Interferon-induced antiviral Mx1 GTPase is associated with components of the SUMO-1 system and promyelocytic leukemia protein nuclear bodies

RNA sequencing provides novel insights into the pathogenesis of naturally occurring myxomatous mitral valve disease stage B1 in beagle dogs

Myxomatous mitral valve disease (MMVD) is the most common cardiovascular disorder in dogs with a high prevalence, accounting for approximately 75% of all canine heart disease cases. MMVD is a complex disease and shows variable progression from mild valve leakage to severe regurgitation, potentially leading to heart failure. However, the molecular mechanisms and age-related changes that govern disease progression, especially at the early stage (B1) before the development of discernable clinical signs, remain poorly understood. In this prospective study, we aimed to compare gene expression differences between blood samples of aged beagle dogs with stage B1 MMVD and those of healthy controls using RNA sequencing. Clinical evaluation was also conducted, which revealed minimal differences in radiographic and echocardiographic measurements despite distinct biomarker variations between the two groups. Comparative transcriptomics revealed differentially expressed genes associated with extracellular matrix remodeling, prostaglandin metabolism, immune modulation, and interferon-related pathways, which bear functional relevance for MMVD. In particular, the top 10 over- and under-expressed genes represent promising candidates for influencing pathogenic changes in MMVD stage B1. Our research findings, which include identified variations in clinical markers and gene expression, enhance our understanding of MMVD. Furthermore, they underscore the need for further research into early diagnosis and treatment strategies, as, to the best of our knowledge, no prior studies have explored the precise molecular mechanisms of stage B1 in MMVD through total RNA sequencing.

An evolutionarily conserved N-terminal leucine is essential for MX1 GTPase antiviral activity against different families of RNA viruses

Myxovirus resistance protein 1 (MX1) and MX2 are homologous, dynamin-like large GTPases, induced upon interferon exposure. Human MX1 (HsMX1) is known to inhibit many viruses, including influenza A virus, by likely acting at various steps of their life cycles. Despite decades of studies, the mechanism(s) of action with which MX1 proteins manage to inhibit target viruses is not fully understood. MX1 proteins are mechano-enzymes and share a similar organization to dynamin, with a GTPase domain and a carboxy-terminal stalk domain, connected by a bundle signaling element. These three elements are known to be essential for antiviral activity. HsMX1 has two unstructured regions, the L4 loop, also essential for antiviral activity, and a short amino (N)-terminal region, which greatly varies between MX1 proteins of different species. The role of this N-terminal domain in antiviral activity is not known. Herein, using mutagenesis, imaging, and biochemical approaches, we demonstrate that the N-terminal domain of HsMX1 is essential for antiviral activity against influenza A virus, Vesicular Stomatitis Virus, and La Crosse virus. Furthermore, we pinpoint a highly conserved leucine within this region, which is absolutely crucial for human, mouse, and bat MX1 protein antiviral activity. Importantly, mutation of this leucine does not compromise GTPase activity or oligomerization capabilities but does modify MX1 protein subcellular localization. The discovery of this essential and highly conserved residue defines this region as key for antiviral activity and may reveal insights as to the mechanism(s) of action of MX1 proteins.

Knockout of p53 leads to a significant increase in ALV-J replication

Avian leukemia is a common malignant disease, and and its regulatory mechanism is complex. As the most extensive tumor suppressor gene in cancer research, p53 can control multiple functions such as that of DNA repair, induction of apoptosis, cell cycle arrest and so on. In view of the diversity associated with varied function of p53, this study analyzed the possible effect of gene on ALV-J replication and its regulatory mechanism. We successfully constructed a p53 knockout DF-1 cell line (p53-KO-DF-1 cells) by using CRISPR-Cas9 system. When ALV-J was co-infected with DF-1 and p53-KO-DF-1 cells, it was found that compared with wild-type DF-1 cells, the viral copy number of p53-KO-DF-1 cells infected with ALV-J increased significantly 48 h after infection, whereas the expression of innate immune factors such as Il-2,TNF- α, IFN- γ and MX1 decreased significantly. Detection of p53-related tumor genes indicated that after p53 deletion, the expression of c-myc, bcl-2, and bak increased significantly, while the expression of p21 and p27 was noted to be decreased. The cell cycle distribution and apoptosis of the 2 cell lines was detected by flow cytometry analysis. The results showed that p53 knockout prevented G0/G1 and G2 M phase arrest induced by ALV-J, and substantially decreased the rate of apoptosis. Overall, the results indicated that p53 gene can effectively inhibits ALV-J replication by regulating important cellular processes, and p53 gene related proteins involved in cell cycle activity may function as the key targets for the prevention and treatment of ALV-J.

Interplay between RNA Viruses and Promyelocytic Leukemia Nuclear Bodies

Promyelocytic leukemia nuclear bodies (PML NBs) are nuclear membrane-less sub structures that play a critical role in diverse cellular pathways including cell proliferation, DNA damage, apoptosis, transcriptional regulation, stem cell renewal, alternative lengthening of telomeres, chromatin organization, epigenetic regulation, protein turnover, autophagy, intrinsic and innate antiviral immunity. While intrinsic and innate immune functions of PML NBs or PML NB core proteins are well defined in the context of nuclear replicating DNA viruses, several studies also confirm their substantial roles in the context of RNA viruses. In the present review, antiviral activities of PML NBs or its core proteins on diverse RNA viruses that replicate in cytoplasm or the nucleus were discussed. In addition, viral counter mechanisms that reorganize PML NBs, and specifically how viruses usurp PML NB functions in order to create a cellular environment favorable for replication and pathogenesis, are also discussed.

Murine GFP-Mx1 forms nuclear condensates and associates with cytoplasmic intermediate filaments: Novel antiviral activity against VSV

Type I and III interferons induce expression of the "myxovirus resistance proteins" MxA in human cells and its ortholog Mx1 in murine cells. Human MxA forms cytoplasmic structures, whereas murine Mx1 forms nuclear bodies. Whereas both HuMxA and MuMx1 are antiviral toward influenza A virus (FLUAV) (an orthomyxovirus), only HuMxA is considered antiviral toward vesicular stomatitis virus (VSV) (a rhabdovirus). We previously reported that the cytoplasmic human GFP-MxA structures were phase-separated membraneless organelles ("biomolecular condensates"). In the present study, we investigated whether nuclear murine Mx1 structures might also represent phase-separated biomolecular condensates. The transient expression of murine GFP-Mx1 in human Huh7 hepatoma, human Mich-2H6 melanoma, and murine NIH 3T3 cells led to the appearance of Mx1 nuclear bodies. These GFP-MuMx1 nuclear bodies were rapidly disassembled by exposing cells to 1,6-hexanediol (5%, w/v), or to hypotonic buffer (40-50 mosm), consistent with properties of membraneless phase-separated condensates. Fluorescence recovery after photobleaching (FRAP) assays revealed that the GFP-MuMx1 nuclear bodies upon photobleaching showed a slow partial recovery (mobile fraction: ∼18%) suggestive of a gel-like consistency. Surprisingly, expression of GFP-MuMx1 in Huh7 cells also led to the appearance of GFP-MuMx1 in 20-30% of transfected cells in a novel cytoplasmic giantin-based intermediate filament meshwork and in cytoplasmic bodies. Remarkably, Huh7 cells with cytoplasmic murine GFP-MuMx1 filaments, but not those with only nuclear bodies, showed antiviral activity toward VSV. Thus, GFP-MuMx1 nuclear bodies comprised phase-separated condensates. Unexpectedly, GFP-MuMx1 in Huh7 cells also associated with cytoplasmic giantin-based intermediate filaments, and such cells showed antiviral activity toward VSV.

The important roles of protein SUMOylation in the occurrence and development of leukemia and clinical implications

Leukemia is a severe malignancy of the hematopoietic system, which is characterized by uncontrolled proliferation and dedifferentiation of immature hematopoietic precursor cells in the lymphatic system and bone marrow. Leukemia is caused by alterations of the genetic and epigenetic regulation of processes underlying hematologic malignancies, including SUMO modification (SUMOylation). Small ubiquitin-like modifier (SUMO) proteins covalently or noncovalently conjugate and modify a large number of target proteins via lysine residues. SUMOylation is a small ubiquitin-like modification that is catalyzed by the SUMO-specific activating enzyme E1, the binding enzyme E2, and the ligating enzyme E3. SUMO is covalently linked to substrate proteins to regulate the cellular localization of target proteins and the interaction of target proteins with other biological macromolecules. SUMOylation has emerged as a critical regulatory mechanism for subcellular localization, protein stability, protein-protein interactions, and biological function and thus regulates normal life activities. If the SUMOylation process of proteins is affected, it will cause a cellular reaction and ultimately lead to various diseases, including leukemia. There is growing evidence showing that a large number of proteins are SUMOylated and that SUMOylated proteins play an important role in the occurrence and development of various types of leukemia. Targeting the SUMOylation of proteins alone or in combination with current treatments might provide powerful targeted therapeutic strategies for the clinical treatment of leukemia.

Biomolecular condensates in cell biology and virology: Phase-separated membraneless organelles (MLOs)

Membraneless organelles (MLOs) in the cytoplasm and nucleus in the form of 2D and 3D phase-separated biomolecular condensates are increasingly viewed as critical in regulating diverse cellular functions. These functions include cell signaling, immune synapse function, nuclear transcription, RNA splicing and processing, mRNA storage and translation, virus replication and maturation, antiviral mechanisms, DNA sensing, synaptic transmission, protein turnover and mitosis. Components comprising MLOs often associate with low affinity; thus cell integrity can be critical to the maintenance of the full complement of respective MLO components. Phase-separated condensates are typically metastable (shape-changing) and can undergo dramatic, rapid and reversible assembly and disassembly in response to cell signaling events, cell stress, during mitosis, and after changes in cytoplasmic "crowding" (as observed with condensates of the human myxovirus resistance protein MxA). Increasing evidence suggests that neuron-specific aberrations in phase-separation properties of RNA-binding proteins (e.g. FUS and TDP-43) and others (such as the microtubule-binding protein tau) contribute to the development of degenerative neurological diseases (e.g. amyotrophic lateral sclerosis, frontotemporal lobar degeneration, and Alzheimer's disease). Thus, studies of liquid-like phase separation (LLPS) and the formation, structure and function of MLOs are of considerable importance in understanding basic cell biology and the pathogenesis of human diseases.

SUMO and Cytoplasmic RNA Viruses: From Enemies to Best Friends

SUMO is a ubiquitin-like protein that covalently binds to lysine residues of target proteins and regulates many biological processes such as protein subcellular localization or stability, transcription, DNA repair, innate immunity, or antiviral defense. SUMO has a critical role in the signaling pathway governing type I interferon (IFN) production, and among the SUMOylation substrates are many IFN-induced proteins. The overall effect of IFN is increasing global SUMOylation, pointing to SUMO as part of the antiviral stress response. Viral agents have developed different mechanisms to counteract the antiviral activities exerted by SUMO, and some viruses have evolved to exploit the host SUMOylation machinery to modify their own proteins. The exploitation of SUMO has been mainly linked to nuclear replicating viruses due to the predominant nuclear localization of SUMO proteins and enzymes involved in SUMOylation. However, SUMOylation of numerous viral proteins encoded by RNA viruses replicating at the cytoplasm has been lately described. Whether nuclear localization of these viral proteins is required for their SUMOylation is unclear. Here, we summarize the studies on exploitation of SUMOylation by cytoplasmic RNA viruses and discuss about the requirement for nuclear localization of their proteins.

Equine Mx1 Restricts Influenza A Virus Replication by Targeting at Distinct Site of its Nucleoprotein

Interferon-mediated host factors myxovirus (Mx) proteins are key features in regulating influenza A virus (IAV) infections. Viral polymerases are essential for viral replication. The Mx1 protein has been known to interact with viral nucleoprotein (NP) and PB2, resulting in the influence of polymerase activity and providing interspecies restriction. The equine influenza virus has evolved as an independent lineage to influenza viruses from other species. We estimated the differences in antiviral activities between human MxA (huMxA) and equine Mx1 (eqMx1) against a broad range of IAV strains. We found that huMxA has antiviral potential against IAV strains from non-human species, whereas eqMx1 could only inhibit the polymerase activity of non-equine species. Here, we demonstrated that NP is the main target of eqMx1. Subsequently, we found adaptive mutations in the NP of strains A/equine/Jilin/1/1989 (H3N8_JL89) and A/chicken/Zhejiang/DTID-ZJU01/2013 (H7N9_ZJ13) that confer eqMx1 resistance and sensitivity respectively. A substantial reduction in Mx1 resistance was observed for the two mutations G34S and H52N in H3N8_JL89 NP. Thus, eqMx1 is an important dynamic force in IAV nucleoprotein evolution. We, therefore, suggest that the amino acids responsible for Mx1 resistance should be regarded as a robust indicator for the pandemic potential of lately evolving IAVs.

Executable pathway analysis using ensemble discrete-state modeling for large-scale data

Pathway analysis is widely used to gain mechanistic insights from high-throughput omics data. However, most existing methods do not consider signal integration represented by pathway topology, resulting in enrichment of convergent pathways when downstream genes are modulated. Incorporation of signal flow and integration in pathway analysis could rank the pathways based on modulation in key regulatory genes. This implementation can be facilitated for large-scale data by discrete state network modeling due to simplicity in parameterization. Here, we model cellular heterogeneity using discrete state dynamics and measure pathway activities in cross-sectional data. We introduce a new algorithm, Boolean Omics Network Invariant-Time Analysis (BONITA), for signal propagation, signal integration, and pathway analysis. Our signal propagation approach models heterogeneity in transcriptomic data as arising from intercellular heterogeneity rather than intracellular stochasticity, and propagates binary signals repeatedly across networks. Logic rules defining signal integration are inferred by genetic algorithm and are refined by local search. The rules determine the impact of each node in a pathway, which is used to score the probability of the pathway's modulation by chance. We have comprehensively tested BONITA for application to transcriptomics data from translational studies. Comparison with state-of-the-art pathway analysis methods shows that BONITA has higher sensitivity at lower levels of source node modulation and similar sensitivity at higher levels of source node modulation. Application of BONITA pathway analysis to previously validated RNA-sequencing studies identifies additional relevant pathways in in-vitro human cell line experiments and in-vivo infant studies. Additionally, BONITA successfully detected modulation of disease specific pathways when comparing relevant RNA-sequencing data with healthy controls. Most interestingly, the two highest impact score nodes identified by BONITA included known drug targets. Thus, BONITA is a powerful approach to prioritize not only pathways but also specific mechanistic role of genes compared to existing methods. BONITA is available at: https://github.com/thakar-lab/BONITA.

The 125th Lys and 145th Thr Amino Acids in the GTPase Domain of Goose Mx Confer Its Antiviral Activity against the Tembusu Virus

The Tembusu virus (TMUV) is an avian pathogenic flavivirus that causes a highly contagious disease and catastrophic losses to the poultry industry. The myxovirus resistance protein (Mx) of innate immune effectors is a key antiviral “workhorse” of the interferon (IFN) system. Although mammalian Mx resistance against myxovirus and retrovirus was witnessed for decades, whether or not bird Mx has anti-flavivirus activity remains unknown. In this study, we found that the transcription of goose Mx (goMx) was obviously driven by TMUV infection, both in vivo and in vitro, and that the titers and copies of TMUV were significantly reduced by goMx overexpression. In both primary (goose embryo fibroblasts, GEFs) and passaged cells (baby hamster kidney cells, BHK21, and human fetal kidney cells, HEK 293T), it was shown that goMx was mainly located in the cytoplasm, and sporadically distributed in the nucleus. The intracellular localization of this protein is attributed to the predicted bipartite nuclear localization signal (NLS; 30 residues: the 441st–471st amino acids of goMx). Intuitively, it seems that the cells with a higher level of goMx expression tend to have lower TMUV loads in the cytoplasm, as determined by an immunofluorescence assay. To further explore the antiviral determinants, a panel of variants was constructed. Two amino acids at the 125th (Lys) and 145th (Thr) positions in GTP-binding elements, not in the L4 loop (40 residues: the 532nd–572nd amino acids of goMx), were vital for the antiviral function of goMx against TMUV in vitro. These findings will contribute to our understanding of the functional significance of the antiviral system in aquatic birds, and the development of goMx could be a valuable therapeutic agent against TMUV.

The Discovery of the Antiviral Resistance Gene Mx: A Story of Great Ideas, Great Failures, and Some Success

The discovery of the Mx gene-dependent, innate resistance of mice against influenza virus was a matter of pure chance. Although the subsequent analysis of this antiviral resistance was guided by straightforward logic, it nevertheless led us into many blind alleys and was full of surprising turns and twists. Unexpectedly, this research resulted in the identification of one of the first interferon-stimulated genes and provided a new view of interferon action. It also showed that in many species, MX proteins have activities against a broad range of viruses. To this day, Mx research continues to flourish and to provide insights into the never-ending battle between viruses and their hosts.

Addition of an EGFP-tag to the N-terminal of influenza virus M1 protein impairs its ability to accumulate in ND10

A previous report demonstrated that influenza virus infection induces accumulation of EGFP-tagged M1 protein (EGFP-M1) in the sub-nuclear domain ND10. Here, we show that the transfection of four viral protein (NP, PB2, PB1, PA) expression vectors and eight RNA segment expression vectors induced the formation of nuclear dots of EGFP-M1 as seen in virus infections. Omission of the segment 7 RNA expression vector, however, abolished the nuclear dots of EGFP-M1. This result suggests an essential role for authentic M1 protein and/or M2 protein, both of which are encoded in segment 7, in the formation of nuclear dots of EGFP-M1. Co-expression of M1 protein but not M2 protein with EGFP-M1 induced the formation of nuclear dots of EGFP-M1. The dots co-localized with PML protein, which is an indicator of ND10. When only M1 protein was expressed, immunostaining of M1 protein clearly revealed the nuclear dots and their colocalization with PML protein. These results demonstrate that the accumulation in ND10 is an intrinsic characteristic of M1 protein and EGFP addition abolishes this characteristic. The addition of EGFP to M1 protein induced a defect in M1 protein.

Oas1b-dependent Immune Transcriptional Profiles of West Nile Virus Infection in the Collaborative Cross

The oligoadenylate-synthetase (Oas) gene locus provides innate immune resistance to virus infection. In mouse models, variation in the Oas1b gene influences host susceptibility to flavivirus infection. However, the impact of Oas variation on overall innate immune programming and global gene expression among tissues and in different genetic backgrounds has not been defined. We examined how Oas1b acts in spleen and brain tissue to limit West Nile virus (WNV) susceptibility and disease across a range of genetic backgrounds. The laboratory founder strains of the mouse Collaborative Cross (CC) (A/J, C57BL/6J, 129S1/SvImJ, NOD/ShiLtJ, and NZO/HlLtJ) all encode a truncated, defective Oas1b, whereas the three wild-derived inbred founder strains (CAST/EiJ, PWK/PhJ, and WSB/EiJ) encode a full-length OAS1B protein. We assessed disease profiles and transcriptional signatures of F1 hybrids derived from these founder strains. F1 hybrids included wild-type Oas1b (F/F), homozygous null Oas1b (N/N), and heterozygous offspring of both parental combinations (F/N and N/F). These mice were challenged with WNV, and brain and spleen samples were harvested for global gene expression analysis. We found that the Oas1b haplotype played a role in WNV susceptibility and disease metrics, but the presence of a functional Oas1b allele in heterozygous offspring did not absolutely predict protection against disease. Our results indicate that Oas1b status as wild-type or truncated, and overall Oas1b gene dosage, link with novel innate immune gene signatures that impact specific biological pathways for the control of flavivirus infection and immunity through both Oas1b-dependent and independent processes.

Mx GTPases: dynamin-like antiviral machines of innate immunity

The Mx dynamin-like GTPases are key antiviral effector proteins of the type I and type III interferon (IFN) systems. They inhibit several different viruses by blocking early steps of the viral replication cycle. We focus on new structural and functional insights and discuss recent data revealing that human MxA (MX1) provides a safeguard against introduction of avian influenza A viruses (FLUAV) into the human population. The related human MxB (MX2) serves as restriction factor for HIV-1 and other primate lentiviruses.

MxA interacts with and is modified by the SUMOylation machinery

Mx proteins are evolutionarily conserved dynamin-like large GTPases involved in viral resistance triggered by types I and III interferons. The human MxA is a cytoplasmic protein that confers resistance to a large number of viruses. The MxA protein is also known to self-assembly into high molecular weight homo-oligomers. Using a yeast two-hybrid screen, we identified 27 MxA binding partners, some of which are related to the SUMOylation machinery. The interaction of MxA with Small-Ubiquitin MOdifier 1 (SUMO1) and Ubiquitin conjugating enzyme 9 (Ubc9) was confirmed by co-immunoprecipitation and co-localization by confocal microscopy. We identified one SUMO conjugation site at lysine 48 and two putative SUMO interacting motifs (SIMa and SIMb). We showed that MxA interacts with the EIL loop of SUMO1 in a SIM-independent manner via its CID-GED domain. The yeast two-hybrid mapping also revealed that Ubc9 binds to the MxA GTPase domain. Mutation in the putative SIMa and SIMb, which are located in the GTPase binding domain, reduced MxA antiviral activity. In addition, we showed that MxA can be conjugated to SUMO2 or SUMO3 at lysine 48 and that the SUMOylation-deficient mutant of MxA (MxAK48R) retained its capacity to oligomerize and to inhibit Vesicular Stomatitis Virus (VSV) and Influenza A Virus replication, suggesting that MxA SUMOylation is not essential for its antiviral activity.

Intracellular immunity: finding the enemy within--how cells recognize and respond to intracellular pathogens

Historically, once a cell became infected, it was considered to be beyond all help. By this stage, the invading pathogen had breached the innate defenses and was beyond the reach of the humoral arm of the adaptive immune response. The pathogen could still be removed by cell-mediated immunity (e.g., by NK cells or cytotoxic T lymphocytes), but these mechanisms necessitated the destruction of the infected cell. However, in recent years, it has become increasingly clear that many cells possess sensor and effector mechanisms for dealing with intracellular pathogens. Most of these mechanisms are not restricted to professional immune cells nor do they all necessitate the destruction of the host. In this review, we examine the strategies that cells use to detect and destroy pathogens once the cell membrane has been penetrated.

Transfer of the amino-terminal nuclear envelope targeting domain of human MX2 converts MX1 into an HIV-1 resistance factor

The myxovirus resistance 2 (MX2) protein of humans has been identified recently as an interferon (IFN)-inducible inhibitor of human immunodeficiency virus type 1 (HIV-1) that acts at a late postentry step of infection to prevent the nuclear accumulation of viral cDNA (C. Goujon et al., Nature 502:559-562, 2013, http://dx.doi.org/10.1038/nature12542; M. Kane et al., Nature 502:563-566, 2013, http://dx.doi.org/10.1038/nature12653; Z. Liu et al., Cell Host Microbe 14:398-410, 2013, http://dx.doi.org/10.1016/j.chom.2013.08.015). In contrast, the closely related human MX1 protein, which suppresses infection by a range of RNA and DNA viruses (such as influenza A virus [FluAV]), is ineffective against HIV-1. Using a panel of engineered chimeric MX1/2 proteins, we demonstrate that the amino-terminal 91-amino-acid domain of MX2 confers full anti-HIV-1 function when transferred to the amino terminus of MX1, and that this fusion protein retains full anti-FluAV activity. Confocal microscopy experiments further show that this MX1/2 fusion, similar to MX2 but not MX1, can localize to the nuclear envelope (NE), linking HIV-1 inhibition with MX accumulation at the NE. MX proteins are dynamin-like GTPases, and while MX1 antiviral function requires GTPase activity, neither MX2 nor MX1/2 chimeras require this attribute to inhibit HIV-1. This key discrepancy between the characteristics of MX1- and MX2-mediated viral resistance, together with previous observations showing that the L4 loop of the stalk domain of MX1 is a critical determinant of viral substrate specificity, presumably reflect fundamental differences in the mechanisms of antiviral suppression. Accordingly, we propose that further comparative studies of MX proteins will help illuminate the molecular basis and subcellular localization requirements for implementing the noted diversity of virus inhibition by MX proteins.

IMPORTANCE

Interferon (IFN) elicits an antiviral state in cells through the induction of hundreds of IFN-stimulated genes (ISGs). The human MX2 protein has been identified as a key effector in the suppression of HIV-1 infection by IFN. Here, we describe a molecular genetic approach, using a collection of chimeric MX proteins, to identify protein domains of MX2 that specify HIV-1 inhibition. The amino-terminal 91-amino-acid domain of human MX2 confers HIV-1 suppressor capabilities upon human and mouse MX proteins and also promotes protein accumulation at the nuclear envelope. Therefore, these studies correlate the cellular location of MX proteins with anti-HIV-1 function and help establish a framework for future mechanistic analyses of MX-mediated virus control.

Safety Evaluation of Hypoallergenic Wheat Flour by Using a DNA Microarray

The effect of the consumption of hypoallergenic wheat flour (HWF) on the expression of a wide spectrum of genes was analyzed by using a DNA microarray. Gene expression profiles in the liver and intestines of rats fed on a diet composed mainly of HWF were compared with those of rats fed on a normal flour-based diet. Among the eight thousand transcripts represented on the GeneChip microarray, no more than 30 genes exhibited up- or down-regulation of two-fold or more after one week or two months of HWF consumption. No adverse effects were apparent. Up-regulation of some of the genes known to respond to the interferon-gamma signal was apparent in the one-week experiment, which may be related to possible oral immunotolerance resulting from HWF feeding. This DNA microarray technology presents an efficient method for evaluating the safety of foods.

Pandemic influenza A viruses escape from restriction by human MxA through adaptive mutations in the nucleoprotein

The interferon-induced dynamin-like MxA GTPase restricts the replication of influenza A viruses. We identified adaptive mutations in the nucleoprotein (NP) of pandemic strains A/Brevig Mission/1/1918 (1918) and A/Hamburg/4/2009 (pH1N1) that confer MxA resistance. These resistance-associated amino acids in NP differ between the two strains but form a similar discrete surface-exposed cluster in the body domain of NP, indicating that MxA resistance evolved independently. The 1918 cluster was conserved in all descendent strains of seasonal influenza viruses. Introduction of this cluster into the NP of the MxA-sensitive influenza virus A/Thailand/1(KAN-1)/04 (H5N1) resulted in a gain of MxA resistance coupled with a decrease in viral replication fitness. Conversely, introduction of MxA-sensitive amino acids into pH1N1 NP enhanced viral growth in Mx-negative cells. We conclude that human MxA represents a barrier against zoonotic introduction of avian influenza viruses and that adaptive mutations in the viral NP should be carefully monitored.

Interferon-inducible protein Mx1 inhibits influenza virus by interfering with functional viral ribonucleoprotein complex assembly

Mx1 is a GTPase that is part of the antiviral response induced by type I and type III interferons in the infected host. It inhibits influenza virus infection by blocking viral transcription and replication, but the molecular mechanism is not known. Polymerase basic protein 2 (PB2) and nucleoprotein (NP) were suggested to be the possible target of Mx1, but a direct interaction between Mx1 and any of the viral proteins has not been reported. We investigated the interplay between Mx1, NP, and PB2 to identify the mechanism of Mx1's antiviral activity. We found that Mx1 inhibits the PB2-NP interaction, and the strength of this inhibition correlated with a decrease in viral polymerase activity. Inhibition of the PB2-NP interaction is an active process requiring enzymatically active Mx1. We also demonstrate that Mx1 interacts with the viral proteins NP and PB2, which indicates that Mx1 protein has a direct effect on the viral ribonucleoprotein complex. In a minireplicon system, avian-like NP from swine virus isolates was more sensitive to inhibition by murine Mx1 than NP from human influenza A virus isolates. Likewise, murine Mx1 displaced avian NP from the viral ribonucleoprotein complex more easily than human NP. The stronger resistance of the A/H1N1 pandemic 2009 virus against Mx1 also correlated with reduced inhibition of the PB2-NP interaction. Our findings support a model in which Mx1 interacts with the influenza ribonucleoprotein complex and interferes with its assembly by disturbing the PB2-NP interaction.

Interferon-induced antiviral protein MxA interacts with the cellular RNA helicases UAP56 and URH49

Mx proteins are a family of large GTPases that are induced exclusively by interferon-α/β and have a broad antiviral activity against several viruses, including influenza A virus (IAV). Although the antiviral activities of mouse Mx1 and human MxA have been studied extensively, the molecular mechanism of action remains largely unsolved. Because no direct interaction between Mx proteins and IAV proteins or RNA had been demonstrated so far, we addressed the question of whether Mx protein would interact with cellular proteins required for efficient replication of IAV. Immunoprecipitation of MxA revealed its association with two closely related RNA helicases, UAP56 and URH49. UAP56 and its paralog URH49 play an important role in IAV replication and are involved in nuclear export of IAV mRNAs and prevention of dsRNA accumulation in infected cells. In vitro binding assays with purified recombinant proteins revealed that MxA formed a direct complex with the RNA helicases. In addition, recombinant mouse Mx1 was also able to bind to UAP56 or URH49. Furthermore, the complex formation between cytoplasmic MxA and UAP56 or URH49 occurred in the perinuclear region, whereas nuclear Mx1 interacted with UAP56 or URH49 in distinct dots in the nucleus. Taken together, our data reveal that Mx proteins exerting antiviral activity can directly bind to the two cellular DExD/H box RNA helicases UAP56 and URH49. Moreover, the observed subcellular localization of the Mx-RNA helicase complexes coincides with the subcellular localization, where human MxA and mouse Mx1 proteins act antivirally. On the basis of these data, we propose that Mx proteins exert their antiviral activity against IAV by interfering with the function of the RNA helicases UAP56 and URH49.

Influenza A virus interacts extensively with the cellular SUMOylation system during infection

SUMOylation, the post-translational conjugation of the Small Ubiquitin-like MOdifier (SUMO) to a target protein, regulates a wide array of cellular processes and plays important roles for numerous viruses during infection. However, the relevance of the cellular SUMOylation system for influenza virus infection remains mostly unexplored. We previously reported that the non-structural protein of influenza A virus NS1 is a bona fide SUMO target. Here we determine that at least four additional influenza virus proteins, namely PB1, NP, M1, and NS2, are also authentic SUMO targets, and provide data supporting that PB1, NP, and M1 are SUMOylated during viral infection. The functional relevance of SUMOylation for these proteins is supported by the observation that, despite no apparent changes in the cellular levels of the E1 and E2 SUMO enzymes, influenza viral infection leads to a global increase in cellular SUMOylation. This increase, characterized by the appearance of two new SUMOylated proteins of ∼70kDa and ∼52kDa of molecular weight, is dependent upon viral replication and cannot be recreated by interferon stimulation alone. Altogether, these observations indicate that influenza A virus interacts extensively with the cellular SUMOylation system during infection and suggest that SUMOylation plays an important role during influenza virus infection, potentially contributing to the functional diversity exhibited by influenza viral proteins.

Human MxA protein: an interferon-induced dynamin-like GTPase with broad antiviral activity

The human myxovirus resistance protein 1 (MxA) is a key mediator of the interferon-induced antiviral response against a wide range of viruses. MxA expression is tightly regulated by type I and type III interferons, requires signal transducer and activator of transcription 1 signaling, and is not inducible directly by viruses or other stimuli. MxA shares many properties with the dynamin superfamily of large GTPases. It consists of 3 domains, namely, an N-terminal GTPase domain that binds and hydrolyses GTP, a middle domain mediating self-assembly, and a carboxy-terminal GTPase effector domain. Like dynamin, MxA has the ability to self-assemble into highly ordered oligomers and to form ring-like structures around liposomes, inducing liposome tubulation. The structural details of MxA oligomerization have recently been elucidated, providing new insights into the antiviral mechanism of this mechanochemical enzyme. The structural and functional data suggest that MxA targets the nucleoprotein of MxA-sensitive viruses. Thus, MxA may form oligomeric rings around tubular nucleocapsid structures, thereby inhibiting their transcriptional and replicative function. Here we briefly review the most salient features of MxA expression and antiviral function.

Quantitative proteomic analyses of influenza virus-infected cultured human lung cells

Because they are obligate intracellular parasites, all viruses are exclusively and intimately dependent upon host cells for replication. Viruses, in turn, induce profound changes within cells, including apoptosis, morphological changes, and activation of signaling pathways. Many of these alterations have been analyzed by gene arrays, which measure the cellular "transcriptome." Until recently, it has not been possible to extend comparable types of studies to globally examine all the host cellular proteins, which are the actual effector molecules. We have used stable isotope labeling by amino acids in cell culture (SILAC), combined with high-throughput two-dimensional (2-D) high-performance liquid chromatography (HPLC)/mass spectrometry, to determine quantitative differences in host proteins after infection of human lung A549 cells with human influenza virus A/PR/8/34 (H1N1) for 24 h. Of the 4,689 identified and measured cytosolic protein pairs, 127 were significantly upregulated at >95% confidence, 153 were significantly downregulated at >95% confidence, and a total of 87 proteins were upregulated or downregulated more than 5-fold at >99% confidence. Gene ontology and pathway analyses indicated differentially regulated proteins and included those involved in host cell immunity and antigen presentation, cell adhesion, metabolism, protein function, signal transduction, and transcription pathways.

Mx gene diversity and influenza association among five wild dabbling duck species (Anas spp.) in Alaska

Mx (myxovirus-resistant) proteins are induced by interferon and inhibit viral replication as part of the innate immune response to viral infection in many vertebrates. Influenza A virus appears to be especially susceptible to Mx antiviral effects. We characterized exon 13 and the 3' UTR of the Mx gene in wild ducks, the natural reservoir of influenza virus and explored its potential relevance to influenza infection. We observed a wide range of intra- and interspecies variations. Total nucleotide diversity per site was 0.0014, 0.0027, 0.0044, 0.0051, and 0.0061 in mallards, northern shovelers, northern pintails, American wigeon, and American green-winged teals, respectively. There were 61 haplotypes present across all five species and four were shared among species. Additionally, we observed a significant association between Mx haplotype and influenza infection status in northern shovelers. However, we found no evidence of balancing or diversifying selection in this region of the Mx gene. Characterization of the duck Mx gene is an important step in understanding how the gene may affect disease resistance or susceptibility in wild populations. Furthermore, given that waterfowl act as a natural reservoir for influenza virus, the Mx gene could be an important determinant in the ecology of the virus.

Herpes simplex virus-1 induces expression of a novel MxA isoform that enhances viral replication

MxA is an antiviral protein induced by interferon (IFN)-α/β that is known to inhibit the replication of many RNA viruses. In these experiments, the 76-kDa MxA protein expressed in IFN-α-treated cells was shown to have antiviral activity against herpes simplex virus-1 (HSV-1), a human DNA virus. However, MxA was expressed as a 56-kDa protein in HSV-1-infected cells in the absence of IFN-α. This previously unrecognized MxA isoform was produced from an alternatively spliced MxA transcript that had a deletion of Exons 14-16 and a frame shift altering the C-terminus. The variant MxA (varMxA) isoform was associated with HSV-1 regulatory proteins and virions in nuclear replication compartments. varMxA expression enhanced HSV-1 infection as shown by a reduction in infectious virus titers from cells in which MxA had been inhibited by RNA interference and by an increase in HSV-1 titers when the 56-kDa varMxA was expressed constitutively. Thus, the human MxA gene encodes two MxA isoforms, which are expressed differentially depending on whether the stimulus is IFN-α or HSV-1. These findings show that alternative splicing of cellular mRNA can result in expression of a novel isoform of a host defense gene that supports instead of restricting viral infection.

Dynamin-like MxA GTPase: structural insights into oligomerization and implications for antiviral activity

The interferon-inducible MxA GTPase is a key mediator of cell-autonomous innate immunity against a broad range of viruses such as influenza and bunyaviruses. MxA shares a similar domain structure with the dynamin superfamily of mechanochemical enzymes, including an N-terminal GTPase domain, a central middle domain, and a C-terminal GTPase effector domain. Recently, crystal structures of a GTPase domain dimer of dynamin 1 and of the oligomerized stalk of MxA (built by the middle and GTPase effector domains) were determined. These data provide exciting insights into the architecture and antiviral function of the MxA oligomer. Moreover, the structural knowledge paves the way for the development of novel antiviral drugs against influenza and other highly pathogenic viruses.

PML nuclear bodies

PML nuclear bodies are matrix-associated domains that recruit an astonishing variety of seemingly unrelated proteins. Since their discovery in the early 1960s, PML bodies have fascinated cell biologists because of their beauty and their tight association with cellular disorders. The identification of PML, a gene involved in an oncogenic chromosomal translocation, as the key organizer of these domains drew instant interest onto them. The multiple levels of PML body regulation by a specific posttranslational modification, sumoylation, have raised several unsolved issues. Functionally, PML bodies may sequester, modify or degrade partner proteins, but in many ways, PML bodies still constitute an enigma.

Viral parkinsonism

Parkinson's disease is a debilitating neurological disorder that affects 1-2% of the adult population over 55 years of age. For the vast majority of cases, the etiology of this disorder is unknown, although it is generally accepted that there is a genetic susceptibility to any number of environmental agents. One such agent may be viruses. It has been shown that numerous viruses can enter the nervous system, i.e. they are neurotropic, and induce a number of encephalopathies. One of the secondary consequences of these encephalopathies can be parkinsonism, that is both transient as well as permanent. One of the most highlighted and controversial cases of viral parkinsonism is that which followed the 1918 influenza outbreak and the subsequent induction of von Economo's encephalopathy. In this review, we discuss the neurological sequelae of infection by influenza virus as well as that of other viruses known to induce parkinsonism including Coxsackie, Japanese encephalitis B, St. Louis, West Nile and HIV viruses.

The Mx GTPase family of interferon-induced antiviral proteins

Mx proteins are interferon-induced members of the dynamin superfamily of large GTPases. They inhibit a wide range of viruses by blocking an early stage of the replication cycle. Studies in genetically defined mouse strains highlight their powerful action in early antiviral host defence.

Early blood profiles of virus infection in a monkey model for Lassa fever

Acute arenavirus disease in primates, like Lassa hemorrhagic fever in humans, begins with flu-like symptoms and leads to death approximately 2 weeks after infection. Our goal was to identify molecular changes in blood that are related to disease progression. Rhesus macaques (Macaca mulatta) infected intravenously with a lethal dose of lymphocytic choriomeningitis virus (LCMV) provide a model for Lassa virus infection of humans. Blood samples taken before and during the course of infection were used to monitor gene expression changes that paralleled disease onset. Changes in blood showed major disruptions in eicosanoid, immune response, and hormone response pathways. Approximately 12% of host genes alter their expression after LCMV infection, and a subset of these genes can discriminate between virulent and non-virulent LCMV infection. Major transcription changes have been given preliminary confirmation by quantitative PCR and protein studies and will be valuable candidates for future validation as biomarkers for arenavirus disease.

Interferon-induced Mx proteins in antiviral host defense

Mx proteins are key components of the antiviral state induced by interferons in many species. They belong to the class of dynamin-like large guanosine triphosphatases (GTPases) known to be involved in intracellular vesicle trafficking and organelle homeostasis. Mx GTPases share structural and functional properties with dynamin, such as self-assembly and association with intracellular membranes. A unique property of some Mx GTPases is their antiviral activity against a wide range of RNA viruses, including influenza viruses and members of the bunyavirus family. These viruses are inhibited at an early stage in their life cycle, soon after host cell entry and before genome amplification. The mouse Mx1 GTPase accumulates in the cell nucleus where it associates with components of the PML nuclear bodies and inhibits influenza and Thogoto viruses known to replicate in the nucleus. The human MxA GTPase accumulates in the cytoplasm and is partly associated with a COP-I-positive subcompartment of the endoplasmic reticulum. This membrane compartment seems to provide an interaction platform that facilitates viral target recognition. In the case of bunyaviruses, MxA recognizes the viral nucleocapsid protein and interferes with its role in viral genome replication. In the case of Thogoto virus, MxA recognizes the viral nucleoprotein and prevents the incoming viral nucleocapsids from being transported into the nucleus, the site of viral transcription and replication. In both cases, GTP-binding and carboxy-terminal effector functions of MxA are required for target recognition. In general, Mx GTPases appear to detect viral infection by sensing nucleocapsid-like structures. As a consequence, these viral components are trapped and sorted to locations where they become unavailable for the generation of new virus particles.

The Interferon-Inducible GTPases

Mammalian cells respond to interferons (IFNs) secreted during infection by the transcriptional upregulation of as many as a thousand genes. This remarkable transition prepares cells and organisms for resistance to infection, and many IFN-regulated gene products are players in well-understood resistance programs. Oddly, however, many of the most abundantly induced proteins are GTPases whose functions are not well understood. Here we review the progress that has been made toward understanding the roles of individual GTPase families in disease resistance and the hints of common mechanisms that are now available.

Interferon: The pathways of discovery

This historical account covers 50 years of seminal research work on interferon done since its discovery in 1957. Topics related to molecular structure, production and action of interferons are considered from the viewpoint of how our insights have expanded and deepened within the context of evolving tools and general knowledge in cellular and molecular biology. Lines of thought that linked each discovery to the next are expounded.

Gene expression in mice infected with West Nile virus strains of different neurovirulence

West Nile virus causes febrile illness in humans with a proportion of cases progressing to meningoencephalitis, encephalitis, hepatitis, and death. Isolates of the virus fall into two genetic lineages, with differences in neuroinvasiveness for mice occurring between strains within both lineages. We used DNA microarrays to compare gene expression in mice infected peripherally with seven lineage 1 and 2 strains confirmed to be of either high or low neuroinvasiveness in mice and associated with severe or benign infection in humans and birds. The 4 strains with highest neuroinvasiveness induced increased expression of 47 genes in the brain, 111 genes in the liver, and 70 genes in the spleen, relative to the 3 least neuroinvasive strains. Genes involved in interferon signaling pathways, protein degradation, T-cell recruitment, MHC class I and II antigen presentation, and apoptosis were identified that may have both pathogenic and protective effects, but increased expression of certain acute proteins, central nervous system specific proteins and proteins associated with T-cell hepatitis, implicate mechanisms related to exalted virulence.

topors, a p53 and topoisomerase I-binding RING finger protein, is a coactivator of p53 in growth suppression induced by DNA damage

The RING family zinc-finger protein topors (topoisomerase I-binding protein) binds not only topoisomerase I, but also p53 and the AAV-2 Rep78/68 proteins. topors maps to human chromosome 9p21, which contains candidate tumor suppressor genes implicated in small cell lung cancers. In this study, we isolated the murine counterpart of topors and investigated its impact on p53 function. The deduced amino-acid sequence of mouse topors exhibits extensive similarity to human topors. Overexpressed myc-tagged topors associates with and stabilizes p53, and enhances the p53-dependent transcriptional activities of p21(Waf1), MDM2 and Bax promoters and elevates endogenous p21(Waf1) mRNA levels. Overexpression of topors consequently results in the suppression of cell growth by cell cycle arrest and/or by the induction of apoptosis. Taken together, these studies identify topors as a positive regulator of p53. The expression of topors is induced by exposure to the genotoxic reagents cisplatin and camptothecin, a DNA topoisomerase I inhibitor. We therefore postulate that topors mediates p53-dependent cellular responses induced by DNA damage, suggesting its physiological role as a tumor suppressor.

MxA, a member of the dynamin superfamily, interacts with the ankyrin-like repeat domain of TRPC

Mammalian transient receptor potential canonical channels have been proposed as the molecular entities associated with calcium entry activity in nonexcitable cells. Amino acid sequence analyses of TRPCs revealed the presence of ankyrin-like repeat domains, one of the most common protein-protein interaction motifs. Using a yeast two-hybrid interaction assay, we found that the second ankyrin-like repeat domain of TRPC6 interacted with MxA, a member of the dynamin superfamily. Using a GST pull-down and co-immunoprecipitation assay, we showed that MxA interacted with TRPC1, -3, -4, -5, -6, and -7. Overexpression of MxA in HEK293T cells slightly increased endogenous calcium entry subsequent to stimulation of G(q) protein-coupled receptors or store depletion by thapsigargin. Co-expression of MxA with TRPC6 enhanced agonist-induced or OAG-induced calcium entry activity. GTP binding-defective MxA mutants had only a minor potentiating effect on OAG-induced TRPC6 activity. However, a MxA mutant that could bind GTP but that lacked GTPase activity produced the same effect as MxA on OAG-induced TRPC6 activity. These results indicated that MxA interacted specifically with the second ankyrin-like repeat domain of TRPCs and suggested that monomeric MxA regulated the activity of TRPC6 by a mechanism requiring GTP binding. Additional results showed that an increase in the endogenous expression of MxA, induced by a treatment with interferon alpha, regulated the activity of TRPC6. The study clearly identified MxA as a new regulatory protein involved in Ca2+ signaling.

SUMO protein modification

SUMO (small ubiquitin-related modifier) family proteins are not only structurally but also mechanistically related to ubiquitin in that they are posttranslationally attached to other proteins. As ubiquitin, SUMO is covalently linked to its substrates via amide (isopeptide) bonds formed between its C-terminal glycine residue and the epsilon-amino group of internal lysine residues. The enzymes involved in the reversible conjugation of SUMO are similar to those mediating the ubiquitin conjugation. Since its discovery in 1996, SUMO has received a high degree of attention because of its intriguing and essential functions, and because its substrates include a variety of biomedically important proteins such as tumor suppressor p53, c-jun, PML and huntingtin. SUMO modification appears to play important roles in diverse processes such as chromosome segregation and cell division, DNA replication and repair, nuclear protein import, protein targeting to and formation of certain subnuclear structures, and the regulation of a variety of processes including the inflammatory response in mammals and the regulation of flowering time in plants.

IFN-inducible GTPases and immunity to intracellular pathogens

By eliciting host antimicrobial programs in nearly all nucleated cells interferons (IFNs) help orchestrate the innate immune response of mammals to a diverse array of microbial pathogens. Recent work has highlighted the complexity of this transcriptional repertoire and the emergence of several families of IFN-inducible guanosine 5' triphosphatases (GTPases)--p47, guanylate-binding protein (GBP), Mx and very large inducible GTPases (VLIG)--that subsume pathogen-specific roles. Such specificity arises from a combination of both the type and timing of inductive stimuli, target-cell population, subcellular binding partners and the infectious agent encountered. Evolution of different GTPase families to combat compartmentalized versus cytosolic pathogens reveals a hitherto unexpected level of intracellular discrimination during vertebrate host defense.

Nuclear body movement is determined by chromatin accessibility and dynamics

Promyelocytic leukemia (PML) and Cajal bodies are mobile subnuclear organelles, which are involved in activities like RNA processing, transcriptional regulation, and antiviral defense. A key parameter in understanding their biological functions is their mobility. The diffusion properties of PML and Cajal bodies were compared with a biochemically inactive body formed by aggregates of murine Mx1 by using single-particle tracking methods. The artificial Mx1-yellow fluorescent protein body showed a very similar mobility compared with PML and Cajal bodies. The data are described quantitatively by a mechanism of nuclear body movement consisting of two components: diffusion of the body within a chromatin corral and its translocation resulting from chromatin diffusion. This finding suggests that the body mobility reflects the dynamics and accessibility of the chromatin environment, which might target bodies to specific nuclear subcompartments where they exert their biological function.

Mx1 GTPase accumulates in distinct nuclear domains and inhibits influenza A virus in cells that lack promyelocytic leukaemia protein nuclear bodies

The interferon-induced murine Mx1 GTPase is a nuclear protein. It specifically inhibits influenza A viruses at the step of primary transcription, a process known to occur in the nucleus of infected cells. However, the exact mechanism of inhibition is still poorly understood. The Mx1 GTPase has previously been shown to accumulate in distinct nuclear dots that are spatially associated with promyelocytic leukaemia protein (PML) nuclear bodies (NBs), but the significance of this association is not known. Here it is reported that, in cells lacking PML and, as a consequence, PML NBs, Mx1 still formed nuclear dots. These dots were indistinguishable from the dots observed in wild-type cells, indicating that intact PML NBs are not required for Mx1 dot formation. Furthermore, Mx1 retained its antiviral activity against influenza A virus in these PML-deficient cells, which were fully permissive for influenza A virus. Nuclear Mx proteins from other species showed a similar subnuclear distribution. This was also the case for the human MxA GTPase when this otherwise cytoplasmic protein was translocated into the nucleus by virtue of a foreign nuclear localization signal. Human MxA and mouse Mx1 do not interact or form heterooligomers. Yet, they co-localized to a large degree when co-expressed in the nucleus. Taken together, these findings suggest that Mx1 dots represent distinct nuclear domains (‘Mx nuclear domains’) that are frequently associated with, but functionally independent of, PML NBs.

Characterization of Atlantic halibut (Hippoglossus hippoglossus) Mx protein expression

Mx proteins are antiviral GTPases that are induced by type I interferons in vertebrates. An Atlantic halibut Mx cDNA (HHMx) was recently cloned. In this work, a polyclonal antiserum against HHMx protein was generated that detected a 71 kDa protein in the nuclei of Chinook salmon embryo cells transfected with the HHMx cDNA. Mx protein expression in organs of halibut was studied by immunoblot analysis after injection with the double-stranded RNA poly I:C or infectious pancreatic necrosis virus. Poly I:C stimulated increased Mx protein expression in liver, kidney, heart, spleen, gills and intestine. The Mx protein level in liver reached a maximum after 3 days and remained elevated for 14 days after treatment. IPNV infection resulted in increased Mx protein in liver from 4 to at least 35 days. Immunocytochemical detection of Mx proteins in blood smears from poly I:C treated halibut indicated that a cytoplasmic Mx form might exist in this species. Detection of Mx proteins in blood leukocytes could thus work as an early non-lethal test for viral infections.

Inhibitor of kappaB kinase is required to activate a subset of interferon gamma-stimulated genes

IkappaB kinase (IKK), discovered as the major activator of NF-kappaB, plays additional roles in signaling. By using mouse embryo fibroblasts (MEFs) lacking both the alpha and beta subunits of IKK, we find that these proteins are required for induction of a major subset of IFNgamma-stimulated genes and that this requirement is independent of NF-kappaB activation. Furthermore, there is no defect in IFNgamma-stimulated signal transducer and activator of transcription 1 (Stat1) activation or function in the IKKalpha/beta-null MEFs. Therefore, although activated Stat1 dimers are necessary for the activation of these genes in response to IFNgamma, they are not sufficient. These results reveal an important additional pathway for IFNgamma-stimulated gene expression in which an NF-kappaB-independent function of IKK is required.

Dynamin interacts with members of the sumoylation machinery

Dynamin is a GTP-binding protein whose oligomerization-dependent assembly around the necks of lipid vesicles mediates their scission from parent membranes. Dynamin is thus directly involved in the regulation of endocytosis. Sumoylation is a post-translational protein modification whereby the ubiquitin-like modifier Sumo is covalently attached to lysine residues on target proteins by a process requiring the concerted action of an activating enzyme (ubiquitin-activating enzyme), a conjugating enzyme (ubiquitin carrier protein), and a ligating enzyme (ubiquitin-protein isopeptide ligase). Here, we show that dynamin interacts with Sumo-1, Ubc9, and PIAS-1, all of which are members of the sumoylation machinery. Ubc9 and PIAS-1 are known ubiquitin carrier protein and ubiquitin-protein isopeptide ligase enzymes, respectively, for the process of sumoylation. We have identified the coiled-coil GTPase effector domain (GED) of dynamin as the site on dynamin that interacts with Sumo-1, Ubc9, and PIAS-1. Although we saw no evidence of covalent Sumo-1 attachment to dynamin, Sumo-1 and Ubc9 are shown here to inhibit the lipid-dependent oligomerization of dynamin. Expression of Sumo-1 and Ubc9 in mammalian cells down-regulated the dynamin-mediated endocytosis of transferrin, whereas dynamin-independent fluid-phase uptake was not affected. Furthermore, using high resolution NMR spectroscopy, we have identified amino acid residues on Sumo-1 that directly interact with the GED of dynamin. The results suggest that the GED of dynamin may serve as a scaffold that concentrates the sumoylation machinery in the vicinity of potential acceptor proteins.

SUMO modification of proteins other than transcription factors

A wide range of eukaryotic proteins has been shown to be sumoylated. Most, but not all of these proteins are nuclear. In all cases documented so far, sumoylation has been shown to occur on lysine residues. In general these are located within the consensus sequence psiKxE, although there are some exceptions to this. The role of sumoylation has been investigated for a number of identified targets. Unlike the situation with ubiquitination, sumoylation does not appear to target proteins for proteasome-mediated degradation. In contrast, the effect of SUMO modification appears to depend on the target protein and includes roles in altering protein activity, protein-protein interactions or protein localisation.

Maneuvering for advantage: the genetics of mouse susceptibility to virus infection

Genetic studies of host susceptibility to infection contribute to our understanding of an organism's response to pathogens at the immunological, cellular, and molecular levels. In this review we describe how the study of host genetics in mouse models has helped our understanding of host defense mechanisms against viral infection, and how this knowledge can be extended to human infections. We focus especially on the innate mechanisms that function as the host's first line of defense against infection. We also discuss the main issues that confront this field, as well as its future.

The homeodomain-interacting kinase PKM (HIPK-2) modifies ND10 through both its kinase domain and a SUMO-1 interaction motif and alters the posttranslational modification of PML

Homeodomain-interacting protein kinases (HIPK-1, -2, and -3) are a family of enzymes that have been implicated in the phosphorylation and repression of homeodomain-containing transcription factors. HIPK-2 has been found to interact with the SUMO-1-conjugating enzyme Ubc9 and can be covalently modified by SUMO-1. It has also been shown to interact with and phosphorylate p53 and to form punctate speckles in the nucleus of which a proportion colocalize with PML nuclear bodies (ND10). We have previously shown that the hamster equivalent of HIPK-2 (named PKM) interacts with the interferon-induced antiviral GTPase Mx1 and associates with ND10 in interferon-treated cells. Given the connections between the interferon response pathway, constituents of ND10, and SUMO-1-conjugated proteins, we have studied the effects of exogenously expressed PKM on endogenous ND10 proteins. We found that PKM induces structural changes in ND10 that can be attributed both to its kinase activity and to the presence of a functional SUMO-1 interaction motif in the C-terminal half of the protein. The changes in the localization of PML, Sp100, and hDaxx induced by exogenous PKM or fragments thereof correlate with changes in the posttranslationally modified species of PML. We propose that PKM is able to modify ND10 structure by inducing changes in the posttranslational modification of PML and by interacting with SUMO-1 modification pathways.