O‐GlcNAcylation

Improves KEAP1’s function to degrade NRF2

we identified KEAP1 (also known as KLHL19), the primary negative regulator of NRF2, as a direct substrate of OGT. We show that O‐GlcNAcylation of KEAP1 at serine 104 is required for the efficient ubiquitination and degradation of NRF2. Interestingly, O‐GlcNAc levels and NRF2 activation co‐vary in response to glucose fluctuations, indicating that KEAP1 O‐GlcNAcylation links nutrient sensing to downstream stress resistance. (R1)

• Transcriptomics show that O‐GlcNAc transferase (OGT) inhibition activates the NRF2 pathway.
• O‐GlcNAcylation of the KEAP1 adaptor is required to restrain NRF2 via ubiquitin‐dependent proteolysis.
• Ser104 O‐GlcNAcylation of KEAP1 promotes its productive interaction with CUL3 ubiquitin ligase.
• Glucose deprivation reduces KEAP1 O‐GlcNAcylation and activates the NRF2 transcription factor. (R1)

Note to self: reduced O‐GlcNAcylation of KEAP1 increases NRF2 activation. This correlates with glucose deprivation.

Reduced O-GlcNAcylation is associated with Alzheimer’s disease

Impaired O-GlcNAcylation appears to be most strongly associated with the pathogenesis of Alzheimer’s disease (AD). Several lines of evidence support a link between decreased O-GlcNAcylation and AD:

1. Key AD-related proteins are O-GlcNAcylated and this modification regulates their functions:

• Tau, which forms neurofibrillary tangles in AD, is O-GlcNAcylated. Reduced O-GlcNAcylation of tau is associated with increased phosphorylation and aggregation into toxic tangles[6][10][12].
• The amyloid precursor protein (APP) that generates amyloid-beta (Aβ) peptides is also O-GlcNAcylated. Impaired O-GlcNAcylation may alter APP processing to promote the amyloidogenic pathway and Aβ accumulation[10][12].

2. O-GlcNAcylation is decreased in AD brains:

• Post-mortem studies found reduced global O-GlcNAcylation levels in AD brains compared to controls[6][10][11].
• Tau aggregates in AD lack O-GlcNAc, suggesting a deficiency of this modification may enable tangle formation[8][12].

3. Increasing O-GlcNAcylation is protective in AD models:

• Pharmacologically elevating O-GlcNAc levels reduces pathological aggregation of tau and Aβ in cell and animal models of AD[6][8][10].
• Boosting O-GlcNAcylation protects neurons against Aβ and tau toxicity and improves cognitive function in AD mice[6][10][11].

4. Impaired brain glucose metabolism may drive O-GlcNAc deficits in AD:

• Glucose hypometabolism occurs early in AD and the biosynthesis of O-GlcNAc is coupled to glucose availability[8][9].
• Disrupted glucose utilization in the AD brain could thus lead to impaired protein O-GlcNAcylation[8][9][12].

Mechanistically, deficient O-GlcNAcylation may contribute to AD pathogenesis by enabling the hyperphosphorylation and aggregation of tau, promoting amyloidogenic APP processing, dysregulating signaling pathways like ERK, disrupting synaptic function, and increasing neuronal vulnerability to stress[6][7][10][11].

While most research has focused on AD, some studies suggest altered O-GlcNAcylation may also play a role in other neurodegenerative conditions like Parkinson’s and Huntington’s diseases, potentially by regulating the aggregation and toxicity of α-synuclein and huntingtin proteins respectively[4][6][8].

In summary, impaired protein O-GlcNAcylation, likely resulting from brain glucose dysregulation, appears to be an important pathogenic factor in AD that enables the development of multiple disease hallmarks. Pharmacological inhibition of O-GlcNAc removal to restore levels of this modification has shown promise as a potential disease-modifying therapeutic strategy for AD in preclinical studies[8][10][12]. However, more research is needed to fully define the mechanisms involved and establish the therapeutic potential of targeting impaired O-GlcNAcylation in AD and related neurodegenerative disorders.

Citations: [1] https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6568225/ [2] https://www.nature.com/articles/nrm.2017.22 [3] https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8639716/ [4] https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8639716/ [5] https://www.frontiersin.org/articles/10.3389/fnagi.2023.1155630/full [6] https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8639716/ [7] https://www.jbc.org/article/S0021-9258%2823%2902439-0/fulltext [8] https://pubs.rsc.org/en/content/articlelanding/2014/cs/c4cs00038b [9] https://www.ncbi.nlm.nih.gov/pmc/articles/PMC9700112/ [10] https://pubmed.ncbi.nlm.nih.gov/31894464/ [11] https://www.nature.com/articles/s12276-021-00709-5 [12] https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4263855/

OGT / OGA complex localization

Within the nucleus and particularly at sites of transcription, the two O-GlcNAc cycling enzymes are often found within the same complex. Paradoxically, although OGT is mostly nuclear, it is excluded from the nucleolus, and O-GlcNAcase, which is mostly cytosolic, is highly enriched within the nucleolus. (R2)