Ezrin-radixin-moesin family proteins are involved in parvovirus replication and spreading

The Expression and Function of the Small Nonstructural Proteins of Adeno-Associated Viruses (AAVs)

Adeno-associated viruses (AAVs) are small, non-enveloped viruses that package a single-stranded (ss)DNA genome of 4.7 kilobases (kb) within their T = 1 icosahedral capsid. AAVs are replication-deficient viruses that require a helper virus to complete their life cycle. Recombinant (r)AAVs have been utilized as gene delivery vectors for decades in gene therapy applications. So far, six rAAV-based gene medicines have been approved by the US FDA. The 4.7 kb ssDNA genome of AAV encodes nine proteins, including three viral structural/capsid proteins, VP1, VP2, and VP3; four large nonstructural proteins (replication-related proteins), Rep78/68 and Rep52/40; and two small nonstructural proteins. The two nonstructured proteins are viral accessory proteins, namely the assembly associated protein (AAP) and membrane-associated accessory protein (MAAP). Although the accessory proteins are conserved within AAV serotypes, their functions are largely obscure. In this review, we focus on the expression strategy and functional properties of the small nonstructural proteins of AAVs.

How Viruses Hijack and Modify the Secretory Transport Pathway

Eukaryotic cells contain dynamic membrane-bound organelles that are constantly remodeled in response to physiological and environmental cues. Key organelles are the endoplasmic reticulum, the Golgi apparatus and the plasma membrane, which are interconnected by vesicular traffic through the secretory transport route. Numerous viruses, especially enveloped viruses, use and modify compartments of the secretory pathway to promote their replication, assembly and cell egression by hijacking the host cell machinery. In some cases, the subversion mechanism has been uncovered. In this review, we summarize our current understanding of how the secretory pathway is subverted and exploited by viruses belonging to Picornaviridae, Coronaviridae, Flaviviridae,Poxviridae, Parvoviridae and Herpesviridae families.

Non-viral gene delivery of the oncotoxic protein NS1 for treatment of hepatocellular carcinoma

Hepatocellular carcinoma (HCC) is related to increasing incidence rates and poor clinical outcomes due to lack of efficient treatment options and emerging resistance mechanisms. The aim of the present study is to exploit a non-viral gene therapy enabling the expression of the parvovirus-derived oncotoxic protein NS1 in HCC. This anticancer protein interacts with different cellular kinases mediating a multimodal host-cell death. Lipoplexes (LPX) designed to deliver a DNA expression plasmid encoding NS1 are characterized using a comprehensive set of in vitro assays. The mechanisms of cell death induction are assessed and phosphoinositide-dependent kinase 1 (PDK1) is identified as a potential predictive biomarker for a NS1-LPX-based gene therapy. In an HCC xenograft mouse model, NS1-LPX therapeutic approach results in a significant reduction in tumor growth and extended survival. Data provide convincing evidence for future studies using a targeted NS1 gene therapy for PDK1 overexpressing HCC.

Vibrio vulnificus RtxA1 cytotoxin targets filamin A to regulate PAK1- and MAPK-dependent cytoskeleton reorganization and cell death

Cytoskeletal rearrangement and acute cytotoxicity occur in Vibrio vulnificus-infected host cells. RtxA1 toxin, a multifunctional autoprocessing repeats-in-toxin (MARTX), is essential for the pathogenesis of V. vulnificus and the programmed necrotic cell death. In this study, HeLa cells expressing RtxA1 amino acids 1491–1971 fused to GFP were observed to be rounded. Through yeast two-hybrid screening and subsequent immunoprecipitation validation assays, we confirmed the specific binding of a RtxA1_1491–1971 fragment with host-cell filamin A, an actin cross-linking scaffold protein. Downregulation of filamin A expression decreased the cytotoxicity of RtxA1 toward host cells. Furthermore, the phosphorylation of JNK and p38 MAPKs was induced by the RtxA1-filamin A interaction during the toxin-mediated cell death. However, the phosphorylation of these MAPKs was not observed during the RtxA1 intoxication of filamin A-deficient M2 cells. In addition, the depletion of pak1, which appeared to be activated by the RtxA1-filamin A interaction, inhibited RtxA1-induced phosphorylation of JNK and p38, and the cells treated with a pak1 inhibitor exhibited decreased RtxA1-mediated cytoskeletal rearrangement and cytotoxicity. Thus, the binding of filamin A by the RtxA1_1491–1971 domain appears to be a requisite to pak1-mediated MAPK activation, which contributes to the cytoskeletal reorganization and host cell death.

H-1 Parvovirus as a Cancer-Killing Agent: Past, Present, and Future

The rat protoparvovirus H-1PV is nonpathogenic in humans, replicates preferentially in cancer cells, and has natural oncolytic and oncosuppressive activities. The virus is able to kill cancer cells by activating several cell death pathways. H-1PV-mediated cancer cell death is often immunogenic and triggers anticancer immune responses. The safety and tolerability of H-1PV treatment has been demonstrated in early clinical studies in glioma and pancreatic carcinoma patients. Virus treatment was associated with surrogate signs of efficacy including immune conversion of tumor microenvironment, effective virus distribution into the tumor bed even after systemic administration, and improved patient overall survival compared with historical control. However, monotherapeutic use of the virus was unable to eradicate tumors. Thus, further studies are needed to improve H-1PV's anticancer profile. In this review, we describe H-1PV's anticancer properties and discuss recent efforts to improve the efficacy of H-1PV and, thereby, the clinical outcome of H-1PV-based therapies.

Two Sides of the Coin: Ezrin/Radixin/Moesin and Merlin Control Membrane Structure and Contact Inhibition

The merlin-ERM (ezrin, radixin, moesin) family of proteins plays a central role in linking the cellular membranes to the cortical actin cytoskeleton. Merlin regulates contact inhibition and is an integral part of cell–cell junctions, while ERM proteins, ezrin, radixin and moesin, assist in the formation and maintenance of specialized plasma membrane structures and membrane vesicle structures. These two protein families share a common evolutionary history, having arisen and separated via gene duplication near the origin of metazoa. During approximately 0.5 billion years of evolution, the merlin and ERM family proteins have maintained both sequence and structural conservation to an extraordinary level. Comparing crystal structures of merlin-ERM proteins and their complexes, a picture emerges of the merlin-ERM proteins acting as switchable interaction hubs, assembling protein complexes on cellular membranes and linking them to the actin cytoskeleton. Given the high level of structural conservation between the merlin and ERM family proteins we speculate that they may function together.

Platinum-induced mitochondrial DNA mutations confer lower sensitivity to paclitaxel by impairing tubulin cytoskeletal organization

Development of chemoresistance is a cogent clinical issue in oncology, whereby combination of anticancer drugs is usually preferred also to enhance efficacy. Paclitaxel (PTX), combined with carboplatin, represents the standard first-line chemotherapy for different types of cancers. We here depict a double-edge role of mitochondrial DNA (mtDNA) mutations induced in cancer cells after treatment with platinum. MtDNA mutations were positively selected by PTX, and they determined a decrease in the mitochondrial respiratory function, as well as in proliferative and tumorigenic potential, in terms of migratory and invasive capacity. Moreover, cells bearing mtDNA mutations lacked filamentous tubulin, the main target of PTX, and failed to reorient the Golgi body upon appropriate stimuli. We also show that the bioenergetic and cytoskeletal phenotype were transferred along with mtDNA mutations in transmitochondrial hybrids, and that this also conferred PTX resistance to recipient cells. Overall, our data show that platinum-induced deleterious mtDNA mutations confer resistance to PTX, and confirm what we previously reported in an ovarian cancer patient treated with carboplatin and PTX who developed a quiescent yet resistant tumor mass harboring mtDNA mutations.

Human genome-wide RNAi screen reveals host factors required for enterovirus 71 replication

Enterovirus 71 (EV71) is a neurotropic enterovirus without antivirals or vaccine, and its host-pathogen interactions remain poorly understood. Here we use a human genome-wide RNAi screen to identify 256 host factors involved in EV71 replication in human rhabdomyosarcoma cells. Enrichment analyses reveal overrepresentation in processes like mitotic cell cycle and transcriptional regulation. We have carried out orthogonal experiments to characterize the roles of selected factors involved in cell cycle regulation and endoplasmatic reticulum-associated degradation. We demonstrate nuclear egress of CDK6 in EV71 infected cells, and identify CDK6 and AURKB as resistance factors. NGLY1, which co-localizes with EV71 replication complexes at the endoplasmatic reticulum, supports EV71 replication. We confirm importance of these factors for EV71 replication in a human neuronal cell line and for coxsackievirus A16 infection. A small molecule inhibitor of NGLY1 reduces EV71 replication. This study provides a comprehensive map of EV71 host factors and reveals potential antiviral targets.

Enterovirus 71 (EV71) infection causes a spectrum of symptoms including neurological disease. To improve our understanding of EV71-host interactions, Wu et al. here perform a genome-wide RNAi screen, which implicates cell cycle regulation and ER-associated degradation as important factors in EV71 replication.

Late Maturation Steps Preceding Selective Nuclear Export and Egress of Progeny Parvovirus

Although the mechanism is not well understood, growing evidence indicates that the nonenveloped parvovirus minute virus of mice (MVM) may actively egress before passive release through cell lysis. We have dissected the late maturation steps of the intranuclear progeny with the aims of confirming the existence of active prelytic egress and identifying critical capsid rearrangements required to initiate the process. By performing anion-exchange chromatography (AEX), we separated intranuclear progeny particles by their net surface charges. Apart from empty capsids (EC), two distinct populations of full capsids (FC) arose in the nuclei of infected cells. The earliest population of FC to appear was infectious but, like EC, could not be actively exported from the nucleus. Further maturation of this early population, involving the phosphorylation of surface residues, gave rise to a second, late population with nuclear export potential. While capsid surface phosphorylation was strictly associated with nuclear export capacity, mutational analysis revealed that the phosphoserine-rich N terminus of VP2 (N-VP2) was dispensable, although it contributed to passive release. The reverse situation was observed for the incoming particles, which were dephosphorylated in the endosomes. Our results confirm the existence of active prelytic egress and reveal a late phosphorylation event occurring in the nucleus as a selective factor for initiating the process.

IMPORTANCE

In general, the process of egress of enveloped viruses is active and involves host cell membranes. However, the release of nonenveloped viruses seems to rely more on cell lysis. At least for some nonenveloped viruses, an active process before passive release by cell lysis has been reported, although the underlying mechanism remains poorly understood. By using the nonenveloped model parvovirus minute virus of mice, we could confirm the existence of an active process of nuclear export and further characterize the associated capsid maturation steps. Following DNA packaging in the nucleus, capsids required further modifications, involving the phosphorylation of surface residues, to acquire nuclear export potential. Inversely, those surface residues were dephosphorylated on entering capsids. These spatially controlled phosphorylation-dephosphorylation events concurred with the nuclear export-import potential required to complete the infectious cycle.

Tumor Selectivity of Oncolytic Parvoviruses: From in vitro and Animal Models to Cancer Patients

Oncolytic virotherapy of cancer is among the innovative modalities being under development and especially promising for targeting tumors, which are resistant to conventional treatments. Presently, at least a dozen of viruses, belonging to nine different virus families, are being tested within the frames of various clinical studies in cancer patients. Continuously growing preclinical evidence showing that the autonomous rat parvovirus H-1 (H-1PV) is able to kill tumor cells that resist conventional treatments and to achieve a complete cure of various human tumors in animal models argues for its inclusion in the arsenal of oncolytic viruses with an especially promising bench to bedside translation potential. Oncolytic parvovirus safe administration to humans relies on the intrinsic preference of these agents for quickly proliferating, metabolically, and biochemically disturbed tumor versus normal cells (tumor selectivity or oncotropism). The present review summarizes and discusses (i) preclinical evidence of H-1PV innocuousness for normal cells and healthy tissues in vitro and in animals, respectively, (ii) toxicological assessments of H-1PV mono- or combined therapy in tumor-bearing virus-permissive animal models, as well as (iii) historical results of experimental infection of human cancer patients with H-1PV. Altogether, these data argue against a risk of H-1PV inducing significant toxic effects in human patients. This highly favorable safety profile allowed the translation of H-1PV preclinical research into a Phase I/IIa clinical trial being currently in progress.

PKCη/Rdx-driven phosphorylation of PDK1: a novel mechanism promoting cancer cell survival and permissiveness for parvovirus-induced lysis

The intrinsic oncotropism and oncosuppressive activities of rodent protoparvoviruses (PVs) are opening new prospects for cancer virotherapy. Virus propagation, cytolytic activity, and spread are tightly connected to activation of the PDK1 signaling cascade, which delays stress-induced cell death and sustains functioning of the parvoviral protein NS1 through PKC(η)-driven modifications. Here we reveal a new PV-induced intracellular loop-back mechanism whereby PKCη/Rdx phosphorylates mouse PDK1:S138 and activates it independently of PI3-kinase signaling. The corresponding human PDK1phosphoS135 appears as a hallmark of highly aggressive brain tumors and may contribute to the very effective targeting of human gliomas by H-1PV. Strikingly, although H-1PV does not trigger PDK1 activation in normal human cells, such cells show enhanced viral DNA amplification and NS1-induced death upon expression of a constitutively active PDK1 mimicking PDK1phosphoS135. This modification thus appears as a marker of human glioma malignant progression and sensitivity to H-1PV-induced tumor cell killing.

The H-1 protoparvovirus (H-1PV) is the first replication-competent member of the Parvoviridae family to undergo a phase I/IIa clinical trial in patients suffering from glioblastoma multiforme. Although the intrinsic oncotropism and oncolytic activity of protoparvoviruses are well known, the underlying molecular mechanisms remain elusive. Here we identify a PV-induced intracellular loop-back mechanism that promotes PV replication and cytotoxicity through PI3-kinase-independent stimulation of PDK1 and of the PKC and PKB/Akt1 downstream kinases. This mechanism involves PKCη/Rdx-mediated phosphorylation of PDK1 (at S138 in mouse or S135 in human). Interestingly, this phosphorylation appears as a hallmark of highly aggressive brain tumors. Although H-1PV does not promote it in normal human cells, experimentally administered activated PDK1 variants were able to sensitize these cells to virus infection. These data lead us to propose PDK1phosphoS135 as a new candidate marker for monitoring tumor progression and responsiveness to oncolytic parvovirotherapy, particularly in the case of highly aggressive brain tumors. Furthermore, the sensitivity of PDK1phosphoS135-positive cell lines to inhibitors of PKCη/Rdx argues for considering this complex as a potential target for anticancer drug development.

Viral genes as oncolytic agents for cancer therapy

Many viruses have the ability to modulate the apoptosis, and to accomplish it; viruses encode proteins which specifically interact with the cellular signaling pathways. While some viruses encode proteins, which inhibit the apoptosis or death of the infected cells, there are viruses whose encoded proteins can kill the infected cells by multiple mechanisms, including apoptosis. A particular class of these viruses has specific gene(s) in their genomes which, upon ectopic expression, can kill the tumor cells selectively without affecting the normal cells. These genes and their encoded products have demonstrated great potential to be developed as novel anticancer therapeutic agents which can specifically target and kill the cancer cells leaving the normal cells unharmed. In this review, we will discuss about the viral genes having specific cancer cell killing properties, what is known about their functioning, signaling pathways and their therapeutic applications as anticancer agents.

p38MAPK, Rho/ROCK and PKC pathways are involved in influenza-induced cytoskeletal rearrangement and hyperpermeability in PMVEC via phosphorylating ERM

Severe influenza infections are featured by acute lung injury, a syndrome of pulmonary microvascular leak. A growing number of evidences have shown that the pulmonary microvascular endothelial cells (PMVEC) are critical target of influenza virus, promoting microvascular leak. It is reported that there are multiple mechanisms by which influenza virus could elicit increased pulmonary endothelial permeability, in both direct and indirect manners. Ezrin/radixin/moesin family proteins, the linkers between plasma membrane and actin cytoskeleton, have been reported to be involved in cell adhesion, motility and may modulate endothelial permeability. Studies have also shown that ERM is phosphorylated in response to various stimuli via p38MAPK, Rho/ROCK or PKC pathways. However, it is unclear that whether influenza infection could induce ERM phosphorylation and its relocalization. In the present study, we have found that there are cytoskeletal reorganization and permeability increases in the course of influenza virus infection, accompanied by upregulated levels of p-ERM. p-ERM's aggregation along the periphery of PMVEC upon influenza virus infection was detected via confocal microscopy. Furthermore, we sought to determine the role of p38MAPK, Rho/ROCK and PKC pathways in ERM phosphorylation as well as their involvement in influenza virus-induced endothelial malfunction. The activation of p38MAPK, Rho/ROCK and PKC pathways upon influenza virus stimulation were observed, as evidenced by the evaluation of phosphorylated p38 (p-p38), phosphorylated MKK (p-MKK) in p38MAPK pathway, ROCK1 in Rho/ROCK pathway and phosphorylated PKC (p-PKC) in PKC pathway. We also showed that virus-induced ERM phosphorylation was reduced by using p38MAPK inhibitor, SB203580 (20 μM), Rho/ROCK inhibitor, Y27632 (20 μM), PKC inhibitor, LY317615 (10 μM). Additionally, influenza virus-induced F-actin reorganization and hyperpermeability were attenuated by pretreatment with SB203580, Y27632 and LY317615. Taken together, we provide the first evidence that p38MAPK, Rho/ROCK and PKC are involved in influenza-induced cytoskeletal changes and permeability increases in PMVEC via phosphorylating ERM.

Parvoviruses: Small Does Not Mean Simple

Parvoviruses are small, rugged, nonenveloped protein particles containing a linear, nonpermuted, single-stranded DNA genome of ∼5 kb. Their limited coding potential requires optimal adaptation to the environment of particular host cells, where entry is mediated by a variable program of capsid dynamics, ultimately leading to genome ejection from intact particles within the host nucleus. Genomes are amplified by a continuous unidirectional strand-displacement mechanism, a linear adaptation of rolling circle replication that relies on the repeated folding and unfolding of small hairpin telomeres to reorient the advancing fork. Progeny genomes are propelled by the viral helicase into the preformed capsid via a pore at one of its icosahedral fivefold axes. Here we explore how the fine-tuning of this unique replication system and the mechanics that regulate opening and closing of the capsid fivefold portals have evolved in different viral lineages to create a remarkably complex spectrum of phenotypes.

Differential replication of two porcine parvovirus strains in bovine cell lines ensues from initial DNA processing and NS1 expression

Porcine parvovirus (PPV) is a small DNA virus with restricted coding capacity. The 5 kb genome expresses three major non-structural proteins (NS1, NS2 and SAT), and two structural proteins (VP1 and VP2). These few viral proteins are pleiotropic and interact with cellular components throughout viral replication. In this regard, very few cell lines have been shown to replicate the virus efficiently. Cell lines were established from a primary culture of bovine cells that allowed allotropic variants of PPV to be distinguished. Three cell lines were differentially sensitive to infection by two prototype PPV strains, NADL-2 and Kresse. In the first cell line (D10), infection was restricted early in the infectious cycle and was not productive. Infection of the second cell line (G11) was 1000 times less efficient with the NADL-2 strain compared with porcine cells, while production of infectious virus of the Kresse strain was barely detectable. Restriction points in these cells were the initial generation of DNA replication intermediates and NS1 production. Infection with chimeras between NADL-2 and Kresse showed that residues outside the previously described allotropic determinant were also partially responsible for the restriction to Kresse replication in G11 cells. F4 cells were permissive to both strains, although genome replication and infectious virus production were lower than in the porcine cells used for comparison. These results highlight the dependent nature of parvovirus tropism on host factors and suggest that cells from a non-host origin can fully support a productive infection by both strains.

Tumor suppressing properties of rodent parvovirus NS1 proteins and their derivatives

Cancer chemotherapy with monospecific agents is often hampered by the rapid development of tumor resistance to the drug used. Therefore, combination treatments aiming at several different targets are sought. Viral regulatory proteins, modified or not, appear ideal for this purpose because of their multimodal killing action against neoplastically transformed cells. The large nonstructural protein NS1 of rodent parvoviruses is an excellent candidate for an anticancer agent, shown to interfere specifically with cancer cell growth and survival. The present review describes the structure, functions, and regulation of the multifunctional protein NS1, its specific interference with cell processes and cell protein activities, and what is known so far about the mechanisms underlying NS1 interference with cancer growth. It further outlines prospects for the development of new, multimodal cancer toxins and their potential applications.

Vesicular transport of progeny parvovirus particles through ER and Golgi regulates maturation and cytolysis

Progeny particles of non-enveloped lytic parvoviruses were previously shown to be actively transported to the cell periphery through vesicles in a gelsolin-dependent manner. This process involves rearrangement and destruction of actin filaments, while microtubules become protected throughout the infection. Here the focus is on the intracellular egress pathway, as well as its impact on the properties and release of progeny virions. By colocalization with cellular marker proteins and specific modulation of the pathways through over-expression of variant effector genes transduced by recombinant adeno-associated virus vectors, we show that progeny PV particles become engulfed into COPII-vesicles in the endoplasmic reticulum (ER) and are transported through the Golgi to the plasma membrane. Besides known factors like sar1, sec24, rab1, the ERM family proteins, radixin and moesin play (an) essential role(s) in the formation/loading and targeting of virus-containing COPII-vesicles. These proteins also contribute to the transport through ER and Golgi of the well described analogue of cellular proteins, the secreted Gaussia luciferase in absence of virus infection. It is therefore likely that radixin and moesin also serve for a more general function in cellular exocytosis. Finally, parvovirus egress via ER and Golgi appears to be necessary for virions to gain full infectivity through post-assembly modifications (e.g. phosphorylation). While not being absolutely required for cytolysis and progeny virus release, vesicular transport of parvoviruses through ER and Golgi significantly accelerates these processes pointing to a regulatory role of this transport pathway.

Previously, it was thought that non-enveloped lytic parvoviruses were released through a lytic burst of cells at the end of infection. However, recent work demonstrated that these small non-enveloped single-stranded DNA viruses are actively transported through vesicles from the nucleus, the site of replication and assembly, to the cell periphery. The current investigation demonstrates that progeny particles become engulfed into COPII-vesicles in the endoplasmic reticulum (ER) and are transported through the Golgi to the plasma membrane (PM). ERM family proteins radixin and moesin appear to play an essential role in this cellular secretion pathway. While passing through ER and Golgi cisternae, PVs maturate through post-assembly modifications, which significantly increase the infectivity of progeny virions. Finally, the vesicular transport of parvoviral particles was shown to regulate virus-induced cytolysis, thereby accelerating the further release and spread of progeny virions. As rodent PVs are currently viewed as oncolytic agents for cancer virotherapy, it is important to further investigate the mechanism of PV egress — not only to improve the spreading of these agents through the tumor mass, but also to optimize the induction of an anti-tumor immune response upon virus — induced cytolysis.

Comparative proteomic analysis of HIV-1 particles reveals a role for Ezrin and EHD4 in the Nef-dependent increase of virus infectivity

Nef is a human immunodeficiency virus type 1 (HIV-1) auxiliary protein that plays an important role in virus replication and the onset of acquired immunodeficiency. Although known functions of Nef might explain its contribution to HIV-1-associated pathogenesis, how Nef increases virus infectivity is still an open question. In vitro, Nef-deleted viruses have a defect that prevents efficient completion of early steps of replication. We have previously shown that this restriction is not due to the absence of Nef in viral particles. Rather, a loss of function in virus-producing cells accounts for the lower infectivity of nef-deleted viruses compared to wild-type (WT) viruses. Here we used DiGE and iTRAQ to identify differences between the proteomes of WT and nef-deleted viruses. We observe that glucosidase II is enriched in WT virions, whereas Ezrin, ALG-2, CD81, and EHD4 are enriched in nef-deleted virions. Functional analysis shows that glucosidase II, ALG-2, and CD81 have no or only Nef-independent effect on infectivity. In contrast, Ezrin and EHD4 are involved in the ability of Nef to increase virus infectivity (referred to thereafter as Nef potency). Indeed, simultaneous Ezrin and EHD4 depletion in SupT1 and 293T virus-producing cells result in an ∼30 and ∼70% decrease of Nef potency, respectively. Finally, while Ezrin behaves as an inhibitory factor counteracted by Nef, EHD4 should be considered as a cofactors required by Nef to increase virus infectivity.

Ezrin interacts with the SARS coronavirus Spike protein and restrains infection at the entry stage

BACKGROUND

Entry of Severe Acute Respiratory Syndrome coronavirus (SARS-CoV) and its envelope fusion with host cell membrane are controlled by a series of complex molecular mechanisms, largely dependent on the viral envelope glycoprotein Spike (S). There are still many unknowns on the implication of cellular factors that regulate the entry process.

METHODOLOGY/PRINCIPAL FINDINGS

We performed a yeast two-hybrid screen using as bait the carboxy-terminal endodomain of S, which faces the cytosol during and after opening of the fusion pore at early stages of the virus life cycle. Here we show that the ezrin membrane-actin linker interacts with S endodomain through the F1 lobe of its FERM domain and that both the eight carboxy-terminal amino-acids and a membrane-proximal cysteine cluster of S endodomain are important for this interaction in vitro. Interestingly, we found that ezrin is present at the site of entry of S-pseudotyped lentiviral particles in Vero E6 cells. Targeting ezrin function by small interfering RNA increased S-mediated entry of pseudotyped particles in epithelial cells. Furthermore, deletion of the eight carboxy-terminal amino acids of S enhanced S-pseudotyped particles infection. Expression of the ezrin dominant negative FERM domain enhanced cell susceptibility to infection by SARS-CoV and S-pseudotyped particles and potentiated S-dependent membrane fusion.

CONCLUSIONS/SIGNIFICANCE

Ezrin interacts with SARS-CoV S endodomain and limits virus entry and fusion. Our data present a novel mechanism involving a cellular factor in the regulation of S-dependent early events of infection.

An in-frame deletion in the NS protein-coding sequence of parvovirus H-1PV efficiently stimulates export and infectivity of progeny virions

An in-frame, 114-nucleotide-long deletion that affects the NS-coding sequence was created in the infectious molecular clone of the standard parvovirus H-1PV, thereby generating Del H-1PV. The plasmid was transfected and further propagated in permissive human cell lines in order to analyze the effects of the deletion on virus fitness. Our results show key benefits of this deletion, as Del H-1PV proved to exhibit (i) higher infectivity (lower particle-to-infectivity ratio) in vitro and (ii) enhanced tumor growth suppression in vivo compared to wild-type H-1PV. This increased infectivity correlated with an accelerated egress of Del H-1PV progeny virions in producer cells and with an overall stimulation of the viral life cycle in subsequently infected cells. Indeed, virus adsorption and internalization were significantly improved with Del H-1PV, which may account for the earlier appearance of viral DNA replicative forms that was observed with Del H-1PV than wild-type H-1PV. We hypothesize that the internal deletion within the NS2 and/or NS1 protein expressed by Del H-1PV results in the stimulation of some step(s) of the viral life cycle, in particular, a maturation step(s), leading to more efficient nuclear export of infectious viral particles and increased fitness of the virus produced.

Proteins selectively killing tumor cells

All human cells have a genetic program that upon activation will cause cell death, named apoptosis. Cancer cells can grow due to unbalances in proliferation, cell cycle regulation and their apoptosis machinery: genomic mutations resulting in non-functional pro-apoptosis proteins or over-expression of anti-apoptosis proteins form the basis of tumor formation. Surprisingly, lessons learned from viruses show that cancer cannot be regarded simply as the opposite of apoptosis. For instance, adenovirus can only transform cells when both its anti- and pro-apoptotic proteins are produced. Oncolytic viruses are known to replicate selectively in tumor cells resulting in cell death. Proteins derived from viruses, i.e. chicken anemia virus (CAV)-derived apoptosis-inducing protein (apoptin), adenovirus early region 4 open reading frame (E4orf4) and parvovirus-H1 derived non-structural protein 1 (NS1), the human alpha-lactalbumin made lethal to tumor cells (HAMLET), which is present in human milk or the human cytokines melanoma differentiation-associated gene-7 (mda-7) and tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) have all the ability to induce tumor-selective apoptosis. The tumor-selective apoptosis-inducing proteins seem to interact with transforming survival processes, which can become redirected by these proteins into cell death. Transformation-related processes have been identified, which seem to be crucial for the tumor-selectively killing activity of these proteins. For instance, the transformation-related protein phosphatase 2A (PP2A) plays a role in the induction of tumor-selective apoptosis. The proteins mda-7, TRAIL and HAMLET are already successfully tested in first clinical trials. Proteins harboring tumor-selective apoptosis characteristics represent, therefore, a therapeutic potential and a tool for unraveling tumor-related processes. Fundamental molecular and (pre)clinical therapeutic studies of the various tumor-selective apoptosis-inducing proteins apoptin, E4orf4, HAMLET, mda-7, NS1, TRAIL and related proteins will be discussed.