DETECTION OF 2 FORMS OF GP330 - THEIR ROLE IN HEYMANN NEPHRITIS

Heymann nephritis is characterized by glomerular immune deposits that contain a glycoprotein called gp330. The deposits are believed to resu lt from shedding of immune complexes formed on podocytes. Complexes ar e also shed from proximal tubule cells, when antibodies combine with g p330 on the cell surface. We performed the present study to investigat e what portion of the gp330 molecule is shed, using a rabbit antiserum against a peptide deduced to be in the cytoplasmic domain of gp330, a s well as a rabbit antiserum and two monoclonal antibodies that recogn ize extracellular epitopes of gp330. The anti-cytoplasmic peptide anti serum precipitated from Fx1A (a crude renal cortical membrane preparat ion), a protein with a mass of about 440 kd that was reactive with two monoclonal anti-gp330 antibodies. (In our experiments, the protein ca lled gp330 generally has a mass estimated to be about 440 kd.) The ant i-cytoplasmic peptide antiserum also reacted with a truncated gp330 pr otein produced in transfected COS cells. Immunohistochemical studies s howed that all the antibodies recognized the same group of epithelial cells. However, as seen in immunoultrastructural studies of proximal t ubules, the anti-cytoplasmic peptide antiserum reacted only with compo nents at the base of microvilli, whereas the anti-gp330 ectodomain ant ibodies identified material not only at the base, but over the surface of microvilli as well. In rats with Heymann nephritis, glomerular dep osits and material shed into tubule lumens reacted with antibodies aga inst extracellular epitopes of gp330, but not with the anticytoplasmic peptide antiserum. We propose that there are two forms of gp330 on th e cell surface of proximal renal tubules. One form is restricted to co ated pit regions at the base of microvilli and has a cytoplasmic domai n containing a sequence deduced from a partial complementary DNA encod ing gp330. The other form is present over microvilli (and possibly at the base of microvilli as well) and lacks the cytoplasmic domain deduc ed from the complementary DNA. The complexes that are shed in Heymann nephritis contain either a portion of gp330 cleaved from the full-leng th molecule or a form of gp330 that lacks the cytoplasmic domain.