Abstract
Key points: LAT4 is a broadly expressed uniporter selective for essential branched chain amino acids, methionine and phenylalanine, which are involved in epithelial transport. Its global deletion leads to an early malnutrition-like phenotype and death within 10 days after birth. Here, we tested the impact of deleting LAT4 selectively in the mouse intestine. This affected slightly the absorption of amino acids (AAs) and delayed gastrointestinal motility; however, it had no major phenotypic effect, even when combined with aromatic AA uniporter TAT1 knockout (KO). Conversely, kidney tubule-selective deletion of LAT4 led to a substantial aminoaciduria that strongly increased under a high protein diet. Combining a partial tubular LAT4 deletion with TAT1 KO implicated their synergistic action on AA reabsorption. These results show that LAT4 plays an important role for kidney AA reabsorption, but that its functional role in intestinal AA absorption is largely dispensable. Amino acid (AA) transporter LAT4 (Slc43a2) functions as facilitated diffusion uniporter for essential neutral AAs and is highly expressed at the basolateral membrane of small intestine (SI) and kidney tubule epithelia. Previously, we showed that LAT4 global knockout (KO) mice were born at the expected Mendelian ratio but died within 10 days. Their failure to gain weight and a severe malnutrition-like phenotype contrasted with apparently normal feeding, suggesting a severe intestinal AA absorption defect. In the present study, using conditional global and tissue-specific LAT4 KO mouse models, we nullified this hypothesis, demonstrating that the selective lack of intestinal LAT4 does not impair postnatal development, although it leads to an absorption defect accompanied by delayed gastrointestinal motility. Kidney tubule-specific LAT4 KO led to a substantial aminoaciduria as a result of a reabsorption defect of AAs transported by LAT4 and of other AAs that are substrates of the antiporter LAT2, demonstrating, in vivo, the functional co-operation of these two transporters. The major role played by basolateral uniporters in the kidney was further supported by the observation that, in mice lacking TAT1, another neutral AA uniporter, a partial LAT4 KO led to a synergistic increase of urinary AA loss. Surprisingly in the SI, the same combined KO induced no major effect, suggesting yet unknown compensatory mechanisms. Taken together, the lethal malnutrition-like phenotype observed previously in LAT4 global KO pups is suggested to be the consequence of a combinatorial effect of LAT4 deletion in the SI, kidney and presumably other tissues.