tau was a better substrate for cAMP-dependent protein kinase than the deglycosylated tau. These results suggest that the glycosylation of tau is an early abnormality that can facilitate the subsequent abnormal hyperphosphorylation of tau in AD brain.

, in which cytoplasmic TDP-43 was increased. These findings suggest that TDP-43 promotes tau exon 10 inclusion and 4R-tau expression and that disease-related changes of TDP-43, truncations and mutations, affect its function in tau exon 10 splicing, possibly because of TDP-43 mislocalization to the cytoplasm.

In the brains of individuals with Alzheimer's disease (AD) and chronic traumatic encephalopathy, tau pathology is accompanied usually by intracellular aggregation of transactive response DNA-binding protein 43 (TDP-43). However, the role of TDP-43 in tau pathogenesis is not understood. Here, we investigated the role of TDP-43 in tau expression in vitro and in vivo. We found that TDP-43 suppressed tau expression by promoting its mRNA instability through the UG repeats of its 3΄-untranslated region (3΄-UTR). The C-terminal region of TDP-43 was required for this function. Neurodegenerative diseases-causing TDP-43 mutations affected tau mRNA instability differentially, in that some promoted and others did not significantly affect tau mRNA instability. The expression levels of tau and TDP-43 were inverse in the frontal cortex and the cerebellum. Accompanied with cytoplasmic accumulation of TDP-43, tau expression was elevated in TDP-43M337V transgenic mouse brains. The level of TDP-43, which is decreased in AD brains, was found to correlate negatively with the tau level in human brain. Our findings indicate that TDP-43 suppresses tau expression by promoting the instability of its mRNA. Down-regulation of TDP-43 may be involved in the tau pathology in AD and related neurodegenerative disorders.