Amino Acid Transport Across the Mammalian Intestine

Dynamic Changes in Postprandial Plasma Free Amino Acid Levels of the Hepatic Portal, Hepatic, and Jugular Veins in the Healthy Pre-Ruminant Calves

Sufficient amino acids (AAs) supply is crucial in growing animals to maintain the rapid skeletal muscle protein synthesis and healthy growth. Liver is known to be the major organ that plays a central role in AA metabolism. Seeing as few studies have been made to investigate the dynamic changes of postprandial AAs over a short time interval before and after the liver, a first attempt was made to investigate the changes in postprandial free AA levels over eight time points with short interval in plasma, collected simultaneously from the hepatic portal, hepatic, and jugular veins, to better understand the intrahepatic, pre- and post-hepatic AA metabolisms. AAs absorption and uptake by liver occurred soon after feeding and most of the AAs peaked at 30 min postprandial. Two postprandial peaks of the plasma total free AAs, essential AAs (EAAs), and non-essential AAs were observed in hepatic portal and hepatic veins, indicating that two phases of digestion and absorption of AAs may occur in the gastrointestinal tract of the pre-ruminant calves. Individual free AAs showed three main AA transition profiles over time. The total EAA concentration at 240 min postprandial was significantly higher (p < 0.05) in the HPV than in the JV.

Regionalization of intestinal microbiota and metabolites in the small intestine of the Bactrian camel

Introduction

Peyer's patches (PPs) are crucial antigen-inductive sites of intestinal mucosal immunity. Prior research indicated that, in contrast to other ruminants, PPs in the small intestine of Bactrian camels are found in the duodenum, jejunum, and ileum and display polymorphism. Using this information, we analyzed the microbial and metabolic characteristics in various segments of the Bactrian camel's small intestine to further elucidate how the immune system varies across different regions.

Methods

In this study, the microbiota and metabolite of 36 intestinal mucosal samples, including duodenal (D-PPs), jejunal (J-PPs), and ileal PPs (I-PPs), were profiled for six Bactrian camels using 16S rRNA gene sequencing and liquid chromatography with tandem mass spectrometry (LC-MS/MS). To confirm meaningful associations, we conducted connection analyses on the significantly different objects identified in each group's results. ELISA was used to analyze the levels of IgA, IgG, and IgM in the same tissues.

Results

The microbiota and metabolite profiles of J-PPs and I-PPs were found to be similar, whereas those of D-PPs were more distinct. In J-PPs and I-PPs, the dominant bacterial genera included Clostridium, Turicibacter, and Shigella. In contrast, D-PPs had a significant increase in the abundance of Prevotella, Fibrobacter, and Succinobacter. Regarding the metabolomics, D-PPs exhibited high levels of polypeptides, acetylcholine, and histamine. On the other hand, J-PPs and I-PPs were characterized by an enrichment of free amino acids, such as L-arginine, L-glutamic acid, and L-serine. These metabolic differences mainly involve amino acid production and metabolic processes. Furthermore, the distribution of intestinal immunoglobulins highlighted the specificity of D-PPs. Our results indicated that proinflammatory microbes and metabolites were significantly enriched in D-PPs. In contrast, J-PPs and I-PPs contained substances that more effectively enhance immune responses, as evidenced by the differential distribution of IgA, IgG, and IgM.

Discussion

The intestinal microenvironment of Bactrian camels displays distinct regional disparities, which we propose are associated with variations in immunological function throughout different segments of the small intestine. This study highlights the specific traits of the intestinal microbiota and metabolites in Bactrian camels, offering a valuable reference for understanding the relationship between regional intestinal immunity and the general health and disease of the host.

Epithelial metabolism as a rheostat for intestinal inflammation and malignancy

The gut epithelium protects the host from a potentially hostile environment while allowing nutrient uptake that is vital for the organism. To maintain this delicate task, the gut epithelium has evolved multilayered cellular functions ranging from mucus production to hormone release and orchestration of mucosal immunity. Here, we review the execution of intestinal epithelial metabolism in health and illustrate how perturbation of epithelial metabolism affects experimental gut inflammation and tumorigenesis. We also discuss the impact of environmental factors and host-microbe interactions on epithelial metabolism in the context of inflammatory bowel disease and colorectal cancer. Insights into epithelial metabolism hold promise to unravel mechanisms of organismal health that may be therapeutically exploited in humans in the future.

Regulation of Enterocyte Brush Border Membrane Primary Na-Absorptive Transporters in Human Intestinal Organoid-Derived Monolayers

In the small intestine, sodium (Na) absorption occurs primarily via two apical transporters, Na-hydrogen exchanger 3 (NHE3) and Na-glucose cotransporter 1 (SGLT1). The two primary Na-absorptive pathways were previously shown to compensatorily regulate each other in rabbit and rat intestinal epithelial cells. However, whether NHE3 and SGLT1 regulate one another in normal human enterocytes is unknown, mainly due to a lack of appropriate experimental models. To investigate this, we generated 2D enterocyte monolayers from human jejunal 3D organoids and used small interfering RNAs (siRNAs) to knock down NHE3 or SGLT1. Molecular and uptake studies were performed to determine the effects on NHE3 and SGLT1 expression and activity. Knockdown of NHE3 by siRNA in enterocyte monolayers was verified by qPCR and Western blot analysis and resulted in reduced NHE3 activity. However, in NHE3 siRNA-transfected cells, SGLT1 activity was significantly increased. siRNA knockdown of SGLT1 was confirmed by qPCR and Western blot analysis and resulted in reduced SGLT1 activity. However, in SGLT1 siRNA-transfected cells, NHE3 activity was significantly increased. These results demonstrate for the first time the functionality of siRNA in patient-derived organoid monolayers. Furthermore, they show that the two primary Na absorptive pathways in human enterocytes reciprocally regulate one another.

Effect of bile salts on intestinal epithelial function in gilthead seabream (Sparus aurata)

In the context of modern aquaculture, the effort to reduce the reliance on fishmeal/marine ingredients in fish diets has led to the exploration of plant-based protein sources as potential substitutes, a dietary shift that disrupts the bile acid profile in fish. Therefore, bile salts are being sought as additives. However, artificially increased intestinal levels of bile acids may significantly impact mucosal function. Therefore, here, we explored the regulatory role in the intestine of gilthead sea bream (Sparus aurata) of (i) chenodeoxycholic acid (CDC), (ii) a mixture formed by two bile acids, 3% cholic acid and 97% deoxycholic acid (MIX), and (iii) a conjugated bile salt sodium taurocholate (TC) in Ussing chambers with the epithelial voltage clamp technique. We tested the bile salts in a 50-500 μg/ml concentration range, and all of them promoted ion absorption. Yet, clear concentration-dependent and more pronounced effects on the ion transport were observed in the posterior intestine. On the other hand, bile salts had no or minor effects on tissue resistance. However, there are indications that the MIX could have adverse effects at high concentrations (500 μg/ml), promoting a threefold increase in tissue permeability measured using FITC-dextran (4 kD) regardless of the intestinal region, thus suggesting an alteration in intestinal permeability at high bile salt concentrations. The findings from our study emphasize the importance of considering intestinal function when contemplating the possible use of a particular bile salt as a dietary supplement. It appears that bile salts, whether acting individually or in combination, play a pivotal role in orchestrating nutrient absorption by influencing the function of epithelial ion transport. However further research is needed to fully grasp the region-dependent nuances of bile salt effects on ion transport and the ultimate consequences for nutrient absorption in the context of fish aquaculture.

The amino acid transporter SLC16A10 promotes melanogenesis by facilitating the transportation of phenylalanine

Phenylalanine is a crucial amino acid in the process of melanogenesis. However, the exact mechanism by which it is transported into melanocytes has not been disclosed. The aim of this study was to identify and examine the key transporters that are responsible for phenylalanine transportation and evaluate their significance in melanogenesis. The amino acid transporter SLC16A10 was found to be up-regulated in both melasma (GSE72140) and sun-exposed skin (GSE67098). The protein levels of SLC16A10 were proportional to the melanin content in melanocytic nevi, indicating that SLC16A10 was related to melanogenesis. After SLC16A10 overexpression, melanin increased significantly in MNT1 cells. Meanwhile, the expression of melanogenesis-related proteins such as TYR and TYRP1 increased, while their RNA levels did not change. Transcriptomics data indicated that SLC16A10 can enhance the function of ribosome. Furthermore, targeted metabolomics data and ELISA results demonstrated SLC16A10 mainly affected the transport of phenylalanine into the cells. Then, phenylalanine was added to the cell culture medium after SLC16A10 overexpression, melanin synthesis in cells furtherly increased, which verified that SLC16A10 enhances melanogenesis by promoting the uptake of phenylalanine. Finally, we found that SLC16A10 expression increased after UVB irradiation. Knockdown SLC16A10 reduced UVB-induced melanin production and phenylalanine uptake by cells. In summary, SLC16A10 enhances melanogenesis by promoting the uptake of phenylalanine, and upregulation SLC16A10 is likely responsible for the UVB-induced hyperpigmentation as well.

Animal board invited review: Heritability of nitrogen use efficiency in fattening pigs: Current state and possible directions

Pork, an important component of human nutrition worldwide, contributes considerably to anthropogenic nitrogen and greenhouse gas emissions. Reducing the environmental impact of pig production is therefore essential. This can be achieved through system-level strategies, such as optimising resource use, improving manure management and recycling leftovers from human food production, and at the individual animal level by maintaining pig health and fine-tuning dietary protein levels to individual requirements. Breeding, coupled with nutritional strategies, offers a lasting solution to improve nitrogen use efficiency (NUE) - the ratio of nitrogen retained in the body to nitrogen ingested. With a heritability as high as 0.54, incorporating NUE into breeding programmes appears promising. Nitrogen use efficiency involves multiple tissues and metabolic processes, and is influenced by the environment and individual animal characteristics, including its genetic background. Heritable genetic variation in NUE may therefore occur in many different processes, including the central nervous regulation of feed intake, the endocrine system, the gastrointestinal tract where digestion and absorption take place, and the composition of the gut microbiome. An animal's postabsorptive protein metabolism might also harbour important genetic variation, especially in the maintenance requirements of tissues and organs. Precise phenotyping, although challenging and costly, is essential for successful breeding. Various measurement techniques, such as imaging techniques and mechanistic models, are being explored for their potential in genetic analysis. Despite the difficulties in phenotyping, some studies have estimated the heritability and genetic correlations of NUE. These studies suggest that direct selection for NUE is more effective than indirect methods through feed efficiency. The complexity of NUE indicates a polygenic trait architecture, which has been confirmed by genome-wide association studies that have been unable to identify significant quantitative trait loci. Building sufficiently large reference populations to train genomic prediction models is an important next step. However, this will require the development of truly high-throughput phenotyping methods. In conclusion, breeding pigs with higher NUE is both feasible and necessary but will require increased efforts in high-throughput phenotyping and improved genome annotation.

L-valine is a powerful stimulator of GLP-1 secretion in rodents and stimulates secretion through ATP-sensitive potassium channels and voltage-gated calcium channels

BACKGROUND

We previously reported that, among all the naturally occurring amino acids, L-valine is the most powerful luminal stimulator of glucagon-like peptide 1 (GLP-1) release from the upper part of the rat small intestine. This makes L-valine an interesting target for nutritional-based modulation of GLP-1 secretion. However, the molecular mechanism of L-valine-induced secretion remains unknown.

METHODS

We aimed to investigate the effect of orally given L-valine in mice and to identify the molecular details of L-valine stimulated GLP-1 release using the isolated perfused rat small intestine and GLUTag cells. In addition, the effect of L-valine on hormone secretion from the distal intestine was investigated using a perfused rat colon.

RESULTS

Orally given L-valine (1 g/kg) increased plasma levels of active GLP-1 comparably to orally given glucose (2 g/kg) in male mice, supporting that L-valine is a powerful stimulator of GLP-1 release in vivo (P > 0.05). Luminal L-valine (50 mM) strongly stimulated GLP-1 release from the perfused rat small intestine (P < 0.0001), and inhibition of voltage-gated Ca2+-channels with nifedipine (10 μM) inhibited the GLP-1 response (P < 0.01). Depletion of luminal Na+ did not affect L-valine-induced GLP-1 secretion (P > 0.05), suggesting that co-transport of L-valine and Na+ is not important for the depolarization necessary to activate the voltage-gated Ca2+-channels. Administration of the KATP-channel opener diazoxide (250 μM) completely blocked the L-valine induced GLP-1 response (P < 0.05), suggesting that L-valine induced depolarization arises from metabolism and opening of KATP-channels. Similar to the perfused rat small intestine, L-valine tended to stimulate peptide tyrosine-tyrosine (PYY) and GLP-1 release from the perfused rat colon.

CONCLUSIONS

L-valine is a powerful stimulator of GLP-1 release in rodents. We propose that intracellular metabolism of L-valine leading to closure of KATP-channels and opening of voltage-gated Ca2+-channels are involved in L-valine induced GLP-1 secretion.

Iterative assessment of a sports rehydration beverage containing a novel amino acid formula on water uptake kinetics

PURPOSE

Rapid gastric emptying and intestinal absorption of beverages is essential for rapid rehydration, and certain amino acids (AA) may augment fluid delivery. Three sugar-free beverages, containing differing AA concentrations (AA + PZ), were assessed for fluid absorption kinetics against commercial sugar-free (PZ, GZ) and carbohydrate-containing (GTQ) beverages.

METHODS

Healthy individuals (n = 15-17 per study) completed three randomised trials. Three beverages (550-600 mL) were ingested in each study (Study 1: AA + PZ [17.51 g/L AA], PZ, GZ; Study 2: AA + PZ [6.96 g/L AA], PZ, GZ; Study 3: AA + PZ [3.48 g/L AA], PZ, GTQ), containing 3.000 g deuterium oxide (D2O). Blood samples were collected pre-, 2-min, 5-min, and every 5-min until 60-min post-ingestion to quantify maximal D2O enrichment (Cmax), time Cmax occurred (Tmax) and area under the curve (AUC).

RESULTS

Study 1: AUC (AA + PZ: 15,184 ± 3532 δ‰ vs. VSMOW; PZ: 17,328 ± 3153 δ‰ vs. VSMOW; GZ: 17,749 ± 4204 δ‰ vs. VSMOW; P ≤ 0.006) and Tmax (P ≤ 0.005) were lower for AA + PZ vs. PZ/GZ. Study 2: D2O enrichment characteristics were not different amongst beverages (P ≥ 0.338). Study 3: Cmax (AA + PZ: 440 ± 94 δ‰ vs. VSMOW; PZ: 429 ± 83 δ‰ vs. VSMOW; GTQ: 398 ± 81 δ‰ vs. VSMOW) was greater (P = 0.046) for AA + PZ than GTQ, with no other differences (P ≥ 0.106).

CONCLUSION

The addition of small amounts of AA (3.48 g/L) to a sugar-free beverage increased fluid delivery to the circulation compared to a carbohydrate-based beverage, but greater amounts (17.51 g/L) delayed delivery.

Induction of tomato plant biochemical immune responses by the synthesized zinc oxide nanoparticles against wilt-induced Fusarium oxysporum

The current study used zinc oxide nanoparticles (ZnO-NPs) to protect the tomato plant against Fusarium wilt. Gamma rays were used to synthesize ZnO-NPs, and the designed ZnO-NPs were characterized using high-resolution transmission electron microscopy (HRTEM), scanning electron microscope (SEM), dynamic light scattering (DLS), energy-dispersive X-ray spectroscopy (EDX), and ultraviolet-visible (UV-Vis.) spectroscopy. We found that the 20 kGy dose is the most effective for ZnO-NPs synthesis, with the highest O.D. = 1.65 (diluted 3 times) at 400 nm. The scale of ZnO-NPs ranged from 10.45 to 75.25 nm with an average diameter of 40.20 nm. The results showed that the designed ZnO-NPs showed promising activity as a potent inducer of plant physiological immunity against Fusarium wilt disease. Likewise, ZnO-NPs significantly reduced the wilt disease symptoms incidence by 28.57% and high protection by 67.99% against F. oxysporum. Additionally, infected tomato plants treated with ZnO-NPs show improved shoot length (44.71%), root length (40.0%), number of leaves (60.0 %), chlorophyll a (36.93%), chlorophyll b (16.46%), and carotenoids (21.87%) versus infected plants. Notably, in the treatment of tomato seedlings, the beneficial effects of ZnO-NPs extended to increase not only in osmolyte contents but also total phenol contents in comparison with control plants. In conclusion, the designed ZnO-NPs can control Fusarium wilt disease and improve and develop biochemical compounds responsible for defense against fusarial infection.

Low crude protein formulation with supplemental amino acids for its impacts on intestinal health and growth performance of growing-finishing pigs

BACKGROUND

Low crude protein (CP) formulations with supplemental amino acids (AA) are used to enhance intestinal health, reduce costs, minimize environmental impact, and maintain growth performance of pigs. However, extensive reduction of dietary CP can compromise growth performance due to limited synthesis of non-essential AA and limited availability of bioactive compounds from protein supplements even when AA requirements are met. Moreover, implementing a low CP formulation can increase the net energy (NE) content in feeds causing excessive fat deposition. Additional supplementation of functional AA, coupled with low CP formulation could further enhance intestinal health and glucose metabolism, improving nitrogen utilization, and growth performance. Three experiments were conducted to evaluate the effects of low CP formulations with supplemental AA on the intestinal health and growth performance of growing-finishing pigs.

METHODS

In Exp. 1, 90 pigs (19.7 ± 1.1 kg, 45 barrows and 45 gilts) were assigned to 3 treatments: CON (18.0% CP, supplementing Lys, Met, and Thr), LCP (16.0% CP, supplementing Lys, Met, Thr, Trp, and Val), and LCPT (16.1% CP, LCP + 0.05% SID Trp). In Exp. 2, 72 pigs (34.2 ± 4.2 kg BW) were assigned to 3 treatments: CON (17.7% CP, meeting the requirements of Lys, Met, Thr, and Trp); LCP (15.0% CP, meeting Lys, Thr, Trp, Met, Val, Ile, and Phe); and VLCP (12.8% CP, meeting Lys, Thr, Trp, Met, Val, Ile, Phe, His, and Leu). In Exp. 3, 72 pigs (54.1 ± 5.9 kg BW) were assigned to 3 treatments and fed experimental diets for 3 phases (grower 2, finishing 1, and finishing 2). Treatments were CON (18.0%, 13.8%, 12.7% CP for 3 phases; meeting Lys, Met, Thr, and Trp); LCP (13.5%, 11.4%, 10.4% CP for 3 phases; meeting Lys, Thr, Trp, Met, Val, Ile, and Phe); and LCPG (14.1%, 12.8%, 11.1% CP for 3 phases; LCP + Glu to match SID Glu with CON). All diets had 2.6 Mcal/kg NE.

RESULTS

In Exp. 1, overall, the growth performance did not differ among treatments. The LCPT increased (P < 0.05) Claudin-1 expression in the duodenum and jejunum. The LCP and LCPT increased (P < 0.05) CAT-1, 4F2hc, and B0AT expressions in the jejunum. In Exp. 2, overall, the VLCP reduced (P < 0.05) G:F and BUN. The LCP and VLCP increased (P < 0.05) the backfat thickness (BFT). In Exp. 3, overall, growth performance and BFT did not differ among treatments. The LCPG reduced (P < 0.05) BUN, whereas increased the insulin in plasma. The LCP and LCPG reduced (P < 0.05) the abundance of Streptococcaceae, whereas the LCP reduced (P < 0.05) Erysipelotrichaceae, and the alpha diversity.

CONCLUSIONS

When implementing low CP formulation, CP can be reduced by supplementation of Lys, Thr, Met, Trp, Val, and Ile without affecting the growth performance of growing-finishing pigs when NE is adjusted to avoid increased fat deposition. Supplementation of Trp above the requirement or supplementation of Glu in low CP formulation seems to benefit intestinal health as well as improved nitrogen utilization and glucose metabolism.

Beyond protein synthesis: the emerging role of arginine in poultry nutrition and host-microbe interactions

Arginine is a functional amino acid essential for various physiological processes in poultry. The dietary essentiality of arginine in poultry stems from the absence of the enzyme carbamoyl phosphate synthase-I. The specific requirement for arginine in poultry varies based on several factors, such as age, dietary factors, and physiological status. Additionally, arginine absorption and utilization are also influenced by the presence of antagonists. However, dietary interventions can mitigate the effect of these factors affecting arginine utilization. In poultry, arginine is utilized by four enzymes, namely, inducible nitric oxide synthase arginase, arginine decarboxylase and arginine: glycine amidinotransferase (AGAT). The intermediates and products of arginine metabolism by these enzymes mediate the different physiological functions of arginine in poultry. The most studied function of arginine in humans, as well as poultry, is its role in immune response. Arginine exerts immunomodulatory functions primarily through the metabolites nitric oxide (NO), ornithine, citrulline, and polyamines, which take part in inflammation or the resolution of inflammation. These properties of arginine and arginine metabolites potentiate its use as a nutraceutical to prevent the incidence of enteric diseases in poultry. Furthermore, arginine is utilized by the poultry gut microbiota, the metabolites of which might have important implications for gut microbial composition, immune regulation, metabolism, and overall host health. This comprehensive review provides insights into the multifaceted roles of arginine and arginine metabolites in poultry nutrition and wellbeing, with particular emphasis on the potential of arginine in immune regulation and microbial homeostasis in poultry.

Assessing seromuscular layer and serosa removal on intestinal permeability measurements in weaned piglet everted sac segments

The integrity of the intestinal barrier is crucial for regulating the passage of pathogens and toxins, while facilitating nutrient absorption. The everted gut sac technique, an ex-vivo technique, can be used to study interventions on barrier function. This cost-effective approach utilizes relatively large gut segments to study specific intestinal regions. Typically, intact (non-stripped) intestinal segments are used, but their use may underestimate permeability due to the medial positioning of blood vessels relative to the seromuscular layer and serosa. However, removing these layers risks physical damage, resulting in an overestimation of intestinal permeability. Therefore, we investigated the impact of stripping jejunal segments on permeability to fluorescein isothiocyanate-dextran (FITC, 4 kDa) and tetramethylrhodamine isothiocyanate-dextran (TRITC, 40 kDa), and on the absorption of glucose, lysine, and methionine in jejunal segments from 80 piglets at 8 d postweaning. Piglets were subjected to either high or low sanitary housing conditions and diets provoking intestinal protein fermentation or not, expected to influence intestinal permeability. Stripping of the seromuscular layer and serosa increased the passage of 4 kDa FITC-dextran (stripped vs. non-stripped; 1.1 vs. 0.9 pmol/cm2/min, P < 0.001), glucose (40.0 vs. 19.1 pmol/cm2/min, P < 0.001), lysine (2.5 vs. 2.0 nmol/cm2/min, P < 0.001), and methionine (4.1 vs. 2.7 pmol/cm2/min, P < 0.001). As permeability increased, the differences in methionine passage between stripped and non-stripped intestinal segments also increased (slope = 1.30, P = 0.009). The coefficients of variation were comparable between stripped and non-stripped intestines (over all treatments, stripped vs. non-stripped 38% vs. 40%). Stripping, by isolating mucosal processes without introducing additional variation, is thus recommended for studies on intestinal permeability or absorption.

Increase in Paracellular Leakage of Amino Acids Mediated by Aging-Induced Reduction of Claudin-4 Expression

BACKGROUND

Claudins (CLDNs), major components of tight junctions, control paracellular permeabilities of mineral ions and wastes. The absorption of nutrients including glucose and amino acids (AAs) is regulated by intestinal epithelial cells. However, the role of CLDNs is not fully understood.

OBJECTIVES

The purpose of this study was to clarify the effect of AA deprivation on the expression of AA transporters and CLDNs, as well as the role of CLDNs in the regulation of paracellular AA fluxes.

METHODS

The messenger RNA and protein expression of various CLDNs were examined by real-time quantitative polymerase chain reaction and Western blot analyses, respectively. The AA selectivity of CLDNs was estimated using liquid chromatography-tandem mass spectrometry (LC-MS) analysis.

RESULTS

The expression levels of some AA transporters, CLDN4, and CLDN15 were increased by AA deprivation in normal mouse colon-derived MCE301 cells. The expression of AA transporters and CLDN15 in the mouse colon was positively correlated with aging but the expression of CLDN4 was not. The AA deprivation-induced elevation of CLDN4 expression was inhibited by MHY1485, a mammalian target of rapamycin (mTOR) activator. Furthermore, CLDN4 expression was increased by rapamycin, an mTOR inhibitor. mTOR may be involved in the transcriptional activation of CLDN4. The fluxes of AAs from the basal to apical compartments were decreased and increased by CLDN4 overexpression and silencing, respectively. LC-MS analysis showed that the fluxes of all AAs, especially Lys, His, and Arg, were enhanced by CLDN4 silencing.

CONCLUSIONS

CLDN4 is suggested to form a paracellular barrier to AAs, especially alkaline AAs, which is attenuated with aging.

Intestinal Atp8b1 dysfunction causes hepatic choline deficiency and steatohepatitis

Choline is an essential nutrient, and its deficiency causes steatohepatitis. Dietary phosphatidylcholine (PC) is digested into lysoPC (LPC), glycerophosphocholine, and choline in the intestinal lumen and is the primary source of systemic choline. However, the major PC metabolites absorbed in the intestinal tract remain unidentified. ATP8B1 is a P4-ATPase phospholipid flippase expressed in the apical membrane of the epithelium. Here, we use intestinal epithelial cell (IEC)-specific Atp8b1-knockout (Atp8b1IEC-KO) mice. These mice progress to steatohepatitis by 4 weeks. Metabolomic analysis and cell-based assays show that loss of Atp8b1 in IEC causes LPC malabsorption and thereby hepatic choline deficiency. Feeding choline-supplemented diets to lactating mice achieves complete recovery from steatohepatitis in Atp8b1IEC-KO mice. Analysis of samples from pediatric patients with ATP8B1 deficiency suggests its translational potential. This study indicates that Atp8b1 regulates hepatic choline levels through intestinal LPC absorption, encouraging the evaluation of choline supplementation therapy for steatohepatitis caused by ATP8B1 dysfunction.

Growth, Cannibalism, and 5-TH Metabolism in Pufferfish (Takifugu obscurus ♀ × Takifugu rubripes): The Role of Graded Levels of Dietary Tryptophan

The objective of this study was to investigate the potential effect of graded levels of tryptophan on the growth, cannibalism, and 5-hydroxytryptpamine (5-TH) metabolism in pufferfish (Takifugu obscurus ♀ × Takifugu rubripes ♂). A 63-day feeding trial was performed wherein pufferfish were fed four diets. Three experimental diets were formulated with various levels of tryptophan based on the control diet. Four diets were named as T1, T2, T3, and T4, corresponding to 4.30, 7.80, 14.90, and 23.70 g kg-1 tryptophan of dry diet. Final body weight, weight gain, and specific growth rate were similar between the T1 and T4 groups, but exhibited a significantly increased trend compared to the T2 group. Although survival rate was not affected by various levels of dietary tryptophan, intraspecific cannibalism was significantly reduced in the group fed with highest level of tryptophan (T4). For free amino acid in brain, the concentration of tryptophan was the highest in the T3 group and the lowest in the T2 group, while phenylalanine, tyrosine, and methionine showed an opposite trend between those two groups. The levels of dietary tryptophan not only affected the expression of aromatic amino acid transporter TAT1, but also affected the expression of B0AT1, B0AT2, and 4F2hc in intestine, as well as B0AT1, y+LAT1, and LAT2 in brain. The activity of tryptophan hydroxylase (TPH) in serum increased with the increase of dietary tryptophan, and the expression of TPH1 in brain upregulated in the excessive tryptophan groups (T2, T3, and T4). MAO activity in serum as well as its gene expression in brain and intestine showed a decreased trend in the T4 group. In conclusion, excessive tryptophan (23.70 g kg-1 of dry diet, corresponding to 50.3 g kg-1 of dietary protein) in feed could mitigate cannibalistic behavior of pufferfish and promote the growth, and the reason for this effect might affect the metabolism of 5-TH in vivo.

Experimental evolution of metabolism under nutrient restriction: enhanced amino acid catabolism and a key role of branched-chain amino acids

Periodic food shortage is a common ecological stressor for animals, likely to drive physiological and metabolic adaptations to alleviate its consequences, particularly for juveniles that have no option but to continue to grow and develop despite undernutrition. Here we study changes in metabolism associated with adaptation to nutrient shortage, evolved by replicate Drosophila melanogaster populations maintained on a nutrient-poor larval diet for over 240 generations. In a factorial metabolomics experiment we showed that both phenotypic plasticity and genetically-based adaptation to the poor diet involved wide-ranging changes in metabolite abundance; however, the plastic response did not predict the evolutionary change. Compared to nonadapted larvae exposed to the poor diet for the first time, the adapted larvae showed lower levels of multiple free amino acids in their tissues-and yet they grew faster. By quantifying accumulation of the nitrogen stable isotope 15N we show that adaptation to the poor diet led to an increased use of amino acids for energy generation. This apparent "waste" of scarce amino acids likely results from the trade-off between acquisition of dietary amino acids and carbohydrates observed in these populations. The three branched-chain amino acids (leucine, isoleucine, and valine) showed a unique pattern of depletion in adapted larvae raised on the poor diet. A diet supplementation experiment demonstrated that these amino acids are limiting for growth on the poor diet, suggesting that their low levels resulted from their expeditious use for protein synthesis. These results demonstrate that selection driven by nutrient shortage not only promotes improved acquisition of limiting nutrients, but also has wide-ranging effects on how the nutrients are used. They also show that the abundance of free amino acids in the tissues does not, in general, reflect the nutritional condition and growth potential of an animal.

Regulation of Paracellular Fluxes of Amino Acids by Claudin-8 in Normal Mouse Intestinal MCE301 Cells

The ingested proteins are catabolized to di/tri-peptides and amino acids (AAs), which are absorbed through various transporters in the small intestinal and colonic epithelial cells. Tight junctions (TJs) are formed between neighboring cells and restrict paracellular fluxes to mineral ions and aqueous molecules. However, it is unknown whether the TJs are implicated in the control of paracellular fluxes to AAs. The paracellular permeability is controlled by claudins (CLDNs), which comprise a family of over 20 members. Here, we found that CLDN8 expression is decreased by AAs deprivation in normal mouse colon-derived MCE301 cells. The reporter activity of CLDN8 was not significantly changed by AAs deprivation, whereas the stability of CLDN8 protein was decreased. MicroRNA analysis showed that AAs deprivation increases the expression of miR-153-5p which targets CLDN8. The AAs deprivation-induced decline of CLDN8 expression was reversed by a miR-153-5p inhibitor. The CLDN8 silencing enhanced the paracellular fluxes to AAs, especially middle molecular size AAs. The expression levels of colonic CLDN8 and miR-153-5p in aged mice were lower and higher than those in young mice, respectively. We suggest that AAs deprivation downregulates CLDN8-dependent barrier function, mediated by the elevation of miR-153-5p expression in the colon, in order to enhance the AAs absorption.

Amino acid profiles, amino acid sensors and transporters expression and intestinal microbiota are differentially altered in goats infected with Haemonchus contortus

Infection with the nematode Haemonchus contortus causes host malnutrition and gastrointestinal injuries. The objective of this study was to investigate the effects of H. contortus infection on gastrointestinal contents of free amino acids (AA), the expression of AA transporters and microbiota with a focus on amino acid metabolism. Twenty-four Xiangdong black goats (13 ± 1.5 kg, 6 months old) were randomly assigned into the control group (n = 8) and the infected group (n = 16). The results showed that H. contortus infection increased (P < 0.05) the free AA contents in jejunum and ileum digesta. The concentrations of blood threonine, phenylalanine and tyrosine were lower (P < 0.05) in the infected group as compared to the control group. In the jejunum and ileum epithelium, H. contortus infection significantly (P < 0.05) down-regulated the expression of AA transporter b^0,+AT/rBAT and B0AT1, but up-regulated (P < 0.05) the expression of transporter CAT2 and xCT. Furthermore, microbiota in both jejunum (Bifidobacteriaceae, Lachnospiraceae, Bacteroidaceae, Enterobacteriaceae, and Micrococcaceae) and ileum (Acidaminococcaceae, Desulfovibrionaceae, Bacteroidaceae, and Peptostreptococcaceae) were also altered at the family level by H. contortus infection. The  commensal bacteria of jejunum showed a close correlation with amino acids, AA transporters, and amino acid metabolism, especially cystine. In conclusion, H. contortus infection affected the intestinal AA contents and the expression of intestinal AA transporters, suggesting altered AA metabolism and absorption, which were accompanied by changes in the relative abundances of gut bacteria that mediate amino acid metabolism.

Drosophila as a diet discovery tool for treating amino acid disorders

Amino acid disorders (AADs) are a large group of rare inherited conditions that collectively impact one in 6500 live births, often resulting in rapid neurological decline and death during infancy. For several AADs, including phenylketonuria, dietary modification prevents physiological deterioration and ameliorates symptoms. Despite this remarkable potential for treatment success, dietary therapy for most AADs remains largely unexplored. Although animal models have provided novel insights into AAD mechanisms, few have been used for therapeutic diet discovery. Here, we find that of all the animal models, Drosophila is particularly well suited for nutrigenomic disease modelling, having amino acid pathways conserved with humans, exceptional genetic tractability, and the unique availability of a synthetic customisable diet.

Apparent jejunal amino acid digestibility, gut morphology, and the expression of intestinal amino acid transporters in pigs fed protein or free amino acids

Dietary amino acids (AA) supplied as protein or in free form are not only digested and absorbed at different rates but can also induce differences in the intestinal physiology of pigs. We compared the apparent jejunal AA digestibility, intestinal morphology, and gene expression of AA transporters of pigs fed diets providing different forms of AA. Thirty growing pigs (33.7 ± 4.1 kg) were fed one of three experimental diets that provided AA either as protein from feather meal (INT), as free AA and small peptides obtained by extensive acid hydrolysis of feathers (HYD), or as a mix of individual purified AA with the same AA profile as HYD (FAA). Pigs were fed the same quantity of feed, energy, and AA. After 14 d, pigs were slaughtered 3 h after feeding a meal with indigestible markers. Digesta and tissue were collected from different sections of the small intestine. Jejunal digesta was used to measure apparent jejunal digestibility of AA. Samples of the duodenum, jejunum, and ileum were used to measure intestinal morphology and the gene expression of intestinal AA transporters. The measured apparent jejunal digestibility of AA of INT was lower compared to HYD and FAA (P < 0.05). The apparent jejunal digestibility of Cys, Gly, His, Met, and Pro was lower for FAA compared to HYD (P < 0.05). This may be due to the small peptides in HYD, which are absorbed faster than individual AA. The villi area in the ileum of HYD fed pigs was the highest (P < 0.05) among the treatments, which may be associated with the reabsorption of endogenous proteins, which occurs mostly in the ileum. In the duodenum, HYD and FAA had lower expression of PepT1 (P < 0.01) probably due to the rapid transit time of digesta compared to INT fed pigs. Pigs fed HYD expressed more ASCT2 (P = 0.02) and CAT-1 (P = 0.04) in the jejunum compared to the pigs fed the other diets. The expression of these transporters along the intestine depended on the relative abundance of readily absorbable dietary AA. Results showed that dietary AA form can have an influence on the morphology and on the expression of different AA transporters along the different sections of the small intestine.

Post-exercise rehydration: Comparing the efficacy of three commercial oral rehydration solutions

Introduction

This study compared the efficacy of three commercial oral rehydration solutions (ORS) for restoring fluid and electrolyte balance, after exercise-induced dehydration.

Method

Healthy, active participants (N = 20; ♀ = 3; age ∼27 y, V˙O₂peak ∼52 ml/kg/min) completed three randomised, counterbalanced trials whereby intermittent exercise in the heat (∼36°C, ∼50% humidity) induced ∼2.5% dehydration. Subsequently, participants rehydrated (125% fluid loss in four equal aliquots at 0, 1, 2, 3 h) with a glucose-based (G-ORS), sugar-free (Z-ORS) or amino acid-based sugar-free (AA-ORS) ORS of varying electrolyte composition. Urine output was measured hourly and capillary blood samples collected pre-exercise, 0, 2 and 5 h post-exercise. Sodium, potassium, and chloride concentrations in urine, sweat, and blood were determined.

Results

Net fluid balance peaked at 4 h and was greater in AA-ORS (141 ± 155 ml) and G-ORS (101 ± 195 ml) than Z-ORS (-47 ± 208 ml; P ≤ 0.010). Only AA-ORS achieved positive sodium and chloride balance post-exercise, which were greater for AA-ORS than G-ORS and Z-ORS (P ≤ 0.006), as well as for G-ORS than Z-ORS (P ≤ 0.007) from 1 to 5 h.

Conclusion

when provided in a volume equivalent to 125% of exercise-induced fluid loss, AA-ORS produced comparable/superior fluid balance and superior sodium/chloride balance responses to popular glucose-based and sugar-free ORS.

Diet-induced gut dysbiosis and inflammation: Key drivers of obesity-driven NASH

Sucrose, the primary circulating sugar in plants, contains equal amounts of fructose and glucose. The latter is the predominant circulating sugar in animals and thus the primary fuel source for various tissue and cell types in the body. Chronic excessive energy intake has, however, emerged as a major driver of obesity and associated pathologies including nonalcoholic fatty liver diseases (NAFLD) and the more severe nonalcoholic steatohepatitis (NASH). Consumption of a high-caloric, western-style diet induces gut dysbiosis and inflammation resulting in leaky gut. Translocation of gut-derived bacterial content promotes hepatic inflammation and ER stress, and when either or both of these are combined with steatosis, it can cause NASH. Here, we review the metabolic links between diet-induced changes in the gut and NASH. Furthermore, therapeutic interventions for the treatment of obesity and liver metabolic diseases are also discussed with a focus on restoring the gut-liver axis.

It Ain't Over 'Til It's Over: SARS CoV-2 and Post-infectious Gastrointestinal Dysmotility

The ongoing pandemic resulting from severe acute respiratory syndrome-caused by coronavirus 2 (SARS-CoV-2)-has posed a multitude of healthcare challenges of unprecedented proportions. Intestinal enterocytes have the highest expression of angiotensin-converting enzyme-2 (ACE2), which functions as the key receptor for SARS-CoV-2 entry into cells. As such, particular interest has been accorded to SARS-CoV-2 and how it manifests within the gastrointestinal system. The acute and chronic alimentary clinical implications of infection are yet to be fully elucidated, however, the gastrointestinal consequences from non-SARS-CoV-2 viral GI tract infections, coupled with the generalized nature of late sequelae following COVID-19 disease, would predict that motility disorders are likely to be seen in these patients. Determination of the chronic effects of COVID-19 disease, herein defined as GI disease which is persistent or recurrent more than 3 months following recovery from the acute respiratory illness, will require comprehensive investigations comprising combined endoscopic- and motility-based evaluation. It will be fascinating to ascertain whether the specific post-COVID-19 phenotype is hypotonic or hypertonic in nature and to identify the most vulnerable target portions of the gut. A specific biological hypothesis is that motility disorders may result from SARS-CoV-2-induced angiotensin-converting enzyme 2 (ACE2) depletion. Since SARS-CoV-2 is known to exhibit direct neuronal tropism, the potential also exists for the development of neurogenic motility disorders. This review aims to explore some of the potential pathophysiologic mechanisms underlying motility dysfunction as it relates to ACE2 and thereby aims to provide the foundation for mechanism-based potential therapeutic options.

Efficacy and pharmacokinetics of betaine in CBS and cblC deficiencies: a cross-over randomized controlled trial

BACKGROUND

Betaine is an "alternate" methyl donor for homocysteine remethylation catalyzed by betaine homocysteine methyltransferase (BHMT), an enzyme mainly expressed in the liver and kidney. Betaine has been used for more than 30 years in pyridoxine non-responsive cystathionine beta-synthase (pnrCBS) and cobalamin C (cblC) deficiencies to lower the hyperhomocysteinemia, although little is known about the optimal therapeutic dosage and its pharmacokinetic in these patients.

AIMS

We compared 2 betaine doses (100 mg/kg/day vs. 250 mg/kg/day) in children affected by pnrCBS or cblC deficiencies. We also measured the pharmacokinetics parameters after a single dose of betaine (100 or 250 mg/kg) in these patients.

METHODS

We conducted a prospective, randomized, crossover clinical trial with blinded evaluation. The primary outcome was the equivalence of total plasma homocysteine (tHcy) concentrations upon one-month oral treatment with betaine at 100 versus 250 mg/kg/day.

RESULTS

Eleven patients completed the study (5 pnrCBS and 6 cblC). tHcy concentrations were equivalent after a one-month treatment period for the two betaine dosages. Multivariate analysis showed a significant effect of betaine dose on methionine (Met) (p = 0.01) and S-adenosylmethionine (SAM) concentrations (p = 0.006).

CONCLUSIONS

Our analysis shows that there is no overt benefit to increasing betaine dosage higher than 100 mg/kg/day to lower tHcy concentrations in pnrCBS and cblC deficiencies. However, increasing betaine up to 250 mg/kg/d could benefit cblC patients through the increase of methionine and SAM concentrations, as low Met and SAM concentrations are involved in the pathophysiology of this disease. In contrast, in pnrCBS deficiency, betaine doses higher than 100 mg/kg/day could be harmful to these patients with pre-existing hypermethioninemia.

TRIAL REGISTRATION

Clinical Trials, NCT02404337. Registered 23 May 2015-prospectively registered, https://clinicaltrials.gov .

Biology of Peptide Transporter 2 in Mammals: New Insights into Its Function, Structure and Regulation

Peptide transporter 2 (PepT2) in mammals plays essential roles in the reabsorption and conservation of peptide-bound amino acids in the kidney and in maintaining neuropeptide homeostasis in the brain. It is also of significant medical and pharmacological significance in the absorption and disposing of peptide-like drugs, including angiotensin-converting enzyme inhibitors, β-lactam antibiotics and antiviral prodrugs. Understanding the structure, function and regulation of PepT2 is of emerging interest in nutrition, medical and pharmacological research. In this review, we provide a comprehensive overview of the structure, substrate preferences and localization of PepT2 in mammals. As PepT2 is expressed in various organs, its function in the liver, kidney, brain, heart, lung and mammary gland has also been addressed. Finally, the regulatory factors that affect the expression and function of PepT2, such as transcriptional activation and posttranslational modification, are also discussed.

The digestive systems of carnivorous plants

To survive in the nutrient-poor habitats, carnivorous plants capture small organisms comprising complex substances not suitable for immediate reuse. The traps of carnivorous plants, which are analogous to the digestive systems of animals, are equipped with mechanisms for the breakdown and absorption of nutrients. Such capabilities have been acquired convergently over the past tens of millions of years in multiple angiosperm lineages by modifying plant-specific organs including leaves. The epidermis of carnivorous trap leaves bears groups of specialized cells called glands, which acquire substances from their prey via digestion and absorption. The digestive glands of carnivorous plants secrete mucilage, pitcher fluids, acids, and proteins, including digestive enzymes. The same (or morphologically distinct) glands then absorb the released compounds via various membrane transport proteins or endocytosis. Thus, these glands function in a manner similar to animal cells that are physiologically important in the digestive system, such as the parietal cells of the stomach and intestinal epithelial cells. Yet, carnivorous plants are equipped with strategies that deal with or incorporate plant-specific features, such as cell walls, epidermal cuticles, and phytohormones. In this review, we provide a systematic perspective on the digestive and absorptive capacity of convergently evolved carnivorous plants, with an emphasis on the forms and functions of glands.

A double-blind clinical trial to compare the efficacy and safety of a multiple amino acid-based ORS with the standard WHO-ORS in the management of non-cholera acute watery diarrhea in infants and young children: "VS002A" trial protocol

BACKGROUND

Diarrhea is the second deadliest disease for under-five children globally and the situation is more serious in developing countries. Oral rehydration solution (ORS) is being used as a standard treatment for acute watery diarrhea for a long time. Our objective is to compare the efficacy of amino acid-based ORS "VS002A" compared to standard glucose-based WHO-ORS in infants and young children suffering from acute non-cholera watery diarrhea.

METHODS

It is a randomized, double-blind, two-cell clinical trial at Dhaka Hospital of icddr,b. A total of 312 male children aged 6-36 months old with acute non-bloody watery diarrhea are included in this study. Intervention arm participants get amino acid-based ORS (VS002A) and the control arm gets standard glucose-based WHO-ORS. The primary efficacy endpoint is the duration of diarrhea in the hospital.

DISCUSSION

Oral rehydration therapy (ORT) with the present ORS formulation has certain limitations - it does not reduce the volume, frequency, or duration of diarrhea. Additionally, the failure of present standard ORS to significantly reduce stool output likely contributes to the relatively limited use of ORS by mothers as they do not feel that ORS is helping their child recover from the episode of diarrhea. Certain neutral amino acids (e.g., glycine, L-alanine, L-glutamine) can enhance the absorption of sodium ions and water from the gut. By using this concept, a shelf-stable, sugar-free amino acid-based hydration medicinal food named 'VS002A' that effectively rehydrates, and improves the barrier function of the bowel following infections targeting the gastrointestinal tract has been developed. If the trial shows significant benefits of VS002A use, this may provide evidence to support consideration of the use of VS002A in the present WHO diarrhea management guidelines. Conversely, if there is no evidence of benefit, these results will reaffirm the current guidelines.

TRIAL REGISTRATION

ClinicalTrials.gov NCT04677296 . Registered on December 21, 2020.

Impaired Amino Acid Metabolism and Its Correlation with Diabetic Kidney Disease Progression in Type 2 Diabetes Mellitus

Background: Metabolomics is useful in elucidating the progression of diabetes; however, the follow-up changes in metabolomics among health, diabetes mellitus, and diabetic kidney disease (DKD) have not been reported. This study was aimed to reveal metabolomic signatures in diabetes development and progression. Methods: In this cross-sectional study, we compared healthy (n = 30), type 2 diabetes mellitus (T2DM) (n = 30), and DKD (n = 30) subjects with the goal of identifying gradual altering metabolites. Then, a prospective study was performed in T2DM patients to evaluate these altered metabolites in the onset of DKD. Logistic regression was conducted to predict rapid eGFR decline in T2DM subjects using altered metabolites. The prospective association of metabolites with the risk of developing DKD was examined using logistic regression and restricted cubic spline regression models. Results: In this cross-sectional study, impaired amino acid metabolism was the main metabolic signature in the onset and development of diabetes, which was characterized by increased N-acetylaspartic acid, L-valine, isoleucine, asparagine, betaine, and L-methionine levels in both the T2DM and DKD groups. These candidate metabolites could distinguish the DKD group from the T2DM group. In the follow-up study, higher baseline levels of L-valine and isoleucine were significantly associated with an increased risk of rapid eGFR decline in T2DM patients. Of these, L-valine and isoleucine were independent risk factors for the development of DKD. Notably, nonlinear associations were also observed for higher baseline levels of L-valine and isoleucine, with an increased risk of DKD among patients with T2DM. Conclusion: Amino acid metabolism was disturbed in diabetes, and N-acetylaspartic acid, L-valine, isoleucine, asparagine, betaine, and L-methionine could be biomarkers for the onset and progression of diabetes. Furthermore, high levels of L-valine and isoleucine may be risk factors for DKD development.

Feeding Thai Native Sheep Molasses Either Alone or in Combination with Urea-Fermented Sugarcane Bagasse: The Effects on Nutrient Digestibility, Rumen Fermentation, and Hematological Parameters

The purpose of this study was to find out how adding molasses to fermented sugarcane bagasse (FSB) alone or in combination with urea affected sheep’s rumen fermentation, hematological parameters, and ability to digest nutrients. Four Thai native sheep with an initial body weight (BW) of 20.87 ± 1.95 kg and 11 ± 1.0 months old were assigned to a 4 × 4 Latin square design with 4 periods of 14-d adaptation and 7 d of sample collection. Each treatment received a different combination of experimental roughage as follows: FSB without additives (T1), FSB + 10% molasses (T2), FSB + 20% molasses (T3), and FSB + 10% molasses + 3% urea (T4). The concentrate diet was fed twice daily at 2% BW, while roughage sources were provided ad libitum for each treatment. The crude protein (CP) digestibility in the T2 and T3 groups was higher (p < 0.05) than in the FSB group without additions, with the T4 group having the highest (p < 0.05). Although there were no significant differences in blood glucose, packed cell volume, ruminal pH, ammonia−nitrogen (NH3-N), propionic acid, or acetic acid, the plasma urea nitrogen (PUN) at 0 h was highest in the T4 group (p < 0.05) compared with the other groups. However, the proportion of butyric acid tended to be higher in all FSB groups with additives. Thus, the current experiment concluded that the addition of molasses alone or in combination with urea had positive effects on pH and LAB population, and including both together in FSB improved the CP digestibility of sheep. In conclusion, FSB with 10% molasses and 3% urea might be used as an alternate roughage source for ruminants without affecting the animal’s ruminal fermentation or hematological parameters.

Revisiting the concept of incretin and enteroendocrine L-cells as type 2 diabetes mellitus treatment

The significant growth in type 2 diabetes mellitus (T2DM) prevalence strikes a common threat to the healthcare and economic systems globally. Despite the availability of several anti-hyperglycaemic agents in the market, none can offer T2DM remission. These agents include the prominent incretin-based therapy such as glucagon-like peptide-1 receptor (GLP-1R) agonists and dipeptidyl peptidase-4 inhibitors that are designed primarily to promote GLP-1R activation. Recent interest in various therapeutically useful gastrointestinal hormones in T2DM and obesity has surged with the realisation that enteroendocrine L-cells modulate the different incretins secretion and glucose homeostasis, reflecting the original incretin definition. Targeting L-cells offers promising opportunities to mimic the benefits of bariatric surgery on glucose homeostasis, bodyweight management, and T2DM remission. Revising the fundamental incretin theory is an essential step for therapeutic development in this area. Therefore, the present review explores enteroendocrine L-cell hormone expression, the associated nutrient-sensing mechanisms, and other physiological characteristics. Subsequently, enteroendocrine L-cell line models and the latest L-cell targeted therapies are reviewed critically in this paper. Bariatric surgery, pharmacotherapy and new paradigm of L-cell targeted pharmaceutical formulation are discussed here, offering both clinician and scientist communities a new common interest to push the scientific boundary in T2DM therapy.

Role of dietary amino acids and microbial metabolites in the regulation of pig intestinal health

With the rapid development of sequencing technology, research on pigs has focused on intestinal microbes. Accumulating evidence suggests that the metabolites of intestinal microbes are the key medium for interactions between microbes and the host. Amino acid metabolism is involved in the growth and immune processes of pigs. The gut microbes of pigs are heavily involved in the metabolism of amino acids in their hosts. Here, we review the latest relevant literature. Research findings show that microbial metabolites, such as indoles, short-chain fatty acids, and ammonia, play a key role in gut health. Moreover, we summarize the effects of amino acids on the structure of the gut microbial community and the metabolism of amino acids by pig gut microbes. Evidence shows that microbial amino acid metabolites act as signal molecules in the intestine and play an important role in the intestinal health of pigs.

The Possible Mechanism of Physiological Adaptation to the Low-Se Diet and Its Health Risk in the Traditional Endemic Areas of Keshan Diseases

Selenium is an essential trace element for humans and animals. As with oxygen and sulfur, etc., it belongs to the sixth main group of the periodic table of elements. Therefore, the corresponding amino acids, such as selenocysteine (Sec), serine (Ser), and cysteine (Cys), have similar spatial structure, physical, and chemical properties. In this review, we focus on the neglected but key role of serine in a possible mechanism of the physiological adaptation to Se-deficiency in human beings with an adequate intake of dietary protein: the insertion of Cys in place of Sec during the translation of selenoproteins dependent on the Sec insertion sequence element in the 3'UTR of mRNA at the UGA codon through a novel serine-dependent pathway for the de novo synthesis of the Cys-tRNA^[Ser]Sec, similar to Sec-tRNA^[Ser]Sec. We also discuss the important roles of serine in the metabolism of selenium directly or indirectly via GSH, and the maintenance of selenium homostasis regulated through the methylation modification of Sec-tRNA^[Ser]Sec at the position 34U by SAM. Finally, we propose a hypothesis to explain why Keshan disease has gradually disappeared in China and predict the potential health risk of the human body in the physiological adaptation state of low selenium based on the results of animal experiments.

Molecular mechanism of nutrient uptake in developing embryos of oviparous cloudy catshark (Scyliorhinus torazame)

Forms of embryonic nutrition are highly diverse in cartilaginous fishes, which contain oviparity, yolk-sac viviparity and several types of matrotrophic viviparity (histotrophy, oophagy, and placentotrophy). The molecular mechanisms of embryonic nutrition are poorly understood in these animals as few species are capable of reproducing in captivity. Oviparous cartilaginous fishes solely depend on yolk nutrients for their growth and development. In the present study, we compared the contribution to embryonic nutrition of the embryonic intestine with the yolk sac membrane (YSM). RNA-seq analysis was performed on the embryonic intestine and YSM of the oviparous cloudy catshark Scyliorhinus torazame to identify candidate genes involved in nutrient metabolism to further the understanding of nutrient utilization of developing embryos. RNA-seq discovery was subsequently confirmed by quantitative PCR analysis and we identified increases in several amino acid transporter genes (slc3a1, slc6a19, slc3a2, slc7a7) as well as genes involved in lipid absorption (apob and mtp) in the intestine after ‘pre-hatching’, which is a developmental event marked by an early opening of the egg case about 4 months before hatching. Although a reciprocal decrease in the nutritional role of YSM was expected after the intestine became functional, we observed similar increases in gene expression among amino acid transporters, lipid absorption molecules, and lysosomal cathepsins in the extraembryonic YSM in late developmental stages. Ultrastructure of the endodermal cells of YSM showed that yolk granules were incorporated by endocytosis, and the number of granules increased during development. Furthermore, the digestion of yolk granules in the YSM and nutrient transport through the basolateral membrane of the endodermal cells appeared to be enhanced after pre-hatching. These findings suggest that nutrient digestion and absorption is highly activated in both intestine and YSM after pre-hatching in catshark embryos, which supports the rapid growth at late developmental stages.

Ca2+-Permeable Channels/Ca2+ Signaling in the Regulation of Ileal Na+/Gln Co-Transport in Mice

Oral glutamine (Gln) has been widely used in gastrointestinal (GI) clinical practice, but it is unclear if Ca²⁺ regulates intestinal Gln transport, although both of them are essential nutrients for mammals. Chambers were used to determine Gln (25 mM)-induced I_sc through Na⁺/Gln co-transporters in the small intestine in the absence or the presence of selective activators or blockers of ion channels and transporters. Luminal but not serosal application of Gln induced marked intestinal I_sc, especially in the distal ileum. Lowering luminal Na⁺ almost abolished the Gln-induced ileal I_sc, in which the calcium-sensitive receptor (CaSR) activation were not involved. Ca²⁺ removal from both luminal and serosal sides of the ileum significantly reduced Gln- I_sc. Blocking either luminal Ca²⁺ entry via the voltage-gated calcium channels (VGCC) or endoplasmic reticulum (ER) release via inositol 1,4,5-triphosphate receptor (IP₃R) and ryanodine receptor (RyR) attenuated the Gln-induced ileal I_sc, Likewise, blocking serosal Ca²⁺ entry via the store-operated Ca²⁺ entry (SOCE), TRPV1/2 channels, and Na⁺/Ca²⁺ exchangers (NCX) attenuated the Gln-induced ileal I_sc. In contrast, activating TRPV1/2 channels enhanced the Gln-induced ileal I_sc. We concluded that Ca²⁺ signaling is critical for intestinal Gln transport, and multiple plasma membrane Ca²⁺-permeable channels and transporters play roles in this process. The Ca²⁺ regulation of ileal Na⁺/Gln transport expands our understanding of intestinal nutrient uptake and may be significant in GI health and disease.

Changes to gut amino acid transporters and microbiome associated with increased E/I ratio in Chd8+/- mouse model of ASD-like behavior

Autism spectrum disorder (ASD), a group of neurodevelopmental disorders characterized by social communication deficits and stereotyped behaviors, may be associated with changes to the gut microbiota. However, how gut commensal bacteria modulate brain function in ASD remains unclear. Here, we used chromodomain helicase DNA-binding protein 8 (CHD8) haploinsufficient mice as a model of ASD to elucidate the pathways through which the host and gut microbiota interact with each other. We found that increased levels of amino acid transporters in the intestines of the mouse model of ASD contribute to the high level of serum glutamine and the increased excitation/inhibition (E/I) ratio in the brain. In addition, elevated α-defensin levels in the haploinsufficient mice resulted in dysregulation of the gut microbiota characterized by a reduced abundance of Bacteroides. Furthermore, supplementation with Bacteroides uniformis improved the ASD-like behaviors and restored the E/I ratio in the brain by decreasing intestinal amino acid transport and the serum glutamine levels. Our study demonstrates associations between changes in the gut microbiota and amino acid transporters, and ASD-like behavioral and electrophysiology phenotypes, in a mouse model.

Protein Quality in Perspective: A Review of Protein Quality Metrics and Their Applications

For design of healthy and sustainable diets and food systems, it is important to consider not only the quantity but also the quality of nutrients. This is particularly important for proteins, given the large variability in amino acid composition and digestibility between dietary proteins. This article reviews measurements and metrics in relation to protein quality, but also their application. Protein quality methods based on concentrations and digestibility of individual amino acids are preferred, because they do not only allow ranking of proteins, but also assessment of complementarity of protein sources, although this should be considered only at a meal level and not a diet level. Measurements based on ileal digestibility are preferred over those on faecal digestibility to overcome the risk of overestimation of protein quality. Integration of protein quality on a dietary level should also be done based on measurements on an individual amino acid basis. Effects of processing, which is applied to all foods, should be considered as it can also affect protein quality through effects on digestibility and amino acid modification. Overall, protein quality data are crucial for integration into healthy and sustainable diets, but care is needed in data selection, interpretation and integration.

Amino acid transporters as modulators of glucose homeostasis

Amino acids modulate glucose homeostasis. Cytosolic levels of amino acids are regulated by amino acid transporters, modulating insulin release, protein synthesis, cell proliferation, cell fate, and metabolism. In β-cells, amino acid transporters modulate incretin-stimulated insulin release. In the liver, amino acid transporters provide glutamine and alanine for gluconeogenesis. Intestinal amino acid transporters facilitate the intake of amino acids causing protein restriction when inactive. Adipocyte development is regulated by amino acid transporters through activation of mechanistic target of rapamycin (mTORC1) and amino acid-related metabolites. The accumulation and metabolism of branched-chain amino acids (BCAAs) in muscle depends on transporters. The integration between amino acid metabolism and transport is critical for the maintenance and function of tissues and cells involved in glucose homeostasis.

Unconventional Functions of Amino Acid Transporters: Role in Macropinocytosis (SLC38A5/SLC38A3) and Diet-Induced Obesity/Metabolic Syndrome (SLC6A19/SLC6A14/SLC6A6)

Amino acid transporters are expressed in mammalian cells not only in the plasma membrane but also in intracellular membranes. The conventional function of these transporters is to transfer their amino acid substrates across the lipid bilayer; the direction of the transfer is dictated by the combined gradients for the amino acid substrates and the co-transported ions (Na⁺, H⁺, K⁺ or Cl⁻) across the membrane. In cases of electrogenic transporters, the membrane potential also contributes to the direction of the amino acid transfer. In addition to this expected traditional function, several unconventional functions are known for some of these amino acid transporters. This includes their role in intracellular signaling, regulation of acid–base balance, and entry of viruses into cells. Such functions expand the biological roles of these transporters beyond the logical amino acid homeostasis. In recent years, two additional unconventional biochemical/metabolic processes regulated by certain amino acid transporters have come to be recognized: macropinocytosis and obesity. This adds to the repertoire of biological processes that are controlled and regulated by amino acid transporters in health and disease. In the present review, we highlight the unusual involvement of selective amino acid transporters in macropinocytosis (SLC38A5/SLC38A3) and diet-induced obesity/metabolic syndrome (SLC6A19/SLC6A14/SLC6A6).

Xenogeneic-Free Human Intestinal Organoids for Assessing Intestinal Nutrient Absorption

Since many nutrients, including the three major ones of glucose, dipeptides, and cholesterol, are mainly absorbed in the small intestine, the assessment of their effects on intestinal tissue is important for the study of food absorption. However, cultured intestinal cell lines, such as Caco-2 cells, or animal models, which differ from normal human physiological conditions, are generally used for the evaluation of intestinal absorption and digestion. Therefore, it is necessary to develop an alternative in vitro method for more accurate analyses. In this study, we demonstrate inhibitory effects on nutrient absorption through nutrient transporters using three-dimensional xenogeneic-free human intestinal organoids (XF-HIOs), with characteristics of the human intestine, as we previously reported. We first show that the organoids absorbed glucose, dipeptide, and cholesterol in a transporter-dependent manner. Next, we examine the inhibitory effect of natural ingredients on the absorption of glucose and cholesterol. We reveal that glucose absorption was suppressed by epicatechin gallate or nobiletin, normally found in green tea catechin or citrus fruits, respectively. In comparison, cholesterol absorption was not inhibited by luteolin and quercetin, contained in some vegetables. Our findings highlight the usefulness of screening for the absorption of functional food substances using XF-HIOs.

Analysis of L-leucine amino acid transporter species activity and gene expression by human blood brain barrier hCMEC/D3 model reveal potential LAT1, LAT4, B0AT2 and y+LAT1 functional cooperation

In the CNS, amino acid (AA) neurotransmitters and neurotransmitter precursors are subject to tight homeostatic control mediated by blood-brain barrier (BBB) solute carrier amino acid transporters (AATs). Since the BBB is composed of multiple closely apposed cell types and opportunities for human in vivo studies are limited, we used in vitro and computational approaches to investigate human BBB AAT activity and regulation. Quantitative real-time PCR (qPCR) of the human BBB endothelial cell model hCMEC/D3 (D3) was used to determine expression of selected AAT, tight junction (TJ), and signal transduction (ST) genes under various culture conditions. L-leucine uptake data were interrogated with a computational model developed by our group for calculating AAT activity in complex cell cultures. This approach is potentially applicable to in vitro cell culture drug studies where multiple "receptors" may mediate observed responses. Of 7 Leu AAT genes expressed by D3 only the activity of SLC7A5-SLC3A2/LAT1-4F2HC (LAT1), SLC43A2/LAT4 (LAT4) and sodium-dependent AATs, SLC6A15/B⁰AT2 (B⁰AT2), and SLC7A7/y⁺LAT1 (y⁺LAT1) were calculated to be required for Leu uptake. Therefore, D3 Leu transport may be mediated by a potentially physiologically relevant functional cooperation between the known BBB AAT, LAT1 and obligatory exchange (y⁺LAT1), facilitative diffusion (LAT4), and sodium symporter (B⁰AT2) transporters.

Present and Future: Crosstalks Between Polycystic Ovary Syndrome and Gut Metabolites Relating to Gut Microbiota

Polycystic ovary syndrome (PCOS) is a common disease, affecting 8%-13% of the females of reproductive age, thereby compromising their fertility and long-term health. However, the pathogenesis of PCOS is still unclear. It is not only a reproductive endocrine disease, dominated by hyperandrogenemia, but also is accompanied by different degrees of metabolic abnormalities and insulin resistance. With a deeper understanding of its pathogenesis, more small metabolic molecules, such as bile acids, amino acids, and short-chain fatty acids, have been reported to be involved in the pathological process of PCOS. Recently, the critical role of gut microbiota in metabolism has been focused on. The gut microbiota-related metabolic pathways can significantly affect inflammation levels, insulin signaling, glucose metabolism, lipid metabolism, and hormonal secretions. Although the abnormalities in gut microbiota and metabolites might not be the initial factors of PCOS, they may have a significant role in the pathological process of PCOS. The dysbiosis of gut microbiota and disturbance of gut metabolites can affect the progression of PCOS. Meanwhile, PCOS itself can adversely affect the function of gut, thereby contributing to the aggravation of the disease. Inhibiting this vicious cycle might alleviate the symptoms of PCOS. However, the role of gut microbiota in PCOS has not been fully explored yet. This review aims to summarize the potential effects and modulative mechanisms of the gut metabolites on PCOS and suggests its potential intervention targets, thus providing more possible treatment options for PCOS in the future.

Branched-chain amino acids govern the high learning ability phenotype in Tokai high avoider (THA) rats

To fully understand the mechanisms governing learning and memory, animal models with minor interindividual variability and higher cognitive function are required. THA rats established by crossing those with high learning capacity exhibit excellent learning and memory abilities, but the factors underlying their phenotype are completely unknown. In the current study, we compare the hippocampi of parental strain Wistar rats to those of THA rats via metabolomic analysis in order to identify molecules specific to the THA rat hippocampus. Higher branched-chain amino acid (BCAA) levels and enhanced activation of BCAA metabolism-associated enzymes were observed in THA rats, suggesting that acetyl-CoA and acetylcholine are synthesized through BCAA catabolism. THA rats maintained high blood BCAA levels via uptake of BCAAs in the small intestine and suppression of BCAA catabolism in the liver. Feeding THA rats with a BCAA-reduced diet decreased acetylcholine levels and learning ability, thus, maintaining high BCAA levels while their proper metabolism in the hippocampus is the mechanisms underlying the high learning ability in THA rats. Identifying appropriate BCAA nutritional supplements and activation methods may thus hold potential for the prevention and amelioration of higher brain dysfunction, including learning disabilities and dementia.

Protective Effects of Lactobacillus plantarum Lac16 on Clostridium perfringens Infection-Associated Injury in IPEC-J2 Cells

Clostridium perfringens (C. perfringens) causes intestinal injury through overgrowth and the secretion of multiple toxins, leading to diarrhea and necrotic enteritis in animals, including pigs, chickens, and sheep. This study aimed to investigate the protective effects of Lactobacillus plantarum (L. plantarum) Lac16 on C. perfringens infection-associated injury in intestinal porcine epithelial cell line (IPEC-J2). The results showed that L. plantarum Lac16 significantly inhibited the growth of C. perfringens, which was accompanied by a decrease in pH levels. In addition, L. plantarum Lac16 significantly elevated the mRNA expression levels of host defense peptides (HDPs) in IPEC-J2 cells, decreased the adhesion of C. perfringens to IPEC-J2 cells, and attenuated C. perfringens-induced cellular cytotoxicity and intestinal barrier damage. Furthermore, L. plantarum Lac16 significantly suppressed C. perfringens-induced gene expressions of proinflammatory cytokines and pattern recognition receptors (PRRs) in IPEC-J2 cells. Moreover, L. plantarum Lac16 preincubation effectively inhibited the phosphorylation of p65 caused by C. perfringens infection. Collectively, probiotic L. plantarum Lac16 exerts protective effects against C. perfringens infection-associated injury in IPEC-J2 cells.

Expression and function of SLC38A5, an amino acid-coupled Na+/H+ exchanger, in triple-negative breast cancer and its relevance to macropinocytosis

Metabolic reprogramming in cancer necessitates increased amino acid uptake, which is accomplished by up-regulation of specific amino acid transporters. However, not all tumors rely on any single amino acid transporter for this purpose. Here, we report on the differential up-regulation of the amino acid transporter SLC38A5 in triple-negative breast cancer (TNBC). The up-regulation is evident in TNBC tumors, conventional and patient-derived xenograft TNBC cell lines, and a mouse model of spontaneous TNBC mammary tumor. The up-regulation is confirmed by functional assays. SLC38A5 is an amino acid-dependent Na+/H+ exchanger which transports Na+ and amino acids into cells coupled with H+ efflux. Since cell-surface Na+/H+ exchanger is an established inducer of macropinocytosis, an endocytic process for cellular uptake of bulk fluid and its components, we examined the impact of SLC38A5 on macropinocytosis in TNBC cells. We found that the transport function of SLC38A5 is coupled to the induction of macropinocytosis. Surprisingly, the transport function of SLC38A5 is inhibited by amilorides, the well-known inhibitors of Na+/H+ exchanger. Down-regulation of SLC38A5 in TNBC cells attenuates serine-induced macropinocytosis and reduces cell proliferation significantly as assessed by multiple methods, but does not induce cell death. The Cancer Genome Atlas database corroborates SLC38A5 up-regulation in TNBC. This represents the first report on the selective expression of SLC38A5 in TNBC and its role as an inducer of macropinocytosis, thus revealing a novel, hitherto unsuspected, function for an amino acid transporter that goes beyond amino acid delivery but is still relevant to cancer cell nutrition and proliferation.

Effects of low-crude protein diets supplemented with arginine, glutamine, threonine, and methionine on regulating nutrient absorption, intestinal health, and growth performance of Eimeria-infected chickens

The study was conducted to evaluate the effects of low crude protein diets supplemented with arginine, glutamine, methionine, and/or threonine on apparent ileal digestibility of amino acids, intestinal morphology, intestinal permeability, gene expression of nutrient transporters, and tight junction proteins of broiler chickens challenged with mixed Eimeria spp. A total of five hundred seventy-six, 12-day-old male broiler chickens were allocated into 8 treatments, and 6 replicate cages of 12 chickens per cage. This experiment included a nonchallenged control (NC) fed regular corn-soybean meal-based diet (Regular diet, 19% crude protein), an Eimeria-challenged control (CC) fed Regular diet, an Eimeria challenge group fed low-crude protein diet (LCP, 16% crude protein), 4 Eimeria challenge groups fed low-crude protein diet supplemented with 0.75% arginine, glutamine, methionine, and threonine, respectively (ARG, GLN, MET, and THR), and an Eimeria challenge group fed low-crude protein diet with 0.75% supplemented arginine, glutamine, methionine, and threonine collectively as a combination group (COMB). On d 14, birds in the challenge groups were gavaged with a mixed Eimeria spp. solution containing 12,500 oocysts of E. maxima, 12,500 oocysts of E. tenella, and 62,500 oocysts of E. acervulina. The results showed that the Eimeria challenge reduced overall growth performance, but the LCP had no adverse impacts on intestinal health and growth of Eimeria-infected birds compared to the CC. Additionally, supplementation of crystalline arginine, glutamine, methionine, and threonine improved the apparent ileal digestibility of these specific amino acids on 6 dpi. Moreover, the THR treatment increased villus height in the duodenum. The ARG treatment decreased intestinal permeability and gene expression of amino acid transporters, whereas the GLN and THR treatments both reversed adverse effects of coccidiosis on gene expression of tight junction protein (claudin 1). However, the MET and COMB treatments exacerbated infection severity of coccidiosis. In summary, adding 0.75% of arginine, glutamine, or threonine in a low crude protein diet can improve the intestinal health of birds challenged with a mild coccidia infection.

Amino Acid Homeostasis in Mammalian Cells with a Focus on Amino Acid Transport

Amino acid homeostasis is maintained by import, export, oxidation, and synthesis of nonessential amino acids, and by the synthesis and breakdown of protein. These processes work in conjunction with regulatory elements that sense amino acids or their metabolites. During and after nutrient intake, amino acid homeostasis is dominated by autoregulatory processes such as transport and oxidation of excess amino acids. Amino acid deprivation triggers processes such as autophagy and the execution of broader transcriptional programs to maintain plasma amino acid concentrations. Amino acid transport plays a crucial role in the absorption of amino acids in the intestine, the distribution of amino acids across cells and organs, the recycling of amino acids in the kidney, and the recycling of amino acids after protein breakdown.

Organic cation transporters (OCTs/OCTNs) in human primary alveolar epithelial cells

Alveolar epithelium, besides exerting a key role in gas exchange and surfactant production, plays important functions in host defense and inflammation. Pathological conditions associated to alveolar dysfunction include Acute Respiratory Distress Syndrome (ARDS), asthma, chronic obstructive pulmonary disease (COPD) and idiopathic pulmonary fibrosis (IPF). The use of predictive in vitro models of human alveolar epithelium is nowadays required for the study of disease mechanisms, as well as of pharmacokinetic parameters of pulmonary drugs delivery. Here, we employed a novel 3D model of human alveoli, namely EpiAlveolar™, consisting of primary alveolar epithelial cells, pulmonary endothelial cells and fibroblasts, that reflects properly the in vivo-like conditions. In EpiAlveolar™ we performed a characterization of Organic Cation Transporters (OCTs and OCTNs) expression and activity and we found that OCTN2, OCT1 and OCT3 are expressed on the basolateral membrane; instead, ATB^0,+ transporter for cationic and neutral amino acids, which shares with OCTN2 the affinity for carnitine as substrate, is readily detectable and functional at the apical side. We also show that these transporters differentially interact with anticholinergic drugs. Overall, our findings reveal close similarities of EpiAlveolar™ with the tracheal/bronchial epithelium (EpiAirway™ model) and entrust this alveolar tissue as a potential tool for the screening of biopharmaceuticals molecules.

Coordinated action of multiple transporters in the acquisition of essential cationic amino acids by the intracellular parasite Toxoplasma gondii

Intracellular parasites of the phylum Apicomplexa are dependent on the scavenging of essential amino acids from their hosts. We previously identified a large family of apicomplexan-specific plasma membrane-localized amino acid transporters, the ApiATs, and showed that the Toxoplasma gondii transporter TgApiAT1 functions in the selective uptake of arginine. TgApiAT1 is essential for parasite virulence, but dispensable for parasite growth in medium containing high concentrations of arginine, indicating the presence of at least one other arginine transporter. Here we identify TgApiAT6-1 as the second arginine transporter. Using a combination of parasite assays and heterologous characterisation of TgApiAT6-1 in Xenopus laevis oocytes, we demonstrate that TgApiAT6-1 is a general cationic amino acid transporter that mediates both the high-affinity uptake of lysine and the low-affinity uptake of arginine. TgApiAT6-1 is the primary lysine transporter in the disease-causing tachyzoite stage of T. gondii and is essential for parasite proliferation. We demonstrate that the uptake of cationic amino acids by TgApiAT6-1 is ‘trans-stimulated’ by cationic and neutral amino acids and is likely promoted by an inwardly negative membrane potential. These findings demonstrate that T. gondii has evolved overlapping transport mechanisms for the uptake of essential cationic amino acids, and we draw together our findings into a comprehensive model that highlights the finely-tuned, regulated processes that mediate cationic amino acid scavenging by these intracellular parasites.

The causative agent of toxoplasmosis, Toxoplasma gondii, is a versatile intracellular parasite that can proliferate within nucleated cells of warm-blooded organisms. In order to survive, T. gondii parasites must scavenge the cationic amino acids lysine and arginine from their hosts. In a previous study, we demonstrated that a plasma membrane-localized protein called TgApiAT1 facilitates the uptake of arginine into the parasite. We found that parasites lacking TgApiAT1 could proliferate when cultured in medium containing high concentrations of arginine, suggesting the existence of an additional uptake pathway for arginine. In the present study, we demonstrate that this second uptake pathway is mediated by TgApiAT6-1, a protein belonging to the same solute transporter family as TgApiAT1. We show that TgApiAT6-1 is the major lysine transporter of the parasite, and that it is critical for parasite proliferation. Furthermore, we demonstrate that TgApiAT6-1 can transport arginine into parasites under conditions in which arginine concentrations are high and lysine concentrations are comparatively lower. These data support a model for the finely-tuned acquisition of essential cationic amino acids that involves multiple transporters, and which likely contributes to these parasites being able to survive and proliferate within a wide variety of host cell types.