Chloride anion transporters inhibit growth of methicillin-resistant Staphylococcus aureus (MRSA) in vitro

Unveiling the potent activity of a synthetic ion transporter against multidrug-resistant Gram-positive bacteria and biofilms

The increasing prevalence of drug-resistant infections caused by Gram-positive bacteria poses a significant threat to public healthcare. These pathogens exhibit not only smart resistance mechanisms but also form impenetrable biofilms on various surfaces, rendering them resilient to conventional therapies. In this study, we present the potent antibacterial activity of a synthetic ion transporter T against multi-drug resistant (MDR) Gram-positive pathogens, with minimum inhibitory concentration (MIC) values ranging from 0.5 to 2 μg mL-1. The compound demonstrates high selectivity with negligible toxicity towards mammalian cells (HC50 = 810 μg mL-1). It exhibits fast killing kinetics, completely eliminating >5 log bacterial cells within 12 h. Moreover, the compound displays efficacy against both planktonic bacteria and preformed biofilms of methicillin-resistant S. aureus (MRSA), reducing the bacterial burden within the biofilm by 2 log. Mechanistic investigations reveal that the ion transporter depolarizes the bacterial membrane potential and enhances membrane permeability. Additionally, it generates reactive oxygen species, contributing to its bactericidal activity. Notably, MRSA did not exhibit detectable resistance to the ion transporter even after serial passaging for 10 days. Collectively, this novel class of ion transporter holds promise as a therapeutic candidate for combating infections caused by multi-drug resistant Gram-positive bacteria.

Pyridyl-Linked Hetero Hydrazones: Transmembrane H+/Cl- Symporters with Efficient Antibacterial Activity

The development of potent antibacterial agents has become increasingly difficult as bacteria continue to evolve and develop resistance to antibiotics. It is therefore imperative to find effective antimicrobial agents that can address the evolving challenges posed by infectious diseases and antimicrobial resistance. Using artificial transmembrane ion transporters is an emerging and promising avenue to address this issue. We report pyridyl-linked hetero hydrazones as highly efficient transmembrane HCl symporters. These compounds offer an appropriate HCl binding site through cooperative protonation, followed by recognition of chloride ions. HCl transport by these compounds inhibits the growth of different Gram-negative bacterial strains with high efficacy by affecting the cell envelope homeostasis. This specific class of compounds holds substantial promise in the ongoing pursuit of developing highly efficient antibacterial agents.

A chloride-responsive molecular switch: driving ion transport and empowering antibacterial properties

A molecular switch was developed to recognize and transport Cl- across lipid bilayers. The XRD-crystal structure and NOESY NMR spectra of a potent 4-aminoquinazoline analogue confirmed Cl--induced conformation changes. Systematic biophysical studies revealed that the quinazoline moiety forms cooperative interactions of H+ and Cl- ions with the thiourea moiety, resulting in the transport of H+/Cl- across the membranes. A pH-dependent analysis revealed that the transport of Cl- by the potent compound increased in an acidic environment. The potent compound could also transport H+/Cl- across Gram-positive bacteria, leading to antibacterial activities.

Stimulus-Controlled Anion Binding and Transport by Synthetic Receptors

Anionic species are omnipresent and involved in many important biological processes. A large number of artificial anion receptors has therefore been developed. Some of these are capable of mediating transmembrane transport. However, where transport proteins can respond to stimuli in their surroundings, creation of synthetic receptors with stimuli-responsive functions poses a major challenge. Herein, we give a full overview of the stimulus-controlled anion receptors that have been developed thus far, including their application in membrane transport. In addition to their potential operation as membrane carriers, the use of anion recognition motifs in forming responsive membrane-spanning channels is discussed. With this review article, we intend to increase interest in transmembrane transport among scientists working on host-guest complexes and dynamic functional systems in order to stimulate further developments.

A potent fluorescent transmembrane HCl transporter perturbs cellular pH and promotes cancer cell death

A series of fluorescent coumarin bis-ureas 1-4 have been synthesised, and their anion transport properties studied. The compounds function as highly potent HCl co-transport agents in lipid bilayer membranes. Single crystal X-ray diffraction of compound 1 showed antiparallel stacking of the coumarin rings, stabilised by hydrogen bonds. Binding studies, using ¹H-NMR titration, showed moderate chloride binding in DMSO-d₆/0.5% with 1 : 1 binding mode (for transporter 1) and 1 : 2 binding mode (host: guest, for transporters 2-4). We examined the cytotoxicity of compounds 1-4 against three cancer cell lines, lung adenocarcinoma (A549), colon adenocarcinoma (SW620) and breast adenocarcinoma (MCF-7). The most lipophilic transporter, 4 showed a cytotoxic effect against all three cancer cell lines. Cellular fluorescence studies showed compound 4 crossed the plasma membrane and localised in the cytoplasm after a short time. Interestingly, compound 4, lacking any lysosome targeting groups, was co-localised with LysoTracker Red at 4 and 8 h in the lysosome. Cellular anion transport of compound 4 was assessed by measuring intracellular pH and showed a decrease in cellular pH, which may be due to the capacity of transporter 4 to co-transport HCl across biological membranes, as evidenced by the liposomal studies.

GltS regulates biofilm formation in methicillin-resistant Staphylococcus aureus

Biofilm-based infection is a major healthcare burden. Methicillin-resistant Staphylococcus aureus (MRSA) is one of major organisms responsible for biofilm infection. Although biofilm is induced by a number of environmental signals, the molecule responsible for environmental sensing is not well delineated. Here we examined the role of ion transporters in biofilm formation and found that the sodium-glutamate transporter gltS played an important role in biofilm formation in MRSA. This was shown by gltS transposon mutant as well as its complementation. The lack of exogenous glutamate also enhanced biofilm formation in JE2 strain. The deficiency of exogenous glutamate intake accelerated endogenous glutamate/glutamine production, which led to the activation of the urea cycle. We also showed that urea cycle activation was critical for biofilm formation. In conclusion, we showed that gltS was a critical regulator of biofilm formation by controlling the intake of exogenous glutamate. An intervention to target glutamate intake may be a potential useful approach against biofilm.

pH-Dependent transport of amino acids across lipid bilayers by simple monotopic anion carriers

The transport of amino acids across lipid membranes is vital for the proper functioning of every living cell. In spite of that, examples of synthetic transporters that can facilitate amino acid transport are rare. This is mainly because at physiological conditions amino acids predominantly exist as highly polar zwitterions and proper shielding of their charged termini, which is necessary for fast diffusion across lipophilic membranes, requires complex and synthetically challenging heteroditopic receptors. Here we report the first simple monotopic anion receptor, dithioamide 1, that efficiently transports a variety of natural amino acids across lipid bilayers at physiological pH. Mechanistic studies revealed that the receptor rapidly transports deprotonated amino acids, even though at pH 7.4 these forms account for less than 3% of the total amino acid concentration. We also describe a new fluorescent assay for the selective measurement of the transport of deprotonated amino acids into liposomes. The new assay allowed us to study the pH-dependence of amino acid transport and elucidate the mechanism of transport by 1, as well as to explain its exceptionally high activity. With the newly developed assay we screened also four other representative examples of monotopic anion transporters, of which two showed promising activity. Our results imply that heteroditopic receptors are not necessary for achieving high amino acid transport activities and that many of the previously reported anionophores might be active amino acid transporters. Based on these findings, we propose a new strategy for the development of artificial amino acid transporters with improved properties.

Dissecting transmembrane bicarbonate transport by 1,8-di(thio)amidocarbazoles

Synthetic ionophores able to transport bicarbonate and chloride anions across lipid bilayers are appealing for their wide range of potential biological applications. We have studied the bicarbonate and chloride transport by carbazoles with two amido/thioamido groups using a bicarbonate-sensitive europium(III) probe in liposomes and found a highly remarkable transporter concentration dependence. This can be explained by a combination of two distinct transport mechanisms: HCO₃^-/Cl^- exchange and a combination of unassisted CO₂ diffusion and HCl transport, of which the respective contributions were quantified. The compounds studied were found to be highly potent HCl transporters. Based on the mechanistic insights on anion transport, we have tested the antimicrobial activity of these compounds and found a good correlation with their ion transport properties and a high activity against Gram-positive bacteria.

Recent Advances in Bioactive Artificial Ionophores

Several life-threatening diseases, also known as 'Channelopathies' are linked to irregularities in ion transport proteins. Significant research efforts have fostered the development of artificial transport systems that facilitates to restore the functions of impaired natural transport proteins. Indeed, a few of these artificial ionophores demonstrate the rare combination of transmembrane ion transport and important biological activity, offering early promises of suitability in 'channel replacement therapy'. In this review, structural facets and functions of both cationophores and anionophores are discussed. Ionophores that are toxic to various bacteria and yeast, could be exploited as antimicrobial agent. Nevertheless, few non-toxic ionophores offer the likelihood of treating a wide range of genetic diseases caused by the gene mutations. In addition, their ability to disrupt cellular homeostasis and to alter lysosomal pH endow ionophores as promising candidates for cancer treatment. Overall, critically outlining the advances in artificial ionophores in terms of in vitro ion transport, possible modes of action and biological activities enables us to propose possible future roadmaps in this research area.

Chemical synthesis, molecular docking and MepA efflux pump inhibitory effect by 1,8-naphthyridines sulfonamides

This study aimed to evaluate the antibacterial activity and to verify, in silico and in vitro, the inhibition of efflux mechanisms using a series of synthesized 1,8-naphthyridines sulfonamides against Staphylococcus aureus strains carrying MepA efflux pumps. The chemical synthesis occurred through the thermolysis of the Meldrum's acid adduct. The sulfonamide derivatives were obtained by the sulfonylation of 2-amino-5‑chloro-1,8-naphthyridine with commercial benzenesulfonyl chloride. Antibacterial activity was assessed by the broth microdilution test. Efflux pump inhibitory capacity was evaluated in silico by molecular docking and in vitro by analyzing synergistic effects on ciprofloxacin and ethidium bromide (EtBr) and by EtBr fluorescence emission assays. The following 1,8-naphthyridines were synthesized: 4-methyl-N-(5‑chloro-1,8-naphthyridin-2-yl)-benzenesulfonamide (Compound 10a); 2,5-dichloro-N-(5‑chloro-1,8-naphthyridin-2-yl)-benzenesulfonamide (Compound 10b); 4-fluoro-N-(5‑chloro-1,8-naphthyridin-2-yl)-benzenesulfonamide (Compound 10c); 2,3,4-trifluoro-N-(5‑chloro-1,8-naphthyridin-2-yl)-benzenesulfonamide (Compound 10d); 3-trifluoromethyl-N-(5‑chloro-1,8-naphthyridin-2-yl)-benzenesulfonamide (Compound 10e); 4‑bromo-2,5-difluoro-N-(5‑chloro-1,8-naphthyridin-2-yl)-benzenesulfonamide (Compound 10f). The 1,8-naphthyridines derivatives associated with sulfonamides did not show antibacterial activity. However, they showed a favorable pharmacokinetic profile with possible MepA efflux pump inhibitory action, demonstrated in molecular docking. In addition to the promising results in reducing the concentration of intracellular EtBr. 1,8-naphthyridines act as putative agents in the inhibitory action of the MepA efflux pump.

Advances in anion transport and supramolecular medicinal chemistry

Advances in anion transport by synthetic supramolecular systems are discussed in this article. Developments in the design of discrete molecular carriers for anions and supramolecular anion channels are reviewed followed by an overview of the use of these systems in biological systems as putative treatments for diseases such as cystic fibrosis and cancer.

A Class of Potent Inhibitors of the HIV-1 Nucleocapsid Protein Based on Aminopyrrolic Scaffolds

The HIV-1 nucleocapsid protein 7 (NC) is a potential target for effective antiretroviral therapy due to its central role in virus replication, mainly linked to nucleic acid (NA) chaperone activity, and low susceptibility to drug resistance. By screening a compounds library, we identified the aminopyrrolic compound CN14_17, a known carbohydrate binding agent, that inhibits the NC chaperone activity in the low micromolar range. Different from most of available NC inhibitors, CN14_17 fully prevents the NC-induced annealing of complementary NA sequences. Using fluorescence assays and isothermal titration calorimetry, we found that CN14_17 competes with NC for the binding to NAs, preferentially targeting single-stranded sequences. Molecular dynamics simulations confirmed that binding to cTAR occurs preferably within the guanosine-rich single stranded sequence. Finally, CN14_17 exhibited antiretroviral activity in the low micromolar range, although with a moderate therapeutic index. Overall, CN14_17 might be the progenitor of a new promising class of NC inhibitors.

Oxyanion transport across lipid bilayers: direct measurements in large and giant unilamellar vesicles

A simple di(thioamido)carbazole 1 serves as a potent multispecific transporter for various biologically relevant oxyanions, such as drugs, metabolites and model organic phosphate. The transport kinetics of a wide range of oxyanions can be easily quantified by a modified lucigenin assay in both large and giant unilamellar vesicles.

Biological applications of synthetic anion transporters

Transmembrane transport of anions by small molecules has recently been used to reduce the viability of cancer cells and fight against antibiotic-resistant and clinically relevant bacterial strains.

A highly-selective chloride microelectrode based on a mercuracarborand anion carrier

The chloride gradient plays an important role in regulating cell volume, membrane potential, pH, secretion, and the reversal potential of inhibitory glycine and GABA_A receptors. Measurement of intracellular chloride activity, \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${{\boldsymbol{a}}}_{{\boldsymbol{Cl}}}^{{\boldsymbol{i}}}$$\end{document}aCli, using liquid membrane ion-selective microelectrodes (ISM), however, has been limited by the physiochemical properties of Cl⁻ ionophores which have caused poor stability, drift, sluggish response times, and interference from other biologically relevant anions. Most importantly, intracellular \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\bf{HC}}{{\bf{O}}}_{{\bf{3}}}^{-}$$\end{document}HCO3− may be up to 4 times more abundant than Cl⁻ (e.g. skeletal muscle) which places severe constraints on the required selectivity of a Cl⁻ – sensing ISM. Previously, a sensitive and highly-selective Cl⁻ sensor was developed in a polymeric membrane electrode using a trinuclear Hg(II) complex containing carborane-based ligands, [9]-mercuracarborand-3, or MC3 for short. Here, we have adapted the use of the MC3 anion carrier in a liquid membrane ion-selective microelectrode and show the MC3-ISM has a linear Nernstian response over a wide range of a_Cl (0.1 mM to 100 mM), is highly selective for Cl⁻ over other biological anions or inhibitors of Cl⁻ transport, and has a 10% to 90% settling  time of 3  sec. Importantly, over the physiological range of a_Cl (1 mM to 100 mM) the potentiometric response of the MC3-ISM is insensitive to \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\bf{HC}}{{\bf{O}}}_{{\bf{3}}}^{-}$$\end{document}HCO3− or changes in pH. Finally, we demonstrate the biological application of an MC3-ISM by measuring intracellular a_Cl, and the response to an external Cl-free challenge, for an isolated skeletal muscle fiber.

Anion carriers as potential treatments for cystic fibrosis: transport in cystic fibrosis cells, and additivity to channel-targeting drugs

Defective anion transport is a hallmark of the genetic disease cystic fibrosis (CF). One approach to restore anion transport to CF cells utilises alternative pathways for transmembrane anion transport, including artificial anion carriers (anionophores). Here, we screened 22 anionophores for biological activity using fluorescence emission from the halide-sensitive yellow fluorescent protein. Three compounds possessed anion transport activity similar to or greater than that of a bis-(p-nitrophenyl)ureidodecalin previously shown to have promising biological activity. Anion transport by these anionophores was concentration-dependent and persistent. All four anionophores mediated anion transport in CF cells, and their activity was additive to rescue of the predominant disease-causing variant F508del-CFTR using the clinically-licensed drugs lumacaftor and ivacaftor. Toxicity was variable but minimal at the lower end. The results provide further evidence that anionophores, by themselves or together with other treatments that restore anion transport, offer a potential therapeutic strategy for CF.

Small molecule anion transporters display in vitro antimicrobial activity against clinically relevant bacterial strains

Highly active transmembrane anion transporters have demonstrated their activity against antibiotic-resistant and clinically relevant bacterial strains.

Diphenylethylenediamine-Based Potent Anionophores: Transmembrane Chloride Ion Transport and Apoptosis Inducing Activities

Synthetic anion transporters have been recognized as one of the potential therapeutic agents for the treatment of diseases including cystic fibrosis, myotonia, and epilepsy that originate due to the malfunctioning of natural Cl^- ion transport systems. Recent studies showed that the synthetic Cl^- ion transporters can also disrupt cellular ion-homeostasis and induce apoptosis in cancer cell lines, leading to a revived attention for synthetic Cl^- ion transporters. Herein, we report the development of conformationally controlled 1,2-diphenylethylenediamine-based bis(thiourea) derivatives as a new class of selective Cl^- ion carrier. The strong Cl^- ion binding properties ( K_d = 3.87-6.66 mM) of the bis(thiourea) derivatives of diamine-based compounds correlate well with their transmembrane anion transport activities (EC₅₀ = 2.09-4.15 nM). The transport of Cl^- ions via Cl^-/NO₃^- antiport mechanism was confirmed for the most active molecule. Perturbation of Cl^- ion homeostasis by this anion carrier induces cell death by promoting the caspase-mediated intrinsic pathway of apoptosis.

A well-defined unimolecular channel facilitates chloride transport

A unimolecular ion channel was optimized by functionalization with a new type of rigid-rod oligomer. The macrocycle pendant endows chloride selectivity and the fluorescence feature and suitable length of the rod facilitates the visual insertion of channels into the lipid bilayer, resulting in efficient ion transport with an EC₅₀ value of 0.36 μM.

A ruthenium(iii) complex derived from N,N′-bis(salicylidene)ethylenediamine as a chemosensor for the selective recognition of acetate and its interaction with cells for bio-imaging: experimental and theoretical studies

A ruthenium(iii) complex ofN,N′-bis(salicylidene)ethylenediamine (L¹) was used as chemosensor for the recognition of acetate in cells for bio-imaging.

Synthesis of a dimeric 3α-hydroxy-7α,12α-diamino-5β-cholan-24-oate conjugate and its derivatives, and the effect of lipophilicity on their anion transport efficacy

A dimeric 3α-hydroxy-7α,12α-diamino-5β-cholan-24-oate conjugate and its derivatives were synthesized, and lipophilicity was found to significantly affect their anion transport efficacy.

Anion transport and supramolecular medicinal chemistry

New approaches to the transmembrane transport of anions are discussed in this review.

Tris-ureas as transmembrane anion transporters

Tris ureas can mediate the exchange of Cl⁻/NO₃⁻ and also Cl⁻/HCO₃⁻ across bilayer membranes.