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

Beyond SGLT2: proximal tubule transporters as potential drug targets for chronic kidney disease

The kidneys produce daily about 180 liters of urine but only about 2 liters are excreted. The proximal tubule plays an important role in reabsorbing the majority of filtered urine and many metabolites such as sugars, amino acids, salts or phosphate that are contained in this large volume. Reabsorption of these important metabolites is mediated by a diverse group of highly specialized transport proteins. Another group of transport proteins in the proximal tubule is responsible for the active secretion of metabolic waste products or toxins and drugs into urine. All these transporters have in common that they are directly linked to kidney metabolism and indirectly to whole-body metabolism and functions. In recent years, it has become evident that modulation of these transporters may influence the onset, progression and consequences of kidney disease. This review summarizes recent developments in this field and discusses some examples of drugs already in clinical use or in development. The examples include inhibitors of sugar transporters (SGLT2 inhibitors) that are successfully used in patients with kidney disease, diabetes or heart failure. Likewise, indirect inhibitors (acetazolamide) of an transporter absorbing sodium in exchange for protons (NHE3) are used mostly in patients with heart failure or for prevention of high altitude disease, while direct inhibitors show promise in preclinical studies to reduce damage in episodes of acute kidney disease or high blood pressure. Modulators of transporters mediating the excretion of urate have been used in patients with gout and are also discussed to prevent kidney disease. Novel drugs in development target transporters for phosphate, amino acids, or toxin and drug excretion and may be helpful for specific conditions associated with kidney disease. The advantages and challenges associated with these (novel) drugs targeting proximal tubule transport are discussed.

ABSTRACT

The proximal tubule is responsible for reabsorbing about 60% of filtered solutes and water and is critical for the secretion of metabolic waste products, drugs and toxins. A large number of highly specialized ion channels and transport proteins belonging to the SLC and ABC transporter families are involved. Their activity is directly or indirectly linked to ATP consumption and requires large quantities of energy and oxygen supply. Moreover, the activity of these transporters is often coupled to the movement of Na+ ions thus influencing also salt and water balance, as well as transport and regulatory processes in downstream segments. Because of their relevance for systemic ion balance, for renal metabolism or for affecting regulatory processes, proximal tubule transporters are attractive targets for existing drug and for novel strategies to reduce kidney disease progression or to alleviate the consequences of decreased kidney function. In this review, the relevance of some major proximal tubule transport systems as drug targets in individuals with chronic kidney disease (CKD) is discussed. Inhibitors of the sodium-glucose cotransporter 2, SGLT2, are now part of standard therapy in patients with CKD and/or heart failure. Also, indirect inhibition of Na+/H+-exchangers by carbonic anhydrase inhibitors and uricosuric drugs have been used for decades. Inhibition of phosphate and amino acid transporters have recently been proposed as novel principles to remove excess phosphate or to protect the proximal tubule metabolically, respectively. In addition, organic cation and anion transporters involved in drug and toxin excretion may serve as targets of new drugs. The advantages and challenges associated with (novel) drugs targeting proximal tubule transport are discussed.

Cefadroxil Targeting of SLC15A2/PEPT2 Protects From Colistin Nephrotoxicity

Acute kidney injury (AKI) and chronic kidney disease (CKD) are considered interconnected syndromes, as AKI episodes may accelerate CKD progression, and CKD increases the risk of AKI. Genome-wide association studies (GWAS) may identify novel actionable therapeutic targets. Human GWAS for AKI or CKD were combined with murine AKI transcriptomics data sets to identify 13 (ACACB, ACSM5, CNDP1, DPEP1, GATM, SLC6A12, AGXT2L1, SLC15A2, CTSS, ICAM1, ITGAX, ITGAM, and PPM1J) potentially actionable therapeutic targets to modulate kidney disease severity across species and the AKI-CKD spectrum. Among them, SLC15A2, encoding the cell membrane proton-coupled peptide transporter 2, was prioritized for data mining and functional intervention studies in vitro and in vivo because of its known function to transport nephrotoxic drugs such as colistin and the possibility for targeting with small molecules already in clinical use, such as cefadroxil. Data mining disclosed that SLC15A2 was upregulated in the tubulointerstitium of human CKD, including diabetic nephropathy, and the upregulation was localized to proximal tubular cells. Colistin elicited cytotoxicity and proinflammatory response in cultured human and murine proximal tubular cells that was decreased by concomitant exposure to cefadroxil. In proof-of-concept in vivo studies, cefadroxil protected from colistin nephrotoxicity in mice. The GWAS association of SLC15A2 with human kidney disease may be actionable and related to the modifiable transport of nephrotoxins causing repeated subclinical episodes of AKI and/or chronic nephrotoxicity.

Orphan lysosomal solute carrier MFSD1 facilitates highly selective dipeptide transport

Orphan solute carrier (SLC) represents a group of membrane transporters whose exact functions and substrate specificities are not known. Elucidating the function and regulation of orphan SLC transporters is not only crucial for advancing our knowledge of cellular and molecular biology but can potentially lead to the development of new therapeutic strategies. Here, we provide evidence for the biological function of a ubiquitous orphan lysosomal SLC, the Major Facilitator Superfamily Domain-containing Protein 1 (MFSD1), which has remained phylogenetically unassigned. Targeted metabolomics revealed that dipeptides containing either lysine or arginine residues accumulate in lysosomes of cells lacking MFSD1. Whole-cell patch-clamp electrophysiological recordings of HEK293-cells expressing MFSD1 on the cell surface displayed transport affinities for positively charged dipeptides in the lower mM range, while dipeptides that carry a negative net charge were not transported. This was also true for single amino acids and tripeptides, which MFSD1 failed to transport. Our results identify MFSD1 as a highly selective lysosomal lysine/arginine/histidine-containing dipeptide exporter, which functions as a uniporter.

Proton-Coupled Oligopeptide Transport (Slc15) in the Brain: Past and Future Research

This mini-review describes the role of the solute carrier (SLC)15 family of proton-coupled oligopeptide transporters (POTs) and particularly Pept2 (Slc15A2) and PhT1 (Slc15A4) in the brain. That family transports endogenous di- and tripeptides and peptidomimetics but also a number of drugs. The review focuses on the pioneering work of David E. Smith in the field in identifying the impact of PepT2 at the choroid plexus (the blood-CSF barrier) as well as PepT2 and PhT1 in brain parenchymal cells. It also discusses recent findings and future directions in relation to brain POTs including cellular and subcellular localization, regulatory pathways, transporter structure, species differences and disease states.

Feasibility of Transport of 26 Biologically Active Ultrashort Peptides via LAT and PEPT Family Transporters

The aim of this work is to verify the possibility of transport of 26 biologically active ultrashort peptides (USPs) into cells via LAT and PEPT family transporters. Molecular modeling and computer-assisted docking of peptide ligands revealed that the size and structure of ligand-binding sites of the amino acid transporters LAT1, LAT2, and of the peptide transporter PEPT1 are sufficient for the transport of the 26 biologically active di-, tri-, and tetra-peptides. Comparative analysis of the binding of all possible di- and tri-peptides (8400 compounds) at the binding sites of the LAT and PEPT family transporters has been carried out. The 26 biologically active USPs systematically showed higher binding scores to LAT1, LAT2, and PEPT1, as compared with di- and tri-peptides, for which no biological activity has been established. This indicates an important possible role which LAT and PEPT family transporters may play in a variety of biological activities of the 26 biologically active peptides under investigation in this study. Most of the 26 studied USPs were found to bind to the LAT1, LAT2, and PEPT1 transporters more efficiently than the known substrates or inhibitors of these transporters. Peptides ED, DS, DR, EDR, EDG, AEDR, AEDL, KEDP, and KEDG, and peptoids DS7 and KE17 with negatively charged Asp^- or Glu^- amino acid residues at the N-terminus and neutral or positively charged residues at the C-terminus of the peptide are found to be the most effective ligands of the transporters under investigation. It can be assumed that the antitumor effect of the KE, EW, EDG, and AEDG peptides could be associated with their ability to inhibit the LAT1, LAT2, and PEPT1 amino acid transporters. The data obtained lead to new prospects for further study of the mechanisms of transport of USP-based drugs into the cell and design of new antitumor drugs.

Biology of Peptide Transporter 2 in Mammals: New Insights into Its Function, Structure and Regulation

Peptide transporter 2 (PepT2) in mammals plays essential roles in the reabsorption and conservation of peptide-bound amino acids in the kidney and in maintaining neuropeptide homeostasis in the brain. It is also of significant medical and pharmacological significance in the absorption and disposing of peptide-like drugs, including angiotensin-converting enzyme inhibitors, β-lactam antibiotics and antiviral prodrugs. Understanding the structure, function and regulation of PepT2 is of emerging interest in nutrition, medical and pharmacological research. In this review, we provide a comprehensive overview of the structure, substrate preferences and localization of PepT2 in mammals. As PepT2 is expressed in various organs, its function in the liver, kidney, brain, heart, lung and mammary gland has also been addressed. Finally, the regulatory factors that affect the expression and function of PepT2, such as transcriptional activation and posttranslational modification, are also discussed.

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.

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.

Crystal nephropathy and amoxicillin: insights from international spontaneous reporting systems

BACKGROUND

A substantial increase in amoxicillin-induced crystal nephropathy was recently reported in France. Our study aims to further characterize this safety issue from a worldwide perspective.

METHODS

We queried both the FDA Adverse Event Reporting System (FAERS) and the Eudravigilance databases, and performed disproportionality analysis, selecting only adverse events (AEs) related to crystal nephropathy where amoxicillin or amoxicillin/clavulanic acid were reported as suspect. In FAERS, the reporting odds ratios were calculated and deemed significant by the lower limit of the 95% confidence interval (LL95%CI) > 1, selecting all other drugs/events recorded in FAERS as comparator. Deduplication followed by case-by-case assessment and comparison between French and non-French cases were also performed in both databases.

RESULTS

Overall, 57,754 and 84,764 AE reports with amoxicillin or amoxicillin/clavulanic acid were recorded in FAERS and Eudravigilance, respectively, with France accounting for 18.7% and 22.0% of cases. Specific AEs of interest were retrieved in 144 and 239 cases, respectively. Increased reporting was found in FAERS for crystalluria (N = 99; LL95%CI 53.18), crystal nephropathy (24; 27.01), medication crystal in urine present (9; 92.00), crystal urine (8; 11.90), and crystal urine present (4; 1.57). In FAERS and Eudravigilance databases, reports were classified as serious in 98.8% and 91.2% of cases, respectively. Acute kidney injury (AKI) was found in 81.2% and 71.1% of patients. Amoxicillin was mainly given intravenously, and a dose ≥ 12 g/day was administered in 50.0% and 19.7% of cases in the FAERS and Eudravigilance databases, respectively.

CONCLUSION

Although causal association cannot be firmly inferred, a consistent signal of crystal nephropathy with amoxicillin emerged, especially in France. Clinicians should monitor patients for possible early AKI onset, especially when dealing with intravenous administration of daily doses > 12 g.

Identification of a Six-Gene SLC Family Signature With Prognostic Value in Patients With Lung Adenocarcinoma

Given the importance of solute carrier (SLC) proteins in maintaining cellular metabolic homeostasis and that their dysregulation contributes to cancer progression, here we constructed a robust SLC family signature for lung adenocarcinoma (LUAD) patient stratification. Transcriptomic profiles and relevant clinical information of LUAD patients were downloaded from the TCGA and GEO databases. SLC family genes differentially expressed between LUAD tissues and adjacent normal tissues were identified using limma in R. Of these, prognosis-related SLC family genes were further screened out and used to construct a novel SLC family-based signature in the training cohort. The accuracy of the prognostic signature was assessed in the testing cohort, the entire cohort, and the external GSE72094 cohort. Correlations between the prognostic signature and the tumor immune microenvironment and immune cell infiltrates were further explored. We found that seventy percent of SLC family genes (279/397) were differentially expressed between LUAC tissues and adjacent normal. Twenty-six genes with p-values < 0.05 in univariate Cox regression analysis and Kaplan-Meier survival analysis were regarded as prognosis-related SLC family genes, six of which were used to construct a prognostic signature for patient classification into high- and low-risk groups. Kaplan-Meier survival analysis in all internal and external cohorts revealed a better overall survival for patients in the low-risk group than those in the high-risk group. Univariate and multivariate Cox regression analyses indicated that the derived risk score was an independent prognostic factor for LUAD patients. Moreover, a nomogram based on the six-gene signature and clinicopathological factors was developed for clinical application. High-risk patients had lower stromal, immune, and ESTIMATE scores and higher tumor purities than those in the low-risk group. The proportions of infiltrating naive CD4 T cells, activated memory CD4 T cells, M0 macrophages, resting dendritic cells, resting mast cells, activated mast cells, and eosinophils were significantly different between the high- and low-risk prognostic groups. In all, the six-gene SLC family signature is of satisfactory accuracy and generalizability for predicting overall survival in patients with LUAD. Furthermore, this prognostics signature is related to tumor immune status and distinct immune cell infiltrates in the tumor microenvironment.

Finding a cell-permeable compound to inhibit inflammatory cytokines: Uptake, biotransformation, and anti-cytokine activity of javamide-I/-II esters

AIM

Currently, there is limited information available about cell-permeability and anti-cytokine activity of javamide-I/-II esters in monocyte/macrophage-like cells. Therefore, the aim of this study was to investigate their cell-permeability and anti-cytokine activity in the cells.

MATERIALS AND METHODS

The uptake of javamide-I/-II and esters was studied in THP-1 cells and PBMCs. Also, kinetic and inhibition studies were conducted using THP-1 cells. Western Blot was performed to determine the level of ATF-2 phosphorylation in THP-1 cells, and ELISA assays were carried out to measure TNF-alpha, MCP-1, IL-1beta and IL-8 levels in PBMCs.

KEY FINDINGS

In THP-1 cells, the uptake of javamide-I/-II esters was significantly higher than javamide-I/-II (P < 0.001), and the K_m for javamide-I ester was 27 μM. Also, the uptake of the esters was inhibited by PepT2 substrate/blocker. In THP-1 cells, javamide-I/-II esters were also biotransformed into javamide-I/-II. Furthermore, javamide-I ester could inhibit ATF-2 phosphorylation better than javamide-I in the cells, suggesting that the ester could be transported inside the cells better than javamide-I. Similarly, javamide-I/-II esters were found to be transported and biotransformed in PBMCs involved in inflammation processes. As anticipated, the esters were found to inhibit TNF-alpha and MCP-1 significantly in PBMCs (P < 0.005). Especially, javamide-I ester inhibited TNF-alpha, MCP-1, IL-1beta and IL-8 with IC₅₀ values of 1.79, 0.88, 0.91 and 2.57 μM in PBMCs.

SIGNIFICANCE

Javamide-I/-II esters can be transported, biotransformed and inhibit inflammatory cytokines significantly in monocyte/macrophage-like cells, suggesting that they may be utilized as a potent cell-permeable carrier to inhibit inflammatory cytokines in the cells.

CHEMICAL COMPOUNDS

Javamide-I, javamide-I-O-methyl ester, javamide-II, javamide-II-O-methyl ester, tryptophan, coumaric acid, caffeic acid, GlySar, enalapril.

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.

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".

Transporter-mediated drug-drug interactions: advancement in models, analytical tools, and regulatory perspective

Drug-drug interactions mediated by transporters are a serious clinical concern hence a tremendous amount of work has been done on the characterization of the transporter-mediated proteins in humans and animals. The underlying mechanism for the transporter-mediated drug-drug interaction is the induction or inhibition of the transporter which is involved in the cellular uptake and efflux of drugs. Transporter of the brain, liver, kidney, and intestine are major determinants that alter the absorption, distribution, metabolism, excretion profile of drugs, and considerably influence the pharmacokinetic profile of drugs. As a consequence, transporter proteins may affect the therapeutic activity and safety of drugs. However, mounting evidence suggests that many drugs change the activity and/or expression of the transporter protein. Accordingly, evaluation of drug interaction during the drug development process is an integral part of risk assessment and regulatory requirements. Therefore, this review will highlight the clinical significance of the transporter, their role in disease, possible cause underlying the drug-drug interactions using analytical tools, and update on the regulatory requirement. The recent in-silico approaches which emphasize the advancement in the discovery of drug-drug interactions are also highlighted in this review. Besides, we discuss several endogenous biomarkers that have shown to act as substrates for many transporters, which could be potent determinants to find the drug-drug interactions mediated by transporters. Transporter-mediated drug-drug interactions are taken into consideration in the drug approval process therefore we also provided the extrapolated decision trees from in-vitro to in-vivo, which may trigger the follow-up to clinical studies.

Effects of Short-Term Fasting on mRNA Expression of Ghrelin and the Peptide Transporters PepT1 and 2 in Atlantic Salmon (Salmo salar)

Food intake is a vital process that supplies necessary energy and essential nutrients to the body. Information regarding luminal composition in the gastrointestinal tract (GIT) collected through mechanical and nutrient sensing mechanisms are generally conveyed, in both mammals and fish, to the hypothalamic neurocircuits. In this context, ghrelin, the only known hormone with an orexigenic action, and the intestinal peptide transporters 1 and 2, involved in absorption of dietary di- and tripeptides, exert important and also integrated roles for the nutrient uptake. Together, both are potentially involved in signaling pathways that control food intake originating from different segments of the GIT. However, little is known about the role of different paralogs and their response to fasting. Therefore, after 3 weeks of acclimatization, 12 Atlantic salmon (Salmo salar) post-smolt were fasted for 4 days to explore the gastrointestinal response in comparison with fed control (n = 12). The analysis covered morphometric (weight, length, condition factor, and wet content/weight fish %), molecular (gene expression variations), and correlation analyses. Such short-term fasting is a common and recommended practice used prior to any handling in commercial culture of the species. There were no statistical differences in length and weight but a significant lower condition factor in the fasted group. Transcriptional analysis along the gastrointestinal segments revealed a tendency of downregulation for both paralogous genes slc15a1a and slc15a1b and with significant lowered levels in the pyloric ceca for slc15a1a and in the pyloric ceca and midgut for slc15a1b. No differences were found for slc15a2a and slc15a2b (except a higher expression of the fasted group in the anterior midgut), supporting different roles for slc15 paralogs. This represents the first report on the effects of fasting on slc15a2 expressed in GIT in teleosts. Transcriptional analysis of ghrelin splicing variants (ghrl-1 and ghrl-2) showed no difference between treatments. However, correlation analysis showed that the mRNA expression for all genes (restricted to segment with the highest levels) were affected by the residual luminal content. Overall, the results show minimal effects of 4 days of induced fasting in Atlantic salmon, suggesting that more time is needed to initiate a large GIT response.

Choroid Plexus and Drug Removal Mechanisms

Timely and efficient removal of xenobiotics and metabolites from the brain is crucial in maintaining the homeostasis and normal function of the brain. The choroid plexus (CP) forms the blood-cerebrospinal fluid barrier and vitally removes drugs and wastes from the brain through several co-existing clearance mechanisms. The CP epithelial (CPE) cells synthesize and secrete the cerebrospinal fluid (CSF). As the CSF passes through the ventricular and subarachnoid spaces and eventually drains into the general circulation, it collects and removes drugs, toxins, and metabolic wastes from the brain. This bulk flow of the CSF serves as a default and non-selective pathway for the removal of solutes and macromolecules from the brain interstitium. Besides clearance by CSF bulk flow, the CPE cells express several multispecific membrane transporters to actively transport substrates from the CSF side into the blood side. In addition, several phase I and II drug-metabolizing enzymes are expressed in the CPE cells, which enzymatically inactivate a broad spectrum of reactive or toxic substances. This review summarizes our current knowledge of the functional characteristics and key contributors to the various clearance pathways in the CP-CSF system, overviewing recent developments in our understanding of CSF flow dynamics and the functional roles of CP uptake and efflux transporters in influencing CSF drug concentrations.

Molecular and Metabolic Mechanisms Underlying Selective 5-Aminolevulinic Acid-Induced Fluorescence in Gliomas

Simple Summary

5-aminolevulinic acid (5-ALA) is a medication that produces fluorescence in certain cancers, which enables surgeons to visualize tumor margins during surgery. Gliomas are brain tumors that can be difficult to fully resect due to their infiltrative nature. In this review we explored what is known about the mechanism of 5-ALA, recent discoveries that increase our understanding of that mechanism, and potential targets to increase fluorescence in lower grade gliomas.

Abstract

5-aminolevulinic acid (5-ALA) is a porphyrin precursor in the heme synthesis pathway. When supplied exogenously, certain cancers consume 5-ALA and convert it to the fluorogenic metabolite protoporphyrin IX (PpIX), causing tumor-specific tissue fluorescence. Preoperative administration of 5-ALA is used to aid neurosurgical resection of high-grade gliomas such as glioblastoma, allowing for increased extent of resection and progression free survival for these patients. A subset of gliomas, especially low-grade tumors, do not accumulate PpIX intracellularly or readily fluoresce upon 5-ALA administration, making gross total resection difficult to achieve in diffuse lesions. We review existing literature on 5-ALA metabolism and PpIX accumulation to explore potential mechanisms of 5-ALA-induced glioma tissue fluorescence. Targeting the heme synthesis pathway and understanding its dysregulation in malignant tissues could aid the development of adjunct therapies to increase intraoperative fluorescence after 5-ALA treatment.

Phylogeny of Slc15 family and response to Aeromonas hydrophila infection following Lactococcus lactis dietary supplementation in Cyprinus carpio

Solute carrier 15 family (Slc15) are membrane proteins that utilize the proton gradient and negative membrane protential for the transmembrane transporter of di-/tripeptide and peptide-mimetic molecules, in addition, they also play important roles in immunoreaction. In this study, 10 Slc15 genes were identified in the common carp genome database. Comparative genomics analysis showed considerable expansion of the Slc15 genes and verified the four-round whole genome duplication (WGD) event in common carp. Phylogenetic analysis revealed all Slc15 genes of common carp were clustered into orthologous groups indicating the highly conservative during evolution. Besides, the tissues and temporal expression examined by RT-PCR and qRT-PCR showed that most of the Slc15 genes had a narrow tissue distribution and exhibited tissue-specific expression patterns. Expression divergences were observed between these copies proving function divergence after the WGD. Then, we investigated the dietary supplementation effects of three Lactococcus lactis strains on the expression of Slc15 genes in common carp infected by A. hydrophila to find an effective way to treat aquatic diseases. Almost all of the Slc15 genes had an increased expression trend in the early post-challenge stage, and reached the highest expression level at 12h post-challenge. Then, the expression level showed a bluff descent at the last two stages and the expression level reached the lowest at 48 h post-challenge. Slc15 genes expression is actively up-regulated when stimulated by inflammatory factors, which can "amplify" immune signals, and improve the body's defense against foreign invasion in the early stage of the inflammatory response. So activation of the Slc15 genes may be an effective way for infectious disease treatment. As expected, three strains improved the expression of Slc15 genes variously compared with the control/infection groups. The strain 3 of L. lactis had a better induction of Slc15 genes compared with strain 1 and strain 2. It might be applied as a potential activation of Slc15 genes for disease treatment and adding befitting L. lactis may be a good way to protect aquatilia from bacillosis.

Membrane Transporters in Human Parotid Gland-Targeted Proteomics Approach

Salivary glands provide secretory functions, including secretion of xenobiotics and among them drugs. However, there is no published information about protein abundance of drug transporters measured using reliable protein quantification methods. Therefore, mRNA expression and absolute protein content of clinically relevant ABC (n = 6) and SLC (n = 15) family member transporters in the human parotid gland, using the qRT-PCR and liquid chromatography‒tandem mass spectrometry (LC−MS/MS) method, were studied. The abundance of nearly all measured proteins ranged between 0.04 and 0.45 pmol/mg (OCT3 > MRP1 > PEPT2 > MRP4 > MATE1 > BCRP). mRNAs of ABCB1, ABCC2, ABCC3, SLC10A1, SLC10A2, SLC22A1, SLC22A5, SLC22A6, SLC22A7, SLC22A8, SLCO1A2, SLCO1B1, SLCO1B3 and SLCO2B1 were not detected. The present study provides, for the first time, information about the protein abundance of membrane transporters in the human parotid gland, which could further be used to define salivary bidirectional transport (absorption and secretion) mechanisms of endogenous compounds and xenobiotics.

Intranasal carnosine attenuates transcriptomic alterations and improves mitochondrial function in the Thy1-aSyn mouse model of Parkinson's disease

Mitochondrial dysfunction plays a central role in the pathogenesis of neurodegenerative diseases such as Parkinson's disease (PD). This study was designed to determine whether the dipeptide carnosine, which has been shown to protect against oxidative stress and mitochondrial dysfunction, would provide a beneficial effect on mitochondrial function in the Thy1-aSyn mouse model of PD. Thy1-aSyn mice, which overexpress wild-type human alpha-synuclein (aSyn), exhibit progressive non-motor and motor deficits as early as 2 months of age. Two-month old Thy1-aSyn mice and wild-type littermates were randomly assigned to treatment groups with intranasal (IN) and drinking water carnosine, with controls receiving 10 μl of sterile waster intranasally or carnosine-free drinking water, respectively. After two months of treatment, mice were euthanized, and the midbrain was dissected for the evaluation of the gene expression and mitochondrial function. Transcriptional deficiencies associated with the aSyn overexpression in Thy1-aSyn mice were related to ribosomal and mitochondrial function. These deficiencies were attenuated by IN carnosine administration, which increased the expression of mitochondrial genes and enhanced mitochondrial function. These results suggest a potential neuroprotective role for IN-carnosine in PD patients.

Exercise alters and β-alanine combined with exercise augments histidyl dipeptide levels and scavenges lipid peroxidation products in human skeletal muscle

Carnosine and anserine are dipeptides synthesized from histidine and β-alanine by carnosine synthase (ATPGD1). These dipeptides, present in high concentration in the skeletal muscle, form conjugates with lipid peroxidation products such as 4-hydroxy trans-2-nonenal (HNE). Although skeletal muscle levels of these dipeptides could be elevated by feeding β-alanine, it is unclear how these dipeptides and their conjugates are affected by exercise training with or without β-alanine supplementation. We recruited twenty physically active men, who were allocated to either β-alanine or placebo-feeding group matched for VO₂ peak, lactate threshold, and maximal power (W_max). Participants completed 2 weeks of conditioning phase followed by 1 week of exercise testing (CPET) and a single session followed by 6 weeks of high intensity interval training (HIIT). Analysis of muscle biopsies showed that the levels of carnosine and ATPGD1 expression were increased after CPET and decreased following a single session and 6 weeks of HIIT. Expression of ATPGD1 and levels of carnosine were increased upon β-alanine-feeding after CPET, while ATPGD1 expression decreased following a single session of HIIT. The expression of fiber type markers myosin heavy chain (MHC) I and IIa remained unchanged after CPET. Levels of carnosine, anserine, carnosine-HNE, carnosine-propanal and carnosine-propanol were further increased after 9 weeks of β-alanine supplementation and exercise training, but remained unchanged in the placebo-fed group. These results suggest that carnosine levels and ATPGD1 expression fluctuates with different phases of training. Enhancing carnosine levels by β-alanine feeding could facilitate the detoxification of lipid peroxidation products in the human skeletal muscle.

Organic and Peptidyl Constituents of Snake Venoms: The Picture Is Vastly More Complex Than We Imagined

Small metabolites and peptides in 17 snake venoms (Elapidae, Viperinae, and Crotalinae), were quantified using liquid chromatography-mass spectrometry. Each venom contains >900 metabolites and peptides. Many small organic compounds are present at levels that are probably significant in prey envenomation, given that their known pharmacologies are consistent with snake envenomation strategies. Metabolites included purine nucleosides and their bases, neurotransmitters, neuromodulators, guanidino compounds, carboxylic acids, amines, mono- and disaccharides, and amino acids. Peptides of 2⁻15 amino acids are also present in significant quantities, particularly in crotaline and viperine venoms. Some constituents are specific to individual taxa, while others are broadly distributed. Some of the latter appear to support high anabolic activity in the gland, rather than having toxic functions. Overall, the most abundant organic metabolite was citric acid, owing to its predominance in viperine and crotaline venoms, where it chelates divalent cations to prevent venom degradation by venom metalloproteases and damage to glandular tissue by phospholipases. However, in terms of their concentrations in individual venoms, adenosine, adenine, were most abundant, owing to their high titers in Dendroaspis polylepis venom, although hypoxanthine, guanosine, inosine, and guanine all numbered among the 50 most abundant organic constituents. A purine not previously reported in venoms, ethyl adenosine carboxylate, was discovered in D. polylepis venom, where it probably contributes to the profound hypotension caused by this venom. Acetylcholine was present in significant quantities only in this highly excitotoxic venom, while 4-guanidinobutyric acid and 5-guanidino-2-oxopentanoic acid were present in all venoms.

Expression analysis of human solute carrier (SLC) family transporters in nasal mucosa and RPMI 2650 cells

With nearly 400 members, the solute-linked carrier (SLC) superfamily is one of the most important gene classes concerning the disposition of drugs and the transport of physiological substrates in the human body. The mapping of related transport proteins is already well advanced for the intestines, kidneys and liver, but it has recently been brought into focus for various respiratory epithelia. The aim of this study was to evaluate the expression of several SLC transporters in differently cultured RPMI 2650 cells, as well as in specimens of the human nasal mucosa. The expression profiles of PEPT2, OATP1A2, OATP4C1, OCT2, OCTN1 and OCTN2 were investigated at the gene and protein levels by performing RT-PCR, western blot analysis and immunohistological staining. Uptake assays using appropriate substrates and inhibitory substances were performed to compare the activity of peptide, organic anion and organic cation transporters, respectively, among the three models. Expression of the six SLC transporters under investigation was confirmed at the mRNA and protein levels in human nasal mucosa ex vivo as well as in RPMI 2650 cells grown under different culture conditions. The functionality was almost equal among all of the models for the PEPT and OCT(N) transporters, while the functional activity of the OATP transporters was more pronounced for both in vitro models than for excised nasal tissue. Despite negligible variations in transporter capacities, the RPMI 2650 cell cultures and freshly isolated human nasal epithelium showed nearly comparable expression patterns for the examined SLC proteins. Therefore, in vitro models based on the RPMI 2650 cell line could provide helpful data during the preclinical investigation of intranasally administered drug formulations and in the development of strategies to target nasal drug transporters for either local or systemic drug delivery.

Identification of two novel genes SLC15A2 and SLCO1B3 associated with maintenance dose variability of warfarin in a Chinese population

Warfarin is a commonly prescribed and effective oral anticoagulant. Genetic polymorphisms associated with warfarin metabolism and sensitivity have been implicated in the wide inter-individual dose variation that is observed. Several algorithms integrating patients’ clinical characteristics and genetic polymorphism information have been explored to predict warfarin dose. However, most of these algorithms could explain only over half of the variation in a warfarin maintenance dose, suggesting that additional genetic factors may exist and need to be identified. Here, a drug absorption, distribution, metabolism and excretion (ADME) Core Panel Kit-based pharmacogenetic study was performed to screen for warfarin dose-associated SNP sites in Han-Chinese population patients taking warfarin therapy, and the screen was followed by pyrosequencing-based validation. Finally, we confirmed that the common variant rs9923231 in VKORC1 and two novel genes, SLC15A2 (rs1143671 and rs1143672) and SLCO1B3 (rs4149117 and rs7311358), are associated with the warfarin maintenance dose. As has been shown for those carriers with the variant rs9923231 in VKORC1, it was suggested that those subjects with homozygous minor alleles in those four SNPs should take a lower warfarin dose than those carrying the wild type alleles. Together with the established predictor rs9923231 in VKORC1, those four novel variants on SLC15A2 and SLCO1B3 should be considered as useful biomarkers for warfarin dose adjustment in clinical practice in Han-Chinese populations.

Pulmonary Pharmacokinetics of Colistin following Administration of Dry Powder Aerosols in Rats

Colistin has been administered via nebulization for the treatment of respiratory tract infections. Recently, dry powder inhalation (DPI) has attracted increasing attention. The current study aimed to investigate the pharmacokinetics (PK) of colistin in epithelial lining fluid (ELF) and plasma following DPI and intravenous (i.v.) administration in healthy Sprague-Dawley rats. Rats were given colistin as DPI intratracheally (0.66 and 1.32 mg base/kg of body weight) or i.v. injection (0.66 mg base/kg). Histopathological examination of lung tissue was performed at 24 h. Colistin concentrations in both ELF and plasma were quantified, and a population PK model was developed and compared to a previously published PK model of nebulized colistin in rats. A two-compartment structural model was developed to describe the PK of colistin in both ELF and plasma following pulmonary or i.v. administration. The model-estimated clearance from the central plasma compartment was 0.271 liter/h/kg (standard error [SE] = 2.51%). The transfer of colistin from the ELF compartment to the plasma compartment was best described by a first-order rate constant (clearance of colistin from the ELF compartment to the plasma compartment = 4.03 × 10^-4 liter/h/kg, SE = 15%). DPI appeared to have a higher rate of absorption (time to the maximum concentration in plasma after administration of colistin by DPI, ≤10 min) than nebulization (time to the maximum concentration in plasma after administration of colistin by nebulization, 20 to 30 min), but the systemic bioavailabilities by the two routes of administration were similar (∼46.5%, SE = 8.43%). Histopathological examination revealed no significant differences in inflammation in lung tissues between the two treatments. Our findings suggest that colistin DPI is a promising alternative to nebulization considering the similar PK and safety profiles of the two forms of administration. The PK and histopathological information obtained is critical for the development of optimal aerosolized colistin regimens with activity against lung infections caused by Gram-negative bacteria.

Aerosolized Polymyxin B for Treatment of Respiratory Tract Infections: Determination of Pharmacokinetic-Pharmacodynamic Indices for Aerosolized Polymyxin B against Pseudomonas aeruginosa in a Mouse Lung Infection Model

Pulmonary administration of polymyxins is increasingly used for the treatment of respiratory tract infections caused by multidrug-resistant Gram-negative bacteria, such as those in patients with cystic fibrosis. However, there is a lack of pharmacokinetics (PK), pharmacodynamics (PD), and toxicity data of aerosolized polymyxin B to inform rational dosage selection. The PK and PD of polymyxin B following pulmonary and intravenous dosing were examined in neutropenic infected mice, and the data were analyzed by a population PK model. Dose fractionation study was performed for total daily doses between 2.06 and 24.8 mg base/kg of weight against Pseudomonas aeruginosa ATCC 27853, PAO1, and FADDI-PA022 (MIC of 1 mg/liter for all three strains). Histopathological examination of the lung was undertaken at 24 h posttreatment in both healthy and neutropenic infected mice. A two-compartment PK model was required for both epithelial lining fluid (ELF) and plasma drug exposure. The model consisted of central and peripheral compartments and was described by bidirectional first-order distribution clearance. The ratio of the area under the curve to the MIC (AUC/MIC) was the most predictive PK/PD index to describe the antimicrobial efficacy of aerosolized polymyxin B in treating lung infections in mice (R² of 0.70 to 0.88 for ELF and 0.70 to 0.87 for plasma). The AUC/MIC targets associated with bacteriostasis against the three P. aeruginosa strains were 1,326 to 1,506 in ELF and 3.14 to 4.03 in plasma. Histopathological results showed that polymyxin B aerosols significantly reduced lung inflammation and preserved lung epithelial integrity. This study highlights the advantageous PK/PD characteristics of pulmonary delivery of polymyxin B over intravenous administration in achieving high drug exposure in ELF.

Biogenic Aldehydes as Therapeutic Targets for Cardiovascular Disease

Aldehydes are continuously formed in biological systems through enzyme-dependent and spontaneous oxidation of lipids, glucose, and primary amines. These highly reactive, biogenic electrophiles can become toxic via covalent modification of proteins, lipids and DNA. Thus, agents that scavenge aldehydes through conjugation have therapeutic value for a number of major cardiovascular diseases. Several commonly-prescribed drugs (e.g., hydralazine) have been shown to have potent aldehyde-conjugating properties which may contribute to their beneficial effects. Herein, we briefly describe the major sources and toxicities of biogenic aldehydes in cardiovascular system, and provide an overview of drugs that are known to have aldehyde-conjugating effects. Some compounds of phytochemical origin, and histidyl-dipeptides with emerging therapeutic value in this area are also discussed.

Species Differences in Human and Rodent PEPT2-Mediated Transport of Glycylsarcosine and Cefadroxil in Pichia Pastoris Transformants

The proton-coupled oligopeptide transporter PEPT2 (SLC15A2) plays an important role in the disposition of di/tripeptides and peptide-like drugs in kidney and brain. However, unlike PEPT1 (SLC15A1), there is little information about species differences in the transport of PEPT2-mediated substrates. The purpose of this study was to determine whether PEPT2 exhibited a species-dependent uptake of glycylsarcosine (GlySar) and cefadroxil using yeast Pichia pastoris cells expressing cDNA from human, mouse, and rat. In such a system, the functional activity of PEPT2 was evaluated with [3H]GlySar as a function of time, pH, substrate concentration, and specificity, and with [3H]cefadroxil as a function of concentration. We observed that the uptake of GlySar was pH-dependent with an optimal uptake at pH 6.5 for all three species. Moreover, GlySar showed saturable uptake kinetics, with Km values in human (150.6 µM) > mouse (42.8 µM) ≈ rat (36.0 µM). The PEPT2-mediated uptake of GlySar in yeast transformants was specific, being inhibited by di/tripeptides and peptide-like drugs, but not by amino acids and nonsubstrate compounds. Cefadroxil also showed a saturable uptake profile in all three species, with Km values in human (150.8 μM) > mouse (15.6 μM) ≈ rat (11.9 μM). These findings demonstrated that the PEPT2-mediated uptake of GlySar and cefadroxil was specific, species dependent, and saturable. Furthermore, based on the Km values, mice appeared similar to rats but both were less than optimal as animal models in evaluating the renal reabsorption and pharmacokinetics of peptides and peptide-like drugs in humans.

A Variant of Peptide Transporter 2 Predicts the Severity of Porphyria-Associated Kidney Disease

CKD occurs in most patients with acute intermittent porphyria (AIP). During AIP, δ-aminolevulinic acid (ALA) accumulates and promotes tubular cell death and tubulointerstitial damage. The human peptide transporter 2 (PEPT2) expressed by proximal tubular cells mediates the reabsorption of ALA, and variants of PEPT2 have different affinities for ALA. We tested the hypothesis that PEPT2 genotypes affect the severity and prognosis of porphyria-associated kidney disease. We analyzed data from 122 individuals with AIP who were followed from 2003 to 2013 and genotyped for PEPT2 At last follow-up, carriers of the PEPT2*1*1 genotype (higher affinity variant) exhibited worse renal function than carriers of the lower affinity variants PEPT2*1/*2 and PEPT2*2/*2 (mean±SD eGFR: 54.4±19.1, 66.6±23.8, and 78.1±19.9 ml/min per 1.73 m², respectively). Change in eGFR (mean±SD) over the 10-year period was -11.0±3.3, -2.4±1.9, and 3.4±2.6 ml/min per 1.73 m² for PEPT2*1/*1, PEPT2*1*2, and PEPT*2*2*2 carriers, respectively. At the end of follow-up, 68% of PEPT2*1*1 carriers had an eGFR<60 ml/min per 1.73 m², compared with 37% of PEPT2*1*2 carriers and 15% of PEPT2*2*2 carriers. Multiple regression models including all confounders indicated that the PEPT2*1*1 genotype independently associated with an eGFR<60 ml/min per 1.73 m² (odds ratio, 6.85; 95% confidence interval, 1.34 to 46.20) and an annual decrease in eGFR of >1 ml/min per 1.73 m² (odds ratio, 3.64; 95% confidence interval, 1.37 to 9.91). Thus, a gene variant is predictive of the severity of a chronic complication of AIP. The therapeutic value of PEPT2 inhibitors in preventing porphyria-associated kidney disease warrants investigation.

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

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

Substrates of the human oligopeptide transporter hPEPT2

Oligopeptide transporters serve important functions in nutrition and pharmacology. In particular, these transporters help maintain the homeostasis of peptides. The peptide-transporter PEPT2 is a high-affinity and low-capacity type oligopeptide transporter from the proton-coupled oligopeptide transporter family. PEPT2 has recently received attention because of its potential application in targeted drug delivery. PEPT2 is widely distributed in kidney, central nervous system, and lung of organisms. In general, all dipeptides, tripeptides, and peptide-like drugs such as β-lactam antibiotics and angiotensin-converting enzyme inhibitors could be mediated and transported as a substrate of PEPT2. The design of many extant drugs and prodrugs is based on the substrate structure of PEPT2 to accelerate absorption via peptide transporters. Thus, this paper summarizes the substrate features of PEPT2 to promote the rational design of drugs and prodrugs that target peptide transporters.

Population pharmacokinetic modeling of cefadroxil renal transport in wild-type and Pept2 knockout mice

1. Cefadroxil is a broad-spectrum β-lactam antibiotic that is widely used in the treatment of various infectious diseases. Currently, poor understanding of the drug's pharmacokinetic profiles and disposition mechanism(s) prevents determining optimal dosage regimens and achieving ideal antibacterial responses in patients. In the present retrospective study, we developed a population pharmacokinetic model of cefadroxil in wild-type and Pept2 knockout mice using the nonlinear mixed effect modeling (NONMEM) approach. 2. Cefadroxil pharmacokinetics were best described by a two-compartment model, with both saturable and nonsaturable elimination processes to/from the central compartment. Through this modeling approach, pharmacokinetic parameters in wild-type and Pept2 knockout mice were well estimated, respectively, as follows: volume of central compartment V1 (3.43 versus 4.23 mL), volume of peripheral compartment V2 (5.98 versus 8.61 mL), intercompartment clearance Q (0.599 versus 0.586 mL/min) and linear elimination rate constant K10 (0.111 versus 0.070 min(-1)). Moreover, the secretion kinetics (i.e. V(m1) = 17.6 nmoL/min and K(m1) = 37.1 µM) and reabsorption kinetics (i.e. V(m2) = 15.0 nmoL/min and K(m2) = 27.1 µM) of cefadroxil were quantified in kidney, for the first time, under in vivo conditions. 3. Our model provides a unique tool to quantitatively predict the dose-dependent nonlinear disposition of cefadroxil, as well as the potential for transporter-mediated drug interactions.

USP18 Sensitivity of Peptide Transporters PEPT1 and PEPT2

USP18 (Ubiquitin-like specific protease 18) is an enzyme cleaving ubiquitin from target proteins. USP18 plays a pivotal role in antiviral and antibacterial immune responses. On the other hand, ubiquitination participates in the regulation of several ion channels and transporters. USP18 sensitivity of transporters has, however, never been reported. The present study thus explored, whether USP18 modifies the activity of the peptide transporters PEPT1 and PEPT2, and whether the peptide transporters are sensitive to the ubiquitin ligase Nedd4-2. To this end, cRNA encoding PEPT1 or PEPT2 was injected into Xenopus laevis oocytes without or with additional injection of cRNA encoding USP18. Electrogenic peptide (glycine-glycine) transport was determined by dual electrode voltage clamp. As a result, in Xenopus laevis oocytes injected with cRNA encoding PEPT1 or PEPT2, but not in oocytes injected with water or with USP18 alone, application of the dipeptide gly-gly (2 mM) was followed by the appearance of an inward current (I_gly-gly). Coexpression of USP18 significantly increased I_gly-gly in both PEPT1 and PEPT2 expressing oocytes. Kinetic analysis revealed that coexpression of USP18 increased maximal I_gly-gly. Conversely, overexpression of the ubiquitin ligase Nedd4-2 decreased I_gly-gly. Coexpression of USP30 similarly increased I_gly-gly in PEPT1 expressing oocytes. In conclusion, USP18 sensitive cellular functions include activity of the peptide transporters PEPT1 and PEPT2.

Downregulation of peptide transporters PEPT1 and PEPT2 by oxidative stress responsive kinase OSR1

BACKGROUND/AIMS

OSR1 (oxidative-stress-responsive kinase 1) participates in the regulation of renal tubular ion transport, cell volume and blood pressure. Whether OSR1 contributes to the regulation of organic solute transport remained; however, elusive. The present study thus explored the OSR1 sensitivity of the peptide transporters PEPT1 and PEPT2.

METHODS

cRNA encoding PEPT1 or PEPT2 were injected into Xenopus oocytes without or with additional injection of cRNA encoding wild-type OSR1, WNK1 insensitive inactive (T185A)OSR1, constitutively active (T185E)OSR1, and catalytically inactive (D164A)OSR1. Electrogenic peptide (glycine-glycine) transport was determined by dual electrode voltage clamp, the abundance of hemagglutinin-tagged PEPT2 (PEPT2-HA) by chemiluminescence.

RESULTS

In Xenopus oocytes injected with cRNA encoding PEPT1 or PEPT2, but not in oocytes injected with water, the dipeptide gly-gly (2 mM) generated an appreciable inward current (I(gly-gly)). Coexpression of OSR1 significantly decreased Igly-gly in both PEPT1 and PEPT2 expressing oocytes. The effect of OSR1 coexpression on Igly-gly in PEPT1 expressing oocytes was mimicked by coexpression of (T185E)OSR1, but not of (D164A)OSR1 or (T185A)OSR1. Kinetic analysis revealed that coexpression of OSR1 decreased maximal Igly-gly. OSR1 further decreased the PEPT2-HA protein abundance in the cell membrane.

CONCLUSION

OSR1 has the capacity to downregulate the peptide transporters PEPT1 and PEPT2 by decreasing the carrier protein abundance in the cell membrane.

The role of drug transporters in the kidney: lessons from tenofovir

Tenofovir disoproxil fumarate, the prodrug of nucleotide reverse transcriptase inhibitor tenofovir, shows high efficacy and relatively low toxicity in HIV patients. However, long-term kidney toxicity is now acknowledged as a modest but significant risk for tenofovir-containing regimens, and continuous use of tenofovir in HIV therapy is currently under question by practitioners and researchers. Co-morbidities (hepatitis C, diabetes), low body weight, older age, concomitant administration of potentially nephrotoxic drugs, low CD4 count, and duration of therapy are all risk factors associated with tenofovir-associated tubular dysfunction. Tenofovir is predominantly eliminated via the proximal tubules of the kidney, therefore drug transporters expressed in renal proximal tubule cells are believed to influence tenofovir plasma concentration and toxicity in the kidney. We review here the current evidence that the actions, pharmacogenetics, and drug interactions of drug transporters are relevant factors for tenofovir-associated tubular dysfunction. The use of creatinine and novel biomarkers for kidney damage, and the role that drug transporters play in biomarker disposition, are discussed. The lessons learnt from investigating the role of transporters in tenofovir kidney elimination and toxicity can be utilized for future drug development and clinical management programs.

SPAK dependent regulation of peptide transporters PEPT1 and PEPT2

BACKGROUND/AIMS

SPAK (STE20-related proline/alanine-rich kinase) is a powerful regulator of renal tubular ion transport and blood pressure. Moreover, SPAK contributes to the regulation of cell volume. Little is known, however, about a role of SPAK in the regulation or organic solutes. The present study thus addressed the influence of SPAK on the peptide transporters PEPT1 and PEPT2.

METHODS

To this end, cRNA encoding PEPT1 or PEPT2 were injected into Xenopus laevis oocytes without or with additional injection of cRNA encoding wild-type, SPAK, WNK1 insensitive inactive (T233A)SPAK, constitutively active (T233E)SPAK, and catalytically inactive (D212A)SPAK. Electrogenic peptide (glycine-glycine) transport was determined by dual electrode voltage clamp and PEPT2 protein abundance in the cell membrane by chemiluminescence. Intestinal electrogenic peptide transport was estimated from peptide induced current in Ussing chamber experiments of jejunal segments isolated from gene targeted mice expressing SPAK resistant to WNK-dependent activation (spak(tg/tg)) and respective wild-type mice (spak(+/+)).

RESULTS

In PEPT1 and in PEPT2 expressing oocytes, but not in oocytes injected with water, the dipeptide gly-gly (2 mM) generated an inward current, which was significantly decreased following coexpression of SPAK. The effect of SPAK on PEPT1 was mimicked by (T233E)SPAK, but not by (D212A)SPAK or (T233A)SPAK. SPAK decreased maximal peptide induced current of PEPT1. Moreover, SPAK decreased carrier protein abundance in the cell membrane of PEPT2 expressing oocytes. In intestinal segments gly-gly generated a current, which was significantly higher in spak(tg/tg) than in spak(+/+) mice.

CONCLUSION

SPAK is a powerful regulator of peptide transporters PEPT1 and PEPT2.

Divergent developmental expression and function of the proton-coupled oligopeptide transporters PepT2 and PhT1 in regional brain slices of mouse and rat

This study evaluated the developmental gene and protein expression of proton-coupled oligopeptide transporters (POTs: peptide transporter, PepT1 and PepT2; peptide-histidine transporter, PhT1 and PhT2) in different regions of rodent brain, and the age-dependent uptake of a POT substrate, glycylsarcosine (GlySar), in brain slices. Slices were obtained from cerebral cortex, cerebellum and hippocampus of wildtype and PepT2 null mice, and from rats at different ages. Gene and protein expression were determined by real-time PCR and immunoblot analyses. Brain slice uptakes of radiolabeled glycylsarcosine were determined in the absence and presence of excess unlabeled glycylsarcosine or l-histidine, the latter being an inhibitor of PhT1/2 but not PepT1/2. As PepT2 and PhT1 transcripts were abundantly expressed in all three regions of mouse brain, little to no expression was observed for PepT1 and PhT2. PhT1 protein was present in brain regions of adult but not neonatal mice and expression levels increased with age in rats. Glycylsarcosine uptake, inhibition and transporter dominance did not show regional brain or species differences. However, there were clear age-related differences in functional activity, with PepT2 dominating in neonatal mice and rats, and PhT1 dominating in adult rodents. These developmental changes may markedly impact the neural activity of both endogenous and exogenous (drug) peptides/mimetics. Developmental gene and protein expression of peptide transporters was evaluated in various regions of rodent brain, along with age-dependent uptake of dipeptide. We found marked changes in protein expression and functional activity of PhT1 and PepT2, the former predominating in adult and the latter in neonate. These developmental changes may markedly impact the neural activity of endogenous and exogenous peptides/mimetics.

Proton-coupled oligopeptide transporter family SLC15: physiological, pharmacological and pathological implications

Mammalian members of the proton-coupled oligopeptide transporter family (SLC15) are integral membrane proteins that mediate the cellular uptake of di/tripeptides and peptide-like drugs. The driving force for uphill electrogenic symport is the chemical gradient and membrane potential which favors proton uptake into the cell along with the peptide/mimetic substrate. The peptide transporters are responsible for the absorption and conservation of dietary protein digestion products in the intestine and kidney, respectively, and in maintaining homeostasis of neuropeptides in the brain. They are also responsible for the absorption and disposition of a number of pharmacologically important compounds including some aminocephalosporins, angiotensin-converting enzyme inhibitors, antiviral prodrugs, and others. In this review, we provide updated information on the structure-function of PepT1 (SLC15A1), PepT2 (SLC15A2), PhT1 (SLC15A4) and PhT2 (SLC15A3), and their expression and localization in key tissues. Moreover, mammalian peptide transporters are discussed in regard to pharmacogenomic and regulatory implications on host pharmacology and disease, and as potential targets for drug delivery. Significant emphasis is placed on the evolving role of these peptide transporters as elucidated by studies using genetically modified animals. Whenever possible, the relevance of drug-drug interactions and regulatory mechanisms are evaluated using in vivo studies.

Metabolome-wide association study of neovascular age-related macular degeneration

PURPOSE

To determine if plasma metabolic profiles can detect differences between patients with neovascular age-related macular degeneration (NVAMD) and similarly-aged controls.

METHODS

Metabolomic analysis using liquid chromatography with Fourier-transform mass spectrometry (LC-FTMS) was performed on plasma samples from 26 NVAMD patients and 19 controls. Data were collected from mass/charge ratio (m/z) 85 to 850 on a Thermo LTQ-FT mass spectrometer, and metabolic features were extracted using an adaptive processing software package. Both non-transformed and log2 transformed data were corrected using Benjamini and Hochberg False Discovery Rate (FDR) to account for multiple testing. Orthogonal Partial Least Squares-Discriminant Analysis was performed to determine metabolic features that distinguished NVAMD patients from controls. Individual m/z features were matched to the Kyoto Encyclopedia of Genes and Genomes database and the Metlin metabolomics database, and metabolic pathways associated with NVAMD were identified using MetScape.

RESULTS

Of the 1680 total m/z features detected by LC-FTMS, 94 unique m/z features were significantly different between NVAMD patients and controls using FDR (q = 0.05). A comparison of these features to those found with log2 transformed data (n = 132, q = 0.2) revealed 40 features in common, reaffirming the involvement of certain metabolites. Such metabolites included di- and tripeptides, covalently modified amino acids, bile acids, and vitamin D-related metabolites. Correlation analysis revealed associations among certain significant features, and pathway analysis demonstrated broader changes in tyrosine metabolism, sulfur amino acid metabolism, and amino acids related to urea metabolism.

CONCLUSIONS

These data suggest that metabolomic analysis can identify a panel of individual metabolites that differ between NVAMD cases and controls. Pathway analysis can assess the involvement of certain metabolic pathways, such as tyrosine and urea metabolism, and can provide further insight into the pathophysiology of AMD.

Organic anion transporter 3 interacts selectively with lipophilic β-lactam antibiotics

Transporters are major determinants of the disposition of xenobiotics and endogenous chemicals in the body. Organic anion transporter 3 (Oat3) functions in the kidney and brain to remove metabolic waste, toxins, and drugs, and thus transports diverse chemicals. Some β-lactam antibiotics interact with Oat3, and penicillin G exhibits a strong dependence on Oat3 for renal elimination. However, over 80 β-lactams exist, and many have not been assessed for an interaction with Oat3. Moreover, β-lactams continue to receive U.S. Food and Drug Administration approval. This study identified new β-lactam-Oat3 interactions, provided a head-to-head comparison with Oat1, and characterized the physicochemical determinants of affinity for Oat3. Cells expressing mouse Oat3 (mOat3) and Oat1 (mOat1), and human OAT3 (hOAT3) were used to test inhibitors, and high-performance liquid chromatography (HPLC) was used to measure transport. Of 26 β-lactams tested, 12 were clear inhibitors of Oat3, and 14 exhibited poor interactions. Inhibitors exhibited a nearly identical rank-order of potency against mOat3 and hOAT3. Oat1 demonstrated a poor interaction with most β-lactams. The majority of Oat3 inhibitors were substrates, and there were clear physicochemical differences between inhibitors and noninhibitors. That is, inhibitors had nearly 40% fewer hydrogen bond donors (P < 0.001), a lower total polar surface area (P < 0.05), and greater lipophilicity (LogP of inhibitors, +1.41; noninhibitors, -1.54; P < 0.001). Pharmacophore mapping revealed a prohibitive hydrogen bond donor group in noninhibitors adjacent to a hydrophobic moiety that was important for binding to Oat3. These findings indicate that Oat3 recognizes lipophilic β-lactams more readily. Moreover, this study has potential implications for designing β-lactams to avoid renal accumulation or brain efflux via Oat3.

Expression Profiling of Solute Carrier Gene Families at the Blood-CSF Barrier

The choroid plexus (CP) is a highly vascularized tissue in the brain ventricles and acts as the blood-cerebrospinal fluid (CSF) barrier (BCSFB). A main function of the CP is to secrete CSF, which is accomplished by active transport of small ions and water from the blood side to the CSF side. The CP also supplies the brain with certain nutrients, hormones, and metal ions, while removing metabolites and xenobiotics from the CSF. Numerous membrane transporters are expressed in the CP in order to facilitate the solute exchange between the blood and the CSF. The solute carrier (SLC) superfamily represents a major class of transporters in the CP that constitutes the molecular mechanisms for CP function. Recently, we systematically and quantitatively examined Slc gene expression in 20 anatomically comprehensive brain areas in the adult mouse brain using high-quality in situ hybridization data generated by the Allen Brain Atlas. Here we focus our analysis on Slc gene expression at the BCSFB using previously obtained data. Of the 252 Slc genes present in the mouse brain, 202 Slc genes were found at detectable levels in the CP. Unsupervised hierarchical cluster analysis showed that the CP Slc gene expression pattern is substantially different from the other 19 analyzed brain regions. The majority of the Slc genes in the CP are expressed at low to moderate levels, whereas 28 Slc genes are present in the CP at the highest levels. These highly expressed Slc genes encode transporters involved in CSF secretion, energy production, and transport of nutrients, hormones, neurotransmitters, sulfate, and metal ions. In this review, the functional characteristics and potential importance of these Slc transporters in the CP are discussed, with particular emphasis on their localization and physiological functions at the BCSFB.

Transforming dietary peptides in promising lead compounds: the case of bioavailable carnosine analogs

The ability of carnosine to prevent advanced glycoxidation end products (AGEs) and advanced lipoxidation end products (ALEs) formation, on the one hand, and the convincing evidence that these compounds act as pathogenetic factors, on the other hand, strongly support carnosine as a promising therapeutic agent for oxidative-based diseases. The mechanism/s by which carnosine inhibits AGEs and ALEs is still under investigation but an emerging hypothesis is that carnosine acts by deactivating the AGEs and ALEs precursors and in particular the reactive carbonyl species (RCS) generated by both lipid and sugar oxidation. The ability of carnosine to inhibit AGEs and ALEs formation and the corresponding biological effects has been demonstrated in several in vitro studies and in some animal models. However, such effects are in line of principle, limited in humans, due to the effect of serum carnosinase (absent in rodents), which catalyzes the carnosine hydrolysis to its constitutive amino acids. Such a limitation has prompted a great interest in the design of carnosine derivatives, which maintaining (or improving) the reactivity with RCS, are more resistant to carnosinase. The present paper intends to critically review the most recent studies oriented to obtaining carnosine derivatives, optimized in terms of reactivity with RCS, selectivity (no reaction with physiological aldehydes) and the pharmacokinetic profile (mainly through an enhanced resistance to carnosinase hydrolysis). The review also includes a brief description of AGEs and ALEs as drug targets and the evidence so far reported regarding the ability of carnosine as inhibitor of AGEs and ALEs formation and the proposed reaction mechanisms.

Characterizing the impact of renal impairment on the clinical pharmacology of biologics

Similar to small-molecule drugs, there is also concern for protein-based therapeutics about their clinical use in patients with renal impairment including renal insufficiency and end-stage renal disease, which may modulate the efficacy and/or safety profile of these compounds. Theoretical considerations and clinical evidence suggest that the kidneys play a relevant role in the catabolism and thus elimination of only those protein therapeutics that have a size below the cutoff for glomerular filtration of approximately 60 kDa. Thus, the effect of renal impairment on protein therapeutics seems to be predictable and only relevant for compounds below this molecular weight cutoff. This is supported by clinical evidence that shows a lack of effect of renal impairment on large proteins such as monoclonal antibodies, whereas smaller proteins below the cutoff such as interleukin-10, growth hormone, erythropoietin, and anakinra experience a gradual decrease of their clearance and increase of their systemic exposure with increasing severity of renal impairment. Thus, dedicated renal impairment studies are warranted in the clinical development program of protein therapeutics that undergo glomerular filtration to establish the scientific rationale for their safe and efficacious use in patients with renal insufficiency.