Age-Related Adenosine Triphosphate (ATP) Increase in Proximal Tubules and Its Potential Effect on AKI to CKD Transition

Background: The development of effective treatment for the transition of acute kidney injury (AKI) to chronic kidney disease (CKD) remains a significant challenge. We previously reported that ATP dynamics in proximal tubules (PTs) serve as a predictor of kidney fibrosis in murine models. Given that elderly patients constitute a substantial proportion of AKI patients, we focused on ATP metabolisms in aged kidney and its impact on AKI to CKD transition. Methods: In vivo ATP imaging was conducted in the kidneys of young (8-12 weeks old) and aged (>24 months old) mice that systemically express FRET-based ATP biosensor (GO-ATeam2 mice) using multiphoton microscopy, both prior to and during ischemia reperfusion injury (IRI). Ex-vivo ATP imaging was performed on kidney slices of GO-ATeam2 mice using multiphoton microscopy. Pathological changes in the kidneys were assessed on day1 and on day14 after IRI, and correlation between ATP levels in PTs and severities of these pathological changes were assessed. Results: In vivo ATP imaging in healthy condition revealed significant increase in ATP levels in PTs, distal tubules, and collecting ducts of aged kidneys. This elevation in proximal tubular ATP was also observed in ex-vivo ATP imaging, suggesting that the increase is independent of systemic regulation, such as blood components and blood pressure. Although ATP dynamics in PTs during IRI were not significantly different between two groups, baseline ATP levels in PTs were significantly correlated with Hvcr1 (coding KIM1, PT injury marker) expression on day 1. To assess the impact of increased basal ATP in aged PTs on AKI to CKD transition, pathological changes on day14 after IRI were assessed. Histological analysis showed that aged kidneys exhibited higher percentages of injured tubules and smaller fibrotic region compared to young kidneys, indicating impaired recovery processes from acute injury. Interestingly, basal ATP levels in PTs were inversely correlated with the expressions of fibrotic markers, including Acta2, Col1a1, and Fn1. Additionally, the expression of Emr1, a macrophage maker, was strongly correlated with fibrotic marker expressions and inversely correlated with basal ATP levels in PTs. Conclusion: Aged kidneys demonstrated elevated proximal tubular ATP levels, which were associated with impaired recovery processes following IRI. Funding: Government Support – Non-U.S.