VAPB Regulates Fatty Acid Oxidation via Maintaining the Integrity of Mitochondria-Associated Endoplasmic Reticulum Membranes in Diabetic Kidney Disease (DKD)

Background: Defective fatty acid oxidation (FAO) in renal tubular epithelial cells plays an important role in the progression of diabetic kidney disease (DKD). Contact between lipid droplets and mitochondria facilitates mitochondrial FAO. We performed transcriptome-wide association studies (TWAS) and identified vesicle-associated membrane protein-associated protein B (VAPB)，an ER-anchored protein that mediates tethering between ER and mitochondria, as a potential causative gene for DKD. In this project, we aim to investigate whether VAPB affects FAO in DKD and to elucidate the underlying mechanism. Methods: VAPB adenovirus or negative control was delivered to the kidneys of male db/db mice and littermate db/m mice at 12 weeks of age by intra-parenchymal injection, and kidneys were harvested 4 weeks after injection. Primary renal tubular cells were transfected with VAPB adenovirus or negative control and then exposed to the high glucose (30mM) treatment for the indicated time. Results: VAPB expression was found to be decreased in renal tubules of patients with DKD and db/db mice. Inducing overexpression of VAPB attenuated tubular injury and lipid droplet accumulation, and restored expression level of FAO-related enzymes (PPAR-α，ACADL) both in vivo and in vitro. Mechanistically, analysis of VAPB binding partners and membrane contact sites revealed that mitoguardin2 (MIGA2), a mitochondria outer membrane protein and lipid droplet binding protein, may interact with VAPB to maintain the integrity of mitochondria-associated endoplasmic reticulum membranes (MAMs).We found that high glucose reduces the binding of VAPB to MIGA2, disrupts the integrity of MAMs, and increases the distance between lipid droplets and mitochondria in vitro. Conclusion: We proposed that VAPB may regulate fatty acid oxidation in renal tubules in DKD by maintaining the integrity of MAMs through interaction with MIGA2, which promotes contact between lipid droplets and mitochondria.