Primary structure and expression analysis of human UDP-N-acetyl-glucosamine-2-epimerase/N-acetylmannosamine kinase, the bifunctional enzyme in neuraminic acid biosynthesis

Glycation Interferes with the Activity of the Bi-Functional UDP-N-Acetylglucosamine 2-Epimerase/N-Acetyl-mannosamine Kinase (GNE)

Mutations in the gene coding for the bi-functional UDP-N-acetylglucosamine 2-epimerase/N-acetylmannosamine kinase (GNE), the key enzyme of the sialic acid biosynthesis, are responsible for autosomal-recessive GNE myopathy (GNEM). GNEM is an adult-onset disease with a yet unknown exact pathophysiology. Since the protein appears to work adequately for a certain period of time even though the mutation is already present, other effects appear to influence the onset and progression of the disease. In this study, we want to investigate whether the late onset of GNEM is based on an age-related effect, e.g., the accumulation of post-translational modifications (PTMs). Furthermore, we also want to investigate what effect on the enzyme activity such an accumulation would have. We will particularly focus on glycation, which is a PTM through non-enzymatic reactions between the carbonyl groups (e.g., of methylglyoxal (MGO) or glyoxal (GO)) with amino groups of proteins or other biomolecules. It is already known that the levels of both MGO and GO increase with age. For our investigations, we express each domain of the GNE separately, treat them with one of the glycation agents, and determine their activity. We demonstrate that the enzymatic activity of the N-acetylmannosamine kinase (GNE-kinase domain) decreases dramatically after glycation with MGO or GO-with a remaining activity of 13% ± 5% (5 mM MGO) and 22% ± 4% (5 mM GO). Whereas the activity of the UDP-N-acetylglucosamine 2-epimerase (GNE-epimerase domain) is only slightly reduced after glycation-with a remaining activity of 60% ± 8% (5 mM MGO) and 63% ± 5% (5 mM GO).

Sialin mediates submandibular gland regeneration ability by affecting polysialic acid synthesis

OBJECTIVES

Sialin is a multifunctional molecule with a well-described role in physiological equilibrium regulation. The aim of this study was to elucidate the role of sialin in salivary glands regeneration.

MATERIALS AND METHODS

Submandibular gland duct ligation/deligation of rat was performed to develop a rat model of submandibular gland regeneration. Phenotype changes were investigated using western blotting and quantitative real-time polymerase chain reaction, as well as immunohistochemical staining. LV-slc17a5-RNAi vectors were injected into the submandibular glands via retroductal instillation to establish a stable sialin knockdown model.

RESULTS

Submandibular gland tissue structure could completely restore 28 days after duct deligation, when the duct had been ligated for 7 days. The expression of sialin, polysialic acid, and polysialyltransferase IV was significantly increased on day 0 after duct deligation, and it returned to the level of the control group at day 28. Moreover, sialin knockdown could weakened gland regeneration by reducing polysialic acid synthesis. Supplementing drinking water with polysialic acid precursors (ManNAc) in drinking water could partially rescue submandibular gland regeneration in sialin knockdown rats.

CONCLUSION

These data indicated that sialin was vital for submandibular gland regeneration which mediated the process of gland regeneration by affecting the polysialic acid synthesis.

Tissue-specific isoform expression of GNE gene in human tissues

Mutations in the sialic acid biosynthesis enzyme GNE lead to a late-onset, debilitating neuromuscular disorder, GNE myopathy, characterized by progressive skeletal muscle weakness. The mechanisms responsible for skeletal muscle specificity, late-onset, and disease progression are unknown. Our main aim is to understand the reason for skeletal muscle-specific phenotype. To answer this question, we have analyzed the expression profile of the GNE gene and its multiple mRNA variants in different human tissues. A combinatorial approach encompassing bioinformatics tools and molecular biology techniques was used. NCBI, Ensembl, and GTEx were used for data mining. The expression analysis of GNE and its variants was performed with cDNA tissue panel using PCR and targeted RNA-seq. Among nine different GNE isoforms reported in this study, transcript variants 1, X1, and X2 were not tissue specific. Transcript variants 1, 6, X1, and X2, were found in skeletal muscles suggesting their possible role in GNE myopathy. In the current study, we present new data about GNE expression patterns in human tissues. Our results suggest that there may be a link between tissue-specific pathology and isoform pattern in skeletal muscles, which could provide clues for the development of new treatment strategies for GNE myopathy.

Glycomaterials to Investigate the Functional Role of Aberrant Glycosylation in Glioblastoma

Glioblastoma (GBM) is a stage IV astrocytoma that carries a dismal survival rate of ≈10 months postdiagnosis and treatment. The highly invasive capacity of GBM and its ability to escape therapeutic challenges are key factors contributing to the poor overall survival rate. While current treatments aim to target the cancer cell itself, they fail to consider the significant role that the GBM tumor microenvironment (TME) plays in promoting tumor progression and therapeutic resistance. The GBM tumor glycocalyx and glycan-rich extracellular matrix (ECM), which are important constituents of the TME have received little attention as therapeutic targets. A wide array of aberrantly modified glycans in the GBM TME mediate tumor growth, invasion, therapeutic resistance, and immunosuppression. Here, an overview of the landscape of aberrant glycan modifications in GBM is provided, and the design and utility of 3D glycomaterials are discussed as a tool to evaluate glycan-mediated GBM progression and therapeutic efficacy. The development of alternative strategies to target glycans in the TME can potentially unveil broader mechanisms of restricting tumor growth and enhancing the efficacy of tumor-targeting therapeutics.

Biological Functions and Analytical Strategies of Sialic Acids in Tumor

Sialic acids, a subset of nine carbon acidic sugars, often exist as the terminal sugars of glycans on either glycoproteins or glycolipids on the cell surface. Sialic acids play important roles in many physiological and pathological processes via carbohydrate-protein interactions, including cell-cell communication, bacterial and viral infections. In particular, hypersialylation in tumors, as well as their roles in tumor growth and metastasis, have been widely described. Recent studies have indicated that the aberrant sialylation is a vital way for tumor cells to escape immune surveillance and keep malignance. In this article, we outline the present state of knowledge on the metabolic pathway of human sialic acids, the function of hypersialylation in tumors, as well as the recent labeling and analytical techniques for sialic acids. It is expected to offer a brief introduction of sialic acid metabolism and provide advanced analytical strategies in sialic acid studies.

Chemical and biological methods for probing the structure and functions of polysialic acids

Owing to its poly-anionic charge and large hydrodynamic volume, polysialic acid (polySia) attached to neural cell adhesion molecule regulates axon–axon and axon–substratum interactions and signalling, particularly, in the development of the central nervous system (CNS). Expression of polySia is spatiotemporally regulated by the action of two polysialyl transferases, namely ST8SiaII and ST8SiaIV. PolySia expression peaks during late embryonic and early post-natal period and maintained at a steady state in adulthood in neurogenic niche of the brain. Aberrant polySia expression is associated with neurological disorders and brain tumours. Investigations on the structure and functions, over the past four decades, have shed light on the physiology of polySia. This review focuses on the biological, biochemical, and chemical tools available for polySia engineering. Genetic knockouts, endo-neuraminidases that cleave polySia, antibodies, exogenous expression, and neuroblastoma cells have provided deep insights into the ability of polySia to guide migration of neuronal precursors in neonatal brain development, neuronal clustering, axonal pathway guidance, and axonal targeting. Advent of metabolic sialic acid engineering using ManNAc analogues has enabled reversible and dose-dependent modulation polySia in vitro and ex vivo. In vivo, ManNAc analogues readily engineer the sialoglycans in peripheral tissues, but show no effect in the brain. A recently developed carbohydrate-neuroactive hybrid strategy enables a non-invasive access to the brain in living animals across the blood–brain barrier. A combination of recent advances in CNS drugs and imaging with ManNAc analogues for polySia modulation would pave novel avenues for understanding intricacies of brain development and tackling the challenges of neurological disorders.

Identification of an Alu element-mediated deletion in the promoter region of GNE in siblings with GNE myopathy

Background

GNE myopathy is a rare genetic disease characterized by progressive muscle atrophy and weakness. It is caused by biallelic mutations in the GNE gene that encodes for the bifunctional enzyme, uridine diphosphate (UDP)‐N‐acetylglucosamine (GlcNAc) 2‐epimerase/N‐acetylmannosamine (ManNAc) kinase. Typical characteristics of GNE myopathy include progressive myopathy, first involving anterior tibialis muscle and sparing the quadriceps, and rimmed vacuoles on muscle biopsy. Identifying biallelic mutations by sequencing of the GNE gene confirms the diagnosis of GNE myopathy. In a subset of patients, diagnostic confirmation is challenged by the identification of mutations in only one allele, suggesting mutations in deep intronic regions or regulatory regions.

Methods

We performed targeted sequencing and copy number variant (CNV) analysis of GNE in two siblings who clinically presented with GNE myopathy. Further molecular and biochemical studies were done to characterize the effect of a previously uncharacterized GNE mutation.

Results

We report two siblings of Indian descent with characteristic features of GNE myopathy, including progressive skeletal muscle weakness initially involving the anterior tibialis, and rimmed vacuoles on muscle biopsy, in which a heterozygous mutation, p.Val727Met, was identified in both affected siblings, but no other deleterious variants in either coding region or exon–intron boundaries of the gene. Subsequent insertion/deletion analysis identified a novel 11.3‐kb deletion (Chr9 [GRCh37]: g.36257583_36268910del) encompassing the GNE promoter region, with breakpoints residing in Alu repeats. Gene expression analysis revealed reduced GNEmRNA and protein levels, confirming decreased expression of the deleted allele harboring the deletion.

Conclusions

We have identified GNE as one of the genes susceptible to Alu‐mediated recombination. Our findings suggest that the deletion may encompass the promoter or another region necessary for GNE expression. In patients with typical manifestations of GNE myopathy and a single GNE variant identified, copy number variant (CNV) analysis may be useful in arriving at the diagnosis.

Substantial deficiency of free sialic acid in muscles of patients with GNE myopathy and in a mouse model

GNE myopathy (GNEM), also known as hereditary inclusion body myopathy (HIBM), is a late- onset, progressive myopathy caused by mutations in the GNE gene encoding the enzyme responsible for the first regulated step in the biosynthesis of sialic acid (SA). The disease is characterized by distal muscle weakness in both the lower and upper extremities, with the quadriceps muscle relatively spared until the late stages of disease. To explore the role of SA synthesis in the disease, we conducted a comprehensive and systematic analysis of both free and total SA levels in a large cohort of GNEM patients and a mouse model. A sensitive LC/MS/MS assay was developed to quantify SA in serum and muscle homogenates. Mean serum free SA level was 0.166 μg/mL in patients and 18% lower (p<0.001) than that of age-matched control samples (0.203 μg/mL). In biopsies obtained from patients, mean free SA levels of different muscles ranged from 0.046–0.075 μg/μmol Cr and were markedly lower by 72–85% (p<0.001) than free SA from normal controls. Free SA was shown to constitute a small fraction (3–7%) of the total SA pool in muscle tissue. Differences in mean total SA levels in muscle from patients compared with normal controls were less distinct and more variable between different muscles, suggesting a small subset of sialylation targets could be responsible for the pathogenesis of GNEM. Normal quadriceps had significantly lower levels of free SA (reduced by 39%) and total SA (reduced by 53%) compared to normal gastrocnemius. A lower SA requirement for quadriceps may be linked to the reported quadriceps sparing in GNEM. Analysis of SA levels in Gne^M743T/M743T mutant mice corroborated the human study results. These results show that serum and muscle free SA is severely reduced in GNEM, which is consistent with the biochemical defect in SA synthesis associated with GNE mutations. These results therefore support the approach of reversing SA depletion as a potential treatment for GNEM patients.

Metabolic Glycoengineering with N-Acyl Side Chain Modified Mannosamines

In metabolic glycoengineering (MGE), cells or animals are treated with unnatural derivatives of monosaccharides. After entering the cytosol, these sugar analogues are metabolized and subsequently expressed on newly synthesized glycoconjugates. The feasibility of MGE was first discovered for sialylated glycans, by using N-acyl-modified mannosamines as precursor molecules for unnatural sialic acids. Prerequisite is the promiscuity of the enzymes of the Roseman-Warren biosynthetic pathway. These enzymes were shown to tolerate specific modifications of the N-acyl side chain of mannosamine analogues, for example, elongation by one or more methylene groups (aliphatic modifications) or by insertion of reactive groups (bioorthogonal modifications). Unnatural sialic acids are incorporated into glycoconjugates of cells and organs. MGE has intriguing biological consequences for treated cells (aliphatic MGE) and offers the opportunity to visualize the topography and dynamics of sialylated glycans in vitro, ex vivo, and in vivo (bioorthogonal MGE).

Genetic defects in the hexosamine and sialic acid biosynthesis pathway

BACKGROUND

Congenital disorders of glycosylation are caused by defects in the glycosylation of proteins and lipids. Classically, gene defects with multisystem disease have been identified in the ubiquitously expressed glycosyltransferases required for protein N-glycosylation. An increasing number of defects are being described in sugar supply pathways for protein glycosylation with tissue-restricted clinical symptoms.

SCOPE OF REVIEW

In this review, we address the hexosamine and sialic acid biosynthesis pathways in sugar metabolism. GFPT1, PGM3 and GNE are essential for synthesis of nucleotide sugars uridine diphosphate N-acetylglucosamine (UDP-GlcNAc) and cytidine-5'-monophospho-N-acetylneuraminic acid (CMP-sialic acid) as precursors for various glycosylation pathways. Defects in these enzymes result in contrasting clinical phenotypes of congenital myasthenia, immunodeficiency or adult-onset myopathy, respectively. We therefore discuss the biochemical mechanisms of known genetic defects in the hexosamine and CMP-sialic acid synthesis pathway in relation to the clinical phenotypes.

MAJOR CONCLUSIONS

Both UDP-GlcNAc and CMP-sialic acid are important precursors for diverse protein glycosylation reactions and for conversion into other nucleotide-sugars. Defects in the synthesis of these nucleotide sugars might affect a wide range of protein glycosylation reactions. Involvement of multiple glycosylation pathways might contribute to disease phenotype, but the currently available biochemical information on sugar metabolism is insufficient to understand why defects in these pathways present with tissue-specific phenotypes.

GENERAL SIGNIFICANCE

Future research on the interplay between sugar metabolism and different glycosylation pathways in a tissue- and cell-specific manner will contribute to elucidation of disease mechanisms and will create new opportunities for therapeutic intervention. This article is part of a Special Issue entitled "Glycans in personalised medicine" Guest Editor: Professor Gordan Lauc.

Mutation update for GNE gene variants associated with GNE myopathy

The GNE gene encodes the rate-limiting, bifunctional enzyme of sialic acid biosynthesis, uridine diphosphate-N-acetylglucosamine 2-epimerase/N-acetylmannosamine kinase (GNE). Biallelic GNE mutations underlie GNE myopathy, an adult-onset progressive myopathy. GNE myopathy-associated GNE mutations are predominantly missense, resulting in reduced, but not absent, GNE enzyme activities. The exact pathomechanism of GNE myopathy remains unknown, but likely involves aberrant (muscle) sialylation. Here, we summarize 154 reported and novel GNE variants associated with GNE myopathy, including 122 missense, 11 nonsense, 14 insertion/deletions, and seven intronic variants. All variants were deposited in the online GNE variation database (http://www.dmd.nl/nmdb2/home.php?select_db=GNE). We report the predicted effects on protein function of all variants well as the predicted effects on epimerase and/or kinase enzymatic activities of selected variants. By analyzing exome sequence databases, we identified three frequently occurring, unreported GNE missense variants/polymorphisms, important for future sequence interpretations. Based on allele frequencies, we estimate the world-wide prevalence of GNE myopathy to be ∼4-21/1,000,000. This previously unrecognized high prevalence confirms suspicions that many patients may escape diagnosis. Awareness among physicians for GNE myopathy is essential for the identification of new patients, which is required for better understanding of the disorder's pathomechanism and for the success of ongoing treatment trials.

Non-specific accumulation of glycosphingolipids in GNE myopathy

BACKGROUND

UDP-GlcNAc 2-epimerase/ManNAc 6-kinase (GNE) is a bifunctional enzyme responsible for the first committed steps in the synthesis of sialic acid, a common terminal monosaccharide in both protein and lipid glycosylation. GNE mutations are responsible for a rare autosomal recessive neuromuscular disorder, GNE myopathy (also called hereditary inclusion body myopathy). The connection between the impairment of sialic acid synthesis and muscle pathology in GNE myopathy remains poorly understood.

METHODS

Glycosphingolipid (GSL) analysis was performed by HPLC in multiple models of GNE myopathy, including patients' fibroblasts and plasma, control fibroblasts with inhibited GNE epimerase activity through a novel imino sugar, and tissues of Gne(M712T/M712T) knock-in mice.

RESULTS

Not only neutral GSLs, but also sialylated GSLs, were significantly increased compared to controls in all tested models of GNE myopathy. Treatment of GNE myopathy fibroblasts with N-acetylmannosamine (ManNAc), a sialic acid precursor downstream of GNE epimerase activity, ameliorated the increased total GSL concentrations.

CONCLUSION

GNE myopathy models have increased total GSL concentrations. ManNAc supplementation results in decrease of GSL levels, linking abnormal increase of total GSLs in GNE myopathy to defects in the sialic acid biosynthetic pathway. These data advocate for further exploring GSL concentrations as an informative biomarker, not only for GNE myopathy, but also for other disorders of sialic acid metabolism.

Gne depletion during zebrafish development impairs skeletal muscle structure and function

GNE Myopathy is a rare recessively inherited neuromuscular disorder caused by mutations in the GNE gene, which codes for the key enzyme in the metabolic pathway of sialic acid synthesis. The process by which GNE mutations lead to myopathy is not well understood. By in situ hybridization and gne promoter-driven fluorescent transgenic fish generation, we have characterized the spatiotemporal expression pattern of the zebrafish gne gene and have shown that it is highly conserved compared with the human ortholog. We also show the deposition of maternal gne mRNA and maternal GNE protein at the earliest embryonic stage, emphasizing the critical role of gne in embryonic development. Injection of morpholino (MO)-modified antisense oligonucleotides specifically designed to knockdown gne, into one-cell embryos lead to a variety of phenotypic severity. Characterization of the gne knockdown morphants showed a significantly reduced locomotor activity as well as distorted muscle integrity, including a reduction in the number of muscle myofibers, even in mild or intermediate phenotype morphants. These findings were further confirmed by electron microscopy studies, where large gaps between sarcolemmas were visualized, although normal sarcomeric structures were maintained. These results demonstrate a critical novel role for gne in embryonic development and particularly in myofiber development, muscle integrity and activity.

N-acetylglucosamine (GlcNAc)-inducible gene GIG2 is a novel component of GlcNAc metabolism in Candida albicans

Candida albicans is an opportunistic fungal pathogen that resides in the human body as a commensal and can turn pathogenic when the host is immunocompromised. Adaptation of C. albicans to host niche-specific conditions is important for the establishment of pathogenicity, where the ability of C. albicans to utilize multiple carbon sources provides additional flexibility. One alternative sugar is N-acetylglucosamine (GlcNAc), which is now established as an important carbon source for many pathogens and can also act as a signaling molecule. Although GlcNAc catabolism has been well studied in many pathogens, the importance of several enzymes involved in the formation of metabolic intermediates still remains elusive. In this context, microarray analysis was carried out to investigate the transcriptional responses induced by GlcNAc under different conditions. A novel gene that was highly upregulated immediately following the GlcNAc catabolic genes was identified and was named GIG2 (GlcNAc-induced gene 2). This gene is regulated in a manner distinct from that of the GlcNAc-induced genes described previously in that GlcNAc metabolism is essential for its induction. Furthermore, this gene is involved in the metabolism of N-acetylneuraminate (sialic acid), a molecule equally important for initial host-pathogen recognition. Mutant cells showed a considerable decrease in fungal burden in mouse kidneys and were hypersensitive to oxidative stress conditions. Since GIG2 is also present in many other fungal and enterobacterial genomes, targeted inhibition of its activity would offer insight into the treatment of candidiasis and other fungal or enterobacterial infections.

The key enzyme of the sialic acid metabolism is involved in embryoid body formation and expression of marker genes of germ layer formation

The bi-functional enzyme UDP-N-acetyl-2-epimerase/N-acetylmannosamine kinase (GNE) is the key enzyme of the sialic acid biosynthesis. Sialic acids are negatively charged nine carbon amino sugars and are found on most glycoproteins and many glycolipids in terminal positions, where they are involved in a variety of biological important molecular interactions. Inactivation of the GNE by homologous recombination results in early embryonic lethality in mice. Here, we report that GNE-deficient embryonic stem cells express less differentiation markers compared to wild-type embryonic stem cells. As a result, GNE-deficient embryonic stem cells fail to form proper embryoid bodies (EB) within the first day of culture. However, when culturing these cells in the presence of sialic acids for three days, also GNE-deficient embryonic stem cells form normal EBs. In contrast, when culturing these cells in sialic acid reduced medium, GNE-deficient embryonic stem cells proliferate faster and form larger EBs without any change in the expression of markers of the germ layers.

UDP-GlcNAc 2-Epimerase/ManNAc Kinase (GNE): A Master Regulator of Sialic Acid Synthesis

UDP-N-acetylglucosamine 2-epimerase/N-acetylmannosamine kinase is the key enzyme of sialic acid biosynthesis in vertebrates. It catalyzes the first two steps of the cytosolic formation of CMP-N-acetylneuraminic acid from UDP-N-acetylglucosamine. In this review we give an overview of structure, biochemistry, and genetics of the bifunctional enzyme and its complex regulation. Furthermore, we will focus on diseases related to UDP-N-acetylglucosamine 2-epimerase/N-acetylmannosamine kinase.

Properties of BoAGE2, a second N-acetyl-D-glucosamine 2-epimerase from Bacteroides ovatus ATCC 8483

N-Acyl-D-Glucosamine 2-epimerase (AGE) catalyzes the reversible epimerization between N-acetyl-D-mannosamine (ManNAc) and N-acetyl-D-glucosamine (GlcNAc). Bacteroides ovatus ATCC 8483 shows 3 putative genes for AGE activity (BACOVA_00274, BACOVA_01795 and BACOVA_01816). The BACOVA_00274 gene encodes an AGE (BoAGE1) with strong similarity to the AGE previously characterized in Bacteroides fragilis. Interestingly, the BACOVA_01816 gene (BoAGE2) shares 57% identity with Anabaena sp. CH1 AGE, but has an extra 27-amino acid tag sequence in the N-terminal. When cloned and expressed in Escherichia coli Rosetta (DE3)pLys, BACOVA_01816 was able to convert ManNAc into GlcNAc and vice versa. It was stable over a broad range of pHs and its activity was enhanced by ATP (20 μM). The incubation with ATP stabilized its structure, raising its melting temperature by about 8 °C. In addition, the catalytic efficiency for ManNAc synthesis was higher than that for GlcNAc synthesis. These characteristics make BoAGE2 a promising biocatalyst for sialic acid production using cheap GlcNAc as starting material. BoAGE2 could be considered a Renin-binding Protein and its interaction with renin was studied for the first time in a prokaryotic AGE. Surprisingly, renin activated BoAGE2, an effect which is contrary to that described for mammalian AGE and unrelated with the unique N-terminal tag, since a mutant without this tag was also activated by renin. When BoAGE2 sequence was compared with other related (real and putative) AGE described in the databases, it was seen that AGE enzymes can be divided in 3 different groups. The relationship between these groups is also discussed.

Identification, tissue distribution, and molecular modeling of novel human isoforms of the key enzyme in sialic acid synthesis, UDP-GlcNAc 2-epimerase/ManNAc kinase

UDP-GlcNAc 2-epimerase/ManNAc kinase (GNE) catalyzes the first two committed steps in sialic acid synthesis. In addition to the three previously described human GNE isoforms (hGNE1-hGNE3), our database and polymerase chain reaction analysis yielded five additional human isoforms (hGNE4-hGNE8). hGNE1 is the ubiquitously expressed major isoform, while the hGNE2-hGNE8 isoforms are differentially expressed and may act as tissue-specific regulators of sialylation. hGNE2 and hGNE7 display a 31-residue N-terminal extension compared to hGNE1. On the basis of similarities to kinases and helicases, this extension does not seem to hinder the epimerase enzymatic active site. hGNE3 and hGNE8 contain a 55-residue N-terminal deletion and a 50-residue N-terminal extension compared to hGNE1. The size and secondary structures of these fragments are similar, and modeling predicted that these modifications do not affect the overall fold compared to that of hGNE1. However, the epimerase enzymatic activity of GNE3 and GNE8 is likely absent, because the deleted fragment contains important substrate binding residues in homologous bacterial epimerases. hGNE5-hGNE8 have a 53-residue deletion, which was assigned a role in substrate (UDP-GlcNAc) binding. Deletion of this fragment likely eliminates epimerase enzymatic activity. Our findings imply that GNE is subject to evolutionary mechanisms to improve cellular functions, without increasing the number of genes. Our expression and modeling data contribute to elucidation of the complex functional and regulatory mechanisms of human GNE and may contribute to further elucidating the pathology and treatment strategies of the human GNE-opathies sialuria and hereditary inclusion body myopathy.

Biochemical characterization of the M712T-mutation of the UDP-N-acetylglucosamine 2-epimerase/N-acetyl-mannosaminekinase in hereditary inclusion body myopathy

Hereditary inclusion body myopathy is a neuromuscular disorder characterized by muscle weakness with a late onset and slow progression. It is caused by mutations of the gene encoding UDP-N-acetylglucosamine-2-epimerase/N-acetylmannosamine kinase (GNE). One of the most frequent mutations is an exchange of methionine to threonine at position 712 (M712T). Here we analyzed wildtype (wt) and M712T-mutated (M712T) GNE. We identified threonine 712 as an additional possible phosphorylation site and found by two-dimensional gel-electrophoresis a lower isoelectric point compared to wt-GNE. This lower isoelectric point could be partially reversed back to the wildtype isoelectric point after treatment with protein phosphatase. Furthermore, in contrast to wt-GNE, a significant fraction of M712T-GNE was in the insoluble fraction. Finally, by using bimolecular fluorescence complementation we demonstrate that the M712T mutation does not disrupt the formation of GNE-oligomers.

Features and applications of microbial sugar epimerases

Sugar (carbohydrate) epimerases catalyze the reversible conversion of a sugar epimer into its counterpart form. More than 20 types of sugar epimerase have been reported to date, and their biological properties, catalytic mechanisms, and 3D structures are very diverse among them. Recently, microbial sugar epimerases have been characterized in detail. This review surveys the catalytic aspects of microbial epimerases, which are relevant for production of bioactive mono- and oligosaccharides.

Regulation and pathophysiological implications of UDP-GlcNAc 2-epimerase/ManNAc kinase (GNE) as the key enzyme of sialic acid biosynthesis

The key enzyme for the biosynthesis ofN-acetylneuraminic acid, from which all other sialic acids are formed, is the bifunctional enzyme UDP-N-acetylglucosamine 2-epimerase/N-acetylmannosamine kinase (GNE). GNE is a highly conserved protein found throughout the animal kingdom. Its highest expression is seen in the liver and placenta. GNE is regulated by a variety of biochemical means, including tetramerization promoted by the substrate UDP-GlcNAc, phosphorylation by protein kinase C and feedback inhibition by CMP-Neu5Ac, which is defect in the human disease sialuria. GNE knock-out in mice leads to embryonic lethality, emphasizing the crucial role of this key enzyme for sialic acid biosynthesis. The metabolic capacity to synthesize sialic acid and CMP-sialic acid upon ManNAc loads is amazingly high. An additional characteristic of GNE is its interaction with proteins involved in the regulation of development, which might play a crucial role in the hereditary inclusion body myopathy. Due to the importance of increased concentrations of tumor-surface sialic acid, first attempts to find inhibitors of GNE have been successful.

Safety and in vivo expression of a GNE-transgene: a novel treatment approach for hereditary inclusion body myopathy-2

Hereditary inclusion body myopathy-2 (HIBM2) is an adult-onset, muscular disease caused by mutations in the GNE gene. HIBM2-associated GNE mutations causing hyposialyation have been proposed to contribute to reduced muscle function in patients with HIBM2, though the exact cause of this disease is unknown. In the current studies we examined pre-clinical in vivo toxicity, and expression of the plasmid-based, CMV driven wild-type GNE plasmid vector. The plasmid vector was injected intramuscularly (IM) or systemically (IV) into BALB/c mice, following encapsulation in a cationic liposome (DOTAP:Cholesterol). Single IM injections of the GNE-lipoplex at 40 μg did not produce overt toxicity or deaths, indicating that the no observable adverse effect level (NOAEL) dose for IM injection was ≥40 μg. Single intravenous (IV) infusion of GNE-lipoplex was lethal in 33% of animals at 100 μg dose, with a small proportion of animals in the 40 μg cohort demonstrating transient toxicity. Thus the NOAEL dose by the IV route was greater than 10 μg and less than or equal to 40 μg. Real-time RT-qPCR analysis demonstrated recombinant human GNE mRNA expression in 100% of muscle tissues that received IM injection of 40 μg GNE-lipoplex, at 2 weeks. These results indicate that GNE-lipoplex gene transfer is safe and can produce durable transgene expression in treated muscles. Our findings support future exploration of the clinical efficacy of GNE-lipoplex for experimental gene therapy of HIBM2.

Biochemical characterization of human and murine isoforms of UDP-N-acetylglucosamine 2-epimerase/N-acetylmannosamine kinase (GNE)

The bifunctional enzyme UDP-N-acetylglucosamine 2-epimerase/N-acetylmannosamine kinase (GNE) is the key enzyme for the biosynthesis of sialic acids, terminal components of glycoconjugates associated with a variety of physiological and pathological processes. Different protein isoforms of human and mouse GNE, deriving from splice variants, were predicted recently: GNE1 represents the GNE protein described in several studies before, GNE2 and GNE3 are proteins with extended and deleted N-termini, respectively. hGNE2, recombinantly expressed in insect and mamalian cells, displayed selective reduction of UDP-GlcNAc 2-epimerase activity by the loss of its tetrameric state, which is essential for full enzyme activity. hGNE3, which had to be expressed in Escherichia coli, only possessed kinase activity, whereas mGNE1 and mGNE2 showed no significant differences. Our data therefore suggest a role of GNE1 in basic supply of cells with sialic acids, whereas GNE2 and GNE3 may have a function in fine-tuning of the sialic acid pathway.

Prediction of three different isoforms of the human UDP-N-acetylglucosamine 2-epimerase/N-acetylmannosamine kinase

The bifunctional enzyme UDP-N-acetylglucosamine 2-epimerase/N-acetylmannosamine kinase (GNE) is the key enzyme of the biosynthesis of sialic acids, terminal components of glycoconjugates associated with a variety of cellular processes. Two novel isoforms of human GNE, namely GNE2 and GNE3, which possess extended and deleted N-termini, respectively, were characterized. GNE2 was also found in other species like apes, rodents, chicken or fish, whereas GNE3 seems to be restricted to primates. Both, GNE2 and GNE3, displayed tissue specific expression patterns, therefore may contribute to the complex regulation of sialic acid metabolism.

Evidence for dynamic interplay of different oligomeric states of UDP-N-acetylglucosamine 2-epimerase/N-acetylmannosamine kinase by biophysical methods

The bifunctional enzyme UDP-N-acetylglucosamine 2-epimerase/N-acetylmannosamine kinase (GNE) is a key enzyme for the biosynthesis of sialic acids, the terminal sugars of glycoconjugates associated with a variety of physiological and pathological processes such as cell adhesion, development, inflammation and cancer. In this study, we characterized rat GNE by different biophysical methods, analytical ultracentrifugation, dynamic light-scattering and size-exclusion chromatography, all revealing the native hydrodynamic behavior and molar mass of the protein. We show that GNE is able to reversibly self-associate into different oligomeric states including monomers, dimers and tetramers. Additionally, it forms non-specific aggregates of high molecular mass, which cannot be unequivocally assigned a distinct size. Our results also indicate that ligands of the epimerase domain of the bifunctional enzyme, namely UDP-N-acetylglucosamine and CMP-N-acetylneuraminic acid, stabilize the protein against aggregation and are capable of modulating the quaternary structure of the protein. The presence of UDP-N-acetylglucosamine strongly favors the tetrameric state, which therefore likely represents the active state of the enzyme in cells.

The Central Cavity from the (Alpha/Alpha)6 Barrel Structure of Anabaena sp. CH1 N-Acetyl-d-glucosamine 2-Epimerase Contains Two Key Histidine Residues for Reversible Conversion

N-acetyl-D-glucosamine 2-epimerase (GlcNAc 2-epimerase) catalyzes the reversible epimerization between N-acetyl-D-glucosamine (GlcNAc) and N-acetyl-D-mannosamine (ManNAc). We report here the 2.0 A resolution crystal structure of the GlcNAc 2-epimerase from Anabaena sp. CH1. The structure demonstrates an (alpha/alpha)(6) barrel fold, which shows structural homology with porcine GlcNAc 2-epimerase as well as a number of glycoside hydrolase enzymes and other sugar-metabolizing enzymes. One side of the barrel structure consists of short loops involved in dimer interactions. The other side of the barrel structure is comprised of long loops containing six short beta-sheets, which enclose a putative central active-site pocket. Site-directed mutagenesis of conserved residues near the N-terminal region of the inner alpha helices shows that R57, H239, E308, and H372 are strictly required for activity. E242 and R375 are also essential in catalysis. Based on the structure and kinetic analysis, H239 and H372 may serve as the key active site acid/base catalysts. These results suggest that the (alpha/alpha)(6) barrel represents a steady fold for presenting active-site residues in a cleft at the N-terminal ends of the inner alpha helices, thus forming a fine-tuned catalytic site in GlcNAc 2-epimerase.

The collapsin response mediator protein 1 (CRMP-1) and the promyelocytic leukemia zinc finger protein (PLZF) bind to UDP-N-acetylglucosamine 2-epimerase/N-acetylmannosamine kinase (GNE), the key enzyme of sialic acid biosynthesis

Sialic acids (Sia) are expressed as terminal sugars in many glycoconjugates. They are involved in a variety of cell-cell interactions and therefore play an important role during development and regeneration. UDP-N-acetylglucosamine 2-epimerase/N-acetylmannosamine kinase (GNE) is the key enzyme in the de novo synthesis of Sia and it is a regulator of cell surface sialylation. Inactivation of GNE in mice results in early embryonic lethality. Mutations in the GNE gene are of clinical relevance in hereditary inclusion body myopathy, but these mutations do not necessarily decrease the enzymatic activity of GNE. In this study, we searched for novel function of the GNE protein beside its enzymatic function in the Sia biosynthesis. We here report the identification of novel GNE-interacting proteins. Using a human prey matrix we identified four proteins interacting with GNE in a yeast two-hybrid assay. For two of them, the collapsin response mediator protein 1 and the promyelocytic leukemia zinc finger protein, we could verify protein-protein interaction with GNE.

Abnormal glycosylation with hypersialylated O-glycans in patients with Sialuria

Sialuria is an inborn error of metabolism characterized by coarse face, hepatomegaly and recurrent respiratory tract infections. The genetic defect in this disorder results in a loss of feedback control of UDP-N-acetylglucosamine 2-epimerase/N-acetylmannosamine-kinase by CMP-N-acetylneuraminic acid (CMP-NeuAc) resulting in a substantial overproduction of cytoplasmic free sialic acid. This study addresses fibroblast CMP-NeuAc levels and N- and O-glycan sialylation of serum proteins from Sialuria patients. CMP-NeuAc levels were measured with HPLC in fibroblasts. Isoelectric focusing (IEF) of serum transferrin and of apolipoprotein C-III (apoC-III) was performed on serum of three Sialuria patients. Isoforms of these proteins can be used as specific markers for the biosynthesis of N- and core 1 O-glycans. Furthermore, total N- and O-linked glycans from serum proteins were analyzed by HPLC. HPLC showed a clear overproduction of CMP-NeuAc in fibroblasts of a Sialuria patient. Minor changes were found for serum N-glycans and hypersialylation was found for core 1 O-glycans on serum apoC-III and on total serum O-glycans in Sialuria patients. HPLC showed an increased ratio of disialylated over monosialylated core 1 O-glycans. The hypersialylation of core 1 O-glycans is due to the increase of NeuAcalpha2,6-containing structures (mainly NeuAcalpha2-3Galbeta1-3[NeuAcalpha2-6]GalNAc). This may relate to KM differences between GalNAc-alpha2,6-sialyltransferase and alpha2,3-sialyltransferases. This is the first study demonstrating that the genetic defect in Sialuria results in a CMP-NeuAc overproduction. Subsequently, increased amounts of alpha2,6-linked NeuAc were found on serum core 1 O-glycans from Sialuria patients. N-glycosylation of serum proteins seems largely unaffected. Sialuria is the first metabolic disorder presenting with hypersialylated O-glycans.

Distal myopathy with rimmed vacuoles in a case of opercular syndrome

We report the case of a 30-year-old man with opercular syndrome who developed distal myopathy with rimmed vacuoles (DMRV). Muscle biopsy showed variation in fiber size and scattered fibers with rimmed vacuoles. The identification of a homozygous c. 1714G>C (p. V572L) mutation in the GNE gene genetically confirmed the diagnosis of DMRV, which is thought to be identical to hereditary inclusion body myopathy (HIBM). Our results indicate the possibility that other organs such as the central nervous system could be affected in DMRV/HIBM, although bilateral opercular lesions might have been caused by destructive events either in utero or in the perinatal period.

Mechanisms in protein O-glycan biosynthesis and clinical and molecular aspects of protein O-glycan biosynthesis defects: a review

BACKGROUND

Genetic diseases that affect the biosynthesis of protein O-glycans are a rapidly growing group of disorders. Because this group of disorders does not have a collective name, it is difficult to get an overview of O-glycosylation in relation to human health and disease. Many patients with an unsolved defect in N-glycosylation are found to have an abnormal O-glycosylation as well. It is becoming increasingly evident that the primary defect of these disorders is not necessarily localized in one of the glycan-specific transferases, but can likewise be found in the biosynthesis of nucleotide sugars, their transport to the endoplasmic reticulum (ER)/Golgi, and in Golgi trafficking. Already, disorders in O-glycan biosynthesis form a substantial group of genetic diseases. In view of the number of genes involved in O-glycosylation processes and the increasing scientific interest in congenital disorders of glycosylation, it is expected that the number of identified diseases in this group will grow rapidly over the coming years.

CONTENT

We first discuss the biosynthesis of protein O-glycans from their building blocks to their secretion from the Golgi. Subsequently, we review 24 different genetic disorders in O-glycosylation and 10 different genetic disorders that affect both N- and O-glycosylation. The key clinical, metabolic, chemical, diagnostic, and genetic features are described. Additionally, we describe methods that can be used in clinical laboratory screening for protein O-glycosylation biosynthesis defects and their pitfalls. Finally, we introduce existing methods that might be useful for unraveling O-glycosylation defects in the future.

Use of a cell-free system to determine UDP-N-acetylglucosamine 2-epimerase and N-acetylmannosamine kinase activities in human hereditary inclusion body myopathy

Hereditary inclusion body myopathy (HIBM) is an autosomal recessive neuromuscular disorder associated with mutations in uridine diphosphate (UDP)-N-acetylglucosamine (GlcNAc) 2-epimerase (GNE)/N-acetylmannosamine (ManNAc) kinase (MNK), the bifunctional and rate-limiting enzyme of sialic acid biosynthesis. We developed individual GNE and MNK enzymatic assays and determined reduced activities in cultured fibroblasts of patients, with HIBM harboring missense mutations in either or both the GNE and MNK enzymatic domains. To assess the effects of individual mutations on enzyme activity, normal and mutated GNE/MNK enzymatic domains were synthesized in a cell-free in vitro transcription-translation system and subjected to the GNE and MNK enzymatic assays. This cell-free system was validated for both GNE and MNK activities, and it revealed that mutations in one enzymatic domain (in GNE, G135V, V216A, and R246W; in MNK, A631V, M712T) affected not only that domain's enzyme activity, but also the activity of the other domain. Moreover, studies of the residual enzyme activity associated with specific mutations revealed a discrepancy between the fibroblasts and the cell-free systems. Fibroblasts exhibited higher residual activities of both GNE and MNK than the cell-free system. These findings add complexity to the tightly regulated system of sialic acid biosynthesis. This cell-free approach can be applied to other glycosylation pathway enzymes that are difficult to evaluate in whole cells because their substrate specificities overlap with those of ancillary enzymes.

Domain-specific characteristics of the bifunctional key enzyme of sialic acid biosynthesis, UDP-N-acetylglucosamine 2-epimerase/N-acetylmannosamine kinase

UDP-N-acetylglucosamine 2-epimerase/N-acetylmannosamine kinase is a bifunctional enzyme, which initiates and regulates sialic acid biosynthesis. Sialic acids are important compounds of mammalian glycoconjugates, mediating several biological processes, such as cell-cell or cell-matrix interactions. In order to characterize the function of UDP-N-acetylglucosamine 2-epimerase/N-acetylmannosamine kinase, a number of deletion mutants were generated, lacking either parts of the N-terminal epimerase or the C-terminal kinase domain. N-terminal deletion of only 39 amino acids results in a complete loss of epimerase activity. Deletions in the C-terminal part result in a reduction or complete loss of kinase activity, depending on the size of the deletion. Deletions at either the N- or the C-terminus also result in a reduction of the other enzyme activity. These results indicate that a separate expression of both domains is possible, but that a strong intramolecular dependency of the two domains has arisen during evolution of the enzyme. N-terminal, as well as C-terminal, mutants tend to form trimers, in addition to the hexameric structure of the native enzyme. These results and yeast two-hybrid experiments show that structures required for dimerization are localized within the kinase domain, and a potential trimerization site is possibly located in a region between the two domains. In conclusion, our results reveal that the activities, as well as the oligomeric structure, of this bifunctional enzyme seem to be organized and regulated in a complex manner.

Localization of UDP-GlcNAc 2-epimerase/ManAc kinase (GNE) in the Golgi complex and the nucleus of mammalian cells

The bifunctional enzyme UDP-N-acetylglucosamine-2-epimerase/N-acetylmannosamine kinase (GNE) is essential for early embryonic development and catalyzes the rate limiting step in sialic acid biosynthesis. Although epimerase and kinase activities have been attributed to GNE, little is known about the regulation, differential expression, and subcellular localization of GNE in vivo. Mutations in GNE cause a rare inherited muscle disorder in humans called hereditary inclusion body myopathy (HIBM). However, the role of GNE in HIBM pathogenesis has not been defined yet. Here, we show that the GNE protein is expressed in various mammalian cells and tissues with highest levels found in cancer cells and liver. In human skeletal muscle, GNE protein is developmentally regulated: high levels are found in immature myoblasts but low levels in mature skeletal muscle. The GNE protein colocalizes with resident proteins of the Golgi compartment in a variety of human cells including muscle. Drug-induced disruption of the Golgi and subsequent recovery reveals co-distribution of GNE along with Golgi-targeted proteins. This subcellular localization of GNE is in good agreement with its established role as the key enzyme of sialic acid biosynthesis, since the sialylation of glycoconjugates takes place in the Golgi complex. Surprisingly, GNE is also detected in the nucleus. Upon nocodazole treatment, GNE redistributes to the cytoplasm suggesting that GNE may act as a nucleocytoplasmic shuttling protein. A regulatory role for GNE shifting between the nuclear and the Golgi compartment is proposed. Further insight into GNE regulation may promote the understanding of HIBM pathogenesis.

Characterization of ligand binding to the bifunctional key enzyme in the sialic acid biosynthesis by NMR: I. Investigation of the UDP-GlcNAc 2-epimerase functionality

The bifunctional enzyme UDP-N-acetylglucosamine 2-epimerase/N-acetylmannosamine kinase is the key enzyme for the biosynthesis of sialic acids. As terminal components of glycoconjugates, sialic acids are associated with a variety of pathological processes such as inflammation and cancer. For the first time, this study reveals characteristics of the interaction of the epimerase site of the enzyme with its natural substrate, UDP-N-acetylglucosamine (UDP-GlcNAc) and derivatives thereof at atomic resolution. Saturation transfer difference NMR experiments were crucial in obtaining ligand binding epitopes and to rank ligands according to their binding affinities. Employing a fragment based approach, it was possible to assign the major component of substrate recognition to the UDP moiety. In particular, the binding epitopes of the uridine moieties of UMP, UDP, UDP-GalNAc, and UDP-GlcNAc are rather similar, suggesting that the binding mode of the UDP moiety is the same in all cases. In contrast, the hexopyranose units of UDP-GlcNAc and UDP-GalNAc display small differences reflecting the inability of the enzyme to process UDP-GalNAc. Surprisingly, saturation transfer difference NMR titrations show that UDP has the largest binding affinity to the epimerase site and that at least one phosphate group is required for binding. Consequently, this study provides important new data for rational drug design.

UDP-N-acetylglucosamine 2-epimerase/N-acetylmannosamine kinase, functionally expressed in and purified from Escherichia coli, yeast, and insect cells

UDP-GlcNAc 2-epimerase/ManNAc kinase is the key enzyme of sialic acid biosynthesis in mammals. Its functional expression is a prerequisite for early embryogenesis and for the synthesis of several cell recognition motifs in adult organism. This bifunctional enzyme is involved in the development of different diseases like sialuria or hereditary inclusion body myopathy. For a detailed understanding of the enzyme, large amounts of the pure active protein are needed. Different heterologous cell systems were therefore analyzed for the enzyme, which was found to be functionally expressed in Escherichia coli, the yeast strains Saccharomyces cerevisiae and Pichia pastoris, and insect cells. In all these cell types, the expressed enzyme displayed both epimerase and kinase activities. In E. coli, up to 2mg protein/l cell culture was expressed, in yeast cells only 0.4mg/L, while up to 100mg/L, were detected in insect cells. In all three cell systems, insoluble protein aggregates were also observed. Purification from E. coli resulted in 100microg/L pure and structurally intact protein. For insect cells, purification methods were established which resulted in up to 50mg/L pure, soluble, and active protein. In summary, expression and purification of the UDP-GlcNAc 2-epimerase/ManNAc kinase in Sf-900 cells can yield the milligram amounts of protein required for structural characterization of the enzyme. However, the easier expression in E. coli and yeast provides sufficient quantities for enzymatic and kinetic characterization.

Increased expression of the selectin ligand sialyl–Lewisx by biochemical engineering of sialic acids

Sialylation of glycoproteins and glycolipids plays an important role during development, regeneration and pathogenesis of several diseases. The precursor of all physiological sialic acids is N-acetyl-d-mannosamine. Using N-propanoyl mannosamine, a novel precursor of sialic acid, we showed earlier that sialic acids with a prolonged N-acyl side chain (e.g., N-propanoyl neuraminic acid) are incorporated into cell surface glycoconjugates. In this study, we report the structural and functional consequences of the incorporation of the nonphysiological sialic acid, N-propanoyl neuraminic acid, into glycoconjugates of HL60-I cells. These cells do not express UDP-GlcAc-2-epimerase, the key enzyme of the biosynthesis of N-acetyl-d-mannosamine. Therefore, they do not express sialyl-Lewis(x) structures and consequently do not bind to selectins. Application of N-acetyl-d-mannosamine leads to the expression of sialyl-Lewis(x) structures and to binding to selectins. Surprisingly, incorporation of N-propanoyl neuraminic acid into glycoconjugates of these cells leads to a dramatic increase of sialyl-Lewis(x) structures and to increased adhesion to selectins.

Reduction of UDP-N-acetylglucosamine 2-epimerase/N-acetylmannosamine kinase activity and sialylation in distal myopathy with rimmed vacuoles

Distal myopathy with rimmed vacuoles is an autosomal recessive muscle disease with preferential involvement of the tibialis anterior that spares the quadriceps muscles in young adulthood. In a Japanese patient with distal myopathy with rimmed vacuoles, we identified pathogenic mutations in the gene encoding the bifunctional enzyme UDP-GlcNAc 2-epimerase/ManNAc kinase, which catalyzes the initial two steps in the biosynthesis of sialic acid. In this study, we demonstrated the relationship between the genetic mutations and enzymatic activities using an in vitro expression assay system. Furthermore, we also showed that the levels of sialic acid in muscle and primary cultured cells from DMRV patients were reduced to 60-75% of control. The reactivities to lectins were also variable in some myofibers, suggesting that hyposialylation and abnormal glycosylation in muscles may contribute to the focal accumulations of autophagic vacuoles, amyloid deposits, or both in patient muscle tissue. The addition of ManNAc and NeuAc to primary cultured cells normalized sialylation levels, thus demonstrating the therapeutic potential of these compounds for this disease.

Epigenetically mediated loss of UDP-GlcNAc 2-epimerase/ManNAc kinase expression in hyposialylated cell lines

The bifunctional enzyme UDP-GlcNAc 2-epimerase/ManNAc kinase is the key enzyme in sialic acid biosynthesis. Loss of UDP-GlcNAc 2-epimerase activity results in a hyposialylated phenotype as shown for two human hematopoietic cell lines that lack the specific mRNA. We found that treatment with the DNA methylation inhibitor 5'-aza-2'-deoxycytidine (5-aza-dC) restored the UDP-GlcNAc 2-epimerase/ManNAc kinase mRNA, as well as enzyme activity and cell surface sialylation. Increase of UDP-GlcNAc 2-epimerase activity by 5-aza-dC treatment was also found for a rat Morris hepatoma cell line. These results indicate a regulation of UDP-GlcNAc 2-epimerase/ManNAc kinase expression on the transcriptional level by DNA methylation.

GlcNAc 2-epimerase can serve a catabolic role in sialic acid metabolism

Sialic acid is a major determinant of carbohydrate-receptor interactions in many systems pertinent to human health and disease. N-Acetylmannosamine (ManNAc) is the first committed intermediate in the sialic acid biosynthetic pathway; thus, the mechanisms that control intracellular ManNAc levels are important regulators of sialic acid production. UDP-GlcNAc 2-epimerase and GlcNAc 2-epimerase are two enzymes capable of generating ManNAc from UDP-GlcNAc and GlcNAc, respectively. Whereas the former enzyme has been shown to direct metabolic flux toward sialic acid in vivo, the function of the latter enzyme is unclear. Here we study the effects of GlcNAc 2-epimerase expression on sialic acid production in cells. A key tool we developed for this study is a cell-permeable, small molecule inhibitor of GlcNAc 2-epimerase designed based on mechanistic principles. Our results indicate that, unlike UDP-GlcNAc 2-epimerase, which promotes biosynthesis of sialic acid, GlcNAc 2-epimerase can serve a catabolic role, diverting metabolic flux away from the sialic acid pathway.

Unfolding story of inclusion-body myositis and myopathies: role of misfolded proteins, amyloid-beta, cholesterol, and aging

Sporadic inclusion-body myositis and hereditary inclusion-body myopathies are progressive muscle diseases leading to severe disability. We briefly summarize their clinical pictures and pathologic diagnostic criteria and discuss the latest advances in illuminating their pathogenic mechanism(s). We emphasize how different etiologies might lead to the strikingly similar pathology and possibly similar pathogenic cascade. On the basis of our research, several processes seem to be important in relation to the still speculative pathogenesis, including (a) increased transcription and accumulation of amyloid-beta precursor protein and accumulation of its proteolytic fragment amyloid-beta; (b) abnormal accumulation of components related to lipid metabolism, for example, cholesterol, accumulation of which is possibly owing to its abnormal trafficking; (c) oxidative stress; (d) accumulations of other Alzheimer's disease-related proteins; and (e) a milieu of muscle cellular aging in which these changes occur. We discuss a potentially very important role of unfolded and/or misfolded proteins as a possible mechanism in the formations of the inclusion bodies and other abnormalities.

Magnesium may help patients with recessive hereditary inclusion body myopathy, a pathological review

Recently, bi-allelic mutations in the gene coding for the bi-functional enzyme UDP-N-acetylglucosamine 2-epimerase/N-acetylmannosamine kinase (GNE/MNK), symbol GNE or GLCNE (MIM: 603824), were associated with the recessively inherited phenotype of IBM2 (MIM: 600737). All patients tested so far have bi-allelic missense mutation(s) of epimerase and/or kinase domains of GNE gene, which clearly explains the recessive inheritance pattern of this phenotype. Single allelic mutations of codons 263-266 of GNE have been implicated as the cause of French type sialuria (MIM: 269921). The dominantly inherited French type sialuria seems to result from defective allosteric feedback inhibitory regulation of GNE/MNK by cytidine monophosphate-N-acetylneuraminic acid (CMP-NANA), resulting in overproduction of cytosolic N-acetylneuraminic acid, and massive urinary excretion of free sialic acid. Because GNE is relatively weakly expressed in skeletal muscle cells, and involvement of other organs are not clinically evident in patients affected with IBM2, it is likely that the missense mutation(s) found in these patients cause a partial reduction of the efficiency of either the epimerase or the kinase activity of this enzyme. Therapeutic dietary modifications are recommended including reduction of ethanol consumption, avoidance of excess selenium, copper, and zinc, and dietary promotion of magnesium (Mg(2+)), which is an essential co-factor for this enzyme.

Inclusion-body myositis and myopathies: different etiologies, possibly similar pathogenic mechanisms

PURPOSE OF REVIEW

Sporadic inclusion-body myositis (s-IBM) and hereditary inclusion body myopathies are progressive muscle diseases that lead to severe disability. We discuss recent advances in illuminating their pathogenic mechanism(s).

RECENT FINDINGS

We emphasize how different etiologies might lead to the strikingly similar pathology and possibly similar pathogenic cascade. Our basic hypothesis is that over-expression of amyloid-beta precursor protein within aging muscle fibers is an early upstream event causing the subsequent pathogenic cascade. On the basis of our research, several processes seem to be important in relation to the still speculative pathogenesis: (a) increased transcription and accumulation of amyloid-beta precursor protein, and accumulation of its proteolytic fragment Abeta; (b) accumulations of phosphorylated tau and other Alzheimer-related proteins; (c) accumulation of cholesterol and low-density lipoprotein receptors, the cholesterol accumulation possibly due to its abnormal trafficking; (d) oxidative stress; and (e) a milieu of muscle cellular aging in which these changes occur. We discuss unfolded and/or misfolded proteins as a possible mechanism in formation of the inclusion bodies and their consequences. The remarkable pathologic similarities between s-IBM muscle and Alzheimer disease brain are discussed.

SUMMARY

Unfolding knowledge of the various pathogenetic aspects of the s-IBMs and hereditary inclusion body myopathies may lead to new therapeutic avenues.

Sialylation is essential for early development in mice

Sialic acids are widely expressed as terminal carbohydrates on glycoconjugates of eukaryotic cells. Sialylation is crucial for a variety of cellular functions, such as cell adhesion or signal recognition, and regulates the biological stability of glycoproteins. The key enzyme of sialic acid biosynthesis is the bifunctional UDP-N-acetylglucosamine-2-epimerase/N-acetylmannosamine kinase (UDP-GlcNAc 2-epimerase), which catalyzes the first two steps of sialic acid biosynthesis in the cytosol. We report that inactivation of the UDP-GlcNAc 2-epimerase by gene targeting causes early embryonic lethality in mice, thereby emphasizing the fundamental role of this bifunctional enzyme and sialylation during development. The need of UDP-GlcNAc 2-epimerase for a defined sialylation process is exemplified with the polysialylation of the neural cell adhesion molecule in embryonic stem cells.

Metabolic selection of glycosylation defects in human cells

Changes in glycosylation are often associated with disease progression, but the genetic and metabolic basis of these events is rarely understood in detail at a molecular level. We describe a metabolism-based approach to the selection of mutants in glycoconjugate biosynthesis that provides insight into regulatory mechanisms for oligosaccharide expression and metabolic flux. Unnatural intermediates are used to challenge a specific pathway, and cell surface expression of their metabolic products provides a readout of flux in that pathway and a basis for selecting genetic mutants. The approach was applied to the sialic acid metabolic pathway in human cells, yielding novel mutants with phenotypes related to the inborn metabolic defect sialuria and metastatic tumor cells.

Protein kinase C phosphorylates and regulates UDP-N-acetylglucosamine-2-epimerase/N-acetylmannosamine kinase

UDP-N-acetylglucosamine-2-epimerase/N-acetylmannosamine kinase (UDP-GlcNAc 2-epimerase) is the key enzyme in the de novo synthesis pathway of neuraminic acid, which is widely expressed as a terminal carbohydrate residue on glycoconjugates. UDP-GlcNAc 2-epimerase is a bifunctional enzyme and catalyzes the first two steps of neuraminic acid synthesis in the cytosol, the conversion of UDP-N-acetylglucosamine to ManAc and the phosphorylation to ManAc-6-phosphate. So far, regulation of this essential enzyme by posttranslational modification has not been shown. Since UDP-N-acetylglucosamine is a cytosolic protein containing eight conserved motifs for protein kinase C (PKC), we investigated whether its enzymatic activity might be regulated by phosphorylation by PKC. We showed that UDP-GlcNAc 2-epimerase interacts with several isoforms of PKC in mouse liver and is phosphorylated in vivo. Furthermore, PKC phosphorylates UDP-GlcNAc 2-epimerase and this phosphorylation results in an upregulation of the UDP-GlcNAc 2-epimerase enzyme activity.

Selective loss of either the epimerase or kinase activity of UDP-N-acetylglucosamine 2-epimerase/N-acetylmannosamine kinase due to site-directed mutagenesis based on sequence alignments

N-Acetylneuraminic acid is the most common naturally occurring sialic acid, as well as being the biosynthetic precursor of this group of compounds. UDP-GlcNAc 2-epimerase/N-acetylmannosamine kinase has been shown to be the key enzyme of N-acetylneuraminic acid biosynthesis in rat liver, and it is a regulator of cell surface sialylation. The N-terminal region of this bifunctional enzyme displays sequence similarities with prokaryotic UDP-GlcNAc 2-epimerases, whereas the sequence of its C-terminal region is similar to sequences of members of the sugar kinase superfamily. High level overexpression of active enzyme was established by using the baculovirus/Sf9 system. For functional characterization, site-directed mutagenesis was performed on different conserved amino acid residues. The histidine mutants H45A, H110A, H132A, H155A, and H157A showed a drastic loss of epimerase activity with almost unchanged kinase activity. Conversely, the mutants D413N, D413K, and R420M in the putative kinase active site lost their kinase activity but retained their epimerase activity. To estimate the structural perturbation effect due to site-directed mutagenesis, the oligomeric state of all mutants was determined by gel filtration analysis. The mutants D413N, D413K, and R420M as well as H45A were shown to form a hexamer like the wild-type enzyme, indicating little influence of mutation on protein folding. Histidine mutants H155A and H157A formed mainly trimeric enzyme with small amounts of hexamer. Oligomerization of mutants H110A and H132A was also significantly different from that of the wild-type enzyme. Therefore the loss of epimerase activity in mutants H110A, H132A, H155A, and H157A can largely be attributed to incorrect protein folding. In contrast, the mutation site of mutant H45A seems to be involved directly in the epimerization process, and the amino acids Asp-413 and Arg-420 of UDP-GlcNAc 2-epimerase/N-acetylmannosamine kinase are essential for the phosphorylation process. The fact that either epimerase or kinase activity are lost selectively provides evidence for the existence of two active sites working quite independently.