Liposome Disruption Assay to Examine Lytic Properties of Biomolecules

Zn(II) and Cu(II) Coordination Enhances the Antimicrobial Activity of Piscidin 3, but Not That of Piscidins 1 and 2

Piscidins, antimicrobial peptides isolated from fish, are potent against a variety of human pathogens; they show minimum inhibitory concentration values comparable to those of commercially used antimicrobials. Piscidins 1 and 2 are generally more effective than piscidin 3 when applied alone; the contrary is observed for their metal complexes: Zn(II) and Cu(II) coordination does not enhance the efficacy of piscidins 1 and 2, while a moderate enhancement is observed for piscidin 3. All three piscidins bind Cu(II) in a so-called albumin-like binding mode, while for Zn(II) complexes, two coordination modes are observed: piscidins 1 and 2 bind Zn(II) by imidazole nitrogens from His4, His11, and His17 side chains; piscidin 3 coordinates Zn(II) by His3, His4, and His11 imidazole nitrogens and additionally supports the interaction, formed by carbonyl oxygen from His4. Most likely, the high antimicrobial activity of piscidin complexes is due to neither the stability of their complexes nor the change in their secondary structure. Copper(II) complexes with piscidins 1 and 2 can form hydroxyl radicals, which could be responsible for the antimicrobial membrane damaging activity of these complexes. Clearly, a different mechanism (most likely an intercellular targeted one) is observed for piscidin 3 metal complexes; in most cases, the coordination of Cu(II) and Zn(II) enhances the antimicrobial potency of piscidin 3, showing that not only piscidin 3 alone but also its metal complexes have a different mode of action than piscidins 1 and 2.

Engineered Hybrid Vesicles and Cellular Internalization in Mammary Cancer Cells

Extracellular vesicles play an important role in intercellular communication, with the potential to serve as biomaterials for nanocarriers. Combining such extracellular vesicles and liposomes results in advanced drug delivery carriers. In this study, we attempted to fabricate hybrid vesicles using a membrane fusion method and incorporated an anticancer drug. As a result, we successfully prepared nanosized uniform hybrid vesicles and evaluated their physicochemical characteristics and intracellular uptake mechanisms via endocytosis in various cell lines. Compared to liposomes, the hybrid vesicles showed better physical properties and a relatively higher reduction in cell viability, which was presumably dependent on the specific cell type. These findings suggest that fusion-based hybrid vesicles offer a novel strategy for delivering therapeutic agents and provide insights into the types of extracellular vesicles that are useful in fabricating hybrid vesicles to develop an advanced drug delivery system.

Naturally acquired antibodies against Plasmodium vivax pre-erythrocytic stage vaccine antigens inhibit sporozoite invasion of human hepatocytes in vitro

In Plasmodium vivax, the most studied vaccine antigens are aimed at blocking merozoite invasion of erythrocytes and disease development. Very few studies have evaluated pre-erythrocytic (PE) stage antigens. The P. vivax circumsporozoite protein (CSP), is considered the leading PE vaccine candidate, but immunity to CSP is short-lived and variant specific. Thus, there is a need to identify other potential candidates to partner with CSP in a multivalent vaccine to protect against infection and disease. We hypothesize that sporozoite antigens important for host cell infection are considered potential targets. In this study, we evaluated the magnitude and quality of naturally acquired antibody responses to four P. vivax PE antigens: sporozoite surface protein 3 (SSP3), sporozoite protein essential for traversal 1 (SPECT1), cell traversal protein of ookinetes and sporozoites (CelTOS) and CSP in plasma of P. vivax infected patients from Thailand. Naturally acquired antibodies to these antigens were prevalent in the study subjects, but with significant differences in magnitude of IgG antibody responses. About 80% of study participants had antibodies to all four antigens and only 2% did not have antibodies to any of the antigens. Most importantly, these antibodies inhibited sporozoite infection of hepatocytes in vitro. Significant variations in magnitude of antigen-specific inhibitory antibody responses were observed with individual samples. The highest inhibitory responses were observed with anti-CelTOS antibodies, followed by anti-SPECT1, SSP3 and CSP antibodies respectively. These data highlight the vaccine potential of these antigens in protecting against hepatocyte infection and the need for a multi-valent pre-erythrocytic vaccine to prevent liver stage development of P. vivax sporozoites.

A Biomimetic Multiparametric Assay to Characterise Anti-Amyloid Drugs

Alzheimer's disease (AD) is the most widespread form of senile dementia worldwide and represents a leading socioeconomic problem in healthcare. Although it is widely debated, the aggregation of the amyloid β peptide (Aβ) is linked to the onset and progression of this neurodegenerative disease. Molecules capable of interfering with specific steps in the fibrillation process remain of pharmacological interest. To identify such compounds, we have set up a small molecule screening process combining multiple experimental methods (UV and florescence spectrometry, ITC, and ATR-FTIR) to identify and characterise potential modulators of Aβ1-42 fibrillation through the description of the biochemical interactions (molecule-membrane Aβ peptide). Three known modulators, namely bexarotene, Chicago sky blue and indomethacin, have been evaluated through this process, and their modulation mechanism in the presence of a biomembrane has been described. Such a well-adapted physico-chemical approach to drug discovery proves to be an undeniable asset for the rapid characterisation of compounds of therapeutic interest for Alzheimer's disease. This strategy could be adapted and transposed to search for modulators of other amyloids such as tau protein.

A small-molecule membrane fluidizer re-sensitizes methicillin-resistant Staphylococcus aureus (MRSA) to β-lactam antibiotics

Novel antibacterial agents and strategies are urgently needed to fight against the ongoing global antibiotic resistance problem. While natural products remain the main source in antibiotic discovery, synthetic antibacterials provide an attractive alternative and may evade the ancient antibiotic resistance. Herein, we report a small molecule that re-sensitizes methicillin-resistant Staphylococcus aureus to β-lactam antibiotics with extremely low potential for resistance development. It belongs to a new class of broad-spectrum antibacterials, trypyricins, which share similar structural characteristics and mechanism of action to the cationic antimicrobial peptides. Mechanistic studies indicated that trypyricins fluidize and disrupt bacterial cytoplasmic membrane. These results suggested that trypyricins represent a promising new class of antibacterials and may be further developed as antibiotic adjuvants to fight against resistant bacteria in the clinic.

Pore-Forming Proteins: From Pore Assembly to Structure by Quantitative Single-Molecule Imaging

Pore-forming proteins (PFPs) play a central role in many biological processes related to infection, immunity, cancer, and neurodegeneration. A common feature of PFPs is their ability to form pores that disrupt the membrane permeability barrier and ion homeostasis and generally induce cell death. Some PFPs are part of the genetically encoded machinery of eukaryotic cells that are activated against infection by pathogens or in physiological programs to carry out regulated cell death. PFPs organize into supramolecular transmembrane complexes that perforate membranes through a multistep process involving membrane insertion, protein oligomerization, and finally pore formation. However, the exact mechanism of pore formation varies from PFP to PFP, resulting in different pore structures with different functionalities. Here, we review recent insights into the molecular mechanisms by which PFPs permeabilize membranes and recent methodological advances in their characterization in artificial and cellular membranes. In particular, we focus on single-molecule imaging techniques as powerful tools to unravel the molecular mechanistic details of pore assembly that are often obscured by ensemble measurements, and to determine pore structure and functionality. Uncovering the mechanistic elements of pore formation is critical for understanding the physiological role of PFPs and developing therapeutic approaches.

Naturally acquired antibodies to gametocyte antigens are associated with reduced transmission of Plasmodium vivax gametocytes to Anopheles arabiensis mosquitoes

Naturally acquired antibodies may reduce the transmission of Plasmodium gametocytes to mosquitoes. Here, we investigated associations between antibody prevalence and P. vivax infectivity to mosquitoes. A total of 368 microscopy confirmed P. vivax symptomatic patients were passively recruited from health centers in Ethiopia and supplemented with 56 observations from asymptomatic P. vivax parasite carriers. Direct membrane feeding assays (DMFA) were performed to assess mosquito infectivity; for selected feeds these experiments were also performed after replacing autologous plasma with malaria naïve control serum (n=61). The prevalence of antibodies against 6 sexual stage antigens (Pvs47, Pvs48/45, Pvs230, PvsHAP2, Pvs25 and PvCelTOS) and an array of asexual antigens was determined by ELISA and multiplexed bead-based assays. Gametocyte (ρ< 0.42; p = 0.0001) and parasite (ρ = 0.21; p = 0.0001) densities were positively associated with mosquito infection rates. Antibodies against Pvs47, Pvs230 and Pvs25 were associated with 23 and 34% reductions in mosquito infection rates (p<0.0001), respectively. Individuals who showed evidence of transmission blockade in serum-replacement DMFAs (n=8) were significantly more likely to have PvsHAP2 or Pvs47 antibodies. Further studies may demonstrate causality for the observed associations, improve our understanding of the natural transmission of P. vivax and support vaccine development.

BefA, a microbiota-secreted membrane disrupter, disseminates to the pancreas and increases β cell mass

Microbiome dysbiosis is a feature of diabetes, but how microbial products influence insulin production is poorly understood. We report the mechanism of BefA, a microbiome-derived protein that increases proliferation of insulin-producing β cells during development in gnotobiotic zebrafish and mice. BefA disseminates systemically by multiple anatomic routes to act directly on pancreatic islets. We detail BefA's atomic structure, containing a lipid-binding SYLF domain, and demonstrate that it permeabilizes synthetic liposomes and bacterial membranes. A BefA mutant impaired in membrane disruption fails to expand β cells, whereas the pore-forming host defense protein, Reg3, stimulates β cell proliferation. Our work demonstrates that membrane permeabilization by microbiome-derived and host defense proteins is necessary and sufficient for β cell expansion during pancreas development, potentially connecting microbiome composition with diabetes risk.

Ganglioside-Enriched Phospholipid Vesicles Induce Cooperative Aβ Oligomerization and Membrane Disruption

A major hallmark of Alzheimer's disease (AD) is the accumulation of extracellular aggregates of amyloid-β (Aβ). Structural polymorphism observed among Aβ fibrils in AD brains seem to correlate with the clinical subtypes suggesting a link between fibril polymorphism and pathology. Since fibrils emerge from a templated growth of low-molecular-weight oligomers, understanding the factors affecting oligomer generation is important. Membrane lipids are key factors to influence early stages of Aβ aggregation and oligomer generation, which cause membrane disruption. We have previously demonstrated that conformationally discrete Aβ oligomers can be generated by modulating the charge, composition, and chain length of lipids and surfactants. Here, we extend our studies into liposomal models by investigating Aβ oligomerization on large unilamellar vesicles (LUVs) of total brain extracts (TBE), reconstituted lipid rafts (LRs), or 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC). Varying the vesicle composition by specifically increasing the amount of GM1 gangliosides as a constituent, we found that only GM1-enriched liposomes induce the formation of toxic, low-molecular-weight oligomers. Furthermore, we found that the aggregation on liposome surface and membrane disruption are highly cooperative and sensitive to membrane surface characteristics. Numerical simulations confirm such a cooperativity and reveal that GM1-enriched liposomes form twice as many pores as those formed in the absence GM1. Overall, this study uncovers mechanisms of cooperativity between oligomerization and membrane disruption under controlled lipid compositional bias, and refocuses the significance of the early stages of Aβ aggregation in polymorphism, propagation, and toxicity in AD.

Implications of conformational flexibility, lipid binding, and regulatory domains in cell traversal-protein CelTOS for apicomplexan migration

Malaria and other apicomplexan-caused diseases affect millions of humans, agricultural animals, and pets. Cell traversal is a common feature used by multiple apicomplexan parasites to migrate through host cells and can be exploited to develop therapeutics against these deadly parasites. Here, we provide insights into the mechanism of the Cell-traversal protein for ookinetes and sporozoites (CelTOS), a conserved cell-traversal protein in apicomplexan parasites and malaria vaccine candidate. CelTOS has previously been shown to form pores in cell membranes to enable traversal of parasites through cells. We establish roles for the distinct protein regions of Plasmodium vivax CelTOS and examine the mechanism of pore formation. We further demonstrate that CelTOS dimer dissociation is required for pore formation, as disulfide bridging between monomers inhibits pore formation, and this inhibition is rescued by disulfide-bridge reduction. We also show that a helix-destabilizing amino acid, Pro127, allows CelTOS to undergo significant conformational changes to assemble into pores. The flexible C terminus of CelTOS is a negative regulator that limits pore formation. Finally, we highlight that lipid binding is a prerequisite for pore assembly as mutation of a phospholipids-binding site in CelTOS resulted in loss of lipid binding and abrogated pore formation. These findings identify critical regions in CelTOS and will aid in understanding the egress mechanism of malaria and other apicomplexan parasites as well as have implications for studying the function of other essential pore-forming proteins.

Lipid tails modulate antimicrobial peptide membrane incorporation and activity

Membrane disrupting antimicrobial peptides (AMPs) are often amphipathic peptides that interact directly with lipid bilayers. AMPs are generally thought to interact mostly with lipid head groups, but it is less clear how the lipid alkyl chain length and saturation modulate interactions with membranes. Here, we used native mass spectrometry to measure the stoichiometry of three different AMPs-LL-37, indolicidin, and magainin-2-in lipid nanodiscs. We also measured the activity of these AMPs in unilamellar vesicle leakage assays. We found that LL-37 formed specific hexamer complexes but with different intermediates and affinities that depended on the bilayer thickness. LL-37 was also most active in lipid bilayers containing longer, unsaturated lipids. In contrast, indolicidin incorporated to a higher degree into more fluid lipid bilayers but was more active with bilayers with thinner, less fluid lipids. Finally, magainin-2 incorporated to a higher degree into bilayers with longer, unsaturated alkyl chains and showed more activity in these same conditions. Together, these data show that higher amounts of peptide incorporation generally led to higher activity and that AMPs tend to incorporate more into longer unsaturated lipid bilayers. However, the activity of AMPs was not always directly related to amount of peptide incorporated.

Zn-Enhanced Asp-Rich Antimicrobial Peptides: N-Terminal Coordination by Zn(II) and Cu(II), Which Distinguishes Cu(II) Binding to Different Peptides

The antimicrobial activity of surfactant-associated anionic peptides (SAAPs), which are isolated from the ovine pulmonary surfactant and are selective against the ovine pathogen Mannheimia haemolytica, is strongly enhanced in the presence of Zn(II) ions. Both calorimetry and ITC measurements show that the unique Asp-only peptide SAAP3 (DDDDDDD) and its analogs SAAP2 (GDDDDDD) and SAAP6 (GADDDDD) have a similar micromolar affinity for Zn(II), which binds to the N-terminal amine and Asp carboxylates in a net entropically-driven process. All three peptides also bind Cu(II) with a net entropically-driven process but with higher affinity than they bind Zn(II) and coordination that involves the N-terminal amine and deprotonated amides as the pH increases. The parent SAAP3 binds Cu(II) with the highest affinity; however, as shown with potentiometry and absorption, CD and EPR spectroscopy, Asp residues in the first and/or second positions distinguish Cu(II) binding to SAAP3 and SAAP2 from their binding to SAAP6, decreasing the Cu(II) Lewis acidity and suppressing its square planar amide coordination by two pH units. We also show that these metal ions do not stabilize a membrane disrupting ability nor do they induce the antimicrobial activity of these peptides against a panel of human pathogens.

Rapid Production and Purification of Dye-Loaded Liposomes by Electrodialysis-Driven Depletion

Liposomes are spherical-shaped vesicles that enclose an aqueous milieu surrounded by bilayer or multilayer membranes formed by self-assembly of lipid molecules. They are intensively exploited as either model membranes for fundamental studies or as vehicles for delivery of active substances in vivo and in vitro. Irrespective of the method adopted for production of loaded liposomes, obtaining the final purified product is often achieved by employing multiple, time consuming steps. To alleviate this problem, we propose a simplified approach for concomitant production and purification of loaded liposomes by exploiting the Electrodialysis-Driven Depletion of charged molecules from solutions. Our investigations show that electrically-driven migration of charged detergent and dye molecules from solutions that include natural or synthetic lipid mixtures leads to rapid self-assembly of loaded, purified liposomes, as inferred from microscopy and fluorescence spectroscopy assessments. In addition, the same procedure was successfully applied for incorporating PEGylated lipids into the membranes for the purpose of enabling long-circulation times needed for potential in vivo applications. Dynamic Light Scattering analyses and comparison of electrically-formed liposomes with liposomes produced by sonication or extrusion suggest potential use for numerous in vitro and in vivo applications.

Streptococcus pneumoniae pneumolysin and neuraminidase A convert high-density lipoproteins into pro-atherogenic particles

High-density lipoproteins (HDLs) are a group of different subpopulations of sialylated particles that have an essential role in the reverse cholesterol transport (RCT) pathway. Importantly, changes in the protein and lipid composition of HDLs may lead to the formation of particles with reduced atheroprotective properties. Here, we show that Streptococcus pneumoniae pneumolysin (PLY) and neuraminidase A (NanA) impair HDL function by causing chemical and structural modifications of HDLs. The proteomic, lipidomic, cellular, and biochemical analysis revealed that PLY and NanA induce significant changes in sialic acid, protein, and lipid compositions of HDL. The modified HDL particles have reduced cholesterol acceptor potential from activated macrophages, elevated levels of malondialdehyde adducts, and show significantly increased complement activating capacity. These results suggest that accumulation of these modified HDL particles in the arterial intima may present a trigger for complement activation, inflammatory response, and thereby promote atherogenic disease progression.

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S. pneumoniae molecules PLY and NanA target human high-density lipoprotein (HDL).

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These interactions result in major modifications in the HDL proteome and lipidome.

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Microbially modified HDL activates humoral and cell-mediated innate immune responses.

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The activated immune response mediates formation of pro-atherogenic epitopes on HDL.

Techniques for studying membrane pores

Pore-forming proteins (PFPs) are of special interest because of the association of their activity with the disruption of the membrane impermeability barrier and cell death. They generally convert from a monomeric, soluble form into transmembrane oligomers that induce the opening of membrane pores. The study of pore formation in membranes with molecular detail remains a challenging endeavor because of its highly dynamic and complex nature, usually involving diverse oligomeric structures with different functionalities. Here we discuss current methods applied for the structural and functional characterization of PFPs at the individual vesicle and cell level. We highlight how the development of high-resolution and single-molecule imaging techniques allows the analysis of the structural organization of protein oligomers and pore entities in lipid membranes.

A Cell-Free Screen for Bacterial Membrane Disruptors Identifies Mefloquine as a Novel Antibiotic Adjuvant

Antibacterial discovery efforts have lagged far behind the need for new antibiotics. An approach that has gained popularity recently is targeting bacterial phospholipid membranes. We leveraged the differences between bacterial and mammalian phospholipid compositions to develop a high-throughput screen that identifies agents that selectively disrupt bacterial membranes while leaving mammalian membranes intact. This approach was used to screen 4480 compounds representing a subset of the Maybridge HitFinderTM V.11 Collection and the Prestwick Chemical Drug Library®. The screen identified 35 "positives" (0.8% hit rate) that preferentially damage bacterial model membranes. Among these, an antimalarial compound, mefloquine, and an aminoglycoside, neomycin, were identified. Further investigation of mefloquine's activity against Staphylococcus aureus showed that it has little antibiotic activity on its own but can alter membrane fluidity, thereby potentiating a β-lactam antibiotic, oxacillin, against both methicillin-susceptible and methicillin-resistant S. aureus. This study indicates that our cell-free screening approach is a promising platform for discovering bacterial membrane disruptors as antibacterials antibiotic adjuvants.

Liposome Fusion Mediated by Hydrophobic Magnetic Nanoparticles Stabilized with Oleic Acid and Modulated by an External Magnetic Field

Membrane fusion is considered relevant in countless scientific areas and biotechnological processes, ranging from vital life events to biomedicine, pharmaceuticals, and materials engineering, among others. In this study, we employed hydrophobic oleic acid (OA)-coated magnetite (Fe₃O₄) nanoparticles (MNP-OA) as a platform to induce the fusion of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphatidylcholine liposomes [large unilamellar vesicles (LUVs)] in a colloidal dispersion. This fusion was monitored through dynamic light scattering, turbidimetry, and fluorescence assay using the well-known Tb/dipicolinic acid (DPA) complex formation assay. MNP-OA have shown to be able to induce fusion with the mixing of liposomal inner content with direct dependence on the nanoparticle concentration added to the LUVs. Moreover, changes in the permeability of the liposome bilayer, upon the addition of MNP-OA to liposomes, were evaluated by studying the leakage of carboxyfluorescein and of the co-encapsulated Tb/DPA complex. These assays allowed us to determine that MNP-OA did not significantly modify liposome permeability during the fusion process. Transmission electron microscopy and confocal microscopy revealed that MNP-OA remained embedded in the lipid bilayer without producing membrane rupture, liposome deformation, or destruction. In addition, we evaluated the effect of applying a low-intensity magnetic field to the LUVs/MNP-OA system and observed that the nanoparticles considerably increased their fusogenic activity under this external stimulus, as well as they are capable of responding to low magnetic fields of around 0.45 mT. These results revealed the potential of hydrophobic magnetic nanoparticles, stabilized with OA, to act as a fusogen, thus representing a valuable tool for biotechnological applications.

Biochemical determinants of the IGFBP-3-hyaluronan interaction

IGFBP‐3, the most abundant IGFBP and the main carrier of insulin‐like growth factor I (IGF‐I) in the circulation, can bind IGF‐1 with high affinity, which attenuates IGF/IGF‐IR interactions, thereby resulting in antiproliferative effects. The C‐terminal domain of insulin‐like growth factor‐binding protein‐3 (IGFBP‐3) is known to contain an 18‐basic amino acid motif capable of interacting with either humanin (HN) or hyaluronan (HA). We previously showed that the 18‐amino acid IGFBP‐3 peptide is capable of binding either HA or HN with comparable affinities to the full‐length IGFBP‐3 protein and that IGFBP‐3 can compete with the HA receptor, CD44, for binding HA. Blocking the interaction between HA and CD44 reduced viability of A549 human lung cancer cells. In this study, we set out to better characterize IGFBP‐3‐HA interactions. We show that both stereochemistry and amino acid identity are important determinants of the interaction between the IGFBP‐3 peptide and HA and for the peptide's ability to exert its cytotoxic effects. Binding of IGFBP‐3 to either HA or HN was unaffected by glycosylation or reduction of IGFBP‐3, suggesting that the basic 18‐amino acid residue sequence of IGFBP‐3 remains accessible for interaction with either HN or HA upon glycosylation or reduction of the full‐length protein. Removing N‐linked oligosaccharides from CD44 increased its ability to compete with IGFBP‐3 for binding HA, while reduction of CD44 rendered the protein relatively ineffective at blocking IGFBP‐3‐HA interactions. We conclude that both deglycosylation and disulfide bond formation are important for CD44 to compete with IGFBP‐3 for binding HA.

Effect of Pegylation and Targeting Moieties on the Ultrasound-Mediated Drug Release from Liposomes

The use of targeted liposomes encapsulating chemotherapy drugs enhances the specific targeting of cancer cells, thus reducing the side effects of these drugs and providing patient-friendly chemotherapy treatment. Targeted pegylated (stealth) liposomes have the ability to safely deliver their loaded drugs to the cancer cells by targeting specific receptors overly expressed on the surface of these cells. Applying ultrasound as an external stimulus will safely trigger drug release from these liposomes in a controlled manner. In this study, we investigated the release kinetics of the model drug "calcein" from targeted liposomes sonicated with low-frequency ultrasound (20 kHz). Our results showed that pegylated liposomes were more sonosensitive compared to nonpegylated liposomes. A comparison of the effect of three targeting moieties conjugated to the surface of pegylated liposomes, namely human serum albumin (HSA), transferrin (Tf) and arginylglycylaspartic acid (RGD), on calcein release kinetics was conducted. The fluorescent results showed that HSA-PEG and Tf-PEG liposomes were more sonosensitive (showing higher calcein release following the exposure to pulsed LFUS) compared to the control pegylated liposomes, thus adding more acoustic benefits to their targeting efficacy.

Membrane Pore Formation by Peptides Studied by Fluorescence Techniques

Pore formation in cellular membranes by pathogen-derived proteins is a mechanism utilized by a set of microbes to exert their cytotoxic effect. On the other hand, the host cells have developed a defense mechanism to produce antimicrobial peptides to kill the pathogens by a similar, membrane perforation mechanism. Furthermore, certain endogenous proteins or peptides kill the parent cells through membrane permeabilization. Analysis of the molecular details of membrane pore formation is often conducted using artificial systems, such as bilayer lipid membranes and synthetic peptides. This chapter describes two fluorescence-based methods to study peptide-induced membrane leakage. One method involves preparation of lipid vesicles loaded with a fluorophore (e.g., calcein or carboxyfluorescein) at a self-quenching concentration. If the externally added peptide forms relatively large pores (≥1 nm in diameter), the fluorophore leaks out and undergoes dequenching, resulting in time-dependent increase in fluorescence. The other method is designed to monitor smaller pores (<1 nm in diameter). It involves preparation of vesicles in a Ca²⁺-less buffer, containing a Ca²⁺-dependent fluorophore, such as Quin-2. Removal of external Quin-2 by a desalting column and addition of an appropriate concentration of CaCl₂ externally sequesters Quin-2 and Ca²⁺ ions by the vesicle membrane. Addition of the pore-forming peptide to these vesicles results in membrane permeabilization, Ca²⁺ influx and binding to Quin-2. In both cases, the kinetics of the increase of fluorescence and its equilibrium levels allow quantitative analysis of the pore formation mechanism.

Mechanism of High-Level Daptomycin Resistance in Corynebacterium striatum

Daptomycin, a last-line-of-defense antibiotic for treating Gram-positive infections, is experiencing clinical failure against important infectious agents, including Corynebacterium striatum The recent transition of daptomycin to generic status is projected to dramatically increase availability, use, and clinical failure. Here we confirm the genetic mechanism of high-level daptomycin resistance (HLDR; MIC = >256 µg/ml) in C. striatum, which evolved within a patient during daptomycin therapy, a phenotype recapitulated in vitro In all 8 independent cases tested, loss-of-function mutations in phosphatidylglycerol synthase (pgsA2) were necessary and sufficient for high-level daptomycin resistance. Through lipidomic and biochemical analysis, we demonstrate that daptomycin's activity is dependent on the membrane phosphatidylglycerol (PG) concentration. Until now, the verification of PG as the in vivo target of daptomycin has proven difficult since tested cell model systems were not viable without membrane PG. C. striatum becomes daptomycin resistant at a high level by removing PG from the membrane and changing the membrane composition to maintain viability. This work demonstrates that loss-of-function mutation in pgsA2 and the loss of membrane PG are necessary and sufficient to produce high-level resistance to daptomycin in C. striatumIMPORTANCE Antimicrobial resistance threatens the efficacy of antimicrobial treatment options, including last-line-of-defense drugs. Understanding how this resistance develops can help direct antimicrobial stewardship efforts and is critical to designing the next generation of antimicrobial therapies. Here we determine how Corynebacterium striatum, a skin commensal and opportunistic pathogen, evolved high-level resistance to a drug of last resort, daptomycin. Through a single mutation, this pathogen was able to remove the daptomycin's target, phosphatidylglycerol (PG), from the membrane and evade daptomycin's bactericidal activity. We found that additional compensatory changes were not necessary to support the removal of PG and replacement with phosphatidylinositol (PI). The ease with which C. striatum evolved high-level resistance is cause for alarm and highlights the importance of screening new antimicrobials against a wide range of clinical pathogens which may harbor unique capacities for resistance evolution.

Membrane Lipid Screen to Identify Molecular Targets of Biomolecules

Proteins that bind to and disrupt cell membranes may target specific phospholipids. Here we describe a protocol to identify the lipid targets of proteins and biomolecules. First, we describe a screen to identify lipids in membranes that are specifically bound by the biomolecule of interest. Second, we describe a method for determining if the presence of these lipids within membranes is necessary for membrane disruption. The methods described here were used to determine that the malaria vaccine candidate CelTOS disrupts cell membranes by specifically targeting phosphatidic acid (Jimah et al., 2016). This protocol has a companion protocol: 'Liposome disruption assay to examine lytic properties of biomolecules' which can be applied to examine the ability of the biomolecule to disrupt membranes composed of the lipid target identified by following this protocol (Jimah et al., 2017).

Malaria parasite CelTOS targets the inner leaflet of cell membranes for pore-dependent disruption

Apicomplexan parasites contain a conserved protein CelTOS that, in malaria parasites, is essential for traversal of cells within the mammalian host and arthropod vector. However, the molecular role of CelTOS is unknown because it lacks sequence similarity to proteins of known function. Here, we determined the crystal structure of CelTOS and discovered CelTOS resembles proteins that bind to and disrupt membranes. In contrast to known membrane disruptors, CelTOS has a distinct architecture, specifically binds phosphatidic acid commonly present within the inner leaflet of plasma membranes, and potently disrupts liposomes composed of phosphatidic acid by forming pores. Microinjection of CelTOS into cells resulted in observable membrane damage. Therefore, CelTOS is unique as it achieves nearly universal inner leaflet cellular activity to enable the exit of parasites from cells during traversal. By providing novel molecular insight into cell traversal by apicomplexan parasites, our work facilitates the design of therapeutics against global pathogens.