Amino acid transport by small intestinal, hepatic, and pancreatic epithelia

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.

Boronotyrosine, a Borylated Amino Acid Mimetic with Enhanced Solubility, Tumor Boron Delivery, and Retention for the Re-emerging Boron Neutron Capture Therapy Field

Boron neutron capture therapy (BNCT) is a re-emerging binary cellular level cancer intervention that occurs through the interaction of a cancer-specific 10boron (10B) drug and neutrons. We created a new 10B drug, 3-borono-l-tyrosine (BTS), that improves on the characteristics of the main historical BNCT drug 4-borono-l-phenylalanine (BPA). BTS has up to 4 times greater uptake in vitro than BPA and increased cellular retention. Like BPA, BTS uptake is mediated by the l-type amino acid transporter-1 (LAT1) but is less sensitive to natural amino acid competition. BTS can be formulated and bolus dosed at much higher levels than BPA, resulting in 2-3 times greater boron delivery in vivo. Fast blood clearance and greater tumor boron delivery result in superior tumor-to-blood ratios. BTS boron delivery appears to correlate with LAT1 expression. BTS is a promising boron delivery drug that has the potential to improve modern BNCT interventions.

An amino acid metabolism-based seventeen-gene signature correlates with the clinical outcome and immune features in pancreatic cancer

Background: Pancreatic cancer is an aggressive tumor with a low 5-year survival rate and primary resistance to most therapy. Amino acid (AA) metabolism is highly correlated with tumor growth, crucial to the aggressive biological behavior of pancreatic cancer; nevertheless, the comprehensive predictive significance of genes that regulate AA metabolism in pancreatic cancer remains unknown. Methods: The mRNA expression data downloaded from The Cancer Genome Atlas (TCGA) were derived as the training cohort, and the GSE57495 cohort from Gene Expression Omnibus (GEO) database was applied as the validation cohort. Random survival forest (RSF) and the least absolute shrinkage and selection operator (LASSO) regression analysis were employed to screen genes and construct an AA metabolism-related risk signature (AMRS). Kaplan-Meier analysis and receiver operating characteristic (ROC) curve were performed to assess the prognostic value of AMRS. We performed genomic alteration analysis and explored the difference in tumor microenvironment (TME) landscape associated with KRAS and TP53 mutation in both high- and low-AMRS groups. Subsequently, the relationships between AMRS and immunotherapy and chemotherapy sensitivity were evaluated. Results: A 17-gene AA metabolism-related risk model in the TCGA cohort was constructed according to RSF and LASSO. After stratifying patients into high- and low-AMRS groups based on the optimal cut-off value, we found that high-AMRS patients had worse overall survival (OS) in the training cohort (a median OS: 13.1 months vs. 50.1 months, p < 0.0001) and validation cohort (a median OS: 16.2 vs. 30.5 months, p = 1e-04). Genetic mutation analysis revealed that KRAS and TP53 were significantly more mutated in high-AMRS group, and patients with KRAS and TP53 alterations had significantly higher risk scores than those without. Based on the analysis of TME, low-AMRS group displayed significantly higher immune score and more enrichment of T Cell CD8⁺ cells. In addition, high-AMRS-group exhibited higher TMB and significantly lower tumor immune dysfunction and exclusion (TIDE) score and T Cells dysfunction score, which suggested a higher sensitive to immunotherapy. Moreover, high-AMRS group was also more sensitive to paclitaxel, cisplatin, and docetaxel. Conclusion: Overall, we constructed an AA-metabolism prognostic model, which provided a powerful prognostic predictor for the clinical treatment of pancreatic cancer.

Loss of LAT1 sex-dependently delays recovery after caerulein-induced acute pancreatitis

BACKGROUND

The expression of amino acid transporters is known to vary during acute pancreatitis (AP) except for LAT1 (slc7a5), the expression of which remains stable. LAT1 supports cell growth by importing leucine and thereby stimulates mammalian target of rapamycin (mTOR) activity, a phenomenon often observed in cancer cells. The mechanisms by which LAT1 influences physiological and pathophysiological processes and affects disease progression in the pancreas are not yet known.

AIM

To evaluate the role of LAT1 in the development of and recovery from AP.

METHODS

AP was induced with caerulein (cae) injections in female and male mice expressing LAT1 or after its knockout (LAT1 Cre/LoxP). The development of the initial AP injury and its recovery were followed for seven days after cae injections by daily measuring body weight, assessing microscopical tissue architecture, mRNA and protein expression, protein synthesis, and enzyme activity levels, as well as by testing the recruitment of immune cells by FACS and ELISA.

RESULTS

The initial injury, evaluated by measurements of plasma amylase, lipase, and trypsin activity, as well as the gene expression of dedifferentiation markers, did not differ between the groups. However, early metabolic adaptations that support regeneration at later stages were blunted in LAT1 knockout mice. Especially in females, we observed less mTOR reactivation and dysfunctional autophagy. The later regeneration phase was clearly delayed in female LAT1 knockout mice, which did not regain normal expression of the pancreas-specific differentiation markers recombining binding protein suppressor of hairless-like protein (rbpjl) and basic helix-loop-helix family member A15 (mist1). Amylase mRNA and protein levels remained lower, and, strikingly, female LAT1 knockout mice presented signs of fibrosis lasting until day seven. In contrast, pancreas morphology had returned to normal in wild-type littermates.

CONCLUSION

LAT1 supports the regeneration of acinar cells after AP. Female mice lacking LAT1 exhibited more pronounced alterations than male mice, indicating a sexual dimorphism of amino acid metabolism.

Untargeted analysis of the serum metabolome in cats with exocrine pancreatic insufficiency

Exocrine pancreatic insufficiency (EPI) causes chronic digestive dysfunction in cats, but its pathogenesis and pathophysiology are poorly understood. Untargeted metabolomics is a promising analytic methodology that can reveal novel metabolic features and biomarkers of clinical disease syndromes. The purpose of this preliminary study was to use untargeted analysis of the serum metabolome to discover novel aspects of the pathobiology of EPI in cats. Serum samples were collected from 5 cats with EPI and 8 healthy controls. The diagnosis of EPI was confirmed by measurement of subnormal serum feline trypsin-like immunoreactivity (fTLI). Untargeted quantification of serum metabolite utilized ultra-high-performance liquid chromatography-tandem mass spectroscopy. Cats with EPI had significantly increased serum quantities of long-chain fatty acids, polyunsaturated fatty acids, mevalonate pathway intermediates, and endocannabinoids compared with healthy controls. Diacylglycerols, phosphatidylethanolamines, amino acid derivatives, and microbial metabolites were significantly decreased in cats with EPI compared to healthy controls. Diacyclglycerols and amino acid metabolites were positively correlated, and sphingolipids and long-chain fatty acids were negatively correlated with serum fTLI, respectively. These results suggest that EPI in cats is associated with increased lipolysis of peripheral adipose stores, dysfunction of the mevalonate pathway, and altered amino acid metabolism. Differences in microbial metabolites indicate that feline EPI is also associated with enteric microbial dysbiosis. Targeted studies of the metabolome of cats with EPI are warranted to further elucidate the mechanisms of these metabolic derangements and their influence on the pathogenesis and pathophysiology of EPI in cats.

LINGO3 regulates mucosal tissue regeneration and promotes TFF2 dependent recovery from colitis

Aim: Recovery of damaged mucosal surfaces following inflammatory insult requires diverse regenerative mechanisms that remain poorly defined. Previously, we demonstrated that the reparative actions of Trefoil Factor 3 (TFF3) depend upon the enigmatic receptor, leucine rich repeat and immunoglobulin-like domain containing nogo receptor 2 (LINGO2). This study examined the related orphan receptor LINGO3 in the context of intestinal tissue damage to determine whether LINGO family members are generally important for mucosal wound healing and maintenance of the intestinal stem cell (ISC) compartment needed for turnover of mucosal epithelium.Methods and Results: We find that LINGO3 is broadly expressed on human enterocytes and sparsely on discrete cells within the crypt niche, that contains ISCs. Loss of function studies indicate that LINGO3 is involved in recovery of normal intestinal architecture following dextran sodium sulfate (DSS)-induced colitis, and that LINGO3 is needed for therapeutic action of the long acting TFF2 fusion protein (TFF2-Fc), including a number of signaling pathways critical for cell proliferation and wound repair. LINGO3-TFF2 protein-protein interactions were relatively weak however and LINGO3 was only partially responsible for TFF2 induced MAPK signaling suggesting additional un-identified components of a receptor complex. However, deficiency in either TFF2 or LINGO3 abrogated budding/growth of intestinal organoids and reduced expression of the intestinal ISC gene leucine-rich repeat-containing G-protein coupled receptor 5 (LGR5), indicating homologous roles for these proteins in tissue regeneration, possibly via regulation of ISCs in the crypt niche.Conclusion: We propose that LINGO3 serves a previously unappreciated role in promoting mucosal wound healing.

B0AT1 Amino Acid Transporter Complexed With SARS-CoV-2 Receptor ACE2 Forms a Heterodimer Functional Unit: In Situ Conformation Using Radiation Inactivation Analysis

The SARS-CoV-2 receptor, angiotensin-converting enzyme-2 (ACE2), is expressed at levels of greatest magnitude in the small intestine as compared with all other human tissues. Enterocyte ACE2 is coexpressed as the apical membrane trafficking partner obligatory for expression and activity of the B0AT1 sodium-dependent neutral amino acid transporter. These components are assembled as an [ACE2:B0AT1]2 dimer-of-heterodimers quaternary complex that putatively steers SARS-CoV-2 tropism in the gastrointestinal (GI) tract. GI clinical symptomology is reported in about half of COVID-19 patients, and can be accompanied by gut shedding of virion particles. We hypothesized that within this 4-mer structural complex, each [ACE2:B0AT1] heterodimer pair constitutes a physiological "functional unit." This was confirmed experimentally by employing purified lyophilized enterocyte brush border membrane vesicles exposed to increasing doses of high-energy electron radiation from a 16 MeV linear accelerator. Based on radiation target theory, the results indicated the presence of Na+-dependent neutral amino acid influx transport activity functional unit with target size molecular weight 183.7 ± 16.8 kDa in situ in intact apical membranes. Each thermodynamically stabilized [ACE2:B0AT1] heterodimer functional unit manifests the transport activity within the whole ∼345 kDa [ACE2:B0AT1]2 dimer-of-heterodimers quaternary structural complex. The results are consistent with our prior molecular docking modeling and gut-lung axis approaches to understanding COVID-19. These findings advance understanding the physiology of B0AT1 interaction with ACE2 in the gut, and thereby contribute to translational developments designed to treat or mitigate COVID-19 variant outbreaks and/or GI symptom persistence in long-haul postacute sequelae of SARS-CoV-2.

A Review on the Role of Food-Derived Bioactive Molecules and the Microbiota-Gut-Brain Axis in Satiety Regulation

Obesity is a chronic disease resulting from an imbalance between energy intake and expenditure. The growing relevance of this metabolic disease lies in its association with other comorbidities. Obesity is a multifaceted disease where intestinal hormones such as cholecystokinin (CCK), glucagon-like peptide 1 (GLP-1), and peptide YY (PYY), produced by enteroendocrine cells (EECs), have a pivotal role as signaling systems. Receptors for these hormones have been identified in the gut and different brain regions, highlighting the interconnection between gut and brain in satiation mechanisms. The intestinal microbiota (IM), directly interacting with EECs, can be modulated by the diet by providing specific nutrients that induce environmental changes in the gut ecosystem. Therefore, macronutrients may trigger the microbiota-gut-brain axis (MGBA) through mechanisms including specific nutrient-sensing receptors in EECs, inducing the secretion of specific hormones that lead to decreased appetite or increased energy expenditure. Designing drugs/functional foods based in bioactive compounds exploiting these nutrient-sensing mechanisms may offer an alternative treatment for obesity and/or associated metabolic diseases. Organ-on-a-chip technology represents a suitable approach to model multi-organ communication that can provide a robust platform for studying the potential of these compounds as modulators of the MGBA.

Amino Acids in Intestinal Physiology and Health

Dietary protein digestion is an efficient process resulting in the absorption of amino acids by epithelial cells, mainly in the jejunum. Some amino acids are extensively metabolized in enterocytes supporting their high energy demand and/or production of bioactive metabolites such as glutathione or nitric oxide. In contrast, other amino acids are mainly used as building blocks for the intense protein synthesis associated with the rapid epithelium renewal and mucin production. Several amino acids have been shown to support the intestinal barrier function and the intestinal endocrine function. In addition, amino acids are metabolized by the gut microbiota that use them for their own protein synthesis and in catabolic pathways releasing in the intestinal lumen numerous metabolites such as ammonia, hydrogen sulfide, branched-chain amino acids, polyamines, phenolic and indolic compounds. Some of them (e.g. hydrogen sulfide) disrupts epithelial energy metabolism and may participate in mucosal inflammation when present in excess, while others (e.g. indole derivatives) prevent gut barrier dysfunction or regulate enteroendocrine functions. Lastly, some recent data suggest that dietary amino acids might regulate the composition of the gut microbiota, but the relevance for the intestinal health remains to be determined. In summary, amino acid utilization by epithelial cells or by intestinal bacteria appears to play a pivotal regulator role for intestinal homeostasis. Thus, adequate dietary supply of amino acids represents a key determinant of gut health and functions.

Absorptive transport of amino acids by the rat colon

The capacity of the colon to absorb microbially produced amino acids (AAs) and the underlying mechanisms of AA transport are incompletely defined. We measured the profile of 16 fecal AAs along the rat ceco-colonic axis and compared unidirectional absorptive AA fluxes across mucosal tissues isolated from the rat jejunum, cecum, and proximal colon using an Ussing chamber approach, in conjunction with ¹H-NMR and ultra-performance liquid chromatography-mass spectrometry chemical analyses. Passage of stool from cecum to midcolon was associated with segment-specific changes in fecal AA composition and a decrease in total AA content. Simultaneous measurement of up to 16 AA fluxes under native luminal conditions, with correction for endogenous AA release, demonstrated absorptive transfer of AAs across the cecum and proximal colon at rates comparable (30-80%) to those across the jejunum, with significant Na⁺-dependent and H⁺-stimulated components. Expression profiling of 30 major AA transporter genes by quantitative PCR revealed comparatively high levels of transcripts for 20 AA transporters in the cecum and/or colon, with the levels of 12 exceeding those in the small intestine. Our results suggest a more detailed model of major apical and basolateral AA transporters in rat colonocytes and provide evidence for a previously unappreciated transfer of AAs across the colonic epithelium that could link the prodigious metabolic capacities of the luminal microbiota, the colonocytes, and the body tissues.NEW & NOTEWORTHY This study provides evidence for a previously unappreciated transfer of microbially generated amino acids across the colonic epithelium under physiological conditions that could link the prodigious metabolic capacities of the luminal microbiota, the colonocytes, and the body tissues. The segment-specific expression of at least 20 amino acid transporter genes along the colon provides a detailed mechanistic basis for uniport, heteroexchange, Na⁺-cotransport, and H⁺-cotransport components of colonic amino acid absorption.

Non-alcoholic fatty liver disease alters expression of genes governing hepatic nitrogen conversion

BACKGROUND & AIMS

We recently showed that the functional capacity for ureagenesis is deficient in non-alcoholic fatty liver disease (NAFLD) patients. The aim of this study was to assess expression of urea cycle-related genes to elucidate a possible gene regulatory basis to the functional problem.

METHODS

Liver mRNA expression analyses within the gene pathway governing hepatic nitrogen conversion were performed in 20 non-diabetic, biopsy-proven NAFLD patients (8 simple steatosis; 12 non-alcoholic steatohepatitis [NASH]) and 12 obese and 14 lean healthy individuals. Sixteen NAFLD patients were included for gene expression validation. Relationship between gene expressions and functional capacity for ureagenesis was described.

RESULTS

Gene expression of most urea cycle-related enzymes were downregulated in NAFLD vs both control groups; markedly so for the urea cycle flux-generating carbamoyl phosphate synthetase (CPS1) (~3.5-fold, P < .0001). In NASH, CPS1 downregulation paralleled the deficit in ureagenesis (P = .03). Additionally, expression of several genes involved in amino acid uptake and degradation, and the glucagon receptor gene, were downregulated in NAFLD. Conversely, glutamine synthetase (GS) expression increased >1.5-fold (P ≤ .03), inversely related to CPS1 expression (P = .004).

CONCLUSIONS

NAFLD downregulated the expression of urea cycle-related genes. Downregulation of urea cycle flux-generating CPS1 correlated with the loss of functional capacity for ureagenesis in NASH. On gene level, these changes coincided with an increase in the major ammonia scavenging enzyme GS. The effects seemed related to a fatty liver as such rather than NASH or obesity. The findings support gene regulatory mechanisms involved in the deficient ureagenesis of NAFLD, but it remains unexplained how hepatocyte fat accumulation exerts these effects.

Unusual proteins in Giardia duodenalis and their role in survival

The capacity of the parasite Giardia duodenalis to perform complex functions with minimal amounts of proteins and organelles has attracted increasing numbers of scientists worldwide, trying to explain how this parasite adapts to internal and external changes to survive. One explanation could be that G. duodenalis evolved from a structurally complex ancestor by reductive evolution, resulting in adaptation to its parasitic lifestyle. Reductive evolution involves the loss of genes, organelles, and functions that commonly occur in many parasites, by which the host renders some structures and functions redundant. However, there is increasing data that Giardia possesses proteins able to perform more than one function. During recent decades, the concept of moonlighting was described for multitasking proteins, which involves only proteins with an extra independent function(s). In this chapter, we provide an overview of unusual proteins in Giardia that present multifunctional properties depending on the location and/or parasite requirement. We also discuss experimental evidence that may allow some giardial proteins to be classified as moonlighting proteins by examining the properties of moonlighting proteins in general. Up to date, Giardia does not seem to require the numerous redundant proteins present in other organisms to accomplish its normal functions, and thus this parasite may be an appropriate model for understanding different aspects of moonlighting proteins, which may be helpful in the design of drug targets.

Metabolism of the neurotoxic amino acid β-N-methylamino-L-alanine in human cell culture models

Human dietary exposure to the environmental neurotoxin β-N-methylamino-L-alanine (BMAA) has been implicated in an increased risk of developing sporadic neurodegenerative diseases like Alzheimer's and amyotrophic lateral sclerosis. Evidence suggests that humans are exposed to BMAA globally, but very little is known about BMAA metabolism in mammalian systems, let alone in humans. The most plausible, evidence-based mechanisms of BMAA toxicity rely on the metabolic stability of the amino acid and that, following ingestion, it enters the circulatory system unmodified. BMAA crosses from the intestinal lumen into the circulatory system, and the small intestine and liver are the first sites for dietary amino acid metabolism. Both tissues have substantial amino acid metabolic needs, which are largely fulfilled by dietary amino acids. Metabolism of BMAA in these tissues has been largely overlooked, yet is important in gauging the true human exposure risk. Here we investigate the potential for BMAA metabolism by the human liver and small intestine, using in vitro cell systems. Data show that BMAA metabolism via common proteinogenic amino acid metabolic pathways is negligible, and that in the presence of other amino acids cellular uptake of BMAA is substantially reduced. These data suggest that the majority of ingested BMAA remains unmodified following passage through the small intestine and liver. This not only supports oral BMAA exposure as a plausible exposure route to toxic doses of BMAA, but also supports previous notions that protein deficient diets or malnutrition may increase an individual's susceptibility to BMAA absorption and subsequent toxicity.

Regulation of Intestinal Epithelial Cells Properties and Functions by Amino Acids

Intestinal epithelial cells (IECs) line the surface of intestinal epithelium, where they play important roles in the digestion of food, absorption of nutrients, and protection of the human body from microbial infections, and others. Dysfunction of IECs can cause diseases. The development, maintenance, and functions of IECs are strongly influenced by external nutrition, such as amino acids. Amino acids play important roles in regulating the properties and functions of IECs. In this article, we briefly reviewed the current understanding of the roles of amino acids in the regulation of IECs' properties and functions in physiological state, including in IECs homeostasis (differentiation, proliferation, and renewal), in intestinal epithelial barrier structure and functions, and in immune responses. We also summarized some important findings on the effects of amino acids supplementation (e.g., glutamine and arginine) in restoring IECs' and intestine functions in some diseased states. These findings will further our understanding of the important roles of amino acids in the homeostasis of IECs and could potentially help identify novel targets and reagents for the therapeutic interventions of diseases associated with dysfunctional IECs.

Spatial mRNA Expression and Response to Fasting and Refeeding of Neutral Amino Acid Transporters slc6a18 and slc6a19a in the Intestinal Epithelium of Mozambique tilapia

The mRNA expressions of the epithelial neutral amino acid transporters slc6a18 and slc6a19a in the five segments (HL, PMC, GL, DMC, and TS) of the intestine of Mozambique tilapia, and their responses to fasting and refeeding were investigated for a better understanding of the functional and nutritional characteristics of slc6a18 and slc6a19a. Although both slc6a18 and slc6a19a were expressed mainly in the intestine, these genes showed opposing spatial distributions along the intestine. The slc6a18 was mainly expressed in the middle (GL) and posterior (DMC and TS) intestines, while slc6a19a was specifically expressed in the anterior intestine (HL and PMC). Large decreases of amino acid concentrations from the HL to GL imply that amino acids are mainly absorbed before reaching the GL, suggesting an important role of slc6a19a in the absorption. Moreover, substantial amounts of some neutral amino acids with the isoelectric point close to 6 remain in the GL. These are most likely the remaining unabsorbed amino acids or those from of amino acid antiporters which release neutral amino acids in exchange for uptake of its substrates. These amino acids were diminished in the TS, suggesting active absorption in the posterior intestine. This suggests that slc6a18 is essential to complete the absorption of neutral amino acids. At fasting, significant downregulation of slc6a19a expression was observed from the initial up to day 2 and became stable from day 4 to day 14 in the HL and PMC suggesting that slc6a19a expression reflects nutritional condition in the intestinal lumen. Refeeding stimulates slc6a19a expression, although expressions did not exceed the initial level within 3 days after refeeding. The slc6a18 expression was decreased during fasting in the GL but no significant change was observed in the DMC. Only a transient decrease was observed at day 2 in the TS. Refeeding did not stimulate slc6a18 expression. Results in this study suggest that Slc6a18 and Slc6a19 have different roles in the intestine, and that both of these contribute to establish the efficient neutral amino acid absorption system in the tilapia.

Expression of apical Na(+)-L-glutamine co-transport activity, B(0)-system neutral amino acid co-transporter (B(0)AT1) and angiotensin-converting enzyme 2 along the jejunal crypt-villus axis in young pigs fed a liquid formula

Gut apical amino acid (AA) transport activity is high at birth and during suckling, thus being essential to maintain luminal nutrient-dependent mucosal growth through providing AA as essential metabolic fuel, substrates and nutrient stimuli for cellular growth. Because system-B(0) Na(+)-neutral AA co-transporter (B(0)AT1, encoded by the SLC6A19 gene) plays a dominant role for apical uptake of large neutral AA including L-Gln, we hypothesized that high apical Na(+)-Gln co-transport activity, and B(0)AT1 (SLC6A19) in co-expression with angiotensin-converting enzyme 2 (ACE2) were expressed along the entire small intestinal crypt-villus axis in young animals via unique control mechanisms. Kinetics of Na(+)-Gln co-transport activity in the apical membrane vesicles, prepared from epithelial cells sequentially isolated along the jejunal crypt-villus axis from liquid formula-fed young pigs, were measured with the membrane potential being clamped to zero using thiocyanate. Apical maximal Na(+)-Gln co-transport activity was much higher (p < 0.05) in the upper villus cells than in the middle villus (by 29 %) and the crypt (by 30 %) cells, whereas Na(+)-Gln co-transport affinity was lower (p < 0.05) in the upper villus cells than in the middle villus and the crypt cells. The B(0)AT1 (SLC6A19) mRNA abundance was lower (p < 0.05) in the crypt (by 40-47 %) than in the villus cells. There were no significant differences in B(0)AT1 and ACE2 protein abundances on the apical membrane among the upper villus, the middle villus and the crypt cells. Our study suggests that piglet fast growth is associated with very high intestinal apical Na(+)-neutral AA uptake activities via abundantly co-expressing B(0)AT1 and ACE2 proteins in the apical membrane and by transcribing the B(0)AT1 (SLC6A19) gene in the epithelia along the entire crypt-villus axis.

In vivo and in vitro evaluations of intestinal gabapentin absorption: effect of dose and inhibitors on carrier-mediated transport

PURPOSE

Gabapentin exhibits saturable absorption kinetics, however, it remains unclear which transporters that are involved in the intestinal transport of gabapentin. Thus, the aim of the current study was to explore the mechanistic influence of transporters on the intestinal absorption of gabapentin by both in vivo and in vitro investigations

METHODS

Pharmacokinetic parameters were determined following a range of intravenous (5-100 mg/kg) and oral doses (10-200 mg/kg) in rats. Transepithelial transport (50 μM-50 mM) and apical uptake of gabapentin (0.01-50 mM) were investigated in Caco-2 cells. The effect of co-application of the LAT-inhibitor, BCH, and the b(0,+)-substrate, L-lysine, on intestinal transport of gabapentin was evaluated in vivo and in vitro.

RESULTS

Gabapentin showed dose-dependent oral absorption kinetics and dose-independent disposition kinetics. Co-application of BCH inhibited intestinal absorption in vivo and apical uptake in vitro, whereas no effect was observed following co-application of L-lysine.

CONCLUSIONS

The present study shows for the first time that BCH was capable of inhibiting intestinal absorption of gabapentin in vivo. Furthermore, in Caco-2 cell experiments BCH inhibited apical uptake of gabapentin. These findings may imply that a BCH-sensitive transport-system was involved in the apical and possibly the basolateral transport of gabapentin across the intestinal wall.

Long-term osmotic regulation of amino acid transport systems in mammalian cells

Mammalian cells accumulate organic osmolytes, either to adapt to permanent osmotic changes or to mediate cell volume increase in cell cycle progression. Amino acids may serve as osmolytes in a great variety of cells. System A, a transport system for neutral amino acids, is induced after hypertonic shock by a mechanism which requires protein synthesis and gene transcription. Indirect evidence supports the view that system A activity increases due to the interaction of pre-existing A carriers with putative activating proteins. The intracellular accumulation of most neutral amino acids after hypertonic shock depends, exclusively, on the increase in system A activity. Long-term activation of system A is dependent on the integrity of cytoskeletal structures, but in a different way depending on whether cells are polarized or not.

Amino acid transporters expression in acinar cells is changed during acute pancreatitis

Pancreatic acinar cells accumulate amino acids against a marked concentration gradient to synthesize digestive enzymes. Thus, the function of acinar cells depends on amino acid uptake mediated by active transport. Despite the importance of this process, pancreatic amino acid transporter expression and cellular localization is still unclear. We screened mouse pancreas for the expression of genes encoding amino acid transporters. We showed that the most highly expressed transporters, namely sodium dependent SNAT3 (Slc38a3) and SNAT5 (Slc38a5) and sodium independent neutral amino acids transporters LAT1 (Slc7a5) and LAT2 (Slc7a8), are expressed in the basolateral membrane of acinar cells. SNAT3 and SNAT5, LAT1 and LAT2 are expressed in acinar cells. Additional evidence that these transporters are expressed in mature acinar cells was gained using acinar cell culture and acute pancreatitis models. In the acute phase of pancreatic injury, when acinar cell loss occurs, and in an acinar cell culture model, which mimics changes occurring during pancreatitis, SNAT3 and SNAT5 are strongly down-regulated. LAT1 and LAT2 were down-regulated only in the in vitro model. At protein level, SNAT3 and SNAT5 expression was also reduced during pancreatitis. Expression of other amino acid transporters was also modified in both models of pancreatitis. The subset of transporters with differential expression patterns during acute pancreatitis might be involved in the injury/regeneration phases. Further expression, localization and functional studies will follow to better understand changes occurring during acute pancreatitis. These findings provide insight into pancreatic amino acid transport in healthy pancreas and during acute pancreatitis injury.

Intestinal absorption of amino acids in the Pacific bluefin tuna (Thunnus orientalis): in vitro lysine-arginine interaction using the everted intestine system

The interaction between lysine (Lys) and arginine (Arg) in the proximal intestinal region of Pacific bluefin tuna (Thunnus orientalis) was evaluated using the everted intestine method. This in vitro intestinal system has been shown to be an effective tool for studying the nutrient absorption without the need to handle the tuna fish in marine cages as needed for digestibility and amino acid (AA) absorption. We used a factorial design with two sets of variables: low and high Lys concentration (10 and 75 mM) and four different Arg concentrations (3, 10, 20, and 30 mM). Both amino acids were dissolved in marine Ringer solution with a basal amino acidic composition consisting of a tryptone solution (9 mg mL(-1)). No interaction was observed between the absorption of Lys and Arg during the first 10 min of the experiment when low concentration of Lys and Arg was used in the hydrolyzate solution. However, there seemed to be a positive effect on Lys absorption when both amino acids were at high concentrations (30 and 75 mM, respectively). This type of studies will led us to test different formulations and/or additives to better understand the efficiency of AA supplementation as an alternative to in situ studies that are difficult to follow to design with the Pacific Bluefin Tuna.

Re-print of "Intestinal luminal nitrogen metabolism: role of the gut microbiota and consequences for the host"

Alimentary and endogenous proteins are mixed in the small intestinal lumen with the microbiota. Although experimental evidences suggest that the intestinal microbiota is able to incorporate and degrade some of the available amino acids, it appears that the microbiota is also able to synthesize amino acids raising the view that amino acid exchange between the microbiota and host can proceed in both directions. Although the net result of such exchanges remains to be determined, it is likely that a significant part of the amino acids recovered from the alimentary proteins are used by the microbiota. In the large intestine, where the density of bacteria is much higher than in the small intestine and the transit time much longer, the residual undigested luminal proteins and peptides can be degraded in amino acids by the microbiota. These amino acids cannot be absorbed to a significant extent by the colonic epithelium, but are precursors for the synthesis of numerous metabolic end products in reactions made by the microbiota. Among these products, some like short-chain fatty acids and organic acids are energy substrates for the colonic mucosa and several peripheral tissues while others like sulfide and ammonia can affect the energy metabolism of colonic epithelial cells. More work is needed to clarify the overall effects of the intestinal microbiota on nitrogenous compound metabolism and consequences on gut and more generally host health.

Continuous enteral administration can enable normal amino acid absorption in rats with methotrexate-induced gastrointestinal mucositis

It is unknown what feeding strategy to use during chemotherapy-induced gastrointestinal mucositis, which causes weight loss and possibly malabsorption. To study the absorptive capacity of amino acids during mucositis, we determined the plasma availability of enterally administered amino acids (AA), their utilization for protein synthesis, and the preferential side of the intestine for AA uptake in rats with and without methotrexate (MTX)-induced mucositis. Four days after injection with MTX (60 mg/kg) or saline (controls), rats received a primed, continuous dual-isotope infusion (intraduodenal and intravenous) of labeled L-leucine, L-lysine, L-phenylalanine, L-threonine, and L-methionine. We collected blood samples, assessed jejunal histology, and determined labeled AA incorporation in proximal and distal small intestinal mucosa, plasma albumin, liver, and thigh muscle. MTX-induced mucositis was confirmed by histology. The median systemic availability of all AA except for leucine was similar in MTX-treated rats and in controls. However, the individual availability of all AA differed substantially within the group of MTX-treated rats, ranging from severely reduced (<10% of intake) to not different from controls (>40% of intake in 5 of 9 rats). More AA originating from basolateral uptake than those originating from apical uptake were used for intestinal protein synthesis in MTX-treated rats (≥420% more, P < 0.05). We conclude that continuous enteral administration can enable normal AA absorption in rats with MTX-induced mucositis. The intestine prefers basolateral AA uptake to meet its need for AA for protein synthesis during mucositis.

Differences in portal flow rates of amino acids and liver composition between rats fed casein or lactalbumin

The portal appearance rates and net rates of amino acids' absorption were studied in rats fed semi-synthetic diets containing either casein or lactalbumin (CAS and LA, respectively) as the only protein sources. Rats were pre-adapted to the experimental diets for 5 days prior to the absorption studies. Rats fed the LA diet had higher (p < 0.05) portal vein concentrations of free essential amino acids than those fed the CAS diet at 0, 60, 105 and 150 min after feeding. Portal and arterial concentrations of arginine, leucine, tryptophan, lysine and methionine were higher (p < 0.05) in rats fed LA at most time points tested, while concentrations of tyrosine were higher (p < 0.05) in CAS fed rats. When portal flow rates were compared, values for arginine, threonine, alanine, leucine, tryptophan and lysine were higher (p < 0.05) in LA at most time points tested, while proline, tyrosine and valine were higher (p < 0.05) for CAS fed rats after 60 and 105 min feeding. Portal blood flow varied (p < 0.05) with time in rats fed protein-free or LA diets, and was higher (p < 0.05) than that of CAS at 105 min. Intestinal net rates of absorption of tyrosine, valine, leucine and lysine were higher (p < 0.05) for LA fed rats as compared to those fed CAS at most time points tested, while alanine and proline net rates were higher (p < 0.05) for CAS fed rats at 60, 105 and 150 min. Amounts of protein in stomach contents of rats fed the CAS diet were significantly higher (p < 0.05) than those in LA fed rats at 60, 105 and 150 min after feeding. The relative liver weight of the rats fed the CAS diet was lower (p < 0.05) than that of animals fed the LA diet. Lower (p < 0.05) liver glycogen and lipid contents were determined in rats fed CAS diet respect to LA or protein-free fed rats. Results indicate that dietary and plasma amino acids profile are only partially related, and that under normal feeding conditions amino acids from CAS and LA are absorbed at different rates, which is likely to affect liver composition and metabolism.

Roux-en-Y gastric bypass alters small intestine glutamine transport in the obese Zucker rat

The metabolic effects of Roux-en-Y gastric bypass (RYGB) are caused by postsurgical changes in gastrointestinal anatomy affecting gut function. Glutamine is a critical gut nutrient implicated in regulating glucose metabolism as a substrate for intestinal gluconeogenesis. The present study examines the effects of obesity and RYGB on intestinal glutamine transport and metabolism. First, lean and obese Zucker rats (ZRs) were compared. Then the effects of RYGB and sham surgery with pair feeding (PF) in obese ZRs were studied. Segments of small intestine (biliopancreatic limb, Roux limb, and common channel) mucosa were harvested and brush border membrane vesicles (BBMVs) were isolated on postoperative day 28. Glutamine transporter activity and abundance, B(0)AT1 protein, and mRNA levels were measured. Levels of glutaminase, cytosolic phosphoenolpyruvate carboxykinase (PEPCK-C), and glucose-6-phosphatase (G6Pase) were measured to assess glutamine metabolism and intestinal gluconeogenesis. Obesity increased glutamine transport and B(0)AT1 expression throughout the intestine. RYGB increased glutamine transport activity in the biliopancreatic (3.8-fold) and Roux limbs (1.4-fold) but had no effect on the common channel. The relative abundance of B(0)AT1 mRNA and protein were increased in the biliopancreatic (6-fold) and Roux limbs (10-fold) after RYGB (P < 0.05 vs. PF), but not the common channel. Glutaminase levels were increased, whereas the relative abundance of PEPCK-C and G6Pase were decreased in all segments of intestine after RYGB. RYGB selectively increased glutamine absorption in biliopancreatic and Roux limbs by a mechanism involving increased B(0)AT1 expression. Post-RYGB glutaminase levels were increased, but the reductions in PEPCK-C and G6Pase suggest that RYGB downregulates intestinal gluconeogenesis.

Some insights into energy metabolism for osmoregulation in fish

A sufficient and timely energy supply is a prerequisite for the operation of iono- and osmoregulatory mechanisms in fish. Measurements of whole-fish or isolated-gill (or other organs) oxygen consumption have demonstrated regulation of the energy supply during acclimation to different osmotic environments, and such regulation is dependent on species, the situation of acclimation or acclimatization, and life habits. Carbohydrate metabolism appears to play a major role in the energy supply for iono- and osmoregulation, and the liver is the major source supplying carbohydrate metabolites to osmoregulatory organs. Compared with carbohydrates, the roles of lipids and proteins remain largely unclear. Energy metabolite translocation was recently found to occur between fish gill ionocytes and neighboring glycogen-rich (GR) cells, indicating the physiological significance of a local energy supply for gill ion regulatory mechanisms. Spatial and temporal relationships between the liver and other osmoregulatory and non-osmoregulatory organs in partitioning the energy supply for ion regulatory mechanisms during salinity challenges were also proposed. A novel glucose transporter was found to specifically be expressed and function in gill ionocytes, providing the first cue for investigating energy translocation among gill cells. Advanced molecular physiological approaches can be used to examine energy metabolism relevant to a particular cell type (e.g., gill ionocytes), and functional genomics may also provide another powerful approach to explore new metabolic pathways related to fish ion regulation.

Chemotherapy does not influence intestinal amino acid uptake in children

Chemotherapy will frequently induce intestinal damage (mucositis). Enteral nutrition is then often withheld for fear of impaired intestinal absorption as shown in animal models. There is no clinical evidence, however, that absorption is indeed compromised during chemotherapy-induced mucositis. The aim of this study was to evaluate systemic availability of dietary amino acids (leucine) during chemotherapy-induced mucositis. We studied eight childhood cancer patients (age 1.5-16 y) on 2 d, i.e. the day before chemotherapy and 3-5 d after. Chemotherapy-induced oral mucositis and diarrhea were scored on a World Health Organization toxicity scale. Stable isotope tracers were used to measure first-pass splanchnic leucine uptake and whole-body leucine kinetics. Patients showed increased mucositis and/or diarrhea toxicity scores (p < 0.0001) after chemotherapy. Systemic availability of enterally administered leucine was not significantly affected by chemotherapy (before 60%, after 90%, p = 0.46). Interestingly, five patients already showed a negative leucine balance before chemotherapy. In conclusion, most children receiving chemotherapy are already catabolic before start of a new cycle of chemotherapy. Amino acid transport as measured by leucine uptake in the intestine is not affected by chemotherapy-induced mucositis.

Deciphering the mechanisms of intestinal imino (and amino) acid transport: The redemption of SLC36A1

The absorption of zwitterionic imino and amino acids, and related drugs, is an essential function of the small intestinal epithelium. This review focuses on the physiological roles of transporters recently identified at the molecular level, in particular SLC36A1, by identifying how they relate to the classical epithelial imino and amino acid transporters characterised in mammalian small intestine in the 1960s-1990s. SLC36A1 transports a number of D- and L-imino and amino acids, beta- and gamma-amino acids and orally-active neuromodulatory and antibacterial agents. SLC36A1 (or PAT1) functions as a proton-coupled imino and amino acid symporter in cooperation with the Na+/H+ exchanger NHE3 (SLC9A3) to produce the imino acid carrier identified in rat small intestine in the 1960s but subsequently ignored because of confusion with the IMINO transporter. However, it is the sodium/imino and amino acid cotransporter SLC6A20 which corresponds to the betaine carrier (identified in hamster, 1960s) and IMINO transporter (identified in rabbit and guinea pig, 1980s). This review summarises evidence for expression of SLC36A1 and SLC6A20 in human small intestine, highlights the differences in functional characteristics of the imino acid carrier and IMINO transporter, and explains the confusion surrounding these two distinct transport systems.

The tegument surface membranes of the human blood parasite Schistosoma mansoni: a proteomic analysis after differential extraction

The blood fluke Schistosoma mansoni can live for years in the hepatic portal system of its human host and so must possess very effective mechanisms of immune evasion. The key to understanding how these operate lies in defining the molecular organisation of the exposed parasite surface. The adult worm is covered by a syncytial tegument, bounded externally by a plasma membrane and overlain by a laminate secretion, the membranocalyx. In order to determine the protein composition of this surface, the membranes were detached using a freeze/thaw technique and enriched by sucrose density gradient centrifugation. The resulting preparation was sequentially extracted with three reagents of increasing solubilising power. The extracts were separated by 2-DE and their protein constituents were identified by MS/MS, yielding predominantly cytosolic, cytoskeletal and membrane-associated proteins, respectively. After extraction, the final pellet containing membrane-spanning proteins was processed by liquid chromatographic techniques before MS. Transporters for sugars, amino acids, ions and other solutes were found together with membrane enzymes and proteins concerned with membrane structure. The proteins identified were categorised by their function and putative location on the basis of their homology with annotated proteins in other organisms.

Amino acid transporters ASCT2 and LAT1 in cancer: partners in crime?

Relative to other neutral amino acid transporters, the expression levels of ASCT2 and LAT1, are coordinately elevated in a wide spectrum of primary human cancers, suggesting that they are frequently co-opted to support the "tumor metabolome". Each has recently been shown to play important roles in the growth and survival of cancer cell lines, making them potential targets for cancer therapy. The properties and putative relationship of these two amino acid exchangers are discussed in the context of their demonstrated utility in cancer biology, including cellular growth and survival signaling and integrated links to the mammalian target-of-rapamycin (mTOR) kinase.

The hepatic amino acid system A transport activity, is up-regulated in obese Zucker rats

The utilization of L-alanine by liver is dependent on amino acid uptake from blood. This uptake, mainly mediated by the A transport system, may be regulated by different nutritional and physiologic conditions. The regulation of this transport system by diets with different protein content was tested in lean and obese Zucker rats. High-protein (HP) and low-protein (LP) diets led to changes in the rats' growth patterns, especially in lean animals. However, homeostasis was relatively well maintained, as seen in plasma values, in spite of the increased urea production in the HP groups and increased triacylglycerides in the LP groups. The obese animals took up L-alanine at a higher rate than the lean animals. Obesity led to the emergence of a high-affinity component (K(M) approximately 0.1-0.2 mM) in the transport system, which was not dependent on the protein content of the diet. This component has a 10-fold increase in affinity for L-alanine, but with an approximately 3- to 5-fold reduction in maximal velocity of transport.

H+/amino acid transporter 1 (PAT1) is the imino acid carrier: An intestinal nutrient/drug transporter in human and rat

BACKGROUND AND AIMS

Amino acid (and related drug) absorption across the human small intestinal wall is an essential intestinal function. Despite the revelation of a number of mammalian genomes, the molecular identity of the classic Na(+)-dependent imino acid transporter (identified functionally in the 1960s) remains elusive. The aims of this study were to determine whether the recently isolated complementary DNA hPAT1 (human proton-coupled amino acid transporter 1), or solute carrier SLC36A1, represents the imino acid carrier; the Na(+) -dependent imino acid transport function measured at the brush-border membrane of intact intestinal epithelia results from a close functional relationship between human proton-coupled amino acid transporter-1 and N(+) /H(+) exchanger 3 (NHE3).

METHODS

PAT1 function was measured in isolation ( Xenopus laevis oocytes) and in intact epithelia (Caco-2 cell monolayers and rat small intestine) by measurement of amino acid and/or H(+) influx. Tissue and membrane expression of PAT1 were determined by reverse-transcription polymerase chain reaction and immunohistochemistry.

RESULTS

PAT1-specific immunofluorescence was localized exclusively to the luminal membrane of Caco-2 cells and human and rat small intestine. The substrate specificity of hPAT1 is identical to that of the imino acid carrier. In intact epithelia, PAT1-mediated amino acid influx is reduced under conditions in which NHE3 is inactive.

CONCLUSIONS

The identification in intact epithelia of a cooperative functional relationship between PAT1 (H(+) /amino acid symport) and NHE3 (N(+) /H(+) exchange) explains the apparent Na + dependence of the imino acid carrier in studies with mammalian intestine. hPAT1 is the high-capacity imino acid carrier localized at the small intestinal luminal membrane that transports nutrients (imino/amino acids) and orally active neuromodulatory agents (used to treat affective disorders).

Effect of chronic renal failure with metabolic acidosis on alanine metabolism in isolated liver cells

BACKGROUND & AIMS

Decreased ureagenesis and gluconeogenesis from alanine have been reported during chronic renal failure in rat. This study addressed the respective roles of plasma-membrane transport and intracellular metabolism in these abnormalities of alanine pathways.

METHODS

In hepatocytes isolated from uremic and control rats, we investigated: (1) the influence of uremia on gluconeogenesis and ureagenesis during incubations with alanine; (2) the kinetics of alanine plasma-membrane transport; (3) the relationships between intracellular alanine concentrations and its metabolism. Plasma-membrane alanine transport was assessed after addition of alanine (2 mM) by measuring its intracellular accumulation from 0 to 10 min, in the presence of a transaminase inhibitor. Alanine metabolism was studied in perifused hepatocytes by measuring intracellular alanine concentration together with urea, glucose and lactate production in the presence of increasing concentrations of alanine (0-8 mM).

RESULTS

Uremic rats showed decreased plasma bicarbonate. Uremia induced (P<0.05) a decrease in both gluconeogenesis (36%) and ureagenesis (22%). Alanine plasma-membrane transport decreased by 20% during uremia. During perifusions, uremia induced a 30-40% decrease in urea, glucose, and lactate production without modifying intracellular alanine concentration.

CONCLUSIONS

In uremic rats with acidosis, hepatocyte alanine utilization was impaired at both plasma-membrane transport and intracellular transamination steps.

Expression of apical membrane L-glutamate transporters in neonatal porcine epithelial cells along the small intestinal crypt-villus axis

Enteral l-glutamate is extensively utilized as an oxidative fuel by the gut mucosa in the neonate. To identify major uptake pathways and to understand uptake regulation, we examined transport kinetics and molecular identities of apical membrane l-glutamate transporters in epithelial cells sequentially isolated along the small intestinal crypt-villus axis from milk protein-fed, 16-day-old pigs. The distended intestinal sac method was used to isolate 12 sequential cell fractions from the tip villus to the bottom crypt. Initial rates and kinetics of l-glutamate uptake were measured with l-[G-(3)H]glutamate by fast filtration in apical membrane vesicles prepared by Mg(2+) precipitation and differential centrifugation, with membrane potential clamped by SCN(-). Initial l-glutamate uptake results suggested the presence of B(o) and X(AG)(-) transport systems, but the X(AG)(-) system was predominant for uptake across the apical membrane. Kinetic data suggested that l-glutamate uptake through the X(AG)(-) system was associated with higher maximal transport activity but lower transporter affinity in crypt than in villus cells. Molecular identity of the X(AG)(-) glutamate transporter, based on immunoblot and RT-PCR analysis, was primarily the defined excitatory amino acid carrier (EAAC)-1. EAAC-1 expression was increased with cell differentiation and regulated at transcription and translation levels from crypt to upper villus cells. In conclusion, efficiency and capacity of luminal l-glutamate uptake across the apical membrane are regulated by changing expression of the X(AG)(-) system transporter gene EAAC-1 at transcription and translation levels as well as maximal uptake activity and transporter affinity along the intestinal crypt-villus axis in the neonate.

Hartnup disorder is caused by mutations in the gene encoding the neutral amino acid transporter SLC6A19

Hartnup disorder (OMIM 234500) is an autosomal recessive abnormality of renal and gastrointestinal neutral amino acid transport noted for its clinical variability. We localized a gene causing Hartnup disorder to chromosome 5p15.33 and cloned a new gene, SLC6A19, in this region. SLC6A19 is a sodium-dependent and chloride-independent neutral amino acid transporter, expressed predominately in kidney and intestine, with properties of system B(0). We identified six mutations in SLC6A19 that cosegregated with disease in the predicted recessive manner, with most affected individuals being compound heterozygotes. The disease-causing mutations that we tested reduced neutral amino acid transport function in vitro. Population frequencies for the most common mutated SLC6A19 alleles are 0.007 for 517G --> A and 0.001 for 718C --> T. Our findings indicate that SLC6A19 is the long-sought gene that is mutated in Hartnup disorder; its identification provides the opportunity to examine the inconsistent multisystemic features of this disorder.

Growth hormone and epidermal growth factor together enhance amino acid transport systems B0,+ and A in remnant small intestine after massive enterectomy

BACKGROUND

Sodium-dependent brush-border nutrient transport is decreased 2 weeks after massive enterectomy. This down-regulation is ameliorated by a 1-week infusion of parenteral growth hormone (GH) and epidermal growth factor (EGF) started 1 week after resection. We hypothesize that glutamine (GLN) transport will be enhanced by earlier and longer growth factor infusion, with differential effects on the Na(+)-dependent GLN transport systems A, B(0,+), and B(0)/ASCT2.

MATERIALS AND METHODS

New Zealand White rabbits underwent 70% small bowel resection then immediately received parenteral EGF, GH, both EGF and GH, or neither for 2 weeks. Na(+)-dependent 3H-GLN uptake by jejunal and ileal brush-border membrane vesicles was measured and the contribution of systems A, B(0,+), and B(0) was then determined by competitive inhibition. Data were analyzed using one-way analysis of variance.

RESULTS

In nonresected animals, the relative contribution of the systems was similar in jejunum (A 9%, B(0,+) 20%, and B(0) 71%) and ileum (A 13%, B(0,+) 27%, and B(0) 60%). Na(+)-dependent GLN uptake was reduced by one half in resected untreated controls, primarily because of decreased B(0) activity. EGF or GH alone did not affect Na(+)-dependent GLN transport, but, as a combination, there was increased uptake in the residual ileum and jejunum by 144% and 150%, respectively, over resected controls (P < 0.05). This was twice that achieved by delayed and shorter-duration combination treatment. This augmentation was a result of a 6.1-8.2-fold increase in system A as well as a 3.8-3.9-fold enhancement of system B(0,+) activity in remnant ileum and jejunum (P < 0.01).

CONCLUSIONS

Parenteral EGF and GH, given in combination for 2 weeks immediately after massive enterectomy, synergistically enhance GLN uptake by systems A and B(0,+).

Growth hormone and epidermal growth factor together enhance amino acid transport systems B(0,+) and A in remnant small intestine after massive enterectomy

BACKGROUND

Sodium-dependent brush border nutrient transport is decreased 2 weeks after massive enterectomy. This downregulation is ameliorated by a 1-week infusion of parenteral growth hormone (GH) and epidermal growth factor (EGF) started 1 week after resection. We hypothesized that glutamine (GLN) transport would be enhanced by earlier and longer growth factor infusion, with differential effects on the Na(+)-dependent GLN transport systems A, B(0,+), and B0/ASCT2.

MATERIALS AND METHODS

New Zealand White rabbits underwent 70% small bowel resection then immediately received parenteral EGF, GH, both, or neither for 2 weeks. Na(+)-dependent 3H-GLN uptake by jejunal and ileal brush-border membrane vesicles was measured and the contribution of systems A, B(0,+), and B0 then determined by competitive inhibition. Data were analyzed using one-way analysis of variance.

RESULTS

In nonresected animals, the relative contribution of the systems was similar in jejunum (A, 9%, B(0,+), 20%; and B0, 71%) and ileum (A, 13%; B(0,+), 27%; and B0, 60%). Na(+)-dependent GLN uptake was reduced by half in resected, untreated controls, primarily because of decreased B(0) activity. EGF or GH alone did not affect Na(+)-dependent GLN transport, but as a combination, increased uptake in the residual ileum and jejunum by 144% and 150%, respectively, over resected controls (P<0.05). This was twice that achieved by delayed and shorter-duration combination treatment. This augmentation was due to a 6.1- to 8.2-fold increase in system A as well as a 3.8- to 3.9-fold enhancement of system B(0,+) activity in remnant ileum and jejunum (P<0.01).

CONCLUSIONS

Parenteral EGF and GH, given in combination for 2 weeks immediately after massive enterectomy, synergistically enhance GLN uptake by systems A and B(0,+).

Mucosal defences against Giardia

Giardia lamblia (syn. G. duodenalis or G. intestinalis), the causative agent of giardiasis, is one of the most common causes worldwide of intestinal infections in humans. Symptomatic infection is characterized by diarrhoea, epigastric pain, nausea, vomiting, and weight loss, yet many infections are asymptomatic. The protozoan, unicellular parasite resides in the lumen and attaches to the epithelium and overlying mucus layers but does not invade the mucosa and causes little or no mucosal inflammation. Giardiasis is normally transient, indicating the existence of effective host defences, although re-infections can occur, which may be related to differences in infecting parasites and/or incomplete immune protection. Mucosal defences against Giardia must act in the small intestinal lumen in the absence of induction by classical inflammatory mediators. Secretory IgA antibodies have a central role in anti-giardial defence. B cell-independent mechanisms also exist and can contribute to eradication of the parasite, although their identity and physiological importance are poorly understood currently. Possible candidates are nitric oxide, antimicrobial peptides such as Paneth cell alpha-defensins, and lactoferrin. Elucidation of the key anti-giardial effector mechanisms will be important for selecting the best adjuvants in the rational development of vaccination strategies against Giardia.

Effect of soybean meal particle size on amino acid and energy digestibility in grower-finisher swine

A study was conducted using the ileal digestibility technique with grower-finisher pigs to evaluate the effects of particle size reduction of soybean meal (SBM) on amino acid and energy digestibility. Soybean meal was processed through a hammer mill to achieve average particle sizes of 900, 600, 300, and 150 microm. The treatments included the use of two soybean meal sources and soy protein concentrate. One source of SBM was ground to four different particle sizes (i.e., 949, 600, 389, 185); a second source was a common SBM source used in other trials (i.e., 800 microm). The soy protein concentrate had an average particle size of 385 microm. A low-protein (5% casein) diet was fed to determine endogenous amino acid losses. This experiment was conducted in a 7 x 7 Latin Square design in two replicates using 14 crossbred barrows ([Landrace x Yorkshire] x Duroc) that averaged 28 kg BW and 60 d of age. Animals were surgically fitted with a T-cannula at the distal ileum. Treatment diets were fed in meal form, initially at 0.09 kg BW(0.75) and at graded increases at each subsequent period. Pigs within replicate were fed a constant quantity of their treatment diet for a 5-d adjustment period followed by a 2-d collection of ileal digesta samples. Apparent and true digestibility of amino acids was calculated by use of chromic oxide (0.5%) as an indigestible marker. Apparent digestibility of isoleucine, methionine, phenylalanine, and valine increased linearly (P < 0.05) as particle size decreased. True digestibility of isoleucine, methionine, phenylalanine, and valine increased linearly (P < 0.05) as particle size decreased. When the essential amino acids were averaged, apparent digestibility increased (P < 0.10) from 83.5% to 84.9% as particle size decreased, whereas, nonessential amino acid digestibility increased only slightly (P > 0.15). Essential amino acid true digestibility increased numerically from 91.0% to 92.4% as particle size decreased. Energy digestibility was not affected by particle size (P > 0.15). These results suggest that a reduction in particle size of soybean meal resulted in a small increase in the digestibility of its amino acids with the essential amino acids being affected more than the nonessential amino acids. The largest improvement in digestibility, however, was obtained when the particle size was reduced to 600 microm.

Hartnup disorder: polymorphisms identified in the neutral amino acid transporter SLC1A5

Hartnup disorder is an inborn error of renal and gastrointestinal neutral amino acid transport. The cloning and functional characterization of the 'system B0' neutral amino acid transporter SLC1A5 led to it being proposed as a candidate gene for Hartnup disorder. Linkage analysis performed at 19q13.3, the chromosomal position of SLC1A5, was suggestive of an association with the Hartnup phenotype in some families. However, SLC1A5 was not linked to the Hartnup phenotype in other families. Linkage analysis also excluded an alternative candidate region at 11q13 implicated by a putative mouse model for Hartnup disorder. Sequencing of the coding region of SLC1A5 in Hartnup patients revealed two coding region polymorphisms. These mutations did not alter the predicted amino acid sequence of SLC1A5 and were considered unlikely to play a role in Hartnup disorder. There were no mutations in splice sites flanking each exon. Quantitative RT-PCR of SLC1A5 messenger RNA in affected and unaffected subjects did not support systemic differences in expression as an explanation for Hartnup disorder. In the six unrelated Hartnup pedigrees studied, examination of linkage at 19q13.3, polymorphisms in the coding sequence and quantitation of expression of SLC1A5 did not suffice to explain the defect in neutral amino acid transport.

Na(+)-dependent neutral amino acid transporter ATB(0) is a rabbit epithelial cell brush-border protein

System B(0) activity accounts for the majority of intestinal and kidney luminal neutral amino acid absorption. An amino acid transport system, called ATB(0) (also known as ASCT2), with functional characteristics similar to those of system B(0), has been recently cloned. We generated polyclonal antibodies to human and rabbit ATB(0) COOH-terminal peptides and used Western blot analysis to detect ATB(0) protein in rabbit tissues, rabbit ileal brush-border membrane vesicles (BBMV), and HeLa cells transfected with plasmids containing ATB(0) cDNAs. Immunohistochemistry was used to localize ATB(0) in rabbit kidney and intestine. In Western blots of rabbit tissues, ATB(0) was a broad smear of 78- to 85-kDa proteins. In transfected HeLa cells, ATB(0) appeared as a smear consisting of 57- to 65-kDa proteins. The highest expression was found in the kidney. ATB(0) was enriched in rabbit ileal BBMV and in HeLa cells transfected with ATB(0) cDNAs. In the kidney and in the intestine, ATB(0) was confined to the brush-border membrane (BBM) of the proximal tubular cell and of the enterocyte, respectively. Tissue and intracellular distribution of ATB(0) protein parallels that of system B(0) activity. ATB(0) protein could be the transporter responsible for system B(0) in the BBM of epithelial cells.

Multiple transport pathways for dibasic amino acids in the larval midgut of the silkworm Bombyx mori

The transport pathways for dibasic amino acids were investigated in brush border membrane vesicles (BBMV) from the anterior-middle (AM) and posterior (P) regions of Bombyx mori midgut. In the absence of K(+), a low-affinity saturable transport of arginine in both AM- and P-BBMV (K(m) 1.01 mM, V(max) 4.07 nmol/7s/mg protein and K(m) 1.38 mM, V(max) 2.26 nmol/7s/mg protein, respectively) was detected. Arginine influx was dependent on the membrane electrical potential (Deltapsi) and increased raising the alkalinity of the external medium from pH 7.2 to 10.6. Competition experiments indicated the following order of substrate affinity: arginine, homoarginine, N(G)-monomethylarginine, N(G)-nitroarginine>lysine>>ornithine>cysteine>methionine. Leucine, valine and BCH (2-amino-2-norbornanecarboxylic acid) did not inhibit arginine influx. In the presence of external K(+), the influx of arginine as a function of arginine concentration fitted to a complex saturation kinetics compatible with both a low-affinity and a high-affinity component. The latter (K(m) 0.035 mM, V(max) 2.54 nmol/7s/mg protein) was fully characterized. The influx rate had an optimum at pH 8.8, was strongly affected by Deltapsi and was homogeneous along the midgut. The substrate affinity rank was: homoarginine>arginine, N(G)-monomethylarginine>>cysteine, lysine>>N(G)-nitroarginine>ornithine>methionine. Leucine and amino acids with a hydrophobic side chain were not accepted. This system is also operative in the absence of potassium, with the same order of specificity but a very low activity. Lysine influx is mediated by two more transport systems, the leucine uniport and the K(+)/leucine symport specific for amino acids with a hydrophobic side chain that recognizes lysine at extravesicular pH values (pH(out)) exceeding 9. Both the uniport and the symport differ from the cationic transport systems so far identified in mammals because they are unaffected by N-ethylmaleimide, have no significant affinity for neutral amino acids in the presence of the cation and show a striking difference in their optimum pH.

Growth factors regulation of rabbit sodium-dependent neutral amino acid transporter ATB0 and oligopeptide transporter 1 mRNAs expression after enteretomy

BACKGROUND

Sucessful intestinal adaptation after massive enterectomy is dependent on increased efficiency of nutrient transport. However, midgut resection (MGR) in rabbits induces an initial decrease in sodium-dependent brush border neutral amino acid transport, whereas parenteral epidermal growth factor (EGF) and growth hormone (GH) reverse this downregulation. We investigated intestinal amino acid transporter B0 (ATB0) and oligopeptide transporter 1 (PEPT 1) mRNA expression after resection and in response to EGF and/or GH.

METHODS

Rabbits underwent anesthesia alone (control) or proximal, midgut, and distal resections. Full-thickness intestine was harvested from all groups on postoperative day (POD) 7, and on POD 14 from control and MGR rabbits. A second group of MGR rabbits received EGF and/or GH for 7 days, beginning 7 days after resection. ATB0 and PEPT 1 mRNA levels were determined by Northern blot analysis.

RESULTS

In control animals, ileal ATB0 mRNA abundance was three times higher than jejunal mRNA, whereas PEPT 1 mRNA expression was similar. By 7 and 14 days after MGR, jejunal ATB0 mRNA abundance was decreased by 50% vs control jejunum. A 50% decrease in jejunal PEPT 1 message was delayed until 14 days after MGR. Treatment with EGF plus GH did not alter ATB0 mRNA expression but doubled PEPT 1 mRNA in the jejunum.

CONCLUSION

The site of resection, time postresection, and growth factors treatment differentially influence ATB0 and PEPT 1 mRNA expression. Enhanced sodium-dependent brush border neutral amino acid transport with GH plus EGF administration is independent of increased ATB0 mRNA expression in rabbit small intestine after enterectomy.

Glycine supply to human enterocytes mediated by high-affinity basolateral GLYT1

BACKGROUND & AIMS

Intestinal glycine transport is involved in nutrient absorption and enterocyte homeostasis, particularly for glutathione synthesis. The primary aim of this study was to characterize the mechanism of postabsorptive (basolateral) glycine acquisition by the enterocyte.

METHODS

Assimilation of [(14)C]glycine was studied in human enterocytic Caco-2 cells, and expression of the glycine transporter GLYT1 was examined in Caco-2 cells and human intestine by reverse-transcription polymerase chain reaction, immunoblotting, and immunohistochemistry. The regulation of glycine transport in Caco-2 cells by phorbol-ester-induced protein kinase C activation was investigated.

RESULTS

Basolateral glycine uptake into Caco-2 cells is predominantly Na(+) and Cl(-) dependent and is 4-fold greater than apical uptake. The dominant Na(+)- and Cl(-)-dependent mechanism was characterized by a restricted inhibition profile, selectively sensitive to sarcosine, with an apparent Michaelis constant of 40-80 micromol/L, indicating system GLY. Consistent with these functional data, molecular techniques detected expression of GLYT1 messenger RNA and protein in the human intestine and Caco-2 cells. Protein kinase C activation reduced maximum velocity for GLYT1-mediated glycine uptake without effect on the Michaelis constant. The reduction in functional activity was independent of a measured protein kinase C-induced decrease in GLYT1 messenger RNA levels.

CONCLUSIONS

Enterocytes express GLYT1 along the length of the crypt-villus axis, where it mediates high-affinity basolateral glycine uptake.

Characteristics of L-glutamine transport during Caco-2 cell differentiation

Glutamine is the main fuel of intestinal epithelial cells, as well as a precursor for the intense nucleotide biosynthesis which arises with the rapid turnover of enterocytes. In order to determine whether glutamine uptake may vary as a function of metabolic demand, glutamine transport across the brush-border membrane of differentiating Caco-2 cells has been investigated. The uptake of L-[(3)H]glutamine was measured between day 7 and day 21 post-seeding. Kinetic analysis with glutamine concentrations ranging from 6.25 microM to 12.8 mM revealed the involvement of high affinity Na(+)-dependent (K(t)=110 microM) and low affinity Na(+)-independent (K(t)=900 microM) transport components at day 7. Both components were partially inhibited by L-lysine in a competitive fashion, suggesting that four different systems were responsible for glutamine uptake: B(0), B(0,+), b(0,+) and L. All four systems were present during the differentiation process, with systems L and B(0) being responsible for up to 80% of glutamine uptake. Caco-2 cell differentiation was associated with a marked decrease in L-glutamine uptake, which affected both the Na(+)-dependent and the Na(+)-independent components. In contrast to glucose uptake, the development of L-glutamine uptake across the brush-border membrane of Caco-2 cells may reflect an adjustment to cell metabolic demand rather than the progressive appearance of a vectorial transport process.

Nutritional regulation of nucleoside transporter expression in rat small intestine

BACKGROUND & AIMS

Concentrative nucleoside transporters CNT1 (pyrimidine preferring) and CNT2 (purine preferring) may be involved in the uptake of nucleoside-derived drugs used in antiviral and chemical therapies. The possibility that nucleoside carrier isoform expression is modulated by nutrient availability has been studied.

METHODS

CNT1 and CNT2 tissue distribution was determined by Western blot analysis. The effect of 48-hour starvation on CNT expression was then studied. Nucleoside transporter expression and uptake activity were measured in jejunal brush border plasma membrane vesicles from fed and starved rats. The expression of nucleoside transporters was later determined in a second model of nutrient deficiency: rats fed a purified diet with or without nucleotides for 10 days.

RESULTS

CNT1 and CNT2 nucleoside transporters were expressed in a wider variety of tissues than expected from messenger RNA distribution analysis. CNT1 was sensitive to nutrient availability in small intestine and, accordingly, jejunal brush border membrane vesicles from 48-hour-fasted rats showed increased expression of CNT1 and enhanced Na(+)-dependent thymidine and gemcitabine uptake. This effect was mimicked by feeding semipurified diets lacking nucleotides.

CONCLUSIONS

Substrate availability modulates nucleoside transporter expression (CNT1) in rat jejunum in vivo.