H+-coupled nutrient, micronutrient and drug transporters in the mammalian small intestine

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

BACKGROUND

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

METHODS

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

RESULTS

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

CONCLUSIONS

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

Metabolic Signature of Warburg Effect in Cancer: An Effective and Obligatory Interplay between Nutrient Transporters and Catabolic/Anabolic Pathways to Promote Tumor Growth

Aerobic glycolysis in cancer cells, originally observed by Warburg 100 years ago, which involves the production of lactate as the end product of glucose breakdown even in the presence of adequate oxygen, is the foundation for the current interest in the cancer-cell-specific reprograming of metabolic pathways. The renewed interest in cancer cell metabolism has now gone well beyond the original Warburg effect related to glycolysis to other metabolic pathways that include amino acid metabolism, one-carbon metabolism, the pentose phosphate pathway, nucleotide synthesis, antioxidant machinery, etc. Since glucose and amino acids constitute the primary nutrients that fuel the altered metabolic pathways in cancer cells, the transporters that mediate the transfer of these nutrients and their metabolites not only across the plasma membrane but also across the mitochondrial and lysosomal membranes have become an integral component of the expansion of the Warburg effect. In this review, we focus on the interplay between these transporters and metabolic pathways that facilitates metabolic reprogramming, which has become a hallmark of cancer cells. The beneficial outcome of this recent understanding of the unique metabolic signature surrounding the Warburg effect is the identification of novel drug targets for the development of a new generation of therapeutics to treat cancer.

A Novel Peptide Prevents Enterotoxin- and Inflammation-Induced Intestinal Fluid Secretion by Stimulating Sodium-Hydrogen Exchanger 3 Activity

BACKGROUND & AIMS

Acute diarrheal diseases are the second most common cause of infant mortality in developing countries. This is contributed to by lack of effective drug therapy that shortens the duration or lessens the volume of diarrhea. The epithelial brush border sodium (Na+)/hydrogen (H+) exchanger 3 (NHE3) accounts for a major component of intestinal Na+ absorption and is inhibited in most diarrheas. Because increased intestinal Na+ absorption can rehydrate patients with diarrhea, NHE3 has been suggested as a potential druggable target for drug therapy for diarrhea.

METHODS

A peptide (sodium-hydrogen exchanger 3 stimulatory peptide [N3SP]) was synthesized to mimic the part of the NHE3 C-terminus that forms a multiprotein complex that inhibits NHE3 activity. The effect of N3SP on NHE3 activity was evaluated in NHE3-transfected fibroblasts null for other plasma membrane NHEs, a human colon cancer cell line that models intestinal absorptive enterocytes (Caco-2/BBe), human enteroids, and mouse intestine in vitro and in vivo. N3SP was delivered into cells via a hydrophobic fluorescent maleimide or nanoparticles.

RESULTS

N3SP uptake stimulated NHE3 activity at nmol/L concentrations under basal conditions and partially reversed the reduced NHE3 activity caused by elevated adenosine 3',5'-cyclic monophosphate, guanosine 3',5'-cyclic monophosphate, and Ca2+ in cell lines and in in vitro mouse intestine. N3SP also stimulated intestinal fluid absorption in the mouse small intestine in vivo and prevented cholera toxin-, Escherichia coli heat-stable enterotoxin-, and cluster of differentiation 3 inflammation-induced fluid secretion in a live mouse intestinal loop model.

CONCLUSIONS

These findings suggest pharmacologic stimulation of NHE3 activity as an efficacious approach for the treatment of moderate/severe diarrheal diseases.

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 impact of advanced age on gastrointestinal characteristics that are relevant to oral drug absorption: An AGePOP review

The purpose of this review is to summarize the current knowledge on three physiological determinants of oral drug absorption, i.e., gastric emptying, volumes and composition of luminal fluids, and intestinal permeability, in the advanced age population, so that potential knowledge gaps and directions for further research efforts are identified. Published data on gastric emptying rates in older people are conflicting. Also, there are significant knowledge gaps, especially on gastric motility and emptying rates of drugs and of non-caloric fluids. Compared with younger adults, volumes of luminal contents seem to be slightly smaller in older people. Our understanding on the impact of advanced age on luminal physicochemical characteristics is, at best, very limited, whereas the impact of (co)morbidities and geriatric syndromes in the advanced age population has not been addressed to date. The available literature on the effect of advanced age on intestinal permeability is limited, and should be approached with caution, primarily due to the limitations of the experimental methodologies used.

Iron metabolism: pathways and proteins in homeostasis

Iron is essential to human survival. The biological role and trafficking of this trace essential inorganic element which is also a potential toxin is constantly being researched and unfolded. Vital for oxygen transport, DNA synthesis, electron transport, neurotransmitter biosynthesis and present in numerous other heme and non-heme enzymes the physiological roles are immense. Understanding the molecules and pathways that regulate this essential element at systemic and cellular levels are of importance in improving therapeutic strategies for iron related disorders. This review highlights the progress in understanding the metabolism and trafficking of iron along with the pathophysiology of iron related disorders.

Regulation of nutrient and electrolyte absorption in human organoid-derived intestinal epithelial cell monolayers

Recently developed human intestinal epithelial 3D organoid cultures are a useful cell culture model to study intestinal transport physiology. From these, 2D monolayer cultures can be generated in which apical transporters are exposed to the medium, thereby better facilitating in vitro investigation of intestinal absorption processes. However, whether nutrient and electrolyte absorption can be physiologically regulated in human organoid-derived monolayers has not been determined. Constitutive nitric oxide (cNO) is known to regulate multiple gastrointestinal physiological functions. Previous studies using in vivo and in vitro mammalian animal models indicate that enhanced intracellular cNO differentially regulates the two primary apical Na transporters in small intestinal epithelial cells. Here, we generated human jejunal organoid-derived monolayers to determine whether apical nutrient and electrolyte transporter function is regulated by cNO in human enterocytes. Western blot analysis and immunocytochemical staining showed that organoid-derived 2D cultures express markers of enterocyte differentiation and form intact monolayers of apical-basal polarized epithelial cells. Uptake studies demonstrated that jejunal monolayers exhibit functional activity of Na-glucose cotransporter 1 (SGLT1; SLC5A1) and Na-H exchanger 3 (NHE3; SLC9A3). In response to physiological increases in cNO, the two primary apical Na transporters were differentially regulated in human intestinal organoid-derived monolayers, across multiple human specimens. An increase in cNO stimulated SGLT1, while NHE3 was inhibited. These results are similar to what is seen in vivo and in vitro in different animal intestinal models. Thus, human jejunal organoid-derived monolayers are an ideal in vitro model to better understand how intestinal nutrient absorption is regulated.

The NHE3 Inhibitor Tenapanor Prevents Intestinal Obstructions in CFTR-Deleted Mice

Mutations in the CFTR chloride channel result in intestinal obstructive episodes in cystic fibrosis (CF) patients and in CF animal models. In this study, we explored the possibility of reducing the frequency of obstructive episodes in cftr^−/− mice through the oral application of a gut-selective NHE3 inhibitor tenapanor and searched for the underlying mechanisms involved. Sex- and age-matched cftr^+/+ and cftr^−/− mice were orally gavaged twice daily with 30 mg kg⁻¹ tenapanor or vehicle for a period of 21 days. Body weight and stool water content was assessed daily and gastrointestinal transit time (GTT) once weekly. The mice were sacrificed when an intestinal obstruction was suspected or after 21 days, and stool and tissues were collected for further analysis. Twenty-one day tenapanor application resulted in a significant increase in stool water content and stool alkalinity and a significant decrease in GTT in cftr^+/+ and cftr^−/− mice. Tenapanor significantly reduced obstructive episodes to 8% compared to 46% in vehicle-treated cftr^−/− mice and prevented mucosal inflammation. A decrease in cryptal hyperproliferation, mucus accumulation, and mucosal mast cell number was also observed in tenapanor- compared to vehicle-treated, unobstructed cftr^−/− mice. Overall, oral tenapanor application prevented obstructive episodes in CFTR-deficient mice and was safe in cftr^+/+ and cftr^−/− mice. These results suggest that tenapanor may be a safe and affordable adjunctive therapy in cystic fibrosis patients to alleviate constipation and prevent recurrent DIOS.

Cryo-EM Structure of an Atypical Proton-Coupled Peptide Transporter: Di- and Tripeptide Permease C

Proton-coupled Oligopeptide Transporters (POTs) of the Major Facilitator Superfamily (MFS) mediate the uptake of short di- and tripeptides in all phyla of life. POTs are thought to constitute the most promiscuous class of MFS transporters, with the potential to transport more than 8400 unique substrates. Over the past two decades, transport assays and biophysical studies have shown that various orthologues and paralogues display differences in substrate selectivity. The E. coli genome codes for four different POTs, known as Di- and tripeptide permeases A-D (DtpA-D). DtpC was shown previously to favor positively charged peptides as substrates. In this study, we describe, how we determined the structure of the 53 kDa DtpC by cryogenic electron microscopy (cryo-EM), and provide structural insights into the ligand specificity of this atypical POT. We collected and analyzed data on the transporter fused to split superfolder GFP (split sfGFP), in complex with a 52 kDa Pro-macrobody and with a 13 kDa nanobody. The latter sample was more stable, rigid and a significant fraction dimeric, allowing us to reconstruct a 3D volume of DtpC at a resolution of 2.7 Å. This work provides a molecular explanation for the selectivity of DtpC, and highlights the value of small and rigid fiducial markers such as nanobodies for structure determination of low molecular weight integral membrane proteins lacking soluble domains.

Transcriptome Analysis of the Marine Nematode Litoditis marina in a Chemically Defined Food Environment with Stearic Acid Supplementation

Stearic acid represents one of the most abundant fatty acids in the Western diet and profoundly regulates health and diseases of animals and human beings. We previously showed that stearic acid supplementation promoted development of the terrestrial model nematode Caenorhabditis elegans in chemically defined CeMM food environment. However, whether stearic acid regulates development of other nematodes remains unknown. Here, we found that dietary supplementation with stearic acid could promote the development of the marine nematode Litoditis marina, belonging to the same family as C. elegans, indicating the conserved roles of stearic acid in developmental regulation. We further employed transcriptome analysis to analyze genome-wide transcriptional signatures of L. marina with dietary stearic acid supplementation. We found that stearic acid might promote development of L. marina via upregulation of the expression of genes involved in aminoacyl-tRNA biosynthesis, translation initiation and elongation, ribosome biogenesis, and transmembrane transport. In addition, we observed that the expression of neuronal signaling-related genes was decreased. This study provided important insights into how a single fatty acid stearic acid regulates development of marine nematode, and further studies with CRISPR genome editing will facilitate demonstrating the molecular mechanisms underlying how a single metabolite regulates animal development and health.

The Role of Gut Microbiota and Metabolites in Obesity-Associated Chronic Gastrointestinal Disorders

The gut microbiota is a complex community of microorganisms that has become a new focus of attention due to its association with numerous human diseases. Research over the last few decades has shown that the gut microbiota plays a considerable role in regulating intestinal homeostasis, and disruption to the microbial community has been linked to chronic disease conditions such as inflammatory bowel disease (IBD), colorectal cancer (CRC), and obesity. Obesity has become a global pandemic, and its prevalence is increasing worldwide mostly in Western countries due to a sedentary lifestyle and consumption of high-fat/high-sugar diets. Obesity-mediated gut microbiota alterations have been associated with the development of IBD and IBD-induced CRC. This review highlights how obesity-associated dysbiosis can lead to the pathogenesis of IBD and CRC with a special focus on mechanisms of altered absorption of short-chain fatty acids (SCFAs)

Spatial discordances between mRNAs and proteins in the intestinal epithelium

The use of transcriptomes as reliable proxies for cellular proteomes is controversial. In the small intestine, enterocytes operate for 4 days as they migrate along villi, which are highly graded microenvironments. Spatial transcriptomics have demonstrated profound zonation in enterocyte gene expression, but how this variability translates to protein content is unclear. Here we show that enterocyte proteins and messenger RNAs along the villus axis are zonated, yet often spatially discordant. Using spatial sorting with zonated surface markers, together with a Bayesian approach to infer protein translation and degradation rates from the combined spatial profiles, we find that, while many genes exhibit proteins zonated toward the villus tip, mRNA is zonated toward the villus bottom. Finally, we demonstrate that space-independent protein synthesis delays can explain many of the mRNA-protein discordances. Our work provides a proteomic spatial blueprint of the intestinal epithelium, highlighting the importance of protein measurements for inferring cell states in tissues that operate outside of steady state.

VARIDT 2.0: structural variability of drug transporter

The structural variability data of drug transporter (DT) are key for research on precision medicine and rational drug use. However, these valuable data are not sufficiently covered by the available databases. In this study, a major update of VARIDT (a database previously constructed to provide DTs' variability data) was thus described. First, the experimentally resolved structures of all DTs reported in the original VARIDT were discovered from PubMed and Protein Data Bank. Second, the structural variability data of each DT were collected by literature review, which included: (a) mutation-induced spatial variations in folded state, (b) difference among DT structures of human and model organisms, (c) outward/inward-facing DT conformations and (d) xenobiotics-driven alterations in the 3D complexes. Third, for those DTs without experimentally resolved structural variabilities, homology modeling was further applied as well-established protocol to enrich such valuable data. As a result, 145 mutation-induced spatial variations of 42 DTs, 1622 inter-species structures originating from 292 DTs, 118 outward/inward-facing conformations belonging to 59 DTs, and 822 xenobiotics-regulated structures in complex with 57 DTs were updated to VARIDT (https://idrblab.org/varidt/ and http://varidt.idrblab.net/). All in all, the newly collected structural variabilities will be indispensable for explaining drug sensitivity/selectivity, bridging preclinical research with clinical trial, revealing the mechanism underlying drug-drug interaction, and so on.

PEPT1 is essential for the growth of pancreatic cancer cells: a viable drug target

PEPT1 is a proton-coupled peptide transporter that is up-regulated in PDAC cell lines and PDXs, with little expression in the normal pancreas. However, the relevance of this up-regulation to cancer progression and the mechanism of up-regulation have not been investigated. Herein, we show that PEPT1 is not just up-regulated in a large panel of PDAC cell lines and PDXs but is also functional and transport-competent. PEPT2, another proton-coupled peptide transporter, is also overexpressed in PDAC cell lines and PDXs, but is not functional due to its intracellular localization. Using glibenclamide as a pharmacological inhibitor of PEPT1, we demonstrate in cell lines in vitro and mouse xenografts in vivo that inhibition of PEPT1 reduces the proliferation of the cancer cells. These findings are supported by genetic knockdown of PEPT1 with shRNA, wherein the absence of the transporter significantly attenuates the growth of cancer cells, both in vitro and in vivo, suggesting that PEPT1 is critical for the survival of cancer cells. We also establish that the tumor-derived lactic acid (Warburg effect) in the tumor microenvironment supports the transport function of PEPT1 in the maintenance of amino acid nutrition in cancer cells by inducing MMPs and DPPIV to generate peptide substrates for PEPT1 and by generating a H+ gradient across the plasma membrane to energize PEPT1. Taken collectively, these studies demonstrate a functional link between PEPT1 and extracellular protein breakdown in the tumor microenvironment as a key determinant of pancreatic cancer growth, thus identifying PEPT1 as a potential therapeutic target for PDAC.

Functional role of serine 318 of the proton-coupled folate transporter in methotrexate transport

This study revealed the importance of serine 318 (S318) residue for proton-coupled folate transporter (PCFT, SLC46A1) functioning. Substitution of S318 with arginine or lysine impaired transport of methotrexate (MTX), but substitution with alanine (has a simple side chain structure), or cysteine (structurally similar to serine), had no significant effect on MTX transport. The initial uptake rate of MTX by S318A and S318C mutant at pH 5.0, followed by Michaelis-Menten kinetics with a K_m value of approximately 2.3 μM (for S318A) and 2.9 μM (for S318C), was similar to that of the wild-type. The normalized V_max value of the S318A mutant, calculated by dividing the V_max value by the Western blot protein band's relative intensity, was approximately 2-fold greater than that of the wild-type. The normalized V_max value of the S318C mutant was approximately 0.8-fold smaller than the wild-type. Results obtained showed that the substitution of S318 with basic amino acid residues results in the loss of transport activity, even though PCFT mutants are expressed at the cell membrane. Alternatively, the substitution of S318 with neutral amino acids did not significantly affect the transport function of PCFT.

Increased Bioavailability of β-Alanine by a Novel Controlled-Release Powder Blend Compared to a Slow-Release Tablet

BACKGROUND

β-Alanine is a sport supplement with increasing popularity due to its consistent ability to improve physical performance, with the downside of requiring several weeks of supplementation as imposed to the maximum daily and single dose tolerated without side effects (i.e., paresthesia). To date, the only alternative to overcome this problem has been use of a sustained-release tablet, while powders are the most commonly used format to deliver several grams of amino acids in a single dose. In this study we assessed the bioavailability, pharmacokinetics and paresthesia effect of β-alanine after administration in a novel controlled-released powder blend (test) versus a sustained-release tablet (reference).

METHODS

Twelve subjects (25.6 ± 3.2 y, 50% female) participated in a randomized, single-blind, crossover study. Each participant was administered orally the test (β-alanine 8 g, l-histidine 300 mg, carnosine 100 mg) or the reference product (10 tablets to reach β-alanine 8 g, Zinc 20 mg) with a 1-week washout period. β-Alanine plasma concentrations (0-8 h) were determined by LC-MS/MS and model-independent pharmacokinetic analysis was carried out. Paresthesia intensity was evaluated using a Visual Analog Score (VAS) and the categorical Intensity Sensory Score (ISS).

RESULTS

The CMAX and AUC0→∞ increased 1.6- and 2.1-fold (both p < 0.001) in the test product, respectively, which yielded 2.1-fold higher bioavailability; Ka decreased in the test (0.0199 ± 0.0107 min-1) versus the reference (0.0299 ± 0.0121 min-1) product (p = 0.0834) as well as V/F and Cl/F (both p < 0.001); MRT0→last increased in the test (143 ± 19 min) versus reference (128 ± 16 min) formulation (p = 0.0449); t1/2 remained similar (test: 63.5 ± 8.7 min, reference: 68.9 ± 9.8 min). Paresthesia EMAX increased 1.7-fold using the VAS (p = 0.086) and the ISS (p = 0.009). AUEC increased 1.9-fold with the VAS (p = 0.107) and the ISS (p = 0.019) reflecting scale intrinsic differences. Pharmacokinetic-pharmacodynamic analysis showed a clockwise hysteresis loop without prediction ability between CMAX, AUC0→∞ and EMAX or AUEC. No side effects were reported (except paresthesia).

CONCLUSIONS

The novel controlled-release powder blend shows 100% higher bioavailability of β-alanine, opening a new paradigm that shifts from chronic to short or mid-term supplementation strategies to increase carnosine stores in sports nutrition.

The Transporter-Mediated Cellular Uptake and Efflux of Pharmaceutical Drugs and Biotechnology Products: How and Why Phospholipid Bilayer Transport Is Negligible in Real Biomembranes

Over the years, my colleagues and I have come to realise that the likelihood of pharmaceutical drugs being able to diffuse through whatever unhindered phospholipid bilayer may exist in intact biological membranes in vivo is vanishingly low. This is because (i) most real biomembranes are mostly protein, not lipid, (ii) unlike purely lipid bilayers that can form transient aqueous channels, the high concentrations of proteins serve to stop such activity, (iii) natural evolution long ago selected against transport methods that just let any undesirable products enter a cell, (iv) transporters have now been identified for all kinds of molecules (even water) that were once thought not to require them, (v) many experiments show a massive variation in the uptake of drugs between different cells, tissues, and organisms, that cannot be explained if lipid bilayer transport is significant or if efflux were the only differentiator, and (vi) many experiments that manipulate the expression level of individual transporters as an independent variable demonstrate their role in drug and nutrient uptake (including in cytotoxicity or adverse drug reactions). This makes such transporters valuable both as a means of targeting drugs (not least anti-infectives) to selected cells or tissues and also as drug targets. The same considerations apply to the exploitation of substrate uptake and product efflux transporters in biotechnology. We are also beginning to recognise that transporters are more promiscuous, and antiporter activity is much more widespread, than had been realised, and that such processes are adaptive (i.e., were selected by natural evolution). The purpose of the present review is to summarise the above, and to rehearse and update readers on recent developments. These developments lead us to retain and indeed to strengthen our contention that for transmembrane pharmaceutical drug transport "phospholipid bilayer transport is negligible".

Effect of Tea Tree Oil on the Expression of Genes Involved in the Innate Immune System in Goat Rumen Epithelial Cells

Simple Summary

Subacute rumen acidosis (SARA) often causes significant losses on commercial farms. SARA is mainly caused by endotoxin (LPS) produced by the lysis of Gram-negative bacteria, which causes an inflammatory response. To alleviate the inflammatory response mediated by LPS, it is important to improve animal production performance. Tea tree oil (TTO) is a plant extract that possesses good bactericidal and anti-inflammatory effects. According to this study, LPS can significantly induce inflammatory responses in goat rumen epithelial cells (GRECs), while the addition of TTO could markedly mitigate inflammatory responses mediated by LPS in GRECs. Therefore, it may be useful for the treatment of SARA.

Abstract

In subacute rumen acidosis (SARA), the rumen epithelium is frequently attacked by endotoxin (LPS), which is caused by the lysis of dead Gram-negative bacteria. However, the rumen epithelium innate immune system can actively respond to the infection. Previous studies have demonstrated that tea tree oil (TTO) has good bactericidal and anti-inflammatory effects. Therefore, the aim of this study was to investigate the effect of TTO on the expression of genes involved in the inflammatory cytokines in goat rumen epithelial cells (GRECs) triggered by LPS. Our study shows that rumen epithelial cells isolated from goat rumen tissue can be cultured in vitro in 0.25% trypsin for a long time. These cells were identified as epithelial cells by the expression of cytokeratin 18, monocarboxylate transporter 4 (MCT4), Na[+]/H[+] hydrogen exchanger 1 (NHE1), putative anion transporter 1 (PAT1), vH⁺ ATPase B subunit (vH⁺ ATPase), and anion exchanger 2 (AE2). The mRNA expression of IL-1β, IL-6, TNF-α, TLR-2, NF-κB, CXCL6 and CXCL8 genes was significantly increased when LPS was used compared to untreated controls. In addition, mRNA expression of IL-1β, IL-6, TNF-α, TLR-2, NF-κB, CXCL8, CXCL6 and interferon-induced protein with tetratricopeptide repeats 3 (IFIT3) genes was also significantly higher in the LPS group compared to the 0.05% TTO group. However, the expression of IL-1β, IL-6, TNF-α, TLR-2, CXCL6 and IFIT3 genes was significantly lower in the LPS and 0.05% TTO group compared to the 1 μg/mL LPS group. These results suggest that TTO can inhibit LPS-induced inflammatory cytokines expression in GRECs.

Intestinal GLUT5 and FAT/CD36 transporters and blood glucose are reduced by a carotenoid/MUFA-rich oil in high-fat fed mice

AIMS

Intestinal nutrient absorption plays a vital role in developing obesity, and nutrient transporters expressed in the enterocytes facilitate this process. Moreover, previous studies have shown that specific foods and diets can affect their cell levels. Herein, we investigated the effects of pequi oil (PO), which is high in several bioactive compounds, on intestinal nutrient transporter levels as well as on intestinal morphology and metabolic biomarkers.

MAIN METHODS

Groups of male C57BL/6 mice were fed either a standard (C) or a high-fat diet (HFD) and pequi oil (CP and HFDP with PO by gavage at 150 mg/day) for eight weeks. Food intake and body weight were monitored, serum metabolic biomarkers, intestinal transporter levels and histological analyses were performed.

KEY FINDINGS

PO increased caloric intake without increasing body or fat mass regardless of diet. The HFD group treated with PO reduced fasting blood glucose and villus width. PO did not affect GLUT2, L-FABP, FATP4, NPC1L1, NHE3 or PEPT1 content in CP or HFDP groups. GLUT5 and FAT/CD36 levels were reduced in both CP and HFDP.

SIGNIFICANCE

Our data suggest that PO attenuated monosaccharide and fatty acid absorption, contributing to lower fasting glycemia and higher food intake without affecting body weight or visceral fat of high-fat feed mice.

Intestinal Calcium Absorption

In this article, we focus on mammalian calcium absorption across the intestinal epithelium in normal physiology. Intestinal calcium transport is essential for supplying calcium for metabolism and bone mineralization. Dietary calcium is transported across the mucosal epithelia via saturable transcellular and nonsaturable paracellular pathways, both of which are under the regulation of 1,25-dihydroxyvitamin D₃ and several other endocrine and paracrine factors, such as parathyroid hormone, prolactin, 17β-estradiol, calcitonin, and fibroblast growth factor-23. Calcium absorption occurs in several segments of the small and large intestine with varying rates and capacities. Segmental heterogeneity also includes differential expression of calcium transporters/carriers (e.g., transient receptor potential cation channel and calbindin-D_9k ) and the presence of favorable factors (e.g., pH, luminal contents, and gut motility). Other proteins and transporters (e.g., plasma membrane vitamin D receptor and voltage-dependent calcium channels), as well as vesicular calcium transport that probably contributes to intestinal calcium absorption, are also discussed. © 2021 American Physiological Society. Compr Physiol 11:1-27, 2021.

Human ferroportin mediates proton-coupled active transport of iron

As the sole iron exporter in humans, ferroportin controls systemic iron homeostasis through exporting iron into the blood plasma. The molecular mechanism of how ferroportin exports iron under various physiological settings remains unclear. Here we found that purified ferroportin incorporated into liposomes preferentially transports Fe2+ and exhibits lower affinities of transporting other divalent metal ions. The iron transport by ferroportin is facilitated by downhill proton gradients at the same direction. Human ferroportin is also capable of transporting protons, and this activity is tightly coupled to the iron transport. Remarkably, ferroportin can conduct active transport uphill against the iron gradient, with favorable charge potential providing the driving force. Targeted mutagenesis suggests that the iron translocation site is located at the pore region of human ferroportin. Together, our studies enhance the mechanistic understanding by which human ferroportin transports iron and suggest that a combination of electrochemical gradients regulates iron export.

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.

Human gastrointestinal epithelia of the esophagus, stomach, and duodenum resolved at single-cell resolution

The upper gastrointestinal tract, consisting of the esophagus, stomach, and duodenum, controls food transport, digestion, nutrient uptake, and hormone production. By single-cell analysis of healthy epithelia of these human organs, we molecularly define their distinct cell types. We identify a quiescent COL17A1high KRT15high stem/progenitor cell population in the most basal cell layer of the esophagus and detect substantial gene expression differences between identical cell types of the human and mouse stomach. Selective expression of BEST4, CFTR, guanylin, and uroguanylin identifies a rare duodenal cell type, referred to as BCHE cell, which likely mediates high-volume fluid secretion because of continual activation of the CFTR channel by guanylin/uroguanylin-mediated autocrine signaling. Serotonin-producing enterochromaffin cells in the antral stomach significantly differ in gene expression from duodenal enterochromaffin cells. We, furthermore, discover that the histamine-producing enterochromaffin-like cells in the oxyntic stomach express the luteinizing hormone, yet another member of the enteroendocrine hormone family.

Inhibition of Na+ /H+ exchanger isoform 3 improves gut fluidity and alkalinity in cystic fibrosis transmembrane conductance regulator-deficient and F508del mutant mice

BACKGROUND AND PURPOSE

Constipation and intestinal obstructive episodes are major health problems in cystic fibrosis (CF) patients. Three FDA-approved drugs against constipation-prone irritable bowel syndrome were tested for their ability to increase luminal fluidity and alkalinity in cystic fibrosis transmembrane conductance regulator (CFTR) null (cftr^-/- ) and F508del mutant (F508del^mut/mut ) murine intestine.

EXPERIMENTAL APPROACH

Guanylate cyclase C agonist linaclotide, PGE₁ analogue lubiprostone and intestine-specific NHE3 inhibitor tenapanor were perfused through a ~3 cm jejunal, proximal or mid-distal colonic segment in anaesthetized cftr^-/- , F508del^mut/mut and WT mice. Net fluid balance was determined gravimetrically and alkaline output by pH-stat back titration.

KEY RESULTS

Basal jejunal fluid absorptive rates were significantly higher and basal HCO₃ ^- output was significantly lower in cftr^-/- and F508del^mut/mut compared to WT mice. In cftr^-/- and F508del^mut/mut mice, all three drugs significantly inhibited the fluid absorptive rate and increased alkaline output in the jejunum and tenapanor and lubiprostone, but not linaclotide, in the colon. After tenapanor pre-incubation, linaclotide elicited a robust fluid secretory response in WT jejunum, while no further change in absorptive rates was observed in cftr^-/- and F508del^mut/mut jejunum, suggesting that the increase in gut fluidity and alkalinity by linaclotide in CF gut is mediated via NHE3 inhibition. Lubiprostone also inhibited fluid absorption in cftr^-/- and F508del^mut/mut jejunum via NHE3 inhibition but had a residual NHE3-independent effect.

CONCLUSION AND IMPLICATIONS

Linaclotide, lubiprostone and tenapanor reduced fluid absorption and increased alkaline output in the CF gut. Their application may ameliorate constipation and reduce obstructive episodes in CF patients.

The therapeutic importance of acid-base balance

Baking soda and vinegar have been used as home remedies for generations and today we are only a mouse-click away from claims that baking soda, lemon juice, and apple cider vinegar are miracles cures for everything from cancer to COVID-19. Despite these specious claims, the therapeutic value of controlling acid-base balance is indisputable and is the basis of Food and Drug Administration-approved treatments for constipation, epilepsy, metabolic acidosis, and peptic ulcers. In this narrative review, we present evidence in support of the current and potential therapeutic value of countering local and systemic acid-base imbalances, several of which do in fact involve the administration of baking soda (sodium bicarbonate). Furthermore, we discuss the side effects of pharmaceuticals on acid-base balance as well as the influence of acid-base status on the pharmacokinetic properties of drugs. Our review considers all major organ systems as well as information relevant to several clinical specialties such as anesthesiology, infectious disease, oncology, dentistry, and surgery.

PacBio single molecule long-read sequencing provides insight into the complexity and diversity of the Pinctada fucata martensii transcriptome

BACKGROUND

The pearl oyster Pinctada fucata martensii is an economically valuable shellfish for seawater pearl production, and production of pearls depends on its growth. To date, the molecular mechanisms of the growth of this species remain poorly understood. The transcriptome sequencing has been considered to understanding of the complexity of mechanisms of the growth of P. f. martensii. The recently released genome sequences of P. f. martensii, as well as emerging Pacific Bioscience (PacBio) single-molecular sequencing technologies, provide an opportunity to thoroughly investigate these molecular mechanisms.

RESULTS

Herein, the full-length transcriptome was analysed by combining PacBio single-molecule long-read sequencing (PacBio sequencing) and Illumina sequencing. A total of 20.65 Gb of clean data were generated, including 574,561 circular consensus reads, among which 443,944 full-length non-chimeric (FLNC) sequences were identified. Through transcript clustering analysis of FLNC reads, 32,755 consensus isoforms were identified, including 32,095 high-quality consensus sequences. After removing redundant reads, 16,388 transcripts were obtained, and 641 fusion transcripts were derived by performing fusion transcript prediction of consensus sequences. Alternative splicing analysis of the 16,388 transcripts was performed after accounting for redundancy, and 9097 gene loci were detected, including 1607 new gene loci and 14,946 newly discovered transcripts. The original boundary of 11,235 genes on the chromosomes was corrected, 12,025 complete open reading frame sequences and 635 long non-coding RNAs (LncRNAs) were predicted, and functional annotation of 13,482 new transcripts was achieved. Two thousand three hundred eighteen alternative splicing events were detected. A total of 228 differentially expressed transcripts (DETs) were identified between the largest (L) and smallest (S) pearl oysters. Compared with the S, the L showed 99 and 129 significantly up-and down-regulated DETs, respectively. Six of these DETs were further confirmed by quantitative real-time RT-PCR (RT-qPCR) in independent experiment.

CONCLUSIONS

Our results significantly improve existing gene models and genome annotations, optimise the genome structure, and in-depth understanding of the complexity and diversity of the differential growth patterns of P. f. martensii.

Elucidating the H+ Coupled Zn2+ Transport Mechanism of ZIP4; Implications in Acrodermatitis Enteropathica

Cellular Zn²⁺ homeostasis is tightly regulated and primarily mediated by designated Zn²⁺ transport proteins, namely zinc transporters (ZnTs; SLC30) that shuttle Zn²⁺ efflux, and ZRT-IRT-like proteins (ZIPs; SLC39) that mediate Zn²⁺ influx. While the functional determinants of ZnT-mediated Zn²⁺ efflux are elucidated, those of ZIP transporters are lesser understood. Previous work has suggested three distinct molecular mechanisms: (I) HCO3^- or (II) H⁺ coupled Zn²⁺ transport, or (III) a pH regulated electrodiffusional mode of transport. Here, using live-cell fluorescent imaging of Zn²⁺ and H⁺, in cells expressing ZIP4, we set out to interrogate its function. Intracellular pH changes or the presence of HCO3^- failed to induce Zn²⁺ influx. In contrast, extracellular acidification stimulated ZIP4 dependent Zn²⁺ uptake. Furthermore, Zn²⁺ uptake was coupled to enhanced H⁺ influx in cells expressing ZIP4, thus indicating that ZIP4 is not acting as a pH regulated channel but rather as an H⁺ powered Zn²⁺ co-transporter. We further illustrate how this functional mechanism is affected by genetic variants in SLC39A4 that in turn lead to Acrodermatitis enteropathica, a rare condition of Zn²⁺ deficiency.

The peptide transporter 1a of the zebrafish Danio rerio, an emerging model in nutrigenomics and nutrition research: molecular characterization, functional properties, and expression analysis

Background

Peptide transporter 1 (PepT1, alias Slc15a1) mediates the uptake of dietary di/tripeptides in all vertebrates. However, in teleost fish, more than one PepT1-type transporter might function, due to specific whole genome duplication event(s) that occurred during their evolution leading to a more complex paralogue gene repertoire than in higher vertebrates (tetrapods).

Results

Here, we describe a novel di/tripeptide transporter in the zebrafish (Danio rerio), i.e., the zebrafish peptide transporter 1a (PepT1a; also known as Solute carrier family 15 member a1, Slc15a1a), which is a paralogue (78% similarity, 62% identity at the amino acid level) of the previously described zebrafish peptide transporter 1b (PepT1b, alias PepT1; also known as Solute carrier family 15 member 1b, Slc15a1b). Also, we report a basic analysis of the pept1a (slc15a1a) mRNA expression levels in zebrafish adult tissues/organs and embryonic/early larval developmental stages. As assessed by expression in Xenopus laevis oocytes and two-electrode voltage clamp measurements, zebrafish PepT1a, as PepT1b, is electrogenic, Na⁺-independent, and pH-dependent and functions as a low-affinity system, with K_0.5 values for Gly-Gln at − 60 mV of 6.92 mmol/L at pH 7.6 and 0.24 mmol/L at pH 6.5 and at − 120 mV of 3.61 mmol/L at pH 7.6 and 0.45 mmol/L at pH 6.5. Zebrafish pept1a mRNA is highly expressed in the intestine and ovary of the adult fish, while its expression in early development undergoes a complex trend over time, with pept1a mRNA being detected 1 and 2 days post-fertilization (dpf), possibly due to its occurrence in the RNA maternal pool, decreasing at 3 dpf (~ 0.5-fold) and increasing above the 1–2 dpf levels at 4 to 7 dpf, with a peak (~ 7-fold) at 6 dpf.

Conclusions

We show that the zebrafish PepT1a-type transporter is functional and co-expressed with pept1b (slc15a1b) in the adult fish intestine. Its expression is also confirmed during the early phases of development when the yolk syncytial layer is present and yolk protein resorption processes are active. While completing the missing information on PepT1-type transporters function in the zebrafish, these results open to future investigations on the similar/differential role(s) of PepT1a/PepT1b in zebrafish and teleost fish physiology.

Lactate/GPR81 signaling and proton motive force in cancer: Role in angiogenesis, immune escape, nutrition, and Warburg phenomenon

Reprogramming of biochemical pathways is a hallmark of cancer cells, and generation of lactic acid from glucose/glutamine represents one of the consequences of such metabolic alterations. Cancer cells export lactic acid out to prevent intracellular acidification, not only increasing lactate levels but also creating an acidic pH in extracellular milieu. Lactate and protons in tumor microenvironment are not innocuous bystander metabolites but have special roles in promoting tumor-cell proliferation and growth. Lactate functions as a signaling molecule by serving as an agonist for the G-protein-coupled receptor GPR81, involving both autocrine and paracrine mechanisms. In the autocrine pathway, cancer cell-generated lactate activates GPR81 on cancer cells; in the paracrine pathway, cancer cell-generated lactate activates GPR81 on immune cells, endothelial cells, and adipocytes present in tumor stroma. The end result of GPR81 activation is promotion of angiogenesis, immune evasion, and chemoresistance. The acidic pH creates an inwardly directed proton gradient across the cancer-cell plasma membrane, which provides driving force for proton-coupled transporters in cancer cells to enhance supply of selective nutrients. There are several molecular targets in the pathways involved in the generation of lactic acid by cancer cells and its role in tumor promotion for potential development of novel anticancer therapeutics.

Fatty acids - from energy substrates to key regulators of cell survival, proliferation and effector function

Recent advances in immunology and cancer research show that fatty acids, their metabolism and their sensing have a crucial role in the biology of many different cell types. Indeed, they are able to affect cellular behaviour with great implications for pathophysiology. Both the catabolic and anabolic pathways of fatty acids present us with a number of enzymes, receptors and agonists/antagonists that are potential therapeutic targets, some of which have already been successfully pursued. Fatty acids can affect the differentiation of immune cells, particularly T cells, as well as their activation and function, with important consequences for the balance between anti- and pro-inflammatory signals in immune diseases, such as rheumatoid arthritis, psoriasis, diabetes, obesity and cardiovascular conditions. In the context of cancer biology, fatty acids mainly provide substrates for energy production, which is of crucial importance to meet the energy demands of these highly proliferating cells. Fatty acids can also be involved in a broader transcriptional programme as they trigger signals necessary for tumorigenesis and can confer to cancer cells the ability to migrate and generate distant metastasis. For these reasons, the study of fatty acids represents a new research direction that can generate detailed insight and provide novel tools for the understanding of immune and cancer cell biology, and, more importantly, support the development of novel, efficient and fine-tuned clinical interventions. Here, we review the recent literature focusing on the involvement of fatty acids in the biology of immune cells, with emphasis on T cells, and cancer cells, from sensing and binding, to metabolism and downstream effects in cell signalling.

Factors inhibiting intestinal calcium absorption: hormones and luminal factors that prevent excessive calcium uptake

Besides the two canonical calciotropic hormones, namely parathyroid hormone and 1,25-dihydroxyvitamin D [1,25(OH)2D3], there are several other endocrine and paracrine factors, such as prolactin, estrogen, and insulin-like growth factor that have been known to directly stimulate intestinal calcium absorption. Generally, to maintain an optimal plasma calcium level, these positive regulators enhance calcium absorption, which is indirectly counterbalanced by a long-loop negative feedback mechanism, i.e., through calcium-sensing receptor in the parathyroid chief cells. However, several lines of recent evidence have revealed the presence of calcium absorption inhibitors present in the intestinal lumen and extracellular fluid in close vicinity to enterocytes, which could also directly compromise calcium absorption. For example, luminal iron, circulating fibroblast growth factor (FGF)-23, and stanniocalcin can decrease calcium absorption, thereby preventing excessive calcium uptake under certain conditions. Interestingly, the intestinal epithelial cells themselves could lower their rate of calcium uptake after exposure to high luminal calcium concentration, suggesting a presence of an ultra-short negative feedback loop independent of systemic hormones. The existence of neural regulation is also plausible but this requires more supporting evidence. In the present review, we elaborate on the physiological significance of these negative feedback regulators of calcium absorption, and provide evidence to show how our body can efficiently restrict a flood of calcium influx in order to maintain calcium homeostasis.

Physiology of Electrolyte Transport in the Gut: Implications for Disease

We now have an increased understanding of the genetics, cell biology, and physiology of electrolyte transport processes in the mammalian intestine, due to the availability of sophisticated methodologies ranging from genome wide association studies to CRISPR-CAS technology, stem cell-derived organoids, 3D microscopy, electron cryomicroscopy, single cell RNA sequencing, transgenic methodologies, and tools to manipulate cellular processes at a molecular level. This knowledge has simultaneously underscored the complexity of biological systems and the interdependence of multiple regulatory systems. In addition to the plethora of mammalian neurohumoral factors and their cross talk, advances in pyrosequencing and metagenomic analyses have highlighted the relevance of the microbiome to intestinal regulation. This article provides an overview of our current understanding of electrolyte transport processes in the small and large intestine, their regulation in health and how dysregulation at multiple levels can result in disease. Intestinal electrolyte transport is a balance of ion secretory and ion absorptive processes, all exquisitely dependent on the basolateral Na⁺ /K⁺ ATPase; when this balance goes awry, it can result in diarrhea or in constipation. The key transporters involved in secretion are the apical membrane Cl^- channels and the basolateral Na⁺ -K⁺ -2Cl^- cotransporter, NKCC1 and K⁺ channels. Absorption chiefly involves apical membrane Na⁺ /H⁺ exchangers and Cl^- /HCO₃ ^- exchangers in the small intestine and proximal colon and Na⁺ channels in the distal colon. Key examples of our current understanding of infectious, inflammatory, and genetic diarrheal diseases and of constipation are provided. © 2019 American Physiological Society. Compr Physiol 9:947-1023, 2019.

Screening of PI3K-Akt-targeting Drugs for Silkworm against Bombyx mori Nucleopolyhedrovirus

Bombyx mori nucleopolyhedrovirus (BmNPV) is the most prevalent threat to silkworms. Hence, there is a need for antiviral agents in sericulture. The PI3K-Akt pathway is essential for the efficient replication of the baculovirus. In an attempt to screen antiviral drugs against BmNPV, we summarized the commercial compounds targeting PI3K-Akt and selected the following seven oral drugs for further analyses: afuresertib, AZD8835, AMG319, HS173, AS605240, GDC0941, and BEZ235. Cell viability assay revealed that the cytotoxicity of these drugs at 10 µM concentration was not strong. Viral fluorescence observation and qPCR analysis showed that these candidate drugs significantly inhibited BmNPV in BmE cells. Only AMG319 and AZD8835 inhibited viral proliferation in silkworm larvae. The mortality of AZD8835-treated silkworms was lower than that of the control silkworms. Western blotting showed that AMG319 and AZD8835 decreased p-Akt expression after BmNPV infection. These results suggest that AZD8835 has application potential in sericulture.

A glucose-insulin-potassium solution improves glucose intake in hypoxic cardiomyocytes by a differential expression of glucose transporters in a metabolic syndrome model

Among the last consequences of metabolic syndrome are cardiovascular complications such as infarcts. The hypoxic heart switches its lipid-based metabolism to carbohydrates, and a glucose-insulin-potassium (GIK) solution can be the metabolic support to protect the organ. Due to the physiology and cardiac risks associated with the metabolic syndrome, we studied the effect of GIK solution during hypoxia in a metabolic syndrome model by observing the participation of glucose transporters (GLUTs). The metabolic syndrome characteristics were established by giving a 30% sucrose drinking solution to Wistar rats for 24 weeks. The GIK solution's effect on myocyte glucose uptake during hypoxia and oxygenation was observed using a colorimetric method, and Western blot technique visualized the GLUT participation. Oxygenated control myocytes consumed 1.7 +/- 0.2 μg of glucose per gram of fresh tissue per hour using the GLUT1, and during hypoxia, they incorporated 41.1% more glucose by GLUT1 and GLUT4. The GIK solution improved glucose uptake in oxygenation by 70.5% through GLUT1. In hypoxia, the uptake was 21% more than the hypoxic control group and by both GLUTs too. Oxygenated metabolic syndrome myocytes uptake was similar to control cells but achieved by both carriers in oxygenation and hypoxia. Also, the GIK solution had a better response in both oxygenation (113%) and hypoxia (71%). Despite the metabolic energy disorders of this syndrome, the GIK solution protects cardiomyocytes, in conditions of hypoxia, through the modulation of both GLUTs. So, this solution can be considered a useful resource during a heart attack in cases of metabolic syndrome.

Amino Acid Transport Across the Mammalian Intestine

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

Loss of MYO5B Leads to Reductions in Na+ Absorption With Maintenance of CFTR-Dependent Cl- Secretion in Enterocytes

BACKGROUND & AIMS

Inactivating mutations in MYO5B cause microvillus inclusion disease (MVID), but the physiological cause of the diarrhea associated with this disease is unclear. We investigated whether loss of MYO5B results in aberrant expression of apical enterocyte transporters.

METHODS

We studied alterations in apical membrane transporters in MYO5B-knockout mice, as well as mice with tamoxifen-inducible, intestine-specific disruption of Myo5b (VilCreERT2;Myo5bflox/flox mice) or those not given tamoxifen (controls). Intestinal tissues were collected from mice and analyzed by immunostaining, immunoelectron microscopy, or cultured enteroids were derived. Functions of brush border transporters in intestinal mucosa were measured in Ussing chambers. We obtained duodenal biopsy specimens from individuals with MVID and individuals without MVID (controls) and compared transporter distribution by immunocytochemistry.

RESULTS

Compared to intestinal tissues from littermate controls, intestinal tissues from MYO5B-knockout mice had decreased apical localization of SLC9A3 (also called NHE3), SLC5A1 (also called SGLT1), aquaporin (AQP) 7, and sucrase isomaltase, and subapical localization of intestinal alkaline phosphatase and CDC42. However, CFTR was present on apical membranes of enterocytes from MYO5B knockout and control mice. Intestinal biopsies from patients with MVID had subapical localization of NHE3, SGLT1, and AQP7, but maintained apical CFTR. After tamoxifen administration, VilCreERT2;Myo5bflox/flox mice lost apical NHE3, SGLT1, DRA, and AQP7, similar to germline MYO5B knockout mice. Intestinal tissues from VilCreERT2;Myo5bflox/flox mice had increased CFTR in crypts and CFTR localized to the apical membranes of enterocytes. Intestinal mucosa from VilCreERT2;Myo5bflox/flox mice given tamoxifen did not have an intestinal barrier defect, based on Ussing chamber analysis, but did have decreased SGLT1 activity and increased CFTR activity.

CONCLUSIONS

Although trafficking of many apical transporters is regulated by MYO5B, trafficking of CFTR is largely independent of MYO5B. Decreased apical localization of NHE3, SGLT1, DRA, and AQP7 might be responsible for dysfunctional water absorption in enterocytes of patients with MVID. Maintenance of apical CFTR might exacerbate water loss by active secretion of chloride into the intestinal lumen.

Methionine to cystine ratio in the total sulfur amino acid requirements and sulfur amino acid metabolism using labelled amino acid approach for broilers

Background

Assuming that part of Methionine (Met) is converted into Cystine (Cys), but ignoring the rates with which such phenomenon occurs may lead to an excessive supply of Met in poultry diets. Such inconvenient could be easily avoided with the knowledge of the ideal Met:Cys/Total sulfur amino acids (TSAA) ratio and the rates of Met conversion into Cys.

Results

Met sources did not affect performance. Met:Cys/TSAA ideal ratio was determined using curvilinear-plateau regression model. Both optimum body weight gain and feed conversion ratio were estimated in 1007 g/day and 1.49, respectively, at 52% Met/TSAA ratio. Feed intake was not affected by Met:Cys/TSAA ratios. In the labelled amino acid assay, the rates with which Met was converted into Cys ranged from 27 to 43% in response to changes in Met:Cys/TSAA ratios, being higher at 56:44.

Conclusion

Based on performance outcomes, the minimum concentration of Met relative to Cys in diets for broilers from 14 to 28 d of age based on a TSAA basis, is 52% (52:48 Met:Cys/TSAA). The outcomes from labelled amino acid assay indicate that highest the Met supply in diets, the highest is its conversion into Cys.

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.

Short communication: Characterization of gene expression profiles related to yak milk protein synthesis during the lactation cycle

This research assessed the gene expression patterns related to the synthesis of milk in yak, which is characterized by high fat and protein content but low yield. The yak (Bos grunniens) is one of the most crucial domestic animals in Tibetan life; however, the genetic and molecular factors underlying yak milk protein synthesis remain understudied. Yak mammary biopsies harvested during late-pregnancy (d -15) through the end of subsequent lactation (d 1, 15, 30, 60, 180, and 240) were used to evaluate gene expression via real-time quantitative PCR. The expression pattern of 41 genes encompassing multiple pathways integral to milk protein synthesis including insulin, mammalian target of rapamycin (mTOR), 5' AMP-activated protein kinase, Jak2-Stat5 signaling, and the expression of glucose and AA transporters was evaluated. Our results confirmed that most upregulated genes increased from d -15 and peaked at d 30 or 60 and then remained relatively highly expressed. Specifically, there was an increased expression of mTOR-related amino acid transporters (SLC1A5, SLC7A5, and SLC36A1), glucose transporters (SLC2A1, SLC2A3, and SLC2A8), Jak2-Stat5 pathway (ELF5), and insulin signaling pathway components (IRS1, PDPK1, and AKT1). For activation of proteins synthesis, MTOR was significantly increased only at d 1. Among inhibitors of mTOR signaling, TSC1 and PRKAA2 were significantly upregulated during lactation. The RPL23 was downregulated among ribosomal components. In conclusion, a critical role for AA and glucose transporters and insulin signaling through mTOR for regulation of yak milk protein synthesis was revealed in this study of the yak mammary gland.

Mistargeting of a truncated Na-K-2Cl cotransporter in epithelial cells

We recently reported the case of a young patient with multisystem failure carrying a de novo mutation in SLC12A2, the gene encoding the Na-K-2Cl cotransporter-1 (NKCC1). Heterologous expression studies in nonepithelial cells failed to demonstrate dominant-negative effects. In this study, we examined expression of the mutant cotransporter in epithelial cells. Using Madin-Darby canine kidney (MDCK) cells grown on glass coverslips, permeabilized support, and Matrigel, we show that the fluorescently tagged mutant cotransporter is expressed in cytoplasm and at the apical membrane and affects epithelium integrity. Expression of the mutant transporter at the apical membrane also results in the mislocalization of some of the wild-type transporter to the apical membrane. This mistargeting is specific to NKCC1 as the Na⁺-K⁺-ATPase remains localized on the basolateral membrane. To assess transporter localization in vivo, we created a mouse model using CRISPR/cas9 that reproduces the 11 bp deletion in exon 22 of Slc12a2. Although the mice do not display an overt phenotype, we show that the colon and salivary gland expresses wild-type NKCC1 abundantly at the apical pole, confirming the data obtained in cultured epithelial cells. Enough cotransporter must remain, however, on the basolateral membrane to participate in saliva secretion, as no significant decrease in saliva production was observed in the mutant mice.

Novel drug-resistance mechanisms of pemetrexed-treated non-small cell lung cancer

Pemetrexed (PEM) improves the overall survival of patients with advanced non-small cell lung cancer (NSCLC) when administered as maintenance therapy. However, PEM resistance often appears during the therapy. Although thymidylate synthase is known to be responsible for PEM resistance, no other mechanisms have been investigated in detail. In this study, we explored new drug resistance mechanisms of PEM-treated NSCLC using two combinations of parental and PEM-resistant NSCLC cell lines from PC-9 and A549. PEM increased the apoptosis cells in parental PC-9 and the senescent cells in parental A549. However, such changes were not observed in the respective PEM-resistant cell lines. Quantitative RT-PCR analysis revealed that, besides an increased gene expression of thymidylate synthase in PEM-resistant PC-9 cells, the solute carrier family 19 member1 (SLC19A1) gene expression was markedly decreased in PEM-resistant A549 cells. The siRNA-mediated knockdown of SLC19A1 endowed the parental cell lines with PEM resistance. Conversely, PEM-resistant PC-9 cells carrying an epidermal growth factor receptor (EGFR) mutation acquired resistance to a tyrosine kinase inhibitor erlotinib. Although erlotinib can inhibit the phosphorylation of EGFR and Erk, it is unable to suppress the phosphorylation of Akt in PEM-resistant PC-9 cells. Additionally, PEM-resistant PC-9 cells were less sensitive to the PI3K inhibitor LY294002 than parental PC-9 cells. These results indicate that SLC19A1 negatively regulates PEM resistance in NSCLC, and that EGFR-tyrosine-kinase-inhibitor resistance was acquired with PEM resistance through Akt activation in NSCLC harboring EGFR mutations.

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.

Effect of short-chain fatty acids on the expression of genes involved in short-chain fatty acid transporters and inflammatory response in goat jejunum epithelial cells

Short-chain fatty acids (SCFAs) produced by microbial fermentation of dietary fibers are utilized by intestinal epithelial cells to provide an energy source for the ruminant. Although the regulation of mRNA expression and inflammatory response involved in SCFAs is established in other animals and tissues, the underlying mechanisms of the inflammatory response by SCFAs in goat jejunum epithelial cells (GJECs) have not been understood. Therefore, the objective of the study is to investigate the underlying mechanisms of the effects of SCFAs on SCFA transporters and inflammatory response in GJECs. These results showed that the acetate, butyrate, and SCFA concentration were markedly reduced in GJECs (p < 0.01). In addition, the propionate concentration was significantly decreased in GJECs (p < 0.05). The mRNA abundance of monocarboxylate transporter 1 (MCT1), MCT4, NHE1, and putative anion transporter 1 (PAT1) was elevated (p < 0.05) by 20 mM SCFAs at pH 7.4 compared with exposure to the pH group. The anion exchanger 2 (AE2) was increased (p < 0.05) by 20 mM SCFAs at pH 6.2. The mRNA abundance of vH⁺ ATPase B subunit (vH⁺ ATPase) was attenuated by SCFAs. For inflammatory responses, IL-1β and TNF-α were increased with SCFAs (p < 0.05). In addition, IκBα involved in NF-κB signaling pathways was disrupted by SCFAs. Consistently, p-p65 signaling molecule was enhanced by adding SCFAs. However, IL-6 was attenuated by adding SCFAs (p < 0.05). Furthermore, p-ErK1/2 mitogen-activated protein kinase (MAPK) signaling pathway was downregulated by adding SCFAs. In conclusion, these novel findings demonstrated that mRNA abundance involved in SCFA absorption is probably associated to SCFAs and pH value, and mechanism of the inflammatory response by SCFAs may be involved in NF-κB and p-ErK1/2 MAPK signaling pathways in GJECs. These pathways may mediate protective inflammation response in GJECs.

Interactions of aliskiren, a direct renin inhibitor, with antiepileptic drugs in the test of maximal electroshock in mice

Experimental studies showed that certain angiotensin-converting enzyme inhibitors and angiotensin AT₁ receptor antagonists can decrease seizure severity in rodents. Additionally, some of these blockers of the renin-angiotensin system have been documented to enhance the anticonvulsant activity of antiepileptic drugs against maximal electroshock-induced seizures. The aim of the current study was to investigate the effect of aliskiren, a direct renin inhibitor and a novel antihypertensive drug, on the protective action of numerous antiepileptic drugs (carbamazepine, valproate, clonazepam, phenobarbital, oxcarbazepine, lamotrigine, topiramate and pregabalin) in the test of maximal electroshock in mice. The examined drugs were administered intraperitoneally. Aliskiren up to a dose of 75mg/kg did not affect the threshold for electroconvulsions, however, aliskiren (75mg/kg) enhanced the anticonvulsant action of clonazepam and valproate. Following aliskiren treatment, a higher brain concentration of valproate was noted, suggesting a pharmacokinetic interaction. In the rota-rod test, the concomitant treatment with aliskiren (50 or 75mg/kg) and clonazepam (22.6mg/kg) impaired motor coordination while clonazepam (22.6mg/kg) alone showed strong tendency towards this impairment. The combination of aliskiren (75mg/kg) with phenobarbital (25.5mg/kg) caused long-term memory deficits in the passive avoidance task. This study shows that there are no negative interactions between aliskiren and the examined antiepileptic drugs as concerns their anticonvulsant activity. Aliskiren even potentiated the anticonvulsant action of clonazepam and valproate against maximal electroshock. The impact of aliskiren alone on seizure activity or on the anticonvulsant and adverse activity of antiepileptic drugs needs further evaluation in other animal models of seizures.

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

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