Targeted renal knockdown of Na/H exchanger regulatory factor produces uric acid nephrolithiasis in .

Nephrolithiasis is one of the most common kidney diseases, with poorly understood pathophysiology, but experimental study has been hindered by lack of experimentally tractable models. is a useful model organism for renal diseases because of genetic and functional similarities of Malpighian (renal) tubules with the human kidney. Here, we demonstrated function of the sex-determining region Y protein-interacting protein-1 () gene, an ortholog of human Na/H exchanger regulatory factor (), in Malpighian tubules and its impact on nephrolithiasis. Abundant birefringent calculi were observed in mutant flies, and the phenotype was also observed in renal stellate cell-specific RNA interference knockdown in otherwise normal flies, confirming a renal etiology. This phenotype was abolished in mutant flies (which model human xanthinuria) and by the xanthine oxidase inhibitor allopurinol, suggesting that the calculi were of uric acid. This was confirmed by direct biochemical assay for urate. Stones rapidly dissolved when the tubule was bathed in alkaline media, suggesting that knockdown was acidifying the tubule. SIP1 was shown to collocate with Na/H exchanger isoform 2 (NHE2) and with moesin in stellate cells. Knockdown of specifically to the stellate cells also increased renal uric acid stone formation, and so a model was developed in which SIP1 normally regulates NHE2 activity and luminal pH, ultimately leading to uric acid stone formation. renal tubules may thus offer a useful model for urate nephrolithiasis.