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

Marine-Derived Metabolites Act as Promising Antifungal Agents

The incidence of invasive fungal diseases (IFDs) is on the rise globally, particularly among immunocompromised patients, leading to significant morbidity and mortality. Current clinical antifungal agents, such as polyenes, azoles, and echinocandins, face increasing resistance from pathogenic fungi. Therefore, there is a pressing need for the development of novel antifungal drugs. Marine-derived secondary metabolites represent valuable resources that are characterized by varied chemical structures and pharmacological activities. While numerous compounds exhibiting promising antifungal activity have been identified, a comprehensive review elucidating their specific underlying mechanisms remains lacking. In this review, we have compiled a summary of antifungal compounds derived from marine organisms, highlighting their diverse mechanisms of action targeting various fungal cellular components, including the cell wall, cell membrane, mitochondria, chromosomes, drug efflux pumps, and several biological processes, including vesicular trafficking and the growth of hyphae and biofilms. This review is helpful for the subsequent development of antifungal drugs due to its summary of the antifungal mechanisms of secondary metabolites from marine organisms.

Deciphering the Biological Activities of Antifungal Agents with Chemical Probes

The growing number of fungal infections caused by pathogens resistant to one or more classes of antifungal drugs emphasizes the threat that these microorganisms pose to animal and human health and global food security. Open questions remain regarding the mechanisms of action of the limited repertoire of antifungal agents, making it challenging to rationally develop more efficacious therapeutics. In recent years, the use of chemical biology approaches has resolved some of these questions and has provided new promising concepts to guide the design of antifungal agents. By focusing on examples from studies carried out in recent years, this minireview describes the key roles that probes based on antifungal agents and their derivatives have played in uncovering details about their activities, in detecting resistance, and in characterizing the interactions between these agents and their targets.

Marine natural products targeting the eukaryotic cell membrane

The cell membrane, with high fluidity and alternative curvatures, maintains the robust integrity to distinguish inner and outer space of cells or organelles. Lipids are the main components of the cell membrane, but their functions are largely unknown. Even the visualization of lipids is not straightforward since modification of lipids often hampers its correct physical properties. Many natural products target cell membranes, some of which are used as pharmaceuticals and/or research tools. They show specific recognition on lipids, and thus exhibit desired pharmacological effects and unique biological phenotypes. This review is a catalog of marine natural products that target eukaryotic cell membranes. Chemical structures, biological activities, and molecular mechanisms are summarized. I hope that this review will be helpful for readers to notice the potential of marine natural products in the exploration of the function of lipids and the druggability of eukaryotic cell membranes.

Cellular cholesterol and how to find it

Cholesterol is an essential component of eukaryotic cellular membranes. Information about its subcellular localization and transport pathways inside cells are key for the understanding and treatment of cholesterol-related diseases. In this review we give an overview over the most commonly used methods that contributed to our current understanding of subcellular cholesterol localization and transport routes. First, we discuss methods that provide insights into cholesterol metabolism based on readouts of downstream effects such as esterification. Subsequently, we focus on the use of cholesterol-binding molecules as probes that facilitate visualization and quantification of sterols inside of cells. Finally, we explore different analogues of cholesterol which, when taken up by living cells, are integrated and transported in a similar fashion as endogenous sterols. Taken together, we highlight the challenges and advantages of each method such that researchers studying aspects of cholesterol transport may choose the most pertinent approach for their problem.

Genomic and Targeted Approaches Unveil the Cell Membrane as a Major Target of the Antifungal Cytotoxin Amantelide A

Amantelide A, a polyhydroxylated macrolide isolated from a marine cyanobacterium, displays broad-spectrum activity against mammalian cells, bacterial pathogens, and marine fungi. We conducted comprehensive mechanistic studies to identify the molecular targets and pathways affected by amantelide A. Our investigations relied on chemical structure similarities with compounds of known mechanisms, yeast knockout mutants, yeast chemogenomic profiling, and direct biochemical and biophysical methods. We established that amantelide A exerts its antifungal action by binding to ergosterol-containing membranes followed by pore formation and cell death, a mechanism partially shared with polyene antifungals. Binding assays demonstrated that amantelide A also binds to membranes containing epicholesterol or mammalian cholesterol, thus suggesting that the cytotoxicity to mammalian cells might be due to its affinity to cholesterol-containing membranes. However, membrane interactions were not completely dependent on sterols. Yeast chemogenomic profiling suggested additional direct or indirect effects on actin. Accordingly, we performed actin polymerization assays, which suggested that amantelide A also promotes actin polymerization in cell-free systems. However, the C-33 acetoxy derivative amantelide B showed a similar effect on actin dynamics in vitro but no significant activity against yeast. Overall, these studies suggest that the membrane effects are the most functionally relevant for amantelide A mechanism of action.

Chemical diversity and mode of action of natural products targeting lipids in the eukaryotic cell membrane

Covering: up to 2019Nature furnishes bioactive compounds (natural products) with complex chemical structures, yet with simple, sophisticated molecular mechanisms. When natural products exhibit their activities in cells or bodies, they first have to bind or react with a target molecule in/on the cell. The cell membrane is a major target for bioactive compounds. Recently, our understanding of the molecular mechanism of interactions between natural products and membrane lipids progressed with the aid of newly-developed analytical methods. New technology reconnects old compounds with membrane lipids, while new membrane-targeting molecules are being discovered through the screening for antimicrobial potential of natural products. This review article focuses on natural products that bind to eukaryotic membrane lipids, and includes clinically important molecules and key research tools. The chemical diversity of membrane-targeting natural products and the molecular basis of lipid recognition are described. The history of how their mechanism was unveiled, and how these natural products are used in research are also mentioned.

The Perpendicular Orientation of Amphotericin B Methyl Ester in Hydrated Lipid Bilayers Supports the Barrel-Stave Model

The clinically important antibiotic amphotericin B (AmB) is a membrane-active natural product that targets membrane sterol. The antimicrobial activity of AmB is generally attributed to its membrane permeabilization, which occurs when a pore is formed across a lipid bilayer. In this study, the molecular orientation of AmB was investigated using solid-state nuclear magnetic resonance (NMR) to better understand the mechanism of antifungal activity. The methyl ester of AmB (AME) labeled with NMR isotopes, d₃-AME, and its fluorinated and/or ¹³C-labeled derivatives were prepared. All of the AmB derivatives showed similar membrane-disrupting activities and ultraviolet spectra in phospholipid liposomes, suggesting that their molecular assemblies in membranes closely mimic those of AmB. Solid-state ²H NMR measurements of d₃-AME in a hydrated membrane showed that the mobility of AME molecules depends on concentration and temperature. At a 1:5:45 AME:Erg:dimyristoylphosphatidylcholine ratio, AME became sufficiently mobilized to observe the motional averaging of quadrupole coupling. On the basis of the rotational averaging effect of ¹⁹F chemical shift anisotropy, ²H quadrupolar splitting, and ¹³C-¹⁹F dipolar coupling of 14β-F-AMEs, we deduced that the molecular axis of AME is predominantly parallel to the normal of a lipid bilayer. This result supports the barrel-stave model as a molecular assembly of AmB in membranes.

Recent updates of marine antimicrobial peptides

Antimicrobial peptides are group of proteins showing broad-spectrum antimicrobial activity that have been known to be powerful agents against a variety of pathogens. This class of compounds contributed to solving the microbial resistance dilemma that limited the use of many potent antimicrobial agents. The marine environment is known to be one of the richest sources for antimicrobial peptides, yet this environment is not fully explored. Hence, the scientific research attention should be directed toward the marine ecosystem as enormous amount of useful discoveries could be brought to the forefront. In the current article, the marine antimicrobial peptides reported from mid 2012 to 2017 have been reviewed.

Membrane permeabilizing action of amphidinol 3 and theonellamide A in raft-forming lipid mixtures

Amphidinol 3 (AM3) and theonellamide A (TNM-A) are potent antifungal compounds produced by the dinoflagellate Amphidinium klebsii and the sponge Theonella spp., respectively. Both of these metabolites have been demonstrated to interact with membrane lipids ultimately resulting in a compromised bilayer integrity. In this report, the activity of AM3 and TNM-A in ternary lipid mixtures composed of 1-palmitoyl-2-oleoyl-sn-glycerol-3-phosphocholine (POPC):brain sphingomyelin:cholesterol at a mole ratio of 1:1:1 or 3:1:1 exhibiting lipid rafts coexistence is presented. It was found that AM3 has a more extensive membrane permeabilizing activity compared with TNM-A in these membrane mimics, which was almost complete at 15 μM. The extent of their activity nevertheless is similar to the previously reported binary system of POPC and cholesterol, suggesting that phase separation has neither beneficial nor detrimental effects in their ability to disrupt the lipid bilayer.

Sterol-recognition ability and membrane-disrupting activity of Ornithogalum saponin OSW-1 and usual 3-O-glycosyl saponins

OSW-1 is a structurally unique steroidal saponin isolated from the bulbs of Ornithogalum saundersiae, and has exhibited highly potent and selective cytotoxicity in tumor cell lines. This study aimed to investigate the molecular mechanism for the membrane-permeabilizing activity of OSW-1 in comparison with those of other saponins by using various spectroscopic approaches. The membrane effects and hemolytic activity of OSW-1 were markedly enhanced in the presence of membrane cholesterol. Binding affinity measurements using fluorescent cholestatrienol and solid-state NMR spectroscopy of a 3-d-cholesterol probe suggested that OSW-1 interacts with membrane cholesterol without forming large aggregates while 3-O-glycosyl saponin, digitonin, forms cholesterol-containing aggregates. The results suggest that OSW-1/cholesterol interaction is likely to cause membrane permeabilization and pore formation without destroying the whole membrane integrity, which could partly be responsible for its highly potent cell toxicity.

Marine sponge cyclic peptide theonellamide A disrupts lipid bilayer integrity without forming distinct membrane pores

Theonellamides (TNMs) are antifungal and cytotoxic bicyclic dodecapeptides derived from the marine sponge Theonella sp. These peptides specifically bind to 3β-hydroxysterols, resulting in 1,3-β-D-glucan overproduction and membrane damage in yeasts. The inclusion of cholesterol or ergosterol in phosphatidylcholine membranes significantly enhanced the membrane affinity of theonellamide A (TNM-A) because of its direct interaction with 3β-hydroxyl groups of sterols. To better understand TNM-induced membrane alterations, we investigated the effects of TNM-A on liposome morphology. (31)P nuclear magnetic resonance (NMR) and dynamic light scattering (DLS) measurements revealed that the premixing of TNM-A with lipids induced smaller vesicle formation. When giant unilamellar vesicles were incubated with exogenously added TNM-A, confocal micrographs showed dynamic changes in membrane morphology, which were more frequently observed in cholesterol-containing than sterol-free liposomes. In conjunction with our previous data, these results suggest that the membrane action of TNM-A proceeds in two steps: 1) TNM-A binds to the membrane surface through direct interaction with sterols and 2) accumulated TNM-A modifies the local membrane curvature in a concentration-dependent manner, resulting in dramatic membrane morphological changes and membrane disruption.

Detectors for evaluating the cellular landscape of sphingomyelin- and cholesterol-rich membrane domains

Although sphingomyelin and cholesterol are major lipids of mammalian cells, the detailed distribution of these lipids in cellular membranes remains still obscure. However, the recent development of protein probes that specifically bind sphingomyelin and/or cholesterol provides new information about the landscape of the lipid domains that are enriched with sphingomyelin or cholesterol or both. Here, we critically summarize the tools to study distribution and dynamics of sphingomyelin and cholesterol. This article is part of a Special Issue entitled: The cellular lipid landscape edited by Tim P. Levine and Anant K. Menon.

Susceptibility of outer hair cells to cholesterol chelator 2-hydroxypropyl-β-cyclodextrine is prestin-dependent

Niemann-Pick type C1 disease (NPC1) is a fatal genetic disorder caused by impaired intracellular cholesterol trafficking. Recent studies reported ototoxicity of 2-hydroxypropyl- β-cyclodextrin (HPβCD), a cholesterol chelator and the only promising treatment for NPC1. Because outer hair cells (OHCs) are the only cochlear cells affected by HPβCD, we investigated whether prestin, an OHC-specific motor protein, might be involved. Single, high-dose administration of HPβCD resulted in OHC death in prestin wildtype (WT) mice whereas OHCs were largely spared in prestin knockout (KO) mice in the basal region, implicating prestin's involvement in ototoxicity of HPβCD. We found that prestin can interact with cholesterol in vitro, suggesting that HPβCD-induced ototoxicity may involve disruption of this interaction. Time-lapse analysis revealed that OHCs isolated from WT animals rapidly deteriorated upon HPβCD treatment while those from prestin-KOs tolerated the same regimen. These results suggest that a prestin-dependent mechanism contributes to HPβCD ototoxicity.

A dive into membrane dynamics with sponge peptides

In this issue of Chemistry & Biology, Arita et al. (2015) report that theonellamides can specifically recognize cholesterol in liquid-disordered environment, modulate membrane order, and change cell morphology and thus may serve as probes to unveil the enigmatic nature of cell membranes.

Marine natural products

This review covers the literature published in 2013 for marine natural products (MNPs), with 982 citations (644 for the period January to December 2013) referring to compounds isolated from marine microorganisms and phytoplankton, green, brown and red algae, sponges, cnidarians, bryozoans, molluscs, tunicates, echinoderms, mangroves and other intertidal plants and microorganisms. The emphasis is on new compounds (1163 for 2013), together with the relevant biological activities, source organisms and country of origin. Reviews, biosynthetic studies, first syntheses, and syntheses that lead to the revision of structures or stereochemistries, have been included.

Mitsunobu mischief: neighbor-directed histidine N(τ)-alkylation provides access to peptides containing selectively functionalized imidazolium heterocycles

There are few methodologies that yield peptides containing His residues with selective N(τ), N(π)-bis-alkylated imidazole rings. We have found that, under certain conditions, on-resin Mitsunobu coupling of alcohols with peptides having a N(π)-alkylated His residue results in selective and high-yield alkylation of the imidazole N(τ) nitrogen. The reaction requires the presence of a proximal phosphoric, carboxylic or sulfonic acid, and proceeds through an apparent intramolecular mechanism involving Mitsunobu intermediates. These transformations have particular application to phosphopeptides, where "charge masking" of one phosphoryl anionic charge by the cationic histidine imidazolium ion is now possible. This chemistry opens selective access to peptides containing differentially functionalized imidazolium heterocycles, which provide access to new classes of peptides and peptide mimetics.

Balance between exocytosis and endocytosis determines the efficacy of sterol-targeting antibiotics

Antifungals targeting membrane ergosterol are longstanding, yet indispensable drugs in clinical use. However, the mechanisms by which the cellular membrane domains recognized by these antibiotics are generated remain largely unknown. Here, we demonstrate that the balance between endocytosis and exocytosis in membrane trafficking is a critical factor in the action of sterol-targeting antibiotics. When fission yeast cells were treated with manumycin A, cellular binding and the action of the antifungals filipin, amphotericin B, and theonellamides, all of which are ergosterol-binders, were abolished. Additionally, manumycin A treatment attenuated Cdc42 activity and inhibited exocytosis, while endocytosis was only moderately suppressed. Similar defects in membrane trafficking could be reproduced by heat shock and genetic perturbation, which also abolished the action of the antibiotics. We propose that exocytosis and endocytosis respectively supply and internalize the specific plasma membrane domains recognized by sterol-targeting antibiotics.

Direct and stereospecific interaction of amphidinol 3 with sterol in lipid bilayers

Amphidinol 3 (AM3), a polyhydroxy-polyene metabolite from the dinoflagellate Amphidinium klebsii, possesses potent antifungal activity. Although AM3 permeabilizes phospholipid membranes only in the presence of sterol, the detailed molecular basis by which AM3 recognizes sterols in membranes remains unknown. Here, we investigated the molecular interaction between sterols and AM3 in membranes from the viewpoint of stereospecific molecular recognition using ergosterol, cholesterol, and epicholesterol, which is the 3-OH epimer of cholesterol. Dye leakage assays, surface plasmon resonance experiments, (2)H and (31)P NMR measurements, and microscopic observations revealed that AM3 directly interacts with membrane sterols through the strict molecular recognition of the stereochemistry of the sterol 3-OH group. The direct interaction enhances the membrane binding efficiency of AM3, which subsequently permeabilizes membranes without altering membrane integrity.

Theonellamide G, a potent antifungal and cytotoxic bicyclic glycopeptide from the Red Sea marine sponge Theonella swinhoei

In our search for bioactive metabolites from marine organisms, we have investigated the polar fraction of the organic extract of the Red Sea sponge Theonella swinhoei. Successive chromatographic separations and final HPLC purification of the potent antifungal fraction afforded a new bicyclic glycopeptide, theonellamide G (1). The structure of the peptide was determined using extensive 1D and 2D NMR and high-resolution mass spectral determinations. The absolute configuration of theonellamide G was determined by chemical degradation and 2D NMR spectroscopy. Theonellamide G showed potent antifungal activity towards wild and amphotericin B-resistant strains of Candida albicans with IC₅₀ of 4.49 and 2.0 μM, respectively. Additionally, it displayed cytotoxic activity against the human colon adenocarcinoma cell line (HCT-16) with IC₅₀ of 6.0 μM. These findings provide further insight into the chemical diversity and biological activities of this class of compounds.

Structure and biological activity of 8-deoxyheronamide C from a marine-derived Streptomyces sp.: heronamides target saturated hydrocarbon chains in lipid membranes

Polyene macrolactams are a class of microbial metabolites, many of which show potent biological activities with unidentified modes of action. Here we report that 8-deoxyheronamide C, a new 20-membered polyene macrolactam from a marine-derived actinomycete Streptomyces sp., is a unique membrane binder. 8-Deoxyheronamide C showed a characteristic sensitivity profile against fission yeast sterol mutant cells, indicating that the metabolite targets cell membranes. We detected tight physical interaction between heronamides including 8-deoxyheronamide C and heronamide C and saturated hydrocarbon chains in lipid membranes using surface plasmon resonance experiments. We further show that heronamides induced abnormal cell wall morphology in fission yeast probably by perturbing the structure of membrane microdomains. This work will accelerate the biological and medical investigation of polyene macrolactams.

Interaction kinetics of liposome-incorporated unsaturated fatty acids with fatty acid-binding protein 3 by surface plasmon resonance

The role of heart-type fatty acid-binding protein (FABP3) in human physiology as an intracellular carrier of fatty acids (FAs) has been well-documented. In this study, we aimed to develop an analytical method to study real-time interaction kinetics between FABP3 immobilized on the sensor surface and unsaturated C18 FAs using surface plasmon resonance (SPR). To establish the conditions for SPR experiments, we used an FABP3-selective inhibitor 4-(2-(1-(4-bromophenyl)-5-phenyl-1H-pyrazol-3-yl)-phenoxy)-butyric acid. The affinity index thus obtained was comparable to that reported previously, further supporting the usefulness of the SPR-based approach for evaluating interactions between FABPs and hydrophobic ligands. A pseudo-first-order affinity of FABP3 to K(+) petroselinate (C18:1 Δ6 cis), K(+) elaidate (C18:1 Δ9 trans), and K(+) oleate (C18:1 Δ9 cis) was characterized by the dissociation constant (K(d)) near micromolar ranges, whereas K(+) linoleate (C18:2 Δ9,12 cis/cis) and K(+) α-linolenate (C18:3 Δ9,12,15 cis/cis/cis) showed a higher affinity to FABP3 with Kd around 1 × 10(-6)M. Interactions between FAPB3 and C18 FAs incorporated in large unilamellar vesicles consisting of 1,2-dimyristoyl-sn-glycero-3-phosphocholine and FAs (5:1 molar ratio) were also analysed. Control DMPC liposomes without FA showed only marginal binding to FABP3 immobilized on a sensor chip while liposome-incorporated FA revealed significant responses in sensorgrams, demonstrating that the affinity of FAs to FABP3 could be evaluated by using the liposome-incorporated analytes. Significant affinity to FABP3 was observed for monounsaturated fatty acids (K(d) in the range of 1 × 10(-7)M). These experiments demonstrated that highly hydrophobic compounds in a liposome-incorporated form could be subjected to SPR experiments for kinetic analysis.

Visualization of sterol-rich membrane domains with fluorescently-labeled theonellamides

Cholesterol plays important roles in biological membranes. The cellular location where cholesterol molecules work is prerequisite information for understanding their dynamic action. Bioimaging probes for cholesterol molecules would be the most powerful means for unraveling the complex nature of lipid membranes. However, only a limited number of chemical or protein probes have been developed so far for cytological analysis. Here we show that fluorescently-labeled derivatives of theonellamides act as new sterol probes in mammalian cultured cells. The fluorescent probes recognized cholesterol molecules and bound to liposomes in a cholesterol-concentration dependent manner. The probes showed patchy distribution in the plasma membrane, while they stained specific organelle in the cytoplasm. These data suggest that fTNMs will be valuable sterol probes for studies on the role of sterols in the biological membrane under a variety of experimental conditions.