Expression of PEPT2 peptide transporter mRNA and protein in glial cells of rat dorsal root ganglia

The Emerging Roles of γ-Glutamyl Peptides Produced by γ-Glutamyltransferase and the Glutathione Synthesis System

L-γ-Glutamyl-L-cysteinyl-glycine is commonly referred to as glutathione (GSH); this ubiquitous thiol plays essential roles in animal life. Conjugation and electron donation to enzymes such as glutathione peroxidase (GPX) are prominent functions of GSH. Cellular glutathione balance is robustly maintained via regulated synthesis, which is catalyzed via the coordination of γ-glutamyl-cysteine synthetase (γ-GCS) and glutathione synthetase, as well as by reductive recycling by glutathione reductase. A prevailing short supply of L-cysteine (Cys) tends to limit glutathione synthesis, which leads to the production of various other γ-glutamyl peptides due to the unique enzymatic properties of γ-GCS. Extracellular degradation of glutathione by γ-glutamyltransferase (GGT) is a dominant source of Cys for some cells. GGT catalyzes the hydrolytic removal of the γ-glutamyl group of glutathione or transfers it to amino acids or to dipeptides outside cells. Such processes depend on an abundance of acceptor substrates. However, the physiological roles of extracellularly preserved γ-glutamyl peptides have long been unclear. The identification of γ-glutamyl peptides, such as glutathione, as allosteric modulators of calcium-sensing receptors (CaSRs) could provide insights into the significance of the preservation of γ-glutamyl peptides. It is conceivable that GGT could generate a new class of intercellular messaging molecules in response to extracellular microenvironments.

Critical Roles of the Cysteine-Glutathione Axis in the Production of γ-Glutamyl Peptides in the Nervous System

γ-Glutamyl moiety that is attached to the cysteine (Cys) residue in glutathione (GSH) protects it from peptidase-mediated degradation. The sulfhydryl group of the Cys residue represents most of the functions of GSH, which include electron donation to peroxidases, protection of reactive sulfhydryl in proteins via glutaredoxin, and glutathione conjugation of xenobiotics, whereas Cys-derived sulfur is also a pivotal component of some redox-responsive molecules. The amount of Cys that is available tends to restrict the capacity of GSH synthesis. In in vitro systems, cystine is the major form in the extracellular milieu, and a specific cystine transporter, xCT, is essential for survival in most lines of cells and in many primary cultivated cells as well. A reduction in the supply of Cys causes GPX4 to be inhibited due to insufficient GSH synthesis, which leads to iron-dependent necrotic cell death, ferroptosis. Cells generally cannot take up GSH without the removal of γ-glutamyl moiety by γ-glutamyl transferase (GGT) on the cell surface. Meanwhile, the Cys-GSH axis is essentially common to certain types of cells; primarily, neuronal cells that contain a unique metabolic system for intercellular communication concerning γ-glutamyl peptides. After a general description of metabolic processes concerning the Cys-GSH axis, we provide an overview and discuss the significance of GSH-related compounds in the nervous system.

Transporter-Mediated Drug Delivery

Transmembrane transport of small organic and inorganic molecules is one of the cornerstones of cellular metabolism. Among transmembrane transporters, solute carrier (SLC) proteins form the largest, albeit very diverse, superfamily with over 400 members. It was recognized early on that xenobiotics can directly interact with SLCs and that this interaction can fundamentally determine their efficacy, including bioavailability and intertissue distribution. Apart from the well-established prodrug strategy, the chemical ligation of transporter substrates to nanoparticles of various chemical compositions has recently been used as a means to enhance their targeting and absorption. In this review, we summarize efforts in drug design exploiting interactions with specific SLC transporters to optimize their therapeutic effects. Furthermore, we describe current and future challenges as well as new directions for the advanced development of therapeutics that target SLC transporters.

GPCRs in Intracellular Compartments: New Targets for Drug Discovery

The architecture of eukaryotic cells is defined by extensive membrane-delimited compartments, which entails separate metabolic processes that would otherwise interfere with each other, leading to functional differences between cells. G protein-coupled receptors (GPCRs) are the largest class of cell surface receptors, and their signal transduction is traditionally viewed as a chain of events initiated from the plasma membrane. Furthermore, their intracellular trafficking, internalization, and recycling were considered only to regulate receptor desensitization and cell surface expression. On the contrary, accumulating data strongly suggest that GPCRs also signal from intracellular compartments. GPCRs localize in the membranes of endosomes, nucleus, Golgi and endoplasmic reticulum apparatuses, mitochondria, and cell division compartments. Importantly, from these sites they have shown to orchestrate multiple signals that regulate different cell pathways. In this review, we summarize the current knowledge of this fascinating phenomenon, explaining how GPCRs reach the intracellular sites, are stimulated by the endogenous ligands, and their potential physiological/pathophysiological roles. Finally, we illustrate several mechanisms involved in the modulation of the compartmentalized GPCR signaling by drugs and endogenous ligands. Understanding how GPCR signaling compartmentalization is regulated will provide a unique opportunity to develop novel pharmaceutical approaches to target GPCRs and potentially lead the way towards new therapeutic approaches.

Mechanisms of Neuronal Damage in Acute Hepatic Porphyrias

Porphyrias are a group of congenital and acquired diseases caused by an enzymatic impairment in the biosynthesis of heme. Depending on the specific enzyme involved, different types of porphyrias (i.e., chronic vs. acute, cutaneous vs. neurovisceral, hepatic vs. erythropoietic) are described, with different clinical presentations. Acute hepatic porphyrias (AHPs) are characterized by life-threatening acute neuro-visceral crises (acute porphyric attacks, APAs), featuring a wide range of neuropathic (central, peripheral, autonomic) manifestations. APAs are usually unleashed by external "porphyrinogenic" triggers, which are thought to cause an increased metabolic demand for heme. During APAs, the heme precursors δ-aminolevulinic acid (ALA) and porphobilinogen (PBG) accumulate in the bloodstream and urine. Even though several hypotheses have been developed to explain the protean clinical picture of APAs, the exact mechanism of neuronal damage in AHPs is still a matter of debate. In recent decades, a role has been proposed for oxidative damage caused by ALA, mitochondrial and synaptic ALA toxicity, dysfunction induced by relative heme deficiency on cytochromes and other hemeproteins (i.e., nitric oxide synthases), pyridoxal phosphate functional deficiency, derangements in the metabolic pathways of tryptophan, and other factors. Since the pathway leading to the biosynthesis of heme is inscribed into a complex network of interactions, which also includes some fundamental processes of basal metabolism, a disruption in any of the steps of this pathway is likely to have multiple pathogenic effects. Here, we aim to provide a comprehensive review of the current evidence regarding the mechanisms of neuronal damage in AHPs.

Glioblastoma research: US and international networking achievements

Being the most aggressive type of brain tumor, glioblastoma is estimated to be diagnosed in about 12,400 new cases in 2017. The diagnosis is dramatic to patients and relatives and leaves open many unanswered questions for them. One is the big question why there is no cure as in other tumors. This review illustrates the US and global research efforts that have been made over the past century. It demonstrates the great magnitude of energy invested by US clinicians and scientists but undoubtedly, more research is needed and funding by NIH and other sources should be continued on the same level.

Transcriptional down-regulation of suppressor of cytokine signaling (SOCS)-3 in chronic obstructive pulmonary disease

BACKGROUND

Tobacco is a leading environmental factor in the initiation of respiratory diseases and causes chronic obstructive pulmonary disease (COPD). Suppressor of cytokine signaling (SOCS) family members are involved in the pathogenesis of many inflammatory diseases and SOCS-3 has been shown to play an important role in the regulation, onset and maintenance of airway allergic inflammation indicating that SOCS-3 displays a potential therapeutic target for anti-inflammatory respiratory drugs development. Since chronic obstructive pulmonary disease (COPD) is also characterized by inflammatory changes and airflow limitation, the present study assessed the transcriptional expression of SOCS-3 in COPD.

METHODS

Real-time PCR was performed to assess quantitative changes in bronchial biopsies of COPD patients in comparison to unaffected controls.

RESULTS

SOCS-3 was significantly down-regulated in COPD at the transcriptional level while SOCS-4 and SOCS-5 displayed no change.

CONCLUSIONS

It can be concluded that the presently observed inhibition of SOCS-3 mRNA expression may be related to the dysbalance of cytokine signaling observed in COPD.

Expression of VPAC1 in a murine model of allergic asthma

Vasoactive intestinal polypeptide (VIP) is a putative neurotransmitter of the inhibitory non-adrenergic non-cholinergic nervous system and influences the mammalian airway function in various ways. Hence known for bronchodilatory, immunomodulatory and mucus secretion modulating effects by interacting with the VIP receptors VPAC1 and VPAC2, it is discussed to be a promising target for pharmaceutical intervention in common diseases such as COPD and bronchial asthma. Here we examined the expression and transcriptional regulation of VPAC1 in the lungs of allergic mice using an ovalbumin (OVA) -induced model of allergic asthma. Mice were sensitized to OVA and challenged with an OVA aerosol. In parallel a control group was sham sensitized with saline. VPAC1 expression was examined using RT-PCR and real time-PCR studies were performed to quantify gene transcription. VPAC1 mRNA expression was detected in all samples of OVA-sensitized and challenged animals and control tissues. Further realtime analysis did not show significant differences at the transcriptional level.Although the present studies did not indicate a major transcriptional regulation of VPAC1 in states of allergic airway inflammation, immunomodulatory effects of VPAC1 might still be present due to regulations at the translational level.

Nanoparticles and cars - analysis of potential sources

Urban health is potentially affected by particle emissions. The potential toxicity of nanoparticles is heavily debated and there is an enormous global increase in research activity in this field. In this respect, it is commonly accepted that nanoparticles may also be generated in processes occurring while driving vehicles. So far, a variety of studies addressed traffic-related particulate matter emissions, but only few studies focused on potential nanoparticles.Therefore, the present study analyzed the literature with regard to nanoparticles and cars. It can be stated that, to date, only a limited amount of research has been conducted in this area and more studies are needed to 1) address kind and sources of nanoparticles within automobiles and to 2) analyse whether there are health effects caused by these nanoparticles.

Tobacco smoke particles and indoor air quality (ToPIQ) - the protocol of a new study

Environmental tobacco smoke (ETS) is a major contributor to indoor air pollution. Since decades it is well documented that ETS can be harmful to human health and causes premature death and disease. In comparison to the huge research on toxicological substances of ETS, less attention was paid on the concentration of indoor ETS-dependent particulate matter (PM). Especially, investigation that focuses on different tobacco products and their concentration of deeply into the airways depositing PM-fractions (PM10, PM2.5 and PM1) must be stated. The tobacco smoke particles and indoor air quality study (ToPIQS) will approach this issue by device supported generation of indoor ETS and simultaneously measurements of PM concentration by laser aerosol spectrometry. Primarily, the ToPIQ study will conduct a field research with focus on PM concentration of different tobacco products and within various microenvironments. It is planned to extend the analysis to basic research on influencing factors of ETS-dependent PM concentration.

Car indoor air pollution - analysis of potential sources

The population of industrialized countries such as the United States or of countries from the European Union spends approximately more than one hour each day in vehicles. In this respect, numerous studies have so far addressed outdoor air pollution that arises from traffic. By contrast, only little is known about indoor air quality in vehicles and influences by non-vehicle sources.Therefore the present article aims to summarize recent studies that address i.e. particulate matter exposure. It can be stated that although there is a large amount of data present for outdoor air pollution, research in the area of indoor air quality in vehicles is still limited. Especially, knowledge on non-vehicular sources is missing. In this respect, an understanding of the effects and interactions of i.e. tobacco smoke under realistic automobile conditions should be achieved in future.

Role and relevance of PEPT2 in drug disposition, dynamics, and toxicity

Pept2 knockout mice are an important tool to evaluate the evolving role and relevance of this proton-coupled oligopeptide transporter beyond drug disposition, where the transporter also modulates the pharmacodynamic and toxicodynamic effects of drug substrates. Our in vivo studies with glycylsarcosine in Pept2 knockout mice have established "proof of concept" that PEPT2 can have a significant effect on dipeptide disposition. Subsequent studies with the aminocephalosporin antibiotic cefadroxil have shown relevance to pharmacology and infectious disease. Finally, studies with the endogenous peptidomimetic 5-aminolevulinic acid have demonstrated relevance to toxicology in the framework of porphyria- and lead-induced neurotoxicity. These studies have consistently demonstrated the dual action of PEPT2 with respect to its apical localization in choroid plexus epithelium and kidney in: 1) effluxing substrates from CSF into choroid plexus, thereby affecting regional pharmacokinetics in brain; and 2) reabsorbing substrates from renal tubular fluid into proximal tubules, thereby affecting systemic pharmacokinetics and exposure. Moreover, these studies have shown that the regional effect of PEPT2 in limiting substrate concentrations in the CSF is more dramatic than its effect in increasing systemic exposure. In the case of 5-aminolevulinic acid, such regional modulation of drug disposition translates directly into significant changes in neurotoxicity.

Structural requirements for the substrates of the H+/peptide cotransporter PEPT2 determined by three-dimensional quantitative structure-activity relationship analysis

The renal type H(+)/peptide cotransporter PEPT2 has a substantial influence on the in vivo disposition of dipeptides and tripeptides as well as peptide-like drugs within the body, particularly in kidney, lung, and the brain. The comparative molecular similarity indices analysis (CoMSIA) method was applied to identify those regions in the substrate structures that are responsible for recognition and for differences in affinity. We have developed a comprehensive 3D quantitative structure-activity relationship (3D-QSAR) model based on 83 compounds that is able to explain and predict the binding affinities of new PEPT2 substrates. This 3D-QSAR model possesses a high predictive power (q(2) = 0.755; r(2) = 0.893). An additional 3D-QSAR model based on the same compounds was generated and correlated with affinity data of the intestinal H(+)/peptide cotransporter PEPT1. By comparing the CoMSIA contour plots, differences in selectivity between the intestinal and the renal type peptide carrier become evident.

The renal type H+/peptide symporter PEPT2: structure-affinity relationships

The H(+)/peptide cotransporter PEPT2 is expressed in a variety of organs including kidney, lung, brain, mammary gland, and eye. PEPT2 substrates are di- and tripeptides as well as peptidomimetics, such as beta-lactam antibiotics. Due to the presence of PEPT2 at the bronchial epithelium, the aerosolic administration of peptide-like drugs might play a major role in future treatment of various pulmonary and systemic diseases. Moreover, PEPT2 has a significant influence on the in vivo disposition and half-life time of peptide-like drugs within the body, particularly in kidney and brain. PEPT2 is known to have similar but not identical structural requirements for substrate recognition and transport compared to PEPT1, its intestinal counterpart. In this review we compiled available affinity constants of 352 compounds, measured at different mammalian tissues and expression systems and compare the data whenever possible with those of PEPT1.

Novel strategies of aerosolic pharmacotherapy

The pulmonary administration of drugs plays a crucial role in the management of various respiratory and systemic diseases. While the cellular properties of airway epithelial cells offer a great potential to deliver drugs into the lungs or the circulation, only little is known about the exact transport pathways. Recently, the high-affinity proton-coupled drug and peptide transporter PEPT2 was identified in the human respiratory tract. The expression of transporter mRNA and protein was localized to the airway epithelium and alveolar type II pneumocytes. In addition, transport studies revealed transporter-mediated uptake of substrates into epithelial cells indicating that the transporter is the molecular basis for the transport of peptides and peptidomimetic drugs in pulmonary epithelial cells. Since genotype analysis revealed no significant differences amongst different transporter genotypes concerning expression and function, the transporter displays an interesting novel target for pulmonary delivery of drugs.

Novel concepts of neuropeptide-based drug therapy: vasoactive intestinal polypeptide and its receptors

Chronic inflammatory airway diseases such as bronchial asthma or chronic obstructive pulmonary disease (COPD) are major contributors to the global burden of disease. Although inflammatory cells play the central role in the pathogenesis of the diseases, recent observations indicate that also resident respiratory cells represent important targets for pulmonary drug development. Especially targeting airway neuromediators offers a possible mechanism by which respiratory diseases may be treated in the future. Among numerous peptide mediators such as tachykinins, calcitonin gene-related peptide, neurotrophins or opioids, vasoactive intestinal polypeptide (VIP) is one of the most abundant molecules found in the respiratory tract. In human airways, it influences many respiratory functions via the receptors VPAC1, VPAC2 and PAC1. VIP-expressing nerve fibers are present in the tracheobronchial smooth muscle layer, submucosal glands and in the walls of pulmonary and bronchial arteries and veins. Next to its strong bronchodilator effects, VIP potently relaxes pulmonary vessels, and plays a pivotal role in the mediation of immune mechanisms. A therapy utilizing the respiratory effects of VIP would offer potential benefits in the treatment of obstructive and inflammatory diseases and long acting VIP-based synthetic non-peptide compounds may represent a novel target for drug development.

Cytosolic COOH terminus of the peptide transporter PEPT2 is involved in apical membrane localization of the protein

The peptide transporter PEPT2 is a polytopic transmembrane protein that mediates the cellular uptake of di- and tripeptides and a variety of peptidomimetics. It is widely expressed in mammalian tissues, including kidney, lung, mammary gland, choroid plexus, and glia cells. In renal tubular cells, PEPT2 is exclusively found at the apical membrane. The molecular mechanisms underlying this polarized expression and targeting to the brush-border membrane are not known. We have explored the role of the 36 COOH-terminal amino acid residues in PEPT2 trafficking and apical expression. EGFP-tagged PEPT2 wild-type transporter and various truncated and mutant proteins were expressed in the polarized proximal tubule cell lines SKPT and OK, and the cellular distribution of the fusion proteins was assessed using confocal microscopy. Whereas deletion of the last seven amino acids (delC7) did not alter PEPT2 surface expression, deletion of the next residue (delC8) or up to 30 terminal amino acids resulted in impaired apical expression and distinct accumulation of mutant proteins in endosomal and lysosomal vesicles. Truncation of more amino acids (delC36) containing tyrosine-based motifs led to a rather diffuse intracellular distribution pattern. Mutations introduced at isoleucine-720 (I720A) and leucine-722 (I722A) also caused an impaired surface appearance. Internalization assays revealed a higher endocytotic rate of the PEPT2 mutants I720A, L722A, and delC36. Our data suggest that a three-amino acid stretch (INL) and tyrosine-based motifs within the COOH tail of PEPT2 are involved in PEPT2's apical membrane localization and membrane steady-state level.

Functional expression of the peptide transporter PEPT2 in the mammalian enteric nervous system

The peptide transporter PEPT2 mediates transmembrane uptake of small peptides. So far, its expression has not been evidenced in the gastrointestinal tract. We have investigated peptide transport activity in the neuromuscular layers of the gastrointestinal tract by using the fluorescent tracer-dipeptide beta-Ala-Lys-Nepsilon-7-amino-4-methyl-coumarin-3-acetic acid (Ala-Lys-AMCA). Whole-mount preparations from mouse, rat, and guinea pig stomach and small and large intestine were incubated with Ala-Lys-AMCA in the presence or absence of the uptake-inhibitors L-histidine, D-phenylalanyl-L-alanine (D-Phe-Ala), glycyl-L-sarcosine (Gly-Sar), glycyl-L-glutamine (Gly-Gln), benzylpenicillin, and cefadroxil. Fluorescence microscopy revealed that Ala-Lys-AMCA specifically accumulated in both ganglionic layers of the enteric nervous system (ENS) in all regions and species studied. This could be inhibited by Gly-Sar, D-Phe-Ala, Gly-Gln, and cefadroxil, but not by free histidine and benzylpenicillin, indicating uptake via PEPT2. Accordingly, dipeptide uptake was completely abolished in PEPT2-deficient mice. Reverse transcriptase-polymerase chain reaction analysis detected a PEPT2-specific transcript in extracts from the ganglionic ENS layers of mouse small and large intestine, further proving that enteric dipeptide transport activity is specifically mediated via PEPT2. The cellular site of dipeptide uptake was immunohistochemically localized to enteric glial cells and tissue-resident macrophages. In addition, dipeptide uptake occurred in a neurochemically defined subset of neurons in the guinea pig ENS. Our results constitute the first functional evidence for dipeptide transport activity in the ENS. PEPT2-mediated dipeptide transport in enteric glia could contribute to the clearance of neuropeptides in the ENS. In addition, the fluorophore-coupled dipeptide uptake via PEPT2 is a novel vital marker for glial cells in the ENS.

Hypothyroidism induces expression of the peptide transporter PEPT2

The kidney is a target organ for thyroid hormone action and a variety of renal transport processes are altered in response to impaired thyroid functions. To investigate the effect of thyroid hormone on the expression of the renal proximal tubular high-affinity-type H+-peptide cotransporter (PEPT2) in rats, hypothyroidism was induced in animals by administration of methimazole (0.05%) via drinking water. After 7 weeks of treatment, hypothyroidism was confirmed by determining serum free T3 and free T4 concentrations. Northern blotting was used to examine the expression of PEPT2 mRNA in kidney tissues from hypothyroid rats compared to control rats. Hypothyroidism resulted in an increased level of total renal PEPT2 mRNA (121.1±3.3% vs. control 100±2.8%; p=0.008). The mRNA results were confirmed by immuno-blotting, which demonstrated significantly increased protein levels (162% vs. control 100%; p<0.01). Immunohistochemistry also revealed increased PEPT2 protein levels in the proximal tubules of treated compared to non-treated rats. In summary, PEPT2 is the first proximal tubule transporter protein that shows increased expression in states of hypothyreosis. As PEPT2 reabsorbs filtered di- and tripeptides and peptide-like drugs, the present findings may have important implications in nutritional amino acid homeostasis and for drug dynamics in states of altered thyroid function.

Comparison of human and monkey peptide transporters: PEPT1 and PEPT2

Human proton-dependent peptide transporters, PEPT1 and PEPT2, mediate the cellular uptake of di- and tripeptides as well as a variety of drug molecules. Although PEPT1 and PEPT2 have been cloned from many species, there are no data available for monkey, an important pharmacological and preclinical species in drug development. In this study, it was first verified that monkey intestine transports a model dipeptide, Gly-Sar, in a proton-dependent manner (0.30 +/- 0.05 pmol cm(-2) s(-1) at pH 6.0 and 0.10 +/- 0.03 pmol cm(-2) s(-1) at pH 7.4) in the absorptive direction, presumably by monkey PEPT1. RT-PCR and rapid amplification of cDNA ends (RACE) were then used to clone monkey PEPT1 and PEPT2. Monkey PEPT1 (2127 bp and 708 amino acids) was found to be >94 and > 92% identical to human PEPT1 at the cDNA and amino acid level, respectively. Monkey PEPT2 (2190 bp and 729 amino acids) was found to be > 97% identical to human PEPT2 at both the cDNA and amino acid levels. Functional comparison of human and monkey peptide transporters expressed in HeLa cells suggested that functionalities of PEPT1 and PEPT2 were largely conserved in terms of Gly-Sar uptake kinetics and inhibitor specificity (for most tested substrates). Finally, Northern and RT-PCR analyses revealed some differences in tissue mRNA levels of peptide transporters between human and monkey.

[11C]Glycylsarcosine: synthesis and in vivo evaluation as a PET tracer of PepT2 transporter function in kidney of PepT2 null and wild-type mice

[11C]Glycylsarcosine (Gly-Sar) was synthesized as a potential radiotracer to investigate the localization and in vivo function of the peptide transporter PepT2 in mouse kidney. Its C-11 labeled diketopiperazine derivative, [11C]cyclo(Gly-Sar) [1-methylpiperazine-2,5-dione], was also evaluated as a potential tracer. [11C]Gly-Sar exhibited rapid initial uptake into kidneys with slow clearance from the medulla, consistent with uptake and retention of the radiotracer through the actions of PepT2. In contrast, the corresponding cyclized dipeptide [11C]cyclo(Gly-Sar) showed rapid clearance and accumulation only in the renal pelvis region. Involvement of PepT2 in reabsorption and delayed clearance of [11C]Gly-Sar was confirmed using the PepT2 knockout mouse, where rapid renal elimination of [11C]Gly-Sar and the absence of radioactivity in medulla were observed. This study demonstrates using in vivo imaging technique that PepT2 is primarily responsible for renal tubular active reabsorption of Gly-Sar, and provides a new tool for studying tubular peptide reabsorption and clearance.

Molecular mechanisms of severe acute respiratory syndrome (SARS)

Severe acute respiratory syndrome (SARS) is a new infectious disease caused by a novel coronavirus that leads to deleterious pulmonary pathological features. Due to its high morbidity and mortality and widespread occurrence, SARS has evolved as an important respiratory disease which may be encountered everywhere in the world. The virus was identified as the causative agent of SARS due to the efforts of a WHO-led laboratory network. The potential mutability of the SARS-CoV genome may lead to new SARS outbreaks and several regions of the viral genomes open reading frames have been identified which may contribute to the severe virulence of the virus. With regard to the pathogenesis of SARS, several mechanisms involving both direct effects on target cells and indirect effects via the immune system may exist. Vaccination would offer the most attractive approach to prevent new epidemics of SARS, but the development of vaccines is difficult due to missing data on the role of immune system-virus interactions and the potential mutability of the virus. Even in a situation of no new infections, SARS remains a major health hazard, as new epidemics may arise. Therefore, further experimental and clinical research is required to control the disease.

Smads as intracellular mediators of airway inflammation

Transforming growth factor-beta (TGF-beta) plays an important role in the pathogenesis of allergic asthma and other airway diseases. Signals from the activated TGF-beta receptor complex are transduced to the nucleus of airway cells by Smad proteins, which represent a family of transcription factors that have recently been implicated to play a major role as intracellular mediators of inflammation. The Smad family consists of the receptor-regulated Smads, a common pathway Smad, and inhibitory Smads. Receptor-regulated Smads (R-Smads) are phosphorylated by the TGF-beta type Ireceptor. They include Smad2 and Smad3, which are recognized by TGF-beta and activin receptors, and Smads 1, 5, 8, and 9, which are recognized by bone morphogenetic protein (BMP) receptors. Smad4 is a common pathway Smad, which is also defined as cooperating Smad (co-Smad) and is not phosphorylated by the TGF-beta type I receptor. Inhibitory Smads(anti-Smads) include Smad6 and Smad7, which down-regulate TGF-beta signaling. To date, the Smads are the only TGF-beta receptor substrates with a demonstrated ability to propagate signals and with regard to the growing number of investigations of Smad-mediated effects in the airways, Smads may prove to be an important target for future development of new therapeutic strategies for asthma and chronic obstructive pulmonary disease.

Immunolocalization of the Proton-Coupled Oligopeptide Transporter PEPT2 in Developing Rat Brain

This study examined the tissue distribution, cellular localization, and developmental expression of the PEPT2 protein in rat brain. Immunoblot and immunocytochemistry analyses were performed with specific rat PEPT1 and PEPT2 antisera developed in our laboratory. Rats were examined from fetus (gestation for 17 days) to adult (day 75). On immunoblot analysis, the PEPT2 protein was detected in cerebral cortex, olfactory bulb, basal ganglia, cerebellum, and hindbrain sections of adult brain, with the strongest signals in cerebral cortex. No PEPT1 protein was found in brain. Expression levels of the PEPT2 protein in cerebral cortex were maximal in the fetus and declined rapidly with advancing age. Adult protein levels were approximately 14% of that observed in fetus. In immunofluorescence experiments, the strongest PEPT2 signals were observed in epithelial cells of the choroid plexus for both adult and neonate brains. The PEPT2 protein was exclusively expressed on the apical membrane (CSF-facing) of choroid plexus epithelia. In double labeling experiments, PEPT2 immunoreactivity in adult brain colocalized with NeuN, a neuronal marker, but not with GFAP, an astrocyte marker. In contrast, in neonatal brain, PEPT2 immunoreactivity colocalized with both GFAP and NeuN. These findings demonstrate that the PEPT2 protein is found throughout the brain. The apical expression of PEPT2 in choroid plexus suggests that it is involved in the export of neuropeptides, peptide fragments, and peptide-like drugs from cerebrospinal fluid. PEPT2 may also play a role in the regulation of neuropeptide concentrations in extracellular fluid, especially during early development.

Direct visualization of peptide uptake activity in the central nervous system of the rat

Carrier-mediated transport of small peptides and peptidomimetics offers the opportunity for a targeted drug delivery across cell membranes in the central nervous system (CNS). This process is mediated by the proton-coupled transporter PEPT2 which is expressed in glial and choroid plexus cells. In the present studies, an uptake assay was established to visualize directly peptide uptake in intact rat brain slices. Accumulation of a reporter molecule, the fluorophore-labeled dipeptide derivative D-Ala-L-Lys-AMCA, was found in plexus choroideus and glial cells and uptake was inhibited by prototypical PEPT2 substrates, such as glycyl-L-glutamine and cefadroxil. The presence of PEPT2 was confirmed by RT-PCR and Northern blot analysis. This first CNS peptide and drug transport-visualizing assay may be used to examine new compounds which are carried by the proton-driven CNS peptide transporter.

Molecular mechanisms of pulmonary peptidomimetic drug and peptide transport

The aerosolic administration of peptidomimetic drugs could play a major role in the future treatment of various pulmonary and systemic diseases, because rational drug design offers the potential to specifically generate compounds that are transported efficiently into the epithelium by distinct carrier proteins such as the peptide transporters. From the two presently known peptide transporters, PEPT1 and PEPT2, which have been cloned from human tissues, the high-affinity transporter PEPT2 is expressed in the respiratory tract epithelium. The transporter is an integral membrane protein with 12 membrane-spanning domains and mediates electrogenic uphill peptide and peptidomimetic drug transport by coupling of substrate translocation to a transmembrane electrochemical proton gradient serving as driving force. In human airways, PEPT2 is localized to bronchial epithelium and alveolar type II pneumocytes, and transport studies revealed that both peptides and peptidomimetic drugs such as antibiotic, antiviral, and antineoplastic drugs are carried by the system. PEPT2 is also responsible for the transport of delta-aminolevulinic acid, which is used for photodynamic therapy and the diagnostics of pulmonary neoplasms. Based on the recent progress in understanding the structural requirements for substrate binding and transport, PEPT2 becomes a target for a rational drug design that may lead to a new generation of respiratory drugs and prodrugs that can be delivered to the airways via the peptide transporter.

Functional characterization of brain peptide transporter in rat cerebral cortex: identification of the high-affinity type H+/peptide transporter PEPT2

In this report, we studied the functional characteristics of a brain peptide transporter using synaptosomes prepared from rat cerebral cortex. Crude synaptosomes (P(2) fraction) were prepared from cerebral cortices in male Wistar rats. Uptake of [14C]glycylsarcosine (Gly-Sar), a substrate for H(+)/oligopeptide transporters PEPT1 and PEPT2, and [3H]histidine, a substrate for peptide/histidine transporters PHT1 and PHT2, was measured at 37 degrees C by a rapid filtration technique. The uptake of [14C]Gly-Sar into synaptosomes was stimulated by an inwardly directed H(+)-gradient. The uptake system exhibited a Michaelis-Menten constant (K(t)) of 110+/-20 microM for Gly-Sar. This value is comparable to the K(t) value for Gly-Sar uptake via the high-affinity H(+)/peptide transporter PEPT2. The H(+)-dependent uptake of [14C]Gly-Sar into synaptosomes was inhibited by di- and tripeptides and beta-lactam antibiotics, but was unaffected by amino acids glycine and histidine. In particular, kyotorphin (Tyr-Arg) completely inhibited Gly-Sar uptake with the K(i) value of 29+/-14 microM. These uptake properties of the brain peptide transporter (i.e., the K(t) value for Gly-Sar uptake and the K(i) value of kyotorphin for Gly-Sar uptake) are very similar to those of PEPT2. RT-PCR and Western blotting analyses revealed that PEPT2 is actually expressed in the cerebral cortex in rat. These results indicate that a H(+)-coupled high affinity peptide transport system is functionally expressed in the cerebral cortex and that this transport system is identical to PEPT2.

An update on renal peptide transporters

The brush-border membrane of renal epithelial cells contains PEPT1 and PEPT2 proteins that are rheogenic carriers for short-chain peptides. The carrier proteins display a distinct surface expression pattern along the proximal tubule, suggesting that initially di- and tripeptides, either filtered or released by surface-bound hydrolases from larger oligopeptides, are taken up by the low-affinity but high-capacity PEPT1 transporter and then by PEPT2, which possesses a higher affinity but lower transport capacity. Both carriers transport essentially all possible di- and tripeptides and numerous structurally related drugs. A unique feature of the mammalian peptide transporters is the capability of proton-dependent electrogenic cotransport of all substrates, regardless of their charge, that is achieved by variable coupling in proton movement along with the substrate down the transmembrane potential difference. This review focuses on the postcloning research efforts to understand the molecular physiology of peptide transport processes in renal tubules and summarizes available data on the underlying genes, protein structures, and transporter function as derived from studies in heterologous expression systems.

Toxic rhinitis-induced changes of human nasal mucosa innervation

Irritative toxic rhinitis is a nasal disorder induced by chemical compounds like ozone, formaldehyde, nickel, chrome, solvents and tobacco smoke. These noxious stimuli may have effects on the nasal innervation leading to a cascade of neuro-immune interactions and an augmentation of the symptoms. Here we examined changes in the neuropeptide content of mucosal parasympathetic, sympathetic and sensory nerves of patients with toxic rhinitis caused by chronic cigarette smoke exposure. Semiquantitative immunohistochemistry using antibodies against calcitonin gene-related peptide (CGRP), substance P (SP), neuropeptide tyrosine (NPY), and vasoactive intestinal peptide (VIP) was carried out on cryostat sections of human nasal mucosa obtained from normal subjects and patients with toxic rhinitis and revealed significant differences between both groups. Toxic rhinitis patients had significantly elevated expression scores for VIP (2.83 +/- 0.31 vs 1.27 +/- 0.47 control group) and NPY (3.17 +/- 0.31 vs 0.91 +/- 0.37 control group) revealing an increase of mediators in distinct subpopulations of airway nerves. In summary, the present studies indicate a differential participation of subclasses of mucosal nerves in the pathophysiology of toxic rhinitis. Airway innervation may have a major role in the pathophysiology of toxic rhinitis associated with chronic cigarette smoke exposure.

Renal assimilation of short chain peptides: visualization of tubular peptide uptake

PURPOSE

Renal assimilation of short chain peptides plays an important role in systemic protein metabolism and amino acid homeostasis. The transepithelial peptide transport across the apical membrane of tubular cells is mediated almost exclusively by pH-dependent H(+)-peptide symport pathways. The current study was designed to identify by visualization functional peptide transport activity along the nephron structures.

METHODS

Visualization of peptide uptake was achieved by using the fluorescent dipeptide derivative D-Ala-Lys-AMCA and unlabelled cefadroxil and glycylglutamine as transport competitors to demonstrate specificity. To confirm these assays, rat specific cRNA probes were synthesized and non-isotopic high-resolution in-situ-hybridization and northern blot analysis were carried out to demonstrate the expression of the high-affinity peptide transporter PEPT2.

RESULTS

The reporter molecule was accumulated by cells of the proximal tubulus but not in glomerular or endothelial cells. Inhibition studies revealed competitive inhibition of D-Ala-Lys-AMCA uptake by the betalactam cefadroxil and the dipeptide glycylglutamine. The control organs intestine and spleen did not show uptake of the systemically administered molecule. Non-isotopic mRNA in-situ-hybridization, using an antisense probe for rat PEPT2 confirmed up-take assays by identifying PEPT2 expression throughout segments of the straight proximal tubule at the inner cortex and outer stripe.

CONCLUSIONS

We demonstrate for the first time renal in vivo transport activity of a dipeptide that allows cells that participate in peptide reabsorption to be visualized. This functional assay may be used to investigate renal peptide transport mechanisms and test new compounds that are transported via proton-driven peptide transporters.

Mammalian peptide transporters as targets for drug delivery

Peptide transporters are integral plasma membrane proteins that mediate the cellular uptake of dipeptides and tripeptides in addition to a variety of peptidomimetics. The carriers, which occur predominantly in the brush-border membranes of epithelial cells of the small intestine, lung, choroid plexus and kidney, contribute to absorption, distribution and elimination of their substrates. The cellular uptake of peptides and peptidomimetics involves the cotransport of protons down an inwardly directed, electrochemical proton gradient that provides the driving force and causes the electrogenicity of the translocation step. Peptide transporters represent excellent targets for the delivery of pharmacologically active compounds because their substrate-binding site can accommodate a wide range of molecules of differing size, hydrophobicity and charge.

Peptide transport in the mammary gland: expression and distribution of PEPT2 mRNA and protein

The lactating mammary gland utilizes free plasma amino acids as well as those derived by hydrolysis from circulating short-chain peptides for protein synthesis. Apart from the major route of amino acid nitrogen delivery to the gland by the various transporters for free amino acids, it has been suggested that dipeptides may also be taken up in intact form to serve as a source of amino acids. The identification of peptide transporters in the mammary gland may therefore provide new insights into protein metabolism and secretion by the gland. The expression and distribution of the high-affinity type proton-coupled peptide transporter PEPT2 were investigated in rat lactating mammary gland as well as in human epithelial cells derived from breast milk. By use of RT-PCR, PEPT2 mRNA was detected in rat mammary gland extracts and human milk epithelial cells. The expression pattern of PEPT2 mRNA revealed a localization in epithelial cells of ducts and glands by nonisotopic high resolution in situ hybridization. In addition, immunohistochemistry was carried out and showed transporter immunoreactivity in the same epithelial cells of the glands and ducts. In addition, two-electrode voltage clamp recordings using PEPT2-expressing Xenopus laevis oocytes demonstrated positive inward currents induced by selected dipeptides that may play a role in aminonitrogen handling in mammalian mammary gland. Taken together, these data suggest that PEPT2 is expressed in mammary gland epithelia, in which it may contribute to the reuptake of short-chain peptides derived from hydrolysis of milk proteins secreted into the lumen. Whereas PEPT2 also transports a variety of drugs, such as selected beta-lactams, angiotensin-converting enzyme inhibitors, and antiviral and anticancer metabolites, their efficient reabsorption via PEPT2 may reduce the burden of xenobiotics in milk.

In vitro models to study hepatotoxicity

Drug discovery and development consists of a series of processes starting with the demonstration of pharmacological effects in experimental cell and animal models and ending with drug safety and efficacy studies in patients. A main limitation is often the unacceptable level of toxicity with the liver as the primary target organ. Therefore, approaches to study hepatic toxicity in the early phase of drug discovery represent an important step towards rational drug development. A variety of in vitro liver models have been developed in the past years. Next to their use in drug development, they can also be applied to study environmental toxins and their hepatotoxicity. The 3 main approaches are ex vivo isolated and perfused organ models, precision-cut liver slices and cell culture models. Although the advantage of whole organ perfusions is based on the assessment of physiologic parameters such as bile production and morphologic parameters such as tissue histology, cell culture models can be efficiently used to assess cellular metabolism, cytotoxicity and genotoxicity. The advantage of precision-cut liver slices is based on the juxtaposition of cellular assays and tissue morphology. None of these models can be compared as they all focus on different fields of hepatoxicology. For the future, the ideal setup for testing the hepatic toxicity of a new compound could of primary studies in cell or slice cultures to assess cellular effects and secondary studies using ex vivo perfused organs to examine gross organ function parameters and histology.

Simultaneous detection of receptor mRNA and ligand protein in human skin tissues

BACKGROUND

In situ hybridization techniques allow a cell-type-specific messenger RNA (mRNA) analysis in complex tissues such as human skin.

METHODS

To evaluate both the expression of mRNA and protein within the same tissue section, we developed a protocol of combined non-radioactive in situ hybridization and immunohistochemistry for use in dermatohistopathology. To validate the technique, we assessed the distribution of vasoactive intestinal polypeptide (VIP) protein and its receptor, VPAC2 mRNA, in human skin samples.

RESULTS

Simultaneous detection of VPAC2 mRNA and VIP immunoreactivity led to abundant staining for both signals in a variety of cell types. There was marked staining for VPAC2 mRNA in epidermal cells, with most pronounced hybridization signals found in keratinocytes of the basal layer and in glandular cells surrounded by VIP-immunoreactive nerve fibers. Hair follicle cells next to VIP-positive fibers also exhibited hybridization signals. Specific staining was also detected in endothelial and mononuclear cells. The findings of simultaneous in situ hybridization and immunohistochemistry were identical to results obtained by the single application of both methods.

CONCLUSIONS

The combination of in situ hybridization and immunohistochemistry appears to be a promising technique to assess the expression of both protein and mRNA in skin samples and may be used for various purposes in experimental and clinical dermatopathology.

Distribution and function of the peptide transporter PEPT2 in normal and cystic fibrosis human lung

BACKGROUND

Aerosol administration of peptide based drugs has an important role in the treatment of various pulmonary and systemic diseases. The characterisation of pulmonary peptide transport pathways can lead to new strategies in aerosol drug treatment.

METHODS

Immunohistochemistry and ex vivo uptake studies were established to assess the distribution and activity of the beta-lactam transporting high affinity proton coupled peptide transporter PEPT2 in normal and cystic fibrosis human airway tissue.

RESULTS

PEPT2 immunoreactivity in normal human airways was localised to cells of the tracheal and bronchial epithelium and the endothelium of small vessels. In peripheral lung immunoreactivity was restricted to type II pneumocytes. In sections of cystic fibrosis lung a similar pattern of distribution was obtained with signals localised to endothelial cells, airway epithelium, and type II pneumocytes. Functional ex vivo uptake studies with fresh lung specimens led to an uptake of the fluorophore conjugated dipeptide derivative D-Ala-L-Lys-AMCA into bronchial epithelial cells and type II pneumocytes. This uptake was competitively inhibited by dipeptides and cephalosporins but not ACE inhibitors, indicating a substrate specificity as described for PEPT2.

CONCLUSIONS

These findings provide evidence for the expression and function of the peptide transporter PEPT2 in the normal and cystic fibrosis human respiratory tract and suggest that PEPT2 is likely to play a role in the transport of pulmonary peptides and peptidomimetics.

Functional and molecular characterization of a peptide transporter in the rat PC12 neuroendocrine cell line

We have studied functional properties of peptide transport in the pheochromocytoma neuroendocrine cell line from rat. The neutral peptide D-Phe-L-Ala (resistant to hydrolysis) is a good substrate for uptake into these cells. Transport is substantially inhibited by diethylpyrocarbonate pretreatment and is stimulated by external acidification. It is sodium-independent and, unexpectedly, insensitive to membrane potential. Peptide uptake is inhibited by a wide variety of other di- and tripeptides but not by amino acids. The neuropeptide kyotorphin (opioid dipeptide (L-Tyr-L-Arg)) inhibits uptake of labelled peptide and trans-stimulates efflux showing that it is a transported substrate. These findings are discussed in relation to the molecular basis and physiological role of this transport system.