Co-expression of foreign proteins tethered to HIV-1 envelope glycoprotein on the cell surface by introducing an intervening second membrane-spanning domain

Sperm induction of somatic cell-cell fusion as a novel functional test

The fusion of mammalian gametes requires the interaction between IZUMO1 on the sperm and JUNO on the oocyte. We have recently shown that ectopic expression of mouse IZUMO1 induces cell-cell fusion and that sperm can fuse to fibroblasts expressing JUNO. Here, we found that the incubation of mouse sperm with hamster fibroblasts or human epithelial cells in culture induces the fusion between these somatic cells and the formation of syncytia, a pattern previously observed with some animal viruses. This sperm-induced cell-cell fusion requires a species-matching JUNO on both fusing cells, can be blocked by an antibody against IZUMO1, and does not rely on the synthesis of new proteins. The fusion is dependent on the sperm's fusogenic capacity, making this a reliable, fast, and simple method for predicting sperm function during the diagnosis of male infertility.

Interaction of the Hemagglutinin Stalk Region with Cell Adhesion Molecule (CADM) 1 and CADM2 Mediates the Spread between Neurons and Neuropathogenicity of Measles Virus with a Hyperfusogenic Fusion Protein

Measles virus (MeV), the causative agent of measles, is an enveloped RNA virus of the family Paramyxoviridae, which remains an important cause of childhood morbidity and mortality. MeV has two envelope glycoproteins, the hemagglutinin (H) and fusion (F) proteins. During viral entry or virus-mediated fusion between infected cells and neighboring susceptible cells, the head domain of the H protein initially binds to its receptors, signaling lymphocytic activation molecule family member 1 (SLAM) and nectin-4, and then the stalk region of the H protein transmits the fusion-triggering signal to the F protein. MeV may persist in the human brain and cause a fatal neurodegenerative disease, subacute sclerosing panencephalitis (SSPE). Recently, we showed, using in vitro cell culture, that cell adhesion molecule (CADM) 1 and CADM2 are host factors that trigger hyperfusogenic mutant F proteins, causing cell-to-cell fusion and the transfer of the MeV genome between neurons. Unlike conventional receptors, CADM1 and CADM2 interact in cis (on the same membrane) with the H protein and then trigger membrane fusion. Here, we show that alanine substitutions in part of the stalk region (positions 171-175) abolish the ability of the H protein to mediate membrane fusion triggered by CADM1 and CADM2, but not by SLAM. The recombinant hyperfusogenic MeV carrying this mutant H protein loses its ability to spread in primary mouse neurons as well as its neurovirulence in experimentally infected suckling hamsters. These results indicate that CADM1 and CADM2 are key molecules for MeV propagation in the brain and its neurovirulence in vivo. IMPORTANCE Measles is an acute febrile illness with skin rash. Despite the availability of highly effective vaccines, measles is still an important cause of childhood morbidity and mortality in many countries. The World Health Organization estimates that more than 120,000 people died from measles worldwide in 2021. Measles virus (MeV), the causative agent of measles, can also cause a fatal progressive neurological disorder, subacute sclerosing panencephalitis (SSPE), several years after acute infection. There is currently no effective treatment for this disease. In this study, using recombinant MeVs with altered receptor usage patterns, we show that cell adhesion molecule (CADM) 1 and CADM2 are host factors critical for MeV spread in neurons and its neurovirulence. These findings further our understanding of the molecular mechanism of MeV neuropathogenicity.

Collective fusion activity determines neurotropism of an en bloc transmitted enveloped virus

Measles virus (MeV), which is usually non-neurotropic, sometimes persists in the brain and causes subacute sclerosing panencephalitis (SSPE) several years after acute infection, serving as a model for persistent viral infections. The persisting MeVs have hyperfusogenic mutant fusion (F) proteins that likely enable cell-cell fusion at synapses and "en bloc transmission" between neurons. We here show that during persistence, F protein fusogenicity is generally enhanced by cumulative mutations, yet mutations paradoxically reducing the fusogenicity may be selected alongside the wild-type (non-neurotropic) MeV genome. A mutant F protein having SSPE-derived substitutions exhibits lower fusogenicity than the hyperfusogenic F protein containing some of those substitutions, but by the wild-type F protein coexpression, the fusogenicity of the former F protein is enhanced, while that of the latter is nearly abolished. These findings advance the understanding of the long-term process of MeV neuropathogenicity and provide critical insight into the genotype-phenotype relationships of en bloc transmitted viruses.

Reduction of Cell Fusion by Deletion in the Hypervariable Region of the Spike Protein of Mouse Hepatitis Virus

Deletions in the spike gene of mouse hepatitis virus (MHV) produce several variants with diverse biological characteristics, highlighting the significance of the spike gene in viral pathogenesis. In this study, we characterized the JHM-X strain, which has a deletion in the hypervariable region (HVR) of the spike gene, compared with the cl-2 strain, which has a full spike gene. Cytopathic effects (CPEs) induced by the two strains revealed that the size of the CPE produced by cl-2 is much greater than that produced by JHM-X in delayed brain tumor (DBT) cells. Thus, this finding explains the greater fusion activity of cl-2 than JHM-X in cultured cells, and we speculate that the deletion region of the spike protein is involved in the fusion activity differences. In contrast with the fusion activity, a comparison of the virus growth kinetics revealed that the titer of JHM-X was approximately 100 times higher than that of cl-2. We found that the deletion region of the spike protein was involved in fusion activity differences, whereas cl-2 produced significantly higher luciferase activity than JHM-X upon similar expression levels of the spike protein. However, the reason behind the growth difference is still unknown. Overall, we discovered that deletion in the HVR of the spike gene could be involved in the fusion activity differences between the two strains.

In silico identification and experimental validation of cellular uptake and intracellular labeling by a new cell penetrating peptide derived from CDN1

Bioactive therapeutic molecules are generally impermeable to the cell membrane, hindering their utility and efficacy. A group of peptides called cell-penetrating peptides (CPPs) were found to have the capability of transporting different types of cargo molecules across the cell membrane. Here, we identified a short peptide named P2, which has a higher proportion of basic residues than the CDN1 (cyclin-dependent kinase inhibitor 1) protein it is derived from, and we used bioinformatic analysis and experimental validation to confirm the penetration property of peptide P2. We found that peptide P2 can efficiently enter different cell lines in a concentration-dependent manner. The endocytosis pathway, especially receptor-related endocytosis, may be involved in the process of P2 penetration. Our data also showed that peptide P2 is safe in cultured cell lines and red blood cells. Lastly, peptide P2 can efficiently deliver self-labeling protein HaloTag into cells for imaging. Our study illustrates that peptide P2 is a promising imaging agent delivery vehicle for future applications.

Short-stalk isoforms of CADM1 and CADM2 trigger neuropathogenic measles virus-mediated membrane fusion by interacting with the viral hemagglutinin

Measles virus (MeV), an enveloped RNA virus in the family Paramyxoviridae, usually causes acute febrile illness with skin rash, but in rare cases persists in the brain, causing a progressive neurological disorder, subacute sclerosing panencephalitis (SSPE). MeV bears two envelope glycoproteins, the hemagglutinin (H) and fusion (F) proteins. The H protein possesses a head domain that initially mediates receptor binding and a stalk domain that subsequently transmits the fusion-triggering signal to the F protein. We have recently shown that cell adhesion molecule 1 (CADM1, also known as IGSF4A, Necl-2, SynCAM1) and CADM2 (also known as IGSF4D, Necl-3, SynCAM2) are host factors enabling cell-cell membrane fusion mediated by hyperfusogenic F proteins of neuropathogenic MeVs as well as MeV spread between neurons lacking the known receptors. CADM1 and CADM2 interact in cis with the H protein on the same cell membrane, triggering hyperfusogenic F protein-mediated membrane fusion. Multiple isoforms of CADM1 and CADM2 containing various lengths of their stalk regions are generated by alternative splicing. Here we show that only short-stalk isoforms of CADM1 and CADM2 predominantly expressed in the brain induce hyperfusogenic F protein-mediated membrane fusion. While the known receptors interact in trans with the H protein through its head domain, these isoforms can interact in cis even with the H protein lacking the head domain and trigger membrane fusion, presumably through its stalk domain. Thus, our results unveil a new mechanism of viral fusion triggering by host factors. Importance Measles, an acute febrile illness with skin rash, is still an important cause of childhood morbidity and mortality worldwide. Measles virus (MeV), the causative agent of measles, may also cause a progressive neurological disorder, subacute sclerosing panencephalitis (SSPE), several years after acute infection. The disease is fatal, and no effective therapy is available. Recently, we have reported that cell adhesion molecule 1 (CADM1) and CADM2 are host factors enabling MeV cell-to-cell spread in neurons. These molecules interact in cis with the MeV attachment protein on the same cell membrane, triggering the fusion protein and causing membrane fusion. CADM1 and CADM2 are known to exist in multiple splice isoforms. In this study, we report that their short-stalk isoforms can induce membrane fusion by interacting in cis with the viral attachment protein independently of its receptor-binding head domain. This finding may have important implications for cis-acting fusion triggering by host factors.

CADM1 and CADM2 Trigger Neuropathogenic Measles Virus-Mediated Membrane Fusion by Acting in cis

Measles virus (MeV), an enveloped RNA virus in the family Paramyxoviridae, is still an important cause of childhood morbidity and mortality worldwide. MeV usually causes acute febrile illness with skin rash, but in rare cases persists in the brain, causing a progressive neurological disorder, subacute sclerosing panencephalitis (SSPE). The disease is fatal, and no effective therapy is currently available. Although transsynaptic cell-to-cell transmission is thought to account for MeV propagation in the brain, neurons do not express the known receptors for MeV. Recent studies have shown that hyperfusogenic changes in the MeV fusion (F) protein play a key role in MeV propagation in the brain. However, how such mutant viruses spread in neurons remains unexplained. Here, we show that cell adhesion molecule 1 (CADM1; also known as IGSF4A, Necl-2, and SynCAM1) and CADM2 (also known as IGSF4D, Necl-3, SynCAM2) are host factors that enable MeV to cause membrane fusion in cells lacking the known receptors and to spread between neurons. During enveloped virus entry, a cellular receptor generally interacts in trans with the attachment protein on the envelope. However, CADM1 and CADM2 interact in cis with the MeV attachment protein on the same cell membrane, causing the fusion protein triggering and membrane fusion. Knockdown of CADM1 and CADM2 inhibits syncytium formation and virus transmission between neurons that are both mediated by hyperfusogenic F proteins. Thus, our results unravel the molecular mechanism (receptor-mimicking cis-acting fusion triggering) by which MeV spreads transsynaptically between neurons, thereby causing SSPE. IMPORTANCE Measles virus (MeV), an enveloped RNA virus, is the causative agent of measles, which is still an important cause of childhood morbidity and mortality worldwide. Persistent MeV infection in the brain causes a fatal progressive neurological disorder, subacute sclerosing panencephalitis (SSPE), several years after acute infection. However, how MeV spreads in neurons, which are mainly affected in SSPE, remains largely unknown. In this study, we demonstrate that cell adhesion molecule 1 (CADM1) and CADM2 are host factors enabling MeV spread between neurons. During enveloped virus entry, a cellular receptor generally interacts in trans with the attachment protein on the viral membrane (envelope). Remarkably, CADM1 and CADM2 interact in cis with the MeV attachment protein on the same membrane, triggering the fusion protein and causing membrane fusion, as viral receptors usually do in trans. Careful screening may lead to more examples of such "receptor-mimicking cis-acting fusion triggering" in other viruses.

Discovery of New Potent anti-MERS CoV Fusion Inhibitors

Middle East respiratory syndrome coronavirus (MERS-CoV), capable of zoonotic transmission, has been associated with emerging viral pneumonia in humans. In this study, a set of highly potent peptides were designed to prevent MERS-CoV fusion through competition with heptad repeat domain 2 (HR2) at its HR1 binding site. We designed eleven peptides with stronger estimated HR1 binding affinities than the wild-type peptide to prevent viral fusion with the cell membrane. Eight peptides showed strong inhibition of spike-mediated MERS-CoV cell-cell fusion with IC50 values in the nanomolar range (0.25–2.3 µM). Peptides #4–6 inhibited 95–98.3% of MERS-CoV plaque formation. Notably, peptide four showed strong inhibition of MERS-CoV plaques formation with EC50 = 0.302 µM. All peptides demonstrated safe profiles without cytotoxicity up to a concentration of 10 μM, and this cellular safety, combined with their anti-MERS-CoV antiviral activity, indicate all peptides can be regarded as potential promising antiviral agents.

Discovery of New Fusion Inhibitor Peptides against SARS-CoV-2 by Targeting the Spike S2 Subunit

A novel coronavirus, severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2), caused a worldwide pandemic. Our aim in this study is to produce new fusion inhibitors against SARS-CoV-2, which can be the basis for developing new antiviral drugs. The fusion core comprising the heptad repeat domains (HR1 and HR2) of SARS-CoV-2 spike (S) were used to design the peptides. A total of twelve peptides were generated, comprising a short or truncated 24-mer (peptide #1), a long 36-mer peptide (peptide #2), and ten peptide #2 analogs. In contrast to SARS-CoV, SARS-CoV-2 S-mediated cell-cell fusion cannot be inhibited with a minimal length, 24-mer peptide. Peptide #2 demonstrated potent inhibition of SARS-CoV-2 S-mediated cell-cell fusion at 1 µM concentration. Three peptide #2 analogs showed IC50 values in the low micromolar range (4.7-9.8 µM). Peptide #2 inhibited the SARS-CoV-2 pseudovirus assay at IC50=1.49 µM. Given their potent inhibition of viral activity and safety and lack of cytotoxicity, these peptides provide an attractive avenue for the development of new prophylactic and therapeutic agents against SARS-CoV-2.

Membrane Protein Production and Purification from Escherichia coli and Sf9 Insect Cells

A major obstacle to studying membrane proteins by biophysical techniques is the difficulty in producing sufficient amounts of materials for functional and structural studies. To overexpress the target membrane protein heterologously, especially an eukaryotic protein, a key step is to find the optimal host expression system and perform subsequent expression optimization. In this chapter, we describe protocols for screening membrane protein production using bacterial and insect cells, solubilization screening, large-scale production, and commonly used affinity chromatography purification methods. We discuss general optimization conditions, such as promoters and tags, and describe current techniques that can be used in any laboratory without specialized expensive equipment. Especially for insect cells, GFP fusions are particularly useful for localization and in-gel fluorescence detection of the proteins on SDS-PAGE. We give detailed protocols that can be used to screen the best expression and purification conditions for membrane protein study.

The Antimalarial Compound Atovaquone Inhibits Zika and Dengue Virus Infection by Blocking E Protein-Mediated Membrane Fusion

Flaviviruses bear class II fusion proteins as their envelope (E) proteins. Here, we describe the development of an in vitro quantitative mosquito-cell-based membrane-fusion assay for the E protein using dual split proteins (DSPs). The assay does not involve the use of live viruses and allows the analysis of a membrane-fusion step independent of other events in the viral lifecycle, such as endocytosis. The progress of membrane fusion can be monitored continuously by measuring the activities of Renilla luciferase derived from the reassociation of DSPs during cell fusion. We optimized the assay to screen an FDA-approved drug library for a potential membrane fusion inhibitor using the E protein of Zika virus. Screening results identified atovaquone, which was previously described as an antimalarial agent. Atovaquone potently blocked the in vitro Zika virus infection of mammalian cells with an IC₉₀ of 2.1 µM. Furthermore, four distinct serotypes of dengue virus were also inhibited by atovaquone with IC₉₀ values of 1.6–2.5 µM, which is a range below the average blood concentration of atovaquone after its oral administration in humans. These findings make atovaquone a likely candidate drug to treat illnesses caused by Zika as well as dengue viruses. Additionally, the DSP assay is useful to study the mechanism of membrane fusion in Flaviviruses.

Anti-Chikungunya Virus Monoclonal Antibody That Inhibits Viral Fusion and Release

Chikungunya fever, a mosquito-borne disease manifested by fever, rash, myalgia, and arthralgia, is caused by chikungunya virus (CHIKV), which belongs to the genus Alphavirus of the family Togaviridae Anti-CHIKV IgG from convalescent patients is known to directly neutralize CHIKV, and the state of immunity lasts throughout life. Here, we examined the epitope of a neutralizing mouse monoclonal antibody against CHIKV, CHE19, which inhibits viral fusion and release. In silico docking analysis showed that the epitope of CHE19 was localized in the viral E2 envelope and consisted of two separate segments, an N-linker and a β-ribbon connector, and that its bound Fab fragment on E2 overlapped the position that the E3 glycoprotein originally occupied. We showed that CHIKV-E2 is lost during the viral internalization and that CHE19 inhibits the elimination of CHIKV-E2. These findings suggested that CHE19 stabilizes the E2-E1 heterodimer instead of E3 and inhibits the protrusion of the E1 fusion loop and subsequent membrane fusion. In addition, the antigen-bound Fab fragment configuration showed that CHE19 connects to the CHIKV spikes existing on the two individual virions, leading us to conclude that the CHE19-CHIKV complex was responsible for the large virus aggregations. In our subsequent filtration experiments, large viral aggregations by CHE19 were trapped by a 0.45-μm filter. This virion-connecting characteristic of CHE19 could explain the inhibition of viral release from infected cells by the tethering effect of the virion itself. These findings provide clues toward the development of effective prophylactic and therapeutic monoclonal antibodies against the Alphavirus infection.IMPORTANCE Recent outbreaks of chikungunya fever have increased its clinical importance. Neither a specific antiviral drug nor a commercial vaccine for CHIKV infection are available. Here, we show a detailed model of the docking between the envelope glycoprotein of CHIKV and our unique anti-CHIKV-neutralizing monoclonal antibody (CHE19), which inhibits CHIKV membrane fusion and virion release from CHIKV-infected cells. Homology modeling of the neutralizing antibody CHE19 and protein-protein docking analysis of the CHIKV envelope glycoprotein and CHE19 suggested that CHE19 inhibits the viral membrane fusion by stabilizing the E2-E1 heterodimer and inhibits virion release by facilitating the formation of virus aggregation due to the connecting virions, and these predictions were confirmed by experiments. Sequence information of CHE19 and the CHIKV envelope glycoprotein and their docking model will contribute to future development of an effective prophylactic and therapeutic agent.

Small Molecule Inhibitors of Middle East Respiratory Syndrome Coronavirus Fusion by Targeting Cavities on Heptad Repeat Trimers

Middle East Respiratory Syndrome Coronavirus (MERS-CoV) is a newly emerging viral disease with fatal outcomes. However, no MERS-CoV-specific treatment is commercially available. Given the absence of previous structure-based drug discovery studies targeting MERS-CoV fusion proteins, this set of compounds is considered the first generation of MERS-CoV small molecule fusion inhibitors. After a virtual screening campaign of 1.56 million compounds followed by cell-cell fusion assay and MERS-CoV plaques inhibition assay, three new compounds were identified. Compound numbers 22, 73, and 74 showed IC₅₀ values of 12.6, 21.8, and 11.12 μM, respectively, and were most effective at the onset of spike-receptor interactions. The compounds exhibited safe profiles against Human embryonic kidney cells 293 at a concentration of 20 μM with no observed toxicity in Vero cells at 10 μM. The experimental results are accompanied with predicted favorable pharmacokinetic descriptors and drug-likeness parameters. In conclusion, this study provides the first generation of MERS-CoV fusion inhibitors with potencies in the low micromolar range.

The Anticoagulant Nafamostat Potently Inhibits SARS-CoV-2 S Protein-Mediated Fusion in a Cell Fusion Assay System and Viral Infection In Vitro in a Cell-Type-Dependent Manner

Although infection by SARS-CoV-2, the causative agent of coronavirus pneumonia disease (COVID-19), is spreading rapidly worldwide, no drug has been shown to be sufficiently effective for treating COVID-19. We previously found that nafamostat mesylate, an existing drug used for disseminated intravascular coagulation (DIC), effectively blocked Middle East respiratory syndrome coronavirus (MERS-CoV) S protein-mediated cell fusion by targeting transmembrane serine protease 2 (TMPRSS2), and inhibited MERS-CoV infection of human lung epithelium-derived Calu-3 cells. Here we established a quantitative fusion assay dependent on severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) S protein, angiotensin I converting enzyme 2 (ACE2) and TMPRSS2, and found that nafamostat mesylate potently inhibited the fusion while camostat mesylate was about 10-fold less active. Furthermore, nafamostat mesylate blocked SARS-CoV-2 infection of Calu-3 cells with an effective concentration (EC)50 around 10 nM, which is below its average blood concentration after intravenous administration through continuous infusion. On the other hand, a significantly higher dose (EC50 around 30 mM) was required for VeroE6/TMPRSS2 cells, where the TMPRSS2-independent but cathepsin-dependent endosomal infection pathway likely predominates. Together, our study shows that nafamostat mesylate potently inhibits SARS-CoV-2 S protein-mediated fusion in a cell fusion assay system and also inhibits SARS-CoV-2 infection in vitro in a cell-type-dependent manner. These findings, together with accumulated clinical data regarding nafamostat's safety, make it a likely candidate drug to treat COVID-19.

Weak cis and trans Interactions of the Hemagglutinin with Receptors Trigger Fusion Proteins of Neuropathogenic Measles Virus Isolates

Measles virus (MeV) is an enveloped RNA virus bearing two envelope glycoproteins, the hemagglutinin (H) and fusion (F) proteins. Upon receptor binding, the H protein triggers conformational changes of the F protein, causing membrane fusion and subsequent virus entry. MeV may persist in the brain, infecting neurons and causing fatal subacute sclerosing panencephalitis (SSPE). Since neurons do not express either of the MeV receptors, signaling lymphocytic activation molecule (SLAM; also called CD150) and nectin-4, how MeV propagates in neurons is unknown. Recent studies have shown that specific substitutions in the F protein found in MeV isolates from SSPE patients are critical for MeV neuropathogenicity by rendering the protein unstable and hyperfusogenic. Recombinant MeVs possessing the F proteins with such substitutions can spread in primary human neurons and in the brains of mice and hamsters and induce cell-cell fusion in cells lacking SLAM and nectin-4. Here, we show that receptor-blind mutant H proteins that have decreased binding affinities to receptors can support membrane fusion mediated by hyperfusogenic mutant F proteins, but not the wild-type F protein, in cells expressing the corresponding receptors. The results suggest that weak interactions of the H protein with certain molecules (putative neuron receptors) trigger hyperfusogenic F proteins in SSPE patients. Notably, where cell-cell contacts are ensured, the weak cis interaction of the H protein with SLAM on the same cell surface also could trigger hyperfusogenic F proteins. Some enveloped viruses may exploit such cis interactions with receptors to infect target cells, especially in cell-to-cell transmission.IMPORTANCE Measles virus (MeV) may persist in the brain, causing incurable subacute sclerosing panencephalitis (SSPE). Because neurons, the main target in SSPE, do not express receptors for wild-type (WT) MeV, how MeV propagates in the brain is a key question for the disease. Recent studies have demonstrated that specific substitutions in the MeV fusion (F) protein are critical for neuropathogenicity. Here, we show that weak cis and trans interactions of the MeV attachment protein with receptors that are not sufficient to trigger the WT MeV F protein can trigger the mutant F proteins from neuropathogenic MeV isolates. Our study not only provides an important clue to understand MeV neuropathogenicity but also reveals a novel viral strategy to expand cell tropism.

Cell-cell and virus-cell fusion assay-based analyses of alanine insertion mutants in the distal α9 portion of the JRFL gp41 subunit from HIV-1

Membrane fusion is the first essential step in HIV-1 replication. The gp41 subunit of HIV-1 envelope protein (Env), a class I fusion protein, achieves membrane fusion by forming a structure called a six-helix bundle composed of N- and C-terminal heptad repeats. We have recently shown that the distal portion of the α9 helix in the C-terminal heptad repeat of X4-tropic HXB2 Env plays a critical role in the late-stage membrane fusion and viral infection. Here, we used R5-tropic JRFL Env and constructed six alanine insertion mutants, 641+A to 646+A, in the further distal portion of α9 where several glutamine residues are conserved (the number corresponds to the position of the inserted alanine in JRFL Env). 644+A showed the most severe defect in syncytia formation. Decreased fusion pore formation activity, revealed by a dual split protein assay, was observed in all mutants except 641+A. Sequence analysis and substitution of inserted 644A with Gln revealed that the glutamine residue at position 644 that forms complex hydrogen-bond networks with other polar residues on the surface of the six-helix bundle is critical for cell-cell fusion. We also developed a split NanoLuc® (Nluc) reporter-based assay specific to the virus-cell membrane fusion step to analyze several of the mutants. Interestingly syncytia-competent mutants failed to display Nluc activities. In addition to defective fusion activity, a reduction of Env incorporation into virions may further contribute to differences in cell-cell and virus-cell fusions.

Six-helix bundle completion in the distal C-terminal heptad repeat region of gp41 is required for efficient human immunodeficiency virus type 1 infection

Background

The native pre-fusion structure of gp120/gp41 complex of human immunodeficiency virus type 1 was recently revealed. In the model, the helices of gp41 (α6, α7, α8, and α9) form a four-helix collar underneath trimeric gp120. Gp41 is a class I fusion protein and mediates membrane fusion by forming a post-fusion structure called the six-helix bundle (6HB). The comparison of the pre- and post-fusion structures revealed the large conformational changes in gp41 during the antiparallel packing of the N- and C-terminal heptad repeats (NHRs and CHRs) in membrane fusion. Several mutagenesis studies of gp41 performed in the past were interpreted based on 6HB, the only available structure at that time. To obtain an insight about the current pre-fusion structural model and conformational changes during membrane fusion, alanine insertion mutagenesis of the NHR, CHR and connecting loop regions of HXB2 gp41 was performed. The effects of mutations on biosynthesis and membrane fusion were analyzed by immunoblotting and fusion assays, respectively. The extent of membrane fusion was evaluated by split luciferase-based pore formation and syncytia formation assays, respectively.

Results

Consistent with the current structural model, drastic negative effects of mutations on biosynthesis and membrane fusion were observed for NHR, loop, and proximal regions of CHR (up to amino acid position 643). The insertions in α9 after it leaves the four-helix collar were tolerable for biosynthesis. These CHR mutants showed varying effects on membrane fusion. Insertion at position 644 or 645 resulted in poor pore and syncytia formation. Efficient pore and syncytia formation almost similar to that of the wild type was observed for insertion at position 647, 648 or 649. However, recovery of virus infectivity was only observed for the insertions beyond position 648.

Conclusions

The mutagenesis data for HXB2 gp41 is in agreement with the recent pre-fusion structure model. The virus infection data suggested that fusion pores sufficiently large enough for the release of the virus genome complex are formed after the completion of 6HB beyond position 648.

Electronic supplementary material

The online version of this article (10.1186/s12977-018-0410-9) contains supplementary material, which is available to authorized users.

Both Sphingomyelin and Cholesterol in the Host Cell Membrane Are Essential for Rubella Virus Entry

Rubella virus (RuV) causes a systemic infection, and transplacental fetal infection causes congenital rubella syndrome. In this study, we showed that treatment of cells with sphingomyelinase inhibited RuV infection. Assays using inhibitors of serine palmitoyl transferase and ceramide transport protein demonstrated the contribution of sphingomyelin (SM) to RuV infection. Compelling evidence for direct binding of RuV to lipid membranes at neutral pH was obtained using liposome coflotation assays. The absence of either SM or cholesterol (Chol) abrogated the RuV-liposome interaction. SM and Chol (SM/Chol) were also critical for RuV binding to erythrocytes and lymphoid cells. Removal of Ca²⁺ from the assay buffer or mutation of RuV envelope E1 protein Ca²⁺-binding sites abrogated RuV binding to liposomes, erythrocytes, and lymphoid cells. However, RuV bound to various nonlymphoid adherent cell lines independently of extracellular Ca²⁺ or SM/Chol. Even in these adherent cell lines, both the E1 protein Ca²⁺-binding sites and cellular SM/Chol were essential for the early stage of RuV infection, possibly affecting envelope-membrane fusion in acidic compartments. Myelin oligodendrocyte glycoprotein (MOG) has recently been identified as a cellular receptor for RuV. However, RuV bound to MOG-negative cells in a Ca²⁺-independent manner. Collectively, our data demonstrate that RuV has two distinct binding mechanisms: one is Ca²⁺ dependent and the other is Ca²⁺ independent. Ca²⁺-dependent binding observed in lymphoid cells occurs by the direct interaction between E1 protein fusion loops and SM/Chol-enriched membranes. Clarification of the mechanism of Ca²⁺-independent RuV binding is an important next step in understanding the pathology of RuV infection.IMPORTANCE Rubella has a significant impact on public health as infection during early pregnancy can result in babies being born with congenital rubella syndrome. Even though effective rubella vaccines are available, rubella outbreaks still occur in many countries. We studied the entry mechanism of rubella virus (RuV) and found that RuV binds directly to the host plasma membrane in the presence of Ca²⁺ at neutral pH. This Ca²⁺-dependent binding is specifically directed to membranes enriched in sphingomyelin and cholesterol and is critical for RuV infection. Importantly, RuV also binds to many cell lines in a Ca²⁺-independent manner. An unidentified RuV receptor(s) is involved in this Ca²⁺-independent binding. We believe that the data presented here may aid the development of the first anti-RuV drug.

Identification of Nafamostat as a Potent Inhibitor of Middle East Respiratory Syndrome Coronavirus S Protein-Mediated Membrane Fusion Using the Split-Protein-Based Cell-Cell Fusion Assay

Middle East respiratory syndrome (MERS) is an emerging infectious disease associated with a relatively high mortality rate of approximately 40%. MERS is caused by MERS coronavirus (MERS-CoV) infection, and no specific drugs or vaccines are currently available to prevent MERS-CoV infection. MERS-CoV is an enveloped virus, and its envelope protein (S protein) mediates membrane fusion at the plasma membrane or endosomal membrane. Multiple proteolysis by host proteases, such as furin, transmembrane protease serine 2 (TMPRSS2), and cathepsins, causes the S protein to become fusion competent. TMPRSS2, which is localized to the plasma membrane, is a serine protease responsible for the proteolysis of S in the post-receptor-binding stage. Here, we developed a cell-based fusion assay for S in a TMPRSS2-dependent manner using cell lines expressing Renilla luciferase (RL)-based split reporter proteins. S was stably expressed in the effector cells, and the corresponding receptor for S, CD26, was stably coexpressed with TMPRSS2 in the target cells. Membrane fusion between these effector and target cells was quantitatively measured by determining the RL activity. The assay was optimized for a 384-well format, and nafamostat, a serine protease inhibitor, was identified as a potent inhibitor of S-mediated membrane fusion in a screening of about 1,000 drugs approved for use by the U.S. Food and Drug Administration. Nafamostat also blocked MERS-CoV infection in vitro Our assay has the potential to facilitate the discovery of new inhibitors of membrane fusion of MERS-CoV as well as other viruses that rely on the activity of TMPRSS2.

The V1 region of gp120 is preferentially selected during SIV/HIV transmission and is indispensable for envelope function and virus infection

A transmission bottleneck occurs during each human immunodeficiency virus (HIV) transmission event, which allows only a few viruses to establish new infection. However, the genetic characteristics of the transmitted viruses that are preferentially selected have not been fully elucidated. Here, we analyzed amino acids changes in the envelope protein during simian immunodeficiency virus (SIV)/HIV deep transmission history and current HIV evolution within the last 15-20 years. Our results confirmed that the V1V2 region of gp120 protein, particularly V1, was preferentially selected. A shorter V1 region was preferred during transmission history, while during epidemic, HIV may evolve to an expanded V1 region gradually and thus escape immune recognition. We then constructed different HIV-1 V1 mutants using different HIV-1 subtypes to elucidate the role of the V1 region in envelope function. We found that the V1 region, although highly variable, was indispensable for virus entry and infection, probably because V1 deletion mutants exhibited impaired processing of gp160 into mature gp120 and gp41. Additionally, the V1 region affected Env incorporation. These results indicated that the V1 region played a critical role in HIV transmission and infection.

Dual Split Protein (DSP) Assay to Monitor Cell-Cell Membrane Fusion

Fusion between viral and cellular membranes is the essential first step in infection of enveloped viruses. This step is mediated by viral envelope glycoproteins (Env) that recognize cellular receptors. The membrane fusion between the effector cells expressing viral Env and the target cells expressing its receptors can be monitored by several methods. We have recently developed a pair of chimeric reporter protein composed of split Renilla luciferase (RL) and split GFP. We named this reporter dual split protein (DSP), since it recovers both RL and GFP activities upon self reassociation. By using DSP, pore formation and content mixing between the effector and target cells can be monitored upon the recovery of RL and GFP activities after the membrane fusion. This quick assay provides quantitative as well as spatial information about membrane fusion mediated by viral Env.

Using a split luciferase assay (SLA) to measure the kinetics of cell-cell fusion mediated by herpes simplex virus glycoproteins

Herpes simplex virus (HSV) entry and cell-cell fusion require the envelope proteins gD, gH/gL and gB. We propose that receptor-activated conformational changes to gD activate gH/gL, which then triggers gB (the fusogen) into an active form. To study this dynamic process, we have adapted a dual split protein assay originally developed to study the kinetics of human immunodeficiency virus (HIV) mediated fusion. This assay uses a chimera of split forms of renilla luciferase (RL) and green fluorescent protein (GFP). Effector cells are co-transfected with the glycoproteins and one of the split reporters. Receptor-bearing target cells are transfected with the second reporter. Co-culture results in fusion and restoration of RL, which can convert a membrane permeable substrate into a luminescent product, thereby enabling one to monitor initiation and extent of fusion in live cells in real time. Restoration of GFP can also be studied by fluorescence microscopy. Two sets of split reporters have been developed: the original one allows one to measure fusion kinetics over hours whereas the more recent version was designed to enhance the sensitivity of RL activity allowing one to monitor both initiation and rates of fusion in minutes. Here, we provide a detailed, step-by-step protocol for the optimization of the assay (which we call the SLA for split luciferase assay) using the HSV system. We also show several examples of the power of this assay to examine both the initiation and kinetics of cell-cell fusion by wild type forms of gD, gB, gH/gL of both serotypes of HSV as well as the effect of mutations and antibodies that alter the kinetics of fusion. The SLA can be applied to other viral systems that carry out membrane fusion.

HaloTag technology: a versatile platform for biomedical applications

Exploration of protein function and interaction is critical for discovering links among genomics, proteomics, and disease state; yet, the immense complexity of proteomics found in biological systems currently limits our investigational capacity. Although affinity and autofluorescent tags are widely employed for protein analysis, these methods have been met with limited success because they lack specificity and require multiple fusion tags and genetic constructs. As an alternative approach, the innovative HaloTag protein fusion platform allows protein function and interaction to be comprehensively analyzed using a single genetic construct with multiple capabilities. This is accomplished using a simplified process, in which a variable HaloTag ligand binds rapidly to the HaloTag protein (usually linked to the protein of interest) with high affinity and specificity. In this review, we examine all current applications of the HaloTag technology platform for biomedical applications, such as the study of protein isolation and purification, protein function, protein–protein and protein–DNA interactions, biological assays, in vitro cellular imaging, and in vivo molecular imaging. In addition, novel uses of the HaloTag platform are briefly discussed along with potential future applications.

The V4 and V5 Variable Loops of HIV-1 Envelope Glycoprotein Are Tolerant to Insertion of Green Fluorescent Protein and Are Useful Targets for Labeling

The mature human immunodeficiency virus type 1 (HIV-1) envelope glycoprotein (Env) comprises the non-covalently associated gp120 and gp41 subunits generated from the gp160 precursor. Recent structural analyses have provided quaternary structural models for gp120/gp41 trimers, including the variable loops (V1-V5) of gp120. In these models, the V3 loop is located under V1/V2 at the apical center of the Env trimer, and the V4 and V5 loops project outward from the trimeric protomers. In addition, the V4 and V5 loops are predicted to have less movement upon receptor binding during membrane fusion events. We performed insertional mutagenesis using a GFP variant, GFPOPT, placed into the variable loops of HXB2 gp120. This allowed us to evaluate the current structural models and to simultaneously generate a GFP-tagged HIV-1 Env, which was useful for image analyses. All GFP-inserted mutants showed similar levels of whole-cell expression, although certain mutants, particularly V3 mutants, showed lower levels of cell surface expression. Functional evaluation of their fusogenicities in cell-cell and virus-like particle-cell fusion assays revealed that V3 was the most sensitive to the insertion and that the V1/V2 loops were less sensitive than V3. The V4 and V5 loops were the most tolerant to insertion, and certain tag proteins other than GFPOPT could also be inserted without functional consequences. Our results support the current structural models and provide a GFPOPT-tagged Env construct for imaging studies.