Murine renal organic anion transporters mOAT1 and mOAT3 facilitate the transport of neuroactive tryptophan metabolites

Abstract

Tryptophan metabolites such as kynurenate (KYNA), xanthurenate (XA), and quinolinate are considered to have an important impact on many physiological processes, especially brain function. Many of these metabolites are secreted with the urine. Because organic anion transporters (OATs) facilitate the renal secretion of weak organic acids, we investigated whether the secretion of bioactive tryptophan metabolites is mediated by OAT1 and OAT3, two prominent members of the OAT family. Immunohistochemical analyses of the mouse kidneys revealed the expression of OAT1 to be restricted to the proximal convoluted tubule (representing S1 and S2 segments), whereas OAT3 was detected in almost all parts of the nephron, including macula densa cells. In the mouse brain, OAT1 was found to be expressed in neurons of the cortex cerebri and hippocampus as well as in the ependymal cell layer of the choroid plexus. Six tryptophan metabolites, including the bioactive substances KYNA, XA, and the serotonin metabolite 5-hydroxyindol acetate inhibited [(3)H]p-aminohippurate (PAH) or 6-carboxyfluorescein (6-CF) uptake by 50-85%, demonstrating that these compounds interact with OAT1 as well as with OAT3. Half-maximal inhibition of mOAT1 occurred at 34 muM KYNA and 15 muM XA, and it occurred at 8 muM KYNA and 11.5 muM XA for mOAT3. Quinolinate showed a slight but significant inhibition of [(3)H]PAH uptake by mOAT1 and no alteration of 6-CF uptake by mOAT3. [(14)C]-Glutarate (GA) uptake was examined for both transporters and demonstrated differences in the transport rate for this substrate by a factor of 4. Trans-stimulation experiments with GA revealed that KYNA and XA are substrates for mOAT1. Our results support the idea that OAT1 and OAT3 are involved in the secretion of bioactive tryptophan metabolites from the body. Consequently, they are crucial for the regulation of central nervous system tryptophan metabolite concentration.

Identification of a New Urate and High Affinity Nicotinate Transporter, hOAT10 (SLC22A13)

“…The concept of basolateral prostaglandin E 2 release in macula densa cells via OAT3 may also be realized in CCD cells. Prostaglandin E 2 regulates adenylate cyclase activity of CCD cells, leading to a down-regulation of water transport by aquaporin 2 (20,39). Based on our findings, we have to consider organic anion transport (reabsorption as well as secretion) additionally in CCD.…”

“…Most of these are highly expressed in the kidneys (OAT1-OAT4, Oat5, Oat v 1, and URAT1), whereas Oat6 is only expressed in the olfactory bulb, and the recently identified human OAT7 is currently the only liver-specific OAT. The substrate panel of OATs includes important endogenous compounds like urate (15)(16)(17), prostaglandins (18), neurotransmitter and tryptophan metabolites (19,20), sulfated steroids, and a long list of drugs such as anti-viral drugs, antibiotics, antidotes (21), or diuretics (22)(23)(24).…”

Identification of xanthurenic acid 8- O -β- d -glucoside and xanthurenic acid 8- O -sulfate as human natriuretic hormones

“…Xanthurenic acid is believed to an play important role in many biological processes, and thus it is possible that NH and NH-1, in addition to their natriuretic properties, may have other biological functions as well (24,25). However, a more detailed understanding of these compounds' physiological role will have to await further study.…”

Overlapping in vitro and in vivo specificities of the organic anion transporters OAT1 and OAT3 for loop and thiazide diuretics

“…The classic basolateral OAT system, long referred to as the p-aminohippurate (PAH) transporter, was originally identified as novel kidney transporter (NKT) (24), which was proposed to function in renal organic ion secretion and was later directly shown to function in this capacity and named OAT1 (7,34). The other primary candidate for basolateral OA uptake is OAT3, originally identified as Roct [reduced in osteosclerosis (oc) transporter] (4), and several immunohistochemical studies have localized both OAT1 and OAT3 to the basolateral membrane of the proximal tubule of human, rat, and mouse (1,8,15,16,20,23,26,35). When expressed in Xenopus oocytes or epithelial cell lines, OAT1 couples OA entry to dicarboxylate exit and was trans-stimulated by preloading with PAH (34).…”