Slit Diaphragms: Junctions That Never Sleep

The podocyte slit diaphragm is central to the function of the glomerular filtration barrier. Defects in the slit diaphragm caused by heritable genetic mutations result in glomerular proteinuria and FSGS, a major cause of CKD and kidney failure. Genetic analysis of affected individuals has identified over 50 susceptibility genes that when mutated, cause primary or secondary FSGS.1 A major aim now is to deduce from the genetic analysis an expanded set of cellular pathways and protein assemblies that are central to the proper functioning of the podocyte and define entry points for rational, patient-specific therapeutic approaches. The discovery of diverse cellular pathways underlying apparently similar pathologic features (proteinuria, FSGS) has prompted a redefinition of heritable glomerular proteinuria as "podocytopathies."1 Genetic causes of podocytopathies cluster into multiple subgroups, including extracellular matrix and matrix binding proteins, structural and signaling elements of the slit diaphragm itself, the cytoskeleton, mitochondrial function, nuclear transcription and mRNA transport, cell signaling, the cilium, and the lysosome.1 A relatively recent addition to this already extensive list of cellular pathways in podocytopathies is the endocytosis pathway and its associated vesicle sorting proteins. Electron micrographs of the glomerular filtration barrier give the impression of an apparently stable, intricate structure for slit diaphragms. We now know that this static picture is misleading and that the slit diaphragm is a highly dynamic intercellular junction with constant turnover of proteins, like neph1, nephrin, and podocin. Membrane components of the junction are steadily endocytosed and either degraded or recycled back to the foot process junctions.2 To what end, one might wonder. Endocytic turnover of nephrin is regulated in part by phosphorylation. Adaptor proteins recognizing different phosphoforms of Nephrin include Nck, β-arrestin, and ShcA.23–4 These proteins can either stimulate or inhibit Nephrin endocytosis with consequent effects on slit diaphragm function. Mutations in genes that influence vesicle sorting (Inverted Formin 2, INF2) can also lead to failed recycling of nephrin and depletion of slit diaphragms with resulting proteinuria.5 These pathways link nephrin endocytosis to nephrin signaling and confer a dynamic remodeling capacity on slit diaphragms. Endocytosis may also constitute a form of quality control on slit diaphragm proteins and maintenance of the glomerular filter. Two distinct pathways of Nephrin endocytosis have been defined: dynamin-mediated and flotillin/raft-associated endocytic uptake. Recent elegant studies utilizing the Drosophila nephrocyte model of podocytes have shown that dynamin-mediated uptake outside of the slit diaphragm membrane domain restricts the anatomic localization of the junctions, whereas flotillin/raft-associated nephrin uptake is associated with processing nephrin for recycling and dissociation of proteins bound to nephrin.6 An intriguing feature of this model is that nephrin endocytosis and recycling may keep the filter from clogging by clearing off nephrin-bound protein in low-pH endocytic compartments prior to reinsertion in slit diaphragms.6 In this issue of JASN, Milosavljevic et al.7 make further use of the Drosophila nephrocyte model of podocytes to show that the Ras11GAP TBC1 Domain Family Member 8B (TBC1D8B) plays a key role in nephrin endocytic processing. The Drosophila nephrocyte is a fascinating evolutionary relation to the podocyte. These cells have ridge-like surface infoldings linked at their surface by slit diaphragm junctions composed of Drosophila Neph1 and Nephrin orthologs (kirre and sns). Although they do not pass filtrate to a tubule like the glomerular basement membrane, they do show key features of podocytes, including size-selective filtration and active endocytosis.8 These features combined with the rich toolbox of genetic and imaging approaches in the fly make them a valuable model for analyzing homeostasis of the slit diaphragm junction. Prior studies identified TBC1D8B as a cause of human FSGS and demonstrated TBC1D8B interaction with and regulation of Ras-related protein RAB-11 (RAB11), an effector of vesicle trafficking.9,10 By generating a Drosophila mutant in the fly TBC1D8B ortholog, Milosavljevic et al.7 elegantly show a nephrocyte-specific phenotype affecting slit diaphragm turnover. The fly excels at compound gain and loss of function approaches that allow Milosavljevic et al.7 to show that Tbc1d8b is required for rapid turnover and Ras-related protein Rab-5-mediated nephrin endocytosis, identifying a broader role for TBC1D8B beyond acting as an RAB11 Gtpase Activating Protein. By expressing mammalian TBC1D8B alleles in Drosophila and scoring rescue of the mutant phenotype, they also characterize two novel human TBC1D8B sequence variants as pathogenic, highlighting the value of genetic animal models for determining the significance of DNA sequence variants in human genetic disease. Pathway-specific models of podocytopathies and FSGS such as this also highlight the potential in using simpler genetic systems, such as the fly or the zebrafish, as discovery platforms to screen patient-specific therapies for FSGS. Disclosures Drummond reports Ownership Interest: ABBOTT LABORATORIES Editas,Crispr therapeutics; Honoraria: NIH; and Advisory or Leadership Role: Imagine Institute Paris. Funding None.