Developmental changes in glycosaminoglycans during skeletal muscle cell differentiation in culture

Reduced Glycosaminoglycan Sulfation Diminishes the Agrin Signal Transduction Pathway

Proteoglycans consist of a protein core complexed to glycosaminoglycan (GAG) side chains, are abundant in skeletal muscle cell membranes and basal lamina, and have important functions in neuromuscular synapse development. Treatment with chlorate results in the undersulfation of heparan sulfate and chondroitin sulfate GAGs in cell culture. In addition, chlorate treatment decreases the frequency of spontaneous acetylcholine receptor (AChR) clustering in skeletal muscle cell culture. AChRs and other molecules cluster to form the postsynaptic component of neuromuscular synapses. Chlorate treatment is shown here to decrease the frequency of agrin-induced AChR clustering and agrin-induced tyrosine phosphorylation of the AChR beta-subunit. These data suggest that reduced GAG chain sulfation decreases the frequency of AChR clustering by diminishing the agrin signal transduction pathway.

Augmented synthesis and differential localization of heparan sulfate proteoglycans in Duchenne muscular dystrophy

Muscular dystrophies are characterized by continuous cycles of degeneration and regeneration that result in extensive fibrosis and a progressive diminution of muscle mass. Cell surface heparan sulfate proteoglycans are found almost ubiquitously on the surface and in the extracellular matrix (ECM) of mammalian cells. These macromolecules interact with a great variety of ligands, including ECM constituents, adhesion molecules, and growth factors. In this study, we evaluated the expression and localization of three heparan sulfate proteoglycans in the biopsies of Duchenne muscular dystrophy (DMD) patients. Through SDS-PAGE analyses followed by specific identification of heparitinase-digested proteins with an anti-Delta-heparan sulfate specific monoclonal antibodies, we observed an increase of three forms of heparan sulfate proteoglycans, corresponding to perlecan, syndecan-3, and glypican-1. Immunohistochemistry analyses indicated a differential localization for these proteoglycans: glypican-1 and perlecan were found mainly associated to ECM structures, while syndecan-3 was associated to muscle fibers. These results suggest that the amount of specific heparan sulfate proteoglycans is augmented in skeletal muscle in DMD patients presenting a differential localization.

Effects of collagen gel configuration on behavior of vascular smooth muscle cells in vitro: association with vascular morphogenesis

The growth, behavior, and contractile protein expression of rabbit aortic smooth muscle cells (SMC) grown on, between layers, or within a collagen gel was investigated by confocal laser scanning fluorescence microscopy and Western analysis. SMC grown on collagen gel behaved similarly to those on conventional culture dishes. However, when a second layer of collagen was overlaid, cells underwent an elongated quiescent phase before onset of proliferation and a more than threefold lower logarithmic growth rate was observed. These cells self-organized into a network with ring-like structures. With increasing culture time, some of the rings developed into funnel-like, incomplete or complete tubular structures. If a tubular template preexisted within the gel, the SMC established a cylinder-shaped tube with several circularly arranged muscular layers (similar to an artery wall). This behavior mimicked endothelial cells during angiogenesis in vitro. A similar phenomenon occurred in cultures in which SMC were randomly mixed in a collagen gel, but here their behavior and morphology varied with their position within the gel. Western blot analysis showed that the SMC differentiation marker, smooth muscle myosin heavy chain-2 (SM-2), rapidly decreased, disappearing by day 10 in SMC grown on collagen, but was still detectable until day 25 in cells cultured between or within the same gel. These findings indicate that like endothelial cells, vascular SMC can display blood vessel formation behavior in vitro when an appropriate three-dimensional matrix environment is provided to keep them in a relatively higher-differentiated and low-proliferative state.

Antisense inhibition of syndecan-3 expression during skeletal muscle differentiation accelerates myogenesis through a basic fibroblast growth factor-dependent mechanism

Syndecan-3 is a member of a family of transmembrane proteoglycans that posses highly homologous cytoplasmic and transmembrane domains and function as extracellular matrix receptors and low-affinity receptors for signaling molecules such as basic fibroblasts growth factor (FGF-2). Syndecan-3 is transiently expressed in developing limb bud and absent in adult skeletal muscle. In this study we investigated the expression of syndecan-3 and its role on FGF-2-dependent inhibition of myogenesis. Syndecan-3 expression was down-regulated during skeletal muscle differentiation of C(2)C(12) myoblasts, as determined by Northern blot analyses and immunoprecipitation. To probe the function of syndecan-3 during myogenesis, C(2)C(12) myoblasts were stably transfected with a plasmid coding for antisense syndecan-3 mRNA. The resulting inhibition of syndecan-3 expression caused accelerated skeletal muscle differentiation, as determined by expression of creatine kinase and myosin and myoblast fusion. Expression of a master transcription factor for muscle differentiation, myogenin, was also accelerated in antisense syndecan-3-transfected myoblasts compared with control transfected and wild type cells. Reduced expression of syndecan-3 resulted in a 13-fold decrease in sensitivity to FGF-2-dependent inhibition of myogenin expression. Addition of heparin partially reversed this effect. These results demonstrate that syndecan-3 expression is down-regulated during differentiation and the level of expression of membrane-bound heparan sulfate on myoblast surface is critical for fine modulation of responsiveness to FGF-2. These findings strongly suggest a role for syndecan-3 in regulation of skeletal muscle terminal differentiation.

Dynamic expression of proteoglycans during chicken skeletal muscle development and maturation

Skeletal muscle development is a complex process in which cell migration and adhesion play important roles. Because these cellular activities involve cell surface and extracellular matrix molecules, proteoglycan analysis was performed for developing chick skeletal muscle. Proteoglycans are macromolecular conjugates of protein and carbohydrate found in the extracellular matrix and at the cell surface. In developing muscle, both in vivo and in vitro, there is a development-related progression from synthesis of primarily large proteoglycans at earlier stages to mainly small proteoglycans at later stages. This progression was demonstrated by radiolabeling developing muscle and extracting and characterizing the proteoglycans. The large proteoglycans synthesized earlier in myogenesis have been identified as the large chondroitin sulfate proteoglycan, versican. Among the small proteoglycans synthesized at later stages is the small dermatan sulfate proteoglycan, decorin. Immunolocalization of these proteoglycans shows that versican is initially present in pericellular locations around developing myotubes, whereas decorin is observed in the epimysium early in development, and then its distribution gradually spreads to also include the perimysium and endomysium. Studies of regenerating muscle show that there is a recapitulation of the embryonic pattern of proteoglycan synthesis, which, coupled with the results from embryonic muscle development, suggests a role for versican in some early aspect of myogenesis.

Syndecan-1 expression inhibits myoblast differentiation through a basic fibroblast growth factor-dependent mechanism

Expression of syndecan-1, a cell-surface heparan sulfate proteoglycan, is down-regulated during skeletal muscle differentiation (Larraín, J., Cizmeci-Smith, G., Troncoso, V., Stahl, R. C., Carey, D. J., and Brandan, E. (1997) J. Biol. Chem. 272, 18418-18424). We examined the role of syndecan-1 in basic fibroblast growth factor (bFGF)-dependent inhibition of myogenesis. C2C12 myoblasts were stably transfected with an expression plasmid containing the rat syndecan-1 coding region cDNA. Constitutive syndecan-1 expression resulted in a strongly diminished capacity of the transfected clones to differentiate and to express skeletal muscle-specific markers such as fusion, creatine kinase, and myosin. The expression of myogenin, a master transcription factor for muscle differentiation, was also reduced and delayed. Analysis of the induction of a myogenin promoter-driven reporter revealed that syndecan-1 expression resulted in a 6-7-fold increase in sensitivity to bFGF-dependent inhibition of myogenin expression. Transfecting the cells with a plasmid containing myogenin cDNA reversed the inhibition of myogenin transcriptional activation and myosin expression in syndecan-1-transfected cells; however, cell fusion was not observed. These results demonstrate that syndecan-1 expression enhances cell responsiveness to bFGF and inhibits myoblast fusion and suggest that muscle terminal differentiation is regulated by syndecan-1 expression.

Differential heparin inhibition of skeletal muscle alpha-dystroglycan binding to laminins

The laminin binding properties of alpha-dystroglycan purified from rabbit skeletal muscle membranes were examined. In a solid phase microtiter assay, 125I-laminin (laminin-1) bound to purified alpha-dystroglycan in a specific and saturable manner with a half-maximal concentration of 8 nM. The binding of 125I- alpha-dystroglycan to native laminin and merosin (a mixture of laminin-2 and -4) was also compared using the solid phase assay. The absolute binding of 125I- alpha-dystroglycan to laminin (6955 +/- 250 cpm/well) was similar to that measured for merosin (7440 +/- 970 cpm/well). However, inclusion of 1 mg/ml heparin in the incubation medium inhibited 125I-alpha-dystroglycan binding to laminin by 84 +/- 4.3% but inhibited 125I-alpha-dystroglycan binding to merosin by only 17 +/- 5.2%. Similar results were obtained with heparan sulfate, while de-N-sulfated heparin, hyaluronic acid, and chondroitin sulfate had no differential effect. These results were confirmed by iodinated laminin and merosin overlay of electrophoretically separated and blotted dystrophin-glycoprotein complex. In contrast to the results obtained with skeletal muscle alpha-dystroglycan, both laminin and merosin binding to purified brain alpha-dystroglycan was significantly inhibited by heparin. Our data support the possibility that one or more heparan sulfate proteoglycans may specifically modulate the interaction of alpha-dystroglycan with different extracellular matrix proteins in skeletal muscle.

Decorin and a large dermatan sulfate proteoglycan in bovine striated muscle

Proteoglycans were extracted and isolated from adult bovine muscle tissue by dissociative extraction followed by density gradient centrifugation, gel chromatography and ion-exchange chromatography. Two proteoglycans were characterized; one of large molecular size (PG-L) and one of small molecular size (PG-S). The recovery of PG-L and PG-S was 33% and 67%, respectively. By cellulose acetate electrophoresis before and after treatment with chondroitinase AC and ABC both samples were shown to carry predominantly dermatan sulfate chains. The large proteoglycan was recognized with an antibody against a large dermatan sulfate proteoglycan from bovine sclera, whereas the small was recognized by an antibody against decorin from bovine sclera. Chondroitinase ABC treatment of PG-S followed by SDS-PAGE showed a core protein with a molecular weight of 45 kDa, which also reacted with the decorin antibody. Amino-acid analysis of both PG-L and PG-S revealed an amino-acid composition closely similar, although not identical, to the large dermatan sulfate proteoglycan from bovine sclera and decorin, respectively. Immunohistochemical analyses of muscle tissue sections showed that decorin and the large dermatan sulfate proteoglycan are present in the perimysium layers of muscle tissue, although although with a somewhat different pattern of distribution. Decorin was, in addition, found in the endomysium.

Identity of the core proteins of the large chondroitin sulphate proteoglycans synthesized by skeletal muscle and prechondrogenic mesenchyme

Large, chondroitin sulphate-containing proteoglycans are synthesized by three prominent tissue in the embryonic chick limb. One of these proteoglycans is aggrecan, the phenotype-specific proteoglycan of cartilage. Another, PG-M, is produced by prechondrogenic mesenchymal cells. The third, M-CSPG, is made by developing skeletal muscle cells. While the carbohydrate components of PG-M and M-CSPG share some similarities, both of these proteoglycans clearly have different carbohydrate moieties from those of aggrecan. To compare these three proteoglycans at another level, their core protein structures were analysed in three ways: by the presence or absence of monoclonal antibody epitopes, by one-dimensional peptide display of the cyanogen bromide-cleaved core proteins and by electron microscopic imaging of the molecules. Monoclonal antibodies whose epitopes are present in aggrecan core protein were tested with core protein preparations from M-CSPG and PG-M. One of these, 7D1, recognizes both PG-M and M-CSPG, while another, 1C6, shows no reactivity for the non-cartilage proteoglycans. The absence of 1C6 reactivity is of interest, as its epitope is in a region of the aggrecan core protein known to have a functional homologue in the core proteins of PG-M and M-CSPG. The cyanogen bromide-fragmented peptide pattern of M-CSPG is the same as that of PG-M, and both are different from that of aggrecan. The aggrecan pattern has one prominent large band (molecular mass 130 kDa), some less prominent large bands (molecular mass 70-100 kDa) and several smaller bands. In contrast, the PG-M and M-CSPG patterns show no bands with molecular masses > 73 kDa, and the smaller bands (molecular mass < 40 kDa) have a different pattern to that of the smaller bands from aggrecan. The electron microscopic images of aggrecan show a core protein with one end having two globular regions separated by a short linear segment; adjacent to this is a long linear segment, which sometimes contains a third globular region at the end of the core protein opposite the end with the double-globe structure. M-CSPG and PG-M core proteins never show images with the double-globe structure. Instead, one end of the molecule has a single globular domain, and a second globular region is variably present at the opposite end of the core protein. Thus, by all three methods, the core proteins of PG-M and M-CSPG appear to be the same and both differ from the core protein of aggrecan.

Proteoglycan synthesis by clonal skeletal muscle cells during in vitro myogenesis: differences detected in the types and patterns from primary cultures

Proteoglycan synthesis by two clonal murine skeletal muscle cell lines, G8-1 and C2, was examined. Cultures of skeletal muscle cells at both the myoblast and myotube stages were radiolabeled using [35S]sulfate as a precursor. The proteoglycans of the cell layer and medium were separately extracted and isolated by ion exchange chromatography on DEAE-Sephacel followed by gel filtration chromatography on Sepharose CL-2B. The cell layer proteoglycans eluted from Sepharose CL-20 as a single peak with a Kav of 0.66 and contained glycosaminoglycan chains with an average molecular weight of 20,000. The glycosaminoglycan chains were composed of nearly equal mixtures of chondroitin sulfate and heparan sulfate with the exception that C2 myoblast cultures contained larger amounts of heparan sulfate. Of interest, this line differentiates more rapidly in our laboratory than G8-1. The medium proteoglycans also eluted from Sepharose CL-2B as a single peak with a Kav of 0.66 but contained glycosaminoglycan chains with an average molecular weight of 32,000. Based upon enzymatic and chemical analysis, the medium glycosaminoglycan chains were composed of a mixture of chondroitin sulfate (71-80%) and heparin sulfate (19-22%). Following chondroitinase ABC digestion, the predominant disaccharide released from all glycosaminoglycan fractions was chondroitin-4-sulfate. When the extracted cell layer proteoglycans were chromatographed on Sepharose CL-28 in the absence of detergent, a small but consistent proportion (14-18%) eluted in the void volume, suggesting the association of at least a portion of this proteoglycan with cellular lipid. These differences distinguish proteoglycan metabolism in fusing clonal lines from primary muscle cell cultures suggesting their utility in evaluating the contribution of these macromolecules in myogenesis.

Glycosaminoglycan variants in the C2 muscle cell line

Using a replica technique, we have isolated and characterized five genetic variants of the C2 mouse muscle cell line that are defective in incorporation of radiolabeled sulfate into glycosaminoglycans (GAGs). The variants incorporate free sulfate into GAGs at 5-20% of wild-type levels. None of the variants is defective in sulfate transport across the cell membrane, and in no case could the deficit in incorporation of sulfate be reversed by addition of an artificial initiator of GAG biosynthesis, p-nitrophenyl beta-D-xyloside. Analysis of the incorporation of [3H]glucosamine into GAGs by the variants revealed three different patterns: one variant incorporated [3H]glucosamine at the wild-type level; one, S27, at a severely reduced level; and three at intermediate levels. Four of the five variants showed marked deficits in their ability to differentiate and fuse. The remaining variant, S27, formed multinucleated myotubes and expressed acetylcholine receptor with a normal time course. Differentiation of the first four variants could not be restored by addition of exogenous GAGs or extracellular matrix. Because of the important roles that GAGs and proteoglycans are thought to play in the differentiation of muscle, these genetic variants should serve as useful tools in functional analyses of these molecules.

Histochemical analysis of newly synthesized and accumulated sulfated glycosaminoglycans during musculogenesis in the embryonic chick leg

The leg musculature from 11, 14, and 17 day chick embryos was analyzed histochemically to investigate the temporal and spatial distribution of various types of sulfated glycosaminoglycans present during skeletal muscle development. Types of glycans were identified by selective degradation with specific glycosidases and nitrous acid coupled with Alcian blue staining procedures for sulfated polyanions and with [35S]sulfate autoradiography. On day 11, radiolabeled chondroitin sulfate glycosaminoglycans are localized extracellularly in both the myogenic and connective tissue cell populations. By day 17, incorporation of [35S]sulfate into chondroitin sulfate is substantially reduced, although Alcian blue-stained chondroitin sulfate molecules are still detectable. With increasing age and developmental state of the tissues, radiolabeled and stained dermatan sulfate and heparan sulfate progressively increase in relative quantity compared to chondroitin sulfate both in muscle and in associated connective tissue elements. These changes in glycosaminoglycans correlate well with similar changes previously determined biochemically and further document the alterations in extracellular matrix components during embryonic skeletal myogenesis.

Structural characterization of chick embryonic skeletal muscle chondroitin sulfate proteoglycan

Embryonic chick skeletal muscle has been shown to synthesize a distinct proteoglycan of large size with relatively large, highly 6-sulfated chondroitin sulfate glycosaminoglycans. Further analysis of these proteoglycans indicates that tryptic digestion gives rise to fragments with an average of two chondroitin sulfate chains per peptide. The skeletal muscle chondroitin sulfate proteoglycan also contains oligosaccharides whose characteristics suggest the presence of both O-linked and N-linked oligosaccharides. These characteristics include the average hydrodynamic size of the oligosaccharides as well as their localization. Approximately 10% of the putative O-linked oligosaccharides reside on the same tryptic fragments which contain the chondroitin sulfate chains, while the presumptive N-linked oligosaccharides appear to be present at sites distant from the chondroitin sulfate. Further support for this identification comes from radioisotopic labeling with [3H]mannose, which is incorporated exclusively into the putative N-linked oligosaccharides. Some of the O-linked oligosaccharides which are not in close apposition to the chondroitin sulfate seem to occur in clusters. The skeletal muscle chondroitin sulfate proteoglycan has the ability to interact in a link protein-stabilized fashion with hyaluronic acid. This ability as well as the estimated number of chondroitin sulfate chains per cluster and the estimated number of oligosaccharides per chondroitin sulfate chain have implications about the structure of the core protein of the skeletal muscle proteoglycan. The information presented is used to construct a model of these molecules; with this detailed model, attention can now be directed at other aspects of the skeletal muscle chondroitin sulfate proteoglycan, such as its role in myogenesis.

Partial purification and characterization of detergent-solubilized N-sulfotransferase activity associated with calf brain microsomes

Calf brain 3'-phosphoadenosine 5'-phosphosulfate (PAPS):proteoheparan sulfate (PHS) N-sulfotransferase activity is solubilized by extracting salt-washed microsomes with 1% Cutscum. A protocol is described for the partial purification of the sulfotransferase activity utilizing: (1) diethylaminoethyl (DEAE)-Sephacel, (2) heparin-Sepharose CL-6B, and (3) 3',5'-ADP-agarose as chromatographic supports. Sulfotransferase activity was followed by using 3'-phosphoadenosine 5'-phospho[35S]sulfate and endogenous acceptors in heat-inactivated microsomes as exogenous substrates. Two chromatographically distinct fractions (ST1 and ST2) of sulfotransferase activity are resolved on DEAE-Sephacel. Both sulfotransferase activities have been partially purified and characterized. An apparent purification of the two N-sulfotransferase fractions of 22- to 29-fold, relative to the microsomal activity, is achieved by this procedure. Since ST1 appears to represent approximately 24% of the total microsomal activity, a purification of 89-fold has been estimated for this fraction. Neither sulfotransferase activity was stimulated by MnCl2, MgCl2, or CaCl2 added at 10 mM, nor inhibited by the presence of 10 mM EDTA. ST1 and ST2 are optimally active at pH 7.5-8. Apparent Km values for PAPS of 2.3 microM and 0.9 microM have been determined for ST1 and ST2, respectively. ST1 exhibits N-sulfotransferase activity primarily and is inhibited by phosphatidylserine whereas the ST2 fraction contains a mixture of N- and O-sulfotransferase activity and is stimulated by phosphatidylserine, phosphatidylcholine, and lysophosphatidylcholine. The detection of two chromatographically distinct sulfotransferase activities raises the possibility that N-sulfation of proteoheparan sulfates could be catalyzed by more than one enzyme, and that N-sulfation and O-sulfation of proteoglycans are catalyzed by separate enzymes in nervous tissue.

Heparin inhibits skeletal muscle growth in vitro

Heparin or heparan sulfate proteoglycan (HeSPG), but not chondroitin sulfate or hyaluronic acid, exerts a pronounced inhibitory effect on muscle growth in vitro, as determined by total protein, myosin accumulation or synthesis, and [3H]thymidine incorporation studies. Primary muscle fibroblast culture growth is also inhibited by heparin but to a substantially lesser degree compared to muscle (30% and over 90% inhibition of growth, respectively). Heparin-induced inhibition of skeletal muscle growth is a consequence of its interaction with a growth factor(s) present in the media used to support myogenesis; heparin-Sepharose column absorbed horse serum can support muscle growth only in the presence of added heparin-binding growth factors like fibroblast growth factor (FGF) or chicken muscle growth factor (CMGF). Furthermore, heparin prevents the binding of iodinated FGF to the myoblast surface. We also show that the extent of muscle growth is a function of the relative amounts of heparin and FGF in culture. Finally, we provide evidence indicating that FGF can combine with endogenously occurring heparin-like components: immobilized FGF binds sodium-[35S]sulfate labeled components secreted in muscle culture conditioned medium, an interaction inhibited by anti-HeSPG antibodies or heparin, but not by other sulfated glycosaminoglycans. Since heparin binding growth factors not only stimulate myoblast proliferation but also actively inhibit the onset of muscle differentiation (G. Spitzz, D. Roman, and A. Strauss (1986). J. Biol. Chem. 261, 9483-9488), their interaction with naturally occurring heparin-like components may be an important physiological mechanism for modulating muscle growth and differentiation in development and regeneration.

Chondroitin, chondroitin 6-sulphate, chondroitin 4-sulphate and dermatan sulphate proteoglycans in normal and pathological human muscle

Chondroitin, chondroitin 6-sulphate, chondroitin 4-sulphate and dermatan sulphate proteoglycans were immunolocalized by monoclonal antibodies applied to human muscle sections digested with chondroitinase. In normal muscle the 4 proteoglycans presented a different extracellular localization: unsulphated chondroitin sulphate (chondroitin) was present in the endomysium and around capillaries, chondroitin 6-sulphate in the basal membrane zone, chondroitin 4-sulphate in the vessel adventitia, in the endomysium around capillaries and, to a lesser degree, in the perimysium, dermatan sulphate in the perimysium and, to a lesser extent, in the vessel adventitia and in the endomysium around capillaries. The enlarged endomysium of pathological muscle contained chondroitin and chondroitin 4-sulphate. Chondroitin 6-sulphate and dermatan sulphate did not seem present in the increased connective tissue. No peculiar pattern was observed in the various neuromuscular diseases studied. The specific extracellular distribution, the different biochemical composition and the different ability to bind to other extracellular components suggest a different biological role of these compounds. Chondroitin 6-sulphate is a component of a highly specialized extracellular structure, namely basal membrane. Chondroitin and chondroitin 4-sulphate participate in the composition of actively changing extracellular matrix such as the endomysium in pathological muscle. On the other hand, dermatan sulphate is a constituent of the perimysium that is a more static extracellular structure.

Proteoglycan synthesis by primary chick skeletal muscle during in vitro myogenesis

The proteoglycans synthesized by primary chick skeletal muscle during in vitro myogenesis were compared with those of muscle-specific fibroblasts. Cultures of skeletal muscle cells and muscle fibroblasts were separately labeled using [35S] sulfate as a precursor. The proteoglycans of the cell layer and medium were separately extracted and isolated by ion-exchange chromatography on DEAE-Sephacel followed by gel filtration chromatography on Sepharose CL-2B. Two cell layer-associated proteoglycans synthesized both by skeletal muscle cells and muscle fibroblasts were identified. The first, a high molecular weight proteoglycan, eluted from Sepharose CL-2B with a Kav of 0.07 and contained exclusively chondroitin sulfate chains with an average molecular weight greater than 50,000. The second, a relatively smaller proteoglycan, eluted from Sepharose CL-2B with a Kav of 0.61 and contained primarily heparan sulfate chains with an average molecular weight of 16,000. Two labeled proteoglycans were also found in the medium of both skeletal muscle and muscle fibroblasts. A high molecular weight proteoglycan was found with virtually identical properties to that of the high molecular weight chondroitin sulfate proteoglycan of the cell layer. A second, smaller proteoglycan had a similar monomer size (Kav of 0.63) to the cell layer heparan sulfate proteoglycan, but differed from it in that this molecule contained primarily chondroitin sulfate chains with an average molecular weight of 32,000. Studies on the distribution of these proteoglycans in muscle cells during in vitro myogenesis demonstrated that a parallel increase in the relative amounts of the smaller proteoglycans occurred in both the cell layer and medium compared to the large chondroitin sulfate proteoglycan in each compartment. In contrast, muscle-derived fibroblasts displayed a constant ratio of the small proteoglycans of the cell layer and medium fractions, compared to the larger chondroitin sulfate proteoglycan of the respective fraction as a function of cell density. Our results support the concept that proteoglycan synthesis is under developmental regulation during skeletal myogenesis.

Biosynthesis of heparan sulfate proteoglycans of developing chick breast skeletal muscle in vitro

Previous studies have reported an increase in heparan sulfate glycosaminoglycan (HSGAG) during skeletal muscle differentiation in culture. We have investigated this phenomenon further in relation to the heparan sulfate proteoglycans (HSPG) produced by myogenic cultures. Pulse-chase analysis indicated an approx. 3-fold increase in heparan sulfate synthesis in myotube cultures over that in proliferating or aligning myoblast cultures. Muscle fibroblast culture heparan sulfate synthesis was higher than that of myoblasts but was lower than myotubes. The turnover rates appeared to be the same for all stages of development, with a t1/2 of approx. 5 h. Enrichment for heparan sulfate by Sepharose CL-4B and DEAE-Sephacel chromatography indicated an increase in the hydrodynamic size of the proteoglycan produced by myotubes over that from myoblasts, with a shift in Kav from 0.14-0.19 to 0.07. Fibroblasts synthesized the smallest proteoglycan, with a Kav of 0.22. All of the proteoglycans contained similar sized glycosaminoglycan chains with an estimated molecular weight of 30,000-40,000. Localization of the heparan sulfate proteoglycan in myotube cultures by trypsin sensitivity indicated much of the intact proteoglycan to be closely associated with the cell surface, while internalized material appeared in a degraded form.

Differential redistribution of lectin receptor classes on clonal rat myotubes and myoblasts

To evaluate the relative mobilities of cell surface glycoconjugates during myogenesis we have studied the redistribution of fluorescein-conjugated plant lectins on L6 rat myogenic cells. Previous experiments had demonstrated that the receptors for the lectins soybean agglutinin (SBA), wheat germ agglutinin, concanavalin A and Lens culinaris agglutinin all were relatively uniformly distributed on both myoblasts and myotubes, and that SBA receptors were capable of rapid redistribution on myotubes but not myoblasts at 4 degrees C (Sawyer & Akeson, 1983). Here we show that when SBA-labelled myoblasts are incubated at 37 degrees C, or for extended times at 4 degrees C, the lectin aggregates as on myotubes. So it appears that SBA-binding components show a quantitative rather than qualitative change in their mobility during L6 differentiation. In addition, the redistribution of the three other lectins on myoblasts and myotubes was either less prominent (i.e. showing fewer apparent surface clusters) or occurred less rapidly than with SBA. None of these three lectins showed striking differences in mobility between myoblasts and myotubes. Thus, it appears that SBA binds to a subset of surface glycoconjugates that is relatively highly mobile, and that this mobility is specifically enhanced with differentiation.

Immunohistochemical localization of chondroitin sulfate in normal and pathological human muscle

The immunohistological localization of chondroitin sulfate (CS) has been studied in normal and pathological human muscle. The bovine nasal cartilage proteoglycan digested with chondroitinase ABC (BNC-PG-Ch ABC) has been utilized for the production of a rabbit polyclonal antiserum. In vitro studies showed that the antiserum binds to the unsaturated disaccharide that remains attached to the core protein after digestion of the CS chains with chondroitinase ABC (Ch ABC). As the disaccharide is created specifically by Ch ABC digestion of the CS chains, the antiserum allows the immunolocalization of CS on tissue sections digested with Ch ABC. The immunohistochemical study on normal and pathological muscle demonstrated a localization of CS in all the extracellular structures: endomysium, perimysium, muscle spindle capsule and intrafusal space. In pathological conditions, the CS was raised in all the cases with increased connective tissue, showing a pattern comparable to that obtained with fibronectin and collagen III. None of the pathological conditions displayed any peculiar character of CS distribution. This finding does not support a primary role for CS in the pathogenesis of muscular dystrophy.

Glycosaminoglycan metabolism of HL-60 cells during differentiation induction by tetradecanoylphorbol-13-acetate

Many biochemical responses to phorbol ester differentiation inducers have been reported, including alterations in synthesis of specific gene products such as glycoproteins. Stage-specific glycosaminoglycan changes have previously been associated with the differentiation process, including a dramatic reduction in cellular chondroitin 4-sulfate during human myeloid leukemia cell maturation induced by 12-O-tetradecanoylphorbol-13-acetate (TPA). We have demonstrated that treatment of HL-60 human promyelocytic leukemia cells with 4-methyl-umbelliferyl-beta-D-xyloside increases precursor incorporation into glycosaminoglycans linked to beta-D-xyloside, rather than core protein, eliminating the need for core protein and xylosyltransferase. Therefore, these beta-D-xyloside-treated cells were used to study the decreased glycosaminoglycan production during TPA-induced HL-60 differentiation. Exposure of these pretreated HL-60 cells to TPA, which induces macrophage-like maturation, resulted in a 70% reduction of incorporation of [35S]sulfate into cell-associated glycosaminoglycans. Thus, even in HL-60 cells in which glycosaminoglycan production is maximally stimulated by beta-D-xyloside, TPA is a strong inhibitor of free glycosaminoglycan chain production, and this biochemical effect is associated with other features of leukocyte maturation.

Electron microscopic characterization of chick embryonic skeletal muscle proteoglycans

In this article, proteoglycans from embryonic chick leg muscle are quantitatively and qualitatively compared with day 8 high density cell culture cartilage proteoglycans by electron microscopy of proteoglycan-cytochrome c monolayers. The visualized proteoglycan profiles were separated into four categories according to shape, size, and complexity. The two major categories were further characterized by lengths of core proteins, lengths of side projections, and distance between side projections. Two large proteoglycans are identifiable in spread leg muscle preparations. One group has a core protein (mean length of 205 nm) from which extend long thin side projections that we interpret to be groups of chondroitin sulfate glycosaminoglycans with a mean length of 79 nm. This large chondroitin sulfate proteoglycan is the only type found in muscle cultures as determined both biochemically in the past and now by electron microscopy and is referred to as muscle proteoglycan. The second large proteoglycan has a mean core protein length of 250 nm and side projections that are visibly shorter (mean length of 38 nm) and thicker than those of the muscle proteoglycan. This group is referred to as the mesenchymal proteoglycan since its biosynthetic origin is still uncertain. We compare these two profiles with the chick cartilage chondroitin sulfate proteoglycan that has a mean core protein length of 202 nm and side projections with a mean length of 50 nm. The data presented here substantiate the earlier biochemical characterization of these noncartilage proteoglycans and establish the unique structural features of the muscle proteoglycan as compared with the similar profiles of the cartilage and mesenchymal proteoglycans.

Change of hyaluronic acid synthesis during differentiation of myogenic cells and its relation to transformation of myoblasts by Rous sarcoma virus

Hyaluronic acid synthesis was examined in cultures of differentiating chick embryo muscle cells before, during and after fusion. Prior to fusion, hyaluronic acid was synthesized and secreted into the medium, but once fusion began this synthesis was reduced significantly. Synthesis then increased again after completion of fusion. Thus, production of hyaluronic acid was lowest at the time of or right before cell fusion. When myoblasts were transformed by Rous sarcoma virus (RSV), a higher amount of hyaluronic acid was synthesized, and cells were not able to fuse. The turnover rate of hyaluronic acid might be different between myotubes and RSV-transformed myoblasts. The addition of exogenous hyaluronic acid to myoblast cultures resulted in the partial inhibition of fusion. The effect was reversible because fusion took place after removal of the exogenous hyaluronic acid. These observations suggest that hyaluronic acid plays an important role in the differentiation of myogenic cells, and that elevated hyaluronic acid synthesis may partly be the reason for inhibition of myotube formation upon transformation by Rous sarcoma virus.

Cell surface glycosaminoglycans (GAGs) of cultured chick fibroblasts. Modifications in relation to the stage of embryo development

We have investigated the changes in glycosaminoglycan (GAG) composition between cultured fibroblasts derived from 8- and 16-day chick embryos. GAG composition has been studied after [3H]glucosamine and [35S]sulfate labeling. Both the 8- and 16-day embryo fibroblasts were found to contain hyaluronic acid (HA), dermatan sulfate (DS), heparan sulfate (HS) and chondroitin sulfates (CS), the latter being the major component in 8- and 16-day cells. These four GAGs were quantified after their separation using cellulose acetate electrophoresis. The amounts of HA and CS were respectively shown to increase 2-fold and 4-fold between the 8th and 16th day of development, whereas the amounts of HS and DS resp. diminished 2.5-fold and 1.2-fold. These results show that the relative proportions of the different GAGs alter during embryo development. The fibroblasts from 8-day-old embryos detached more rapidly from the culture dishes than the cells from 16-day-old embryos when treated with trypsin. However, this difference was not directly related to the different GAG content.

Glucocorticoid regulation of glycosaminoglycan accumulation in murine fibroblasts

The effects of Dexamethasone (Dex) on Glycosaminoglycan (GAG) accumulation were examined in the murine fibroblast line C3H/10T1/2. Confluent cultures were treated with the hormone and then labelled with either [3H]acetate or [3H]glucosamine and analyzed for [3H]GAG content. Dex could inhibit the accumulation of [3H]GAG in a dose dependent manner with a half maximal response achieved at approximately 5nM and a maximal response at approximately 100nM. Higher concentrations failed to influence precursor incorporation further. The response was seen after 6 hours of hormone exposure and was nearly maximal by 10 hours. The incorporation of [3H]leucine into protein was unaffected by Dex treatment. 11 alpha hydrocortisone, an inactive steroid, failed to elicit a cellular response. Thus it appears that glucocorticoids regulate GAG accumulation in a rodent fibroblast line as they do in primary human skin fibroblast cultures.

The pericellular boundary layer modulates acetylcholine receptor stability in cultured myotubes

Degradation of acetylcholine receptors in cultured chicken myotubes was measured by release into the medium of radioactivity from 125I-labeled alpha-bungarotoxin. Disturbance of the pericellular boundary layer by stirring of the culture medium shortened the half-life of receptor in the membrane from 24 to 12 h. The effect could not be explained by dissociation of toxin-receptor complexes or by conditioning of the bulk phase of the medium. The rates of synthesis and degradation of total cell protein and the degradation of lactoperoxidase-iodinated surface protein were not affected by medium stirring. The loss of glucosamine-labeled material from the cells was enhanced by stirring, however, and this resulted entirely from the increased shedding of high molecular weight glycosubstances from the cells. Cells in stirred cultures contained lower levels of surface coat material stainable with colloidal thorium. These results indicate that glycosubstances of the pericellular matrix protect ACh receptors from degradation.

Culturing chick muscle cells on glycosaminoglycan substrates: attachment and differentiation

The effects of different glycosaminoglycans (GAGs) on myogenesis were tested by culturing embryonic chick myoblasts on tissue culture dishes to which either hyaluronic acid (HA) or chondroitin sulfate (ChS) was covalently bound. Both in cell number and in apparent cell type distribution, the population of cells bound to GAGs is similar to that on gelatin and significantly different from that observed with uncoated dishes. When plated on ChS, myoblasts proliferate, align, and fuse at a rate similar to cells plated on gelatin. The final fused cells appear as sheets rather than long, thin myotubes. On HA, the cells proliferate but are inhibited from differentiation. The extent of inhibition is dependent on the amount of HA present. The inhibition of myogenesis is maintained through four subcultures on HA, but can be reversed at any time by culturing cells on gelatin. These experiments indicate that different GAGs have different effects on myogenesis and that HA can actively inhibit the process.

Isolation and chemical characterization of a melanoma-associated proteoglycan antigen

Many melanoma-associated antigens have been identified by monoclonal antibodies. One of these monoclonal antibodies, O1-94-45, binds only to melanomas, nevus cells, some astrocytomas, and fetal epitheloid cells. There are approximately 100,000 cell surface antigens per melanoma cell with an association constant of 3 X 10(8) M-1. The antigen is efficiently extracted from the membrane only in the presence of detergent and is, therefore, bound by hydrophobic forces. However, it is also shed into the culture supernatant during normal cell growth. The two components of the O1-95-45 antigen are a chondroitin sulfate proteoglycan (CSP, greater than 500,000 Da) and a glycoprotein gp260 (260,000 Da, pI 6.9). CSP contains chondroitin sulfate and N-linked and O-linked oligosaccharides. Only N-linked saccharides were associated with gp260. The antigenic site is expressed on both components and is heat-sensitive. Since the CSP was converted to gp260 by chondroitinase, the protein cores of the two molecules are the same or similar. For more detailed study the O1-95-45 antigen was purified by immunoaffinity chromatography. The amino acid composition of the purified antigen was relatively polar with an unusually high Leu content and low Lys content. Initial attempts to sequence the antigen were unsuccessful probably due to a blocked N-terminus. CSP and gp260 were partially separated by gel filtration chromatography, and both were found to carry the O1-95-45 antigenic determinant. Three other monoclonal antibodies were found to bind the purified antigen at a site or sites different from the O1-95-45 epitope and one other monoclonal antibody may bind at the same site. Two of these antibodies were used for a double determinant immunoassay.

Regulation of glycosaminoglycan synthesis by thyroid hormone in vitro

Human skin fibroblasts synthesize and accumulate glycosaminoglycans (GAG). Recently, we reported that fibroblasts incubated in thyroid hormone-deficient media accumulate more GAG than do cultures incubated in the same media enriched with 0.1 muM triiodothyronine (T(3)) (1981. Endocrinology. 108: 2397). The current study characterizes that enhanced accumulation. Confluent cultures were maintained in thyroid hormone-deficient media without or with added T(3), labeled with [(3)H]acetate and analyzed for total [(3)H]GAG and [(3)H]hyaluronic acid content. Addition of T(3) to thyroid hormone-depleted media consistently inhibited the incorporation of [(3)H]acetate into GAG by 28-60% in fibroblast cultures from four different normal human donors. Maximal inhibitory effect was observed within 3 d after hormone addition at concentrations > 1 nM. 73% of the maximal inhibitory effect was observed in the presence of physiologic concentrations of T(3) (0.16 nM total T(3) or 1.4 pM free T(3)). The following observations indicated that T(3) inhibition of [(3)H]GAG accumulation is most likely due to a decrease in GAG synthesis rather than to changes in the acetate pool or GAG degradation: (a) Addition of 0, 100, 500, and 2,500 muM unlabeled acetate progressively decreased [(3)H]acetate incorporation into GAG, up to 80%, without altering the further inhibitory effect of T(3) (35-40%); (b). A similar effect of T(3) on GAG (32% inhibition) was observed using [(3)H]glucosamine as substrate; (c) T(3) decreased hyaluronate synthetase activity by 32%; and (d) There was no effect of T(3) on GAG degradation in a pulse-chase experiment. The effect of T(3) on [(3)H]GAG accumulation appears to be quite specific, since the hormone had no effect on the incorporation of [(3)H]leucine into trichloroacetic acid-precipitable material.Thus, thyroid hormone inhibits GAG accumulation in a dose-, time-dependent, and reversible manner. This inhibition is apparently due to specific effects on the rate of macromolecular synthesis.

Maturation of the extracellular material of the semilunar heart values in the mouse. A histochemical analysis of collagen and mucopolysaccharides

The developmental changes of collagen and mucopolysaccharides in the semilunar values of the mouse were studied during the embryonic, fetal and postnatal period. The valvular collagen was investigated using Van Gieson and Sirius red-polarization microscopy methods. These procedures showed that the establishment of the fibrosa layer of the cusps does not occur until the second week of the postnatal period. The nature and distribution of the valvular mucopolysaccharides were investigated by staining with Alcian blue at specific pH values and at various critical electrolyte concentrations, with the appropriate enzymatic controls using Streptomyces and testicular hyaluronidase. The results show that hyaluronate and chondroitin 4- and 6-sulphate are the major components during the embryonic and fetal period. In the older fetal stages and during the postnatal period the relative amount of hyaluronate decreases, while chondroitin sulphate increases. It is concluded from this study that the maturation of the valves occurs over a long period of the postnatal life.