Transcript quantification of intestinal neutral amino acids and oligopeptides transporters in European sea bass (Dicentrarchus labrax) reared in fresh water and fed fish and plant protein sources.

Oligopeptides and free amino acids are absorbed along the intestinal
tract by specialized membrane transporter proteins. Di- and tri-peptides
are transported via the H+/coupled peptide transporter
(PEPT1), which is located in the brush border membrane of intestinal
epithelial cells. Free amino acids are absorbed by a variety of Na+-
dependent and independent membrane transporters, frequently
referred to as “systems”. Among the amino acid transporting proteins,
the neutral amino acid transporter SLC6A19, also called system
B(0) neutral amino acid transporter AT1 (B0AT1), is an integral plasma
membrane protein responsible for the uptake of a broad range of
neutral amino acids across the apical membrane of enterocytes and
renal cells. In humans, the B0 system is associated with a severe
neutral aminoaciduria known as Hartnup disorder. The objective of
the present study was to examine the effect of diets with descending
fish meal (FM) inclusion levels and the addition of salt to the diet
containing the lowest FM level on growth performances, food conversion
ratio, and intestinal SLC6A19 and PEPT1 transcript levels, in
freshwater adapted European sea bass (Dicentrarchus labrax). We
have firstly isolated by molecular cloning and sequencing a fulllength
cDNA representing the neutral aminoacid transporter
SLC6A19 in sea bass. The cDNA sequence was deposited in GenBank
database (accession no. KC812315). The 12 transmembrane
domains and the de novo prediction of the three-dimensional structure
of SLC6A19 protein (634 amino acids) are presented. We then
analysed diet-induced changes in the mRNA copies of SLC6A19 and
PEPT1 genes in different portions of sea bass intestine using realtime
RT-PCR. Sea bass were fed for 6 weeks on different diets, with ascending levels of fat or descending levels of FM, which was
replaced with vegetable meal. The salt-enriched diet was prepared by
adding 3% NaCl to the diet containing 10% FM. SLC6A19 mRNAs in
anterior and posterior intestine of sea bass were not modulated by
dietary protein sources and salt supplementation. Conversely, including
salt in the diet containing low FM percentage, upregulated the
mRNA copies of PEPT1 in hindgut. Fish growth correlated positively
with the content of FM in the diets. Interestingly, the addition of salt
to the diet containing 10% FM improved food intake, as well as specific
growth rate and food conversion ratio.