The EXT1/EXT2 tumor suppressors: catalytic activities and role in heparan sulfate biosynthesis

Subcellular imaging of MGAT1/MGAT2 homo- and heteromers in living cells using bioluminescence microscopy

Protein-protein interactions (PPIs) play fundamental roles in many biological processes including the functioning of glycosylation machineries present in the endoplasmic reticulum (ER) and Golgi apparatus of mammalian cells. For the last couple of years, we have been successfully employing the most advanced version of the split luciferase complementation assay, termed NanoBiT, to demonstrate PPIs between solute carrier 35 (SLC35) family members with nucleotide sugar transporting activity and functionally related glycosyltransferases. NanoBiT has several unmatched advantages as compared with other strategies for studying PPIs. Firstly, the tendency of the free luciferase fragments to spontaneously associate is strongly reduced. As a consequence, the fragments of the reconstituted luciferase may dissociate upon the disruption of the PPI of interest. Secondly, the recombinant fusion proteins are expressed at low (near-endogenous) levels. Both of these features significantly minimize the possibility of obtaining false positive results. In this study we pushed the boundaries of this already powerful technique even further by coupling it with bioluminescence imaging of PPIs. Specifically, we visualized homo- and heterologous complexes formed by MGAT1 and MGAT2 glycosylation enzymes tagged with NanoBiT fragments and demonstrated ER-to-Golgi transitions between enzyme homo- and heteromers.

Mutations in Glycosyltransferases and Glycosidases: Implications for Associated Diseases

Glycosylation, a crucial and the most common post-translational modification, coordinates a multitude of biological functions through the attachment of glycans to proteins and lipids. This process, predominantly governed by glycosyltransferases (GTs) and glycoside hydrolases (GHs), decides not only biomolecular functionality but also protein stability and solubility. Mutations in these enzymes have been implicated in a spectrum of diseases, prompting critical research into the structural and functional consequences of such genetic variations. This study compiles an extensive dataset from ClinVar and UniProt, providing a nuanced analysis of 2603 variants within 343 GT and GH genes. We conduct thorough MTR score analyses for the proteins with the most documented variants using MTR3D-AF2 via AlphaFold2 (AlphaFold v2.2.4) predicted protein structure, with the analyses indicating that pathogenic mutations frequently correlate with Beta Bridge secondary structures. Further, the calculation of the solvent accessibility score and variant visualisation show that pathogenic mutations exhibit reduced solvent accessibility, suggesting the mutated residues are likely buried and their localisation is within protein cores. We also find that pathogenic variants are often found proximal to active and binding sites, which may interfere with substrate interactions. We also incorporate computational predictions to assess the impact of these mutations on protein function, utilising tools such as mCSM to predict the destabilisation effect of variants. By identifying these critical regions that are prone to disease-associated mutations, our study opens avenues for designing small molecules or biologics that can modulate enzyme function or compensate for the loss of stability due to these mutations.

Structural basis for heparan sulfate co-polymerase action by the EXT1-2 complex

Heparan sulfate (HS) proteoglycans are extended (-GlcAβ1,4GlcNAcα1,4-)n co-polymers containing decorations of sulfation and epimerization that are linked to cell surface and extracellular matrix proteins. In mammals, HS repeat units are extended by an obligate heterocomplex of two exostosin family members, EXT1 and EXT2, where each protein monomer contains distinct GT47 (GT-B fold) and GT64 (GT-A fold) glycosyltransferase domains. In this study, we generated human EXT1-EXT2 (EXT1-2) as a functional heterocomplex and determined its structure in the presence of bound donor and acceptor substrates. Structural data and enzyme activity of catalytic site mutants demonstrate that only two of the four glycosyltransferase domains are major contributors to co-polymer syntheses: the EXT1 GT-B fold β1,4GlcA transferase domain and the EXT2 GT-A fold α1,4GlcNAc transferase domain. The two catalytic sites are over 90 Å apart, indicating that HS is synthesized by a dissociative process that involves a single catalytic site on each monomer.

Haploinsufficiency of EXT1 and Heparan Sulphate Deficiency Associated with Hereditary Multiple Exostoses in a Pakistani Family

Background and Objectives: Hereditary multiple exostoses (HME) is a disease characterized by cartilage-capped bony protuberances at the site of growth plates of long bones. Functional mutations in the exostosin genes (EXT1 and EXT2) are reported to affect the hedgehog signalling pathways leading to multiple enchondromatosis. However, the exact role of each EXT protein in the regulation of heparan sulphate (HS) chain elongation is still an enigma. In this study, a Pakistani family with HME is investigated to find out the genetic basis of the disease. Materials and Methods: Genotyping of eight members of the family by amplifying microsatellite markers, tightly linked to the EXT1 and EXT2 genes. Results: The study revealed linkage of the HME family to the EXT1 locus 8q24.1. Sanger sequencing identified a heterozygous deletion (c.247Cdel) in exon 1 of EXT1, segregating with the disease phenotype in the family. In silico analysis predicted a shift in the frame causing an early stop codon (p.R83GfsX52). The predicted dwarf protein constituting 134 amino acids was functionally aberrant with a complete loss of the catalytic domain at the C-terminus. Interestingly, an alternative open reading frame 3 (ORF3) caused by the frame shift is predicted to encode a protein sequence, identical to the wild type and containing the catalytic domain, but lacking the first 100 amino acids of the wild-type EXT1 protein. Conclusion: Consequently, haploinsufficiency could be the cause of HME in the investigated family as the mutated copy of EXT1 is ineffective for EXT-1/2 complex formation. The predicted ORF3 protein could be of great significance in understanding several aspects of HME pathogenesis.

Structural basis for matriglycan synthesis by the LARGE1 dual glycosyltransferase

LARGE1 is a bifunctional glycosyltransferase responsible for generating a long linear polysaccharide termed matriglycan that links the cytoskeleton and the extracellular matrix and is required for proper muscle function. This matriglycan polymer is made with an alternating pattern of xylose and glucuronic acid monomers. Mutations in the LARGE1 gene have been shown to cause life-threatening dystroglycanopathies through the inhibition of matriglycan synthesis. Despite its major role in muscle maintenance, the structure of the LARGE1 enzyme and how it assembles in the Golgi are unknown. Here we present the structure of LARGE1, obtained by a combination of X-ray crystallography and single-particle cryo-EM. We found that LARGE1 homo-dimerizes in a configuration that is dictated by its coiled-coil stem domain. The structure shows that this enzyme has two canonical GT-A folds within each of its catalytic domains. In the context of its dimeric structure, the two types of catalytic domains are brought into close proximity from opposing monomers to allow efficient shuttling of the substrates between the two domains. Together, with putative retention of matriglycan by electrostatic interactions, this dimeric organization offers a possible mechanism for the ability of LARGE1 to synthesize long matriglycan chains. The structural information further reveals the mechanisms in which disease-causing mutations disrupt the activity of LARGE1. Collectively, these data shed light on how matriglycan is synthesized alongside the functional significance of glycosyltransferase oligomerization.

Structure of the human heparan sulfate polymerase complex EXT1-EXT2

Heparan sulfates are complex polysaccharides that mediate the interaction with a broad range of protein ligands at the cell surface. A key step in heparan sulfate biosynthesis is catalyzed by the bi-functional glycosyltransferases EXT1 and EXT2, which generate the glycan backbone consisting of repeating N-acetylglucosamine and glucuronic acid units. The molecular mechanism of heparan sulfate chain polymerization remains, however, unknown. Here, we present the cryo-electron microscopy structure of human EXT1-EXT2, which reveals the formation of a tightly packed hetero-dimeric complex harboring four glycosyltransferase domains. A combination of in vitro and in cellulo mutational studies is used to dissect the functional role of the four catalytic sites. While EXT1 can catalyze both glycosyltransferase reactions, our results indicate that EXT2 might only have N-acetylglucosamine transferase activity. Our findings provide mechanistic insight into heparan sulfate chain elongation as a nonprocessive process and lay the foundation for future studies on EXT1-EXT2 function in health and disease.

OUP accepted manuscript

NDST1 (glucosaminyl N-deacetylase/N-sulfotransferase) is a key enzyme in heparan sulfate (HS) biosynthesis, where it is responsible for HS N-deacetylation and N-sulfation. In addition to the full length human enzyme of 882 amino acids, here designated NDST1A, a shorter form containing 825 amino acids (NDST1B) is synthesized after alternative splicing of the NDST1 mRNA. NDST1B is mostly expressed at a low level, but increased amounts are seen in several types of cancer where it is associated with shorter survival. In this study, we aimed at characterizing the enzymatic properties of NDST1B and its effect on HS biosynthesis. Purified recombinant NDST1B lacked both N-deacetylase and N-sulfotransferase activities. Interestingly, HEK293 cells overexpressing NDST1B synthesized HS with reduced sulfation and altered domain structure. Fluorescence resonance energy transfer-microscopy demonstrated that both NDST1A and NDST1B had the capacity to interact with the HS copolymerase subunits EXT1 and EXT2 and also to form NDST1A/NDST1B dimers. Since lysates from cells overexpressing NDST1B contained less NDST enzyme activity than control cells, we suggest that NDST1B works in a dominant negative manner, tentatively by replacing the active endogenous NDST1 in the enzyme complexes taking part in biosynthesis.

Biological role of heparan sulfate in osteogenesis: A review

Heparan sulfate (HS) is extensively expressed in cells, for example, cell membrane and extracellular matrix of most mammalian cells and tissues, playing a key role in the growth and development of life by maintaining homeostasis and implicating in the etiology and diseases. Recent studies have revealed that HS is involved in osteogenesis via coordinating multiple signaling pathways. The potential effect of HS on osteogenesis is a complicated and delicate biological process, which involves the participation of osteocytes, chondrocytes, osteoblasts, osteoclasts and a variety of cytokines. In this review, we summarized the structural and functional characteristics of HS and highlighted the molecular mechanism of HS in bone metabolism to provide novel research perspectives for the further medical research.

CRISPR Tackles Emerging Viral Pathogens

Understanding the dynamic relationship between viral pathogens and cellular host factors is critical to furthering our knowledge of viral replication, disease mechanisms and development of anti-viral therapeutics. CRISPR genome editing technology has enhanced this understanding, by allowing identification of pro-viral and anti-viral cellular host factors for a wide range of viruses, most recently the cause of the COVID-19 pandemic, SARS-CoV-2. This review will discuss how CRISPR knockout and CRISPR activation genome-wide screening methods are a robust tool to investigate the viral life cycle and how other class 2 CRISPR systems are being repurposed for diagnostics.

ER-Golgi dynamics of HS-modifying enzymes via vesicular trafficking is a critical prerequisite for the delineation of HS biosynthesis

The cell surface and extracellular matrix polysaccharide, heparan sulfate (HS) conveys chemical information to control crucial biological processes. HS chains are synthesized in a non-template driven process mainly in the Golgi apparatus, involving a large number of enzymes capable of subtly modifying its substitution pattern, hence, its interactions and biological effects. Changes in the localization of HS-modifying enzymes throughout the Golgi were found to correlate with changes in the structure of HS, rather than protein expression levels. Following BFA treatment, the HS-modifying enzymes localized preferentially in COPII vesicles and at the trans-Golgi. Shortly after heparin treatment, the HS-modifying enzyme moved from cis to trans-Golgi, which coincided with increased HS sulfation. Finally, it was shown that COPI subunits and Sec24 gene expression changed. Collectively, these findings demonstrate that knowledge of the ER-Golgi dynamics of HS-modifying enzymes via vesicular trafficking is a critical prerequisite for the complete delineation of HS biosynthesis.

Mutation spectrum of EXT1 and EXT2 in the Saudi patients with hereditary multiple exostoses

BACKGROUND

Hereditary Multiple Exostoses (HME), also known as Multiple Osteochondromas (MO) is a rare genetic disorder characterized by multiple benign cartilaginous bone tumors, which are caused by mutations in the genes for exostosin glycosyltransferase 1 (EXT1) and exostosin glycosyltransferase 2 (EXT2). The genetic defects have not been studied in the Saudi patients.

AIM OF STUDY

We investigated mutation spectrum of EXT1 and EXT2 in 22 patients from 17 unrelated families.

METHODS

Genomic DNA was extracted from peripheral leucocytes. The coding regions and intron-exon boundaries of both EXT1 and EXT2 genes were screened for mutations by PCR-sequencing analysis. Gross deletions were analyzed by MLPA analysis.

RESULTS

EXT1 mutations were detected in 6 families (35%) and 3 were novel mutations: c.739G > T (p. E247*), c.1319delG (p.R440Lfs*4), and c.1786delA (p.S596Afs*25). EXT2 mutations were detected in 7 families (41%) and 3 were novel mutations: c.541delG (p.D181Ifs*89), c.583delG (p.G195Vfs*75), and a gross deletion of approximately 10 kb including promoter and exon 1. Five patients from different families had no family history and carried de novo mutations (29%, 5/17). No EXT1 and EXT2 mutations were found in the remaining four families. In total, EXT1 and EXT2 mutations were found in 77% (13/17) of Saudi HME patients.

CONCLUSION

EXT1 and EXT2 mutations contribute significantly to the pathogenesis of HME in the Saudi population. In contrast to high mutation rate in EXT 1 (65%) and low mutation rate in EXT2 (25%) in other populations, the frequency of EXT2 mutations are much higher (41%) and comparable to that of EXT1 among Saudi patients. De novo mutations are also common and the six novel EXT1/EXT2 mutations further expands the mutation spectrum of HME.

Heparan Sulfate Proteoglycans Biosynthesis and Post Synthesis Mechanisms Combine Few Enzymes and Few Core Proteins to Generate Extensive Structural and Functional Diversity

Glycosylation is a common and widespread post-translational modification that affects a large majority of proteins. Of these, a small minority, about 20, are specifically modified by the addition of heparan sulfate, a linear polysaccharide from the glycosaminoglycan family. The resulting molecules, heparan sulfate proteoglycans, nevertheless play a fundamental role in most biological functions by interacting with a myriad of proteins. This large functional repertoire stems from the ubiquitous presence of these molecules within the tissue and a tremendous structural variety of the heparan sulfate chains, generated through both biosynthesis and post synthesis mechanisms. The present review focusses on how proteoglycans are “gagosylated” and acquire structural complexity through the concerted action of Golgi-localized biosynthesis enzymes and extracellular modifying enzymes. It examines, in particular, the possibility that these enzymes form complexes of different modes of organization, leading to the synthesis of various oligosaccharide sequences.

Mutational spectrum and clinical signatures in 114 families with hereditary multiple osteochondromas: insights into molecular properties of selected exostosin variants

Hereditary multiple osteochondromas (HMO) is a rare autosomal dominant skeletal disorder, caused by heterozygous variants in either EXT1 or EXT2, which encode proteins involved in the biogenesis of heparan sulphate. Pathogenesis and genotype-phenotype correlations remain poorly understood. We studied 114 HMO families (158 affected individuals) with causative EXT1 or EXT2 variants identified by Sanger sequencing, or multiplex ligation-dependent probe amplification and qPCR. Eighty-seven disease-causative variants (55 novel and 32 known) were identified including frameshift (42%), nonsense (32%), missense (11%), splicing (10%) variants and genomic rearrangements (5%). Informative clinical features were available for 42 EXT1 and 27 EXT2 subjects. Osteochondromas were more frequent in EXT1 as compared to EXT2 patients. Anatomical distribution of lesions showed significant differences based on causative gene. Microscopy analysis for selected EXT1 and EXT2 variants verified that EXT1 and EXT2 mutants failed to co-localize each other and loss Golgi localization by surrounding the nucleus and/or assuming a diffuse intracellular distribution. In a cell viability study, cells expressing EXT1 and EXT2 mutants proliferated more slowly than cells expressing wild-type proteins. This confirms the physiological relevance of EXT1 and EXT2 Golgi co-localization and the key role of these proteins in the cell cycle. Taken together, our data expand genotype-phenotype correlations, offer further insights in the pathogenesis of HMO and open the path to future therapies.

The Interferon-Inducible Proteoglycan Testican-2/SPOCK2 Functions as a Protective Barrier against Virus Infection of Lung Epithelial Cells

Proteoglycans function not only as structural components of the extracellular compartment but also as regulators of various cellular events, including cell migration, inflammation, and infection. Many microbial pathogens utilize proteoglycans to facilitate adhesion and invasion into host cells. Here we report a secreted form of a novel heparan sulfate proteoglycan that functions against virus infection. The expression of SPOCK2/testican-2 was significantly induced in virus-infected lungs or in interferon (IFN)-treated alveolar lung epithelial cells. Overexpression from a SPOCK2 expression plasmid alone or the treatment of cells with recombinant SPOCK2 protein efficiently blocked influenza virus infection at the step of viral attachment to the host cell and entry. Moreover, mice treated with purified SPOCK2 were protected against virus infection. Sialylated glycans and heparan sulfate chains covalently attached to the SPOCK2 core protein were critical for its antiviral activity. Neuraminidase (NA) of influenza virus cleaves the sialylated moiety of SPOCK2, thereby blocking its binding to the virus. Our data suggest that IFN-induced SPOCK2 functions as a decoy receptor to bind and block influenza virus infection, thereby restricting entry of the infecting virus into neighboring cells.IMPORTANCE Here we report a novel proteoglycan protein, testican-2/SPOCK2, that prevents influenza virus infection. Testican-2/SPOCK2 is a complex type of secreted proteoglycan with heparan sulfate GAG chains attached to the core protein. SPOCK2 expression is induced upon virus infection or by interferons, and the protein is secreted to an extracellular compartment, where it acts directly to block virus-cell attachment and entry. Treatment with purified testican-2/SPOCK2 protein can efficiently block influenza virus infection in vitro and in vivo We also identified the heparan sulfate moiety as a key regulatory module for this inhibitory effect. Based on its mode of action (cell attachment/entry blocker) and site of action (extracellular compartment), we propose testican-2/SPOCK2 as a potential antiviral agent that can efficiently control influenza virus infection.

Loss of the Heparan Sulfate Proteoglycan Glypican5 Facilitates Long-Range Sonic Hedgehog Signaling

As a morphogen, Sonic Hedgehog (Shh) mediates signaling at a distance from its sites of synthesis. After secretion, Shh must traverse a distance through the extracellular matrix (ECM) to reach the target cells and activate the Hh response. ECM proteins, in particular, the heparan sulfate proteoglycans (HSPGs) of the glypican family, have both negative and positive effects on Shh signaling, all attributed to their ability to bind Shh. Using mouse embryonic stem cell-derived mosaic tissues with compartments that lack the glycosyltransferases Exostosin1 and Exostosin2, or the HSPG core protein Glypican5, we show that Shh accumulates around its source cells when they are surrounded by cells that have a mutated ECM. This accumulation of Shh is correlated with an increased noncell autonomous Shh response. Our results support a model in which Shh presented on the cell surface accumulates at or near ECM that lacks HSPGs, possibly due to the absence of these Shh sequestering molecules. Stem Cells 2019;37:899-909.

Imaging specific cellular glycan structures using glycosyltransferases via click chemistry

Heparan sulfate (HS) is a polysaccharide fundamentally important for biologically activities. T/Tn antigens are universal carbohydrate cancer markers. Here, we report the specific imaging of these carbohydrates using a mesenchymal stem cell line and human umbilical vein endothelial cells (HUVEC). The staining specificities were demonstrated by comparing imaging of different glycans and validated by either removal of target glycans, which results in loss of signal, or installation of target glycans, which results in gain of signal. As controls, representative key glycans including O-GlcNAc, lactosaminyl glycans and hyaluronan were also imaged. HS staining revealed novel architectural features of the extracellular matrix (ECM) of HUVEC cells. Results from T/Tn antigen staining suggest that O-GalNAcylation is a rate-limiting step for O-glycan synthesis. Overall, these highly specific approaches for HS and T/Tn antigen imaging should greatly facilitate the detection and functional characterization of these biologically important glycans.

The exostosin family of glycosyltransferases: mRNA expression profiles and heparan sulphate structure in human breast carcinoma cell lines

Breast cancer remains a leading cause of cancer-related mortality in women. In recent years, regulation of genes involved in heparan sulphate (HS) biosynthesis have received increased interest as regulators of breast cancer cell adhesion and invasion. The exostosin (EXT) proteins are glycosyltransferases involved in elongation of HS, a regulator of intracellular signaling, cell–cell interactions, and tissue morphogenesis. The EXT family contains five members: EXT1, EXT2, and three EXT-like (EXTL) members: EXTL1, EXTL2, and EXTL3. While the expression levels of these enzymes change in tumor cells, little is known how this changes the structure and function of HS. In the present study, we investigated gene expression profiles of the EXT family members, their glycosyltransferase activities and HS structure in the estrogen receptor (ER), and progesterone receptor (PR) positive MCF7 cells, and the ER, PR, and human epidermal growth factor receptor-2 (HER2) negative MDA-MB-231 and HCC38 epithelial breast carcinoma cell lines. The gene expression profiles for MDA-MB-231 and HCC38 cells were very similar. In both cell lines EXTL2 was found to be up-regulated whereas EXT2 was down-regulated. Interestingly, despite having similar expression of HS elongation enzymes the two cell lines synthesized HS chains of significantly different lengths. Furthermore, both MDA-MB-231 and HCC38 exhibited markedly decreased levels of HS 6-O-sulphated disaccharides. Although the gene expression profiles of the elongation enzymes did not correlate with the length of HS chains, our results indicated specific differences in EXT enzyme levels and HS fine structure characteristic of the carcinogenic properties of the breast carcinoma cells.

Palovarotene Inhibits Osteochondroma Formation in a Mouse Model of Multiple Hereditary Exostoses

Multiple hereditary exostoses (MHE), also known as multiple osteochondromas (MO), is an autosomal dominant disorder characterized by the development of multiple cartilage-capped bone tumors (osteochondromas). The large majority of patients with MHE carry loss-of-function mutations in the EXT1 or EXT2 gene, which encodes a glycosyltransferase essential for heparan sulfate (HS) biosynthesis. Increasing evidence suggests that enhanced bone morphogenetic protein (BMP) signaling resulting from loss of HS expression plays a role in osteochondroma formation in MHE. Palovarotene (PVO) is a retinoic acid receptor γ selective agonist, which is being investigated as a potential drug for fibrodysplasia ossificans progressiva (FOP), another genetic bone disorder with features that overlap with those of MHE. Here we show that PVO inhibits osteochondroma formation in the Fsp1^Cre ;Ext1^flox/flox model of MHE. Four-week daily treatment with PVO starting at postnatal day (P) 14 reduced the number of osteochondromas that develop in these mice by up to 91% in a dose-dependent manner. An inhibition of long bone growth observed in animals treated from P14 was almost entirely abrogated by delaying the initiation of treatment to P21. We also found that PVO attenuates BMP signaling in Fsp1^Cre ;Ext1^flox/flox mice and that aberrant chondrogenic fate determination of Ext1-deficient perichondrial progenitor cells in these mice is restored by PVO. Together, the present data support further preclinical and clinical investigations of PVO as a potential therapeutic agent for MHE. © 2017 American Society for Bone and Mineral Research.

Exostosin-like 2 regulates FGF2 signaling by controlling the endocytosis of FGF2

BACKGROUND

Heparan sulfate proteoglycans are ubiquitously expressed on cell surfaces and in extracellular matrices, and are engaged in heparin-binding growth factor-related signal transduction. Thus, changes in the amounts, structures, and chain lengths of heparan sulfate have profound effects on aspects of cell growth controlled by heparin-binding growth factors such as FGF2. Exostosin glycosyltransferases (EXT1, EXT2, EXTL1, EXTL2, and EXTL3) control heparan sulfate biosynthesis, and the expression levels of their genes regulate the amounts, chain lengths, and sulfation patterns of heparan sulfate. Unlike EXT1, EXT2, and EXTL3, EXTL2 functions chain termination of heparan sulfate. Here, we examined the importance of EXTL2 in FGF2-dependent signaling.

METHODS

We investigated heparan sulfate biosynthesis and FGF2 signaling using four cell lines, EXT1-deficient cells, EXT2-, EXTL2-, or EXTL3-knockdown cells, by HPLC, qRT-PCR, flow cytometry, and western blotting.

RESULTS

Reduced expression of either EXT1, EXT2, or EXTL3 decreased heparan sulfate biosynthesis, and consequently suppressed the FGF2-dependent proliferation of mouse L fibroblasts. In contrast, although knockdown of EXTL2 increased the amounts of heparan sulfate, FGF2-dependent proliferation was significantly inhibited because the increased heparan sulfate enhanced the incorporation of FGF2 into the cells.

CONCLUSIONS

EXTL2 controls FGF2 signaling through regulation of heparan sulfate biosynthesis in a manner distinct from that of other exostosins.

GENERAL SIGNIFICANCE

This study provides new insights into the regulatory mechanisms of FGF2 signaling by EXTL2.

Aberrant perichondrial BMP signaling mediates multiple osteochondromagenesis in mice

Multiple hereditary exostoses (MHE) is characterized by the development of numerous benign bony tumors (osteochondromas). Although it has been well established that MHE is caused by mutations in EXT1 and EXT2, which encode glycosyltransferase essential for heparan sulfate (HS) biosynthesis, the cellular origin and molecular mechanisms of MHE remain elusive. Here, we show that in Ext1 mutant mice, osteochondromas develop from mesenchymal stem cell-like progenitor cells residing in the perichondrium, and we show that enhanced BMP signaling in these cells is the primary signaling defect that leads to osteochondromagenesis. We demonstrate that progenitor cells in the perichondrium, including those in the groove of Ranvier, highly express HS and that Ext1 ablation targeted to the perichondrium results in the development of osteochondromas. Ext1-deficient perichondrial progenitor cells show enhanced BMP signaling and increased chondrogenic differentiation both in vitro and in vivo. Consistent with the functional role for enhanced BMP signaling in osteochondromagenesis, administration of the small molecule BMP inhibitor LDN-193189 suppresses osteochondroma formation in two MHE mouse models. Together, our results demonstrate a role for enhanced perichondrial BMP signaling in osteochondromagenesis in mice, and they suggest the possibility of pharmacological treatment of MHE with BMP inhibitors.

Targeted Next-Generation Sequencing Newly Identifies Mutations in Exostosin-1 and Exostosin-2 Genes of Patients with Multiple Osteochondromas

Multiple osteochondromas (MO) is one of the most common benign bone tumors in humans with an autosomal dominant hereditary mode. MO is a genetic heterogeneity disease with variable number and size of osteochondromas, as well as changeable number and location of diseased bones. Mutations in Exostosin-1/Exostosin-2 (EXT1/EXT2) genes are the main molecular basis of MO. EXT1 and EXT2 genes encode exostosin 1 and exostosin 2, respectively, both of which are transmembrane glycosyltransferases that elongate the chains of heparin sulfate (HS) at HS proteoglycans (HSPGs). HSPGs are considered to be involved in regulating the proliferation and differentiation of chondrocytes. Owing to large size of EXT1/EXT2 genes and lack of mutation hotspots, molecular diagnosis of MO is challenging. Here, we applied targeted next-generation sequencing (t-NGS) in mutation screening of EXT1/EXT2 genes for 10 MO patients. The results were compared and validated with Sanger sequencing. Overall, nine mutations identified by t-NGS were confirmed with Sanger sequencing, excluding two variants of false positive, suggesting the reliability of mutation screening by t-NGS. The nine mutations identified by t-NGS include two missense mutations (EXT1: c.1088G>A and c.2120C>T), one splicing mutation (EXT2: c.744-1G>T), and six nonsense mutations (EXT1: c.351C>G, c.1121G>A, and c.1843_1846dup; EXT2: c.67C>T, c.561delG, and c.575T>A). In summary, our paper provides the primary data of the application of t-NGS in MO molecular diagnosis, including six newly identified mutations (EXT1: c.1843_1846dup, c.1088G>A, c.351C>G, and c.2120C>T and EXT2: c.744-1G>T and c.575T>A), which further enrich the mutation database of MO from the Chinese population.

EXTL3 mutations cause skeletal dysplasia, immune deficiency, and developmental delay

Volpi et al. demonstrate that hypomorphic EXTL3 mutations cause abnormalities of heparan sulfate composition, affect signaling in response to growth factors and cytokines, and perturb thymopoiesis, resulting in a novel genetic disease associating skeletal dysplasia, T cell immunodeficiency, and neurodevelopmental delay.

We studied three patients with severe skeletal dysplasia, T cell immunodeficiency, and developmental delay. Whole-exome sequencing revealed homozygous missense mutations affecting exostosin-like 3 (EXTL3), a glycosyltransferase involved in heparan sulfate (HS) biosynthesis. Patient-derived fibroblasts showed abnormal HS composition and altered fibroblast growth factor 2 signaling, which was rescued by overexpression of wild-type EXTL3 cDNA. Interleukin-2–mediated STAT5 phosphorylation in patients’ lymphocytes was markedly reduced. Interbreeding of the extl3-mutant zebrafish (box) with Tg(rag2:green fluorescent protein) transgenic zebrafish revealed defective thymopoiesis, which was rescued by injection of wild-type human EXTL3 RNA. Targeted differentiation of patient-derived induced pluripotent stem cells showed a reduced expansion of lymphohematopoietic progenitor cells and defects of thymic epithelial progenitor cell differentiation. These data identify EXTL3 mutations as a novel cause of severe immune deficiency with skeletal dysplasia and developmental delay and underline a crucial role of HS in thymopoiesis and skeletal and brain development.

Functional Requirements for Heparan Sulfate Biosynthesis in Morphogenesis and Nervous System Development in C. elegans

The regulation of cell migration is essential to animal development and physiology. Heparan sulfate proteoglycans shape the interactions of morphogens and guidance cues with their respective receptors to elicit appropriate cellular responses. Heparan sulfate proteoglycans consist of a protein core with attached heparan sulfate glycosaminoglycan chains, which are synthesized by glycosyltransferases of the exostosin (EXT) family. Abnormal HS chain synthesis results in pleiotropic consequences, including abnormal development and tumor formation. In humans, mutations in either of the exostosin genes EXT1 and EXT2 lead to osteosarcomas or multiple exostoses. Complete loss of any of the exostosin glycosyltransferases in mouse, fish, flies and worms leads to drastic morphogenetic defects and embryonic lethality. Here we identify and study previously unavailable viable hypomorphic mutations in the two C. elegans exostosin glycosyltransferases genes, rib-1 and rib-2. These partial loss-of-function mutations lead to a severe reduction of HS levels and result in profound but specific developmental defects, including abnormal cell and axonal migrations. We find that the expression pattern of the HS copolymerase is dynamic during embryonic and larval morphogenesis, and is sustained throughout life in specific cell types, consistent with HSPGs playing both developmental and post-developmental roles. Cell-type specific expression of the HS copolymerase shows that HS elongation is required in both the migrating neuron and neighboring cells to coordinate migration guidance. Our findings provide insights into general principles underlying HSPG function in development.

During animal development, cells and neurons navigate long distances to reach their final target destinations. Migrating cells are guided by extracellular molecular cues, and cellular responses to these cues are regulated by heparan sulfate proteoglycans. Heparan sulfate proteoglycans are proteins with long heparan sulfate polysaccharide chains attached. Here we identify and study previously unavailable viable mutants that disrupt the elongation of the heparan sulfate chains in the nematode C. elegans. Our analysis shows that these HS-chain-elongation mutations affect the development of the nervous system as they result in misguided migrations of neurons and axons. Furthermore, we find that heparan sulfate chain elongation occurs in numerous cell types during development and that the coordinated production of heparan sulfate proteoglycans, in both the migrating cell and neighboring tissues, ensures proper migration. Our findings highlight the critical roles of heparan sulfate proteoglycans in nervous system development and the evolutionary conservation of the molecular mechanisms driving guided migrations.

Enzyme overexpression - an exercise toward understanding regulation of heparan sulfate biosynthesis

Biosynthesis of heparan sulfate (HS) involves conversion of D-glucuronic acid (GlcA) to L-iduronic acid (IdoA) units catalyzed by glucuronyl C5-epimerase (Hsepi). IdoA units are the favored substrate for 2-O-sulfotransferase (2OST). We used HEK293 cells as a model to investigate the effects of overexpression of these enzymes on HS structure. Overexpression of Hsepi alone resulted in an unexpected increase in HS chain length. A Hsepi point-mutant (Y168A), devoid of catalytic activity, failed to affect chain length. Moreover, the effect of Hsepi overexpression on HS chain length was abolished by simultaneous overexpression of 2OST. These findings raise novel aspects on regulation of HS biosynthesis. We propose a hypothetical enzyme-binding protein (EBP) with distinct, specific and partly overlapping binding sites, the interactions of which will determine levels of enzymes available to the biosynthetic process.

The role of heparan sulphate in development: the ectodermal story

Heparan sulphate (HS) is ubiquitously expressed and is formed of repeating glucosamine and glucuronic/iduronic acid units which are generally highly sulphated. HS is found in tissues bound to proteins forming HS proteoglycans (HSPGs) which are present on the cell membrane or in the extracellular matrix. HSPGs influence a variety of biological processes by interacting with physiologically important proteins, such as morphogens, creating storage pools, generating morphogen gradients and directly mediating signalling pathways, thereby playing vital roles during development. This review discusses the vital role HS plays in the development of tissues from the ectodermal lineage. The ectodermal layer differentiates to form the nervous system (including the spine, peripheral nerves and brain), eye, epidermis, skin appendages and tooth enamel.

Heparan Sulfate: Biosynthesis, Structure, and Function

Heparan sulfate (HS) proteoglycans (PGs) are ubiquitously expressed on cell surfaces and in the extracellular matrix of most animal tissues, having essential functions in development and homeostasis, as well as playing various roles in disease processes. The functions of HSPGs are mainly dependent on interactions between the HS-side chains with a variety of proteins including cytokines, growth factors, and their receptors. In a given HS polysaccharide, negatively charged sulfate and carboxylate groups are arranged in various types of domains, generated through strictly regulated biosynthetic reactions and with enormous potential for structural variability. The mode of HS-protein interactions is assessed through binding experiments using saccharides of defined composition in vitro, signaling assays in cell models where HS structures are manipulated, and targeted disruption of genes for biosynthetic enzymes in animals (mouse, zebrafish, Drosophila, and Caenorhabditis elegans) followed by phenotype analysis. Whereas some protein ligands appear to require strictly defined HS structure, others bind to variable saccharide domains without apparent dependence on distinct saccharide sequence. These findings raise intriguing questions concerning the functional significance of regulation in HS biosynthesis and the potential for development of therapeutics targeting HS-protein interactions.

Deletion of exon 8 from the EXT1 gene causes multiple osteochondromas (MO) in a family with three affected members

Multiple osteochondromas (also called hereditary multiple exostoses) is an autosomal dominant disorder characterized by multiple cartilaginous tumors, which are caused by mutations in the genes for exostosin-1 (EXT1) and exostosin-2 (EXT2). The goal of this study was to elucidate the genetic alterations in a family with three affected members. Isolation of RNA from the patients’ blood followed by reverse transcription and PCR amplification of selected fragments showed that the three patients lack a specific region of 90 bp from their EXT1 mRNA. This region corresponds to the sequence of exon 8 from the EXT1 gene. No splice site mutation was found around exon 8. However, long-range PCR amplification of the region from intron 7 to intron 8 indicated that the three patients contain a deletion of 4318 bp, which includes exon 8 and part of the flanking introns. There is evidence that the deletion was caused by non-homologous end joining because the breakpoints are not located within a repetitive element, but contain multiple copies of the deletion hotspot sequence TGRRKM. Exon 8 encodes part of the active site of the EXT1 enzyme, including the DXD signature of all UDP-sugar glycosyltransferases. It is conceivable that the mutant protein exerts a dominant negative effect on the activity of the EXT glycosyltransferase since it might interact with normal copies of the enzyme to form an inactive hetero-oligomeric complex. We suggest that sequencing of RNA might be superior to exome sequencing to detect short deletions of a single exon.

Glycosyltransferase complexes in eukaryotes: long-known, prevalent but still unrecognized

Glycosylation is the most common and complex cellular modification of proteins and lipids. It is critical for multicellular life and its abrogation often leads to a devastating disease. Yet, the underlying mechanistic details of glycosylation in both health and disease remain unclear. Partly, this is due to the complexity and dynamicity of glycan modifications, and the fact that not all the players are taken into account. Since late 1960s, a vast number of studies have demonstrated that glycosyltransferases typically form homomeric and heteromeric complexes with each other in yeast, plant and animal cells. To propagate their acceptance, we will summarize here accumulated data for their prevalence and potential functional importance for glycosylation focusing mainly on their mutual interactions, the protein domains mediating these interactions, and enzymatic activity changes that occur upon complex formation. Finally, we will highlight the few existing 3D structures of these enzyme complexes to pinpoint their individual nature and to emphasize that their lack is the main obstacle for more detailed understanding of how these enzyme complexes interact and function in a eukaryotic cell.

Identification of a novel frameshift mutation of the EXT2 gene in a family with multiple osteochondroma

Multiple osteochondroma (MO), also known as multiple hereditary exostoses, is an autosomal dominant skeletal disorder with characteristic multiple cartilage-capped tumours (osteochondromas or exostoses) growing outward from the metaphyseal region of the long tubular bones. Mutations in exostosin glycosyltransferase 1 (EXT1) or EXT2 are the most commonly associated mutations with MO and are responsible for 70–95% of cases. In the present study, a genetic analysis was performed on a large family with MO using polymerase chain reaction and direct DNA sequencing of the entire coding regions of EXT1 and EXT2. Sanger sequencing identified a novel heterozygous frameshift mutation, c.119_120delCT (p.Thr40ArgfsX15), in exon 2 of the EXT2 gene in the proband and all other affected individuals, while this deleterious mutation was not detected in the healthy family members and normal controls. The c.119_120delCT mutation is located in the transmembrane region of the EXT2 protein and results in a truncated EXT2 protein lacking 665 amino acids at the C-terminus, which includes the critical exostosin and glycosyltransferase family 64 domains. Thus, the present study identified a novel causative frameshift mutation in EXT2 from a large MO family. This study is useful for extending the known mutational spectrum of EXT2, for understanding the genetic basis of MO in the patients studied, and for further application of mutation screening in the genetic counseling and subsequent prenatal diagnosis of this family.

"GAG-ing with the neuron": The role of glycosaminoglycan patterning in the central nervous system

Proteoglycans (PGs) are a diverse family of proteins that consist of one or more glycosaminoglycan (GAG) chains, covalently linked to a core protein. PGs are major components of the extracellular matrix (ECM) and play critical roles in development, normal function and damage-response of the central nervous system (CNS). GAGs are classified based on their disaccharide subunits, into the following major groups: chondroitin sulfate (CS), heparan sulfate (HS), heparin (HEP), dermatan sulfate (DS), keratan sulfate (KS) and hyaluronic acid (HA). All except HA are modified by sulfation, giving GAG chains specific charged structures and binding properties. While significant neuroscience research has focused on the role of one PG family member, chondroitin sulfate proteoglycan (CSPG), there is ample evidence in support of a role for the other PGs in regulating CNS function in normal and pathological conditions. This review discusses the role of all the identified PG family members (CS, HS, HEP, DS, KS and HA) in normal CNS function and in the context of pathology. Understanding the pleiotropic roles of these molecules in the CNS may open the door to novel therapeutic strategies for a number of neurological conditions.

Reduced Expression of EXTL2, a Member of the Exostosin (EXT) Family of Glycosyltransferases, in Human Embryonic Kidney 293 Cells Results in Longer Heparan Sulfate Chains

Heparan sulfate proteoglycans are ubiquitously located on cell surfaces and in the extracellular matrices. The negatively charged heparan sulfate chains interact with a multitude of different proteins, thereby influencing a variety of cellular and developmental processes, for example cell adhesion, migration, tissue morphogenesis, and differentiation. The human exostosin (EXT) family of genes contains five members: the heparan sulfate polymerizing enzymes, EXT1 and EXT2, and three EXT-like genes, EXTL1, EXTL2, and EXTL3. EXTL2 has been ascribed activities related to the initiation and termination of heparan sulfate chains. Here we further investigated the role of EXTL2 in heparan sulfate chain elongation by gene silencing and overexpression strategies. We found that siRNA-mediated knockdown of EXTL2 in human embryonic kidney 293 cells resulted in increased chain length, whereas overexpression of EXTL2 in the same cell line had little or no effect on heparan sulfate chain length. To study in more detail the role of EXTL2 in heparan sulfate chain elongation, we tested the ability of the overexpressed protein to catalyze the in vitro incorporation of N-acetylglucosamine and N-acetylgalactosamine to oligosaccharide acceptors resembling unmodified heparan sulfate and chondroitin sulfate precursor molecules. Analysis of the generated products revealed that recombinant EXTL2 showed weak ability to transfer N-acetylgalactosamine to heparan sulfate precursor molecules but also, that EXTL2 exhibited much stronger in vitro N-acetylglucosamine-transferase activity related to elongation of heparan sulfate chains.

Golgi UDP-GlcNAc:polypeptide O-α-N-Acetyl-d-glucosaminyltransferase 2 (TcOGNT2) regulates trypomastigote production and function in Trypanosoma cruzi

All life cycle stages of the protozoan parasite Trypanosoma cruzi are enveloped by mucin-like glycoproteins which, despite major changes in their polypeptide cores, are extensively and similarly O-glycosylated. O-Glycan biosynthesis is initiated by the addition of αGlcNAc to Thr in a reaction catalyzed by Golgi UDP-GlcNAc:polypeptide O-α-N-acetyl-d-glucosaminyltransferases (ppαGlcNAcTs), which are encoded by TcOGNT1 and TcOGNT2. We now directly show that TcOGNT2 is associated with the Golgi apparatus of the epimastigote stage and is markedly downregulated in both differentiated metacyclic trypomastigotes (MCTs) and cell culture-derived trypomastigotes (TCTs). The significance of downregulation was examined by forced continued expression of TcOGNT2, which resulted in a substantial increase of TcOGNT2 protein levels but only modestly increased ppαGlcNAcT activity in extracts and altered cell surface glycosylation in TCTs. Constitutive TcOGNT2 overexpression had no discernible effect on proliferating epimastigotes but negatively affected production of both types of trypomastigotes. MCTs differentiated from epimastigotes at a low frequency, though they were apparently normal based on morphological and biochemical criteria. However, these MCTs exhibited an impaired ability to produce amastigotes and TCTs in cell culture monolayers, most likely due to a reduced infection frequency. Remarkably, inhibition of MCT production did not depend on TcOGNT2 catalytic activity, whereas TCT production was inhibited only by active TcOGNT2. These findings indicate that TcOGNT2 downregulation is important for proper differentiation of MCTs and functioning of TCTs and that TcOGNT2 regulates these functions by using both catalytic and noncatalytic mechanisms.

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 splice mutation and mRNA decay of EXT2 provoke hereditary multiple exostoses

BACKGROUND

Hereditary multiple exostoses (HME) is an autosomal dominant disease. The classical paradigm of mutation screening seeks to relate alterations in the exostosin glycosyltransferase genes, EXT1 and EXT2, which are responsible for over 70% of HME cases. However, the pathological significance of the majority of these mutations is often unclear.

METHODS

In a Chinese family with HME, EXT1 and EXT2 genes were screened by direct sequencing. The consequence of a detected mutant was predicted by in silico analysis and confirmed by mRNA analysis. The EXT1 and EXT2 mRNA and protein levels and the HS patterns in the HME patients were compared with those in healthy controls.

RESULTS

A heterozygous transition (c.743+1G>A) in the EXT2 gene, which co-segregated with the HME phenotype in this family, was identified. The G residue at position +1 in intron 4 of EXT2 was predicted to be a 5' donor splice site. The mRNA analysis revealed an alternative transcript with a cryptic splice site 5 bp downstream of the wild-type site, which harbored a premature stop codon. However, the predicted truncated protein was not detected by western blot analysis. Decay of the mutant mRNA was shown by clone sequencing and quantification analysis. The corresponding downregulation of the EXT2 mRNA will contribute to the abnormal EXT1/EXT2 ratio and HS pattern that were detected in the patients with HME.

CONCLUSION

The heterozygous mutation c.743+1G>A in the EXT2 gene causes HME as a result of abnormal splicing, mRNA decay, and the resulting haploinsufficiency of EXT2.

A visualizable chain-terminating inhibitor of glycosaminoglycan biosynthesis in developing zebrafish

Heparan sulfate (HS) and chondroitin sulfate (CS) glycosaminoglycans (GAG) are proteoglycan-associated polysaccharides with essential functions in animals. They have been studied extensively by genetic manipulation of biosynthetic enzymes, but chemical tools for probing GAG function are limited. HS and CS possess a conserved xylose residue that links the polysaccharide chain to a protein backbone. Here we report that, in zebrafish embryos, the peptide-proximal xylose residue can be metabolically replaced with a chain-terminating 4-azido-4-deoxyxylose (4-XylAz) residue by administration of UDP-4-azido-4-deoxyxylose (UDP-4-XylAz). UDP-4-XylAz disrupted both HS and CS biosynthesis and caused developmental abnormalities reminiscent of GAG biosynthesis and laminin mutants. The azide substituent of protein-bound 4-XylAz allowed for rapid visualization of the organismal sites of chain termination in vivo through bioorthogonal reaction with fluorescent cyclooctyne probes. UDP-4-XylAz therefore complements genetic tools for studies of GAG function in zebrafish embryogenesis.

Genetic alterations in chondrosarcomas - keys to targeted therapies?

BACKGROUND

Chondrosarcomas are malignant tumors of chondrocytes and represent the second most common type of primary bone tumors. Within the context of normal chondrogenesis, this review summarizes results from recent research outlining the key molecular changes that occur during the development of this sarcoma type.

RESULTS

Current data support the notion that a two-hit scenario, common to many tumors, also underlies chondrosarcoma formation. First, early-stage mutations alter the normal proliferation and differentiation of chondrocytes, thereby predisposing them to malignant transformation. These early-stage mutations, found in both benign cartilaginous lesions and chondrosarcomas, include alterations affecting the IHH/PTHrP and IDH1/IDH2 pathways. As they are not observed in malignant cells, mutations in the EXT1 and EXT2 genes are considered early-stage events providing an environment that alters IHH/PTHrP signaling, thereby inducing mutations in adjacent cells. Due to normal cell cycle control that remains active, a low rate of malignant transformation is seen in benign cartilaginous lesions with early-stage mutations. In contrast, late-stage mutations, seen in most malignant chondrosarcomas, appear to induce malignant transformation as they are not found in benign cartilaginous lesions. These late-stage mutations primarily involve cell cycle pathway regulators including p53 and pRB, two genes that are also known to be implicated in numerous other human tumor types.

CONCLUSIONS

Now the key genetic alterations involved in both early and late stages of chondrosarcoma development have been identified, focus should be shifted to the identification of druggable molecular targets for the design of novel chondrosarcoma-specific therapies.

The role of syndecan-1 in cellular signaling and its effects on heparan sulfate biosynthesis in mesenchymal tumors

Proteoglycans (PGs) and in particular the syndecans are involved in the differentiation process across the epithelial-mesenchymal axis, principally through their ability to bind growth factors and modulate their downstream signaling. Malignant tumors have individual proteoglycan profiles, which are closely associated with their differentiation and biological behavior, mesenchymal tumors showing a different profile from that of epithelial tumors. Syndecan-1 is the main syndecan of epithelial malignancies, whereas in sarcomas its expression level is generally low, in accordance with their mesenchymal phenotype and highly malignant behavior. This proteoglycan is often overexpressed in adenocarcinoma cells, whereas mesothelioma and fibrosarcoma cells express syndecan-2 and syndecan-4 more abundantly. Increased expression of syndecan-1 in mesenchymal tumors changes the tumor cell morphology to an epithelioid direction whereas downregulation results in a change in shape from polygonal to spindle-like morphology. Although syndecan-1 plays major roles on the cell-surface, there are also intracellular functions, which are not very well studied. On the functional level, syndecan-1 affects mesenchymal tumor cell proliferation, adhesion, migration and motility, and the effect varies with the different domains of the core protein. Syndecan-1 may exert stimulatory or inhibitory effects, depending on the concentration of various mitogens, enzymes, and signaling molecules, the ratio between the shed and membrane-associated syndecan-1 and histological grade of the tumour. Growth factor signaling seems to be delicately controlled by regulatory loops involving the syndecan expression levels and their sulfation patterns. Overexpression of syndecan-1 modulates the biosynthesis and sulfation of heparan sulfate and it also affects the expression of other PGs. On transcriptomic level, syndecan-1 modulation results in profound effects on genes involved in regulation of cell growth.

The exostosin family: proteins with many functions

Heparan sulfates are complex sulfated molecules found in abundance at cell surfaces and in the extracellular matrix. They bind to and influence the activity of a variety of molecules like growth factors, proteases and morphogens and are thus involved in various cell-cell and cell-matrix interactions. The mammalian EXT proteins have glycosyltransferase activities relevant for HS chain polymerization, however their exact role in this process is still confusing. In this review, we summarize current knowledge about the biochemical activities and some proposed functions of the members of the EXT protein family and their roles in human disease.

Expression of chondroitin/dermatan sulfate glycosyltransferases during early zebrafish development

BACKGROUND

Chondroitin/dermatan sulfate (CS/DS) proteoglycans present in the extracellular matrix have important structural and regulatory functions.

RESULTS

Six human genes have previously been shown to catalyze CS/DS polymerization. Here we show that one of these genes, chpf, is represented by two copies in the zebrafish genome, chpfa and chpfb, while the other five human CS/DS glycosyltransferases csgalnact1, csgalnact2, chpf2, chsy1, and chsy3 all have single zebrafish orthologues. The putative zebrafish CS/DS glycosyltransferases are spatially and temporally expressed. Interestingly, overlapping expression of multiple glycosyltransferases coincides with high CS/DS deposition. Finally, whereas the relative levels of the related polysaccharide HS reach steady-state at around 2 days post fertilization, there is a continued relative increase of the CS amounts per larvae during the first 6 days of development, matching the increased cartilage formation.

CONCLUSIONS

There are 7 CS/DS glycosyltransferases in zebrafish, which, based on homology, can be divided into the CSGALNACT, CHSY, and CHPF families. The overlap between intense CS/DS production and the expression of multiple CS/DS glycosyltransferases suggests that efficient CS/DS biosynthesis requires a combination of several glycosyltransferases.

Glycosaminoglycans in the blood of hereditary multiple exostoses patients: Half reduction of heparan sulfate to chondroitin sulfate ratio and the possible diagnostic application

Hereditary multiple exostoses (HME) is an autosomal dominant skeletal disorder with wide variation in clinical phenotype and is caused by heterogeneous germline mutations in two of the Ext genes, EXT-1 and EXT-2, which encode ubiquitously expressed glycosyltransferases involved in the polymerization of heparan sulfate (HS) chains. To examine whether the Ext mutation could affect HS structures and amounts in HME patients being heterozygous for the Ext genes, we collected blood from patients and healthy individuals, separated it into plasma and cellular fractions and then isolated glycosaminoglycans (GAGs) from those fractions. A newly established method consisting of a combination of selective ethanol precipitation of GAGs, digestion of GAGs recovered on the filter-cup by direct addition of heparitinase or chondroitinase reaction solution and subsequent high-performance liquid chromatography of the unsaturated disaccharide products enabled the analysis using the least amount of blood (200 µL). We found that HS structures of HME patients were almost similar to those of controls in both plasma and cellular fractions. However, interestingly, although both the amounts of HS and chondroitin sulfate (CS) varied depending on the different individuals, the amounts of HS in both the plasma and cellular fractions of HME patient samples were decreased and the ratios of HS to CS (HS/CS) of HME patient samples were almost half those of healthy individuals. The results suggest that HME patients' blood exhibited reduced HS amounts and HS/CS ratios, which could be used as a diagnostic biomarker for HME.

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.

EXTL2, a member of the EXT family of tumor suppressors, controls glycosaminoglycan biosynthesis in a xylose kinase-dependent manner

Mutant alleles of EXT1 or EXT2, two members of the EXT gene family, are causative agents in hereditary multiple exostoses, and their gene products function together as a polymerase in the biosynthesis of heparan sulfate. EXTL2, one of three EXT-like genes in the human genome that are homologous to EXT1 and EXT2, encodes a transferase that adds not only GlcNAc but also N-acetylgalactosamine to the glycosaminoglycan (GAG)-protein linkage region via an α1,4-linkage. However, both the role of EXTL2 in the biosynthesis of GAGs and the biological significance of EXTL2 remain unclear. Here we show that EXTL2 transfers a GlcNAc residue to the tetrasaccharide linkage region that is phosphorylated by a xylose kinase 1 (FAM20B) and thereby terminates chain elongation. We isolated an oligosaccharide from the mouse liver, which was not detected in EXTL2 knock-out mice. Based on structural analysis by a combination of glycosidase digestion and 500-MHz (1)H NMR spectroscopy, the oligosaccharide was found to be GlcNAcα1-4GlcUAβ1-3Galβ1-3Galβ1-4Xyl(2-O-phosphate), which was considered to be a biosynthetic intermediate of an immature GAG chain. Indeed, EXTL2 specifically transferred a GlcNAc residue to a phosphorylated linkage tetrasaccharide, GlcUAβ1-3Galβ1-3Galβ1-4Xyl(2-O-phosphate). Remarkably, the phosphorylated linkage pentasaccharide generated by EXTL2 was not used as an acceptor for heparan sulfate or chondroitin sulfate polymerases. Moreover, production of GAGs was significantly higher in EXTL2 knock-out mice than in wild-type mice. These results indicate that EXTL2 functions to suppress GAG biosynthesis that is enhanced by a xylose kinase and that the EXTL2-dependent mechanism that regulates GAG biosynthesis might be a "quality control system" for proteoglycans.

Xylosyl- and glucuronyltransferase functions of LARGE in α-dystroglycan modification are conserved in LARGE2

LARGE-dependent modification enables α-dystroglycan (α-DG) to bind to its extracellular matrix ligands. Mutations in the LARGE gene and several others involved in O-mannosyl glycan synthesis have been identified in congenital and limb-girdle muscular dystrophies that are characterized by perturbed glycosylation and reduced ligand-binding affinity of α-DG. LARGE is a bifunctional glycosyltransferase that alternately transfers xylose and glucuronic acid, thereby generating the heteropolysaccharides on α-DG that confer its ligand binding. Although the LARGE paralog LARGE2 (also referred to as GYLTL1B) has likewise been shown to enhance the functional modification of α-DG in cultured cells, its enzymatic activities have not been identified. Here, we report that LARGE2 is also a bifunctional glycosyltransferase and compare its properties with those of LARGE. By means of a high-performance liquid chromatography-based enzymatic assay, we demonstrate that like LARGE, LARGE2 has xylosyltransferase (Xyl-T) and glucuronyltransferase (GlcA-T) activities, as well as polymerizing activity. Notably, however, the pH optima of the Xyl-T and GlcA-T of LARGE2 are distinct from one another and also from those of LARGE. Our results suggest that LARGE and LARGE2 catalyze the same glycosylation reactions for the functional modification of α-DG, but that they have different biochemical properties.

Heparan sulfate biosynthesis: regulation and variability

Nearly all vertebrate cells have been shown to express heparan sulfate proteoglycans (HSPGs) at the cell surface. The HSPGs bind to many secreted signaling proteins, including numerous growth factors, cytokines, and morphogens, to affect their tissue distribution and signaling. The heparan sulfate (HS) chains may have variable length and may differ with regard to both degree and pattern of sulfation. As the sulfation pattern of HS chains in most cases will determine if an interaction with a potential ligand will take place, as well as the affinity of the interaction, a key to understanding the function of HSPGs is to clarify how HS biosynthesis is regulated in different biological contexts. This review provides an introduction to the current understanding of HS biosynthesis and its regulation, and identifies research areas where more knowledge is needed to better understand how the HS biosynthetic machinery works.

Heparin biosynthesis

Heparin and heparan sulfate share the same polysaccharide backbone structure but differ in sulfation degree and expression pattern. Whereas heparan sulfate is found in virtually all cells of the human body, heparin expression is restricted to mast cells, where it has a function in storage of granular components such as histamine and mast cell specific proteases. Although differing in charge and sulfation pattern, current knowledge indicates that the same pathway is used for synthesis of heparin and heparan sulfate, with a large number of different enzymes taking part in the process. At present, little is known about how the individual enzymes are coordinated and how biosynthesis is regulated. These questions are addressed in this chapter together with a review of the basic enzymatic steps involved in initiation, elongation, and modification of the polysaccharides.

Lowered expression of heparan sulfate/heparin biosynthesis enzyme N-deacetylase/n-sulfotransferase 1 results in increased sulfation of mast cell heparin

Deficiency of the heparan sulfate biosynthesis enzyme N-deacetylase/N-sulfotransferase 1 (NDST1) in mice causes severely disturbed heparan sulfate biosynthesis in all organs, whereas lack of NDST2 only affects heparin biosynthesis in mast cells (MCs). To investigate the individual and combined roles of NDST1 and NDST2 during MC development, in vitro differentiated MCs derived from mouse embryos and embryonic stem cells, respectively, have been studied. Whereas MC development will not occur in the absence of both NDST1 and NDST2, lack of NDST2 alone results in the generation of defective MCs. Surprisingly, the relative amount of heparin produced in NDST1(+/-) and NDST1(-/-) MCs is higher (≈30%) than in control MCs where ≈95% of the (35)S-labeled glycosaminoglycans produced is chondroitin sulfate. Lowered expression of NDST1 also results in a higher sulfate content of the heparin synthesized and is accompanied by increased levels of stored MC proteases. A model of the GAGosome, a hypothetical Golgi enzyme complex, is used to explain the results.

Cell surface heparan sulfate chains regulate local reception of FGF signaling in the mouse embryo

Heparan sulfate (HS) proteoglycans modulate the activity of multiple growth factors on the cell surface and extracellular matrix. However, it remains unclear how the HS chains control the movement and reception of growth factors into targeted receiving cells during mammalian morphogenetic processes. Here, we found that HS-deficient Ext2 null mutant mouse embryos fail to respond to fibroblast growth factor (FGF) signaling. Marker expression analyses revealed that cell surface-tethered HS chains are crucial for local retention of FGF4 and FGF8 ligands in the extraembryonic ectoderm. Fine chimeric studies with single-cell resolution and expression studies with specific inhibitors for HS movement demonstrated that proteolytic cleavage of HS chains can spread FGF signaling to adjacent cells within a short distance. Together, the results show that spatiotemporal expression of cell surface-tethered HS chains regulate the local reception of FGF-signaling activity during mammalian embryogenesis.

Glycosaminoglycan binding facilitates entry of a bacterial pathogen into central nervous systems

Certain microbes invade brain microvascular endothelial cells (BMECs) to breach the blood-brain barrier (BBB) and establish central nervous system (CNS) infection. Here we use the leading meningitis pathogen group B Streptococcus (GBS) together with insect and mammalian infection models to probe a potential role of glycosaminoglycan (GAG) interactions in the pathogenesis of CNS entry. Site-directed mutagenesis of a GAG-binding domain of the surface GBS alpha C protein impeded GBS penetration of the Drosophila BBB in vivo and diminished GBS adherence to and invasion of human BMECs in vitro. Conversely, genetic impairment of GAG expression in flies or mice reduced GBS dissemination into the brain. These complementary approaches identify a role for bacterial-GAG interactions in the pathogenesis of CNS infection. Our results also highlight how the simpler yet genetically conserved Drosophila GAG pathways can provide a model organism to screen candidate molecules that can interrupt pathogen-GAG interactions for future therapeutic applications.

Streptococcus agalactiae (Group B Streptococcus, GBS) is a leading cause of meningitis in human newborn infants. The bacterial and host factors that allow this pathogen to cross the blood-brain barrier (BBB) and cause central nervous system (CNS) infection are not well understood. Here we demonstrate that GBS expresses a specific protein on its surface that can bind to sugar molecules known as glycosaminoglycans (GAGs) on the surface of brain capillary cells, initiating infection of the BBB. Fruit flies or mice genetically engineered to have reduced GAGs showed decreased dissemination of GBS into the brain tissues following experimental infection. Our results identify a role for bacterial-GAG interactions in the pathogenesis of newborn meningitis and highlight how the simpler yet genetically conserved fruit fly GAG biosynthetic pathways make the fruit fly a good model organism to screen candidate molecules that can interrupt pathogen-GAG interactions for future therapeutic applications.