SLC transporters ASCT2, B⁰AT1‐like, y⁺LAT1, and LAT4‐like associate with methionine electrogenic and radio‐isotope flux kinetics in rainbow trout intestine

Abstract

Methionine (Met) is an important building block and metabolite for protein biosynthesis. However, the mechanism behind its absorption in the fish gut has not been elucidated. Here, we describe the fundamental properties of Met transport along trout gut at µmol/L and mmol/L concentration. Both electrogenic and unidirectional DL‐[14C]Met flux were employed to characterize Met transporters in Ussing chambers. Exploiting the differences in gene expression between diploid (2N) and triploid (3N) and intestinal segment as tools, allowed the association between gene and methionine transport. Specifically, three intestinal segments including pyloric caeca (PC), midgut (MG), and hindgut (HG) were assessed. Results at 0–150 µmol/L concentration demonstrated that the DL‐Met was most likely transported by apical transporter ASCT2 (SLC1A5) and recycled by basolateral transporter y+LAT1 (SLC7A7) due to five lines of observation: (1) lack of Na+‐independent kinetics, (2) low expression of B0AT2‐like gene, (3) Na+‐dependent, high‐affinity (K m, µmol/L ranges) kinetics in DL‐[14C]Met flux, (4) association mRNA expression with the high‐affinity kinetics and (5) electrogenic currents induced by Met. Results at 0.2–20 mmol/L concentration suggested that the DL‐Met transport is likely transported by B0AT1‐like (SLC6A19‐like) based on gene expression, Na+‐dependence and low‐affinity kinetics (K m, mmol/L ranges). Similarly, genomic and gene expression analysis suggest that the basolateral exit of methionine was primarily through LAT4‐like transporter (SLC43A2‐like). Conclusively, DL‐Met uptake in trout gut was most likely governed by Na+‐dependent apical transporters ASCT2 and B0AT1‐like and released through basolateral LAT4‐like, with some recycling through y+LAT1. A comparatively simpler model than that previously described in mammals.

Metabolic and nutritional responses of Nile tilapia juveniles to dietary methionine sources

“…Similarly, in broiler chicks fed 14 C-DL-Met or 14 C-MHA a higher amount of tracer was present in the excrements of 14 C-MHA fed birds than in excrements of birds fed the 14 C-DL-Met (39,40) . DL-Met and MHA uptake is mediated by Na-dependent transporters across the brush border membrane (9,10) ; however, MHA is also partly transported by diffusion (41) . In rainbow trout, it has been demonstrated that MHA presents a slower intestinal absorption rate than DL-Met (10) .…”

“…MHA chemical structure is similar to that of Met; however, it contains a hydroxyl group instead of the amino group. Recently, studies in rainbow trout (Oncorhynchus mykiss) suggested that DL-Met and MHA uptake in the gut apical surface is facilitated by Na-dependent transporters and mediated by proton-independent transporters across the basolateral membrane (9,10) . Data comparing the intestinal flux rates of DL-Met and MHA suggest a faster intestinal transport of the former synthetic source (8,10,11) .…”

Interactions of Environmental Chemicals and Natural Products With ABC and SLC Transporters in the Digestive System of Aquatic Organisms

“…In fish, SLCs play an important role in organismal development and transport of endogenous compounds to allow for proper development of swim bladders ( Kim et al, 2020 ), inner ears ( Liu et al, 2015 ), kidneys ( Schoels et al, 2021 ), and normal development of embryos ( Takesono et al, 2012 ). In addition, SLCs play a crucial role in intestinal absorption and transport of amino acids and other physiological compounds, including thyroid hormones ( Arjona et al, 2011 ; Muzzio et al, 2014 ), iron ( Cooper et al, 2007 ), zinc ( Yan et al, 2012 ; Jiang et al, 2014 ), coenzyme-A ( Kim et al, 2020 ), and amino acids and peptides ( Romano et al, 2006 , 2014 ; Verri et al, 2010 , 2017 ; Margheritis et al, 2013 ; Rimoldi et al, 2015 ; Song et al, 2017 ; To et al, 2019 ; Vacca et al, 2019 ). Several SLC transporters have been shown to be involved in intestinal methionine absorption as well as iron transport and acid-base regulation in the gills ( Cooper et al, 2007 ; Ivanis et al, 2008 ; To et al, 2019 ).…”

Dietary electrolyte balance of Atlantic salmon (Salmo salar) freshwater feeds: Impact on osmoregulation, mineral metabolism and performance in seawater

“…The dEB of the feeds can alter the acid secretion in the stomach and has been shown to improve protein digestibility when fed high dEB levels in tilapia (Saravanan et al, 2013). Further, the role of Na + dependent mechanisms in the uptake of amino acids can be a plausible reason for improved protein digestibility (Subramaniam et al, 2020;To et al, 2019), which could indirectly influence Zn uptake via amino acid uptake pathways (Prabhu et al, 2018;Glover and Hogstrand, 2002). The concurrent contrary effects in plasma concentrations of Mn and Cu also point towards altered Zn absorption by change in dEB, as Zn has antagonistic effects on Cu and Mn absorption (Clearwater et al, 2002;Knox et al, 1984;Knox et al, 1982).…”