Tissue specificity of retrovirus expression in inoculated avian embryos revealed by in situ hybridization to whole-body section

Resistance to avian leukosis virus lymphomagenesis occurs subsequent to proviral c-myc integration

Most chicken strains are highly susceptible to avian leukosis virus (ALV) induction of bursal lymphoma, involving proviral integration within the c-myc proto-oncogene, while certain strains are genetically resistant to lymphomagenesis. A nested PCR assay was developed to analyse the appearance of proviral c-myc integrations after ALV infection of lymphoma-susceptible birds, and to determine whether these integrations arise in lymphoma-resistant birds. Proviral c-myc integrations are detected in bursa and other tissues from 6 day-old lymphoma-susceptible birds infected as embryos. The abundance of bursal cells carrying these integrations increases roughly 40-fold by 35 days of age, indicating that these cells hyperproliferate within the bursal environment. Bursal cells with proviral c-myc integrations also arise soon after infection of lymphoma-resistant embryos. However, these cells expand much more slowly than cells from lymphoma-susceptible birds. Both strains show the same rate of viral infection, so that resistance to lymphomagenesis occurs at a step subsequent to proviral c-myc integration. Proviral c-erbB gene integrations arise at the same frequency in bursa and other tissues of both strains, and they do not increase in abundance during development. These findings indicate that the mechanism of resistance to lymphomagenesis involves specific inhibition of cells with proviral c-myc integrations within the bursa.

The VBP and a1/EBP leucine zipper factors bind overlapping subsets of avian retroviral long terminal repeat CCAAT/enhancer elements

Two long terminal repeat (LTR) enhancer-binding proteins which may regulate high rates of avian leukosis virus (ALV) LTR-enhanced c-myc transcription during bursal lymphomagenesis have been identified (A. Ruddell, M. Linial, and M. Groudine, Mol. Cell. Biol. 9:5660-5668, 1989). The genes encoding the a1/EBP and a3/EBP binding factors were cloned by expression screening of a lambda gt11 cDNA library from chicken bursal lymphoma cells. The a1/EBP cDNA encodes a novel leucine zipper transcription factor (W. Bowers and A. Ruddell, J. Virol. 66:6578-6586, 1992). The partial a3/EBP cDNA clone encodes amino acids 84 to 313 of vitellogenin gene-binding protein (VBP), a leucine zipper factor that binds the avian vitellogenin II gene promoter (S. Iyer, D. Davis, and J. Burch, Mol. Cell. Biol. 11:4863-4875, 1991). Multiple VBP mRNAs are expressed in B cells in a pattern identical to that previously observed for VBP in other cell types. The LTR-binding activities of VBP, a1/EBP, and B-cell nuclear extract protein were compared and mapped by gel shift, DNase I footprinting, and methylation interference assays. The purified VBP and a1/EBP bacterial fusion proteins bind overlapping but distinct subsets of CCAAT/enhancer elements in the closely related ALV and Rous sarcoma virus (RSV) LTR enhancers. Protein binding to these CCAAT/enhancer elements accounts for most of the labile LTR enhancer-binding activity observed in B-cell nuclear extracts. VBP and a1/EBP could mediate the high rates of ALV and RSV LTR-enhanced transcription in bursal lymphoma cells and many other cell types.

Patterns of integration and expression of retroviral, non-replicative vectors in avian embryos: embryo developmental stage and virus subgroup envelope modulate tissue-tropism

We previously demonstrated that Avian Leukemia Viruses (ALV) carrying the v-myc gene specifically induce two types of tumors, cardiomyocytic tumors when the virus is injected before embryonic day 3 (E3), skin tumors when the virus is injected at E3 or E5. Aiming to elucidate the mechanisms which determine this time-dependent change in target, we infected chick and quail embryos at E3 and E5 with replication-deficient, lacZ gene-carrying, ALV-based viruses produced by a packaging cell line. Three constructs driven by 3 different Long Terminal Repeats (LTRs) were tested and yielded similar results. When the constructs were inoculated at E3 and the lacZ gene product revealed 5 days later, around 70% of the embryos carried lacZ+ clones in the heart, around 50% had positive clones in the skin anywhere on the body, while a few embryos displayed clones in internal organs (liver, stomach, lungs). Immunocytological identification of the heart cell type(s) expressing the virus revealed that the only cells infected were cardiomyocytes. When the constructs were inoculated at E5, no lacZ+ clones appeared in the heart but all were located in the cephalic skin. In order to examine the relationship between viral integration and expression, DNA of different organs or tissues from lacZ stained embryos was analyzed by PCR. A tight correlation between integration and expression in the heart and in the skin was revealed in most cases. In contrast, a significant PCR signal was often detected in the liver or the stomach despite weak or absent expression as revealed by lacZ+ clones. We then investigated the influence of envelope glycoprotein subgroups on the tropism of these constructs. The lacZ vector driven by RAV-2 LTRs was packaged as subgroups A, B or E viral particles. The A subgroup, used in the part of the study described above, infects both chick and quail while the B and E subgroups are specific for chick or quail respectively. These B and E subgroups induced lacZ+ clones in the heart (after E3 injection) while no clones or only a few were detected in the skin either after E3 or E5 injection. The following conclusions can be drawn: 1) cardiomyocytes are at E3 the major target for integration and expression of ALV-derived viruses in vivo; 2) targets change rapidly with embryonic age; and 3) tissue-specific infections depend on the envelope subgroup, thus presumably on the presence of the cognate receptor.(ABSTRACT TRUNCATED AT 400 WORDS)

Mutation of the C/EBP binding sites in the Rous sarcoma virus long terminal repeat and gag enhancers

Several C/EBP binding sites within the Rous sarcoma virus (RSV) long terminal repeat (LTR) and gag enhancers were mutated, and the effect of these mutations on viral gene expression was assessed. Minimal site-specific mutations in each of three adjacent C/EBP binding sites in the LTR reduced steady-state viral RNA levels. Double mutation of the two 5' proximal LTR binding sites resulted in production of 30% of wild-type levels of virus. DNase I footprinting analysis of mutant DNAs indicated that the mutations blocked C/EBP binding at the affected sites. Additional C/EBP binding sites were identified upstream of the 3' LTR and within the 5' end of the LTRs. Point mutations in the RSV gag intragenic enhancer region, which blocked binding of C/EBP at two of three adjacent C/EBP sites, also reduced virus production significantly. Nuclear extracts prepared from both chicken embryo fibroblasts (CEFs) and chicken muscle contained proteins binding to the same RSV DNA sites as did C/EBP, and mutations that prevented C/EBP binding also blocked binding of these chicken proteins. It appears that CEFs and chicken muscle contain distinct proteins binding to these RSV DNA sites; the CEF binding protein was heat stable, as is C/EBP, while the chicken muscle protein was heat sensitive.

Replication-competent retroviral vectors encoding alkaline phosphatase reveal spatial restriction of viral gene expression/transduction in the chick embryo

Replication-competent avian retroviruses, capable of transducing and expressing up to 2 kb of nonviral sequences, are now available to effect widespread gene transfer in chicken (chick) embryos (S. H. Hughes, J. J. Greenhouse, C. J. Petropoulos, and P. Sutrave, J. Virol. 61:3004-3012, 1987). We have constructed novel avian retroviral vectors that encode human placental alkaline phosphatase as a marker whose expression can be histochemically monitored. These vectors have been tested for expression by introducing them into the embryonic chick nervous system. They have revealed that the expression of retrovirally transduced genes can be spatially and temporally limited without the need for tissue-specific promoters. By varying the site and time of infection, targeted gene transfer can be confined to selected populations of neural cells over the course of several days, a time window that is sufficient for many key developmental processes. The capability of differentially infecting specific target populations may avoid confounding variables such as detrimental effects of a transduced gene on processes unrelated to the cells or tissue of interest. These vectors and methods thus should be useful in studies of the effect of transduced genes on the development of various organs and tissues during avian embryogenesis. In addition, the vectors will facilitate studies aimed at an understanding of viral infection and expression patterns.

Replication-competent retroviral vectors encoding alkaline phosphatase reveal spatial restriction of viral gene expression/transduction in the chick embryo

Replication-competent avian retroviruses, capable of transducing and expressing up to 2 kb of nonviral sequences, are now available to effect widespread gene transfer in chicken (chick) embryos (S. H. Hughes, J. J. Greenhouse, C. J. Petropoulos, and P. Sutrave, J. Virol. 61:3004-3012, 1987). We have constructed novel avian retroviral vectors that encode human placental alkaline phosphatase as a marker whose expression can be histochemically monitored. These vectors have been tested for expression by introducing them into the embryonic chick nervous system. They have revealed that the expression of retrovirally transduced genes can be spatially and temporally limited without the need for tissue-specific promoters. By varying the site and time of infection, targeted gene transfer can be confined to selected populations of neural cells over the course of several days, a time window that is sufficient for many key developmental processes. The capability of differentially infecting specific target populations may avoid confounding variables such as detrimental effects of a transduced gene on processes unrelated to the cells or tissue of interest. These vectors and methods thus should be useful in studies of the effect of transduced genes on the development of various organs and tissues during avian embryogenesis. In addition, the vectors will facilitate studies aimed at an understanding of viral infection and expression patterns.

Packaging cells for avian leukosis virus-based vectors with various host ranges

Using our previously described Haydée semipackaging cell line (F. L. Cosset, C. Legras, Y. Chebloune, P. Savatier, P. Thoraval, J. L. Thomas, J. Samarut, V. M. Nigon, and G. Verdier, J. Virol. 64:1070-1078, 1990) which produces avian leukosis virus gag and pol proteins, we have constructed packaging cells with subgroups B, C, and E envelope specificities. This allows us to produce helper-free avian leukosis virus particles carrying the lacZ reporter gene and the A, B, C, or E subgroup specificities. Titers of the recombinant lacZ virus are shown to be dependent upon the type of the env subgroup and the target avian cell.

Appropriate in vivo expression of a muscle-specific promoter by using avian retroviral vectors for gene transfer [corrected]

The promoter regions of the chicken skeletal muscle alpha-actin (alpha sk-actin) and the cytoplasmic beta-actin genes were linked to the bacterial chloramphenicol acetyltransferase (CAT) gene. Replication-competent retroviral vectors were used to introduce these two actin/CAT cassettes into the chicken genome. Chickens infected with retroviruses containing the alpha sk-actin promoter expressed high levels of CAT activity in striated muscle (skeletal muscle and heart); much lower levels of CAT activity were produced in the other nonmuscle tissues. In contrast, chickens infected with retroviruses containing the beta-actin promoter linked to the CAT gene expressed low levels of CAT activity in many different tissue types and with no discernible tissue specificity. Data are presented to demonstrate that the high levels of CAT activity that were detected in the skeletal muscle of chickens infected with the retrovirus containing the alpha sk-actin promoter/CAT cassette were not due to preferential infectivity, integration, or replication of the retrovirus vector in the striated muscles of these animals.

Avian retroviral long terminal repeats bind CCAAT/enhancer-binding protein

DNA-protein interactions involving enhancer and promoter sequences within the U3 regions of several avian retroviral long terminal repeats (LTRs) were studied by DNase I footprinting. The rat CCAAT/enhancer-binding protein, C/EBP, bound to all four viral LTRs examined. The Rous sarcoma virus binding site corresponded closely to the 5' limit of the LTR enhancer; nucleotides -225 to -188 were protected as a pair of adjacent binding domains. The Fujinami sarcoma virus LTR bound C/EBP at a single site at nucleotides -213 to -195. C/EBP also bound to the promoter region of the enhancerless Rous-associated virus-0 LTR at nucleotides -77 to -57. The avian myeloblastosis virus LTR bound C/EBP at three sites: nucleotides -262 to -246, -154 to -134, and -55 to -39. We have previously observed binding of C/EBP to an enhancer in the gag gene of avian retroviruses. A heat-treated nuclear extract from chicken liver bound to all of the same retroviral sequences as did C/EBP. Alignment of the avian retroviral binding sequences with the published binding sites for C/EBP in two CCAAT boxes and in the simian virus 40, polyoma, and murine sarcoma virus enhancers suggested TTGNNGCTAATG as a consensus sequence for binding of C/EBP. When two bases of this consensus sequence were altered by site-specific mutagenesis of the Rous sarcoma virus LTR, binding of the heat-stable chicken protein was eliminated.

Avian retroviral long terminal repeats bind CCAAT/enhancer-binding protein

DNA-protein interactions involving enhancer and promoter sequences within the U3 regions of several avian retroviral long terminal repeats (LTRs) were studied by DNase I footprinting. The rat CCAAT/enhancer-binding protein, C/EBP, bound to all four viral LTRs examined. The Rous sarcoma virus binding site corresponded closely to the 5' limit of the LTR enhancer; nucleotides -225 to -188 were protected as a pair of adjacent binding domains. The Fujinami sarcoma virus LTR bound C/EBP at a single site at nucleotides -213 to -195. C/EBP also bound to the promoter region of the enhancerless Rous-associated virus-0 LTR at nucleotides -77 to -57. The avian myeloblastosis virus LTR bound C/EBP at three sites: nucleotides -262 to -246, -154 to -134, and -55 to -39. We have previously observed binding of C/EBP to an enhancer in the gag gene of avian retroviruses. A heat-treated nuclear extract from chicken liver bound to all of the same retroviral sequences as did C/EBP. Alignment of the avian retroviral binding sequences with the published binding sites for C/EBP in two CCAAT boxes and in the simian virus 40, polyoma, and murine sarcoma virus enhancers suggested TTGNNGCTAATG as a consensus sequence for binding of C/EBP. When two bases of this consensus sequence were altered by site-specific mutagenesis of the Rous sarcoma virus LTR, binding of the heat-stable chicken protein was eliminated.

Targets of v-myc tumorigenesis in the avian embryo depend on time and not on site of retroviral infection

The present study extends our previous data, showing that the v-myc oncogene induces heart tumors and skin anomalies in young avian embryos [Saule et al., Proc. Natl. Acad. Sci. USA 84, 7982-7986 (1987)]. We now report that the target cells which become transformed are the same, whether the MC29 retrovirus is injected at E3 in various sites of the embryo (coelom, heart, brain, lateral plate mesoderm) or deposited on the embryo. Furthermore we confirm, in the quail, the time-specific pattern previously observed in the chick. In the quail, the incidence of heart tumors falls from 100% to 28% when injection is delayed from E3 to E4. By contrast, the incidence of skin anomalies rises from 30% to 64% when injection is delayed from E3 to E4. The skin defect, which consists of the presence of bell-shaped cornified feathers, could be assigned to hyperkeratinization of the epidermis. Both the dermis and the epidermis displayed hyperproliferation, whereas skin muscle hypertrophy during the embryonic period could not be confirmed. The presence of myc gene products was investigated using an antibody that recognizes both the c- and v-myc proteins. In the skin of control embryos, nuclei were well stained at E12-E13. At E14 the signal had disappeared. In abnormal skin patches from infected embryos, the antibody still marked heavily epidermal and dermal nuclei at E18. Finally we injected MC29 through the chorioallantoic vein in E10 chickens. No tumors were found during embryonic life, but 81% of the chickens developed tumors of hemopoietic or endothelial origin from the 14th posthatching day onwards. Studies of MC29 integration sites demonstrated that these tumors were derived from only a few transformed cells. Thus, contrasting with in vitro experiments, in vivo this virus has a restricted number of targets varying with the time of injection.

pp60v-src tyrosine kinase is expressed and active in sarcoma-free avian embryos microinjected with Rous sarcoma virus

Early embryonic avian tissue is resistant to transformation by Rous sarcoma virus. To determine the nature of this resistance, we examined the expression and properties of the Rous sarcoma virus transforming protein pp60v-src, in infected embryonic chicken limbs in ovo. Lysates from Rous sarcoma virus-infected limbs contained the viral structural protein p19gag, as detected by immunoblot analysis, and showed pp60v-src kinase activity in vitro. Immunoblot analysis of lysates with anti-phosphotyrosine antibodies revealed a number of phosphotyrosine-containing proteins present in lysates of Rous sarcoma virus-infected embryos but not in lysates of control, uninfected embryos. Anti-phosphotyrosine immunoreactivity was observed in frozen sections in the same cell types that expressed pp60v-src and p19gag. These studies demonstrate that pp60v-src is co-expressed with viral structural determinants in infected embryonic avian tissue. Furthermore, pp60v-src is active in ovo as a tyrosine-specific phosphotransferase, despite the apparent lack of sarcoma induction. The localization pattern of the major src gene substrate p36 (calpactin I) was compared with that of p19gag by double-label immunofluorescence and found to be generally nonoverlapping. These observations are consistent with the concept that the induction of tumors in ovo requires complementation between viral determinants and host factors. These host factors, which may be critical substrates of pp60v-src, are subject to developmental regulation in the avian embryo.

Transfer and expression of the bacterial NPT-II gene in chick embryos using a Schmidt-Ruppin retrovirus vector

In an effort to introduce foreign genes into chickens, the bacterial neomycin phosphotransferase (NPT-II) gene was cloned into an infectious avian retroviral vector derived from the Schmidt-Ruppin A strain of RSV. The NPT-II gene was stable in the vector during passage in vitro and infected cells were resistant to G418. Fertilized chicken embryos were inoculated with the recombinant virus on day 0 and screened on day 20 for the NPT-II gene in blood cell DNA. Approximately 12% of the embryos were positive for the NPT-II gene. Screening of DNA from the brain, muscle, liver and foot of the positive embryos indicated that the NPT-II gene copy number could vary in a single embryo. However, some embryos had nearly equal NPT-II copy number in each tissue examined. To determine the expression of the bacterial gene, tissue extracts from the positive embryos were assayed for NPT-II activity. The results indicated that NPT-II activity varied depending on the tissue, with activity being highest in muscle and foot regardless of NPT-II gene copy number.