Aqueous solubilization of transmembrane peptide sequences with retention of membrane insertion and function

Branched Amphipathic Peptide Capsules: Different Ratios of the Two Constituent Peptides Direct Distinct Bilayer Structures, Sizes, and DNA Transfection Efficiency

Branched amphipathic peptide capsules (BAPCs) are biologically derived, bilayer delimited, nanovesicles capable of being coated by or encapsulating a wide variety of solutes. The vesicles and their cargos are readily taken up by cells and become localized in the perinuclear region of cells. When BAPCs are mixed with DNA, the BAPCs act as cationic nucleation centers around which DNA winds. The BAPCs-DNA nanoparticles are capable of delivering plasmid DNA in vivo and in vitro yielding high transfection rates and minimal cytotoxicity. BAPCs share several biophysical properties with lipid vesicles. They are however considerably more stable-resisting disruption in the presence of chaotropes such as urea and guanidinium chloride, anionic detergents, proteases, and elevated temperature (∼95 °C). To date, all of our published results have utilized BAPCs that are composed of equimolar concentrations of the two branched sequences (Ac-FLIVI)₂-K-K₄-CO-NH₂ and (Ac-FLIVIGSII)₂-K-K₄-CO-NH₂. The mixture of sizes was utilized to relieve potential curvature strain in the spherical capsule. In this article, different molar ratios of the two peptides were studied to test whether alternate ratios produced BAPCs with different biological and biophysical properties. Additionally, preparation (annealing) temperature was included as a second variable.

Theoretical and Experimental: The Synthetic and Anion-Binding Properties of Tripodal Salicylaldehyde Derivatives

A series of colorimetric anion probes 1–6 containing OH and NO₂ groups were synthesized, and their recognition properties toward various anions were investigated by visual observation, ultraviolet–visible spectroscopy, fluorescence, ¹H nuclear magnetic resonance titration spectra and theoretical investigation. Nanomaterials of three compounds 2–4 were prepared successfully. Four compounds 3–6 that contain electron-withdrawing substituents showed a high binding ability for AcO⁻. The host–guest complex formed through a 1:1 binding ratio, and color changes were detectable during the recognition process. Theoretical investigation analysis revealed that an intramolecular hydrogen bond existed in the structures of compounds and the roles of molecular frontier orbitals in molecular interplay. These studies suggested that this series of compounds could be used as colorimetric probes to detect of AcO⁻.

Dual-channel detection of Cu(2+) and F(-) with a simple Schiff-based colorimetric and fluorescent sensor

A simple and easily synthesized colorimetric and fluorescent receptor 1, based on 4-diethylaminosalicylaldehyde moieties as a binding and signaling unit, has been synthesized and characterized. The receptor 1 has a selective colorimetric sensing ability for copper (II) ion by changing color from colorless to yellow in aqueous solution, and could be utilized to monitor Cu(II) over a wide pH range of 4-11. In addition, the detection limit (12μM) of 1 for Cu(2+) is much lower than that (30μM) recommended by WHO in drinking water, and its copper complex could be reversible simply through treatment with a proper reagent such as EDTA. Moreover, receptor 1 exhibited both a color change from colorless to yellow and fluorescence enhancement with a red shift upon addition to F(-) in DMSO. The recognition mechanism was attributed to the intermolecular proton transfer between the hydroxyl group of the receptor and the fluoride.

Chromogenic naked-eye detection of copper ion and fluoride

Colorimetric chemosensor was reported for detection of Cu²⁺ and F⁻via the color change from colorless to yellow and to orange.

A diaminomaleonitrile based selective colorimetric chemosensor for copper(ii) and fluoride ions

A chemosensor showed colorimetric sensing for copper(ii) and fluoride by changing color from yellow to colorless and to orange.

Free energy analysis of conductivity and charge selectivity of M2GlyR-derived synthetic channels

Significant progresses have been made in the design, synthesis, modeling and in vitro testing of channel-forming peptides derived from the second transmembrane domain of the α-subunit of the glycine receptor (GlyR). The latest designs, including p22 (KKKKP ARVGL GITTV LTMTT QS), are highly soluble in water with minimal aggregation propensity and insert efficiently into cell membranes to form highly conductive ion channels. The last obstacle to a potential lead sequence for channel replacement treatment of CF patients is achieving adequate chloride selectivity. We have performed free energy simulation to analyze the conductance and charge selectivity of M2GlyR-derived synthetic channels. The results reveal that the pentameric p22 pore is non-selective. Moderate barriers for permeation of both K(+) and Cl(-) are dominated by the desolvation cost. Despite previous evidence suggesting a potential role of threonine side chains in anion selectivity, the hydroxyl group is not a good surrogate of water for coordinating these ions. We have also tested initial ideas of introducing additional rings of positive changes to various positions along the pore to increase anion selectivity. The results support the feasibility of achieving anion selectivity by modifying the electrostatic properties of the pore, but at the same time suggest that the peptide assembly and pore topology may also be dramatically modified, which could abolish the effects of modified electrostatics on anion selectivity. This was confirmed by subsequent two-electrode voltage clamp measurements showing that none of the tested mono-, di- and tri-Dap substituted sequences was selective. The current study thus highlights the importance of controlling channel topology besides modifying pore electrostatics for achieving anion selectivity. Several strategies are now being explored in our continued efforts to design an anion selective peptide channel with suitable biophysical, physiological and pharmacological properties as a potential treatment modality for channel replacement therapy. This article is part of a Special Issue entitled: Interfacially Active Peptides and Proteins. Guest Editors: William C. Wimley and Kalina Hristova.

Effect of diaminopropionic acid (Dap) on the biophysical properties of a modified synthetic channel-forming peptide

Channel replacement therapy, based on synthetic channel-forming peptides (CFPs) with the ability to supersede defective endogenous ion channels, is a novel treatment modality that may augment existing interventions against multiple diseases. Previously, we derived CFPs from the second transmembrane segment of the α-subunit of the glycine receptor, M2GlyR, which forms chloride-selective channels in its native form. The best candidate, NK4-M2GlyR T19R, S22W (p22-T19R, S22W), was water-soluble, incorporated into cell membranes and was nonimmunogenic, but lacked the structural properties for high conductance and anion selectivity when assembled into a pore. Further studies suggested that the threonine residues at positions 13, 17, and 20 line the pore of assembled p22-T19R, S22W, and here we used 2,3-diaminopropionic acid (Dap) substitutions to introduce positive charges to the pore-lining interface of the predicted p22-T19R, S22W channel. Dap-substituted p22-T19R, S22W peptides retained the α-helical secondary structure characteristic of their parent peptide, and induced short-circuit transepithelial currents when exposed to the apical membrane of Madin-Darby canine kidney (MDCK) cells; the sequences containing multiple Dap-substituted residues induced larger currents than the peptides with single or no Dap substitutions. To gain further insights into the effects of Dap residues on the properties of the putative pore, we performed two-electrode voltage clamp electrophysiology on Xenopus oocytes exposed to p22-T19R, S22W or its Dap-modified analogues. We observed that Dap-substituted peptides also induced significantly larger voltage-dependent currents than the parent compound, but there was no apparent change in reversal potential upon replacement of external Na+, Cl- or K+, indicating that these currents remained nonselective. These results suggest that the introduction of positively charged side chains in predicted pore-lining residues does not improve anion-to-cation selectivity, but results in higher conductance, perhaps due to higher oligomerization numbers.

Reversible sol-to-gel transformation of uracil gelators: specific colorimetric and fluorimetric sensor for fluoride ions

A new type of anthracene organogelator based on uracil was obtained using organic aromatic solvents, cyclohexane, DMSO, ethanol, and ethyl acetate. It was further characterized by field-emission scanning electron microscopy and transmission electron microscopy. Specifically, the resulting organogels were demonstrated to be promising colorimetric and fluorescent sensors toward fluoride ions with high sensitivity and selectivity, accompanying the disruption of the gelators. Spectroscopic study and (1)H NMR titration experiment revealed that the deprotonation of the hydrogen atom on the N position of uracil moiety by fluoride ions is responsible for the recognition events, evidenced by immediate transformation from the sol phase to the gel state upon adding a small amount of a proton solvent, methanol. The process is reversible, with zero loss in sensing activity and sol-to-gel transformation ability even after five runs.

Role of bilirubin oxidation products in the pathophysiology of DIND following SAH

Despite intensive research efforts, by our own team and many others, the molecules responsible for acute neurological damage following subarachnoid hemorrhage (SAH) and contributing to delayed ischemic neurological deficit (DIND) have not yet been elucidated. While there are a number of candidate mechanisms, including nitric oxide (NO) scavenging, endothelin-1, protein kinase C (PKC) activation, and rho kinase activation, to name but a few, that have been investigated using animal models and human trials, we are, it seems, no closer to discovering the true nature of this complex and enigmatic pathology. Efforts in our laboratory have focused on the chemical milieu present in hemorrhagic cerebrospinal fluid (CSF) following SAH and the interaction of the environment with the molecules generated by SAH and subsequent events, including NO scavenging, immune response, and clot breakdown. We have identified and characterized a group of molecules formed by the oxidative degradation of bilirubin (a clot breakdown product) and known as BOXes (bilirubin oxidation products). We present a synopsis of the characterization of BOXes as found in human SAH patients' CSF and the multiple signaling pathways by which BOXes act. In summary, BOXes are likely to play an essential role in the etiology of acute brain injury following SAH, as well as DIND.

Small β2-glycoprotein I peptides protect from intestinal ischemia reperfusion injury

Intestinal ischemic events, which are followed by reperfusion, induce significant tissue damage and frequently result in multiple organ failure, with >70% mortality. Upon reperfusion, excessive inflammation leads to exacerbated tissue damage. Previous studies indicated that binding of the serum protein, β2-glycoprotein I, to the endothelium initiates a cascade of inflammatory molecules that is required for damage. We hypothesized that peptides derived from the binding domain (domain V) of β2-glycoprotein I would attenuate ischemia/reperfusion-induced damage and inflammation in a therapeutic manner. Using a mouse model of intestinal ischemia/reperfusion, we administered peptides either prior to ischemia or at clinically relevant time points during reperfusion and evaluated intestinal tissue damage and inflammation after 2 h of reperfusion. We demonstrate that multiple peptides attenuate injury and inflammation in a dose-dependent manner and, perhaps more significantly, are efficacious when administered up to 30 min after the onset of reperfusion. In addition, an all D-amino acid retro-inverso peptide was biologically active. Thus, the β2-glycoprotein I-derived peptides attenuate injury and inflammation when administered in a therapeutic manner in intestinal ischemia/reperfusion injury.

Effect of the synthetic NC-1059 peptide on diffusion of riboflavin across an intact corneal epithelium

PURPOSE

To investigate the effect of the peptide NC-1059 on riboflavin (RF) diffusion across an intact corneal epithelium into the stroma.

METHODS

NC-1059 peptide was synthesized by solid-phase synthesis with 9-fluorenylmethoxycarbonyl chemistry, characterized by reversed-phase HPLC, and matrix-assisted laser desorption ionization time-of-flight mass spectroscopy. The diffusion of RF across embryonic day 18 chick corneal epithelium ex vivo was monitored using confocal microscopy. The depth distributions of RF in the corneal stroma were calculated using a group of linear equations based on the relationship between RF fluorescence intensity and concentration.

RESULTS

Data presented in this study demonstrate that the NC-1059 peptide can transiently open the intact epithelial barrier to allow the permeation of RF into the stroma. The effect of NC-1059 peptide on RF diffusion across the corneal epithelium was concentration and time dependent. The amount of RF reaching a 50-μm depth of chick corneal stoma increased dramatically after exposure to NC-1059 for 10 minutes, reaching a plateau by 30 minutes. The concentrations of RF in the presence of NC-1059 at corneal stromal depths of 50, 100, and 150 μm were significantly higher than in the absence of the peptide, and almost as high as in corneas in which the epithelium first had been physically removed. In addition, a cell viability assay indicated that the NC-1059 peptide did not kill corneal epithelial cells.

CONCLUSIONS

NC-1059 peptide significantly enhances the diffusion of RF across intact corneal epithelium into the stroma.

Structural and biophysical properties of a synthetic channel-forming peptide: designing a clinically relevant anion selective pore

The design, synthesis, modeling and in vitro testing of channel-forming peptides derived from the cys-loop superfamily of ligand-gated ion channels are part of an ongoing research focus. Over 300 different sequences have been prepared based on the M2 transmembrane segment of the spinal cord glycine receptor α-subunit. A number of these sequences are water-soluble monomers that readily insert into biological membranes where they undergo supramolecular assembly, yielding channels with a range of selectivities and conductances. Selection of a sequence for further modifications to yield an optimal lead compound came down to a few key biophysical properties: low solution concentrations that yield channel activity, greater ensemble conductance, and enhanced ion selectivity. The sequence NK(4)-M2GlyR T19R, S22W (KKKKPARVGLGITTVLTMRTQW) addressed these criteria. The structure of this peptide has been analyzed by solution NMR as a monomer in detergent micelles, simulated as five-helix bundles in a membrane environment, modified by cysteine-scanning and studied for insertion efficiency in liposomes of selected lipid compositions. Taken together, these results define the structural and key biophysical properties of this sequence in a membrane. This model provides an initial scaffold from which rational substitutions can be proposed and tested to modulate anion selectivity. This article is part of a Special Issue entitled: Protein Folding in Membranes.

Structural characterization of two pore-forming peptides: consequences of introducing a C-terminal tryptophan

Synthetic channel-forming peptides that can restore chloride conductance across epithelial membranes could provide a novel treatment of channelopathies such as cystic fibrosis. Among a series of 22-residue peptides derived from the second transmembrane segment of the glycine receptor alpha(1)-subunit (M2GlyR), p22-S22W (KKKKP ARVGL GITTV LTMTT QW) is particularly promising with robust membrane insertion and assembly. The concentration to reach one-half maximal short circuit current is reduced to 45 +/- 6 microM from that of 210 +/- 70 microM of peptide p22 (KKKKP ARVGL GITTV LTMTT QS). However, this is accompanied with nearly 50% reduction in conductance. Toward obtaining a molecular level understanding of the channel activities, we combine information from solution NMR, existing biophysical data, and molecular modeling to construct atomistic models of the putative pentameric channels of p22 and p22-S22W. Simulations in membrane bilayers demonstrate that these structural models, even though highly flexible, are stable and remain adequately open for ion conductance. The membrane-anchoring tryptophan residues not only rigidify the whole channel, suggesting increased stability, but also lead to global changes in the pore profile. Specifically, the p22-S22W pore has a smaller opening on average, consistent with lower measured conductance. Direct observation of several incidences of chloride transport suggests several qualitative features of how these channels might selectively conduct anions. The current study thus helps to rationalize the functional consequences of introducing a single C-terminal tryptophan. Availability of these structural models also paves the way for future work to rationally modify and improve M2GlyR-derived peptides toward potential peptide-based channel replacement therapy.

Mode of interaction between the HIV-1-neutralizing monoclonal antibody 2F5 and its epitope

OBJECTIVE

To determine the mechanism of interaction between the HIV-1 gp41-specific broadly neutralizing monoclonal antibody (mAb) 2F5, its epitope in the membrane proximal external region and a domain located in the fusion peptide proximal region in the N-terminal region of gp41. Knowledge of these interactions would be useful for the design of antigens used to induce 2F5-like antibodies.

METHODS

The binding and avidity of the mAb 2F5 were analyzed using enzyme-linked immunosorbent assays, epitope mapping and surface plasmon resonance analysis. Inhibition of virus neutralization by 2F5 was analyzed using peptides corresponding to the gp41 sequence.

RESULTS

Using transmembrane envelope proteins of gammaretroviruses, we had previously induced neutralizing antibodies that recognize two epitopes, one located in the N-terminal part of the transmembrane protein (designated E1) and the other in the C-terminal membrane proximal external region (E2). The E2 epitope corresponds to the mAb 2F5/4E10 epitope in the gp41 of HIV and we have now identified a corresponding E1 domain in gp41. Although 2F5 did not bind directly to E1, the presence of E1 peptides increased the binding of 2F5 to peptides carrying its epitope. Neutralization of HIV-1 by 2F5 was inhibited more effectively by both gp41-derived peptides E1 and E2 together than with the peptide E2 alone.

CONCLUSION

The interaction between the E1 and E2 domains of gp41 increased the efficacy of mAb 2F5 binding to its E2 epitope. Such an interaction may occur after gp41 folding into a six-helix bundle. Antigens containing both domains might be used to induce broadly neutralizing 2F5-like antibodies.

NC-1059: a channel-forming peptide that modulates drug delivery across in vitro corneal epithelium

PURPOSE

The goal of this study was to determine whether a synthetic peptide, NC-1059, can modulate the corneal epithelium to increase the permeation of therapeutic agents across this barrier.

METHODS

An in vitro system employing transformed human corneal epithelial (THCE) cells was optimized for this study. Culture conditions were identified to promote formation of a confluent monolayer that rapidly develops a substantial transepithelial electrical resistance. Electrical parameters were measured with a modified Ussing flux chamber, and solute flux was quantified with fluorescently labeled compounds.

RESULTS

NC-1059 causes a concentration-dependent increase in short-circuit current and an increase in transepithelial electrical conductance when assessed in modified Ussing chambers. The effect of NC-1059 on transepithelial electrical resistance was reversible. To test for paracellular permeability and size exclusion, FITC-labeled dextran ranging in size from 10 to 70 kDa was used. Dextran permeated the corneal cell monolayer in the presence, but not the absence, of NC-1059. Fluorescein sodium and carboxyfluorescein were then used as low molecular weight markers with similar NC-1059-modulated kinetics being observed. Maximum permeation for the fluorescein derivatives occurred 30 to 90 minutes after a 5-minute NC-1059 exposure. A prototypical drug, methotrexate, also exhibited increased permeation in the presence of NC-1059.

CONCLUSIONS

NC-1059 enhances drug permeation across cultured corneal epithelial cell monolayers by transiently affecting the paracellular pathway. Thus, NC-1059 is a lead compound for development of cotherapeutic agents to enhance access and effectiveness of ophthalmic compounds.

Epithelial barrier modulation by a channel forming peptide

NC-1059 is a synthetic channel-forming peptide that provides for ion transport across, and transiently reduces the barrier integrity of, cultured epithelial monolayers derived from canine kidney (MDCK cells). Experiments were conducted to determine whether epithelial cells derived from other sources were similarly affected. Epithelial cells derived from human intestine (T-84), airway (Calu-3), porcine intestine (IPEC-J2) and reproductive duct (PVD9902) were grown on permeable supports. Basal short circuit current (Isc) was <3 microA cm(-2) for T-84, IPEC-J2 and PVD9902 cell monolayers and <8 microA cm(-2) for Calu-3 cells. Apical NC-1059 exposure caused, in all cell types, an increase in Isc to >15 microA cm(-2), indicative of net anion secretion or cation absorption, which was followed by an increase in transepithelial conductance (in mS cm(-2): T-84, 1.6 to 62; PVD9902, 0.2 to 51; IPEC-J2, 0.3 to 26; Calu-3, 2.3 to 13). These results are consistent with the peptide affecting transcellular ion movement, with a likely effect also on the paracellular route. NC-1059 exposure increased dextran permeation when compared to basal permeation, which documents an effect on the paracellular pathway. In order to evaluate membrane ion channels, experiments were conducted to study the dose dependence and stability of the NC-1059-induced membrane conductance in Xenopus laevis oocytes. NC-1059 induced a dose-dependent increase in oocyte membrane conductance that remained stable for greater than 2 h. The results demonstrate that NC-1059 increases transcellular conductance and paracellular permeation in a wide range of epithelia. These effects might be exploited to promote drug delivery across barrier epithelia.

Immunity to a self-derived, channel-forming peptide in the respiratory tract

The channel-forming peptide NC-1130 was generated based on the amino acid sequence of the M2 segment of the spinal cord alpha-subunit of the glycine receptor and has been proposed as a therapeutic agent for anion channelopathies such as cystic fibrosis. Lysine adduction and amino acid substitutions at positions T19R and S22W of the peptide improved its performance as an ion channel. However, these modifications generated an altered self, potentially making this NC-1130 peptide immunogenic, which could preclude the repeated use of NC-1130 as a therapeutic agent. To measure the ability of NC-1130 to induce an immune response, it was administered nasally with or without cholera toxin (CT). The NC-1130 peptide, when given alone without adjuvant, induced very little peptide-specific immunity based on analyses of peptide-specific antibodies by enzyme-linked immunosorbent assay and enzyme-linked immunospot assay, induction of cytokine production, and delayed-type hypersensitivity (DTH) responses. The administration of NC-1130 with the mucosal adjuvant CT induced peptide-specific immunoglobulin G (IgG) antibodies and DTH responses and a Th2-dominant cytokine response. The coadministration of the strong mucosal adjuvant CT induced a systemic NC-1130-specific IgG response but not a mucosal peptide-specific antibody response. The lack of peptide-specific immunity and specifically mucosal immunity should allow repeated NC-1130 peptide applications to epithelial surfaces to correct anion channelopathies.

Design of 11-residue peptides with unusual biophysical properties: induced secondary structure in the absence of water

A series of oligopeptides with beta-forming and adhesive properties, were synthesized and analyzed for adhesion shear strength, secondary structure, and association properties. The sequences contained related hydrophobic core segments varying in length from 5 to 12 residues and flanked by di- or tri-lysine segments. Three remarkable peptides consisting of just 11 residues with hydrophobic core sequences of FLIVI, IGSII, and IVIGS flanked by three lysine residues gave the highest dry adhesion shear strength and displayed unusual biophysical properties in the presence and absence of water. KKKFLIVIKKK had its highest adhesion strength at 2% (w/v) at pH 12.0 and showed the highest adhesion strength after exposure to water (water resistance). Both KKKIGSIIKKK and KKKIVIGSKKK, at 4% (w/v) at pH 12.0, displayed nearly identical dry shear strength values to that with the FLIVI core sequence. The peptide with IGSII core, however, displayed a lower water resistance and the latter, IVIGS, showed no water resistance, completely delaminating upon soaking in water. These are the smallest peptides with adhesive properties reported to date and show remarkable adhesion strength even at lower concentrations of 0.2% (w/v), which corresponds to 1.6 mM. The FLIVI containing peptide adopted a beta-sheet secondary structure in water while the IGSII- and IVIGS-containing sequences folded similarly only in the absence of water. Analytical ultracentrifugation studies showed that when the FLIVI sequence adopts beta-structure in aqueous solution, it associates into a large molecular weight assembly. The random coils of IGSII and IVIGS showed no tendency to associate at any pH.

Synthesis and Ion Sensing Properties of New Colorimetric and Fluorimetric Chemosensors Based on Bithienyl-Imidazo-Anthraquinone Chromophores

Novel colorimetric receptors for fluoride ion sensing containing anthraquinone as a chromogenic signaling unit and imidazo-2,2'-bithiophene binding sites are reported. Well-defined color change was observed upon addition of fluoride ions to acetonitrile solutions of receptors 2. Compounds 2a-c, deprotonated after fluoride ion addition, were studied as metal ion chemosensors in the presence of Zn(II), Hg(II), and Cu(II) in acetonitrile solutions, especially compound 2a which displayed a marked change from pink to yellow-gold colors upon complexation.

Synthetic, biologically active amphiphilic peptides

Amphiphilic peptides typically consist of a peptide portion that may be 5-25 (or more) amino acids in length. The hydrophobic portion may be a single fatty acid residue, but can also be more elaborate. The main focus of this article lies on the family of synthetic anion binders (SATs) of the general structure (R(1))(2)N-COCH(2)OCH(2)CO-(Aaa)(n)-OR(3). The most-common R(1) group is the octadecyl (C(18)H(37)) group. The most studied peptide sequence in this family is (Gly)(3)-Pro-(Gly)(3), although different sequences (and longer and shorter peptides) have been prepared as well. The C-terminal ester residue providing the most effective anion release from liposomes is heptyl (C(7)H(15)), although many others have been examined. The compound (C(18)H(37))(2)N-COCH(2)OCH(2)CO-(Gly)(3)-Pro-(Gly)(3)-OBn (Bn=benzyl) was found to mediate Cl(-) transport in mouse epithelial cells.

Dipyrrolyl-Functionalized Bipyridine-Based Anion Receptors for Emission-Based Selective Detection of Dihydrogen Phosphate

New cationic anion receptors, based on the use of pyrrole-substituted bipyridine and coordinated to transition metals, are described. Specifically, polypyridine-ruthenium and -rhodium cores have been functionalized to generate an anion binding site. The design was chosen to probe the influence of the pyrrole-to-pyrrole separation on anion-binding affinities and selectivities; this distance is greater in the new systems of this report (receptors 1 and 2) relative to that present in related dipyrrolyl quinoxaline based receptors 3 and 4. Solution-phase anion-binding studies, carried out by means of (1)H NMR spectroscopic titrations in [D(6)]DMSO and isothermal titration calorimetry (ITC) in DMSO, reveal that 1 and 2 bind most simple anions with substantially higher affinity than either 3 or 4. In the case of chloride anion, structural studies, carried out by means of single-crystal X-ray diffraction analyses, are consistent with the solution-phase results and reveal that receptors 1 and 2 are both able to stabilize complexes with this halide anion in the solid state.

Biologically active, synthetic ion transporters

The compelling chemical goal of modeling protein channel behavior has led to synthetic compounds that are true ion channels. Although they largely lack the selectivity and sophistication of highly evolved proteins, they successfully perform a variety of biological functions. This tutorial review describes these novel structures and their activity in living systems. Different channel structures show antibacterial to anticancer activity when tested against a variety of cell types.

First Fluorescent Sensor for Fluoride Based on 2-Ureido-4[1H]-pyrimidinone Quadruple Hydrogen-Bonded AADD Supramolecular Assembly

A simple, highly selective, neutral, fluorescent sensor for fluoride anions is reported. It is based on 2-ureido-4[1H]-pyrimidinone quadruple hydrogen-bonded AADD supramolecular assembly, and its assembling and disassembling processes are also able to respond to external stimuli reversibly.

Redesigning channel-forming peptides: amino acid substitutions that enhance rates of supramolecular self-assembly and raise ion transport activity

Three series of 22-residue peptides derived from the transmembrane M2 segment of the glycine receptor alpha1-subunit (M2GlyR) have been designed, synthesized, and tested to determine the plasticity of a channel-forming sequence and to define whether channel pores with enhanced conductive properties could be created. Sixteen sequences were examined for aqueous solubility, solution-association tendency, secondary structure, and half-maximal concentration for supramolecular assembly, channel activity, and ion transport properties across epithelial monolayers. All peptides interact strongly with membranes: associating with, inserting across, and assembling to form homooligomeric bundles when in micromolar concentrations. Single and double amino acid replacements involving arginine and/or aromatic amino acids within the final five C-terminal residues of the peptide cause dramatic effects on the concentration dependence, yielding a range of K1/2 values from 36 +/- 5 to 390 +/- 220 microM for transport activity. New water/lipid interfacial boundaries were established for the transmembrane segment using charged or aromatic amino acids, thus limiting the peptides' ability to move perpendicularly to the plane of the bilayer. Formation of discrete water/lipid interfacial boundaries appears to be necessary for efficient supramolecular assembly and high anion transport activity. A peptide sequence is identified that may show efficacy in channel replacement therapy for channelopathies such as cystic fibrosis.

Synthesis and initial characterization of FGFR3 transmembrane domain: consequences of sequence modifications

Receptor Tyrosine Kinases (RTKs) conduct biochemical signals via lateral dimerization in the plasma membrane, and defects in their dimerization lead to unregulated signaling and disease. RTK transmembrane (TM) domains are proposed to play an important role in the process, underscored by the finding that single amino acids mutations in the TM domains can induce pathological phenotypes. Therefore, many important questions pertaining to the mode of signal transduction and the mechanism of pathology induction could be answered by studying the chemical-physical basis behind RTK TM domain dimerization and the interactions of RTK TM domains with lipids in model bilayer systems. As a first step towards this goal, here we report the synthesis of the TM domain of fibroblast growth factor receptor 3 (FGFR3), an RTK that is crucial for skeletal development. We have used solid phase peptide synthesis to produce two peptides: one corresponding to the membrane embedded segment and the naturally occurring flanking residues at the N- and C-termini (TMwt), and a second one in which the flanking residues have been substituted with diLysines at the termini (TMKK). We have demonstrated that the hydrophobic FGFR3 TM domain can be synthesized for biophysical studies with high yield. The protocol presented in the paper can be applied to the synthesis of other RTK TM domains. As expected, the Lys flanks decrease the hydrophobicity of the TM domain, such that TMKK elutes much earlier than TMwt during reverse phase HPLC purification. The Lysines have no effect on peptide solubility in SDS and on peptide secondary structure, but they abolish peptide dimerization on SDS gels. These results suggest that caution should be exercised when modifying RTK TM domains to render them more manageable for biophysical studies.

Activity and structural comparisons of solution associating and monomeric channel-forming peptides derived from the glycine receptor m2 segment

A number of channel-forming peptides derived from the second transmembrane (TM) segment (M2) of the glycine receptor alpha(1) subunit (M2GlyR), including the 22-residue sequence NK(4)-M2GlyR p22 wild type (WT) (KKKKPARVGLGITTVLTMTTQS), induce anion permeation across epithelial cell monolayers. In vitro assays suggest that this peptide or related sequences might function as a candidate for ion channel replacement therapy in treating channelopathies such as cystic fibrosis (CF). The wild-type sequence forms soluble associations in water that diminish its efficacy. Introduction of a single substitution S22W at the C-terminus, NK(4)-M2GlyR p22 S22W, eliminates the formation of higher molecular weight associations in solution. The S22W peptide also reduces the concentration of peptide required for half-maximal anion transport induced across Madin-Darby canine kidney cells (MDCK) monolayers. A combination of 2D double quantum filtered correlation spectroscopy (DQF-COSY), total correlation spectroscopy (TOCSY), nuclear Overhauser effect spectroscopy (NOESY), and rotating frame nuclear Overhauser effect spectroscopy (ROESY) data were recorded for both the associating WT and nonassociating S22W peptides and used to compare the primary structures and to assign the secondary structures. High-resolution structural studies were recorded in the solvent system (40% 2,2,2-Trifluoroethanol (TFE)/water), which gave the largest structural difference between the two peptides. Nuclear Overhauser effect crosspeak intensity provided interproton distances and the torsion angles were measured by spin-spin coupling constants. These constraints were put into the DYANA modeling program to generate a group of structures. These studies yielded energy-minimized structures for this mixed solvent environment. Structure for both peptides is confined to the 15-residue transmembrane segments. The energy-minimized structure for the WT peptide shows a partially helical extended structure. The S22W peptide adopts a bent conformation forming a hydrophobic pocket by hydrophobic interactions.

Channel-forming peptide modulates transepithelial electrical conductance and solute permeability

NC-1059, a synthetic channel-forming peptide, transiently increases transepithelial electrical conductance (g(TE)) and ion transport (as indicated by short-circuit current) across Madin-Darby canine kidney (MDCK) cell monolayers in a time- and concentration-dependent manner when apically exposed. g(TE) increases from <2 to >40 mS/cm(2) over the low to middle micromolar range. Dextran polymer (9.5 but not 77 kDa) permeates the monolayer following apical NC-1059 exposure, suggesting that modulation of the paracellular pathway accounts for changes in g(TE). However, concomitant alterations in junctional protein localization (zonula occludens-1, occludin) and cellular morphology are not observed. Effects of NC-1059 on MDCK g(TE) occur in nominally Cl(-)- and Na(+)-free apical media, indicating that permeation by these ions is not required for effects on g(TE), although two-electrode voltage-clamp assays with Xenopus oocytes suggest that both Cl(-) and Na(+) permeate NC-1059 channels with a modest Cl(-) permselectivity (P(Cl):P(Na) = 1.3). MDCK monolayers can be exposed to multiple NC-1059 treatments over days to weeks without diminution of response, alteration in the time course, or loss of responsiveness to physiological and pharmacological secretagogues. Together, these results suggest that NC-1059 represents a valuable tool to investigate tight junction regulation and may be a lead compound for therapeutic interventions.

Efficient Fluoride-Selective Fluorescent Host: Experiment and Theory

A new naphthalene derivative containing a urea group at the 1,8-position of naphthalene was synthesized and showed a unique absorption and fluorescence peak with fluoride ion. Calculations suggested that a new peak was attributed to the increased anionic character of urea nitrogen due to the strongly charged hydrogen bonding between fluoride and amide protons of the urea. The fluoride selectivity among halides (F(-), Cl(-), Br(-)) comes from the fact that the fluoride approaches much closer to the amide protons than other halides and resides in the cavity with fast dynamics. The nature of electronic transitions that were analyzed from the calculations by the collective electronic oscillator method also supports the anionic nature of the complex between host and fluoride.

Expression of an artificial Cl- channel in microperfused renal proximal tubules

To better understand the process of fluid movement driven by Cl- conductance, a Cl- channel-forming peptide was delivered to the luminal membrane of microperfused rabbit renal proximal tubules. When the peptide (NK4-M2GlyR) was perfused, a significant new conductance was observed within 3 min and stabilized at 10 min. Alteration of the ion composition revealed it to be a Cl(-)-specific conductance. Reabsorption of Cl- (JCl) was increased by NK4-M2GlyR, but not by a scramble NK4-M2GlyR sequence, suggesting that the active peptide formed de novo Cl- channels in the luminal membrane of the perfused tubules. In the presence of the peptide, reabsorption of fluid (Jv) was dramatically increased and JNa and JCa were concomitantly increased. We propose that introduction of the new Cl- conductance in the luminal membrane leads to a coordinated efflux of water across the membrane and an increase in cation translocation via the paracellular pathway, resulting in an increase in Jv. This novel method could prove useful in characterizing mechanisms of fluid transport driven by Cl- gradients.

Synthetic membrane transporters

An increasing number of synthetic compounds have been shown to facilitate ion and polar molecule transport across bilayer membranes. Most notably, recent advances in anion transport have yielded synthetic chloride channels and phospholipid translocases. Attention has also turned to the ability of short amino acid sequences to transport peptides and proteins across cellular membranes.

Distinct structural elements that direct solution aggregation and membrane assembly in the channel-forming peptide M2GlyR

Restoration of chloride conductance via the introduction of an anion selective pore, formed by a channel-forming peptide, has been hypothesized as a novel treatment modality for patients with cystic fibrosis (CF). Delivery of these peptide sequences to airway cells from an aqueous environment in the absence of organic solvents is paramount. New highly soluble COOH- and NH(2)-terminal truncated peptides, derived from the second transmembrane segment of the glycine receptor alpha-subunit (M2GlyR), were generated, with decreasing numbers of amino acid residues. NH(2)-terminal lysyl-adducted truncated peptides with lengths of 22, 25, and 27 amino acid residues are equally able to stimulate short circuit current (I(SC)). Peptides with as few as 16 amino acid residues are able to stimulate I(SC), although to a lesser degree. In contrast, COOH-terminal truncated peptides show greatly reduced induced I(SC) values for all peptides fewer than 27 residues in length and show no measurable activity for peptides fewer than 21 residues in length. CD spectra for both the NH(2)- and COOH-truncated peptides have random structure in aqueous solution, and those sequences that stimulated the highest maximal I(SC) are predominantly helical in 40% trifluoroethanol. Peptides with a decreased propensity to form helical structures in TFE also failed to stimulate I(SC). Palindromic peptide sequences based on both the NH(2)- and COOH-terminal halves of M2GlyR were synthesized to test roles of the COOH- and NH(2)-terminal halves of the molecule in solution aggregation and channel forming ability. On the basis of the study presented here, there are distinct, nonoverlapping regions of the M2GlyR sequence that define solution aggregation and membrane channel assembly. Peptides that eliminate solution aggregation with complete retention of channel forming activity were generated.

Phenanthroline complexes bearing fused dipyrrolylquinoxaline anion recognition sites: efficient fluoride anion receptors

Novel anion recognition host molecules, tris-1,10-phenanthroline cobalt(III) and bis-2,2'-bipyridine mono-1,10-phenanthroline ruthenium(II) complexes bearing fused dipyrrolylquinoxaline moieties have been synthesized. As determined by UV-vis spectroscopic and electrochemical studies, these metal complexes bind fluoride with high affinity in polar media both in absolute terms and relative to the metal-free phenanthroline dipyrrolylquinozaline precursor from which they are derived (fluoride is bound to the tris-1,10-phenanthroline cobalt(III) dipyrrolylquinoxaline system with a 1:1 binding constant of 54 000 M-1 in DMSO). The large observed binding constants are ascribed to two factors, (i) the presence of a phenanthroline-coordinated cationic charge that decreases the electron density on the pyrrole NH protons and (ii) pure electrostatic effects.

Visual detection of specific, native interactions between soluble and microbead-tethered alpha-helices from membrane proteins

Using peptides tethered to polymer microbeads, we have developed a technique for measuring the interactions between the transmembrane alpha-helices of membrane proteins and for screening combinatorial libraries of peptides for members that interact with specific helices from membrane proteins. The method was developed using the well-characterized homodimerization sequence of the membrane-spanning alpha-helix from the erythrocyte membrane protein glycophorin A (GPA). As a control, we also tested a variant with a dimer-disrupting alteration of a critical glycine residue to leucine. To test for detectable, native interactions between detergent-solubilized and microbead-tethered alpha-helices, we incubated fluorescent dye-labeled GPA analogues in sodium dodecyl sulfate solution with microbeads that contained covalently attached GPA analogues. When the dye-labeled peptide in solution and the bead-tethered peptide both contained the native glycophorin A sequence, the microbeads readily accumulated the dye through lateral peptide-peptide interactions and were visibly fluorescent under UV light. When either the peptide in solution or the peptide attached to the beads contained the glycine to leucine change, the beads did not accumulate any dye. The usefulness of this method for screening tethered peptide libraries was tested by incubating dye-labeled, native sequence peptides in detergent solution with a few native sequence beads plus an excess of beads containing the variant glycine to leucine sequence. When the dye-labeled peptide in solution was present at a concentration of > or =2 microM, the few native sequence beads were visually distinguishable from the others because of their bright fluorescence. Using this model system, we have shown that it is possible to visually detect specific, native interactions between alpha-helices from membrane proteins using peptides tethered to polymer microbeads. It will thus be possible to use this method to measure the specific lateral interactions that drive the folding and organization of membrane proteins and to screen combinatorial libraries of peptides for members that interact with them.

Synthetic chloride channel restores glutathione secretion in cystic fibrosis airway epithelia

Cystic fibrosis (CF), an inherited disease characterized by defective epithelial Cl- transport, damages lungs via chronic inflammation and oxidative stress. Glutathione, a major antioxidant in the epithelial lung lining fluid, is decreased in the apical fluid of CF airway epithelia due to reduced glutathione efflux (Gao L, Kim KJ, Yankaskas JR, and Forman HJ. Am J Physiol Lung Cell Mol Physiol 277: L113-L118, 1999). The present study examined the question of whether restoration of chloride transport would also restore glutathione secretion. We found that a Cl- channel-forming peptide (N-K4-M2GlyR) and a K+ channel activator (chlorzoxazone) increased Cl- secretion, measured as bumetanide-sensitive short-circuit current, and glutathione efflux, measured by high-performance liquid chromatography, in a human CF airway epithelial cell line (CFT1). Addition of the peptide alone increased glutathione secretion (181 +/- 8% of the control value), whereas chlorzoxazone alone did not significantly affect glutathione efflux; however, chlorzoxazone potentiated the effect of the peptide on glutathione release (359 +/- 16% of the control value). These studies demonstrate that glutathione efflux is associated with apical chloride secretion, not with the CF transmembrane conductance regulator per se, and the defect of glutathione efflux in CF can be overcome pharmacologically.

NH(2)-terminal modification of a channel-forming peptide increases capacity for epithelial anion secretion

A synthetic, channel-forming peptide, derived from the alpha-subunit of the glycine receptor (M2GlyR), has been synthesized and modified by adding four lysine residues to the NH(2) terminus (N-K(4)-M2GlyR). In Ussing chamber experiments, apical N-K(4)-M2GlyR (250 microM) increased transepithelial short-circuit current (I(sc)) by 7.7 +/- 1.7 and 10.6 +/- 0.9 microA/cm(2) in Madin-Darby canine kidney and T84 cell monolayers, respectively; these values are significantly greater than those previously reported for the same peptide modified by adding the lysines at the COOH terminus (Wallace DP, Tomich JM, Iwamoto T, Henderson K, Grantham JJ, and Sullivan LP. Am J Physiol Cell Physiol 272: C1672-C1679, 1997). N-K(4)-M2GlyR caused a concentration-dependent increase in I(sc) (k([1/2]) = 190 microM) that was potentiated two- to threefold by 1-ethyl-2-benzimidazolinone. N-K(4)-M2GlyR-mediated increases in I(sc) were insensitive to changes in apical cation species. Pharmacological inhibitors of endogenous Cl(-) conductances [glibenclamide, diphenylamine-2-dicarboxylic acid, 5-nitro-2-(3-phenylpropylamino)benzoic acid, 4,4'-dinitrostilben-2,2'-disulfonic acid, indanyloxyacetic acid, and niflumic acid] had little effect on N-K(4)-M2GlyR-mediated I(sc). Whole cell membrane patch voltage-clamp studies revealed an N-K(4)-M2GlyR-induced anion conductance that exhibited modest outward rectification and modest time- and voltage-dependent activation. Planar lipid bilayer studies yielded results indicating that N-K(4)-M2GlyR forms a 50-pS anion conductance with a k([1/2]) for Cl(-) of 290 meq. These results indicate that N-K(4)-M2GlyR forms an anion-selective channel in epithelial monolayers and shows therapeutic potential for the treatment of hyposecretory disorders such as cystic fibrosis.

Neurotoxicity of the putative transmembrane domain of the prion protein

It has been shown recently that the generation of an abnormal transmembrane form of the prion protein ((Ctm)PrP) is involved in the neurodegeneration process during inherited and infectious prion diseases but a causative relationship has never been established. We wanted to know if and how the proposed transmembrane domain of PrP could induce neuronal dysfunction. Thus, we investigated the neurotoxic properties of two peptides whose sequences are encompassed within this domain. We show that PrP peptides 118-135 and 105-132 as well as an amidated more soluble peptide 105-132 induce the death of pure cortical neurons originating from normal and PrP knockout mice. This can be correlated with the high propensity of these peptides to insert stably into and to destabilize cell membranes. Through this study, we have identified a novel mechanism of neurotoxicity for PrP, which directly involves membrane perturbation; this mechanism is independent of fibril formation and probably corresponds to the effect of the transmembrane insertion of (Ctm)PrP.

A synthetic peptide based on a glycine-gated chloride channel induces a novel chloride conductance in isolated epithelial cells

CK(4)-M2GlyR, an aqueous soluble peptide derived from the transmembrane M2 segment of the glycine-gated Cl(-) channel found in postsynaptic membranes of the central nervous system, has previously been shown to increase transepithelial Cl(-) and fluid secretion of epithelial monolayers. The goal of this study was to determine whether CK(4)-M2GlyR exerts these effects via formation of a novel chloride conductance pathway, modulation of endogenous chloride channel activity, or a combination of these effects. Ionic currents were recorded from isolated epithelial cells before and after treatment with the peptide using the whole-cell configuration of the patch-clamp technique. CK(4)-M2GlyR increased whole-cell Cl(-) currents in all epithelial cell lines that were studied, including: Madin-Darby canine kidney cells, a human colonic epithelial cell line (T84), and airway epithelial cells derived from a human cystic fibrosis patient (IB3-1). No evidence was found for modulation of endogenous Cl(-) channels by CK(4)-M2GlyR based on both the electrophysiological properties of the observed currents and the pharmacological profile of the CK(4)-M2GlyR-induced current. These results suggest that CK(4)-M2GlyR increases Cl(-) permeability in epithelial cells directly, by forming a distinct conduction pathway in cell membranes.