Orphan Transporter SLC6A18 Is Renal Neutral Amino Acid Transporter B0AT3

Abstract

The orphan transporter Slc6a18 (XT2) is highly expressed at the luminal membrane of kidney proximal tubules and displays ϳ50% identity with Slc6a19 (B 0 AT1), which is the main neutral amino acid transporter in both kidney and small intestine. As yet, the amino acid transport function of XT2 has only been experimentally supported by the urinary glycine loss observed in xt2 null mice. We report here that in Xenopus laevis oocytes, co-expressed ACE2 (angiotensin-converting enzyme 2) associates with XT2 and reveals its function as a Na ؉ -and Cl ؊ -dependent neutral amino acid transporter. In contrast to its association with ACE2 observed in Xenopus laevis oocytes, our experiments with ace2 and collectrin null mice demonstrate that in vivo it is Collectrin, a smaller homologue of ACE2, that is required for functional expression of XT2 in kidney. To assess the function of XT2 in vivo, we reanalyzed its knock-out mouse model after more than 10 generations of backcrossing into C57BL/6 background. In addition to the previously published glycinuria, we observed a urinary loss of several other amino acids, in particular ␤-branched and small neutral ones. Using telemetry, we confirmed the previously described link of XT2 absence with hypertension but only in physically restrained animals. Taken together, our data indicate that the formerly orphan transporter XT2 functions as a sodium and chloride-dependent neutral amino acid transporter that we propose to rename B 0 AT3.

Impaired Nutrient Signaling and Body Weight Control in a Na+ Neutral Amino Acid Cotransporter (Slc6a19)-deficient Mouse

“…Thus, glycine transport is most likely mediated by B 0 AT1 and B 0 AT3. This is consistent with the glycinuria and minor neutral aminoaciduria displayed by Slc6a18 nullizygous mice (16,36) as well as the lack of inhibition of proline uptake by glycine observed in this study. The Na ϩ dependence and MeAib sensitivity of proline uptake indicates that it was most likely mediated by the IMINO transporter (37).…”

“…Mice lacking collectrin (Tmem27) have a similar renal phenotype as humans with Hartnup disorder but lack the intestinal phenotype (9). Ace2 nullizygous mice have a more complex phenotype including cardiac deficiencies and glomerulosclerosis but exhibit normal urine amino acid levels (15,16).…”

Collectrin and ACE2 in renal and intestinal amino acid transport

“…As for B 0 AT1, B 0 AT3 requires an accessory protein to be correctly expressed at the membrane. 12,18,19 B 0 AT3 appears to be required for the reabsorption of tubular amino acids leftover by B 0 AT1 in the early proximal kidney tubule segments or amino acids that have leaked back through the paracellular pathway. 20 In a knockout mouse model of slc6a18, an abnormal excretion of several neutral amino acids was observed, especially glycine and L-glutamine.…”

“…The mice otherwise develop normally and are viable but have been shown to develop high blood pressure under stress conditions. 18,21 In humans, the connection between single nucleotide polymorphism of SLC6A18, high blood pressure or myocardial infarction has been examined and remained inconclusive. 22,23 Glyt1 (Slc6a9).…”

Ligand-gated Ion Channels

“…By contrast PROT (SLC6A7), which is expressed only in brain in association with a subset of excitatory nerve terminals, shows specificity for the transport of L-proline. Endogenous substrates Leu, met, iso, val > asn, phe, ala, ser > thr, gly, pro Pro > ala, val, met, leu > iso, thr, asn, ser, phe > gly Ala, gly > met, phe, leu, his, gln (Vanslambrouck et al 2010) Predicted stoichiometry 1 Na: 1 amino acid (Böhmer et al, 2005) 1 Na: 1 amino acid Na + -and Cl --dependent transport (Singer et al, 2009) Systematic name SLC6A16 SLC6A17 SLC6A20…”