The Role of Protein Degradation Systems for Glomerular Protein Homeostasis

• Applicant: Professorin Dr. Catherine Meyer-Schwesinger
• Subject Area: Nephrology
• Term: from 2019 to 2024
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 429327206

Three types of glomerular cells, which form a functional syncytium, achieve renal filtration. Of these, podocytes and endothelial cells are separated by the glomerular basement membrane and ultimately compose the three-layered glomerular filtration barrier (GFB), which is thought to impart both size-selective and charge-selective properties. Mesangial cells occupy the space inbetween the GFB to provide structural support and to indirectly participate in filtration by reducing the glomerular surface area by contraction. Typical for glomerular injury is the deposition of proteins originating from the plasma in form of subepithelial, subendothelial and mesangial deposits. Under physiologic conditions glomerular deposition of protein is neglectable even though permeability of the GFB to plasma protein is only partial, suggesting that preventive mechanisms must exist. Impairment of these mechanisms could result in pathologic glomerular protein deposition, which in turn would affect glomerular function. The basis of glomerular protein homeostasis is unknown partly due to the complexity of protein degradation systems and due to the complexity of the glomerular syncytium. In this proposal we hypothesize, that protein uptake and intracellular degradation through the two major degradative systems, namely the ubiquitin-proteasomal system (UPS) and the autophagosomal lysosomal system (ALS) interplay in glomerular cells and contribute to maintain the integrity of the glomerular filter in a cell-specific manner. Preliminary investigations demonstrate that intra- and extracellular protein homeostasis of mesangial and endothelial cells predominantly depends on an intact ALS, whereas podocytes in first line depend on the UPS. Thereby, inhibition of the UPS promotes subepithelial IgG deposition and proteinuria whereas ALS inhibition results in mesangial IgG deposition. To dissect the unknown physiologic and pathophysiologic importance of the UPS and ALS for the intra- and extracellular protein homeostasis of glomerular cells we defined three aims which will be addressed with novel techniques of glomerular cell-specific analyses. (1) Define the differential cell-specific basal activity of the UPS and the ALS in glomerular cells. (2) Comparative analyses of the consequence of clinically used proteasomal inhibitors on the protein homeostasis of glomerular cells. (3) Comparative analyses of the consequence of genetic cell-specific proteasomal and lysosomal impairment for the unchallenged glomerular syncytium and after IgG exposition. The results will provide the missing basis for i.e. understanding the mechanisms of glomerular protein deposition, for understanding conflicting data on adverse drug effects of proteasomal inhibitors, and of the different glomerular phenotypes of mutations involving UPS and ALS proteins.

• DFG Programme: Research Grants