Expression of porcine intestinal nutrient transporters along crypt-villus axis and during postnatal development
This research was conducted to investigate the expression of porcine intestinal nutrient transporters along the neonatal crypt-villus axis and during the postnatal development. First, we examined the transport kinetics of Na+-glucose co-tranporter 1 (SGLT1) and Na+-dependent neutral amino acid (AA) transporter B0AT1 and then the protein and mRNA abundances of SGLT1, B0AT1 and Na+-dependent neutral AA exchanger ASCT2 along the jejunal crypt-villus axis in the neonatal pig and the potential mechanisms associated with their regulations. Our results suggested that: 1) high levels of apical maximal SGLT1 and B0AT1 uptake activities were shown to exist along the entire jejunal crypt-villus axis in the neonatal pig; 2) there were no significant differences in the SGLT1, B0AT1 and ASCT2 protein abundances in spite of their different mRNA abundances among the crypt-villus axis, suggesting unique posttranscriptional regulatory mechanisms; and 3) global protein translational efficiency, as assessed by examining some of the key protein translational initiation and elongation factors, was higher in the crypt cells than in the upper villus cells, likely playing a regulatory role for maintaining apical nutrient transporter abundances in crypt cells of the neonate. Second, we further examined the protein and mRNA abundances of jejunal neutral AA transporters B0AT1 and ASCT2 and acidic AA transporter EAAC1 during the postnatal development in pigs at the ages of d 1, 4, 6, 12, 20, 28 (1-wk post-weaning), and 70 (mature gut at grower phase), respectively. Our results showed that the jejunal apical B0AT1, ASCT2 and EAAC1 protein abundances were dramatically decreased during the postnatal development and were likely regulated at both the transcriptional and post-transcriptional levels. These substantial decreases in the small intestinal apical Na+-dependent AA transporter abundances may contribute to increased intestinal microbial catabolism of AA, which may be partially responsible for the reduced whole body efficiency of nitrogen utilization during the postnatal growth in pigs. Collectively, our results suggest that apical nutrient transporters SGLT1, B0AT1 and ASCT2 are abundantly expressed along the entire jejunal crypt-villus axis in the neonatal pig, whereas abundances of jejunal apical AA transporters EAAC1, B0AT1 and ASCT2 declined substantially during the postnatal growth in pigs.