CNDP2 drives renal tubular fibrosis in diabetic kidney disease via a sulfur-containing amino acids-mTOR signaling axis

BACKGROUND: Diabetic kidney disease (DKD) is a leading cause of end-stage renal disease, with proximal tubule fibrosis being a key pathological feature. Our previous study identified significant upregulation of cytosolic nonspecific dipeptidase 2 (CNDP2) in the renal tubules of a DKD mouse model, yet its functional role remains unclear. OBJECTIVE: This study aimed to investigate the role of CNDP2 in renal tubular fibrosis during DKD, focusing specifically on the "sulfur-containing amino acids (SAAs) metabolism-mammalian target of rapamycin (mTOR)" signaling axis. METHODS: We employed integrated in vivo and in vitro models. Kidney- specific Cndp2 knockdown was achieved using an adeno-associated virus vector, and the CNDP2 inhibitor bestatin was used for pharmacological intervention. Renal function, fibrosis, the Ragulator-Rag GTPase-mTOR signaling pathway, amino acid profiles, and ultrastructural changes were assessed. A dietary intervention restricting SAAs was also applied. RESULTS: CNDP2 was specifically upregulated in renal tubules under DKD conditions. Both genetic knockdown and pharmacological inhibition of CNDP2 significantly improved renal function and attenuated fibrosis. Mechanistically, CNDP2 hydrolyzes dipeptides, leading to elevated levels of SAAs (cysteine/cystine). This promotes the activation of the Ragulator-Ras-related GTPase (Rag) complex, resulting in subsequent hyperactivation of mTOR signaling and driving tubular fibrosis. Notably, dietary restriction of SAAs similarly ameliorated DKD pathology. CONCLUSION: CNDP2 drives renal tubular fibrosis in DKD by activating mTOR signaling through disruption of SAAs metabolism. Our findings reveal a novel "CNDP2-SAAs-mTOR" pathway, identifying CNDP2 as a promising therapeutic target for DKD intervention.