Cholesterol dependence of collagen and echovirus 1 trafficking along the novel α2β1 integrin internalization pathway

Enterovirus Replication Organelles and Inhibitors of Their Formation

Enteroviral replication reorganizes the cellular membrane. Upon infection, viral proteins and hijacked host factors generate unique structures called replication organelles (ROs) to replicate their viral genomes. ROs promote efficient viral genome replication, coordinate the steps of the viral replication cycle, and protect viral RNA from host immune responses. More recent researches have focused on the ultrastructure structures, formation mechanism, and functions in the virus life cycle of ROs. Dynamic model of enterovirus ROs structure is proposed, and the secretory pathway, the autophagy pathway, and lipid metabolism are found to be associated in the formation of ROs. With deeper understanding of ROs, some compounds have been found to show inhibitory effects on viral replication by targeting key proteins in the process of ROs formation. Here, we review the recent findings concerning the role, morphology, biogenesis, formation mechanism, and inhibitors of enterovirus ROs.

Extracellular Albumin and Endosomal Ions Prime Enterovirus Particles for Uncoating That Can Be Prevented by Fatty Acid Saturation

There is limited information about the molecular triggers leading to the uncoating of enteroviruses under physiological conditions. Using real-time spectroscopy and sucrose gradients with radioactively labeled virus, we show at 37°C, the formation of albumin-triggered, metastable uncoating intermediate of echovirus 1 without receptor engagement. This conversion was blocked by saturating the albumin with fatty acids. High potassium but low sodium and calcium concentrations, mimicking the endosomal environment, also induced the formation of a metastable uncoating intermediate of echovirus 1. Together, these factors boosted the formation of the uncoating intermediate, and the infectivity of this intermediate was retained, as judged by end-point titration. Cryo-electron microscopy reconstruction of the virions treated with albumin and high potassium, low sodium, and low calcium concentrations resulted in a 3.6-Å resolution model revealing a fenestrated capsid showing 4% expansion and loss of the pocket factor, similarly to altered (A) particles described for other enteroviruses. The dimer interface between VP2 molecules was opened, the VP1 N termini disordered and most likely externalized. The RNA was clearly visible, anchored to the capsid. The results presented here suggest that extracellular albumin, partially saturated with fatty acids, likely leads to the formation of the infectious uncoating intermediate prior to the engagement with the cellular receptor. In addition, changes in mono- and divalent cations, likely occurring in endosomes, promote capsid opening and genome release.IMPORTANCE There is limited information about the uncoating of enteroviruses under physiological conditions. Here, we focused on physiologically relevant factors that likely contribute to opening of echovirus 1 and other B-group enteroviruses. By combining biochemical and structural data, we show that, before entering cells, extracellular albumin is capable of priming the virus into a metastable yet infectious intermediate state. The ionic changes that are suggested to occur in endosomes can further contribute to uncoating and promote genome release, once the viral particle is endocytosed. Importantly, we provide a detailed high-resolution structure of a virion after treatment with albumin and a preset ion composition, showing pocket factor release, capsid expansion, and fenestration and the clearly visible genome still anchored to the capsid. This study provides valuable information about the physiological factors that contribute to the opening of B group enteroviruses.

Human platelet antigens in disease

Platelets have various functions and participate in primary hemostasis, inflammation, and immune responses. Human platelet antigens (HPAs) are alloantigens expressed on the platelet membrane. Each HPA represent one of six platelet glycoproteins GPIIb, GPIIIa, GPIa, GPIbα, GPIbβ, and CD109, and six biallelic systems are grouped. A single nucleotide polymorphism (SNP) in the gene sequence causes a single amino acid substitution of relevant platelet glycoprotein with the exception of HPA-14bw. High-throughput next-generation sequencing-based method has been developed, which enable accurately identification of HPA polymorphisms. The roles of HPA in disease were reviewed. HPAs mediate platelet-microorganism and platelet-malignant cell interactions, and they also participate in pathogenesis of hemorrhagic fever with renal syndrome and infective endocarditis. The exploration of HPA polymorphisms in association with disease susceptibility of individuals will benefit prevention or management of disease.

ECM turnover-stimulated gene delivery through collagen-mimetic peptide-plasmid integration in collagen

Gene therapies have great potential in regenerative medicine; however, clinical translation has been inhibited by low stability and limited transfection efficiencies. Herein, we incorporate collagen-mimetic peptide (CMP)-linked polyplexes in collagen scaffolds to increase DNA stability by up to 400% and enable tailorable in vivo transgene expression at 100-fold higher levels and 10-fold longer time periods. These improvements were directly linked to a sustained interaction between collagen and polyplexes that persisted during cellular remodeling, polyplex uptake, and intracellular trafficking. Specifically, incorporation of CMPs into polyethylenimine (PEI) polyplexes preserved serum-exposed polyplex-collagen activity over a period of 14days, with 4 orders-of-magnitude more intact DNA present in CMP-modified polyplex-collagen relative to unmodified polyplex-collagen after a 10day incubation under cell culture conditions. CMP-modification also altered endocytic uptake, as indicated by gene silencing studies showing a nearly 50% decrease in transgene expression in response to caveolin-1 silencing in modified samples versus only 30% in unmodified samples. Furthermore, cellular internalization studies demonstrated that polyplex-collagen association persisted within cells in CMP polyplexes, but not in unmodified polyplexes, suggesting that CMP linkage to collagen regulates intracellular transport. Moreover, experiments in an in vivo repair model showed that CMP modification enabled tailoring of transgene expression from 4 to 25days over a range of concentrations. Overall, these findings demonstrate that CMP decoration provides substantial improvements in gene retention, altered release kinetics, improved serum-stability, and improved gene activity in vivo. This versatile technique has great potential for multiple applications in regenerative medicine.

STATEMENT OF SIGNIFICANCE

In this work, we demonstrate a novel approach for stably integrating DNA into collagen scaffolds to exploit the natural process of collagen remodelling for high efficiency non-viral gene delivery. The incorporation of CMPs into DNA polyplexes, coupled with the innate affinity between CMPs and collagen, not only permitted improved control over polyplex retention and release, but also provided a series of substantial and highly unique benefits via the stable and persistent linkage between CMP-polyplexes and collagen fragments. Specifically, CMP-modification of polyplexes was demonstrated to (i) control release for nearly a month, (ii) improve vector stability under physiological-like conditions, and (iii) provide ligands able to efficiently transfer genes via endocytic collagen pathways. These unique properties overcome key barriers inhibiting non-viral gene therapy.

Integrin beta and receptor for activated protein kinase C are involved in the cell entry of Bombyx mori cypovirus

Receptor-mediated endocytosis using a β1 integrin-dependent internalization was considered as the primary mechanism for the initiation of mammalian reovirus infection. Bombyx mori cypovirus (BmCPV) is a member of Reoviridae family which mainly infects the midgut epithelium of silkworm; the cell entry of BmCPV is poorly explored. In this study, co-immunoprecipitation (Co-IP), virus overlay protein binding assay (VOPBA), and BmCPV-protein interaction on the polyvinylidene difluoride membrane (BmCPV-PI-PVDF) methods were employed to screen the interacting proteins of BmCPV, and several proteins including integrin beta and receptor for activated protein kinase C (RACK1) were identified as the candidate interacting proteins for establishing the infection of BmCPV. The infectivity of BmCPV was investigated in vivo and in vitro by RNA interference (RNAi) and antibody blocking methods, and the results showed that the infectivity of BmCPV was significantly reduced by either small interfering RNA-mediated silencing of integrin beta and RACK1 or antibody blocking of integrin beta and RACK1. The expression level of integrin beta or RACK1 is not the highest in the silkworm midgut which is a principal target tissue of BmCPV, suggesting that the molecules other than integrin beta or RACK1 might play a key role in determining the tissue tropism of BmCPV infection. The establishment of BmCPV infection depends on other factors, and these factors interacted with integrin beta and RACK1 to form receptor complex for the cell entry of BmCPV.

A Novel Open and Infectious Form of Echovirus 1

One of the hallmarks of enterovirus genome delivery is the formation of an uncoating intermediate particle. Based on previous studies of mostly heated picornavirus particles, intermediate particles were shown to have externalized the innermost capsid protein (VP4) and exposed the N terminus of VP1 and to have reduced infectivity. Here, in addition to the native and intact particle type, we have identified another type of infectious echovirus 1 (E1) particle population during infection. Our results show that E1 is slightly altered during entry, which leads to the broadening of the major virion peak in the sucrose gradient. In contrast, CsCl gradient separation revealed that in addition to the light intact and empty particles, a dense particle peak appeared during infection in cells. When the broad peak from the sucrose gradient was subjected to a CsCl gradient, it revealed light and dense particles, further suggesting that the shoulder represents the dense particle. The dense particle was permeable to SYBR green II, it still contained most of its VP4, and it was able to bind to its receptor α2β1 integrin and showed high infectivity. A thermal assay further showed that the α2β1 integrin binding domain (I-domain) stabilized the virus particle. Finally, heating E1 particles to superphysiological temperatures produced more fragile particles with aberrant ultrastructural appearances, suggesting that they are distinct from the dense E1 particles. These results describe a more open and highly infectious E1 particle that is naturally produced during infection and may represent a novel form of an uncoating intermediate.

IMPORTANCE

In this paper, we have characterized a possible uncoating intermediate particle of E1 that is produced in cells during infection. Before releasing their genome into the host cytosol, enteroviruses go through structural changes in their capsid, forming an uncoating intermediate particle. It was shown previously that structural changes can be induced by receptor interactions and, in addition, by heating the native virion to superphysiological temperatures. Here, we demonstrate that an altered, still infectious E1 particle is found during infection. This particle has a more open structure, and it cannot be formed by heating. It still contains the VP4 protein and is able to bind to its receptor and cause infection. Moreover, we show that in contrast to some other enteroviruses, the receptor-virion interaction has a stabilizing effect on E1. This paper highlights the differences between enterovirus species and further increases our understanding of various uncoating forms of enteroviruses.

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

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

Infectious Entry Pathway of Enterovirus B Species

Enterovirus B species (EV-B) are responsible for a vast number of mild and serious acute infections. They are also suspected of remaining in the body, where they cause persistent infections contributing to chronic diseases such as type I diabetes. Recent studies of the infectious entry pathway of these viruses revealed remarkable similarities, including non-clathrin entry of large endosomes originating from the plasma membrane invaginations. Many cellular factors regulating the efficient entry have recently been associated with macropinocytic uptake, such as Rac1, serine/threonine p21-activated kinase (Pak1), actin, Na/H exchanger, phospholipace C (PLC) and protein kinase Cα (PKCα). Another characteristic feature is the entry of these viruses to neutral endosomes, independence of endosomal acidification and low association with acidic lysosomes. The biogenesis of neutral multivesicular bodies is crucial for their infection, at least for echovirus 1 (E1) and coxsackievirus A9 (CVA9). These pathways are triggered by the virus binding to their receptors on the plasma membrane, and they are not efficiently recycled like other cellular pathways used by circulating receptors. Therefore, the best “markers” of these pathways may be the viruses and often their receptors. A deeper understanding of this pathway and associated endosomes is crucial in elucidating the mechanisms of enterovirus uncoating and genome release from the endosomes to start efficient replication.

Molecular mechanisms of Ebola virus pathogenesis: focus on cell death

Ebola virus (EBOV) belongs to the Filoviridae family and is responsible for a severe disease characterized by the sudden onset of fever and malaise accompanied by other non-specific signs and symptoms; in 30–50% of cases hemorrhagic symptoms are present. Multiorgan dysfunction occurs in severe forms with a mortality up to 90%. The EBOV first attacks macrophages and dendritic immune cells. The innate immune reaction is characterized by a cytokine storm, with secretion of numerous pro-inflammatory cytokines, which induces a huge number of contradictory signals and hurts the immune cells, as well as other tissues. Other highly pathogenic viruses also trigger cytokine storms, but Filoviruses are thought to be particularly lethal because they affect a wide array of tissues. In addition to the immune system, EBOV attacks the spleen and kidneys, where it kills cells that help the body to regulate its fluid and chemical balance and that make proteins that help the blood to clot. In addition, EBOV causes liver, lungs and kidneys to shut down their functions and the blood vessels to leak fluid into surrounding tissues. In this review, we analyze the molecular mechanisms at the basis of Ebola pathogenesis with a particular focus on the cell death pathways induced by the virus. We also discuss how the treatment of the infection can benefit from the recent experience of blocking/modulating cell death in human degenerative diseases.

Cholesterol shuttling is important for RNA replication of coxsackievirus B3 and encephalomyocarditis virus

Picornaviruses are a family of positive-strand RNA viruses that includes important human and animal pathogens. Upon infection, picornaviruses induce an extensive remodelling of host cell membranes into replication organelles (ROs), which is critical for replication. Membrane lipids and lipid remodelling processes are at the base of RO formation, yet their involvement remains largely obscure. Recently, phosphatidylinositol-4-phosphate was the first lipid discovered to be important for the replication of a number of picornaviruses. Here, we investigate the role of the lipid cholesterol in picornavirus replication. We show that two picornaviruses from distinct genera that rely on different host factors for replication, namely the enterovirus coxsackievirus B3 (CVB3) and the cardiovirus encephalomyocarditis virus (EMCV), both recruited cholesterol to their ROs. Although CVB3 and EMCV both required cholesterol for efficient genome replication, the viruses appeared to rely on different cellular cholesterol pools. Treatments that altered the distribution of endosomal cholesterol inhibited replication of both CVB3 and EMCV, showing the importance of endosomal cholesterol shuttling for the replication of these viruses. Summarizing, we here demonstrate the importance of cholesterol homeostasis for efficient replication of CVB3 and EMCV.

Swine interferon-induced transmembrane protein, sIFITM3, inhibits foot-and-mouth disease virus infection in vitro and in vivo

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Swine IFITM3 (sIFITM3) shares the conserved functional domains and amino acid residues with its human ortholog.

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sIFITM3 restricts FMDV infection in BHK cells.

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sIFITM3 disrupts FMDV viral attachment to the host cell surface.

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sIFITM3 protects suckling mice from FMDV challenge.

The interferon-induced transmembrane protein 3 (IFITM3) is a widely expressed potent antiviral effector of the host innate immune system. It restricts a diverse group of pathogenic, enveloped viruses, by interfering with endosomal fusion. In this report, the swine IFITM3 (sIFITM3) gene was cloned. It shares the functionally conserved CD225 domain and multiple critical amino acid residues (Y19, F74, F77, R86 and Y98) with its human ortholog, which are essential for antiviral activity. Ectopic expression of sIFITM3 significantly inhibited non-enveloped foot-and-mouth disease virus (FMDV) infection in BHK-21 cells. Furthermore, sIFITM3 blocked FMDV infection at early steps in the virus life cycle by disrupting viral attachment to the host cell surface. Importantly, inoculation of 2-day-old suckling mice with a plasmid expressing sIFITM3 conferred protection against lethal challenge with FMDV. These results suggest that sIFITM3 is a promising antiviral agent and that can safeguard the host from infection with FMDV.

Coxsackievirus A9 infects cells via nonacidic multivesicular bodies

Coxsackievirus A9 (CVA9) is a member of the human enterovirus B species in the Enterovirus genus of the family Picornaviridae. According to earlier studies, CVA9 binds to αVβ3 and αVβ6 integrins on the cell surface and utilizes β2-microglobulin, dynamin, and Arf6 for internalization. However, the structures utilized by the virus for internalization and uncoating are less well understood. We show here, based on electron microscopy, that CVA9 is found in multivesicular structures 2 h postinfection (p.i.). A neutral red labeling assay revealed that uncoating occurs mainly around 2 h p.i., while double-stranded RNA is found in the cytoplasm after 3 h p.i. The biogenesis of multivesicular bodies (MVBs) is crucial for promoting infection, as judged by the strong inhibitory effect of the wild-type form of Hrs and dominant negative form of VPS4 in CVA9 infection. CVA9 infection is dependent on phospholipase C at the start of infection, whereas Rac1 is especially important between 1 and 3 h p.i., when the virus is in endosomes. Several lines of evidence implicate that low pH does not play a role in CVA9 infection. The infection is not affected by Bafilomycin A1. In addition, CVA9 is not targeted to acidic late endosomes or lysosomes, and the MVBs accumulating CVA9 have a neutral pH. Thus, CVA9 is the second enterovirus demonstrated so far, after echovirus 1, that can trigger neutral MVBs, which are important for virus infection.

IMPORTANCE

We demonstrate here that the enterovirus coxsackievirus A9 (CVA9) uses a nonclathrin and nonacidic pathway to infect cells. CVA9 does not accumulate in conventional late endosomes or lysosomes. We found that inhibitors of phospholipase C (PLC), Rac1, and the Na(+)/H(+) exchanger decreased CVA9 infection. The PLC inhibitor acts on early entry, the Rac1 inhibitor acts between 1 and 3 h, when the virus is in endosomes, and the Na(+)/H(+) exchange inhibitor acts during various steps during the virus life cycle. The infection depends on the formation of novel neutral multivesicular bodies (MVBs), which accumulate CVA9 during the first hours of entry. Thus, CVA9 is the second enterovirus demonstrated so far, after echovirus 1, that can trigger formation of neutral MVBs. The data show that these enteroviruses favor nonacidic conditions and complex MVBs to promote virus infection.