The Alphavirus E2 Membrane-Proximal Domain Impacts Capsid Interaction and Glycoprotein Lattice Formation

Inhibitors of the Structural and Nonstructural Proteins of Alphaviruses

The Alphavirus genus includes viruses that cause encephalitis due to neuroinvasion and viruses that cause arthritis due to acute and chronic inflammation. There is no approved therapeutic for alphavirus infections, but significant efforts are ongoing, more so in recent years, to develop vaccines and therapeutics for alphavirus infections. This review article highlights some of the major advances made so far to identify small molecules that can selectively target the structural and the nonstructural proteins in alphaviruses with the expectation that persistent investigation of an increasingly expanding chemical space through a variety of structure-based design and high-throughput screening strategies will yield candidate drugs for clinical studies. While most of the works discussed are still in the early discovery to lead optimization stages, promising avenues remain for drug development against this family of viruses.

Structural constraints link differences in neutralization potency of human anti-Eastern equine encephalitis virus monoclonal antibodies

Selection and development of monoclonal antibody (mAb) therapeutics against pathogenic viruses depends on certain functional characteristics. Neutralization potency, or the half-maximal inhibitory concentration (IC50) values, is an important characteristic of candidate therapeutic antibodies. Structural insights into the bases of neutralization potency differences between antiviral neutralizing mAbs are lacking. In this report, we present cryo-electron microscopy (EM) reconstructions of three anti-Eastern equine encephalitis virus (EEEV) neutralizing human mAbs targeting overlapping epitopes on the E2 protein, with greater than 20-fold differences in their respective IC50 values. From our structural and biophysical analyses, we identify several constraints that contribute to the observed differences in the neutralization potencies. Cryo-EM reconstructions of EEEV in complex with these Fab fragments reveal structural constraints that dictate intravirion or intervirion cross-linking of glycoprotein spikes by their IgG counterparts as a mechanism of neutralization. Additionally, we describe critical features for the recognition of EEEV by these mAbs including the epitope-paratope interaction surface, occupancy, and kinetic differences in on-rate for binding to the E2 protein. Each constraint contributes to the extent of EEEV inhibition for blockade of virus entry, fusion, and/or egress. These findings provide structural and biophysical insights into the differences in mechanism and neutralization potencies of these antibodies, which help inform rational design principles for candidate vaccines and therapeutic antibodies for all icosahedral viruses.

Emerging chikungunya virus variants at the E1-E1 inter-glycoprotein spike interface impact virus attachment and Inflammation

Chikungunya virus (CHIKV) is a re-emerging arthropod-borne alphavirus and a serious threat to human health. Therefore, efforts toward elucidating how this virus causes disease and the molecular mechanisms underlying steps of the viral replication cycle are crucial. Using an in vivo transmission system that allows intra-host evolution, we identified an emerging CHIKV variant carrying a mutation in the E1 glycoprotein (V156A) in the serum of mice and saliva of mosquitoes. E1 V156A has since emerged in humans during an outbreak in Brazil, co-occurring with a second mutation, E1 K211T, suggesting an important role for these residues in CHIKV biology. Given the emergence of these variants, we hypothesized that they function to promote CHIKV infectivity and subsequent disease. Here, we show that E1 V156A and E1 K211T modulate virus attachment and fusion and impact binding to heparin, a homolog of heparan sulfate, a key entry factor on host cells. These variants also exhibit differential neutralization by anti-glycoprotein monoclonal antibodies, suggesting structural impacts on the particle that may be responsible for altered interactions at the host membrane. Finally, E1 V156A and E1 K211T exhibit increased titers in an adult arthritic mouse model and induce increased foot-swelling at the site of injection. Taken together, this work has revealed new roles for E1 where discrete regions of the glycoprotein are able to modulate cell attachment and swelling within the host. IMPORTANCE Alphaviruses represent a growing threat to human health worldwide. The re-emerging alphavirus chikungunya virus (CHIKV) has rapidly spread to new geographic regions in the last several decades, causing overwhelming outbreaks of disease, yet there are no approved vaccines or therapeutics. The CHIKV glycoproteins are key determinants of CHIKV adaptation and virulence. In this study, we identify and characterize the emerging E1 glycoprotein variants, V156A and K211T, that have since emerged in nature. We demonstrate that E1 V156A and K211T function in virus attachment to cells, a role that until now has been only attributed to specific residues of the CHIKV E2 glycoprotein. We also demonstrate E1 V156A and K211T to increase foot-swelling of the ipsilateral foot in mice infected with these variants. Observing that these variants and other pathogenic variants occur at the E1-E1 inter-spike interface, we highlight this structurally important region as critical for multiple steps during CHIKV infection. Together, these studies further defines the function of E1 in CHIKV infection and can inform the development of therapeutic or preventative strategies.

A Naturally Occurring Polymorphism in the Base of Sudan Virus Glycoprotein Decreases Glycoprotein Stability in a Species-Dependent Manner

Sudan virus (SUDV) is one of five filoviruses that compose the genus Ebolavirus that has been responsible for episodic outbreaks in Central Africa. While the SUDV glycoprotein (GP) structure has been solved, GP residues that affect SUDV entry have not been extensively examined; many of the entry characteristics of SUDV GP are inferred from studies with the Zaire Ebola virus (EBOV) GP. Here, we investigate the effect on virus entry of a naturally occurring polymorphism in SUDV GP. Two of the earliest SUDV isolates contain glutamine at residue 95 (Q95) within the base region of GP1, whereas more recent SUDV isolates and GPs from all other ebolaviruses carry lysine at this position (K95). A K95Q change dramatically decreased titers of pseudovirions bearing SUDV GP, whereas the K95Q substitution in EBOV GP had no effect on titer. We evaluated virus entry to identify SUDV GP Q95-specific entry defects. The presence of Q95 in either EBOV or SUDV GP resulted in enhanced sensitivity of GP to proteolytic processing, yet this could not account for the SUDV-specific decrease in GP Q95 infectivity. We found that SUDV GP Q95 pseudovirions were more sensitive to imipramine, a GP-destabilizing antiviral. In contrast, SUDV GP K95 was more stable, requiring elevated temperatures to inhibit virus infection. Thus, the residue present at GP 95 has a critical role in stabilizing the SUDV glycoprotein, whereas this polymorphism has no effect on EBOV GP stability. These results provide novel insights into filovirus species-specific GP structure that affects virus infectivity. IMPORTANCE Filovirus outbreaks are associated with significant morbidity and mortality. Understanding the structural constraints of filoviral GPs that control virus entry into cells is critical for rational development of novel antivirals to block infection. Here, we identify a naturally occurring glutamine (Q) to lysine (K) polymorphism at residue 95 as a critical determinant of Sudan virus GP stability but not Zaire Ebola virus GP stability. We propose that glutamine at residue 95 in Sudan virus GP mediates decreased virus entry, thereby reducing infectivity. Our findings highlight a unique structural characteristic of Sudan virus GP that affects GP-mediated functionality. Further, it provides a cautionary note for the development of future broad-spectrum filovirus antivirals.

Therapeutic alphavirus cross-reactive E1 human antibodies inhibit viral egress

Alphaviruses cause severe arthritogenic or encephalitic disease. The E1 structural glycoprotein is highly conserved in these viruses and mediates viral fusion with host cells. However, the role of antibody responses to the E1 protein in immunity is poorly understood. We isolated E1-specific human monoclonal antibodies (mAbs) with diverse patterns of recognition for alphaviruses (ranging from Eastern equine encephalitis virus [EEEV]-specific to alphavirus cross-reactive) from survivors of natural EEEV infection. Antibody binding patterns and epitope mapping experiments identified differences in E1 reactivity based on exposure of epitopes on the glycoprotein through pH-dependent mechanisms or presentation on the cell surface prior to virus egress. Therapeutic efficacy in vivo of these mAbs corresponded with potency of virus egress inhibition in vitro and did not require Fc-mediated effector functions for treatment against subcutaneous EEEV challenge. These studies reveal the molecular basis for broad and protective antibody responses to alphavirus E1 proteins.

Identification and evaluation of antiviral potential of thymoquinone, a natural compound targeting Chikungunya virus capsid protein

Capsid protein (CP) of Chikungunya virus (CHIKV) is a multifunctional protein with a conserved hydrophobic pocket that plays a crucial role in the capsid assembly and virus budding process. This study demonstrates antiviral activity of thymoquinone (TQ), a natural compound targeting the hydrophobic pocket of CP. The binding of TQ to the hydrophobic pocket of CHIKV CP was analysed by structure-based molecular docking, isothermal titration calorimetry and fluorescence spectroscopy. The binding constant K_D obtained for TQ was 27 μM. Additionally, cell-based antiviral studies showed that TQ diminished CHIKV replication with an EC50 value 4.478 μM. Reduction in viral RNA copy number and viral replication as assessed by the qRT-PCR and immunofluorescence assay, confirmed the antiviral potential of TQ. Our study reveals that TQ is an effective antiviral targeting the hydrophobic pocket of CHIKV CP and may serve as the basis for development of a broad-spectrum therapy against alphaviral diseases.

A molecular understanding of alphavirus entry

Alphaviruses cause severe human illnesses including persistent arthritis and fatal encephalitis. As alphavirus entry into target cells is the first step in infection, intensive research efforts have focused on elucidating aspects of this pathway, including attachment, internalization, and fusion. Herein, we review recent developments in the molecular understanding of alphavirus entry both in vitro and in vivo and how these advances might enable the design of therapeutics targeting this critical step in the alphavirus life cycle.