Canine distemper virus infection requires cholesterol in the viral envelope

Pseudorabies virus upregulates low-density lipoprotein receptors to facilitate viral entry

Pseudorabies virus (PRV) is the causative agent of Aujeszky's disease in pigs. The low-density lipoprotein receptor (LDLR) is a transcriptional target of the sterol-regulatory element-binding proteins (SREBPs) and participates in the uptake of LDL-derived cholesterol. However, the involvement of LDLR in PRV infection has not been well characterized. We observed an increased expression level of LDLR mRNA in PRV-infected 3D4/21, PK-15, HeLa, RAW264.7, and L929 cells. The LDLR protein level was also upregulated by PRV infection in PK-15 cells and in murine lung and brain. The treatment of cells with the SREBP inhibitor, fatostatin, or with SREBP2-specific small interfering RNA prevented the PRV-induced upregulation of LDLR expression as well as viral protein expression and progeny virus production. This suggested that PRV activated SREBPs to induce LDLR expression. Furthermore, interference in LDLR expression affected PRV proliferation, while LDLR overexpression promoted it. This indicated that LDLR was involved in PRV infection. The study also demonstrated that LDLR participated in PRV invasions. The overexpression of LDLR or inhibition of proprotein convertase subtilisin/kexin type 9 (PCSK9), which binds to LDLR and targets it for lysosomal degradation, significantly enhanced PRV attachment and entry. Mechanistically, LDLR interacted with PRV on the plasma membrane, and pretreatment of cells with LDLR antibodies was able to neutralize viral entry. An in vivo study indicated that the treatment of mice with the PCSK9 inhibitor SBC-115076 promoted PRV proliferation. The data from the study indicate that PRV hijacks LDLR for viral entry through the activation of SREBPs.IMPORTANCEPseudorabies virus (PRV) is a herpesvirus that primarily manifests as fever, pruritus, and encephalomyelitis in various domestic and wild animals. Owing to its lifelong latent infection characteristics, PRV outbreaks have led to significant financial setbacks in the global pig industry. There is evidence that PRV variant strains can infect humans, thereby crossing the species barrier. Therefore, gaining deeper insights into PRV pathogenesis and developing updated strategies to contain its spread are critical. This study posits that the low-density lipoprotein receptor (LDLR) could be a co-receptor for PRV infection. Hence, strategies targeting LDLR may provide a promising avenue for the development of effective PRV vaccines and therapeutic interventions.

Lipid compartments and lipid metabolism as therapeutic targets against coronavirus

Lipids perform a series of cellular functions, establishing cell and organelles' boundaries, organizing signaling platforms, and creating compartments where specific reactions occur. Moreover, lipids store energy and act as secondary messengers whose distribution is tightly regulated. Disruption of lipid metabolism is associated with many diseases, including those caused by viruses. In this scenario, lipids can favor virus replication and are not solely used as pathogens' energy source. In contrast, cells can counteract viruses using lipids as weapons. In this review, we discuss the available data on how coronaviruses profit from cellular lipid compartments and why targeting lipid metabolism may be a powerful strategy to fight these cellular parasites. We also provide a formidable collection of data on the pharmacological approaches targeting lipid metabolism to impair and treat coronavirus infection.

Emodin and rhapontigenin inhibit the replication of African swine fever virus by interfering with virus entry

Africa swine fever (ASF) is a highly pathogenic contagion caused by African swine fever virus (ASFV), which not only affects the development of domestic pig industry, but also causes huge losses to the world agricultural economy. Vaccine development targeting ASFV remains elusive, which leads to severe difficulties in disease prevention and control. Emodin (EM) and rhapontigenin (RHAG), which are extracted from the dried rhizome of Polygonum knotweed, have various biological properties such as anti-neoplastic and anti-bacterial activities, but no studies have reported that they have anti-ASFV effects. This study discovered that EM and RHAG at different concentrations had a significant dose-dependent inhibitory effect on the ASFV GZ201801 strain in porcine alveolar macrophages (PAMs), and at the specified concentration, EM and RHAG showed continuous inhibition at 24 h, 48 h and 72 h. Not only did they strongly impact virion attachment and internalization, but also inhibit the early stages of ASFV replication. Further research proved that the expression level of Rab 7 protein was reduced by EM and RHAG, and treatments with EM and RHAG induced the accumulation of free cholesterol in endosomes and inhibited endosomal acidification, which prevented the virus from escaping and shelling from late endosomes. This study summarized the application of EM and RHAG in inhibiting ASFV replication in-vitro. Similarly, EM and RHAG targeted Rab 7 in the viral endocytosis pathway, inhibited viral infection, and induced the accumulation of cholesterol in the endosomes and the acidification of the endosomes to inhibit uncoating. A reference could be made to the results of this study when developing antiviral drugs and vaccines.

Development of a monoclonal antibody recognizing novel linear neutralizing epitope on H protein of canine distemper virus vaccine strains (America-1 genotype)

Canine distemper virus (CDV) is an economically important virus responsible for canine distemper (CD), a highly contagious disease that afflicts various animal species worldwide. The hemagglutinin (H) protein is the major neutralizing target of virus. Therefore, it is often considered as immunogen to prepare neutralizing antibodies. The accurate identification of neutralizing epitope will provide important antigenic information and extend the knowledge of mechanisms of virus neutralization. In this study, we generated a neutralizing monoclonal antibody (mAb) 4C6 against CDV H protein, and defined the minimal linear epitope 238DIEREFDT245, which was highly conserved in America-1 genotype of CDV strains (vaccines). The mAb 4C6 could not react with a CDV strain that had two substitutions of D238Y and R241G in the epitope, which appeared in most CDV strains of the other genotypes. Besides, a few different amino acid mutations in the epitope were also included. Collectively, the epitope 238DIEREFDT245 was variable in the other genotypes of CDV strains. The epitope 238DIEREFDT245 was exposed to the surface of CDV H protein, showing good antigenicity. These data will provide insights into structure, function and antigenicity of H protein and lay the foundation for the development of diagnostic technologies and vaccine design for CDV.

The H protein of attenuated canine distemper virus is degraded via endoplasmic reticulum-associated protein degradation

Canine distemper (CD) caused by canine distemper virus (CDV) is considered a highly contagious and acutely febrile disease in various animals around the world. Endoplasmic reticulum-associated protein degradation (ERAD) is an important biological effect induced by endoplasmic reticulum (ER) stress (ERS) for the degradation of unfolded/misfolded proteins in the ER of cells. CDV H glycoprotein is translocated into the ER for post-translational modifications. The effects of CDV H and ER on each other are unclear. In this study, we found that CDV H protein induced ERS through the PERK-mediated signaling pathway. The inhibition of ERS by 4-Phenylbutyric acid (4-PBA) increased the H protein amounts of an attenuated CDV, which was reduced by dithiothreitol (DTT)-induced ERS. Further, the H protein levels were increased when ERAD was inhibited by using Eeyarestatin I or interfering E3 ligase Hrd1 in ERAD, suggesting that the attenuated CDV H protein is degraded via ERAD. ERAD involved ubiquitin-dependent proteasome degradation (UPD) and/or autophagic-lysosome degradation (ALD). The attenuated CDV H protein was ubiquitinated and significantly increased after treatment with UPD inhibitor MG132 but not ALD inhibitor chloroquine (CQ), suggesting that ERAD degrading the attenuated CDV H protein selectively depends on UPD. Moreover, the inhibition of the degradation of CDV H protein with 4-PBA or MG132 treatment increased viral replication, whereas treatment with DTT promoting degradation of H protein was found to reduce viral replication. These findings suggest that the degradation of CDV H protein via ERAD negatively affects viral replication and provide a new idea for developing CDV prevention and control strategies.

Aminotransferasas y perfil lipídico en pacientes ecuatorianos con infección activa por virus dengue

El dengue es la arbovirosis con mayor incidencia a nivel mundial. Aproximadamente 100 millones de casos de dengue con signos de alarma y entre 250.000 y 500.000 casos de dengue grave, se registran anualmente. En Ecuador, en los últimos cuatro años se han registrado 83.472 casos de dengue. Estudios previos evidencian un incremento de los casos que cursan con disfunción hepática. El objetivo de este estudio fue analizar la asociación entre los niveles séricos de las enzimas aspartato aminotransferasa y alanino aminotransferasa y el perfil lipídico en pacientes con infección confirmada de Dengue. Se estudiaron 110 pacientes seleccionados sin distingo de edad, género o procedencia, cuyo diagnóstico fue confirmado virológica y serológicamente. Se incluyó un grupo control seronegativo al virus. En el perfil lipídico se evidenciaron diferencias significativas (p<0,003) en los valores de colesterol total y en infecciones secundarias; mientras que la frecuencia de elevación de ambas aminotransferasas fue alta en pacientes con dengue, no obstante, al comparar cuantitativamente los valores séricos no arrojaron cambios significativos, ni asociación. Se confirma la endemicidad del dengue, los cambios en el perfil lipídico, sin embargo, es evidente la necesidad de estudios poblacionales tomando en cuenta la genética de las poblaciones

Overcoming the Barrier of the Respiratory Epithelium during Canine Distemper Virus Infection

Canine distemper virus (CDV) is a highly contagious pathogen and is known to enter the host via the respiratory tract and disseminate to various organs. Current hypotheses speculate that CDV uses the homologous cellular receptors of measles virus (MeV), SLAM and nectin-4, to initiate the infection process. For validation, here, we established the well-differentiated air-liquid interface (ALI) culture model from primary canine tracheal airway epithelial cells. By applying the green fluorescent protein (GFP)-expressing CDV vaccine strain and recombinant wild-type viruses, we show that cell-free virus infects the airway epithelium mainly via the paracellular route and only after prior disruption of tight junctions by pretreatment with EGTA; this infection was related to nectin-4 but not to SLAM. Remarkably, when CDV-preinfected DH82 cells were cocultured on the basolateral side of canine ALI cultures grown on filter supports with a 1.0-μm pore size, cell-associated CDV could be transmitted via cell-to-cell contact from immunocytes to airway epithelial cultures. Finally, we observed that canine ALI cultures formed syncytia and started to release cell-free infectious viral particles from the apical surface following treatment with an inhibitor of the JAK/STAT signaling pathway (ruxolitinib). Our findings show that CDV can overcome the epithelial barrier through different strategies, including infection via immunocyte-mediated transmission and direct infection via the paracellular route when tight junctions are disrupted. Our established model can be adapted to other animals for studying the transmission routes and the pathogenicity of other morbilliviruses. IMPORTANCE Canine distemper virus (CDV) is not only an important pathogen of carnivores, but it also serves as a model virus for analyzing measles virus pathogenesis. To get a better picture of the different stages of infection, we used air-liquid interface cultures to analyze the infection of well-differentiated airway epithelial cells by CDV. Applying a coculture approach with DH82 cells, we demonstrated that cell-mediated infection from the basolateral side of well-differentiated epithelial cells is more efficient than infection via cell-free virus. In fact, free virus was unable to infect intact polarized cells. When tight junctions were interrupted by treatment with EGTA, cells became susceptible to infection, with nectin-4 serving as a receptor. Another interesting feature of CDV infection is that infection of well-differentiated airway epithelial cells does not result in virus egress. Cell-free virions are released from the cells only in the presence of an inhibitor of the JAK/STAT signaling pathway. Our results provide new insights into how CDV can overcome the barrier of the airway epithelium and reveal similarities and some dissimilarities compared to measles virus.

Evaluation of Newcastle disease virus vaccine effectiveness in dogs with neurological signs of canine distemper

Canine Distemper Virus (CDV) is the cause of a highly lethal infectious disease affecting a broad range of carnivores. Despite using various treatments, there is still no effective treatment, especially in the neurological form of distemper. The aim of this study was to evaluate the therapeutic effect of injecting Newcastle disease vaccine into the subarachnoid space of dogs with neurological form of distemper. The dogs that had symptoms of nervous distemper, particularly myoclonus, were included in the plan. After anesthetizing of dogs, 0.10 to 1.00 mL of cerebrospinal fluid (CSF) were removed and, 0.10 to 0.50 mL of the prepared Newcastle solution were injected into their subarachnoid space. Another 0.50 to 1.00 mL of normal saline was then injected to remove the needle from the vaccine. The live attenuated LaSota or B1 vaccine was used in this study. Rapid kit tests and reverse transcription polymerase chain reaction (RT-PCR) assays were used to diagnose of the disease. Dogs were monitored for up to 3 to 24 months during that time they were evaluated for improvement or worsening of clinical symptoms. Out of nine dogs in which distemper were diagnosed with different tests, one dog recovered completely and another dog recovered greatly. Therefore, the overall recovery rate was 22.20%. It is concluded that administration of Newcastle vaccine into the subarachnoid space of dogs with nervous distemper causes at least 22.20% improvement and does not cause specific side effects and can be used to treat affected dogs.

Roles of Cholesterol in Early and Late Steps of the Nipah Virus Membrane Fusion Cascade

Cholesterol has been implicated in various viral life cycle steps for different enveloped viruses, including viral entry into host cells, cell-cell fusion, and viral budding from infected cells. Enveloped viruses acquire their membranes from their host cells. Although cholesterol has been associated with the binding and entry of various enveloped viruses into cells, cholesterol's exact function in the viral-cell membrane fusion process remains largely elusive, particularly for the paramyxoviruses. Furthermore, paramyxoviral fusion occurs at the host cell membrane and is essential for both virus entry (virus-cell fusion) and syncytium formation (cell-cell fusion), central to viral pathogenicity. Nipah virus (NiV) is a deadly member of the Paramyxoviridae family, which also includes Hendra, measles, mumps, human parainfluenza, and various veterinary viruses. The zoonotic NiV causes severe encephalitis, vasculopathy, and respiratory symptoms, leading to a high mortality rate in humans. We used NiV as a model to study the role of membrane cholesterol in paramyxoviral membrane fusion. We used a combination of methyl-beta cyclodextrin (MβCD), lovastatin, and cholesterol to deplete or enrich cell membrane cholesterol outside cytotoxic concentrations. We found that the levels of cellular membrane cholesterol directly correlated with the levels of cell-cell fusion induced. These phenotypes were paralleled using NiV/vesicular stomatitis virus (VSV)-pseudotyped viral infection assays. Remarkably, our mechanistic studies revealed that cholesterol reduces an early F-triggering step but enhances a late fusion pore formation step in the NiV membrane fusion cascade. Thus, our results expand our mechanistic understanding of the paramyxoviral/henipaviral entry and cell-cell fusion processes.IMPORTANCE Cholesterol has been implicated in various steps of the viral life cycle for different enveloped viruses. Nipah virus (NiV) is a highly pathogenic enveloped virus in the Henipavirus genus within the Paramyxoviridae family, capable of causing a high mortality rate in humans and high morbidity in domestic and agriculturally important animals. The role of cholesterol for NiV or the henipaviruses is unknown. Here, we show that the levels of cholesterol influence the levels of NiV-induced cell-cell membrane fusion during syncytium formation and virus-cell membrane fusion during viral entry. Furthermore, the specific role of cholesterol in membrane fusion is not well defined for the paramyxoviruses. We show that the levels of cholesterol affect an early F-triggering step and a late fusion pore formation step during the membrane fusion cascade. Thus, our results expand our mechanistic understanding of the viral entry and cell-cell fusion processes, which may aid the development of antivirals.

Current Understanding of the Role of Cholesterol in the Life Cycle of Alphaviruses

Enveloped viruses rely on different lipid classes present in cell membranes to accomplish several steps of their life cycle in the host. Particularly for alphaviruses, a medically important group of arboviruses, which are part of the Togaviridae family, cholesterol seems to be a critical lipid exploited during infection, although its relevance may vary depending on which stage of the virus life cycle is under consideration and whether infection takes place in vertebrate or invertebrate hosts. In this review, the role of cholesterol in both early and late events of alphavirus infection and how viral replication may affect cholesterol metabolism are summarized, taking into account studies on Old World and New World alphaviruses in different cell lines. Moreover, the importance of cholesterol for the structural stability of alphavirus particles is also discussed, shedding light on the role played by this lipid when they leave the host cell.

Cholesterol-rich lipid rafts both in cellular and viral membrane are critical for caprine parainfluenza virus type3 entry and infection in host cells

Cholesterol-rich lipid rafts have been shown to play important roles in the life cycle of various non-enveloped and enveloped viruses. Deletion of cholesterol from lipid rafts could influence different steps of viral replication cycle including entry, infection, assembly and release. Caprine parainfluenza virus type3 (CPIV3) is a newly identified member of Paramyxoviridae family. CPIV3 is highly prevalence and threatened the goat industry in China. The infection mechanism of CPIV3 is under exploring and still not fully understood, the roles of cholesterol and lipid rafts for CPIV3 infection remains unclear. In this study, we investigated the association of cholesterol and lipid rafts with CPIV3 during the different viral replication stages (binding, entry and infection) in two cells [MDBK and goat bronchial epithelial (GBE) cells]. Methyl-β- cyclodextrin (MβCD) was used to deplete cholesterol from cell and viral membranes. The results showed that MβCD treatment significantly inhibited CPIV3 entry and infection in these two cells with a dose-dependent manner, but didn't impair the binding of CPIV3. Addition of exogenous cholesterol to the cells after MβCD treatment restored the viral infection. In addition, treatment of MβCD only before virus-entry showed inhibitory effect in MDBK cells. Depletion of cholesterol from virion envelop also decreased the entry and infection of CPIV3 in the two cells. Furthermore, lipid rafts isolation test indicated that viral proteins (HN and N) co-localized with lipid rafts during infection in MDBK and GBE cells. Viral N protein co-localized with caveolin-1 (the marker of lipid rafts) in these two cells both at the entry and infection steps, as detected by con-focal laser scanning microscopy test. In conclusion, the results presented here demonstrated that cholesterol rich lipid rafts play an important role in CPIV3 life cycle. The findings give new insights on understanding of the mechanism of CPIV3 infection and provide a new anti-CPIV3 strategy.

Tropism and molecular pathogenesis of canine distemper virus

Background

Canine distemper virus (CDV), currently termed Canine morbillivirus, is an extremely contagious disease that affects dogs. It is identified as a multiple cell tropism pathogen, and its host range includes a vast array of species. As a member of Mononegavirales, CDV has a negative, single-stranded RNA genome, which encodes eight proteins.

Main body

Regarding the molecular pathogenesis, the hemagglutinin protein (H) plays a crucial role both in the antigenic recognition and the viral interaction with SLAM and nectin-4, the host cells’ receptors. These cellular receptors have been studied widely as CDV receptors in vitro in different cellular models. The SLAM receptor is located in lymphoid cells; therefore, the infection of these cells by CDV leads to immunosuppression, the severity of which can lead to variability in the clinical disease with the potential of secondary bacterial infection, up to and including the development of neurological signs in its later stage.

Conclusion

Improving the understanding of the CDV molecules implicated in the determination of infection, especially the H protein, can help to enhance the biochemical comprehension of the difference between a wide range of CDV variants, their tropism, and different steps in viral infection. The regions of interaction between the viral proteins and the identified host cell receptors have been elucidated to facilitate this understanding. Hence, this review describes the significant molecular and cellular characteristics of CDV that contribute to viral pathogenesis.

Electronic supplementary material

The online version of this article (10.1186/s12985-019-1136-6) contains supplementary material, which is available to authorized users.

Canine respiratory coronavirus employs caveolin-1-mediated pathway for internalization to HRT-18G cells

Canine respiratory coronavirus (CRCoV), identified in 2003, is a member of the Coronaviridae family. The virus is a betacoronavirus and a close relative of human coronavirus OC43 and bovine coronavirus. Here, we examined entry of CRCoV into human rectal tumor cells (HRT-18G cell line) by analyzing co-localization of single virus particles with cellular markers in the presence or absence of chemical inhibitors of pathways potentially involved in virus entry. We also targeted these pathways using siRNA. The results show that the virus hijacks caveolin-dependent endocytosis to enter cells via endocytic internalization.

Omega-3 fatty acids, membrane remodeling and cancer prevention

Proteins are often credited as the macromolecule responsible for performing critical cellular functions, however lipids have recently garnered more attention as our understanding of their role in cell function and human health becomes more apparent. Although cellular membranes are the lipid environment in which many proteins function, it is now apparent that protein and lipid assemblies can be organized to form distinct micro- or nanodomains that facilitate signaling events. Indeed, it is now appreciated that cellular function is partly regulated by the specific spatiotemporal lipid composition of the membrane, down to the nanosecond and nanometer scale. Furthermore, membrane composition is altered during human disease processes such as cancer and obesity. For example, an increased rate of lipid/cholesterol synthesis in cancerous tissues has long been recognized as an important aspect of the rewired metabolism of transformed cells. However, the contribution of lipids/cholesterol to cellular function in disease models is not yet fully understood. Furthermore, an important consideration in regard to human health is that diet is a major modulator of cell membrane composition. This can occur directly through incorporation of membrane substrates, such as fatty acids, e.g., n-3 polyunsaturated fatty acids (n-3 PUFA) and cholesterol. In this review, we describe scenarios in which changes in membrane composition impact human health. Particular focus is placed on the importance of intrinsic lipid/cholesterol biosynthesis and metabolism and extrinsic dietary modification in cancer and its effect on plasma membrane properties.

Cholesterol and host cell surface proteins contribute to cell-cell fusion induced by the Burkholderia type VI secretion system 5

Following escape into the cytoplasm of host cells, Burkholderia pseudomallei and the related species Burkholderia thailandensis employ the type VI secretion system 5 (T6SS-5) to induce plasma membrane fusion with an adjacent host cell. This process leads to the formation of multinucleated giant cells and facilitates bacterial access to an uninfected host cell in a direct manner. Despite its importance in virulence, the mechanism of the T6SS-5 and the role of host cell factors in cell-cell fusion remain elusive. To date, the T6SS-5 is the only system of bacterial origin known to induce host-cell fusion. To gain insight into the nature of T6SS-5-stimulated membrane fusion, we investigated the contribution of cholesterol and proteins exposed on the host cell surface, which were shown to be critically involved in virus-mediated giant cell formation. In particular, we analyzed the effect of host cell surface protein and cholesterol depletion on the formation of multinucleated giant cells induced by B. thailandensis. Acute protease treatment of RAW264.7 macrophages during infection with B. thailandensis followed by agarose overlay assays revealed a strong reduction in the number of cell-cell fusions compared with EDTA treated cells. Similarly, proteolytic treatment of specifically infected donor cells or uninfected recipient cells significantly decreased multinucleated giant cell formation. Furthermore, modulating host cell cholesterol content by acute cholesterol depletion from cellular membranes by methyl- β-cyclodextrin treatment or exogenous addition of cholesterol impaired the ability of B. thailandensis to induce cell-cell fusions. The requirement of physiological cholesterol levels suggests that the membrane organization or mechanical properties of the lipid bilayer influence the fusion process. Altogether, our data suggest that membrane fusion induced by B. pseudomallei and B. thailandensis involves a complex interplay between the T6SS-5 and the host cell.

Cellular cholesterol is required for porcine nidovirus infection

Porcine reproductive and respiratory syndrome virus (PRRSV) and porcine epidemic diarrhea virus (PEDV) are porcine nidoviruses that are considered emerging and re-emerging viral pathogens of pigs that pose a significant economic threat to the global pork industry. Although cholesterol is known to affect the replication of a broad range of viruses in vitro, its significance and role in porcine nidovirus infection remains to be elucidated. Therefore, the present study was conducted to determine whether cellular or/and viral cholesterol levels play a role in porcine nidovirus infection. Our results showed that depletion of cellular cholesterol by treating cells with methyl-β-cyclodextrin (MβCD) dose-dependently suppressed the replication of both nidoviruses. Conversely, cholesterol depletion from the viral envelope had no inhibitory effect on porcine nidovirus production. The addition of exogenous cholesterol to MβCD-treated cells moderately restored the infectivity of porcine nidoviruses, indicating that the presence of cholesterol in the target cell membrane is critical for viral replication. The antiviral activity of MβCD on porcine nidovirus infection was found to be predominantly exerted when used as a treatment pre-infection or prior to the viral entry process. Furthermore, pharmacological sequestration of cellular cholesterol efficiently blocked both virus attachment and internalization and, accordingly, markedly affected subsequent post-entry steps of the replication cycle, including viral RNA and protein biosynthesis and progeny virus production. Taken together, our data indicate that cell membrane cholesterol is required for porcine nidovirus entry into cells, and pharmacological drugs that hamper cholesterol-dependent virus entry may have antiviral potential against porcine nidoviruses.

Cholesterol-rich lipid rafts play a critical role in bovine parainfluenza virus type 3 (BPIV3) infection

Lipid rafts are specialized lipid domains enriched in cholesterol and sphingolipid, which can be utilized in the lifecycle of numerous enveloped viruses. Bovine parainfluenza virustype3 (BPIV3) entry to cell is mediated by receptor binding and membrane fusion, but how lipid rafts in host cell membrane and BPIV3 envelope affect virus infection remains unclear. In this study, we investigated the role of lipid rafts in the different stages of BPIV3 infection. The MDBK cells were treated by methyl-β-cyclodextrin (MβCD) to disrupt cellular lipid raft, and the virus infection was determined. The results showed that MβCD significantly inhibited BPIV3 infection in a dose-dependent manner, but didn't block the binding of virus to the cell membrane. Whereas, the MDBK cells treated by MβCD after virus-entry had no effects on the virus infection, to suggest that BPIV3 infection was associated with lipid rafts in cell membrane during viral entry stage. To further confirm lipid rafts in viral envelope also affected BPIV3 infection, we treated BPIV3 with MβCD to determine the virus titer. We found that disruption of the viral lipid raft caused a significant reduction of viral yield. Cholesterol reconstitution experiment showed that BPIV3 infection was successfully restored by cholesterol supplementation both in cellular membrane and viral envelope, which demonstrated that cholesterol-rich lipid rafts played a critical role in BPIV3 infection. These findings provide insights on our understanding of the mechanism of BPIV3 infection and imply that lipid raft might be a good potential therapeutic target to prevent virus infection.

Cholesterol reducing agents inhibit assembly of type I parainfluenza viruses

Many enveloped RNA viruses utilize lipid rafts for the assembly of progeny virions, but the role of cholesterol, a major component of rafts, on paramyxovirus budding and virion formation is controversial. In this study, we analyzed the effects of FDA-approved cholesterol-reducing agents, gemfibrozil and lovastatin, on raft formation and assembly of human parainfluenza virus type 1 (hPIV1) and Sendai virus (SeV). Treatment of the human airway epithelial A549 cells with the agents, especially when combined, significantly decreased production of infectious hPIV1 and SeV. Mechanistic analysis indicated that depletion of cellular cholesterol reduced cell surface accumulation of envelope glycoproteins and association of viral matrix and nucleocapsids with raft membrane, which resulted in impaired virus budding and release from the cells. These results indicate that cellular cholesterol is required for assembly and formation of type 1 parainfluenza viruses and suggest that cholesterol could be an attractive target for antiviral agents against hPIV1.

Differential effect of cholesterol on type I and II feline coronavirus infection

Feline infectious peritonitis (FIP) is a fatal disease of domestic and wild felidae that is caused by feline coronavirus (FCoV). FCoV has been classified into types I and II. Since type I FCoV infection is dominant in the field, it is necessary to develop antiviral agents and vaccines against type I FCoV infection. However, few studies have been conducted on type I FCoV. Here, we compare the effects of cholesterol on types I and II FCoV infections. When cells were treated methyl-β-cyclodextrin (MβCD) and inoculated with type I FCoV, the infection rate decreased significantly, and the addition of exogenous cholesterol to MβCD-treated cells resulted in the recovery of the infectivity of type I FCoV. Furthermore, exogenous cholesterol increased the infectivity of type I FCoV. In contrast, the addition of MβCD and exogenous cholesterol had little effect on the efficiency of type II FCoV infection. These results strongly suggest that the dependence of infection by types I and II FCoV on cholesterol differs.

Critical role of the lipid rafts in caprine herpesvirus type 1 infection in vitro

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A dose-dependent fall of CpHV.1 yeld was observed after MβCD treatment of MDBK cells.

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Cholesterol is mainly required during virus entry rather than during post-entry stage.

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Alteration of the lipid composition of the viral envelope affected virus entry.

The fusion machinery for herpesvirus entry in the host cells involves the interactions of viral glycoproteins with cellular receptors, although additional viral and cellular domains are required. Extensive areas of the plasma membrane surface consist of lipid rafts organized into cholesterol-rich microdomains involved in signal transduction, protein sorting, membrane transport and in many processes of viruses infection. Because of the extraction of cholesterol leads to disorganization of lipid microdomains and to dissociation of proteins bound to the lipid rafts, we investigated the effect of cholesterol depletion by methyl-β-cyclodextrin (MβCD) on caprine herpesvirus 1 (CpHV.1) in three important phases of virus infection such as binding, entry and post-entry. MβCD treatment did not prejudice virus binding to cells, while a dose-dependent reduction of the virus yield was observed at the virus entry stage, and 30 mM MβCD reduced infectivity evidently. Treatment of MDBK after virus entry revealed a moderate inhibitory effect suggesting that cholesterol is mainly required during virus entry rather than during the post-entry stage. Alteration of the envelope lipid composition affected virus entry and a noticeable reduction in virus infectivity was detected in the presence of 15 mM MβCD. Considering that the recognition of a host cell receptor is a crucial step in the start-up phase of infection, these data are essential for the study of CpHV.1 pathogenesis. To date virus receptors for CpHV.1 have not yet been identified and further investigations are required to state that MβCD treatment affects the expression of the viral receptors.

Role of the lipid rafts in the life cycle of canine coronavirus

Coronaviruses are enveloped RNA viruses that have evolved complex relationships with their host cells, and modulate their lipid composition, lipid synthesis and signalling. Lipid rafts, enriched in sphingolipids, cholesterol and associated proteins, are special plasma membrane microdomains involved in several processes in viral infections. The extraction of cholesterol leads to disorganization of lipid microdomains and to dissociation of proteins bound to lipid rafts. Because cholesterol-rich microdomains appear to be a general feature of the entry mechanism of non-eneveloped viruses and of several coronaviruses, the purpose of this study was to analyse the contribution of lipids to the infectivity of canine coronavirus (CCoV). The CCoV life cycle is closely connected to plasma membrane cholesterol, from cell entry to viral particle production. The methyl-β-cyclodextrin (MβCD) was employed to remove cholesterol and to disrupt the lipid rafts. Cholesterol depletion from the cell membrane resulted in a dose-dependent reduction, but not abolishment, of virus infectivity, and at a concentration of 15 mM, the reduction in the infection rate was about 68 %. MβCD treatment was used to verify if cholesterol in the envelope was required for CCoV infection. This resulted in a dose-dependent inhibitory effect, and at a concentration of 9 mM MβCD, infectivity was reduced by about 73 %. Since viral entry would constitute a target for antiviral strategies, inhibitory molecules interacting with viral and/or cell membranes, or interfering with lipid metabolism, may have strong antiviral potential. It will be interesting in the future to analyse the membrane microdomains in the CCoV envelope.

Critical role of cellular cholesterol in bovine rotavirus infection

Background

Bovine rotavirus (BRV) is a non-enveloped dsRNA virus that cause neonatal calf diarrhea. Lipid rafts are cholesterol-enrich membrane mircodomains that play a vital role in many cellular processes. In this study, the effect of cellular cholesterol depletion on infection of MA-104 cells with bovine rotavirus was investigated.

Results

We demonstrated that cholesterol depletion of the plasma membrane by MβCD had no effect on BRV binding to cells but significantly impaired BRV entry in a dose-dependent manner and the effect was partially reversed by addition of exogenous cholesterol, suggesting the reduction of BRV infection by MβCD was specifically due to cholesterol depletion. Cholesterol depletion after virus entry did not reduce BRV replication, whereas affected virus assembly.

Conclusions

Taken together, our results demonstrate that cell membrane cholesterol is essential to BRV infectivity.

Requirement of cholesterol in the viral envelope for dengue virus infection

The role of cholesterol in the virus envelope or in the cellular membranes for dengue virus (DENV) infection was examined by depletion with methyl-beta-cyclodextrin (MCD) or nystatin. Pretreatment of virions with MCD or nystatin significantly reduced virus infectivity in a dose-dependent manner. By contrast, pre-treatment of diverse human cell lines with MCD or nystatin did not affect DENV infection. The four DENV serotypes were similarly inactivated by cholesterol-extracting drugs and infectivity was partially rescued when virion suspensions were treated with MCD in the presence of bovine serum. The addition of serum or exogenous water-soluble cholesterol after MCD treatment did not produce a reversion of MCD inactivating effect. Furthermore, virion treatment with extra cholesterol exerted also a virucidal effect. Binding and uptake of cholesterol-deficient DENV into the host cell were not impaired, whereas the next step of fusion between virion envelope and endosome membrane leading to virion uncoating and release of nucleocapsids to the cytoplasm appeared to be prevented, as determined by the retention of capsid protein in cells infected with MCD inactivated-DENV virions. Thereafter, the infection was almost completely inhibited, given the failure of viral RNA synthesis and viral protein expression in cells infected with MCD-treated virions. These data suggest that envelope cholesterol is a critical factor in the fusion process for DENV entry.

Cholesterol dependence of Newcastle Disease Virus entry

Lipid rafts are membrane microdomains enriched in cholesterol, sphingolipids, and glycolipids that have been implicated in many biological processes. Since cholesterol is known to play a key role in the entry of some other viruses, we investigated the role of cholesterol and lipid rafts in the host cell plasma membrane in Newcastle Disease Virus (NDV) entry. We used methyl-β-cyclodextrin (MβCD) to deplete cellular cholesterol and disrupt lipid rafts. Our results show that the removal of cellular cholesterol partially reduces viral binding, fusion and infectivity. MβCD had no effect on the expression of sialic acid containing molecule expression, the NDV receptors in the target cell. All the above-described effects were reversed by restoring cholesterol levels in the target cell membrane. The HN viral attachment protein partially localized to detergent-resistant membrane microdomains (DRMs) at 4°C and then shifted to detergent-soluble fractions at 37°C. These results indicate that cellular cholesterol may be required for optimal cell entry in NDV infection cycle.

Differential infectious entry of human influenza A/NWS/33 virus (H1N1) in mammalian kidney cells

In this report we focused our interest on the early events of the replication cycle of NWS/33 human influenza A (NWS) virus in MDCK (canine), LLC-MK2 (simian), and NSK (swine) kidney cells, with different susceptibility upon infection. We have previously demonstrated that actin organization induces restriction to viral replication during the early stages of NWS virus infection in simian kidney cells. To explore how cell endocytic mechanisms are hijacked by NWS virus and may modulate the outcome of viral infection, the effect of drugs affecting selectively the entry via clathrin-coated pits, caveolar/raft-dependent endocytosis and macropinocytosis was analyzed. Results point to critical differences in terms of internalization pathways exploited by NWS virus to enter the examined cell models. Moreover, we show that some ways of entry do not allow an effective virus internalization, depending on the cell type. Understanding how specific cell functions/components may regulate early phases of viral replication allows us to deepen our knowledge on influenza virus infection and provides new insights for anti-viral researches.

Cholesterol is important for a post-adsorption step in the entry process of transmissible gastroenteritis virus

Cholesterol is a major constituent of detergent-resistant membrane microdomains (DRMs). We localized transmissible gastroenteritis virus (TGEV) spike (S) protein in DRMs in the viral envelope. Though S protein was not solubilized by cold non-ionic detergents, this behavior was unchanged when cholesterol was depleted from viral membrane by methyl-β-cyclodextrin (MβCD) and the protein did not comigrate with cellular DRM marker proteins in flotation analyses. Therefore, the S protein is not anchored in the viral membrane DRMs as they are known to occur in the plasma membrane. Cholesterol depletion from viral membrane may not affect the adsorption process as neither the sialic acid binding activity nor the binding to aminopeptidase N was reduced post-MβCD treatment. Reduced infectivity of cholesterol-depleted TGEV was observed only when the adsorption process occurred at 37 °C but not when the virus was applied at 4 °C. Cholesterol is important for a post-adsorption step, allowing membrane rearrangements that facilitate virus entry.

Cholesterol dependence of pseudorabies herpesvirus entry

Lipid rafts are special microdomains in the plasma membrane. They are enriched in sphingolipids and cholesterol, playing critical roles in many biological processes. The purpose of this study is to analyze the requirement of cholesterol, a crucial component of lipid rafts for cell infection by pseudorabies virus (PrV). Cholesterol of plasma membrane or viral envelope was depleted with methyl-beta-cyclodextrin (MβCD), and the infectivity of three strains of PrV was determined with plaque assays. The effect of adding cholesterol to MβCD-treated cells and viruses on cell infection was analyzed. Furthermore, effect of post-adsorption cholesterol depletion on PrV infection was investigated. We show that cholesterol depletion of either the plasma membrane or the viral membrane by MβCD significantly impaired the infectivity of PrV strains Kaplan, Becker, and Bartha K-61. The virus was shown to have lower cholesterol content and to respond to lower MβCD concentrations. Exogenous cholesterol added to either MβCD-treated cells or virions partially restored the virus infectivity. Optimal PrV infection requires cholesterol in viral and plasma membranes.

Paramyxovirus assembly and budding: building particles that transmit infections

The paramyxoviruses define a diverse group of enveloped RNA viruses that includes a number of important human and animal pathogens. Examples include human respiratory syncytial virus and the human parainfluenza viruses, which cause respiratory illnesses in young children and the elderly; measles and mumps viruses, which have caused recent resurgences of disease in developed countries; the zoonotic Hendra and Nipah viruses, which have caused several outbreaks of fatal disease in Australia and Asia; and Newcastle disease virus, which infects chickens and other avian species. Like other enveloped viruses, paramyxoviruses form particles that assemble and bud from cellular membranes, allowing the transmission of infections to new cells and hosts. Here, we review recent advances that have improved our understanding of events involved in paramyxovirus particle formation. Contributions of viral matrix proteins, glycoproteins, nucleocapsid proteins, and accessory proteins to particle formation are discussed, as well as the importance of host factor recruitment for efficient virus budding. Trafficking of viral structural components within infected cells is described, together with mechanisms that allow for the selection of specific sites on cellular membranes for the coalescence of viral proteins in preparation of bud formation and virion release.

Measles virus M protein-driven particle production does not involve the endosomal sorting complex required for transport (ESCRT) system

Assembly and budding of enveloped RNA viruses rely on viral matrix (M) proteins and host proteins involved in sorting and vesiculation of cellular cargoes, such as the endosomal sorting complex required for transport (ESCRT). The measles virus (MV) M protein promotes virus-like particle (VLP) production, and we now show that it shares association with detergent-resistant or tetraspanin-enriched membrane microdomains with ebolavirus VP40 protein, yet accumulates less efficiently at the plasma membrane. Unlike VP40, which recruits ESCRT components via its N-terminal late (L) domain and exploits them for particle production, the M protein does this independently of this pathway, as (i) ablation of motifs bearing similarity to canonical L domains did not affect VLP production, (ii) it did not redistribute Tsg101, AIP-1 or Vps4A to the plasma membrane, and (iii) neither VLP nor infectious virus production was sensitive to inhibition by dominant-negative Vps4A. Importantly, transfer of the VP40 L domain into the MV M protein did not cause recruitment of ESCRT proteins or confer sensitivity of VLP release to Vps4A, indicating that MV particle production occurs independently of and cannot be routed into an ESCRT-dependent pathway.

Characterization of Lassa virus glycoprotein oligomerization and influence of cholesterol on virus replication

Mature glycoprotein spikes are inserted in the Lassa virus envelope and consist of the distal subunit GP-1, the transmembrane-spanning subunit GP-2, and the signal peptide, which originate from the precursor glycoprotein pre-GP-C by proteolytic processing. In this study, we analyzed the oligomeric structure of the viral surface glycoprotein. Chemical cross-linking studies of mature glycoprotein spikes from purified virus revealed the formation of trimers. Interestingly, sucrose density gradient analysis of cellularly expressed glycoprotein showed that in contrast to trimeric mature glycoprotein complexes, the noncleaved glycoprotein forms monomers and oligomers spanning a wide size range, indicating that maturation cleavage of GP by the cellular subtilase SKI-1/S1P is critical for formation of the correct oligomeric state. To shed light on a potential relation between cholesterol and GP trimer stability, we performed cholesterol depletion experiments. Although depletion of cholesterol had no effect on trimerization of the glycoprotein spike complex, our studies revealed that the cholesterol content of the viral envelope is important for the infectivity of Lassa virus. Analyses of the distribution of viral proteins in cholesterol-rich detergent-resistant membrane areas showed that Lassa virus buds from membrane areas other than those responsible for impaired infectivity due to cholesterol depletion of lipid rafts. Thus, derivation of the viral envelope from cholesterol-rich membrane areas is not a prerequisite for the impact of cholesterol on virus infectivity.

Borna disease virus requires cholesterol in both cellular membrane and viral envelope for efficient cell entry

Borna disease virus (BDV), the prototypic member of the family Bornaviridae within the order Mononegavirales, provides an important model for the investigation of viral persistence within the central nervous system (CNS) and of associated brain disorders. BDV is highly neurotropic and enters its target cell via receptor-mediated endocytosis, a process mediated by the virus surface glycoprotein (G), but the cellular factors and pathways determining BDV cell tropism within the CNS remain mostly unknown. Cholesterol has been shown to influence viral infections via its effects on different viral processes, including replication, budding, and cell entry. In this work, we show that cell entry, but not replication and gene expression, of BDV was drastically inhibited by depletion of cellular cholesterol levels. BDV G-mediated attachment to BDV-susceptible cells was cholesterol independent, but G localized to lipid rafts (LR) at the plasma membrane. LR structure and function critically depend on cholesterol, and hence, compromised structural integrity and function of LR caused by cholesterol depletion likely inhibited the initial stages of BDV cell internalization. Furthermore, we also show that viral-envelope cholesterol is required for BDV infectivity.

Hepatitis B virus infection is dependent on cholesterol in the viral envelope

The viral and cellular determinants leading to binding and entry of hepatitis B virus (HBV) are still not fully understood. We found that HBV infection of primary hepatocyte cultures is dependent on the presence of cholesterol in the viral envelope. Extraction of cholesterol from HBV purified from plasma of HBV-infected patients with methyl-beta-cyclodextrin (MbetaCD) leads to a strongly reduced level of infection. The cholesterol-depleted virions showed higher buoyant density (1.23 versus 1.17 g ml(-1)), a smaller diameter (39 versus 48 nm), but maintained particle integrity, antigenicity and ability to bind to hepatocytes. Although addition of exogenous cholesterol and cholesterol analogues restored the physical appearance of cholesterol-depleted virions, infectivity was only regained by cholesterol add-back. Infectivity of HBV produced from cell culture in the presence of inhibitors of cholesterol-synthesis is severely impaired. Interestingly, cholesterol extraction from cellular membranes, incubation with filipin and the protein tyrosine kinase inhibitor genistein showed no effect on HBV infection, excluding a role of lipid rafts for the infection process of HBV. In summary, presence of cholesterol within the viral envelope is not important for viral binding, but indispensable for the entry process of HBV and might be important for a later step in viral uptake, e.g. fusion in a yet unknown compartment.

Importance of cholesterol-rich membrane microdomains in the interaction of the S protein of SARS-coronavirus with the cellular receptor angiotensin-converting enzyme 2

Cholesterol present in the plasma membrane of target cells has been shown to be important for the infection by SARS-CoV. We show that cholesterol depletion by treatment with methyl-β-cyclodextrin (mβCD) affects infection by SARS-CoV to the same extent as infection by vesicular stomatitis virus-based pseudotypes containing the surface glycoprotein S of SARS-CoV (VSV-ΔG-S). Therefore, the role of cholesterol for SARS-CoV infection can be assigned to the S protein and is unaffected by other coronavirus proteins. There have been contradictory reports whether or not angiotensin-converting enzyme 2 (ACE2), the cellular receptor for SARS-CoV, is present in detergent-resistant membrane domains. We found that ACE2 of both Vero E6 and Caco-2 cells co-purifies with marker proteins of detergent-resistant membranes supporting the notion that cholesterol-rich microdomains provide a platform facilitating the efficient interaction of the S protein with the cellular receptor ACE2. To understand the involvement of cholesterol in the initial steps of the viral life cycle, we applied a cell-based binding assay with cells expressing the S protein and cells containing angiotensin-converting enzyme 2 (ACE2). Alternatively, we used a soluble S protein as interaction partner. Depletion of cholesterol from the ACE2-expressing cells reduced the binding of S-expressing cells by 50% whereas the binding of soluble S protein was not affected. This result suggests that optimal infection requires a multivalent interaction between viral attachment protein and cellular receptors.

Importance of cholesterol for infection of cells by transmissible gastroenteritis virus

In this study, we addressed the question whether cholesterol is important for transmissible gastroenteritis virus (TGEV), a porcine coronavirus, in the initiation of an infection. We found that cholesterol depletion from the cellular membrane by methyl-β-cyclodextrin (MβCD) significantly impaired the efficiency of TGEV infection. Infectivity was also reduced after depleting cholesterol from the viral envelope. This finding is surprising because coronaviruses bud from a pre-Golgi compartment which is expected to be low in cholesterol compared to the plasma membrane. Addition of exogenous cholesterol resulted in a restoration of the infectivity confirming our conclusion that efficient TGEV infection requires cholesterol in both the viral and the cellular membranes. Our data raise the possibility that the viral and cellular proteins involved in the entry process may be associated with cholesterol-rich membrane microdomains.