Project Summary Uric Acid (UA) nephrolithiasis constitutes 8-10% of kidney stones in the United States. Our group has previously characterized several distinct features of idiopathic UA nephrolithiasis (IUAN): (1) The principal underlying abnormality is an unduly acidic urine which increases the risk of UA precipitation. (2) The low urine pH is due to the combined effect of increased net acid excretion (NAE) and reduced ammonium (NH4+) excretion. (3) Patients with type 2 diabetes and/or obesity are at increased risk for IUAN. Our preliminary data suggest that (1) renal fat accumulation (steatosis) occurs in humans, and is associated with low NH4+/NAE, (2) steatosis in an animal model is associated with aciduria and impaired NH4+ excretion, features of IUAN, (3) steatosis in a cell culture model results in lipotoxicity which manifests as reduced NH4+ secretion, and (4) low urinary pH is necessary but not sufficient for UA stone formation, suggesting the presence of unknown promoters or the absence of inhibitors of UA crystallization. We hypothesize that three defects are present in UA stone formers: (1) Steatosis of the kidney which impairs renal NH4+ excretion, resulting in an acidic urine at any given acid load. (2) Increased endogenous organic acid production and NAE. (3) In IUAN patients, the lack of a urinary inhibitor and/or the presence of a promoter may additionally account for UA crystallization. Aim 1 of this proposal will determine the functional consequences of renal steatosis by correlating kidney fat content with urinary acid-base parameters in human subjects and in animal models of generalized and kidney- specific lipotoxicity. Aim 2 will evaluate the outcome of reversing renal steatosis in humans, animals, and cell culture systems. Aim 3 will characterize the urinary physicochemical background accounting for the formation of UA stones in IUAN patients using classical physicochemical techniques. This proposal lays the foundation for the novel concept of renal lipotoxicity, identify its pathophysiologic role in uric acid stone formation, characterize the urinary physicochemical background accounting for the formation of uric acid stones, and open new avenues for improved diagnosis and treatment of uric acid nephrolithiasis.