mRNA expression of amino acid transporters, aminopeptidase, and the di- and tri-peptide transporter PepT1 in the intestine and liver of posthatch broiler chicks

Amino acid (AA) transporter proteins are responsible for the movement of amino acids in and out of cells. Aminopeptidase cleaves AAs from the N-terminus of polypeptides making them available for transport, while PepT1 is a di- and tripeptide transporter. In the intestine, these proteins are present on the brush border and basolateral membranes of enterocytes, and are essential for the uptake of AAs into enterocytes and their release into circulation. The purpose of this study was to determine the level of transcription of these genes after hatch in 3 regions of the small intestine, the ceca, and liver. Heritage broiler chicks (n = 5) were sampled at day after hatch and days 3, 5, 7, 10, 12, 14, 17, and 21 posthatch, and mRNA expression level was measured using absolute quantitation. The small intestine (duodenum, jejunum, and ileum) expressed the largest quantities of each gene tested. The expression in the ceca and liver was 1 to 3 orders of magnitude less than that of the small intestine. The expression of basolateral transporters in the small intestine was more constant over days posthatch than the expression of brush border transporters. In the ceca the expression of the brush border transporters decreased over the sampling period, while expression of basolateral genes was relatively constant. In the liver the expression of Na+ independent cationic and zwitterionic amino acid transporter (b(o,+) AT), Na+ independent cationic amino acid transporter 2 (CAT2), excitatory amino acid transporter 3 (EAAT3), and the heavy chain corresponding to the b(o,+) system (rBAT) significantly decreased at 12 days posthatch; however, the expression of Na+ independent cationic and Na+ dependent neutral amino acid transporter 1 (y(+) LAT1), Na+ coupled neutral amino acid transporter 1; (SNAT1), and Na+ coupled neutral amino acid transporter 2 (SNAT2) significantly increased at day 5 posthatch compared to day 1 and these levels remained throughout the rest of the sampling period. The current results suggest that at 1 day posthatch chicks are capable of AA processing and transport in the intestine as well as the liver. Additionally the ability of the ceca in transporting AA from the lumen may decrease with age. The liver should be capable of amino acid transport, but its capabilities may be more specific since the expression of several transporters in this organ is either absent or very low.