The Human SLC7A5 (LAT1): The Intriguing Histidine/Large Neutral Amino Acid Transporter and Its Relevance to Human Health

Abstract

SLC7A5, known as LAT1, belongs to the APC superfamily and forms a heterodimeric amino acid transporter interacting with the glycoprotein CD98 (SLC3A2) through a conserved disulfide. The complex is responsible for uptake of essential amino acids in crucial body districts such as placenta and blood brain barrier. LAT1/CD98 heterodimer has been studied over the years to unravel the transport mechanism and the role of each subunit. Studies conducted in intact cells demonstrated that LAT1/CD98 mediates a Na⁺ and pH independent antiport of amino acids. Some novel insights into the function of LAT1 derived from studies conducted in proteoliposomes reconstituted with the recombinant human LAT1. Using this experimental tool, it has been demonstrated that the preferred substrate is histidine and that CD98 is not required for transport being, plausibly, involved in routing LAT1 to the plasma membrane. Since a 3D structure of LAT1 is not available, homology models have been built on the basis of the AdiC transporter from E.coli. Crucial residues for substrate recognition and gating have been identified using a combined approach of bioinformatics and site-directed mutagenesis coupled to functional assays. Over the years, the interest around LAT1 increased because this transporter is involved in important human diseases such as neurological disorders and cancer. Therefore, LAT1 became an important pharmacological target together with other nutrient membrane transporters. Moving from knowledge on structure/function relationships, two cysteine residues, lying on the substrate binding site, have been exploited for designing thiol reacting covalent inhibitors. Some lead compounds have been characterized whose efficacy has been tested in a cancer cell line.

Keywords: LAT1; SLC7 family; drug design; histidine; molecular docking; pro-drugs; proteoliposomes.

Figure 1

Schematic representation of human SLC7A5 (A) and SLC3A2 (B) genes according to RCh38.p7 genome assembly. Intronic and exonic sequences are depicted in blue and red, respectively. UTR sequence is indicated in dark gray. Predicted UTR sequences or isoforms are in transparency. For each transcript, the relative Genbank accession number is indicated.

Figure 2

Representation of LAT1/CD98 and its substrates. The protein heterodimer is asymmetrically inserted in the cell membrane. The double-face feature of LAT1 is indicated by two-color design of the model. The blue half represents the specificity toward amino acids which are colored in blue (Essential Amino Acids) and light blue (Non-Essential Amino Acids). The size is suggestive of the higher or the lower specificity toward the amino acids. The red half represents the known specificity toward non-amino acid substrates: hormones, drugs and inhibitors. The reactivity of –SH group(s) of the protein toward Hg²⁺ is also represented. Glycosylation of CD98 is depicted as “antennas”.

Figure 3

The Essential Amino Acids distribution network through LAT1. Interplay among blood and epithelial polarized cells of intestine (apical membrane depicted as brush-border and basolateral membrane in contact with blood), placenta, BBB (Blood Brain Barrier) and other tissues. Essential Amino Acids are absorbed through intestine using Na⁺ dependent transporters (indicated in gray) to allow their accumulation with high capacity. The same amino acids are conveyed to blood by other transporters present in the basolateral membrane of gut cells (indicated in light brown). From blood, Essential Amino Acids are accumulated in BBB and in placenta cells using LAT1 with an antiport reaction. In these cells, LAT1 is localized at both sides of cells allowing the flux of amino acids to Brain and Fetus, respectively. In other tissues, LAT1 is not highly expressed but plays the role of distributing Essential Amino Acids used both for signaling and metabolic purposes thanks also to alternative localization in lysosome membrane. These aspects became more important in tissues where LAT1 expression is strongly increased as in case of cancers. LAT1 is depicted as in Figure 2 by two-colored half. Flux of Essential Amino Acids are indicated by blue arrows (from blood to tissues) and by gray arrows (from tissues to blood).