Reconstitution of human atlastin fusion activity reveals autoinhibition by the C terminus

ER network formation depends on membrane fusion by the atlastin (ATL) GTPase. In humans, three paralogs are differentially expressed with divergent N- and C-terminal extensions, but their respective roles remain unknown. This is partly because, unlike Drosophila ATL, the fusion activity of human ATLs has not been reconstituted. Here, we report successful reconstitution of fusion activity by the human ATLs. Unexpectedly, the major splice isoforms of ATL1 and ATL2 are each autoinhibited, albeit to differing degrees. For the more strongly inhibited ATL2, autoinhibition mapped to a C-terminal α-helix is predicted to be continuous with an amphipathic helix required for fusion. Charge reversal of residues in the inhibitory domain strongly activated its fusion activity, and overexpression of this disinhibited version caused ER collapse. Neurons express an ATL2 splice isoform whose sequence differs in the inhibitory domain, and this form showed full fusion activity. These findings reveal autoinhibition and alternate splicing as regulators of atlastin-mediated ER fusion.

M protein of subacute sclerosing panencephalitis virus, synergistically with the F protein, plays a crucial role in viral neuropathogenicity

Subacute sclerosing panencephalitis (SSPE) is a rare fatal neurodegenerative disease caused by a measles virus (MV) variant, SSPE virus, that accumulates mutations during long-term persistent infection of the central nervous system (CNS). Clusters of mutations identified around the matrix (M) protein in many SSPE viruses suppress productive infectious particle release and accelerate cell–cell fusion, which are features of SSPE viruses. It was reported, however, that these defects of M protein function might not be correlated directly with promotion of neurovirulence, although they might enable establishment of persistent infection. Neuropathogenicity is closely related to the character of the viral fusion (F) protein, and amino acid substitution(s) in the F protein of some SSPE viruses confers F protein hyperfusogenicity, facilitating viral propagation in the CNS through cell–cell fusion and leading to neurovirulence. The F protein of an SSPE virus Kobe-1 strain, however, displayed only moderately enhanced fusion activity and required additional mutations in the M protein for neuropathogenicity in mice. We demonstrated here the mechanism for the M protein of the Kobe-1 strain supporting the fusion activity of the F protein and cooperatively inducing neurovirulence, even though each protein, independently, has no effect on virulence. The occurrence of SSPE has been estimated recently as one in several thousand in children who acquired measles under the age of 5 years, markedly higher than reported previously. The probability of a specific mutation (or mutations) occurring in the F protein conferring hyperfusogenicity and neuropathogenicity might not be sufficient to explain the high frequency of SSPE. The induction of neurovirulence by M protein synergistically with moderately fusogenic F protein could account for the high frequency of SSPE.

Mechanism of Cross-Resistance to Fusion Inhibitors Conferred by the K394R Mutation in Respiratory Syncytial Virus Fusion Protein

The fusion glycoprotein (F) is essential for respiratory syncytial virus (RSV) entry and has become an attractive target for anti-RSV drug development. Despite the promising prospect of RSV F inhibitors, issues of drug resistance remain challenging. In this study, we established a dual-luciferase protocol for RSV fusion inhibitor discovery. A small-molecule inhibitor, salvianolic acid R (LF-6), was identified to inhibit virus-cell and cell-cell fusion mediated by RSV F protein. Sequence analysis of the resultant resistant viruses identified a K394R mutation in the viral F protein. The K394R mutant virus also conferred cross-resistance to multiple RSV fusion inhibitors, including several inhibitors undergoing clinical trials. Our study further showed that K394R mutation not only increased the triggering rate of F protein in prefusion conformation but also enhanced fusion activity of F protein, both of which were positively correlated with the resistance to fusion inhibitors. Moreover, the K394R mutation also showed cooperative effects with other escape mutations to increase the fusion activity of F protein. By substitution of K394 into different amino acids, we found that K394R or K394H substitution resulted in hyperfusiogenic F proteins, whereas F variants with other substitutions exhibited lower fusion activity. Both K394R and K394H in F protein exhibited cross-resistance to RSV fusion inhibitors. Collectively, these findings reveal a positive correlation existed between membrane fusion activity of F protein and the resistance of corresponding inhibitors. All the results demonstrate that K394R in F protein confers cross-resistance to fusion inhibitors through destabilizing F protein and increasing its membrane fusion activity. IMPORTANCE Respiratory syncytial virus (RSV) causes serious respiratory tract disease in children and the elderly. Therapeutics against RSV infection are urgently needed. This study reports the discovery of a small-molecule inhibitor of RSV fusion glycoprotein by using a dual-luciferase protocol. The escape mutation (K394R) of this compound also confers cross-resistance to multiple RSV fusion inhibitors that have been reported previously, including two candidates currently in clinical development. The combination of K394R with other escape mutations can increase the resistance of F protein to these inhibitors through destabilizing F protein and enhancing the membrane fusion activity of F protein. By amino acid deletion or substitution, we found that positively charged residue at the 394 th site is crucial for the fusion ability of F protein as well as for the cross-resistance against RSV fusion inhibitors. These results reveal the mechanism of cross-resistance conferred by K394R mutation and the possible cross-resistance risk of RSV fusion inhibitors.

SARS-CoV-2 Spreads through Cell-to-Cell Transmission

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is a highly transmissible coronavirus responsible for the global COVID-19 pandemic. Herein we provide evidence that SARS-CoV-2 spreads through cell-cell contact in cultures, mediated by the spike glycoprotein. SARS-CoV-2 spike is more efficient in facilitating cell-to-cell transmission than SARS-CoV spike, which reflects, in part, their differential cell-cell fusion activity. Interestingly, treatment of cocultured cells with endosomal entry inhibitors impairs cell-to-cell transmission, implicating endosomal membrane fusion as an underlying mechanism. Compared with cell-free infection, cell-to-cell transmission of SARS-CoV-2 is refractory to inhibition by neutralizing antibody or convalescent sera of COVID-19 patients. While ACE2 enhances cell-to-cell transmission, we find that it is not absolutely required. Notably, despite differences in cell-free infectivity, the variants of concern (VOC) B.1.1.7 and B.1.351 have similar cell-to-cell transmission capability. Moreover, B.1.351 is more resistant to neutralization by vaccinee sera in cell-free infection, whereas B.1.1.7 is more resistant to inhibition by vaccine sera in cell-to-cell transmission. Overall, our study reveals critical features of SARS-CoV-2 spike-mediated cell-to-cell transmission, with important implications for a better understanding of SARS-CoV-2 spread and pathogenesis.

Synthesis, Biophysical Characterization, and Anti–HIV-1 Fusion Activity of DNA Quadruplex-based Inhibitors with Dipeptide MT Hook Conjugation

Background: Human immunodeficiency virus type-1 (HIV-1) infection is the reason for the epidemic of acquired immunodeficiency syndrome (AIDS). Developing HIV-1 fusion inhibitors gained increasing attention as they took effect in the early stage of HIV-1 infecting cells. DNA G-quadruplex-based inhibitors had been found to interact with HIV-1 envelope glycoprotein, showing anti–HIV-1 fusion activity. C-peptide derived molecules with Met-Thr terminal also showed potent anti-fusion activity, the Met-Thr dipeptide adopted a hook-like structure (termed MT hook) in the hydrophobic pocket to "anchor" inhibitors to the N-terminal heptad repeat (NHR) of HIV-1 envelope glycoprotein gp41. Objective: Our work was to conjugate MT hooks to the 5'-terminal ends of DNA quadruplex-based inhibitor and demonstrate its biophysical characterization and anti–HIV-1 fusion activity. Methods: A 6-aminohexanol phosphonamidite was utilized in solid synthesis for the conjunction of oligodeoxynucleotide and MT dipeptide. Hydrophobic groups were introduced by a nucleoside analogue from the base site. Circular dichroism spectrum and native polyacrylamide gel electrophoresis were used to confirm the helix formation. A cell-cell fusion assay was carried out to test the anti-fusion activity. Results: The conjugate G1 showed improved anti-cell-cell fusion activity than quadruplex without MT hook. The MT hook did not affect the oligodeoxynucleotide (ODN) G-quadruplex assembly. It was also proved that G1 could effectively interfere with endogenous 6-helical bundle (6HB) formation between the N-terminal heptad repeat N36 (NHR) and the C-terminal heptad repeat C34 (CHR) during virus fusion course. Conclusion: In this work, conjugate of DNA-oligopeptide were successfully synthesized. The conjugation of MT hook did improve the anti-fusion activity of DNA G-quadruplex-based inhibitors. Our results can add information regarding on structure-activity relationships of DNA helix-based inhibitors and provide a reference for the follow-up experimental studies.

Impairment of SARS-CoV-2 spike glycoprotein maturation and fusion activity by the broad-spectrum anti-infective drug nitazoxanide

ABSTRACTThe emergence of the highly-pathogenic severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2), the causative agent of COVID-19 (coronavirus disease-2019), has caused an unprecedented global health crisis, as well as societal and economic disruption. The SARS-CoV-2 spike (S), a surface-anchored trimeric class-I fusion glycoprotein essential for entry into host cells, represents a key target for developing vaccines and therapeutics capable of blocking virus invasion. The emergence of several SARS-CoV-2 spike variants that facilitate virus spread and may affect the efficacy of recently developed vaccines, creates great concern and highlights the importance of identifying antiviral drugs to reduce SARS-CoV-2-related morbidity and mortality. Nitazoxanide, a thiazolide originally developed as an antiprotozoal agent with recognized broad-spectrum antiviral activity in-vitro and in clinical studies, was recently shown to be effective against several coronaviruses, including SARS-CoV-2. Using biochemical and pseudovirus entry assays, we now demonstrate that nitazoxanide interferes with the SARS-CoV-2 spike biogenesis, hampering its maturation at an endoglycosidase H-sensitive stage, and hindering its fusion activity in human cells. Besides membrane fusion during virus entry, SARS-CoV-2 S-proteins in infected cells can also trigger receptor-dependent formation of syncytia, observed in-vitro and in COVID-19 patients tissues, facilitating viral dissemination between cells and possibly promoting immune evasion. Utilizing two different quantitative cell-cell fusion assays, we show that nitazoxanide is effective in inhibiting syncytia formation mediated by different SARS-CoV-2 spike variants in human lung, liver and intestinal cells. The results suggest that nitazoxanide may represent a useful tool in the fight against COVID-19 infections, inhibiting SARS-CoV-2 replication and preventing spike-mediated syncytia formation.

Functional Analysis of the Fusion and Attachment Glycoproteins of Mojiang Henipavirus

Mojiang virus (MojV) is the first henipavirus identified in a rodent and known only by sequence data, whereas all other henipaviruses have been isolated from bats (Hendra virus, Nipah virus, Cedar virus) or discovered by sequence data from material of bat origin (Ghana virus). Ephrin-B2 and -B3 are entry receptors for Hendra and Nipah viruses, but Cedar virus can utilize human ephrin-B1, -B2, -A2 and -A5 and mouse ephrin-A1. However, the entry receptor for MojV remains unknown, and its species tropism is not well characterized. Here, we utilized recombinant full-length and soluble forms of the MojV fusion (F) and attachment (G) glycoproteins in membrane fusion and receptor tropism studies. MojV F and G were functionally competent and mediated cell–cell fusion in primate and rattine cells, albeit with low levels and slow fusion kinetics. Although a relative instability of the pre-fusion conformation of a soluble form of MojV F was observed, MojV F displayed significantly greater fusion activity when heterotypically paired with Ghana virus G. An exhaustive investigation of A- and B-class ephrins indicated that none serve as a primary receptor for MojV. The MojV cell fusion phenotype is therefore likely the result of receptor restriction rather than functional defects in recombinant MojV F and G glycoproteins.

Antiviral Screen against Canine Distemper Virus-Induced Membrane Fusion Activity

Canine distemper virus (CDV), a close relative of the human pathogen measles virus (MeV), is an enveloped, negative sense RNA virus that belongs to the genus Morbillivirus and causes severe diseases in dogs and other carnivores. Although the vaccination is available as a preventive measure against the disease, the occasional vaccination failure highlights the importance of therapeutic alternatives such as antivirals against CDV. The morbilliviral cell entry system relies on two interacting envelope glycoproteins: the attachment (H) and fusion (F) proteins. Here, to potentially discover novel entry inhibitors targeting CDV H, F and/or the cognate receptor: signaling lymphocyte activation molecule (SLAM) proteins, we designed a quantitative cell-based fusion assay that matched high-throughput screening (HTS) settings. By screening two libraries of small molecule compounds, we successfully identified two membrane fusion inhibitors (F2736-3056 and F2261-0043). Although both inhibitors exhibited similarities in structure and potency with the small molecule compound 3G (an AS-48 class morbilliviral F-protein inhibitor), F2736-3056 displayed improved efficacy in blocking fusion activity when a 3G-escape variant was employed. Altogether, we present a cell-based fusion assay that can be utilized not only to discover antiviral agents against CDV but also to dissect the mechanism of morbilliviral-mediated cell-binding and cell-to-cell fusion activity.