Serine mediates protection against calcium oxalate nephrolithiasis by attenuating tubular epithelial cell apoptosis through the SDSL-IDH2 axis.

BACKGROUND: Calcium oxalate calculi constitute the predominant form of kidney stones, yet the molecular pathways governing their formation are not fully understood. Growing evidence suggests that these crystalline deposits can trigger apoptotic pathways in renal tubular epithelial cells. Within this pathological context, various amino acid metabolic pathways significantly influence cellular viability. This study seeks to elucidate the involvement of serine metabolism in endoplasmic reticulum stress (ERS), reactive oxygen species (ROS) production, and the ensuing apoptosis in renal tubular epithelial cells, along with delineating its mechanistic basis. METHODS: We first collected serum samples from renal calculus patients and healthy controls, followed by metabolic sequencing. Comparative analysis revealed a significant reduction (depletion) of serine in the patient's serum. We then investigated the impact of serine metabolism on stone-induced injury progression and its underlying mechanism. Initially, HK-2 cells were treated with varying concentrations of serine and stimulated with oxalate. Cell viability was assessed using the CCK-8 assay, while relevant proteins were expressed. Additionally, reactive oxygen species (ROS) levels were detected by immunofluorescence, and apoptosis was evaluated via TUNEL staining. Based on these results, the optimal Serine concentration was selected for subsequent experiments. Subsequently, KEGG pathway analysis and a review of pertinent literature were conducted to identify serine metabolism pathways potentially involved in this process. The erapeutic potential of serine was assessed in vitro and in a mouse model. A key enzyme implicated in the aforementioned processes was screened out, and its expression levels across different groups were examined. To further elucidate the mechanism, a plasmid encoding shRNA was constructed to knock down the upstream gene of this key enzyme. Following identical oxalate stimulation conditions, experiments-including CCK-8 assay, measurement of related protein expression, ROS immunofluorescence, and TUNEL staining-were repeated to validate the functional role of the pathway. RESULTS: Metabolic analysis of serum samples demonstrated a significant reduction in serine levels in nephrolithiasis patients compared to healthy controls. Administration of an optimal serine concentration significantly alleviated oxalate-mediated cytotoxicity and renal injury in vivo. Moreover, serine supplementation activated the SDSL-IDH2 axis, reducing ERS, ROS production, and apoptotic cell death. SDSL inhibition significantly exacerbated ERS, ROS, and apoptosis levels, even in serine presence. CONCLUSION: Our findings demonstrate that serine protects against ERS, ROS, and apoptosis across experimental models, which is mediated through the activation of the SDSL-IDH2 pathway.