H/dipeptide absorption across the human intestinal epithelium is controlled indirectly via a functional Na/H exchanger

The TWK-26 potassium channel governs nutrient absorption in the C. elegans intestine

Ion channels are necessary for proper water and nutrient absorption in the intestine, which supports cellular metabolism and organismal growth. While a role for Na + co-transporters and pumps in intestinal nutrient absorption is well defined, how individual K + uniporters function to maintain ion homeostasis is poorly understood. Using Caenorhabditis elegans , we show that a gain-of-function mutation in twk-26 , which encodes a two-pore domain K + ion channel orthologous to human KCNK3, facilitates nutrient absorption and suppresses the metabolic and developmental defects displayed by impaired intestinal MAP Kinase (MAPK) signaling. Mutations in drl-1 and flr-4, which encode two components of this MAPK pathway, cause severe growth defects, reduced lipid storage, and a dramatic increase in autophagic lysosomes, which mirror dietary restriction phenotypes. Additionally, these MAPK mutants display structural defects of the intestine and an impaired defecation motor program. We find that activation of TWK-26 reverses the dietary restriction-like state of the MAPK mutants by restoring intestinal nutrient absorption without correcting the intestinal bloating or defecation defects. This study provides unique insight into the mechanisms by which intestinal K + ion channels support intestinal metabolic homeostasis.

Linking microbial genes to plasma and stool metabolites uncovers host-microbial interactions underlying ulcerative colitis disease course

Understanding the role of the microbiome in inflammatory diseases requires the identification of microbial effector molecules. We established an approach to link disease-associated microbes to microbial metabolites by integrating paired metagenomics, stool and plasma metabolomics, and culturomics. We identified host-microbial interactions correlated with disease activity, inflammation, and the clinical course of ulcerative colitis (UC) in the Predicting Response to Standardized Colitis Therapy (PROTECT) pediatric inception cohort. In severe disease, metabolite changes included increased dipeptides and tauro-conjugated bile acids (BAs) and decreased amino-acid-conjugated BAs in stool, whereas in plasma polyamines (N-acetylputrescine and N1-acetylspermidine) increased. Using patient samples and Veillonella parvula as a model, we uncovered nitrate- and lactate-dependent metabolic pathways, experimentally linking V. parvula expansion to immunomodulatory tryptophan metabolite production. Additionally, V. parvula metabolizes immunosuppressive thiopurine drugs through xdhA xanthine dehydrogenase, potentially impairing the therapeutic response. Our findings demonstrate that the microbiome contributes to disease-associated metabolite changes, underscoring the importance of these interactions in disease pathology and treatment.

Ion channels and transporters regulate nutrient absorption in health and disease

Ion channels and transporters are ubiquitously expressed on cell membrane, which involve in a plethora of physiological process such as contraction, neurotransmission, secretion and so on. Ion channels and transporters is of great importance to maintaining membrane potential homeostasis, which is essential to absorption of nutrients in gastrointestinal tract. Most of nutrients are electrogenic and require ion channels and transporters to absorb. This review summarizes the latest research on the role of ion channels and transporters in regulating nutrient uptake such as K+ channels, Ca2+ channels and ion exchangers. Revealing the mechanism of ion channels and transporters associated with nutrient uptake will be helpful to provide new methods to diagnosis and find potential targets for diseases like diabetes, inflammatory bowel diseases, etc. Even though some of study still remain ambiguous and in early stage, we believe that ion channels and transporters will be novel therapeutic targets in the future.

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

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

Intestinal Amino Acid Transport and Metabolic Health

Amino acids derived from protein digestion are important nutrients for the growth and maintenance of organisms. Approximately half of the 20 proteinogenic amino acids can be synthesized by mammalian organisms, while the other half are essential and must be acquired from the nutrition. Absorption of amino acids is mediated by a set of amino acid transporters together with transport of di- and tripeptides. They provide amino acids for systemic needs and for enterocyte metabolism. Absorption is largely complete at the end of the small intestine. The large intestine mediates the uptake of amino acids derived from bacterial metabolism and endogenous sources. Lack of amino acid transporters and peptide transporter delays the absorption of amino acids and changes sensing and usage of amino acids by the intestine. This can affect metabolic health through amino acid restriction, sensing of amino acids, and production of antimicrobial peptides.

Tissue-wide metabolomics reveals wide impact of gut microbiota on mice metabolite composition

The essential role of gut microbiota in health and disease is well recognized, but the biochemical details that underlie the beneficial impact remain largely undefined. To maintain its stability, microbiota participates in an interactive host-microbiota metabolic signaling, impacting metabolic phenotypes of the host. Dysbiosis of microbiota results in alteration of certain microbial and host metabolites. Identifying these markers could enhance early detection of certain diseases. We report LC-MS based non-targeted metabolic profiling that demonstrates a large effect of gut microbiota on mammalian tissue metabolites. It was hypothesized that gut microbiota influences the overall biochemistry of host metabolome and this effect is tissue-specific. Thirteen different tissues from germ-free (GF) and conventionally-raised (MPF) C57BL/6NTac mice were selected and their metabolic differences were analyzed. Our study demonstrated a large effect of microbiota on mammalian biochemistry at different tissues and resulted in statistically-significant modulation of metabolites from multiple metabolic pathways (p ≤ 0.05). Hundreds of molecular features were detected exclusively in one mouse group, with the majority of these being unique to specific tissue. A vast metabolic response of host to metabolites generated by the microbiota was observed, suggesting gut microbiota has a direct impact on host metabolism.

Na+/H+ exchangers differentially contribute to midgut fluid sodium and proton concentration in the sea urchin larva

Regulation of ionic composition and pH is a requisite of all digestive systems in the animal kingdom. Larval stages of the marine superphylum Ambulacraria, including echinoderms and hemichordates, were demonstrated to have highly alkaline conditions in their midgut with the underlying epithelial transport mechanisms being largely unknown. Using ion-selective microelectrodes, the present study demonstrated that pluteus larvae of the purple sea urchin have highly alkaline pH (pH ∼9) and low [Na+] (∼120 mmol l-1) in their midgut fluids, compared with the ionic composition of the surrounding seawater. We pharmacologically investigated the role of Na+/H+ exchangers (NHE) in intracellular pH regulation and midgut proton and sodium maintenance using the NHE inhibitor 5-(n-ethyl-n-isopropyl)amiloride (EIPA). Basolateral EIPA application decreased midgut pH while luminal application via micro-injections increased midgut [Na+], without affecting pH. Immunohistochemical analysis demonstrated a luminal localization of NHE-2 (SpSlc9a2) in the midgut epithelium. Specific knockdown of spslc9a2 using Vivo-Morpholinos led to an increase in midgut [Na+] without affecting pH. Acute acidification experiments in combination with quantitative PCR analysis and measurements of midgut pH and [Na+] identified two other NHE isoforms, Spslc9a7 and SpSlc9a8, which potentially contribute to the regulation of [Na+] and pH in midgut fluids. This work provides new insights into ion regulatory mechanisms in the midgut epithelium of sea urchin larvae. The involvement of NHEs in regulating pH and Na+ balance in midgut fluids shows conserved features of insect and vertebrate digestive systems and may contribute to the ability of sea urchin larvae to cope with changes in seawater pH.

Metabolomic shifts associated with heat stress in coral holobionts

Understanding the response of the coral holobiont to environmental change is crucial to inform conservation efforts. The most pressing problem is "coral bleaching," usually precipitated by prolonged thermal stress. We used untargeted, polar metabolite profiling to investigate the physiological response of the coral species Montipora capitata and Pocillopora acuta to heat stress. Our goal was to identify diagnostic markers present early in the bleaching response. From the untargeted UHPLC-MS data, a variety of co-regulated dipeptides were found that have the highest differential accumulation in both species. The structures of four dipeptides were determined and showed differential accumulation in symbiotic and aposymbiotic (alga-free) populations of the sea anemone Aiptasia (Exaiptasia pallida), suggesting the deep evolutionary origins of these dipeptides and their involvement in symbiosis. These and other metabolites may be used as diagnostic markers for thermal stress in wild coral.

Enteroendocrine cells couple nutrient sensing to nutrient absorption by regulating ion transport

The ability to absorb ingested nutrients is an essential function of all metazoans and utilizes a wide array of nutrient transporters found on the absorptive enterocytes of the small intestine. A unique population of patients has previously been identified with severe congenital malabsorptive diarrhea upon ingestion of any enteral nutrition. The intestines of these patients are macroscopically normal, but lack enteroendocrine cells (EECs), suggesting an essential role for this rare population of nutrient-sensing cells in regulating macronutrient absorption. Here, we use human and mouse models of EEC deficiency to identify an unappreciated role for the EEC hormone peptide YY in regulating ion-coupled absorption of glucose and dipeptides. We find that peptide YY is required in the small intestine to maintain normal electrophysiology in the presence of vasoactive intestinal polypeptide, a potent stimulator of ion secretion classically produced by enteric neurons. Administration of peptide YY to EEC-deficient mice restores normal electrophysiology, improves glucose and peptide absorption, diminishes diarrhea and rescues postnatal survival. These data suggest that peptide YY is a key regulator of macronutrient absorption in the small intestine and may be a viable therapeutic option to treat patients with electrolyte imbalance and nutrient malabsorption.

Enteroendocrine cells (EECs) are specialized gastrointestinal cells that have a role in nutrient sensing and hormone secretion. Here the authors show that peptide YY from EECs regulates nutrient absorption in intestinal organoids.

Segment-specific effects of resveratrol on porcine small intestinal dipeptide absorption depend on the mucosal pH and are due to different mechanisms: potential roles of different transport proteins and protein kinases

Numerous beneficial features of the polyphenol resveratrol (RSV) have been demonstrated in several tissues and cell culture models. There is also evidence, that RSV impairs intestinal nutrient transport but the underlying mechanisms are not understood. The aim of the present study was to evaluate whether RSV has also an impact on the H⁺-coupled transport of peptides via the peptide transporter 1 (PepT1) and to characterize RSV mediated changes in the apical abundance of nutrients transport proteins and protein kinases that may be involved. RSV decreased the H⁺-coupled transport of peptides in the porcine small intestines in a pH and location specific manner (jejunum vs ileum) as measured in Ussing chamber experiments. The comparison of the effects of RSV with the effects of the cAMP/PKA-activating agent forskolin indicates that different mechanisms may be responsible in the intestinal segments. Additionally, it seems that the transport of peptides and glucose in the jejunum are inhibited via the same mechanism while there might be two mechanisms involved in the ileum. Functional data and protein expression data indicate, that, besides PepT1, the activity of the Na⁺/H⁺-exchanger 3 (NHE3) may be involved. Protein kinase A (PKA) and AMP-activated kinase (AMPK) are both activated by RSV while the extracellular regulated kinase (ERK) and the serum and glucocorticoid induced kinase (SGK) are widely unaffected. Although PKA and AMPK are activated, AMPK seems not to be related to the effects of RSV. Additionally, both the functional data and the protein expression data reveal some interesting pH- and segment-specific differences.

Acquisition of alanyl-alanine in an Agnathan: Characteristics of dipeptide transport across the hindgut of the Pacific hagfish Eptatretus stoutii

This study used ³ H-_L -alanyl-_L -alanine to demonstrate dipeptide uptake using in vitro gut sacs prepared from the hindgut of the Pacific hagfish Eptatretus stoutii. Concentration-dependent kinetic analysis resulted in a sigmoidal distribution with a maximal (± SE) uptake rate (J_max -like) of 70 ± 3 nmol cm^-2 h^-1 and an affinity constant (K_m -like) of 1072 ± 81 μM. Addition of high alanine concentrations to transport assays did not change dipeptide transport rates, indicating that hydrolysis of the dipeptide in mucosal solutions and subsequent uptake via apical amino acid transporters was not occurring, which was further supported by a K_m distinct from that of amino acid transport. Transport occurred independent of mucosal pH, but uptake was reduced by 42% in low mucosal sodium. This may implicate cooperation between peptide transporters and sodium-proton exchangers, previously demonstrated in several mammalian and teleost species. Finally, apical _L -alanyl-_L -alanine uptake rates (i.e., mucosal disappearance) were significantly increased following a meal, demonstrating regulation of uptake. Overall, this examination of dipeptide acquisition in the earliest extant Agnathan suggests evolutionarily conserved mechanisms of transport between hagfish and later-diverging vertebrates such as teleosts and mammals.

Luminal Na+ homeostasis has an important role in intestinal peptide absorption in vivo

Intestinal cell line studies indicated luminal Na⁺ homeostasis is essential for proton-coupled peptide absorption, because the driving force of PepT1 activity is supported by the apical Na⁺/H⁺ exchanger NHE3. However, there is no direct evidence demonstrating the importance of in vivo luminal Na⁺ for peptide absorption in animal experiments. To investigate the relationship between luminal Na⁺ homeostasis and peptide absorption, we took advantage of claudin 15-deficient (cldn15^-/-) mice, whereby Na⁺ homeostasis is disrupted. We quantitatively assessed the intestinal segment responsible for peptide absorption using radiolabeled nonhydrolyzable dipeptide (glycylsarcosine, Gly-Sar) and nonabsorbable fluid phase marker polyethylene glycol (PEG) 4000 in vivo. In wild-type (WT) mice, the concentration ratio of Gly-Sar to PEG 4000 decreased in the upper jejunum, suggesting the upper jejunum is responsible for peptide absorption. Gly-Sar absorption was decreased in the jejunum of cldn15^-/- mice. To elucidate the mechanism underlining these impairments, a Gly-Sar-induced short-circuit ( I_sc) current was measured. In WT mice, increments of Gly-Sar-induced I_sc were inhibited by the luminal application of a NHE3-specific inhibitor S3226 in a dose-dependent fashion. In contrast to in vivo experiments, robust Gly-Sar-induced I_sc increments were observed in the jejunal mucosa of cldn15^-/- mice. Gly-Sar-induced I_sc was inhibited by S3226 or a reduction of luminal Na⁺ concentration, which mimics low luminal Na⁺ concentrations in vivo . Our study demonstrates that luminal Na⁺ homeostasis is important for peptide absorption in native epithelia and that there is a cooperative functional relationship between PepT1 and NHE3. NEW & NOTEWORTHY Our study is the first to demonstrate that luminal Na⁺ homeostasis is important for proton-coupled peptide absorption in in vivo animal experiments.

Physiology of the Gut: Experimental Models for Investigating Intestinal Fluid and Electrolyte Transport

Once thought to be exclusively an absorptive tissue, the intestine is now recognized as an important secretory tissue, playing a key role in body ion and fluid homeostasis. Given the intestine's role in fluid homeostasis, it is not surprising that important clinical pathologies arise from imbalances in fluid absorption and secretion. Perhaps the most important examples of this can be seen in enterotoxigenic secretory diarrheas with extreme fluid secretion, and Cystic Fibrosis with little or no fluid secretion. A mechanistic understanding of the cellular pathways regulating ion and fluid transport has been obtained from a variety of approaches and model systems. These have ranged from the intact intestine to a single intestinal epithelial cell type. Although for many years a reductionist approach has held sway for investigating intestinal transport, the growing realization that physiologic processes should really be examined within a physiological context has seen a marked increase in studies using models that are essentially mini-intestines in a dish. The aim of this chapter is to provide a historical context for our understanding of intestinal ion and fluid transport, and to highlight the model systems that have been used to acquire this knowledge.

Dietary salt levels affect digestibility, intestinal gene expression, and the microbiome, in Nile tilapia (Oreochromis niloticus)

Nile tilapia (Oreochromis niloticus) is the world’s most widely cultured fish species. Therefore, its nutritional physiology is of great interest from an aquaculture perspective. Studies conducted on several fish species, including tilapia, demonstrated the beneficial effects of dietary salt supplementation on growth; however, the mechanism behind these beneficial effects is still not fully understood. The fish intestine is a complex system, with functions, such as nutrient absorption, ion equilibrium and acid-base balance that are tightly linked and dependent on each other's activities and products. Ions are the driving force in the absorption of feed components through pumps, transporters and protein channels. In this study, we examined the impact of 5% increase in dietary NaCl on protein, lipid, ash and dry matter digestibility, as well as on the expression of intestinal peptide transporters (PepTs) and ion pumps (Na⁺/K⁺-ATPase, V-H⁺-ATPase, N⁺/H⁺-Exchanger) in Nile tilapia. In addition, effects on the gut microbiome were evaluated. Our results show that dietary salt supplementation significantly increased digestibility of all measured nutritional components, peptide transporters expression and ion pumps activity. Moreover, changes in the gut microbial diversity were observed, and were associated with lipid digestibility and Na⁺/K⁺-ATPase expression.

Intestinal Nucleoside Transporters: Function, Expression, and Regulation

The gastrointestinal tract is the absorptive organ for nutrients found in foods after digestion. Nucleosides and, to a lesser extent nucleobases, are the late products of nucleoprotein digestion. These metabolites are absorbed by nucleoside (and nucleobase) transporter (NT) proteins. NTs are differentially distributed along the gastrointestinal tract showing also polarized expression in epithelial cells. Concentrative nucleoside transporters (CNTs) are mainly located at the apical side of enterocytes, whereas equilibrative nucleoside transporters (ENTs) facilitate the basolateral efflux of nucleosides and nucleobases to the bloodstream. Moreover, selected nucleotides and the bioactive nucleoside adenosine act directly on intestinal cells modulating purinergic signaling. NT-polarized insertion is tightly regulated. However, not much is known about the modulation of intestinal NT function in humans, probably due to the lack of appropriate cell models retaining CNT functional expression. Thus, the possibility of nutritional regulation of intestinal NTs has been addressed using animal models. Besides the nutrition-related role of NT proteins, orally administered drugs also need to cross the intestinal barrier, this event being a major determinant of drug bioavailability. In this regard, NT proteins might also play a role in pharmacology, thereby allowing the absorption of nucleoside- and nucleobase-derived drugs. The relative broad selectivity of these membrane transporters also suggests clinically relevant drug-drug interactions when using combined therapies. This review focuses on all these physiological and pharmacological aspects of NT protein biology. © 2017 American Physiological Society. Compr Physiol 8:1003-1017, 2018.

Function, Regulation, and Pathophysiological Relevance of the POT Superfamily, Specifically PepT1 in Inflammatory Bowel Disease

Mammalian members of the proton-coupled oligopeptide transporter family are integral membrane proteins that mediate the cellular uptake of di/tripeptides and peptide-like drugs and couple substrate translocation to the movement of H⁺ , with the transmembrane electrochemical proton gradient providing the driving force. Peptide transporters are responsible for the (re)absorption of dietary and/or bacterial di- and tripeptides in the intestine and kidney and maintaining homeostasis of neuropeptides in the brain. These proteins additionally contribute to absorption of a number of pharmacologically important compounds. In this overview article, we have provided updated information on the structure, function, expression, localization, and activities of PepT1 (SLC15A1), PepT2 (SLC15A2), PhT1 (SLC15A4), and PhT2 (SLC15A3). Peptide transporters, in particular, PepT1 are discussed as drug-delivery systems in addition to their implications in health and disease. Particular emphasis has been placed on the involvement of PepT1 in the physiopathology of the gastrointestinal tract, specifically, its role in inflammatory bowel diseases. © 2018 American Physiological Society. Compr Physiol 8:731-760, 2018.

Oligopeptide Transport in Rat Lung Alveolar Epithelial Cells is Mediated by Pept2

PURPOSE

Studies were conducted in primary cultured rat alveolar epithelial cell monolayers to characterize peptide transporter expression and function.

METHODS

Freshly isolated rat lung alveolar epithelial cells were purified and cultured on permeable support with and without keratinocyte growth factor (KGF). Messenger RNA and protein expression of Pept1 and Pept2 in alveolar epithelial type I- and type II-like cell monolayers (±KGF, resp.) were examined by RT-PCR and Western blotting. ³H-Glycyl-sarcosine (³H-gly-sar) transmonolayer flux and intracellular accumulation were evaluated in both cell types.

RESULTS

RT-PCR showed expression of Pept2, but not Pept1, mRNA in both cell types. Western blot analysis revealed presence of Pept2 protein in type II-like cells, and less in type I-like cells. Bi-directional transmonolayer ³H-gly-sar flux lacked asymmetry in transport in both types of cells. Uptake of ³H-gly-sar from apical fluid of type II-like cells was 7-fold greater than that from basolateral fluid, while no significant differences were observed from apical vs. basolateral fluid of type I-like cells.

CONCLUSIONS

This study confirms the absence of Pept1 from rat lung alveolar epithelium in vitro. Functional Pept2 expression in type II-like cell monolayers suggests its involvement in oligopeptide lung disposition, and offers rationale for therapeutic development of di/tripeptides, peptidomimetics employing pulmonary drug delivery.

Expression and localization of VPAC1, the major receptor of vasoactive intestinal peptide along the length of the intestine

Vasoactive intestinal peptide (VIP) is an endogenous neuropeptide with a broad array of physiological functions in many organs including the intestine. Its actions are mediated via G protein-coupled receptors, and vasoactive intestinal peptide receptor 1 (VPAC1) is the key receptor responsible for majority of VIP's biological activity. The distribution of VPAC1 along the length of the gastrointestinal tract and its subcellular localization in intestinal epithelial cells have not been fully characterized. The current studies were undertaken to determine VPAC1 distribution and localization so that VIP-based therapies can be targeted to specific regions of the intestine. The results indicated that the mRNA levels of VPAC1 showed an abundance pattern of colon > ileum > jejunum in the mouse intestine. In parallel, the VPAC1 protein levels were higher in the mouse colon, followed by the ileum and jejunum. Immunofluorescence studies in mouse colon demonstrated that the receptor was specifically localized to the luminal surface, as was evident by colocalization with the apical marker villin but not with the basolateral marker Na+/K+-ATPase. In the human intestine, VPAC1 mRNA expression exhibited a distribution similar to that in mouse intestine and was highest in the sigmoid colon. Furthermore, in the human colon, VPAC1 also showed predominantly apical localization. The physiological relevance of the expression and apical localization of VPAC1 remains elusive. We speculate that apical VPAC1 in intestinal epithelial cells may have relevance in recognizing secreted peptides in the intestinal lumen and therefore supports the feasibility of potential therapeutic and targeting use of VIP formulations via oral route to treat gastrointestinal diseases.NEW & NOTEWORTHY These studies for the first time present comprehensive data on the relative characterization of vasoactive intestinal peptide (VIP) receptors in the intestinal mucosa. Vasoactive intestinal peptide receptor 1 (VPAC1) was identified as the predominant receptor with higher levels in the colon compared with the small intestine and was mainly localized to the apical membrane. In addition, the findings in the human tissues were consistent with VPAC1 expression in the mouse intestine and open possibilities to target colonic tissues with VIP for treating diseases such as inflammatory bowel disease.

Tenapanor administration and the activity of the H+ -coupled transporter PepT1 in healthy volunteers

AIM

Tenapanor (RDX5791/AZD1722), an inhibitor of gastrointestinal Na⁺ /H⁺ exchanger NHE3, is being evaluated for the treatment of patients with constipation-predominant irritable bowel syndrome and the treatment of hyperphosphataemia in patients with chronic kidney disease on dialysis. By reducing intestinal H⁺ secretion, inhibition of NHE3 by tenapanor could indirectly affect H⁺ -coupled transporter activity, leading to drug-drug interactions. We investigated the effect of tenapanor on the activity of the H⁺ -coupled peptide transporter PepT1 via assessment of the pharmacokinetics of cefadroxil - a compound transported by PepT1 - in healthy volunteers.

METHODS

In this open-label, two-period crossover, phase 1 study (NCT02140281), 28 volunteers received in random order: a single dose of cefadroxil 500 mg for 1 day; and tenapanor 15 mg twice daily over 4 days followed by single doses of both cefadroxil 500 mg and tenapanor 15 mg on day 5. There was a 4-day washout between treatment periods.

RESULTS

Cefadroxil exposure was similar when administered alone or in combination with tenapanor {geometric least-squares mean ratios [(cefadroxil + tenapanor)/cefadroxil] (90% confidence interval): area under the concentration-time curve 93.3 (90.6-96.0)%; maximum concentration in plasma 95.9 (89.8-103)%}. Tenapanor treatment caused a softening of stool consistency and an increase in stool frequency, consistent with its expected pharmacodynamic effect. No safety concerns were identified and tenapanor was not detected in plasma.

CONCLUSIONS

These results suggest that tenapanor 15 mg twice daily does not have a clinically relevant impact on the activity of the H⁺ -coupled transporter PepT1 in humans. This may guide future research on drug-drug interactions involving NHE3 inhibitors.

Salinity-Dependent Shift in the Localization of Three Peptide Transporters along the Intestine of the Mozambique Tilapia (Oreochromis mossambicus)

The peptide transporter (PepT) systems are well-known for their importance to protein absorption in all vertebrate species. These symporters use H⁺ gradient at the apical membrane of the intestinal epithelial cells to mediate the absorption of small peptides. In fish, the intestine is a multifunctional organ, involved in osmoregulation, acid-base regulation, and nutrient absorption. Therefore, we expected environmental stimuli to affect peptide absorption. We examined the effect of three environmental factors; salinity, pH and feeding, on the expression, activity and localization of three PepT transporters (PepT1a, PepT1b, PepT2) along the intestine of the Mozambique tilapia (Oreochromis mossambicus). Quantitative real time PCR (qPCR) analysis demonstrated that the two PepT1 variants are typical to the proximal intestinal section while PepT2 is typical to the distal intestinal sections. Immunofluorescence analysis with custom-made antibodies supported the qPCR results, localized both transporters on the apical membrane of enterocytes and provided the first evidence for the participation of PepT2 in nutrient absorption. This first description of segment-specific expression and localization points to a complementary role of the different peptide transporters, corresponding to the changes in nutrient availability along the intestine. Both gene expression and absorption activity assays showed that an increase in water salinity shifted the localization of the PepT genes transcription and activity down along the intestinal tract. Additionally, an unexpected pH effect was found on the absorption of small peptides, with increased activity at higher pH levels. This work emphasizes the relationships between different functions of the fish intestine and how they are affected by environmental conditions.

Na+/H+ exchanger 3 inhibitor diminishes the amino-acid-enhanced transepithelial calcium transport across the rat duodenum

Na⁺/H⁺ exchanger (NHE)-3 is important for intestinal absorption of nutrients and minerals, including calcium. The previous investigations have shown that the intestinal calcium absorption is also dependent on luminal nutrients, but whether aliphatic amino acids and glucose, which are abundant in the luminal fluid during a meal, similarly enhance calcium transport remains elusive. Herein, we used the in vitro Ussing chamber technique to determine epithelial electrical parameters, i.e., potential difference (PD), short-circuit current (Isc), and transepithelial resistance, as well as ⁴⁵Ca flux in the rat duodenum directly exposed on the mucosal side to glucose or various amino acids. We found that mucosal glucose exposure led to the enhanced calcium transport, PD, and Isc, all of which were insensitive to NHE3 inhibitor (100 nM tenapanor). In the absence of mucosal glucose, several amino acids (12 mM in the mucosal side), i.e., alanine, isoleucine, leucine, proline, and hydroxyproline, markedly increased the duodenal calcium transport. An inhibitor for NHE3 exposure on the mucosal side completely abolished proline- and leucine-enhanced calcium transport, but not transepithelial transport of both amino acids themselves. In conclusion, glucose and certain amino acids in the mucosal side were potent stimulators of the duodenal calcium absorption, but only amino-acid-enhanced calcium transport was NHE3-dependent.

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

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

siRNA capsulated brain-targeted nanoparticles specifically knock down OATP2B1 in mice: a mechanism for acute morphine tolerance suppression

Regulating main brain-uptake transporter of morphine may restrict its tolerance generation, then modify its antinociception. In this study, more than 2 fold higher intracellular uptake concentrations for morphine and morphine-6-glucuronide (M6G) were observed in stable expression cells, HEK293-hOATP2B1 than HEK293-MOCK. Specifically, the K_m value of morphine to OATP2B1 (57.58 ± 8.90 μM) is 1.4-time more than that of M6G (80.31 ± 21.75 μM); Cyclosporine A (CsA), an inhibitor of OATP2B1, can inhibit their intracellular accumulations with IC₅₀ = 3.90 ± 0.50 μM for morphine and IC₅₀ = 6.04 ± 0.86 μM for M6G, respectively. To further investigate the role of OATP2B1 in morphine brain transport and tolerance, the novel nanoparticles of DGL-PEG/dermorphin capsulated siRNA (OATP2B1) were applied to deliver siRNA into mouse brain. Along with OATP2B1 depressed, a main reduction was found for each of morphine or M6G in cerebrums or epencephalons of acute morphine tolerance mice. Furthermore, calcium/calmodulin-dependent protein kinase IIα (CaMKIIα) in mouse prefrontal cortex (mPFC) underwent dephosphorylation at Thr286. In conclusion, OATP2B1 downregulation in mouse brain can suppress tolerance via blocking morphine and M6G brain transport. These findings might help to improve the pharmacological effects of morphine.

Intestinal brush-border Na+/H+ exchanger-3 drives H+-coupled iron absorption in the mouse

Divalent metal-ion transporter-1 (DMT1), the principal mechanism by which nonheme iron is taken up at the intestinal brush border, is energized by the H(+)-electrochemical potential gradient. The provenance of the H(+) gradient in vivo is unknown, so we have explored a role for brush-border Na(+)/H(+) exchanger (NHE) isoforms by examining iron homeostasis and intestinal iron handling in mice lacking NHE2 or NHE3. We observed modestly depleted liver iron stores in NHE2-null (NHE2(-/-)) mice stressed on a low-iron diet but no change in hematological or blood iron variables or the expression of genes associated with iron metabolism compared with wild-type mice. Ablation of NHE3 strongly depleted liver iron stores, regardless of diet. We observed decreases in blood iron variables but no overt anemia in NHE3-null (NHE3(-/-)) mice on a low-iron diet. Intestinal expression of DMT1, the apical surface ferrireductase cytochrome b reductase-1, and the basolateral iron exporter ferroportin was upregulated in NHE3(-/-) mice, and expression of liver Hamp1 (hepcidin) was suppressed compared with wild-type mice. Absorption of (59)Fe from an oral dose was substantially impaired in NHE3(-/-) compared with wild-type mice. Our data point to an important role for NHE3 in generating the H(+) gradient that drives DMT1-mediated iron uptake at the intestinal brush border.

Calcineurin homologous proteins regulate the membrane localization and activity of sodium/proton exchangers in C. elegans

Calcineurin B homologous proteins (CHP) are N-myristoylated, EF-hand Ca(2+)-binding proteins that bind to and regulate Na(+)/H(+) exchangers, which occurs through a variety of mechanisms whose relative significance is incompletely understood. Like mammals, Caenorhabditis elegans has three CHP paralogs, but unlike mammals, worms can survive CHP loss-of-function. However, mutants for the CHP ortholog PBO-1 are unfit, and PBO-1 has been shown to be required for proton signaling by the basolateral Na(+)/H(+) exchanger NHX-7 and for proton-coupled intestinal nutrient uptake by the apical Na(+)/H(+) exchanger NHX-2. Here, we have used this genetic model organism to interrogate PBO-1's mechanism of action. Using fluorescent tags to monitor Na(+)/H(+) exchanger trafficking and localization, we found that loss of either PBO-1 binding or activity caused NHX-7 to accumulate in late endosomes/lysosomes. In contrast, NHX-2 was stabilized at the apical membrane by a nonfunctional PBO-1 protein and was only internalized following its complete loss. Additionally, two pbo-1 paralogs were identified, and their expression patterns were analyzed. One of these contributed to the function of the excretory cell, which acts like a kidney in worms, establishing an alternative model for testing the role of this protein in membrane transporter trafficking and regulation. These results lead us to conclude that the role of CHP in Na(+)/H(+) exchanger regulation differs between apical and basolateral transporters. This further emphasizes the importance of proper targeting of Na(+)/H(+) exchangers and the critical role of CHP family proteins in this process.

Transporters at CNS barrier sites: obstacles or opportunities for drug delivery?

The blood-brain barrier (BBB) and blood-cerebrospinal fluid (BCSF) barriers are critical determinants of CNS homeostasis. Additionally, the BBB and BCSF barriers are formidable obstacles to effective CNS drug delivery. These brain barrier sites express putative influx and efflux transporters that precisely control permeation of circulating solutes including drugs. The study of transporters has enabled a shift away from "brute force" approaches to delivering drugs by physically circumventing brain barriers towards chemical approaches that can target specific compounds of the BBB and/or BCSF barrier. However, our understanding of transporters at the BBB and BCSF barriers has primarily focused on understanding efflux transporters that efficiently prevent drugs from attaining therapeutic concentrations in the CNS. Recently, through the characterization of multiple endogenously expressed uptake transporters, this paradigm has shifted to the study of brain transporter targets that can facilitate drug delivery (i.e., influx transporters). Additionally, signaling pathways and trafficking mechanisms have been identified for several endogenous BBB/BCSF transporters, thereby offering even more opportunities to understand how transporters can be exploited for optimization of CNS drug delivery. This review presents an overview of the BBB and BCSF barrier as well as the many families of transporters functionally expressed at these barrier sites. Furthermore, we present an overview of various strategies that have been designed and utilized to deliver therapeutic agents to the brain with a particular emphasis on those approaches that directly target endogenous BBB/BCSF barrier transporters.

Functional involvement of RFVT3/SLC52A3 in intestinal riboflavin absorption

Riboflavin, also known as vitamin B2, is transported across the biological membrane into various organs by transport systems. Riboflavin transporter RFVT3 is expressed in the small intestine and has been suggested to localize in the apical membranes of the intestinal epithelial cells. In this study, we investigated the functional involvement of RFVT3 in riboflavin absorption using intestinal epithelial T84 cells and mouse small intestine. T84 cells expressed RFVT3 and conserved unidirectional riboflavin transport corresponding to intestinal absorption. Apical [(3)H]riboflavin uptake was pH-dependent in T84 cells. This uptake was not affected by Na(+) depletion at apical pH 6.0, although it was significantly decreased at apical pH 7.4. The [(3)H]riboflavin uptake from the apical side of T84 cells was prominently inhibited by the RFVT3 selective inhibitor methylene blue and significantly decreased by transfection of RFVT3-small-interfering RNA. In the gastrointestinal tract, RFVT3 was expressed in the jejunum and ileum. Mouse jejunal and ileal permeabilities of [(3)H]riboflavin were measured by the in situ closed-loop method and were significantly reduced by methylene blue. These results strongly suggest that RFVT3 would functionally be involved in riboflavin absorption in the apical membranes of intestinal epithelial cells.

The peptide transporter PEPT1 is expressed in distal colon in rodents and humans and contributes to water absorption

The peptide transporter PEPT1, expressed in the brush border membrane of enterocytes, mediates the uptake of di- and tripeptides from luminal protein digestion in the small intestine. PEPT1 was proposed not to be expressed in normal colonic mucosa but may become detectable in inflammatory states such as Crohn's disease or ulcerative colitis. We reassessed colonic expression of PEPT1 by performing a systematic analysis of PEPT1 mRNA and protein levels in healthy colonic tissues in mice, rats, and humans. Immunofluorescence analysis of different mouse strains (C57BL/6N, 129/Sv, BALB/c) demonstrated the presence of PEPT1 in the distal part of the colon but not in proximal colon. Rat and human intestines display a similar distribution of PEPT1 as found in mice. However, localization in human sigmoid colon revealed immunoreactivity present at low levels in apical membranes but substantial staining in distinct intracellular compartments. Functional activity of PEPT1 in colonic tissues from mice was assessed in everted sac preparations using [¹⁴C]Gly-Sar and found to be 5.7-fold higher in distal compared with proximal colon. In intestinal tissues from Pept1-/- mice, no [¹⁴C]Gly-Sar transport was detectable but feces samples revealed significantly higher water content than in wild-type mice, suggesting that PEPT1 contributes to colonic water absorption. In conclusion, our studies unequivocally demonstrate the presence of PEPT1 protein in healthy distal colonic epithelium in mice, rats, and humans and proved that the protein is functional and contributes to electrolyte and water handling in mice.

The SLC36 family of proton-coupled amino acid transporters and their potential role in drug transport

Members of the solute carrier (SLC) 36 family are involved in transmembrane movement of amino acids and derivatives. SLC36 consists of four members. SLC36A1 and SLC36A2 both function as H(+) -coupled amino acid symporters. SLC36A1 is expressed at the luminal surface of the small intestine but is also commonly found in lysosomes in many cell types (including neurones), suggesting that it is a multipurpose carrier with distinct roles in different cells including absorption in the small intestine and as an efflux pathway following intralysosomal protein breakdown. SLC36A1 has a relatively low affinity (K(m) 1-10 mM) for its substrates, which include zwitterionic amino and imino acids, heterocyclic amino acids and amino acid-based drugs and derivatives used experimentally and/or clinically to treat epilepsy, schizophrenia, bacterial infections, hyperglycaemia and cancer. SLC36A2 is expressed at the apical surface of the human renal proximal tubule where it functions in the reabsorption of glycine, proline and hydroxyproline. SLC36A2 also transports amino acid derivatives but has a narrower substrate selectivity and higher affinity (K(m) 0.1-0.7 mM) than SLC36A1. Mutations in SLC36A2 lead to hyperglycinuria and iminoglycinuria. SLC36A3 is expressed only in testes and is an orphan transporter with no known function. SLC36A4 is widely distributed at the mRNA level and is a high-affinity (K(m) 2-3 µM) transporter for proline and tryptophan. We have much to learn about this family of transporters, but from current knowledge, it seems likely that their function will influence the pharmacokinetic profiles of amino acid-based drugs by mediating transport in both the small intestine and kidney.

Calmodulin kinase II constitutively binds, phosphorylates, and inhibits brush border Na+/H+ exchanger 3 (NHE3) by a NHERF2 protein-dependent process

The epithelial brush border (BB) Na(+)/H(+) exchanger 3 (NHE3) accounts for most renal and intestinal Na(+) absorption. Ca(2+)/calmodulin-dependent protein kinase II (CaMKII) inhibits NHE3 activity under basal conditions in intact intestine, acting in the BB, but the mechanism is unclear. We now demonstrate that in both PS120 fibroblasts and polarized Caco-2BBe cells expressing NHE3, CaMKII inhibits basal NHE3 activity, because the CaMKII-specific inhibitors KN-93 and KN-62 stimulate NHE3 activity. This inhibition requires NHERF2. CaMKIIγ associates with NHE3 between aa 586 and 605 in the NHE3 C terminus in a Ca(2+)-dependent manner, with less association when Ca(2+) is increased. CaMKII inhibits NHE3 by an effect on its turnover number, not changing surface expression. Back phosphorylation demonstrated that NHE3 is phosphorylated by CaMKII under basal conditions. This overall phosphorylation of NHE3 is not affected by the presence of NHERF2. Amino acids downstream of NHE3 aa 690 are required for CaMKII to inhibit basal NHE3 activity, and mutations of the three putative CaMKII phosphorylation sites downstream of aa 690 each prevented KN-93 stimulation of NHE3 activity. These studies demonstrate that CaMKIIγ is a novel NHE3-binding protein, and this association is reduced by elevated Ca(2+). CaMKII inhibits basal NHE3 activity associated with phosphorylation of NHE3 by effects requiring aa downstream of NHE3 aa 690 and of the CaMKII-binding site on NHE3. CaMKII binding to and phosphorylation of the NHE3 C terminus are parts of the physiologic regulation of NHE3 that occurs in fibroblasts as well as in the BB of an intestinal Na(+)-absorptive cell.

A glutathione peroxidase, intracellular peptidases and the TOR complexes regulate peptide transporter PEPT-1 in C. elegans

The intestinal peptide transporter PEPT-1 in Caenorhabditis elegans is a rheogenic H⁺-dependent carrier responsible for the absorption of di- and tripeptides. Transporter-deficient pept-1(lg601) worms are characterized by impairments in growth, development and reproduction and develop a severe obesity like phenotype. The transport function of PEPT-1 as well as the influx of free fatty acids was shown to be dependent on the membrane potential and on the intracellular pH homeostasis, both of which are regulated by the sodium-proton exchanger NHX-2. Since many membrane proteins commonly function as complexes, there could be proteins that possibly modulate PEPT-1 expression and function. A systematic RNAi screening of 162 genes that are exclusively expressed in the intestine combined with a functional transport assay revealed four genes with homologues existing in mammals as predicted PEPT-1 modulators. While silencing of a glutathione peroxidase surprisingly caused an increase in PEPT-1 transport function, silencing of the ER to Golgi cargo transport protein and of two cytosolic peptidases reduced PEPT-1 transport activity and this even corresponded with lower PEPT-1 protein levels. These modifications of PEPT-1 function by gene silencing of homologous genes were also found to be conserved in the human epithelial cell line Caco-2/TC7 cells. Peptidase inhibition, amino acid supplementation and RNAi silencing of targets of rapamycin (TOR) components in C. elegans supports evidence that intracellular peptide hydrolysis and amino acid concentration are a part of a sensing system that controls PEPT-1 expression and function and that involves the TOR complexes TORC1 and TORC2.

pH-sensitive interaction of HMG-CoA reductase inhibitors (statins) with organic anion transporting polypeptide 2B1

The human organic anion transporting polypeptide 2B1 (OATP2B1, SLCO2B1) is ubiquitously expressed and may play an important role in the disposition of xenobiotics. The present study aimed to examine the role of OATP2B1 in the intestinal absorption and tissue uptake of 3-hydroxy-3-methylglutaryl-Coenzyme A (HMG-CoA) reductase inhibitors (statins). We first investigated the functional affinity of statins to the transporter as a function of extracellular pH, using OATP2B1-transfeced HEK293 cells. The results indicate that OATP2B1-mediated transport is significant for rosuvastatin, fluvastatin and atorvastatin, at neutral pH. However, OATP2B1 showed broader substrate specificity as well as enhanced transporter activity at acidic pH. Furthermore, uptake at acidic pH was diminished in the presence of proton ionophore, suggesting proton gradient as the driving force for OATP2B1 activity. Notably, passive transport rates are predominant or comparable to active transport rates for statins, except for rosuvastatin and fluvastatin. Second, we studied the effect of OATP modulators on statin uptake. At pH 6.0, OATP2B1-mediated transport of atorvastatin and cerivastatin was not inhibitable, while rosuvastatin transport was inhibited by E-3-S, rifamycin SV and cyclosporine with IC(50) values of 19.7 ± 3.3 μM, 0.53 ± 0.2 μM and 2.2 ± 0.4 μM, respectively. Rifamycin SV inhibited OATP2B1-mediated transport of E-3-S and rosuvastatin with similar IC(50) values at pH 6.0 and 7.4, suggesting that the inhibitor affinity is not pH-dependent. Finally, we noted that OATP2B1-mediated transport of E-3-S, but not rosuvastatin, is pH sensitive in intestinal epithelial (Caco-2) cells. However, uptake of E-3-S and rosuvastatin by Caco-2 cells was diminished in the presence of proton ionophore. The present results indicate that OATP2B1 may be involved in the tissue uptake of rosuvastatin and fluvastatin, while OATP2B1 may play a significant role in the intestinal absorption of several statins due to their transporter affinity at acidic pH.

Amino acid absorption and homeostasis in mice lacking the intestinal peptide transporter PEPT1

The intestinal peptide transporter PEPT1 mediates the uptake of di- and tripeptides derived from dietary protein breakdown into epithelial cells. Whereas the transporter appears to be essential to compensate for the reduced amino acid delivery in patients with mutations in amino acid transporter genes, such as in cystinuria or Hartnup disease, its physiological role in overall amino acid absorption is still not known. To assess the quantitative importance of PEPT1 in overall amino acid absorption and metabolism, PEPT1-deficient mice were studied by using brush border membrane vesicles, everted gut sacs, and Ussing chambers, as well as by transcriptome and proteome analysis of intestinal tissue samples. Neither gene expression nor proteome profiling nor functional analysis revealed evidence for any compensatory changes in the levels and/or function of transporters for free amino acids in the intestine. However, most plasma amino acid levels were increased in Pept1(-/-) compared with Pept1(+/+) animals, suggesting that amino acid handling is altered. Plasma appearance rates of (15)N-labeled amino acids determined after intragastric administration of a low dose of protein remained unchanged, whereas administration of a large protein load via gavage revealed marked differences in plasma appearance of selected amino acids. PEPT1 seems, therefore, important for overall amino acid absorption only after high dietary protein intake when amino acid transport processes are saturated and PEPT1 can provide additional absorption capacity. Since renal amino acid excretion remained unchanged, elevated basal concentrations of plasma amino acids in PEPT1-deficient animals seem to arise mainly from alterations in hepatic amino acid metabolism.

Hijacking solute carriers for proton-coupled drug transport

The physiological role of mammalian solute carrier (SLC) proteins is to mediate transmembrane movement of electrolytes, nutrients, micronutrients, vitamins, and endogenous metabolites from one cellular compartment to another. Many transporters in the small intestine, kidney, and solid tumors are H(+)-coupled, driven by local H(+)-electrochemical gradients, and transport numerous drugs. These transporters include PepT1 and PepT2 (SLC15A1/2), PCFT (SLC46A1), PAT1 (SLC36A1), OAT10 (SLC22A13), OATP2B1 (SLCO2B1), MCT1 (SLC16A1), and MATE1 and MATE2-K (SLC47A1/2).

Evidence for two different broad-specificity oligopeptide transporters in intestinal cell line Caco-2 and colonic cell line CCD841

Recently the existence of two different Na(+)-coupled oligopeptide transport systems has been described in mammalian cells. These transport systems are distinct from the previously known H(+)/peptide cotransporters PEPT1 and PEPT2, which transport only dipeptides and tripeptides. To date, the only peptide transport system known to exist in the intestine is PEPT1. Here we investigated the expression of the Na(+)-coupled oligopeptide transporters in intestinal cell lines, using the hydrolysis-resistant synthetic oligopeptides deltorphin II and [d-Ala(2),d-Leu(5)]enkephalin (DADLE) as model substrates. Caco-2 cells and CCD841 cells, both representing epithelial cells from human intestinal tract, were able to take up these oligopeptides. Uptake of deltorphin II was mostly Na(+) dependent, with more than 2 Na(+) involved in the uptake process. In contrast, DADLE uptake was only partially Na(+) dependent. The uptake of both peptides was also influenced by H(+) and Cl(-), although to a varying degree. The processes responsible for the uptake of deltorphin II and DADLE could be differentiated not only by their Na(+) dependence but also by their modulation by small peptides. Several dipeptides and tripeptides stimulated deltorphin II uptake but inhibited DADLE uptake. These modulating small peptides were, however, not transportable substrates for the transport systems that mediate deltorphin II or DADLE uptake. These two oligopeptide transport systems were also able to take up several nonopioid oligopeptides, consisting of 9-17 amino acids. This represents the first report on the existence of transport systems in intestinal cells that are distinct from PEPT1 and capable of transporting oligopeptides consisting of five or more amino acids.

Reciprocal regulation of the primary sodium absorptive pathways in rat intestinal epithelial cells

Sodium absorption in the mammalian small intestine occurs predominantly by two primary pathways that include Na/H exchange (NHE3) and Na-glucose cotransport (SGLT1) on the brush border membrane (BBM) of villus cells. However, whether NHE3 and SGLT1 function together to regulate intestinal sodium absorption is unknown. Nontransformed small intestinal epithelial cells (IEC-18) were transfected with either NHE3 or SGLT1 small interfering RNAs (siRNAs) and were grown in confluent monolayers on transwell plates to measure the effects on Na absorption. Uptake studies were performed as well as molecular studies to determine the effects on NHE3 and SGLT1 activity. When IEC-18 monolayers were transfected with silencing NHE3 RNA, the cells demonstrated decreased NHE3 activity as well as decreased NHE3 mRNA and protein. However, in NHE3 siRNA-transected cells, SGLT1 activity, mRNA, and protein in the BBM were significantly increased. Thus, inhibition of NHE3 expression regulates the expression and function of SGLT1 in the BBM of intestinal epithelial cells. In addition, IEC-18 cells transected with silencing SGLT1 RNA demonstrated an inhibition of Na-dependent glucose uptake and a decrease in SGLT1 activity, mRNA, and protein levels. However, in these cells, Na/H exchange activity was significantly increased. Furthermore, NHE3 mRNA and protein levels were also increased. Therefore, the inhibition of SGLT1 expression stimulates the transcription and function of NHE3 and vice versa in the BBM of intestinal epithelial cells. Thus this study demonstrates that the major sodium absorptive pathways together function to regulate sodium absorption in epithelial cells.

Gene ablation for PEPT1 in mice abolishes the effects of dipeptides on small intestinal fluid absorption, short-circuit current, and intracellular pH

PEPT1 function in mouse intestine has not been assessed by means of electrophysiology and methods to assess its role in intracellular pH and fluid homeostasis. Therefore, the effects of the dipeptide glycilsarcosin (Gly-Sar) on jejunal fluid absorption and villous enterocyte intracellular pH (pH(i)) in vivo, as well as on enterocyte[(14)C]Gly-Sar uptake, short-circuit current (I(sc)) response, and enterocyte pH(i) in vitro were determined in wild-type and PEPT1-deficient mice and in mice lacking PEPT1. Immunohistochemistry for PEPT1 failed to detect any protein in enterocyte apical membranes in Slc15a1(-/-) animals. Saturable Gly-Sar uptake in Slc15a1(-/-) everted sac preparations was no longer detectable. Similarly, Gly-Sar-induced jejunal I(sc) response in vitro was abolished. The dipeptide-induced increase in fluid absorption in vivo was also abolished in animals lacking PEPT1. Since PEPT1 acts as an acid loader in enterocytes, enterocyte pH(i) was measured in vivo by two-photon microscopy in SNARF-4-loaded villous enterocytes of exteriorized jejuni in anesthetized mice, as well as in BCECF-loaded enterocytes of microdissected jejunal villi. Gly-Sar-induced pH(i) decrease was no longer observed in the absence of PEPT1. A reversal of the proton gradient across the luminal membrane did not significantly diminish Gly-Sar-induced I(sc) response, whereas a depolarization of the apical membrane potential by high K(+) or via Na(+)-K(+)-ATPase inhibition strongly diminished Gly-Sar-induced I(sc) responses. This study demonstrates for the first time that proton-coupled electrogenic dipeptide uptake in the native small intestine, mediated by PEPT1, relies on the negative apical membrane potential as the major driving force and contributes significantly to intestinal fluid transport.

Putative anion transporter-1 (Pat-1, Slc26a6) contributes to intracellular pH regulation during H+-dipeptide transport in duodenal villous epithelium

The majority of dietary amino acids are absorbed via the H(+)-di-/tripeptide transporter Pept1 of the small intestine. Proton influx via Pept1 requires maintenance of intracellular pH (pH(i)) to sustain the driving force for peptide absorption. The apical membrane Na(+)/H(+) exchanger Nhe3 plays a major role in minimizing epithelial acidification during H(+)-di-/tripeptide absorption. However, the contributions of HCO(3)(-)-dependent transporters to this process have not been elucidated. In this study, we investigate the role of putative anion transporter-1 (Pat-1), an apical membrane anion exchanger, in epithelial pH(i) regulation during H(+)-peptide absorption. Using wild-type (WT) and Pat-1(-) mice, Ussing chambers were employed to measure the short-circuit current (I(sc)) associated with Pept1-mediated glycyl-sarcosine (Gly-Sar) absorption. Microfluorometry was used to measure pH(i) and Cl(-)/HCO(3)(-) exchange in the upper villous epithelium. In CO(2)/HCO(3)(-)-buffered Ringers, WT small intestine showed significant Gly-Sar-induced I(sc) and efficient pH(i) regulation during pharmacological inhibition of Nhe3 activity. In contrast, epithelial acidification and reduced I(sc) response to Gly-Sar exposure occurred during pharmacological inhibition of Cl(-)/HCO(3)(-) exchange and in the Pat-1(-) intestine. Pat-1 interacts with carbonic anhydrase II (CAII), and studies using CAII(-) intestine or the pharmacological inhibitor methazolamide on WT intestine resulted in increased epithelial acidification during Gly-Sar exposure. Increased epithelial acidification during Gly-Sar exposure also occurred in WT intestine during inhibition of luminal extracellular CA activity. Measurement of Cl(-)/HCO(3)(-) exchange in the presence of Gly-Sar revealed an increased rate of Cl(-)(OUT)/HCO(3)(-)(IN) exchange that was both Pat-1 dependent and CA dependent. In conclusion, Pat-1 Cl(-)/HCO(3)(-) exchange contributes to pH(i) regulation in the villous epithelium during H(+)-dipeptide absorption, possibly by providing a HCO(3)(-) import pathway.

Pharmaceutical and pharmacological importance of peptide transporters

Peptide transport is currently a prominent topic in membrane research. The transport proteins involved are under intense investigation because of their physiological importance in protein absorption and also because peptide transporters are possible vehicles for drug delivery. Moreover, in many tissues peptide carriers transduce peptidic signals across membranes that are relevant in information processing. The focus of this review is on the pharmaceutical relevance of the human peptide transporters PEPT1 and PEPT2. In addition to their physiological substrates, both carriers transport many β-lactam antibiotics, valaciclovir and other drugs and prodrugs because of their sterical resemblance to di- and tripeptides. The primary structure, tissue distribution and substrate specificity of PEPT1 and PEPT2 have been well characterized. However, there is a dearth of knowledge on the substrate binding sites and the three-dimensional structure of these proteins. Until this pivotal information becomes available by X-ray crystallography, the development of new drug substrates relies on classical transport studies combined with molecular modelling. In more than thirty years of research, data on the interaction of well over 700 di- and tripeptides, amino acid and peptide derivatives, drugs and prodrugs with peptide transporters have been gathered. The aim of this review is to put the reports on peptide transporter-mediated drug uptake into perspective. We also review the current knowledge on pharmacogenomics and clinical relevance of human peptide transporters. Finally, the reader's attention is drawn to other known or proposed human peptide-transporting proteins.

Loss of the apical V-ATPase a-subunit VHA-6 prevents acidification of the intestinal lumen during a rhythmic behavior in C. elegans

In Caenorhabditis elegans, oscillations of intestinal pH contribute to the rhythmic defecation behavior, but the acid-base transport mechanisms that facilitate proton movement are not well understood. Here, we demonstrate that VHA-6, an intestine-specific a-subunit of the H(+)-K(+)-ATPase complex (V-ATPase), resides in the apical membrane of the intestinal epithelial cells and is required for luminal acidification. Disruption of the vha-6 gene led to early developmental arrest; the arrest phenotype could be complemented by expression of a fluorescently labeled vha-6 transgene. To study the contribution of vha-6 to pH homeostasis in larval worms, we used a partial reduction of function through postembryonic single-generation RNA interference. We demonstrate that the inability to fully acidify the intestinal lumen coincides with a defect in pH recovery of the intestinal epithelial cells, suggesting that VHA-6 is essential for proton pumping following defecation. Moreover, intestinal dipeptide accumulation and fat storage are compromised by the loss of VHA-6, suggesting that luminal acidification promotes nutrient uptake in worms, as well as in mammals. Since acidified intracellular vesicles and autofluorescent storage granules are indistinguishable between the vha-6 mutant and controls, it is likely that the nutrient-restricted phenotype is due to a loss of plasma membrane V-ATPase activity specifically. These data establish a simple genetic model for proton pump-driven acidification. Since defecation occurs at 45-s intervals in worms, this model represents an opportunity to study acute regulation of V-ATPase activity on a short time scale and may be useful in the study of alternative treatments for acid-peptic disorders.

The role of the NHERF family of PDZ scaffolding proteins in the regulation of salt and water transport

The four members of the NHERF (Na(+)/H(+) exchanger regulatory factor) family of PDZ adapter proteins bind to a variety of membrane transporters and receptors and modulate membrane expression, mobility, interaction with other proteins, and the formation of signaling complexes. All four family members are expressed in the intestine. The CFTR (cystic fibrosis transmembrane regulator) anion channel and the Na(+)/H(+) exchanger NHE3 (Na/H exchanger- isoform 3) are two prominent binding partners to this PDZ-adapter family, which are also known key players in the regulation of intestinal electrolyte and fluid transport. Experiments in heterologous expression systems have provided a number of mechanistic models how NHERF protein interactions can affect the function of their targets at the molecular level. Recently, NHERF1, 2, and 3 knockout mice have become available, and this review summarizes the reports on electrolyte and fluid transport regulation in the native intestine of these mice.

How the intestinal peptide transporter PEPT-1 contributes to an obesity phenotype in Caenorhabditits elegans

BACKGROUND

Amino acid absorption in the form of di- and tripeptides is mediated by the intestinal proton-coupled peptide transporter PEPT-1 (formally OPT-2) in Caenorhabditits elegans. Transporter-deficient animals (pept-1(lg601)) show impaired growth, slowed postembryonal development and major changes in amino acid status.

PRINCIPAL FINDINGS

Here we demonstrate that abolished intestinal peptide transport also leads to major metabolic alterations that culminate in a two fold increase in total body fat content. Feeding of C. elegans with [U-(13)C]-labelled E. coli revealed a decreased de novo synthesis of long-chain fatty acids in pept-1(lg601) and reduced levels of polyunsaturated fatty acids. mRNA profiling revealed increased transcript levels of enzymes/transporters needed for peroxisomal beta-oxidation and decreased levels for those required for fatty acid synthesis, elongation and desaturation. As a prime and most fundamental process that may account for the increased fat content in pept-1(lg601) we identified a highly accelerated absorption of free fatty acids from the bacterial food in the intestine.

CONCLUSIONS

The influx of free fatty acids into intestinal epithelial cells is strongly dependent on alterations in intracellular pH which is regulated by the interplay of PEPT-1 and the sodium-proton exchanger NHX-2. We here provide evidence for a central mechanism by which the PEPT-1/NHX-2 system strongly influences the in vivo fat content of C. elegans. Loss of PEPT-1 decreases intestinal proton influx leading to a higher uptake of free fatty acids with fat accumulation whereas loss of NHX-2 causes intracellular acidification by the PEPT-1 mediated proton/dipeptide symport with an almost abolished uptake of fatty acids and a lean phenotype.

Ouabain-induced stimulation of sodium-hydrogen exchange in rat optic nerve astrocytes

Sodium-dependent transporters are inhibited indirectly by the Na-K-ATPase inhibitor ouabain. Here we report stimulation of sodium-hydrogen exchange (NHE) in ouabain-treated cells. BCECF was used to measure cytoplasmic pH in cultured rat optic nerve astrocytes. Ammonium chloride was applied to acid load the cells. On removal of ammonium chloride, cytoplasmic pH fell abruptly, then gradually recovered toward baseline. Ouabain (1 microM) did not change cell sodium content, but the rate of pH recovery increased by 68%. Ouabain speeded pH recovery both in the presence and absence of bicarbonate. In bicarbonate-free medium, dimethylamiloride, an NHE inhibitor, eliminated the effect of 1 microM ouabain on pH recovery. Western blot analysis showed an NHE1 immunoreactive band but not NHE2, NHE3, or NHE4. Immunoprecipitation studies showed phosphorylation of NHE1 in cells treated with 1 microM ouabain. Ouabain evoked an increase of cAMP, and the effect of 1 microM ouabain on pH recovery was abolished by H-89, a protein kinase A inhibitor. 8-Bromoadenosine-cAMP increased the pH recovery rate, and this recovery was not further increased by ouabain. Although 1 microM ouabain did not alter cytoplasmic calcium concentration, it stimulated calcium entry after store depletion, a response abolished by 2-APB. Ouabain-induced stimulation of pH recovery was suppressed by inhibitors of capacitative calcium entry, SKF-96365, and 2-APB, as well as the cytoplasmic calcium chelator BAPTA. The cAMP increase in ouabain-treated cells was abolished by BAPTA and 2-APB. Taken together, the results are consistent with increased capacitative calcium entry and subsequent cAMP-PKA-dependent stimulation of NHE1 in ouabain-treated cells.