Antiviral Susceptibility of Highly Pathogenic Avian Influenza A(H5N1) Viruses Circulating Globally in 2022-2023

The antiviral susceptibility of currently circulating (2022-2023) highly pathogenic avian influenza (HPAI) A(H5N1) viruses was assessed by genotypic and phenotypic approaches. The frequency of neuraminidase (NA) and polymerase acidic (PA) substitutions associated with reduced inhibition by NA inhibitors (NAIs) (21/2698, 0.78%) or by the PA inhibitor baloxavir (14/2600, 0.54%) was low. Phenotypic testing of 22 clade 2.3.2.1a and 2.3.4.4b viruses revealed broad susceptibility to NAIs and baloxavir concluding that most contemporary HPAI A(H5N1) viruses retain susceptibility to antiviral drugs. Novel NA-K432E and NA-T438I substitutions (N2 numbering) were identified at elevated frequencies (104/2698, 3.85%) and caused reduced zanamivir and peramivir inhibition.

Cancer-Associated Gangliosides as a Therapeutic Target for Host Defense Peptide Mimics

Aberrant levels of glycolipids expressed on cellular surfaces are characteristic of different types of cancers. The oligomer of acylated lysine (OAK) mimicking antimicrobial peptides displays in vitro activity against human and murine melanoma cell lines with upregulated GD3 and GM3 gangliosides. Herein, we demonstrate the capability of OAK to intercalate into the sialo-oligosaccharides of DPPC/GD3 and DPPC/GM3 lipid monolayers using X-ray scattering. The lack of insertion into monolayers containing phosphatidylserine suggests that the mechanism of action by OAKs against glycosylated lipid membranes is not merely driven by charge effects. The fluorescence microscopy data demonstrates the membrane-lytic activity of OAK. Understanding the molecular basis for selectivity toward GD3 and GM3 gangliosides by antimicrobial lipopeptides will contribute to the development of novel therapies to cure melanoma and other malignancies.

Structural Changes in Films of Pulmonary Surfactant Induced by Surfactant Vesicles

When compressed by the shrinking alveolar surface area during exhalation, films of pulmonary surfactant in situ reduce surface tension to levels at which surfactant monolayers collapse from the surface in vitro. Vesicles of pulmonary surfactant added below these monolayers slow collapse. X-ray scattering here determined the structural changes induced by the added vesicles. Grazing incidence X-ray diffraction on monolayers of extracted calf surfactant detected an ordered phase. Mixtures of dipalmitoyl phosphatidylcholine and cholesterol, but not the phospholipid alone, mimic that structure. At concentrations that stabilize the monolayers, vesicles in the subphase had no effect on the unit cell, and X-ray reflection showed that the film remained monomolecular. The added vesicles, however, produced a concentration-dependent increase in the diffracted intensity. These results suggest that the enhanced resistance to collapse results from enlargement by the additional material of the ordered phase.

The Structural Role of Gangliosides: Insights from X-ray Scattering on Model Membranes

Background:

Gangliosides are an essential component of eukaryotic plasma membranes implicated in multiple physiological processes. Little is known about molecular mechanisms underlying the distribution and functions of membrane gangliosides. The overwhelmingly complex organization of glycocalyx impedes the structural analysis on cell surface and the interplay between the lipid components. Advanced X-ray analytical tools applicable to studying biological interfaces call for the simplistic models that mimic ganglioside-enriched cellular membranes.

Objective:

To summarize the mechanistic evidences of ganglioside interactions with lipid environment and biologically active ligands using high-resolution synchrotron X-ray scattering.

Methods:

A comprehensive review of studies published over the last decade was done to discuss recent accomplishments and future trends.

Results:

Langmuir monolayers represent an adequate model system to assess the effect of gangliosides on membrane structure. Grazing incidence X-ray diffraction reveals a condensation effect by gangliosides on zwitterionic phospholipids with the cooperative packing of sialo- and phosphate groups. In turn, the arrangement of negatively charged lipids in ganglioside mixture remains unchanged due to the stretched conformation of carbohydrate moieties. Upon interaction with biological ligands, such as cholera toxin and galectins, the ganglioside redistribution within the ordered regions of monolayer follows distinct mechanistic patterns. The cholera toxin pentamer attached to the oligosaccharide core induces local transition from oblique to the hexagonal lattice resulting in phase coexistence. The incorporation of the A subunit responsible for endocytosis is further promoted by the acidic environment characteristic for endosomal space. X-ray reflectivity shows in-plane orientation of galectin dimers with the spatial mismatch between the lectin binding sites and ganglioside carbohydrates to perturb ceramide alkyl chains. Recent data also demonstrate sialic acid groups to be potential targets for novel peptide mimicking anticancer therapeutics.

Conclusion:

Coupled with surface X-ray scattering, the membrane mimetic approach allows for better understanding the biological role of gangliosides and their potential applications.

Salmonella Membrane Structural Remodeling Increases Resistance to Antimicrobial Peptide LL-37

Gram-negative bacteria are protected from their environment by an outer membrane that is primarily composed of lipopolysaccharides (LPSs). Under stress, pathogenic serotypes of Salmonella enterica remodel their LPSs through the PhoPQ two-component regulatory system that increases resistance to both conventional antibiotics and antimicrobial peptides (AMPs). Acquired resistance to AMPs is contrary to the established narrative that AMPs circumvent bacterial resistance by targeting the general chemical properties of membrane lipids. However, the specific mechanisms underlying AMP resistance remain elusive. Here we report a 2-fold increase in bacteriostatic concentrations of human AMP LL-37 for S. enterica with modified LPSs. LPSs with and without chemical modifications were isolated and investigated by Langmuir films coupled with grazing-incidence X-ray diffraction (GIXD) and specular X-ray reflectivity (XR). The initial interactions between LL-37 and LPS bilayers were probed using all-atom molecular dynamics simulations. These simulations suggest that initial association is nonspecific to the type of LPS and governed by hydrogen bonding to the LPS outer carbohydrates. GIXD experiments indicate that the interactions of the peptide with monolayers reduce the number of crystalline domains but greatly increase the typical domain size in both LPS isoforms. Electron densities derived from XR experiments corroborate the bacteriostatic values found in vitro and indicate that peptide intercalation is reduced by LPS modification. We hypothesize that defects at the liquid-ordered boundary facilitate LL-37 intercalation into the outer membrane, whereas PhoPQ-mediated LPS modification protects against this process by having innately increased crystallinity. Since induced ordering has been observed with other AMPs and drugs, LPS modification may represent a general mechanism by which Gram-negative bacteria protect against host innate immunity.

Peptoid drug discovery and optimization via surface X-ray scattering

Synthetic polymers mimicking antimicrobial peptides have drawn considerable interest as potential therapeutics. N-substituted glycines, or peptoids, are recognized by their in vivo stability and ease of synthesis. Peptoids are thought to act primarily on the negatively charged lipids that are abundant in bacterial cell membranes. A mechanistic understanding of lipid-peptoid interaction at the molecular level will provide insights for rational design and optimization of peptoids. Here, we highlight recent studies that utilize synchrotron liquid surface X-ray scattering to characterize the underlying peptoid interactions with bacterial and eukaryotic membranes. Cellular membranes are highly complex, and difficult to characterize at the molecular level. Model systems including Langmuir monolayers, are used in these studies to reduce system complexity. The general workflow of these systems and the corresponding data analysis techniques are presented alongside recent findings. These studies investigate the role of peptoid physicochemical characteristics on membrane activity. Specifically, the roles of cationic charge, conformational constraint via macrocyclization, and hydrophobicity are shown to correlate their membrane interactions to biological activities in vitro. These structure-activity relationships have led to new insights into the mechanism of action by peptoid antimicrobials, and suggest optimization strategies for future therapeutics based on peptoids.

Hydrophobic interactions modulate antimicrobial peptoid selectivity towards anionic lipid membranes

Hydrophobic interactions govern specificity for natural antimicrobial peptides. No such relationship has been established for synthetic peptoids that mimic antimicrobial peptides. Peptoid macrocycles synthesized with five different aromatic groups are investigated by minimum inhibitory and hemolytic concentration assays, epifluorescence microscopy, atomic force microscopy, and X-ray reflectivity. Peptoid hydrophobicity is determined using high performance liquid chromatography. Disruption of bacterial but not eukaryotic lipid membranes is demonstrated on the solid supported lipid bilayers and Langmuir monolayers. X-ray reflectivity studies demonstrate that intercalation of peptoids with zwitterionic or negatively charged lipid membranes is found to be regulated by hydrophobicity. Critical levels of peptoid selectivity are demonstrated and found to be modulated by their hydrophobic groups. It is suggested that peptoids may follow different optimization schemes as compared to their natural analogues.

Cyclization Improves Membrane Permeation by Antimicrobial Peptoids

The peptidomimetic approach has emerged as a powerful tool for overcoming the inherent limitations of natural antimicrobial peptides, where the therapeutic potential can be improved by increasing the selectivity and bioavailability. Restraining the conformational flexibility of a molecule may reduce the entropy loss upon its binding to the membrane. Experimental findings demonstrate that the cyclization of linear antimicrobial peptoids increases their bactericidal activity against Staphylococcus aureus while maintaining high hemolytic concentrations. Surface X-ray scattering shows that macrocyclic peptoids intercalate into Langmuir monolayers of anionic lipids with greater efficacy than for their linear analogues. It is suggested that cyclization may increase peptoid activity by allowing the macrocycle to better penetrate the bacterial cell membrane.

Modification of Salmonella Lipopolysaccharides Prevents the Outer Membrane Penetration of Novobiocin

Small hydrophilic antibiotics traverse the outer membrane of Gram-negative bacteria through porin channels. Large lipophilic agents traverse the outer membrane through its bilayer, containing a majority of lipopolysaccharides in its outer leaflet. Genes controlled by the two-component regulatory system PhoPQ modify lipopolysaccharides. We isolate lipopolysaccharides from isogenic mutants of Salmonella sp., one lacking the modification, the other fully modified. These lipopolysaccharides were reconstituted as monolayers at the air-water interface, and their properties, as well as their interaction with a large lipophilic drug, novobiocin, was studied. X-ray reflectivity showed that the drug penetrated the monolayer of the unmodified lipopolysaccharides reaching the hydrophobic region, but was prevented from this penetration into the modified lipopolysaccharides. Results correlate with behavior of bacterial cells, which become resistant to antibiotics after PhoPQ-regulated modifications. Grazing incidence x-ray diffraction showed that novobiocin produced a striking increase in crystalline coherence length, and the size of the near-crystalline domains.

Guanidino groups greatly enhance the action of antimicrobial peptidomimetics against bacterial cytoplasmic membranes

Antimicrobial peptides or their synthetic mimics are a promising class of potential new antibiotics. Herein we assess the effect of the type of cationic side chain (i.e., guanidino vs. amino groups) on the membrane perturbing mechanism of antimicrobial α-peptide-β-peptoid chimeras. Langmuir monolayers composed of 1,2-dipalmitoyl-sn-glycero-3-phosphatidylglycerol (DPPG) were used to model cytoplasmic membranes of both Gram-positive and Gram-negative bacteria, while lipopolysaccharide Kdo2-lipid A monolayers were mimicking the outer membrane of Gram-negative species. We report the results of the measurements using an array of techniques, including high-resolution synchrotron surface X-ray scattering, epifluorescence microscopy, and in vitro antimicrobial activity to study the molecular mechanisms of peptidomimetic interaction with bacterial membranes. We found guanidino group-containing chimeras to exhibit greater disruptive activity on DPPG monolayers than the amino group-containing analogues. However, this effect was not observed for lipopolysaccharide monolayers where the difference was negligible. Furthermore, the addition of the nitrobenzoxadiazole fluorophore did not reduce the insertion activity of these antimicrobials into both model membrane systems examined, which may be useful for future cellular localization studies.

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The theoretic concepts and social merits are dealt with of the scarcely known Bulgarian doctors--struggling philosophers-marxists (Dr. Peter Genov, 1880-1923, and dr. Stamen Iliev, 1883-1923). Their bold and never retreating struggle against the bourgeois philosophy and ideology in the field of natural history and sociology is outlined. Both are defensors and supporters of the philosophical and methodological basis of marxism-leninsm during one of the most difficult periods in the developent of Bulgarian and international labour and communist movements.

Replication studies in longevity: puzzling findings in Danish centenarians at the 3'APOB-VNTR locus

In Danes we replicated the 3'APOB-VNTR gene/longevity association study previously carried out in Italians, by which the Small alleles (less than 35 repeats) had been identified as frailty alleles for longevity. In Danes, neither genotype nor allele frequencies differed between centenarians and 20-64-year-old subjects. However, when Danish and Italian data were compared, a significant difference (p = 0.0004) was found between the frequencies of Small alleles in youths, which disappeared in centenarians (p = 0.290). Furthermore, the demographic-genetic approach revealed in Danes a significant gene-sex interaction relevant to Long alleles (more than 37 repeats). The different findings in Denmark and Italy suggest that gene/longevity associations are population-specific, and heavily affected by the population-specific genetic and environmental history.