ph-dependent hydrophobicity profile of hemagglutinin of influenza virus and its possible relevance in virus fusion

Escaping the endosome: assessing cellular trafficking mechanisms of non-viral vehicles

Non-viral vehicles hold therapeutic promise in advancing the delivery of a variety of cargos in vitro and in vivo, including small molecule drugs, biologics, and especially nucleic acids. However, their efficacy at the cellular level is limited by several delivery barriers, with endolysosomal degradation being most significant. The entrapment of vehicles and their cargo in the acidified endosome prevents access to the cytosol, nucleus, and other subcellular compartments. Understanding the factors that contribute to uptake and intracellular trafficking, especially endosomal entrapment and release, is key to overcoming delivery obstacles within cells. In this review, we summarize and compare experimental techniques for assessing the extent of endosomal escape of a variety of non-viral vehicles and describe proposed escape mechanisms for different classes of lipid-, polymer-, and peptide-based delivery agents. Based on this evaluation, we present forward-looking strategies utilizing information gained from mechanistic studies to inform the rational design of efficient delivery vehicles.

Development of a Stable Liquid Formulation for Live Attenuated Influenza Vaccine

Live attenuated influenza vaccine (LAIV) is considered one of the most effective vaccines and can be manufactured quickly and inexpensively to counter seasonal or pandemic influenza. Lyophilization is widely used in vaccine production. However, it requires a longer production cycle and large-scale equipment, thus posing a considerable financial burden for developing countries. A potential solution is the development of liquid LAIV, which can increase the yield and reduce the cost of production. In this study, influential factors of LAIV, such as potential stabilizing excipients and pH, were optimized by an orthogonal design. We found that pH is the most critical factor for the stability of LAIV; salt concentration and initial virus titer are also important for LAIV stability. With these data, we developed a liquid formulation consisting of 2.5% sucrose, 0.1% monosodium glutamate, 1% arginine, and 0.5% human serum albumin, with pH ranging from 6.2 to 6.9 (optimum pH 6.5-6.7), for optimal production of monovalent or trivalent LAIVs. This liquid formulation has the potential to considerably improve vaccine production capacity to compensate for the immense shortfall in influenza vaccines globally.

Endosomal Escape and Cytosolic Penetration of Macromolecules Mediated by Synthetic Delivery Agents

Cell delivery reagents often exploit the endocytic pathway as a route of cell entry. Once endocytosed, these reagents must overcome endosomal entrapment to ensure the release of their macromolecular cargo into the cytosol of cells. In this review, we describe several examples of prototypical synthetic reagents that are capable of endosomal escape and examine their mechanisms of action, their efficiencies, and their effects on cells. Although these delivery systems are chemically distinct, some commonalities in how they interact with cellular membranes can be inferred. This, in turn, sheds some light on the process of endosomal escape, and may help guide the development and optimization of next-generation delivery tools.

Improving the endosomal escape of cell-penetrating peptides and their cargos: strategies and challenges

Cell penetrating peptides (CPPs) can deliver cell-impermeable therapeutic cargos into cells. In particular, CPP-cargo conjugates tend to accumulate inside cells by endocytosis. However, they often remain trapped inside endocytic organelles and fail to reach the cytosolic space of cells efficiently. In this review, the evidence for CPP-mediated endosomal escape is discussed. In addition, several strategies that have been utilized to enhance the endosomal escape of CPP-cargos are described. The recent development of branched systems that display multiple copies of a CPP is presented. The use of viral or synthetic peptides that can disrupt the endosomal membrane upon activation by the low pH of endosomes is also discussed. Finally, we survey how CPPs labeled with chromophores can be used in combination with light to stimulate endosomal lysis. The mechanisms and challenges associated with these intracellular delivery methodologies are discussed.

Modification of the cytoplasmic domain of influenza virus hemagglutinin affects enlargement of the fusion pore

The fusion activity of chimeras of influenza virus hemagglutinin (HA) (from A/fpv/Rostock/34; subtype H7) with the transmembrane domain (TM) and/or cytoplasmic tail (CT) either from the nonviral, nonfusogenic T-cell surface protein CD4 or from the fusogenic Sendai virus F-protein was studied. Wild-type or chimeric HA was expressed in CV-1 cells by the transient T7-RNA-polymerase vaccinia virus expression system. Subsequently, the fusion activity of the expression products was monitored with red blood cells or ghosts as target cells. To assess the different steps of fusion, target cells were labeled with the fluorescent membrane label octadecyl rhodamine B-chloride (R18) (membrane fusion) and with the cytoplasmic fluorophores calcein (molecular weight [MW], 623; formation of small aqueous fusion pore) and tetramethylrhodamine-dextran (MW, 10,000; enlargement of fusion pore). All chimeric HA/F-proteins, as well as the chimera with the TM of CD4 and the CT of HA, were able to mediate the different steps of fusion very similarly to wild-type HA. Quite differently, chimeric proteins with the CT of CD4 were strongly impaired in mediating pore enlargement. However, membrane fusion and formation of small pores were similar to those of wild-type HA, indicating that the conformational change of the ectodomain and earlier fusion steps were not inhibited. Various properties of the CT which may affect pore enlargement are considered. We surmise that the hydrophobicity of the sequence adjacent to the transmembrane domain is important for pore dilation.

Structure and topology of the influenza virus fusion peptide in lipid bilayers

The secondary structure of a 20-amino acid length synthetic peptide corresponding to the N terminus of the second subunit of hemagglutinin (HA2) of influenza virus A/PR8/34 and its interaction with phospholipid bilayers are investigated using ESR, Fourier transform infrared (FTIR), and CD spectroscopy. N-terminal spin labeling of the peptide did not affect the secondary structure of the peptide either in solution or when bound to liposomes as revealed by FTIR and CD spectroscopy. ESR spectra show that the mobility of the labeled peptide is dramatically restricted in the presence of phosphatidylcholine liposomes, suggesting a strong binding to the lipid membranes. The N terminus of the peptide penetrates into the membrane and is located within the hydrophobic core. We find an oblique insertion of the peptide into the lipid bilayer with an angle of about 45 degrees between helix axis and membrane plane using FTIR spectroscopy. No gross changes of the peptide's orientation, motion, and secondary structure were observed between pH 7.4 and pH 5.0. A model of the insertion of the fusion sequence of HA2 into a lipid bilayer is presented taking into account recent investigations on the low pH conformation of HA2 (Bullough, P. A., Hughson, F. M., Skehel, J. J., and Wiley, D. C. (1994) Nature 371, 37-43).

Target cell membrane sialic acid modulates both binding and fusion activity of influenza virus

Influenza virus binds to cell surface sialic acid receptors, and following endocytosis fuses with the endosome membrane at low pH. Whether sialic acid plays a role in the virus-cell membrane fusion step is not known. We investigated the effect of the removal of cell membrane sialic acid on the fusion activity of influenza virus (A/PR/8/34 strain) toward human T lymphocytic leukemia (CEM) cells at low pH. Fusion was monitored by fluorescence dequenching of octadecylrhodamine incorporated in the virus membrane. Removal of sialic acid by neuraminidase resulted in a drastic reduction in both viral binding and fusion. The association of the virus with neuraminidase-treated cells was enhanced at pH 5, compared to that at neutral pH, probably due to the unfolding of the hemagglutinin and the resulting increase in viral surface hydrophobicity, but the fusion capacity of the virus was reduced significantly. The results were analysed with a mass-action kinetic model which could explain and predict the kinetics of fusion. Our results indicate that binding of influenza virus to sialic acid residues on the cell surface leads to rapid and extensive fusion and partially inhibits the low pH-induced viral inactivation.

pH-dependent binding of the fluorophore bis-ANS to influenza virus reflects the conformational change of hemagglutinin

Binding of the fluorophore 1,1'-bis(4-anilino)naphthalene-5,5'-disulfonic acid (bis-ANS) to influenza virus A/PR 8/34 is strongly enhanced at low pH. Binding is accompanied by a significant increase in fluorescence intensity. The binding and the fluorescence increase are associated with the low-pH induced conformational change of the viral spike protein, hemagglutinin, exposing hydrophobic binding sites. The data indicate that in addition to the hydrophobic N-terminus of HA2 other hydrophobic sequences of the HA ectodomain become accessible to bis-ANS at low pH. It is shown that the time course of the fluorescence increase of bis-ANS at low pH is determined by the conformational change of HA. The application of this assay for continuously monitoring the kinetics of the structural alteration in HA is discussed and its relevance for elucidating the temporal relationship between the conformational change of HA and virus-membrane fusion is outlined.