Establishment and analysis of SLC22A12 (URAT1) knockout mouse

A System for Discovering Novel Uricosurics Targeting Urate Transporter 1 Based on In Vitro and In Vivo Modeling

Hyperuricemia has become a global burden with the increasing prevalence and risk of associated metabolic disorders and cardiovascular diseases. Uricosurics act as a vital urate-lowering therapy by promoting uric acid excretion via the kidneys. However, potent and safe uricosurics are still in urgent demand for use in the clinic. In this study, we aimed to establish in vitro and in vivo models to aid the discovery of novel uricosurics, and to search for potent active compounds, especially targeting urate transporter 1 (URAT1), the major urate transporter in the kidney handling uric acid homeostasis. As a result, for preliminary screening, the in vitro URAT1 transport activity was assessed using a non-isotopic uric acid uptake assay in hURAT1-stably expressed HEK293 cells. The in vivo therapeutic effect was evaluated in a subacute hyperuricemic mouse model (sub-HUA) and further confirmed in a chronic hyperuricemic mouse model (Ch-HUA). By utilizing these models, compound CC18002 was obtained as a potent URAT1 inhibitor, with an IC50 value of 1.69 μM, and favorable uric acid-lowering effect in both sub-HUA and Ch-HUA mice, which was comparable to that of benzbromarone at the same dosage. Moreover, the activity of xanthine oxidoreductase, the key enzyme catalyzing uric acid synthesis, was not altered by CC18002 treatment. Taken together, we have developed a novel screening system, including a cell model targeting URAT1 and two kinds of mouse models, for the discovery of novel uricosurics. Utilizing this system, compound CC18002 was investigated as a candidate URAT1 inhibitor to treat hyperuricemia.

Vitamin C transporter SVCT1 serves a physiological role as a urate importer: functional analyses and in vivo investigations

Uric acid, the end product of purine metabolism in humans, is crucial because of its anti-oxidant activity and a causal relationship with hyperuricemia and gout. Several physiologically important urate transporters regulate this water-soluble metabolite in the human body; however, the existence of latent transporters has been suggested in the literature. We focused on the Escherichia coli urate transporter YgfU, a nucleobase-ascorbate transporter (NAT) family member, to address this issue. Only SLC23A proteins are members of the NAT family in humans. Based on the amino acid sequence similarity to YgfU, we hypothesized that SLC23A1, also known as sodium-dependent vitamin C transporter 1 (SVCT1), might be a urate transporter. First, we identified human SVCT1 and mouse Svct1 as sodium-dependent low-affinity/high-capacity urate transporters using mammalian cell-based transport assays. Next, using the CRISPR-Cas9 system followed by the crossing of mice, we generated Svct1 knockout mice lacking both urate transporter 1 and uricase. In the hyperuricemic mice model, serum urate levels were lower than controls, suggesting that Svct1 disruption could reduce serum urate. Given that Svct1 physiologically functions as a renal vitamin C re-absorber, it could also be involved in urate re-uptake from urine, though additional studies are required to obtain deeper insights into the underlying mechanisms. Our findings regarding the dual-substrate specificity of SVCT1 expand the understanding of urate handling systems and functional evolutionary changes in NAT family proteins.

New insight into the management of renal excretion and hyperuricemia: Potential therapeutic strategies with natural bioactive compounds

Hyperuricemia is the result of increased production and/or underexcretion of uric acid. Hyperuricemia has been epidemiologically associated with multiple comorbidities, including metabolic syndrome, gout with long-term systemic inflammation, chronic kidney disease, urolithiasis, cardiovascular disease, hypertension, rheumatoid arthritis, dyslipidemia, diabetes/insulin resistance and increased oxidative stress. Dysregulation of xanthine oxidoreductase (XOD), the enzyme that catalyzes uric acid biosynthesis primarily in the liver, and urate transporters that reabsorb urate in the renal proximal tubules (URAT1, GLUT9, OAT4 and OAT10) and secrete urate (ABCG2, OAT1, OAT3, NPT1, and NPT4) in the renal tubules and intestine, is a major cause of hyperuricemia, along with variations in the genes encoding these proteins. The first-line therapeutic drugs used to lower serum uric acid levels include XOD inhibitors that limit uric acid biosynthesis and uricosurics that decrease urate reabsorption in the renal proximal tubules and increase urate excretion into the urine and intestine via urate transporters. However, long-term use of high doses of these drugs induces acute kidney disease, chronic kidney disease and liver toxicity. Therefore, there is an urgent need for new nephroprotective drugs with improved safety profiles and tolerance. The current systematic review summarizes the characteristics of major urate transporters, the mechanisms underlying the pathogenesis of hyperuricemia, and the regulation of uric acid biosynthesis and transport. Most importantly, this review highlights the potential mechanisms of action of some naturally occurring bioactive compounds with antihyperuricemic and nephroprotective potential isolated from various medicinal plants.

OAT10/SLC22A13 Acts as a Renal Urate Re-Absorber: Clinico-Genetic and Functional Analyses With Pharmacological Impacts

Dysfunctional missense variant of organic anion transporter 10 (OAT10/SLC22A13), rs117371763 (c.1129C>T; p.R377C), is associated with a lower susceptibility to gout. OAT10 is a urate transporter; however, its physiological role in urate handling remains unclear. We hypothesized that OAT10 could be a renal urate re-absorber that will be a new molecular target of urate-lowering therapy like urate transporter 1 (URAT1, a physiologically-important well-known renal urate re-absorber) and aimed to examine the effect of OAT10 dysfunction on renal urate handling. For this purpose, we conducted quantitative trait locus analyses of serum urate and fractional excretion of uric acid (FE_UA) using samples obtained from 4,521 Japanese males. Moreover, we performed immunohistochemical and functional analyses to assess the molecular properties of OAT10 as a renal urate transporter and evaluated its potential interaction with urate-lowering drugs. Clinico-genetic analyses revealed that carriers with the dysfunctional OAT10 variant exhibited significantly lower serum urate levels and higher FE_UA values than the non-carriers, indicating that dysfunction of OAT10 increases renal urate excretion. Given the results of functional assays and immunohistochemical analysis demonstrating the expression of human OAT10 in the apical side of renal proximal tubular cells, our data indicate that OAT10 is involved in the renal urate reabsorption in renal proximal tubules from urine. Additionally, we found that renal OAT10 inhibition might be involved in the urate-lowering effect of losartan and lesinurad which exhibit uricosuric effects; indeed, losartan, an approved drug, inhibits OAT10 more strongly than URAT1. Accordingly, OAT10 can be a novel potential molecular target for urate-lowering therapy.

Genetic Association Between SLC22A12 Variants and Susceptibility to Hyperuricemia: A Meta-Analysis

Aims: Gout is a form of inflammatory arthritis characterized by the deposition of monosodium urate crystals. An important risk factor for gout is hyperuricemia. The relationship between SLC22A12 gene variants and the susceptibility to hyperuricemia has been reported, but these findings have been inconsistent. Thus, we aimed to assess the relationship between SLC22A12 gene variants and hyperuricemia susceptibility through a meta-analysis. Methods: The meta-analysis was performed by searching PubMed, Embase, Web of Science, and Chinese National Knowledge Infrastructure (CNKI) databases. The relationship between hyperuricemia risk and the SLC22A12 rs11602903, rs524023, rs3825018, rs3825016, rs11231825, rs7932775, rs893006, and rs475688 variants was assessed by odds ratios and 95% confidence intervals. Results: In total, 20 eligible publications with 4817 cases and 6819 controls were included in the meta-analysis. Hyperuricemia risk was significantly associated with the SLC22A12 alleles rs3825018, rs7932775, and rs475688 under both the dominant and recessive models and with rs3825016 under the allelic and dominant models. Conclusions: Under the allelic model SLC22A12 rs3825018 and rs3825016 were risk factors for hyperuricemia and gout as was rs7932775 under dominant and recessive models, while the SLC22A12 rs475688 was protective against hyperuricemia under both dominant and recessive models.

Xanthine Oxidoreductase Inhibitors Suppress the Onset of Exercise-Induced AKI in High HPRT Activity Urat1-Uox Double Knockout Mice

BACKGROUND

Hereditary renal hypouricemia type 1 (RHUC1) is caused by URAT1/SLC22A12 dysfunction, resulting in urolithiasis and exercise-induced AKI (EIAKI). However, because there is no useful experimental RHUC1 animal model, the precise pathophysiologic mechanisms underlying EIAKI have yet to be elucidated. We established a high HPRT activity Urat1-Uox double knockout (DKO) mouse as a novel RHUC1 animal model for investigating the cause of EIAKI and the potential therapeutic effect of xanthine oxidoreductase inhibitors (XOIs).

METHODS

The novel Urat1-Uox DKO mice were used in a forced swimming test as loading exercise to explore the onset mechanism of EIAKI and evaluate related purine metabolism and renal injury parameters.

RESULTS

Urat1-Uox DKO mice had uricosuric effects and elevated levels of plasma creatinine and BUN as renal injury markers, and decreased creatinine clearance observed in a forced swimming test. In addition, Urat1-Uox DKO mice had increased NLRP3 inflammasome activity and downregulated levels of Na⁺-K⁺-ATPase protein in the kidney, as Western blot analysis showed. Finally, we demonstrated that topiroxostat and allopurinol, XOIs, improved renal injury and functional parameters of EIAKI.

CONCLUSIONS

Urat1-Uox DKO mice are a useful experimental animal model for human RHUC1. The pathogenic mechanism of EIAKI was found to be due to increased levels of IL-1β via NLRP3 inflammasome signaling and Na⁺-K⁺-ATPase dysfunction associated with excessive urinary urate excretion. In addition, XOIs appear to be a promising therapeutic agent for the treatment of EIAKI.

Modulation of Urate Transport by Drugs

BACKGROUND

Serum urate (SU) levels in primates are extraordinarily high among mammals. Urate is a Janus-faced molecule that acts physiologically as a protective antioxidant but provokes inflammation and gout when it precipitates at high concentrations. Transporters play crucial roles in urate disposition, and drugs that interact with urate transporters either by intention or by accident may modulate SU levels. We examined whether in vitro transporter interaction studies may clarify and predict such effects.

METHODS

Transporter interaction profiles of clinically proven urate-lowering (uricosuric) and hyperuricemic drugs were compiled from the literature, and the predictive value of in vitro-derived cut-offs like C_max/IC₅₀ on the in vivo outcome (clinically relevant decrease or increase of SU) was assessed.

RESULTS

Interaction with the major reabsorptive urate transporter URAT1 appears to be dominant over interactions with secretory transporters in determining the net effect of a drug on SU levels. In vitro inhibition interpreted using the recommended cut-offs is useful at predicting the clinical outcome.

CONCLUSIONS

In vitro safety assessments regarding urate transport should be done early in drug development to identify candidates at risk of causing major imbalances. Attention should be paid both to the inhibition of secretory transporters and inhibition or trans-stimulation of reabsorptive transporters, especially URAT1.

The Lactate Receptor HCA1 Is Present in the Choroid Plexus, the Tela Choroidea, and the Neuroepithelial Lining of the Dorsal Part of the Third Ventricle

The volume, composition, and movement of the cerebrospinal fluid (CSF) are important for brain physiology, pathology, and diagnostics. Nevertheless, few studies have focused on the main structure that produces CSF, the choroid plexus (CP). Due to the presence of monocarboxylate transporters (MCTs) in the CP, changes in blood and brain lactate levels are reflected in the CSF. A lactate receptor, the hydroxycarboxylic acid receptor 1 (HCA1), is present in the brain, but whether it is located in the CP or in other periventricular structures has not been studied. Here, we investigated the distribution of HCA1 in the cerebral ventricular system using monomeric red fluorescent protein (mRFP)-HCA1 reporter mice. The reporter signal was only detected in the dorsal part of the third ventricle, where strong mRFP-HCA1 labeling was present in cells of the CP, the tela choroidea, and the neuroepithelial ventricular lining. Co-labeling experiments identified these cells as fibroblasts (in the CP, the tela choroidea, and the ventricle lining) and ependymal cells (in the tela choroidea and the ventricle lining). Our data suggest that the HCA1-containing fibroblasts and ependymal cells have the ability to respond to alterations in CSF lactate in body-brain signaling, but also as a sign of neuropathology (e.g., stroke and Alzheimer's disease biomarker).

Sex Differences in Urate Handling

Hyperuricemia, or elevated serum urate, causes urate kidney stones and gout and also increases the incidence of many other conditions including renal disease, cardiovascular disease, and metabolic syndrome. As we gain mechanistic insight into how urate contributes to human disease, a clear sex difference has emerged in the physiological regulation of urate homeostasis. This review summarizes our current understanding of urate as a disease risk factor and how being of the female sex appears protective. Further, we review the mechanisms of renal handling of urate and the significant contributions from powerful genome-wide association studies of serum urate. We also explore the role of sex in the regulation of specific renal urate transporters and the power of new animal models of hyperuricemia to inform on the role of sex and hyperuricemia in disease pathogenesis. Finally, we advocate the use of sex differences in urate handling as a potent tool in gaining a further understanding of physiological regulation of urate homeostasis and for presenting new avenues for treating the constellation of urate related pathologies.

The SLC transporter in nutrient and metabolic sensing, regulation, and drug development

The prevalence of metabolic diseases is growing worldwide. Accumulating evidence suggests that solute carrier (SLC) transporters contribute to the etiology of various metabolic diseases. Consistent with metabolic characteristics, the top five organs in which SLC transporters are highly expressed are the kidney, brain, liver, gut, and heart. We aim to understand the molecular mechanisms of important SLC transporter-mediated physiological processes and their potentials as drug targets. SLC transporters serve as ‘metabolic gate’ of cells and mediate the transport of a wide range of essential nutrients and metabolites such as glucose, amino acids, vitamins, neurotransmitters, and inorganic/metal ions. Gene-modified animal models have demonstrated that SLC transporters participate in many important physiological functions including nutrient supply, metabolic transformation, energy homeostasis, tissue development, oxidative stress, host defense, and neurological regulation. Furthermore, the human genomic studies have identified that SLC transporters are susceptible or causative genes in various diseases like cancer, metabolic disease, cardiovascular disease, immunological disorders, and neurological dysfunction. Importantly, a number of SLC transporters have been successfully targeted for drug developments. This review will focus on the current understanding of SLCs in regulating physiology, nutrient sensing and uptake, and risk of diseases.

Xanthine oxidoreductase knockout mice with high HPRT activity were not rescued by NAD+ replenishment

Although xanthinuria is nonfatal in human, xanthine oxidoreductase knockout (Xor-KO) mice have only a short lifespan. Hypoxanthine phosphoribosyltransferase activity (HPRT) in human and wild mice is higher than in laboratory mice. The aim of this study was to investigate the underlying mechanisms that give rise to the longer lifespan of high-HPRT/Xor-KO mice. Before Xor-KO mice die, urinary excretion of hypoxanthine increased with a corresponding decrease in excretion of xanthine. The switch of excretion from xanthine to hypoxanthine might be a cause of death for Xor-KO mice, suggesting inhibition of NAD⁺-dependent IMP dehydrogenase. Because hypoxanthine inhibits the synthesis of nicotinamide mononucleotide (NMN), a precursor of NAD⁺, the accumulation of hypoxanthine in Xor-KO mice may cause a depletion in the levels of NAD⁺. Moreover, urinary excretion of urate in high-HPRT/Uox-KO/Xor-KO mice means urate derived from gut microbiota is absorbed by the intestine. Likewise, over excretion of oxypurine in mice may be caused by intestinal absorption of oxypurine. For NAD⁺ replenishment, oral supplementation with 1% L-tryptophan, an alternative precursor of NAD⁺, resulted in a recovery of body weight gain in high-HPRT/Uox-KO/Xor-KO mice. In conclusion, the death of Xor-KO mice by renal failure seems to be caused by a depletion in NAD⁺ levels due to the intracellular accumulation of hypoxanthine. NAD⁺ replenishment by oral supplementation of NMN or tryptophan was complicated by the effect of gut microbiota and failed to rescue high-HPRT/Xor-KO mice. The attenuation of intestinal absorption of oxypurines seems to be necessary to avoid hypoxanthine accumulation and over excretion of oxypurine.

Mouse models for human hyperuricaemia: a critical review

Hyperuricaemia (increased serum urate concentration) occurs mainly in higher primates, including in humans, because of inactivation of the gene encoding uricase during primate evolution. Individuals with hyperuricaemia might develop gout - a painful inflammatory arthritis caused by monosodium urate crystal deposition in articular structures. Hyperuricaemia is also associated with common chronic diseases, including hypertension, chronic kidney disease, type 2 diabetes and cardiovascular disease. Many mouse models have been developed to investigate the causal mechanisms for hyperuricaemia. These models are highly diverse and can be divided into two broad categories: mice with genetic modifications (genetically induced models) and mice exposed to certain environmental factors (environmentally induced models; for example, pharmaceutical or dietary induction). This Review provides an overview of the mouse models of hyperuricaemia and the relevance of these models to human hyperuricaemia, with an emphasis on those models generated through genetic modifications. The challenges in developing and comparing mouse models of hyperuricaemia and future research directions are also outlined.

Soluble Uric Acid Promotes Atherosclerosis via AMPK (AMP-Activated Protein Kinase)-Mediated Inflammation

OBJECTIVE

Uric acid is supposed but not yet determined to be associated with atherosclerosis. Uric acid is released from damaged cells to form urate crystal, which is recognized by the immune system to produce IL (interleukin)-1. Danger signals and IL-1 have been shown to play an important role in atherosclerosis. We determined whether the physiological level of soluble uric acid promotes inflammation and develops atherosclerosis. Approach and Results: The secretion of IL-1β from human peripheral blood mononuclear cells mediated by NLRP3 (NACHT, LRR, and PYD domain-containing protein 3) inflammasome was promoted by physiological levels in serum uric acid. This augmentation of inflammation was mediated by the regulation of the AMPK (AMP-activated protein kinase)-mTOR (mammalian target of rapamycin) mitochondrial reactive oxygen species and HIF-1α (hypoxia-inducible factor-1α) pathway. In both of uricase transgenic and xanthine oxidase inhibitor-treated mice, decreased levels of uric acid resulted in the activation of AMPK and attenuation of the development of atherosclerotic plaques. Further, acute uric acid reduction by the administration of benzbromarone in healthy humans for 2 weeks significantly decreased plasma IL-18-an inflammasome-dependent cytokine.

CONCLUSIONS

The data indicate that the development of atherosclerosis and inflammation is promoted by uric acid in vivo. Moreover, the lowering of uric acid levels attenuated inflammation via the activation of the AMPK pathway. This study provides mechanistic evidence of uric acid-lowering therapies for atherosclerosis.

Learning Physiology From Inherited Kidney Disorders

The identification of genes causing inherited kidney diseases yielded crucial insights in the molecular basis of disease and improved our understanding of physiological processes that operate in the kidney. Monogenic kidney disorders are caused by mutations in genes coding for a large variety of proteins including receptors, channels and transporters, enzymes, transcription factors, and structural components, operating in specialized cell types that perform highly regulated homeostatic functions. Common variants in some of these genes are also associated with complex traits, as evidenced by genome-wide association studies in the general population. In this review, we discuss how the molecular genetics of inherited disorders affecting different tubular segments of the nephron improved our understanding of various transport processes and of their involvement in homeostasis, while providing novel therapeutic targets. These include inherited disorders causing a dysfunction of the proximal tubule (renal Fanconi syndrome), with emphasis on epithelial differentiation and receptor-mediated endocytosis, or affecting the reabsorption of glucose, the handling of uric acid, and the reabsorption of sodium, calcium, and magnesium along the kidney tubule.

Insulin stimulates uric acid reabsorption via regulating urate transporter 1 and ATP-binding cassette subfamily G member 2

Accumulating data indicate that renal uric acid (UA) handling is altered in diabetes and by hypoglycemic agents. In addition, hyperinsulinemia is associated with hyperuricemia and hypouricosuria. However, the underlying mechanisms remain unclear. In this study, we aimed to investigate how diabetes and hypoglycemic agents alter the levels of renal urate transporters. In insulin-depleted diabetic rats with streptozotocin treatment, both UA excretion and fractional excretion of UA were increased, suggesting that tubular handling of UA is altered in this model. In the membrane fraction of the kidney, the expression of urate transporter 1 (URAT1) was significantly decreased, whereas that of ATP-binding cassette subfamily G member 2 (ABCG2) was increased, consistent with the increased renal UA clearance. Administration of insulin to the diabetic rats decreased UA excretion and alleviated UA transporter-level changes, while sodium glucose cotransporter 2 inhibitor (SGLT2i) ipragliflozin did not change renal UA handling in this model. To confirm the contribution of insulin in the regulation of urate transporters, normal rats received insulin and separately, ipragliflozin. Insulin significantly increased URAT1 and decreased ABCG2 levels, resulting in increased UA reabsorption. In contrast, the SGLT2i did not alter URAT1 or ABCG2 levels, although blood glucose levels were similarly reduced. Furthermore, we found that insulin significantly increased endogenous URAT1 levels in the membrane fraction of NRK-52E cells, the kidney epithelial cell line, demonstrating the direct effects of insulin on renal UA transport mechanisms. These results suggest a previously unrecognized mechanism for the anti-uricosuric effects of insulin and provide novel insights into the renal UA handling in the diabetic state.

Urat1-Uox double knockout mice are experimental animal models of renal hypouricemia and exercise-induced acute kidney injury

Renal hypouricemia (RHUC) is a hereditary disease characterized by a low level of plasma urate but with normal urinary urate excretion. RHUC type 1 is caused by mutations of the urate transporter URAT1 gene (SLC22A12). However, the plasma urate levels of URAT1 knockout mice are no different from those of wild-type mice. In the present study, a double knockout mouse, in which the URAT1 and uricase (Uox) genes were deleted (Urat1-Uox-DKO), were used as an experimental animal model of RHUC type 1 to investigate RHUC and excise-induced acute kidney injury (EIAKI). Mice were given a variable content of allopurinol for one week followed by HPLC measurement of urate and creatinine concentrations in spot urine and blood from the tail. The urinary excretion of urate in Urat1-Uox-DKO mice was approximately 25 times higher than those of humans. With allopurinol, the plasma urate levels of Urat1-Uox-DKO mice were lower than those of Uox-KO mice. There were no differences in the urinary urate excretions between Urat1-Uox-DKO and Uox-KO mice administered with 9 mg allopurinol /100 g feed. In the absence of allopurinol, plasma creatinine levels of some Urat1-Uox-DKO mice were higher than those of Uox-KO mice. Consequently, hypouricemia and normouricosuria may indicate that the Urat1-Uox-DKO mouse administered with allopurinol may represent a suitable animal model of RHUC type 1. Urat1-Uox-DKO mice without allopurinol exhibited acute kidney injury, thus providing additional benefit as a potential animal model for EIAKI. Finally, our data indicate that allopurinol appears to provide prophylactic effects for EIAKI.

Uricosuric agents decrease the plasma urate level in rats by concomitant treatment with topiroxostat, a novel xanthine oxidoreductase inhibitor

OBJECTIVES

The aim of this study was to establish the rat model for evaluating hypouricemic effects by some uricosuric agents.

METHODS

Rats were made hyperuricemic by subcutaneous administration of potassium oxonate, a uricase inhibitor, or made hypouricemic by oral administration of topiroxostat, a xanthine oxidoreductase inhibitor. Furthermore, rats were co-treated with topiroxostat and inosine, a urate precursor. In each condition, hypouricemic effects by uricosuric agents were examined.

KEY FINDINGS

In potassium oxonate-treated rats, treatment with uricosuric agents such as FYU-981, F12859 and probenecid showed no hypouricemic effect. On the other hand, in topiroxostat-treated rats, uricosuric agents remarkably lowered plasma urate level compared with topiroxostat treatment alone, with a dose dependency of 30 and 100 mg/kg for FYU-981 and F12859 each. The decrease in the plasma urate level observed in the topiroxostat-treated rats disappeared by further co-treatment with inosine.

CONCLUSIONS

Effects of uricosuric agents on the plasma urate level in rats were sensitive to the rate of urate formation. Induction of slower urate formation by topiroxostat provides valuable model for evaluation of hypouricemic effects by uricosuric agents in rats.

False in vitro and in vivo elevations of uric acid levels in mouse blood

Uric acid (UA) levels in mouse blood have been reported to range widely from 0.1 μM to 760 μM. The aim of this study was to demonstrate false in vitro and in vivo elevations of UA levels in mouse blood. Male ICR mice were anesthetized with pentobarbital (breathing mice) or sacrificed with overdose ether (non-breathing mice). Collected blood was dispensed into MiniCollect® tubes and incubated in vitro for 0 or 30 min at room temperature. After separation of plasma or serum, the levels of UA and hypoxanthine were determined using HPLC. From the non-incubated plasma of breathing mice, the true value of UA level in vivo was 13.5±1.4 μM. However, UA levels in mouse blood increased by a factor of 3.9 following incubation in vitro. This "false in vitro elevation" of UA levels in mouse blood after blood sampling was inhibited by allopurinol, a xanthine oxidase inhibitor. Xanthine oxidase was converted to UA in mouse serum from hypoxanthine which was released from blood cells during incubation. Plasma UA levels from non-breathing mice were 19 times higher than those from breathing mice. This "false in vivo elevation" of UA levels before blood sampling was inhibited by pre-treatment with phentolamine, an α-antagonist. Over-anesthesia with ether might induce α-vasoconstriction and ischemia and thus degrade intracellular ATP to UA. For the accurate measurement of UA levels in mouse blood, the false in vitro and in vivo elevations of UA level must be avoided by immediate separation of plasma after blood sampling from anesthetized breathing mice.

The organic anion transporter (OAT) family: a systems biology perspective

The organic anion transporter (OAT) subfamily, which constitutes roughly half of the SLC22 (solute carrier 22) transporter family, has received a great deal of attention because of its role in handling of common drugs (antibiotics, antivirals, diuretics, nonsteroidal anti-inflammatory drugs), toxins (mercury, aristolochic acid), and nutrients (vitamins, flavonoids). Oats are expressed in many tissues, including kidney, liver, choroid plexus, olfactory mucosa, brain, retina, and placenta. Recent metabolomics and microarray data from Oat1 [Slc22a6, originally identified as NKT (novel kidney transporter)] and Oat3 (Slc22a8) knockouts, as well as systems biology studies, indicate that this pathway plays a central role in the metabolism and handling of gut microbiome metabolites as well as putative uremic toxins of kidney disease. Nuclear receptors and other transcription factors, such as Hnf4α and Hnf1α, appear to regulate the expression of certain Oats in conjunction with phase I and phase II drug metabolizing enzymes. Some Oats have a strong selectivity for particular signaling molecules, including cyclic nucleotides, conjugated sex steroids, odorants, uric acid, and prostaglandins and/or their metabolites. According to the "Remote Sensing and Signaling Hypothesis," which is elaborated in detail here, Oats may function in remote interorgan communication by regulating levels of signaling molecules and key metabolites in tissues and body fluids. Oats may also play a major role in interorganismal communication (via movement of small molecules across the intestine, placental barrier, into breast milk, and volatile odorants into the urine). The role of various Oat isoforms in systems physiology appears quite complex, and their ramifications are discussed in the context of remote sensing and signaling.

Macrophage migration inhibitory factor is a possible candidate for the induction of microalbuminuria in diabetic db/db mice

Preventing the onset of microalbuminuria in diabetic nephropathy is a problem that needs urgent rectification. The use of a mouse model for diabetes is vital in this regard. For example, db/db mice exhibit defects in the leptin receptor Ob-Rb sub-type, while the ob/ob strain exhibits defects in the leptin ligand. These mouse strains demonstrate type 2 diabetes, either with or without microalbuminuria, respectively. The purpose of the present study was to use DNA microarray technology to screen for the gene responsible for the onset of diabetic microalbuminuria. Using Affymetrix Mouse Gene ST 1.0 arrays, microarray analysis was performed using total RNA from the kidneys of ob control, ob/ob, db/m, and db/db mice. Microarray and quantitative reverse transcription-polymerase chain reaction (RT-PCR) indicated that transcription of the macrophage migration inhibitory factor (MIF) gene was significantly enhanced in the kidneys of db/db mice. Western blotting showed that levels of MIF protein was enhanced in the kidneys of both diabetic db/db and ob/ob mice. On the other hand, elevation of urinary MIF excretion detected by enzyme-linked immunosorbent assay (ELISA) was only in db/db mice and preceded the onset of microalbuminuria. Immunofluorescence studies revealed that MIF was expressed in mouse kidney glomeruli. While MIF expression was enhanced in the diabetic kidneys of both mouse strains, the elevated secretion from db/db mouse kidneys may be responsible for initiating the onset of microalbuminuria in diabetic nephropathy.

Ependymal cells of the mouse brain express urate transporter 1 (URAT1)

BACKGROUND

Elevated uric acid (UA) is commonly associated with gout and it is also a known cardiovascular disease risk factor. In contrast to such deleterious effects, UA possesses neuroprotective properties in the brain and elucidating the molecular mechanisms involved may have significant value regarding the therapeutic treatment of neurodegenerative disease. However, it is not yet fully established how UA levels are regulated in the brain. In this study, we investigated the distribution of mouse urate transporter 1 (URAT1) in the brain. URAT1 is a major reabsorptive urate transporter predominantly found in the kidney.

METHODS

Immunohistochemistry of wild type and URAT1 knockout mouse brain using paraffin or frozen sections and a rabbit polyclonal anti-mouse URAT1 antibody were employed.

RESULTS

Antibody specificity was confirmed by the lack of immunostaining in brain tissue from URAT1 knockout mice. URAT1 was distributed throughout the ventricular walls of the lateral ventricle, dorsal third ventricle, ventral third ventricle, aqueduct, and fourth ventricle, but not in the non-ciliated tanycytes in the lower part of the ventral third ventricle. URAT1 was localized to the apical membrane, including the cilia, of ependymal cells lining the wall of the ventricles that separates cerebrospinal fluid (CSF) and brain tissue.

CONCLUSION

In this study, we report that URAT1 is expressed on cilia and the apical surface of ventricular ependymal cells. This is the first report to demonstrate expression of the urate transporter in ventricular ependymal cells and thus raises the possibility of a novel urate transport system involving CSF.

Renal transport of uric acid: evolving concepts and uncertainties

In addition to its role as a metabolic waste product, uric acid has been proposed to be an important molecule with multiple functions in human physiologic and pathophysiologic processes and may be linked to human diseases beyond nephrolithiasis and gout. Uric acid homeostasis is determined by the balance between production, intestinal secretion, and renal excretion. The kidney is an important regulator of circulating uric acid levels by reabsorbing about 90% of filtered urate and being responsible for 60% to 70% of total body uric acid excretion. Defective renal handling of urate is a frequent pathophysiologic factor underpinning hyperuricemia and gout. Despite tremendous advances over the past decade, the molecular mechanisms of renal urate transport are still incompletely understood. Many transport proteins are candidate participants in urate handling, with URAT1 and GLUT9 being the best characterized to date. Understanding these transporters is increasingly important for the practicing clinician as new research unveils their physiologic characteristics, importance in drug action, and genetic association with uric acid levels in human populations. The future may see the introduction of new drugs that act specifically on individual renal urate transporters for the treatment of hyperuricemia and gout.

The increased protein level of URAT1 was observed in obesity/metabolic syndrome model mice

To elucidate the mechanism of obesity/metabolic syndrome-related hyperuricemia, this study aimed to determine the expression levels of transport systems for urate absorption (Urat1, Smct1, Glut9) and urate secretion (Abcg2). The kidneys of two obesity models in mice were used: 1) leptin-deficient mice (ob/ob mice) and 2) Quick fat diet model. 1) 8-week-old male ob/ob mice demonstrated the increased protein levels of Slc22a12 (Urat1), Slc2a9 (Glut9), and Abcg2 (Abcg2) and a decreased protein level of Slc5a8 (Smct1). However, no significant changes in the mRNA levels of these genes were observed. 2) C57BL/6 mice were fed with a Quick fat diet (crude fat content: 13.6%) from the age of 24 to 28 weeks (Quick fat diet group). The average body weights of the Quick fat diet group were heavier than those of the control group fed with a normal diet (crude fat content: 4.8%). The mRNA levels of Slc22a12, Slc2a9, Abcg2, or Slc5a8 did not change significantly in both groups. The protein levels of Slc22a12 (Urat1) and Abcg2 (Abcg2) increased significantly in the Quick fat diet group. Those of Slc2a9 (Glut9) and Slc5a8 (Smct1) were not changed significantly in the Quick fat diet group. In conclusion, the Quick fat diet enhanced the protein levels of Urat1 and Abcg2 without any changes in their mRNA transcription levels. The cause of obesity/metabolic syndrome-associated hyperuricemia appears to be associated with the urate reabsorption transporter Urat1 protein enhanced by fat.

The effect of female hormones upon urate transport systems in the mouse kidney

Increased levels of serum urate in postmenopausal women are thought to be caused by a change in renal urate elimination associated with the loss of female hormones. In this study, we investigated the regulation of renal urate transporter expression by female hormones using ovariectomized mice with or without hormone replacement. Estradiol suppressed the protein levels of urate reabsorptive transporters urate transporter 1 and glucose transporter 9 (Urat1 and Glut9), and that of urate efflux transporter ATP-binding cassette sub-family G member 2 (Abcg2). Progesterone suppressed protein levels of sodium-coupled monocarboxylate transporter 1 (Smct1). However, neither estradiol nor progesterone influenced the respective levels of mRNA.