Interaction of an adenovirus 14.7-kilodalton protein inhibitor of tumor necrosis factor alpha cytolysis with a new member of the GTPase superfamily of signal transducers

B Chromosomes’ Sequences in Yellow-Necked Mice Apodemus flavicollis—Exploring the Transcription

B chromosomes (Bs) are highly polymorphic additional chromosomes in the genomes of many species. Due to the dispensability of Bs and the lack of noticeable phenotypic effects in their carriers, they were considered genetically inert for a long time. Recent studies on Bs in Apodemus flavicollis revealed their genetic composition, potential origin, and spatial organization in the interphase nucleus. Surprisingly, the genetic content of Bs in this species is preserved in all studied samples, even in geographically distinct populations, indicating its biological importance. Using RT-PCR we studied the transcription activity of three genes (Rraga, Haus6, and Cenpe) previously identified on Bs in A. flavicollis. We analysed mRNA isolated from spleen tissues of 34 animals harboring different numbers of Bs (0–3).The products of transcriptional activity of the analysed sequences differ in individuals with and without Bs. We recorded B-genes and/or genes from the standard genome in the presence of Bs, showing sex-dependent higher levels of transcriptional activity. Furthermore, the transcriptional activity of Cenpe varied with the age of the animals differently in the group with and without Bs. With aging, the amount of product was only found to significantly decrease in B carriers. The potential biological significance of all these differences is discussed in the paper.

Expression Profiling Analysis Reveals Key MicroRNA-mRNA Interactions in Early Retinal Degeneration in Retinitis Pigmentosa

Purpose

The aim of this study was to identify differentially expressed microRNAs (miRNAs) that might play an important role in the etiology of retinal degeneration in a genetic mouse model of retinitis pigmentosa (rd10 mice) at initial stages of the disease.

Methods

miRNAs–mRNA interaction networks were generated for analysis of biological pathways involved in retinal degeneration.

Results

Of more than 1900 miRNAs analyzed, we selected 19 miRNAs on the basis of (1) a significant differential expression in rd10 retinas compared with control samples and (2) an inverse expression relationship with predicted mRNA targets involved in biological pathways relevant to retinal biology and/or degeneration. Seven of the selected miRNAs have been associated with retinal dystrophies, whereas, to our knowledge, nine have not been previously linked to any disease.

Conclusions

This study contributes to our understanding of the etiology and progression of retinal degeneration.

Dysfunction of Optineurin in Amyotrophic Lateral Sclerosis and Glaucoma

Neurodegenerative disorders, including amyotrophic lateral sclerosis (ALS), frontotemporal dementia, and glaucoma, affect millions of people worldwide. ALS is caused by the loss of motor neurons in the spinal cord, brainstem, and brain, and genetic mutations are responsible for 10% of all ALS cases. Glaucoma is characterized by the loss of retinal ganglion cells and is the most common cause of irreversible blindness. Interestingly, mutations in OPTN, encoding optineurin, are associated with both ALS and glaucoma. Optineurin is a highly abundant protein involved in a wide range of cellular processes, including the inflammatory response, autophagy, Golgi maintenance, and vesicular transport. In this review, we summarize the role of optineurin in cellular mechanisms implicated in neurodegenerative disorders, including neuroinflammation, autophagy, and vesicular trafficking, focusing in particular on the consequences of expression of mutations associated with ALS and glaucoma. This review, therefore showcases the impact of optineurin dysfunction in ALS and glaucoma.

An E3-14.7K peptide that promotes microtubules-mediated transport of plasmid DNA increases polyplexes transfection efficiency

Chemical vectors as cationic polymers and cationic lipids are promising alternatives to viral vectors for gene therapy. Beside endosome escape and nuclear import, plasmid DNA (pDNA) migration in the cytosol toward the nuclear envelope is also regarded as a limiting step for efficient DNA transfection with non-viral vectors. Here, the interaction between E3-14.7K and FIP-1 to favor migration of pDNA along microtubules is exploited. E3-14.7K is an early protein of human adenoviruses that interacts via FIP-1 (Fourteen.7K Interacting Protein 1) protein with the light-chain components of the human microtubule motor protein dynein (TCTEL1). This peptide is conjugated with pDNA and mediates interaction of pDNA in vitro with isolated microtubules as well as with microtubules in cellulo. Videomicroscopy and tracking treatment of images clearly demonstrate that P79-98/pDNA conjugate exhibits a linear transport with large amplitude along microtubules upon 2 h transfection with polyplexes whereas control pDNA conjugate exhibits small non-directional movements in the cytoplasm. Remarkably, P79-98/peGFP polyplexes enhance by a factor 2.5 (up to 76%) the number of transfected cells. The results demonstrate, for the first time, that the transfection efficiency of polyplexes can be drastically increased when the microtubules migration of pDNA is facilitated by a peptide allowing pDNA docking to TCTEL1. This is a real breakthrough in the non viral gene delivery field that opens hope to build artificial viruses.

Genetic susceptibility to feline infectious peritonitis in Birman cats

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Feline infectious peritonitis (FIP) is caused by a mutant enteric coronavirus.

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Susceptibility to FIP appears to be heritable in pedigreed cats.

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Birman cats suffer a high incidence of FIP and are highly inbred.

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Heritable factors were interrogated in Birman cats by genome wide association study.

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Five relevant candidate genes were strongly associated with FIP susceptibility.

Genetic factors are presumed to influence the incidence of feline infectious peritonitis (FIP), especially among pedigreed cats. However, proof for the existence of such factors has been limited and mainly anecdotal. Therefore, we sought evidence for genetic susceptibility to FIP using feline high density single nucleotide polymorphism (SNP) arrays in a genome-wide association study (GWAS). Birman cats were chosen for GWAS because they are highly inbred and suffer a high incidence of FIP. DNA from 38 Birman cats that died of FIP and 161 healthy cats from breeders in Denmark and USA were selected for genotyping using 63K SNPs distributed across the feline genome. Danish and American Birman cats were closely related and the populations were therefore combined and analyzed in two manners: (1) all cases (FIP) vs. all controls (healthy) regardless of age, and (2) cases 1½ years of age and younger (most susceptible) vs. controls 2 years of age and older (most resistant). GWAS of the second cohort was most productive in identifying significant genome-wide associations between case and control cats. Four peaks of association with FIP susceptibility were identified, with two being identified on both analyses. Five candidate genes ELMO1, RRAGA, TNFSF10, ERAP1 and ERAP2, all relevant to what is known about FIP virus pathogenesis, were identified but no single association was fully concordant with the disease phenotype. Difficulties in doing GWAS in cats and interrogating complex genetic traits were discussed.

The critical protein interactions and structures that elicit growth deregulation in cancer and viral replication

One of the greatest challenges in biomedicine is to define the critical targets and network interactions that are subverted to elicit growth deregulation in human cells. Understanding and developing rational treatments for cancer requires a definition of the key molecular targets and how they interact to elicit the complex growth deregulation phenotype. Viral proteins provide discerning and powerful probes to understand both how cells work and how they can be manipulated using a minimal number of components. The small DNA viruses have evolved to target inherent weaknesses in cellular protein interaction networks to hijack the cellular DNA and protein replication machinery. In the battle to escape the inevitability of senescence and programmed cell death, cancers have converged on similar mechanisms, through the acquisition and selection of somatic mutations that drive unchecked cellular replication in tumors. Understanding the dynamic mechanisms through which a minimal number of viral proteins promote host cells to undergo unscheduled and pathological replication is a powerful strategy to identify critical targets that are also disrupted in cancer. Viruses can therefore be used as tools to probe the system-wide protein-protein interactions and structures that drive growth deregulation in human cells. Ultimately this can provide a path for developing system context-dependent therapeutics. This review will describe ongoing experimental approaches using viruses to study pathways deregulated in cancer, with a particular focus on viral cellular protein-protein interactions and structures.

The large and small isoforms of human papillomavirus type 16 E6 bind to and differentially affect procaspase 8 stability and activity

Human papillomavirus type 16 (HPV-16) has developed numerous ways to modulate host-initiated immune mechanisms. The HPV-16 E6 oncoprotein, for example, can modulate the cellular level, and consequently the activity, of procaspase 8, thus modifying the cellular response to cytokines of the tumor necrosis factor family. E6 from HPV-16, but not E6 from the low-risk types 6b and 11, alters the cellular level of procaspase 8 in a dose-dependent manner. Both the large and small (E6*) isoforms of E6, which originate by way of alternate splicing, can modulate procaspase 8 stability. Intriguingly, although both isoforms bind to procaspase 8, the large isoform accelerates the degradation of procaspase 8 while the small isoform stabilizes it. Binding leads to a change in the ability of procaspase 8 to bind either to itself or to FADD (Fas-associated death domain), with the large version of E6 able to inhibit this binding while the small isoform does not. Consistent with this model, knockdown of the large version of E6 by small interfering RNA leads to increases in the levels of procaspase 8 and its binding to both itself and FADD. Thus, these alternatively spliced isoforms can modulate both the level and the activity of procaspase 8 in opposite directions.

Inhibition of TNF receptor 1 internalization by adenovirus 14.7K as a novel immune escape mechanism

The adenoviral protein E3-14.7K (14.7K) is an inhibitor of TNF-induced apoptosis, but the molecular mechanism underlying this protective effect has not yet been explained exhaustively. TNF-mediated apoptosis is initiated by ligand-induced recruitment of TNF receptor-associated death domain (TRADD), Fas-associated death domain (FADD), and caspase-8 to the death domain of TNF receptor 1 (TNFR1), thereby establishing the death-inducing signaling complex (DISC). Here we report that adenovirus 14.7K protein inhibits ligand-induced TNFR1 internalization. Analysis of purified magnetically labeled TNFR1 complexes from murine and human cells stably transduced with 14.7K revealed that prevention of TNFR1 internalization resulted in inhibition of DISC formation. In contrast, 14.7K did not affect TNF-induced NF-kappaB activation via recruitment of receptor-interacting protein 1 (RIP-1) and TNF receptor-associated factor 2 (TRAF-2). Inhibition of endocytosis by 14.7K was effected by failure of coordinated temporal and spatial assembly of essential components of the endocytic machinery such as Rab5 and dynamin 2 at the site of the activated TNFR1. Furthermore, we found that the same TNF defense mechanisms were instrumental in protecting wild-type adenovirus-infected human cells expressing 14.7K. This study describes a new molecular mechanism implemented by a virus to escape immunosurveillance by selectively targeting TNFR1 endocytosis to prevent TNF-induced DISC formation.

Mechanisms of E3 Modulation of Immune and Inflammatory Responses

Adenoviruses contain genes that have evolved to control the host immune and inflammatory responses; however, it is not clear whether these genes function primarily to facilitate survival of the virus during acute infection or during its persistent phase. These issues have assumed greater importance as the use of adenoviruses as vectors for gene therapy has been expanded. This review will focus on the mechanism of immune evasion mediated by the proteins encoded within the early region 3 (E3) transcription region, which affect the functions of a number of cell surface receptors including Fas, intracellular cell signaling events involving NF-kappaB, and the secretion of pro-inflammatory molecules such as chemokines. The successful use of E3 genes in facilitating allogeneic transplantation and in preventing autoimmune diabetes in several transgenic mouse models will also be described.

A novel human nucleolar protein, Nop132, binds to the G proteins, RRAG A/C/D

RRAG A (Rag A)/Gtr1p is a member of the Ras-like small G protein family that genetically interacts with RCC1, a guanine nucleotide exchange factor for RanGTPase. RRAG A/Gtr1p forms a heterodimer with other G proteins, RRAG C and RRAG D/Gtr2p, in a nucleotide-independent manner. To further elucidate the function of RRAG A/Gtr1p, we isolated a protein that interacts with RRAG A. This protein is a novel nucleolar protein, Nop132. Nop132 is associated with the GTP form, but not the GDP form, of RRAG A, suggesting that RRAG A might regulate Nop132 function. Nop132 is also associated with RRAG C and RRAG D. The Nop132 amino acid sequence is similar to the Saccharomyces cerevisiae nucleolar Nop8p, which is associated with Gtr1p, Gtr2p, and Nip7p. Nop132 also interacts with human Nip7 and is colocalized with RRAG A, RRAG C, and Nip7. RNA interference knockdown of Nop132 inhibited cell growth of HeLa cells.

Subversion of host defense mechanisms by adenoviruses

Adenoviruses (Ads) cause acute and persistent infections. Alike the much more complex herpesviruses, Ads encode numerous immunomodulatory functions. About a third of the viral genome is devoted to counteract both the innate and the adaptive antiviral immune response. Immediately upon infection, E1A blocks interferon-induced gene expression and the VA-RNA inhibits interferon-induced PKR activity. At the same time, E1A reprograms the cell for DNA synthesis and induces the intrinsic cellular apoptosis program that is interrupted by E1B/19K and E1B/55K proteins, the latter inhibits p53-mediated apoptosis. Most other viral stealth functions are encoded by a separate transcription units, E3. Several E3 products prevent death receptor-mediated apoptosis. E3/14.7K seems to interfere with the cytolytic and pro-inflammatory activities of TNF while E3/10.4K and 14.5K proteins remove Fas and TRAIL receptors from the cell surface by inducing their degradation in lysosomes. These and other functions that may afect granule-mediated cell death might drastically limit lysis by NK cells and cytotoxic T cells (CTL). Moreover, Ads interfere with recognition of infected cell by CTL. The paradigmatic E3/19K protein subverts antigen presentation by MHC class I molecules by inhibiting their transport to the cell surface. In concert, these viral countermeasures ensure prolonged survival in the infected host and, as a consequence, facilitate transmission. Elucidating the molecular mechanisms of Ad-mediated immune evasion has stimulated corresponding research on other viruses. This knowledge will also be instrumental for designing better vectors for gene therapy and vaccination, and may lead to a more rational treatment of life-threatening Ad infections, e.g. in transplantation patients.

Kaposi's sarcoma-associated herpesvirus mitochondrial K7 protein targets a cellular calcium-modulating cyclophilin ligand to modulate intracellular calcium concentration and inhibit apoptosis

On viral infection, infected cells can become the target of host immune responses or can go through a programmed cell death process, called apoptosis, as a defense mechanism to limit the ability of the virus to replicate. To prevent this, viruses have evolved elaborate mechanisms to subvert the apoptotic process. Here, we report the identification of a novel antiapoptotic K7 protein of Kaposi's sarcoma-associated herpesvirus (KSHV) which expresses during lytic replication. The KSHV K7 gene encodes a small mitochondrial membrane protein, and its expression efficiently inhibits apoptosis induced by a variety of apoptogenic agents. The yeast two-hybrid screen has demonstrated that K7 targets cellular calcium-modulating cyclophilin ligand (CAML), a protein that regulates the intracellular Ca(2+) concentration. Similar to CAML, K7 expression significantly enhances the kinetics and amplitudes of the increase in intracellular Ca(2+) concentration on apoptotic stimulus. Mutational analysis showed that K7 interaction with CAML is required for its function in the inhibition of apoptosis. This indicates that K7 targets cellular CAML to increase the cytosolic Ca(2+) response, which consequently protects cells from mitochondrial damage and apoptosis. This is a novel viral antiapoptosis strategy where the KSHV mitochondrial K7 protein targets a cellular Ca(2+)-modulating protein to confer resistance to apoptosis, which allows completion of the viral lytic replication and, eventually, maintenance of persistent infection in infected host.

Adenovirus RIDbeta subunit contains a tyrosine residue that is critical for RID-mediated receptor internalization and inhibition of Fas- and TRAIL-induced apoptosis

The adenovirus-encoded receptor internalization and degradation (RID) protein (previously named E3-10.4K/14.5K), which is composed of RIDalpha and RIDbeta subunits, down-regulates a number of cell surface receptors in the tumor necrosis factor (TNF) receptor superfamily, namely Fas, TRAIL receptor 1, and TRAIL receptor 2. Down-regulation of these "death" receptors protects adenovirus-infected cells from apoptosis induced by the death receptor ligands Fas ligand and TRAIL. RID also down-regulates certain tyrosine kinase cell surface receptors, especially the epidermal growth factor receptor (EGFR). RID-mediated Fas and EGFR down-regulation occurs via endocytosis of the receptors into endosomes followed by transport to and degradation within lysosomes. However, the molecular interactions underlying this function of RID are unknown. To investigate the molecular determinants of RIDbeta that are involved in receptor down-regulation, mutations within the cytoplasmic tail of RIDbeta were constructed and the mutant proteins were analyzed for their capacity to internalize and degrade Fas and EGFR and to protect cells from death receptor ligand-induced apoptosis. The results demonstrated the critical nature of a tyrosine residue near the RIDbeta C terminus; mutation of this residue to alanine abolished RID function. Mutating the tyrosine to phenylalanine did not abolish the function of RID, arguing that phosphorylation of the tyrosine is not required for function. These data suggest that this tyrosine residue forms part of a tyrosine-based sorting signal (Yxxphi). Additional mutations that target another potential sorting motif and several possible protein-protein interaction motifs had no discernible effect on RID function. It was also demonstrated that mutation of serine 116 to alanine eliminated phosphorylation of RIDbeta but did not affect any of the functions of RID that were examined. These results suggest a model in which the tyrosine-based sorting signal in RID plays a role in RID's ability to down-regulate receptors.

Construction and characterization of E1-minus replication-defective adenovirus vectors that express E3 proteins from the E1 region

Previous research has indicated that the adenovirus protein complex named RID, derived from the E3 transcription unit, functions to remove the receptors named Fas/Apo1/CD95 (Fas) and epidermal growth factor receptor (EGFR) from the surface of cells. (The RID complex is composed of the RIDalpha and RIDbeta polypeptides, previously named 10.4K and 14.5K, respectively.) In response to RID, Fas and EGFR appear to be internalized into endosomes and degraded in lysosomes. Fas is a death receptor in the tumor necrosis factor (TNF) receptor superfamily. RID inhibits apoptosis via the Fas pathway, presumably because RID gets rid of Fas. Earlier work further showed that another adenovirus E3-coded protein, E3-14.7K, inhibits apoptosis induced by TNF. Most of the above studies have been conducted using viable virus mutants that lack one or more of the genes for RID, E3-14.7K, or E1B-19K (this protein, coded by the E1B transcription unit, also inhibits apoptosis via the TNF and Fas pathways). Some studies have also been conducted with the genes for RID or E3-14.7K transiently or stably transfected into cells. We now report a new approach to studying the E3 genes. We have constructed four E1-minus replication-defective vectors that have all the E3 genes deleted from their natural position and then reinserted, in different permutations, into the deleted E1 region under control of the cytomegalovirus immediate early promoter. Vector Ad/RID only has the genes for RIDalpha and RIDbeta. Vector Ad/14.7K only has the gene for E3-14.7K. Vector Ad/RID/14.7K only has the genes for RIDalpha, RIDbeta, and E3-14.7K. Vector Ad/E3 has all E3 genes, but there are two missense mutations in the gene for Adenovirus Death Protein. These vectors expressed RID and/or E3-14.7K, as expected. The RID-expressing vectors forced the internalization and degradation of Fas and EGFR, and they inhibited apoptosis induced through the Fas pathway. These vectors should be useful reagents to study the E3 proteins.

Adenoviral inhibitors of apoptotic cell death

Adenoviruses (Ads) are endemic in the human population and the well-studied group C Ads typically cause an acute infection in the respiratory epithelium. A growing body of evidence suggests that these viruses also establish a persistent infection. The Ad genome encodes several proteins that counteract the host anti-viral mechanisms, which function to limit viral infections. This review describes the adenovirus immuno-regulatory proteins and how they function to block apoptosis of infected cells. In addition to facilitating the successful completion of the viral replication cycle and spread of progeny virus, these functions may help maintain the virus in a persistent state.

Constitutive expression of the adenovirus E3-14.7K protein does not prolong adenovirus vector DNA persistence but protects mice against lipopolysaccharide-induced acute hepatitis

First-generation adenovirus vectors, deleted in the E1 and E3 regions of the genome, induce a strong inflammatory response that affects persistence of vector DNA in transduced organs and causes toxicity in the host. Wild-type adenovirus encodes a number of proteins that are nonessential for viral propagation in vitro but that dampen the inflammatory and immune responses mounted by the host during infection. The adenovirus E3 region-encoded 14.7K protein inhibits tumor necrosis factor alpha (TNF-alpha)-induced apoptosis and arachidonic acid synthesis. To evaluate the impact of constitutive expression of the 14.7K protein on vector-induced pathology, toxicity, and DNA persistence, we constructed vectors that contain a cytomegalovirus promoter-driven 14.7K expression cassette. Although these vectors inhibit TNF-alpha-induced apoptosis in vitro, they do not show better vector DNA persistence, or lower inflammation or pathology than E3-deleted first-generation vector in mouse models. However, the 14.7K protein is functional in mice because animals injected intravenously with a 14.7K-constitutive vector were fully protected against a lethal dose of lipopolysaccharide 5 days after vector administration. These results open new applications for the E3-encoded 14.7K protein, which can be used to protect organs against inflammatory reactions and TNF-alpha-mediated apoptosis.

Characterization of Ad5 E3-14.7K, an adenoviral inhibitor of apoptosis: structure, oligomeric state, and metal binding

The adenovirus E3-14.7K protein, expressed early in the life cycle of human adenoviruses to protect the virus from the antiviral response of host cells, inhibits cell death mediated by TNF-alpha and FasL receptors. To better understand its role in cell death inhibition, we have sought to characterize the biophysical properties of the protein from adenovirus serotype 5 (Ad5 E3-14.7K, or simply 14.7K) through a variety of approaches. To obtain sufficient quantities of recombinantly expressed protein for biophysical characterization, we explored the use of various expression constructs and chaperones; fusion to MBP was by far the most effective at generating soluble protein. Using limited proteolysis, mass spectrometry, and protein-protein interaction assays, we demonstrate that the C-terminal two-thirds of the protein, predicted to be composed of five beta-strands and one alpha-helix, is highly structured and binds its putative cellular receptors. Furthermore, using atomic absorption and ultraviolet/visible spectroscopies, we have studied the metal binding properties of the protein, providing insight into the observation that cysteine/serine mutants of 14.7K lack in vivo antiapoptotic activity. Lastly, results from size exclusion chromatography, dynamic light scattering, sucrose gradient sedimentation, chemical crosslinking, and electron microscopy experiments revealed that 14.7K exists in a stable high-order oligomeric state (nonamer) in solution.

Inhibition of TRAIL-induced apoptosis and forced internalization of TRAIL receptor 1 by adenovirus proteins

Tumor necrosis factor (TNF)-related apoptosis-inducing ligand (TRAIL) induces apoptosis through two receptors, TRAIL-R1 (also known as death receptor 4) and TRAIL-R2 (also known as death receptor 5), that are members of the TNF receptor superfamily of death domain-containing receptors. We show that human adenovirus type 5 encodes three proteins, named RID (previously named E3-10.4K/14.5K), E3-14.7K, and E1B-19K, that independently inhibit TRAIL-induced apoptosis of infected human cells. This conclusion was derived from studies using wild-type adenovirus, adenovirus replication-competent mutants that lack one or more of the RID, E3-14.7K, and E1B-19K genes, and adenovirus E1-minus replication-defective vectors that express all E3 genes, RID plus E3-14.7K only, RID only, or E3-14.7K only. RID inhibits TRAIL-induced apoptosis when cells are sensitized to TRAIL either by adenovirus infection or treatment with cycloheximide. RID induces the internalization of TRAIL-R1 from the cell surface, as shown by flow cytometry and indirect immunofluorescence for TRAIL-R1. TRAIL-R1 was internalized in distinct vesicles which are very likely to be endosomes and lysosomes. TRAIL-R1 is degraded, as indicated by the disappearance of the TRAIL-R1 immunofluorescence signal. Degradation was inhibited by bafilomycin A1, a drug that prevents acidification of vesicles and the sorting of receptors from late endosomes to lysosomes, implying that degradation occurs in lysosomes. RID was also shown previously to internalize and degrade another death domain receptor, Fas, and to prevent apoptosis through Fas and the TNF receptor. RID was shown previously to force the internalization and degradation of the epidermal growth factor receptor. E1B-19K was shown previously to block apoptosis through Fas, and both E1B-19K and E3-14.7K were found to prevent apoptosis through the TNF receptor. These findings suggest that the receptors for TRAIL, Fas ligand, and TNF play a role in limiting virus infections. The ability of adenovirus to inhibit killing through these receptors may prolong acute and persistent infections.

Gene delivery from the E3 region of replicating human adenovirus: evaluation of the E3B region

Successful therapies for cancer need to deal with the complexity associated with the human tumor. Studies of tumor and viral biology have progressed to a point where replicating viruses are now being engineered as potential treatments for human cancers. The complex nature of human cancers dictates that successful treatments will require combination therapies. To this end, we have focused on developing the gene delivery capacity of the replicating adenovirus, using the non-essential E3 region transcription unit as a target site for therapeutic transgene insertions. Utilizing the endogenous expression machinery of the E3 region (promoter, splicing, polyA) we show that a therapeutic transgene, TNF, is efficiently expressed from the E3B region and with exclusive late gene expression kinetics. Potential clinical applications are discussed.

Immunomodulatory functions encoded by the E3 transcription unit of adenoviruses

Persistent viruses have evolved multiple strategies to escape the host immune system. One important prerequisite for efficient viral reproduction in the face of an ongoing immune response is prevention of premature lysis of infected cells. A number of viruses achieve this goal by interfering with antigen presentation and recognition of infected cells by cytotoxic T cells (CTL). Another viral strategy aims to block apoptosis triggered by host defense mechanisms. Both types of strategies seem to be realized by human adenoviruses (Ads). The early transcription unit E3 of Ads encodes proteins that inhibit antigen presentation by MHC class I molecules as well as apoptosis induced by tumor necrosis factor alpha (TNF-alpha) and Fas ligand (FasL). Here, we will describe the organization of the E3 regions of different Ad subgroups and compare the structure and functions of the known immunomodulatory E3 proteins.

Subversion and piracy: DNA viruses and immune evasion

During the co-evolution of viruses with their vertebrate hosts, the DNA viruses have acquired an impressive array of immunomodulatory genes to combat host immune responses and their hosts have developed a sophisticated immune system to contain virus infections. In order to replicate, the viruses have evolved mechanisms to inhibit key host anti-virus responses that include apoptosis, interferon production, chemokine production, inflammatory cytokine production, and the activity of cytotoxic T-cells, natural killer cells and antibody. In addition, some of the viruses encode cytokine or chemokine homologues that recruit or expand cell numbers for infection or that subvert the host cellular response from a protective response to a benign one. The specificity of the viral immunomodulatory molecules reflects the life cycle and the pathogenesis of the viruses. Herpesviruses achieve latency in host cells by inducing cell survival and protecting infected cells from immune recognition. This involves interference with cell signal transduction pathways. Many of the viral immunomodulatory proteins are homologues of host proteins that appear to have been pirated from the host and reassorted in the virus genomes. Some of these have unique functions and indicate novel or important aspects of both viral pathogenesis and host immunity to viruses. The specific example of orf virus infection of sheep is described.

Novel G proteins, Rag C and Rag D, interact with GTP-binding proteins, Rag A and Rag B

Rag A/Gtr1p are G proteins and are known to be involved in the RCC1-Ran pathway. We employed the two-hybrid method using Rag A as the bait to identify proteins binding to Rag A, and we isolated two novel human G proteins, Rag C and Rag D. Rag C demonstrates homology with Rag D (81.1% identity) and with Gtr2p of Saccharomyces cerevisiae (46.1% identity), and it belongs to the Rag A subfamily of the Ras family. Rag C and Rag D contain conserved GTP-binding motifs (PM-1, -2, and -3) in their N-terminal regions. Recombinant glutathione S-transferase fusion protein of Rag C efficiently bound to both [(3)H]GTP and [(3)H]GDP. Rag A was associated with both Rag C and Rag D in their C-terminal regions where a potential leucine zipper motif and a coiled-coil structure were found. Rag C and D were associated with both the GDP and GTP forms of Rag A. Both Rag C and Rag D changed their subcellular localization, depending on the nucleotide-bound state of Rag A. In a similar way, the disruption of S. cerevisiae GTR1 resulted in a change in the localization of Gtr2p.

Identification and characterization of a 30K protein (Ad4E3-30K) encoded by the E3 region of human adenovirus type 4

Human adenovirus type 4 (Ad4), the sole member of subgroup E, contains an open reading frame in the E3 region predicted to encode a unique 30-kDa protein (named Ad4E3-30K). Ad4E3-30K is predicted to be an integral membrane protein containing an N-terminal signal sequence, a lumenal domain, a transmembrane domain near the C-terminus, and a 37-amino-acid cytoplasmic tail. To determine whether this protein is expressed, rabbit polyclonal antisera were raised against 30K-containing fusion proteins expressed in bacteria. A 30K protein was detected by immunoprecipitation from cell-free translation products and from Ad4-infected A549 cells radiolabeled in the presence of tunicamycin. The protein was detected at only low levels in infected cells. It was not synthesized by a mutant with a large E3 deletion that includes the Ad4E3-30K gene. This mutant grows as well as wild-type Ad4 in culture. Features of Ad4E3-30K were characterized in different transient expression systems. The protein underwent glycosylation by addition of approximately six asparagine-linked oligosaccharides. These glycans were sensitive to endoglycosidase H, indicating that they were either high-mannose or hybrid types, but not complex types, and that the protein did not pass through the Golgi apparatus. Immunofluorescence staining of transfected cells revealed that Ad4E3-30K was localized primarily in the endoplasmic reticulum and nuclear envelope.

Viral subversion of the immune system

This review describes the diverse array of pathways and molecular targets that are used by viruses to elude immune detection and destruction. These include targeting of pathways for major histocompatibility complex-restricted antigen presentation, apoptosis, cytokine-mediated signaling, and humoral immune responses. The continuous interactions between host and pathogens during their coevolution have shaped the immune system, but also the counter measures used by pathogens. Further study of their interactions should improve our ability to manipulate and exploit the various pathogens.

The prodomain of caspase-1 enhances Fas-mediated apoptosis through facilitation of caspase-8 activation

Caspase-1 (interleukin-1beta converting enzyme) is produced in the form of a latent precursor, which is cleaved to yield a prodomain in addition to the p20 and p10 subunits. It has been established that the (p20/p10)(2) heterotetramer processes the latent precursor of interleukin-1beta into an active form during apoptosis, but the function of the residual prodomain of caspase-1 (Pro-C1) has not been established. To evaluate the involvement of Pro-C1 in apoptosis, a Pro-C1 expression vector was transfected into the HeLa cell line, which is susceptible to Fas-mediated apoptosis. Expression of recombinant Pro-C1 in HeLa cells enhanced apoptosis mediated by Fas, but not etoposide-induced apoptosis. This enhancement of Fas-mediated apoptosis was abolished by inhibitors of caspase-8 (Ile-Glu-Thr-Asp-fluoromethyl ketone) and caspase-3 (Asp-Glu-Val-Asp-aldehyde) but was only slightly diminished by an inhibitor of caspase-1 (acetyl-Tyr-Val-Ala-Asp-chloromethyl ketone). During apoptosis induced by an agonistic anti-Fas antibody, the activation of caspase-8 and caspase-3 was more pronounced and occurred more rapidly in HeLa/Pro-C1 cells than in the empty vector transfectant (HeLa/vec) cells; in contrast, caspase-1 was not activated in either HeLa/Pro-C1 or HeLa/vec cells. These results demonstrate an additional and novel function for caspase-1 in which Pro-C1 acts to enhance Fas-mediated apoptosis, most probably through facilitation of the activation of caspase-8.

An adenovirus inhibitor of tumor necrosis factor alpha-induced apoptosis complexes with dynein and a small GTPase

Adenoviruses (Ad) code for immunoregulatory and cytokine regulatory proteins, one of which is the early region 3, 14.7-kDa protein (Ad E3-14.7K), which has been shown to inhibit tumor necrosis factor alpha-induced apoptosis. In an effort to understand the mechanism of action of Ad E3-14.7K, we previously searched for cell proteins with which it interacted. Three Ad E3-14.7K-interacting proteins (FIP-1, -2, and -3) were isolated. FIP-1 is a small GTPase which was used in this report as bait in the yeast two-hybrid system to find other interacting cell targets. The search resulted in the isolation of a protein, which we called GIP-1 (GTPase-interacting protein) that subsequently was shown to be identical to one of the light-chain components of human dynein (TCTEL1). FIP-1 was able to bind both TCTEL1 and Ad E3-14.7K simultaneously and was necessary to form a complex in which the viral protein was associated with a microtubule-binding motor protein. The functional significance of these interactions is discussed with respect to the steps of the Ad life cycle which are microtubule associated.

Activation of apoptosis and its inhibition

The induction of apoptosis, or controlled cell death, by various stimuli has been shown to activate a cascade of endoproteases, called caspases, that cleave numerous cellular proteins necessary for cellular homeostasis. This review discusses this family of proteases together with a variety of mammalian and viral regulatory proteins that act to control this activation.

A portrait of the Bcl-2 protein family: life, death, and the whole picture

The Bcl-2 family of proteins are important regulators of cell death. They are comprised of two opposing factions, the proapoptotic versus the antiapoptotic members. Both are required for normal development and cellular homeostasis of the immune system and other tissues. However, in certain circumstances they may participate in the development of disease.

Immune responses to adenoviruses: viral evasion mechanisms and their implications for the clinic

Adenoviruses encode proteins that block responses to interferons, intrinsic cellular apoptosis, killing by CD8(+) cytotoxic T lymphocytes and killing by the death ligands TNF, Fas ligand and TRAIL. The viral proteins are believed to prolong acute and persistent adenovirus infections. The proteins may prove useful in protecting adenovirus gene therapy vectors and transplanted cells from the immune system.

Saccharomyces cerevisiae putative G protein, Gtr1p, which forms complexes with itself and a novel protein designated as Gtr2p, negatively regulates the Ran/Gsp1p G protein cycle through Gtr2p

Prp20p and Rna1p are GDP/GTP exchanging and GTPase-activating factors of Gsp1p, respectively, and their mutations, prp20-1 and rna1-1, can both be suppressed by Saccharomyces cerevisiae gtr1-11. We found that gtr1-11 caused a single amino acid substitution in Gtr1p, forming S20L, which is a putative GDP-bound mutant protein, while Gtr1p has been reported to bind to GTP alone. Consistently, gtr1-S20N, another putative GDP-bound mutant, suppressed both prp20-1 and rna1-1. On the other hand, gtr1-Q65L, a putative GTP-bound mutant, was inhibitory to prp20-1 and rna1-1. Thus, the role that Gtr1p plays in vivo appears to depend upon the nucleotide bound to it. Our data suggested that the GTP-bound Gtr1p, but not the GDP-bound Gtr1p, interacts with itself through its C-terminal tail. S. cerevisiae possesses a novel gene, GTR2, which is homologous to GTR1. Gtr2p interacts with itself in the presence of Gtr1p. The disruption of GTR2 suppressed prp20-1 and abolished the inhibitory effect of gtr1-Q65L on prp20-1. This finding, taken together with the fact that Gtr1p-S20L is a putative, inactive GDP-bound mutant, implies that Gtr1p negatively regulates the Ran/Gsp1p GTPase cycle through Gtr2p.

The regulation of apoptosis by microbial pathogens

In the past few years, there has been remarkable progress unraveling the mechanism and significance of eukaryotic programmed cell death (PCD), or apoptosis. Not surprisingly, it has been discovered that numerous, unrelated microbial pathogens engage or circumvent the host's apoptotic program. In this chapter, we briefly summarize apoptosis, emphasizing those studies which assist the reader in understanding the subsequent discussion on PCD and pathogens. We then examine the relationship between virulent bacteria and apoptosis. This section is organized to reflect both common and diverse mechanisms employed by bacteria to induce PCD. A short discussion of parasites and fungi is followed by a detailed description of the interaction of viral pathogens with the apoptotic machinery. Throughout the review, apoptosis is considered within the broader contexts of pathogenesis, virulence, and host defense. Our goals are to update the reader on this rapidly expanding field and identify topics in the current literature which demand further investigation.

Immune evasion by adenoviruses

Adenovirus is a human pathogen that infects mainly respiratory and gastrointestinal epithelia. While the pathology caused by this virus is generally not life threatening in immunocompetent individuals, there is a large literature describing its ability to establish a persistent infection. These persistent infections typically occur in apparently healthy individuals with no outward signs of disease. Such a long term and benign interaction between virus and immune system requires adenoviruses to dampen host antiviral effector mechanisms that would otherwise eliminate the virus and cause immune-mediated pathology to the host. Adenovirus devotes a significant portion of its genome to gene products whose sole function seems to be the modulation of host immune responses. This review focuses on what is currently understood about how these immunomodulatory mechanisms work and how they might play a role in maintaining the virus in a persistent state.

Identification of a cell protein (FIP-3) as a modulator of NF-kappaB activity and as a target of an adenovirus inhibitor of tumor necrosis factor alpha-induced apoptosis

FIP-3 (14.7K interacting protein) was discovered during a search for cell proteins that could interact with an adenovirus protein (Ad E3-14.7K) that had been shown to prevent tumor necrosis factor (TNF)-alpha-induced cytolysis. FIP-3, which contains leucine zippers and a zinc finger domain, inhibits both basal and induced transcriptional activity of NF-kappaB and causes a late-appearing apoptosis with unique morphologic manifestations. Ad E3-14.7K can partially reverse apoptotic death induced by FIP-3. FIP-3 also was shown to bind to other cell proteins, RIP and NIK, which previously had been described as essential components of TNF-alpha-induced NF-kappaB activation. In addition, FIP-3 inhibited activation of NF-kappaB induced by TNF-alpha, the TNFR-1 receptor, RIP, NIK, and IKKbeta, as well as basal levels of endogenous NF-kappaB in 293 cells. Because the activation of NF-kappaB has been shown to inhibit apoptosis, FIP-3 appears both to activate a cell-death pathway and to inhibit an NF-kappaB-dependent survival mechanism.

Inhibition of NF-kappaB activation in combination with bcl-2 expression allows for persistence of first-generation adenovirus vectors in the mouse liver

NF-kappaB is a key regulator of the innate antiviral immune response, due in part to its transcriptional activation of cytokines and adhesion molecules, which, in turn, function in chemotaxis and activation of inflammatory cells. We reported earlier that viral gene expression in hepatocytes transduced with first-generation (E1-deleted) adenoviruses induced NF-kappaB activation, elevation of serum cytokines, and hepatocellular apoptosis during the first days postinfusion. These events did not occur in mice infused with an adenovirus vector deleted for E1, E2, E3, and late gene expression. In the present study, we used an adenovirus expressing an IkappaBalpha supersuppressor (Ad.IkappaBM) and bcl-2 transgenic mice to unravel the role of virus-induced NF-kappaB activation and apoptosis in the clearance of recombinant adenovirus vectors from the liver. The combined action of IkappaBM and Bcl-2 allowed for vector persistence in livers of C57BL/6 x C3H mice. In the absence of Bcl-2, IkappaBM expression in mouse livers significantly reduced NF-kappaB activation, cytokine expression, leukocyte infiltration, and the humoral immune response against the transgene product; however, this was not sufficient to prevent the decline of vector DNA in transduced cells. Infusion of Ad.IkappaBM caused extended apoptosis predominantly in periportal liver regions, indicating that NF-kappaB activation may protect transduced hepatocytes from apoptosis induced by adenovirus gene products. To confer vector persistence, bcl-2 transgene expression was required to block virus-induced apoptosis if NF-kappaB protection was inactivated by IkappaBM. Expression of gene products involved in early stages of apoptotic pathways was up-regulated in response to virus infusion in bcl-2 transgenic mice, which may represent a compensatory effect. Our study supports the idea that the suppression of innate defense mechanisms improves vector persistence.

Lung-specific expression of adenovirus E3-14.7K in transgenic mice attenuates adenoviral vector-mediated lung inflammation and enhances transgene expression

Herein, we report that the adenovirus E3-14.7K protein inhibits the inflammatory response to adenovirus in transgenic mice in which the E3-14.7K gene was selectively expressed in the respiratory epithelium, using the human surfactant protein C (SP-C) promoter. E3-14.7K mRNA and protein were detected specifically in the lungs of SPC/E3-14.7K transgenic mice. Responses of the transgenic mice to Av1Luc1, an E1-E3-deleted Ad vector encoding the luciferase reporter gene, were examined, including vector transgene expression and lung inflammation. In wild-type mice, luciferase activity declined rapidly and was lost 14 days following Av1Luc1 administration. The loss of luciferase activity was associated with pulmonary infiltration by macrophages and lymphocytes. In heterozygous SPC/E3-14.7K mice, luciferase activity was increased by 7 days compared with control littermates, and pulmonary infiltration by macrophages was decreased. In homozygous (+/+) SPC/E3-14.7K mice, luciferase activity was increased 7, 14, and 21 days following administration compared with wild-type mice, and lung inflammation was markedly reduced. After Av1Luc1 administration, PCNA staining of bronchiolar and alveolar respiratory epithelial cells was decreased in SPC/E3-14.7K transgenic mice, indicating decreased epithelial cell proliferation, a finding consistent with the observed reduction in inflammation. CD4 and CD8 lymphocyte populations were only mildly altered, while humoral responses to adenoviral vectors were unchanged in the SPC/E3-14.7K mice. The E3-14.7K protein expressed selectively in respiratory epithelial cells suppresses Ad-induced pulmonary epithelial cell cytotoxicity and lung inflammation in vivo and prolongs reporter gene expression.

Apoptosis regulators from DNA viruses

The past year has seen an considerable expansion in knowledge about the field of apoptosis modulators expressed by DNA viruses. These diverse classes of virus-encoded regulators include caspase inhibitors, signal transduction effectors, Bcl-2 homologs, cell cycle control proteins, transcriptional regulators, reactive oxide scavengers, kinases, 'death factors' and novel host-range proteins.

Interaction of an adenovirus E3 14.7-kilodalton protein with a novel tumor necrosis factor alpha-inducible cellular protein containing leucine zipper domains

Early region 3 (E3) of group C human adenoviruses (Ad) encodes several inhibitors of tumor necrosis factor alpha (TNF-alpha) cytolysis, including an E3 14.7-kDa protein (E3-14.7K) and a heterodimer containing two polypeptides of 10.4 and 14.5 kDa. To understand the mechanism by which the viral proteins inhibit TNF-alpha functions, the E3-14.7K protein was used to screen a HeLa cell cDNA library to search for interacting proteins in the yeast two-hybrid system. A novel protein containing multiple leucine zipper domains without any significant homology with any known protein was identified and has been named FIP-2 (for 14.7K-interacting protein). FIP-2 interacted with E3-14.7K both in vitro and in vivo. It colocalized with Ad E3-14.7K in the cytoplasm, especially near the nuclear membrane, and caused redistribution of the viral protein. FIP-2 by itself does not cause cell death; however, it can reverse the protective effect of E3-14.7K on cell killing induced by overexpression of the intracellular domain of the 55-kDa TNF receptor or by RIP, a death protein involved in the TNF-alpha and Fas apoptosis pathways. Deletion analysis indicates that the reversal effect of FIP-2 depends on its interaction with E3-14.7K. Three major mRNA forms of FIP-2 have been detected in multiple human tissues, and expression of the transcripts was induced by TNF-alpha treatment in a time-dependent manner in two different cell lines. FIP-2 has consensus sequences for several potential posttranslational modifications. These data suggest that FIP-2 is one of the cellular targets for Ad E3-14.7K and that its mechanism of affecting cell death involves the TNF receptor, RIP, or a downstream molecule affected by either of these two molecules.