Platelet-derived growth factor receptor b (PDGFR) is expressed in injured glomeruli and in activated cultured mesangial cells. Activation of PDGFR stimulates mesangial cell proliferation and migration, phenotypes manifest in many glomerular diseases including mesangioproliferative glomerulonephritis (GN). We recently demonstrated that PDGFR-stimulated phosphatidylinositol 3 kinase (PI 3K) activity is necessary for proliferation and migration of cultured mesangial cells. A serine threonine kinase, Akt, has been identified as a downstream target of PI 3K. Our hypothesis is that Akt regulates mesangial cell activation which includes proliferation and migration of these cells during glomerular injury. We propose to characterize pathways by which Akt functions in cultured mesangial cells and in vivo in a model of anti-Thy-1-induced GN in rats. Akt kinase activity will be determined during progression of GN. The role of Akt kinase in mesangial cell activation will be determined by examining the effect of dominant negative and constitutively active versions of this protein on mesangial cell proliferation and migration. Proteins that regulate Akt kinase activity or represent substrates for this enzyme will be identified using a yeast two-hybrid protein-protein interaction strategy. Open reading frames of interacting proteins will be determined by nucleotide sequencing. Characterization of these proteins will be carried out by raising antipeptide and GST-fusion protein antibodies. Regulation of the Akt activity by these Akt-associated proteins will be studied in vitro and in cultured mesangial cells. The role of these proteins in pathways involving Akt and regulating mesangial cell proliferation and migration will be determined. Our preliminary data indicate that cross-talk between PDGFR tyrosine kinase and bone morphogenetic protein receptor serine threonine kinases exists in mesangial cells. We have recently demonstrated that activation of receptor serine threonine kinase by bone morphogenetic protein 2 (BMP-2), a member of TGFb superfamily, inhibits PDGF-induced DNA synthesis in the absence of matrix expansion. This inhibition is due to inhibition of PDGF-induced Erk1/2 type of MAPK (mitogen-activated protein kinase). In our second specific aim, we will use BMP-2 in a therapeutic approach to treat mesangioproliferative GN in rats. An adenovirus vector expressing BMP-2 will be constructed. Adenovirus-mediated gene transfer and engineered mesangial cell vectors will be used to express BMP-2 in vivo to inhibit mesangial cell proliferation in GN, without inducing extracellular matrix expansion. Activities of PI 3 kinase, MAPK and Akt will be determined in the glomerular lysates from vector-targeted animals. These studies will identify important signaling mechanisms involved in glomerular pathology and help to establish effective therapeutic modalities for treatment of proliferative forms of GN.
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