Modulating the Influenza A Virus-Target Membrane Fusion Interface With Synthetic DNA-Lipid Receptors

Single VLP lipid-mixing measurements confirm off-pathway state in dengue virus fusion mechanism

Dengue virus (DENV) is the causative agent of dengue fever and exerts a substantial healthcare burden worldwide. Like other flaviviruses, DENV must undergo membrane fusion with the host cell in order to initiate infection. This membrane fusion occurs following acidification during endocytosis and is pH dependent. Here, we interrogate whether the mechanism of DENV fusion contains an off-pathway state, such has been reported previously for two other flaviviruses - Zika virus and West Nile virus. To do this, we utilize single particle lipid mixing measurements of DENV virus-like particles (VLPs) to tethered liposomes, together with computational modeling inspired by chemical kinetics. By observing and then modeling the pH dependence of single VLP fusion kinetics, we provide evidence that the DENV fusion mechanism must contain an off-pathway state. Measuring the proportion of VLPs undergoing hemi-fusion over time, we also demonstrate that the off-pathway state appears to be slowly reversible over tens of minutes, at least for some virions. Additionally, we find that late endosomal anionic lipids do not appear to influence the off-pathway mechanism to any great extent. In conjunction with the prior reports on Zika virus and West Nile virus, this work indicates that an off-pathway fusion state may be a feature of flavivirus fusion more broadly. We also note that the platform and mechanistic model described in this study may be useful in elucidating the mechanism of action of small molecule inhibitors of flavivirus fusion developed by our group and others.

Single Particle Analysis of H3N2 Influenza Entry Differentiates the Impact of the Sialic Acids (Neu5Ac and Neu5Gc) on Virus Binding and Membrane Fusion

Entry of influenza A viruses (IAVs) into host cells is initiated by binding to sialic acids (Sias), their primary host cell receptor, followed by endocytosis and membrane fusion to release the viral genome into the cytoplasm of the host cell. Host tropism is affected by these entry processes, with a primary factor being receptor specificity. Sias exist in several different chemical forms, including the hydroxylated N-glycolylneuraminic acid (Neu5Gc), which is found in many hosts; however, it has not been clear how modified Sias affect viral binding and entry. Neu5Gc is commonly found in many natural influenza hosts, including pigs and horses, but not in humans or ferrets. Here, we engineered HEK293 cells to express the hydoxylase gene (CMAH) that converts Neu5Ac to Neu5Gc, or knocked out the Sia-CMP transport gene (SLC35A1), resulting in cells that express 95% Neu5Gc or minimal level of Sias, respectively. H3N2 (X-31) showed significantly reduced infectivity in Neu5Gc-rich cells compared to wild-type HEK293 (>95% Neu5Ac). To determine the effects on binding and fusion, we generated supported lipid bilayers (SLBs) derived from the plasma membranes of these cells and carried out single particle microscopy. H3N2 (X-31) exhibited decreased binding to Neu5Gc-containing SLBs, but no significant difference in H3N2 (X-31)'s fusion kinetics to either SLB type, suggesting that reduced receptor binding does not affect subsequent membrane fusion. This finding suggests that for this virus to adapt to host cells rich in Neu5Gc, only receptor affinity changes are required without further adaptation of virus fusion machinery. IMPORTANCE Influenza A virus (IAV) infections continue to threaten human health, causing over 300,000 deaths yearly. IAV infection is initiated by the binding of influenza glycoprotein hemagglutinin (HA) to host cell sialic acids (Sias) and the subsequent viral-host membrane fusion. Generally, human IAVs preferentially bind to the Sia N-acetylneuraminic acid (Neu5Ac). Yet, other mammalian hosts, including pigs, express diverse nonhuman Sias, including N-glycolylneuraminic acid (Neu5Gc). The role of Neu5Gc in human IAV infections in those hosts is not well-understood, and the variant form may play a role in incidents of cross-species transmission and emergence of new epidemic variants. Therefore, it is important to investigate how human IAVs interact with Neu5Ac and Neu5Gc. Here, we use membrane platforms that mimic the host cell surface to examine receptor binding and membrane fusion events of human IAV H3N2. Our findings improve the understanding of viral entry mechanisms that can affect host tropism and virus evolution.

DNA-Tethered Lipid Membrane Formation via Solvent-Assisted Self-Assembly

DNA-tethered lipid bilayers have been used in many studies, based on the controllable and well-defined properties of DNA tethers. However, their application has been limited, because it is difficult to cover a wide range of surfaces and achieve electrical insulation. We implemented an existing method, where a DNA hybrid chip on a silica or glass surface supports a lipid membrane using solvent-assisted self-assembly. The formation of a continuous lipid bilayer was confirmed through the change in quartz crystal microbalance dissipation results, depending on the presence or absence of DNA hybrids. The fluidity of the DNA-tethered lipid membranes was analyzed using a fluorescence microscope. The electrochemical analysis demonstrated the versatility of this new technique, which can be used for sensor or electrode surface modification for biosensors or bioelectronics.

Lipid Membrane Interface Viewpoint: From Viral Entry to Antiviral and Vaccine Development

Membrane-enveloped viruses are responsible for most viral pandemics in history, and more effort is needed to advance broadly applicable countermeasures to mitigate the impact of future outbreaks. In this Perspective, we discuss how biosensing techniques associated with lipid model membrane platforms are contributing to improving our mechanistic knowledge of membrane fusion and destabilization that is closely linked to viral entry as well as vaccine and antiviral drug development. A key benefit of these platforms is the simplicity of interpreting the results which can be complemented by other techniques to decipher more complicated biological observations and evaluate the biophysical functionalities that can be correlated to biological activities. Then, we introduce exciting application examples of membrane-targeting antivirals that have been refined over time and will continue to improve based on biophysical insights. Two ways to abrogate the function of viral membranes are introduced here: (1) selective disruption of the viral membrane structure and (2) alteration of the membrane component. While both methods are suitable for broadly useful antivirals, the latter also has the potential to produce an inactivated vaccine. Collectively, we emphasize how biosensing tools based on membrane interfacial science can provide valuable information that could be translated into biomedicines and improve their selectivity and performance.

Recent Developments in Single-Virus Fusion Assay

Viral infection is a global health hazard. A crucial step in the infection cycle of enveloped viruses is the fusion of viral and host cellular membranes, which permits the transfer of the viral genome to the host cells. Membrane fusion is a ubiquitous process involved in sperm-egg fusion, exocytosis, vesicular trafficking, and viral entry to host cells. While different protein machineries catalyze the diverse fusion processes, the essential step, i.e., merging of two lipid bilayers against a kinetic energy barrier, is the same. Therefore, viral fusion machineries/pathways are not only the sites for antiviral drug development but also serve as model fusogens. Ensemble-based spectroscopic approaches or bulk fusion assays have yielded valuable insights regarding the fusion processes. However, due to the stochastic nature of the fusion events, ensemble-based assays do not permit synchronization of all the fusion events, and the molecular steps leading to fusion pore opening cannot be resolved entirely and correlated with the structural changes in viral fusion proteins. Several single-virus fusion assays have been developed to circumvent these issues. The review describes the recent advancements in single-virus/particle fusion assays using the Influenza virus as a paradigm.