Glomerular epithelial alterations resulting from sialic acid surface coat removal

Abstract

An in vitro model system for studying kidney glomeruli was used to investigate the effects of glomerular epithelial sialic acid surface coat removal on the fine structure of these cells. The glomeruli exposed on the surfaces of rat kidney slices were incubated in either oxygenated normothermic Tyrode's balanced salt solution or culture medium, each with and without the enzyme neuraminidase. After 6 hours of incubation in Tyrode's or culture medium alone, the only significant alteration of the glomerular epithelium was an increase in number and length of free surface microvilli and an increase in cytoplasmic lipid inclusions. After 3 hours of incubation in these solutions plus neuraminidase (0.067 U/ml of solution), there was a significant reduction in the glomerular sialic acid surface coat as determined by the colloidal iron staining technique. Coincident with this sialic acid removal, there was a loss of podocyte foot processes and filtration slits and the formation of numerous knoblike projections of the plasmalemma over the glomerular epithelium. By transmission electron microscopy, it appeared that junctional complexes had formed between many podocyte processes and that these processes had retracted toward the cell body. Also noted was the displacement of filtration slit diaphragms away from the glomerular basal lamina and an apparent extensive folding of these diaphragms. Samples examined after 6 hours of incubation in neuraminidase appeared similar to the 3 hour samples except that most of the knobby projections of the glomerular epithelial plasmalemma were gone. The foregoing fine structural alterations resulting from glomerular epithelial sialic acid surface coat removal closely parallel changes exhibited by thse cells during puromycin aminonucleoside nephrosis. The possible significance of these fine structural changes in the etiology of glomerular disease is discussed.