Cholesterol Alters the Orientation and Activity of the Influenza Virus M2 Amphipathic Helix in the Membrane

Cholesterol: A friend to viruses

Cholesterol is a key life-sustaining molecule which regulates membrane fluidity and serves as a signaling mediator. Cholesterol homeostasis is closely related to various pathological conditions including tumor, obesity, atherosclerosis, Alzheimer's disease and viral infection. Viral infection disrupts host cholesterol homeostasis, facilitating their own survival. Meanwhile, the host cells strive to reduce cholesterol accessibility to limit viral infection. This review focuses on the regulation of cholesterol metabolism and the role of cholesterol in viral infection, specifically providing an overview of cholesterol as a friend to promote viral entry, replication, assembly, release and immune evasion, which might inspire valuable thinking for pathogenesis and intervention of viral infection.

The Influenza A Virus Replication Cycle: A Comprehensive Review

Influenza A virus (IAV) is the primary causative agent of influenza, colloquially called the flu. Each year, it infects up to a billion people, resulting in hundreds of thousands of human deaths, and causes devastating avian outbreaks with worldwide losses worth billions of dollars. Always present is the possibility that a highly pathogenic novel subtype capable of direct human-to-human transmission will spill over into humans, causing a pandemic as devastating if not more so than the 1918 influenza pandemic. While antiviral drugs for influenza do exist, they target very few aspects of IAV replication and risk becoming obsolete due to antiviral resistance. Antivirals targeting other areas of IAV replication are needed to overcome this resistance and combat the yearly epidemics, which exact a serious toll worldwide. This review aims to summarise the key steps in the IAV replication cycle, along with highlighting areas of research that need more focus.

The Functionality of Membrane-Inserting Proteins and Peptides: Curvature Sensing, Generation, and Pore Formation

Proteins and peptides with hydrophobic and amphiphilic segments are responsible for many biological functions. The sensing and generation of membrane curvature are the functions of several protein domains or motifs. While some specific membrane proteins play an essential role in controlling the curvature of distinct intracellular membranes, others participate in various cellular processes such as clathrin-mediated endocytosis, where several proteins sort themselves at the neck of the membrane bud. A few membrane-inserting proteins form nanopores that permeate selective ions and water to cross the membrane. In addition, many natural and synthetic small peptides and protein toxins disrupt the membrane by inducing nonspecific pores in the membrane. The pore formation causes cell death through the uncontrolled exchange between interior and exterior cellular contents. In this article, we discuss the insertion depth and orientation of protein/peptide helices, and their role as a sensor and inducer of membrane curvature as well as a pore former in the membrane. We anticipate that this extensive review will assist biophysicists to gain insight into curvature sensing, generation, and pore formation by membrane insertion.

Influenza A Virus Infection Alters Lipid Packing and Surface Electrostatic Potential of the Host Plasma Membrane

The pathogenesis of influenza A viruses (IAVs) is influenced by several factors, including IAV strain origin and reassortment, tissue tropism and host type. While such factors were mostly investigated in the context of virus entry, fusion and replication, little is known about the viral-induced changes to the host lipid membranes which might be relevant in the context of virion assembly. In this work, we applied several biophysical fluorescence microscope techniques (i.e., Förster energy resonance transfer, generalized polarization imaging and scanning fluorescence correlation spectroscopy) to quantify the effect of infection by two IAV strains of different origin on the plasma membrane (PM) of avian and human cell lines. We found that IAV infection affects the membrane charge of the inner leaflet of the PM. Moreover, we showed that IAV infection impacts lipid-lipid interactions by decreasing membrane fluidity and increasing lipid packing. Because of such alterations, diffusive dynamics of membrane-associated proteins are hindered. Taken together, our results indicate that the infection of avian and human cell lines with IAV strains of different origins had similar effects on the biophysical properties of the PM.

Propranolol induces large-scale remodeling of lipid bilayers: tubules, patches, and holes

Herein, we report fluorescence microscopy analysis of the interaction between propranolol (PPN), a beta-adrenergic blocking agent, and planar supported lipid bilayers (SLBs), as model membranes. The results indicate that PPN can remarkably promote largescale remodeling in SLBs with various lipid compositions. It was found that PPN insertion induces the formation of long microtubules that can retract into hemispherical caps on the surface of the bilayer. These transformations are dynamic, partially reversible, and dependent upon the drug concentration. Quantitative analysis revealed a three-step model for PPN-lipid bilayer interaction, with the first step involving interfacial electrostatic adsorption, the second step centered on hydrophobic insertion, and the third step associated with membrane disruption and hole formation. By introducing cholesterol, phosphoethanolamine, phosphatidylglycerol, and phosphatidylserine lipids into the phosphocholine SLBs, it was illustrated that both the chemistry of the lipid headgroups and the packing of lipid acyl chains can substantially affect the particular steps in the interactions between PPN and lipid bilayers. Our findings may help to elucidate the possible mechanisms of PPN interaction with lipid membranes, the toxic behavior and overdosage scenarios of beta-blockers, and provide valuable information for drug development and modification.

Molecular origins of asymmetric proton conduction in the influenza M2 channel

The M2 proton channel of influenza A is embedded into the viral envelope and allows acidification of the virion when the external pH is lowered. In contrast, no outward proton conductance is observed when the internal pH is lowered, although outward current is observed at positive voltage. Residues Trp41 and Asp44 are known to play a role in preventing pH-driven outward conductance, but the mechanism for this is unclear. We investigate this issue using classical molecular dynamics simulations with periodic proton hops. When all key His37 residues are neutral, inward proton movement is much more facile than outward movement if the His are allowed to shuttle the proton. The preference for inward movement increases further as the charge on the His37 increases. Analysis of the trajectories reveals three factors accounting for this asymmetry. First, in the outward direction, Asp44 traps the hydronium by strong electrostatic interactions. Secondly, Asp44 and Trp41 orient the hydronium with the protons pointing inward, hampering outward Grotthus hopping. As a result, the effective barrier is lower in the inward direction. Trp41 adds to the barrier by weakly H-bonding to potential H+ acceptors. Finally, for charged His, the H3O+ in the inner vestibule tends to get trapped at lipid-lined fenestrations of the cone-shaped channel. Simulations qualitatively reproduce the experimentally observed higher outward conductance of mutants. The ability of positive voltage, unlike proton gradient, to induce an outward current appears to arise from its ability to bias H3O+ and the waters around it toward more H-outward orientations.

Cholesterol and M2 Rendezvous in Budding and Scission of Influenza A Virus

The cholesterol of the host cell plasma membrane and viral M2 protein plays a crucial role in multiple stages of infection and replication of the influenza A virus. Cholesterol is required for the formation of heterogeneous membrane microdomains (or rafts) in the budozone of the host cell that serves as assembly sites for the viral components. The raft microstructures act as scaffolds for several proteins. Cholesterol may further contribute to the mechanical forces necessary for membrane scission in the last stage of budding and help to maintain the stability of the virus envelope. The M2 protein has been shown to cause membrane scission in model systems by promoting the formation of curved lipid bilayer structures that, in turn, can lead to membrane vesicles budding off or scission intermediates. Membrane remodeling by M2 is intimately linked with cholesterol as it affects local lipid composition, fluidity, and stability of the membrane. Thus, both cholesterol and M2 protein contribute to the efficient and proper release of newly formed influenza viruses from the virus-infected cells.

Cholesterol Binds the Amphipathic Helix of IFITM3 and Regulates Antiviral Activity

The interferon-induced transmembrane (IFITM) proteins broadly inhibit the entry of diverse pathogenic viruses, including Influenza A virus (IAV), Zika virus, HIV-1, and SARS coronaviruses by inhibiting virus-cell membrane fusion. IFITM3 was previously shown to disrupt cholesterol trafficking, but the functional relationship between IFITM3 and cholesterol remains unclear. We previously showed that inhibition of IAV entry by IFITM3 is associated with its ability to promote cellular membrane rigidity, and these activities are functionally linked by a shared requirement for the amphipathic helix (AH) found in the intramembrane domain (IMD) of IFITM3. Furthermore, it has been shown that the AH of IFITM3 alters lipid membranes in vitro in a cholesterol-dependent manner. Therefore, we aimed to elucidate the relationship between IFITM3 and cholesterol in more detail. Using a fluorescence-based in vitro binding assay, we found that a peptide derived from the AH of IFITM3 directly interacted with the cholesterol analog, NBD-cholesterol, while other regions of the IFITM3 IMD did not, and native cholesterol competed with this interaction. In addition, recombinant full-length IFITM3 protein also exhibited NBD-cholesterol binding activity. Importantly, previously characterized mutations within the AH of IFITM3 that strongly inhibit antiviral function (F63Q and F67Q) disrupted AH structure in solution, inhibited cholesterol binding in vitro, and restricted bilayer insertion in silico. Our data suggest that direct interactions with cholesterol may contribute to the inhibition of membrane fusion pore formation by IFITM3. These findings may facilitate the design of therapeutic peptides for use in broad-spectrum antiviral therapy.

Understanding the conformational changes in the influenza B M2 ion channel at various protonation states

The characterization of influenza (A/B M2) ion channels is very important as they are potential binding sites for the drugs. We report the all-atom molecular dynamics study of the influenza B M2 ion channel in the presence of explicit solvent and lipid bilayers using the high resolution solid-state NMR structures. The importance of the various protonation states of histidine in the activation of the ion channel is discussed. The conformational changes at the closed and the open structures clearly show that the increase in tilt angle is necessary for the activation of the ion channel. Additionally, the free energy surfaces of the eight systems show the importance of the protonation state of the histidine residues in the activation of the influenza B M2 ion channel. The protonation of the histidine residues increases the tilt angle and the intra-helix distance which is evident from the superimposition of the structures corresponding to the maxima and the minima in the free energy landscape. The findings imply differences in the singly protonated and double protonated conformational states of BM2 ion channel and provide insights to help further studies of these ion channels as the drug targets for the influenza virus.

YWHAG inhibits influenza a virus replication by suppressing the release of viral M2 protein

Influenza A virus (IAV) poses a serious threat to human life and property. The IAV matrix protein 2 (M2) is significant in viral budding. Increasing studies have proven the important roles of host factors in IAV replication. In this study, immunoprecipitation combined with mass spectrometry revealed that the host protein tyrosine 3-monooxygenase/tryptophan 5-monooxygenase activation protein gamma (YWHAG), which belongs to the 14-3-3 protein scaffold family, interacts with M2. Their interactions were further confirmed by co-immunoprecipitation (Co-IP), immunofluorescence, and confocal microscopy of virus-infected HeLa cells. Moreover, we constructed YWHAG-KO and YWHAG-overexpressing cells and found that YWHAG knockout significantly increased viral production, whereas its overexpression reduced the titer of virus progeny. Therefore, YWHAG is a negative regulatory factor during IAV infection. Further, YWHAG knockout or overexpression had no effect on the binding, entry, or viral RNA replication in the early stages of the virus life cycle. On the contrary, it impaired the release of virions at the plasma membrane as determined using transmission electron microscopy and suppressed the M2-mediated budding of the influenza virus. Importantly, the H158F mutation of YWHAG was found to affect interaction with M2 and its budding. Collectively, our work demonstrates that YWHAG is a novel cellular regulator that targets and mediates the interaction and release of M2.

Clustering of tetrameric influenza M2 peptides in lipid bilayers investigated by 19F solid-state NMR

The influenza M2 protein forms a drug-targeted tetrameric proton channel to mediate virus uncoating, and carries out membrane scission to enable virus release. While the proton channel function of M2 has been extensively studied, the mechanism by which M2 catalyzes membrane scission is still not well understood. Previous fluorescence and electron microscopy studies indicated that M2 tetramers concentrate at the neck of the budding virus in the host plasma membrane. However, molecular evidence for this clustering is scarce. Here, we use ¹⁹F solid-state NMR to investigate M2 clustering in phospholipid bilayers. By mixing equimolar amounts of 4F-Phe47 labeled M2 peptide and CF₃-Phe47 labeled M2 peptide and measuring F-CF₃ cross peaks in 2D ¹⁹F¹⁹F correlation spectra, we show that M2 tetramers form nanometer-scale clusters in lipid bilayers. This clustering is stronger in cholesterol-containing membranes and phosphatidylethanolamine (PE) membranes than in cholesterol-free phosphatidylcholine and phosphatidylglycerol membranes. The observed correlation peaks indicate that Phe47 sidechains from different tetramers are less than ~2 nm apart. ¹H¹⁹F correlation peaks between lipid chain protons and fluorinated Phe47 indicate that Phe47 is more deeply inserted into the lipid bilayer in the presence of cholesterol than in its absence, suggesting that Phe47 preferentially interacts with cholesterol. Static ³¹P NMR spectra indicate that M2 induces negative Gaussian curvature in the PE membrane. These results suggest that M2 tetramers cluster at cholesterol- and PE-rich regions of cell membranes to cause membrane curvature, which in turn can facilitate membrane scission in the last step of virus budding and release.

Dietary Cholesterol Causes Inflammatory Imbalance and Exacerbates Morbidity in Mice Infected with Influenza A Virus

Influenza is a common cause of pneumonia-induced hospitalization and death, but how host factors function to influence disease susceptibility or severity has not been fully elucidated. Cellular cholesterol levels may affect the pathogenesis of influenza infection, as cholesterol is crucial for viral entry and replication, as well as immune cell proliferation and function. However, there is still conflicting evidence on the extent to which dietary cholesterol influences cholesterol metabolism. In this study, we examined the effects of a high-cholesterol diet in modulating the immune response to influenza A virus (IAV) infection in mice. Mice were fed a standard or a high-cholesterol diet for 5 wk before inoculation with mouse-adapted human IAV (Puerto Rico/8/1934), and tissues were collected at days 0, 4, 8, and 16 postinfection. Cholesterol-fed mice exhibited dyslipidemia characterized by increased levels of total serum cholesterol prior to infection and decreased triglycerides postinfection. Cholesterol-fed mice also displayed increased morbidity compared with control-fed mice, which was neither a result of immunosuppression nor changes in viral load. Instead, transcriptomic analysis of the lungs revealed that dietary cholesterol caused upregulation of genes involved in viral-response pathways and leukocyte trafficking, which coincided with increased numbers of cytokine-producing CD4⁺ and CD8⁺ T cells and infiltrating dendritic cells. Morbidity as determined by percent weight loss was highly correlated with numbers of cytokine-producing CD4⁺ and CD8⁺ T cells as well as granulocytes. Taken together, dietary cholesterol promoted IAV morbidity via exaggerated cellular immune responses that were independent of viral load.

Deciphering the Assembly of Enveloped Viruses Using Model Lipid Membranes

The cell plasma membrane is mainly composed of phospholipids, cholesterol and embedded proteins, presenting a complex interface with the environment. It maintains a barrier to control matter fluxes between the cell cytosol and its outer environment. Enveloped viruses are also surrounded by a lipidic membrane derived from the host-cell membrane and acquired while exiting the host cell during the assembly and budding steps of their viral cycle. Thus, model membranes composed of selected lipid mixtures mimicking plasma membrane properties are the tools of choice and were used to decipher the first step in the assembly of enveloped viruses. Amongst these viruses, we choose to report the three most frequently studied viruses responsible for lethal human diseases, i.e., Human Immunodeficiency Type 1 (HIV-1), Influenza A Virus (IAV) and Ebola Virus (EBOV), which assemble at the host-cell plasma membrane. Here, we review how model membranes such as Langmuir monolayers, bicelles, large and small unilamellar vesicles (LUVs and SUVs), supported lipid bilayers (SLBs), tethered-bilayer lipid membranes (tBLM) and giant unilamellar vesicles (GUVs) contribute to the understanding of viral assembly mechanisms and dynamics using biophysical approaches.

Hydrophobic Residues at the Intracellular Domain of the M2 Protein Play an Important Role in Budding and Membrane Integrity of Influenza Virus

M2 plays a crucial role in the influenza virus life cycle. However, the function of the C-terminal intracellular domain of M2 protein remains largely unclear.

Measuring Membrane Penetration Depths and Conformational Changes in Membrane Peptides and Proteins

The structural organization and dynamic nature of the biomembrane components are important determinants for numerous cellular functions. Particularly, membrane proteins are critically important for various physiological functions and are important drug targets. The mechanistic insights on the complex functionality of membrane lipids and proteins can be elucidated by understanding the interplay between structure and dynamics. In this regard, membrane penetration depth represents an important parameter to obtain the precise depth of membrane-embedded molecules that often define the conformation and topology of membrane probes and proteins. In this review, we discuss about the widely used fluorescence quenching-based methods (parallax method, distribution analysis, and dual-quencher analysis) to accurately determine the membrane penetration depths of fluorescent probes that are either membrane-embedded or attached to lipids and proteins. Further, we also discuss a relatively novel fluorescence quenching method that utilizes tryptophan residue as the quencher, namely the tryptophan-induced quenching, which is sensitive to monitor small-scale conformational changes (short distances of < 15 Å) and useful in mapping distances in proteins. We have provided numerous examples for the benefit of readers to appreciate the importance and applicability of these simple yet powerful methods to study membrane proteins.

Endosomal Cholesterol in Viral Infections - A Common Denominator?

Cholesterol has gained tremendous attention as an essential lipid in the life cycle of virtually all viruses. These seem to have developed manifold strategies to modulate the cholesterol metabolism to the side of lipid uptake and de novo synthesis. In turn, affecting the cholesterol homeostasis has emerged as novel broad-spectrum antiviral strategy. On the other hand, the innate immune system is similarly regulated by the lipid and stimulated by its derivatives. This certainly requires attention in the design of antiviral strategies aiming to decrease cellular cholesterol, as evidence accumulates that withdrawal of cholesterol hampers innate immunity. Secondly, there are exceptions to the rule of the abovementioned virus-induced metabolic shift toward cholesterol anabolism. It therefore is of interest to dissect underlying regulatory mechanisms, which we aimed for in this minireview. We further collected evidence for intracellular cholesterol concentrations being less important in viral life cycles as compared to the spatial distribution of the lipid. Various routes of cholesterol trafficking were found to be hijacked in viral infections with respect to organelle-endosome contact sites mediating cholesterol shuttling. Thus, re-distribution of cellular cholesterol in the context of viral infections requires more attention in ongoing research. As a final aim, a pan-antiviral treatment could be found just within the transport and re-adjustment of local cholesterol concentrations. Thus, we aimed to emphasize the importance of the regulatory roles the endosomal system fulfils herein and hope to stimulate research in this field.