Nancy B. Schwartz

Sulfation of chondroitin sulfate proteoglycans is necessary for proper Indian hedgehog signaling in the developing growth plate

In contrast to the functional role of heparan sulfate proteoglycans (HSPGs), the importance of chondroitin sulfate proteoglycans (CSPGs) in modulating signaling pathways involving hedgehog proteins, wingless-related proteins and fibroblast growth factors remains unclear. To elucidate the importance of sulfated CSPGs in signaling paradigms required for endochondral bone formation, the brachymorphic (bm) mouse was used as a model for undersulfated CSPGs. The bm mouse exhibits a postnatal chondrodysplasia caused by a mutation in the phosphoadenosine phosphosulfate (PAPS) synthetase (Papss2) gene, leading to reduced levels of PAPS and undersulfated proteoglycans. Biochemical analysis of the glycosaminoglycan (GAG) content in bm cartilage via sulfate labeling and fluorophore-assisted carbohydrate electrophoresis revealed preferential undersulfation of chondroitin chains (CS) and normal sulfation of heparan sulfate chains. In situ hybridization and immunohistochemical analysis of bm limb growth plates showed diminished Indian hedgehog (Ihh) signaling and abnormal Ihh protein distribution in the extracellular matrix. Consistent with the decrease in hedgehog signaling, BrdU incorporation exhibited a significant reduction in chondrocyte proliferation. Direct measurements of Ihh binding to defined GAG chains demonstrated that Ihh interacts with CS, particularly chondroitin-4-sulfate. Furthermore, co-immunoprecipitation experiments showed that Ihh binds to the major cartilage CSPG aggrecan via its CS chains. Overall, this study demonstrates an important function for CSPGs in modulating Ihh signaling in the developing growth plate, and highlights the importance of carbohydrate sulfation in regulating growth factor signaling.

Defect in 3'-phosphoadenosine 5'-phosphosulfate synthesis in brachymorphic mice. I. Characterization of the defect.

Biosynthesis of the undersulfated proteoglycan found in brachymorphic mouse (bm/ bm) cartilage has been investigated. Similar amounts of cartilage proteoglycan core protein, as measured by radioimmune inhibition assay, and comparable activity levels of four of the glycosyltransferases requisite for synthesis of chondroitin sulfate chains were found in cartilage homogenates from neonatal bm/bm and normal mice, suggesting normal production of glycosylated core protein acceptor for sulfation. When incubated with 35S-labeled 3′-phosphoadenosine 5′-phosphosulfate (PAPS), bm/bm cartilage extracts showed a higher than control level of sulfotransferase activity. In contrast, when synthesis was initiated from ATP and 35SO42−, mutant cartilage extracts showed lower incorporation of 35SO42− into endogenous chondroitin sulfate proteoglycan (19% of control level) and greatly reduced formation of PAPS (10% of control level). Results from coincubations of normal and mutant cartilage extracts exhibited intermediate levels of sulfate incorporation into PAPS and endogenous acceptors, suggesting the absence of an inhibitor for sulfate-activating enzymes or sulfotransferases. Degradation rates of 35S]PAPS and of 35S-labeled adenosine 5′-phosphosulfate (APS) were comparable in bm/bm and normal cartilage extracts. Specific assays for both ATP sulfurylase (sulfate adenylyltransferase; ATP:sulfate adenylyltransferase, EC 2.7.7.4) and APS kinase (adenylylsulfate kinase; ATP:adenylylsulfate 3′-phosphotransferase, EC 2.7.1.25) showed decreases in the former (50% of control) and the latter (10–15% of control) enzyme activities in bm/bm cartilage extracts. Both enzyme activities were reduced to intermediate levels in extracts of cartilage from heterozygous brachymorphic mice (ATP-sulfurylase, 80% of control; APS kinase, 40–70% of control). Furthermore, the moderate reduction in ATP sulfurylase activity in bm/bm cartilage extracts was accompanied by increased lability to freezing and thawing of the residual activity of this enzyme. These results indicate that under-sulfation of chondroitin sulfate proteoglycan in bm/bm cartilage is due to a defect in synthesis of the sulfate donor (PAPS), resulting from diminished activities of both ATP sulfurylase and APS kinase, although the reduced activity of the latter enzyme seems to be primarily responsible for the defect in PAPS synthesis.

Proteoglycans in brain development

Proteoglycans, as part of the extracellular or cell-surface milieu of most tissues and organ systems, play important roles in morphogenesis by modulating cell-matrix or cell-cell interactions, cell adhesiveness, or by binding and presenting growth and differentiation factors. Chondroitin sulfate proteoglycans which constitute the major population of proteoglycans in the central nervous system may influence formation of neuronal nuclei, establishment of boundaries for axonal growth and act as modulators of neuronal outgrowth during brain development, as well as during regeneration after injury. There is a paucity of information on the role of chondroitin sulfate proteoglycans in central nervous system organogenesis. In the chick embryo, aggrecan has a regionally specific and developmentally regulated expression profile during brain development. By Northern and Western blot analysis, aggrecan expression is first detected in chick brain on embryonic day 7 (E7), increases from E7 to E13, declines markedly after E16, and is not evident in hatchling brains. The time course and pattern of aggrecan expression observed in ventricular zone cells suggested that it might play a role in gliogenesis. We have analyzed the role of aggrecan during brain development using a aggrecan-deficient model, nanomelia. In nanomelic chicks, expression and levels of neurocan and brevican is not affected, indicating a non-redundant role for these members of the aggrecan gene family. Our analysis of the aggrecan-deficient model found a severely altered phenotype which affects cell behavior in a neuronal culture paradigm and expression of astrocytic markers in vivo. Taken together our results suggest a function for aggrecan in the specification of a sub-set of glia precursors that might give rise to astrocytes in vivo. Published in 2004.

Stimulation of Chondroitin Sulfate Proteoglycan Production by Chondrocytes in Monolayer

Chondrocytes in monolayer undergo morphological and biochemical changes which culminate in the establishment of cartilage nodules in vitro. Chondroitin sulfate or heparin, added to the culture media of these cells, stimulates the production of chondroitin sulfate proteoglycan over the entire period of culture with a maximum effect during the log phase of growth. In addition, a lag of 2-3 hours is required before an increase in sulfate incorporation into polysaccharide is observed. The responsiveness of chondrocytes is influenced by several factors, such as cell density, conditioned media and enzyme treatment. Furthermore, puromycin abolishes the endogenous as well as the stimulated synthesis, demonstrating the necessity for core protein synthesis in both synthetic processes. Addition of beta-D-xylosides (which presumably act as initiators of chondroitin sulfate polysaccharide synthesis) and chondroitin sulfate, concurrently, stimulate sulfate incorporation to levels higher than either agent alone, indicating that these compounds act by different mechanisms.

Domain Organization, Genomic Structure, Evolution, and Regulation of Expression of the Aggrecan Gene Family

Proteoglycans are complex macromolecules, consisting of a polypeptide backbone to which are covalently attached one or more glycosaminoglycan chains. Molecular cloning has allowed identification of the genes encoding the core proteins of various proteoglycans, leading to a better understanding of the diversity of proteoglycan structure and function, as well as to the evolution of a classification of proteoglycans on the basis of emerging gene families that encode the different core proteins. One such family includes several proteoglycans that have been grouped with aggrecan, the large aggregating chondroitin sulfate proteoglycan of cartilage, based on a high number of sequence similarities within the N- and C-terminal domains. Thus far these proteoglycans include versican, neurocan, and brevican. It is now apparent that these proteins, as a group, are truly a gene family with shared structural motifs on the protein and nucleotide (mRNA) levels, and with nearly identical genomic organizations. Clearly a common ancestral origin is indicated for the members of the aggrecan family of proteoglycans. However, differing patterns of amplification and divergence have also occurred within certain exons across species and family members, leading to the class-characteristic protein motifs in the central carbohydrate-rich region exclusively. Thus the overall domain organization strongly suggests that sequence conservation in the terminal globular domains underlies common functions, whereas differences in the central portions of the genes account for functional specialization among the members of this gene family.

Biosynthesis and regulation of expression of proteoglycans.

Proteoglycans are a family of complex macromolecules characterized by the presence of one or more glycosaminoglycan chains covalently linked to a polypeptide backbone. Although originally named and categorized on the basis of the glycosaminoglycan substituent, increasingly they are being identified as members of gene families that encode their different core proteins. Proteoglycans are found predominantly in the extracellular matrix (ECM) or associated with the cell surface of most eucaryotic cells where they bind to other matrix- and cell-associated components. Their ability to be so interactive stems in large part from their structural diversity, which arises from variations in polysaccharide type, size and composition as well as core protein primary sequence, domain arrangement, degree of substitution and distribution of polysaccharide chains. Considering the complexity of proteoglycan molecules, often having modular core protein domains and posttranslational modifications that vary with developmental setting, the various steps of synthesis and processing are most likely highly regulated. Furthermore, regulation of proteoglycan expression is even more complex as they frequently are expressed transiently by multiple cell types and in different developmental time frames. Elucidation of cell- and developmental-specific control elements which regulate the expression of these complex macromolecular families are only beginning.

Defective PAPS-synthesis in epiphyseal cartilage from brachymorphic mice

Abstract Activity levels of sulfotransferases, requisite for the sulfation of chondroitin sulfate proteoglycan, were measured in cell-free homogenates prepared from neonatal epiphyseal cartilage of normal C57B1/6J or homozygous brachymorphic mice. In the presence of [35S]-PAPS only or [35S]-PAPS plus an exogenous sulfate acceptor, comparable amounts of 35 SO 4 2− were incorporated into chondroitin sulfate by the normal and mutant types of cartilage. In contrast, the mutant cartilage catalyzed the conversion of only 30% of the 35 SO 4 2− into chondroitin sulfate as compared to normal mouse cartilage when synthesis was initiated from ATP and H235SO4. These results suggest that the production of an undersulfated proteoglycan which has previously been reported in brachymorphic mice (Orkin, R.W. et al . (1976) Devel. Biol. 50 , 82–94) may result from a defect in the synthesis of the sulfate donor PAPS.

Defect in 3′-phosphoadenosine 5′-phosphosulfate synthesis in brachymorphic mice: II. Tissue distribution of the defect☆

Abstract The tissue distribution of the defective PAPS synthetic pathway in homozygous brachymorphic mice ( bm bm ) has been investigated using four different criteria: (i) incorporation of 35SO42− into adenosine 5′-phosphosulfate (APS), 3′-phosphoadenosine 5′-phosphosulfate (PAPS), and endogenous macromolecular acceptors, (ii) APS kinase (adenylylsulfate kinase; ATP:adenylylsulfate 3′-phosphotransferase, EC 2.7.1.25) activity, (iii) ATP sulfurylase (sulfate adenylyltransferase; ATP:sulfate adenylyltransferase, EC 2.7.7.4) activity, (iv) thermostability of ATP sulfurylase. With respect to the first three criteria, the results indicate that liver is affected as profoundly as cartilage ( K. Sugahara and N. B. Schwartz, Arch. Biochem. Biophys. (1982) 214, 589–601 ). In contrast, skin and brain show no differences between normal and mutant. Kidney is significantly, but only moderately, affected. The results from thermostability studies demonstrate that ATP sulfurylase activity is more labile in bm bm cartilage, liver, and kidney, but not in skin or brain, supporting the above-observed distribution of the defect. Therefore, the present results indicate a multiple, but not universal, tissue distribution of the defective PAPS synthetic pathway in bm bm mice. Furthermore, these findings support the suggestion that ATP sulfurylase as well as APS kinase is defective in brachymorphic mice.

An improved method of sequential alcian blue and ammoniacal silver staining of chondroitin sulfate proteoglycan in polyacrylamide gels

Partially deglycosylated chondroitin sulfate proteoglycan (CSPG) or peptide fragments obtained from CSPG are not readily detectable in gels by staining with Alcian blue 8GX or ammoniacal silver using the technique of Oakley et al. (B. Oakley, D. Kirsh, and N. Morris (1980) Anal. Biochem. 105, 361). Sequencial staining with both reagents allows visualization of intact CSPG or peptides derived from proteoglycans in polyacrylamide gels at protein concentrations as low as 2 ng/mm2, or glucuronic acid and galactosamine concentrations of 1 ng/mm2 or less. This method is significantly more sensitive and has broader applicability than that described by H. Min and M. Cowman (1986) Anal. Biochem. 155, 275) for staining glycosaminoglycan fragments in polyacrylamide gels.

Location of xylosyltransferase in the cisternae of the rough endoplasmic reticulum of embryonic cartilage cells.

Purified antibodies were prepared to UDP-D-xylose: core protein xylosyltransferase, the enzyme which initiates the formation of chondroitin sulfate chains in the course of proteoglycan biosynthesis in cartilage. The purified antibodies were conjugated to ferritin with a two-step glutaraldehyde procedure, and conjugates were then used to locate xylosyltransferase in fragments of embryonic cartilage cells. The results indicated that the enzyme is located within the cisternae of the rough endoplasmic reticulum. The distribution of the enzyme was similar to that of prolyl hydroxylase in the same cell fragments, suggesting that procollagen synthesis and initiation of chondroitin sulfate chains occur in the same regions of the rough endoplasmic reticulum.