Understanding the Protective Role of Podocyte-Specific ApoJ in Diabetic Kidney Disease

Background: Mechanisms that confer resistance to progression of diabetic kidney disease (DKD) are unclear. While podocyte loss occurs early in DKD, proximal tubule (PT) injury ultimately determines DKD progression. ApoJ/Clusterin, a secreted molecular chaperone, is increased in human and mouse models of diabetic kidney disease (DKD). However, the role of ApoJ in mediating the progression of DKD remains understudied. Here, we investigate the mechanism by which podocyte-specific ApoJ preconditions the proximal tubule to attenuate the progression of DKD. Methods: Human podocytes with ApoJ overexpression were generated using lentiviral method and the podocyte secretome was collected. Uninephrectomy + streptozotocin was used as a diabetic mouse model with the respective control mice. Pharmacological inhibition of CaMK1d, using STO-609, was conducted in primary PT cells and diabetic mice. Oxygen consumption rate (OCR) was measured using a seahorse analyzer. PAS, H&E, picrosirus red, immunofluorescence staining, qPCR and western blot were performed. Results: Primary PT cells treated with the secretome from podocytes overexpressing ApoJ demonstrated an increase in OCR and pDrp1 levels with a decrease in mitochondrial fragmentation under high glucose (HG) conditions compared to control conditions. Inhibition of Lrp2/megalin using cilastatin mitigated this increase in OCR, suggesting that podocyte-specific ApoJ binds to Lrp2. ApoJ also colocalizes with Lrp2 by immunostaining in the apical surface of the proximal tubule in diabetic mice. IP for calmodulin demonstrated that ApoJ interacts with CaMK1d. Camk1d-/- as well as treatment with STO-609 reduced pDrp1 levels, OCR, and CaMK1d kinase activity with an increase in mitochondrial fragmentation and cell injury in primary PT cells. Similarly, STO-609-treated diabetic mice exacerbated PT injury as compared to VEH-treated diabetic mice. Conclusion: These data suggest that under diabetic conditions, the podocyte-specific ApoJ preconditions the PT against injury by undergoing PT uptake via Lrp2, leading to CaMK1d-mediated restoration of mitochondrial function to prevent DKD progression. Funding: NIDDK Support, Veterans Affairs Support