Mechanistic roles for altered O-GlcNAcylation in neurodegenerative disorders

Abstract

Neurodegenerative diseases such as Alzheimer's and Parkinson's remain highly prevalent and incurable disorders. A major challenge in fully understanding and combating the progression of these diseases is the complexity of the network of processes that lead to progressive neuronal dysfunction and death. An ideal therapeutic avenue is conceivably one that could address many if not all of these multiple misregulated mechanisms. Over the years, chemical intervention for the up-regulation of the endogenous posttranslational modification (PTM) O-GlcNAc has been proposed as a potential strategy to slow down the progression of neurodegeneration. Through the development and application of tools that allow dissection of the mechanistic roles of this PTM, there is now a growing body of evidence that O-GlcNAc influences a variety of important neurodegeneration-pertinent mechanisms, with an overall protective effect. As a PTM that is appended onto numerous proteins that participate in protein quality control and homeostasis, metabolism, bioenergetics, neuronal communication, inflammation, and programmed death, O-GlcNAc has demonstrated beneficence in animal models of neurodegenerative diseases, and its up-regulation is now being pursued in multiple clinical studies.

Keywords: O-GlcNAc; neurodegeneration; protein aggregation.

Figure 1.. Chemical approaches to decipher roles of O-GlcNAc in neurodegeneration.

a. O-GlcNAc is a posttranslational modification that is regulated by the enzymatic activity of the cycling enzymes OGT and OGA, and the availability of the sugar donor UDP-GlcNAc produced from the hexosamine biosynthetic pathway (HBP). Inhibitors of the HBP enzyme GFAT or of OGT (in red dotted circles) lower O-GlcNAc levels in cells or animals, while inhibitors of OGA (in blue dotted circle) increase O-GlcNAc. b. Native and expressed protein ligations enable the preparation of homogeneously O-GlcNAcylated proteins. This involves the chemoselective reaction between a peptide fragment bearing a C-terminal thioester and another fragment with an N-terminal thiol (such as that in cysteines). These fragments can be prepared by recombinant expression or solid-phase peptide synthesis (SPPS), with SPPS also enabling precise installation of O-GlcNAcylated serine/ threonine residues.

Figure 2.. The process of protein aggregation.

Soluble, monomeric proteins implicated in NDs can slowly undergo a nucleation step to form compact β-sheet rich oligomers. These oligomers undergo molecular rearrangements and structural transitions to generate protofibrillar seeds. Protofibrils can then catalyze rapid amplification of protein aggregates through a much faster extension step. Mature amyloid fibers are routinely found in extracellular deposits called plaques in AD, or in intracellular inclusions such as neurofibrillary tangles (NFTs) in AD or Lewy bodies (LBs) in PD.

Figure 3.. O-GlcNAc is a multifaceted modulator of protein aggregation.

a. The amyloid precursor protein (APP) can undergo two alternative processing mechanisms through the action of ɑ-, β-, and γ- secretases. The amyloidogenic pathway is pertinent in AD as it generates Aβ peptides that are highly prone towards aggregation. APP O-GlcNAcylation at Thr576 as well as O-GlcNAcylation of a γ-secretase subunit are known to favor the non-amyloidogenic pathway. b. Tau and ɑSyn are O-GlcNAc modified at multiple sites. O-GlcNAcylation of these two proteins have been shown to inhibit both nucleation and extension steps of amyloid formation. c. Small heat shock proteins (sHSPs) bearing a C-terminal IXI motif are O-GlcNAc-modified at this regulator region. O-GlcNAc-modifications near or at the IXI motif inhibits its interaction with the conserved ɑ-crystallin domain (ACD) which bears a well-characterized binding cleft for sequestration of Aβ, tau, or αSyn aggregation intermediates. O-GlcNAcylated sHSPs exhibits better anti-amyloidogenic chaperone activities against the aggregation of Aβ and αSyn.

Figure 4.. O-GlcNAc influences multiple mechanisms in neurodegeneration.

a. O-GlcNAc participates in various forms of crosstalk with phosphorylation. Some modification sites can be modified by both types of PTM, leading to direct reciprocal relationship wherein the presence of one prevents the addition of the other. The presence of a PTM on one site can also affect a proximal or distal modification site. Specific O-GlcNAc sites on tau and αSyn are known to inhibit phosphorylation at different sites, and vice versa. b. Cell-to-cell propagation of neurons occur through release of aggregates by an affected neuron, followed by uptake of surrounding neurons. The process of endocytosis of exogenous aggregates is inhibited by upregulation of O-GlcNAc. c. Necroptosis is a programmed form of cell death that is initiated by the formation of a RIPK1-RIPK3 complex that signals downstream toward inflammation and necrosis. O-GlcNAc inhibits the formation of the RIPK1-RIPK3 complex, resulting in reduced neuroinflammation and necroptosis.