Chondroitin beta-1,4-N-acetylgalactosaminyltransferase-1 missense mutations are associated with neuropathies

Chondroitin sulfate in tissue remodeling: Therapeutic implications for pulmonary fibrosis

Fibrosis is characterized by the deposition of extracellular matrix (ECM) proteins, while idiopathic pulmonary fibrosis (IPF) is a chronic respiratory disease characterized by dysregulated tissue repair and remodeling. Anti-inflammatory drugs, such as corticosteroids and immunosuppressants, and antifibrotic drugs, like pirfenidone and nintedanib, are used in IPF therapy. However, their limited effects suggest that single mediators are inadequate to control IPF. Therefore, therapies targeting the multifactorial cascades that regulate tissue remodeling in fibrosis could provide alternate solutions. ECM molecules have been shown to modulate various biological functions beyond tissue structure support and thus, could be developed into novel therapeutic targets for modulating tissue remodeling. Among ECM molecules, glycosaminoglycans (GAG) are linear polysaccharides consisting of repeated disaccharides, which regulate cell-matrix interactions. Chondroitin sulfate (CS), one of the major GAGs, binds to multifactorial mediators in the ECM and reportedly participates in tissue remodeling in various diseases; however, to date, its biological functions have drawn considerably less attention than other GAGs, like heparan sulfate. In the present review, we discuss the involvement and regulation of CS in tissue remodeling and pulmonary fibrotic diseases, its role in pulmonary fibrosis, and the therapeutic approaches targeting CS.

Bone and connective tissue disorders caused by defects in glycosaminoglycan biosynthesis: a panoramic view

Glycosaminoglycans (GAGs) are a heterogeneous family of linear polysaccharides that constitute the carbohydrate moiety covalently attached to the protein core of proteoglycans, macromolecules present on the cell surface and in the extracellular matrix. Several genetic disorders of bone and connective tissue are caused by mutations in genes encoding for glycosyltransferases, sulfotransferases and transporters that are responsible for the synthesis of sulfated GAGs. Phenotypically, these disorders all reflect alterations in crucial biological functions of GAGs in the development, growth and homoeostasis of cartilage and bone. To date, up to 27 different skeletal phenotypes have been linked to mutations in 23 genes encoding for proteins involved in GAG biosynthesis. This review focuses on recent genetic, molecular and biochemical studies of bone and connective tissue disorders caused by GAG synthesis defects. These insights and future research in the field will provide a deeper understanding of the molecular pathogenesis of these disorders and will pave the way for developing common therapeutic strategies that might be targeted to a range of individual phenotypes.

High-throughput DNA methylation analysis in anorexia nervosa confirms TNXB hypermethylation

OBJECTIVES

Patients with anorexia nervosa (AN) are ideally suited to identify differentially methylated genes in response to starvation.

METHODS

We examined high-throughput DNA methylation derived from whole blood of 47 females with AN, 47 lean females without AN and 100 population-based females to compare AN with both controls. To account for different cell type compositions, we applied two reference-free methods (FastLMM-EWASher, RefFreeEWAS) and searched for consensus CpG sites identified by both methods. We used a validation sample of five monozygotic AN-discordant twin pairs.

RESULTS

Fifty-one consensus sites were identified in AN vs. lean and 81 in AN vs. population-based comparisons. These sites have not been reported in AN methylation analyses, but for the latter comparison 54/81 sites showed directionally consistent differential methylation effects in the AN-discordant twins. For a single nucleotide polymorphism rs923768 in CSGALNACT1 a nearby site was nominally associated with AN. At the gene level, we confirmed hypermethylated sites at TNXB. We found support for a locus at NR1H3 in the AN vs. lean control comparison, but the methylation direction was opposite to the one previously reported.

CONCLUSIONS

We confirm genes like TNXB previously described to comprise differentially methylated sites, and highlight further sites that might be specifically involved in AN starvation processes.

Roles of CSGalNAcT1, a key enzyme in regulation of CS synthesis, in neuronal regeneration and plasticity

Chondroitin sulfate (CS) is a sulfated glycosaminoglycan composed of a long chain of repeating disaccharide units that are attached to core proteins, resulting in CS proteoglycans (CSPGs). In the mature brain, CS is concentrated in perineuronal nets (PNNs), which are extracellular structures that surround synapses and regulate synaptic plasticity. In addition, CS is rapidly synthesized after CNS injury to create a physical and chemical barrier that inhibits axon growth. Most previous studies used a bacterial CS-degrading enzyme to investigate the physiological roles of CS. Recent studies have shown that CS is synthesized by more than 15 enzymes, all of which have been characterized in vitro. Here we focus on one of those enzymes, CSGalNAcT1 (T1). We produced T1 knockout mice (KO), which show extensive axon regeneration following spinal cord injury, as well as the loss of onset of ocular dominance plasticity. These results from T1KO mice suggest important roles for extracellular CS in the brain regarding neuronal plasticity and axon regeneration.

Chondroitin Sulfate Is Required for Onset and Offset of Critical Period Plasticity in Visual Cortex

Ocular dominance plasticity is easily observed during the critical period in early postnatal life. Chondroitin sulfate (CS) is the most abundant component in extracellular structures called perineuronal nets (PNNs), which surround parvalbumin-expressing interneurons (PV-cells). CS accumulates in PNNs at the critical period, but its function in earlier life is unclear. Here, we show that initiation of ocular dominance plasticity was impaired with reduced CS, using mice lacking a key CS-synthesizing enzyme, CSGalNAcT1. Two-photon in vivo imaging showed a weaker visual response of PV-cells with reduced CS compared to wild-type mice. Plasticity onset was restored by a homeoprotein Otx2, which binds the major CS-proteoglycan aggrecan and promotes its further expression. Continuous CS accumulation together with Otx2 contributed bidirectionally to both onset and offset of plasticity, and was substituted by diazepam, which enhances GABA function. Therefore, CS and Otx2 may act as common inducers of both onset and offset of the critical period by promoting PV-cell function throughout the lifetime.

Biosynthesis of glycosaminoglycans: associated disorders and biochemical tests

Glycosaminoglycans (GAG) are long, unbranched heteropolymers with repeating disaccharide units that make up the carbohydrate moiety of proteoglycans. Six distinct classes of GAGs are recognized. Their synthesis follows one of three biosynthetic pathways, depending on the type of oligosaccharide linker they contain. Chondroitin sulfate, dermatan sulfate, heparan sulfate, and heparin sulfate contain a common tetrasaccharide linker that is O-linked to specific serine residues in core proteins. Keratan sulfate can contain three different linkers, either N-linked to asparagine or O-linked to serine/threonine residues in core proteins. Finally, hyaluronic acid does not contain a linker and is not covalently attached to a core protein. Most inborn errors of GAG biosynthesis are reported in small numbers of patients. To date, in 20 diseases, convincing evidence for pathogenicity has been presented for mutations in a total of 16 genes encoding glycosyltransferases, sulfotransferases, epimerases or transporters. GAG synthesis defects should be suspected in patients with a combination of characteristic clinical features in more than one connective tissue compartment: bone and cartilage (short long bones with or without scoliosis), ligaments (joint laxity/dislocations), and subepithelial (skin, sclerae). Some produce distinct clinical syndromes. The commonest laboratory tests used for this group of diseases are analysis of GAGs, enzyme assays, and molecular testing. In principle, GAG analysis has potential as a general first-line diagnostic test for GAG biosynthesis disorders.

GlcUAβ1-3Galβ1-3Galβ1-4Xyl(2-O-phosphate) is the preferred substrate for chondroitin N-acetylgalactosaminyltransferase-1

A deficiency in chondroitin N-acetylgalactosaminyltransferase-1 (ChGn-1) was previously shown to reduce the number of chondroitin sulfate (CS) chains, leading to skeletal dysplasias in mice, suggesting that ChGn-1 regulates the number of CS chains for normal cartilage development. Recently, we demonstrated that 2-phosphoxylose phosphatase (XYLP) regulates the number of CS chains by dephosphorylating the Xyl residue in the glycosaminoglycan-protein linkage region of proteoglycans. However, the relationship between ChGn-1 and XYLP in controlling the number of CS chains is not clear. In this study, we for the first time detected a phosphorylated tetrasaccharide linkage structure, GlcUAβ1-3Galβ1-3Galβ1-4Xyl(2-O-phosphate), in ChGn-1(-/-) growth plate cartilage but not in ChGn-2(-/-) or wild-type growth plate cartilage. In contrast, the truncated linkage tetrasaccharide GlcUAβ1-3Galβ1-3Galβ1-4Xyl was detected in wild-type, ChGn-1(-/-), and ChGn-2(-/-) growth plate cartilage. Consistent with the findings, ChGn-1 preferentially transferred N-acetylgalactosamine to the phosphorylated tetrasaccharide linkage in vitro. Moreover, ChGn-1 and XYLP interacted with each other, and ChGn-1-mediated addition of N-acetylgalactosamine was accompanied by rapid XYLP-dependent dephosphorylation during formation of the CS linkage region. Taken together, we conclude that the phosphorylated tetrasaccharide linkage is the preferred substrate for ChGn-1 and that ChGn-1 and XYLP cooperatively regulate the number of CS chains in growth plate cartilage.

Human genetic disorders and knockout mice deficient in glycosaminoglycan

Glycosaminoglycans (GAGs) are constructed through the stepwise addition of respective monosaccharides by various glycosyltransferases and maturated by epimerases and sulfotransferases. The structural diversity of GAG polysaccharides, including their sulfation patterns and sequential arrangements, is essential for a wide range of biological activities such as cell signaling, cell proliferation, tissue morphogenesis, and interactions with various growth factors. Studies using knockout mice of enzymes responsible for the biosynthesis of the GAG side chains of proteoglycans have revealed their physiological functions. Furthermore, mutations in the human genes encoding glycosyltransferases, sulfotransferases, and related enzymes responsible for the biosynthesis of GAGs cause a number of genetic disorders including chondrodysplasia, spondyloepiphyseal dysplasia, and Ehlers-Danlos syndromes. This review focused on the increasing number of glycobiological studies on knockout mice and genetic diseases caused by disturbances in the biosynthetic enzymes for GAGs.

A chondroitin synthase-1 (ChSy-1) missense mutation in a patient with neuropathy impairs the elongation of chondroitin sulfate chains initiated by chondroitin N-acetylgalactosaminyltransferase-1

BACKGROUND

Previously, we identified two missense mutations in the chondroitin N-acetylgalactosaminyltransferase-1 gene in patients with neuropathy. These mutations are associated with a profound decrease in chondroitin N-acetylgalactosaminyltransferase-1 enzyme activity. Here, we describe a patient with neuropathy who is heterozygous for a chondroitin synthase-1 mutation. Chondroitin synthase-1 has two glycosyltransferase activities: it acts as a GlcUA and a GalNAc transferase and is responsible for adding repeated disaccharide units to growing chondroitin sulfate chains.

METHODS

Recombinant wild-type chondroitin synthase-1 enzyme and the F362S mutant were expressed. These enzymes and cells expressing them were then characterized.

RESULTS

The mutant chondroitin synthase-1 protein retained approximately 50% of each glycosyltransferase activity relative to the wild-type chondroitin synthase-1 protein. Furthermore, unlike chondroitin polymerase comprised of wild-type chondroitin synthase-1 protein, the non-reducing terminal 4-O-sulfation of GalNAc residues synthesized by chondroitin N-acetylgalactosaminyltransferase-1 did not facilitate the elongation of chondroitin sulfate chains when chondroitin polymerase that consists of the mutant chondroitin synthase-1 protein was used as the enzyme source.

CONCLUSIONS

The chondroitin synthase-1 F362S mutation in a patient with neuropathy resulted in a decrease in chondroitin polymerization activity and the mutant protein was defective in regulating the number of chondroitin sulfate chains via chondroitin N-acetylgalactosaminyltransferase-1. Thus, the progression of peripheral neuropathies may result from defects in these regulatory systems.

GENERAL SIGNIFICANCE

The elongation of chondroitin sulfate chains may be tightly regulated by the cooperative expression of chondroitin synthase-1 and chondroitin N-acetylgalactosaminyltransferase-1 in peripheral neurons and peripheral neuropathies may result from synthesis of abnormally truncated chondroitin sulfate chains.

Human genetic disorders caused by mutations in genes encoding biosynthetic enzymes for sulfated glycosaminoglycans

A number of genetic disorders are caused by mutations in the genes encoding glycosyltransferases and sulfotransferases, enzymes responsible for the synthesis of sulfated glycosaminoglycan (GAG) side chains of proteoglycans, including chondroitin sulfate, dermatan sulfate, and heparan sulfate. The phenotypes of these genetic disorders reflect disturbances in crucial biological functions of GAGs in human. Recent studies have revealed that mutations in genes encoding chondroitin sulfate and dermatan sulfate biosynthetic enzymes cause various disorders of connective tissues. This minireview focuses on growing glycobiological studies of recently described genetic diseases caused by disturbances in biosynthetic enzymes for sulfated GAGs.

Bone Dysplasia as a Key Feature in Three Patients with a Novel Congenital Disorder of Glycosylation (CDG) Type II Due to a Deep Intronic Splice Mutation in TMEM165

Three patients belonging to two families presented with a psychomotor-dysmorphism syndrome including postnatal growth deficiency and major spondylo-, epi-, and metaphyseal skeletal involvement. Other features were muscular hypotrophy, fat excess, partial growth hormone deficiency, and, in two of the three patients, episodes of unexplained fever. Additional investigations showed mild to moderate increases of serum transaminases (particularly of aspartate transaminase (AST)), creatine kinase (CK), and lactate dehydrogenase (LDH), as well as decreased coagulation factors VIII, IX, XI, and protein C. Diagnostic work-up revealed a type 2 serum transferrin isoelectrofocusing (IEF) pattern and a cathodal shift on apolipoprotein C-III IEF pointing to a combined N- and O-glycosylation defect. Known glycosylation disorders with similar N-glycan structures lacking galactose and sialic acid were excluded. Through a combination of homozygosity mapping and expression profiling, a deep intronic homozygous mutation (c.792 + 182G>A) was found in TMEM165 (TPARL) in the three patients. TMEM165 is a gene of unknown function, possibly involved in Golgi proton/calcium transport. Here we present a detailed clinical description of the three patients with this mutation. The TMEM165 deficiency represents a novel type of CDG (TMEM165-CDG). This disorder enlarges the group of CDG caused by deficiencies in proteins that are not specifically involved in glycosylation but that have functions in the organization and homeostasis of the intracellular compartments and the secretory pathway, like COG-CDG and ATP6V0A2-CDG.

Chondroitin 4-O-sulfotransferase-2 regulates the number of chondroitin sulfate chains initiated by chondroitin N-acetylgalactosaminyltransferase-1

Recently, it has been shown that a deficiency in ChGn-1 (chondroitin N-acetylgalactosaminyltransferase-1) reduced the numbers of CS (chondroitin sulfate) chains, leading to skeletal dysplasias in mice. Although these results indicate that ChGn-1 regulates the number of CS chains, the mechanism mediating this regulation is not clear. ChGn-1 is thought to initiate CS biosynthesis by transferring the first GalNAc (N-acetylgalactosamine) to the tetrasaccharide in the protein linkage region of CS. However, in vitro chondroitin polymerization does not occur on the non-reducing terminal GalNAc-linkage pentasaccharide structure. In the present study we show that several different heteromeric enzyme complexes composed of different combinations of four chondroitin synthase family members synthesized more CS chains when a GalNAc-linkage pentasaccharide structure with a non-reducing terminal 4-O-sulfation was the CS acceptor. In addition, C4ST-2 (chondroitin 4-O-sulfotransferase-2) efficiently transferred sulfate from 3′-phosphoadenosine 5′-phosphosulfate to position 4 of non-reducing terminal GalNAc-linkage residues, and the number of CS chains was regulated by the expression levels of C4ST-2 and of ChGn-1. Taken together, the results of the present study indicate that C4ST-2 plays a key role in regulating levels of CS synthesized via ChGn-1.

Faulty initiation of proteoglycan synthesis causes cardiac and joint defects

Proteoglycans are a major component of extracellular matrix and contribute to normal embryonic and postnatal development by ensuring tissue stability and signaling functions. We studied five patients with recessive joint dislocations and congenital heart defects, including bicuspid aortic valve (BAV) and aortic root dilatation. We identified linkage to chromosome 11 and detected a mutation (c.830G>A, p.Arg277Gln) in B3GAT3, the gene coding for glucuronosyltransferase-I (GlcAT-I). The enzyme catalyzes an initial step in the synthesis of glycosaminoglycan side chains of proteoglycans. Patients' cells as well as recombinant mutant protein showed reduced glucuronyltransferase activity. Patient fibroblasts demonstrated decreased levels of dermatan sulfate, chondroitin sulfate, and heparan sulfate proteoglycans, indicating that the defect in linker synthesis affected all three lines of O-glycanated proteoglycans. Further studies demonstrated that GlcAT-I resides in the cis and cis-medial Golgi apparatus and is expressed in the affected tissues, i.e., heart, aorta, and bone. The study shows that reduced GlcAT-I activity impairs skeletal as well as heart development and results in variable combinations of heart malformations, including mitral valve prolapse, ventricular septal defect, and bicuspid aortic valve. The described family constitutes a syndrome characterized by heart defects and joint dislocations resulting from altered initiation of proteoglycan synthesis (Larsen-like syndrome, B3GAT3 type).