Institute of Physiology University of Zurich, Zurich, Switzerland.
NCCR Kidney. CH, University of Zurich, Zurich, Switzerland.
J Physiol. 2020 Nov;598(22):5109-5132. doi: 10.1113/JP280234. Epub 2020 Sep 16.
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.
LAT4 是一种广泛表达的单转运蛋白，对必需的支链氨基酸（蛋氨酸和苯丙氨酸）具有选择性，这些氨基酸参与上皮细胞的转运。LAT4 的全局缺失会导致类似于早期营养不良的表型，并在出生后 10 天内死亡。在这里，我们测试了选择性地在小鼠肠道中缺失 LAT4 的影响。这稍微影响了氨基酸（AA）的吸收并延迟了胃肠道蠕动；然而，即使与芳香族 AA 单转运蛋白 TAT1 缺失（KO）结合，它也没有主要的表型影响。相反，肾脏管状选择性缺失 LAT4 导致大量氨基酸尿，在高蛋白饮食下强烈增加。部分管状 LAT4 缺失与 TAT1 KO 的组合暗示它们在 AA 重吸收方面具有协同作用。这些结果表明，LAT4 在肾脏 AA 重吸收中起重要作用，但在肠道 AA 吸收中的功能作用在很大程度上是可有可无的。
氨基酸（AA）转运蛋白 LAT4（Slc43a2）作为必需中性 AA 的易化扩散单转运蛋白发挥作用，在小肠（SI）和肾脏管状上皮的基底外侧膜上高度表达。以前，我们发现 LAT4 全局敲除（KO）小鼠以预期的孟德尔比例出生，但在 10 天内死亡。它们无法增加体重和严重的类似于营养不良的表型与明显正常的喂养形成对比，表明存在严重的肠道 AA 吸收缺陷。在本研究中，使用条件全局和组织特异性 LAT4 KO 小鼠模型，我们否定了这一假设，表明选择性缺乏肠道 LAT4 不会损害产后发育，尽管它会导致吸收缺陷伴随着胃肠道蠕动延迟。肾脏管状特异性 LAT4 KO 导致大量氨基酸尿，这是由于 LAT4 转运的 AA 以及其他作为反向转运蛋白 LAT2 底物的 AA 的重吸收缺陷所致，在体内证明了这两种转运蛋白的功能合作。基底外侧单转运体在肾脏中的主要作用还得到了以下观察结果的支持：在缺乏另一种中性 AA 单转运蛋白 TAT1 的小鼠中，部分 LAT4 KO 导致尿 AA 丢失协同增加。令人惊讶的是，在 SI 中，相同的联合 KO 没有引起主要影响，这表明存在未知的代偿机制。总之，以前在 LAT4 全局 KO 幼崽中观察到的类似于致命的营养不良的表型被认为是 LAT4 在 SI、肾脏和可能其他组织中缺失的组合效应的结果。
