Synergistic Effects of Endoplasmic Reticulum-Associated Degradation and Autophagy in Podocyte Function

Background: Podocytes are specialized epithelial cells, key to the kidney glomerular filtration barrier, through which the blood is filtered by forming interdigitating foot processes with neighboring podocytes. The slit diaphragm is a specialized cell junction composed of multiple proteins including nephrin to connect foot processes. Impaired biosynthesis of these proteins leads to foot process effacement and renal failure. Endoplasmic reticulum (ER)-associated degradation (ERAD) and autophagy are two major protein quality-control machineries to maintain protein homeostasis. However, the crosstalk between these two quality-control systems has not yet been investigated in the podocyte function. Methods: The podocyte-specific Sel1L and Atg7 double knock-out mice (DKO) were generated. Mouse body weight and survival curves were recorded. Urine samples from mice at 1, 3, and 5 weeks of age were collected to assess the abundance of albumin. Kidneys from wild-type (WT), Sel1L knock-out, Atg7 knock-out and DKO mice were collected for histology examination by H&E staining, ultrastructure observation via scanning electron microscopy (SEM) and transmission electron microscopy (TEM), as well as determination of abundance and cellular location by immunofluorescence staining and western blot. Results: DKO mice exhibited severe early onset renal failure at 3 weeks of age and premature death as early as 6 weeks of age, while Sel1L knock-out mice died with a median life span of ~14 weeks. Atg7 knock-out mice were normal like WT mice until 1 year of age. At 3 weeks of age, large protein casts were only observed in the renal tubules of DKO mice. SEM and TEM revealed that DKO mice developed effaced foot process much earlier than Sel1L knock-out mice. Mechanismlly, nephrin, an ERAD substrate, accumulated in the ER of Sel1L-deficient podocytes. In the absence of ERAD, ATG7 deficiency increased nephrin accumulation in the ER, further impairing its trafficking to the cell membrane. Conclusion: ERAD and autophagy synergistically regulate podocyte function. Deficiencies in ERAD and autophagy lead to podocyte foot process effacement and subsequent renal failure by affecting nephrin maturation and cell membrane trafficking. Funding: NIDDK Support