Functional characterization of Atlantic salmon (Salmo salar L.) PepT2 transporters

Abstract

The high-affinity/low-capacity system Slc15a2 (PepT2) is responsible for the reuptake of di/tripeptides from the renal proximal tubule, but it also operates in many other tissues and organs. Information regarding PepT2 in teleost fish is limited and, to date, functional data are available from the zebrafish (Danio rerio) only. Here, we report the identification of two slc15a2 genes in the Atlantic salmon (Salmo salar) genome, namely slc15a2a and slc15a2b. The two encoded PepT2 proteins share 87% identity and resemble both structurally and functionally the canonical vertebrate PepT2 system. The mRNA tissue distribution analyses reveal a widespread distribution of slc15a2a transcripts, being more abundant in the brain and gills, while slc15a2b transcripts are mainly expressed in the kidney and the distal part of the gastrointestinal tract. The function of the two transporters was investigated by heterologous expression in Xenopus laevis oocytes and two-electrode voltage-clamp recordings of transport and presteady-state currents. Both PepT2a and PepT2b in the presence of Gly-Gln elicit pH-dependent and Na + independent inward currents. The biophysical and kinetic analysis of the recorded currents defined the transport properties, confirming that the two Atlantic salmon PepT2 proteins behave as high-affinity/low-capacity transporters. The recent structures and the previous kinetic schemes of rat and human PepT2 qualitatively account for the characteristics of the two Atlantic salmon proteins. This study is the first to report on the functional expression of two PepT2-type transporters that operate in the same vertebrate organism as a result of (a) gene duplication process(es).

Peptide transporters in teleost fish: a look into the distant past for a more sustainable future

“…Most importantly, the biological, physiological and ecological effects of sustained exposure of fish to runoff antibiotics, as well as how this is contributing to the worldwide antibiotic resistance crisis, remain to be systematically investigated. As industrialization presses on, work in the field of fish transport physiology as that presented here by Vacca et al (2022) is crucial, not merely for a deeper understanding of the cellular and pharmacological mechanisms involved, but perhaps to ignite a tidal change regarding sustainable manufacturing, use and disposal of pharmaceuticals.…”

“…While much is known about the pharmacology, molecular mechanisms and structure of the human orthologues (see for example Killer et al, 2021;Sala-Rabanal et al, 2008), less attention has been paid to lower vertebrates, in particular fish. In this issue of The Journal of Physiology, Vacca et al (2022) help to bridge this knowledge gap by using a combination of phylogenetic, biochemical and biophysical approaches to characterize two distinct genes of the Atlantic salmon (Salmo salar) PepT2. Data are put forward that these transporters are expressed, thus potentially serving a physiological purpose, in fish epithelial and neural tissues.…”

“…Critically, this work puts the spotlight on how by-products of human activity may be adding to the ongoing environmental emergency: β-lactams, such as penicillin derivatives and cephalosporins, which are among the most heavily used antibiotics in global healthcare and industrial land farming, are increasingly polluting streams, lakes and oceans, from where they may find their way into the fish bloodstream through gill, intestine or kidney PepTs. Vacca et al (2022) focus primarily on cultured salmon, and periodic food safety screenings, such as those performed by the Norwegian Institute of Marine Research, ensure that levels of contaminants in farmed seafood are negligible (see, for example, https://www.hi.no/en/hi/nettrapporter/ rapport-fra-havforskningen-en-2021-40); however, less is known about wild-caught fish and shellfish, which are a major part of the diet in many parts of the world. Pharmacokinetics data seem to suggest β-lactam antibiotics do not significantly accumulate in seafood of human consumption, such as the gilthead sea bream (Sparus aurata) (Katharios et al, 2004), thereby questioning the likelihood that they may end up on our dinner plate.…”

“…In this issue of The Journal of Physiology , Vacca et al. (2022) help to bridge this knowledge gap by using a combination of phylogenetic, biochemical and biophysical approaches to characterize two distinct genes of the Atlantic salmon ( Salmo salar ) PepT2. Data are put forward that these transporters are expressed, thus potentially serving a physiological purpose, in fish epithelial and neural tissues.…”

“…Vacca et al. (2022) focus primarily on cultured salmon, and periodic food safety screenings, such as those performed by the Norwegian Institute of Marine Research, ensure that levels of contaminants in farmed seafood are negligible (see, for example, https://www.hi.no/en/hi/nettrapporter/rapport-fra-havforskningen-en-2021-40); however, less is known about wild‐caught fish and shellfish, which are a major part of the diet in many parts of the world. Pharmacokinetics data seem to suggest β‐lactam antibiotics do not significantly accumulate in seafood of human consumption, such as the gilthead sea bream ( Sparus aurata ) (Katharios et al., 2004), thereby questioning the likelihood that they may end up on our dinner plate.…”