Disturbed Ca2+ kinetics in N-deacetylase/N-sulfotransferase-1 defective myotubes

Physiology and Pathophysiology of Heparan Sulfate in Animal Models: Its Biosynthesis and Degradation

Heparan sulfate (HS) is a type of glycosaminoglycan that plays a key role in a variety of biological functions in neurology, skeletal development, immunology, and tumor metastasis. Biosynthesis of HS is initiated by a link of xylose to Ser residue of HS proteoglycans, followed by the formation of a linker tetrasaccharide. Then, an extension reaction of HS disaccharide occurs through polymerization of many repetitive units consisting of iduronic acid and N-acetylglucosamine. Subsequently, several modification reactions take place to complete the maturation of HS. The sulfation positions of N-, 2-O-, 6-O-, and 3-O- are all mediated by specific enzymes that may have multiple isozymes. C5-epimerization is facilitated by the epimerase enzyme that converts glucuronic acid to iduronic acid. Once these enzymatic reactions have been completed, the desulfation reaction further modifies HS. Apart from HS biosynthesis, the degradation of HS is largely mediated by the lysosome, an intracellular organelle with acidic pH. Mucopolysaccharidosis is a genetic disorder characterized by an accumulation of glycosaminoglycans in the body associated with neuronal, skeletal, and visceral disorders. Genetically modified animal models have significantly contributed to the understanding of the in vivo role of these enzymes. Their role and potential link to diseases are also discussed.

Perlecan/Hspg2 deficiency impairs bone's calcium signaling and associated transcriptome in response to mechanical loading

Perlecan, a heparan sulfate proteoglycan, acts as a mechanical sensor for bone to detect external loading. Deficiency of perlecan increases the risk of osteoporosis in patients with Schwartz-Jampel Syndrome (SJS) and attenuates loading-induced bone formation in perlecan deficient mice (Hypo). Considering that intracellular calcium [Ca²⁺]_i is an ubiquitous messenger controlling numerous cellular processes including mechanotransduction, we hypothesized that perlecan deficiency impairs bone's calcium signaling in response to loading. To test this, we performed real-time [Ca²⁺]_i imaging on in situ osteocytes of adult murine tibiae under cyclic loading (8N). Relative to wild type (WT), Hypo osteocytes showed decreases in the overall [Ca²⁺]_i response rate (-58%), calcium peaks (-33%), cells with multiple peaks (-53%), peak magnitude (-6.8%), and recovery speed to baseline (-23%). RNA sequencing and pathway analysis of tibiae from mice subjected to one or seven days of unilateral loading demonstrated that perlecan deficiency significantly suppressed the calcium signaling, ECM-receptor interaction, and focal adhesion pathways following repetitive loading. Defects in the endoplasmic reticulum (ER) calcium cycling regulators such as Ryr1/ryanodine receptors and Atp2a1/Serca1 calcium pumps were identified in Hypo bones. Taken together, impaired calcium signaling may contribute to bone's reduced anabolic response to loading, underlying the osteoporosis risk for the SJS patients.

A highly efficient tree structure for the biosynthesis of heparan sulfate accounts for the commonly observed disaccharides and suggests a mechanism for domain synthesis

The form of the biosynthetic pathway of the biologically and medically important polysaccharides heparan sulfate (HS) and the closely related heparin remain obscure despite significant progress characterising the biosynthetic machinery. Considering possible biosynthetic schemes using a graph approach and applying known constraints of enzyme order and specificity, a previously unreported system with a highly efficient tree structure emerged with two features: (1) All commonly occurring HS disaccharides could be synthesised through a common route, the major branch. (2) The least common disaccharides also occurred on a separate common branch, termed here the minor branch. This suggested that the relative abundance of these two sets of structures were the result of the specificity of a single enzyme (HS epimerase) acting at an early point in the scheme, to convert GlcA-GlcNS to IdoA-GlcNS in preference to converting GlcA-GlcNAc to IdoA-GlcNAc. A third key finding was that the common substrates for 3-O-sulfation all lie on the same (major) branch. The proposed scheme is consistent with a wide body of experiments comprising both biochemical data and results from HS biosynthetic enzyme knockout experiments in the literature. The major branch also contains a bifurcation, providing a choice of two distinct backbone geometries with the same charge. Further development of this novel biosynthetic scheme, in which frame shifts in the site of action of the enzymes were permitted to occur, while maintaining their order of action, suggested a mechanism by which domains could be generated, or further modification blocked. The relationship between the proposed pathway and the geometric and charge possibilities it allows were also explored.

Glycosaminoglycan mimetics trigger IP3-dependent intracellular calcium release in myoblasts

Glycosaminoglycans (GAG) are sulfated polysaccharides that play an important role in regulating cell functions. GAG mimetics called RGTAs (for ReGeneraTing Agents) have been shown to stimulate tissue repair. In particular they accelerate myogenesis, in part via their heparin-mimetic property towards growth factors. RGTAs also increase activity of calcium-dependent intracellular protease suggesting an effect on calcium cellular homeostasis. This effect was presently investigated on myoblasts in vitro using one member of the RGTA family molecule named OTR4120. We have shown that OTR4120 or heparin induced transient increases of intracellular calcium concentration ([Ca(2+)]i) in pre-fusing myoblasts from both mouse SolD7 cell line and rat skeletal muscle satellite cells grown in primary culture by mobilising sarcoplasmic reticulum store. This [Ca(2+)]i was not mediated by ryanodine receptors but instead resulted from stimulation of the Inositol-3 phosphate-phospholipase C activation pathway. OTR4120-induced calcium transient was not mediated through an ATP, nor a tyrosine kinase, nor an acetylcholine receptor but principally through serotonin 5-HT2A receptor. This original finding shows that the GAG mimetic can induce calcium signal through serotonin receptors and the IP3 pathway may be relevant to its ability to favour myoblast differentiation. It supports a novel and unexpected function of GAGs in the regulation of calcium homeostasis.

Mice deficient in heparan sulfate N-deacetylase/N-sulfotransferase 1

Ndsts (N-deacetylase/N-sulfotransferases) are enzymes responsible for N-sulfation during heparan sulfate (HS) and heparin biosynthesis. In this review, basic features of the Ndst1 enzyme are covered and a brief description of HS biosynthesis and its regulation is presented. Effects of Ndst1 deficiency on embryonic development are described. These include immature lungs, craniofacial dysplasia and eye developmental defects, branching defect during lacrimal gland development, delayed mineralization of the skeleton, and reduced pericyte recruitment during vascular development. A brief account of the effects of Ndst1 deficiency in selective cell types in adult mice is also given.

Phage Display‐Derived Human Antibodies Against Specific Glycosaminoglycan Epitopes

Glycosaminoglycans (GAGs) are long unbranched polysaccharides, most of which are linked to a core protein to form proteoglycans. Depending on the nature of their backbone, one can discern galactosaminoglycans (chondroitin sulfate [CS] and dermatan sulfate [DS]) and glucosaminoglycans (heparan sulfate [HS], heparin, hyaluronic acid, and keratan sulfate). Modification of the backbone by sulfation, deacetylation, and epimerization results in unique sequences within GAG molecules, which are instrumental in the binding of a large number of proteins. Investigating the exact roles of GAGs has long been hampered by the lack of appropriate tools, but we have successfully implemented phage display technology to generate a large panel of antibodies against CS, DS, HS, and heparin epitopes. These antibodies provide unique and highly versatile tools to study the topography, structure, and function of specific GAG domains. In this chapter, we describe the selection, characterization, and application of antibodies against specific GAG epitopes.

Heparan sulfates in skeletal muscle development and physiology

Recent years have seen an emerging interest in the composition of the skeletal muscle extracellular matrix (ECM) and in the developmental and physiological roles of its constituents. Many cell surface-associated and ECM-embedded molecules occur in highly organized spatiotemporal patterns, suggesting important roles in the development and functioning of skeletal muscle. Glycans are historically underrepresented in the study of skeletal muscle ECM, even though studies from up to 30 years ago have demonstrated specific carbohydrates and glycoproteins to be concentrated in neuromuscular junctions (NMJs). Changes in glycan profile and distribution during myogenesis and synaptogenesis hint at an active involvement of glycoconjugates in muscle development. A modest amount of literature involves glycoconjugates in muscle ion housekeeping, but a recent surge of evidence indicates that glycosylation defects are causal for many congenital (neuro)muscular disorders, rendering glycosylation essential for skeletal muscle integrity. In this review, we focus on a single class of ECM-resident glycans and their emerging roles in muscle development, physiology, and pathology: heparan sulfate proteoglycans (HSPGs), notably their heparan sulfate (HS) moiety.

A synthetic glycosaminoglycan mimetic (RGTA) modifies natural glycosaminoglycan species during myogenesis

Crucial events in myogenesis rely on the highly regulated spatiotemporal distribution of cell surface heparan sulfate proteoglycans to which are associated growth factors, thus creating a specific microenvironment around muscle cells. Most growth factors involved in control of myoblast growth and differentiation are stored in the extracellular matrix through interaction with specific sequences of glycosaminoglycan oligosaccharides, mainly heparan sulfate (HS). Different HS subspecies revealed by specific antibodies, have been shown to provide spatiotemporal regulation during muscle development. We have previously shown that glycosaminoglycan (GAG) mimetics called RGTA (ReGeneraTing Agent), stimulate muscle precursor cell growth and differentiation. These data suggest an important role of GAGs during myogenesis; however, little is yet known about the different species of GAGs synthesized during myogenesis and their metabolic regulation. We therefore quantified GAGs during myogenesis of C2.7 cells and show that the composition of GAG species was modified during myogenic differentiation. In particular, HS levels were increased during this process. In addition, the GAG mimetic RGTA, which stimulated both growth and differentiation of C2.7 cells, increased the total amount of GAG produced by these cells without significantly altering their rate of sulfation. RGTA treatment further enhanced HS levels and changed its sub-species composition. Although mRNA levels of the enzymes involved in HS biosynthesis were almost unchanged during myogenic differentiation, heparanase mRNA levels decreased. RGTA did not markedly alter these levels. Here we show that the effects of RGTA on myoblast growth and differentiation are in part mediated through an alteration of GAG species and provide an important insight into the role of these molecules in normal or pathologic myogenic processes.

Heparan sulfate structure in mice with genetically modified heparan sulfate production

Using a high throughput heparan sulfate (HS) isolation and characterization protocol, we have analyzed HS structure in several tissues from mice/mouse embryos deficient in HS biosynthesis enzymes (N-deacetylase/N-sulfotransferase (NDST)-1, NDST-2, and C5-epimerase, respectively) and in mice lacking syndecan-1. The results have given us new information regarding HS biosynthesis with implications on the role of HS in embryonic development. Our main conclusions are as follows. 1) The HS content, disaccharide composition, and the overall degree of N- and O-sulfation as well as domain organization are characteristic for each individual mouse tissue. 2) Removal of a key biosynthesis enzyme (NDST-1 or C5-epimerase) results in similar structural alterations in all of the tissues analyzed. 3) Essentially no variation in HS tissue structure is detected when individuals of the same genotype are compared. 4) NDST-2, although generally expressed, does not contribute significantly to tissue-specific HS structures. 5) No change in HS structure could be detected in syndecan-1-deficient mice.

Heterogeneity of heparan sulfates in human lung

Heparan sulfates (HS), a class of glycosaminoglycans, are long linear complex polysaccharides covalently attached to a protein core. The HS molecules are made up of repeating disaccharides onto which modification patterns are superimposed. This results in a large structural heterogeneity and forms the basis of specific interactions of HS toward a vast array of proteins, including growth factors and proteases. To study HS heterogeneity in the lung, we used phage display technology to select seven antibodies against human lung HS. Antibodies reacted with HS/heparin, but not with other glycosaminoglycans or polyanions. Sulfate groups were essential for antibody binding. The amino acid sequence of the antibodies was established, the complementarity determining region 3 of the heavy chain containing basic amino acids. The antibodies defined HS epitopes with a characteristic tissue distribution. Antibody EV3A1 primarily stained macrophages. Other antibodies primarily stained basement membranes, but with different preference toward type of basement membrane. Antibody EV3C3 was the only antibody which clearly reacted with bronchiolar epithelial cells. In human lung parenchyma, basic fibroblast growth factor and vascular endothelial growth factor were largely bound by HS. Some antibodies blocked a basic fibroblast growth factor-binding site of HS, and one antibody blocked a vascular endothelial growth factor-binding site of heparin. Taken together, these data suggest a specific role for HS epitopes in human lung. The antibodies obtained may be valuable tools to study HS in pulmonary diseases.