A comparative study of entry modes into C6/36 cells by Semliki Forest and Japanese encephalitis viruses

Dengue Virus Glycosylation: What Do We Know?

In many infectious diseases caused by either viruses or bacteria, pathogen glycoproteins play important roles during the infection cycle, ranging from entry to successful intracellular replication and host immune evasion. Dengue is no exception. Dengue virus glycoproteins, envelope protein (E) and non-structural protein 1 (NS1) are two popular sub-unit vaccine candidates. E protein on the virion surface is the major target of neutralizing antibodies. NS1 which is secreted during DENV infection has been shown to induce a variety of host responses through its binding to several host factors. However, despite their critical role in disease and protection, the glycosylated variants of these two proteins and their biological importance have remained understudied. In this review, we seek to provide a comprehensive summary of the current knowledge on protein glycosylation in DENV, and its role in virus biogenesis, host cell receptor interaction and disease pathogenesis.

Replication of alphaviruses: a review on the entry process of alphaviruses into cells

Alphaviruses are small, enveloped viruses, ~70 nm in diameter, containing a single-stranded, positive-sense, RNA genome. Viruses belonging to this genus are predominantly arthropod-borne viruses, known to cause disease in humans. Their potential threat to human health was most recently exemplified by the 2005 Chikungunya virus outbreak in La Reunion, highlighting the necessity to understand events in the life-cycle of these medically important human pathogens. The replication and propagation of viruses is dependent on entry into permissive cells. Viral entry is initiated by attachment of virions to cells, leading to internalization, and uncoating to release genetic material for replication and propagation. Studies on alphaviruses have revealed entry via a receptor-mediated, endocytic pathway. In this paper, the different stages of alphavirus entry are examined, with examples from Semliki Forest virus, Sindbis virus, Chikungunya virus, and Venezuelan equine encephalitis virus described.

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.

Both E protein glycans adversely affect dengue virus infectivity but are beneficial for virion release

The E protein of most flaviviruses is modified by Asn-linked glycosylation at residue 153/154 and in the case of the four dengue virus (DENV) serotypes by a second glycan at residue 67. However, the absence of E protein glycosylation among numerous natural isolates of different flaviviruses suggests that the glycan, per se, is not critically important in the virus life cycle. Consistent with this notion, we show that ablation of both glycans from the DENV-2 E protein reduces but does not prevent growth of the variant in mammalian and mosquito cells. We found a pronounced and opposing effect of glycan ablation on two stages of the virus growth cycle: infectivity and release. Loss of either of the two DENV E protein glycans markedly enhanced infectivity of variants for mosquito cells at the expense of efficient virion release. The variants also displayed reduced release in mammalian cells, which was more prominent for viruses lacking the Asn 67-linked glycan than for those lacking the Asn 153-linked glycan, without a marked change in infectivity. Mutations, which compensated for the defect in virus morphogenesis associated with ablation of the Asn 67-linked glycan in mammalian cells but interestingly not in mosquito cells, were identified at the glycosylation acceptor motif and a second site in E protein domain II. The dueling influences of infectivity and release on virus growth affected by the glycans may explain the plasticity in E protein glycosylation among the flaviviruses.

Characterization of dengue virus entry into HepG2 cells

Background

Despite infections by the dengue virus being a significant problem in tropical and sub-tropical countries, the mechanism by which the dengue virus enters into mammalian cells remains poorly described.

Methods

A combination of biochemical inhibition, dominant negative transfection of Eps15 and siRNA mediated gene silencing was used to explore the entry mechanism of dengue into HepG2 cells.

Results

Results were consistent with entry via multiple pathways, specifically via clathrin coated pit mediated endocytosis and macropinocytosis, with clathrin mediated endocytosis being the predominant pathway.

Conclusion

We propose that entry of the dengue virus to mammalian cells can occur by multiple pathways, and this opens the possibility of the virus being directed to multiple cellular compartments. This would have significant implications in understanding the interaction of the dengue virus with the host cell machinery.

Conformational changes in Sindbis virions resulting from exposure to low pH and interactions with cells suggest that cell penetration may occur at the cell surface in the absence of membrane fusion

Alphaviruses have the ability to induce cell-cell fusion after exposure to acid pH. This observation has served as an article of proof that these membrane-containing viruses infect cells by fusion of the virus membrane with a host cell membrane upon exposure to acid pH after incorporation into a cell endosome. We have investigated the requirements for the induction of virus-mediated, low pH-induced cell-cell fusion and cell-virus fusion. We have correlated the pH requirements for this process to structural changes they produce in the virus by electron cryo-microscopy. We found that exposure to acid pH was required to establish conditions for membrane fusion but that membrane fusion did not occur until return to neutral pH. Electron cryo-microscopy revealed dramatic changes in the structure of the virion as it was moved to acid pH and then returned to neutral pH. None of these treatments resulted in the disassembly of the virus protein icosahedral shell that is a requisite for the process of virus membrane-cell membrane fusion. The appearance of a prominent protruding structure upon exposure to acid pH and its disappearance upon return to neutral pH suggested that the production of a "pore"-like structure at the fivefold axis may facilitate cell penetration as has been proposed for polio (J. Virol. 74 (2000) 1342) and human rhino virus (Mol. Cell 10 (2002) 317). This transient structural change also provided an explanation for how membrane fusion occurs after return to neutral pH. Examination of virus-cell complexes at neutral pH supported the contention that infection occurs at the cell surface at neutral pH by the production of a virus structure that breaches the plasma membrane bilayer. These data suggest an alternative route of infection for Sindbis virus that occurs by a process that does not involve membrane fusion and does not require disassembly of the virus protein shell.

The membrane proteins of flaviviruses form ion-permeable pores in the target membrane after fusion: identification of the pores and analysis of their possible role in virus infection

Recently, we presented evidence that the E1 fusion protein of the alphavirus Semliki Forest virus forms ion-permeable pores in the target membrane after fusion. We proposed that the homologous fusion proteins of flaviviruses and hepatitis C virus form similar pores. To test this hypothesis for the E fusion protein of flaviviruses, the release of [(3)H]choline from liposomes by the flavivirus West Nile (WN) virus was determined. [(3)H]Choline was released at mildly acid pH. The pH threshold depended on the lipid composition. Release from certain liposomes was activated even at neutral pH. To identify the generation of individual pores, single cells were investigated with the patch-clamp technique. The formation of individual pores during low pH-induced WN virus entry at the plasma membrane occurred within seconds. These experiments were performed in parallel with Semliki Forest virus. The results indicated that, similar to alphavirus infection, infection with flaviviruses via endosomes leads to the formation of ion-permeable pores in the endosome after fusion, which allows the flow of protons from the endosome into the cytoplasm during virus entry. However, in vitro translation experiments of viral cores showed that, in contrast to alphaviruses, which probably need this proton flow for core disassembly, the genome RNA of WN virus present in the viral core is directly accessible for translation. For entry of flaviviruses, therefore, a second pathway for productive infection may exist, in which fusion of the viral membrane is activated at neutral pH by contact with a plasma membrane of appropriate lipid composition.

Prefusion rearrangements resulting in fusion Peptide exposure in Semliki forest virus

Semliki Forest virus (SFV), like many enveloped viruses, takes advantage of the low pH in the endosome to convert into a fusion-competent configuration and complete infection by fusion with the endosomal membrane. Unlike influenza virus, carrying an N-terminal fusion peptide, SFV represents a less-well understood fusion principle involving an endosequence fusion peptide. To explore the series of events leading to a fusogenic configuration of the SFV, we exposed the virus to successive acidification, mimicking endosomal conditions, and followed structural rearrangements at probed sensor surfaces. Thus revealed, the initial phase involves a transient appearance of a non-linear neutralizing antibody epitope in the fusion protein, E1. Concurrent with the disappearance of this epitope, a set of masked sequences in proteins E1 and E2 became exposed. When pH reached 6.0-5.9 the virion transformed into a configuration of enlarged diameter with the fusion peptide optimally exposed. Simultaneously, a partly hidden sequence close to the receptor binding site in E2 became fully uncovered. At this presumably fusogenic stage, maximally 80 fusion peptide-identifying antibody Fab fragments could be bound per virion, i.e. one ligand per three copies of the fusion protein. The phenomena observed are discussed in terms of alphavirus structure and reported functional domains.

Mutations in the NS3 gene and 3'-NCR of Japanese encephalitis virus isolated from an unconventional ecosystem and implications for natural attenuation of the virus

The T1P1 strain of Japanese encephalitis (JE) virus was recently isolated from paddy-free Liu-Chiu Islet in which natural JE antibody has been prevalent. In mouse neuroblastoma-derived Neuro-2a cells, T1P1 appeared significantly lower in virus productivity than another local isolate, CH1392. It implied that this new isolate possesses a characteristic viral replication pattern other than that of CH1392. T1P1 has also shown lower neurovirulence, which was reflected by a significantly higher LD(50) (2.44 x 10(6) PFU) than CH1392 (2.87 x 10(2) PFU). In comparison of the full-length RNA sequences between T1P1 and CH1392, a total of 7 nucleotides, including 1 in preM/M and 2 each in NS3, NS5, and the 3'-end noncoding region (NCR), appeared different. Of them, only the changes in NS3 (position 325, T for CH1392, A for T1P1; and position 364, G for CH1392 and A for T1P1) resulted in substitutions of deduced amino acids. There were two additional nucleotide changes appearing in the 3'-NCR. The amino acids 109 Phe and 122 Glu in NS3 of CH1392 were substituted by Ile and Lys, respectively, in T1P1. The unique growth properties and low virulence of T1P1 presented in this report were likely related to abnormal enzymatic activity due to mutations of the NS3 gene (especially position 364) and possibly to the mutations in the 3'-NCR. The natural attenuation of T1P1 that has been circulating in paddy-free Liu-Chiu Islet may account for the absence of clinical JE cases in past years.

Heparan sulfate proteoglycans initiate dengue virus infection of hepatocytes

Dengue viruses (DEN) cause a broad spectrum of clinical manifestations including potentially life-threatening conditions such as hemorrhagic shock syndrome and less frequently acute hepatitis with liver failure and encephalopathy. In addition, dengue viruses provide a potential model to understand the initiation of hepatocyte infection by the structurally closely related hepatitis C virus (HCV), because this virus at present cannot be grown in cell culture. Although the initial steps of viral infection are a critical determinant of tissue tropism and therefore pathogenesis, little is known about the molecular basis of binding and endocytic trafficking of DEN or of any other flavivirus. Our studies revealed that binding of radiolabeled DEN to the human hepatoma cell line HuH-7 was strictly pH dependent and substantially inhibitable by the glycosaminoglycan heparin. Ligand-blot analysis, performed as a viral overlay assay, showed two heparan sulfate (HS) containing cell-surface binding proteins resolving at 33 and 37 kd. Based on the sensitivity of unprotected virus and the viral binding site on the cell surface to trypsin, viral internalization was quantified as an increase in trypsin protected virus over time. Virus trafficking to the site of degradation was inhibited by pH dissociation of the clathrin coat and dependent on IP(3)-mediated homotypic endosomal fusion. These findings confirm the hypothesis that binding and internalization of DEN by hepatocytes are mediated primarily by HS containing proteoglycans and suggest that flaviviruses traffic the major clathrin-dependent endocytic pathway during infection.

The clathrin endocytic pathway in viral infection

How important is the clathrin-dependent endocytic pathway for entry of viruses into host cells? While it is widely accepted that Semliki Forest virus (SFV), an enveloped virus, requires this pathway there are conflicting data concerning the closely related Sindbis virus, as well as varying results with picornaviruses such as human rhinovirus 14 (HRV 14) and poliovirus. We have examined the entry mode of SFV, Sindbis virus, HRV 14 and poliovirus using a method that identifies single infected cells. This assay takes advantage of the observation that the clathrin-dependent endocytic pathway is specifically and potently arrested by overexpression of dynamin mutants that prevent clathrin-coated pit budding. Using HeLa cells and conditions of low multiplicity of infection to favor use of the most avid pathway of cell entry, it was found that SFV, Sindbis virus and HRV 14 require an active clathrin-dependent endocytic pathway for successful infection. In marked contrast, infection of HeLa cells by poliovirus did not appear to require the clathrin pathway.

The fragmentation of incoming Semliki Forest virus nucleocapsids in mosquito (Aedes albopictus) cells might be coupled to virion uncoating

The fate of Semliki Forest virus (SFV) nucleocapsid, especially the capsid protein (C-protein), was investigated during the early stages of a productive infection in mosquito Aedes albopictus cells. Infection of the cells resulted in a time dependent accumulation of a C-protein derived fragment. This fragmentation of incoming viral nucleocapsid was prevented by NH4Cl, an agent generally used to elevate the pH in acidic intracellular compartments, suggesting that a low intravesicular pH is required for this process. Density gradient analysis of the postnuclear cell lysate demonstrated that the fragmentation was associated with a cellular compartment showing a density of 1.14 +/- 0.02 g/ml. This cellular compartment was devoid from a lysosomal marker enzyme and represented the timely preceding cellular fraction through which SFV passed before encountering a lysosomal fraction. Furthermore, the intracellular distribution of the viral, 3H-uridine-labeled RNA suggested that the same fraction might represent a key cellular compartment in which the separation of the viral RNA from the viral structural proteins is primed. In conclusion, these data lead to the suggestion that the fragmentation of incoming SFV nucleocapsids in Aedes albopictus cells might be the part of the mechanism leading to the release of viral RNA into the cytosol during early stages of productive infection.

Comparison of replication rates and pathogenicities between the SA14 parent and SA14-14-2 vaccine strains of Japanese encephalitis virus in mouse brain neurons

The replication rates and pathogenicities of the SA 14 parent and SA 14-14-2 vaccine strains of Japanese encephalitis (JE) virus in neurons of the mouse brain following intracerebral inoculation were compared. All the mice inoculated with the SA 14 parent strain died within one week postinoculation (p.i.), whereas all the mice inoculated with the SA 14-14-2 vaccine strains survived without showing any signs of central nervous system (CNS) involvement. The virus titers of the mouse brains inoculated with the SA 14 strain reached progressively higher levels until day 5 when the animals died. On the other hand, the virus titers of the mouse brains inoculated with the SA 14-14-2 strain persisted at low levels for several days and could not be detected after 10 days. In the routine electron microscopical study, a majority of neurons in the mouse brains inoculated with the SA 14 strain contained virions and showed characteristic cytopathological changes in connection with viral replication. In the brains inoculated with the SA 14-14-2 strain, however, we failed to find neurons containing virions or showing characteristic cytopathological changes. In the alkaline phosphatase immunostaining of paraffin-embedded sections, a majority of neurons in the brains of mice inoculated with the SA 14 strain stained positively on day 5 p.i., but only a small number of neurons in scattered small foci stained positively in the brains inoculated with the SA 14-14-2 strain. The immunogold staining of Vibratome sections also revealed the identical patterns; moreover, electron microscopical examination of the immunopositive foci of the brain inoculated with the vaccine strain revealed neurons that contained virions in dilated cisternae of rough endoplasmic reticulum (RER), indicating that the SA 14-14-2 strain also replicated, albeit poorly, in neurons. The present results showed that upon intracerebral inoculation into mice the SA 14 parent strain of JE virus grew vigorously in a large number of neurons, killing the animals, while the SA 14-14-2 vaccine strain grew poorly only in a small number of neurons without causing mortality. Possible mechanisms involved in the alteration of pathogenicity between the SA 14 parent virus and the SA 14-14-2 vaccine virus are discussed.

Cytopathology of PC12 cells infected with Japanese encephalitis virus

Infection of a clonal rat pheochromocytoma cell line, PC12, with Japanese encephalitis (JE) virus produced successively higher titers of virus in the culture fluid during the 72-h experimental period. In electron microscopical observation, JE virus entered PC12 cells by direct penetration through the plasma membrane at 2 min postinoculation (p.i.) and caused marked cellular hypertrophy and extensive proliferation of the cellular secretory system including rough endoplasmic reticulum (RER) and Golgi complexes starting 24 h p.i. The proliferating RER of the virally infected cells contained progeny virions and characteristic endoplasmic reticulum vesicles in its cisternae, and the proliferating Golgi complexes contained virions in their saccules. These findings indicated that the proliferation of the cellular secretory system occurred in association with viral replication and maturation in the system. Seventy-two hours p.i., the cellular secretory system of infected PC12 cells showed degenerative changes with vesiculation, disorganization, and dispersion of the Golgi complexes and fragmentation, focal cystic dilation, and dissolution of the RER in the same manner as those seen in the secretory system of JE-virus-infected neurons in the mouse brain. Thus, JE-virus-infected PC12 cells seem to be a suitable neurogenic cell line for the study of the pathogenic mechanism of JE virus. At the same time, the virally infected cells seem to offer an interesting cell model for the study of the morphogenesis of the cellular secretory system.

pH-dependent fusion of tick-borne encephalitis virus with artificial membranes

pH-dependent fusion of TBE virus with artificial membranes was effective at slightly acidic pH with maximum at 6.4. The influence of various changes in E protein conformation on fusion process was studied.

Entry and replication of Japanese encephalitis virus in cultured neurogenic cells

The entry mode and growth pattern of Japanese encephalitis (JE) virus in mouse neuroblastoma N18TG2 cells and mouse neuroblastoma x rat glioma NG108-15 hybrid cells were studied by electron microscopy. At two minutes after inoculation, JE virions adsorbed onto and directly penetrated through the plasma membrane of the hybrid cells, whereas virions did not adsorb nor entered the neuroblastoma cells. Correspondingly, the hybrid cells showed assembling progeny JE virions in the cisternae of rough endoplasmic reticulum (RER) 1 day postinoculation (p.i.) although virions were rarely found on the following days during the experiment. On the other hand, progeny virions did not assemble in the RER cisternae of the neuroblastoma cells throughout the experiment. The morphologic observations, therefore, suggest that (a) the hybrid cells express JE-virus receptors which facilitate the viral attachment onto and entry into the cells, while the neuroblastoma cells do not and (b) JE virus replicates very poorly after the entry into the hybrid cells while it does not replicate at all in the neuroblastoma cells. The virus titrations of the media of the neuroblastoma and hybrid cell cultures showed only titers indicative of residual virus of the inoculum that progressively decreased during the experiment. The present results show therefore that of the two neurogenic cell culture lines studied only the hybrid cell line can be used for the study of viral entry and replication, although it is not suited for virus production. Possible reasons for the poor replication of JE virus in the hybrid cells are discussed.