Glutamylation of Renal Tubular Cells Promotes Renal Fibrosis by Aggravating Mitochondrial Damage

Background: Renal fibrosis is closely related to the dysregulation of glutaminolysis in renal tubular epithelial cells (TECs). However, how glutaminolysis is involved in the fibrotic response of TECs remains unclear. Methods: We investigated the expression patterns of genes related to glutamine catabolism in renal tissues from patients with chronic kidney disease (CKD) and fibrotic mice using publicly available datasets. We further developed in vivo and in vitro models of renal fibrosis. Subsequently, we conducted experiments involving glutamine deprivation and supplementation in both model systems to further assess the effects of glutamine and glutamic acid. Transcriptomic and metabolomic analyses were employed to elucidate the role of glutamine catabolism in the fibrotic response. Additionally, we identified upstream regulatory molecules of key metabolic enzymes using ChIP-seq and ChIP-qPCR, and evaluated their impact on renal fibrosis by targeting these genes. Results: The expression levels of SLC1A5 and GLS were positively correlated with renal function. Glutamine deprivation was found to mitigate TECs fibrotic response in both in vivo and in vitro settings. However, complete deprivation of glutamine resulted in significant cell death. Supplementation with 5 mM and 10 mM concentrations of glutamine or glutamate induced substantial apoptosis, elevated the expression of SLC1A5 and GLS, and triggered fibrotic responses. Metabolomics and transcriptomics analyses revealed a significant accumulation of glutamate during the fibrotic process. Mass spectrometry and subsequent Co-IP assays indicated a significant increase in the glutamylation of OPA1, a protein involved in mitochondrial dynamics, along with a notable reduction in its protein levels during the fibrotic response. ChIP-seq and ChIP-qPCR analyses demonstrated that FOXK1 is involved in the transcriptional regulation of SLC1A5 and SLC7A11. Conclusion: The metabolites resulting from glutaminolysis do not proceed into the tricarboxylic acid cycle. Rather, they accumulate within the cytoplasm and undergo glutamylation modification. This accumulation leads to the degradation of OPA1, exacerbating mitochondrial damage and ultimately promoting renal fibrosis. This process is modulated by FOXK1. Targeting glutamine metabolism and FOXK1 may offer potential therapeutic strategies for the treatment of renal fibrosis.