Immune mediated kidney diseases cause substantial morbidity and often lead to progressive renal failure. Immune complexes (ICs) have been found in podocytes in a variety of immune mediated kidney diseases but it is not known how podocytes handle immune complexes and whether trafficking of immune complexes in podocytes exacerbates glomerulonephritis. The neonatal Fc receptor (FcRn) correctly sorts and traffics ICs in a variety of cells including podocytes. FcRn is also required to traffic immune complexes to the lysosome in dendritic cells for proteolytic processing and presentation on MHC II. In preliminary studies, we have found that podocyte specific knockout of FcRn protects in two models of immune mediated nephritis but that this protection is not conferred by an immune mediated mechanism. While there is no difference in any of the immune parameters examined, we have found that there is significantly less apoptotic cell death and upregulation of apoptotic pathways in FcRn knockout (KO) podocytes treated with immune complexes and significantly decreased apoptosis in the glomeruli of podocyte specific FcRn KO mice after induction of nephrotoxic serum nephritis (NTS). In addition, we have found significantly less lysosomal activation and less upregulation of lysosomal enzymes in FcRn KO podocytes treated with immune complexes. Since FcRn is required to traffic immune complexes to the lysosome, we hypothesize that in immune mediated nephritis, FcRn directed trafficking of ICs to the lysosome results in lysosomal dysfunction, upregulation of the intrinsic apoptotic pathway and podocyte death. To test this hypothesis we will directly examine lysosomal structure and function in WT and FcRn KO podocytes after an immune challenge using both advanced imaging techniques and biochemical methods. We will also examine whether FcRn mediated trafficking of immune complexes to the lysosome results in increased lysosomal permeability and leakage of lysosomal cathepsins into the cytosol causing degradation of mitochondrial membrane integrity, leakage of cytochrome c and upregulation of intrinsic apoptotic pathways. We will test whether podocyte specific knockout of FcRn results in decreased lysosomal activation and improved lysosomal function in podocytes in vivo after induction of immune mediated kidney disease (NTS) and whether this in turn results in less apoptotic cell death, an increased number of viable podocytes and less severe disease as assessed by functional and histologic parameters. Since upregulation of lysosomal cathepsins correlates with more severe lupus nephritis in patients, we will also examine whether podocyte specific knockout of FcRn ameliorates lupus nephritis in a spontaneous mouse model of this disease. Taken together, the proposed work will provide novel insights into FcRn mediated trafficking of immune complexes in podocytes and may allow for the creation of targeted therapies to slow or prevent progression of immune mediated glomerulonephritis.
Research Narrative Immune mediated kidney disease leads to immune complex (IC) deposition in podocytes but it is unknown how podocytes handle ICs. The neonatal Fc receptor (FcRn) plays a key role in correctly sorting and trafficking ICs in other cell types. The goal of this proposal is to examine the role of FcRn in trafficking of ICs to the podocyte lysosome and to determine whether inhibition of the lysosomal response to ICs in podocytes ameliorates immune mediated glomerulonephritis.