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

Functional Characterization of the Lysosomal Peptide/Histidine Transporter PHT1 (SLC15A4) by Solid Supported Membrane Electrophysiology (SSME)

The peptide/histidine transporter PHT1 (SLC15A4) is expressed in the lysosomal membranes of immune cells where it plays an important role in metabolic and inflammatory signaling. PHT1 is an H+-coupled/histidine symporter that can transport a wide range of oligopeptides, including a variety of bacterial-derived peptides. Moreover, it enables the scaffolding of various metabolic signaling molecules and interacts with key regulatory elements of the immune response. Not surprisingly, PHT1 has been implicated in the pathogenesis of autoimmune diseases such as systemic lupus erythematosus (SLE). Unfortunately, the pharmacological development of PHT1 modulators has been hampered by the lack of suitable transport assays. To address this shortcoming, a novel transport assay based on solid-supported membrane-based electrophysiology (SSME) is presented. Key findings of the present SSME studies include the first recordings of electrophysiological properties, a pH dependence analysis, an assessment of PHT1 substrate selectivity, as well as the transport kinetics of the identified substrates. In contrast to previous work, PHT1 is studied in its native lysosomal environment. Moreover, observed substrate selectivity is validated by molecular docking. Overall, this new SSME-based assay is expected to contribute to unlocking the pharmacological potential of PHT1 and to deepen the understanding of its functional properties.

Jejunal transcriptomic profiling of carnosine synthesis precursor-related genes and pathways in slow-growing Korat chicken

Carnosine is a physiologically important molecule in normal human body functions. Chicken meat is an excellent source of carnosine; especially slow-growing Korat chicken (KR) females have a high carnosine content in their meat. The carnosine content of chicken meat can be increased by dietary supplementation of β-alanine (βA) and L-histidine (L-His). Our objective was to reveal the pathways and genes through jejunal transcriptomic profiling related to βA and L-His absorption and transportation. We collected whole jejunum samples from 5 control and 5 experimental KR chicken, fed with 1% βA and 0.5% L-His supplementation. A total of 407 differentially expressed genes (P < 0.05, log2 fold change ≥2) were identified, 272 of which were down-regulated and 135 up-regulated in the group with dietary supplementation compared to the control group. Based on the integrated analysis of the protein-protein interaction network and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway maps, 87 gene ontology terms were identified and 6 KEGG pathways were significantly (P < 0.05) enriched in the jejunum. The analyses revealed 6 key genes, KCND3, OPRM1, CCK, GCG, TRH, and GABBR2, that are related to neuroactive ligand-receptor interaction and the calcium signaling pathway. These findings give insight regarding the molecular mechanism related to carnosine precursor absorption and transportation in the jejunum and help to identify useful molecular markers for improving the carnosine content in slow-growing KR chicken meat.

Peptide Transporter 1-Mediated Dipeptide Transport Promotes Hepatocellular Carcinoma Metastasis by Activating MAP4K4/G3BP2 Signaling Axis

Cancer metastasis is the leading cause of mortality in patients with hepatocellular carcinoma (HCC). To meet the rapid malignant growth and transformation, tumor cells dramatically increase the consumption of nutrients, such as amino acids. Peptide transporter 1 (PEPT1), a key transporter for small peptides, has been found to be an effective and energy-saving intracellular source of amino acids that are required for the growth of tumor cells. Here, the role of PEPT1 in HCC metastasis and its underlying mechanisms is explored. PEPT1 is upregulated in HCC cells and tissues, and high PEPT1 expression is associated with poor prognosis in patients with HCC. PEPT1 overexpression dramatically promoted HCC cell migration, invasion, and lung metastasis, whereas its knockdown abolished these effects both in vitro and in vivo. Mechanistic analysis revealed that high PEPT1 expression increased cellular dipeptides in HCC cells that are responsible for activating the MAP4K4/G3BP2 signaling pathway, ultimately facilitating the phosphorylation of G3BP2 at Thr227 and enhancing HCC metastasis. Taken together, these findings suggest that PEPT1 acts as an oncogene in promoting HCC metastasis through dipeptide-induced MAP4K4/G3BP2 signaling and that the PEPT1/MAP4K4/G3BP2 axis can serve as a promising therapeutic target for metastatic HCC.

SLC15A3 plays a crucial role in pulmonary fibrosis by regulating macrophage oxidative stress

Idiopathic pulmonary fibrosis (IPF) is a fatal and irreversible disease with few effective treatments. Alveolar macrophages (AMs) are involved in the development of IPF from the initial stages due to direct exposure to air and respond to external oxidative damage (a major inducement of pulmonary fibrosis). Oxidative stress in AMs plays an indispensable role in promoting fibrosis development. The oligopeptide histidine transporter SLC15A3, mainly expressed on the lysosomal membrane of macrophages and highly expressed in the lung, has proved to be involved in innate immune and antiviral signaling pathways. In this study, we demonstrated that during bleomycin (BLM)- or radiation-induced pulmonary fibrosis, the recruitment of macrophages induced an increase of SLC15A3 in the lung, and the deficiency of SLC15A3 protected mice from pulmonary fibrosis and maintained the homeostasis of the pulmonary microenvironment. Mechanistically, deficiency of SLC15A3 resisted oxidative stress in macrophages, and SLC15A3 interacted with the scaffold protein p62 to regulate its expression and phosphorylation activation, thereby regulating p62-nuclear factor erythroid 2-related factor 2 (NRF2) antioxidant stress pathway protein, which is related to the production of reactive oxygen species (ROS). Overall, our data provided a novel mechanism for targeting SLC15A3 to regulate oxidative stress in macrophages, supporting the therapeutic potential of inhibiting or silencing SLC15A3 for the precautions and treatment of pulmonary fibrosis.

Simultaneous Prediction Method for Intestinal Absorption and Metabolism Using the Mini-Ussing Chamber System

Many evaluation tools for predicting human absorption are well-known for using cultured cell lines such as Caco-2, MDCK, and so on. Since the combinatorial chemistry and high throughput screening system, pharmacological assay, and pharmaceutical profiling assay are mainstays of drug development, PAMPA has been used to evaluate human drug absorption. In addition, cultured cell lines from iPS cells have been attracting attention because they morphologically resemble human intestinal tissues. In this review, we used human intestinal tissues to estimate human intestinal absorption and metabolism. The Ussing chamber uses human intestinal tissues to directly assay a drug candidate's permeability and determine the electrophysiological parameters such as potential differences (PD), short circuit current (Isc), and resistance (R). Thus, it is an attractive tool for elucidating human intestinal permeability and metabolism. We have presented a novel prediction method for intestinal absorption and metabolism by utilizing a mini-Ussing chamber using human intestinal tissues and animal intestinal tissues, based on the transport index (TI). The TI value was calculated by taking the change in drug concentrations on the apical side due to precipitation and the total amounts accumulated in the tissue (Tcorr) and transported to the basal side (Xcorr). The drug absorbability in rank order, as well as the fraction of dose absorbed (Fa) in humans, was predicted, and the intestinal metabolism of dogs and rats was also predicted, although it was not quantitative. However, the metabolites formation index (MFI) values, which are included in the TI values, can predict the evaluation of intestinal metabolism and absorption by using ketoconazole. Therefore, the mini-Ussing chamber, equipped with human and animal intestinal tissues, would be an ultimate method to predict intestinal absorption and metabolism simultaneously.

Oligopeptide/Histidine Transporter PHT1 and PHT2 - Function, Regulation, and Pathophysiological Implications Specifically in Immunoregulation

The oligopeptide/histidine transporters PHT1 and PHT2, two mammalian solute carrier family 15A proteins, mediate the transmembrane transport of histidine and some di/tripeptides via proton gradient. PHT1 and PHT2 are distributed in a variety of tissues but are preferentially expressed in immune cells and localize to the lysosome-related organelles. Studies have reported the relationships between PHT1/PHT2 and immune diseases. PHT1 and PHT2 participate in the regulation of lysosomal homeostasis and lysosome-associated signaling pathways through their transport and nontransport functions, playing important roles in inflammatory diseases. In this review, we summarize recent research on PHT1 and PHT2, aiming to provide reference for their further biological research and as targets for drug design.

Molecular Insights to the Structure-Interaction Relationships of Human Proton-Coupled Oligopeptide Transporters (PepTs)

Human proton-coupled oligopeptide transporters (PepTs) are important membrane influx transporters that facilitate the cellular uptake of many drugs including ACE inhibitors and antibiotics. PepTs mediate the absorption of di- and tri-peptides from dietary proteins or gastrointestinal secretions, facilitate the reabsorption of peptide-bound amino acids in the kidney, and regulate neuropeptide homeostasis in extracellular fluids. PepT1 and PepT2 have been the most intensively investigated of all PepT isoforms. Modulating the interactions of PepTs and their drug substrates could influence treatment outcomes and adverse effects with certain therapies. In recent studies, topology models and protein structures of PepTs have been developed. The aim of this review was to summarise the current knowledge regarding structure-interaction relationships (SIRs) of PepTs and their substrates as well as the potential applications of this information in therapeutic optimisation and drug development. Such information may provide insights into the efficacy of PepT drug substrates in patients, mechanisms of drug-drug/food interactions and the potential role of PepTs targeting in drug design and development strategies.

Plasticity of the binding pocket in peptide transporters underpins promiscuous substrate recognition

Proton-dependent oligopeptide transporters (POTs) are promiscuous transporters of the major facilitator superfamily that constitute the main route of entry for a wide range of dietary peptides and orally administrated peptidomimetic drugs. Given their clinical and pathophysiological relevance, several POT homologs have been studied extensively at the structural and molecular level. However, the molecular basis of recognition and transport of diverse peptide substrates has remained elusive. We present 14 X-ray structures of the bacterial POT DtpB in complex with chemically diverse di- and tripeptides, providing novel insights into the plasticity of the conserved central binding cavity. We analyzed binding affinities for more than 80 peptides and monitored uptake by a fluorescence-based transport assay. To probe whether all 8400 natural di- and tripeptides can bind to DtpB, we employed state-of-the-art molecular docking and machine learning and conclude that peptides with compact hydrophobic residues are the best DtpB binders.

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.

Development, characterization and in vivo zinc absorption capacity of a novel soy meal hydrolysate-zinc complexes

Background

Zinc is an essential trace element for the human body. Recently, a novel Zn-binding peptide, Lys-Tyr-Lys-Arg-Gln-Arg-Trp (PP), was purified and identified from soy protein hydrolysates with high Zn-binding capacity (83.21 ± 2.65%) by our previous study. The preparation of soy meal hydrolysates (SMHs)-Zn complexes is convenient and low-cost, while PP (Lys-Tyr-Lys-Arg-Gln-Arg-Trp)-Zn complexes have a higher coordination rate but a relatively high cost. The aim of this study was to investigate the effect of soy meal hydrolysates (SMHs)-Zn complexes on zinc absorption in mice model, and synthetic soy peptide (PP)-Zn complexes with high Zn-binding capacity were used as control. Firstly, SMHs were prepared by enzymolysis, and the PP (Lys-Tyr-Lys-Arg-Gln-Arg-Trp) were synthesized based on previous studies. The binding mechanism of soy hydrolysates and zinc was analyzed by spectral analysis. Furthermore, the cytotoxicity of the SMHs-Zn complexes was also studied using the CCK-8 method. The effect of zinc absorption was evaluated based on Zn content, total protein and albumin content, relevant enzyme system, and the PeT1 and ZnT1 mRNA expression levels.

Result

The result showed that zinc was bound with carboxyl oxygen and amino nitrogen atoms on SMHs, with hydrophobic and electrostatic interactions as auxiliary stabilizing forces. SMHs-Zn were proved to have great solubility and a small particle size at different pH values, and it showed a beneficial effect on Caco-2 cells growth. Moreover, it was proved that SMHs-Zn and PP-Zn could increase the levels of zinc and the activity of Zn-related enzymes in mice. SMHs-Zn possessed higher PepT1 and ZnT1 mRNA expression levels than PP-Zn in the small intestine.

Conclusion

SMHs-Zn with a lower Zn-binding capacity had similar effects on zinc absorption in mice as PP-Zn, suggesting that the bioavailability of peptide-zinc complexes in mice was not completely dependent on their Zn-binding capacity, but may also be related to the amino acid composition.

Quercetin Alleviates Inflammation and Energy Deficiency Induced by Lipopolysaccharide in Chicken Embryos

Energy deficiency causes multiple organ dysfunctions after LPS induction. Quercetin is a phenolic compound found in herbal medicines. However, the effects of quercetin in alleviating LPS-induced energy deficiency remain unclear. In the present study, an in vivo LPS-induced inflammation model was established in chicken embryos. Specific pathogen-free chicken embryos (n = 120) were allocated to control, PBS with or without ethanol, quercetin (10, 20, or 40 nmol, respectively), and LPS (125 ng/egg) with or without quercetin groups. Fifteen day old embryonated eggs were injected with the abovementioned solutions via the allantoic cavity. On embryonic day 19, the tissues of the embryos were collected for histopathological examination using frozen oil red O staining, RNA extraction, real-time quantitative polymerase chain reaction, and immunohistochemical investigations. The glycogen and lipid contents in the liver increased after LPS stimulation as compared with the PBS group, whereas quercetin decreased the accumulation as compared with the LPS group. The mRNA expressions of AMPKα1 and AMPKα2 in the duodena, ceca, and livers were upregulated after LPS induction as compared with the PBS group, while quercetin could downregulate these expressions as compared with the LPS group. The immunopositivity of AMPKα2 in the villus, crypt, lamina propria, tunica muscularis, and myenteric plexus in the duodena and in the cytoplasms of hepatocytes significantly increased after LPS induction when compared with the PBS group (p < 0.01), whereas the immunopositivity to AMPKα2 in the quercetin treatment group significantly decreased when compared with the LPS group (p < 0.01 or p < 0.05). The LPS-induced high expressions of transcription factor PPARα and glucose transporter (SGLT1) were blocked by quercetin in the duodena, ceca, and livers. Quercetin treatment improved the LPS-induced decrease in APOA4 in the duodena, ceca, and livers. The mRNA expression of PEPT1 in the duodena and ceca increased after LPS challenge, whereas quercetin could downregulate PEPT1 gene expression. These data demonstrate that quercetin improved the energy deficiency induced by LPS in chicken embryos. The LPS-induced inflammation model was established to avoid the effect of LPS exposure from the environment and intestinal flora. The results form the basis the administration of quercetin pretreatment (in ovo infection) to improve the energy state of chicken embryos and improve the inflammation response.

Pharmacokinetics of Antibacterial Agents in the Elderly: The Body of Evidence

Infections are important factors contributing to the morbidity and mortality among elderly patients. High rates of consumption of antimicrobial agents by the elderly may result in increased risk of toxic reactions, deteriorating functions of various organs and systems and leading to the prolongation of hospital stay, admission to the intensive care unit, disability, and lethal outcome. Both safety and efficacy of antibiotics are determined by the values of their plasma concentrations, widely affected by physiologic and pathologic age-related changes specific for the elderly population. Drug absorption, distribution, metabolism, and excretion are altered in different extents depending on functional and morphological changes in the cardiovascular system, gastrointestinal tract, liver, and kidneys. Water and fat content, skeletal muscle mass, nutritional status, use of concomitant drugs are other determinants of pharmacokinetics changes observed in the elderly. The choice of a proper dosing regimen is essential to provide effective and safe antibiotic therapy in terms of attainment of certain pharmacodynamic targets. The objective of this review is to perform a structure of evidence on the age-related changes contributing to the alteration of pharmacokinetic parameters in the elderly.

The functional and regulatory entities underlying free and peptide-bound amino acid transporters in the bovine mammary gland

Free and peptide-bound AA act as building blocks and key regulators of milk protein. To improve milk protein production, mammary epithelial cells of lactating mammals require extensive AA movement across the plasma membrane via multiple transport systems. Recent studies on bovine mammary cells/tissues have expanded the number of AA transporter systems identified and the knowledge on their contribution to AA utilization for milk protein synthesis and the regulatory machinery. However, in lactating cows, the exact intracellular location of mammary AA transporters and the extent of mammary net AA utilization for milk protein production remain unclear. This review highlights the existing knowledge on various characteristics, such as substrate specificity, kinetics, their effects on AA uptake and utilization, and regulatory mechanism, of recently examined bovine mammary free and peptide-bound AA transporters.

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.

Amyotrophic Lateral Sclerosis Pathoetiology and Pathophysiology: Roles of Astrocytes, Gut Microbiome, and Muscle Interactions via the Mitochondrial Melatonergic Pathway, with Disruption by Glyphosate-Based Herbicides

The pathoetiology and pathophysiology of motor neuron loss in amyotrophic lateral sclerosis (ALS) are still to be determined, with only a small percentage of ALS patients having a known genetic risk factor. The article looks to integrate wider bodies of data on the biological underpinnings of ALS, highlighting the integrative role of alterations in the mitochondrial melatonergic pathways and systemic factors regulating this pathway across a number of crucial hubs in ALS pathophysiology, namely glia, gut, and the muscle/neuromuscular junction. It is proposed that suppression of the mitochondrial melatonergic pathway underpins changes in muscle brain-derived neurotrophic factor, and its melatonergic pathway mimic, N-acetylserotonin, leading to a lack of metabolic trophic support at the neuromuscular junction. The attenuation of the melatonergic pathway in astrocytes prevents activation of toll-like receptor agonists-induced pro-inflammatory transcription factors, NF-kB, and yin yang 1, from having a built-in limitation on inflammatory induction that arises from their synchronized induction of melatonin release. Such maintained astrocyte activation, coupled with heightened microglia reactivity, is an important driver of motor neuron susceptibility in ALS. Two important systemic factors, gut dysbiosis/permeability and pineal melatonin mediate many of their beneficial effects via their capacity to upregulate the mitochondrial melatonergic pathway in central and systemic cells. The mitochondrial melatonergic pathway may be seen as a core aspect of cellular function, with its suppression increasing reactive oxygen species (ROS), leading to ROS-induced microRNAs, thereby altering the patterning of genes induced. It is proposed that the increased occupational risk of ALS in farmers, gardeners, and sportsmen and women is intimately linked to exposure, whilst being physically active, to the widely used glyphosate-based herbicides. This has numerous research and treatment implications.

Transport of Biologically Active Ultrashort Peptides Using POT and LAT Carriers

Ultrashort peptides (USPs), consisting of 2–7 amino-acid residues, are a group of signaling molecules that regulate gene expression and protein synthesis under normal conditions in various diseases and ageing. USPs serve as a basis for the development of drugs with a targeted mechanism of action. The purpose of this review is to systematize the available data on USP transport involving POT and LAT transporters in various organs and tissues under normal, pathological and ageing conditions. The carriers of the POT family (PEPT1, PEPT2, PHT1, PHT2) transport predominantly di- and tripeptides into the cell. Methods of molecular modeling and physicochemistry have demonstrated the ability of LAT1 to transfer not only amino acids but also some di- and tripeptides into the cell and out of it. LAT1 and 2 are involved in the regulation of the antioxidant, endocrine, immune and nervous systems’ functions. Analysis of the above data allows us to conclude that, depending on their structure, di- and tripeptides can be transported into the cells of various tissues by POT and LAT transporters. This mechanism is likely to underlie the tissue specificity of peptides, their geroprotective action and effectiveness in the case of neuroimmunoendocrine system disorders.

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.

Functional characterization of Atlantic salmon (Salmo salar L.) PepT2 transporters

Abstract

The high‐affinity/low‐capacity system Slc15a2 (PepT2) is responsible for the reuptake of di/tripeptides from the renal proximal tubule, but it also operates in many other tissues and organs. Information regarding PepT2 in teleost fish is limited and, to date, functional data are available from the zebrafish (Danio rerio) only. Here, we report the identification of two slc15a2 genes in the Atlantic salmon (Salmo salar) genome, namely slc15a2a and slc15a2b. The two encoded PepT2 proteins share 87% identity and resemble both structurally and functionally the canonical vertebrate PepT2 system. The mRNA tissue distribution analyses reveal a widespread distribution of slc15a2a transcripts, being more abundant in the brain and gills, while slc15a2b transcripts are mainly expressed in the kidney and the distal part of the gastrointestinal tract. The function of the two transporters was investigated by heterologous expression in Xenopus laevis oocytes and two‐electrode voltage‐clamp recordings of transport and presteady‐state currents. Both PepT2a and PepT2b in the presence of Gly‐Gln elicit pH‐dependent and Na⁺ independent inward currents. The biophysical and kinetic analysis of the recorded currents defined the transport properties, confirming that the two Atlantic salmon PepT2 proteins behave as high‐affinity/low‐capacity transporters. The recent structures and the previous kinetic schemes of rat and human PepT2 qualitatively account for the characteristics of the two Atlantic salmon proteins. This study is the first to report on the functional expression of two PepT2‐type transporters that operate in the same vertebrate organism as a result of (a) gene duplication process(es).

Key points

Two slc15a2‐type genes, slc15a2a and slc15a2b coding for PepT2‐type peptide transporters were found in the Atlantic salmon.

slc15a2a transcripts, widely distributed in the fish tissues, are abundant in the brain and gills, while slc15a2b transcripts are mainly expressed in the kidney and distal gastrointestinal tract.

Amino acids involved in vertebrate Slc15 transport function are conserved in PepT2a and PepT2b proteins.

Detailed kinetic analysis indicates that both PepT2a and PepT2b operate as high‐affinity transporters.

The kinetic schemes and structures proposed for the mammalian models of PepT2 are suitable to explain the function of the two Atlantic salmon transporters.

Structural snapshots of human PepT1 and PepT2 reveal mechanistic insights into substrate and drug transport across epithelial membranes

The uptake of peptides in mammals plays a crucial role in nutrition and inflammatory diseases. This process is mediated by promiscuous transporters of the solute carrier family 15, which form part of the major facilitator superfamily. Besides the uptake of short peptides, peptide transporter 1 (PepT1) is a highly abundant drug transporter in the intestine and represents a major route for oral drug delivery. PepT2 also allows renal drug reabsorption from ultrafiltration and brain-to-blood efflux of neurotoxic compounds. Here, we present cryogenic electron microscopy (cryo-EM) structures of human PepT1 and PepT2 captured in four different states throughout the transport cycle. The structures reveal the architecture of human peptide transporters and provide mechanistic insights into substrate recognition and conformational transitions during transport. This may support future drug design efforts to increase the bioavailability of different drugs in the human body.

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.

Specifically Targeted Transport of Plasma Membrane Transporters: From Potential Mechanisms for Regulating Cell Health or Disease to Applications

Normal substrate transport and signal transmission are the premise to ensure the health of biological somatic cells. Therefore, a comprehensive understanding of the molecular mechanism of intercellular substrate transport is of great significance for clinical treatment. In order to better understand the membrane protein through its interaction with receptors, to help maintain a healthy cell and the molecular mechanisms of disease, in this paper, we seek to clarify, first of all, the recognition mechanism for different types of membrane protein receptors; pathogen invasion using the transport pathway involved in the membrane; and the latest specific target sites of various kinds of membrane transport carriers; to provide an explanation and summary of the system. Secondly, the downstream receptor proteins and specific substrates of different membrane transporters were classified systematically; the functional differences of different subclasses and their relationship with intracellular transport disorders were analyzed to further explore the potential relationship between cell transport disorders and diseases. Finally, the paper summarizes the use of membrane transporter-specific targets for drug design and development from the latest research results; it points out the transporter-related results in disease treatment; the application prospects and the direction for drug development and disease treatment providing a new train of thought; also for disease-specific targeted therapy, it provides a certain reference value.

A Novel UPLC-MS/MS Method Identifies Organ-Specific Dipeptide Profiles

BACKGROUND

Amino acids have a central role in cell metabolism, and intracellular changes contribute to the pathogenesis of various diseases, while the role and specific organ distribution of dipeptides is largely unknown.

METHOD

We established a sensitive, rapid and reliable UPLC-MS/MS method for quantification of 36 dipeptides. Dipeptide patterns were analyzed in brown and white adipose tissues, brain, eye, heart, kidney, liver, lung, muscle, sciatic nerve, pancreas, spleen and thymus, serum and urine of C57BL/6N wildtype mice and related to the corresponding amino acid profiles.

RESULTS

A total of 30 out of the 36 investigated dipeptides were detected with organ-specific distribution patterns. Carnosine and anserine were most abundant in all organs, with the highest concentrations in muscles. In liver, Asp-Gln and Ala-Gln concentrations were high, in the spleen and thymus, Glu-Ser and Gly-Asp. In serum, dipeptide concentrations were several magnitudes lower than in organ tissues. In all organs, dipeptides with C-terminal proline (Gly-Pro and Leu-Pro) were present at higher concentrations than dipeptides with N-terminal proline (Pro-Gly and Pro-Leu). Organ-specific amino acid profiles were related to the dipeptide profile with several amino acid concentrations being related to the isomeric form of the dipeptides. Aspartate, histidine, proline and serine tissue concentrations correlated with dipeptide concentrations, when the amino acids were present at the C- but not at the N-terminus.

CONCLUSION

Our multi-dipeptide quantification approach demonstrates organ-specific dipeptide distribution. This method allows us to understand more about the dipeptide metabolism in disease or in healthy state.

Expression of the Tas1r3 and Pept1 genes in the digestive tract of wagyu cattle

Animals have precise recognition systems for amino acids and peptides that regulate their feeding behavior as well as metabolic responses. Because of their particular gastrointestinal structure, ruminants are expected to have unique mechanisms of amino acid regulation in the digestive tract. To better understand these mechanisms in the ruminant digestive tract, the expression of Tas1r3 and Pept1 was studied along the gastrointestinal tract of Japanese Black cattle through quantitative RT-PCR and immunohistochemistry. Tas1r3 mRNA was detected ubiquitously along the gastrointestinal tract, and the most predominant expression was observed in the reticulum. In addition, the presence of Tas1r3 receptor was confirmed in the rumen through immunohistochemistry. The expression level of Pept1 mRNA was higher in the forestomach (rumen, reticulum, and omasum) and small intestine (duodenum) than that in the tongue, and predominant expression was observed in the rumen. By contrast, a negligible amount of Pept1 mRNA was detected in the abomasum and large intestine. Further studies on the roles of Tas1r3 and Pept1 in the digestive tract, in particular, in the four components of the stomach, will help us to understand the mechanisms of amino acids regulation in ruminants and provide the basis for formulating cattle diets to improve the health and productivity of cattle.

Impact of PepT1 deletion on microbiota composition and colitis requires multiple generations

Numerous studies of knockout mice find impacts on microbiota composition that influence host phenotype. However, such differences can vanish when KO mice are compared directly to WT littermates, suggesting these differences do not reflect the genetic deletion per se but microbiota composition drifting over generations. Hence, our hypothesis that absence of di/tri-peptide transporter PepT1 altered microbiota composition resulting in resistance to colitis compelled scrutiny. In this study, we used PepT1^−/− and WT founder mice bred separately for multiple generations. Such mice were then bred to each other to generate F1 PepT1^−/− and WT littermates, which were then bred within their genotype to generate F2, F3, and F4, offspring. Here we report that founder PepT1^−/− mice were, relative to their WT counterparts, resistant to DSS colitis. Such resistance was associated with alterations in gut microbiota, which, when transplanted to germfree mice, was sufficient to transfer resistance to colitis. Such differences were not observed when comparing F1 PepT1^−/− to F1 WT littermates but rather, returned gradually over subsequent generations such that, relative to their F4 WT controls, F4 PepT1^−/− displayed microbiota composition and colitis-resistant phenotype nearly identical to the founder PepT1^−/− mice. Our findings indicate a role for PepT1 in influencing microbiota composition and, consequently, proneness to colitis and cancer. Overall, our study indicates that littermate-controlled experiments can be insufficient for assessing microbiota-dependent phenotypes and prevent a full comprehension of genotype-driven phenomena. Rather, impact of a single genetic alteration on microbiota and host phenotype may take generations to manifest.

A PepT1 mediated medicinal nano-system for targeted delivery of cyclosporine A to alleviate acute severe ulcerative colitis

To effectively alleviate acute severe ulcerative colitis (ASUC), we developed a colon-specific delivery system-PLGA-KPV/MMT/CS multifunctional medicinal nanoparticles loaded with cyclosporine A (CyA). The lysine-proline-valine (KPV) tripeptide, which possesses anti-inflammatory properties and high affinity to peptide transporter 1 (PepT1), can target therapy-related cells (colonic epithelial cells and macrophages) via overexpression of PepT1. Montmorillonite (MMT)/chitosan (CS) coating can reduce CyA leakage in the upper gastrointestinal tract (GIT) and enhance nanoparticle adhesion to the inflamed colon. The bio-distribution demonstrated that nanoparticles can specifically accumulate in the inflamed tissues and can be retained for up to 36 h. After being treated with the CyA-PLGA-KPV/MMT/CS nanoparticles (PKMCN), the mice with DSS-induced ulcerative colitis exhibited significant improvements in body weight, colon length, and disease activity index. Moreover, biochemistry and immunohistochemical analysis showed that the PKMCN treatment group performed as well as the healthy group. Intriguingly, PKMCN without CyA also presented marked therapeutic effects. Our results suggested that PKMCN could be a promising drug delivery system for ASUC therapy by targeting inflamed cells, prolonging curative time, and mitigating colitis.

Amino acid transporters revisited: New views in health and disease

Amino acid transporters (AATs) are membrane-bound transport proteins that mediate transfer of amino acids into and out of cells or cellular organelles. AATs have diverse functional roles ranging from neurotransmission to acid-base balance, intracellular energy metabolism, and anabolic and catabolic reactions. In cancer cells and diabetes, dysregulation of AATs leads to metabolic reprogramming, which changes intracellular amino acid levels, contributing to the pathogenesis of cancer, obesity and diabetes. Indeed, the neutral amino acid transporters (NATs) SLC7A5/LAT1 and SLC1A5/ASCT2 are likely involved in several human malignancies. However, a clinical therapy that directly targets AATs has not yet been developed. The purpose of this review is to highlight the structural and functional diversity of AATs, their diverse physiological roles in different tissues and organs, their wide-ranging implications in human diseases and the emerging strategies and tools that will be necessary to target AATs therapeutically.