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Background and Aims Acute Kidney Injury (AKI) is a syndrome characterized by an acute deterioration of renal function. Due to its high prevalence and poor short-term outcomes, AKI represents a global healthcare issue. Many epidemiologic studies have indicated that the development of Chronic Kidney Disease (CKD) features prominently among the numerous long-term complications of AKI. The pathophysiological basis for this phenomenon has remained unclear so far. Recently, we found that tubular epithelial cells (TEC) undergo endoreplication-mediated hypertrophy after AKI. Endoreplications are incomplete cell cycles that lead to the formation of polyploid cells. Physiologically, polyploidy offers several advantages such as rapid adaptation to stress, compensation for cell loss and enhanced cell function. However, as renal epithelial cells are massively lost after AKI, TEC polyploidy may constitute an effective strategy to sustain a temporary functional recovery of the kidney without restoring tissue integrity potentially leading to CKD. Therefore, we hypothesized that: 1) polyploid TEC are an adaptive stress response required to maintain kidney function after AKI; 2) polyploid TEC are involved in the AKI to CKD progression. Method To address these hypotheses, we employed a series of in vitro and in vivo transgenic models based on the Fluorescence Ubiquitin Cell Cycle Indicator (FUCCI) technology to monitor cell cycle phasing in combination with YAP1 overexpression or downregulation. In the in vivo models, YAP1 overexpressing mice and YAP1 knock-out mice were subjected to unilateral ischemia reperfusion injury (IRI) or glycerol-induced rhabdomyolysis to induce AKI. Polyploid cells have been then characterized by single cell-RNA sequencing analysis, cell sorting, super-resolution STED microscopy and transmission electron microscopy in both mouse and human. Results In vitro, human renal tubular cells undergo polyploidization. The fraction of polyploid cells significantly decreases when YAP1 nuclear translocation is blocked, indicating a possible involvement of YAP1 in regulating TEC polyploidy. After AKI in mice, YAP1 expression and nuclear translocation are significantly enhanced. The inhibition of YAP1 following AKI, reduces the number of polyploid cells impairing kidney function and causing a dramatic reduction of mouse survival. In contrast, YAP1 overexpression leads to an increase in the number of polyploid cells even in the absence of kidney damage (healthy mice). Strikingly, these healthy mice, despite having an increased percentage of polyploid cells, present an unexpected decline of renal function suggesting an association between increased polyploidy and CKD development. Indeed, they develop tubulointerstitial fibrosis acquiring a marked senescent phenotype triggering CKD. Isolation of polyploid cells proved that these cells actively transcribe and secrete pro-fibrotic factors thus confirming their role in CKD progression. Conclusion Collectively, these data suggest that: 1) polyploidization after AKI is required to maintain kidney function allowing survival; 2) polyploid cells are pro-fibrotic leading in the long run to CKD progression.

FC 041TUBULAR EPITHELIAL CELL POLYPLOIDY IS ESSENTIAL TO SURVIVE AKI BUT IT CONTRIBUTES TO CKD PROGRESSION