Characterisation of the H(+)/peptide cotransporter of eel intestinal brush-border membranes

The teleost fish PepT1-type peptide transporters and their relationships with neutral and charged substrates

In teleosts, two PepT1-type (Slc15a1) transporters, i.e., PepT1a and PepT1b, are expressed at the intestinal level. They translocate charged di/tripeptides with different efficiency, which depends on the position of the charged amino acid in the peptide and the external pH. The relation between the position of the charged amino acid and the capability of transporting the dipeptide was investigated in the zebrafish and Atlantic salmon PepT1-type transporters. Using selected charged (at physiological pH) dipeptides: i.e., the negatively charged Asp-Gly and Gly-Asp, and the positively charged Lys-Gly and Gly-Lys and Lys-Met and Met-Lys, transport currents and kinetic parameters were collected. The neutral dipeptide Gly-Gln was used as a reference substrate. Atlantic salmon PepT1a and PepT1b transport currents were similar in the presence of Asp-Gly and Gly-Asp, while zebrafish PepT1a elicited currents strongly dependent on the position of Asp in the dipeptide and zebrafish PepT1b elicited small transport currents. For Lys- and Met-containing dipeptides smaller currents compared to Gly-Gln were observed in PepT1a-type transporters. In general, for zebrafish PepT1a the currents elicited by all tested substrates slightly increased with membrane potential and pH. For Atlantic salmon PepT1a, the transport current increased with negative potential but only in the presence of Met-containing dipeptides and in a pH-dependent way. Conversely, large currents were shown for PepT1b for all tested substrates but Gly-Lys in Atlantic salmon. This shows that in Atlantic salmon PepT1b for Lys-containing substrates the position of the charged dipeptides carrying the Lys residue defines the current amplitudes, with larger currents observed for Lys in the N-terminal position. Our results add information on the ability of PepT1 to transport charged amino acids and show species-specificity in the kinetic behavior of PepT1-type proteins. They also suggest the importance of the proximity of the substrate binding site of residues such as LysPepT1a/GlnPepT1b for recognition and specificity of the charged dipeptide and point out the role of the comparative approach that exploits the natural protein variants to understand the structure and functions of membrane transporters.

Absorption of protein in teleosts: a review

Teleost is a widely diverse group of fishes and so do their feeding habits. From aquaculture points of view, there have been un-interrupted efforts to optimize feeding rates with protein as the chief ingredients in the supplementary diet. However, knowledge on its protein absorption is incomplete so far, to acquire absolute feeding design to mobilize enhanced production of animal-source protein as fish biomass. In this review, the variable protein absorption across digestive tract (DT) in this group of fish has been highlighted. Emphasis is given to outline how DT components, like enterocyte specific absorptive mechanisms, are different in anterior and posterior regions of DT or from the absorptive transporter system. The existence of a transporter-based absorption mechanism brings more variability in the protein absorption in teleosts. At least two such transport systems (Na⁺-dependent and Na⁺-independent) with within-system differences impart more variability to protein absorption. Further, shifting from one stage to another stage of development involves considerable modification of the protein absorptive mechanism in teleosts. Gut microbes may also indirectly facilitate protein absorption in teleosts. Overall, the present review projects a comprehensive understanding of the protein absorption in teleosts that will help to strategize the modulation of feeding technology in fish culture.

Di- and tripeptide transport in vertebrates: the contribution of teleost fish models

Solute Carrier 15 (SLC15) family, alias H⁺-coupled oligopeptide cotransporter family, is a group of membrane transporters known for their role in the cellular uptake of di- and tripeptides (di/tripeptides) and peptide-like molecules. Of its members, SLC15A1 (PEPT1) chiefly mediates intestinal absorption of luminal di/tripeptides from dietary protein digestion, while SLC15A2 (PEPT2) mainly allows renal tubular reabsorption of di/tripeptides from ultrafiltration, SLC15A3 (PHT2) and SLC15A4 (PHT1) possibly interact with di/tripeptides and histidine in certain immune cells, and SLC15A5 has unknown function. Our understanding of this family in vertebrates has steadily increased, also due to the surge of genomic-to-functional information from 'non-conventional' animal models, livestock, poultry, and aquaculture fish species. Here, we review the literature on the SLC15 transporters in teleost fish with emphasis on SLC15A1 (PEPT1), one of the solute carriers better studied amongst teleost fish because of its relevance in animal nutrition. We report on the operativity of the transporter, the molecular diversity, and multiplicity of structural-functional solutions of the teleost fish orthologs with respect to higher vertebrates, its relevance at the intersection of the alimentary and osmoregulative functions of the gut, its response under various physiological states and dietary solicitations, and its possible involvement in examples of total body plasticity, such as growth and compensatory growth. By a comparative approach, we also review the few studies in teleost fish on SLC15A2 (PEPT2), SLC15A4 (PHT1), and SLC15A3 (PHT2). By representing the contribution of teleost fish to the knowledge of the physiology of di/tripeptide transport and transporters, we aim to fill the gap between higher and lower vertebrates.

Ceftiofur pharmacokinetics in Nile tilapia Oreochromis niloticus after intracardiac and intramuscular administrations

Ceftiofur is a broad-spectrum third generation cephalosporin, which acts by inhibiting bacterial cell wall synthesis. It is active against Gram-positive and Gram-negative bacteria such as Aeromonas hydrophila and β-lactamase-producing strains, which are common pathogens in freshwater fish. Ceftiofur pharmacokinetics in Nile tilapia Oreochromis niloticus were studied following single intracardiac (i.c.) or intramuscular (i.m.) administration of ceftiofur sodium (NAXCEL®) in a dose of 5 mg ceftiofur kg-1 body weight. After i.c. injection, ceftiofur plasma concentrations decreased biexponentially, suggesting a 2-compartmental open model. Distribution and elimination half-lives (t0.5(α) and t0.5(β)) were 0.61 ± 0.22 and 0.14 ± 0.03 h mean ±SD, respectively. Elimination constant (Kel) and total body clearances (Cltot) were 3.22 ± 0.48 h-1 and 1.64 ± 0.47 l h-1 kg-1, respectively. Volume of distribution (Vss) and areas under curves (AUC) were 0.12 ± 0.03 l kg-1 and 24.18 ± 8.81 µg ml-1 h, respectively. Following i.m. injection of ceftiofur, plasma concentrations were best described by a 1-compartment open model with a first order absorption; bioavailability was quite high (96.85 ± 23.74%). Plasma maximum concentration (Cmax) was 12.32 ± 6.53 µg ml-1; achieved at time of maximum concentration (Tmax) of 0.74 ± 0.04 h. Absorption and elimination half-lives (t0.5ab and t0.5β) were 0.49 ± 0.06 and 0.53 ± 0.03 h, respectively. In conclusion, i.m. injection of ceftiofur sodium produced extremely high bioavailability with high plasma concentrations that persisted up to 6 h post injection, which may make ceftiofur a useful alternative antibiotic for treatment of brood stock or important ornamental fishes.

Dietary protein hydrolysates and free amino acids affect the spatial expression of peptide transporter PepT1 in the digestive tract of Atlantic cod (Gadus morhua)

The effects of dietary inclusions of size-fractionated peptides and free amino acids (FAAs) on Peptide Transporter 1 (PepT1) mRNA levels were assessed along the length of the intestine of juvenile Atlantic cod (Gadus morhua). Five groups of fish (10-15g) were fed for 46days on diets containing approximately 42% protein, provided either as fish meal (FM, control diet) or as a combination of FM with whole fish hydrolysate (FH), retenate after ultrafiltration of FH (UFR), nanofiltered retenate of FH (NFR), or a mix of FAAs, at a 30% level of FM substitution. PepT1 mRNA expression was assessed in pyloric caeca (S1) and the remainder of the intestine divided into four equally long segments (S2-S5). PepT1 transcripts were found in all segments, indicating that the whole intestine is involved in peptide absorption. Differences in the regional expression profile of PepT1 were found. Under control diet (FM diet) conditions, fish exhibited a reduced expression in S5 compared to S2. In fish fed FAA and UFR diets, PepT1 mRNA levels were higher in S2 and S3 compared to other regions. These data suggest that PepT1 may be variably recruited along the whole intestine, including the most distal part, in response to changes in the luminal protein source content. This adaptive response might be functional to keep a maximal efficiency of protein absorption at the intestinal level.

Atlantic salmon (Salmo salar L.) parr fed genetically modified soybeans and maize: Histological, digestive, metabolic, and immunological investigations

Physiological and health related responses to dietary inclusion of genetically modified (GM) full-fat soybean meal (Roundup Ready; GM-soy) and maize (MON810 Bt-maize; GM-maize), as well as non-parental, untransformed lines (nGM-soy and nGM-maize D2), were evaluated in farmed Atlantic salmon (Salmo salar L.) parr during the first 8 months of feeding. Significant effects of dietary GM presence were only found in intestinal Na+-dependent d-glucose uptake and SGLT1 protein level in the region pyloric caeca in which the highest values were found in the GM-soy, intermediate in the nGM-soy, and lowest in the standard FM fed groups. Data from this study confirm that GM soybeans (RRS) and maize (MON810) at inclusion levels of about 6% appear to be as safe as commercially available nGM soy and maize in diets for Atlantic salmon parr. Results from studies with higher inclusion levels and with non-modified, isogenic or near-isogenic parental lines as control groups are pending.

Is there a compromise between nutrient uptake and gas exchange in the gut of Misgurnus anguillicaudatus, an intestinal air-breathing fish?

The Asian weatherloach, Misgurnus anguillicaudatus (Cobitidae), is a facultative air-breathing teleost fish that makes use of its hindgut or intestine as an accessory air-breathing organ (ABO). The hindgut is highly modified, being well vascularized with intraepithelial capillaries, which makes it well suited for gas exchange. However, the consequences for nutrient uptake, the traditional function of the intestine are unknown. The alimentary canal was examined histologically to assess differences between the fore-, mid- and hindgut regions that have been considered as the digestive, spiral and respiratory zones, respectively. In order to characterise the potential digestive (absorptive) function of the respiratory zone we used semi-quantitative polymerase chain reaction (PCR) to detect the presence of the intestinal Na(+):glucose cotransporter (SGLT1; SLC5A1) and H(+):peptide cotransporter (PEPT1a; SLC15A1) and partially sequenced the SGLT1 and PEPT1a cDNAs. These two transporters play important roles in the absorption of carbohydrate and di-/tripeptides, respectively, in the gut of fishes and other vertebrates and were therefore used as markers for potential nutrient uptake function. We also determined their tissue distributions through semi-quantitative RT-PCR. The effects of diet composition (high protein or high carbohydrate) or fasting on gene expression were also examined. SGLT1 expression was found in kidney, liver, heart, as well as in the three zones of the gut except the most distal part of the hindgut. PEPT1a mRNA was found in heart, brain, liver, and fore- and midgut, but absent in the hindgut. Our results clearly show high expression of SGLT1 (both mRNA and protein by immunolocalization) and PEPT1a (mRNA) in the foregut and midgut correlated with the digestive region of the gut. Modulatory effects of diet on the gene expression for both SGLT1 and PEPT1a were not observed. The presence of SGLT1 transcripts in the respiratory zone of the intestine suggests an overlap in function. However, in the case of PEPT1a, the distal limit was the midgut. Thus, despite its highly modified structure, the hindgut of the loach retains some potential nutrient uptake function.

Transepithelial transport of zinc and L-histidine across perfused intestine of American lobster, Homarus americanus

The intestine of the American lobster, Homarus americanus, was isolated and perfused in vitro with a physiological saline, based on the ion composition of the blood, to characterize the mechanisms responsible for transmural transport of zinc and how the amino acid, L-histidine, affects the net movement of the metal across the tissue. Previous studies with this preparation, focusing on the characteristics of unidirectional mucosa to serosa (M to S) fluxes of (65)Zn(2+) and (3)H-L-histidine, indicated the presence of a brush border co-transport process responsible for simultaneously transferring the metal and amino acid across this tissue as an apparent bis-complex (Zn-[His](2)) using a PEPT-1-like dipeptide carrier mechanism. In addition, both zinc and L-histidine were also transferred toward the blood by separate transporters that were independent of the other substrate. The focus of the present study was to characterize the serosa to mucosa (S to M) flux of (65)Zn(2+) under a variety of conditions, and use these values in conjunction with those from the previous study, to assess the direction and magnitude of net metal movement across the tissue. Transmural S to M transport of (65)Zn(2+) was markedly reduced with the addition of the serosal inhibitors ouabain (32%), excess K(+) (25%), excess Ca(2+) (30%), Cu(2+) (38%), nifedipine (21%), and vanadate (53%). In contrast, this flux was markedly stimulated with the serosal addition of ATP (24%) and excess Na(+) (28%). These results suggest that S to M fluxes of zinc occurred by the combination of the basolateral Na/Ca exchanger (NCX), where zinc replaced calcium, and a basolateral nifedipine-sensitive calcium channel. Transmural M to S (65)Zn(2+) fluxes (5-100 microM) were threefold greater than S to M metal transport, and the addition of luminal L-histidine doubled the net M to S zinc flux over its rate in the absence of the amino acid. The results of this paper and those in its predecessor indicate that zinc transport by the lobster intestine is absorptive and significantly enhanced by luminal amino acids.

Digestive tract ontogeny of Dicentrarchus labrax: implication in osmoregulation

The ontogeny of the digestive tract (DT) and of Na(+)/K(+)-ATPase localization was investigated during the early postembryonic development (from yolk sac larva to juvenile) of the euryhaline teleost Dicentrarchus labrax reared at two salinities: seawater and diluted seawater. Histology, electron microscopy and immunocytochemistry were used to determine the presence and differentiation of ion transporting cells. At hatching, the DT is an undifferentiated straight tube over the yolk sac. At the mouth opening (day 5), it comprises six segments: buccopharynx, esophagus, stomach, anterior intestine, posterior intestine and rectum, well differentiated at the juvenile stage (day 72). The enterocytes displayed ultrastructural features similar to those of mitochondria-rich cells known to be involved in active ion transport. At hatching, ion transporting cells lining the intestine and the rectum exhibited a Na(+)/K(+)-ATPase activity which increased mainly after the larva/juvenile (20 mm) metamorphic transition. The immunofluorescence intensity was dependent upon the stage of development of the gut as well as on the histological configuration of the analyzed segment. The appearance and distribution of enteric ionocytes and the implication of the DT in osmoregulation are discussed.

3H-L-histidine and 65Zn(2+) are cotransported by a dipeptide transport system in intestine of lobster Homarus americanus

The tubular intestine of the American lobster Homarus americanus was isolated in vitro and perfused with a physiological saline whose composition was based on hemolymph ion concentrations and contained variable concentrations of (3)H-l-histidine, (3)H-glycyl-sarcosine and (65)Zn(2+). Mucosa to serosa (M-->S) flux of each radiolabelled substrate was measured by the rate of isotope appearance in the physiological saline bathing the tissue on the serosal surface. Addition of 1-50 micromol l(-1) zinc to the luminal solution containing 1-50 micromol l(-1) (3)H-l-histidine significantly (P<0.01) increased M-->S flux of amino acid compared to controls lacking the metal. The kinetics of M-->S (3)H-l-histidine flux in the absence of zinc followed Michaelis-Menten kinetics (K(m)=6.2+/-0.8 micromol l(-1); J(max) =0.09+/-0.004 pmol cm(-2) min(-1)). Addition of 20 micromol l(-1) zinc to the luminal perfusate increased both kinetic constants (K(m)=19+/-3 micromol l(-1); J(max)=0.28+/-0.02 pmol cm(-2) min(-1)). Addition of both 20 micromol l(-1) zinc and 100 micromol l(-1) l-leucine abolished the stimulatory effect of the metal alone (K(m)=4.5+/-1.7 micromol l(-1); J(max)=0.08+/-0.008 pmol cm(-2) min(-1)). In the absence of l-histidine, M-->S flux of (65)Zn(2+) also followed the Michaelis-Menten relationship and addition of l-histidine to the perfusate significantly (P<0.01) increased both kinetic constants. Addition of either 50 micromol l(-1) Cu(+) or Cu(2+) and 20 micromol l(-1) l-histidine simultaneously abolished the stimulatory effect of l-histidine alone on transmural (65)Zn(2+) transport. Zinc-stimulation of M-->S (3)H-l-histidine flux was significantly (P<0.01) reduced by the addition of 100 micromol l(-1) glycyl-sarcosine to the perfusate, as a result of the dipeptide significantly (P<0.01) reducing both l-histidine transport K(m) and J(max). Transmural transport of (3)H-glycyl-sarcosine was unaffected by the presence of either l-histidine or l-leucine when either amino acid was added to the perfusate alone, but at least a 50% reduction in peptide transport was observed when zinc and either of the amino acids were added simultaneously. These results show that (3)H-l-histidine and (65)Zn(2+) are cotransported across the lobster intestine by a dipeptide carrier protein that binds both substrates in a bis-complex (Zn-[His](2)) resembling the normal dipeptide substrate. In addition, the transmural transports of both substrates may also occur by uncharacterized carrier processes that are independent of one another and appear relatively specific to the solutes used in this study.

The Smallest Vertebrate, Teleost Fish, Can Utilize Synthetic Dipeptide-Based Diets

In vitro studies of brush-border intestinal transport of dipeptides and cytoplasmic hydrolysis in fish suggest that these processes could be key mechanisms in the absorption and utilization of nutrients for growth. However, in vivo experimentation to study the nutritional importance of these processes was needed. We compared three dietary formulations based on free, peptide and protein sources of amino acids. Our results were the first to show that a synthetic dipeptide (PP)-based diet could support growth in the early stages of ontogenesis of a teleost fish, rainbow trout (Oncorhynchus mykiss), whereas a free amino acid (FAA)-based diet failed. We found that fish fed an FAA-based diet had an increased rate of ammonia excretion [1.78 +/- 0.19 mmol NH(3)-N/(kg body wt.h)], compared with fish fed a PP-based diet [1.25 +/- 0.07 mmol NH(3)-N/(kg body wt.h)], suggesting that deamination is involved in the metabolism of dietary FAA. Teleost fish are known to obtain a high proportion of total energy from protein, compared with higher vertebrates. However, we found that feeding trout alevins a PP-based diet increased postprandial oxygen consumption for 2 to 24 h, whereas other treatments decreased 24-h postprandial metabolism. This may indicate that peptide metabolism is less efficient than protein metabolism. Juvenile rainbow trout differed from alevins in their response to FAA- and PP-based diets. These observations strongly suggest that intestinal dietary peptide transport and hydrolysis could support protein synthesis and growth in vertebrates that respond poorly to FAA-based diets. We conclude that nutrient administration may be improved by manipulating dietary peptide composition and peptide/protein ratios, leading to better utilization of synthetic peptides, with nutritional and therapeutic implications for all vertebrates.

Characterisation of intestinal peptide transporter of the Antarctic haemoglobinless teleost Chionodraco hamatus

H(+)/peptide cotransport was studied in brush-border membrane vesicles (BBMV) from the intestine of the haemoglobinless Antarctic teleost Chionodraco hamatus by monitoring peptide-dependent intravesicular acidification with the pH-sensitive dye Acridine Orange. Diethylpyrocarbonate-inhibited intravesicular acidification was specifically achieved in the presence of extravesicular glycyl-L-proline (Gly-L-Pro) as well as of glycyl-L-alanine (Gly-L-Ala) and D-phenylalanyl-L-alanine (D-Phe-L-Ala). H(+)/Gly-L-Pro cotransport displayed saturable kinetics, involving a single carrier system with an apparent substrate affinity (K(m,app)) of 0.806+/-0.161 mmol l(-1). Using degenerated primers from eel and human (PepT1) transporter sequence, a reverse transcription-polymerase chain reaction (RT-PCR) signal was detected in C. hamatus intestine. RT-PCR paralleled kinetic analysis, confirming the hypothesis of the existence of a PepT1-type transport system in the brush-border membranes of icefish intestine. Functional expression of H(+)/peptide cotransport was successfully performed in Xenopus laevis oocytes after injection of poly(A)(+) RNA (mRNA) isolated from icefish intestinal mucosa. Injection of mRNA stimulated D-Phe-L-Ala uptake in a dose-dependent manner and an excess of glycyl-L-glutamine inhibited this transport. H(+)/peptide cotransport in the Antarctic teleost BBMV exhibited a marked difference in temperature optimum with respect to the temperate teleost Anguilla anguilla, the maximal activity rate occurring at approximately 0 degrees C for the former and 25 degrees C for the latter. Temperature dependence of icefish and eel intestinal mRNA-stimulated uptake in the heterologous system (oocytes) was comparable.

Transport of peptidomimetic drugs by the intestinal Di/tri-peptide transporter, PepT1

The apical membrane of small intestinal enterocytes possess an uptake system for di- and tripeptides. The physiological function of the system is to transport small peptides resulting from digestion of dietary protein. Moreover, due to the broad substrate specificity of the system, it is also capable of transporting a number of orally administered peptidomimetic drugs. Absorbed peptides may be hydrolysed in the cells due to the high peptidase activity present in the cytosol. Peptidomimetic drugs may, if resistant to the cellular enzyme activity, pass the basolateral membrane via a basolateral peptide transport mechanism and enter the systemic circulation. As the number of new peptide and peptidomimetic drugs are rapidly increasing, the peptide transport system has gained increasing attention as a possible drug delivery system for small peptides and peptide-like compounds. In this paper we give an updated introduction to the transport system and discuss the substrate characteristics of the di/tri-peptide transporter system with special emphasis on chemically modified substrates and prodrugs.