The glycosylation of nucleocytoplasmic proteins with O-linked N-acetylglucosamine residues (O-GlcNAc) is conserved among metazoans and is particularly abundant within brain. O-GlcNAc is involved in diverse cellular processes ranging from the regulation of gene expression to stress response. Moreover, O-GlcNAc is implicated in various diseases including cancers, diabetes, cardiac dysfunction, and neurodegenerative diseases. Pharmacological inhibition of O-GlcNAcase (OGA), the sole enzyme that removes O-GlcNAc, reproducibly slows neurodegeneration in various Alzheimer's disease (AD) mouse models manifesting either tau or amyloid pathology. These data have stimulated interest in the possibility of using OGA-selective inhibitors as pharmaceuticals to alter the progression of AD. The mechanisms mediating the neuroprotective effects of OGA inhibitors, however, remain poorly understood. Here we show, using a range of methods in neuroblastoma N2a cells, in primary rat neurons, and in mouse brain, that selective OGA inhibitors stimulate autophagy through an mTOR-independent pathway without obvious toxicity. Additionally, OGA inhibition significantly decreased the levels of toxic protein species associated with AD pathogenesis in the JNPL3 tauopathy mouse model as well as the 3×Tg-AD mouse model. These results strongly suggest that OGA inhibitors act within brain through a mechanism involving enhancement of autophagy, which aids the brain in combatting the accumulation of toxic protein species. Our study supports OGA inhibition being a feasible therapeutic strategy for hindering the progression of AD and other neurodegenerative diseases. Moreover, these data suggest more targeted strategies to stimulate autophagy in an mTOR-independent manner may be found within the O-GlcNAc pathway. These findings should aid the advancement of OGA inhibitors within the clinic.

Loss of activity of the lysosomal glycosidase β-glucocerebrosidase (GCase) causes the lysosomal storage disease Gaucher disease (GD) and has emerged as the greatest genetic risk factor for the development of both Parkinson disease (PD) and dementia with Lewy bodies. There is significant interest into how GCase dysfunction contributes to these diseases, however, progress toward a full understanding is complicated by presence of endogenous cellular factors that influence lysosomal GCase activity. Indeed, such factors are thought to contribute to the high degree of variable penetrance of GBA mutations among patients. Robust methods to quantitatively measure GCase activity within lysosomes are therefore needed to advance research in this area, as well as to develop clinical assays to monitor disease progression and assess GCase-directed therapeutics. Here, we report a selective fluorescence-quenched substrate, LysoFQ-GBA, which enables measuring endogenous levels of lysosomal GCase activity within living cells. LysoFQ-GBA is a sensitive tool for studying chemical or genetic perturbations of GCase activity using either fluorescence microscopy or flow cytometry. We validate the quantitative nature of measurements made with LysoFQ-GBA using various cell types and demonstrate that it accurately reports on both target engagement by GCase inhibitors and the GBA allele status of cells. Furthermore, through comparisons of GD, PD, and control patient-derived tissues, we show there is a close correlation in the lysosomal GCase activity within monocytes, neuronal progenitor cells, and neurons. Accordingly, analysis of clinical blood samples using LysoFQ-GBA may provide a surrogate marker of lysosomal GCase activity in neuronal tissue.

Glycosyltransferases carry out important cellular functions in species ranging from bacteria to humans.Despite their essential roles in biology,simple and robust activity assays that can be easily applied to high-throughput screening for inhibitors of these enzymes have been challenging to develop. Herein, we report abead-based strategy to measure the grouptransfer activity of glycosyltransferases sensitively using simple fluorescence measurements,w ithout the need for coupled enzymes or secondary reactions.W ev alidate the performance and accuracy of the assayusing O-GlcNAc transferase (OGT) as am odel system through detailed Michaelis-Menten kinetic analysis of various substrates and inhibitors.O ptimization of this assay and application to high-throughput screening enabled screening for inhibitors of OGT,l eading to an ovel inhibitory scaffold. We believe this assay will prove valuable not only for the study of OGT,b ut also more widely as ag eneral approach for the screening of glycosyltransferases and other group-transfer enzymes.Supportinginformation and the ORCID identification number(s) for the author(s) of this article can be found under: https://doi.

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Within mammals, there are often several functionally related glycoside hydrolases, which makes monitoring their activities problematic. This problem is particularly acute for the enzyme β-glucocerebrosidase (GCase), the malfunction of which is a key driver of Gaucher's disease (GD) and a major risk factor for Parkinson's disease (PD). Humans harbor two other functionally related β-glucosidases known as GBA2 and GBA3, and the currently used fluorogenic substrates are not selective, which has driven the use of complicated subtractive assays involving the use of detergents and inhibitors. Here we describe the preparation of fluorogenic substrates based on the widely used nonselective substrate resorufin β-D-glucopyranoside. Using recombinant enzymes, we show that these substrates are highly selective for GCase. We also demonstrate their value through the analysis of GCase activity in brain tissue homogenates from transgenic mice expressing mutant human GCase and patient fibroblasts expressing mutant GCase. This approach simplifies the analysis of cell and tissue homogenates and should facilitate the analysis of clinical and laboratory tissues and samples.