Suppression of Endoplasmic Reticulum-Associated Degradation Process by Intraglomerular Cross-Talk Between Podocytes and Mesangial Cells Causes Podocyte Injury in Diabetic Kidney Disease

Background: Mesangial lesion and podocyte injury are essential for the progression of diabetic kidney disease (DKD). Although crosstalk between mesangial cells (MCs) and podocytes is recently suggested by single nucleus RNA-sequence analyses, its molecular mechanisms and role on disease progression still remain elusive. Methods: We evaluated the ER stress responses of podocytes stimulated with mesangial cell-cultured medium (MC-sup) under high-glucose condition (HG) in vitro. Then, the effects of an ER-associated protein degradation (ERAD) inhibitor eeyarestatin I (EerI) in cultured podocytes and glomeruli of db/db (type 2 diabetic) mice were also examined by western blotting, immunofluorescence and TUNEL staining. Furthermore, we evaluated the effect of ERAD inhibitor on nephrin phosphorylation of podocytes by flowcytometric analysis and western blotting. Results: In vitro experiments revealed the suppression of the ER-associated degradation (ERAD) pathway and induction of apoptosis in podocytes that were stimulated with the supernatant of mesangial cells cultured in high glucose conditions. In diabetic mice, ERAD inhibition resulted in exacerbated albuminuria, increased apoptosis in podocytes, and reduced nephrin expression associated with the downregulation of ERAD-related biomolecules. Flowcytometry analysis of podocytes isolated from MafB (a transcription factor known to be expressed in macrophages and podocytes)-GFP knock-in mice revealed that ERAD inhibition resulted in decreased nephrin phosphorylation. Decreased nephrin phosphorylation was also confirmed in in vitro experiments. Conclusions: ERAD process has been reported to be important for avoiding ER stress and cellular damages. Our findings suggest that an intraglomerular crosstalk between MCs and podocytes can inhibit physiological ERAD processes and suppress the phosphorylation of nephrin in podocytes, which thereby lead to podocyte injury under diabetic conditions. Therapeutic intervention of the ERAD pathway through the crosstalk between these cells is potentially a novel strategy for DKD.