Evaluation of the cell specificity and sulfate dependence of glomerular extracellular matrix proteoglycan synthesis

Homogeneous cultures of epithelial, endothelial, and mesangial cells from calf glomeruli were radiolabeled with [35S]sulfate in order to evaluate their capacity for the biosynthesis of the proteoglycan (PG) components present in the glomerular extracellular matrix. Although each cell type was observed to incorporate into its matrix predominantly immunologically related heparan sulfate (HS) PGs (M(r) approximately 500 kDa), endothelial and mesangial cells also deposited substantial amounts of PGs with chondroitin sulfate (CS) and dermatan sulfate (DS) chains. The limited capacity of epithelial cells to synthesize PGs other than those containing HS was also evident from the immunologically distinct components (M(r) approximately 300 kDa) shed into the medium which in contrast to those from the endothelial and mesangial cells contained no CS and only small amounts of DS glycosaminoglycans. While the matrix proteoglycan HS chains differed in length depending on cell type, they were similar in containing the six mono- and disulfated disaccharide species previously found in bovine glomerular basement membrane, including the distinctive iduronic-GlcNSO3 (3-SO4) sequences. While the addition of sulfate to medium free of this ion brought about no change in HS PG production by any of the three cell types and the formation of CS and DS chains by epithelial and mesangial cells was unaffected, the formation of CS/DS PGs by endothelial cells was altered to a pronounced extent through the conversion of an undersulfated PG to a more polyanionic molecule. Our findings are consistent with the concept that the glomerular extracellular matrix is made up of two biosynthetically distinct regions (mesangium and basement membrane) and are relevant to an understanding of various diseases affecting the renal filter.