Chronic kidney disease (CKD) is a global public health issue, affecting over 800 million people and ranking as the 9th leading cause of death worldwide, with complications such as cardiovascular disease and muscle wasting contributing to its high mortality. CKD predominantly results from diabetic kidney disease (DKD) and hypertension, yet there are few preventive strategies to intervene prior to the onset of clinical outcomes. Recent evidence suggests that disruptions in renal tubular epithelial cells (TECs), including cellular senescence and alterations in nicotinamide adenine dinucleotide (NAD+) metabolism, play a critical role in disease progression. Understanding these early changes is crucial for developing new therapeutic approaches to promote kidney health. This thesis explores TEC homeostasis through a multidisciplinary approach, integrating human omics data and preclinical models to uncover actionable targets in CKD.

First, we developed a multiparametric flow cytometry assay to capture the complexity of the senescent phenotype in TECs. This method quantifies seven distinct hallmarks of senescence and enables medium/high-throughput screening for potential senotherapeutic interventions.

Next, we identified nicotinamide-N-methyltransferase (NNMT) as a key driver of tubular senescence and fibrosis in CKD. NNMT expression positively correlates with kidney fibrosis and inversely with renal function in human CKD. Spatial transcriptomics of human DKD biopsies showed that NNMT-positive tubules are enriched in senescence signatures and are surrounded by more inflammatory and fibrotic microenvironments. In preclinical early-CKD models of aging and diabetes, NNMT is upregulated and associates with senescence. Mechanistic studies in vitro show that genetic overexpression of NNMT in TECs exacerbates senescence and epithelial-to-mesenchymal transition while its inhibition with a clinically safe small molecule in damaged cells and kidney organoids is protective. Collectively, NNMT represents a clinically relevant biomarker for early CKD and a promising therapeutic target to mitigate tubular senescence and fibrosis, essential factors in CKD progression.

Finally, we investigated trigonelline, a naturally occurring NAD+ precursor, as a potential therapy for mitochondrial dysfunction in DKD. Spatial transcriptomics revealed impaired oxidative phosphorylation in DKD renal tubules, and declining endogenous trigonelline levels were associated with reduced kidney function. In vivo supplementation with trigonelline improved mitochondrial homeostasis, reduced inflammation, and prevented muscle wasting in early-stage DKD mice. Overall, these findings suggest that trigonelline-mediated NAD+ and mitochondrial pathways modulation may offer a nutritional strategy to slow the progression of DKD.

Altogether, this work uncovers novel molecular mechanisms involved in the disruption of the tubular epithelium in early CKD, with promising translational applications aimed at reducing senescence and restoring NAD+ metabolism to improve kidney health.