The Stabilities of the Soluble Ectodomain and Fusion Peptide Hairpins of the Influenza Virus Hemagglutinin Subunit II Protein Are Positively Correlated with Membrane Fusion

Influenza B Virus Receptor Specificity: Closing the Gap between Binding and Tropism

Influenza A and influenza B viruses (FLUAV and FLUBV, respectively) cause significant respiratory disease, hospitalization, and mortality each year. Despite causing at least 25% of the annual disease burden, FLUBV is historically understudied. Unlike FLUAVs, which possess pandemic potential due to their many subtypes and broad host range, FLUBVs are thought to be restricted to only humans and are limited to two lineages. The hemagglutinins (HA) of both influenza types bind glycans terminating in α2,6- or α2,3-sialic acids. For FLUAV, the tropism of human- and avian-origin viruses is well-defined and determined by the terminal sialic acid configuration the HA can accommodate, with avian-origin viruses binding α2,3-linked sialic acids and human-origin viruses binding α2,6-linked sialic acids. In contrast, less is known about FLUBV receptor binding and its impact on host tropism. This review discusses the current literature on FLUBV receptor specificity, HA glycosylation, and their roles in virus tropism, evolution, and infection. While the focus is on findings in the past dozen years, it should be noted that the most current approaches for measuring virus-glycan interactions have not yet been applied to FLUBV and knowledge gaps remain.

Lipid acyl chain protrusion induced by the influenza virus hemagglutinin fusion peptide detected by NMR paramagnetic relaxation enhancement

The glycoprotein spikes of membrane-enveloped viruses include a subunit that catalyzes fusion (joining) of the viral and target cell membranes. For influenza virus, this is subunit 2 of hemagglutinin which has a ∼ 20-residue N-terminal fusion peptide (Fp) region that binds target membrane. An outstanding question is whether there are associated membrane changes important for fusion. Several computational studies have found increased "protrusion" of lipid acyl chains near Fp, i.e. one or more chain carbons are closer to the aqueous region than the headgroup phosphorus. Protrusion may accelerate initial joining of outer leaflets of the two membranes into a stalk intermediate. In this study, higher protrusion probability in membrane with vs. without Fp is convincingly detected by larger Mn2+-associated increases in chain 13C NMR transverse relaxation rates (Γ2's). Data analysis provides a ratio Γ2,neighbor/Γ2,distant for lipids neighboring vs. more distant from the Fp. The calculated ratio depends on the number of Fp-neighboring lipids and the experimentally-derived range of 4 to 24 matches the range of increased protrusion probabilities from different simulations. For samples either with or without Fp, the Γ2 values are well-fitted by an exponential decay as the 13C site moves closer to the chain terminus. The decays correlate with free-energy of protrusion proportional to the number of protruded -CH2 groups, with free energy per -CH2 of ∼0.25 kBT. The NMR data support one major fusion role of the Fp to be much greater protrusion of lipid chains, with highest protrusion probability for chain regions closest to the headgroups.

Design, synthesis, and biological evaluation of novel penindolone derivatives as potential inhibitors of hemagglutinin-mediated membrane fusion

Development and design of anti-influenza drugs with novel mechanisms is of great significance to combat the ongoing threat of influenza A virus (IAV). Hemagglutinin (HA) is regarded as a potential target for the therapy of IAV. Our previous research led to the discovery of penindolone (PND), a new diclavatol indole adduct, as an HA targeting leading compound exhibited anti-IAV activity. To enhance the bioactivity and understand the structure-activity relationships (SARs), 65 PND derivatives were designed and synthesized, and the anti-IAV activities as well as the HA targeting effects were systematically investigated in this study. Among them, compound 5g possessed high affinity to HA and was more effective than PND in terms of inhibiting HA-mediated membrane fusion. Compound 5g may act on the trypsin cleavage site of HA to exhibit a strong inhibition on membrane fusion. In addition, oral administration of 5g can significantly reduce the pulmonary virus titer, attenuate the weight loss, and improve the survival of IAV-infected mice, superior to the effects of PND. These findings suggest that the HA inhibitor 5g has potential to be developed into a novel broad-spectrum anti-IAV agent in the future.

Advanced Strategies for Overcoming Endosomal/Lysosomal Barrier in Nanodrug Delivery

Nanocarriers have therapeutic potential to facilitate drug delivery, including biological agents, small-molecule drugs, and nucleic acids. However, their efficiency is limited by several factors; among which, endosomal/lysosomal degradation after endocytosis is the most important. This review summarizes advanced strategies for overcoming endosomal/lysosomal barriers to efficient nanodrug delivery based on the perspective of cellular uptake and intracellular transport mechanisms. These strategies include promoting endosomal/lysosomal escape, using non-endocytic methods of delivery to directly cross the cell membrane to evade endosomes/lysosomes and making a detour pathway to evade endosomes/lysosomes. On the basis of the findings of this review, we proposed several promising strategies for overcoming endosomal/lysosomal barriers through the smarter and more efficient design of nanodrug delivery systems for future clinical applications.

A large HIV gp41 construct with trimer-of-hairpins structure exhibits V2E mutation-dominant attenuation of vesicle fusion and helicity very similar to V2E attenuation of HIV fusion and infection and supports: (1) hairpin stabilization of membrane apposition with larger distance for V2E; and (2) V2E dominance by an antiparallel β sheet with interleaved fusion peptide strands from two gp41 trimers

There is complete attenuation of fusion and infection mediated by HIV gp160 with gp41 subunit with V2E mutation, and also V2E dominance with WT/V2E mixtures. V2E is at the N-terminus of the ∼25-residue fusion peptide (Fp) which likely binds the target membrane. In this study, large V2E attenuation and dominance were observed for vesicle fusion induced by FP_HM, a large gp41 ectodomain construct with Fp followed by hyperthermostable hairpin with N- and C-helices, and membrane-proximal external region (Mper). FP_HM is a trimer-of-hairpins, the final gp41 structure during fusion. Vesicle fusion and helicity were measured for FP_HM using trimers with different fractions (f's) of WT and V2E proteins. Reductions in FP_HM fusion and helicity vs. fV2E were quantitatively-similar to those for gp160-mediated fusion and infection. Global fitting of all V2E data supports 6 WT gp41 (2 trimers) required for fusion. These data are understood by a model in which the ∼25 kcal/mol free energy for initial membrane apposition is compensated by the thermostable hairpin between the Fp in target membrane and Mper/transmembrane domain in virus membrane. The data support a structural model for V2E dominance with a membrane-bound Fp with antiparallel β sheet and interleaved strands from the two trimers. Relative to fV2E = 0, a longer Fp sheet is stabilized with small fV2E because of salt-bridge and/or hydrogen bonds between E2 on one strand and C-terminal Fp residues on adjacent strands, like R22. A longer Fp sheet results in shorter N- and C-helices, and larger separation during membrane apposition which hinders fusion.

Cell-free protein synthesis of influenza virus hemagglutinin HA2-integrated virosomes for siRNA delivery

It is well known that the difficulty of siRNA therapeutic application is the lack of safe and effective delivery vector. Virosome is a nano vesicle composed of lipid membrane and membrane protein. It retains fusion protein without virus genetic material, and therefore has the reduced immunogenicity compared with viral vector. Virosomes have the potential to deliver protein and nucleic acid drugs, but the traditional preparation method of virosomes is quite limited. In this study, we firstly proposed to synthesize influenza virus hemagglutinin HA2 virosomes by cell-free protein synthesis. In this study, liposomes provided the hydrophobic lipid bilayer environment for the formation of HA2 protein multimer, which inhibited the aggregation of hydrophobic HA2 and improved HA2 protein expression. Chitosan as a rigid core adsorbed siRNA and improved the encapsulation efficiency of siRNA. In conclusion, the cell-free protein synthesis was used to prepare HA2 virosomes, which paves the way for constructing a novel nano vector with high delivery efficiency and biosafety for the delivery of siRNA.

Stability of HA2 Prefusion Structure and pH-Induced Conformational Changes in the HA2 Domain of H3N2 Hemagglutinin

Influenza hemagglutinin is the fusion protein that mediates fusion of the viral and host membranes through a large conformational change upon acidification in the developing endosome. The "spring-loaded" model has long been used to describe the mechanism of hemagglutinin and other type 1 viral glycoproteins. This model postulates a metastable conformation of the HA2 subunit, caged from adopting a lower-free energy conformation by the HA1 subunit. Here, using a combination of biochemical and spectroscopic methods, we study a truncated construct of HA2 (HA2*, lacking the transmembrane domain) recombinantly expressed in Escherichia coli as a model for HA2 without the influence of HA1. Our data show that HA2* folds into a conformation like that of HA2 in full length HA and forms trimers. Upon acidification, HA2* undergoes a conformational change that is consistent with the change from pre- to postfusion HA2 in HA. This conformational change is fast and occurs on a time scale that is not consistent with aggregation. These results suggest that the prefusion conformation of HA2 is stable and the change to the postfusion conformation is due to protonation of HA2 itself and not merely uncaging by HA1.

Rapid 2H NMR Transverse Relaxation of Perdeuterated Lipid Acyl Chains of Membrane with Bound Viral Fusion Peptide Supports Large-Amplitude Motions of These Chains That Can Catalyze Membrane Fusion

An early step in cellular infection by a membrane-enveloped virus like HIV or influenza is joining (fusion) of the viral and cell membranes. Fusion is catalyzed by a viral protein that typically includes an apolar "fusion peptide" (fp) segment that binds the target membrane prior to fusion. In this study, the effects of nonhomologous HIV and influenza fp's on lipid acyl chain motion are probed with ²H NMR transverse relaxation rates (R₂'s) of a perdeuterated DMPC membrane. Measurements were made between 35 and 0 °C, which brackets the membrane liquid-crystalline-to-gel phase transitions. Samples were made with either HIV "GPfp" at pH 7 or influenza "HAfp" at pH 5 or 7. GPfp induces vesicle fusion at pH 7, and HAfp induces more fusion at pH 5 vs 7. GPfp bound to DMPC adopts an intermolecular antiparallel β sheet structure, whereas HAfp is a monomer helical hairpin. The R₂'s of the no peptide and HAfp, pH 7, samples increase gradually as temperature is lowered. The R₂'s of GPfp and HAfp, pH 5, samples have very different temperature dependence, with a ∼10× increase in R₂^CD2 when temperature is reduced from 25 to 20 °C and smaller but still substantial R₂'s at 10 and 0 °C. The large R₂'s with GPfp and HAfp, pH 5, are consistent with large-amplitude motions of lipid acyl chains that can aid fusion catalysis by increasing the population of chains near the aqueous phase, which is the chain location for transition states between membrane fusion intermediates.

A Bimodal Nanosensor for Probing Influenza Fusion Protein Activity Using Magnetic Relaxation

Viral fusion is a critical step in the entry pathway of enveloped viruses and remains a viable target for antiviral exploration. The current approaches for studying fusion mechanisms include ensemble fusion assays, high-resolution cryo-TEM, and single-molecule fluorescence-based methods. While these methods have provided invaluable insights into the dynamic events underlying fusion processes, they come with their own limitations. These often include extensive data and image analysis in addition to experimental time and technical requirements. This work proposes the use of the spin-spin T2 relaxation technique as a sensitive bioanalytical method for the rapid quantification of interactions between viral fusion proteins and lipids in real time. In this study, new liposome-coated iron oxide nanosensors (LIONs), which mimic as magnetic-labeled host membranes, are reported to detect minute interactions occurring between the membrane and influenza's fusion glycoprotein, hemagglutinin (HA). The influenza fusion protein's interaction with the LION membrane is detected by measuring changes in the sensitive spin-spin T2 magnetic relaxation time using a bench-top NMR instrument. More data is gleaned from including the fluorescent dye DiI into the LION membrane. In addition, the effects of environmental factors on protein-lipid interaction that affect fusion such as pH, time of incubation, trypsin, and cholesterol were also examined. Furthermore, the efficacy and sensitivity of the spin-spin T2 relaxation assay in quantifying similar protein/lipid interactions with more native configurations of HA were demonstrated using virus-like particles (VLPs). Shorter domains derived from HA were used to start a reductionist path to identify the parts of HA responsible for the NMR changes observed. Finally, the known fusion inhibitor Arbidol was employed in our spin-spin T2 relaxation-based fusion assay to demonstrate the application of LIONs in real-time monitoring of this aspect of fusion for evaluation of potential fusion inhibitors.

Cardenolides and pentacyclic triterpenes isolated from Acokanthera oblongifolia leaves: their biological activities with molecular docking study

Pentacyclic triterpenes and cardenolides were isolated from Acokanthera oblongifolia leaves. Their chemical structures were determined based on comprehensive 1D and 2D NMR spectroscopy. Their MIC was determined against 12 microorganisms. Their exerted cytotoxicity on the immortalized normal cells, hTERT-RPE1 was assessed by the sulforhodamine-B assay. The viral inhibitory effects of compounds against Newcastle disease virus (NDV) and H5N1 influenza virus IV were evaluated. Four in vitro antioxidant assays were performed in comparison with BHT and trolox and a weak activity was exhibited. Acovenoside A was with potent against H5N1-IV and NDV with IC₅₀ ≤ 3.2 and ≤ 2.1 μg/ml and SI values of 93.75 and 95.23%, respectively, in comparison to ribavirin. Its CC₅₀ record on Vero cells was > 400 and 200 μg/ml, respectively. Acobioside A was the most active compound against a broad range of microbes while Pseudomonas aeruginosa was the most sensitive. Its MIC (0.07 μg/ml) was 1/100-fold of the recorded CC₅₀ (7.1 μg/ml/72 h) against hTERT-RPE1. The molecular docking of compounds on human DNA topoisomerase I (Top1-DNA) and IV glycoprotein hemagglutinin were studied using MOE program. This study has introduced the cardenolides rather than triterpenoids with the best docking score and binding interaction with the active site of the studied proteins.

2H nuclear magnetic resonance spectroscopy supports larger amplitude fast motion and interference with lipid chain ordering for membrane that contains β sheet human immunodeficiency virus gp41 fusion peptide or helical hairpin influenza virus hemagglutinin fusion peptide at fusogenic pH

Enveloped viruses are surrounded by a membrane which is obtained from an infected host cell during budding. Infection of a new cell requires joining (fusion) of the virus and cell membranes. This process is mediated by a monotopic viral fusion protein with a large ectodomain outside the virus. The ectodomains of class I enveloped viruses have a N-terminal "fusion peptide" (fp) domain that is critical for fusion and binds to the cell membrane. In this study, ²H NMR spectra are analyzed for deuterated membrane with fp from either HIV gp41 (GP) or influenza hemagglutinin (HA) fusion proteins. In addition, the HAfp samples are studied at more fusogenic pH 5 and less fusogenic pH 7. GPfp adopts intermolecular antiparallel β sheet structure whereas HAfp is a monomeric helical hairpin. The data are obtained for a set of temperatures between 35 and 0 °C using DMPC-d54 lipid with perdeuterated acyl chains. The DMPC has liquid-crystalline (L_α) phase with disordered chains at higher temperature and rippled gel (P_β') or gel phase (L_β') with ordered chains at lower temperature. At given temperature T, the no peptide and HAfp, pH 7 samples exhibit similar spectral lineshapes. Spectral broadening with reduced temperature correlates with the transition from L_α to P_β' and then L_β' phases. At given T, the lineshapes are narrower for HAfp, pH 5 vs. no peptide and HAfp, pH 7 samples, and even narrower for the GPfp sample. These data support larger-amplitude fast (>10⁵ Hz) lipid acyl chain motion for samples with fusogenic peptides, and peptide interference with chain ordering. The NMR data of the present paper correlate with insertion of these peptides into the hydrocarbon core of the membrane and support a significant fusion contribution from the resultant lipid acyl chain disorder, perhaps because of reduced barriers between the different membrane topologies in the fusion pathway. Membrane insertion and lipid perturbation appear common to both β sheet and helical hairpin peptides.

An Oleanolic Acid Derivative Inhibits Hemagglutinin-Mediated Entry of Influenza A Virus

Influenza A viruses (IAV) have been a major public health threat worldwide, and options for antiviral therapy become increasingly limited with the emergence of drug-resisting virus strains. New and effective anti-IAV drugs, especially for highly pathogenic influenza, with different modes of action, are urgently needed. The influenza virus glycoprotein hemagglutinin (HA) plays critical roles in the early stage of virus infection, including receptor binding and membrane fusion, making it a potential target for the development of anti-influenza drugs. In this study, we show that OA-10, a newly synthesized triterpene out of 11 oleanane-type derivatives, exhibited significant antiviral activity against four different subtypes of IAV (H1N1, H5N1, H9N2 and H3N2) replications in A549 cell cultures with EC₅₀ ranging from 6.7 to 19.6 μM and a negligible cytotoxicity (CC₅₀ > 640 μM). It inhibited acid-induced hemolysis in a dose-dependent manner, with an IC₅₀ of 26 µM, and had a weak inhibition on the adsorption of H5 HA to chicken erythrocytes at higher concentrations (≥40 µM). Surface plasmon resonance (SPR) analysis showed that OA-10 interacted with HA in a dose-dependent manner with the equilibrium dissociation constants (KD) of the interaction of 2.98 × 10^-12 M. Computer-aided molecular docking analysis suggested that OA-10 might bind to the cavity in HA stem region which is known to undergo significant rearrangement during membrane fusion. Our results demonstrate that OA-10 inhibits H5N1 IAV replication mainly by blocking the conformational changes of HA2 subunit required for virus fusion with endosomal membrane. These findings suggest that OA-10 could serve as a lead for further development of novel virus entry inhibitors to prevent and treat IAV infections.

Hydrogen-Deuterium Exchange Supports Independent Membrane-Interfacial Fusion Peptide and Transmembrane Domains in Subunit 2 of Influenza Virus Hemagglutinin Protein, a Structured and Aqueous-Protected Connection between the Fusion Peptide and Soluble Ectodomain, and the Importance of Membrane Apposition by the Trimer-of-Hairpins Structure

The influenza virus hemagglutinin (HA) protein has HA1 and HA2 subunits, which form an initial complex. HA1's bind host cell sialic acids, which triggers endocytosis, HA1/HA2 separation, and HA2-mediated fusion between virus and endosome membranes. We report hydrogen-deuterium exchange mass spectrometry (HDX-MS) on the HA2 subunit without HA1. HA2 contains the fusion peptide (FP), soluble ectodomain (SE), transmembrane domain (TM), and endodomain. FP is a monomer by itself, while SE is a trimer of hairpins that includes an interior bundle of residue 38-105 helices, turns, and residue 154-178 strands packed antiparallel to the bundle. FP and TM extend from the same side of the SE hairpin, and fusion models often depict a FP/TM complex with membrane traversal of both domains that is important for membrane pore expansion. The HDX-MS data of this study do not support this complex and instead support independent FP and TM with respective membrane-interfacial and traversal locations. The data also show a low level of aqueous exposure of the 22-38 segment, consistent with retention of the 23-35 antiparallel β sheet observed in the initial HA1/HA2 complex. We propose the β sheet as a semirigid connector between FP and SE that enables close membrane apposition prior to fusion. The I173E mutant exhibits greater exchange for residues 22-69 and 150-191, consistent with dissociation of SE C-terminal strands from interior N-helices. Similar trends are observed for the G1E mutant as well as less exchange for G1E FP. Fusion is highly impaired with either mutant, which correlates with reduced membrane apposition and, for G1E, FP binding to SE rather than the target membrane.