Neutral proteinase activity produced in vitro by cells of the glomerular mesangium

Cells derived from isolated glomerular tufts of rats were studied in primary tissue culture after the removal of epithelial cells by collagenase treatment. The cultured cells, fusiform or stellate in shape, grew readily over a 12-day period. Immunofluorescence staining was positive for myosin and fibronectin, while negative for Factor VIII, suggesting that the outgrowing cells were derived from the glomerular mesangium. In serum-free culture, these cells produced neutral proteinase activity that occurred as a latent trypsin-activable form (apparent molecular weight range, 78,000 to 100,000 daltons) and in an active form (44,000 to 58,000 daltons). Neutral proteinase activity was inhibited by EDTA and by cysteine, and exhibited a pH optimum of 7.2 to 7.8, characteristic of an extracellularly active metalloendopeptidase. The culture supernate which contained the neutral proteinase activity was capable of degrading purified rat glomerular basement membrane. The release of hydroxyproline-containing fragments from the basement membrane indicated that degradation of the type IV collagen component of the basement membrane was occurring. These findings suggest that the neutral proteinase activity generated by mesangium-derived cells may play a role in the physiologic turnover of glomerular structural proteins in vivo.