Variability of proteoglycan expression in the isolated rat glomerulus

Abstract

The sulphation of proteoglycans in freshly isolated rat glomeruli was studied by biosynthetic labelling with [ 35S]sulphate. At least 75% of the observed sulphation requires de novo synthesis of core protein and proceeds at a constant rate over at least 40% h. Herparan and dermatan sulphate proteoglycans (HSPG and DSPG, respectively) are the two major species produced, with only minor amounts (less than 5%) of chondroitin sulphate labelled under these conditions. Several factors affect the population distribution of labelled material. When glomeruli were obtained from rats 6 weeks of age, HSPG accounted for 75±9% of tissue proteoglycan sulphated over 16 h. When older rats (12–14 weeks) were used, only 32±10% of label was associated with HSPG, DSPG accounting for the remainder. Production of HSPG is sulphate-dependent, increasing relative to DSPG with increasing sulphate, up to physiological concentrations. However, the net charge-density of sulphated material is conserved even at the lowest concentrations of sulphate. This may reflect the importance of electrostatic properties in the function of glomerular proteoglycans. The production of HSPG increases relative to DSPG with time following isolation and this effect is more dramatic in glomeruli from younger rats. However, reciprocal changes in production of HSPG and DSPG sustain a constant rate of sulphation. This phenomenon may arise from interdependency of the glomerular epithelial and mesangial cells with respect to regulation of proteoglycan synthesis.