Uric acid urolithiasis composes up to about 10% of kidney stones and is increasing in prevalence contemporaneously with the escalating prevalence of obesity, metabolic syndrome, and diabetes. Uric acid urolithiasis is the renal manifestation of the multi-systemic disturbances in obesity, metabolic syndrome and diabetes. The primary pathophysiologic feature of uric acid urolithiasis is excessive aciduria. We propose a multi-organ pathogenic model of uric acid urolithiasis traceable to intestinal organic acid generation by the gut microbiota (totality of the microbial fauna) that escapes complete hepatic metabolism due to subtle defects in the liver from steatosis, which then imposes an increased acid load on the kidneys. Defective ammoniagenesis in the kidney from renal steatosis mandates excretion of the excess acid carried by alternative buffers including urate which ultimately leads to uric acid precipitation and stones. In this model, the first step in uric acid urolithiasis starts in the gut and liver. This proposal will use a combination of animal and human studies to test the hypothesis of the gut and liver as pathogenic origin of uric acid kidney stones. We propose three Aims to address: 1. Microbiota-phenotype association. Microbiome (microbial genome) from uric acid stone formers, obese, and lean non-stone-formers will be correlated with plasma and urinary parameters. 2. Microbiota-phenotype causation. Causality will be tested by transplantation of the microbiota from the three groups of humans from Aim 1 into germ-free mice and we will examine whether the appropriate phenotype is conferred. 3. Microbiota-host interaction. We will test the hypothesis that increased acid production from the microbiota is necessary but not sufficient to confer the disease phenotype. Additional host factors need to be invoked; specifically impaired hepatic metabolism of the lumen-derived organic acids from fatty infiltration of hepatocytes. We will use both animals and humans for all Aims drawing expertise from a diverse synergistic team of human researchers, animal pathophysiologists, microbiota biologists, and bioinformaticians to test our model. These experiments are the first to explore the very origin of the acid load in uric acid urolithiasis, and take the study and treatment of uric acid urolithiasis from empirical management by urinary alkalinization to a genuine and thorough elucidation of its pathophysiology, and subsequent definitive therapy. The studies also open up novel lines of investigation of the pathobiology of microbiota-host interaction in the metabolic syndrome.
Uric acid kidney stones are increasing in frequency and occurs almost exclusively in subjects with obesity and diabetes constituting a formidable health care burden, and we propose that uric acid kidney stones is one way by which obesity and diabetes manifest themselves in the kidney. Bacteria in the intestine produces excessive amount of acid and because of liver problems in obesity and diabetes, these acids escape breakdown by the liver and present themselves to the kidney, thus altering the urine chemistry to promote formation of uric acid kidney stones. We will explore this model using a combination of human and animal studies that can potentially lead to new therapies for uric acid kidney stones and new insight into how gut bacteria interact with humans.