Abstract
BackgroundSystem L transporters LAT1 (SLC7A5) and LAT2 (SLC7A8) mediate the uptake of large, neutral amino acids in the human placenta. Many System L substrates are essential amino acids, thus representing crucial nutrients for the growing fetus. Both LAT isoforms are expressed in the human placenta, but the relative contribution of LAT1 and LAT2 to placental System L transport and their subcellular localisation are not well established. Moreover, the influence of maternal body mass index (BMI) on placental System L amino acid transport is poorly understood. Therefore the aims of this study were to determine: i) the relative contribution of the LAT isoforms to System L transport activity in primary human trophoblast (PHT) cells isolated from term placenta; ii) the subcellular localisation of LAT transporters in human placenta; and iii) placental expression and activity of System L transporters in response to maternal overweight/obesity.MethodsSystem L mediated leucine uptake was measured in PHT cells after treatment with si-RNA targeting LAT1 and/or LAT2. The localisation of LAT isoforms was studied in isolated microvillous plasma membranes (MVM) and basal membranes (BM) by Western blot analysis. Results were confirmed by immunohistochemistry in sections of human term placenta. Expression and activity System L transporters was measured in isolated MVM from women with varying pre-pregnancy BMI.ResultsBoth LAT1 and LAT2 isoforms contribute to System L transport activity in primary trophoblast cells from human term placenta. LAT1 and LAT2 transporters are highly expressed in the MVM of the syncytiotrophoblast layer at term. LAT2 is also localised in the basal membrane and in endothelial cells lining the fetal capillaries. Measurements in isolated MVM vesicles indicate that System L transporter expression and activity is not influenced by maternal BMI.ConclusionsLAT1 and LAT2 are present and functional in the syncytiotrophoblast MVM, whereas LAT2 is also expressed in the BM and in the fetal capillary endothelium. In contrast to placental System A and beta amino acid transporters, MVM System L activity is unaffected by maternal overweight/obesity.
Highlights
System L transporters LAT1 (SLC7A5) and LAT2 (SLC7A8) mediate the uptake of large, neutral amino acids in the human placenta
In cells transfected with si-RNA targeting both LAT1 and LAT2, silencing efficiency was similar to single transfections because LAT1 protein expression decreased by 53 % (p < 0.01, N = 3) and LAT2 expression by 22 % (p < 0.01, N = 3; Fig. 2a)
In light of the results on the localisation of L-type Amino acid Transporter (LAT) isoforms (Fig. 3), we studied the expression of System L transporters LAT1 and LAT2 only in isolated microvillous membrane (MVM) vesicles from placentas of women with normal and high Body mass index (BMI)
Summary
System L transporters LAT1 (SLC7A5) and LAT2 (SLC7A8) mediate the uptake of large, neutral amino acids in the human placenta. Many System L substrates are essential amino acids, representing crucial nutrients for the growing fetus Both LAT isoforms are expressed in the human placenta, but the relative contribution of LAT1 and LAT2 to placental System L transport and their subcellular localisation are not well established. System L (Leucine preferring) is the main Na+-independent transporter for branched-chain (such as L-leucine) and aromatic neutral amino acids (including L-phenylalanine) [3, 4], many of which are essential. It is a heterodimer consisting of a light chain (typically L-type Amino acid Transporter LAT1 (SLC7A5) or LAT2 (SLC7A8)) covalently attached to a heavy chain (CD98/4F2hc) [5]. System L transporters exchange with 1:1 stoichiometry their substrate amino acids with intracellular non-essential amino acids (e.g. L-glycine) [6], substrates of accumulative amino acid transporters, such as System A [7]
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