Reciprocal impacts of telomerase activity and ADRN/MES differentiation state in neuroblastoma tumor biology

Telomere maintenance and tumor cell differentiation have been separately implicated in neuroblastoma malignancy. Their mechanistic connection is unclear. We analyzed neuroblastoma cell lines and morphologic subclones representing the adrenergic (ADRN) and mesenchymal (MES) differentiation states and uncovered sharp differences in their telomere protein and telomerase activity levels. Pharmacologic conversion of ADRN into MES cells elicited consistent and robust changes in the expression of telomere-related proteins. Conversely, stringent down-regulation of telomerase activity triggers the differentiation of ADRN into MES cells, which was reversible upon telomerase up-regulation. Interestingly, the MES differentiation state is associated with elevated levels of innate immunity factors, including key components of the DNA-sensing pathway. Accordingly, MES but not ADRN cells can mount a robust response to viral infections in vitro. A gene expression signature based on telomere and cell lineage-related factors can cluster neuroblastoma tumor samples into predominantly ADRN or MES-like groups, with distinct clinical outcomes. Our findings establish a strong mechanistic connection between telomere and differentiation and suggest that manipulating telomeres may suppress malignancy not only by limiting the tumor growth potential but also by inducing tumor cell differentiation and altering its immunogenicity.

ADAM17 stabilizes its interacting partner inactive Rhomboid 2 (iRhom2) but not inactive Rhomboid 1 (iRhom1)

The metalloprotease ADAM17 (a disintegrin and metalloprotease 17) is a key regulator of tumor necrosis factor α (TNFα), interleukin 6 receptor (IL-6R), and epidermal growth factor receptor (EGFR) signaling. ADAM17 maturation and function depend on the seven-membrane-spanning inactive rhomboid-like proteins 1 and 2 (iRhom1/2 or Rhbdf1/2). Most studies to date have focused on overexpressed iRhom1 and -2, so only little is known about the properties of the endogenous proteins. Here, we show that endogenous iRhom1 and -2 can be cell surface-biotinylated on mouse embryonic fibroblasts (mEFs), revealing that endogenous iRhom1 and -2 proteins are present on the cell surface and that iRhom2 also is present on the surface of lipopolysaccharide-stimulated primary bone marrow-derived macrophages. Interestingly, very little, if any, iRhom2 was detectable in mEFs or bone marrow-derived macrophages lacking ADAM17, suggesting that iRhom2 is stabilized by ADAM17. By contrast, the levels of iRhom1 were slightly increased in the absence of ADAM17 in mEFs, indicating that its stability does not depend on ADAM17. These findings support a model in which iRhom2 and ADAM17 are obligate binding partners and indicate that iRhom2 stability requires the presence of ADAM17, whereas iRhom1 is stable in the absence of ADAM17.

Parvovirus B19 integration into human CD36+ erythroid progenitor cells

The pathogenic autonomous human parvovirus B19 (B19V) productively infects erythroid progenitor cells (EPCs). Functional similarities between B19V nonstructural protein (NS1), a DNA binding endonuclease, and the Rep proteins of Adeno-Associated Virus (AAV) led us to hypothesize that NS1 may facilitate targeted nicking of the human genome and B19 vDNA integration. We adapted an integration capture sequencing protocol (IC-Seq) to screen B19V infected human CD36+ EPCs for viral integrants, and discovered 40,000 unique B19V integration events distributed throughout the human genome. Computational analysis of integration patterns revealed strong correlations with gene intronic regions, H3K9me3 sites, and the identification of 41 base pair consensus sequence with an octanucleotide core motif. The octanucleotide core has homology to a single region of B19V, adjacent to the P6 promoter TATA box. We present the first direct evidence that B19V infection of erythroid progenitor cells disrupts the human genome and facilitates viral DNA integration.

Adeno-associated virus type 2 preferentially integrates single genome copies with defined breakpoints

Background

Adeno-associated virus (AAV) serotype 2 prevalently infects humans and is the only described eukaryotic virus that integrates site-preferentially. In a recent high throughput study, the genome wide distribution of AAV-2 integrants was determined using Integrant Capture Sequencing (IC-Seq). Additional insight regarding the integration of AAV-2 into human genomic DNA could be gleaned by low-throughput sequencing of complete viral-chromosomal junctions.

Findings

In this study, 140 clones derived from Integrant-Capture Sequencing were sequenced. 100 met sequence inclusion criteria, and of these 39 contained validated junction sequences. These unique sequences were analyzed to investigate the structure and location of viral-chromosomal junctions.

Conclusions

Overall the low-throughput analysis confirmed the genome wide distribution profile gathered through the IC-Seq analysis. We found no unidentifiable sequence inserted at AAV-2 chromosomal junctions. Assessing both left and right ends of the AAV genome, viral breakpoints predominantly occurred in one hairpin of the inverted terminal repeat and AAV genomes were preferentially integrated as single copies.

Serotype 5 Adenovirus fiber (F7F41S) chimeric vectors incur packaging deficiencies when targeting peptides are inserted into Ad41 short fiber

Adenovirus is a well-established viral gene transfer model system that presents two major hurdles when being considered for cell-specific targeting applications. First is the need to detarget the vector from inherent host binding mechanisms, and second is the need to establish a productive and stable method to retarget the vector to a desired cell receptor. In previous studies we had generated an adenovirus vector platform that lacks the normal targeting attributes derived from the fiber and penton capsid proteins. In the current study we characterized our detargeted Ad5-based vectors (Ad5.F7F41S and Ad5.F7F41SDeltaRGD) as platforms for novel retargeted viruses. The experimental strategy relied on incorporating small peptide ligands into several sites of the Ad 41short fiber knob domain (AB, CD, HI, G and Cterm). Reengineering of Ad41 short fiber resulted either in a bypass to fiber 7 usage, or in a dominant negative packaging/production deficiency phenotype. Under specific growth conditions we could remedy some of the capsid deficiencies and generate high titer viruses. However when examined by Western blot analysis, the resulting viruses were still defective in capsid content. The tandem fiber F7F41S platform has revealed an unanticipated sensitivity of Adenovirus packaging to fiber 41short structural modifications. These studies indicate fiber assembly into an intact virion or fiber influenced capsid stability as a bottleneck to efficient particle production. We also demonstrate that virus particles characterized as mature virions following CsCl banding can vary significantly in capsid protein content. Considering the complexity of virus entry into a target cell, modified "mature virions" may be compromised at the level of transduction not only through the intended modification, but also by virtue of secondary structural packaging conflicts.

Adeno-associated virus type 2 p5 promoter: a rep-regulated DNA switch element functioning in transcription, replication, and site-specific integration

The large Rep proteins, p68 and p78, function as master controllers of the adeno-associated virus type 2 (AAV2) life cycle, involved in transcriptional control, in latency, in rescue, and in viral DNA replication. The p5 promoter may be the nucleic acid complement to the large Rep proteins. It drives expression of the large Rep proteins, it undergoes autoregulation by Rep, it undergoes induction by helper virus, it is a target substrate for Rep-mediated site-specific integration (RMSSI), and it can function as a replicative origin. To better understand the relationship between each of the p5 functions, we have determined the effects of p5 promoter mutations (p5 integration efficiency element, or p5IEE) on transcription, integration, and replication using RMSSI transfection protocols in HeLa cells. The data demonstrate that the organization of the p5 promoter provides a unique platform for regulated AAV2 template transcription and subsequent repression by Rep through direct and indirect mechanisms. The elements of the p5IEE that define its function as a promoter also define its function as a highly optimized substrate for Rep-mediated site-specific integration and replication. The p5 Rep binding element (RBE) is essential in RMSSI and Rep-dependent replication; however, replacement of the p5 RBE with either the AAV2 inverted terminal repeat or the AAVS1 RBE sequence elements neither enhances nor severely compromises RMSSI activity of p5IEE. The RBE by itself or in combination with the YY1+1 initiator/terminal resolution sequence element does not mediate efficient site-specific integration. We found that replication and integration were highly sensitive to sequence manipulations of the p5 TATA/RBE/YY1+1 core structure in a manner that reflects the function of these elements in transcription. The data presented support a model where, depending on the state of the cell (Rep expression and helper virus influences), the p5IEE operates as a transcription/integration switch sequence element.

Sensing infection by adenovirus: Toll-like receptor-independent viral DNA recognition signals activation of the interferon regulatory factor 3 master regulator

Infection with adenovirus vectors (AdV) results in rapid activation of innate immunity, which serves the dual purpose of stimulating inflammatory antiviral host defenses and the adaptive immune system. Viral recognition by macrophages, dendritic cells, and other cell types requires an ability to sense the presence of a foreign molecular pattern by "pattern recognition receptors." The nature of the adenoviral sensor, the target ligand of the sensor, and the downstream antiviral signaling response triggered by virus infection have not been defined for this nonenveloped double-stranded DNA (dsDNA) virus. We have identified four critical links involved in AdV recognition by murine antigen-presenting cells (APC) and primary lung fibroblasts: (i) viral recognition occurs chiefly via a Toll-like receptor (TLR)-independent nucleic acid-sensing mechanism recognizing the viral dsDNA genome, (ii) the intact viral particle and capsid proteins are required for efficient intracellular delivery of the viral genome, (iii) delivery of the viral genome triggers interferon regulatory factor 3 (IRF3) phosphorylation, and (iv) IRF3 activation is the required dominant antiviral signaling pathway used by APC, whereas the "primary" involvement of NF-kappaB, mitogen-activated protein kinase, or Akt pathways is less prominent. In this study we provide the first direct evidence that infection by a dsDNA virus stimulates an IRF3-mediated interferon and proinflammatory response through a TLR-independent DNA-sensing mechanism.

Adenovirus capsid chimeras: fiber terminal exon insertions/gene replacements in the major late transcription unit

The adenovirus major late transcription unit (MLTU) encodes the main structural capsid proteins. Expression from the MLTU is accomplished through alternative mRNA processing and use of a terminal exon coding strategy. The capsid proteins hexon, penton, and fiber contribute to efficient infection by adenovirus, and each contributes in some manner to the antiviral immune response against adenovirus infection. The ability to manipulate these genes affords one the opportunity to "detarget" adenovirus, to retarget adenovirus, and to alter immune recognition. In this chapter, we are presenting a terminal exon-replacement strategy that can be used to genetically manipulate capsid proteins expressed from the MLTU. An emphasis will be placed on manipulations of fiber as an intact terminal exon.

Fiber and penton base capsid modifications yield diminished adenovirus type 5 transduction and proinflammatory gene expression with retention of antigen-specific humoral immunity

Fiber and penton base capsid proteins of adenovirus type 5 (Ad5) mediate a well-characterized two-step entry pathway in permissive tissue culture cell lines. Fiber binds with high affinity to the cell surface coxsackievirus-and-adenovirus receptor (CAR), and penton base facilitates viral internalization by binding alphav integrins through an RGD motif. In vivo, the entry pathway is complicated by interactions of capsid proteins with additional cell surface molecules and blood factors. When administered systemically in mice, adenovirus vectors (Adv) localize primarily to hepatic tissue, resulting in efficient gene transduction and potent activation of the host antiviral immune response. The goal of the present study was to detarget Adv uptake through fiber and penton base capsid protein manipulations and determine how detargeted vectors influence transduction efficiency, inflammatory activation, and activation of the adaptive arm of the immune system. By manipulating fiber and the penton base, we have generated highly detargeted vectors (up to 1,200-fold reduction in transgene expression in vivo) with reduced macrophage stimulatory activity in vitro and in vivo. In spite of the diminished transduction and macrophage activation, the detargeted vectors induce strong neutralizing immunity as well as efficient antitransgene antibody. Three of the modified vectors produce antitransgene humoral immunity at levels that exceed or are equal to that seen with an unmodified Ad5-based vector. The fiber-pseudotyped and penton base constructs with RGD deleted have attributes that could be important enhancements in a number of vaccine applications.

Influence of fiber detargeting on adenovirus-mediated innate and adaptive immune activation

The major adenovirus (Ad) capsid proteins hexon, penton, and fiber influence the efficiency and tropism of gene transduction by Ad vectors. Fiber is the high-affinity receptor binding protein that serves to mediate cell attachment in vitro when using coxsackie-adenovirus receptor (CAR)-containing cell lines. This contrasts with transduction efficiency in macrophages or dendritic cells that lack high concentrations of CAR. To determine how fiber influences gene transduction and immune activation in a murine model, we have characterized Ad type 5 (Ad5) vectors with two classes of chimeric fiber, CAR binding and non-CAR binding. In a systemic infection, Ad5 fiber contributes to DNA localization and vector transduction in hepatic tissue. However, the majority of vector localization is due to Ad5 fiber-specific functions distinct from CAR binding. CAR-directed transduction occurs but at a modest level. In contrast to CAR binding vectors, the F7 and F7F41S non-CAR-binding vectors demonstrate a 2-log decrease in hepatic transduction, with a 10-fold decrease in the amount of vector DNA localizing to the hepatic tissue. To characterize the innate response to early infection using fiber chimeric vectors, intrahepatic cytokine and chemokine mRNAs were quantified 5 hours postinfection. Tumor necrosis factor alpha mRNA levels resulting from Ad5 fiber infections were elevated compared to viruses expressing serotype 7 or 41 fiber. Levels of chemokine mRNA (gamma interferon-inducible protein 10, T-cell activation gene 3, and macrophage inflammatory protein 1beta) were 10- to 20-fold higher with CAR binding vectors (Ad5 and F41T) than with non-CAR-binding vectors (F7 and F7F41S). In spite of quantitative differences in vector localization and innate activation, fiber pseudotyping did not significantly change the outcome of anti-Ad adaptive immunity. All vectors were cleared with the same kinetics as wild-type Ad5 vectors, and each induced neutralizing antibody. Although non-CAR-binding vectors were impaired in transduction by nearly 2 orders of magnitude, the level of antitransgene immunity was the same for each of the vectors. Using primary bone marrow-derived macrophages and dendritic cells, we demonstrate that transduction, induction of cytokine/chemokine, and phenotypic maturation of these antigen-presenting cells are independent of fiber content. Our data support a model where fiber-mediated hepatic localization enhances innate responses to virus infection but minimally impacts on adaptive immunity.

Adeno-associated virus site-specific integration and AAVS1 disruption

Adeno-associated virus (AAV) is a single-stranded DNA virus with a unique biphasic lifestyle consisting of both a productive and a latent phase. Typically, the productive phase requires coinfection with a helper virus, for instance adenovirus, while the latent phase dominates in healthy cells. In the latent state, AAV is found integrated site specifically into the host genome at chromosome 19q13.4 qtr (AAVS1), the only animal virus known to integrate in a defined location. In this study we investigated the latent phase of serotype 2 AAV, focusing on three areas: AAV infection, rescue, and integration efficiency as a function of viral multiplicity of infection (MOI); efficiency of site-specific integration; and disruption of the AAVS1 locus. As expected, increasing the AAV MOI resulted in an increase in the percentage of cells infected, with 80% of cells infected at an MOI of 10. Additional MOI only marginally effected a further increase in percentage of infected cells. In contrast to infection, we found very low levels of integration at MOIs of less than 10. At an MOI of 10, at which 80% of cells are infected, less than 5% of clonal cell lines contained integrated AAV DNA. At an MOI of 100 or greater, however, 35 to 40% of clonal cell lines contained integrated AAV DNA. Integration and the ability to rescue viral genomes were highly correlated. Analysis of integrated AAV indicated that essentially all integrants were AAVS1 site specific. Although maximal integration efficiency approached 40% of clonal cell lines (essentially 50% of infected cells), over 80% of cell lines contained a genomic disruption at the AAVS1 integration locus on chromosome 19 ( approximately 100% of infected cells). Rep expression by itself and in the presence of a plasmid integration substrate was able to mediate this disruption of the AAVS1 site. We further characterized the disruption event and demonstrated that it resulted in amplification of the AAVS1 locus. The data are consistent with a revised model of AAV integration that includes preliminary expansion of a defined region in AAVS1.

Adenovirus-induced maturation of dendritic cells through a PI3 kinase-mediated TNF-alpha induction pathway

Systemic administration of adenovirus and adenovirus vectors induces a robust innate and adaptive immune response in a variety of animal models. In tumor necrosis factor (TNF)(-/-) mice, a diminished immune response to adenovirus (Ad) infection has been attributed to compromised dendritic cell (DC) maturation. In this report, we investigated the mechanisms responsible for Ad-mediated activation and maturation of DC. Ad infection induced high levels of TNF-alpha expression by murine bone marrow-derived DC, comparable to levels observed with lipopolysaccharide exposure. Ad-induced TNF-alpha production was necessary for DC maturation and acts in an autocrine manner. Unlike TNF-alpha production associated with exposure to lipopolysaccharide, Ad induction of TNF-alpha was not dependent on the MyD88 signaling pathway. In contrast, Ad-induced TNF-alpha production and DC maturation were dependent on signaling by phosphoinositide-3-OH kinase (PI3K), as determined by wortmannin and LY294002 blocking experiments. The adenovirus capsid protein penton contains a well characterized arginine-glycine-aspartic acid integrin-binding domain that stimulates PI3K in fibroblast cell lines. When this region of the penton was mutated, TNF-alpha expression and bone marrow-derived DC maturation were attenuated. We propose that integrin-mediated PI3K induction of NF-kappaB activates an autocrine TNF-alpha pathway required for DC maturation in response to Ad.

Transgene Expression After Rep-Mediated Site-Specific Integration into Chromosome 19

We have used a plasmid-based transfection model of the adeno-associated virus (AAV) Rep-mediated site-specific integration (RMSSI) pathway to characterize the stability and expression of a site-specifically integrated transgene (either green fluorescent protein [GFP] or chloramphenicol acetyltransferase [CAT]). Three plasmids containing the AAV p5 integration efficiency element (p5IEE) have been used to study integration and transgene expression in HeLa cells: (1) pRepGFP(itr+) contains both AAV ITRs, rep, and p5IEE and can be used as either a plasmid or rAAV vehicle for integration; (2) pRepGFP(itr-) contains the AAV rep gene and the p5IEE; (3) pAd-p5CAT contains only the 138-bp p5IEE of AAV. The data presented demonstrate that in the absence of drug selection, all three constructs undergo site-specific integration (efficiencies of between 10 and 40% of transduced cell lines). At 6 weeks posttransfection most cell lines that underwent RMSSI also expressed the appropriate transgene product. By 18 weeks posttransfection cell lines that were established with rep in cis to the transgene showed a decline in transgene expression as well as a loss of transgene DNA. In many cell lines, there appears to be transgene-containing DNA that does not contribute to gene expression. Data support a model of gene expression and transgene instability through a Rep-mediated pathway. In contrast to rep-containing cell lines, clonal cell lines containing p5IEECAT (with Rep provided in trans) maintained both the integrated transgene and transgene expression throughout the entire experimental time course (18 weeks).

Subgroup B and F fiber chimeras eliminate normal adenovirus type 5 vector transduction in vitro and in vivo

Altering adenovirus vector (Ad vector) targeting is an important goal for a variety of gene therapy applications and involves eliminating or reducing the normal tropism of a vector and retargeting through a distinct receptor-ligand pathway. The first step of Ad vector infection is high-affinity binding to a target cellular receptor. For the majority of adenoviruses and Ad vectors, the fiber capsid protein serves this purpose, binding to the coxsackievirus and adenovirus receptor (CAR) present on a variety of cell types. In this study we have explored a novel approach to altering Ad type 5 (Ad5) vector targeting based on serotypic differences in fiber function. The subgroup B viruses bind to an unidentified receptor that is distinct from CAR. The subgroup F viruses are the only adenoviruses that express two distinct terminal exons encoding fiber open reading frames. We have constructed chimeric fiber adenoviruses that utilize the tandem fiber arrangement of the subgroup F genome configuration. By taking advantage of serotypic differences in fiber expression, fiber shaft length, and fiber binding efficiency, we have developed a tandem fiber vector that has low binding efficiency for the known fiber binding sites, does not rely on an Ad5-based fiber, and can be grown to high titer using conventional cell lines. Importantly, when characterizing these vectors in vivo, we find the subgroup B system and our optimal tandem fiber system demonstrate reduced liver transduction by over 2 logs compared to an Ad5 fiber vector. These attributes make the tandem fiber vector a useful alternative to conventional strategies for fiber manipulation of adenovirus vectors.

A p5 integration efficiency element mediates Rep-dependent integration into AAVS1 at chromosome 19

Adeno-associated virus (AAV) undergoes site-specific integration into human chromosome 19 through a deletion-substitution mechanism at the well characterized AAVS1 site. We have shown previously that a cis element within the left end of the AAV genome enhances the efficiency of Rep-mediated site-specific integration into chromosome 19 when present in inverted terminal repeat-containing recombinant AAV (rAAV) plasmids. We now demonstrate that a 138-bp cis element, the p5 integration efficiency element (p5IEE), mediates efficient integration. The p5IEE is not only required for efficient site-specific integration, it is also sufficient. Integration mediated by the p5IEE occurs in the absence of the AAV inverted terminal-repeat elements. The data presented in this study demonstrate that the p5IEE is a multifunctional element, serving as the highly regulatable Rep promoter and the primary substrate for targeted integration.

Sensitization of IFN-gamma Jak-STAT signaling during macrophage activation

A general paradigm in signal transduction is ligand-induced feedback inhibition and the desensitization of signaling. We found that subthreshold concentrations of interferon-gamma (IFN-gamma), which did not activate macrophages, increased their sensitivity to subsequent IFN-gamma stimulation; this resulted in increased signal transducer and activator of transcription 1 (STAT1) activation and increased IFN-gamma#150;dependent gene activation. Sensitization of IFN-gamma signaling was mediated by the induction of STAT1 expression by low doses of IFN-gamma that did not effectively induce feedback inhibition. IFN-gamma signaling was sensitized in vivo after IFN-gamma injection, and STAT1 expression was increased after injection of lipopolysaccharide and in rheumatoid arthritis synovial cells. These results identify a mechanism that sensitizes macrophages to low concentrations of IFN-gamma and regulates IFN-gamma responses in acute and chronic inflammation.

Efficient integration of recombinant adeno-associated virus DNA vectors requires a p5-rep sequence in cis

The initial aim of this study was to combine attributes of adeno-associated virus (AAV) and adenovirus (Ad) gene therapy vectors to generate an Ad-AAV hybrid vector allowing efficient site-specific integration with Ad vectors. In executing our experimental strategy, we found that, in addition to the known incompatibility of Rep expression and Ad growth, an equally large obstacle was presented by the inefficiency of the integration event when using traditional recombinant AAV (rAAV) vectors. This study has addressed both of these problems. We have shown that a first-generation Ad can be generated that expresses Rep proteins at levels consistent with those found in wild-type AAV (wtAAV) infections and that Rep-mediated AAV persistence can occur in the presence of first-generation Ad vectors. Our finding that traditional rAAV plasmid vectors lack integration potency compared to wtAAV plasmid constructs (10- to 100-fold differences) was unexpected but led to the discovery of a previously unidentified AAV integration enhancer sequence element which functions in cis to an AAV inverted terminal repeat-flanked target gene. rAAV constructs containing left-end AAV sequence, including the p5-rep promoter sequence, integrate efficiently in a site-specific manner. The identification of this novel AAV integration enhancer element is consistent with previous studies, which have indicated that a high frequency of wtAAV recombinant junction formation occurs in the vicinity of the p5 promoter, and recent studies have demonstrated a role for this region in AAV DNA replication. Understanding the contribution of this element to the mechanism of AAV integration will be critical to the use of AAV vectors for targeted gene transfer applications.

TNF-alpha -dependent maturation of local dendritic cells is critical for activating the adaptive immune response to virus infection

Tumor necrosis factor-alpha (TNF-alpha) is well recognized for its role in mediating innate immune responses. However, the mechanisms of TNF-alpha that influence the adaptive immune response to virus infections are not well understood. In this study, we have investigated the role of TNF-alpha in activating the cellular and humoral responses to systemic viral challenge with recombinant replication-defective adenovirus (rAd). Evaluation of T cell function in TNF-alpha-deficient (TNFKO) mice revealed impaired virus-specific proliferation of T cells derived from the draining lymph nodes of the liver. Analysis of dendritic cells (DC) isolated from local draining lymph nodes after systemic challenge showed that DC from TNFKO mice were relatively immature compared with those from strain-matched wild-type mice. In vitro, TNF-alpha was required to mature DC efficiently during virus-mediated stimulation. Adoptive transfer of primed, mature DC into TNFKO mice restored T cell responses and reconstituted anti-adenovirus antibody responses. Thus, TNF-alpha plays a significant role in the maturation of DC after adenovirus challenge both in vitro and in vivo, highlighting the importance of this innate cytokine in activating adaptive immunity to viral challenge.

Variation in adenovirus transgene expression between BALB/c and C57BL/6 mice is associated with differences in interleukin-12 and gamma interferon production and NK cell activation

The innate immune response against replication-defective adenoviruses (Ad) is poorly defined. We and others have previously observed striking differences in the rate at which the Ad vector itself or the virus encoding a variety of transgenes is eliminated in different mouse strains. Here, we report that Ad infection of BALB/ mice is associated with sixfold-higher levels of serum alanine aminotransferase and that Ad transgenes induce two- to threefold-higher levels of intrahepatic NK cells and NK activity compared to C57BL/6 mice. The increase in NK activation in BALB/c mice was associated with approximately 4-fold higher level of mRNA expression of a newly described NKG2 receptor activator, H-60, as well as increased expression of interleukin-12 and gamma interferon mRNAs in BALB/c mice compared to C57BL/6 mice. NK depletion in BALB/c mice or defective NK function in C3H beige mice extended transgene expression compared to their appropriate controls, and attenuation of NK together with CD8 T-cell function had a synergistic effect. These findings indicate that there are intrinsic differences in the innate immune responses of different mouse strains to Ad and Ad transgenes and that NK cells, in cooperation with CD8 T cells, play a pivotal role in the early extinction of transgene expression in BALB/c mice.

Soluble CD8 attenuates cytotoxic T cell responses against replication-defective adenovirus affording transprotection of transgenes in vivo

The T cell coreceptor, CD8, enhances T cell-APC interactions. Because soluble CD8alpha homodimers can antagonize CD8 T cell activation in vitro, we asked whether secretion of soluble CD8 would effect cytotoxic T cell responses in vivo. Production of soluble CD8 by a replication-defective adenovirus vector allowed persistent virus expression for up to 5 mo in C57BL/6 mice and protected a second foreign transgene from rapid deletion. Soluble CD8 selectively inhibited CD8 T cell proliferation and IFN-gamma production and could also attenuate peptide-specific CD8 T cell responses in vivo. These finding suggest that gene vector delivery of soluble CD8 may have therapeutic applications.

Creation and repair of specific DNA double-strand breaks in vivo following infection with adenovirus vectors expressing Saccharomyces cerevisiae HO endonuclease

To study DNA double-strand break (DSB) repair in mammalian cells, the Saccharomyces cerevisiae HO endonuclease gene, or its recognition site, was cloned into the adenovirus E3 or E1 regions. Analysis of DNA from human A549 cells coinfected with the E3::HO gene and site viruses showed that HO endonuclease was active and that broken viral genomes were detectable 12 h postinfection, increasing with time up to approximately 30% of the available HO site genomes. Leftward fragments of approximately 30 kbp, which contain the packaging signal, but not rightward fragments of approximately 6 kbp, were incorporated into virions, suggesting that broken genomes were not held together tightly after cleavage. There was no evidence for DSB repair in E3::HO virus coinfections. In contrast, such evidence was obtained in E1::HO virus coinfections of nonpermissive cells, suggesting that adenovirus proteins expressed in the permissive E3::HO coinfection can inhibit mammalian DSB repair. To test the inhibitory role of E4 proteins, known to suppress genome concatemer formation late in infection (Weiden and Ginsberg, 1994), A549 cells were coinfected with E3::HO viruses lacking the E4 region. The results strongly suggest that the E4 protein(s) inhibits DSB repair.

Fiber swap between adenovirus subgroups B and C alters intracellular trafficking of adenovirus gene transfer vectors

Following receptor binding and internalization, intracellular trafficking of adenovirus (Ad) among subgroups B and C is different, with significant amounts of Ad serotype 7 (Ad7) (subgroup B) virions found in cytoplasm during the initial hours of infection while Ad5 (subgroup C) virions rapidly translocate to the nucleus. To evaluate the role of the fiber in these differences, we examined intracellular trafficking of Ad5, Ad7, and Ad5f7 (a chimeric vector composed of the Ad5 capsid with the fiber replaced by the Ad7 fiber) by conjugating Ad capsids directly with Cy3 fluorescent dye, permitting the trafficking of the capsids to be examined by fluorescence microscopy. The human lung carcinoma cell line A549 was infected with Cy3-conjugated viruses for 10 min followed by a 1-h incubation. Ad5 virions rapidly translocated to the nucleus (within 1 h of infection), while Ad7 virions were widely distributed in the cytoplasm at the same time point. Interestingly, chimeric Ad5f7 virions behaved similarly to Ad7 but not Ad5. In this regard, the percentages of nuclear localization of Ad5, Ad7, and Ad5f7 at 1 h following infection were 72% +/- 4%, 32% +/- 6%, and 38% +/- 2%, respectively. Consistent with these observations, fluorescence in situ hybridization demonstrated that most of the Ad5 DNA was detected at the nucleus after 1 h, but at the same time point, DNA of Ad7 and Ad5f7 was distributed in both the nucleus and cytoplasm. Quantification of the kinetics of Ad genomic DNA delivery to the nucleus using a fluorogenic probe-based PCR assay (TaqMan PCR) demonstrated that the percentages of nuclear association of Ad5 DNA and Ad5f7 DNA at 1 h postinfection were 80% +/- 13% and 43% +/- 1%, respectively. Although it has been generally accepted that Ad fiber protein mediates attachment of virions to cells and that fibers dissociate during endocytic uptake, these data suggest that in addition to mediating binding to the cell surface, fiber likely modulates intracellular trafficking as well.

Inhibition of tumor necrosis factor alpha by an adenovirus-encoded soluble fusion protein extends transgene expression in the liver and lung

The cellular and humoral immune responses to adenovirus (Ad) remain a major barrier to Ad-mediated gene therapy. We recently reported that mice deficient in tumor necrosis factor alpha (TNF-alpha) or Fas (APO-1, CD95) have prolonged expression of an Ad transgene expressing a foreign protein in the liver. To determine whether blockade of TNF-alpha or Fas would have the same effect in normal mice, we created transgenes that expressed soluble murine CD8 or CD8 fused to the extracellular regions of TNF receptor 1 (TNFR) or Fas and inserted into the left-end region of first-generation (E1/E3-) Ad vectors. Consistent with the results observed in TNF-deficient mice, expression of the TNFR-CD8 fusion protein was prolonged in vivo compared to that of control proteins. Not only did expression of TNFR-CD8 persist in the liver and the lung, but when coadministered with another first-generation vector, the protein provided "transprotection" for the companion vector and transgene. In addition, TNFR-CD8 attenuated the humoral immune response to the Ad. Together, these findings demonstrate that blockade of TNF-alpha is likely to be useful in extending the expression of an Ad-encoded transgene in a gene therapy application.

Modification of the genetic program of human alveolar macrophages by adenovirus vectors in vitro is feasible but inefficient, limited in part by the low level of expression of the coxsackie/adenovirus receptor

Robust expression of genes transferred by adenovirus (Ad) vectors depends upon efficient entry of vectors into target cells. Cells deficient in the coxsackie/adenovirus receptor (CAR) are difficult targets for Ad-mediated gene transfer. We hypothesized that low levels of CAR expression may be responsible, in part, for the relative inefficiency of Ad-mediated gene transfer to human alveolar macrophages (AMs). CAR gene expression was detected in human AMs by reverse transcription-polymerase chain reaction and at low levels by Northern analysis. Indirect immunofluorescence showed specific, low-intensity surface staining for CAR, but at levels below those found on the positive-control A549 human lung epithelial cell line. Consistent with this, AMs expressed Ad vector transgenes 100 to 1,000-fold less efficiently than A549 cells, as assessed using the beta-galactosidase reporter (chemiluminescence assay) and green fluorescent protein (fluorescence microscopy and flow cytometry). At high multiplicity of infection, AMs from an HIV+ individual could be transduced with an AdIFNgamma vector to secrete detectable human interferon-gamma. Ad transgene expression by AMs was blocked by capsid fiber protein, suggesting that CAR is required in the pathway for productive Ad entry into alveolar macrophages. To confirm that Ad transgene expression by AMs is limited by low levels of CAR expression, cells were infected with an Ad vector containing the CAR complementary DNA (cDNA). Enhanced expression of CAR protein was demonstrated by indirect immunofluorescence, and the CAR cDNA-transduced cells showed 5-fold enhancement of subsequent Ad transgene expression. These observations demonstrate that human AMs can be targets for Ad-mediated gene transfer, but that efficiency of transgene expression is limited, at least in part, by low levels of CAR expression.

Construction and characterization of hexon-chimeric adenoviruses: specification of adenovirus serotype

This study has used the strategy of gene replacement to characterize the contribution of the adenovirus (Ad) capsid protein hexon to serotype definition. By replacing the Ad type 5 (Ad5) hexon gene with sequences from Ad2, we have changed the type specificity of the chimeric virus. The type-determining epitopes are primarily associated with loop 1 of hexon and, to a much lesser degree, with loop 2. In spite of the serotype distinctiveness of the chimeric hexon viruses, epitope similarity between the vectors resulted in a low level of cross-reactive neutralizing antibody, which in combination with activated cellular and innate arms of the immune system is sufficient to suppress gene transduction following readministration in vivo.

Construction of an adenovirus type 7a E1A- vector

A strategy for constructing replication-defective adenovirus vectors from non-subgroup C viruses has been successfully demonstrated with adenovirus type 7 strain a (Ad7a) as the prototype. An E1A-deleted Ad7a reporter virus expressing the chloramphenicol acetyltransferase (CAT) gene from the cytomegalovirus promoter enhancer was constructed with DNA fragments isolated from Ad7a, an Ad7a recombination reporter plasmid, and the 293 cell line. The Ad7a-CAT virus particle transduces A549 cells as efficiently as Ad5-based vectors. Intravenous infections in a murine model indicate that the Ad7a-CAT virus infects a variety of tissues, with maximal levels of CAT gene expression found in the liver. The duration of Ad7a-CAT transgene expression in the liver was maximally maintained 2 weeks postinfection, with a decline to baseline activity by the week 4 postinfection. Ad7a-CAT represents the first example of a non-subgroup C E1A- adenovirus gene transfer vector.

Tumor necrosis factor alpha plays a central role in immune-mediated clearance of adenoviral vectors

Adenovirus (Ad) gene transfer vectors are rapidly cleared from infected hepatocytes in mice. To determine which effector mechanisms are responsible for elimination of the Ad vectors, we infected mice that were genetically compromised in immune effector pathways [perforin, Fas, or tumor necrosis factor alpha (TNF-alpha)] with the Ad vector, Ad5-chloramphenicol acetyl transferase (CAT). Mice were sacrificed at 7-60 days postinfection, and the levels of CAT expression in the liver determined by a quantitative enzymatic assay. When the livers of infected mice were harvested 28 days postinfection, the levels of CAT expression revealed that the effectors most important for the elimination of the Ad vector were TNF-alpha > Fas > perforin. TNF-alpha did not have a curative effect on infected hepatocytes, as the administration of TNF-alpha to infected severe combined immunodeficient mice or to infected cultures in vitro had no specific effect on virus persistence. However, TNF-alpha-deficient mice demonstrated a striking reduction in the leukocytic infiltration early on in the infection, suggesting that TNF-alpha deficiency resulted in impaired recruitment of inflammatory cells to the site of inflammation. In addition, the TNF-deficient mice had a significantly reduced humoral immune response to virus infection. These results demonstrate a dominant role of TNF-alpha in elimination of Ad gene transfer vectors. This result is particularly important because viral proteins that disable TNF-alpha function have been removed from most Ad vectors, rendering them highly susceptible to TNF-alpha-mediated elimination.

Sequence-mediated regulation of adenovirus gene expression by repression of mRNA accumulation

Gene expression in complex transcription units can be regulated at virtually every step in the production of mature cytoplasmic mRNA, including transcription initiation, elongation, termination, pre-mRNA processing, nucleus-to-cytoplasm mRNA transport, and alterations in mRNA stability. We have been characterizing alternative poly(A) site usage in the adenovirus major late transcription unit (MLTU) as a model for regulation at the level of pre-mRNA 3'-end processing. The MLTU contains five polyadenylation sites (L1 through L5). The promoter proximal site (L1) functions as the dominant poly(A) site during the early stage of adenovirus infection and in plasmid transfections when multiple poly(A) sites are present at the 3' end of a reporter plasmid. In contrast, stable mRNA processed at all five poly(A) sites is found during the late stage of adenovirus infection, after viral DNA replication has begun. Despite its dominance during early infection, L1 is a comparatively poor substrate for 3'-end RNA processing both in vivo and in vitro. In this study we have investigated the basis for the early L1 dominance. We have found that mRNA containing an unprocessed L1 poly(A) site is compromised in its ability to enter the steady-state pool of stable mRNA. This inhibition, which affects either the nuclear stability or nucleus-to-cytoplasm transport of the pre-mRNA, requires a cis-acting sequence located upstream of the L1 poly(A) site.

Innate immune mechanisms dominate elimination of adenoviral vectors following in vivo administration

To evaluate the contribution of the innate immune component of host defense in clearing the genome of adenovirus (Ad) vectors following in vivo administration, the Ad vectors AdCMV.beta gal (expressing beta-galactosidase) or AdCMV.Null (expressing no gene) were administered intravenously to immunocompetent or immunodeficient mice, and the amount of vector genome was quantified in the liver. Strikingly, 90% of vector DNA was eliminated within 24 hr. There was no increase in vector DNA in other tissues over this period, suggesting that rapid clearance of vector genome resulted from local degradation. After 24 hr, vector elimination was slow, with only 9% of the initial amount of vector genome cleared over the subsequent 3 weeks. Importantly, early phase (0-24 hr) elimination of vector DNA was independent of the transgene and similar in immunocompetent and nude animals. These observations suggest two phases of Ad vector elimination: a previously recognized late, immune-related elimination, and the early, innate immune elimination described in the present study. The early phase of vector loss is, by far, the dominant mechanism, an observation that has implications in developing strategies to maintain persistent expression of the newly transferred gene following in vivo gene therapy.

Circumvention of anti-adenovirus neutralizing immunity by administration of an adenoviral vector of an alternate serotype

Effective gene transfer and expression following repetitive administration of adenoviral (Ad) vectors in experimental animals is limited by anti-Ad neutralizing antibodies. Knowing that anti-Ad humoral immunity is serotype-specific, we hypothesized that anti-Ad neutralizing immunity could be circumvented using Ad vectors of different serotypes (Ad2, Ad5) within the same subgroup (C) to transfer and express beta-glucuronidase (beta glu) in the lung. Sprague-Dawley rats received an intratracheal administration of either Ad2 beta glu or Ad5 beta glu, and, 14 days later, repeat administration of either the same vector or a vector of a different serotype. Analysis of serum and bronchoalveolar lavage fluid following initial vector administration demonstrated systemic and local serotype-specific neutralizing antibodies. For both the Ad2 and Ad5 vectors, beta glu expression 24 hr following the second administration of the same serotype was < 30% of that of naive animals. In contrast, beta glu expression 24 hr following second administration of a different serotype Ad vector was similar to expression at 24 hr of naive animals receiving a single administration (Ad5 beta glu followed by Ad2 beta glu, as well as Ad2 beta glu followed by Ad5 beta glu; p > 0.2 both comparisons). Although the alternative serotype bypassed anti-Ad neutralizing immunity, persistence of expression was reduced compared to that following administration to naive animals. Compatible with this observation, systemic administration of the same vectors to C57B1/6 mice demonstrated induction of cytotoxic T lymphocytes directed against the beta glu transgene, as well as products of the Ad genome. Interestingly, intratracheal administration of vectors with different serotypes and different transgenes to rats resulted in longer expression (but still not normalized) compared to that achieved with vectors of different serotypes but the same transgene. These observations demonstrate that alternate use of Ad vectors from different serotypes within the same subgroup can circumvent anti-Ad humoral immunity to permit effective gene transfer after repeat administration, although the chronicity of expression is limited, likely by cellular immune process directed against both the transgene and viral gene products expressed by the vector.

Adenovirus-mediated gene delivery into neuronal precursors of the adult mouse brain

Precursor cells found in the subventricular zone (SVZ) of the adult brain can undergo cell division and migrate long distances before differentiating into mature neurons. We have investigated the possibility of introducing genes stably into this population of cells. Replication-defective adenoviruses were injected into the SVZ of the lateral ventricle of adult mice. The adenoviruses carried a cDNA for the LacZ reporter or the human p75 neurotrophin receptor, for which species-specific antibodies are available. Injection of the viruses into the SVZ led to efficient labeling of neuronal precursors. Two months after viral injection, infected cells were detected in the olfactory bulb, a significant distance from the site of injection. Labeled periglomerular and granular neurons with extensive dendritic arborization were found in the olfactory bulb. These results demonstrate that foreign genes can be efficiently introduced into neuronal precursor cells. Furthermore, adenovirus-directed infection can lead to long-term stable gene expression in progenitor cells found in the adult central nervous system.

Adenovirus type 5 and 7 capsid chimera: fiber replacement alters receptor tropism without affecting primary immune neutralization epitopes

The efficient uptake of adenovirus into a target cell is a function of adenovirus capsid proteins and their interaction with the host cell. The capsid protein fiber mediates high-affinity attachment of adenovirus to the target cell. Although the cellular receptor(s) for adenovirus is unknown, evidence indicates that a single receptor does not function as the attachment site for each of the 49 different serotypes of adenovirus. Sequence variation of the fiber ligand, particularly in the C- terminal knob domain, is associated with serotype-specific binding specificity. Additionally, this domain of fiber functions as a major serotype determinant. Fiber involvement in cell targeting and its function as a target of the host immune response make the fiber gene an attractive target for manipulation, both from the perspective of adenovirus biology and from the perspective of using adenovirus vectors for gene transfer experiments. We have constructed a defective chimeric adenovirus type 5 (Ad5) reporter virus by replacing the Ad5 fiber gene with the fiber gene from Ad7A. Using the chloramphenicol acetyltransferase reporter gene, we have characterized this virus with respect to infectivity both in vitro and in vivo. We have also characterized the role of antifiber antibody in the host neutralizing immune response to adenovirus infection. Our studies demonstrate that exchange of fiber is a strategy that will be useful in characterizing receptor tropism for different serotypes of adenovirus. Additionally, the neutralizing immune response to Ad5 and Ad7 does not differentiate between two viruses that differ only in their fiber proteins. Therefore, following a primary adenovirus inoculation, antibodies generated against fiber do not constitute a significant fraction of the neutralizing antibody population.

Polarity of TRH receptors in transfected MDCK cells is independent of endocytosis signals and G protein coupling

Information concerning the molecular sorting of G protein-coupled receptors in polarized epithelial cells is limited. Therefore, we have expressed the receptor for thyrotropin-releasing hormone (TRH) in Madin-Darby canine kidney (MDCK) cells by adenovirus-mediated gene transfer to determine its distribution in a model cell system and to begin analyzing the molecular information responsible for its distribution. Equilibrium binding of [methyl-3H]TRH to apical and basolateral surfaces of polarized MDCK cells reveals that TRH receptors are expressed predominantly (>80%) on the basolateral cell surface. Receptors undergo rapid endocytosis following agonist binding; up to 80% are internalized in 15 min. A mutant receptor missing the last 59 residues, C335Stop, is poorly internalized (<10%) but is nevertheless basolaterally expressed (>85%). A second mutant TRH receptor, delta218-263, lacks essentially all of the third intracellular loop and is not coupled to G proteins on binding agonist. This receptor internalizes TRH approximately half as efficiently as wild-type TRH receptors but is nevertheless strongly polarized to the basolateral surface (>90%). These results indicate that molecular sequences responsible for basolateral accumulation of TRH receptors can be segregated from signals for ligand-induced receptor endocytosis and coupling to heterotrimeric G proteins.

Ectopic expression of thyrotropin releasing hormone (TRH) receptors in liver modulates organ function to regulate blood glucose by TRH

Maintenance of blood glucose by the liver is normally initiated by extracellular regulatory molecules such as glucagon and vasopressin triggering specific hepatocyte receptors to activate the cAMP or phosphoinositide signal transduction pathways, respectively. We now show that the normal ligand-receptor regulators of blood glucose in the liver can be bypassed using an adenovirus vector expressing the mouse pituitary thyrotropin releasing hormone receptor (TRHR) cDNA ectopically in rat liver in vivo. The ectopically expressed TRHR links to the phosphoinositide pathway, providing a means to regulate liver function with TRH, an extracellular ligand that does not normally affect hepatic function. Administration of TRH to these animals activates the phosphoinositide pathway, resulting in a sustained rise in blood glucose. It should be possible to use this general strategy to modulate the differentiated functions of target organs in a wide variety of pathologic states.

Circumventing the immune response to adenovirus-mediated gene therapy

Adenovirus-mediated gene transfer experiments have demonstrated an exceptional efficiency of virus uptake and gene expression in a variety of in vivo models. Unfortunately, the efficiency of gene delivery is not accompanied by long-term gene expression. Maximal gene expression peaks during the first week of infection followed by a rapid decline to near baseline levels within several weeks. Data from several laboratories implicate host cellular and humoral immune responses as being responsible for the limited duration of expression and for the inability to successfully readminister a gene using adenovirus vectors. In this study we have examined two strategies which, independently or in combination, circumvent aspects of the host immune response against adenovirus-mediated gene therapy. The first strategy explores induction of immune tolerance in the experimental host as a method to increase the duration of gene expression and as a method to allow readministration of adenovirus expression vectors. Our second strategy is directed at the need to readminister adenoviral vectors to immune competent adult animals. We have demonstrated that a sequential exposure of rats to at least two other adenovirus serotypes does not compromise our ability to successfully administer an Ad5-based CAT expression vector. The characterization of serotype-specific neutralizing response indicates that the construction and use of Ad expression vectors from different serotypes will facilitate a useful adenovirus-based strategy allowing multiple administrations of a target gene.

"Sero-switch" adenovirus-mediated in vivo gene transfer: circumvention of anti-adenovirus humoral immune defenses against repeat adenovirus vector administration by changing the adenovirus serotype

Recombinant, replication-deficient adenovirus (Ad) vectors have been successfully used to transfer and express the normal human cystic fibrosis transmembrane conductance regulator (CFTR) cDNA in vivo in the respiratory epithelium of experimental animals and humans with cystic fibrosis (CF). Since Ad-directed gene expression wanes over time, repeat administration is necessary to achieve an effective treatment for CF. A major hurdle to such a strategy is the possibility that anti-Ad humoral immunity may prevent gene expression in individuals with pre-existing anti-Ad immunity or following repeat administration. One strategy to circumvent such a problem would be alternating the use of Ad vectors belonging to different subgroups. Neutralizing antibodies developed with the administration of one Ad serotype do not cross-react with an Ad belonging to a second serotype in a manner that blocks infection and gene expression. To test this hypothesis, an immunizing dose of wild-type Ad5 (subgroup C), Ad4 (subgroup E), or Ad30 (subgroup D) was administered intratracheally to experimental animals, followed by an intratracheal administration of a replication-deficient subgroup C-derived vector coding for marker genes (chloramphenicol acetyl transferase or beta-galactosidase) or for the normal human CFTR cDNA. As expected, studies with vectors coding for marker genes or for CFTR cDNA demonstrated that airway administration of a vector does not yield efficient gene transfer, if there has been prior recent airway administration of the same Ad subgroup. In contrast, effective expression from the second administration can be achieved with an adenovirus vector belonging to a subgroup different from the first adenovirus administered. These data support the paradigm of alternating Ad vectors derived from different subgroups as strategy to circumvent anti-Ad humoral immunity, thus permitting the use of Ad vectors as a means to treat the respiratory manifestations of CF.

The impact of developmental stage, route of administration and the immune system on adenovirus-mediated gene transfer

Important aspects of successful adenovirus gene transfer include the amount and persistence of gene expression, the ability to readminister virus and the localization of virus-directed gene expression to target organs. Our objective in this study was to use a single recombinant adenovirus bearing a quantifiable reporter gene [chloramphenicol acetyltransferase (CAT)] to establish the parameters which define the limits of adenovirus gene expression in a rat model. First, we determined how the route of virus administration affected the amount, duration and distribution of expression in different tissues and in rats of different developmental stages. All routes resulted in infection of all tissues tested. Surprisingly, the most efficient and widespread gene transfer was achieved by intracardiac muscle injection. The high levels of CAT protein that can be produced in a liver (< or = 1.7 mg) or a heart (< or = 196 micrograms) 5 days after infection suggest that the amount of gene product will not be a limitation in the use of adenovirus. Following peak activity at 5 days after infection, a gradual decline of CAT expression was observed in all tissues assayed; by 80 days neither CAT activity nor adenovirus DNA were detectable. In addition, adults could not be boosted by a second administration of virus, presumably due to the presence of high levels of neutralizing antibodies. The limited persistence of gene expression could be circumvented when virus was injected into neonates. Blocking T lymphocyte expansion by cyclosporine enhanced the persistence of CAT gene product over a 25-day period in heart and lung but not in liver compared with control animals.(ABSTRACT TRUNCATED AT 250 WORDS)

Sequence elements upstream of the 3' cleavage site confer substrate strength to the adenovirus L1 and L3 polyadenylation sites

The adenovirus major late transcription unit is a well-characterized transcription unit which relies heavily on alternative pre-mRNA processing to generate distinct populations of mRNA during the early and late stages of viral infection. In the early stage of infection, two major late transcription unit mRNA transcripts are generated through use of the first (L1) of five available poly(A) sites (L1 through L5). This contrasts with the late stage of infection when as many as 45 distinct mRNAs are generated, with each of the five poly(A) sites being used. In previous work characterizing elements involved in alternative poly(A) site use, we showed that the L1 poly(A) site is processed less efficiently than the L3 poly(A) site both in vitro and in vivo. Because of the dramatic difference in processing efficiency and the role processing efficiency plays in production of steady-state levels of mRNA, we have identified the sequence elements that account for the differences in L1 and L3 poly(A) site processing efficiency. We have found that the element most likely to be responsible for poly(A) site strength, the GU/U-rich downstream element, plays a minor role in the different processing efficiencies observed for the L1 and L3 poly(A) sites. The sequence element most responsible for inefficient processing of the L1 poly(A) site includes the L1 AAUAAA consensus sequence and those sequences which immediately surround the consensus hexanucleotide. This region of the L1 poly(A) site contributes to an inability to form a stable processing complex with the downstream GU/U-rich element. In contrast to the L1 element, the L3 poly(A) site has a consensus hexanucleotide and surrounding sequences which can form a stable processing complex in cooperation with the downstream GU/U-rich element. The L3 poly(A) site is also aided by the presence of sequences upstream of the hexanucleotide which facilitate processing efficiency. The sequence UUCUUUUU, present in the L3 upstream region, is shown to enhance processing efficiency as well as stable complex formation (shown by increased binding of the 64-kDa cleavage stimulatory factor subunit) and acts as a binding site for heterogeneous nuclear ribonucleoprotein C proteins.

Expression of thyrotropin-releasing hormone receptors by adenovirus-mediated gene transfer reveals that thyrotropin-releasing hormone desensitization is cell specific

Biological studies of seven-transmembrane region G protein-coupled receptors have been restricted by available techniques for gene transfer into mammalian cells. We have created a highly efficient adenovirus-based expression vector for the thyrotropin-releasing hormone (TRH) receptor (TRH-R), AdCMVmTRHR, to circumvent difficulties encountered when transient or stable plasmid expression systems are used. We show that infection with AdCMVmTRHR results in fully functional TRH-Rs, which can be expressed in a broad range of mammalian cell types, including those resistant to conventional transient transfection. TRH-Rs can be expressed at high levels, up to 2 x 10(6) receptors/cell. Expression in several cell lines in culture reveals that rapid TRH-R desensitization by TRH and phorbol 12-myristate 13-acetate is cell type specific. The versatility of adenovirus-mediated gene transfer and expression of TRH-Rs not only facilitates in vitro studies of TRH-R biology but also provides a valuable in vivo expression vector capable of extending TRH-R studies to animal model systems.

Thyrotropin-releasing hormone (TRH) receptor number determines the size of the TRH-responsive phosphoinositide pool. Demonstration using controlled expression of TRH receptors by adenovirus mediated gene transfer

We use an adenovirus vector, AdCMVmTRHR, to express thyrotropin-releasing hormone (TRH) receptors (TRH-Rs) to determine whether the size of the hormone-responsive phosphoinositide pool in mammalian cells is directly related to receptor number. Infection of HeLa cells with increasing numbers of AdCMVmTRHR caused time-dependent graded expression of TRH-Rs. Measurement of cytoplasmic free Ca2+ in individual cells permitted quantitation of the fraction of cells responsive to TRH. Infection with 100 AdCMVmTRHR particles/cell or more led to TRH responsiveness in > or = 90% of HeLa cells. Measurement of prelabeled phosphoinositides hydrolyzed during prolonged TRH stimulation assesses the size of the TRH-responsive pool. In cells infected with AdCMVmTRHR for 24 h, the size of the TRH-responsive phosphoinositide pool increased with increasing TRH-R expression. The TRH-responsive pool also increased with time after infection as the number of TRH-Rs increased. Similar observations were made in GHY and KB cells. These data confirm our previous suggestion (Cubitt, A. B., Geras-Raaka, E., and Gershengorn, M. C. (1990) Biochem. J. 271, 331-336) that there are hormone-responsive and -unresponsive pools of cellular phosphoinositides and that the maximal size of the TRH-responsive pool is directly related to the number of TRH-Rs.

Quantitative determination of adenovirus-mediated gene delivery to rat cardiac myocytes in vitro and in vivo

To optimize the use of modified adenoviruses as vectors for gene delivery to the myocardium, we have characterized infection of cultured fetal and adult rat cardiac myocytes in vitro and of adult cardiac myocytes in vivo by using a replication-defective adenovirus carrying the chloramphenicol acetyltransferase (CAT) reporter gene driven by the cytomegalovirus promoter (AdCMVCATgD). In vitro, virtually all fetal or adult cardiocytes express the CAT gene when infected with 1 plaque-forming unit of virus per cell. CAT enzymatic activity can be detected in these cells as early as 4 hr after infection, reaching near-maximal levels at 48 hr. In fetal cells, CAT expression was maintained without a loss in activity for at least 1 week. Using in vitro studies as a guide, we introduced the AdCMVCATgD virus directly into adult rat myocardium and compared the expression results obtained from virus injection with those obtained by direct injection of pAdCMVCATgD plasmid DNA. The amount of CAT activity resulting from adenovirus infection of the myocardium was orders of magnitude higher than that seen from DNA injection and was proportional to the amount of input virus. Immunostaining for CAT protein in cardiac tissue sections following adenovirus injection demonstrated large numbers of positive cells, reaching nearly 100% of the myocytes in many regions of the heart. Expression of genes introduced by adenovirus peaked at 5 days but was still detectable 55 days following infection. Adenoviruses are therefore a very useful tool for high-efficiency gene transfer into the cardiovascular system.

3' RNA processing efficiency plays a primary role in generating termination-competent RNA polymerase II elongation complexes

In several mammalian transcription units, a transcription termination mechanism in which efficient termination is dependent on the presence of an intact 3' RNA processing site has been identified. The mouse beta maj-globin transcription unit is one such example, in which an intact poly(A) site is required for efficient transcription termination. It is now evident that 3' mRNA processing sites are not always processed with the same efficiency. In this study, we characterized several pre-mRNAs as substrates for the 3' mRNA processing reaction of cleavage and polyadenylation. We then determined whether poly(A) sites which vary in processing efficiency support a poly(A) site-dependent termination event. The level of processing efficiency was determined in vitro by assays measuring the efficiency of the pre-mRNA cleavage event and in vivo by the level of poly(A) site-dependent mRNA and gene product expression generated in transient transfection assays. The beta maj globin pre-mRNA is very efficiently processed. This efficient processing correlates with its function in termination assays using recombinant adenovirus termination vectors in nuclear run-on assays. When the beta maj globin poly(A) site was replaced by the L1 poly(A) site of the adenovirus major late transcription unit (Ad-ml), which is a poor processing substrate, termination efficiency decreased dramatically. When the beta maj globin poly(A) site was replaced by the Ad-ml L3 poly(A) site, which is 10- to 20-fold more efficiently processed than the Ad-ml L1 poly(A) site, termination efficiency remained high. Termination is therefore dependent on the yield of the processing event. We then tested chimeric poly(A) sites containing the L3 core AAUAAA but varied downstream GU-rich elements. The change in downstream GU-rich elements affected processing efficiency in a manner which correlated with termination efficiency. These experiments provide evidence that the efficiency of 3' processing complex formation is directly correlated to the efficiency of RNA polymerase II termination at the 3' end of a mammalian transcription unit.

Characterization of the mouse beta maj globin transcription termination region: a spacing sequence is required between the poly(A) signal sequence and multiple downstream termination elements

For the majority of mRNA encoding eukaryotic transcription units, there is little or no knowledge of the elements responsible for transcription termination or how they may interact with RNA polymerase. In this report, we have used recombinant adenovirus reporter vectors to characterize the mouse beta maj globin sequence elements that cause transcription termination. Within the globin 3' termination region, we have identified at least three sequence elements which induce significant levels of transcription termination (> 50%). The smallest functionally active element (64% termination) is 69 bp in length. The natural arrangement of these elements results in a cumulative termination which is greater than 90%. Recognition of the termination elements by RNA polymerase II depends on the presence of a functional poly(A) signal sequence. We demonstrate that efficient transcription termination depends on appropriate spacing between the poly(A) signal sequence and the termination element.

Varied poly(A) site efficiency in the adenovirus major late transcription unit

Regulation of adenovirus major late transcription unit (MLTU) mRNA biosynthesis involves poly(A) site selection between five available sites, L1 through L5. The 5' proximal site completely dominates during early infection, whereas all five sites are used during late infection with L3 being favored slightly over the others. Previous studies have shown this early to late poly(A) switch will occur in the absence of MLTU-specific splicing patterns and hinges in large part on the character of the first poly(A) site. We have used in vitro assays to characterize basic features of the L1 and L3 pre-mRNAs which may help define how processing at poly(A) sites is controlled. We have found that L1 is 5-10-fold less efficient than L3 as a substrate for RNA cleavage. A primary difference between the L1 and L3 sites lies in the kinetics of their use, with cleavage at L3 occurring at twice the rate of cleavage at L1. In addition, L1 is 20-fold less effective than L3 in competing for processing factors. To investigate the sequence elements that contribute to poly(A) site efficiency, we have used competition assays in which the competitor RNAs lack upstream or downstream elements.

Sequences regulating temporal poly(A) site switching in the adenovirus major late transcription unit

Temporal regulation of poly(A) site choice occurs in an adenovirus recombinant encoding a miniature version of the major late transcription unit with two poly(A) sites, L1 and L3. Using deletion mutagenesis, we have looked directly for cis-acting elements regulating poly(A) site choice in this recombinant. From this work, we draw two main conclusions. First, elements other than the AAUAAA and downstream sequences of the L1 poly(A) site are required for temporal regulation of poly(A) site choice during infection. Second, these regions function in two distinct modes during infection. The two regions enhance selection of the L1 poly(A) site in an additive manner during an early infection, but deletion of either element abolishes the switch in poly(A) site choice during a late infection. This work documents the first example of a regulatory element downstream of a core poly(A) region.

Regulation of antibody secretion by hybridoma cells. II. Mechanism of idiotype-induced suppression of antibody secretion by hybridoma cells

We have previously reported that antibody secretion by B.22 hybridoma cells can be suppressed in an MHC-restricted manner, by idiotype-specific T cells. It was shown that T cells of both helper and suppressor phenotypes are involved, and that the suppression is mediated by soluble factors. In the present paper, we have characterized the effects of T-cell-mediated suppression at the level of B.22 antibody mRNA expression and stability. Nuclear run-on analysis comparing suppressed and control B.22 cells indicates no change in the transcription rates of heavy and light chains. Northern blot analysis demonstrates that steady-state levels of heavy and light chain mRNAs are also unchanged. Thus, the suppression of antibody secretion by B.22 cells probably occurs at the levels of translation or secretion.

Regulation of poly(A) site selection in adenovirus

We have investigated the mechanisms involved in the early-to-late RNA-processing switch which regulates the mRNA species generated from the adenovirus major late transcription unit (MLTU). In particular, polyadenylation choice mechanisms were characterized by using a reconstructed adenovirus E1A gene as a site for insertion of MLTU poly(A) regulation signals (L1 and L3). Adenovirus constructs containing the variant poly(A) recognition elements were used to compare E1A poly(A) signal utilization with wild-type MLTU (L1 to L5) utilization. In both early and late stages of infection, either polyadenylation site (L1 or L3) is capable of being utilized when presented as the only operational poly(A) site. In an early infection, a virus which contains multiple elements presented in tandem (L13) uses the first poly(A) site, L1, preferentially (ratio of L1 to L3, 8:1) in both E1A and MLTU loci. Transcription termination is not involved in restricting the utilization of the downstream L3 site. In a late infection, when each of the five MLTU poly(A) sites is used, a switch also occurs for the E1AL13 construct, with utilization of both the L1 and L3 poly(A) sites. The switch from early to late was not the result of altered processing factors in the late infection, as demonstrated by superinfecting the E1AL13 construct into cells which had already entered a late stage of infection. The superinfecting virus gave an L1-only phenotype; therefore, a cis mechanism is involved in adenovirus poly(A) regulation.

A poly(A) addition site and a downstream termination region are required for efficient cessation of transcription by RNA polymerase II in the mouse beta maj-globin gene

Sequence elements within the mouse beta maj-globin transcription unit required for efficient termination of transcription by RNA polymerase II have been delineated. To facilitate nascent-chain analysis of termination, the DNA segment in which transcription ceases was introduced into the adenovirus chromosome within its E1A transcription unit. Two beta-globin DNA elements were required to effect efficient termination: an upstream sequence that includes two poly(A) addition signals and a downstream region previously shown to be where RNA synthesis stops. The role of poly(A) addition in termination was established by introduction of several single base pair substitutions into the AATAAA polyadenylylation motifs. These mutations inhibited both polyadenylylation and termination within the beta-globin DNA segment. Therefore, poly(A) addition appears to be a prerequisite for efficient termination.