Betaine insufficiency and reduced betaine dependent remethylation are associated with hyperhomocysteinemia, oxidative stress and mitochondrial damage in early Polycystic Kidney Disease

Abstract

Polycystic kidney disease (PKD) is the most common monogenic kidney disorder and an important cause of end stage renal disease. Vascular abnormalities are the most important non‐cystic complication and contribute to renal disease progression. Homocysteine (Hcy)‐mediated endothelial dysfunction (ED) and oxidative stress are evident early on, but the underlying mechanisms remain unknown. We hypothesized that abnormalities in Hcy metabolism are responsible for the increased levels, leading to Hcy‐induced ED from early stages of PKD.MethodsWe investigated the Hcy metabolic pathway (transulfuration/remethylation fig. 1) and downstream effects ‐main sources of renal tubular (RT) and endothelial cell (EC) reactive oxygen species (ROS), NADPH oxidase 4, (NOX4) and mitochondria‐ in 4‐week‐old PCK and Sprague‐Dawley (SD) control rats (n=12 each). Kidneys were harvested, frozen in liquid nitrogen, or preserved in formalin for metabolomics and ex‐vivo studies. Twenty‐four‐hour urine and terminal blood samples were collected for metabolite analysis and chemistries. Plasma folic acid, vitamin B12, and Hcy were determined by ELISA. Hcy pathway metabolites (betaine, dimethylglycine) were determined by 1HNMR. Endothelial NO (eNOS) and oxidative stress were assessed by double immunofluorescence staining for CD31/eNOS and CD31/NOX4, respectively. RT and EC mitochondrial structure was assessed by electron microscopy.ResultsSerum creatinine and BUN remained unchanged, yet kidney weight/body weight ratio were increased in PCK. eNOS immunoreactivity was lower in PCK vs SD. Plasma Hcy and urine excretion of methyl donor betaine were elevated in PCK, and Hcy positively correlated with urine betaine. Plasma betaine was similar between the groups, but its tissue concentration was lower in PCK, which may lead to betaine insufficiency, impaired betaine dependent remethylation, and hyperhomocysteinemia. Contrarily, tissue glutathione concentration was higher in PCK vs SD, and plasma folate and vitamin B12 were similar, arguing against a defect in the transsulfuration or folate‐dependent remethylation pathways. RT NOX4 expression and immunoreactivity were increased in PCK compared to SD, in association with renal oxidative stress. PCK had increased number of CD31/NOX4 positive cells in peritubular capillaries, indicating increased EC ROS production. Collecting duct and EC mitochondria were swollen and showed cristae remodeling.ConclusionIn PKD, early elevation in Hcy is likely related to betaine insufficiency and impaired betaine‐dependent remethylation may be responsible for the early elevation of Hcy. NOX4 and mitochondria abnormalities may be important contributors to oxidative stress and disease severity. Our data suggests these may not only be early vascular disease markers, but also play an important pathogenic role in vascular complications associated with PKD, and contribute to renal disease progression.Support or Funding InformationR25‐DK101405, NIDDK P30 DK090728, NIDDK P30 DK 106912This abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.