Environmental and nutritional regulation of expression and function of two peptide transporter (PepT1) isoforms in a euryhaline teleost

Expression and function of the oligopeptide transporter PepT1 in response to changes in environmental salinity have received little study despite the important role that dipeptides play in piscine nutrition. We cloned and sequenced two novel full-length cDNAs that encode Fundulus heteroclitus PepT1-type oligopeptide transporters, and examined their expression and functional properties in freshwater- and seawater-acclimated fish and in response to fasting and re-feeding. Phylogenetic analysis of vertebrate SLC15A1 sequences confirms the presence of two PepT1 isoforms, named SLC15A1a and SLC15A1b, in fish. Similar to other vertebrate SLC15A1s, these isoforms have 12 transmembrane domains, and amino acids essential for PepT1 function are conserved. Expression analysis revealed novel environment-specific expression of the SLC15A1 isoforms in F. heteroclitus, with only SLC15A1b expressed in seawater-acclimated fish, and both isoforms expressed in freshwater-acclimated fish. Fasting and re-feeding induced changes in the expression of SLC15A1a and SLC15A1b mRNA. Short-term fasting resulted in up-regulation of PepT1 mRNA levels, while prolonged fasting resulted in down-regulation. The resumption of feeding resulted in up-regulation of PepT1 above pre-fasted levels. Experiments using the in vitro gut sac technique suggest that the PepT1 isoforms differ in functional characteristics. An increased luminal pH resulted in decreased intestinal dipeptide transport in freshwater-acclimated fish but suggested an increased dipeptide transport in seawater-acclimated fish. Overall, this is the first evidence of multiple isoforms of PepT1 in fish whose expression is environmentally dependent and results in functional differences in intestinal dipeptide transport.