Histidine 39 in the dengue virus type 2 M protein has an important role in virus assembly

Flavivirus prM interacts with MDA5 and MAVS to inhibit RLR antiviral signaling

BACKGROUND

Vector-borne flaviviruses, including tick-borne encephalitis virus (TBEV), Zika virus (ZIKV), West Nile virus (WNV), yellow fever virus (YFV), dengue virus (DENV), and Japanese encephalitis virus (JEV), pose a growing threat to public health worldwide, and have evolved complex mechanisms to overcome host antiviral innate immunity. However, the underlying mechanisms of flavivirus structural proteins to evade host immune response remain elusive.

RESULTS

We showed that TBEV structural protein, pre-membrane (prM) protein, could inhibit type I interferon (IFN-I) production. Mechanically, TBEV prM interacted with both MDA5 and MAVS and interfered with the formation of MDA5-MAVS complex, thereby impeding the nuclear translocation and dimerization of IRF3 to inhibit RLR antiviral signaling. ZIKV and WNV prM was also demonstrated to interact with both MDA5 and MAVS, while dengue virus serotype 2 (DENV2) and YFV prM associated only with MDA5 or MAVS to suppress IFN-I production. In contrast, JEV prM could not suppress IFN-I production. Overexpression of TBEV and ZIKV prM significantly promoted the replication of TBEV and Sendai virus.

CONCLUSION

Our findings reveal the immune evasion mechanisms of flavivirus prM, which may contribute to understanding flavivirus pathogenicity, therapeutic intervention and vaccine development.

The Electrostatic Features of Dengue Virus Capsid Assembly

Dengue virus causes serious diseases and deaths in the world. Understanding the fundamental mechanisms of dengue virus is highly demanded to develop treatments for dengue virus caused diseases. Here, we present a computational work which focused on the stability of dengue viral capsid. The interactions among E proteins on the dengue viral capsid were studied using several computational approaches. It was found that the electrostatic distribution on a single E protein monomer is highly inhomogeneous, which makes an E protein strongly binding with another E protein. This is the reason why all the E proteins form homodimers as the basic units on the whole dengue viral capsids. The pKa calculations of E proteins demonstrated that the folding energy of an E protein is low and stable in the range of pH 6–10, which is different from many other proteins that are stable at certain pH. The pH dependence of binding energy of E protein homodimer shows that the binding energy is low and independent from pH when the pH is also in the range of 6–10. This finding implies that the dengue virus can survive in a wide range of pH, which can explain why the dengue virus is so widely distributed in the world and spreads fast.

Zika M Oligopeptide ZAMP Confers Cell Death-Promoting Capability to a Soluble Tumor-Associated Antigen through Caspase-3/7 Activation

Mosquito-borne Zika virus (ZIKV) is an emerging flavivirus of medical concern associated with neurological disorders. ZIKV utilizes apoptosis as a mechanism of cell killing. The structural M protein may play a role in flavivirus-induced apoptosis. The death-promoting capability of M has been restricted to an oligopeptide representing the residues M-32/40. Here, we evaluated the apoptosis inducing ability of the residues M-31/41 of ZIKV. The ZIKV M oligopeptide was associated to a soluble form of GFP (sGFP) and the resulting sGFP-M31/41 construct was assessed in Huh7 cells. Expression of sGFP-M31/41 can trigger apoptosis in Huh7 cells through caspase-3/7 activation. The translocation of sGFP-M31/41 in the endoplasmic reticulum was a prerequisite for apoptosis induction. The residues M-33/35/38 may play a critical role in the death-promoting activity of sGFP-M31/41. The effect of ZIKV M oligopeptide defined as ZAMP (for Zika Apoptosis M Peptide) on expression of a tumor-associated antigen was assayed on megakaryocyte-potentiating factor (MPF). Expression of MPF-ZAMP construct resulted in caspase-associated apoptosis activation in A549 and Huh7 cells. ZIKV has been proposed as an oncolytic virus for cancer therapy. The ability of the Zika M oligopeptide to confer death-promoting capability to MPF opens up attractive perspectives for ZAMP as an innovative anticancer agent.

Structure-guided paradigm shifts in flavivirus assembly and maturation mechanisms

The flavivirus genus encompasses more than 75 unique viruses, including dengue virus which accounts for almost 390 million global infections annually. Flavivirus infection can result in a myriad of symptoms ranging from mild rash and flu-like symptoms, to severe encephalitis and even hemorrhagic fever. Efforts to combat the impact of these viruses have been hindered due to limited antiviral drug and vaccine development. However, the advancement of knowledge in the structural biology of flaviviruses over the last 25 years has produced unique perspectives for the identification of potential therapeutic targets. With particular emphasis on the assembly and maturation stages of the flavivirus life cycle, it is the goal of this review to comparatively analyze the structural similarities between flaviviruses to provide avenues for new research and innovation.

A Molecular Determinant of West Nile Virus Secretion and Morphology as a Target for Viral Attenuation

West Nile virus (WNV) is a worldwide (re)emerging mosquito-transmitted Flavivirus causing fatal neurological diseases in humans. However, no human vaccine has been yet approved. One of the most effective live-attenuated vaccines was empirically obtained by serial passaging of wild-type yellow fever Flavivirus. However, such an approach is not acceptable nowadays, and the development of a rationally designed vaccine is necessary. Generating molecular infectious clones and mutating specific residues known to be involved in Flavivirus virulence constitute a powerful tool to promote viral attenuation. WNV membrane glycoprotein is thought to carry such essential determinants. Here, we identified two residues of this protein whose substitutions are key to the full and stable attenuation of WNV in vivo, most likely through inhibition of secretion and possible alteration of morphology. Applied to other flaviviruses, this approach should help in designing new vaccines against these viruses, which are an increasing threat to global human health.

West Nile virus (WNV), a member of the Flavivirus genus and currently one of the most common arboviruses worldwide, is associated with severe neurological disease in humans. Its high potential to reemerge and rapidly disseminate makes it a bona fide global public health problem. The surface membrane glycoprotein (M) has been associated with Flavivirus-induced pathogenesis. Here, we identified a key amino acid residue at position 36 of the M protein whose mutation impacts WNV secretion and promotes viral attenuation. We also identified a compensatory site at position M-43 whose mutation stabilizes M-36 substitution both in vitro and in vivo. Moreover, we found that introduction of the two mutations together confers a full attenuation phenotype and protection against wild-type WNV lethal challenge, eliciting potent neutralizing-antibody production in mice. Our study thus establishes the M protein as a new viral target for rational design of attenuated WNV strains.

IMPORTANCE West Nile virus (WNV) is a worldwide (re)emerging mosquito-transmitted Flavivirus causing fatal neurological diseases in humans. However, no human vaccine has been yet approved. One of the most effective live-attenuated vaccines was empirically obtained by serial passaging of wild-type yellow fever Flavivirus. However, such an approach is not acceptable nowadays, and the development of a rationally designed vaccine is necessary. Generating molecular infectious clones and mutating specific residues known to be involved in Flavivirus virulence constitute a powerful tool to promote viral attenuation. WNV membrane glycoprotein is thought to carry such essential determinants. Here, we identified two residues of this protein whose substitutions are key to the full and stable attenuation of WNV in vivo, most likely through inhibition of secretion and possible alteration of morphology. Applied to other flaviviruses, this approach should help in designing new vaccines against these viruses, which are an increasing threat to global human health.

The Conserved Molecular Determinants of Virulence in Dengue Virus

Dengue virus belongs to the Flaviviridae family which also includes viruses such as the Zika, West Nile and yellow fever virus. Dengue virus generally causes mild disease, however, more severe forms of the dengue virus infection, dengue haemorrhagic fever (DHF) and dengue haemorrhagic fever with shock syndrome (DSS) can also occur, resulting in multiple organ failure and even death, especially in children. The only dengue vaccine available in the market, CYD-TDV offers limited coverage for vaccinees from 9-45 years of age and is only recommended for individuals with prior dengue exposure. A number of mutations that were shown to attenuate virulence of dengue virus in vitro and/or in vivo have been identified in the literature. The mutations which fall within the conserved regions of all four dengue serotypes are discussed. This review hopes to provide information leading to the construction of a live attenuated dengue vaccine that is suitable for all ages, irrespective of the infecting dengue serotype and prior dengue exposure.

Characterization of dengue virus 2 growth in megakaryocyte-erythrocyte progenitor cells

Megakaryocyte-erythrocyte progenitor (MEP) cells are potential in vivo targets of dengue virus (DENV); the virus has been found associated with megakaryocytes ex vivo and platelets during DENV-induced thrombocytopenia. We report here that DENV serotype 2 (DENV2) propagates well in human nondifferentiated MEP cell lines (Meg01 and K562). In comparison to virus propagated in Vero cells, viruses from MEP cell lines had similar structure and buoyant density. However, differences in MEP-DENV2 stability and composition were suggested by distinct protein patterns in western blot analysis. Also, antibody neutralization of envelope domain I/II on MEP-DENV2 was reduced relative to that on Vero-DENV2. Infectious DENV2 was produced at comparable kinetics and magnitude in MEP and Vero cells. However, fewer virion structures appeared in electron micrographs of MEP cells. We propose that DENV2 infects and produces virus efficiently in megakaryocytes and that megakaryocyte impairment might contribute to dengue disease pathogenesis.

Intra-epidemic evolutionary dynamics of a Dengue virus type 1 population reveal mutant spectra that correlate with disease transmission

Dengue virus (DENV) is currently the most prevalent mosquito-borne viral pathogen. DENVs naturally exist as highly heterogeneous populations. Even though the descriptions on DENV diversity are plentiful, only a few studies have narrated the dynamics of intra-epidemic virus diversity at a fine scale. Such accounts are important to decipher the reciprocal relationship between viral evolutionary dynamics and disease transmission that shape dengue epidemiology. In the current study, we present a micro-scale genetic analysis of a monophyletic lineage of DENV-1 genotype III (epidemic lineage) detected from November 2012 to May 2014. The lineage was involved in an unprecedented dengue epidemic in Singapore during 2013–2014. Our findings showed that the epidemic lineage was an ensemble of mutants (variants) originated from an initial mixed viral population. The composition of mutant spectrum was dynamic and positively correlated with case load. The close interaction between viral evolution and transmission intensity indicated that tracking genetic diversity through time is potentially a useful tool to infer DENV transmission dynamics and thereby, to assess the epidemic risk in a disease control perspective. Moreover, such information is salient to understand the viral basis of clinical outcome and immune response variations that is imperative to effective vaccine design.

A Single Amino Acid Substitution in the M Protein Attenuates Japanese Encephalitis Virus in Mammalian Hosts

Japanese encephalitis virus (JEV) membrane (M) protein plays important structural roles in the processes of fusion and maturation of progeny virus during cellular infection. The M protein is anchored in the viral membrane, and its ectodomain is composed of a flexible N-terminal loop and a perimembrane helix. In this study, we performed site-directed mutagenesis on residue 36 of JEV M protein and showed that the resulting mutation had little or no effect on the entry process but greatly affected virus assembly in mammalian cells. Interestingly, this mutant virus had a host-dependent phenotype and could develop a wild-type infection in insect cells. Experiments performed on infectious virus as well as in a virus-like particle (VLP) system indicate that the JEV mutant expresses structural proteins but fails to form infectious particles in mammalian cells. Using a mouse model for JEV pathogenesis, we showed that the mutation conferred complete attenuation in vivo. The production of JEV neutralizing antibodies in challenged mice was indicative of the immunogenicity of the mutant virus in vivo. Together, our results indicate that the introduction of a single mutation in the M protein, while being tolerated in insect cells, strongly impacts JEV infection in mammalian hosts.

IMPORTANCE

JEV is a mosquito-transmitted flavivirus and is a medically important pathogen in Asia. The M protein is thought to be important for accommodating the structural rearrangements undergone by the virion during viral assembly and may play additional roles in the JEV infectious cycle. In the present study, we show that a sole mutation in the M protein impairs the JEV infection cycle in mammalian hosts but not in mosquito cells. This finding highlights differences in flavivirus assembly pathways among hosts. Moreover, infection of mice indicated that the mutant was completely attenuated and triggered a strong immune response to JEV, thus providing new insights for further development of JEV vaccines.

Viroporins: structure, function and potential as antiviral targets

The channel-forming activity of a family of small, hydrophobic integral membrane proteins termed 'viroporins' is essential to the life cycles of an increasingly diverse range of RNA and DNA viruses, generating significant interest in targeting these proteins for antiviral development. Viroporins vary greatly in terms of their atomic structure and can perform multiple functions during the virus life cycle, including those distinct from their role as oligomeric membrane channels. Recent progress has seen an explosion in both the identification and understanding of many such proteins encoded by highly significant pathogens, yet the prototypic M2 proton channel of influenza A virus remains the only example of a viroporin with provenance as an antiviral drug target. This review attempts to summarize our current understanding of the channel-forming functions for key members of this growing family, including recent progress in structural studies and drug discovery research, as well as novel insights into the life cycles of many viruses revealed by a requirement for viroporin activity. Ultimately, given the successes of drugs targeting ion channels in other areas of medicine, unlocking the therapeutic potential of viroporins represents a valuable goal for many of the most significant viral challenges to human and animal health.

The I22V and L72S substitutions in West Nile virus prM protein promote enhanced prM/E heterodimerisation and nucleocapsid incorporation

Background

Amino acid substitutions I22V and L72S in the prM protein of West Nile virus Kunjin strain (WNV_KUN) were previously shown to enhance virus secretion and virulence, but a mechanism by which this occurred was not determined.

Findings

Using pulse-chase experiments followed by co-immunoprecipitation with anti-E antibody, we demonstrated that the I22V and L72S substitutions enhanced prM/E heterodimerization for both the E-glycosylated and E-unglycosylated virus. Furthermore, analysis of secreted particles revealed that I22V and L72S substitutions also enhanced nucleocapsid incorporation.

Conclusions

We have demonstrated mechanistically that improved secretion of virus particles in the presence of I22V and L72S substitutions was contributed by more efficient prM/E heterodimerization.

KDEL Receptors Assist Dengue Virus Exit from the Endoplasmic Reticulum

Membrane receptors at the surface of target cells are key host factors for virion entry; however, it is unknown whether trafficking and secretion of progeny virus requires host intracellular receptors. In this study, we demonstrate that dengue virus (DENV) interacts with KDEL receptors (KDELR), which cycle between the ER and Golgi apparatus, for vesicular transport from ER to Golgi. Depletion of KDELR by siRNA reduced egress of both DENV progeny and recombinant subviral particles (RSPs). Coimmunoprecipitation of KDELR with dengue structural protein prM required three positively charged residues at the N terminus, whose mutation disrupted protein interaction and inhibited RSP transport from the ER to the Golgi. Finally, siRNA depletion of class II Arfs, which results in KDELR accumulation in the Golgi, phenocopied results obtained with mutagenized prME and KDELR knockdown. Our results have uncovered a function for KDELR as an internal receptor involved in DENV trafficking.

Post-translational regulation and modifications of flavivirus structural proteins

Flaviviruses are a group of single-stranded, positive-sense RNA viruses that generally circulate between arthropod vectors and susceptible vertebrate hosts, producing significant human and veterinary disease burdens. Intensive research efforts have broadened our scientific understanding of the replication cycles of these viruses and have revealed several elegant and tightly co-ordinated post-translational modifications that regulate the activity of viral proteins. The three structural proteins in particular - capsid (C), pre-membrane (prM) and envelope (E) - are subjected to strict regulatory modifications as they progress from translation through virus particle assembly and egress. The timing of proteolytic cleavage events at the C-prM junction directly influences the degree of genomic RNA packaging into nascent virions. Proteolytic maturation of prM by host furin during Golgi transit facilitates rearrangement of the E proteins at the virion surface, exposing the fusion loop and thus increasing particle infectivity. Specific interactions between the prM and E proteins are also important for particle assembly, as prM acts as a chaperone, facilitating correct conformational folding of E. It is only once prM/E heterodimers form that these proteins can be secreted efficiently. The addition of branched glycans to the prM and E proteins during virion transit also plays a key role in modulating the rate of secretion, pH sensitivity and infectivity of flavivirus particles. The insights gained from research into post-translational regulation of structural proteins are beginning to be applied in the rational design of improved flavivirus vaccine candidates and make attractive targets for the development of novel therapeutics.

Sensing of immature particles produced by dengue virus infected cells induces an antiviral response by plasmacytoid dendritic cells

Dengue virus (DENV) is the leading cause of mosquito-borne viral illness and death in humans. Like many viruses, DENV has evolved potent mechanisms that abolish the antiviral response within infected cells. Nevertheless, several in vivo studies have demonstrated a key role of the innate immune response in controlling DENV infection and disease progression. Here, we report that sensing of DENV infected cells by plasmacytoid dendritic cells (pDCs) triggers a robust TLR7-dependent production of IFNα, concomitant with additional antiviral responses, including inflammatory cytokine secretion and pDC maturation. We demonstrate that unlike the efficient cell-free transmission of viral infectivity, pDC activation depends on cell-to-cell contact, a feature observed for various cell types and primary cells infected by DENV, as well as West Nile virus, another member of the Flavivirus genus. We show that the sensing of DENV infected cells by pDCs requires viral envelope protein-dependent secretion and transmission of viral RNA. Consistently with the cell-to-cell sensing-dependent pDC activation, we found that DENV structural components are clustered at the interface between pDCs and infected cells. The actin cytoskeleton is pivotal for both this clustering at the contacts and pDC activation, suggesting that this structural network likely contributes to the transmission of viral components to the pDCs. Due to an evolutionarily conserved suboptimal cleavage of the precursor membrane protein (prM), DENV infected cells release uncleaved prM containing-immature particles, which are deficient for membrane fusion function. We demonstrate that cells releasing immature particles trigger pDC IFN response more potently than cells producing fusion-competent mature virus. Altogether, our results imply that immature particles, as a carrier to endolysosome-localized TLR7 sensor, may contribute to regulate the progression of dengue disease by eliciting a strong innate response.

Highly conserved residues in the helical domain of dengue virus type 1 precursor membrane protein are involved in assembly, precursor membrane (prM) protein cleavage, and entry

The envelope and precursor membrane (prM) proteins of dengue virus (DENV) are present on the surface of immature virions. During maturation, prM protein is cleaved by furin protease into pr peptide and membrane (M) protein. Although previous studies mainly focusing on the pr region have identified several residues important for DENV replication, the functional role of M protein, particularly the α-helical domain (MH), which is predicted to undergo a large conformational change during maturation, remains largely unknown. In this study, we investigated the role of nine highly conserved MH domain residues in the replication cycle of DENV by site-directed mutagenesis in a DENV1 prME expression construct and found that alanine substitutions introduced to four highly conserved residues at the C terminus and one at the N terminus of the MH domain greatly affect the production of both virus-like particles and replicon particles. Eight of the nine alanine mutants affected the entry of replicon particles, which correlated with the impairment in prM cleavage. Moreover, seven mutants were found to have reduced prM-E interaction at low pH, which may inhibit the formation of smooth immature particles and exposure of prM cleavage site during maturation, thus contributing to inefficient prM cleavage. Taken together, these results are the first report showing that highly conserved MH domain residues, located at 20-38 amino acids downstream from the prM cleavage site, can modulate the prM cleavage, maturation of particles, and virus entry. The highly conserved nature of these residues suggests potential targets of antiviral strategy.

Glutamic acid at residue 125 of the prM helix domain interacts with positively charged amino acids in E protein domain II for Japanese encephalitis virus-like-particle production

Interaction between E and prM proteins in flavivirus-infected cells is a major factor for virus-like particle (VLP) production. The prM helical (prM-H) domain is topologically close to and may interact with domain II of the E protein (EDII). In this study, we investigated prM-H domain amino acid residues facing Japanese encephalitis virus EDII using site-directed mutagenesis to determine their roles in prM-E interaction and VLP production. Our results indicate that negatively charged prM-E125 residue at the prM-H domain affected VLP production via one or more interactions with positively charged E-K93 and E-H246 residues at EDII. Exchanges of oppositely charged residue side chains at prM-E125/E-K93 and prM-E125/E-H246 are recoverable for VLP production. The prM-E125 and E-H246 residues are conserved and that the positive charge of the E-K93 residue is preserved in different flavivirus groups. These findings suggest that the electrostatic attractions of prM-E125, E-K93, and E-H246 residues are important to flavivirus VLP production and that inhibiting these interactions is a potential strategy for blocking flavivirus infections.

IMPORTANCE

Molecular interaction between E and prM proteins of Japanese encephalitis virus is a major driving force for virus-like particle (VLP) production. The current high-resolution structures available for prM-E complexes do not include the membrane proximal stem region of prM. The prM stem region contains an N-terminal loop and a helix domain (prM-H). Since the prM-H domain is topologically close to domain II of the E protein (EDII), this study was to determine molecular interactions between the prM-H domain and EDII. We found that the molecular interactions between prM-E125 residue and positively charged E-K93 and E-H246 residues at EDII are critical for VLP production. More importantly, the prM-E125 and E-H246 residues are conserved and the positive charge of the E-K93 residue is preserved in different flavivirus groups. Our findings help refine the structure and molecular interactions on the flavivirus surface and reveal a potential strategy for blocking flavivirus infections by inhibiting these electrostatic interactions.

Identification of residues in West Nile virus pre-membrane protein that influence viral particle secretion and virulence

The pre-membrane protein (prM) of West Nile virus (WNV) functions as a chaperone for correct folding of the envelope (E) protein, and prevents premature fusion during virus egress. However, little is known about its role in virulence. To investigate this, we compared the amino acid sequences of prM between a highly virulent North American strain (WNV(NY99)) and a weakly virulent Australian subtype (WNV(KUN)). Five amino acid differences occur in WNV(NY99) compared with WNV(KUN) (I22V, H43Y, L72S, S105A and A156V). When expressed in mammalian cells, recombinant WNV(NY99) prM retained native antigenic structure, and was partially exported to the cell surface. In contrast, WNV(KUN) prM (in the absence of the E protein) failed to express a conserved conformational epitope and was mostly retained at the pre-Golgi stage. Substitutions in residues 22 (Ile to Val) and 72 (Leu to Ser) restored the antigenic structure and cell surface expression of WNV(KUN) prM to the same level as that of WNV(NY99), and enhanced the secretion of WNV(KUN) prME particles when expressed in the presence of E. Introduction of the prM substitutions into a WNV(KUN) infectious clone (FLSDX) enhanced the secretion of infectious particles in Vero cells, and enhanced virulence in mice. These findings highlight the role of prM in viral particle secretion and virulence, and suggest the involvement of the L72S and I22V substitutions in modulating these activities.

A conserved region in the prM protein is a critical determinant in the assembly of flavivirus particles

Flaviviruses are assembled to bud into the lumen of the endoplasmic reticulum (ER) and are secreted through the vesicle transport pathway, but the details of the molecular mechanism of virion assembly remain largely unknown. In this study, a highly conserved region in the prM protein was identified among flaviviruses. In the subviral particle (SP) system of tick-borne encephalitis virus (TBEV) and Japanese encephalitis virus, secretion of SPs was impaired by a mutation in the conserved region in the prM protein. Viral proteins were sparse in the Golgi complex and accumulated in the ER. Ultrastructural analysis revealed that long filamentous structures, rather than spherical SPs, were observed in the lumen of the ER as a result of the mutation. The production of infectious virions derived from infectious cDNA of TBEV was also reduced by mutations in the conserved region. Molecular modelling analysis suggested that the conserved region is important for the association of prM-envelope protein heterodimers in the formation of a spike of immature virion. These results are the first demonstration that the conserved region in the prM protein is a molecular determinant for the flavivirus assembly process.

Lectin switching during dengue virus infection

Dengue virus receptors are relatively poorly characterized, but there has been recent interest in 2 C-type lectin molecules, dendritic cell-specific intercellular adhesion molecule 3 (ICAM-3)-grabbing nonintegrin (DC-SIGN) and its close homologue liver/lymph node-specific ICAM-3-grabbing integrin (L-SIGN), which can both bind dengue and promote infection. In this report we have studied the interaction of dengue viruses produced in insect cells, tumor cell lines, and primary human dendritic cells (DCs) with DC-SIGN and L-SIGN. Virus produced in primary DCs is unable to interact with DC-SIGN but remains infectious for L-SIGN-expressing cells. Skin-resident DCs may thus be a site of initial infection by insect-produced virus, but DCs will likely not participate in large-scale virus replication during dengue infection. These results reveal that differential glycosylation of dengue virus envelope protein is highly dependent on cell state and suggest that studies of virus tropism using virus prepared in insect cells or tumor cell lines should be interpreted with caution.

The C-terminal helical domain of dengue virus precursor membrane protein is involved in virus assembly and entry

The role of the α-helical domain (MH) of dengue virus (DENV) precursor membrane protein in replication was investigated by site-directed mutagenesis. Proline substitutions of three residues (120, 123 and 127) at the C-terminus, but not those at the N-terminus of MH domain, reduced the virus-like particles of DENV1, DENV2 and DENV4 detected in supernatants. In a DENV2 replicon trans-packaging system, these three mutations suppressed particles detected; two of them (I123P and V127P) also affected viral entry. In the context of DENV2 genome-length RNA, all three mutations reduced virion assembly and virus spreading in cell culture. Analysis of revertants showed that mutation A120P could partially support viral infection cycle; in contrast, mutations I123P and V127P were lethal, and adaptations of I123P→I123L and V127P→V127L were required to restore the viral infection cycle. These findings demonstrate that the C-terminus of the MH domain is involved in both assembly and entry of DENV.

Recent developments in understanding dengue virus replication

Dengue is the most important cause of mosquito-borne virus diseases in tropical and subtropical regions in the world. Severe clinical outcomes such as dengue hemorrhagic fever and dengue shock syndrome are potentially fatal. The epidemiology of dengue has undergone profound changes in recent years, due to several factors such as expansion of the geographical distribution of the insect vector, increase in traveling, and demographic pressure. As a consequence, the incidence of dengue has increased dramatically. Since mosquito control has not been successful and since no vaccine or antiviral treatment is available, new approaches to this problem are needed. Consequently, an in-depth understanding of the molecular and cellular biology of the virus should be helpful to design efficient strategies for the control of dengue. Here, we review the recently acquired knowledge on the molecular and cell biology of the dengue virus life cycle based on newly developed molecular biology technologies.

Dengue virus life cycle: viral and host factors modulating infectivity

Dengue virus (DENV 1-4) represents a major emerging arthropod-borne pathogen. All four DENV serotypes are prevalent in the (sub) tropical regions of the world and infect 50-100 million individuals annually. Whereas the majority of DENV infections proceed asymptomatically or result in self-limited dengue fever, an increasing number of patients present more severe manifestations, such as dengue hemorrhagic fever and dengue shock syndrome. In this review we will give an overview of the infectious life cycle of DENV and will discuss the viral and host factors that are important in controlling DENV infection.

Characterization of the GXXXG motif in the first transmembrane segment of Japanese encephalitis virus precursor membrane (prM) protein

The interaction between prM and E proteins in flavivirus-infected cells is a major driving force for the assembly of flavivirus particles. We used site-directed mutagenesis to study the potential role of the transmembrane domains of the prM proteins of Japanese encephalitis virus (JEV) in prM-E heterodimerization as well as subviral particle formation. Alanine insertion scanning mutagenesis within the GXXXG motif in the first transmembrane segment of JEV prM protein affected the prM-E heterodimerization; its specificity was confirmed by replacing the two glycines of the GXXXG motif with alanine, leucine and valine. The GXXXG motif was found to be conserved in the JEV serocomplex viruses but not other flavivirus groups. These mutants with alanine inserted in the two prM transmembrane segments all impaired subviral particle formation in cell cultures. The prM transmembrane domains of JEV may play importation roles in prM-E heterodimerization and viral particle assembly.

Efficient assembly and secretion of recombinant subviral particles of the four dengue serotypes using native prM and E proteins

Background

Flavivirus infected cells produce infectious virions and subviral particles, both of which are formed by the assembly of prM and E envelope proteins and are believed to undergo the same maturation process. Dengue recombinant subviral particles have been produced in cell cultures with either modified or chimeric proteins but not using the native forms of prM and E.

Methodology/Principal Findings

We have used a codon optimization strategy to obtain an efficient expression of native viral proteins and production of recombinant subviral particles (RSPs) for all four dengue virus (DV) serotypes. A stable HeLa cell line expressing DV1 prME was established (HeLa-prME) and RSPs were analyzed by immunofluorescence and transmission electron microscopy. We found that E protein is mainly present in the endoplasmic reticulum (ER) where assembly of RSPs could be observed. Biochemical characterization of DV1 RSPs secretion revealed both prM protein cleavage and homodimerization of E proteins before their release into the supernatant, indicating that RSPs undergo a similar maturation process as dengue virus. Pulse chase experiment showed that 8 hours are required for the secretion of DV1 RSPs. We have used HeLa-prME to develop a semi-quantitative assay and screened a human siRNA library targeting genes involved in membrane trafficking. Knockdown of 23 genes resulted in a significant reduction in DV RSP secretion, whereas for 22 others we observed an increase of RSP levels in cell supernatant.

Conclusions/Significance

Our data describe the efficient production of RSPs containing native prM and E envelope proteins for all dengue serotypes. Dengue RSPs and corresponding producing cell lines are safe and novel tools that can be used in the study of viral egress as well as in the development of vaccine and drugs against dengue virus.

Differential modulation of prM cleavage, extracellular particle distribution, and virus infectivity by conserved residues at nonfurin consensus positions of the dengue virus pr-M junction

In the generation of flavivirus particles, an internal cleavage of the envelope glycoprotein prM by furin is required for the acquisition of infectivity. Unlike cleavage of the prM of other flaviviruses, cleavage of dengue virus prM is incomplete in many cell lines; the partial cleavage reflects the influence of residues at furin nonconsensus positions of the pr-M junction, as flaviviruses share basic residues at positions P1, P2, and P4, recognized by furin. In this study, viruses harboring the alanine-scanning and other multiple-point mutations of the pr-M junction were generated, employing a dengue virus background that exhibited 60 to 70% prM cleavage and a preponderance of virion-sized extracellular particles. Analysis of prM and its cleavage products in viable mutants revealed a cleavage-suppressive effect at the conserved P3 Glu residue, as well as the cleavage-augmenting effects at the P5 Arg and P6 His residues, indicating an interplay between opposing modulatory influences mediated by these residues on the cleavage of the pr-M junction. Changes in the prM cleavage level were associated with altered proportions of extracellular virions and subviral particles; mutants with reduced cleavage were enriched with subviral particles and prM-containing virions, whereas the mutant with enhanced cleavage was deprived of these particles. Alterations of virus multiplication were detected in mutants with reduced prM cleavage and were correlated with their low specific infectivities. These findings define the functional roles of charged residues located adjacent to the furin consensus sequence in the cleavage of dengue virus prM and provide plausible mechanisms by which the reduction in the pr-M junction cleavability may affect virus replication.

Reverse genetics and the study of dengue virus

Dengue virus infection causes the most important arthropod-borne disease of humans. Currently, there are no dengue vaccines or antiviral therapies in clinical use, although their development is a global health priority. Using a technique known as ‘reverse genetics’, the dengue virus RNA genome can be manipulated, either by the introduction of specific mutations or the deletion and/or substitution of entire genes. This has led to the production of novel recombinant viruses that have potential as vaccines and the production of noninfectious viral subgenomes (termed replicons) useful for drug screening. Reverse genetics is also an invaluable tool for studying the role of dengue virus RNA elements and proteins in replication and pathogenesis. This review describes the contribution of reverse genetics to dengue virus research to date, highlighting the potential use of this technology in the development of effective control measures against dengue in the future.

A single M protein mutation affects the acid inactivation threshold and growth kinetics of a chimeric flavivirus

Numerous viruses of the Flaviviridae family, including dengue, yellow fever, Japanese encephalitis, and West Nile, cause significant disease in humans and animals. The structure and function of the molecular components of the flavivirus envelope are therefore of significant interest. To our knowledge, a membrane (M) protein mutation which affects the pH at which flavivirus particles are inactivated in vitro has never been reported. Here we show that substitution of proline for glutamine at residue M5 (MQ5P) of a Japanese encephalitis-yellow fever chimera (ChimeriVax-JE) increases its acid sensitivity in vitro by 0.3 pH units (i.e., increases the pH at which virus titer is reduced by 50% from 6.08 to 6.38). In addition, growth kinetics of this mutant virus are accelerated in Vero cells, while neurovirulence and neuroinvasiveness measured in a mouse model are unaffected. A possible interpretation of these observations is that M can modulate the envelope (E) protein function during cell infection.

The four serotypes of dengue recognize the same putative receptors in Aedes aegypti midgut and Ae. albopictus cells

Background

Dengue viruses (DENV) attach to the host cell surface and subsequently enter the cell by receptor-mediated endocytosis. Several primary and low affinity co-receptors for this flavivirus have been identified. However, the presence of these binding molecules on the cell surface does not necessarily render the cell susceptible to infection. Determination of which of them serve as bona fide receptors for this virus in the vector may be relevant to treating DENV infection and in designing control strategies.

Results

(1) Overlay protein binding assay showed two proteins with molecular masses of 80 and 67 kDa (R80 and R67). (2) Specific antibodies against these two proteins inhibited cell binding and infection. (3) Both proteins were bound by all four serotypes of dengue virus. (4) R80 and R67 were purified by affinity chromatography from Ae. aegypti mosquito midguts and from Ae albopictus C6/36 cells. (5) In addition, a protein with molecular mass of 57 kDa was purified by affinity chromatography from the midgut extracts. (6) R80 and R67 from radiolabeled surface membrane proteins of C6/36 cells were immunoprecipitated by antibodies against Ae. aegypti midgut.

Conclusion

Our results strongly suggest that R67 and R80 are receptors for the four serotypes of dengue virus in the midgut cells of Ae. aegypti and in C6/36 Ae. albopictus cells.

Histidine at residue 99 and the transmembrane region of the precursor membrane prM protein are important for the prM-E heterodimeric complex formation of Japanese encephalitis virus

The formation of the flavivirus prM-E complex is an important step for the biogenesis of immature virions, which is followed by a subsequent cleavage of prM to M protein through cellular protease to result in the production and release of mature virions. In this study, the intracellular formation of the prM-E complex of Japanese encephalitis virus was investigated by baculovirus coexpression of prM and E in trans in Sf9 insect cells as analyzed by anti-E antibody immunoprecipitation and sucrose gradient sedimentation analysis. A series of carboxyl-terminally truncated prM mutant baculoviruses was constructed to demonstrate that the truncations of the transmembrane (TM) region resulted in a reduction of the formation of the stable prM-E complex by approximately 40% for the TM1 (at residues 130 to 147 [prM130-147]) truncation and 20% for TM2 (at prM153-167) truncation. Alanine-scanning site-directed mutagenesis on the prM99-103 region indicated that the His99 residue was the critical prM binding element for stable prM-E heterodimeric complex formation. The single amino acid mutation at the His99 residue of prM abolishing the prM-E interaction was not due to reduced expression or different subcellular location of the mutant prM protein involved in prM-E interactions as characterized by pulse-chase labeling and confocal scanning microscopic analysis. Recombinant subviral particles were detected in the Sf9 cell culture supernatants by baculovirus coexpression of prM and E proteins but not with the prM H99A mutant. Sequence alignment analysis was further conducted with different groups of flaviviruses to show that the prM H99 residues are generally conserved. Our findings are the first report to characterize the minimum binding elements of the prM protein that are involved in prM-E interactions of flaviviruses. This information, concerning a molecular framework for the prM protein, is considered to elucidate the structure/function relationship of the prM-E complex synthesis and provide the proper trajectory for flavivirus assembly and maturation.