Published on 19 May 2014
ORLANDO, FL USA (UroToday.com) - Uric acid (UA), and its physiological relevant anion urate, represent the final breakdown products of the purine degradation pathway in humans. This is because humans lack the uricase enzyme. Although human kidneys evolved elaborate mechanisms to reabsorb 90% of the filtered UA, the benefits of high UA levels remain controversial. Both, the efficient UA reabsorption and the missing UA degradation can result in gouty diatheses, with or without urinary stones. The worldwide prevalence of UA kidney stones in humans ranges from 10% in Northern America and up to 40% in Israel. The research group developed a uricase knock down model of the fruit fly Drosophila melanogaster in order to study the cause, effect and treatment of UA urolithiasis.
An uricase knock down (suppression) model of Drosophila melanogaster exploiting the established UAS/GAL4 system was developed. The qRT-PCR technique was used to verify knockdown efficiency of the RNAi and to determine parallel changes of gene expression, e.g., inflammation related genes. Dissections were performed after flies were fed defined dietary conditions with either high or low protein, purines, or sugar. Uric acid stones were visualized and quantified in the gut and Malpighian tubules of dissected animals. In addition to regular measurement of the fly weight, two spectroscopic assays were developed to determine total UA and triglyceride levels. Lipid (fat droplet accumulation) was also documented microscopically.
Analogous to humans, the RNA interference mediated depletion of Drosophila uricase (≥ 95 %) prevents further degradation of UA into allantoin and quintupled the systemic UA levels. In addition, 85% of uricase knockdown flies fed a protein rich diet developed concretions in their gut as verified by dissection. Systemic increase in UA levels was associated with a threefold increase in triglyceride and fat droplet accumulation in the genetically manipulated animals. Allopurinol (xanthine oxidase inhibitor), on the other hand, dramatically decreased stone formation in the uricase knockdown flies by 80%.
The research group showed that uric acid calculi develop rapidly (10 – 14 d) in a Drosophila uricase knock down fly model and stone development could be reduced with allopurinol treatment. Given the functional homology of the human kidney and the fly Malpighian tubule, this novel fly model allows to recreate the process of UA-based stone formation. The parallel increase of systemic UA and triglyceride levels links the disease process to obesity and the metabolic syndrome.
Presented by Sven Lang, MD at the American Urological Association (AUA) Annual Meeting - May 16 - 21, 2014 - Orlando, Florida USA
San Francisco, CA USA
Written by Achim Lusch, MD, University of California (Irvine), and medical writer for UroToday.com