Chemistry, Department of

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.

Despite its central importance as a regulator of cellular physiology, identification and precise mapping of O-linked N-acetylglucosamine (O-GlcNAc) post-translational modification (PTM) sites in proteins by mass spectrometry (MS) remains a considerable technical challenge. This is due in part to cleavage of the glycosidic bond occurring prior to the peptide backbone during collisionally activated dissociation (CAD), which leads to generation of characteristic oxocarbenium ions and impairs glycosite localization. Herein, we leverage CAD-induced oxocarbenium ion generation to trigger ultraviolet photodissociation (UVPD), an alternate high-energy deposition method that offers extensive fragmentation of peptides while leaving the glycosite intact. Upon activation using UV laser pulses, efficient photodissociation of glycopeptides is achieved with production of multiple sequence ions that enable robust and precise localization of O-GlcNAc sites. Application of this method to tryptic peptides originating from O-GlcNAcylated proteins TAB1 and Polyhomeotic confirmed previously reported O-GlcNAc sites in TAB1 (S395 and S396) and uncovered new sites within both proteins. We expect this strategy will complement existing MS/MS methods and be broadly useful for mapping O-GlcNAcylated residues of both proteins and proteomes.

Quinolinic acid (QA), a downstream neurometabolite in the kynurenine pathway, the biosynthetic pathway of tryptophan, is associated with neurodegenerative diseases pathology. Mutations in genes encoding kynurenine pathway enzymes, which control the level of QA production, are linked with elevated risk of developing Parkinson's disease. Recent findings have revealed the accumulation and deposition of QA in post-mortem samples, as well as in cellular models of Alzheimer's disease and related disorders. Furthermore, intrastriatal inoculation of mice with QA results in increased levels of phosphorylated α-synuclein and neurodegenerative pathological and behavioral characteristics. However, the cellular and molecular mechanisms underlying the involvement of QA accumulation in protein aggregation and neurodegeneration remain elusive. We recently established that self-assembled ordered structures are formed by various metabolites and hypothesized that these “metabolite amyloids” may seed amyloidogenic proteins. Here we demonstrate the formation of QA amyloid-like fibrillar assemblies and seeding of α-synuclein aggregation by these nanostructures both in vitro and in cell culture. Notably, α-synuclein aggregation kinetics was accelerated by an order of magnitude. Additional amyloid-like properties of QA assemblies were demonstrated using thioflavin T assay, powder X-ray diffraction and cell apoptosis analysis. Moreover, fluorescently labeled QA assemblies were internalized by neuronal cells and co-localized with α-synuclein aggregates. In addition, we observed cell-to-cell propagation of fluorescently labeled QA assemblies in a co-culture of treated and untreated cells. Our findings suggest that excess QA levels, due to mutations in the kynurenine pathway, for example, may lead to the formation of metabolite assemblies that seed α-synuclein aggregation, resulting in neuronal toxicity and induction of Parkinson's disease.

The synthesis of a series of D-gluco-like configured 4,5,6-trihydroxyazepanes bearing a triazole, a sulfonamide or a fluorinated acetamide moiety at C-3 is described. These synthetic derivatives have been tested for their ability to selectively inhibit the muropeptide recycling glucosaminidase NagZ and to thereby increase sensitivity of Pseudomonas aeruginosa to β-lactams, a pathway with substantial therapeutic potential. While introduction of triazole and sulfamide groups failed to lead to glucosaminidase inhibitors, the NHCOCF3 analog proved to be a selective inhibitor of NagZ over other glucosaminidases including human OGA and lysosomal hexosaminidases HexA and B.

The synthesis of a series of D-gluco-like configured

4,5,6-trihydroxyazepanes bearing a triazole, a sulfonamide or a fluorinated

acetamide moiety at C-3 is described. These synthetic derivatives have been

tested for their ability to selectively inhibit the muropeptide recycling

glucosaminidase NagZ and to thereby increase sensitivity of Pseudomonas

aeruginosa to β-lactams, a pathway with substantial therapeutic potential.

While introduction of triazole and sulfamide groups failed to lead to

glucosaminidase inhibitors, the NHCOCF3 analog proved to be a selective

inhibitor of NagZ over other glucosaminidases including human OGA and

lysosomal hexosaminidases HexA and B

Tunable Förster resonance energy transfer (FRET)-quenched substrates are useful for monitoring the activity of various enzymes within their relevant physiological environments. Development of FRET-quenched substrates for exo-glycosidases, however, has been hindered by their constrained pocket-shaped active sites. Here we report the design of a new class of substrate that overcomes this problem. These Bis-Acetal-Based Substrates (BABS) bear a hemiacetal aglycon leaving group that tethers fluorochromes in close proximity, also positioning them distant from the active site pocket. Following cleavage of the glycosidic bond, the liberated hemiacetal spontaneously breaks down, leading to separation of the fluorophore and quencher. We detail the synthesis and characterization of GlcNAc-BABS, revealing a striking 99.9% quenching efficiency. These substrates are efficiently turned over by the human exo-glycosidase O-GlcNAcase (OGA). We find the hemiacetal leaving group rapidly breaks down, enabling quantitative monitoring of OGA activity. We expect this strategy to be broadly useful for the development of substrate probes for monitoring exo-glycosidases, as well as a range of other enzymes having constrained pocket-shaped active sites.

O-GlcNAc transferase (OGT) catalyzes the installation of N-acetylglucosamine (GlcNAc) O-linked to nucleocytoplasmic proteins (O-GlcNAc) within multicellular eukaryotes. OGT shows surprising tolerance for structural changes in the sugar component of its nucleotide sugar donor substrate, uridine diphosphate N-acetylglucosamine (UDP-GlcNAc). Here, we find that OGT uses UDP-glucose to install O-linked glucose (O-Glc) onto proteins only 25-fold less efficiently than O-GlcNAc. Spurred by this observation, we show that OGT transfers 2-azido-2-deoxy-d-glucose (GlcAz) in vitro from UDP-GlcAz to proteins. Further, feeding cells with per-O-acetyl GlcAz (AcGlcAz), in combination with inhibition or inducible knockout of OGT, shows OGT-dependent modification of nuclear and cytoplasmic proteins with O-GlcAz as detected using microscopy, immunoblot, and proteomics. We find that O-GlcAz is reversible within cells, and an unidentified cellular enzyme exists to cleave O-Glc that can also process O-GlcAz. We anticipate that AcGlcAz will prove to be a useful tool to study the O-GlcNAc modification. We also speculate that, given the high concentration of UDP-Glc within certain mammalian tissues, O-Glc may exist within mammals and serve as a physiologically relevant modification.

Lactate dehydrogenase (LDH) is an essential metabolic enzyme that catalyzes the interconversion of pyruvate and lactate using NADH/NAD + as a co-substrate. Many cancer cells exhibit a glycolytic phenotype known as the Warburg effect, in which elevated LDH levels enhance the conversion of glucose to lactate, making LDH an attractive therapeutic target for oncology. Two known inhibitors of the human muscle LDH isoform, LDHA, designated 1 and 2 , were selected, and their IC 50 values were determined to be 14.4 ± 3.77 and 2.20 ± 0.15 µ M , respectively. The X-ray crystal structures of LDHA in complex with each inhibitor were determined; both inhibitors bind to a site overlapping with the NADH-binding site. Further, an apo LDHA crystal structure solved in a new space group is reported, as well as a complex with both NADH and the substrate analogue oxalate bound in seven of the eight molecules and an oxalate only bound in the eighth molecule in the asymmetric unit. In this latter structure, a kanamycin molecule is located in the inhibitor-binding site, thereby blocking NADH binding. These structures provide insights into LDHA enzyme mechanism and inhibition and a framework for structure-assisted drug design that may contribute to new cancer therapies

Deficiency of the lysosomal glycoside hydrolase glucocerebrosidase (GCase) leads to abnormal accumulation of glucosyl ceramide in lysosomes and the development of the lysosomal storage disease known as Gaucher’s disease. More recently, mutations in the GBA1 gene that encodes GCase have been uncovered as a major genetic risk factor for Parkinson’s disease (PD). Current therapeutic strategies to increase GCase activity in lysosomes involve enzyme replacement therapy (ERT) and molecular chaperone therapy. One challenge associated with developing and optimizing these therapies is the difficulty in determining levels of GCase activity present within the lysosomes of live cells. Indeed, visualizing the activity of endogenous levels of any glycoside hydrolases, including GCase, has proven problematic within live mammalian cells. Here we describe the successful modular design and synthesis of fluorescence-quenched substrates for GCase. The selection of a suitable fluorophore and quencher pair permits the generation of substrates that allow convenient time-dependent monitoring of endogenous GCase activity within cells as well as localization of activity within lysosomes. These efficiently quenched (∼99.9%) fluorescent substrates also permit assessment of GCase inhibition in live cells by either confocal microscopy or high content imaging. Such substrates should enable improved understanding of GCase in situ as well the optimization of small-molecule chaperones for this enzyme. These findings also suggest routes to generate fluorescence-quenched substrates for other mammalian glycoside hydrolases for use in live cell imaging.

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. Here we report a bead-based strategy to sensitively measure the group transfer activity of glycosyltransferases using simple fluorescence measurements, without the need for coupled enzymes or secondary reactions. We validate the performance and accuracy of the assay using O-GlcNAc Transferase (OGT) as a model system through detailed Michaelis- Menten kinetic analysis of various substrates and inhibitors. Optimization of this assay and application to high-throughput screening enabled screening for inhibitors of OGT, leading to a novel inhibitory scaffold. We believe this assay will prove valuable not only for the study of OGT, but also more widely as a general approach for the screening of glycosyltransferases and other group transfer enzymes.

Alzheimer disease (AD) is a growing problem for aging populations worldwide. Despite significant efforts, no therapeutics are available that stop or slow progression of AD, which has driven interest in the basic causes of AD and the search for new therapeutic strategies. Longitudinal studies have clarified that defects in glucose metabolism occur in patients exhibiting Mild Cognitive Impairment (MCI) and glucose hypometabolism is an early pathological change within AD brain. Further, type 2 diabetes mellitus (T2DM) is a strong risk factor for the development of AD. These findings have stimulated interest in the possibility that disrupted glucose regulated signaling within the brain could contribute to the progression of AD. One such process of interest is the addition of O-linked N-acetylglucosamine (O-GlcNAc) residues onto nuclear and cytoplasmic proteins within mammals. O-GlcNAc is notably abundant within brain and is present on hundreds of proteins including several, such as tau and the amyloid precursor protein, which are involved in the pathophysiology AD. The cellular levels of O-GlcNAc are coupled to nutrient availability through the action of just two enzymes. O-GlcNAc transferase (OGT) is the glycosyltransferase that acts to install O-GlcNAc onto proteins and O-GlcNAcase (OGA) is the glycoside hydrolase that acts to remove O-GlcNAc from proteins. Uridine 5′-diphosphate-N-acetylglucosamine (UDP-GlcNAc) is the donor sugar substrate for OGT and its levels vary with cellular glucose availability because it is generated from glucose through the hexosamine biosynthetic pathway (HBSP). Within the brains of AD patients O-GlcNAc levels have been found to be decreased and aggregates of tau appear to lack O-GlcNAc entirely. Accordingly, glucose hypometabolism within the brain may result in disruption of the normal functions of O-GlcNAc within the brain and thereby contribute to downstream neurodegeneration. While this hypothesis remains largely speculative, recent studies using different mouse models of AD have demonstrated the protective benefit of pharmacologically increased brain O-GlcNAc levels. In this review we summarize the state of knowledge in the area of O-GlcNAc as it pertains to AD while also addressing some of the basic biochemical roles of O-GlcNAc and how these might contribute to protecting against AD and other neurodegenerative diseases.