Nerve terminal anchorage protein 1 (TAP-1) is a chondroitin sulfate proteoglycan: biochemical and electron microscopic characterization

The Basement Membrane Proteoglycans Perlecan and Agrin: Something Old, Something New

Several members of the proteoglycan family are integral components of basement membranes; other proteoglycan family members interact with or bind to molecular residents of the basement membrane. Proteoglycans are polyfunctional molecules, for they derive their inherent bioactivity from the amino acid motifs embedded in the core protein structure as well as the glycosaminoglycan (GAG) chains that are covalently attached to the core protein. The presence of the covalently attached GAG chains significantly expands the "partnering" potential of proteoglycans, permitting them to interact with a broad spectrum of targets, including growth factors, cytokines, chemokines, and morphogens. Thus proteoglycans in the basement membrane are poised to exert diverse effects on the cells intimately associated with basement membranes.

The case for extending storage and secretion functions of human mast cell granules to include synthesis

Ultrastructural studies using standard procedures have for years indicated close associations of ribosomes and secretory granules in human mast cells. These descriptive studies have informed new studies, using established and new ultrastructural methods based on different principles, designed to investigate the possible role of RNA metabolism in secretory granules of human mast cells. In aggregate, these studies indicate human mast cell secretory granule associations with ribosomes, the protein synthetic machine of cells, with ribosomal proteins, with RNA, with poly(A)-positive mRNA and with various long-lived, or short-lived, uridine-rich, and poly(A)-poor RNA species with key roles in RNA processing and splicing. These studies indicate that secretory-storage granules in human mast cells may also be synthetic granules.

Ribonuclease-gold labels heparin in human mast cell granules. New use for an ultrastructural enzyme affinity technique

We evaluated an enzyme affinity-gold ultrastructural technique designed to identify RNA-rich structures, based on an RNase-gold (R-G) probe in human mast cells (HMCs). As expected, the R-G technique labeled RNA-containing ribosomes and nucleoli in HMCs. The heparin-rich secretory granules in HMCs were also labeled. Extensive studies revealed that HMCs isolated from lung or skin and sustained in short-term cultures, derived de novo in growth factor-supplemented cord blood cell cultures, or present in vivo in multiple sites all shared this property. We performed a large number of controls designed to examine the HMC granule binding characteristics of gold alone, of irrelevant protein- or enzyme-gold reagents, of the role of charge and enzyme activity after various enzyme digestions, after blocking with macromolecules, after exposure to inhibitors of RNase, of heparin, or to irrelevant enzyme inhibitors, including staining of macromolecule-containing test agar blocks and a variety of combined absorption and digestion experiments of the binding of R-G to HMC granules. These studies established that the R-G method detected heparin in this site in conventionally prepared, well-preserved electron microscopic samples. These findings demonstrate a new use for this enzyme affinity-gold technique in mast cell biology, based on the known property of heparin as an inhibitor of RNase.

Interaction between Alzheimer's disease beta A4 precursor protein (APP) and the extracellular matrix: evidence for the participation of heparan sulfate proteoglycans

The interaction between the Alzheimer amyloid precursor protein (APP) and an intact extracellular matrix (ECM), matrigel, obtained from Engelbreth-Holm-Swarm tumors was evaluated. Based on quantitative analyses of the binding data obtained from solid phase binding assays, two binding sites on the ECM were identified for [125I]-APP (with apparent Kd1 of 1.0 x 10(-11) M and Kd2 of 1.6 x 10(-9) M respectively). Over 70% of [125I]-APP was displaced by heparin and N-desulfated heparin but not by chondroitin sulfate. Pretreatment of matrigel with heparitinase decreased the binding of [125I]-APP by 80%. beta-amyloid peptides (residues 1-40, 1-28, and 1-16) containing a heparin binding domain also displaced 80% of bound [125I]-APP, which was totally displaced by intact APP. The binding of [125I]-APP to matrigel increased by 210% with a decrease in the pH. These observations suggest that [125I]-APP interacts mainly with heparan sulfate proteoglycan present in the ECM. The binding of [125I]-APP to individual ECM components was also analyzed. [125I]-APP was found to bind laminin and collagen type IV but not fibronectin. However, when these ECM constituents were combined, the extent of APP-binding decreased significantly, to levels comparable to those obtained with intact matrigel, suggesting that multiple interactions may occur between ECM constituents and [125I]-APP. The results are discussed in terms of APP function and amyloidogenesis.

PG-M/versican-like proteoglycans are components of large disulfide-stabilized complexes in the axolotl embryo

Large disulfide-stabilized proteoglycan complexes were previously shown to be synthesized by the epidermis of axolotl embryos during stages crucial to subepidermal migration of neural crest cells. We now show that the complexes contain PG-M/versican-like monomers in addition to some other component with low buoyant density. Metabolically 35S-labeled proteoglycans were extracted from epidermal explants and separated by size exclusion chromatography and density equilibrium gradient centrifugation. The complexes, which elute in the void volume on Sepharose CL-2B, were recovered at buoyant density 1.42 g/ml in CsCl gradients, whereas the monomer proteoglycans, which could only be liberated from the complexes by reduction, had a higher buoyant density (1.48 g/ml). The native complexes did not aggregate with hyaluronan. The purified complexes reacted with antibodies against a portion of a cloned PG-M/versican-like axolotl proteoglycan. These antibodies were found to stain the subepidermal matrix of axolotl embryos, suggesting that the proteoglycan complexes are encountered by neural crest cells during subepidermal migration. From Western blot analysis, the core protein of the PG-M/versican-like monomers was found to be of similar size ( approximately 500 kDa) as those of PG-M/versican variants of other species. Another chondroitin sulfate proteoglycan that was present in small amounts in the epidermal extracts was found to be distinctly different from the similarly sized PG-M/versican-like monomers.

Extracellular matrix is required for skeletal muscle differentiation but not myogenin expression

Skeletal muscle cells are a useful model for studying cell differentiation. Muscle cell differentiation is marked by myoblast proliferation followed by progressive fusion to form large multinucleated myotubes that synthesize muscle-specific proteins and contract spontaneously. The molecular analysis of myogenesis has advanced with the identification of several myogenic regulatory factors, including myod1, myd, and myogenin. These factors regulate each other's expression and that of muscle-specific proteins such as the acetylcholine receptor and acetylcholinesterase (AChE). In order to investigate the role of extracellular matrix (ECM) in myogenesis we have cultured myoblasts (C2C12) in the presence or absence of an exogenous ECM (Matrigel). In addition, we have induced differentiation of myoblasts in the presence or absence of Matrigel and/or chlorate, a specific inhibitor of proteoglycan sulfation. Our results indicated that the formation of fused myotubes and expression of AChE was stimulated by Matrigel. Treatment of myoblasts induced to differentiate with chlorate resulted in an inhibition of cell fusion and AChE activity. Chlorate treatment was also found to inhibit the deposition and assembly of ECM components such fibronectin and laminin. The expression of myogenin mRNA was observed when myoblasts were induced to differentiate, but was unaffected by the presence of Matrigel or by culture of the cells in the presence of chlorate. These results suggest that the expression of myogenin is independent of the presence of ECM, but that the presence of ECM is essential for the formation of myotubes and the expression of later muscle-specific gene products.

Proteoglycans are present in the transverse tubule system of skeletal muscle

The transverse tubule system (T-tubule, T-system) of skeletal muscle is a membranous network that penetrates the interior of myofibers. The T-system is continuous with the sarcolemma and therefore provides a path for membrane excitation to reach internal myofibrils. In this study we demonstrate that T-tubules in elasmobranch fish, frog, and rat skeletal muscle contain a matrix of chondroitin sulfate proteoglycans. We used anti-T1, a mouse monoclonal antibody that recognizes a rare chondroitin sulfate epitope, for immunolocalization and biochemical studies. First, we find that T1 immunoreactivity colocalizes with a T-tubule marker, the dihydropyridine receptor alpha 2 subunit, in both frog and fish muscle. Secondly, the distribution of T1 immunoreactivity exactly matches the different distribution of T-tubules in rat and frog muscle. In rat muscle, two bands of T1 immunoreactivity are detected per sarcomere, a distribution that corresponds to the T-tubules located at the two A-I junctions of each sarcomere. In frog muscle, we detect one band of T1 immunoreactivity per sarcomere that corresponds to the one T-tubule per sarcomere located at the Z line. Lastly, we have isolated and biochemically characterized T1 antigenicity from fish skeletal muscle. Like extracellular matrix proteoglycans of cartilage, T1 antigenicity requires denaturing conditions to be solubilized. In fish muscle, two chondroitin sulfate proteoglycans bear T1: a heavily glycosylated proteoglycan with a molecular mass of about 1000 kDa, and a smaller proteoglycan that has a mobility on SDS-PAGE like a protein of molecular mass 280 kDa. We propose that proteoglycans function as structural components in the T-system. The proteoglycans may form a matrix, like the one formed by the cartilage proteoglycans they resemble, that can withstand the cytosolic osmotic pressures present in muscle cells and therefore may prevent the T-tubule from collapsing. We present a quantitative argument in support of this hypothesis.

Expression of chondroitin sulfate and keratan sulfate proteoglycans in the path of growing retinal axons in the developing chick

Previous investigations have identified proteoglycans in the central nervous system during development and have implicated some proteoglycans as axon guidance molecules that act by inhibiting axon extension. The present study investigated the pattern of immunoreactivity for several glycosaminoglycans common to certain proteoglycans relative to growing retinal axons in the developing chick visual system and in retinal explant cultures. Immunostaining for chondroitin-6-sulfate, chondroitin-4-sulfate, and keratan sulfate was observed to colocalize with retinal axons throughout the retinofugal pathway during the entire period of retinal axon growth. The proteoglycan form of collagen IX, however, was only observed in the retina, primarily peripheral to the areas with actively growing axons. The pattern of immunostaining for chondroitin sulfate in tissue sections suggested that the retinal axons might be a source for some of the chondroitin sulfate immunostaining in the developing visual pathway. This was confirmed in that chondroitin sulfate immunostaining was also observed on neurites emanating from cultured retinal explants. These findings indicate that retinal axons grow in the presence of chondroitin sulfate and keratan sulfate proteoglycans and that these proteoglycans in the developing chick visual pathway have functions other than to inhibit axon growth.

Proteoglycans and the acute-phase response in Alzheimer's disease brain

Alzheimer's disease is a dementing disorder affecting increasingly large numbers of individuals in the aging population. The characteristic neuropathologic changes of Alzheimer's disease are the deposition of extracellular amyloid plaques, neurons containing neurofibrillary tangles, and neuronal cell loss. The A4 amyloid peptide is the major constituent of senile plaques. In addition to the A4 peptide, senile plaques contain a variety of molecular species, including proteoglycans and inflammatory components. The presence of proteoglycans in the amyloid deposits of Alzheimer's disease and of systemic amyloidoses suggests that these molecules play an active role in the pathogenesis of amyloidosis. However, the molecular mechanisms that lead to the codeposition of amyloid peptide with proteoglycans is still unknown. Recent evidence suggests that the metabolism of proteoglycans is altered in Alzheimer's disease patients. The acute-phase response observed in the brain of patients affected by Alzheimer's disease may be responsible for this effect. In this article, we discuss the role of proteoglycans in Alzheimer's disease, and the possible interactions between factors involved in brain inflammatory mechanisms and proteoglycans in the pathogenesis of Alzheimer's disease.

Retrograde signaling in the formation and maintenance of the neuromuscular junction

The neuromuscular junction is characterized by precise alignment between the nerve terminal and the postsynaptic apparatus formed by the muscle fiber. Organization of the neuromuscular junction during embryonic development, growth, and maintenance is coordinated by signals exchanged between motor neurons and their target muscle fibers. Identification of proteins such as agrin, likely to represent neuronal agents that direct the organization of the postsynaptic apparatus, has focused attention on characterization of proteins that mediate retrograde signals that regulate the organization and function of the nerve terminal. The results of these studies implicate a role for both adhesive and diffusible signals in coordinating the development, maturation, and maintenance of the motor nerve terminal. The diversity of molecules identified to date that appear to play a role in these processes implies a considerable level of redundancy in the transduction pathway. However, studies of early nerve-muscle interactions suggest that a common feature of many of these retrograde agents is activation of a protein kinase coupled with an increase in cytosolic Ca2+ concentration. While the molecular signals that regulate growth and maintenance of neuromuscular junctions are less well understood it seems likely that similar adhesive and diffusible factors will be involved.

Application of cationic probes for the ultrastructural localization of proteoglycans in basement membranes

The application of cationic probes for the ultrastructural detection of proteoglycans in basement membranes is reviewed. Proteoglycans are highly negatively charged macromolecules due to their glycosaminoglycan side chains. The interaction of cationic probes with proteoglycans is of an electrostatic nature. Methods are discussed to increase the specificity of probes for proteoglycans. The use of phthalocyanin-like dyes such as Cuprolinic blue, according to the critical electrolyte concentration method, results in a selective staining of proteoglycans. Enzymatic or chemical digestions, however, should be done to validate the proteoglycan nature of the dye-positive granules/filaments, and to establish the class of proteoglycan. The value of cationic probes in basement membrane research on development and pathology is discussed. The potential for deducting molecular information from the ultrastructural appearance of stained proteoglycans is indicated.

A protective action of chondroitin sulfate proteoglycans against neuronal cell death induced by glutamate

The role of chondroitin sulfate proteoglycans (CSPGs) on excitotoxic cell death and long-term survival of neurons were investigated in primary cultured neurons of the rat cortex. Soluble CSPGs were prepared from 10-day-old and adult rat brains by the ion-exchange chromatography on DEAE-Sephacel. CSPGs were added to the culture medium on culture day 4, and glutamate neurotoxicity was examined on culture day 7 by both microscopic cell count and measurement of lactate dehydrogenase activity in culture media. The effect on long-term survival was evaluated by counting viable neurons until culture day 28. CSPGs and core proteins, but not glycosaminoglycan chains (GAGs), protected cultured neurons from excitotoxic cell death induced by 24 h exposure to 1 mM glutamate, but CSPGs did not promote the long-term survival of neurons. The neuroprotective effect of CSPGs and core proteins was dose-dependent with ED50 about 10 microM hexuronate and 2 micrograms/ml protein respectively. This effect was not considered to be due to adsorption of glutamate by CSPGs because [3H]glutamate was not adsorbed by CSPGs added to the culture medium. Based on these findings, we suggested that CSPGs may exert their neuroprotective action through molecular interactions with the binding sites on neuronal membrane, neurotrophic factors, or other extracellular matrix molecules and may be involved in the pathogenesis of neuronal cell death in acute pathological conditions and chronic degenerative diseases of the brain.

Claustrin, an antiadhesive neural keratan sulfate proteoglycan, is structurally related to MAP1B

Our laboratory has recently identified a keratan sulfate proteoglycan (KSPG), named claustrin, that inhibits neural cell adhesion and neurite outgrowth in the chick nervous system. Antisera prepared against claustrin were used to screen a cDNA expression library from embryonic day 9 chick brain. Initial characterization of positive cDNAs revealed a high degree of homology to the mouse MAP1B gene, although these cDNAs represent a 5' truncated fragment of MAP1B. Protein sequencing of three peptides derived from a tryptic digest of purified, keratanase-treated claustrin also revealed strong homology to MAP1B, and confirmed the authenticity of the 3.4 kb claustrin cDNA. To further determine the relationship between these two proteins, we used antibodies against MAP1B and KSPGs in immunoblotting and immunohistochemical studies. These studies demonstrated cross-reactivity between MAP1B and claustrin antibodies, and that monoclonal antibodies to cartilage keratan sulfate react with MAP1B in rat nervous tissue, and with claustrin in the chick nervous system. In addition, keratanase treatment of a taxol microtubule fraction from chick or rat brain eliminated MAP1B, as detected by immunoblotting with the MAP5 monoclonal antibody. These results suggest that MAP1B and claustrin are highly related, if not identical, proteins.

Isolation and purification of proteoglycans

Purification of a protein typically involves development of a quantitative assay to track protein integrity (e.g. enzyme activity) during subsequent isolation steps. The generalized procedure involves choosing the source of the protein, defining extraction conditions, developing bulk purification methods followed by refined, more selective methods. The purification of proteoglycans is often complicated by a) limited source quantities, b) necessity of chaotrophic solvents for efficient extraction, c) their large molecular size and d) lack of defined functions to enable purity (i.e. activity, conformation) to be assessed. Because the usual goal of proteoglycan purification is physical characterization (intact molecular weight, core protein and glycosaminoglycan class and size), the problems of a suitable assay and/or native conformation are avoided. The 'assay' for tracking proteoglycan isolation typically utilizes uronic acid content or radiolabel incorporation as a marker. Once extracted from their cellular/extracellular environment, proteoglycans can be isolated by density gradient centrifugation and/or column chromatography techniques. Recent advances in the composition of chromatographic supports have enabled the application of ion-exchange, gel permeation, hydrophobic interaction and affinity chromatography resins using efficient high-pressure liquid chromatography to proteoglycan purification.

A lipid-anchored heparan sulfate proteoglycan is present in the surface of differentiated skeletal muscle cells. Isolation and biochemical characterization

We have investigated the presence of hydrophobic membrane-associated heparan sulfate proteoglycans (HSPG) on the cell surface of differentiated skeletal muscle cells. A HSPG releasable by incubation with a phosphatidylinositol-specific phospholipase c (PtdIns-PLC) was obtained. HSPG were also isolated from Triton X-100 extracts of the cells. The hydrodynamic characteristics of the PtdIns-PLC-releasable and detergent-extracted HSPG were indistinguishable. SDS/PAGE analysis of the PtdIns-PLC-releasable HSPG indicated a molecular mass of 250 kDa. Analysis of proteins immunoprecipitated by specific antibodies against a HSPG isolated from Schwann cells demonstrated that the antisera precipitated an intact HSPG that was present in the pool of proteins released by PtdIns-PLC and by Triton X-100 from [35S]sulfate labeled cells. Nitrous acid degradation of the immunoprecipitated proteins released by PtdIns-PLC from [35S]methionine labeled cells produced a single 67-kDa core protein. Analysis of hydrophobicity of the purified HSPG revealed that only the HSPG obtained from the detergent extract were able to be incorporated into the liposomes whereas the PtdIns-PLC-released HSPG was not. Immunocytolocalization analysis of the differentiated cells indicated that the PtdIns-PLC-releasable HSPG was located on the cell surface. Prior incubation of the cells with PtdIns-PLC significantly reduced the surface staining. Analysis of skeletal-muscle sections of adult rat skeletal muscles indicated that this HSPG localized exclusively at the endomysium. This localization suggest that these HSPG may be acting as a cell receptor for extracellular-matrix (ECM) components.

The SV2 protein of synaptic vesicles is a keratan sulfate proteoglycan

We have determined that synaptic vesicles contain a vesicle-specific keratan sulfate integral membrane proteoglycan. This is a major proteoglycan in electric organ synaptic vesicles. It exists in two forms on sodium dodecyl sulfate-polyacrylamide gel electrophoresis, i.e., the L form, which migrates like a protein with an M(r) of 100,000, and the H form, with a lower mobility that migrates with an M(r) of approximately 250,000. Both forms contain SV2, an epitope located on the cytoplasmic side of the vesicle membrane. In addition to electric organ, we have analyzed the SV2 proteoglycan in vesicle fractions from two other sources, electric fish brain and rat brain. Both the H and L forms of SV2 are present in these vesicles and all are keratan sulfate proteoglycans. Unlike previously studied synaptic vesicle proteins, this proteoglycan contains a marker specific for a single group of neurons. This marker is an antigenically unique keratan sulfate side chain that is specific for the cells innervating the electric organ; it is not found on the synaptic vesicle keratan sulfate proteoglycan in other neurons of the electric fish brain.

Function of proteoglycans in the extracellular matrix

Proteoglycans are glycosylated proteins which have covalently attached highly anionic glycosaminoglycans. Many forms of proteoglycans are present in virtually all extracellular matrices of connective tissues. The major biological function of proteoglycans derives from the physicochemical characteristics of the glycosaminoglycan component of the molecule, which provides hydration and swelling pressure to the tissue enabling it to withstand compressional forces. This function is best illustrated by the most abundant proteoglycan in cartilage tissues, aggrecan. During the past decade, diverse species of proteoglycans have been identified in many connective tissues, on cell surfaces and in intracellular compartments. These proteoglycans have distinct biological functions apart from their hydrodynamic functions, and their involvement in many aspects of cell and tissue activities has been demonstrated. For example, decorin, which is widely distributed in many connective tissues, may have functions in regulating collagen fibril formation and in modifying the activity of transforming growth factor-beta; perlecan, the major heparan sulfate proteoglycan in the glomerular basement membrane, may play an important role as the major anionic site responsible for the charge selectivity in glomerular filtration. Specific interactions between proteoglycans (through both their glycosaminoglycan and core protein components) and macromolecules in the extracellular matrix are the key factors in the functions of proteoglycans. Exciting biological functions of proteoglycans are now gradually emerging.

Isolation and purification of proteoglycans

Purification of a protein typically involves development of a quantitative assay to track protein integrity (e.g. enzyme activity) during subsequent isolation steps. The generalized procedure involves choosing the source of the protein, defining extraction conditions, developing bulk purification methods followed by refined, more selective methods. The purification of proteoglycans is often complicated by a) limited source quantities, b) necessity of chaotropic solvents for efficient extraction, c) their large molecular size and d) lack of defined functions to enable purity (i.e. activity, conformation) to be assessed. Because the usual goal of proteoglycan purification is physical characterization (intact molecular weight, core protein and glycosaminoglycan class and size), the problems of a suitable assay and/or native conformation are avoided. The 'assay' for tracking proteoglycan isolation typically utilizes uronic acid content or radiolabel incorporation as a marker. Once extracted from their cellular/extracellular environment, proteoglycans can be isolated by density gradient centrifugation and/or column chromatography techniques. Recent advances in the composition of chromatographic supports have enabled the application of ion-exchange, gel permeation, hydrophobic interaction and affinity chromatography resins using efficient high-pressure liquid chromatography to proteoglycan purification.

Decorin is specifically solubilized by heparin from the extracellular matrix of rat skeletal muscles

We have previously communicated that heparin co-solubilizes the asymmetric form of acetylcholinesterase (AChE) and a dermatan sulfate proteoglycan from the extracellular matrix (ECM) of rat skeletal muscles. In this report we unequivocally demonstrate by biochemical and immunological analyses that the proteoglycan that is solubilized by heparin from rat skeletal muscle ECM corresponds to decorin. These results support the concept for the role of decorin in the ECM organization.

Tyrosine phosphorylation and acetylcholine receptor cluster formation in cultured Xenopus muscle cells

Aggregation of the nicotinic acetylcholine receptor (AChR) at sites of nerve-muscle contact is one of the earliest events to occur during the development of the neuromuscular junction. The stimulus presented to the muscle by nerve and the mechanisms underlying postsynaptic differentiation are not known. The purpose of this study was to examine the distribution of phosphotyrosine (PY)-containing proteins in cultured Xenopus muscle cells in response to AChR clustering stimuli. Results demonstrated a distinct accumulation of PY at AChR clusters induced by several stimuli, including nerve, the culture substratum, and polystyrene microbeads. AChR microclusters formed by external cross-linking did not show PY colocalization, implying that the accumulation of PY in response to clustering stimuli was not due to the aggregation of basally phosphorylated AChRs. A semi-quantitative determination of the time course for development of PY labeling at bead contacts revealed early PY accumulation within 15 min of contact before significant AChR aggregation. At later stages (within 15 h), the AChR signal came to approximate the PY signal. We have reported the inhibition of bead-induced AChR clustering in response to beads by a tyrphostin tyrosine kinase inhibitor (RG50864) (Peng, H. B., L. P. Baker, and Q. Chen. 1991. Neuron. 6:237-246). RG50864 also inhibited PY accumulation at bead contacts, providing evidence for tyrosine kinase activation in response to the bead stimulus. These results suggest that tyrosine phosphorylation may play an important role in the generative stages of cluster formation, and may involve protein(s) other than or in addition to AChRs.

Decorin, a chondroitin/dermatan sulfate proteoglycan is under neural control in rat skeletal muscle

Proteoglycans (PGs) are abundant components of the extracellular matrices (ECM) of skeletal muscle. We have previously found that the synthesis of skeletal muscle PGs present at the ECM increase after denervation. The experiments reported here were undertaken to identify which PG(s) increase after denervation of rat leg muscles. Incorporation of radioactive sulfate demonstrated the presence of a chondroitin/dermatan sulfate PG of 70-90 kDa in the skeletal muscle ECM, which increased after denervation. The PG has a core protein of 39-45 kDa after treatment with chondroitinase ABC. Antibodies against rat decorin, a chondroitin/dermatan sulfate PG synthesized by various cell types, specifically immunoprecipitated this PG from a mixture of PGs. Immunocytolocalization of this PG indicated that the chondroitin/dermatan sulfate PG accumulates at the perimysium of skeletal muscle after denervation. Finally, Northern blot analysis indicated an increase of muscle transcripts for decorin after denervation. The data reported here suggest that a chondroitin/dermatan sulfate PG present at the skeletal muscle ECM, very similar if not identical to decorin, increases after denervation.

Isolation of proteoglycans synthesized by rat heart: evidence for the presence of several distinct forms

1. The proteoglycans (Ps) synthesized by auricle and ventricle from adult rat heart were studied. 2. Auricle tissue incorporated over two times radioactive sulfate compared to ventricle tissue and the Ps were mainly found in the detergent insoluble fraction. 3. The Ps from both tissues were isolated by ion-exchange chromatography on DEAE-Sephacel, followed by gel filtration on Sepharose CL-6B and SDS-PAGE electrophoresis. 4. Enzymatic and chemical degradation of these Ps suggest that at least three and probably four different species of Ps can be observed in heart tissue. 4. A high molecular weight chondroitin sulfate-P, a high molecular weight heparan sulfate-P, a chondroitin/dermatan sulfate-P of 240-200 kDa and a dermatan sulfate of 115 kDa. 5. This latter P was specifically immunoprecipitated using rat decorin antiserum.

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.

Heparin modulates the composition of the extracellular matrix domain surrounding arterial smooth muscle cells

Heparin and related molecules influence vascular wall structure by their ability to inhibit smooth muscle cell (smc) proliferation and migration. However, little is known as to whether heparin has an effect on the extracellular matrix. In the present study, the effect of heparin on the content and regional distribution of elastin, collagen, and proteoglycans (PGs) in blood vessels following experimental injury was determined. Two groups of rats were subjected to left common carotid balloon injury and were infused with either 0.9% saline or heparin in a saline solution, for 2 weeks. Using a new morphometric method of analysis, the authors determined changes in volumes of elastin, collagen, and PGs contained within an 'extracellular matrix domain (ECM domain),' the average envelope of connective tissue surrounding each smc. Heparin treatment inhibited intimal thickening and decreased the elastin content in the ECM domain in the upper and lower arterial intima. Collagen also was found to be significantly decreased 5.0-fold and 7.6-fold in the ECM domains of upper and lower intima, respectively, of heparin-treated animals. The decrease in both elastin and collagen was balanced by a significant increase in amorphous and filamentous electron-dense material. Heparin also caused a significant 1.8-fold and 1.9-fold increase in the PG content in the ECM domain in the upper and lower intima, respectively. Immunohistochemical analysis, using antibodies to elastin and PG subclasses, supported the morphometric observations. This study has shown that heparin administered in vivo can alter the accumulation and distribution of each of the major vascular ECM components in a specific and differential manner.

A diverse set of developmentally regulated proteoglycans is expressed in the rat central nervous system

Cellular interactions in neural development are influenced by various extracellular proteins, many of which bind glycosaminoglycans or proteoglycans. Precise functions of nervous system proteoglycans remain unknown, in part because neural proteoglycan composition is poorly understood. In this study, 25 putative proteoglycan core proteins were identified in subcellular fractions of rat brain. Levels of many of these varied considerably during development. Membrane-associated proteoglycans included two heparan sulfate proteoglycans (cores of 50 and 59 kd) that are covalently linked to glycosyl-phosphatidylinositol lipid, as well as several that appear to aggregate either with themselves or with copurifying proteins. These data indicate that brain proteoglycans exhibit the abundance, structural diversity, and developmental regulation that would be anticipated for molecules with diverse developmental functions.

Non-specific interaction of proteoglycans with surfaces and matrices

Evidence is presented that reversible non-specific adsorption of proteoglycans (PGs) to surfaces and matrices is an inherent property of the PGs. This adsorption is dependent on the intact PG structure as the glycosaminoglycans (GAGs), which are isolated after papain digestion of the PG show no such non-specific adsorption. The interaction of the PG with surfaces and matrices is also highly dependent on the internal milieu and can be both inhibited and enhanced by such factors as the ionic composition and concentration, pH, detergents and chaotropic reagents such as guanidine hydrochloride (Gdn-HC1). It is suggested that this inherent stickiness of the PGs allows them to function like a reversible fluid adhesant in the connective tissues. This weak binding force thus not only aids in maintaining the integrity of the connective tissues, but its reversible nature may provide for easy movement of other materials through the connective tissue matrix.

Biochemical differentiation of nascent neurite junctions: unilateral localization of adheron components

Previous work showed that adhesive contacts made by growth cone filopodia involve extracellular filaments that can be labeled with an antiserum to adherons (adhesion-promoting complexes in conditioned medium). Here, adheron antigens were found to be differentially expressed by particular cell types and synaptic layers during chick retina development, and this differential expression at the cellular level was retained in culture. When applied to living cells, adheron antibodies characteristically patched at filopodial junctions. Monospecific antibodies to purpurin and a heparan sulphate proteoglycan (HSPG), two components of adherons, labeled a subpopulation of junctions. Most interestingly, anti-purpurin and anti-HSPG bound only to one end of adhesive filaments. Such localization to discrete microdomains provides support for a heterotypic adhesive mechanism in junction formation.

Proteoglycans in the pathogenesis of Alzheimer's disease and other amyloidoses

Proteoglycans and the amyloid P component are two constituents of amyloid that appear to be present regardless of the type of amyloid protein deposited, the extent of amyloid deposition and the tissue or organ involved. This article reviews the literature concerning proteoglycans and/or glycosaminoglycans in amyloidosis and describes recent studies which demonstrate their localization to the characteristic lesions of Alzheimer's disease and the amyloid plaques containing PrP protein in the prion diseases. Additionally, the possible interaction of proteoglycans with various amyloidogenic proteins, including the beta-amyloid protein in Alzheimer's disease is discussed. It is postulated that proteoglycans localized to a number of different amyloids play a common role in the pathogenesis of amyloidosis. Some of these hypothesized roles include 1) inducing amyloidogenic precursor proteins to form amyloid fibrils containing a predominant beta-pleated sheet structure, 2) influencing amyloid deposition to occur at specific anatomical sites within tissues and/or 3) aiding in prevention of amyloid degradation once amyloid has formed.

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.

Membrane-associated chondroitin sulfate proteoglycans of human lung fibroblasts

Cultured human fetal lung fibroblasts produce some chondroitin sulfate proteoglycans that are extracted as an aggregate in chaotropic buffers containing 4 M guanidinium chloride. The aggregated proteoglycans are excluded from Sepharose CL4B and 2B, but become included, eluting with a Kav value of 0.53 from Sepharose CL4B, when Triton X-100 is included in the buffer. Conversely, some of the detergent-extractable chondroitin sulfate proteoglycans can be incorporated into liposomes, suggesting the existence of a hydrophobic membrane-intercalated chondroitin sulfate proteoglycan fraction. Purified preparations of hydrophobic chondroitin sulfate proteoglycans contain two major core protein forms of 90 and 52 kD. A monoclonal antibody (F58-7D8) obtained from the fusion of myeloma cells with spleen cells of BALB/c mice that were immunized with hydrophobic proteoglycans recognized the 90- but not the 52-kD core protein. The epitope that is recognized by the antibody is exposed at the surface of cultured human lung fibroblasts and at the surface of several stromal cells in vivo, but also at the surface of Kupffer cells and of epidermal cells. The core proteins of these small membrane-associated chondroitin sulfate proteoglycans are probably distinct from those previously identified in human fibroblasts by biochemical, immunological, and molecular biological approaches.