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.

Syncytin-1 is an endogenous retroviral envelope glycoprotein specifically expressed in human placenta, where the protein was adopted for its physiological function. After interaction with specific receptors, transmembrane proteins ASCT1 and ASCT2, Syncytin-1 initiates cell-cell fusion leading to formation of multinucleated syncytiotrophoblast, which is essential for feto-maternal nutrients exchange. In this diploma thesis a new cell-cell fusion quantification assay was implemented for characterisation of Syncytin-1 fusion determinants. The assay uses Syncytin-1 and ASCT2 expressed separately with fragments of luciferase in heterologous cell-culture system. The assay enables to specifically quantify cell-cell fusions based on activity of reconstituted luciferase reporter. This study discovered new facts about the role of intracytoplasmic tail of Syncytin-1 in the process of the cell- cell fusion. This specific part of protein contains a tandem motif sensitive to changes in amino acid sequence that led to loss of fusogenic potential of Syncytin-1. It was further confirmed, that the protein Suppressyn works as an inhibitor of cell-cell fusions initiated by Syncytin-1. Suppressyn however does not bind to receptors of Syncytin-1 and the mechanism of its inhibition remains unsolved. Finally, it was demonstrated...