#4344 CHANGES IN ENERGY METABOLISM IN THE KIDNEY WITH TUBULOINTERSTITIAL FIBROSIS: AN EX VIVO WHOLE TISSUE 2D-COSY APPROACH
Authors: Humphries Tyrone , Gobe Glenda , Urquhart Aaron , Vesey David , Ellis Robert , Galloway Graham , Francis Ross 
Published: June 2023

Abstract

Background and Aims
Tubulointerstitial fibrosis is the common pathological manifestation in chronic kidney disease (CKD) and can indicate whether a patient will progress to kidney failure. The metabolic consequences of developing fibrosis are not well understood. This project aimed to investigate metabolic changes in the kidney cortex using an adenine-diet murine model of progressive CKD.

Method
C57Bl/6J mice were treated with an adenine-supplemented diet (0.2% adenine) or control diet for eight weeks. At the end of treatment, kidney function was assessed (serum creatinine, proteinuria), animals were euthanised, and the kidneys removed. A transverse section of the kidney was prepared for histology. Whole kidney cortical tissue was analysed using two-dimensional correlated spectroscopy (2D-COSY), a form of proton nuclear magnetic resonance (NMR) spectroscopy. Biochemical species, including metabolites and lipids, within the tissue were identified and compared with respective histological tubulointerstitial fibrosis level and traditional serum markers of kidney function, plasma creatinine and blood urea nitrogen. A 2D-COSY spectrum can be found in Figure 1A.

Results
The adenine-supplemented diet induced tubulointerstitial fibrosis, mirrored by increases in serum creatinine and blood urea nitrogen, indicating decreased kidney function. The 2D-COSY analysis demonstrated numerous changes in metabolites with parallel increasing levels of fibrosis that were detected. Increases in metabolites belonging to the glycolysis (D-glucose, glucose-6-phosphate and lactate) and pentose-phosphate (myo-inositol, D-ribose-5-phosphate, glucuronic acid) pathways were observed, as well as a decrease in lipid. Additionally, increases in creatinine, creatine and a number of amino acids were also observed. These changes are displayed in Table 1 and Figure 1B.

Conclusion
The primary cell type of the kidney cortex is the proximal tubular epithelial cell. Due to its unique function in water and solute transport, this cell has a very high metabolic demand and utilizes fatty acid oxidation as its primary source of energy. However, when these cells are under pathological stress that contributes to tubulointerstitial fibrosis, such as those present within this animal model of chronic kidney disease, metabolic dysfunction occurs. We observed significant decreases in various lipid signals belonging to fatty acids within the cell. These fatty acids are not being utilised and are instead esterified to triglycerides that accumulate in the kidney, causing lipotoxicity. Increased levels of glucose-6-phosphate, lactate and D-glucose, as well as increased myo-inositol, glucuronic acid D-ribose-5-phosphate are indicative of increased utilisation of the glycolysis and pentose phosphate pathway respectively as primary methods of energy metabolism. Here, we provide evidence that in tubulointerstitial fibrosis within the kidneys leads to a shift in the primary energy pathway used by proximal tubular epithelial cells. The study provides new insights into the changes in biochemical pathways that occur in the kidney during the development of CKD and its progression.