Direct interaction between amphotericin B and ergosterol in lipid bilayers as revealed by 2H NMR spectroscopy

Interaction between the marine sponge cyclic peptide theonellamide A and sterols in lipid bilayers as viewed by surface plasmon resonance and solid-state (2)H nuclear magnetic resonance

Theonellamides (TNMs) are members of a distinctive family of antifungal and cytotoxic bicyclic dodecapeptides isolated from the marine sponge Theonella sp. Recently, it has been shown that TNMs recognize 3β-hydroxysterol-containing membranes, induce glucan overproduction, and damage cellular membranes. However, to date, the detailed mode of sterol binding at a molecular level has not been determined. In this study, to gain insight into the mechanism of sterol recognition of TNM in lipid bilayers, surface plasmon resonance (SPR) experiments and solid-state deuterium nuclear magnetic resonance ((2)H NMR) measurements were performed on theonellamide A (TNM-A). SPR results revealed that the incorporation of 10 mol % cholesterol or ergosterol into 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) membranes significantly enhances the affinity of the peptide for the membrane, particularly in the initial binding to the membrane surface. These findings, together with the fact that binding of TNM-A to epicholesterol (3α-cholesterol)-containing liposomes and pure POPC liposomes was comparably weak, confirmed the preference of the peptide for the 3β-hydroxysterol-containing membranes. To further establish the formation of the complex of TNM-A with 3β-hydroxysterols in lipid bilayers, solid-state (2)H NMR measurements were conducted using deuterium-labeled cholesterol, ergosterol, or epicholesterol. The (2)H NMR spectra showed that TNM-A significantly inhibits the fast rotational motion of cholesterol and ergosterol, but not epicholesterol, therefore verifying the direct complexation between TNM-A and 3β-hydroxysterols in lipid bilayers. This study demonstrates that TNM-A directly recognizes the 3β-OH moiety of sterols, which greatly facilitates its binding to bilayer membranes.

Candida infections and their prevention

Infections caused by Candida species have been increased dramatically worldwide due to the increase in immunocompromised patients. For the prevention and cure of candidiasis, several strategies have been adopted at clinical level. Candida infected patients are commonly treated with a variety of antifungal drugs such as fluconazole, amphotericin B, nystatin, and flucytosine. Moreover, early detection and speciation of the fungal agents will play a crucial role for administering appropriate drugs for antifungal therapy. Many modern technologies like MALDI-TOF-MS, real-time PCR, and DNA microarray are being applied for accurate and fast detection of the strains. However, during prolonged use of these drugs, many fungal pathogens become resistant and antifungal therapy suffers. In this regard, combination of two or more antifungal drugs is thought to be an alternative to counter the rising drug resistance. Also, many inhibitors of efflux pumps have been designed and tested in different models to effectively treat candidiasis. However, most of the synthetic drugs have side effects and biomedicines like antibodies and polysaccharide-peptide conjugates could be better alternatives and safe options to prevent and cure the diseases. Furthermore, availability of genome sequences of Candida   albicans and other non-albicans strains has made it feasible to analyze the genes for their roles in adherence, penetration, and establishment of diseases. Understanding the biology of Candida species by applying different modern and advanced technology will definitely help us in preventing and curing the diseases caused by fungal pathogens.

Possible conformation of amphotericin B dimer in membrane-bound assembly as deduced from solid-state NMR

Aiming for structural analysis of amphotericin B (AmB) ion-channel assemblies in membrane, a covalent dimer was synthesized between (13)C-labled AmB methyl ester and (19)F-labled AmB. The dimer showed slightly weaker but significant biological activities against fungi and red blood cells compared with those of monomeric AmB. Then the dimer was subjected to (13)C{(19)F}REDOR (Rotational-Echo Double Resonance) experiments in hydrated lipid bilayers. The obtained REDOR dephasing effects were explained by two components; a short (13)C/(19)F distance (6.9Å) accounting for 23% of the REDOR dephasing, and a longer one (14Å) comprising the rest of the dephasing. The shorter distance is likely to reflect the formation of barrel-stave ion channel.

Probing amphotericin B single channel activity by membrane dipole modifiers

The effects of dipole modifiers and their structural analogs on the single channel activity of amphotericin B in sterol-containing planar phosphocholine membranes are studied. It is shown that the addition of phloretin in solutions bathing membranes containing cholesterol or ergosterol decreases the conductance of single amphotericin B channels. Quercetin decreases the channel conductance in cholesterol-containing bilayers while it does not affect the channel conductance in ergosterol-containing membranes. It is demonstrated that the insertion of styryl dyes, such as RH 421, RH 237 or RH 160, in bilayers with either cholesterol or ergosterol leads to the increase of the current amplitude of amphotericin B pores. Introduction of 5α-androstan-3β-ol into a membrane-forming solution increases the amphotericin B channel conductance in a concentration-dependent manner. All the effects are likely to be attributed to the influence of the membrane dipole potential on the conductance of single amphotericin B channels. However, specific interactions of some dipole modifiers with polyene-sterol complexes might also contribute to the activity of single amphotericin B pores. It has been shown that the channel dwell time increases with increasing sterol concentration, and it is higher for cholesterol-containing membranes than for bilayers including ergosterol, 6-ketocholestanol, 7-ketocholestanol or 5α-androstan-3β-ol. These findings suggest that the processes of association/dissociation of channel forming molecules depend on the membrane fluidity.

Head-to-tail interaction between amphotericin B and ergosterol occurs in hydrated phospholipid membrane

Amphotericin B (AmB) is thought to exert its antifungal activity by forming an ion-channel assembly in the presence of ergosterol. In the present study we aimed to elucidate the mode of molecular interactions between AmB and ergosterol in hydrated phospholipid bilayers using the rotational echo double resonance (REDOR) spectra. We first performed (13)C{(19)F}REDOR experiments with C14-(19)F-labeled AmB and biosynthetically (13)C-labeled ergosterol and implied that both "head-to-head" and "head-to-tail" orientations occur for AmB-ergosterol interaction in the bilayers. To further confirm the "head-to-tail" pairing, (13)C-labeled ergosterol at the dimethyl terminus (C26/C27) was synthesized and subjected to the REDOR measurements. The spectra unambiguously demonstrated the presence of a "head-to-tail" orientation for AmB-ergosterol pairing. In order to obtain information on the position of the dimethyl terminus of ergosterol in membrane, (13)C{(31)P}REDOR were carried out using the labeled ergosterol and the phosphorus atom of a POPC headgroup. Significant REDOR dephasing was observed at the C26/C27 signal of ergosterol in the presence of AmB, but not in the absence of AmB, clearly indicating that the side-chain terminus of ergosterol in the AmB complex comes close to the bilayer surface.

Synthesis-enabled functional group deletions reveal key underpinnings of amphotericin B ion channel and antifungal activities

Amphotericin B is the archetype for small molecules that form transmembrane ion channels. However, despite extensive study for more than five decades, even the most basic features of this channel structure and its contributions to the antifungal activities of this natural product have remained unclear. We herein report that a powerful series of functional group-deficient probes have revealed many key underpinnings of the ion channel and antifungal activities of amphotericin B. Specifically, in stark contrast to two leading models, polar interactions between mycosamine and carboxylic acid appendages on neighboring amphotericin B molecules are not required for ion channel formation, nor are these functional groups required for binding to phospholipid bilayers. Alternatively, consistent with a previously unconfirmed third hypothesis, the mycosamine sugar is strictly required for promoting a direct binding interaction between amphotericin B and ergosterol. The same is true for cholesterol. Synthetically deleting this appendage also completely abolishes ion channel and antifungal activities. All of these results are consistent with the conclusion that a mycosamine-mediated direct binding interaction between amphotericin B and ergosterol is required for both forming ion channels and killing yeast cells. The enhanced understanding of amphotericin B function derived from these synthesis-enabled studies has helped set the stage for the more effective harnessing of the remarkable ion channel-forming capacity of this prototypical small molecule natural product.

Amphotericin B membrane action: role for two types of ion channels in eliciting cell survival and lethal effects

The formation of aqueous pores by the polyene antibiotic amphotericin B (AmB) is at the basis of its fungicidal and leishmanicidal action. However, other types of nonlethal and dose-dependent biphasic effects that have been associated with the AmB action in different cells, including a variety of survival responses, are difficult to reconcile with the formation of a unique type of ion channel by the antibiotic. In this respect, there is increasing evidence indicating that AmB forms nonaqueous (cation-selective) channels at concentrations below the threshold at which aqueous pores are formed. The main foci of this review will be (1) to provide a summary of the evidence supporting the formation of cation-selective ion channels and aqueous pores by AmB in lipid membrane models and in the membranes of eukaryotic cells; (2) to discuss the influence of membrane parameters such as thickness fluctuations, the type of sterol present and the existence of sterol-rich specialized lipid raft microdomains in the formation process of such channels; and (3) to develop a cell model that serves as a framework for understanding how the intracellular K(+) and Na(+) concentration changes induced by the cation-selective AmB channels enhance multiple survival response pathways before they are overcome by the more sustained ion fluxes, Ca(2+)-dependent apoptotic events and cell lysis effects that are associated with the formation of AmB aqueous pores.