Selenium-based metabolic oligosaccharide engineering strategy for quantitative glycan detection

Ac34deoGlcNAz: A Selective Probe for Identifying O-GlcNAc-Modified Proteins in Prostate Cancer

GlcNAcylation is a prevalent protein modification in eukaryotic cells and increasing evidences indicated that over-expressed O-GlcNAcylation are intimately linked to the development and prognosis of prostate cancer. Thus, exploring this modification in the context of prostate cancer is vital for understanding the underlying mechanisms and hopefully used for future targeted therapies. In this paper, we use our previously established metabolic probes to comprehensively compare the labeling efficiency of O-GlcNAc modified proteins in PC3 cells. Our results demonstrated that all the tested probes were non-toxic to PC3 cells and only Ac4GlcNAz, Ac4GalNAz and Ac34deoGlcNAz exhibited robust labeling signals amongst the probes. Further investigations by western blot and flow cytometry analysis revealed that Ac34deoGlcNAz was a specific and efficient probe for intracelluar protein labeling with negligible S-glyco-modification signal. In addition, proteomic analysis further confirmed that 94 % of the proteins identified by Ac34deoGlcNAz were in the form of O-linked GlcNAc rather than S-glyco-modification, these enriched O-GlcNAcylated proteins were mainly involved in the regulated processes of prostate cancer. Our results here together prove Ac34deoGlcNAz is a safe and reliable probe for metabolic labeling O-GlcNAc modified proteins in prostate cancer, providing a mean to fully exploit the regulatory mechanism of O-GlcNAcylation in the process of prostate cancer.

Investigation of relationships between metabolic chemical reporter structures and S-glyco-modification effects

The approach of metabolic chemical reporters (MCRs) for labeling proteins has been widely used in the past several decades. Nevertheless, artificial side reaction generated with fully protected MCRs, termed S-glyco-modification, occurs with cysteine residues through base-promoted β-elimination and Michael addition, leading to false positives in the proteomic identification. Therefore, next generation of MCRs, including partially protected strategy and modifications on the backbone of monosaccharides, have emerged to improve the labeling efficiency. In this paper, we prepared fifteen kinds of unnatural monosaccharides to investigate the relationships of structures and S-glyco-modification labeling. Our results demonstrated that Ac4GlcNAz and Ac4GalNAz exhibited the most remarkable labeling effects among the detected compounds. Of note, Ac4ManNAz, Ac46AzGlucose and Ac46AzGalactose containing similar structures but did not show similar robust signals as them. Moreover, other modifications on the 1-, 2-, 3-, 4- and 6-site indicated minimal side reactions of S-glyco-modification, raising a possibility that subtle modifications of monosaccharide substrate may alter its role in the process of biosynthesis, for example, by change of electronegativity or enhancement of steric hindrance effects. In conclusion, our discoveries provide a new avenue to choose appropriate probe for selective label proteins in vitro and in vivo without undesired S-glyco-modification.

LRRC25 expression during physiological aging and in mouse models of Alzheimer's disease and iPSC-derived neurons

The leucine-rich repeat-containing protein 25 (LRRC25) is relatively a novel protein with no information on its role in neuronal or brain function. A recent study suggested LRRC25 is a potential risk factor for Alzheimer's disease (AD). As a first step to understanding LRRC25's role in the brain and AD, we found LRRC25 is expressed in both cell membranes and cytoplasm in a punctuate appearance in astrocytes, microglia, and neurons in cell lines as well as mouse brain. We also found that LRRC25 expression is both age- and brain region-dependent and that 1-day-old (1D) pups expressed the least amount of LRRC25 protein compared to adult ages. In the APΔE9 mice, immunoblot quantified LRRC25 protein levels were increased by 166% (**p < 0.01) in the cortex (CX) and by 215% (***p < 0.001) in the hippocampus (HP) relative to wild-type (WT) controls. Both the brainstem (BS) and cerebellum (CB) showed no significant alterations. In the 3xTg mice, only CX showed an increase of LRRC25 protein by 91% (*p < 0.05) when compared to WT controls although the increased trend was noted in the other brain regions. In the AD patient brains also LRRC25 protein levels were increased by 153% (***p < 0.001) when compared to normal control (NC) subjects. Finally, LRRC25 expression in the iPSC-derived neurons quantified by immunofluorescence was increased by 181% (**p < 0.01) in AD-derived neurons when compared to NC-derived neurons. Thus increased LRRC25 protein in multiple models of AD suggests that LRRC25 may play a pathogenic role in either Aβ or tau pathology in AD. The mechanism for the increased levels of LRRC25 in AD is unknown at present, but a previous study showed that LRRC25 levels also increase during neonatal hypoxic-ischemia neuronal damage. Based on the evidence that autophagy is highly dysregulated in AD, the increased LRRC25 levels may be due to decreased autophagic degradation of LRRC25. Increased LRRC25 in turn may regulate the stability or activity of key enzymes involved in either Aβ or hyperphosphorylated tau generation and thus may contribute to increased plaques and neurofibrillary tangles.