Urate, a naturally occurring product of purine metabolism, is a scavenger of biological oxidants implicated in numerous disease processes, as demonstrated by its capacity of neuroprotection. It is present at higher levels in human blood (200 500 microM) than in other mammals, because humans have an effective renal urate reabsorption system, despite their evolutionary loss of hepatic uricase by mutational silencing. The molecular basis for urate handling in the human kidney remains unclear because of difficulties in understanding diverse urate transport systems and species differences. Here we identify the long-hypothesized urate transporter in the human kidney (URAT1, encoded by SLC22A12), a urate anion exchanger regulating blood urate levels and targeted by uricosuric and antiuricosuric agents (which affect excretion of uric acid). Moreover, we provide evidence that patients with idiopathic renal hypouricaemia (lack of blood uric acid) have defects in SLC22A12. Identification of URAT1 should provide insights into the nature of urate homeostasis, as well as lead to the development of better agents against hyperuricaemia, a disadvantage concomitant with human evolution.

The serine/threonine kinase Akt (also called protein kinase B) is well known as an important regulator of cell survival and growth and has also been shown to be required for cell migration in different organisms. However, the mechanism by which Akt functions to promote cell migration is not understood. Here, we identify an Akt substrate, designated Girdin/APE (Akt-phosphorylation enhancer), which is an actin binding protein. Girdin expresses ubiquitously and plays a crucial role in the formation of stress fibers and lamellipodia. Akt phosphorylates serine at position 1416 in Girdin, and phosphorylated Girdin accumulates at the leading edge of migrating cells. Cells expressing mutant Girdin, in which serine 1416 was replaced with alanine, formed abnormal elongated shapes and exhibited limited migration and lamellipodia formation. These findings suggest that Girdin is essential for the integrity of the actin cytoskeleton and cell migration and provide a direct link between Akt and cell motility.

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Abstract. Renal hypouricemia is an inherited and heterogeneous disorder characterized by increased urate clearance (CUA). The authors recently established that urate was reabsorbed via URAT1 on the tubular apical membrane and that mutations in SLC22A12 encoding URAT1 cause renal hypouricemia. This study was undertaken to elucidate and correlate clinical and genetic features of renal hypouricemia. The SLC22A12 gene was sequenced in 32 unrelated idiopathic renal hypouricemia patients, and the relationships of serum urate levels, and CUA/creatinine clearance (Ccr) to SLC22A12 genotype were examined. Uricosuric (probenecid and benzbromarone) and anti-uricosuric drug (pyrazinamide) loading tests were also performed in some patients. Three patients had exercise-induced acute renal failure (9.4%), and four patients had urolithiasis (12.5%). The authors identified eight new mutations and two previously reported mutations that result in loss of function. Thirty patients had SLC22A12 mutations; 24 homozygotes and compound heterozygotes, and 6 heterozygotes. Mutation G774A dominated SLC22A12 mutations (74.1% in 54 alleles). Serum urate levels were significantly lower and CUA/Ccr was significantly higher in heterozygotes compared with healthy subjects; these changes were even more significant in homozygotes and compound heterozygotes. These CUA/Ccr relations demonstrated a gene dosage effect that corresponds with the difference in serum urate levels. In contrast to healthy subjects, the CUA/Ccr of patients with homozygous and compound heterozygous SLC22A12 mutations was unaffected by pyrazinamide, benzbromarone, and probenecid. The findings indicate that SLC22A12 was responsible for most renal hypouricemia and that URAT1 is the primary reabsorptive urate transporter, targeted by pyrazinamide, benzbromarone, and probenecid in vivo.Approximately 90% of all urate that is filtered through the glomerulus is eventually reabsorbed. A four-component hypothesis has been proposed to explain the renal urate transport mechanisms; it includes glomerular filtration, presecretory reabsorption, secretion, and postsecretory reabsorption (1,2). Renal hypouricemia is a common inherited and heterogeneous disorder characterized by impaired tubular urate transport (3). The incidence of renal hypouricemia has been reported to be 0.12 to 0.72% (4,5), and exercise-induced acute renal failure and nephrolithiasis have been reported as complications (6).Renal hypouricemia has been classified into the following five types according to responses to the anti-uricosuric drug pyrazinamide, and the uricosuric drug, probenecid: (a) a presecretory reabsorptive defect with an attenuated response to both pyrazinamide and probenecid (3); (b) a post-secretory reabsorptive defect when pyrazinamide suppressible urate clearance (CUA) is not influenced by probenecid (7); (c) total inhibition of urate reabsorption when pyrazinamide induces elimination of CUA exceeding the rate of glomerular filtration (8); (d) enhanced secretion when the pyrazinamide suppressible CUA ...