Disrupted Glycosylation of Lipids and Proteins Is a Cause of Neurodegeneration

Tobias Moll; Pamela J. Shaw; Johnathan Cooper-Knock

Disclosures

Brain. 2020;143(5):1332-1340. 

In This Article

Abstract and Introduction

Abstract

Glycosyltransferases represent a large family of enzymes that catalyse the biosynthesis of oligosaccharides, polysaccharides, and glycoconjugates. A number of studies have implicated glycosyltransferases in the pathogenesis of neurodegenerative diseases but differentiating cause from effect has been difficult. We have recently discovered that mutations proximal to the substrate binding site of glycosyltransferase 8 domain containing 1 (GLT8D1) are associated with familial amyotrophic lateral sclerosis (ALS). We demonstrated that ALS-associated mutations reduce activity of the enzyme suggesting a loss-of-function mechanism that is an attractive therapeutic target. Our work is the first evidence that isolated dysfunction of a glycosyltransferase is sufficient to cause a neurodegenerative disease, but connection between neurodegeneration and genetic variation within glycosyltransferases is not new. Previous studies have identified associations between mutations in UGT8 and sporadic ALS, and between ST6GAL1 mutations and conversion of mild cognitive impairment into clinical Alzheimer's disease. In this review we consider potential mechanisms connecting glycosyltransferase dysfunction to neurodegeneration. The most prominent candidates are ganglioside synthesis and impaired addition of O-linked β-N-acetylglucosamine (O-GlcNAc) groups to proteins important for axonal and synaptic function. Special consideration is given to examples where genetic mutations within glycosyltransferases are associated with neurodegeneration in recognition of the fact that these changes are likely to be upstream causes present from birth.

Introduction

Glycosyltransferases represent a large family of enzymes that catalyse biosynthesis of oligosaccharides, polysaccharides, and glycoconjugates. Sugar moieties are transferred from activated sugar donors to specific acceptor molecules via the formation of glycosidic bonds (Chuh et al., 2016). Acceptor molecules include other sugars, nucleic acids, lipids, and proteins. Glycosyltransferases reside predominantly within the Golgi apparatus of eukaryotes as type II transmembrane proteins. Over 90 glycosyltransferase families have been described (www.cazy.org/GlycosylTransferases.html). Sequence alignment tools have been useful for predicting glycosyltransferase function, including a metal-binding motif important for configuration of substrate within the active site (Lairson et al., 2008). However, even closely related sequences have been shown to exhibit different catalytic activity (Breton et al., 2006). Glycosyltransferases are classified as either 'retaining' or 'inverting' enzymes according to whether the anomeric bond within the donor substrate is retained or inverted during the sugar transfer.

Neurodegenerative diseases are increasing in frequency, in part due to an ageing population. Despite this, neurodegenerative diseases represent a significant unmet health need without effective treatments or clearly delineated pathogenic mechanisms. Changes in expression levels of glycosyltransferases have been strongly linked with neurodegeneration (Ludemann et al., 2005; Desplats et al., 2007; Schneider 2018), but determining whether these effects are upstream of neurotoxicity is difficult. Two distinct glycosyltransferase-associated mechanisms are prominent: ganglioside synthesis and addition of O-linked β-N-acetylglucosamine to proteins (O-GlcNAcylation). Major gangliosides are sialic acid-containing glycosphingolipids. Within the mammalian brain they are synthesized in the endoplasmic reticulum from a lactosylceramide precursor before remodelling during transit from the cis- to the trans-Golgi network by a series of glycosyltransferase enzymes (Figure 1). Mature gangliosides are expressed on the plasma membrane of most vertebrate cells and within bodily fluids. They are particularly abundant on neuronal and glial cells within the CNS where they are thought to function prominently in cell signalling (Vajn et al., 2013). Altered levels of gangliosides have been reported in animal models of amyotrophic lateral sclerosis (ALS) and in post-mortem CNS tissue from ALS patients (Ariga, 2014; Dodge et al., 2015); similar findings have been reported in Parkinson's disease (Wu et al., 2012) and Alzheimer's disease (Gylys et al., 2007). O-GlcNAcylation occurs predominantly in the brain and is regulated by the glycosyltransferases O-linked N-acetylglucosamine transferase (OGT) and EGF domain-specific O-linked N-acetylglucosamine transferase (EOGT), which attach the O-GlcNAc moiety to acceptor proteins at specific serine/threonine residues via an O-linked glycosidic bond. OGT acts intracellularly whereas EOGT acts extracellularly on secreted and membrane proteins (Figure 2). O-GlcNAcylation of CNS proteins important for axonal and synaptic function is significantly reduced in animal models of neurodegenerative diseases and in patient tissue from diseases including Parkinson's disease, Huntington's disease, Alzheimer's disease and ALS (Liu et al., 2004; Ludemann et al., 2005; Kumar et al., 2014; Frenkel-Pinter et al., 2017).

Figure 1.

Schematic overview of the biosynthesis and function of major gangliosides within the mammalian brain. Lactosylceramide is synthesized at the cytoplasmic leaflet of the endoplasmic reticulum membrane from its ceramide precursor. De novo ceramide is transported to the Golgi apparatus and is converted to glycosphingolipids and sphingomyelin through the addition of saccharides and phosphocholine, respectively. Glycosphingolipids are transported in vesicles to the outer leaflet of the plasma membrane. Sialic acid-enriched glycosphingolipids form gangliosides which are anchored to the membrane via their ceramide-lipid moiety. Four major gangliosides comprise >90% of total gangliosides within the brain. A-series gangliosides (red) are derived from GM3. B-series gangliosides (purple) are synthesized from GM3 by GD3 synthase (St8sia1). G = the 'ganglioside' core; the second letter designates the quantity of sialic acid residues; M = mono; D = di; T = tri. Gangliosides are essential to maintaining neuronal integrity with functions including, but not limited to, increasing the neuroprotective properties of astrocytes, stabilizing interactions between neurons and glia, enhancing neurite outgrowth and negatively regulating neuroinflammation through activation of the complement pathway.

Figure 2.

O-GlcNAcylation is implicated the pathophysiology of neurodegenerative disease. An overview of O-GlcNAcylation, a post-translational modification of O-GlcNAc, which has been implicated in neurodegenerative diseases Huntington's disease, Alzheimer's disease, Parkinson's disease and ALS. O-GlcNAcylation occurs predominantly in the brain and is regulated by the glycosyltransferases OGT and EOGT, which attach the O-GlcNAc moiety to acceptor proteins at specific serine/threonine residues via an O-linked glycosidic bond; OGT acts intracellularly whereas EOGT acts extracellularly on secreted and membrane proteins.

Neurodegenerative diseases exhibit late age of onset and it is therefore assumed that genetic mutations are upstream of disease pathogenesis. As a result, the discovery of neurodegenerative disease-associated DNA mutations is a significant step towards identification of upstream therapeutic targets. We have recently discovered that mutations proximal to the substrate-binding site of glycosyltransferase 8 domain-containing 1 (GLT8D1) disrupt enzyme activity and are associated with familial ALS (Cooper-Knock et al., 2019). Our work is the first evidence that dysfunction of a glycosyltransferase is sufficient to cause a neurodegenerative disease. Our data are consistent with an effect of GLT8D1 mutations on ganglioside synthesis. In support of this mechanism, we have demonstrated by immunocytochemistry that ALS-associated GLT8D1 mutations reduce membrane expression of glycosphingolipids, which include gangliosides, in human cells (unpublished data). Moreover, in this review we summarize previous literature linking genetic changes within glycosyltransferases to neurodegeneration, and provide new evidence that genetic mutations within EOGT are significantly associated with sporadic ALS making this another upstream therapeutic target.

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