Transport of neutral, cationic and anionic amino acids by systems B, b(o,+), X(AG), and ASC in swine small intestine

Absorption of methionine sources in animals-is there more to know?

This literature review evaluates the absorption of methionine (Met) sources such as 2-hydroxy-4-methylthiobutyric acid (HMTBa), its calcium salts (HMTBa-Ca), and DL-methionine (DL-Met) by focusing on the state of knowledge regarding the absorption mechanism, experimental methodology, and factors affecting their absorption. The 2 Met sources differ in mechanism and site of absorption due to differences in their chemical characteristics and enzymatic conversion. This review addresses diffusion- and transport-mediated absorption systems for amino acids and carboxylic compounds, best elucidated by in vitro, ex vivo, and in vivo experimental models. Opportunities and limitations in the use of radioisotopes to depict absorption sites as well as host and microbial metabolism are described. Physiological and environmental conditions that lead to changes in gut absorptive capacity and the impact of Met source absorption are also evaluated. This review concludes that any comparison between HMTBa and DL-Met should consider their different behaviors during the absorption phase. Hence, the chemical characteristics of these 2 molecules entail different absorption sites and mechanisms, from passive absorption in the case of HMTBa and HMTBa-Ca to active transporters for DL-Met, HMTBa, and HMTBa-Ca. In addition, the different conversion modes of these 2 molecules further differentiate their absorption modes. Considering these important differences, it is easier to understand the apparent divergence between the conclusions of existing publications. When comparing these 2 molecules, it is recommended to properly adapt to the conditions under which the absorption of Met sources is evaluated.

SLC transporters ASCT2, B0 AT1-like, y+ LAT1, and LAT4-like associate with methionine electrogenic and radio-isotope flux kinetics in rainbow trout intestine

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.

The Human SLC1A5 Neutral Amino Acid Transporter Catalyzes a pH-Dependent Glutamate/Glutamine Antiport, as Well

ASCT2 is a neutral amino acid transporter, which catalyzes a sodium-dependent obligatory antiport among glutamine and other neutral amino acids. The human ASCT2 over-expressed in Pichia pastoris and reconstituted in proteoliposomes has been employed for identifying alternative substrates of the transporter. The experimental data highlighted that hASCT2 also catalyzes a sodium-dependent antiport of glutamate with glutamine. This unconventional antiport shows a preferred sidedness: glutamate is inwardly transported in exchange for glutamine transported in the counter direction. The orientation of the transport protein in proteoliposomes is the same as in the cell membrane; then, the observed sidedness corresponds to the transport of glutamate from the extracellular to the intracellular compartment. The competitive inhibition exerted by glutamate on the glutamine transport together with the docking analysis indicates that the glutamate binding site is the same as that of glutamine. The affinity for glutamate is lower than that for neutral amino acids, while the transport rate is comparable to that measured for the asparagine/glutamine antiport. Differently from the neutral amino acid antiport that is insensitive to pH, the glutamate/glutamine antiport is pH-dependent with optimal activity at acidic pH on the external (extracellular) side. The stimulation of glutamate transport by a pH gradient suggests the occurrence of a proton flux coupled to the glutamate transport. The proton transport has been detected by a spectrofluorometric method. The rate of proton transport correlates well with the rate of glutamate transport indicating a 1:1 stoichiometry H⁺: glutamate. The glutamate/glutamine antiport is also active in intact HeLa cells. On a physiological point of view, the described antiport could have relevance in some districts in which a glutamate/glutamine cycling is necessary, such as in placenta.

Dietary Supplementation of dl-Methionine Potently Induces Sodium-Dependent l-Methionine Absorption in Porcine Jejunum Ex Vivo

BACKGROUND

Methionine is an essential amino acid (AA) with many fundamental roles. Humans often supplement l-Met, whereas dl-Met and dl-2-hydroxy-4-(methylthio)butanoic acid (dl-HMTBA) are more frequently used to supplement livestock.

OBJECTIVES

The study aimed to investigate whether dietary Met source alters the absorptive capacity for Met isomers in the small intestine of piglets.

METHODS

A total of 27 male 10-wk-old piglets in 3 feeding groups received a diet supplemented with 0.21% dl-Met, 0.21% l-Met, or 0.31% dl-HMTBA to meet the Met + cystine requirement. After ≥10 d, absorptive fluxes of d-Met or l-Met were measured at a physiological concentration of 50 μM and a high concentration of 5 mM in duodenum, middle jejunum, and ileum ex vivo. Data were compared by 2-factor ANOVA.

RESULTS

Across diets, fluxes of both Met isomers at both tested concentrations increased from duodenum to ileum by a factor of ∼2-5.5 (P < 0.05). Pigs supplemented with dl-Met had greater (P < 0.085) absorptive fluxes at 50 μM l-Met (0.50, 2.07, and 3.86 nmol · cm-2 · h-1) and d-Met (0.62, 1.41, and 1.19 nmol · cm-2 · h-1) than did pigs supplemented with dl-HMTBA (l-Met: 0.28, 0.76, and 1.08 nmol · cm-2 · h-1; d-Met: 0.34, 0.58, and 0.64 nmol · cm-2 · h-1) in duodenum, jejunum, and ileum, respectively. Only in jejunum of dl-Met-fed pigs, fluxes at 50 μM l-Met were reduced by the omission of luminal Na+ (from 3.27 to 0.86 nmol · cm-2 · h-1; P < 0.05) and by a cocktail of 22 luminal AAs (to 1.05 nmol · cm-2 · h-1; P < 0.05).

CONCLUSIONS

Dietary supplementation of dl-Met increases the efficiency of l-Met and d-Met absorption at physiologically relevant luminal Met concentrations along the small intestine of pigs, including a very prominent induction of an Na+-dependent transport system with preference for l-Met in the mid-jejunum. Dietary supplementation with dl-Met could be a promising tool to improve the absorption of Met and other AAs.

Amino Acid Transport Across the Mammalian Intestine

The small intestine mediates the absorption of amino acids after ingestion of protein and sustains the supply of amino acids to all tissues. The small intestine is an important contributor to plasma amino acid homeostasis, while amino acid transport in the large intestine is more relevant for bacterial metabolites and fluid secretion. A number of rare inherited disorders have contributed to the identification of amino acid transporters in epithelial cells of the small intestine, in particular cystinuria, lysinuric protein intolerance, Hartnup disorder, iminoglycinuria, and dicarboxylic aminoaciduria. These are most readily detected by analysis of urine amino acids, but typically also affect intestinal transport. The genes underlying these disorders have all been identified. The remaining transporters were identified through molecular cloning techniques to the extent that a comprehensive portrait of functional cooperation among transporters of intestinal epithelial cells is now available for both the basolateral and apical membranes. Mouse models of most intestinal transporters illustrate their contribution to amino acid homeostasis and systemic physiology. Intestinal amino acid transport activities can vary between species, but these can now be explained as differences of amino acid transporter distribution along the intestine. © 2019 American Physiological Society. Compr Physiol 9:343-373, 2019.

Expression of Glutamate Transporters in Mouse Liver, Kidney, and Intestine

Glutamate transport activities have been identified not only in the brain, but also in the liver, kidney, and intestine. Although glutamate transporter distributions in the central nervous system are fairly well known, there are still uncertainties with respect to the distribution of these transporters in peripheral organs. Quantitative information is mostly lacking, and few of the studies have included genetically modified animals as specificity controls. The present study provides validated qualitative and semi-quantitative data on the excitatory amino acid transporter (EAAT)1-3 subtypes in the mouse liver, kidney, and intestine. In agreement with the current view, we found high EAAT3 protein levels in the brush borders of both the distal small intestine and the renal proximal tubules. Neither EAAT1 nor EAAT2 was detected at significant levels in murine kidney or intestine. In contrast, the liver only expressed EAAT2 (but 2 C-terminal splice variants). EAAT2 was detected in the plasma membranes of perivenous hepatocytes. These cells also expressed glutamine synthetase. Conditional deletion of hepatic EAAT2 did neither lead to overt neurological disturbances nor development of fatty liver.

Identification and characterization of rabbit small intestinal villus cell brush border membrane Na-glutamine cotransporter

Glutamine, the primary metabolic fuel for the mammalian small intestinal enterocytes, is primarily assimilated by Na-amino acid cotransporters. Although Na-solute cotransport has been shown to exist in the brush border membrane (BBM) of the absorptive villus cells, the identity of Na-glutamine cotransport in rabbit small intestinal villus cells was unknown. Na-dependent glutamine uptake is present in villus BBM vesicles. An intravesicular proton gradient did not stimulate this Na-dependent glutamine uptake, whereas Li+ did not significantly suppress this uptake. These observations in concert with amino acid substitution studies suggested that Na-glutamine cotransporter in the villus cell BBM was the newly identified cotransporter B0AT1 (SLC6A19). Quantitative real-time PCR identified the message for this cotransporter in villus cells. Thus a full-length cDNA of B0AT1 was cloned and expressed in MDA-MB-231 cells. This expressed cotransporter exhibited characteristics similar to those observed in villus cells from the rabbit small intestine. Antibody was generated for B0AT1 that demonstrated the presence of this cotransporter protein in the villus cell BBM. Kinetic studies defined the kinetic parameters of this cotransporter. Thus this study describes the identification, cloning, and characterization of the Na-amino acid cotransporter responsible for the assimilation of a critical amino acid by the absorptive villus cells in the mammalian small intestine.

Amino acid transport across mammalian intestinal and renal epithelia

The transport of amino acids in kidney and intestine is critical for the supply of amino acids to all tissues and the homeostasis of plasma amino acid levels. This is illustrated by a number of inherited disorders affecting amino acid transport in epithelial cells, such as cystinuria, lysinuric protein intolerance, Hartnup disorder, iminoglycinuria, dicarboxylic aminoaciduria, and some other less well-described disturbances of amino acid transport. The identification of most epithelial amino acid transporters over the past 15 years allows the definition of these disorders at the molecular level and provides a clear picture of the functional cooperation between transporters in the apical and basolateral membranes of mammalian epithelial cells. Transport of amino acids across the apical membrane not only makes use of sodium-dependent symporters, but also uses the proton-motive force and the gradient of other amino acids to efficiently absorb amino acids from the lumen. In the basolateral membrane, antiporters cooperate with facilitators to release amino acids without depleting cells of valuable nutrients. With very few exceptions, individual amino acids are transported by more than one transporter, providing backup capacity for absorption in the case of mutational inactivation of a transport system.

Characterization of mouse amino acid transporter B0AT1 (slc6a19)

The mechanism of the mouse (m)B0AT1 (slc6a19) transporter was studied in detail using two electrode voltage-clamp techniques and tracer studies in the Xenopus oocyte expression system. All neutral amino acids induced inward currents at physiological potentials, but large neutral non-aromatic amino acids were the preferred substrates of mB0AT1. Substrates were transported with K0.5 values ranging from approx. 1 mM to approx. 10 mM. The transporter mediates Na+-amino acid co-transport with a stoichiometry of 1:1. No other ions were involved in the transport mechanism. An increase in the extracellular Na+ concentration reduced the K0.5 for leucine, and vice versa. Moreover, the K0.5 values and Vmax values of both substrates varied with the membrane potential. As a result, K0.5 and Vmax values are a complex function of the concentration of substrate and co-substrate and the membrane potential. A model is presented assuming random binding order and a positive charge associated with the ternary [Na+-substrate-transporter] complex, which is consistent with the experimental data.

The molecular basis of neutral aminoacidurias

Recent success in the molecular cloning and identification of apical neutral amino acid transporters has shed a new light on inherited neutral amino acidurias, such as Hartnup disorder and Iminoglycinuria. Hartnup disorder is caused by mutations in the neutral amino acid transporter B(0) AT1 (SLC6A19). The transporter is found in kidney and intestine, where it is involved in the resorption of all neutral amino acids. The molecular defect underlying Iminoglycinuria has not yet been identified. However, two transporters, the proton amino acid transporter PAT1 (SLC36A1) and the IMINO transporter (SLC6A20) appear to play key roles in the resorption of glycine and proline. A model is presented, involving all three transporters that can explain the phenotypic variability of iminoglycinuria.

Expression of heteromeric amino acid transporters along the murine intestine

Members of the new heterodimeric amino acid transporter family are composed of two subunits, a catalytic multitransmembrane spanning protein (light chain) and a type II glycoprotein (heavy chain). These transporters function as exchangers and thereby extend the transmembrane amino acid transport selectivity to specific amino acids. The heavy chain rBAT associates with the light chain b degrees (,+)AT to form a cystine and cationic amino acid transporter. The other heavy chain, 4F2hc, can interact with seven different light chains to form various transporters corresponding to systems L, y(+)L, asc or x(-)(c). The importance of some of these transporters in intestinal and renal (re)absorption of amino acids is highlighted by the fact that mutations in either the rBAT or b degrees (,+)AT subunit result in cystinuria whereas a defect in the y(+)-LAT1 light chain causes lysinuric protein intolerance. Here we investigated the localization of these transporters in intestine since both diseases are also characterized by altered intestinal amino acid absorption. Real time PCR showed organ-specific expression patterns for all transporter subunit mRNAs along the intestine and Western blotting confirmed these findings on the protein level. Immunohistochemistry demonstrated basolateral coexpression of 4F2hc, LAT2 and y(+)-LAT1 in stomach and small intestine, whereas rBAT and b degrees (,+)AT were found colocalizing on the apical side of small intestine epithelium. In stomach, 4F2hc and LAT2 were localized in H(+)/K(+)-ATPase-expressing parietal cells. The abundant expression of several members of the heterodimeric transporter family along the murine small intestine suggests their involvement in amino acids absorption. Furthermore, strong expression of rBAT, b degrees (,+)AT and y(+)-LAT1 in the small intestine explains the reduced intestinal absorption of some amino acid in patients with cystinuria or lysinuric protein intolerance.

Drug delivery to the retina: challenges and opportunities

Retinal drug delivery is a challenging area in the field of ophthalmic drug delivery. An ideal drug delivery system for the retina and vitreous humor has not yet been found, despite extensive research. Drug delivery to retinal tissue and vitreous via systemic administration is constrained due to the presence of a blood-retinal barrier (BRB) which regulates permeation of substances from blood to the retina. Although intravitreal administration overcomes this barrier, it is associated with several other problems. In recent years, transporter targeted drug delivery has become a clinically significant drug delivery approach for enhancing the bioavailabilities of drug molecules with poor membrane permeability characteristics. Various nutrient transporters, which include peptide, amino acid, folate, monocarboxylic acid transporters and so on, have been reported to be expressed on the retina and BRB. Prodrug derivatisation of drug molecules which target these transporters could result in enhanced ocular bioavailability. Highlighted in this review are various strategies currently employed for drug delivery to the posterior chamber, and novel opportunities that can be exploited to enhance ocular bioavailability of drugs.

Function and structure of heterodimeric amino acid transporters

Heterodimeric amino acid transporters are comprised of two subunits, a polytopic membrane protein (light chain) and an associated type II membrane protein (heavy chain). The heavy chain rbAT (related to b(0,+) amino acid transporter) associates with the light chain b(0,+)AT (b(0,+) amino acid transporter) to form the amino acid transport system b(0,+), whereas the homologous heavy chain 4F2hc interacts with several light chains to form system L (with LAT1 and LAT2), system y(+)L (with y(+)LAT1 and y(+)LAT2), system x (with xAT), or system asc (with asc1). The association of light chains with the two heavy chains is not unambiguous. rbAT may interact with LAT2 and y(+)LAT1 and vice versa; 4F2hc may interact with b(0,+)AT when overexpressed. 4F2hc is necessary for trafficking of the light chain to the plasma membrane, whereas the light chains are thought to determine the transport characteristics of the respective heterodimer. In contrast to 4F2hc, mutations in rbAT suggest that rbAT itself takes part in the transport besides serving for the trafficking of the light chain to the cell surface. Heavy and light subunits are linked together by a disulfide bridge. The disulfide bridge, however, is not necessary for the trafficking of rbAT or 4F2 heterodimers to the membrane or for the functioning of the transporter. However, there is experimental evidence that the disulfide bridge in the 4F2hc/LAT1 heterodimer plays a role in the regulation of a cation channel. These results highlight complex interactions between the different subunits of heterodimeric amino acid transporters and suggest that despite high grades of homology, the interactions between rbAT and 4F2hc and their respective partners may be different.

Glutamine transporter in crypts compensates for loss of villus absorption in bovine cryptosporidiosis

Cryptosporidium parvum infection represents a significant cause of diarrhea in humans and animals. We studied the effect of luminally applied glutamine and the PG synthesis inhibitor indomethacin on NaCl absorption from infected calf ileum in Ussing chambers. Infected ileum displayed a decrease in both mucosal surface area and NaCl absorption. Indomethacin and glutamine or its stable derivative alanyl-glutamine increased the net absorption of Na(+) in infected tissue in an additive manner and to a greater degree than in controls. Immunohistochemical and Western blot studies showed that in control animals neutral amino acid transport system ASC was present in villus and crypts, whereas in infected animals, ASC was strongly present only on the apical border of crypts. These results are consistent with PGs mediating the altered NaCl and water absorption in this infection. Our findings further illustrate that the combined use of a PG synthesis inhibitor and glutamine can fully stimulate Na(+) and Cl(-) absorption despite the severe villous atrophy, an effect associated with increased expression of a Na(+)-dependent amino acid transporter in infected crypts.