Faculty Bibliography

We have characterized the expression of organic anion transporter 6, Oat6 (slc22a20), in olfactory mucosa, as well as its interaction with several odorant organic anions. In situ hybridization reveals diffuse Oat6 expression throughout olfactory epithelium, yet olfactory neurons laser-capture microdissected from either the main olfactory epithelium (MOE) or the vomeronasal organ (VNO) did not express Oat6 mRNA. These data suggest that Oat6 is expressed in non-neuronal cells of olfactory tissue, such as epithelial and/or other supporting cells. We next investigated interaction of Oat6 with several small organic anions that have previously been identified as odortype components in mouse urine. We find that each of these compounds, propionate, 2- and 3-methylbutyrate, benzoate, heptanoate, and 2-ethylhexanoate, inhibits Oat6-mediated uptake of a labeled tracer, estrone sulfate, consistent with their being Oat6 substrates. Previously, we noted defects in the renal elimination of odortype and odortype-like molecules in Oat1 knockout mice. The finding that such molecules interact with Oat6 raises the possibility that odorants secreted into the urine through one OAT-mediated mechanism (Eraly et al., JBC 2006) are transported through the olfactory mucosa through another OAT-mediated mechanism. Oat6 might play a direct or indirect role in olfaction, such as modulation of the availability of odorant organic anions at the mucosal surface for presentation to olfactory neurons or facilitation of delivery to a distal site of chemosensation, among other possibilities that we discuss.

The "classical" organic anion secretory pathway of the renal proximal tubule is critical for the renal excretion of the prototypic organic anion, para-aminohippurate, as well as of a large number of commonly prescribed drugs among other significant substrates. Organic anion transporter 1 (OAT1), originally identified as NKT (Lopez-Nieto, C. E., You, G., Bush, K. T., Barros, E. J. G., Beier, D. R., and Nigam, S. K. (1997) J. Biol. Chem. 272, 6471-6478), has physiological properties consistent with a role in this pathway. However, several other transporters (e.g. OAT2, OAT3, and MRP1) have also been proposed as important PAH transporters on the basis of in vitro studies; therefore, the relative contribution of OAT1 has remained unclear. We have now generated a colony of OAT1 knock-out mice, permitting elucidation of the role of OAT1 in the context of these other potentially functionally redundant transporters. We find that the knock-out mice manifest a profound loss of organic anion transport (e.g. para-aminohippurate) both ex vivo (in isolated renal slices) as well as in vivo (as indicated by loss of renal secretion). In the case of the organic anion, furosemide, loss of renal secretion in the knock-out results in impaired diuretic responsiveness to this drug. These results indicate a critical role for OAT1 in the functioning of the classical pathway. In addition, we have determined the levels of approximately 60 endogenous organic anions in the plasma and urine of wild-type and knock-out mice. This has led to identification of several compounds with significantly higher plasma concentrations and/or lower urinary concentrations in knock-out mice, suggesting the involvement of OAT1 in their renal secretion. We have also demonstrated in xenopus oocytes that some of these compounds interact with OAT1 in vitro. Thus, these latter compounds might represent physiological substrates of OAT1.

Kidney excretion of numerous organic anionic drugs and endogenous metabolites is carried out by a family of multispecific organic anion transporters (OATs). Two closely related transporters, SLC22A6, initially identified by us as NKT and also known as OAT1, and SLC22A8, also known as OAT3 and ROCT, are thought to mediate the initial steps in the transport of organic anionic drugs between the blood and proximal tubule cells of the kidney. Coding region polymorphisms in these genes are infrequent and pairing of these genes in the genome suggests they may be coordinately regulated. Hence, 5' regulatory regions of these genes may be important factors in human variation in organic anionic drug handling. We have analyzed novel single nucleotide polymorphisms in the evolutionarily conserved 5' regulatory regions of the SLC22A6 and SLC22A8 genes (phylogenetic footprints) in an ethnically diverse sample of 96 individuals (192 haploid genomes). Only one polymorphism was found in the SLC22A6 5' regulatory region. In contrast, seven polymorphisms were found in the SLC22A8 5' regulatory region, two of which were common to all ethnic groups studied. Computational analysis permitted phase and haplotype reconstruction. Proximity of these non-coding polymorphisms to transcriptional regulatory elements (including potential sex steroid response elements) suggests a potential influence on the level of transcription of the SLC22A6 and/or SLC22A8 genes and will help define their role in variation in human drug, metabolite and toxin excretion. The clustering of OAT genes in the genome raises the possibility that nucleotide polymorphisms in SLC22A6 could also effect SLC22A8 expression, and vice versa.