Understanding and exploiting the roles of O-GlcNAc in neurodegenerative diseases

Abstract

O-GlcNAc is a common modification found on nuclear and cytoplasmic proteins. Determining the catalytic mechanism of the enzyme O-GlcNAcase (OGA), which removes O-GlcNAc from proteins, enabled the creation of potent and selective inhibitors of this regulatory enzyme. Such inhibitors have served as important tools in helping to uncover the cellular and organismal physiological roles of this modification. In addition, OGA inhibitors have been important for defining the augmentation of O-GlcNAc as a promising disease-modifying approach to combat several neurodegenerative diseases including both Alzheimer's disease and Parkinson's disease. These studies have led to development and optimization of OGA inhibitors for clinical application. These compounds have been shown to be well tolerated in early clinical studies and are steadily advancing into the clinic. Despite these advances, the mechanisms by which O-GlcNAc protects against these various types of neurodegeneration are a topic of continuing interest since improved insight may enable the creation of more targeted strategies to modulate O-GlcNAc for therapeutic benefit. Relevant pathways on which O-GlcNAc has been found to exert beneficial effects include autophagy, necroptosis, and processing of the amyloid precursor protein. More recently, the development and application of chemical methods enabling the synthesis of homogenous proteins have clarified the biochemical effects of O-GlcNAc on protein aggregation and uncovered new roles for O-GlcNAc in heat shock response. Here, we discuss the features of O-GlcNAc in neurodegenerative diseases, the application of inhibitors to identify the roles of this modification, and the biochemical effects of O-GlcNAc on proteins and pathways associated with neurodegeneration.

Keywords: Alzheimer's disease; O-GlcNAc; clinical development; enzyme inhibitors; neurodegeneration; protein synthesis; synuclein; tau.

Figure 1

O-GlcNAc and protein amyloid aggregation.A, O-GlcNAc is a dynamic monosaccharide modification of intracellular proteins mediated by the enzymes O-GlcNAc transferase (OGT) and O-GlcNAcase (OGA). B, neurodegenerative diseases are associated with the progressive formation of amyloid aggregates.

Figure 2

Selected examples of potent small-molecule inhibitors of O-GlcNAcase (OGA).A, OGA uses a two-step catalytic mechanism involving two catalytic aspartate residues acting as general acids/bases. B, the proposed transition state structure used by OGA to facilitate substrate hydrolysis. C, structure of the oxazoline intermediate present in the reaction pathway used by OGA and bicyclic thiazoline inhibitors inspired by this intermediate, including first-in-class clinical OGA inhibitor MK-8719. D, other classes of potent carbohydrate-based OGA inhibitors. E, medicinal chemistry–derived inhibitors of OGA that have advanced to the clinic including the PET agent LSN3316612. PET, positron emission tomography.

Figure 3

Chemical methods for studying O-GlcNAc.A, chemoenzymatic modification enables the robust detection of endogenous O-GlcNAc modifications through a two-step procedure. Yellow square—GalNAz. B, site-specifically O-GlcNAcylated proteins can be prepared from synthetic and recombinant fragments using ligation reactions. O-GlcNAc is denoted by a blue square and GalNAz by a yellow square. EPL, expressed protein ligation; NCL, native chemical ligation.

Figure 4

Consequences of O-GlcNAc on critical proteins in NDDs.A, Tau and α-Syn are O-GlcNAc-modified proteins that form amyloid aggregates in AD and PD, respectively. O-GlcNAc modification generally slows the kinetics of fibril formation. The canonical isoform of Tau (4R2N) is shown. B, sHSPs are ATP-independent chaperones. O-GlcNAc modification of some sHSPs increases chaperone function by inhibiting the ACD–IPV interaction. O-GlcNAc is denoted by a blue square. ACD, α-crystallin domain; AD, Alzheimer’s disease; IPV, Ile–Pro–Val; NDD, neurodegenerative disease; PD, Parkinson’s disease; sHSP, small heat shock protein; α-Syn, α-synuclein.

Figure 5

Roles of O-GlcNAc in autophagy, necroptosis, and APP processing. Selected components of cellular pathways that have been proposed to be perturbed by increased O-GlcNAcylation that are thought to drive the neuroprotective effects of OGA inhibitors in preclinical neurodegeneration disease models. Proteins within each pathway that are known to be O-GlcNAc modified are marked with a blue square, and proteins that are phosphorylated are marked with a yellow circle. APP, amyloid precursor protein; OGA, O-GlcNAcase.