title: Identifying genetic loci for metabolic disorders affecting the renal tract
Creators: name: Ghimire, Saurav
Publisher: name: University of Glasgow
Year: 2019
Subjects: subject: Kidney stones, calcium oxalate monohydrate, stone accumulation, temperature, Sip1, NaPi-T.

description: Introduction and Objectives: Nephrolithiasis is one of the most common renal diseases, but with poorly understood pathophysiology. The current understanding of how genetic, environmental, and metabolic factors act singly or in concert to trigger stone formation remains patchy, and the progress of medical therapy has been very modest. Vertebrate models for nephrolithiasis are limited in their ability to rapidly screen multiple and varied interventions that modulate urinary stone formation. Therefore, we hypothesis that the basic research directed at model systems that elucidate the pathophysiology of stone disease is the best hope for advancing the field and leading to the development of new therapeutic approaches that have the potential to reduce the morbidity, mortality, and cost associated with this disease. Further, most studies have confounded that age and temperature share some contribution in precipitation of kidney stones. We also hypothesise that change in temperature (from colder to warmer, low temperature to higher temperature) may have a greater impact on the formation rate of nephrolithiasis. Methods: A short lifespan, rapid deployment of transgenic lines and conception of rapid stone formation makes Drosophila melanogaster an ideal system to screen large numbers of interventions to assess their effect on kidney stone formation. Knockdown of gene candidates for their ability to vary the formation of urinary stones was employed utilising Drosophila. Microdissection, imaging and quantification facilitate identification and collection of fly stones within the lumen of the Drosophila Malpighian tubules (the functional equivalent of the human renal tubule). Further, UAS-RNAi knockdown efficiency was validated by qPCR . The genes which upon knockdown modulated the concentration of accumulated stones were further studied using various genetic, immunostaining and molecular techniques. In addition to that I also identified role of temperature and age in kidney stone formation by rearing flies at 18, 22, 26 and 29 for different time frame (day 7, day 14, day 21 and day 28) and quantifying accumulated stones during that period. The stones obtained were further validated by genetic, temperature switch, colorimetric and biochemical assays. Results: Calcium oxalate stone formation was associated with the stones accumulated intraluminally in parental lines and gene knockdown or mutant panel of genes fed with sodium oxalate (0.2% for 2 days). An unbiased knockdown screen of more than 20 RNAi lines was performed utilising the GAL4-UAS RNAi system and identified 8 genes that rescued kidney stone accumulation compared to parental lines. Microdissection and microscopy confirmed that five genes decreased stone formation in the fly, including genes Waterwitch (Wtrw), Serine Pyruvate Amino Transferase (Spat), Cinnamon (Cin), Na+-dependent inorganic Phosphate Co-transporter (NaPi-T), Sarcoplasmic Calcium-binding Protein 2 (Scp2) and Secretory Pathway Ca2+/Mn2+-ATPase (SPoCK). From the hits, I showed that mutation of SRY interacting protein (Sip1) in Drosophila MTs Stellate cells resulted in the accumulation of uric acid stones with a particularly notable interaction with Moesin and NHE2. I also demonstrated a combined impact of age and temperature in stone formation. Critically, I confirmed that they cause an accumulation of stones due to variation in expression of phosphate transporters NaPi-T and Picot. Our work also provides more precise insight into the impact of age and temperature in stone formation, given that we have shown that stone accumulation increases with age and in flies reared at high temperature compared to flies reared at a lower temperature. Conclusions: A Drosophila urinary stone model was leveraged to perform large scale genetic screens to identify novel genes that modulate calculi formation. Our hits may now be screened as a candidate gene in future cases with a clinical presentation. Our study documents the first case of uric-acid stone formation caused by mutations of a gene using Drosophila as a model organism. Confirmation of these mutations as a causative factor and that the stones accumulated were uric acid stones was done by genetic, molecular and physiological experiments.