Compromised Glycolysis Contributes to Foot Process Fusion of Podocytes in Diabetic Kidney Disease: Role of Ornithine Catabolism

Introduction: Compromised glycolysis in podocytes contributes to the initiation of diabetic kidney disease (DKD). Podocyte injury is characterized by cytoskeletal remodeling and foot process fusion. Compromised glycolysis in diabetes likely leads to switch of energy supply in podocyte. However, the underlying mechanism by which disturbed energy supply in podocytes affects the cytoskeletal structure of podocytes remains unclear.

Methods: Metabolomic and transcriptomic analyses were performed on the glomeruli of db/db mice to examine the catabolism of glucose, fatty, and amino acids. Ornithine catabolism was targeted in db/db and podocyte-specific pyruvate kinase M2 knockout (PKM2-podoKO) mice. In vitro, expression of ornithine decarboxylase (ODC1) was modulated to investigate the effect of ornithine catabolism on mammalian target of rapamycin (mTOR) signaling and cytoskeletal remodeling in cultured podocytes.

Results: Multi-omic analyses of the glomeruli revealed that ornithine metabolism was enhanced in db/db mice compared with that in db/m mice under compromised glycolytic conditions. Additionally, ornithine catabolism was exaggerated in podocytes of diabetic PKM2-podoKO mice compared with that in diabetic PKM2 mice. In vivo, difluoromethylornithine (DFMO, inhibitor of ODC1) administration reduced urinary albumin excretion and alleviated podocyte foot process fusion in db/db mice. In vitro, 2-deoxy-d-glucose (2-DG) exposure induced mTOR signaling activation and cytoskeletal remodeling in podocytes, which was alleviated by ODC1-knockdown. Mechanistically, a small GTPase Ras homolog enriched in the brain (Rheb), a sensor of mTOR signaling, was activated by exposure to putrescine, a metabolic product of ornithine catabolism.

Conclusion: These findings demonstrate that compromised glycolysis in podocytes under diabetic conditions enhances ornithine catabolism. The metabolites of ornithine catabolism contribute to mTOR signaling activation via Rheb and cytoskeletal remodeling in podocytes in DKD.