Human rhinovirus type 2 is internalized by clathrin-mediated endocytosis

TMEM41B Is an Interferon-Stimulated Gene That Promotes Pseudorabies Virus Replication

Pseudorabies virus (PRV) is a double-stranded DNA virus that causes Aujeszky's disease and is responsible for economic loss worldwide. Transmembrane protein 41B (TMEM41B) is a novel endoplasmic reticulum (ER)-localized regulator of autophagosome biogenesis and lipid mobilization; however, the role of TMEM41B in regulating PRV replication remains undocumented. In this study, PRV infection was found to upregulate TMEM41B mRNA and protein levels both in vitro and in vivo. For the first time, we found that TMEM41B could be induced by interferon (IFN), suggesting that TMEM41B is an IFN-stimulated gene (ISG). While TMEM41B knockdown suppressed PRV proliferation, TMEM41B overexpression promoted PRV proliferation. We next studied the specific stages of the virus life cycle and found that TMEM41B knockdown affected PRV entry. Mechanistically, we demonstrated that the knockdown of TMEM41B blocked PRV-stimulated expression of the key enzymes involved in lipid synthesis. Additionally, TMEM41B knockdown played a role in the dynamics of lipid-regulated PRV entry-dependent clathrin-coated pits (CCPs). Lipid replenishment restored the CCP dynamic and PRV entry in TMEM41B knockdown cells. Together, our results indicate that TMEM41B plays a role in PRV infection via regulating lipid homeostasis. IMPORTANCE PRV belongs to the alphaherpesvirus subfamily and can establish and maintain a lifelong latent infection in pigs. As such, an intermittent active cycle presents great challenges to the prevention and control of PRV disease and is responsible for serious economic losses to the pig breeding industry. Studies have shown that lipids play a crucial role in PRV proliferation. Thus, the manipulation of lipid metabolism may represent a new perspective for the prevention and treatment of PRV. In this study, we report that the ER transmembrane protein TMEM41B is a novel ISG involved in PRV infection by regulating lipid synthesis. Therefore, our findings indicate that targeting TMEM41B may be a promising approach for the development of PRV vaccines and therapeutics.

Pseudorabies Virus Inhibits Expression of Liver X Receptors to Assist Viral Infection

Pseudorabies virus (PRV) is a contagious herpesvirus that causes Aujeszky’s disease and economic losses worldwide. Liver X receptors (LXRs) belong to the nuclear receptor superfamily and are critical for the control of lipid homeostasis. However, the role of LXR in PRV infection has not been fully established. In this study, we found that PRV infection downregulated the mRNA and protein levels of LXRα and LXRβ in vitro and in vivo. Furthermore, we discovered that LXR activation suppressed PRV proliferation, while LXR inhibition promoted PRV proliferation. We demonstrated that LXR activation-mediated reduction of cellular cholesterol was critical for the dynamics of PRV entry-dependent clathrin-coated pits. Replenishment of cholesterol restored the dynamics of clathrin-coated pits and PRV entry under LXR activation conditions. Interestingly, T0901317, an LXR agonist, prevented PRV infection in mice. Our results support a model that PRV modulates LXR-regulated cholesterol metabolism to facilitate viral proliferation.

SGIV Induced and Exploited Cellular De Novo Fatty Acid Synthesis for Virus Entry and Replication

Considerable attention has been paid to the roles of lipid metabolism in virus infection due to its regulatory effects on virus replication and host antiviral immune response. However, few literature has focused on whether lipid metabolism is involved in the life cycle of lower vertebrate viruses. Singapore grouper iridovirus (SGIV) is the causative aquatic virus that extensively causes fry and adult groupers death. Here, the potential roles of cellular de novo fatty acid synthesis in SGIV infection was investigated. SGIV infection not only increased the expression levels of key enzymes in fatty acid synthesis in vivo/vitro, including acetyl-Coenzyme A carboxylase alpha (ACC1), fatty acid synthase (FASN), medium-chain acyl-CoA dehydrogenase (MCAD), adipose triglyceride lipase (ATGL), lipoprotein lipase (LPL) and sterol regulatory element-binding protein-1 (SREBP1), but it also induced the formation of lipid droplets (LDs), suggesting that SGIV altered de novo fatty acid synthesis in host cells. Using the inhibitor and specific siRNA of ACC1 and FASN, we found that fatty acid synthesis was essential for SGIV replication, evidenced by their inhibitory effects on CPE progression, viral gene transcription, protein expression and virus production. Moreover, the inhibitor of fatty acid β-oxidation could also reduce SGIV replication. Inhibition of fatty acid synthesis but not β-oxidation markedly blocked virus entry during the life cycle of SGIV infection. In addition, we also found that inhibition of ACC1 and FASN increased the IFN immune and inflammatory response during SGIV infection. Together, our data demonstrated that SGIV infection in vitro regulated host lipid metabolism and, in that process, cellular fatty acid synthesis might exert crucial roles during SGIV infection via regulating virus entry and host immune response.

The role of lipids in the pathophysiology of coronavirus infections

Coronaviruses, which have been known to cause diseases in animals since the 1930s, utilize cellular components during their replication cycle. Lipids play important roles in viral infection, as coronaviruses target cellular lipids and lipid metabolism to modify their host cells to become an optimal environment for viral replication. Therefore, lipids can be considered as potential targets for the development of antiviral agents. This review provides an overview of the roles of cellular lipids in different stages of the life cycle of coronaviruses.

Involvement of adaptor proteins in clathrin-mediated endocytosis of virus entry

The first step in the initiation of effective viral infection is breaking through the cytomembrane to enter the cell. Clathrin-mediated endocytosis is a key vesicular trafficking process in which a variety of cargo molecules are transported from the outside to the inside of the cell. This process is hijacked by numerous families of enveloped or non-enveloped viruses, which use it to enter host cells, followed by trafficking to their replicating sites. Various adaptor proteins that assist in cargo selection, coat assembly, and clathrin-coated bud maturation are important in this process. Research data documented on the involvement of adaptor proteins, such as AP-2, Eps-15, Epsin1, and AP180/CALM, in the invasion of viruses via the clathrin-mediated endocytosis have provided novel insights into understanding the viral life cycle and have led to the development of novel therapeutics. Here, we summarize the latest discoveries on the role of these adaptor proteins in clathrin-mediated endocytosis of virus entry and also discuss the future trends in this field.

Mechanism of Rhinovirus Immunity and Asthma

The majority of asthma exacerbations in children are caused by Rhinovirus (RV), a positive sense single stranded RNA virus of the Picornavirus family. The host has developed virus defense mechanisms that are mediated by the upregulation of interferon-activated signaling. However, the virus evades the immune system by inducing immunosuppressive cytokines and surface molecules like programmed cell death protein 1 (PD-1) and its ligand (PD-L1) on immunocompetent cells. Initially, RV infects epithelial cells, which constitute a physiologic mucosal barrier. Upon virus entrance, the host cell immediately recognizes viral components like dsRNA, ssRNA, viral glycoproteins or CpG-DNA by host pattern recognition receptors (PRRs). Activation of toll like receptors (TLR) 3, 7 and 8 within the endosome and through MDA-5 and RIG-I in the cytosol leads to the production of interferon (IFN) type I and other antiviral agents. Every cell type expresses IFNAR1/IFNAR2 receptors thus allowing a generalized antiviral activity of IFN type I resulting in the inhibition of viral replication in infected cells and preventing viral spread to non-infected cells. Among immune evasion mechanisms of the virus, there is downregulation of IFN type I and its receptor as well as induction of the immunosuppressive cytokine TGF-β. TGF-β promotes viral replication and is associated with induction of the immunosuppression signature markers LAP3, IDO and PD-L1. This article reviews the recent advances on the regulation of interferon type I expression in association with RV infection in asthmatics and the immunosuppression induced by the virus.

NPC1-regulated dynamic of clathrin-coated pits is essential for viral entry

Viruses utilize cellular lipids and manipulate host lipid metabolism to ensure their replication and spread. Therefore, the identification of lipids and metabolic pathways that are suitable targets for antiviral development is crucial. Using a library of compounds targeting host lipid metabolic factors and testing them for their ability to block pseudorabies virus (PRV) and vesicular stomatitis virus (VSV) infection, we found that U18666A, a specific inhibitor of Niemann-Pick C1 (NPC1), is highly potent in suppressing the entry of diverse viruses including pseudotyped severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). NPC1 deficiency markedly attenuates viral growth by decreasing cholesterol abundance in the plasma membrane, thereby inhibiting the dynamics of clathrin-coated pits (CCPs), which are indispensable for clathrin-mediated endocytosis. Significantly, exogenous cholesterol can complement the dynamics of CCPs, leading to efficient viral entry and infectivity. Administration of U18666A improves the survival and pathology of PRV- and influenza A virus-infected mice. Thus, our studies demonstrate a unique mechanism by which NPC1 inhibition achieves broad antiviral activity, indicating a potential new therapeutic strategy against SARS-CoV-2, as well as other emerging viruses.

Viral Interactions with Adaptor-Protein Complexes: A Ubiquitous Trait among Viral Species

Numerous viruses hijack cellular protein trafficking pathways to mediate cell entry or to rearrange membrane structures thereby promoting viral replication and antagonizing the immune response. Adaptor protein complexes (AP), which mediate protein sorting in endocytic and secretory transport pathways, are one of the conserved viral targets with many viruses possessing AP-interacting motifs. We present here different mechanisms of viral interference with AP complexes and the functional consequences that allow for efficient viral propagation and evasion of host immune defense. The ubiquity of this phenomenon is evidenced by the fact that there are representatives for AP interference in all major viral families, covered in this review. The best described examples are interactions of human immunodeficiency virus and human herpesviruses with AP complexes. Several other viruses, like Ebola, Nipah, and SARS-CoV-2, are pointed out as high priority disease-causative agents supporting the need for deeper understanding of virus-AP interplay which can be exploited in the design of novel antiviral therapies.

Cryo-Electron Microscopy Structure of Seneca Valley Virus Procapsid

Seneca Valley virus (SVV), like some other members of the Picornaviridae, forms naturally occurring empty capsids, known as procapsids. Procapsids have the same antigenicity as full virions, so they present an interesting possibility for the formation of stable virus-like particles. Interestingly, although SVV is a livestock pathogen, it has also been found to preferentially infect tumor cells and is being explored for use as a therapeutic agent in the treatment of small-cell lung cancers. Here we used cryo-electron microscopy to investigate the procapsid structure and describe the transition of capsid protein VP0 to the cleaved forms of VP4 and VP2. We show that the SVV receptor binds the procapsid, as evidence of its native antigenicity. In comparing the procapsid structure to that of the full virion, we also show that a cage of RNA serves to stabilize the inside surface of the virus, thereby making it more acid stable.IMPORTANCE Viruses are extensively studied to help us understand infection and disease. One of the by-products of some virus infections are the naturally occurring empty virus capsids (containing no genome), termed procapsids, whose function remains unclear. Here we investigate the structure and formation of the procapsids of Seneca Valley virus, to better understand how they form, what causes them to form, how they behave, and how we can make use of them. One potential benefit of this work is the modification of the procapsid to develop it for targeted in vivo delivery of therapeutics or to make a stable vaccine against SVV, which could be of great interest to the agricultural industry.

Impairing the function of MLCK, myosin Va or myosin Vb disrupts Rhinovirus B14 replication

Together, the three human rhinovirus (RV) species are the most frequent cause of the common cold. Because of their high similarity with other viral species of the genus Enterovirus, within the large family Picornaviridae, studies on RV infectious activities often offer a less pathogenic model for more aggressive enteroviruses, e.g. poliovirus or EV71. Picornaviruses enter via receptor mediated endocytosis and replicate in the cytosol. Most of them depend on functional F-actin, Rab proteins, and probably motor proteins. To assess the latter, we evaluated the role of myosin light chain kinase (MLCK) and two myosin V isoforms (Va and Vb) in RV-B14 infection. We report that ML-9, a very specific MLCK inhibitor, dramatically reduced RV-B14 entry. We also demonstrate that RV-B14 infection in cells expressing dominant-negative forms of myosin Va and Vb was impaired after virus entry. Using immunofluorescent localization and immunoprecipitation, we show that myosin Va co-localized with RV-B14 exclusively after viral entry (15 min post infection) and that myosin Vb was present in the clusters of newly synthesized RNA in infected cells. These clusters, observed at 180 min post infection, are reminiscent of replication sites. Taken together, these results identify myosin light chain kinase, myosin Va and myosin Vb as new players in RV-B14 infection that participate directly or indirectly in different stages of the viral cycle.

The Unresolved Role of Interferon-λ in Asthma Bronchiale

Asthma bronchiale is a disease of the airways with increasing incidence, that often begins during infancy. So far, therapeutic options are mainly symptomatic and thus there is an increasing need for better treatment and/or prevention strategies. Human rhinoviruses (HRVs) are a major cause of asthma exacerbations and might cause acute wheezing associated with local production of pro-inflammatory mediators resulting in neutrophilic inflammatory response. Viral infections induce a characteristic activation of immune response, e.g., TLR3, 4, 7, 8, 9 in the endosome and their downstream targets, especially MyD88. Moreover, other cytoplasmic pattern recognition molecules (PRMs) like RIG1 and MDA5 play important roles in the activation of interferons (IFNs) of all types. Depending on the stimulation of the different PRMs, the levels of the IFNs induced might differ. Recent studies focused on Type I IFNs in samples from control and asthma patients. However, the administration of type I IFN-α was accompanied by side-effects, thus this possible therapy was abandoned. Type III IFN-λ acts more specifically, as fewer cells express the IFN-λ receptor chain 1. In addition, it has been shown that asthmatic mice treated with recombinant or adenoviral expressed IFN-λ2 (IL–28A) showed an amelioration of symptoms, indicating that treatment with IFN-λ might be beneficial for asthmatic patients.

Inhibition of endocytic pathways impacts cytomegalovirus maturation

Endocytic processes are critical for cellular entry of several viruses; however, the role of endocytosis in cellular trafficking of viruses beyond virus entry is only partially understood. Here, we utilized two laboratory strains (AD169 and Towne) of human cytomegalovirus (HCMV), which are known to use cell membrane fusion rather than endocytosis to enter fibroblasts, in order to study a post-entry role of endocytosis in HCMV life cycle. Upon pharmacological inhibition of dynamin-2 or clathrin terminal domain (TD) ligand association, these strains entered the cells successfully based on the expression of immediate early viral protein. However, both the inhibitors significantly reduced the growth rates and final virus yields of viruses without inhibiting the expression of early to late viral proteins. Clathrin accumulated in the cytoplasmic virus assembly compartment (vAC) of infected cells co-localizing with virus tegument protein pp150 and the formation of vAC was compromised upon endocytic inhibition. Transmission electron micrographs (TEM) of infected cells treated with endocytosis inhibitors showed intact nuclear stages of nucleocapsid assembly but the cytoplasmic virus maturation was greatly compromised. Thus, the data presented here implicate endocytic pathways in HCMV maturation and egress.

ICAM-1 Binding Rhinoviruses Enter HeLa Cells via Multiple Pathways and Travel to Distinct Intracellular Compartments for Uncoating

Of the more than 150 human rhinovirus (RV) serotypes, 89 utilize intercellular adhesion molecule-1 (ICAM-1) for cell entry. These belong either to species A or B. We recently demonstrated that RV-B14 and RV-A89, despite binding this same receptor, are routed into distinct endosomal compartments for release of their RNA into the cytosol. To gain insight into the underlying mechanism we now comparatively investigate the port of entry, temperature-dependence of uncoating, and intracellular routing of RV-B3, RV-B14, RV-A16, and RV-A89 in HeLa cells. The effect of various drugs blocking distinct stages on the individual pathways was determined via comparing the number of infected cells in a TissueFaxs instrument. We found that RV-B14 and RV-A89 enter via clathrin-, dynamin-, and cholesterol-dependent pathways, as well as by macropinocytosis. Drugs interfering with actin function similarly blocked entry of all four viruses, indicating their dependence on a dynamic actin network. However, uniquely, RV-A89 was able to produce progeny when internalized at 20 °C followed by neutralizing the endosomal pH and further incubation at 37 °C. Blocking dynein-dependent endosomal transport prevented uncoating of RV-A16 and RV-A89, but not of RV-B3 and RV-B14, indicative for routing of RV-A16 and RV-A89 into the endocytic recycling compartment for uncoating. Our results call for caution when developing drugs aimed at targeting entry or intracellular trafficking of all rhinovirus serotypes.

ICAM-1 Binding Rhinoviruses A89 and B14 Uncoat in Different Endosomal Compartments

Human rhinovirus A89 (HRV-A89) and HRV-B14 bind to and are internalized by intercellular adhesion molecule 1 (ICAM-1); as demonstrated earlier, the RNA genome of HRV-B14 penetrates into the cytoplasm from endosomal compartments of the lysosomal pathway. Here, we show by immunofluorescence microscopy that HRV-A89 but not HRV-B14 colocalizes with transferrin in the endocytic recycling compartment (ERC). Applying drugs differentially interfering with endosomal recycling and with the pathway to lysosomes, we demonstrate that these two major-group HRVs productively uncoat in distinct endosomal compartments. Overexpression of constitutively active (Rab11-GTP) and dominant negative (Rab11-GDP) mutants revealed that uncoating of HRV-A89 depends on functional Rab11. Thus, two ICAM-1 binding HRVs are routed into distinct endosomal compartments for productive uncoating.

IMPORTANCE

Based on similarity of their RNA genomic sequences, the more than 150 currently known common cold virus serotypes were classified as species A, B, and C. The majority of HRV-A viruses and all HRV-B viruses use ICAM-1 for cell attachment and entry. Our results highlight important differences of two ICAM-1 binding HRVs with respect to their intracellular trafficking and productive uncoating; they demonstrate that serotypes belonging to species A and B, but entering the cell via the same receptors, direct the endocytosis machinery to ferry them along distinct pathways toward different endocytic compartments for uncoating.

Rhinoviruses and Their Receptors: Implications for Allergic Disease

Human rhinoviruses (RVs) are picornaviruses that can cause a variety of illnesses including the common cold, lower respiratory tract illnesses such as bronchitis and pneumonia, and exacerbations of asthma. RVs are classified into three species, RV-A, B, and C, which include over 160 types. They utilize three major types of cellular membrane glycoproteins to gain entry into the host cell: intercellular adhesion molecule 1 (ICAM-1) (the majority of RV-A and all RV-B), low-density lipoprotein receptor (LDLR) family members (12 RV-A types), and cadherin-related family member 3 (CDHR3) (RV-C). CDHR3 is a member of cadherin superfamily of transmembrane proteins with yet unknown biological function, and there is relatively little information available about the mechanisms of RV-C interaction with CDHR3. A coding single nucleotide polymorphism (rs6967330) in CDHR3 could promote RV-C infections and illnesses in infancy, which could in turn adversely affect the developing lung to increase the risk of asthma. Further studies are needed to determine how RV infections contribute to pathogenesis of asthma and to develop the optimal treatment approach to control asthma exacerbations.

Viral entry pathways: the example of common cold viruses

For infection, viruses deliver their genomes into the host cell. These nucleic acids are usually tightly packed within the viral capsid, which, in turn, is often further enveloped within a lipid membrane. Both protect them against the hostile environment. Proteins and/or lipids on the viral particle promote attachment to the cell surface and internalization. They are likewise often involved in release of the genome inside the cell for its use as a blueprint for production of new viruses. In the following, I shall cursorily discuss the early more general steps of viral infection that include receptor recognition, uptake into the cell, and uncoating of the viral genome. The later sections will concentrate on human rhinoviruses, the main cause of the common cold, with respect to the above processes. Much of what is known on the underlying mechanisms has been worked out by Renate Fuchs at the Medical University of Vienna.

Principles of Virus Uncoating: Cues and the Snooker Ball

Viruses are spherical or complex shaped carriers of proteins, nucleic acids and sometimes lipids and sugars. They are metastable and poised for structural changes. These features allow viruses to communicate with host cells during entry, and to release the viral genome, a process known as uncoating. Studies have shown that hundreds of host factors directly or indirectly support this process. The cell provides molecules that promote stepwise virus uncoating, and direct the virus to the site of replication. It acts akin to a snooker player who delivers accurate and timely shots (cues) to the ball (virus) to score. The viruses, on the other hand, trick (snooker) the host, hijack its homeostasis systems, and dampen innate immune responses directed against danger signals. In this review, we discuss how cellular cues, facilitators, and built‐in viral mechanisms promote uncoating. Cues come from receptors, enzymes and chemicals that act directly on the virus particle to alter its structure, trafficking and infectivity. Facilitators are defined as host factors that are involved in processes which indirectly enhance entry or uncoating. Unraveling the mechanisms of virus uncoating will continue to enhance understanding of cell functions, and help counteracting infections with chemicals and vaccines.

Therapeutic and prophylactic activity of itraconazole against human rhinovirus infection in a murine model

Human rhinovirus (HRV) is the most common viral infectious agent in humans and is the predominant cause of the common cold. There is a need for appropriate vaccines or therapeutic agents to treat HRV infection. In this study, we investigated whether itraconazole (ICZ) can protect cells from HRV-induced cytotoxicity. Replication of HRV1B was reduced by ICZ treatment in the lungs of HRV1B- as compared to vehicle-treated mice. The numbers of immune cells, including granulocytes and monocytes, were reduced in bronchoalveolar lavage fluid (BALF) by ICZ administration after HRV1B infection, corresponding to decreased pro-inflammatory cytokine and chemokine levels in BALF. A histological analysis of lung tissue showed that ICZ suppressed inflammation caused by HRV1B infection. Interestingly, pretreatment of mice with ICZ in the form of a nasal spray had potent prophylactic antiviral activity. Cholesterol accumulation in the plasma membrane was observed upon HRV infection; ICZ blocked cholesterol trafficking to the plasma membrane, as well as resulted in its accumulation in subcellular compartments near the nucleus. These findings suggest that ICZ is a potential antiviral agent for the treatment of HRV infection, which can be adopted preventatively as well as therapeutically.

Human rhinovirus-induced inflammatory responses are inhibited by phosphatidylserine containing liposomes

Human rhinovirus (HRV) infections are major contributors to the healthcare burden associated with acute exacerbations of chronic airway disease, such as chronic obstructive pulmonary disease and asthma. Cellular responses to HRV are mediated through pattern recognition receptors that may in part signal from membrane microdomains. We previously found Toll-like receptor signaling is reduced, by targeting membrane microdomains with a specific liposomal phosphatidylserine species, 1-stearoyl-2-arachidonoyl-sn-glycero-3-phospho-L-serine (SAPS). Here we explored the ability of this approach to target a clinically important pathogen. We determined the biochemical and biophysical properties and stability of SAPS liposomes and studied their ability to modulate rhinovirus-induced inflammation, measured by cytokine production, and rhinovirus replication in both immortalized and normal primary bronchial epithelial cells. SAPS liposomes rapidly partitioned throughout the plasma membrane and internal cellular membranes of epithelial cells. Uptake of liposomes did not cause cell death, but was associated with markedly reduced inflammatory responses to rhinovirus, at the expense of only modest non-significant increases in viral replication, and without impairment of interferon receptor signaling. Thus using liposomes of phosphatidylserine to target membrane microdomains is a feasible mechanism for modulating rhinovirus-induced signaling, and potentially a prototypic new therapy for viral-mediated inflammation.

Live Imaging of Cellular Internalization of Single Colloidal Particle by Combined Label-Free and Fluorescence Total Internal Reflection Microscopy

In this work we utilize the combination of label-free total internal reflection microscopy and total internal reflectance fluorescence (TIRM/TIRF) microscopy to achieve a simultaneous, live imaging of single, label-free colloidal particle endocytosis by individual cells. The TIRM arm of the microscope enables label free imaging of the colloid and cell membrane features, while the TIRF arm images the dynamics of fluorescent-labeled clathrin (protein involved in endocytosis via clathrin pathway), expressed in transfected 3T3 fibroblasts cells. Using a model polymeric colloid and cells with a fluorescently tagged clathrin endocytosis pathway, we demonstrate that wide field TIRM/TIRF coimaging enables live visualization of the process of colloidal particle interaction with the labeled cell structure, which is valuable for discerning the membrane events and route of colloid internalization by the cell. We further show that 500 nm in diameter model polystyrene colloid associates with clathrin, prior to and during its cellular internalization. This association is not apparent with larger, 1 μm in diameter colloids, indicating an upper particle size limit for clathrin-mediated endocytosis.

Proteomic analysis of host brain components that bind to infectious particles in Creutzfeldt-Jakob disease

Transmissible encephalopathies (TSEs), such as Creutzfeldt-Jakob disease (CJD) and scrapie, are caused by infectious agents that provoke strain-specific patterns of disease. Misfolded host prion protein (PrP-res amyloid) is believed to be the causal infectious agent. However, particles that are stripped of PrP retain both high infectivity and viral proteins not detectable in uninfected mouse controls. We here detail host proteins bound with FU-CJD agent infectious brain particles by proteomic analysis. More than 98 proteins were differentially regulated, and 56 FU-CJD exclusive proteins were revealed after PrP, GFAP, C1q, ApoE, and other late pathologic response proteins were removed. Stripped FU-CJD particles revealed HSC70 (144× the uninfected control), cyclophilin B, an FU-CJD exclusive protein required by many viruses, and early endosome-membrane pathways known to facilitate viral processing, replication, and spread. Synaptosomal elements including synapsin-2 (at 33×) and AP180 (a major FU-CJD exclusive protein) paralleled the known ultrastructural location of 25 nm virus-like TSE particles and infectivity in synapses. Proteins without apparent viral or neurodegenerative links (copine-3), and others involved in viral-induced protein misfolding and aggregation, were also identified. Human sCJD brain particles contained 146 exclusive proteins, and heat shock, synaptic, and viral pathways were again prominent, in addition to Alzheimer, Parkinson, and Huntington aggregation proteins. Host proteins that bind TSE infectious particles can prevent host immune recognition and contribute to prolonged cross-species transmissions (the species barrier). Our infectious particle strategy, which reduces background sequences by >99%, emphasizes host targets for new therapeutic initiatives. Such therapies can simultaneously subvert common pathways of neurodegeneration.

Rhinoviral infection and asthma: the detection and management of rhinoviruses by airway epithelial cells

Human rhinoviruses (HRV) have been linked to the development of childhood asthma and recurrent acute asthma exacerbations throughout life, and contribute considerably to the healthcare and economic burden of this disease. However, the ability of HRV infections to trigger exacerbations, and the link between allergic status and HRV responsiveness, remains incompletely understood. Whilst the receptors on human airway cells that detect and are utilized by most HRV group A and B, but not C serotypes are known, how endosomal pattern recognition receptors (PRRs) detect HRV replication products that are generated within the cytoplasm remains somewhat of an enigma. In this article, we explore a role for autophagy, a cellular homeostatic process that allows the cell to encapsulate its own cytosolic constituents, as the crucial mechanism controlling this process and regulating the innate immune response of airway epithelial cells to viral infection. We will also briefly describe some of the recent insights into the immune responses of the airway to HRV, focusing on neutrophilic inflammation that is a potentially unwanted feature of the acute response to viral infection, and the roles of IL-1 and Pellinos in the regulation of responses to HRV.

Viral uncoating is directional: exit of the genomic RNA in a common cold virus starts with the poly-(A) tail at the 3'-end

Upon infection, many RNA viruses reorganize their capsid for release of the genome into the host cell cytosol for replication. Often, this process is triggered by receptor binding and/or by the acidic environment in endosomes. In the genus Enterovirus, which includes more than 150 human rhinovirus (HRV) serotypes causing the common cold, there is persuasive evidence that the viral RNA exits single-stranded through channels formed in the protein shell. We have determined the time-dependent emergence of the RNA ends from HRV2 on incubation of virions at 56°C using hybridization with specific oligonucleotides and detection by fluorescence correlation spectroscopy. We report that psoralen UV crosslinking prevents complete RNA release, allowing for identification of the sequences remaining inside the capsid. We also present the structure of uncoating intermediates in which parts of the RNA are condensed and take the form of a rod that is directed roughly towards a two-fold icosahedral axis, the presumed RNA exit point. Taken together, in contrast to schemes frequently depicted in textbooks and reviews, our findings demonstrate that exit of the RNA starts from the 3′-end. This suggests that packaging also occurs in an ordered manner resulting in the 3′-poly-(A) tail becoming located close to a position of pore formation during conversion of the virion into a subviral particle. This directional genome release may be common to many icosahedral non-enveloped single-stranded RNA viruses.

Viral infection requires safe transfer of the viral genome from within the protective protein shell into the host cell's cytosol. For many viruses this happens after uptake into endosomes, where receptor-binding and/or the acidic pH trigger conformational modifications or disassembly of the shell, allowing the nucleic acids to escape. For example, common cold viruses are converted into subviral particles still containing the single-stranded positive sense RNA genome; subsequently, the RNA escapes into the cytoplasm, leaving behind empty capsids. We triggered this process by heating HRV2 to 56°C and found that 3′- and 5′-end emerged with different kinetics. Crosslinking prevented complete RNA egress and upon nuclease digestion only sequences derived from the 5′-end were protected. Part of the RNA remaining within the viral shell adopted a rod-like shape pointing towards one of the two-fold axes where the RNA is presumed to exit in single-stranded form. Egress thus commences with the poly-(A) tail and not with the genome-linked peptide VPg. This suggests that assembly and uncoating are well-coordinated to avoid tangling, kinetic traps, and/or simultaneous exit of the two RNA ends at different sites.

Rhinovirus-Induced Exacerbations of Asthma and COPD

Over the past two decades, increasing evidence has shown that, in patients with chronic airways disease, viral infection is the most common cause of exacerbation. This review will examine the evidence for viral-induced exacerbations of asthma and chronic obstructive lung disease and the potential mechanisms by which viruses cause exacerbations. Attention will be focused on rhinovirus, the most common cause of respiratory exacerbations. Exacerbations due to rhinovirus, which infects relatively few cells in the airway and does not cause the cytotoxicity of other viruses such as influenza or respiratory syncytial virus, are particularly poorly understood. While the innate immune response likely plays a role in rhinovirus-induced exacerbations, its precise role, either adaptive or maladaptive, is debated. Because current treatment strategies are only partially effective, further research examining the cellular and molecular mechanisms underlying viral-induced exacerbations of chronic airways diseases is warranted.

Picornavirus entry

The essential event in picornavirus entry is the delivery of the RNA genome to the cytoplasm of a target cell, where replication occurs. In the past several years progress has been made in understanding the structural changes in the virion important for uncoating and RNA release. In addition, for several viruses the endocytic mechanisms responsible for internalization have been identified, as have the cellular sites at which uncoating occurs. It has become clear that entry is not a passive process, and that viruses initiate specific signals required for entry. And we have begun to recognize that for a given virus, there may be multiple routes of entry, depending on the particular target cell and the receptors available on that cell.

Productive entry pathways of human rhinoviruses

Currently, complete or partial genome sequences of more than 150 human rhinovirus (HRV) isolates are known. Twelve species A use members of the low-density lipoprotein receptor family for cell entry, whereas the remaining HRV-A and all HRV-B bind ICAM-1. HRV-Cs exploit an unknown receptor. At least all A and B type viruses depend on receptor-mediated endocytosis for infection. In HeLa cells, they are internalized mainly by a clathrin- and dynamin-dependent mechanism. Upon uptake into acidic compartments, the icosahedral HRV capsid expands by ~4% and holes open at the 2-fold axes, close to the pseudo-3-fold axes and at the base of the star-shaped dome protruding at the vertices. RNA-protein interactions are broken and new ones are established, the small internal myristoylated capsid protein VP4 is expelled, and amphipathic N-terminal sequences of VP1 become exposed. The now hydrophobic subviral particle attaches to the inner surface of endosomes and transfers its genomic (+) ssRNA into the cytosol. The RNA leaves the virus starting with the poly(A) tail at its 3′-end and passes through a membrane pore contiguous with one of the holes in the capsid wall. Alternatively, the endosome is disrupted and the RNA freely diffuses into the cytoplasm.

Characterization of rhinovirus subviral A particles via capillary electrophoresis, electron microscopy and gas-phase electrophoretic mobility molecular analysis: Part I

During infection, enteroviruses, such as human rhinoviruses (HRVs), convert from the native, infective form with a sedimentation coefficient of 150S to empty subviral particles sedimenting at 80S (B particles). B particles lack viral capsid protein 4 (VP4) and the single-stranded RNA genome. On the way to this end stage, a metastable intermediate particle is observed in the cell early after infection. This subviral A particle still contains the RNA but lacks VP4 and sediments at 135S. Native (150S) HRV serotype 2 (HRV2) as well as its empty (80S) capsid have been well characterized by capillary electrophoresis. In the present paper, we demonstrate separation of at least two forms of subviral A particles on the midway between native virions and empty 80S capsids by CE. For one of these intermediates, we established a reproducible way for its preparation and characterized this particle in terms of its electrophoretic mobility and its appearance in transmission electron microscopy (TEM). Furthermore, the conversion of this intermediate to 80S particles was investigated. Gas-phase electrophoretic mobility molecular analysis (GEMMA) yielded additional insights into sample composition. More data on particle characterization including its protein composition and RNA content (for unambiguous identification of the detected intermediate as subviral A particle) will be presented in the second part of the publication.

Acid-dependent viral entry

Virus infection of host cells requires that entry into the cell results in efficient genome release leading to translation and replication. These initial steps revolving around the entry and genomic release processes are crucial for viral progeny generation. Despite the variety of receptors used by viruses to initiate entry, evidence from both enveloped and non-enveloped viral infections is highlighting the important role played by intracellular acidic compartments in the entry of many viruses. These compartments provide connecting nodes within the endocytic network, presenting multiple viral internalization pathways. Endosomal compartments employing an internal acidic pH can trigger molecular mechanisms leading to disassembly of viral particles, thus providing appropriate genome delivery. Accordingly, viruses have evolved to select optimal intracellular conditions for promoting efficient genome release, leading to propagation of the infectious agent. This review will address the implications of cellular compartment involvement in virus infectious processes, and the roles played by the viruses' own machinery, including pH sensing mechanisms and the methodologies applied for studying acid-dependent viral entry into host cells.

The cell biology of receptor-mediated virus entry

The cell imposes multiple barriers to virus entry. However, viruses exploit fundamental cellular processes to gain entry to cells and deliver their genetic cargo. Virus entry pathways are largely defined by the interactions between virus particles and their receptors at the cell surface. These interactions determine the mechanisms of virus attachment, uptake, intracellular trafficking, and, ultimately, penetration to the cytosol. Elucidating the complex interplay between viruses and their receptors is necessary for a full understanding of how these remarkable agents invade their cellular hosts.

Liposomal nanocontainers as models for viral infection: monitoring viral genomic RNA transfer through lipid membranes

After uptake into target cells, many nonenveloped viruses undergo conformational changes in the low-pH environment of the endocytic compartment. This results in exposure of amphipathic viral peptides and/or hydrophobic protein domains that are inserted into and either disrupt or perforate the vesicular membranes. The viral nucleic acids thereby gain access to the cytosol and initiate replication. We here demonstrate the in vitro transfer of the single-stranded positive-sense RNA genome of human rhinovirus 2 into liposomes decorated with recombinant very-low-density lipoprotein receptor fragments. Membrane-attached virions were exposed to pH 5.4, mimicking the in vivo pH environment of late endosomes. This triggered the release of the RNA whose arrival in the liposomal lumen was detected via in situ cDNA synthesis by encapsulated reverse transcriptase. Subsequently, cDNA was PCR amplified. At a low ratio between virions and lipids, RNA transfer was positively correlated with virus concentration. However, membranes became leaky at higher virus concentrations, which resulted in decreased cDNA synthesis. In accordance with earlier in vivo data, the RNA passes through the lipid membrane without causing gross damage to vesicles at physiologically relevant virus concentrations.

Entry of tiger frog virus (an Iridovirus) into HepG2 cells via a pH-dependent, atypical, caveola-mediated endocytosis pathway

Tiger frog virus (TFV), in the genus Ranavirus of the family Iridoviridae, causes high mortality of cultured tiger frog tadpoles in China. To explore the cellular entry mechanism of TFV, HepG2 cells were treated with drugs that inhibit the main endocytic pathways. We observed that TFV entry was inhibited by NH(4)Cl, chloroquine, and bafilomycin, which can all elevate the pH of acidic organelles. In contrast, TFV entry was not influenced by chlorpromazine or overexpression of a dominant-negative form of Esp15, which inhibit the assembly of clathrin-coated pits. These results suggested that TFV entry was not associated with clathrin-mediated endocytosis, but was related to the pH of acidic organelles. Subsequently, we found that endocytosis of TFV was dependent on membrane cholesterol and was inhibited by the caveolin-1 scaffolding domain peptide. Dynamin and actin were also required for TFV entry. In addition, TFV virions colocalized with the cholera toxin subunit B, indicating that TFV enters as caveola-internalized cargo into the Golgi complex. Taken together, our results demonstrated that TFV entry occurs by caveola-mediated endocytosis with a pH-dependent step. This atypical caveola-mediated endocytosis is different from the clathrin-mediated endocytosis of frog virus 3 (FV3) by BHK cells, which has been recognized as a model for iridoviruses. Thus, our work may help further the understanding of the initial steps of iridovirus infection in lower vertebrates.

Mechanisms of single-stranded phosphorothioate modified antisense oligonucleotide accumulation in hepatocytes

Single-stranded antisense oligonucleotides (SSOs) are used to modulate the expression of genes in animal models and are being investigated as potential therapeutics. To better understand why synthetic SSOs accumulate in the same intracellular location as the target RNA, we have isolated a novel mouse hepatocellular SV40 large T-antigen carcinoma cell line, MHT that maintains the ability to efficiently take up SSOs over several years in culture. Sequence-specific antisense effects are demonstrated at low nanomolar concentrations. SSO accumulation into cells is both time and concentration dependent. At least two distinct cellular pathways are responsible for SSO accumulation in cells: a non-productive pathway resulting in accumulation in lysosomes, and a functional uptake pathway in which the SSO gains access to the targeted RNA. We demonstrate that functional uptake, as defined by a sequence-specific reduction in target mRNA, is inhibited by brefeldin A and chloroquine. Functional uptake is blocked by siRNA inhibitors of the adaptor protein AP2M1, but not by clathrin or caveolin. Furthermore, we document that treatment of mice with an AP2M1 siRNA blocks functional uptake into liver tissue. Functional uptake of SSO appears to be mediated by a novel clathrin- and caveolin-independent endocytotic process.

Alzheimer's disease gene signature says: beware of brain viral infections

Background

Recent findings from a genome wide association investigation in a large cohort of patients with Alzheimer's disease (AD) and non demented controls (CTR) showed that a limited set of genes was in a strong association (p > l0^-5) with the disease.

Presentation of the hypothesis

In this report we suggest that the polymorphism association in 8 of these genes is consistent with a non conventional interpretation of AD etiology.

Nectin-2 (NC-2), apolipoprotein E (APOE), glycoprotein carcinoembryonic antigen related cell adhesion molecule- 16 (CEACAM-16), B-cell lymphoma-3 (Bcl-3), translocase of outer mitochondrial membrane 40 homolog (T0MM-40), complement receptor-1 (CR-l), APOJ or clusterin and C-type lectin domain A family-16 member (CLEC-16A) result in a genetic signature that might affect individual brain susceptibility to infection by herpes virus family during aging, leading to neuronal loss, inflammation and amyloid deposition.

Implications of the hypothesis

We hypothesized that such genetic trait may predispose to AD via complex and diverse mechanisms each contributing to an increase of individual susceptibility to brain viral infections

Uncoating of human rhinoviruses

Human rhinoviruses (HRVs) are a major cause of the common cold. The more than one hundred serotypes, divided into species HRV-A and HRV-B, either bind intercellular adhesion molecule 1 (major group viruses) or members of the low-density lipoprotein receptor (minor group viruses) for cell entry. Some major group HRVs can also access the host cell via heparan sulphate proteoglycans. The cell attachment protein(s) of the recently discovered phylogenetic clade HRV-C is unknown. The respective receptors direct virus uptake via clathrin-dependent or independent endocytosis or via macropinocytosis. Triggered by ICAM-1 and/or the low pH environment in endosomes the virions undergo conformational alterations giving rise to hydrophobic subviral particles. These are handed over from the receptors to the endosomal membrane. According to the current view, the RNA genome is released through an opening at one of the fivefold axes of the icosahedral capsid and crosses the membrane through a pore presumably formed by viral proteins. Alternatively, the membrane may be ruptured allowing subviral particles and RNA to enter the cytosol. Whether a channel is formed or the membrane is disrupted most probably depends on the respective HRV receptor.

Different rotavirus strains enter MA104 cells through different endocytic pathways: the role of clathrin-mediated endocytosis

Rotaviruses, the single most important agents of acute severe gastroenteritis in children, are nonenveloped viruses formed by a three-layered capsid that encloses a genome formed by 11 segments of double-stranded RNA. The mechanism of entry of these viruses into the host cell is not well understood. The best-studied strain, RRV, which is sensitive to neuraminidase (NA) treatment of the cells, uses integrins alpha2 beta1 and alphav beta3 and the heat shock protein hsc70 as receptors and enters MA104 cells through a non-clathrin-, non-caveolin-mediated pathway that depends on a functional dynamin and on the presence of cholesterol on the cell surface. In this work, using a combination of pharmacological, biochemical, and genetic approaches, we compared the entry characteristics of four rotavirus strains known to have different receptor requirements. We chose four rotavirus strains that represent all phenotypic combinations of NA resistance or sensitivity and integrin dependence or independence. We found that even though all the strains share their requirements for hsc70, dynamin, and cholesterol, three of them differ from the simian strain RRV in the endocytic pathway used. The human strain Wa, porcine strain TFR-41, and bovine strain UK seem to enter the cell through clathrin-mediated endocytosis, since treatments that inhibit this pathway block their infectivity; consistent with this entry route, these strains were sensitive to changes in the endosomal pH. The inhibition of other endocytic mechanisms, such as macropinocytosis or caveola-mediated uptake, had no effect on the internalization of the rotavirus strains tested here.

Endocytosis of hepatitis C virus non-enveloped capsid-like particles induces MAPK-ERK1/2 signaling events

Although HCV is an enveloped virus, naked nucleocapsids have been reported in the serum of infected patients. The HCV core particle serves as a protective capsid shell for the viral genome and recombinant in vitro assembled HCV core particles induce strong specific immunity. We investigated the post-binding mechanism of recombinant core particle uptake and its intracellular fate. In hepatic cells, these particles are internalized, most likely in a clathrin-dependent pathway, reaching early to late endosomes and finally lysosomes. The endocytic acidic milieu is implicated in trafficking process. Using specific phosphoantibodies, signaling pathway inhibitors and chemical agents, ERK(1/2) was found to be activated in a sustained way after endocytosis, followed by downstream immediate early genes (c-fos and egr-1) modulation. We propose that the intriguing properties of cellular internalization of HCV non-enveloped particles can induce specific ERK(1/2)-MAPKs events that could be important in HCV life cycle and pathogenesis of HCV infection.

Virus entry by endocytosis

Although viruses are simple in structure and composition, their interactions with host cells are complex. Merely to gain entry, animal viruses make use of a repertoire of cellular processes that involve hundreds of cellular proteins. Although some viruses have the capacity to penetrate into the cytosol directly through the plasma membrane, most depend on endocytic uptake, vesicular transport through the cytoplasm, and delivery to endosomes and other intracellular organelles. The internalization may involve clathrin-mediated endocytosis (CME), macropinocytosis, caveolar/lipid raft-mediated endocytosis, or a variety of other still poorly characterized mechanisms. This review focuses on the cell biology of virus entry and the different strategies and endocytic mechanisms used by animal viruses.

Human rhinovirus 14 enters rhabdomyosarcoma cells expressing icam-1 by a clathrin-, caveolin-, and flotillin-independent pathway

Intercellular adhesion molecule 1 (ICAM-1) mediates binding and entry of major group human rhinoviruses (HRVs). Whereas the entry pathway of minor group HRVs has been studied in detail and is comparatively well understood, the pathway taken by major group HRVs is largely unknown. Use of immunofluorescence microscopy, colocalization with specific endocytic markers, dominant negative mutants, and pharmacological inhibitors allowed us to demonstrate that the major group virus HRV14 enters rhabdomyosarcoma cells transfected to express human ICAM-1 in a clathrin-, caveolin-, and flotillin-independent manner. Electron microscopy revealed that many virions accumulated in long tubular structures, easily distinguishable from clathrin-coated pits and caveolae. Virus entry was strongly sensitive to the Na(+)/H(+) ion exchange inhibitor amiloride and moderately sensitive to cytochalasin D. Thus, cellular uptake of HRV14 occurs via a pathway exhibiting some, but not all, characteristics of macropinocytosis and is similar to that recently described for adenovirus 3 entry via alpha(v) integrin/CD46 in HeLa cells.

Picornaviruses

The picornavirus family consists of a large number of small RNA viruses, many of which are significant pathogens of humans and livestock. They are amongst the simplest of vertebrate viruses comprising a single stranded positive sense RNA genome within a T = 1 (quasi T = 3) icosahedral protein capsid of approximately 30 nm diameter. The structures of a number of picornaviruses have been determined at close to atomic resolution by X-ray crystallography. The structures of cell entry intermediate particles and complexes of virus particles with receptor molecules or antibodies have also been obtained by X-ray crystallography or at a lower resolution by cryo-electron microscopy. Many of the receptors used by different picornaviruses have been identified, and it is becoming increasingly apparent that many use co-receptors and alternative receptors to bind to and infect cells. However, the mechanisms by which these viruses release their genomes and transport them across a cellular membrane to gain access to the cytoplasm are still poorly understood. Indeed, detailed studies of cell entry mechanisms have been made only on a few members of the family, and it is yet to be established how broadly the results of these are applicable across the full spectrum of picornaviruses. Working models of the cell entry process are being developed for the best studied picornaviruses, the enteroviruses. These viruses maintain particle integrity throughout the infection process and function as genome delivery modules. However, there is currently no model to explain how viruses such as cardio- and aphthoviruses that appear to simply dissociate into subunits during uncoating deliver their genomes into the cytoplasm.

Low pH-triggered beta-propeller switch of the low-density lipoprotein receptor assists rhinovirus infection

Minor group human rhinoviruses (HRVs) bind three members of the low-density lipoprotein receptor (LDLR) family: LDLR proper, very-LDLR (VLDLR) and LDLR-related protein (LRP). Whereas ICAM-1, the receptor of major group HRVs actively contributes to viral uncoating, LDLRs are rather considered passive vehicles for cargo delivery to the low-pH environment of endosomes. Since the Tyr-Trp-Thr-Asp beta-propeller domain of LDLR has been shown to be involved in the dissociation of bound LDL via intramolecular competition at low pH, we studied whether it also plays a role in HRV infection. Human cell lines deficient in LDLR family proteins are not available. Therefore, we used CHO-ldla7 cells that lack endogenous LDLR. These were stably transfected to express either wild-type (wt) human LDLR or a mutant with a deletion of the beta-propeller. When HRV2 was attached to the propeller-negative LDLR, a lower pH was required for conversion to subviral particles than when attached to wt LDLR. This indicates that high-avidity receptor binding maintains the virus in its native conformation. HRV2 internalization directed the mutant LDLR but not wt LDLR to lysosomes, resulting in reduced plasma membrane expression of propeller-negative LDLR. Infection assays using a CHO-adapted HRV2 variant showed a delay in intracellular viral conversion and de novo viral synthesis in cells expressing the truncated LDLR. Our data indicate that the beta-propeller attenuates the virus-stabilizing effect of LDLR binding and thereby facilitates RNA release from endosomes, resulting in the enhancement of infection. This is a nice example of a virus exploiting high-avidity multimodule receptor binding with an intrinsic release mechanism.

Internalization of swine vesicular disease virus into cultured cells: a comparative study with foot-and-mouth disease virus

We performed a comparative analysis of the internalization mechanisms used by three viruses causing important vesicular diseases in animals. Swine vesicular disease virus (SVDV) internalization was inhibited by treatments that affected clathrin-mediated endocytosis and required traffic through an endosomal compartment. SVDV particles were found in clathrin-coated pits by electron microscopy and colocalized with markers of early endosomes by confocal microscopy. SVDV infectivity was significantly inhibited by drugs that raised endosomal pH. When compared to foot-and-mouth disease virus (FMDV), which uses clathrin-mediated endocytosis, the early step of SVDV was dependent on the integrity of microtubules. SVDV-productive endocytosis was more sensitive to plasma membrane cholesterol extraction than that of FMDV, and differential cell signaling requirements for virus infection were also found. Vesicular stomatitis virus, a model virus internalized by clathrin-mediated endocytosis, was included as a control of drug treatments. These results suggest that different clathrin-mediated routes are responsible for the internalization of these viruses.

Human rhinovirus type 2 uncoating at the plasma membrane is not affected by a pH gradient but is affected by the membrane potential

The minor receptor group human rhinovirus type 2 enters host cells by endocytosis via members of the low-density-lipoprotein receptor family. In late endosomes, it undergoes a conformational change solely induced by a pH of < or =5.6, resulting in RNA transfer across the endosomal membrane into the cytoplasm. To determine potential driving forces of this process, we investigated whether RNA penetration might depend on the pH gradient and/or the membrane potential between the acidic endosome lumen and the neutral cytoplasm. Since these parameters are difficult to assess in endosomes, we took advantage of the possibility of inducing structural changes, RNA release, and consequently infection from the plasma membrane. To manipulate the pH gradient, cell-bound virus was exposed to membrane-permeant or -impermeant acidic buffers at 4 degrees C, and this was followed by a shift to 34 degrees C in medium containing bafilomycin to prevent RNA release from endosomes. To manipulate the plasma membrane potential, similar experiments were carried out, but these included K(+) diffusion potentials in the presence of the K(+) ionophore valinomycin. We demonstrated that infection does not depend on a pH gradient but is enhanced by plasma membrane hyperpolarization compared to plasma membrane depolarization.

Multiple receptors involved in human rhinovirus attachment to live cells

Minor group human rhinoviruses (HRVs) attach to members of the low-density lipoprotein receptor family and are internalized via receptor-mediated endocytosis. The attachment of HRV2 to the cell surface, the first step in infection, was characterized at the single-molecule level by atomic force spectroscopy. Sequential binding of multiple receptors was evident from recordings of characteristic quantized force spectra, which suggests that multiple receptors bound to the virus in a timely manner. Unbinding forces required to detach the virus from the cell membrane increased within a time frame of several hundred milliseconds. The number of receptors involved in virus binding was determined, and estimates for on-rate, off-rate, and equilibrium binding constant of the interaction between HRV2 and plasma membrane-anchored receptors were obtained.

Oropouche virus entry into HeLa cells involves clathrin and requires endosomal acidification

Oropouche virus (ORO), family Bunyaviridae, is the second most frequent cause of arboviral febrile illness in Brazil. Studies were conducted to understand ORO entry in HeLa cells. Chlorpromazine inhibited early steps of ORO replication cycle, consistent with entry/uncoating. The data indicate that ORO enters HeLa cells by clathrin-coated vesicles, by a mechanism susceptible to endosomal acidification inhibitors. Transmission electron microscopy and immunofluorescence indicated that ORO associates with clathrin-coated pits and can be found in association with late endosomes in a time shorter than 1 h.

Membrane domain-disrupting effects of 4-substitued cholesterol derivatives

A wide range of cellular functions are thought to be regulated not only by the activity of membrane proteins, but also by the local membrane organization, including domains of specific lipid composition. Thus, molecules and drugs targeting and disrupting this lipid pattern, particularly of the plasma membrane, will not only help to investigate the role of membrane domains in cell biology, but might also be interesting candidates for therapy. We have identified three 4-substituted cholesterol derivatives that are able to induce a domain-disrupting effect in model membranes. When applied to giant unilamellar vesicles displaying liquid-ordered-liquid-disordered phase coexistence, extensive reorganization of the membrane can be observed, such as the budding of membrane tubules or changes in the geometry of the domains, to the point of complete abolition of phase separation. In this case, the resulting membranes display a fluidity intermediate between those of liquid-disordered and liquid-ordered phases.