Myxoma virus lacking the pyrin-like protein M013 is sensed in human myeloid cells by both NLRP3 and multiple Toll-like receptors, which independently activate the inflammasome and NF-κB innate response pathways

The myxoma virus (MYXV)-encoded pyrin domain-containing protein M013 coregulates inflammatory responses mediated by both the inflammasome and the NF-κB pathways. Infection of human THP-1 monocytic cells with a MYXV construct deleted for the M013 gene (vMyxM013-KO), but not the parental MYXV, activates both the inflammasome and NF-κB pathways and induces a spectrum of proinflammatory cytokines and chemokines, like interleukin-1β (IL-1β), tumor necrosis factor (TNF), IL-6, and monocyte chemoattractant protein 1. Here, we report that vMyxM013-KO virus-mediated activation of inflammasomes and secretion of IL-1β are dependent on the adaptor protein ASC, caspase-1, and NLRP3 receptor. However, vMyxM013-KO virus-mediated activation of NF-κB signaling, which induces TNF secretion, was independent of ASC, caspase-1, and either the NLRP3 or AIM2 inflammasome receptors. We also report that early synthesis of pro-IL-1β in response to vMyxM013-KO infection is dependent upon the components of the inflammasome complex. Activation of the NLRP3 inflammasome and secretion of IL-1β was also dependent on the release of cathepsin B and production of reactive oxygen species (ROS). By using small interfering RNA screening, we further demonstrated that, among the RIG-I-like receptors (RLRs) and Toll-like receptors (TLRs), only TLR2, TLR6, TLR7, and TLR9 contribute to the NF-κB-dependent secretion of TNF and the inflammasome-dependent secretion of IL-1β in response to vMyxM013-KO virus infection. Additionally, we demonstrate that early triggering of the mitogen-activated protein kinase pathway by vMyxM013-KO virus infection of THP-1 cells plays a critical common upstream role in the coordinate induction of both NF-κB and inflammasome pathways. We conclude that an additional cellular sensor(s)/receptor(s) in addition to the known RLRs/TLRs plays a role in the M013 knockout virus-induced activation of NF-κB pathway signaling, but the activation of inflammasomes entirely depends on sensing by the NLRP3 receptor in response to vMyxM013-KO infection of human myeloid cells.     

Co-Regulation of NF-κB and Inflammasome-Mediated Inflammatory Responses by Myxoma Virus Pyrin Domain-Containing Protein M013

NF-κB and inflammasomes both play central roles in orchestrating anti-pathogen responses by rapidly inducing a variety of early-response cytokines and chemokines following infection. Myxoma virus (MYXV), a pathogenic poxvirus of rabbits, encodes a member of the cellular pyrin domain (PYD) superfamily, called M013. The viral M013 protein was previously shown to bind host ASC-1 protein and inhibit the cellular inflammasome complex that regulates the activation and secretion of caspase 1-regulated cytokines such as IL-1β and IL-18. Here, we report that human THP-1 monocytic cells infected with a MYXV construct deleted for the M013L gene (vMyxM013-KO), in stark contrast to the parental MYXV, rapidly induce high levels of secreted pro-inflammatory cytokines like TNF, IL-6, and MCP-1, all of which are regulated by NF-κB. The induction of these NF-κB regulated cytokines following infection with vMyxM013-KO was also confirmed in vivo using THP-1 derived xenografts in NOD-SCID mice. vMyxM013-KO virus infection specifically induced the rapid phosphorylation of IKK and degradation of IκBα, which was followed by nuclear translocation of NF-κB/p65. Even in the absence of virus infection, transiently expressed M013 protein alone inhibited cellular NF-κB-mediated reporter gene expression and nuclear translocation of NF-κB/p65. Using protein/protein interaction analysis, we show that M013 protein also binds directly with cellular NF-κB1, suggesting a direct physical and functional linkage between NF-κB1 and ASC-1. We further demonstrate that inhibition of the inflammasome with a caspase-1 inhibitor did not prevent the induction of NF-κB regulated cytokines following infection with vMyxM013-KO virus, but did block the activation of IL-1β. Thus, the poxviral M013 inhibitor exerts a dual immuno-subversive role in the simultaneous co-regulation of both the cellular inflammasome complex and NF-κB-mediated pro-inflammatory responses.     

Pharmacological Manipulation of the Akt Signaling Pathway Regulates Myxoma Virus Replication and Tropism in Human Cancer Cells

Viruses have evolved an assortment of mechanisms for regulating the Akt signaling pathway to establish a cellular environment more favorable for viral replication. Myxoma virus (MYXV) is a rabbit-specific poxvirus that encodes many immunomodulatory factors, including an ankyrin repeat-containing host range protein termed M-T5 that functions to regulate tropism of MYXV for rabbit lymphocytes and certain human cancer cells. MYXV permissiveness in these human cancer cells is dependent upon the direct interaction between M-T5 and Akt, which has been shown to induce the kinase activity of Akt. In this study, an array of compounds that selectively manipulate Akt signaling was screened and we show that only a subset of Akt inhibitors significantly decreased the ability of MYXV to replicate in previously permissive human cancer cells. Furthermore, reduced viral replication efficiency was correlated with lower levels of phosphorylated Akt. In contrast, the PP2A-specific phosphatase inhibitor okadaic acid promoted increased Akt kinase activation and rescued MYXV replication in human cancer cells that did not previously support viral replication. Finally, phosphorylation of Akt at residue Thr308 was shown to dictate the physical interaction between Akt and M-T5, which then leads to phosphorylation of Ser473 and permits productive MYXV replication in these human cancer cells. The results of this study further characterize the mechanism by which M-T5 exploits the Akt signaling cascade and affirms this interaction as a major tropism determinant that regulates the replication efficiency of MYXV in human cancer cells.Following viral infection, substantial alterations in cellular physiology often lead to modification of various cellular pathways critical to the success of viral replication. The demands for energy, nutrients, and macromolecular synthesis that accompany viral replication can be substantial; thus, many viruses have evolved elaborate strategies for hijacking key cellular signaling networks necessary to support their demands (9). By the same token, antiviral pathways activated by the virus infection may also need to be blocked or subverted to ensure successful virus replication. Poxviruses possess large double-stranded DNA (dsDNA) genomes that encode multiple gene products that specifically modify or debilitate the various host signaling responses of the infected cell (28). Many of the immunoregulatory factors expressed by poxviruses have been well characterized, and these factors include virokines, viroreceptors, signaling modulators, and inhibitors of various antiviral responses, such as initiation of apoptosis pathways and signaling by protective cytokines, like interferon and tumor necrosis factor (TNF) (42).Myxoma virus (MYXV) is a member of the Leporipoxvirus genus and exhibits a restricted pathogenesis that is limited to rabbits, primarily due to its specific immunomodulation of the immune system of leporids (48). In rabbits (Sylvilagus spp.) of the Americas, MYXV infection results in a benign infection, characterized by a cutaneous fibroma restricted to the site of inoculation (14); however, the same virus causes a rapid systemic and highly lethal infection called myxomatosis in European rabbits (Oryctolagus cuniculus) (15). Although MYXV has a narrow host range in nature and is pathogenic only to European rabbits, the tropism of MYXV has recently been extended to include human tumor cells in vitro (6, 47, 54, 57, 60) and in xenografted mice in vivo (24, 25, 61). The mechanisms that mediate MYXV tropism in human cancer cells are still being investigated, but one signaling requirement has been linked to the state of cellular Akt kinase activity (57). Human cancer cells (called type I) that exhibit high levels of endogenous phosphorylated Akt (Ser473 and Thr308) supported permissive MYXV replication, while cells with no detectable endogenous phosphorylated Akt, which were unaffected by the virus infection, were nonpermissive (type III). A unique subset of cancer cells (type II) were found to be permissive to wild-type MYXV but did not support MYXV replication following the deletion of the viral host range factor M-T5 (vMyxT5KO). These type II cells constitutively expressed only low levels of endogenous phosphorylated Akt (mostly at Thr308), but following infection with permissive MYXV, a significant increase in Akt phosphorylation (particularly at Ser473) was observed. In stark contrast, the endogenous levels of phosphorylated Akt remained essentially unchanged when type II cells were infected with the nonpermissive M-T5 knockout virus MYXV (vMyxT5KO) (57).The host range factor M-T5 is essential for MYXV replication in rabbit primary lymphocytes (RL-5 cells) and for virus pathogenesis in European rabbits (31). Structurally, M-T5 possesses seven ankyrin (ANK) repeats and a carboxyl-terminal PRANC (pox protein repeats of ankyrin C-terminal) motif, which closely resembles a cellular protein motif called the F-box domain (29). Interaction between M-T5 and components of the cellular SCF (Skp-cullin-F-box) ubiquitin ligase complex was shown to protect MYXV-infected cells from cell cycle arrest (19). In MYXV-infected type II human cancer cells, physical interaction between M-T5 and cellular Akt was shown to upregulate the kinase activity of Akt (57). In another study, M-T5 was shown to be functionally interchangeable with the host ANK repeat-containing protein PIKE-A, and activation of Akt by either PIKE-A or the viral M-T5 protein was sufficient to mediate MYXV permissiveness in type II human cancer cells (59). Similarly, addition of the immunosuppressant drug rapamycin was successful at rescuing vMyxT5KO replication in type II cells by upregulating Akt activation through the mTOR pathway (47). The critical role of Akt in the regulation of multiple biological processes makes Akt a central regulator of cellular signaling, and therefore, it is not surprising that many viruses have developed sophisticated strategies for manipulating the activation of Akt (9, 11).The serine/threonine kinase Akt (also called protein kinase B [PKB]) was initially discovered as the cellular homolog of the viral oncogene (v-Akt) carried by the AKT8 retrovirus isolated from a murine T-cell lymphoma (7, 20, 46). There are three isoforms found in mammals (Akt1, -2, and -3), encoded by separate genes but sharing over 80% amino acid sequence identity. Activation of Akt is predominantly dependent upon phosphoinositide 3-kinase (PI3K), which phosphorylates phosphoinositides (PIs) at the D3 position of the inositol ring to generate PI(3,4,5)P₃ (PIP₃). Akt possesses an N-terminal PH (pleckstrin homology) domain that binds PIP₃ to promote its translocation of the plasma membrane. Once localized at the membrane, Akt becomes phosphorylated at residue Thr308 in the activation loop by phosphoinositide-dependent kinase 1 (PDK1) and also within the carboxy terminus at residue Ser473 by mTORC2 (mammalian target of rapamycin complex 2) (2, 49, 50). Phosphorylation of both sites is necessary for full induction of Akt kinase activity. Akt is a key regulator of many important cellular functions, including cell survival, proliferation, glucose metabolism, and protein synthesis. In the majority of human cancer cells, the Akt pathway is either mutated or constitutively activated, contributing to cancer progression through both stimulation of cellular proliferation and inhibition of apoptosis (34, 55).In this study, we screened an array of Akt inhibitor compounds that selectively manipulate the Akt signaling network at some level and report that certain Akt inhibitors significantly blocked MYXV replication in previously permissive type I and II human cancer cells. An additional set of inhibitors selectively inhibited only the replication of MYXV deleted for M-T5 and did not modify the replicative ability of the parental wild-type virus. Furthermore, the decrease in viral replication efficiency was correlated with lower levels of phosphorylated Akt at residues Thr308 and Ser473. In contrast, certain PP2A-specific phosphatase inhibitors, such as okadaic acid, promoted increased Akt kinase activation and rescued MYXV replication in type III human cancer cells that did not previously support viral replication. Finally, we demonstrate that the hemi-phosphorylation of Akt at residue Thr308 dictates physical interaction between Akt and M-T5, which ultimately leads to productive MYXV replication in type II cancer cells. These studies show that activation of the Akt signaling cascade is essential for efficient MYXV replication in human cancer cells and further demonstrate the dynamic role by which M-T5 manipulates Akt signaling to establish a cellular environment more favorable for viral replication.     

M-T5, the ankyrin repeat, host range protein of myxoma virus, activates Akt and can be functionally replaced by cellular PIKE-A

The myxoma virus (MV) ankyrin repeat, host range factor M-T5 has the ability to bind and activate cellular Akt, leading to permissive MV replication in a variety of diverse human cancer cell lines (G. Wang, J. W. Barrett, M. Stanford, S. J. Werden, J. B. Johnston, X. Gao, M. Sun, J. Q. Cheng, and G. McFadden, Proc. Natl. Acad. Sci. USA 103:4640-4645, 2006). The susceptibility of permissive human cancer cells to MV infection is directly correlated with the basal or induced levels of phosphorylated Akt. When M-T5 is deleted from MV, the knockout virus, vMyxT5KO, can no longer productively infect a subset of human cancer cells (designated type II) that exhibit little or no endogenous phosphorylated Akt. In searching for a host counterpart of M-T5, we noted sequence similarity of M-T5 to a recently identified ankyrin repeat cellular binding protein of Akt called PIKE-A. PIKE-A binds and activates the kinase activity of Akt in a GTP-dependent manner and promotes the invasiveness of human cancer cell lines. Here, we demonstrate that transfected PIKE-A is able to rescue the ability of vMyxT5KO to productively infect type II human cancer cells that were previously resistant to infection. Also, cancer cells that were completely nonpermissive for both wild-type and vMyxT5KO infection (called type III) were rendered fully permissive following ectopic expression of PIKE-A. We conclude that the MV M-T5 host range protein is functionally interchangeable with the host PIKE-A protein and that the activation of host Akt by either M-T5 or PIKE-A is critical for the permissiveness of human cancer cells for MV.     

Myxoma virus M064 is a novel member of the poxvirus C7L superfamily of host range factors that controls the kinetics of myxomatosis in European rabbits

The myxoma virus (MYXV) carries three tandem C7L-like host range genes (M062R, M063R, and M064R). However, despite the fact that the sequences of these three genes are similar, they possess very distinctive functions in vivo. The role of M064 in MYXV pathogenesis was investigated and compared to the roles of M062 and M063. We report that M064 is a virulence factor that contributes to MYXV pathogenesis but lacks the host range properties associated with M062 and M063.     

An adenovirus early region 1A protein is required for maximal viral DNA replication in growth-arrested human cells.

Two closely related adenovirus early region 1A proteins are expressed in transformed cells. The smaller of these, which is 243 amino acids in length, is required for the transformation of primary rat cells and for the transformation of immortalized rat cells to anchorage-independent growth. This protein is not required for productive infection of exponentially growing HeLa cells but is required for maximal replication in growth (G0)-arrested human lung fibroblasts (WI-38 cells). To determine the function of this protein in viral replication in these G0-arrested cells, we compared viral early mRNA, early protein, and late protein synthesis after infection with wild type or a mutant which does not express the protein. No differences were found. However, viral DNA synthesis by the mutant was delayed and decreased to 20 to 30% that of wild type in these cells. Viral DNA synthesis was much less defective in growing WI-38 cells, and in the transformed human HeLa cell line it occurred at wild-type levels. Furthermore, the mutant which can express only the 243-amino-acid early region 1A protein induced cellular DNA synthesis in G0-arrested rat cells to the same level as wild-type virus. A mutant which can express only the 289-amino-acid early region 1A protein induced less cellular DNA synthesis in G0-arrested rat cells. We propose that the early region 1A 243-amino-acid protein alters the physiology of arrested permissive cells to allow maximal viral DNA replication. In nonpermissive rodent cells, the 243-amino-acid protein drives G0-arrested cells into S phase. This activity is probably important for the immortalization of primary cells.     

Oncolytic virotherapy synergism with signaling inhibitors: Rapamycin increases myxoma virus tropism for human tumor cells

Myxoma virus is a rabbit-specific poxvirus pathogen that also exhibits a unique tropism for human tumor cells and is dramatically oncolytic for human cancer xenografts. Most tumor cell lines tested are permissive for myxoma infection in a fashion intimately tied to the activation state of Akt kinase. A host range factor of myxoma virus, M-T5, directly interacts with Akt and mediates myxoma virus tumor cell tropism. mTOR is a regulator of cell growth and metabolism downstream of Akt and is specifically inhibited by rapamycin. We report that treatment of nonpermissive human tumor cell lines, which normally restrict myxoma virus replication, with rapamycin dramatically increased virus tropism and spread in vitro. This increased myxoma replication is concomitant with global effects on mTOR signaling, specifically, an increase in Akt kinase. In contrast to the effects on human cancer cells, rapamycin does not increase myxoma virus replication in rabbit cell lines or permissive human tumor cell lines with constitutively active Akt. This indicates that rapamycin increases the oncolytic capacity of myxoma virus for human cancer cells by reconfiguring the internal cell signaling environment to one that is optimal for productive virus replication and suggests the possibility of a potentially therapeutic synergism between kinase signaling inhibitors and oncolytic poxviruses for cancer treatment.     

Myxoma virus M141R expresses a viral CD200 (vOX-2) that is responsible for down-regulation of macrophage and T-cell activation in vivo

M141R is a myxoma virus gene that encodes a cell surface protein with significant amino acid similarity to the family of cellular CD200 (OX-2) proteins implicated in the regulation of myeloid lineage cell activation. The creation of an M141R deletion mutant myxoma virus strain (vMyx141KO) and its subsequent infection of European rabbits demonstrated that M141R is required for the full development of a lethal infection in vivo but is not required for efficient virus replication in susceptible cell lines in vitro. Minor secondary sites of infection were detected in the majority of rabbits infected with the M141R deletion mutant, demonstrating that the M141R protein is not required for the dissemination of virus within the host. When compared to wild-type myxoma virus-infected rabbits, vMyx141KO-infected rabbits showed higher activation levels of both monocytes/macrophages and lymphocytes in situ through assessments of inducible nitric oxide synthase-positive and CD25(+) infiltrating cells in infected and lymphoid tissues. Purified peripheral blood mononuclear cells from vMyx141KO-infected rabbits demonstrated an increased ability to express gamma interferon upon activation by phorbol myristate acetate plus ionomycin compared to cells purified from wild-type myxoma virus-infected rabbits. We concluded that the M141R protein is a bona fide CD200-like immunomodulator protein which is required for the full pathogenesis of myxoma virus in the European rabbit and that its loss from the virus results in increased activation levels of macrophages in infected lesions and draining lymph nodes as well as an increased activation level of circulating T lymphocytes during infection. We propose a model whereby M141R transmits inhibitory signals to tissue macrophages, and possibly resident CD200R(+) dendritic cells, that reduce their ability to antigenically prime lymphocytes and possibly provides anergic signals to T cells directly.     

Comparative analyses of human immunodeficiency virus type 1 (HIV-1) and HIV-2 Vif mutants.

Virion infectivity factor (vif), a gene found in all lentiviruses, plays an essential role in virus replication in certain target cells. We examined the replication competence of the human immunodeficiency virus type 2 (HIV-2) vif mutant in different T-cell lines and primary cells in comparison with that of the HIV-1 vif mutant. Both mutant viruses were unable to replicate in peripheral blood-derived mononuclear cells but replicated with wild-type efficiency in certain T-cell lines, such as SupT1 and MOLT-4/8. These results confirm the importance of vif in the infection of relevant target cells and imply that some cellular factor(s) could compensate for vif function. However, HIV-1 and HIV-2 vif mutant viruses also show differential replications in other cell lines, suggesting either different threshold requirements for the same cellular factor(s) or the involvement of different factors to compensate for vif-1 and vif-2 functions. By cross complementation experiments, we showed that vif-1 and vif-2 have similar functions. Our studies further indicate the existence of two kinds of nonpermissive cells: H9 is unable to complement HIV-1 delta vif but is susceptible to a one-round infection with HIV-1 delta vif produced from permissive cells. In contrast, U937 is nonpermissive for HIV-2 delta vif produced from permissive cells but, once infected, is able to complement the delta vif function. In both types of nonpermissive cells, a step prior to proviral DNA synthesis is affected.     

Nipah virus uses leukocytes for efficient dissemination within a host

Nipah virus (NiV) is a recently emerged zoonotic paramyxovirus whose natural reservoirs are several species of Pteropus fruit bats. NiV provokes a widespread vasculitis often associated with severe encephalitis, with up to 75% mortality in humans. We have analyzed the pathogenesis of NiV infection, using human leukocyte cultures and the hamster animal model, which closely reproduces human NiV infection. We report that human lymphocytes and monocytes are not permissive for NiV and a low level of virus replication is detected only in dendritic cells. Interestingly, despite the absence of infection, lymphocytes could efficiently bind NiV and transfer infection to endothelial and Vero cells. This lymphocyte-mediated transinfection was inhibited after proteolytic digestion and neutralization by NiV-specific antibodies, suggesting that cells could transfer infectious virus to other permissive cells without the requirement for NiV internalization. In NiV-infected hamsters, leukocytes captured and carried NiV after intraperitoneal infection without themselves being productively infected. Such NiV-loaded mononuclear leukocytes transfer lethal NiV infection into naïve animals, demonstrating efficient virus transinfection in vivo. Altogether, these results reveal a remarkable capacity of NiV to hijack leukocytes as vehicles to transinfect host cells and spread the virus throughout the organism. This mode of virus transmission represents a rapid and potent method of NiV dissemination, which may contribute to its high pathogenicity.     

The Myxoma Virus M-T5 Ankyrin Repeat Host Range Protein Is a Novel Adaptor That Coordinately Links the Cellular Signaling Pathways Mediated by Akt and Skp1 in Virus-Infected Cells

Most poxviruses express multiple proteins containing ankyrin (ANK) repeats accounting for a large superfamily of related but unique determinants of poxviral tropism. Recently, select members of this novel family of poxvirus proteins have drawn considerable attention for their potential roles in modulating intracellular signaling networks during viral infection. The rabbit-specific poxvirus, myxoma virus (MYXV), encodes four unique ANK repeat proteins, termed M-T5, M148, M149, and M150, all of which include a carboxy-terminal PRANC domain which closely resembles a cellular protein motif called the F-box domain. Here, we show that each MYXV-encoded ANK repeat protein, including M-T5, interacts directly with the Skp1 component of the host SCF ubiquitin ligase complex, and that the binding of M-T5 to cullin 1 is indirect via binding to Skp1 in the host SCF complex. To understand the significance of these virus-host protein interactions, the various binding domains of M-T5 were mapped. The N-terminal ANK repeats I and II were identified as being important for interaction with Akt, whereas the C-terminal PRANC/F-box-like domain was essential for binding to Skp1. We also report that M-T5 can bind Akt and the host SCF complex (via Skp1) simultaneously in MYXV-infected cells. Finally, we report that M-T5 specifically mediates the relocalization of Akt from the nucleus to the cytoplasm during infection with the wild-type MYXV, but not the M-T5 knockout version of the virus. These results indicate that ANK/PRANC proteins play a critical role in reprogramming disparate cellular signaling cascades to establish a new cellular environment more favorable for virus replication.Myxoma virus (MYXV) is a rabbit-specific poxvirus that has proven to be a useful model system to study the mechanism by which virus-encoded immunoregulatory proteins function to manipulate the various host immune responses during the course of viral infection (50). In its long-term evolutionary host (Sylvilagus sp.), MYXV causes a benign disease localized to the site of inoculation, but when the virus infects European rabbits (Oryctolagus cuniculus), it causes a rapid systemic and highly lethal infection called myxomatosis (13). The success of MYXV as a pathogen can be attributed to the ability of the virus to effectively avoid recognition and clearance by the immune systems of susceptible rabbit hosts. At the level of individual virus-infected cells, poxviruses, like MYXV, are particularly adept at binding and entering most mammalian cells, where they attempt to establish a favorable intracellular environment, which promotes viral replication. Thus, the ability of poxviruses to reconfigure or disable the various host antiviral responses of the infected cell directly dictates the outcome of a viral infection at the cellular level (28). To this end, poxviruses possess a large genomic capacity, and all encode a unique repertoire of immunoregulatory and host-interactive proteins that have evolved to specifically mediate a broad range of cellular processes critical for successful viral replication. To date, a large collection of poxvirus-encoded immunoregulatory proteins have been identified and characterized, including virokines, viroreceptors, signaling modulators, and inhibitors of various antiviral responses, such as apoptotic pathways and interferon signaling (43). More recently, a novel category of poxvirus ankyrin (ANK) repeat proteins have drawn considerable attention for their potential roles in modulating intracellular signaling networks during viral infection (48, 49, 53).With the exception of poxviruses, the ANK motif is not commonly reported in viruses, although numerous examples have been identified in eukaryotic, bacterial, and archaeal proteins (6). The ANK motif, a tandemly repeated consensus module of approximately 33 amino acid residues, has been demonstrated to mediate diverse protein-protein interactions between cellular proteins having a broad spectrum of functional roles (32, 42). Solved crystal structures have revealed a conserved fold structure of the ANK repeat unit, by which each repeat forms a characteristic helix-loop-helix structure with a beta-hairpin/loop region projecting out from the helices at a 90° angle (3, 16, 19, 26). However, the ANK fold appears to be defined by its structure rather than any conserved biological function since there is no specific conserved substrate or binding partner structure that is universally recognized by members of the superfamily.The majority of poxviral ANK repeat-containing proteins also include a conserved carboxy-terminal PRANC (pox protein repeats of ankyrin C terminus) motif, which closely resembles a cellular protein motif called the F-box domain (30). Characterized as substrate adaptors, F-box-containing host proteins function to recruit cellular substrate proteins to the SCF ubiquitin-ligase complex (named after their main components, Skp1, cullin 1 [CUL1], and an F-box protein), where the substrates selected by the complex are ubiquitinated and targeted for degradation by the proteasome (21, 45, 60). The process of selective ubiquitination is an essential regulatory step for many cellular processes, and the human genome encodes more than 70 different F-box proteins, which collectively are thought to specifically target a broad collection of cellular substrates for delivery to the SCF complex to initiate turnover (62).Accounting for the largest family of poxviral proteins, almost all chordopoxviruses encode multiple ANK repeat-containing proteins, some of which have been defined as viral host range or virulence factors (30). For example, canarypox virus encodes 51 ANK repeat proteins, accounting for greater than 20% of the genome; however, most other poxviruses express less than a half dozen ANK repeat proteins (52). MYXV encodes four unique ANK repeat proteins, termed M-T5, M148, M149, and M150, all of which have been described as virulence factors for myxomatosis in rabbits (5, 8, 33). The MYXV host range factor M-T5 was first characterized for its ability to regulate viral tropism within rabbit lymphocytes and, later, some classes of human cancer cell lines (33, 51). In human cancer cells, the direct physical interaction between M-T5 and the host cell Akt was shown to be a key restriction determinant for MYXV tropism in a subset referred to as type II cancer cells (56). Furthermore, M-T5 was shown to be functionally interchangeable with a host ANK repeat protein called PIKE-A, and the activation of Akt by either the viral M-T5 or the host PIKE-A protein was critical for MYXV permissiveness in type II human cancer cells (57). M-T5 was also demonstrated to protect MYXV-infected cells from virus-induced cell cycle arrest, a property which was linked to its ability to interact with a member of the host cell SCF complex called CUL1 (20). Unlike M-T5, no specific host binding partners or target substrates have yet been identified for M148, M149, or M150. However, in tumor necrosis factor alpha (TNF-α)-stimulated cells, M150 was shown to colocalize in the nucleus with NF-κB p65, suggesting that this MYXV protein may modulate the NF-κB pathway (8).In this study, we demonstrate that M-T5, M148, M149, and M150 all have functional carboxy-terminal PRANC/F-box-like domains and that each one can interact directly with the Skp1 component of the host SCF complex. We further examined the various binding domains of M-T5 and identified ANK repeats I and II as being important for interaction with Akt, whereas the PRANC/F-box-like domain was essential for binding to Skp1. We also show that the previously reported interaction of M-T5 with CUL1 was in fact, indirect linking of M-T5 to the host SCF complex via Skp1. More specifically, we investigated the ability of M-T5 to function as a molecular scaffold to link disparate cellular binding partners together within a single complex and report that the viral protein binds Akt and the SCF complex (via Skp1) simultaneously in MYXV-infected cells. Finally, we demonstrate that M-T5 specifically mediates the relocalization of Akt from the nucleus to the cytoplasm during MYXV infection. These results suggest that ANK/PRANC proteins, such as M-T5, play a critical role in reprogramming disparate cellular signaling cascades to establish a new cellular environment more favorable for viral replication.     

Suppression of viral replication by stress-inducible GADD34 protein via the mammalian serine/threonine protein kinase mTOR pathway

GADD34 is a protein that is induced by a variety of stressors, including DNA damage, heat shock, nutrient deprivation, energy depletion, and endoplasmic reticulum stress. Here, we demonstrated that GADD34 induced by vesicular stomatitis virus (VSV) infection suppressed viral replication in wild-type (WT) mouse embryo fibroblasts (MEFs), whereas replication was enhanced in GADD34-deficient (GADD34-KO) MEFs. Enhanced viral replication in GADD34-KO MEFs was reduced by retroviral gene rescue of GADD34. The level of VSV protein expression in GADD34-KO MEFs was significantly higher than that in WT MEFs. Neither phosphorylation of eIF2alpha nor cellular protein synthesis was correlated with viral replication in GADD34-KO MEFs. On the other hand, phosphorylation of S6 and 4EBP1, proteins downstream of mTOR, was suppressed by VSV infection in WT MEFs but not in GADD34-KO MEFs. GADD34 was able to associate with TSC1/2 and dephosphorylate TSC2 at Thr1462. VSV replication was higher in TSC2-null cells than in TSC2-expressing cells, and constitutively active Akt enhanced VSV replication. On the other hand, rapamycin, an mTOR inhibitor, significantly suppressed VSV replication in GADD34-KO MEFs. These findings demonstrate that GADD34 induced by VSV infection suppresses viral replication via mTOR pathway inhibition, indicating that cross talk between stress-inducible GADD34 and the mTOR signaling pathway plays a critical role in antiviral defense.     

Replication of Equine Infectious Anemia Virus in Engineered Mouse NIH 3T3 Cells

We employed the equine lentivirus equine infectious anemia virus (EIAV) to investigate the cellular restrictions for lentivirus replication in murine NIH 3T3 cells. The results of these studies demonstrate that NIH 3T3 cells expressing the EIAV receptor ELR1 and equine cyclin T1 supported productive replication of EIAV and produced infectious virions at levels similar to those found in a reference permissive equine cell line. The studies presented here demonstrate, for the first time, differential levels of restriction for EIAV and human immunodeficiency virus type 1 (HIV-1) replication in murine cells and suggest that these differences can be exploited to reveal critical virus-cell interactions required for HIV-1 assembly and budding of lentivirus particles.     

A genome-wide CRISPR/Cas9 gene knockout screen identifies immunoglobulin superfamily DCC subclass member 4 as a key host factor that promotes influenza virus endocytosis

Influenza virus infection is dependent on host cellular factors, and identification of these factors and their underlying mechanisms can provide important information for the development of strategies to inhibit viral infection. Here, we used a highly pathogenic H5N1 influenza virus to perform a genome-wide CRISPR/Cas9 gene knockout screen in human lung epithelial cells (A549 cells), and found that knockout of transmembrane protein immunoglobulin superfamily DCC subclass member 4 (IGDCC4) significantly reduced the replication of the virus in A549 cells. Further studies showed that IGDCC4 interacted with the viral hemagglutinin protein and facilitated virus internalization into host cells. Animal infection studies showed that replication of H5N1 virus in the nasal turbinates, lungs, and kidneys of IGDCC4-knockout mice was significantly lower than that in the corresponding organs of wild-type mice. Half of the IGDCC4-knockout mice survived a lethal H5N1 virus challenge, whereas all of the wild-type mice died within 11 days of infection. Our study identifies a novel host factor that promotes influenza virus infection by facilitating internalization and provides insights that will support the development of antiviral therapies.     

Parvovirus replication.

The members of the family Parvoviridae are among the smallest of the DNA viruses, with a linear single-stranded genome of about 5 kilobases. Currently the family is divided into three genera, two of which contain viruses of vertebrates and a third containing insect viruses. This review concentrates on the vertebrate viruses, with emphasis on recent advances in our insights into the molecular biology of viral replication. Traditionally the vertebrate viruses have been distinguished by the presence or absence of a requirement for a coinfection with a helper virus before productive infection can occur, hence the notion that the dependoviruses (adeno-associated viruses [AAV]) are defective. Recent data would suggest that not only is there a great deal of structural and genetic organizational similarity between the two types of vertebrate viruses, but also there is significant similarity in the molecular biology of productive replication. What differs is the physiological condition of the host cell that renders it permissive. Healthy dividing cells are permissive for productive replication by autonomous parvoviruses; such cells result in latent infection by dependoviruses. For a cell to become permissive for productive AAV replication, it must have been exposed to toxic conditions which activate a latent AAV genome. Such conditions can be caused by helper-virus infection or exposure to physical (UV light) or chemical (some carcinogens) agents. In this paper the molecular biology of replication is reviewed, with special emphasis on the role of the host and the consequences of viral infection for the host.     

Parvovirus replication.

The members of the family Parvoviridae are among the smallest of the DNA viruses, with a linear single-stranded genome of about 5 kilobases. Currently the family is divided into three genera, two of which contain viruses of vertebrates and a third containing insect viruses. This review concentrates on the vertebrate viruses, with emphasis on recent advances in our insights into the molecular biology of viral replication. Traditionally the vertebrate viruses have been distinguished by the presence or absence of a requirement for a coinfection with a helper virus before productive infection can occur, hence the notion that the dependoviruses (adeno-associated viruses [AAV]) are defective. Recent data would suggest that not only is there a great deal of structural and genetic organizational similarity between the two types of vertebrate viruses, but also there is significant similarity in the molecular biology of productive replication. What differs is the physiological condition of the host cell that renders it permissive. Healthy dividing cells are permissive for productive replication by autonomous parvoviruses; such cells result in latent infection by dependoviruses. For a cell to become permissive for productive AAV replication, it must have been exposed to toxic conditions which activate a latent AAV genome. Such conditions can be caused by helper-virus infection or exposure to physical (UV light) or chemical (some carcinogens) agents. In this paper the molecular biology of replication is reviewed, with special emphasis on the role of the host and the consequences of viral infection for the host.     

Myxoma virus M-T5 protects infected cells from the stress of cell cycle arrest through its interaction with host cell cullin-1

The myxoma virus (MV) M-T5 gene encodes an ankyrin repeat protein that is important for virus replication in cells from several species. Insight was gained into the molecular mechanisms underlying the role of M-T5 as a host range determinant when the cell cycle regulatory protein cullin-1 (cul-1) was identified as a cellular binding partner of M-T5 and found to colocalize with the protein in both nuclear and cytosolic compartments. Consistent with this interaction, infection with wild-type MV (vMyxlac) or a deletion mutant lacking M-T5 (vMyxT5KO) differentially altered cell cycle progression in a panel of permissive and nonpermissive cells. Cells infected with vMyxlac transitioned rapidly out of the G0/G1 phase and preferentially accumulated at the G2/M checkpoint, whereas infection with vMyxT5KO impeded progression through the cell cycle, resulting in a greater percentage of cells retained at G0/G1. Levels of the cul-1 substrate, p27/Kip-1, were selectively increased in cells infected with vMyxT5KO compared to vMyxlac, concurrent with decreased phosphorylation of p27/Kip-1 at Thr187 and decreased ubiquitination. Compared to cells infected with vMyxlac, cell death was increased in vMyxT5KO-infected cells following treatment with diverse stimuli known to induce cell cycle arrest, including infection itself, serum deprivation, and exposure to proteasome inhibitors or double-stranded RNA. Moreover, infection with vMyxlac, but not vMyxT5KO, was sufficient to overcome the G0/G1 arrest induced by these stimuli. These findings suggest that M-T5 regulates cell cycle progression at the G0/G1 checkpoint, thereby protecting infected cells from diverse innate host antiviral responses normally triggered by G0/G1 cell cycle arrest.