Conserved charge of glomerular and mesangial cell proteoglycans: possible role of amino acid-derived sulphate

Preparation and structural determination of dermatan sulfate-derived oligosaccharides

Eight oligosaccharides were prepared from dermatan sulfate (DS) and their structures were elucidated. Porcine intestinal mucosal DS was subjected to controlled depolymerization using chondroitin ABC lyase (chondroitinase ABC). The oligosaccharide mixture formed was fractionated by low-pressure gel permeation chromatography (GPC). Size uniform mixtures of disaccharides, tetrasaccharides, hexasaccharides, octasaccharides, decasaccharides, and dodecasaccharides were obtained. Each size-fractionated mixture was then purified on the basis of charge by repetitive semi-preparative strong-anion-exchange (SAX) high-performance liquid chromatography (HPLC). This approach has led to the isolation of six homogeneous oligosaccharides. The size of the oligosaccharides were determined using GPC-HPLC. Treatment of tetrasaccharide and hexasaccharide fragments with Hg(OAc)2 afforded trisaccharide and pentasaccharide products, respectively. The purity of the oligosaccharides obtained was confirmed by analytical SAX-HPLC, and capillary electrophoresis (CE). The molecular mass and degree of sulfation of the eight purified oligosaccharides were elucidated using electrospray ionization (ESI) mass spectrometry and their structures were established with high field nuclear magnetic resonance (NMR) spectroscopy. These DS-oligosaccharides are currently being used to study for interaction of the DS with biologically important proteins.

The clinical chemistry of inorganic sulfate

Although inorganic sulfate is an essential and ubiquitous anion in human biology, it is infrequently assayed in clinical chemistry today. Serum sulfate is difficult to measure accurately without resorting to physicochemical methods, such as ion chromatography, although many other techniques have been described. It is strongly influenced by a variety of physiological factors, including age, diet, pregnancy, and drug ingestion. Urinary excretion is the principal mechanism of disposal for the excess sulfate produced by sulfur amino acid oxidation, and the kidney is the primary site of regulation. In renal failure, sulfoesters accumulate and hypersulfatemia contributes directly to the unmeasured anion gap characteristic of the condition. In contrast, sulfate in urine is readily assayed by a number of means, particularly nephelometry after precipitation as a barium salt. Sulfate is most commonly assayed today as part of the clinical workup for nephrolithiasis, because sulfate is a major contributor to the ionic strength of urine and alters the equilibrium constants governing saturation and precipitation of calcium salts. Total sulfate deficiency has hitherto not been described, although genetic defects in sulfate transporters have been associated recently with congenital osteochondrodystrophies that may be lethal. New insights into sulfate transport and its hormonal regulation may lead to new clinical applications of sulfate analysis in the future.

Enhanced channelling of sulphate through a rapidly exchangeable sulphate pool in response to stimulated glycosaminoglycan synthesis in pancreatic epithelial cells

The ability of cells to decorate glycosaminoglycans (GAGs) with sulphate in highly specific patterns is important to extracellular matrix biogenesis and placing appropriate glycosulphated ligands on the cell surface. We have examined sulphate metabolism in two pancreatic duct epithelial cell lines - PANC-1 and CFPAC-1 (derived from a cystic fibrosis patient) with a view to understanding how pancreatic cells utilise intracellular sulphate. [35S]Sulphate uptake was rapid and reached near steady state levels within 10 min. However, the intracellular specific activity of [35S]sulphate for PANC-1 and CFPAC-1 reached only 35 and 10%, respectively, of the medium specific activity at 10 min. Therefore, sulphate appears to reside within two compartments; a rapidly exchangeable sulphate pool (RESP) and a slowly exchangeable sulphate pool (SESP). Reducing chloride in the medium, increased the specific activity of [35S]sulphate within cells and increased the size of the inorganic sulphate pool, suggesting that the RESP was enlarged. Sulphate pools were not different in size between the two cell lines in physiological NaCl. Increasing the size of the sulphate pool had no effect on [35S]sulphate:[3H]glucosamine ratios incorporated into glycosaminoglycans (GAGs); however, stimulating the synthesis of GAGs with 4-methylumbelliferyl-beta-d-xyloside, stably elevated [35S]:[3H] ratios. This was due to higher [35S]sulphate incorporation. [35S]Cysteine contributed less than 0.1% of the cells' sulphate requirements. We conclude that in the face of elevated demand for sulphate, pancreatic cells appear to channel a greater proportion through the RESP.

High glucose inhibits effect of ascorbic acid on [35S] sulphate incorporation in mesangial cell and matrix proteoglycan

Expansion of the glomerular mesangium is a consistent finding of diabetic nephropathy. Negatively charged proteoglycans are an integral part of the mesangium and their synthesis and degradation is disturbed in many forms of glomerulosclerosis. The metabolism of ascorbic acid (AA), which plays an important role in extracellular matrix regulation, is known to be abnormal in diabetes. The action of AA has also been shown to be inhibited by high glucose (HG) concentration. In this study we investigated the effect of AA and HG on proteoglycan (PG) synthesis by examining the incorporation of [35S] sulphate into PG in the cellular, matrix and media components of rat mesangial cell (MC) cultures. MC were grown in 9 or 25 mM glucose for 8 days, with and without the addition of AA. Sulphation of PG was measured by adding 50 microCi of [35S] sulphuric acid to the culture medium and precipitating 35S-labelled PG with cetylpyridinium chloride. In this study AA was shown to have a stimulatory effect on the overall incorporation of [35S] sulphate into cell and matrix PG and this was inhibited by 25 mM glucose. Correcting for protein synthesis and specific activity of [35S] sulphate showed that HG inhibits AA stimulation by decreasing sulphation of the individual PG molecules. These findings may be of particular importance in the pathophysiology of nephropathy in diabetes, a condition where AA concentration is already compromised.

Posttranscriptional effects of glucose on proteoglycan expression in mesangial cells

Hyperglycemic conditions are known to increase mRNA and protein levels of several extracellular matrix molecules in cultured mesangial cells, but accompanying increases in proteoglycan mRNA have not been found, and there are discrepant reports of normal or decreased proteoglycan synthesis with or without undersulfation in diabetic kidneys and hyperglycemic cultures. We examined the effects in proliferating cells of glucose on [35S]Sulfate incorporation into heparan and dermatan sulfates and on mRNA levels of decorin, biglycan, and basement membrane perlecan. In both mesangial cells and vascular smooth muscle cells, 30 mmol/L glucose caused a decrease of 15% to 25% in the amount of sulfate incorporated into each proteoglycan in cultures confluent for 1 to 4 days, compared with 10 mmol/L glucose. The effect showed no specificity for the class of proteoglycan and was not a consequence of changes in total protein synthesis, which increased, or cell proliferation, which was unaffected. No decrease in charge density of any of the proteoglycan fractions was observed by ion-exchange chromatography. Therefore, the decrease in labeling was due to a decrease in synthesis and not undersulfation. mRNA levels for biglycan and perlecan increased slightly and transiently, and these changes cannot account for the decreased synthesis. Decorin mRNA was detected only in smooth muscle cells, where it and biglycan were differentially affected by glucose, apparently at the transcriptional level; stabilities of the two messages were unaffected by glucose. Although transforming growth factor-beta 1 (TGF-beta 1) mRNA levels increased in response to glucose, the cytokine did not appear to regulate proteoglycan synthesis, because structural changes in proteoglycans elicited by addition of TGF-beta 1 to the culture medium did not occur in the hyperglycemic cultures. On the other hand, inhibition and downregulation of protein kinase C (PKC), while decreasing net sulfate incorporation into mesangial cell proteoglycans, prevented the effect of high glucose. We conclude that a high glucose concentration causes a general decrease in the synthesis of all classes of proteoglycans at a posttranscriptional level, and can do so without affecting the charge density of individual proteoglycan molecules.

The diastrophic dysplasia gene encodes a novel sulfate transporter: positional cloning by fine-structure linkage disequilibrium mapping

Diastrophic dysplasia (DTD) is a well-characterized autosomal recessive osteochondrodysplasia with clinical features including dwarfism, spinal deformation, and specific joint abnormalities. The disease occurs in most populations, but is particularly prevalent in Finland owing to an apparent founder effect. DTD maps to distal chromosome 5q and, based on linkage disequilibrium studies in the Finnish population, we had previously predicted that the DTD gene should lie about 64 kb away from the CSF1R locus. Here, we report the positional cloning of the DTD gene by fine-structure linkage disequilibrium mapping. The gene lies in the predicted location, approximately 70 kb proximal to CSF1R, and encodes a novel sulfate transporter. Impaired function of its product is likely to lead to undersulfation of proteoglycans in cartilage matrix and thereby to cause the clinical phenotype of the disease. These results demonstrate the power of linkage disequilibrium mapping in isolated populations for positional cloning.

Effects of CdCl2 and Cd-metallothionein on cultured mesangial cells

Cadmium is a potent nephrotoxin known to cause damage to the proximal tubular epithelium in vivo. The renal glomerulus is less frequently a target and is sensitive to Cd in vitro. We have previously presented evidence that the mesangial cell is a major target for Cd2+ toxicity in the isolated glomerulus (D. M. Templeton and N. Chaitu, Toxicology 61, 119-133, 1990). The present study was undertaken to investigate the sensitivity to Cd of rat mesangial cells grown in homogeneous culture. At a concentration of 1 microM, Cd2+ was a potent inducer of its binding protein metallothionein (MT). Cd2+ inhibited DNA synthesis in these cells with an EC50 of 5.4 +/- 0.4 microM, while preinduction of MT with Zn2+ was protective, raising the EC50 to 17.6 +/- 0.7 microM Cd2+. DNA synthesis in these cells is especially sensitive to Cd2+; only at concentrations of 20 microM Cd2+ and higher were significant effects on cell viability, attachment, and protein synthesis observed. Renal function depends in part on synthesis of specialized matrices by glomerular cells. Synthesis of both matrix and secreted proteoglycans was specifically affected by Cd2+ with an EC50 of about 10 microM for proteoglycan sulfation. We also investigated the effects of Cd-MT on these parameters. Contrary to observations that extracellular Cd-MT is a potent nephrotoxin in vivo, we were unable to demonstrate any effects of Cd-MT on DNA and protein synthesis at Cd concentrations below 60 microM in the cultured cells. Nor did Cd-MT at these concentrations affect DNA or protein synthesis in LLC-PK1 cells, a proximal tubule cell line. This was not due to failure of the cells to take up Cd because they accumulated comparable amounts of Cd whether it was provided as CdCl2 or Cd-MT. We conclude that ionic Cd2+ is the most toxic form of this metal to cultured mesangial cells. While these cells respond to micromolar concentrations of Cd2+ by increasing their content of metallothionein, presumably a protective response, only slightly higher levels may impair the regenerative capacity of mesangial cells, in addition to interfering with the specialized function of matrix synthesis.