Uricase-deficient rats with similarly stable serum uric acid to human’s are sensitive model animals for studying hyperuricemia

Uric acid reduces the expression of aquaporins in renal collecting ducts to increase urine output in hyperuricemia

Background

Hyperuricemia (HUA) has attracted wide attention due to its close relationship with gout, hypertension, hypertriglyceridemia, obesity, atherosclerotic heart disease, type 2 diabetes and chronic kidney disease. Clinical observations suggest that people with high levels of serum uric acid (sUA) exhibits impaired urine concentration. We speculate that UA may regulate the expression of AQPs through inflammatory pathways, resulting in impaired renal urine concentration.

Methods and results

We revealed that patients and mice with HUA had a polyuria phenotype and found that the expression of aquaporin 2 (AQP2), AQP3 and AQP4 were significantly reduced in the kidneys of mice with HUA. Similarly, uric acid (UA) treatment markedly suppressed the expression of AQP2, AQP3 and AQP4 in cultured inner medullary collecting duct cells (IMCDs). We observed an increased expression of NF-κB in the kidneys of mice with HUA and in the IMCD cells treated with UA. Blockade of NF-κB by its inhibitor Bay 11-7082 dramatically attenuated UA-suppressed expression of AQP2, AQP3 and AQP4. Furthermore, the luciferase reporter, CHIP and EMSA assays showed that NF-κB can directly bind to the promoter regions of AQP2, AQP3 and AQP4 genes to suppress their transcription.

Conclusion

Our findings demonstrate that UA reduces the expression of AQP2, AQP3 and AQP4 in an NFκB-dependent manner, which contributes to the polyuria phenotype in the subjects with HUA.

Progress in modeling avian hyperuricemia and gout (Review)

Human organ tissue is vulnerable to hyperuricemia (HUA), which negatively impacts quality of life, particularly when it progresses to gout. Chicken uric acid formation and metabolism are similar to human uric acid metabolism; therefore, theoretically, the genesis and progression of human HUA and gout may be similar to those of poultry models. The present review explored HUA and gout and the progress of poultry-induced HUA and gout models. The present study reviewed procedures of modelling chicken gout and HUA and the detection indices and current concerns regarding these models. Notably, In the production of poultry hyperuricemia model, the combined method of water and food induction has a higher success rate and stability. Compared with mice induced HUA and gout models, poultry induced HUA and gout models had less kidney damage, and the models were stable and long-lasting.

Raising serum uric acid with a uricase inhibitor worsens PKD in rat and mouse models

Humans are predisposed to gout because they lack uricase that converts uric acid to allantoin. Rodents have uricase, resulting in low basal serum uric acid. A uricase inhibitor raises serum uric acid in rodents. There were two aims of the study in polycystic kidney disease (PKD): 1) to determine whether increasing serum uric acid with the uricase inhibitor, oxonic acid, resulted in faster cyst growth and 2) to determine whether treatment with the xanthine oxidase inhibitor, oxypurinol, reduced the cyst growth caused by oxonic acid. Orthologous models of human PKD were used: PCK rats, a polycystic kidney and hepatic disease 1 (Pkhd1) gene model of autosomal recessive PKD (ARPKD) and Pkd1RC/RC mice, a hypomorphic Pkd1 gene model. In PCK rats and Pkd1RC/RC mice, oxonic acid resulted in a significant increase in serum uric acid, kidney weight, and cyst index. Mechanisms of increased cyst growth that were investigated were proinflammatory cytokines, the inflammasome, and crystal deposition in the kidney. Oxonic acid resulted in an increase in proinflammatory cytokines in the serum and kidney in Pkd1RC/RC mice. Oxonic acid did not cause activation of the inflammasome or uric acid crystal deposition in the kidney. In Pkd1RC/RC male and female mice analyzed together, oxypurinol decreased the oxonic acid-induced increase in cyst index. In summary, increasing serum uric acid by inhibiting uricase with oxonic acid results in an increase in kidney weight and cyst index in PCK rats and Pkd1RC/RC mice. The effect is independent of inflammasome activation or crystal deposition in the kidney.NEW & NOTEWORTHY This is the first reported study of uric acid measurements and xanthine oxidase inhibition in polycystic kidney disease (PKD) rodents. Raising serum uric acid with a uricase inhibitor resulted in increased kidney weight and cyst index in Pkd1RC/RC mice and PCK rats, elevated levels of proinflammatory cytokines in the serum and kidney in Pkd1RC/RC mice, and no uric acid crystal deposition or activation of the caspase-1 inflammasome in the kidney.

Signaling pathways in uric acid homeostasis and gout: From pathogenesis to therapeutic interventions

Uric acid is a product of purine degradation, and uric acid may have multiple physiologic roles, including the beneficial effects as an antioxidant and neuroprotector, maintenance of blood pressure during low salt ingestion, and modulation of immunity. However, overproduction of metabolic uric acid, and/or imbalance of renal uric acid secretion and reabsorption, and/or underexcretion of extrarenal uric acid, e.g. gut, will contribute to hyperuricemia, which is a common metabolic disease. Long-lasting hyperuricemia can induce the formation and deposition of monosodium urate (MSU) crystals within the joints and periarticular structures. MSU crystals further induce an acute, intensely painful, and sterile inflammation conditions named as gout by NLRP3 inflammasome-mediated cleavage of pro-IL-1β to bioactive IL-1β. Moreover, hyperuricemia and gout are associated with multiple cardiovascular and renal disorders, e.g., hypertension, myocardial infarction, stroke, obesity, hyperlipidemia, type 2 diabetes mellitus and chronic kidney disease. Although great efforts have been made by scientists of modern medicine, however, modern therapeutic strategies with a single target are difficult to exert long-term positive effects, and even some of these agents have severe adverse effects. The Chinese have used the ancient classic prescriptions of traditional Chinese medicine (TCM) to treat metabolic diseases, including gout, by multiple targets, for more than 2200 years. In this review, we discuss the current understanding of urate homeostasis, the pathogenesis of hyperuricemia and gout, and both modern medicine and TCM strategies for this commonly metabolic disorder. We hope these will provide the good references for treating hyperuricemia and gout.

Genes Selectively Expressed in Rat Organs

Background

Understanding organic functions at a molecular level is important for scientists to unveil the disease mechanism and to develop diagnostic or therapeutic methods.

Aims

The present study tried to find genes selectively expressed in 11 rat organs, including the adrenal gland, brain, colon, duodenum, heart, ileum, kidney, liver, lung, spleen, and stomach.

Materials and Methods

Three normal male Sprague-Dawley (SD) rats were anesthetized, their organs mentioned above were harvested, and RNA in the fresh organs was extracted. Purified RNA was reversely transcribed and sequenced using the Solexa high-throughput sequencing technique. The abundance of a gene was measured by the expected value of fragments per kilobase of transcript sequence per million base pairs sequenced (FPKM). Genes in organs with the highest expression level were sought out and compared with their median value in organs. If a gene in the highest expressed organ was significantly different (p < 0.05) from that in the medianly expressed organ, accompanied by q value < 0.05, and accounted for more than 70% of the total abundance, the gene was assumed as the selective gene in the organ.

Results & Discussion

The Kyoto Encyclopedia of Genes and Genomes (KEGG), and Gene Ontology (GO) pathways were enriched by the highest expressed genes. Based on the criterion, 1,406 selective genes were screened out, 1,283 of which were described in the gene bank and 123 of which were waiting to be described. KEGG and GO pathways in the organs were partly confirmed by the known understandings and a good portion of the pathways needed further investigation.

Conclusion

The novel selective genes and organic functional pathways are useful for scientists to unveil the mechanisms of the organs at the molecular level, and the selective genes' products are candidate disease markers for organs.

Dieting alleviates hyperuricemia and organ injuries in uricase-deficient rats via down-regulating cell cycle pathway

Dieting is a basic treatment for lowering hyperuricemia. Here, we aimed to determine the optimal amount of dietary food that lowers serum uric acid (SUA) without modifying the dietary ingredients in rats. Increased SUA was found in food-deprived 45-day-old uricase-deficient rats (Kunming-DY rats), and the optimal amount of dietary food (75% dietary intake) to lower SUA was established by controlling the amount of food given daily from 25% to 100% for 2 weeks. In addition to lowering SUA by approximately 22.5 ± 20.5%, the optimal amount of dietary food given for 2 weeks inhibited urine uric acid excretion, lowered the uric acid content in multiple organs, improved renal function, lowered serum triglyceride, alleviated organ injuries (e.g., liver, kidney and intestinal tract) at the histological level, and down-regulated the Kyoto Encyclopedia of Genes and Genome (KEGG) pathway of the cell cycle (ko04110). Taken together, these results demonstrate that 75% dietary food effectively lowers the SUA level without modifying dietary ingredients and alleviates the injuries resulting from uricase deficiency or hyperuricemia, the mechanism of which is associated with the down-regulation of the cell cycle pathway.

Uricase-Deficient Larval Zebrafish with Elevated Urate Levels Demonstrate Suppressed Acute Inflammatory Response to Monosodium Urate Crystals and Prolonged Crystal Persistence

Gout is caused by elevated serum urate leading to the deposition of monosodium urate (MSU) crystals that can trigger episodes of acute inflammation. Humans are sensitive to developing gout because they lack a functional urate-metabolizing enzyme called uricase/urate oxidase (encoded by the UOX gene). A hallmark of long-standing disease is tophaceous gout, characterized by the formation of tissue-damaging granuloma-like structures ('tophi') composed of densely packed MSU crystals and immune cells. Little is known about how tophi form, largely due to the lack of suitable animal models in which the host response to MSU crystals can be studied in vivo long-term. We have previously described a larval zebrafish model of acute gouty inflammation where the host response to microinjected MSU crystals can be live imaged within an intact animal. Although useful for modeling acute inflammation, crystals are rapidly cleared following a robust innate immune response, precluding analysis at later stages. Here we describe a zebrafish uox null mutant that possesses elevated urate levels at larval stages. Uricase-deficient 'hyperuricemic' larvae exhibit a suppressed acute inflammatory response to MSU crystals and prolonged in vivo crystal persistence. Imaging of crystals at later stages reveals that they form granuloma-like structures dominated by macrophages. We believe that uox^-/- larvae will provide a useful tool to explore the transition from acute gouty inflammation to tophus formation, one of the remaining mysteries of gout pathogenesis.