Role of nucleolin in human parainfluenza virus type 3 infection of human lung epithelial cells

Nucleolin-RNA interaction modulates rotavirus replication

Rotavirus infection is a leading cause of gastroenteritis in children worldwide; the genome of this virus is composed of 11 segments of dsRNA packed in a triple-layered protein capsid. Here, we investigated the role of nucleolin, a protein with diverse RNA-binding domains, in rotavirus infection. Knocking down the expression of nucleolin in MA104 cells by RNA interference resulted in a remarkable 6.3-fold increase in the production of infectious rhesus rotavirus (RRV) progeny, accompanied by an elevated synthesis of viral mRNA and genome copies. Further analysis unveiled an interaction between rotavirus segment 10 (S10) and nucleolin, potentially mediated by G-quadruplex domains on the viral genome. To determine whether the nucleolin-RNA interaction regulates RRV replication, MA104 cells were transfected with AGRO100, a compound that forms G4 structures and selectively inhibits nucleolin-RNA interactions by blocking the RNA-binding domains. Under these conditions, viral production increased by 1.5-fold, indicating the inhibitory role of nucleolin on the yield of infectious viral particles. Furthermore, G4 sequences were identified in all 11 RRV dsRNA segments, and transfection of oligonucleotides representing G4 sequences in RRV S10 induced a significant increase in viral production. These findings show that rotavirus replication is negatively regulated by nucleolin through the direct interaction with the viral RNAs by sequences forming G4 structures.IMPORTANCEViruses rely on cellular proteins to carry out their replicative cycle. In the case of rotavirus, the involvement of cellular RNA-binding proteins during the replicative cycle is a poorly studied field. In this work, we demonstrate for the first time the interaction between nucleolin and viral RNA of rotavirus RRV. Nucleolin is a cellular protein that has a role in the metabolism of ribosomal rRNA and ribosome biogenesis, which seems to have regulatory effects on the quantity of viral particles and viral RNA copies of rotavirus RRV. Our study adds a new component to the current model of rotavirus replication, where cellular proteins can have a negative regulation on rotavirus replication.

Hypoxia inducible factors inhibit respiratory syncytial virus infection by modulation of nucleolin expression

Respiratory syncytial virus (RSV) is a global healthcare problem, causing respiratory illness in young children and elderly individuals. Our knowledge of the host pathways that define susceptibility to infection and disease severity are limited. Hypoxia inducible factors (HIFs) define metabolic responses to low oxygen and regulate inflammatory responses in the lower respiratory tract. We demonstrate a role for HIFs to suppress RSV entry and RNA replication. We show that hypoxia and HIF prolyl-hydroxylase inhibitors reduce the expression of the RSV entry receptor nucleolin and inhibit viral cell-cell fusion. We identify a HIF regulated microRNA, miR-494, that regulates nucleolin expression. In RSV-infected mice, treatment with the clinically approved HIF prolyl-hydroxylase inhibitor, Daprodustat, reduced the level of infectious virus and infiltrating monocytes and neutrophils in the lung. This study highlights a role for HIF-signalling to limit multiple aspects of RSV infection and associated inflammation and informs future therapeutic approaches for this respiratory pathogen.

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.

Nucleolin loss of function leads to aberrant Fibroblast Growth Factor signaling and craniofacial anomalies

Ribosomal RNA (rRNA) transcription and ribosome biogenesis are global processes required for growth and proliferation of all cells, yet perturbation of these processes in vertebrates leads to tissue-specific defects termed ribosomopathies. Mutations in rRNA transcription and processing proteins often lead to craniofacial anomalies; however, the cellular and molecular reasons for these defects are poorly understood. Therefore, we examined the function of the most abundant nucleolar phosphoprotein, Nucleolin (Ncl), in vertebrate development. ncl mutant (ncl-/-) zebrafish present with craniofacial anomalies such as mandibulofacial hypoplasia. We observed that ncl-/- mutants exhibited decreased rRNA synthesis and p53-dependent apoptosis, consistent with a role in ribosome biogenesis. However, we found that Nucleolin also performs functions not associated with ribosome biogenesis. We discovered that the half-life of fgf8a mRNA was reduced in ncl-/- mutants, which perturbed Fgf signaling, resulting in misregulated Sox9a-mediated chondrogenesis and Runx2-mediated osteogenesis. Consistent with this model, exogenous FGF8 treatment significantly rescued the cranioskeletal phenotype in ncl-/- zebrafish, suggesting that Nucleolin regulates osteochondroprogenitor differentiation. Our work has therefore uncovered tissue-specific functions for Nucleolin in rRNA transcription and post-transcriptional regulation of growth factor signaling during embryonic craniofacial development.

Nucleolin: a cell portal for viruses, bacteria, and toxins

The main localization of nucleolin is the nucleolus, but this protein is present in multiple subcellular sites, and it is unconventionally secreted. On the cell surface, nucleolin acts as a receptor for various viruses, some bacteria, and some toxins. Aim of this review is to discuss the characteristics that make nucleolin able to act as receptor or co-receptor of so many and different pathogens. The important features that emerge are its multivalence, and its role as a bridge between the cell surface and the nucleus. Multiple domains, short linear motifs and post-translational modifications confer and modulate nucleolin ability to interact with nucleic acids, with proteins, but also with carbohydrates and lipids. This modular multivalence allows nucleolin to participate in different types of biomolecular condensates and to move to various subcellular locations, where it can act as a kind of molecular glue. It moves from the nucleus to the cell surface and can accompany particles in the reverse direction, from the cell surface into the nucleus, which is the destination of several pathogens to manipulate the cell in their favour.

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.

Respiratory syncytial virus: from pathogenesis to potential therapeutic strategies

Respiratory syncytial virus (RSV) is one of the most important viral pathogens causing respiratory tract infection in infants, the elderly and people with poor immune function, which causes a huge disease burden worldwide every year. It has been more than 60 years since RSV was discovered, and the palivizumab monoclonal antibody, the only approved specific treatment, is limited to use for passive immunoprophylaxis in high-risk infants; no other intervention has been approved to date. However, in the past decade, substantial progress has been made in characterizing the structure and function of RSV components, their interactions with host surface molecules, and the host innate and adaptive immune response to infection. In addition, basic and important findings have also piqued widespread interest among researchers and pharmaceutical companies searching for effective interventions for RSV infection. A large number of promising monoclonal antibodies and inhibitors have been screened, and new vaccine candidates have been designed for clinical evaluation. In this review, we first briefly introduce the structural composition, host cell surface receptors and life cycle of RSV virions. Then, we discuss the latest findings related to the pathogenesis of RSV. We also focus on the latest clinical progress in the prevention and treatment of RSV infection through the development of monoclonal antibodies, vaccines and small-molecule inhibitors. Finally, we look forward to the prospects and challenges of future RSV research and clinical intervention.

Identification of RSV Fusion Protein Interaction Domains on the Virus Receptor, Nucleolin

Nucleolin is an essential cellular receptor to human respiratory syncytial virus (RSV). Pharmacological targeting of the nucleolin RNA binding domain RBD1,2 can inhibit RSV infections in vitro and in vivo; however, the site(s) on RBD1,2 which interact with RSV are not known. We undertook a series of experiments designed to: document RSV-nucleolin co-localization on the surface of polarized MDCK cells using immunogold electron microscopy, to identify domains on nucleolin that physically interact with RSV using biochemical methods and determine their biological effects on RSV infection in vitro, and to carry out structural analysis toward informing future RSV drug development. Results of immunogold transmission and scanning electron microscopy showed RSV-nucleolin co-localization on the cell surface, as would be expected for a viral receptor. RSV, through its fusion protein (RSV-F), physically interacts with RBD1,2 and these interactions can be competitively inhibited by treatment with Palivizumab or recombinant RBD1,2. Treatment with synthetic peptides derived from two 12-mer domains of RBD1,2 inhibited RSV infection in vitro, with structural analysis suggesting these domains are potentially feasible for targeting in drug development. In conclusion, the identification and characterization of domains of nucleolin that interact with RSV provide the essential groundwork toward informing design of novel nucleolin-targeting compounds in RSV drug development.

IGF1R is an entry receptor for respiratory syncytial virus

Pneumonia resulting from infection is one of the leading causes of death worldwide. Pulmonary infection by the respiratory syncytial virus (RSV) is a large burden on human health, for which there are few therapeutic options1. RSV targets ciliated epithelial cells in the airways, but how viruses such as RSV interact with receptors on these cells is not understood. Nucleolin is an entry coreceptor for RSV2 and also mediates the cellular entry of influenza, the parainfluenza virus, some enteroviruses and the bacterium that causes tularaemia3,4. Here we show a mechanism of RSV entry into cells in which outside-in signalling, involving binding of the prefusion RSV-F glycoprotein with the insulin-like growth factor-1 receptor, triggers the activation of protein kinase C zeta (PKCζ). This cellular signalling cascade recruits nucleolin from the nuclei of cells to the plasma membrane, where it also binds to RSV-F on virions. We find that inhibiting PKCζ activation prevents the trafficking of nucleolin to RSV particles on airway organoid cultures, and reduces viral replication and pathology in RSV-infected mice. These findings reveal a mechanism of virus entry in which receptor engagement and signal transduction bring the coreceptor to viral particles at the cell surface, and could form the basis of new therapeutics to treat RSV infection.

Respiratory syncytial virus entry and how to block it

Respiratory syncytial virus (RSV) is a leading cause of lower respiratory tract disease in young children and elderly people. Although the virus was isolated in 1955, an effective RSV vaccine has not been developed, and the only licensed intervention is passive immunoprophylaxis of high-risk infants with a humanized monoclonal antibody. During the past 5 years, however, there has been substantial progress in our understanding of the structure and function of the RSV glycoproteins and their interactions with host cell factors that mediate entry. This period has coincided with renewed interest in developing effective interventions, including the isolation of potent monoclonal antibodies and small molecules and the design of novel vaccine candidates. In this Review, we summarize the recent findings that have begun to elucidate RSV entry mechanisms, describe progress on the development of new interventions and conclude with a perspective on gaps in our knowledge that require further investigation.

Respiratory syncytial virus (RSV) is a leading cause of lower respiratory tract disease in young children and elderly people. In this Review, Battles and McLellan summarize our current understanding of RSV entry, describe progress on the development of new interventions and conclude with a perspective on gaps in our knowledge that require further investigation.

Nucleolin (NCL) inhibits the growth of peste des petits ruminants virus

Peste des petits ruminants (PPR) is a highly contagious disease of small ruminants that is caused by peste des petits ruminants virus (PPRV). To date, the molecular mechanism of PPRV infection is still unclear. It is well known that host proteins might be involved in the pathogenesis process for many viruses. In this study, we first proved that nucleolin (NCL), a highly conserved host factor, interacts with the core domain of PPRV N protein through its C terminus and co-locates with the N protein in the nucleus of cells. To investigate the role of NCL in PPRV infection, the expression level of NCL was inhibited with small interfering RNAs of NCL, and the results showed that PPRV growth was improved. However, the proliferation of PPRV was inhibited when the expression level of NCL was improved. Further analysis indicated that the inhibitory effect of NCL on the PPRV was caused by stimulating the interferon (IFN) pathways in host cells. In summary, our results will help us to understand the mechanism of PPRV infection.

Inhibiting nucleolin reduces inflammation induced by mitochondrial DNA in cardiomyocytes exposed to hypoxia and reoxygenation

BACKGROUND AND PURPOSE

Cellular debris causes sterile inflammation after myocardial infarction. Mitochondria constitute about 30 percent of the human heart. Mitochondrial DNA (mtDNA) is a damage-associated-molecular-pattern that induce injurious sterile inflammation. Little is known about mtDNA's inflammatory signalling pathways in cardiomyocytes and how mtDNA is internalized to associate with its putative receptor, toll-like receptor 9 (TLR9).

EXPERIMENTAL APPROACH

We hypothesized that mtDNA can be internalized in cardiomyocytes and induce an inflammatory response. Adult mouse cardiomyocytes were exposed to hypoxia-reoxygenation and extracellular DNA. Microscale thermophoresis was used to demonstrate binding between nucleolin and DNA.

KEY RESULTS

Expression of the pro-inflammatory cytokines IL-1β and TNFα were upregulated by mtDNA, but not by nuclear DNA (nDNA), in cardiomyocytes exposed to hypoxia-reoxygenation. Blocking the RNA/DNA binding protein nucleolin with midkine reduced expression of IL-1β/TNFα and the nucleolin inhibitor AS1411 reduced interleukin-6 release in adult mouse cardiomyocytes. mtDNA bound 10-fold stronger than nDNA to nucleolin. In HEK293-NF-κB reporter cells, mtDNA induced NF-κB activity in normoxia, while CpG-DNA and hypoxia-reoxygenation, synergistically induced TLR9-dependent NF-κB activity. Protein expression of nucleolin was found in the plasma membrane of cardiomyocytes and inhibition of nucleolin with midkine inhibited cellular uptake of CpG-DNA. Inhibition of endocytosis did not reduce CpG-DNA uptake in cardiomyocytes.

CONCLUSION AND IMPLICATIONS

mtDNA, but not nDNA, induce an inflammatory response in mouse cardiomyocytes during hypoxia-reoxygenation. In cardiomyocytes, nucleolin is expressed on the membrane and blocking nucleolin reduce inflammation. Nucleolin might be a therapeutic target to prevent uptake of immunogenic DNA and reduce inflammation.

LINKED ARTICLES

This article is part of a themed section on Mitochondrial Pharmacology: Featured Mechanisms and Approaches for Therapy Translation. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v176.22/issuetoc.

TLR4 and nucleolin influence cell injury, apoptosis and inflammatory factor expression in respiratory syncytial virus-infected N2a neuronal cells

Respiratory syncytial virus (RSV) infection was recently reported to be associated with central nervous system (CNS) symptoms and neurological complications; however, related studies are very limited. Moreover, the molecular mechanism underlying RSV neuropathogenesis is still unclear. Our previous study revealed that toll-like receptor 4 (TLR4) and nucleolin (C23) could be modulated and that they played a role during RSV infection in mouse neuronal-2a (N2a) cells. In the present study, the effects of silencing of TLR4 and C23 on RSV propagation and N2a cellular responses were examined by using RNA interference technology. Four N2a cell treatment groups were established, namely, a normal control group, RSV control group, TLR4 siRNA + RSV group, and C23 siRNA + RSV group. Expression changes in NeuN protein and colocalization of C23 and TLR4 with RSV F protein were assessed using confocal microscopy. Changes in TLR4 and C23 mRNA expression, TLR4, C23, TLR3, TLR7, and p-NF-κB protein expression, and interleukin (IL)-8, IL-6, and tumor necrosis factor (TNF-α) cytokine secretion was measured using quantitative real-time reverse-transcription polymerase chain reaction, Western blot analysis, and enzyme-linked immunosorbent assay, respectively. RSV titers and the apoptotic status of N2a cells were monitored using plaque formation assays and flow cytometry, respectively. The results indicated that TLR4 and C23 gene knockdown decreased the amount of F protein in RSV-infected N2a cells, inhibited RSV propagation, attenuated N2a neuronal injury, diminished cell apoptosis levels, downregulated TLR3 and TLR7 protein expression, and reduced inflammatory protein expression. Therefore, TLR4 and C23 knockdown influences cell injury, apoptosis and inflammatory protein expression in RSV-infected N2a cells.

The PA-interacting host protein nucleolin acts as an antiviral factor during highly pathogenic H5N1 avian influenza virus infection

Polymerase acidic (PA) protein is a multifunctional regulator of influenza A virus (IAV) replication and pathogenesis. In a previous study, we reported that nucleolin (NCL) is a novel PA-interacting host protein. In this study, we further explored the role of NCL during highly pathogenic H5N1 avian influenza virus infection. We found that depletion of endogenous NCL in mammalian cells by siRNA targeting during H5N1 infection resulted in significantly increased viral polymerase activity, elevated viral mRNA, cRNA and vRNA synthesis, accelerated viral replication, and enhanced apoptosis and necrosis. Moreover, siRNA silencing of NCL significantly exacerbated the inflammatory response, resulting in increased secretion of IL-6, TNF-α, TNF-β, CCL-4, CCL-8, IFN-α, IFN-β and IFN-γ. Conversely, overexpression of NCL significantly decreased IAV replication. Collectively, these data show that NCL acts as a novel potential antiviral factor during H5N1 infection. Further studies exploring the antiviral mechanisms of NCL may accelerate the development of new anti-influenza drugs.

PA-X: a key regulator of influenza A virus pathogenicity and host immune responses

PA-X, a fusion protein belonging to influenza A viruses (IAVs), integrating the N-terminal 191 amino acids of PA gene and the ribosomal frame-shifting product that lengthens out to 41 or 61 amino acids. Since its discovery in 2012, multiple functions have been attributed to this small protein, including a process, where wide-spread protein synthesis in infected host cells is shut down (called host shutoff), and viral replication, polymerase activity, viral-induced cell apoptosis, PA nuclear localization, and virulence are modulated. However, many of its proposed functions may be specific to strain, subtype, host, or cell line. In this review, we start by describing the well-defined global host-shutoff ability of PA-X and the potential mechanisms underlying it. We move on to the role played by PA-X in modulating innate and acquired immune responses in the host. We then systematically discuss the role played by PA-X in modulating the virulence of influenza viruses of different subtypes and host origins, and finish with a general overview of the research advances made in identifying the host cell partners that interact with PA-X. To uncover possible clues about the differential effects of PA-X in modulating viral virulence, we focus on systemically evaluating polymorphisms in PA-X from various viral subtypes and hosts, including avian and human H5N1, H5N6, H9N2, and H7N9 viruses. Finally, we conclude with a proposition regarding the possible future research directions for this important protein.

Inflammasome Antagonism by Human Parainfluenza Virus Type 3 C Protein

Human parainfluenza virus type 3 (HPIV3) is a negative-sense single-stranded RNA virus belonging to the Paramyxoviridae family. HPIV3 is a lung-tropic virus causing airway diseases, including pneumonia, croup, and bronchiolitis, during infancy and childhood. The activation of the inflammasome by pathogens results in the production of proinflammatory cytokines such as interleukin-1β (IL-1β) during infection. Thus, the inflammasome-mediated proinflammatory response plays a critical role in regulating the immune response and virus clearance. The inflammasome is a multimeric protein complex triggering caspase-1 activation. Activated caspase-1 cleaves pro-IL-1β into its mature (and active) secretory form. Our study revealed inflammasome activation in macrophages following HPIV3 infection. Specifically, the activation of the NLRP3/ASC inflammasome resulted in the production of mature IL-1β from HPIV3-infected cells. Furthermore, Toll-like receptor 2 (TLR2) activation (first signal) and potassium efflux (second signal) constituted two cellular events mediating inflammasome activation following HPIV3 infection. During our studies, we surprisingly identified the HPIV3 C protein as an antagonist of inflammasome activation. The HPIV3 C protein is an accessory protein encoded by the open reading frame of the viral phosphoprotein (P) gene. The HPIV3 C protein interacted with the NLRP3 protein and blocked inflammasome activation by promoting the proteasomal degradation of the NLRP3 protein. Thus, our studies report NLRP3/ASC inflammasome activation by HPIV3 via TLR2 signaling and potassium efflux. Furthermore, we have identified HPIV3 C as a viral component involved in antagonizing inflammasome activation.IMPORTANCE Human parainfluenza virus type 3 (HPIV3) is a paramyxovirus that causes respiratory tract diseases during infancy and childhood. Currently, there is no effective vaccine or antiviral therapy for HPIV3. Therefore, in order to develop anti-HPIV3 agents (therapeutics and vaccines), it is important to study the HPIV3-host interaction during the immune response. Inflammasomes play an important role in the immune response. Inflammasome activation by HPIV3 has not been previously reported. Our studies demonstrated inflammasome activation by HPIV3 in macrophages. Specifically, HPIV3 activated the NLRP3/ASC inflammasome by TLR2 activation and potassium efflux. C proteins of paramyxoviruses are accessory proteins encoded by the viral phosphoprotein gene. The role of the C protein in inflammasome regulation was unknown. Surprisingly, our studies revealed that the HPIV3 C protein antagonizes inflammasome activation. In addition, we highlighted for the first time a mechanism utilized by paramyxovirus accessory proteins to block inflammasome activation. The HPIV3 C protein interacted with the NLRP3 protein to trigger the proteasomal degradation of the NLRP3 protein.

The role of nanotechnology in the treatment of viral infections

Infectious diseases are the leading cause of mortality worldwide, with viruses in particular making global impact on healthcare and socioeconomic development. In addition, the rapid development of drug resistance to currently available therapies and adverse side effects due to prolonged use is a serious public health concern. The development of novel treatment strategies is therefore required. The interaction of nanostructures with microorganisms is fast-revolutionizing the biomedical field by offering advantages in both diagnostic and therapeutic applications. Nanoparticles offer unique physical properties that have associated benefits for drug delivery. These are predominantly due to the particle size (which affects bioavailability and circulation time), large surface area to volume ratio (enhanced solubility compared to larger particles), tunable surface charge of the particle with the possibility of encapsulation, and large drug payloads that can be accommodated. These properties, which are unlike bulk materials of the same compositions, make nanoparticulate drug delivery systems ideal candidates to explore in order to achieve and/or improve therapeutic effects. This review presents a broad overview of the application of nanosized materials for the treatment of common viral infections.

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.

Generation and Comprehensive Analysis of Host Cell Interactome of the PA Protein of the Highly Pathogenic H5N1 Avian Influenza Virus in Mammalian Cells

Accumulating data have identified the important roles of PA protein in replication and pathogenicity of influenza A virus (IAV). Identification of host factors that interact with the PA protein may accelerate our understanding of IAV pathogenesis. In this study, using immunoprecipitation assay combined with liquid chromatography-tandem mass spectrometry, we identified 278 human cellular proteins that might interact with PA of H5N1 IAV. Gene Ontology annotation revealed that the identified proteins are highly associated with viral translation and replication. Further KEGG pathway analysis of the interactome profile highlighted cellular pathways associated with translation, infectious disease, and signal transduction. In addition, Diseases and Functions analysis suggested that these cellular proteins are highly related with Organismal Injury and Abnormalities and Cell Death and Survival. Moreover, two cellular proteins (nucleolin and eukaryotic translation elongation factor 1-alpha 1) identified both in this study and others were further validated to interact with PA using co-immunoprecipitation and co-localization assays. Therefore, this study presented the interactome data of H5N1 IAV PA protein in human cells which may provide novel cellular target proteins for elucidating the potential molecular functions of PA in regulating the lifecycle of IAV in human cells.

Polyethylenimine-functionalized carbon nanotubes tagged with AS1411 aptamer for combination gene and drug delivery into human gastric cancer cells

In this project, synergistic cancer cell death was achieved by a targeted delivery system comprising Bcl-xL-specific shRNA and a very low DOX content, which simultaneously activated an intrinsic apoptotic pathway. A modified branched polyethylenimine (PEI 10kDa) was grafted through polyethylene glycol (PEG) linker to carboxylated single-walled carbon nanotubes (SWCNT) to serve as a vehicle for shRNA delivery. The SWNT-PEG-PEI conjugate was covalently attached to AS1411 aptamer as the nucleolin ligand to target the co-delivery system to the tumor cells overexpressing nucleolin receptors on their surface. The final vehicle was eventually obtained after intercalation of DOX with pBcl-xL shRNA-SWCNT-PEG-10-10%PEI-Apt. Cell viability assay, GFP expression and transfection experiment against L929 (-nucleolin) and AGS (+nucleolin) cells illustrated that the tested targeted delivery system inhibited the growth of nucleolin-abundant gastric cancer cells with strong cell selectivity. Subsequently, we illustrated that the combination treatment of the selected shRNAs and DOX had excellent tumoricidal efficacy as verified by MTT assay. Furthermore, very low concentration of DOX, approximately 58-fold lower than its IC50 concentration, was used which could mitigate toxic side effects of DOX. Overall, our work revealed that combination of shRNA-mediated gene-silencing strategy with chemotherapeutic agents constitutes a valuable and safe approach for antitumor activity.

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.

Nucleolin interacts with influenza A nucleoprotein and contributes to viral ribonucleoprotein complexes nuclear trafficking and efficient influenza viral replication

Influenza viruses replicate their single-stranded RNA genomes in the nucleus of infected cells and these replicated genomes (vRNPs) are then exported from the nucleus to the cytoplasm and plasma membrane before budding. To achieve this export, influenza viruses hijack the host cell export machinery. However, the complete mechanisms underlying this hijacking remain not fully understood. We have previously shown that influenza viruses induce a marked alteration of the nucleus during the time-course of infection and notably in the nucleolar compartment. In this study, we discovered that a major nucleolar component, called nucleolin, is required for an efficient export of vRNPs and viral replication. We have notably shown that nucleolin interacts with the viral nucleoprotein (NP) that mainly constitutes vRNPs. Our results suggest that this interaction could allow vRNPs to "catch" the host cell export machinery, a necessary step for viral replication.

Hemagglutinin of influenza A virus binds specifically to cell surface nucleolin and plays a role in virus internalization

The hemagglutinin (HA) protein of influenza A virus initiates cell entry by binding to sialic acids on target cells. In the current study, we demonstrated that in addition to sialic acids, influenza A/Puerto Rico/8/34 H1N1 (PR8) virus HA specifically binds to cell surface nucleolin (NCL). The interaction between HA and NCL was initially revealed with virus overlay protein binding assay (VOPBA) and subsequently verified with co-immunoprecipitation. Importantly, inhibiting cell surface NCL with NCL antibody, blocking PR8 viruses with purified NCL protein, or depleting endogenous NCL with siRNA all substantially reduced influenza virus internalization. We further demonstrated that NCL was a conserved cellular factor required for the entry of multiple influenza A viruses, including H1N1, H3N2, H5N1, and H7N9. Overall, our findings identified a novel role of NCL in influenza virus life cycle and established NCL as one of the host cell surface proteins for the entry of influenza A virus.

Lassa Virus Cell Entry via Dystroglycan Involves an Unusual Pathway of Macropinocytosis

The pathogenic Old World arenavirus Lassa virus (LASV) causes a severe hemorrhagic fever with a high rate of mortality in humans. Several LASV receptors, including dystroglycan (DG), TAM receptor tyrosine kinases, and C-type lectins, have been identified, suggesting complex receptor use. Upon receptor binding, LASV enters the host cell via an unknown clathrin- and dynamin-independent pathway that delivers the virus to late endosomes, where fusion occurs. Here we investigated the mechanisms underlying LASV endocytosis in human cells in the context of productive arenavirus infection, using recombinant lymphocytic choriomeningitis virus (rLCMV) expressing the LASV glycoprotein (rLCMV-LASVGP). We found that rLCMV-LASVGP entered human epithelial cells via DG using a macropinocytosis-related pathway independently of alternative receptors. Dystroglycan-mediated entry of rLCMV-LASVGP required sodium hydrogen exchangers, actin, and the GTPase Cdc42 and its downstream targets, p21-activating kinase-1 (PAK1) and Wiskott-Aldrich syndrome protein (N-Wasp). Unlike other viruses that enter cells via macropinocytosis, rLCMV-LASVGP entry did not induce overt changes in cellular morphology and hardly affected actin dynamics or fluid uptake. Screening of kinase inhibitors identified protein kinase C, phosphoinositide 3-kinase, and the receptor tyrosine kinase human hepatocyte growth factor receptor (HGFR) to be regulators of rLCMV-LASVGP entry. The HGFR inhibitor EMD 1214063, a candidate anticancer drug, showed antiviral activity against rLCMV-LASVGP at the level of entry. When combined with ribavirin, which is currently used to treat human arenavirus infection, EMD 1214063 showed additive antiviral effects. In sum, our study reveals that DG can link LASV to an unusual pathway of macropinocytosis that causes only minimal perturbation of the host cell and identifies cellular kinases to be possible novel targets for therapeutic intervention.

IMPORTANCE

Lassa virus (LASV) causes several hundred thousand infections per year in Western Africa, with the mortality rate among hospitalized patients being high. The current lack of a vaccine and the limited therapeutic options at hand make the development of new drugs against LASV a high priority. In the present study, we uncover that LASV entry into human cells via its major receptor, dystroglycan, involves an unusual pathway of macropinocytosis and define a set of cellular factors implicated in the regulation of LASV entry. A screen of kinase inhibitors revealed HGFR to be a possible candidate target for antiviral drugs against LASV. An HGFR candidate inhibitor currently being evaluated for cancer treatment showed potent antiviral activity and additive drug effects with ribavirin, which is used in the clinic to treat human LASV infection. In sum, our study reveals novel fundamental aspects of the LASV-host cell interaction and highlights a possible candidate drug target for therapeutic intervention.

Nucleolin promotes in vitro translation of feline calicivirus genomic RNA

Feline calicivirus depends on host-cell proteins for its replication. We previously showed that knockdown of nucleolin (NCL), a phosphoprotein involved in ribosome biogenesis, resulted in the reduction of FCV protein synthesis and virus yield. Here, we found that NCL may not be involved in FCV binding and entry into cells, but it binds to both ends of the FCV genomic RNA, and stimulates its translation in vitro. AGRO100, an aptamer that specifically binds and inactivates NCL, caused a strong reduction in FCV protein synthesis. This effect could be reversed by the addition of full-length NCL but not by a ΔrNCL, lacking the N-terminal domain. Consistent with this, FCV infection of CrFK cells stably expressing ΔrNCL led to a reduction in virus protein translation. These results suggest that NCL is part of the FCV RNA translational complex, and that the N-terminal part of the protein is required for efficient FCV replication.

Osteoprotegerin secreted by inflammatory and invasive breast cancer cells induces aneuploidy, cell proliferation and angiogenesis

Background

Osteoprotegerin (OPG) is a glycoprotein that has multifaceted role and is associated with several cancer malignancies like that of bladder carcinoma, gastric carcinoma, prostate cancer, multiple myeloma and breast cancer. Also OPG has been associated with several organ pathologies. The widespread expression of OPG suggests that OPG may have multiple biological activities that are yet to be explored.

Methods

The anchorage-independent sphere cultures of the adherent cells were instrumental in our study as it provided a deeper insight into the complexity of a 3D tumor. Cytokine profiling was performed for OPG’s detection in the microenvironment. ELISA and western blotting were performed to quantify the OPG secretion and measure the protein levels respectively. OPG expression was detected in human breast cancer tissue samples by IHC. To decipher OPG’s role in tumor aggressiveness both recombinant human OPG as well as OPG rich and depleted breast cancer cell conditioned media were tested. Western blotting and MTT assay were performed to detect changes in signaling pathways and proliferation that were induced in presence of OPG. Onset of aneuploidy, in presence of OPG, was measured by cell cycle analysis and western blotting. Finally, human Breast Cancer qBiomarker Copy Number PCR Array was used to detect how OPG remarkably induced gene copy numbers for oncogenic pathway regulators.

Results

SUM149PT and SUM1315M02 cells secrete high levels of the cytokine OPG compared to primary human mammary epithelial cells (HMEC). High expression of OPG was also detected in human breast cancer tissue samples compared to the uninvolved tissue from the same patient. OPG induced proliferation of control HMEC spheres and triggered the onset of aneuploidy in HMEC sphere cultures. OPG induced the expression of aneuploidy related kinases Aurora-A Kinase (IAK-1), Bub1 and BubR1 probably through the receptor activator of nuclear factor kappa-B ligand (RANKL) and syndecan-1 receptors via the Erk, AKT and GSK3(3 signaling pathway. Gene copy numbers for oncogenic pathway regulators such AKT1, Aurora-A Kinase (AURKA or IAK-1), epidermal growth factor receptor (EGFR) and MYC with a reduction in the copy numbers of cyclin dependent kinase inhibitor 2A (CDKN2A), PTEN and DNA topoisomerase 2 alpha (TOP2A) were induced in presence of OPG.

Conclusions

These results highlight the role of OPG in reprogramming normal mammary epithelial cells to a tumorigenic state and suggest promising avenues for treating inflammatory breast cancer as well as highly invasive breast cancer with new therapeutic targets.

Electronic supplementary material

The online version of this article (doi:10.1186/s12885-015-1837-1) contains supplementary material, which is available to authorized users.

New insights on the viral and host factors contributing to the airway pathogenesis caused by the respiratory syncytial virus

The respiratory syncytial virus (RSV) is the most prevalent etiological agent of lower respiratory tract infections and the first cause of hospitalization in infants due to respiratory disease worldwide. However, efforts to develop safe and effective vaccines and antivirals have been challenged by an incomplete understanding of the RSV pathogenesis and the host immune response to RSV infection in the airways. Here, we discuss recent advances in understanding the interaction between RSV and the epithelium to induce pathogenesis in the airways, such as the role of the RSV NS2 protein in the airway epithelium, as well as the events involved in the RSV entry process. In addition, we summarize the cellular factors produced by airway epithelial cells (AECs) in response to RSV infection that lead to the activation of innate and adaptive immune responses, inducing lung inflammation and disease. Further, we discuss the possible contribution of a recently identified cytokine, thymic stromal lymphopoitein (TSLP), in the lung immunopathology caused by RSV.

High throughput proteomic analysis and a comparative review identify the nuclear chaperone, Nucleophosmin among the common set of proteins modulated in Chikungunya virus infection

Global re-emergence of Chikungunya virus (CHIKV) has renewed the interest in its cellular pathogenesis. We subjected CHIKV-infected Human Embryo Kidney cells (HEK293), a widely used cell-based system for CHIKV infection studies, to a high throughput expression proteomics analysis by Liquid Chromatography–tandem mass spectrometry. A total of 1047 differentially expressed proteins were identified in infected cells, consistently in three biological replicates. Proteins involved in transcription, translation, apoptosis and stress response were the major ones among the 209 proteins that had significant up-regulation. In the set of 45 down-regulated proteins, those involved in carbohydrate and lipid metabolism predominated. A STRING network analysis revealed tight interaction of proteins within the apoptosis, stress response and protein synthesis pathways. We short-listed a common set of 30 proteins that can be implicated in cellular pathology of CHIKV infection by comparing our results and results of earlier CHIKV proteomics studies. Modulation of eight proteins selected from this set was re-confirmed at transcript level. One among them, Nucleophosmin, a nuclear chaperone, showed temporal modulation and cytoplasmic aggregation upon CHIKV infection in double immunofluorescence staining and confocal microscopy. The short-listed cellular proteins will be potential candidates for targeted study of the molecular interactions of CHIKV with host cells.

Biological significance

Chikungunya remained as a neglected tropical disease till its re-emergence in 2005 in the La RéUnion islands and subsequently, in India and many parts of South East Asia. These and the epidemics that followed in subsequent years ran an explosive course leading to extreme morbidity and attributed mortality to this originally benign virus infection. Apart from classical symptoms of acute fever and debilitating polyarthralgia lasting for several weeks, a number of complications were documented. These included aphthous-like ulcers and vesiculo-bullous eruptions on the skin, hepatic involvement, central nervous system complications such as encephalopathy and encephalitis, and transplacental transmission. The disease has recently spread to the Americas with its initial documentation in the Caribbean islands. The Asian genotype of this positive-stranded RNA virus of the Alphavirus genus has been attributed in these outbreaks. However, the disease ran a similar course as the one caused by the East, Central and South African (ECSA) genotype in the other parts of the world. Studies have documented a number of mutations in the re-emerging strains of the virus that enhances mosquito adaptability and modulates virus infectivity. This might support the occurrence of fiery outbreaks in the absence of herd immunity in affected population. Several research groups work to understand the pathogenesis of chikungunya and the mechanisms of complications using cellular and animal models. A few proteomics approaches have been employed earlier to understand the protein level changes in the infected cells. Our present study, which couples a high throughput proteomic analysis and a comparative review of these earlier studies, identifies a few critical molecules as hypothetical candidates that might be important in this infection and for future study.

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High throughput expression proteomics analysis in HEK293 cells

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Identified four major cellular pathways affected in Chikungunya virus infection

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Short-listed 30 key proteins modulated by a comparative review

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Confirmed modulation of Nucleophosmin and other selected proteins upon infection

Nucleolin is a receptor for maleylated-bovine serum albumin on macrophages

Scavenger receptors have a broad range of functions that include pathogen clearance, and identification of the scavenger receptor family has been of great benefit to the field of physiology. The shuttling-protein nucleolin has recently been shown to possess scavenger receptor-like activity. We therefore investigated whether or not nucleolin is a receptor for maleylated-bovine serum albumin (maleylated-BSA), which is a common ligand for scavenger receptors. Binding and phagocytosis of native control-BSA by thioglycollate-elicited mouse peritoneal macrophages was weak, but that of maleylated-BSA was strong. Surface plasmon-resonance analysis revealed that nucleolin strongly associated with maleylated-BSA but not control-BSA or maleic anhydride. Further, co-treatment of macrophages with anti-nucleolin antibody, but not control-immunoglobulin G, inhibited binding of maleylated-BSA. In addition, antineoplastic guanine rich oligonucleotide (AGRO), a nucleolin-specific oligonucleotide aptamer, inhibited binding of maleylated-BSA. Further, binding of maleylated-BSA to nucleolin-transfected HEK293 cells was higher than that by control HEK cells. These results indicate that nucleolin is a receptor that enables macrophages to recognize maleylated-BSA.

Nucleolin Acts as a Scavenger Receptor for Acetylated Low-Density Lipoprotein on Macrophages

Although macrophage phagocytoses modified low-density lipoprotein (LDL), excessive accumulation of modified LDL induces macrophage foam cell formation, which is a feature of atherosclerotic plaque. Thus, the identification of scavenger receptor for modified LDL will provide better understanding of an atherosclerotic event. We recently showed that nucleolin expressed on macrophages acts as a scavenger receptor for various endogenous discarded products. Here, we investigated whether or not nucleolin is involved in the uptake of acetylated LDL (AcLDL). In contrast to normal LDL, AcLDL directly bound to immobilized nucleolin. AcLDL exhibited a higher affinity for macrophages than normal LDL. This binding of AcLDL was inhibited by anti-nucleolin antibody and antineoplastic guanine-rich oligonucleotide (AGRO), a nucleolin-specific oligonucleotide aptamer. In addition, AcLDL exhibited a higher affinity for HEK cells transfected with nucleolin than those without. Further, intracellular accumulation of AcLDL was also inhibited by anti-nucleolin antibody. The results of this study suggest that nucleolin expressed on macrophages is a receptor for AcLDL.

Identification of cellular proteins that interact with Newcastle Disease Virus and human Respiratory Syncytial Virus by a two-dimensional virus overlay protein binding assay (VOPBA)

Although it is well documented that the initial attachment receptors for Newcastle Disease Virus (NDV) and Respiratory Syncytial Virus (RSV) are sialic acid-containing molecules and glycosaminoglycans respectively, the exact nature of the receptors for both viruses remains to be deciphered. Moreover, additional molecules at the host cell surface might be involved in the entry mechanism. With the aim of identifying the cellular proteins that interact with NDV and RSV at the cell surface, we performed a virus overlay protein binding assay (VOPBA). Cell membrane lysates were separated by two dimensional (2D) gel electrophoresis and electrotransferred to PVDF membranes, after which they were probed with high viral concentrations. NDV interacted with a Protein Disulfide Isomerase from chicken fibroblasts. In the case of RSV, we detected 15 reactive spots, which were identified as six different proteins, of which nucleolin was outstanding. We discuss the possible role of PDI and nucleolin in NDV and RSV entry, respectively.

Highly sulfated K5 Escherichia coli polysaccharide derivatives inhibit respiratory syncytial virus infectivity in cell lines and human tracheal-bronchial histocultures

Respiratory syncytial virus (RSV) exploits cell surface heparan sulfate proteoglycans (HSPGs) as attachment receptors. The interaction between RSV and HSPGs thus presents an attractive target for the development of novel inhibitors of RSV infection. In this study, selective chemical modification of the Escherichia coli K5 capsular polysaccharide was used to generate a collection of sulfated K5 derivatives with a backbone structure that mimics the heparin/heparan sulfate biosynthetic precursor. The screening of a series of N-sulfated (K5-NS), O-sulfated (K5-OS), and N,O-sulfated (K5-N,OS) derivatives with different degrees of sulfation revealed the highly sulfated K5 derivatives K5-N,OS(H) and K5-OS(H) to be inhibitors of RSV. Their 50% inhibitory concentrations were between 1.07 nM and 3.81 nM in two different cell lines, and no evidence of cytotoxicity was observed. Inhibition of RSV infection was maintained in binding and attachment assays but not in preattachment assays. Moreover, antiviral activity was also evident when the K5 derivatives were added postinfection, both in cell-to-cell spread and viral yield reduction assays. Finally, both K5-N,OS(H) and K5-OS(H) prevented RSV infection in human-derived tracheal/bronchial epithelial cells cultured to form a pseudostratified, highly differentiated model of the epithelial tissue of the human respiratory tract. Together, these features put K5-N,OS(H) and K5-OS(H) forward as attractive candidates for further development as RSV inhibitors.

The molecular biology of nairoviruses, an emerging group of tick-borne arboviruses

The nairoviruses are a rapidly emerging group of tick-borne bunyaviruses that includes pathogens of humans (Crimean-Congo hemorrhagic fever virus [CCHFV]) and livestock (Nairobi sheep disease virus [NSDV], also known as Ganjam virus), as well as a large number of viruses for which the normal vertebrate host has not been established. Studies on this group of viruses have been fairly limited, not least because CCHFV is a BSL4 human pathogen, restricting the number of labs able to study the live virus, while NSDV, although highly pathogenic in naive animals, is not seen as a threat in developed countries, making it a low priority. Nevertheless, recent years have seen significant progress in our understanding of the biology of these viruses, particularly that of CCHFV, and this article seeks to draw together our existing knowledge to generate an overall picture of their molecular biology, underlining areas of particular ignorance for future studies.

Viruses and the nucleolus: the fatal attraction

Viruses are small obligatory parasites and as a consequence, they have developed sophisticated strategies to exploit the host cell's functions to create an environment that favors their own replication. A common feature of most – if not all – families of human and non-human viruses concerns their interaction with the nucleolus. The nucleolus is a multifunctional nuclear domain, which, in addition to its well-known role in ribosome biogenesis, plays several crucial other functions. Viral infection induces important nucleolar alterations. Indeed, during viral infection numerous viral components localize in nucleoli, while various host nucleolar proteins are redistributed in other cell compartments or are modified, and non-nucleolar cellular proteins reach the nucleolus. This review highlights the interactions reported between the nucleolus and some human or animal viral families able to establish a latent or productive infection, selected on the basis of their known interactions with the nucleolus and the nucleolar activities, and their links with virus replication and/or pathogenesis. This article is part of a Special Issue entitled: Role of the Nucleolus in Human Disease.

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Most viruses interact with the nucleolus that plays a major role in virus life cycle.

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Virus/nucleolus interaction is crucial for virus replication and pathogenesis.

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Role of nucleoli in the infection with selected RNA viruses and herpes viruses

Nucleolin interacts with the dengue virus capsid protein and plays a role in formation of infectious virus particles

Dengue virus (DENV) is a mosquito-transmitted flavivirus that can cause severe disease in humans and is considered a reemerging pathogen of significant importance to public health. The DENV capsid (C) protein functions as a structural component of the infectious virion; however, it may have additional functions in the virus replicative cycle. Here, we show that the DENV C protein interacts and colocalizes with the multifunctional host protein nucleolin (NCL). Furthermore, we demonstrate that this interaction can be disrupted by the addition of an NCL binding aptamer (AS1411). Knockdown of NCL with small interfering RNA (siRNA) or treatment of cells with AS1411 results in a significant reduction of viral titers after DENV infection. Western blotting and quantitative RT-PCR (qRT-PCR) analysis revealed no differences in viral RNA or protein levels at early time points postinfection, suggesting a role for NCL in viral morphogenesis. We support this hypothesis by showing that treatment with AS1411 alters the migration characteristics of the viral capsid, as visualized by native electrophoresis. Here, we identify a critical interaction between DENV C protein and NCL that represents a potential new target for the development of antiviral therapeutics.

Role of N-linked glycosylation of the human parainfluenza virus type 3 hemagglutinin-neuraminidase protein

Human parainfluenza virus type 3 (hPIV-3) is a major respiratory tract pathogen that affects infants and young children. The hPIV-3 hemagglutinin-neuraminidase (HN) protein is a multifunctional protein mediating hemadsorption (HAD), neuraminidase (NA), and fusion promotion activities, each of which affects the ability of HN to promote viral fusion and entry. The hPIV-3 HN protein contains four potential sites (N308, N351, N485 and N523) for N-linked glycosylation. Electrophoretic mobility analysis of mutated HN proteins indicated that N308, N351 and N523 sites, but not the N485 site in HN protein, were targeted for the addition of glycans in BHK-21 cells. These functional glycosylation sites were systematically eliminated in various combinations from HN to form a panel of mutants in which the roles of individual carbohydrate chains and groups of carbohydrate chains could be analyzed. Removal of individual or multiple N-glycans on the hPIV-3 HN protein had no effects on transport to the cell surface, expression and NA activity. Single glycosylation site mutants (G1, G2 and G4) not only impaired fusion promotion activity but also reduced HAD activity of HN protein, which was even more obvious for all three double mutants (G12, G14 and G24) and the triple mutant (G124). In addition, every mutant protein retained F-interactive capability that was equal to the wild-type protein capability. Interestingly, the F protein that could be co-immunoprecipitated with the G12 mutated protein or immunoprecipitated with anti-F antibody was not efficiently cleaved. For G14, G24 and G124, little cleaved F protein was detected in co-immuoprecipitation F protein assay and its total amounts where in the cell lysates. The mechanism underlying hPIV-3 HN and F protein remained associated before and after receptor engagement and the strength of the HN-receptor interaction modulated the activation of F the protein which could determine the extent of fusion. Finally, we demonstrated that single or multiple N-glycosylation site mutations inhibited fusion at the earliest stages. Taken together, these results indicated that N-glycosylation of hPIV-3 HN is critical to its receptor recognition activity, cleavage of the F protein, and fusion promotion activity, but had no influence on its interaction with the homologous F protein and NA activity.

Macrophage recognition of cells with elevated calcium is mediated by carbohydrate chains of CD43

Macrophages remove deteriorating cells (those undergoing apoptosis and oxidation) via poly-N-acetyllactosaminyl chains on CD43 caps, a major cell-surface glycoprotein. Unusually high intracellular calcium levels are also deteriorating for cells and tissue. Here we artificially elevated calcium levels in cells and examined the mechanism by which this elevation was resolved by macrophages. Results showed that treatment with the calcium ionophore A23187 and ionomycin induces capping of CD43 on Jurkat cells, which are subsequently recognized and phagocytosed by macrophages, indicating that macrophages regard cells with elevated calcium as targets for removal. Further tests showed that A23187- and ionomycin-treated Jurkat cells did not induce apoptotic changes such as DNA fragmentation or phosphatidylserine expression, indicating that these cells were removed despite still being viable. Jurkat cells pretreated with anti-CD43 antibody or those with poly-N-acetyllactosaminyl chains containing oligosaccharides inhibited macrophage binding, indicating that macrophages recognize the poly-N-acetyllactosaminyl chains on CD43. Binding was also inhibited by treating macrophages with anti-nucleolin antibody, indicating that recognition occurs through nucleolin, a cell-surface receptor. Further, nucleolin-transfected HEK293 cells bound A23187-treated cells, and this binding was inhibited by in the presence of oligosaccharides. Taken together, these results show that elevated calcium levels induce CD43 capping, and macrophages remove the cells if their nucleolin receptors can bind to the poly-N-acetyllactosaminyl chains of capped CD43.

The RSV fusion receptor: not what everyone expected it to be

This article reviews current knowledge about respiratory syncytial virus (RSV) binding and entry into cells. The recent discovery of Nucleolin as a fusion receptor for RSV opens new avenues for developing interventions, while raising questions concerning RSV pathobiology and tropism. We also discuss characteristics of a good RSV drug target.

RNA-binding protein nucleolin in disease

Nucleolin is a multifunctional protein localized primarily in the nucleolus, but also found in the nucleoplasm, cytoplasm and cell membrane. It is involved in several aspects of DNA metabolism, and participates extensively in RNA regulatory mechanisms, including transcription, ribosome assembly, mRNA stability and translation, and microRNA processing. Nucleolin's implication in disease is linked to its ability to associate with target RNAs via its four RNA-binding domains and its arginine/glycin-rich domain. By modulating the post-transcriptional fate of target mRNAs, which typically bear AU-rich and/or G-rich elements, nucleolin has been linked to cellular events that influence disease, notably cell proliferation and protection against apoptotic death. Through its diverse RNA functions, nucleolin is increasingly implicated in pathological processes, particularly cancer and viral infection. Here, we review the RNA-binding activities of nucleolin, its influence on gene expression patterns, and its impact upon diseases. We also discuss the rising interest in targeting nucleolin therapeutically.

Virus entry: old viruses, new receptors

The long-sought entry receptors for rubella, sindbis and respiratory syncytial viruses (RV, SV and RSV), together with the missing measles virus (MV) receptor for infection of epithelial cells, were identified in 2011. These have been major developments in the field of virus entry. In addition, 2011 was rich in new information about the interactions of MV, RSV and phleboviruses with DC-SIGN during infection of dendritic cells, a crucial step allowing the virus to breach the epithelial barrier and gain access to the lymph nodes. This faciliates dissemination to susceptible tissues where it can develop a vigorous and sustained replication, to eventually target specific organs from which it can propagate into the environment and efficiently infect new hosts, closing the merry-go-round of the virus cycle.

Clearance of CD43-capped cells by macrophages: capping alone leads to phagocytosis

Apoptotic cells must be recognized early for phagocytosis to ensure that their toxic contents do not damage neighboring cells. In some cases this is achieved via CD43-capped membrane glycoproteins, the sialylpolylactosaminyl chains of which serve as ligands for phagocytosis by macrophages. However, because many additional changes occur during apoptosis, determining exactly which events are responsible for signaling macrophages to initiate phagocytosis remains a challenge. Here, we examined one clearance mechanism in detail and determined that capping of CD43 alone is sufficient to initiate phagocytosis. We induced macrophage-mediated phagocytosis by using cytochalasin B to artificially cap CD43 on healthy (non-apoptotic) Jurkat cells. Additional experiments confirmed that sialylpolylactosaminyl chains formed through this capping method are a prerequisite for removal, and that nucleolin is the macrophage receptor responsible for their detection. These findings strongly suggest that capping of CD43 presents a sufficient signal for phagocytosis without any additional membrane changes.

Identification of nucleolin as a cellular receptor for human respiratory syncytial virus

Human respiratory syncytial virus (RSV) causes a large burden of disease worldwide. There is no effective vaccine or therapy, and the use of passive immunoprophylaxis with RSV-specific antibodies is limited to high-risk patients. The cellular receptor (or receptors) required for viral entry and replication has yet to be described; its identification will improve understanding of the pathogenesis of infection and provide a target for the development of novel antiviral interventions. Here we show that RSV interacts with host-cell nucleolin via the viral fusion envelope glycoprotein and binds specifically to nucleolin at the apical cell surface in vitro. We observed decreased RSV infection in vitro in neutralization experiments using nucleolin-specific antibodies before viral inoculation, in competition experiments in which virus was incubated with soluble nucleolin before inoculation of cells, and upon RNA interference (RNAi) to silence cellular nucleolin expression. Transfection of nonpermissive Spodoptera frugiperda Sf9 insect cells with human nucleolin conferred susceptibility to RSV infection. RNAi-mediated knockdown of lung nucleolin was associated with a significant reduction in RSV infection in mice (P = 0.0004), confirming that nucleolin is a functional RSV receptor in vivo.

Identification of a putative Crimean-Congo hemorrhagic fever virus entry factor

Entry of enveloped viruses into cells is initiated by binding of their envelope glycoproteins (Envs) to cell surface-associated receptors. The Crimean-Congo hemorrhagic fever virus (CCHFV) has two Envs, Gn and Gc, with poorly understood role in binding to susceptible cells. We expressed codon optimized Gn and Gc, and identified independently folded soluble Env fragments, one of which (Gc residues 180-300) bound CCHFV susceptible cells supposedly by interacting with a putative receptor. This receptor binding domain (RBD) was used to identify its interacting partner by coimmunoprecipitation and mass spectrometry. Thus we identified the human cell surface nucleolin as a putative CCHFV entry factor. Nucleolin was expressed on all susceptible cells tested but not on the surface of cells resistant to CCHFV infection. Further studies are needed to explore the nucleolin function as a plausible CCHFV receptor and the molecular mechanisms of the Gc-nucleolin interactions. The identification of the CCHFV RBD and its binding partner could provide novel targets for therapy and tools for prevention as well as more complete understanding of the mechanisms of CCHFV entry and pathogenesis.

Cell-surface nucleolin is involved in lipopolysaccharide internalization and signalling in alveolar macrophages

C23 (nucleolin) shuttling between the nucleus, cytoplasm and cell surface has been implicated in controlling regulatory processes and may play a role in pathogen infection and autoimmune diseases. It has been reported that cell surface-expressed C23 on THP-1 monocytes is involved in the inflammatory response induced by LPS (lipopolysaccharide). This study investigates whether C23 is a membrane receptor for LPS during LPS-induced AMs (alveolar macrophages) activation. First, using immunofluorescence and microscopy, we detected the expression of C23 on the surface of AMs. Second, using LPS affinity columns, we demonstrated that C23 directly binds to LPS. Third, we found that LPS colocalized with C23 on both the cell surface and in the cytoplasm. Finally, knockdown of C23 expression on the cell surface using siRNA (small interfering RNA) led to significant reductions in the internalization of LPS, in LPS-induced NF-κB (nuclear factor κB)-DNA binding and in the protein expression of TNF (tumour necrosis factor)-α and IL-6 (interleukin-6). These findings provide evidence that cell-surface C23 on AMs may serve as a receptor for LPS and are essential for internalization and transport of LPS. Furthermore, C23 participates in the regulation of LPS-induced inflammation of AMs, which indicates that cell-surface C23 is a new and promising therapeutic target for the treatment of bacterial infections.

Nucleolin-mediated cellular trafficking of DNA nanoparticle is lipid raft and microtubule dependent and can be modulated by glucocorticoid

DNA nanoparticles (DNPs) are nonviral gene transfer vectors with excellent in vivo potential. Previously, we reported that cell surface nucleolin directly binds DNPs, and functions as an important receptor for DNPs. However, the fate of the nucleolin-DNP complex following cellular uptake remains elusive. In this study, we examined the role of lipid rafts in the uptake of DNPs, and found that both nucleolin and DNPs are recovered from the low-density raft fractions of the sucrose gradient. Furthermore, nucleolin colocalizes with, and coimmunoprecipitates with a raft protein, flotillin. Disruption of lipid rafts by depleting membrane cholesterol significantly inhibited DNP transfection, while inhibition of other endocytic pathways had little effect. Following the uptake, the nuclear import of the DNPs required microtubules but not F-actin. By coimmunoprecipitation in conjunction with tandem mass spectrometry, we identified glucocorticoid receptor (GCR) as a nucleolin-associated protein, and confirmed this result by western blot. Cortisone or dexamethasone increased nucleolin's association with GCR, and transfection by DNPs. Finally, we detected the expression of nucleolin on the surface of airway epithelia in vivo. Taken together, our findings shed light on important determinants of DNP trafficking in cells and support the notion that nucleolin is a good target for nonviral gene delivery.

Stat1 nuclear translocation by nucleolin upon monocyte differentiation

BACKGROUND

Members of the signal transducer and activator of transcription (Stat) family of transcription factors traverse the nuclear membrane through a specialized structure, called the nuclear pore complex (NPC), which represents a selective filter for the import of proteins. Karyophilic molecules can bind directly to a subset of proteins of the NPC, collectively called nucleoporins. Alternatively, the transport is mediated via a carrier molecule belonging to the importin/karyopherin superfamily, which transmits the import into the nucleus through the NPC.

METHODOLOGY/PRINCIPAL FINDINGS

In this study, we provide evidence for an alternative Stat1 nuclear import mechanism, which is mediated by the shuttle protein nucleolin. We observed Stat1-nucleolin association, nuclear translocation and specific binding to the regulatory DNA element GAS. Using expression of nucleolin transgenes, we found that the nuclear localization signal (NLS) of nucleolin is responsible for Stat1 nuclear translocation. We show that this mechanism is utilized upon differentiation of myeloid cells and is specific for the differentiation step from monocytes to macrophages.

CONCLUSIONS/SIGNIFICANCE

Our data add the nucleolin-Stat1 complex as a novel functional partner for the cell differentiation program, which is uniquely poised to regulate the transcription machinery via Stat1 and nuclear metabolism via nucleolin.

Non-viral vectors in cystic fibrosis gene therapy: recent developments and future prospects

Gene therapy has been proposed for a wide range of human diseases but few have received the level of attention over such a prolonged period as cystic fibrosis (CF) with over 20 clinical studies undertaken. Following a 10-year interval, clinical trials of an aerosolisable non-viral gene transfer agent have recently been initiated by researchers in the United Kingdom. Here we review the rationale and requirements for effective gene therapy for CF lung disease. The previous non-viral gene therapy trials are discussed and the prospects for the current leading non-viral formulations for CF gene therapy are considered. Factors affecting the selection and design of the plasmid DNA molecule, likely to be of central importance to clinical efficacy, are reviewed and we describe the potential merits of the formulation that has been selected for the forthcoming UK trials.

Involvement of the nucleolus in replication of human viruses

Viruses are intracellular pathogens that have to usurp some of the cellular machineries to provide an optimal environment for their own replication. An increasing number of reports reveal that many viruses induce modifications of nuclear substructures including nucleoli, whether they replicate or not in the nucleus of infected cells. Indeed, during infection of cells with various types of human viruses, nucleoli undergo important morphological modifications. A large number of viral components traffic to and from the nucleolus where they interact with different cellular and/or viral factors, numerous host nucleolar proteins are redistributed in other cell compartments or are modified and some cellular proteins are delocalised in the nucleolus of infected cells. Well‐documented studies have established that several of these nucleolar modifications play a role in some steps of the viral cycle, and also in fundamental cellular pathways. The nucleolus itself is the place where several essential steps of the viral cycle take place. In other cases, viruses divert host nucleolar proteins from their known functions in order to exert new unexpected role(s). Copyright © 2009 John Wiley & Sons, Ltd.