Identification of a role for nucleolin in rabies virus infection

Anti-nucleolin aptamer AS1411: an advancing therapeutic

Targeted therapy is highly desirable, as it allows for selective cytotoxicity on diseased cells without off-target side effects. Nucleolin is a remarkable target for cancer therapy given its high abundance, selective presence on the plasma membrane, and multifaceted influence on the initiation and progression of cancer. Nucleolin is a protein overexpressed on the cell membrane in many tumors and serves as a binding protein for several ligands implicated in angiogenesis and tumorigenesis. Nucleolin is present in the cytoplasm, nucleoplasm, and nucleolus and is used by selected pathogens for cell entry. AS1411 is a guanosine-rich oligonucleotide aptamer that binds nucleolin and is internalized in the tumor cells. AS1411 is well tolerated at therapeutic doses and localizes to tumor cells overexpressing nucleolin. AS1411 has a good safety profile with efficacy in relapsed acute myeloid leukemia and renal cell carcinoma producing mild or moderate side effects. The promising potential of AS1411 is its ability to be conjugated to drugs and nanoparticles. When a drug is bound to AS1411, the drug will localize to tumor cells leading to targeted therapy with fewer systemic side effects than traditional practices. AS1411 can also be bound to nanoparticles capable of detecting nucleolin at concentrations far lower than lab techniques used today for cancer diagnosis. AS1411 has a promising potential to change cancer diagnoses and treatment.

Structural insights into the multifunctionality of rabies virus P3 protein

Viruses form extensive interfaces with host proteins to modulate the biology of the infected cell, frequently via multifunctional viral proteins. These proteins are conventionally considered as assemblies of independent functional modules, where the presence or absence of modules determines the overall composite phenotype. However, this model cannot account for functions observed in specific viral proteins. For example, rabies virus (RABV) P3 protein is a truncated form of the pathogenicity factor P protein, but displays a unique phenotype with functions not seen in longer isoforms, indicating that changes beyond the simple complement of functional modules define the functions of P3. Here, we report structural and cellular analyses of P3 derived from the pathogenic RABV strain Nishigahara (Nish) and an attenuated derivative strain (Ni-CE). We identify a network of intraprotomer interactions involving the globular C-terminal domain and intrinsically disordered regions (IDRs) of the N-terminal region that characterize the fully functional Nish P3 to fluctuate between open and closed states, whereas the defective Ni-CE P3 is predominantly open. This conformational difference appears to be due to the single mutation N226H in Ni-CE P3. We find that Nish P3, but not Ni-CE or N226H P3, undergoes liquid-liquid phase separation and this property correlates with the capacity of P3 to interact with different cellular membrane-less organelles, including those associated with immune evasion and pathogenesis. Our analyses propose that discrete functions of a critical multifunctional viral protein depend on the conformational arrangements of distant individual domains and IDRs, in addition to their independent functions.

Henipaviruses and lyssaviruses target nucleolar treacle protein and regulate ribosomal RNA synthesis

The nucleolus is a common target of viruses and viral proteins, but for many viruses the functional outcomes and significance of this targeting remains unresolved. Recently, the first intranucleolar function of a protein of a cytoplasmically-replicating negative-sense RNA virus (NSV) was identified, with the finding that the matrix (M) protein of Hendra virus (HeV) (genus Henipavirus, family Paramyxoviridae) interacts with Treacle protein within nucleolar subcompartments and mimics a cellular mechanism of the nucleolar DNA-damage response (DDR) to suppress ribosomal RNA (rRNA) synthesis. Whether other viruses utilise this mechanism has not been examined. We report that sub-nucleolar Treacle targeting and modulation is conserved between M proteins of multiple Henipaviruses, including Nipah virus and other potentially zoonotic viruses. Furthermore, this function is also evident for P3 protein of rabies virus, the prototype virus of a different RNA virus family (Rhabdoviridae), with Treacle depletion in cells also found to impact virus production. These data indicate that unrelated proteins of viruses from different families have independently developed nucleolar/Treacle targeting function, but that modulation of Treacle has distinct effects on infection. Thus, subversion of Treacle may be an important process in infection by diverse NSVs, and so could provide novel targets for antiviral approaches with broad specificity.

Nucleolin mediates SARS-CoV-2 replication and viral-induced apoptosis of host cells

Host-oriented antiviral therapeutics are promising treatment options to combat COVID-19 and its emerging variants. However, relatively little is known about the cellular proteins hijacked by SARS-CoV-2 for its replication. Here we show that SARS-CoV-2 induces expression and cytoplasmic translocation of the nucleolar protein, nucleolin (NCL). NCL interacts with SARS-CoV-2 viral proteins and co-localizes with N-protein in the nucleolus and in stress granules. Knockdown of NCL decreases the stress granule component G3BP1, viral replication and improved survival of infected host cells. NCL mediates viral-induced apoptosis and stress response via p53. SARS-CoV-2 increases NCL expression and nucleolar size and number in lungs of infected hamsters. Inhibition of NCL with the aptamer AS-1411 decreases viral replication and apoptosis of infected cells. These results suggest nucleolin as a suitable target for anti-COVID therapies.

Lyssavirus P Protein Isoforms Diverge Significantly in Subcellular Interactions Underlying Mechanisms of Interferon Antagonism

Viral hijacking of microtubule (MT)-dependent transport is well understood, but several viruses also express discrete MT-associated proteins (vMAPs), potentially to modulate MT-dependent processes in the host cell. Specific roles for vMAP-MT interactions include subversion of antiviral responses by P3, an isoform of the P protein of rabies virus (RABV; genus Lyssavirus), which mediates MT-dependent antagonism of interferon (IFN)-dependent signal transducers and activators of transcription 1 (STAT1) signaling. P3 also undergoes nucleocytoplasmic trafficking and inhibits STAT1-DNA binding, indicative of intranuclear roles in a multipronged antagonistic strategy. MT association/STAT1 antagonist functions of P3 correlate with pathogenesis, indicating potential as therapeutic targets. However, key questions remain, including whether other P protein isoforms interact with MTs, the relationship of these interactions with pathogenesis, and the extent of conservation of P3-MT interactions between diverse pathogenic lyssaviruses. Using super-resolution microscopy, live-cell imaging, and immune signaling analyses, we find that multiple P protein isoforms associate with MTs and that association correlates with pathogenesis. Furthermore, P3 proteins from different lyssaviruses exhibit variation in intracellular localization phenotypes that are associated with STAT1 antagonist function, whereby P3-MT association is conserved among lyssaviruses of phylogroup I but not phylogroup II, while nucleocytoplasmic localization varies between P3 proteins of the same phylogroup within both phylogroup I and II. Nevertheless, the divergent P3 proteins retain significant IFN antagonist function, indicative of adaptation to favor different inhibitory mechanisms, with MT interaction important to phylogroup I viruses. IMPORTANCE Lyssaviruses, including rabies virus, cause rabies, a progressive encephalomyelitis that is almost invariably fatal. There are no effective antivirals for symptomatic infection, and effective application of current vaccines is limited in areas of endemicity, such that rabies causes ~59,000 deaths per year. Viral subversion of host cell functions, including antiviral immunity, is critical to disease, and isoforms of the lyssavirus P protein are central to the virus-host interface underpinning immune evasion. Here, we show that specific cellular interactions of P protein isoforms involved in immune evasion vary significantly between different lyssaviruses, indicative of distinct strategies to evade immune responses. These findings highlight the diversity of the virus-host interface, an important consideration in the development of pan-lyssavirus therapeutic approaches.

Seneca Valley Virus 3Cpro Mediates Cleavage and Redistribution of Nucleolin To Facilitate Viral Replication

Seneca Valley virus (SVV) is a recently discovered pathogen that poses a significant threat to the global pig industry. It has been shown that many viruses are reliant on nucleocytoplasmic trafficking of nucleolin (NCL) for their own replication. Here, we demonstrate that NCL, a critical protein component of the nucleolus, is cleaved and translocated out of the nucleoli following SVV infection. Furthermore, our data suggest that SVV 3C protease (3Cpro) is responsible for this cleavage and subsequent delocalization from the nucleoli, and that inactivation of this protease activity abolished this cleavage and translocation. SVV 3Cpro cleaved NCL at residue Q545, and the cleavage fragment (aa 1 to 545) facilitated viral replication, which was similar to the activities described for full-length NCL. Small interfering RNA-mediated knockdown indicated that NCL is required for efficient viral replication and viral protein expression. In contrast, lentivirus-mediated overexpression of NCL significantly enhanced viral replication. Taken together, these results indicate that SVV 3Cpro targets NCL for its cleavage and redistribution, which contributes to efficient viral replication, thereby emphasizing the potential target of antiviral strategies for the control of SVV infection. IMPORTANCE The nucleolus is a subnuclear cellular compartment, and nucleolin (NCL) resides predominantly in the nucleolus. NCL participates in viral replication, translation, internalization, and also serves as a receptor for virus entry. The interaction between NCL and SVV is still unknown. Here, we demonstrate that SVV 3Cpro targets NCL for its cleavage and nucleocytoplasmic transportation, which contributes to efficient viral replication. Our results reveal novel function of SVV 3Cpro and provide further insight into the mechanisms by which SVV utilizes nucleoli for efficient replication.

Molecular Basis of Functional Effects of Phosphorylation of the C-Terminal Domain of the Rabies Virus P Protein

Rabies virus P protein is a multifunctional protein with critical roles in replication and manipulation of host-cell processes, including subversion of immunity. This functional diversity involves interactions of several P protein isoforms with the cell nucleus and microtubules.

Receptors for Respiratory Syncytial Virus Infection and Host Factors Regulating the Life Cycle of Respiratory Syncytial Virus

Respiratory syncytial virus (RSV) is a common cause of lower respiratory tract infections and responsible for a large proportion of mortality in children and the elderly. There are no licensed vaccines available to date. Prophylaxis and therapeutic RSV-specific antibodies are limited to populations at high risk owing to high cost and uncertain clinical value. Receptors and host factors are two determinants important for virus entry and establishment of infection in vivo. The identification and understanding of viral receptors and host factors can help us to gain insight into the pathogenesis of RSV infection. Herein, we reviewed receptors and host factors that have been reported thus far. RSV could bind to CX3C chemokine receptor 1 and heparan sulfate proteoglycans via the G protein, and to nucleolin, insulin-like growth factor-1 receptor, epidermal growth factor, and intercellular adhesion molecule-1 via the F protein. Seven host restriction factors and 13 host factors essential for RSV infection were reviewed. We characterized the functions and their roles in the life cycle of RSV, trying to provide an update on the information of RSV-related receptors and host factors.

Implication of the nuclear trafficking of rabies virus P3 protein in viral pathogenicity

Although the majority of viruses of the family Mononegvirales replicate exclusively in the host cell cytoplasm, many of these viruses encode proteins that traffic between the nucleus and cytoplasm, which is believed to enable accessory functions in modulating the biology of the infected host cell. Among these, the P3 protein of rabies virus localizes to the nucleus through the activity of several specific nuclear localization and nuclear export signals. The major defined functions of P3 are in evasion of interferon (IFN)-mediated antiviral responses, including through inhibition of DNA-binding by IFN-activated STAT1. P3 also localizes to nucleoli and promyelocytic leukemia (PML) nuclear bodies, and interacts with nucleolin and PML protein, indicative of several intranuclear roles. The relationship of P3 nuclear localization with pathogenicity, however, is unresolved. We report that nucleocytoplasmic localization of P3 proteins from a pathogenic RABV strain, Nishigahara (Ni) and a non-pathogenic Ni-derived strain, Ni-CE, differs significantly, with nuclear accumulation defective for Ni-CE-P3. Molecular mapping indicates that altered localization derives from a coordinated effect, including two residue substitutions that independently disable nuclear localization and augment nuclear export signals, collectively promoting nuclear exclusion. Intriguingly, this appears to relate to effects on protein conformation or regulatory mechanisms, rather than direct modification of defined trafficking signal sequences. These data provide new insights into the role of regulated nuclear trafficking of a viral protein in the pathogenicity of a virus that replicates in the cytoplasm.

Definition of the immune evasion-replication interface of rabies virus P protein

Rabies virus phosphoprotein (P protein) is a multifunctional protein that plays key roles in replication as the polymerase cofactor that binds to the complex of viral genomic RNA and the nucleoprotein (N protein), and in evading the innate immune response by binding to STAT transcription factors. These interactions are mediated by the C-terminal domain of P (P_CTD). The colocation of these binding sites in the small globular P_CTD raises the question of how these interactions underlying replication and immune evasion, central to viral infection, are coordinated and, potentially, coregulated. While direct data on the binding interface of the P_CTD for STAT1 is available, the lack of direct structural data on the sites that bind N protein limits our understanding of this interaction hub. The P_CTD was proposed to bind via two sites to a flexible loop of N protein (N_pep) that is not visible in crystal structures, but no direct analysis of this interaction has been reported. Here we use Nuclear Magnetic Resonance, and molecular modelling to show N protein residues, Leu381, Asp383, Asp384 and phosphor-Ser389, are likely to bind to a ‘positive patch’ of the P_CTD formed by Lys211, Lys214 and Arg260. Furthermore, in contrast to previous predictions we identify a single site of interaction on the P_CTD by this N_pep. Intriguingly, this site is proximal to the defined STAT1 binding site that includes Ile201 to Phe209. However, cell-based assays indicate that STAT1 and N protein do not compete for P protein. Thus, it appears that interactions critical to replication and immune evasion can occur simultaneously with the same molecules of P protein so that the binding of P protein to activated STAT1 can potentially occur without interrupting interactions involved in replication. These data suggest that replication complexes might be directly involved in STAT1 antagonism.

For viruses to infect cells and generate progeny, they must be able to mediate replication, while simultaneously evading the innate immune system. Viruses with small genomes often achieve this through multifunctional proteins that have roles in both replication and immune evasion, such as the phosphoprotein (P protein) of rabies virus. P protein is an essential cofactor in genome replication and transcription, dependent on the well-folded C-terminal domain (P_CTD), which binds to the nucleoprotein (N protein) when complexed with RNA. The P_CTD can also bind and antagonize signal transducers and activators of transcription (STAT) proteins, that are essential for activating antiviral mechanisms. Here we show using Nuclear Magnetic Resonance spectroscopy and cell-based assays, that the STAT1-binding and N-binding interfaces are proximal but, nevertheless, it appears that the same molecule of P_CTD can simultaneously bind STAT1 and N protein. These data suggest that P-protein-STAT1 interaction, critical to immune evasion, can occur without interrupting interactions underlying replication, and so replication complexes might be directly involved in STAT1 antagonism.

Rabies Infection: An Overview of Lyssavirus-Host Protein Interactions

Viruses are obligatory intracellular parasites that use cell proteins to take the control of the cell functions in order to accomplish their life cycle. Studying the viral-host interactions would increase our knowledge of the viral biology and mechanisms of pathogenesis. Studies on pathogenesis mechanisms of lyssaviruses, which are the causative agents of rabies, have revealed some important host protein partners for viral proteins, especially for most studied species, i.e. RABV. In this review article, the key physical lyssavirus-host protein interactions, their contributions to rabies infection, and their exploitation are discussed to improve the knowledge about rabies pathogenesis.

Structural Elucidation of Viral Antagonism of Innate Immunity at the STAT1 Interface

To evade immunity, many viruses express interferon antagonists that target STAT transcription factors as a major component of pathogenesis. Because of a lack of direct structural data, these interfaces are poorly understood. We report the structural analysis of full-length STAT1 binding to an interferon antagonist of a human pathogenic virus. The interface revealed by transferred cross-saturation NMR is complex, involving multiple regions in both the viral and cellular proteins. Molecular mapping analysis, combined with biophysical characterization and in vitro/in vivo functional assays, indicates that the interface is significant in disease caused by a pathogenic field-strain lyssavirus, with critical roles for contacts between the STAT1 coiled-coil/DNA-binding domains and specific regions within the viral protein. These data elucidate the potentially complex nature of IFN antagonist/STAT interactions, and the spatial relationship of protein interfaces that mediate immune evasion and replication, providing insight into how viruses can regulate these essential functions via single multifunctional proteins.

Structural comparison of the C-terminal domain of functionally divergent lyssavirus P proteins

Lyssavirus P protein is a multifunctional protein that interacts with numerous host-cell proteins. The C-terminal domain (CTD) of P is important for inhibition of JAK-STAT signaling enabling the virus to evade host immunity. Several regions on the surface of rabies virus P are reported to interact with host factors. Among them, an extended, discrete hydrophobic patch of P CTD is notable. Although structures of P CTD of two strains of rabies virus, and of mokola virus have been solved, the structure of P CTD for Duvenhage virus, which is functionally divergent from these species for immune evasion function, is not known. Here, we analyze the structures of P CTD of Duvenhage and of a distinct rabies virus strain to gain further insight on the nature and potential function of the hydrophobic surface. Molecular contacts in crystals suggest that the hydrophobic patch is important to intermolecular interactions with other proteins, which differ between the lyssavirus species.

Kinome-Wide RNA Interference Screening Identifies Mitogen-Activated Protein Kinases and Phosphatidylinositol Metabolism as Key Factors for Rabies Virus Infection

Rabies virus relies on cellular machinery for its replication while simultaneously evading the host immune response. Despite their importance, little is known about the key host factors required for rabies virus infection. Here, we focused on the human kinome, at the core of many cellular pathways, to unveil a new understanding of the rabies virus infectious cycle and to discover new potential therapeutic targets in a small interfering RNA screening. The mitogen-activated protein kinase pathway and phosphatidylinositol metabolism were identified as prominent factors involved in rabies virus infection, and those findings were further confirmed in human neurons. While bringing a new insight into rabies virus biology, we also provide a new list of host factors involved in rabies virus infection.

Throughout the rabies virus (RABV) infectious cycle, host-virus interactions define its capacity to replicate, escape the immune response, and spread. As phosphorylation is a key regulatory mechanism involved in most cellular processes, kinases represent a target of choice to identify host factors required for viral replication. A kinase and phosphatase small interfering RNA (siRNA) high-content screening was performed on a fluorescent protein-recombinant field isolate (Tha RABV). We identified 57 high-confidence key host factors important for RABV replication with a readout set at 18 h postinfection and 73 with a readout set at 36 h postinfection, including 24 common factors at all stages of the infection. Amongst them, gene clusters of the most prominent pathways were determined. Up to 15 mitogen-activated protein kinases (MAPKs) and effectors, including MKK7 (associated with Jun N-terminal protein kinase [JNK] signalization) and DUSP5, as well as 17 phosphatidylinositol (PI)-related proteins, including PIP5K1C and MTM1, were found to be involved in the later stage of RABV infection. The importance of these pathways was further validated, as small molecules Ro 31-8820 and PD 198306 inhibited RABV replication in human neurons.

IMPORTANCE Rabies virus relies on cellular machinery for its replication while simultaneously evading the host immune response. Despite their importance, little is known about the key host factors required for rabies virus infection. Here, we focused on the human kinome, at the core of many cellular pathways, to unveil a new understanding of the rabies virus infectious cycle and to discover new potential therapeutic targets in a small interfering RNA screening. The mitogen-activated protein kinase pathway and phosphatidylinositol metabolism were identified as prominent factors involved in rabies virus infection, and those findings were further confirmed in human neurons. While bringing a new insight into rabies virus biology, we also provide a new list of host factors involved in rabies virus infection.

1H, 15N and 13C resonance assignments of the C-terminal domain of the P protein of the Nishigahara strain of rabies virus

The C-terminal domain of the P protein of rabies virus is a multifunctional domain that interacts with both viral and host cell proteins. Here we report the ¹H, ¹³C and ¹⁵N chemical shift assignments of this domain from P protein of the Nishigahara strain of rabies virus, a pathogenic laboratory strain well established for studies of virulence functions of rabies virus proteins, including P protein. The data and secondary structure analysis are in good agreement with the reported predominantly helical structure of the same domain from the CVS strain of rabies solved by crystallography. These assignments will enable future solution studies of the interactions of the P protein with viral and host proteins, and the effects of post-translational modifications.

Structure and Function of Negri Bodies

Replication and assembly of many viruses occur in viral factories which are specialized intracellular compartments formed during viral infection. For rabies virus, those viral factories are called Negri bodies (NBs). NBs are cytoplasmic inclusion bodies in which viral RNAs (mRNAs as well as genomic and antigenomic RNAs) are synthesized. NBs are spherical, they can fuse together, and can reversibly deform when encountering a physical barrier. All these characteristics are similar to those of eukaryotic membrane-less liquid organelles which contribute to the compartmentalization of the cell interior. Indeed, the liquid nature of NBs has been confirmed by FRAP experiments. The co-expression of rabies virus nucleoprotein N and phosphoprotein P is sufficient to induce the formation of cytoplasmic inclusions recapitulating NBs properties. Remarkably, P and N have features similar to those of cellular proteins involved in liquid organelles formation: N is an RNA-binding protein and P contains intrinsically disordered domains. An overview of the literature indicates that formation of liquid viral factories by phase separation is probably common among Mononegavirales. This allows specific recruitment and concentration of viral proteins. Finally, as virus-associated molecular patterns recognized by cellular sensors of RNA virus replication are probably essentially present in the viral factory, there should be a subtle interplay (which remains to be characterized) between those liquid structures and the cellular proteins which trigger the innate immune response.

Nucleolin mediates the internalization of rabbit hemorrhagic disease virus through clathrin-dependent endocytosis

Rabbit hemorrhagic disease virus (RHDV) is an important member of the Caliciviridae family and a highly lethal pathogen in rabbits. Although the cell receptor of RHDV has been identified, the mechanism underlying RHDV internalization remains unknown. In this study, the entry and post-internalization of RHDV into host cells were investigated using several biochemical inhibitors and RNA interference. Our data demonstrate that rabbit nucleolin (NCL) plays a key role in RHDV internalization. Further study revealed that NCL specifically interacts with the RHDV capsid protein (VP60) through its N-terminal residues (aa 285-318), and the exact position of the VP60 protein for the interaction with NCL is located in a highly conserved region (472Asp-Val-Asn474; DVN motif). Following competitive blocking of the interaction between NCL and VP60 with an artificial DVN peptide (RRTGDVNAAAGSTNGTQ), the internalization efficiency of the virus was markedly reduced. Moreover, NCL also interacts with the C-terminal residues of clathrin light chain A, which is an important component in clathrin-dependent endocytosis. In addition, the results of animal experiments also demonstrated that artificial DVN peptides protected most rabbits from RHDV infection. These findings demonstrate that NCL is involved in RHDV internalization through clathrin-dependent endocytosis.

Viral regulation of host cell biology by hijacking of the nucleolar DNA-damage response

Recent studies indicate that nucleoli play critical roles in the DNA-damage response (DDR) via interaction of DDR machinery including NBS1 with nucleolar Treacle protein, a key mediator of ribosomal RNA (rRNA) transcription and processing. Here, using proteomics, confocal and single molecule super-resolution imaging, and infection under biosafety level-4 containment, we show that this nucleolar DDR pathway is targeted by infectious pathogens. We find that the matrix proteins of Hendra virus and Nipah virus, highly pathogenic viruses of the Henipavirus genus in the order Mononegavirales, interact with Treacle and inhibit its function, thereby silencing rRNA biogenesis, consistent with mimicking NBS1–Treacle interaction during a DDR. Furthermore, inhibition of Treacle expression/function enhances henipavirus production. These data identify a mechanism for viral modulation of host cells by appropriating the nucleolar DDR and represent, to our knowledge, the first direct intranucleolar function for proteins of any mononegavirus.

Many RNA viruses that replicate in the cytoplasm express proteins that localize to nucleoli, but the nucleolar functions remain largely unknown. Here, the authors show that the Henipavirus matrix protein mimics an endogenous Treacle partner of the DNA-damage response, resulting in suppression of rRNA biogenesis.

Identification of diverse defense mechanisms in trout red blood cells in response to VHSV halted viral replication

Background: It has been described that fish nucleated red blood cells (RBCs) generate a wide variety of immune-related gene transcripts when viruses highly replicate inside them and are their main target cell. The immune response and mechanisms of fish RBCs against viruses targeting other cells or tissues has not yet been explored and is the objective of our study. Methods: Trout RBCs were obtained from peripheral blood, ficoll purified and exposed to Viral Haemorrhagic Septicaemia virus (VHSV). Immune response was evaluated by means of RT-qPCR, flow cytometry, immunofluorescence and isobaric tag for relative and absolute quantification (iTRAQ) protein profiling Results: VHSV N gene transcripts incremented early postexposure and were drastically decreased after 6 hours postexposure (hpe). The expression of the type I interferon ( ifn1) gene was significantly downregulated at early postexposure (3 hpe), together with a gradual downregulation of interferon-inducible mx and pkr genes until 72 hpe. Type I IFN protein was downregulated and interferon-inducible Mx protein was maintained at basal levels. Co-culture assays of RBCs with TSS (stromal cell line from spleen) revealed the IFN crosstalk between both cell types. On the other hand, anti-microbial peptide β-defensin 1 and neutrophil chemotactic factor interleukin 8 were slightly upregulated in VHSV-exposed RBCs Isobaric tag for relative and absolute quantification (iTRAQ) revealed that VHSV exposure can induce a global protein downregulation in trout RBCs, mainly related to RNA stability and proteasome pathways. The antioxidant/antiviral response is also suggested to be involved in the response of trout RBCs to VHSV. Conclusions: A variety of mechanisms are proposed to be implicated in the antiviral response of trout RBCs against VHSV halted infection. Ongoing research is focused on understanding the mechanisms in detail. To our knowledge, this is the first report that implicates fish RBCs in the antiviral response against viruses not targeting RBCs.

New perspectives of physiological and pathological functions of nucleolin (NCL)

Nucleolin (NCL) is a multifunctional protein that mainly localized in the nucleolus, it is also found in the nucleoplasm, cytoplasm and cell membrane. The three main structural domains allow the interaction of NCL with different proteins and RNA sequences. Moreover, specific post-translational modifications and its shuttling property also contribute to its multifunctionality. NCL has been demonstrated to be involved in a variety of aspects such as ribosome biogenesis, chromatin organization and stability, DNA and RNA metabolism, cytokinesis, cell proliferation, angiogenesis, apoptosis regulation, stress response and microRNA processing. NCL has been increasingly implicated in several pathological processes, especially in tumorigenesis and viral infection, which makes NCL a potential target for the development of anti-tumor and anti-viral strategies. In this review, we present an overview on the structure, localizations and various functions of NCL, and further describe how the multiple functions of NCL are correlated to its multiple cellular distributions.

G-quadruplex oligonucleotide AS1411 as a cancer-targeting agent: Uses and mechanisms

BACKGROUND

AS1411 is a 26-mer G-rich DNA oligonucleotide that forms a variety of G-quadruplex structures. It was identified based on its cancer-selective antiproliferative activity and subsequently determined to be an aptamer to nucleolin, a multifunctional protein that preferentially binds quadruplex nucleic acids and which is present at high levels on the surface of cancer cells. AS1411 has exceptionally efficient cellular internalization compared to non-quadruplex DNA sequences.

SCOPE OF REVIEW

Recent developments related to AS1411 will be examined, with a focus on its use for targeted delivery of therapeutic and imaging agents.

MAJOR CONCLUSIONS

Numerous research groups have used AS1411 as a targeting agent to deliver nanoparticles, oligonucleotides, and small molecules into cancer cells. Studies in animal models have demonstrated that AS1411-linked materials can accumulate selectively in tumors following systemic administration. The mechanism underlying the cancer-targeting ability of AS1411 is not completely understood, but recent studies suggest a model that involves: (1) initial uptake by macropinocytosis, a form of endocytosis prevalent in cancer cells; (2) stimulation of macropinocytosis by a nucleolin-dependent mechanism resulting in further uptake; and (3) disruption of nucleolin-mediated trafficking and efflux leading to cargoes becoming trapped inside cancer cells.

SIGNIFICANCE

Human trials have indicated that AS1411 is safe and can induce durable remissions in a few patients, but new strategies are needed to maximize its clinical impact. A better understanding of the mechanisms by which AS1411 targets and kills cancer cells may hasten the development of promising technologies using AS1411-linked nanoparticles or conjugates for cancer-targeted therapy and imaging. This article is part of a Special Issue entitled "G-quadruplex" Guest Editor: Dr. Concetta Giancola and Dr. Daniela Montesarchio.

Mutation of the N-Terminal Region of Chikungunya Virus Capsid Protein: Implications for Vaccine Design

Mosquito-transmitted chikungunya virus (CHIKV) is an arthritogenic alphavirus of the Togaviridae family responsible for frequent outbreaks of arthritic disease in humans. Capsid protein, a structural protein encoded by the CHIKV RNA genome, is able to translocate to the host cell nucleolus. In encephalitic alphaviruses, nuclear translocation induces host cell transcriptional shutoff; however, the role of capsid protein nucleolar localization in arthritogenic alphaviruses remains unclear. Using recombinant enhanced green fluorescent protein (EGFP)-tagged expression constructs and CHIKV infectious clones, we describe a nucleolar localization sequence (NoLS) in the N-terminal region of capsid protein, previously uncharacterized in CHIKV. Mutation of the NoLS by site-directed mutagenesis reduced efficiency of nuclear import of CHIKV capsid protein. In the virus, mutation of the capsid protein NoLS (CHIKV-NoLS) attenuated replication in mammalian and mosquito cells, producing a small-plaque phenotype. Attenuation of CHIKV-NoLS is likely due to disruption of the viral replication cycle downstream of viral RNA synthesis. In mice, CHIKV-NoLS infection caused no disease signs compared to wild-type CHIKV (CHIKV-WT)-infected mice; lack of disease signs correlated with significantly reduced viremia and decreased expression of proinflammatory factors. Mice immunized with CHIKV-NoLS, challenged with CHIKV-WT at 30 days postimmunization, develop no disease signs and no detectable viremia. Serum from CHIKV-NoLS-immunized mice is able to efficiently neutralize CHIKV infection in vitro. Additionally, CHIKV-NoLS-immunized mice challenged with the related alphavirus Ross River virus showed reduced early and peak viremia postchallenge, indicating a cross-protective effect. The high degree of CHIKV-NoLS attenuation may improve CHIKV antiviral and rational vaccine design.

CHIKV is a mosquito-borne pathogen capable of causing explosive epidemics of incapacitating joint pain affecting millions of people. After a series of major outbreaks over the last 10 years, CHIKV and its mosquito vectors have been able to expand their range extensively, now making CHIKV a human pathogen of global importance. With no licensed vaccine or antiviral therapy for the treatment of CHIKV disease, there is a growing need to understand the molecular determinants of viral pathogenesis. These studies identify a previously uncharacterized nucleolar localization sequence (NoLS) in CHIKV capsid protein, begin a functional analysis of site-directed mutants of the capsid protein NoLS, and examine the effect of the NoLS mutation on CHIKV pathogenesis in vivo and its potential to influence CHIKV vaccine design. A better understanding of the pathobiology of CHIKV disease will aid the development of effective therapeutic strategies.

Interaction of Host Nucleolin with Influenza A Virus Nucleoprotein in the Early Phase of Infection Limits the Late Viral Gene Expression

Influenza A virus nucleoprotein, is a multifunctional RNA-binding protein, encoded by segment-5 of the negative sense RNA genome. It serves as a key connector between the virus and the host during virus replication. It continuously shuttles between the cytoplasm and the nucleus interacting with various host cellular factors. In the current study, host proteins interacting with nucleoprotein of Influenza A virus of H1N1 2009 pandemic strain were identified by co-immunoprecipitation studies followed by MALDI-TOF/MS analysis. Here we report the host nucleolin, a major RNA-binding protein of the nucleolus as a novel interacting partner to influenza A virus nucleoprotein. We thus, explored the implications of this interaction in virus life cycle and our studies have shown that these two proteins interact early during infection in the cytoplasm of infected cells. Depletion of nucleolin in A549 cells by siRNA targeting endogenous nucleolin followed by influenza A virus infection, disrupted its interaction with viral nucleoprotein, resulting in increased expression of gene transcripts encoding late viral proteins; matrix (M1) and hemagglutinin (HA) in infected cells. On the contrary, over expression of nucleolin in cells transiently transfected with pEGFP-NCL construct followed by virus infection significantly reduced the late viral gene transcripts, and consequently the viral titer. Altered expression of late viral genes and titers following manipulation of host cellular nucleolin, proposes the functional importance of its interaction with nucleoprotein during influenza A virus infection.

Roles of nuclear trafficking in infection by cytoplasmic negative-strand RNA viruses: paramyxoviruses and beyond

Genome replication and virion production by most negative-sense RNA viruses (NSVs) occurs exclusively in the cytoplasm, but many NSV-expressed proteins undergo active nucleocytoplasmic trafficking via signals that exploit cellular nuclear transport pathways. Nuclear trafficking has been reported both for NSV accessory proteins (including isoforms of the rabies virus phosphoprotein, and V, W and C proteins of paramyxoviruses) and for structural proteins. Trafficking of the former is thought to enable accessory functions in viral modulation of antiviral responses including the type I IFN system, but the intranuclear roles of structural proteins such as nucleocapsid and matrix proteins, which have critical roles in extranuclear replication and viral assembly, are less clear. Nevertheless, nuclear trafficking of matrix protein has been reported to be critical for efficient production of Nipah virus and Respiratory syncytial virus, and nuclear localization of nucleocapsid protein of several morbilliviruses has been linked to mechanisms of immune evasion. Together, these data point to the nucleus as a significant host interface for viral proteins during infection by NSVs with otherwise cytoplasmic life cycles. Importantly, several lines of evidence now suggest that nuclear trafficking of these proteins may be critical to pathogenesis and thus could provide new targets for vaccine development and antiviral therapies.

Dynamic Nucleolar Targeting of Dengue Virus Polymerase NS5 in Response to Extracellular pH

The nucleolar subcompartment of the nucleus is increasingly recognized as an important target of RNA viruses. Here we document for the first time the ability of dengue virus (DENV) polymerase, nonstructural protein 5 (NS5), to accumulate within the nucleolus of infected cells and to target green fluorescent protein (GFP) to the nucleolus of live transfected cells. Intriguingly, NS5 exchange between the nucleus and nucleolus is dynamically modulated by extracellular pH, responding rapidly and reversibly to pH change, in contrast to GFP alone or other nucleolar and non-nucleolar targeted protein controls. The minimal pH-sensitive nucleolar targeting region (pHNTR), sufficient to target GFP to the nucleolus in a pH-sensitive fashion, was mapped to NS5 residues 1 to 244, with mutation of key hydrophobic residues, Leu-165, Leu-167, and Val-168, abolishing pHNTR function in NS5-transfected cells, and severely attenuating DENV growth in infected cells. This is the first report of a viral protein whose nucleolar targeting ability is rapidly modulated by extracellular stimuli, suggesting that DENV has the ability to detect and respond dynamically to the extracellular environment.

IMPORTANCE

Infections by dengue virus (DENV) threaten 40% of the world's population yet there is no approved vaccine or antiviral therapeutic to treat infections. Understanding the molecular details that govern effective viral replication is key for the development of novel antiviral strategies. Here, we describe for the first time dynamic trafficking of DENV nonstructural protein 5 (NS5) to the subnuclear compartment, the nucleolus. We demonstrate that NS5's targeting to the nucleolus occurs in response to acidic pH, identify the key amino acid residues within NS5 that are responsible, and demonstrate that their mutation severely impairs production of infectious DENV. Overall, this study identifies a unique subcellular trafficking event and suggests that DENV is able to detect and respond dynamically to environmental changes.

The importance of immune evasion in the pathogenesis of rabies virus

Rabies is a zoonotic disease caused by the Lyssavirus rabies virus (RABV) that can infect most mammals, including humans, where it has a case-fatality rate of almost 100%. Although preventable by vaccination, rabies causes c. 59,000 human fatalities every year worldwide. Thus, there exists an urgent need to establish an effective therapy and/or improve dissemination of vaccines for humans and animals. These outcomes require greater understanding of the mechanisms of RABV pathogenesis to identify new molecular targets for the development of therapeutics and/or live vaccines with high levels of safety. Importantly, a number of studies in recent years have indicated that RABV specifically suppresses host immunity through diverse mechanisms and that this is a key process in pathogenicity. Here, we review current understanding of immune modulation by RABV, with an emphasis on its significance to pathogenicity and the potential exploitation of this knowledge to develop new vaccines and antivirals.

Contribution of the interaction between the rabies virus P protein and I-kappa B kinase ϵ to the inhibition of type I IFN induction signalling

The P protein of rabies virus (RABV) is known to interfere with the phosphorylation of the host IFN regulatory factor 3 (IRF-3) and to consequently inhibit type I IFN induction. Previous studies, however, have only tested P proteins from laboratory-adapted fixed virus strains, and to the best of our knowledge there is no report about the effect of P proteins from street RABV strains or other lyssaviruses on the IRF-3-mediated type I IFN induction system. In this study, we evaluated the inhibitory effect of P proteins from several RABV strains, including fixed and street virus strains and other lyssaviruses (Lagos bat, Mokola and Duvenhage viruses), on IRF-3 signalling. All P proteins tested inhibited retinoic acid-inducible gene-1 (RIG-I)- and TANK binding kinase 1 (TBK1)-mediated IRF-3-dependent IFN-β promoter activities. On the other hand, the P proteins from the RABV street strains 1088 and HCM-9, but not from fixed strains Nishigahara (Ni) and CVS-11 and other lyssaviruses tested, significantly inhibited I-kappa B kinase ϵ (IKKϵ)-inducible IRF-3-dependent IFN-β promoter activity. Importantly, we revealed that the P proteins from the 1088 and HCM-9 strains, but not from the remaining viruses, interacted with IKKϵ. By using expression plasmids encoding chimeric P proteins from the 1088 strain and Ni strain, we found that the C-terminal region of the P protein is important for the interaction with IKKϵ. These findings suggest that the P protein of RABV street strains may contribute to efficient evasion of host innate immunity.

Variation in the nuclear effects of infection by different human rhinovirus serotypes

Human rhinovirus (HRV) is a positive sense RNA virus, which, despite replicating in the cytoplasm, has a significant impact on nuclear transport and nuclear localization of host proteins. A number of studies have identified differences between HRV serotypes, with respect to host response, protease activity and replicative ability. Here we report the sero-specific effects of two group-A HRV serotypes, the minor group HRV2 and the major group HRV16, on nuclear transport and nuclear protein localization. Using Western analysis, immunofluorescence and real time PCR, we show that HRV2 replicates at a faster rate than HRV16, which correlates with earlier production of viral proteases and disruption of host nuclear transport. There is significant variation in the nuclear effects of different rhinovirus species, which in turn may impact disease progression and patient response.

Why we can prevent, control and possibly treat – but will not eradicate – rabies

ABSTRACT 

Rabies is an acute, progressive viral encephalitis. Despite historical recognition, millions still remain exposed annually. Most fatalities are of children, although this zoonosis is a vaccine-preventable disease. All developed countries interrupted canine transmission and increasingly, Asian and African communities recognize what Latin Americans demonstrated – dog rabies can be eliminated – by mass application of veterinary vaccines. Realistically, rabies is not a candidate for eradication. Management is lacking for major reservoirs, such as bats. Increasing pre-exposure immunization of individuals at risk, simplification of postexposure schedules, enhancing vaccine delivery by alternative routes, development of less expensive biologics and antiviral drugs, may lessen its impact if applied strategically in a One Health context.