Replication-competent retrovirus vectors for the transfer and expression of gene cassettes in avian cells

Measuring transcription factor function with cell type-specific somatic transgenesis in chicken embryos

Retroviral-mediated misexpression in chicken embryos has been a powerful research tool for developmental biologists in the last two decades. In the RCASBP retroviral vectors that are widely used for in vivo somatic transgenesis, a coding sequence of interest is under the transcriptional control of a strong viral promoter in the long terminal repeat. While this has proven to be effective for studying secreted signalling proteins, interpretation of the mechanisms of action of nuclear factors is more difficult using this system since it is not clear whether phenotypic effects are cell-autonomous or not, and therefore whether they represent a function of the endogenous protein. Here, we report the consequences of retroviral expression using the RCANBP backbone, in which the transcription factor Dlx5 is expressed under the control of chondrocyte-specific regulatory sequences from the Col2a1 gene. To our knowledge, this is the first demonstration of a tissue-specific phenotype in the chicken embryo.

Heterologous avian system for quantitative analysis of Syncytin-1 interaction with ASCT2 receptor

Background

Human Syncytin-1 is a placentally-expressed cell surface glycoprotein of retroviral origin. After interaction with ASCT2, its cellular receptor, Syncytin-1 triggers cell–cell fusion and formation of a multinuclear syncytiotrophoblast layer of the placenta. The ASCT2 receptor is a multi-spanning membrane protein containing a protruding extracellular part called region C, which has been suggested to be a retrovirus docking site. Precise identification of the interaction site between ASCT2 and Syncytin-1 is challenging due to the complex structure of ASCT2 protein and the background of endogenous ASCT2 gene in the mammalian genome. Chicken cells lack the endogenous background and, therefore, can be used to set up a system with surrogate expression of the ASCT2 receptor.

Results

We have established a retroviral heterologous chicken system for rapid and reliable assessment of ectopic human ASCT2 protein expression. Our dual-fluorescence system proved successful for large-scale screening of mutant ASCT2 proteins. Using this system, we demonstrated that progressive deletion of region C substantially decreased the amount of ASCT2 protein. In addition, we implemented quantitative assays to determine the interaction of ASCT2 with Syncytin-1 at multiple levels, which included binding of the soluble form of Syncytin-1 to ASCT2 on the cell surface and a luciferase-based assay to evaluate cell–cell fusions that were triggered by Syncytin-1. Finally, we restored the envelope function of Syncytin-1 in a replication-competent retrovirus and assessed the infection of chicken cells expressing human ASCT2 by chimeric Syncytin-1-enveloped virus. The results of the quantitative assays showed that deletion of the protruding region C did not abolish the interaction of ASCT2 with Syncytin-1.

Conclusions

We present here a heterologous chicken system for effective assessment of the expression of transmembrane ASCT2 protein and its interaction with Syncytin-1. The system profits from the absence of endogenous ASCT2 background and implements the quantitative assays to determine the ASCT2-Syncytin-1 interaction at several levels. Using this system, we demonstrated that the protruding region C was essential for ASCT2 protein expression, but surprisingly, not for the interaction with Syncytin-1 glycoprotein.

Graphical abstract

Supplementary Information

The online version contains supplementary material available at 10.1186/s12977-021-00558-0.

Visualizing Rous Sarcoma Virus Genomic RNA Dimerization in the Nucleus, Cytoplasm, and at the Plasma Membrane

Retroviruses are unique in that they package their RNA genomes as non-covalently linked dimers. Failure to dimerize their genomes results in decreased infectivity and reduced packaging of genomic RNA into virus particles. Two models of retrovirus genome dimerization have been characterized: in murine leukemia virus (MLV), genomic RNA dimerization occurs co-transcriptionally in the nucleus, resulting in the preferential formation of genome homodimers; whereas in human immunodeficiency virus (HIV-1), genomic RNA dimerization occurs in the cytoplasm and at the plasma membrane, with a random distribution of heterodimers and homodimers. Although in vitro studies have identified the genomic RNA sequences that facilitate dimerization in Rous sarcoma virus (RSV), in vivo characterization of the location and preferences of genome dimerization has not been performed. In this study, we utilized three single molecule RNA imaging approaches to visualize genome dimers of RSV in cultured quail fibroblasts. The formation of genomic RNA heterodimers within cells was dependent on the presence of the dimerization initiation site (DIS) sequence in the L3 stem. Subcellular localization analysis revealed that heterodimers were present the nucleus, cytoplasm, and at the plasma membrane, indicating that genome dimers can form in the nucleus. Furthermore, single virion analysis revealed that RSV preferentially packages genome homodimers into virus particles. Therefore, the mechanism of RSV genomic RNA dimer formation appears more similar to MLV than HIV-1.

Differential regulation of myc homologs by Wnt/β-Catenin signaling in the early metazoan Hydra

The c‐Myc protein is a transcription factor with oncogenic potential controlling fundamental cellular processes. Homologs of the human c‐myc protooncogene have been identified in the early diploblastic cnidarian Hydra (myc1, myc2). The ancestral Myc1 and Myc2 proteins display the principal design and biochemical properties of their vertebrate derivatives, suggesting that important Myc functions arose very early in metazoan evolution. c‐Myc is part of a transcription factor network regulated by several upstream pathways implicated in oncogenesis and development. One of these signaling cascades is the Wnt/β‐Catenin pathway driving cell differentiation and developmental patterning, but also tumorigenic processes including aberrant transcriptional activation of c‐myc in several human cancers. Here, we show that genetic or pharmacological stimulation of Wnt/β‐Catenin signaling in Hydra is accompanied by specific downregulation of myc1 at mRNA and protein levels. The myc1 and myc2 promoter regions contain consensus binding sites for the transcription factor Tcf, and Hydra Tcf binds to the regulatory regions of both promoters. The myc1 promoter is also specifically repressed in the presence of ectopic Hydra β‐Catenin/Tcf in avian cell culture. We propose that Hydra myc1 is a negative Wnt signaling target, in contrast to vertebrate c‐myc, which is one of the best studied genes activated by this pathway. On the contrary, myc2 is not suppressed by ectopic β‐Catenin in Hydra and presumably represents the structural and functional c‐myc ortholog. Our data implicate that the connection between β‐Catenin‐mediated signaling and myc1 and myc2 gene regulation is an ancestral metazoan feature. Its impact on decision making in Hydra interstitial stem cells is discussed.

BarTeL, a Genetically Versatile, Bioluminescent and Granule Neuron Precursor-Targeted Mouse Model for Medulloblastoma

Medulloblastomas are the most common malignant pediatric brain tumor and have been divided into four major molecular subgroups. Animal models that mimic the principal molecular aberrations of these subgroups will be important tools for preclinical studies and allow greater understanding of medulloblastoma biology. We report a new transgenic model of medulloblastoma that possesses a unique combination of desirable characteristics including, among others, the ability to incorporate multiple and variable genes of choice and to produce bioluminescent tumors from a limited number of somatic cells within a normal cellular environment. This model, termed BarTeL, utilizes a Barhl1 homeobox gene promoter to target expression of a bicistronic transgene encoding both the avian retroviral receptor TVA and an eGFP-Luciferase fusion protein to neonatal cerebellar granule neuron precursor (cGNP) cells, which are cells of origin for the sonic hedgehog (SHH) subgroup of human medulloblastomas. The Barhl1 promoter-driven transgene is expressed strongly in mammalian cGNPs and weakly or not at all in mature granule neurons. We efficiently induced bioluminescent medulloblastomas expressing eGFP-luciferase in BarTeL mice by infection of a limited number of somatic cGNPs with avian retroviral vectors encoding the active N-terminal fragment of SHH and a stabilized MYCN mutant. Detection and quantification of the increasing bioluminescence of growing tumors in young BarTeL mice was facilitated by the declining bioluminescence of their uninfected maturing cGNPs. Inclusion of eGFP in the transgene allowed enriched sorting of cGNPs from neonatal cerebella. Use of a single bicistronic avian vector simultaneously expressing both Shh and Mycn oncogenes increased the medulloblastoma incidence and aggressiveness compared to mixed virus infections. Bioluminescent tumors could also be produced by ex vivo transduction of neonatal BarTeL cerebellar cells by avian retroviruses and subsequent implantation into nontransgenic cerebella. Thus, BarTeL mice provide a versatile model with opportunities for use in medulloblastoma biology and therapeutics.

Anti-Müllerian Hormone Is Required for Chicken Embryonic Urogenital System Growth but Not Sexual Differentiation

In mammals, the primary role of anti-Müllerian hormone (AMH) during development is the regression of Müllerian ducts in males. These structures otherwise develop into fallopian tubes, oviducts, and upper vagina, as in females. This highly conserved function is retained in birds and is supported by the high levels of AMH expression in developing testes. In mammals, AMH expression is controlled partly by the transcription factor, SOX9. However, in the chicken, AMH mRNA expression precedes that of SOX9 , leading to the view that AMH may lie upstream of SOX9 and play a more central role in avian testicular development. To help define the role of AMH in chicken gonad development, we suppressed AMH expression in chicken embryos using RNA interference. In males, AMH knockdown did not affect the expression of key testis pathway genes, and testis cords developed normally. However, a reduction in the size of the mesonephros and gonads was observed, a phenotype that was evident in both sexes. This growth defect occurred as a result of the reduced proliferative capacity of the cells of these tissues, and male gonads also had a significant reduction in germ cell numbers. These data suggest that although AMH does not directly contribute to testicular or ovarian differentiation, it is required in a sex-independent manner for proper cell proliferation and urogenital system growth.

Using the RCAS-TVA system to model human cancer in mice

For successful infection, avian sarcoma leukosis virus subgroup A (ASLV-A) requires its receptor, tumor virus A (TVA), to be present on the surface of target cells. This is the basis of the RCAS-TVA gene delivery system: Mammalian cells lack the gene encoding TVA and are normally resistant to infection by ASLV; however, transgenic targeting of TVA to specific cell types or tissues in the mouse renders these cells uniquely susceptible to infection by ASLV-A-based RCAS viruses. The RCAS-TVA system is a powerful tool for effectively modeling human tumors, including pancreatic, ovarian, and breast cancers, gliomas, and melanomas. RCAS viruses can deliver cDNAs (≤2.8 kb), as well as short hairpin RNAs (shRNAs), microRNAs (miRNAs), and other noncoding RNAs. Compared with traditional transgenic and knockout mice, the RCAS-TVA system has several strengths. First, virus delivery is generally performed postnatally and results in a relatively low infection rate of target cells; the sporadic postnatal expression of the gene of interest mimics the situation in developing human tumors. Second, a single transgenic mouse line can be used to compare the consequences of specific genes on tumor development, with viruses encoding oncogenes or shRNAs targeting specific tumor suppressor genes. TVA mouse strains can also be easily combined with transgenic, knock-in, and knockout mouse models to study cooperating genetic events.

Development of retroviral vectors for tissue-restricted expression in chicken embryonic gonads

The chicken embryo has long been a useful model organism for studying development, including sex determination and gonadal differentiation. However, manipulating gene expression specifically in the embryonic avian gonad has been difficult. The viral vector RCASBP can be readily used for embryo-wide transgene expression; however global mis-expression using this method can cause deleterious off-target effects and embryo-lethality. In an attempt to develop vectors for the over-expression of sequences in chicken embryonic urogenital tissues, the viral vector RCANBP was engineered to contain predicted promoter sequences of gonadal-expressed genes. Several promoters were analysed and it was found that although the SF1 promoter produced a tissue-restricted expression pattern that was highest in the mesonephros and liver, it was also higher in the gonads compared to the rest of the body. The location of EGFP expression from the SF1 promoter overlapped with several key gonad-expressed sex development genes; however expression was generally low-level and was not seen in all gonadal cells. To further validate this sequence the key testis determinant DMRT1 was over-expressed in female embryos, which due to insufficient levels had no effect on gonad development. The female gene aromatase was then over-expressed in male embryos, which disrupted the testis pathway as demonstrated by a reduction in AMH protein. Taken together, although these data showed that the SF1 promoter can be used for functional studies in ovo, a stronger promoter sequence would likely be required for the functional analysis of gonad genes that require high-level expression.

Use of lentiviral vectors to deliver and express bicistronic transgenes in developing chicken embryos

The abilities of lentiviral vectors to carry large transgenes (∼8kb) and to efficiently infect and integrate these genes into the genomes of both dividing and non-dividing cells make them ideal candidates for transport of genetic material into cells and tissues. Given the properties of these vectors, it is somewhat surprising that they have seen only limited use in studies of developing tissues and in particular of the developing nervous system. Over the past several years, we have taken advantage of the large capacity of these vectors to explore the expression characteristics of several dual promoter and 2A peptide bicistronic transgenes in developing chick neural retina, with the goal of identifying transgene designs that reliably express multiple proteins in infected cells. Here we summarize the activities of several of these transgenes in neural retina and provide detailed methodologies for packaging lentivirus and delivering the virus into the developing neural tubes of chicken embryos in ovo, procedures that have been optimized over the course of several years of use in our laboratory. Conditions to hatch injected embryos are also discussed. The chicken-specific techniques will be of highest interest to investigators using avian embryos, development and packaging of lentiviral vectors that reliably express multiple proteins in infected cells should be of interest to all investigators whose experiments demand manipulation and expression of multiple proteins in developing cells and tissues.

Wnt and BMP signaling cooperate with Hox in the control of Six2 expression in limb tendon precursor

The number and shape of limb tendons vary along the proximodistal axis, and the autopod contains more tendons than the zeugopod. The transcription factor Six2 is expressed in the developing tendons, and its expression can be traced back to a group of limb mesenchymal cells that are thought to be tendon precursor cells. We tried to elucidate the mechanism controlling position-specific tendon pattern formation using Six2 as a tendon marker. Six2 expression was always found in cells between the limb cartilage and ectoderm. Administration of BMP-2 or BMP antagonist Noggin to the limb bud, respectively repressed or facilitated Six2 expression. Removal of the ectoderm or administration of the Wnt antagonist sFRP-2 abolished Six2 expression and ectopic Wnt expression induced ectopic Six2 expression. Taken together, Six2 expression is induced in the cells located at the point where cartilage-derived Noggin and ectoderm-derived Wnt signals meet. Misexpression of the autopod-specific Hoxa-13 or Hoxd-13 induced ectopic expression of Six2 in the zeugopodal mesenchymal cells of the chick limb bud. Six2 expression in the dorsal autopodal mesenchyme was not detected in Hoxa-13(-/-);HoxD(del/del) mice, indicating that autopod-specific Hox is required for the regulation of Six2 expression. Misexpression of Wnt in the autopod induced ectopic Six2 expression in the autopod. On the other hand, Wnt misexpression alone never induced Six2 expression in the zeugopod, yet co-misexpression of Hoxa-13 and Wnt in the zeugopod enhanced ectopic Six2 expression. Our results indicate that autopodal Hox genes regulate Six2 expression in the autopodal tendon precursor in cooperation with the factors from cartilage and ectoderm.

Delta-like 1 and lateral inhibition during hair cell formation in the chicken inner ear: evidence against cis-inhibition

The formation of the salt-and-pepper mosaic of hair cells and supporting cells in the sensory epithelia of the inner ear is regulated by Notch signalling and lateral inhibition, but the dynamics of this process and precise mode of action of delta-like 1 (Dll1) in this context are unclear. Here, we transfected the chicken inner ear with a fluorescent reporter that includes elements of the mammalian Hes5 promoter to monitor Notch activity in the developing sensory patches. The Hes5 reporter was active in proliferating cells and supporting cells, and Dll1 expression was highest in prospective hair cells with low levels of Notch activity, which occasionally contacted more differentiated hair cells. To investigate Dll1 functions we used constructs in which Dll1 expression was either constitutive, regulated by the Hes5 promoter, or induced by doxycycline. In support of the standard lateral inhibition model, both continuous and Hes5-regulated expression of Dll1 promoted hair cell differentiation cell-autonomously (in cis) and inhibited hair cell formation in trans. However, some hair cells formed despite contacting Dll1-overexpressing cells, suggesting that some progenitor cells are insensitive to lateral inhibition. This is not due to the cis-inhibition of Notch activity by Dll1 itself, as induction of Dll1 did not cell-autonomously reduce the activity of the Hes5 reporter in progenitor and supporting cells. Altogether, our results show that Dll1 functions primarily in trans to regulate hair cell production but also that additional mechanisms operate downstream of lateral inhibition to eliminate patterning errors in the sensory epithelia of the inner ear.

Temporal and spatial expression of CCN3 during retina development

NOV/CCN3 is one of the founding members of the CCN (Cyr61 CTGF NOV) family. In the avian retina, CCN3 expression is mostly located within the central region of the inner nuclear layer. As retinal development progresses and this retinal layer differentiates and matures, CCN3 expression forms a dorsal-ventral and a central-peripheral gradient. CCN3 is produced by two glial cell types, peripapillary cells and Müller cells, as well as by horizontal, amacrine, and bipolar interneurons. In retinal neurons and Müller cell cultures, CCN3 expression is induced by activated BMP signaling, whereas Notch signaling decreases CCN3 mRNA and protein levels in Müller cells and has no effect in retinal neurons. In Müller cells, the CCN3 expression detected may thus result from a balance between the Notch and BMP signaling pathways.

Rediscovering the chick embryo as a model to study retinal development

The embryonic chick occupies a privileged place among animal models used in developmental studies. Its rapid development and accessibility for visualization and experimental manipulation are just some of the characteristics that have made it a vertebrate model of choice for more than two millennia. Until a few years ago, the inability to perform genetic manipulations constituted a major drawback of this system. However, the completion of the chicken genome project and the development of techniques to manipulate gene expression have allowed this classic animal model to enter the molecular age. Such techniques, combined with the embryological manipulations that this system is well known for, provide a unique toolkit to study the genetic basis of neural development. A major advantage of these approaches is that they permit targeted gene misexpression with extremely high spatiotemporal resolution and over a large range of developmental stages, allowing functional analysis at a level, speed and ease that is difficult to achieve in other systems. This article provides a general overview of the chick as a developmental model focusing more specifically on its application to the study of eye development. Special emphasis is given to the state of the art of the techniques that have made gene gain- and loss-of-function studies in this model a reality. In addition, we discuss some methodological considerations derived from our own experience that we believe will be beneficial to researchers working with this system.

Growth of limb muscle is dependent on skeletal-derived Indian hedgehog

During embryogenesis, muscle and bone develop in close temporal and spatial proximity. We show that Indian Hedgehog, a bone-derived signaling molecule, participates in growth of skeletal muscle. In Ihh(-/-) embryos, skeletal muscle development appears abnormal at embryonic day 14.5 and at later ages through embryonic day 20.5, dramatic losses of hindlimb muscle occur. To further examine the role of Ihh in myogenesis, we manipulated Ihh expression in the developing chick hindlimb. Reduction of Ihh in chicken embryo hindlimbs reduced skeletal muscle mass similar to that seen in Ihh(-/-) mouse embryos. The reduction in muscle mass appears to be a direct effect of Ihh since ectopic expression of Ihh by RCAS retroviral infection of chicken embryo hindlimbs restores muscle mass. These effects are independent of bone length, and occur when Shh is not expressed, suggesting Ihh acts directly on fetal myoblasts to regulate secondary myogenesis. Loss of muscle mass in Ihh null mouse embryos is accompanied by a dramatic increase in myoblast apoptosis by a loss of p21 protein. Our data suggest that Ihh promotes fetal myoblast survival during their differentiation into secondary myofibers by maintaining p21 protein levels.

Cellular processes of v-Src transformation revealed by gene profiling of primary cells--implications for human cancer

Background

Cell transformation by the Src tyrosine kinase is characterized by extensive changes in gene expression. In this study, we took advantage of several strains of the Rous sarcoma virus (RSV) to characterize the patterns of v-Src-dependent gene expression in two different primary cell types, namely chicken embryo fibroblasts (CEF) and chicken neuroretinal (CNR) cells. We identified a common set of v-Src regulated genes and assessed if their expression is associated with disease-free survival using several independent human tumor data sets.

Methods

CEF and CNR cells were infected with transforming, non-transforming, and temperature sensitive mutants of RSV to identify the patterns of gene expression in response to v-Src-transformation. Microarray analysis was used to measure changes in gene expression and to define a common set of v-Src regulated genes (CSR genes) in CEF and CNR cells. A clustering enrichment regime using the CSR genes and two independent breast tumor data-sets was used to identify a 42-gene aggressive tumor gene signature. The aggressive gene signature was tested for its prognostic value by conducting survival analyses on six additional tumor data sets.

Results

The analysis of CEF and CNR cells revealed that cell transformation by v-Src alters the expression of 6% of the protein coding genes of the genome. A common set of 175 v-Src regulated genes (CSR genes) was regulated in both CEF and CNR cells. Within the CSR gene set, a group of 42 v-Src inducible genes was associated with reduced disease- and metastasis-free survival in several independent patient cohorts with breast or lung cancer. Gene classes represented within this group include DNA replication, cell cycle, the DNA damage and stress responses, and blood vessel morphogenesis.

Conclusion

By studying the v-Src-dependent changes in gene expression in two types of primary cells, we identified a set of 42 inducible genes associated with poor prognosis in breast and lung cancer. The identification of these genes provides a set of biomarkers of aggressive tumor behavior and a framework for the study of cancer cells characterized by elevated Src kinase activity.

The RCAS-TVA system for introduction of oncogenes into selected somatic mammary epithelial cells in vivo

We have reported the use of the RCAS-TVA system to model sporadic tumorigenesis upon oncogenic activation in somatic mammary epithelial cells in the mouse. Here we review the advantages of this approach as compared to conventional mouse models with transgenic oncogene expression. We also in detail describe the RCAS-TVA method for introducing genes into somatic mammary epithelial cells engineered to express the avian receptor tva. This method may be particularly useful in modeling oncogenic activation and subsequent tumorigenesis in distinct breast epithelial cell sub-populations, including progenitor cells.

Modeling Brain Tumors Using Avian Retroviral Gene Transfer

RCAS/tv-a is a system for postnatal cell-type-specific gene transfer. It is used for the modeling of gliomas and medulloblastomas. This system provides a combination of lineage tracing from the cell origin with oncogenesis induced by mis-expression of specific genes. The genes that are most potent at inducing tumors are those that encode components of signal transduction and undifferentiated cells are most capable of serving as the cell of origin. The system effectively generates tumors with the histologic characteristics of human disease. Mice bearing RCAS/tv-a-induced brain tumors are currently being used for preclinical trials to understand the biology of therapeutic response in the various cell types that make up gliomas and medulloblastomas.

Targeting Marek's disease virus by RNA interference delivered from a herpesvirus vaccine

Live attenuated herpesvirus vaccines such as herpesvirus of turkey (HVT) have been used since 1970 for the control of Marek's disease (MD), a highly infectious lymphoproliferative disease of poultry. Despite the success of these vaccines in reducing losses from the disease, Marek's disease virus (MDV) strains have shown a continuing increase in virulence, presumably due to the inability of the current vaccines in preventing MDV replication. The highly specific and effective nature of RNA interference (RNAi) makes this technology particularly attractive for new antiviral strategies. In order to exploit the power of RNAi-mediated suppression of MDV replication in vivo delivered through existing vaccines, we engineered recombinant HVT expressing short hairpin RNA (shRNA) against MDV genes gB and UL29. The levels of protection induced by the RNAi-expressing HVT against virulent virus challenge were similar to the parent pHVT3 virus. However, chickens vaccinated with recombinant HVT expressing shRNA showed moderate reduction of challenge virus replication in blood and feather samples. Delivery of RNAi-based gene silencing through live attenuated vaccines for reducing replication of pathogenic viruses is a novel approach for the control of infectious diseases.

Mass spectral data for 64 eluted peptides and structural modeling define peptide binding preferences for class I alleles in two chicken MHC-B haplotypes associated with opposite responses to Marek's disease

In the chicken, resistance to lymphomas that form following infection with oncogenic strains of Marek's herpesvirus is strongly linked to the major histocompatibility complex (MHC)-B complex. MHC-B21 haplotype is associated with lower tumor-related mortality compared to other haplotypes including MHC-B13. The single, dominantly expressed class I gene (BF2) is postulated as responsible for the MHC-B haplotype association. We used mass spectrometry to identify peptides and structural modeling to define the peptide binding preferences of BF2 2101 and BF2 1301 proteins. Endogenous peptides (8-12 residues long) were eluted from affinity-purified BF2 2101 and BF2 1301 proteins obtained from transduced cDNA expressed in RP9 cells, hence expressed in the presence of heterologous TAP. Sequences of individual peptides were identified by mass spectrometry. BF2 2101 peptides appear to be tethered at the binding groove margins with longer peptides arching out but selected by preferred residues at positions P3, P5, and P8: X-X-[AVILFP]-X((1-5))-[AVLFWP]-X((2-3))-[VILFM]. BF2 1301 peptides appear selected for residues at P2, P3, P5, and P8: X-[DE]-[AVILFW]-X((1-2))-[DE]-X-X-[ED]-X((0-4)). Some longer BF2 1301 peptides likely also arch out, but others are apparently accommodated by repositioning of Arg83 so that peptides extend beyond the last preferred residue at P8. Comparisons of these peptides with earlier peptides derived in the presence of homologous TAP transport revealed the same side chain preferences. Scanning of Marek's and other viral proteins with the BF2 2101 motif identified many matches, as did the control human leukocyte antigen A 0201 motif. The BF2 1301 motif is more restricting suggesting that this allele may confer a selective advantage only in infections with a subset of viral pathogens.

Wise promotes coalescence of cells of neural crest and placode origins in the trigeminal region during head development

While most cranial ganglia contain neurons of either neural crest or placodal origin, neurons of the trigeminal ganglion derive from both populations. The Wnt signaling pathway is known to be required for the development of neural crest cells and for trigeminal ganglion formation, however, migrating neural crest cells do not express any known Wnt ligands. Here we demonstrate that Wise, a Wnt modulator expressed in the surface ectoderm overlying the trigeminal ganglion, play a role in promoting the assembly of placodal and neural crest cells. When overexpressed in chick, Wise causes delamination of ectodermal cells and attracts migrating neural crest cells. Overexpression of Wise is thus sufficient to ectopically induce ganglion-like structures consisting of both origins. The function of Wise is likely synergized with Wnt6, expressed in an overlapping manner with Wise in the surface ectoderm. Electroporation of morpholino antisense oligonucleotides against Wise and Wnt6 causes decrease in the contact of neural crest cells with the delaminated placode-derived cells. In addition, targeted deletion of Wise in mouse causes phenotypes that can be explained by a decrease in the contribution of neural crest cells to the ophthalmic lobe of the trigeminal ganglion. These data suggest that Wise is able to function cell non-autonomously on neural crest cells and promote trigeminal ganglion formation.

Transgene expression facilitated by the v-src splice acceptor can impair replication kinetics and lead to genomic instability of Rous sarcoma virus-based vectors

Rous sarcoma virus (RSV) can be used for the simple generation of high-titer replication-competent retroviral (RCR) vectors. Retroviruses undergo frequent genomic recombination, however, and vectors with reduced replication kinetics are rapidly overgrown by mutant forms. Vector design is hence critical to vector efficacy. In this study, two different designs of RSV-based RCR vectors were evaluated. Vectors in which transgene expression was facilitated by the v-src splice acceptor were revealed to have greatly reduced replication kinetics and genomic stability in comparison to vectors in which transgene expression was mediated by an internal ribosome entry site in the 3' untranslated region.

Transforming growth factor-beta and Wnt signals regulate chondrocyte differentiation through Twist1 in a stage-specific manner

We investigated the molecular mechanisms underlying the transition between immature and mature chondrocytes downstream of TGF-beta and canonical Wnt signals. We used two developmentally distinct chondrocyte models isolated from the caudal portion of embryonic chick sternum or chick growth plates. Lower sternal chondrocytes exhibited immature phenotypic features, whereas growth plate-extracted cells displayed a hypertrophic phenotype. TGF-beta significantly induced beta-catenin in immature chondrocytes, whereas it repressed it in mature chondrocytes. TGF-beta further enhanced canonical Wnt-mediated transactivation of the Topflash reporter expression in lower sternal chondrocytes. However, it inhibited Topflash activity in a time-dependent manner in growth plate chondrocytes. Our immunoprecipitation experiments showed that TGF-beta induced Sma- and Mad-related protein 3 interaction with T-cell factor 4 in immature chondrocytes, whereas it inhibited this interaction in mature chondrocytes. Similar results were observed by chromatin immunoprecipitation showing that TGF-beta differentially shifts T-cell factor 4 occupancy on the Runx2 promoter in lower sternal chondrocytes vs. growth plate chondrocytes. To further determine the molecular switch between immature and hypertrophic chondrocytes, we assessed the expression and regulation of Twist1 and Runx2 in both cell models upon treatment with TGF-beta and Wnt3a. We show that Runx2 and Twist1 are differentially regulated during chondrocyte maturation. Furthermore, whereas TGF-beta induced Twist1 in mature chondrocytes, it inhibited Runx2 expression in these cells. Opposite effects were observed upon Wnt3a treatment, which predominates over TGF-beta effects on these cells. Finally, overexpression of chick Twist1 in mature chondrocytes dramatically inhibited their hypertrophy. Together, our findings show that Twist1 may be an important regulator of chondrocyte progression toward terminal maturation in response to TGF-beta and canonical Wnt signaling.

Genetically engineered models have advantages over xenografts for preclinical studies

Mouse models of human cancer are valuable tools for cancer research. Although xenografts and genetically engineered models (GEMs) possess limitations as well as advantages, each system plays a significant role in preclinical testing. Tumor xenografts are easy to use, relatively inexpensive, and reproducible. The main drawback of xenografts is that the genetics and histology of the tumors are frequently not representative of the respective human tumor and, thus far, these models have not been as predictive of therapeutic success as one would like. By contrast, GEMs are histologically and genetically accurate models of human cancer but have disadvantages of heterogeneity with regard to frequency, latency, and growth. These disadvantages are reminiscent of the variable behavior of actual human tumors. Recently, these shortcomings have been partly overcome with the development of anatomic and molecular in vivo imaging techniques such as magnetic resonance imaging and bioluminescence imaging. These new technologies will hopefully support the use of GEMs in preclinical trials and help determine if trials in GEMs are more predicative than xenografts of human responses.

Retinoic acid regulates the expression of dorsoventral topographic guidance molecules in the chick retina

Asymmetric expression of several genes in the early eye anlagen is required for the dorsoventral (DV) and anteroposterior (AP) patterning of the retina. Some of these early patterning genes play a role in determining the graded expression of molecules that are needed to form the retinotectal map. The polarized expression of retinoic acid synthesizing and degrading enzymes along the DV axis in the retina leads to several zones of varied retinoic acid (RA)activity. This is suggestive of RA playing a role in DV patterning of the retina. A dominant-negative form of the retinoic acid receptor α(DNhRARα) was expressed in the chick retina to block RA activity. RA signaling was found to play a role in regulating the expression of EphB2,EphB3 and ephrin B2, three molecules whose graded expression in the retina along the DV axis is important for establishing the correct retinotectal map. Blocking RA signaling by misexpression of a RA degrading enzyme, Cyp26A1 recapitulated some but not all the effects of DNhRARα. It also was found that Vax, a ventrally expressed transcription factor that regulates the expression of the EphB and ephrin B molecules, functions upstream of, or in parallel to, RA. Expression of DNhRARα led to increased levels of RA-synthesizing enzymes and loss of RA-degrading enzymes. Activation of such compensatory mechanisms when RA activity is blocked suggests that RA homeostasis is very strictly regulated in the retina.

Defects in chicken neuroretina misexpressing the BMP antagonist Drm/Gremlin

During eye development, bone morphogenetic proteins (BMPs) exert multiple actions on both early and late patterning and differentiation processes. However, the roles of BMP signaling in retinal differentiation are not well understood. To gain insight into a novel role of BMPs during retinal development, we proceeded to retrovirally directed misexpression of the BMP antagonist Drm/Gremlin in the chicken optic vesicle. This resulted in severe eye defects, characterized by microphthalmia, coloboma and the presence of dark streaks. The latter phenotype corresponds to localized perturbations of the stratified structure of the neuroretina. We show that these retinal disorganizations are characterized by a destruction of neuronal layers associated with axonal pathfinding defects, increased apoptosis and lost of N-cadherin expression. Moreover, whereas neuronal differentiation seems to proceed normally, Müller glial differentiation is impaired in Drm-induced disorganizations. These data suggest a possible role of BMP signaling in the laminar organization of the developing neuroretina.

Identification of the LEDGF/p75 HIV-1 integrase-interaction domain and NLS reveals NLS-independent chromatin tethering

To investigate the basis for the LEDGF/p75 dependence of HIV-1 integrase (IN) nuclear localization and chromatin association, we used cell lines made stably deficient in endogenous LEDGF/p75 by RNAi to analyze determinants of its location in cells and its ability to interact with IN. Deletion of C-terminal LEDGF/p75 residues 340-417 preserved nuclear and chromatin localization but abolished the interaction with IN and the tethering of IN to chromatin. Transfer of this IN-binding domain (IBD) was sufficient to confer HIV-1 IN interaction to GFP. HRP-2, the only other human protein with an identifiable IBD domain, was found to translocate IN to the nucleus of LEDGF/p75(-) cells. However, in contrast to LEDGF/p75, HRP-2 is not chromatin bound and does not tether IN to chromatin. A single classical nuclear localization signal (NLS) in the LEDGF/p75 N-terminal region ((146)RRGRKRKAEKQ(156)) was found by deletion mapping and was shown to be transferable to pyruvate kinase. Four central basic residues in the NLS are critical for its activity. Strikingly, however, stable expression studies with NLS(+/-) and IBD(+/-) mutants revealed that the NLS, although responsible for LEDGF/p75 nuclear import, is dispensable for stable, constitutive nuclear association of LEDGF/p75 and IN. Both wild-type LEDGF/p75 and NLS-mutant LEDGF/p75 remain entirely chromatin associated throughout the cell cycle, and each tethers IN to chromatin. Thus, these experiments reveal stable nuclear sequestration of a transcriptional regulator by chromatin during the nuclear-cytosolic mixing of cell division, which additionally enables stable tethering of IN to chromatin. LEDGF/p75 is a multidomain adaptor protein that interacts with the nuclear import apparatus, lentiviral IN proteins and chromatin by means of an NLS, an IBD and additional chromatin-interacting domains.

Functional knockdown of neuropilin-1 in the developing chick nervous system by siRNA hairpins phenocopies genetic ablation in the mouse

The chick embryo is widely used for the study of vertebrate development, but a general, reliable loss-of-function strategy for the analysis of gene function is currently not available. By using small inhibitory hairpin RNA (siRNA) molecules generated by the mouse U6 promoter, we have applied an RNA interference approach to achieve quantitative knockdown of the neuropilin-1 (Nrp-1) receptor in chick embryos. Functional knockdown was evident in the abolition of Sema3A-induced growth cone collapse in Nrp-1-siRNA but not Nrp-2-siRNA-expressing dorsal root ganglion (DRG) neurons. Two nervous system defects in Nrp-1 mutant mice were phenocopied in embryos treated with Nrp-1 siRNA. First, DRG axons prematurely entered the dorsal horn and projected inappropriately. Second, targeted early migrating neural crest cells destined for the sympathetic chain arrested ectopically within ventral spinal nerve roots. Localized knockdown induced by specific siRNA constructs will allow rapid functional analysis of genes regulating chick neural development whilst circumventing embryonic lethal effects often associated with global gene knockout in the mouse.

Role of the progressive ankylosis gene (ank) in cartilage mineralization

Mineralization of growth plate cartilage is a critical event during endochondral bone formation, which allows replacement of cartilage by bone. Ankylosis protein (Ank), which transports intracellular inorganic pyrophosphate (PP(i)) to the extracellular milieu, is expressed by hypertrophic and, especially highly, by terminally differentiated mineralizing growth plate chondrocytes. Blocking Ank transport activity or ank expression in terminally differentiated mineralizing growth plate chondrocytes led to increases of intra- and extracellular PP(i) concentrations, decreases of alkaline phosphatase (APase) expression and activity, and inhibition of mineralization, whereas treatment of these cells with the APase inhibitor levamisole led to an increase of extracellular PP(i) concentration and inhibition of mineralization. Ank-overexpressing hypertrophic nonmineralizing growth plate chondrocytes showed decreased intra- and extracellular PP(i) levels; increased mineralization-related gene expression of APase, type I collagen, and osteocalcin; increased APase activity; and mineralization. Treatment of Ank-expressing growth plate chondrocytes with a phosphate transport blocker (phosphonoformic acid [PFA]) inhibited uptake of inorganic phosphate (P(i)) and gene expression of the type III Na(+)/P(i) cotransporters Pit-1 and Pit-2. Furthermore, PFA or levamisole treatment of Ank-overexpressing hypertrophic chondrocytes inhibited APase expression and activity and subsequent mineralization. In conclusion, increased Ank activity results in elevated intracellular PP(i) transport to the extracellular milieu, initial hydrolysis of PP(i) to P(i), P(i)-mediated upregulation of APase gene expression and activity, further hydrolysis and removal of the mineralization inhibitor PP(i), and subsequent mineralization.

The development of semicircular canals in the inner ear: role of FGFs in sensory cristae

In the vertebrate inner ear, the ability to detect angular head movements lies in the three semicircular canals and their sensory tissues, the cristae. The molecular mechanisms underlying the formation of the three canals are largely unknown. Malformations of this vestibular apparatus found in zebrafish and mice usually involve both canals and cristae. Although there are examples of mutants with only defective canals, few mutants have normal canals without some prior sensory tissue specification, suggesting that the sensory tissues,cristae, might induce the formation of their non-sensory components, the semicircular canals. We fate-mapped the vertical canal pouch in chicken that gives rise to the anterior and posterior canals, using a fluorescent,lipophilic dye (DiI), and identified a canal genesis zone adjacent to each prospective crista that corresponds to the Bone morphogenetic protein 2 (Bmp2)-positive domain in the canal pouch. Using retroviruses or beads to increase Fibroblast Growth Factors (FGFs) for gain-of-function and beads soaked with the FGF inhibitor SU5402 for loss-of-function experiments,we show that FGFs in the crista promote canal development by upregulating Bmp2. We postulate that FGFs in the cristae induce a canal genesis zone by inducing/upregulating Bmp2 expression. Ectopic FGF treatments convert some of the cells in the canal pouch from the prospective common crus to a canal-like fate. Thus, we provide the first molecular evidence whereby sensory organs direct the development of the associated non-sensory components, the semicircular canals, in vertebrate inner ears.

Purifying Selection Masks the Mutational Flexibility of HIV-1 Reverse Transcriptase

DNA and RNA polymerases share a core architecture composed of three structurally conserved motifs: A, B, and C. Although the amino acid sequences of these motifs are highly conserved between closely related organisms, variation across broader evolutionary distances suggests that only a few residues in each motif are indispensable for polymerase function. To test this, we constructed libraries of human immunodeficiency virus type-1 (HIV-1) containing random single amino acid replacements in motif B of reverse transcriptase (RT), and we used selection in culture to assess RT function. Despite the nearly absolute constancy of motif B in vivo, virus replicating in culture tolerated a range of conservative and nonconservative substitutions at 10 of the 11 amino acid positions examined. These included residues that are invariant across all retroviral subfamilies and highly conversed in diverse retroelements. Several mutants retained wild type infectivity, and serial passage experiments revealed replacements that were neutral or even beneficial to viral fitness. In addition, a number of the selected variants exhibited altered susceptibility to the nucleoside analog inhibitors AZT and 3TC. Taken together, these data indicate that HIV-1 tolerates a range of substitutions at conserved RT residues and that selection against slightly deleterious mutations (purifying selection) in vivo masks a large repertoire of viable phenotypic variants. This mutational flexibility likely contributes to HIV-1 evolution in response to changing selection pressures in infected individuals.

New tools for gene manipulation in chicken embryos

Genomics has changed the pace by which genes are analyzed. Rather than looking at genes one by one, gene expression today is studied at the genome level. Unfortunately, the data we get from microarray analysis do not give us any clues about the function of these genes. Functional analyses are still refractory to large-scale, high-throughput studies, particularly in vertebrates. With the development of in ovo RNAi as a tool for specific gene silencing, the chicken embryo has become an efficient in vivo system to study gene function during development. A major advantage of in ovo RNAi is the fact that the knowledge of a cDNA fragment of the gene of interest is sufficient to get loss-of-function phenotypes. Thus, this new approach is a valuable tool for functional genomics.

Wnt signaling through the beta-catenin pathway is sufficient to maintain, but not restore, anagen-phase characteristics of dermal papilla cells

Dermal papilla cells of the hair follicle can be maintained in an active, hair-inducing state in vitro when cocultured with cells secreting Wnt3a. By inducing cultured dermal papilla cells to secrete Wnt themselves, we demonstrate that this activity is a direct effect of Wnt signaling to dermal papilla cells. We further demonstrate that the effects of Wnt3a are exerted through activation of the beta-catenin signal transduction pathway and do not require alternative Wnt transduction cascades. Once dermal papilla cells have lost hair-inducing properties in vitro, neither treatment with Wnt nor expression of a truncated and activating form of beta-catenin is sufficient to restore these properties to the cultured cells.

Expression of transgenes in primary neurons from chick peripheral and central nervous systems by retroviral infection of early embryos

Many cell biological studies require the expression of transgenes in cells in culture, but it is difficult to obtain uniform, stable, and efficient expression of transgenes in primary neurons. We have approached this problem by adapting from developmental biologists the avian retroviral vector, RCAS. This vector allows the introduction of a transgene by infection early in chick embryonic development. Transgenes that are less than 2.6 kb in size can be cloned through an adapter vector, SLAX 12 NCO, and into the RCAS retroviral vector with relative ease. The vector is then used to produce active virus, and the virus is injected into the neural tube or ventricles of stage 10 embryos. By infecting the neuronal precursor cells while they are still mitotic, the retrovirus and accompanying transgene are introduced into the genome and subsequently spread by replication, shedding of new virus, and infection of other cells. Embryos are incubated from the time of injection until E9-E12 and peripheral and central nervous system neurons are dissected out and grown in culture using standard techniques. In this manner, the majority of the sympathetic and dorsal root ganglion neurons can be induced to express the trangene. A similar result, at lower efficiencies, is obtained for central nervous system neurons.

Opposing roles of C/EBPbeta and AP-1 in the control of fibroblast proliferation and growth arrest-specific gene expression

Chicken embryo fibroblasts (CEF) express several growth arrest-specific (GAS) gene products in G0. In contact-inhibited cells, the expression of the most abundant of these proteins, the p20K lipocalin, is activated at the transcriptional level by C/EBPbeta. In this report, we describe the role of C/EBPbeta in CEF proliferation. We show that the expression of a dominant negative mutant of C/EBPbeta (designated Delta184-C/EBPbeta) completely inhibited p20K expression at confluence and stimulated the proliferation of CEF without inducing transformation. Mouse embryo fibroblasts nullizygous for C/EBPbeta had a proliferative advantage over cells with one or two functional copies of this gene. C/EBP inhibition enhanced the expression of the three major components of AP-1 in cycling CEF, namely c-Jun, JunD, and Fra-2, and stimulated AP-1 activity. In contrast, the over-expression of C/EBPbeta caused a dramatic reduction in the levels of AP-1 proteins. Therefore, C/EBPbeta is a negative regulator of AP-1 expression and activity in CEF. The expression of cyclin D1 and cell proliferation were stimulated by the dominant negative mutant of C/EBPbeta but not in the presence of TAM67, a dominant negative mutant of c-Jun and AP-1. CEF over-expressing c-Jun, and to a lesser extent JunD and Fra-2, did not growth arrest at high cell density and did not express p20K. Therefore, AP-1 interfered with the action of C/EBPbeta at high cell density, indicating that these factors play opposing roles in the control of GAS gene expression and CEF proliferation.

Mechanisms of avian retroviral host range extension

Alpharetroviruses provide a useful system for the study of the molecular mechanisms of host range and receptor interaction. These viruses can be divided into subgroups based on diverse receptor usage due to variability within the two host range determining regions, hr1 and hr2, in their envelope glycoprotein SU (gp85). In previous work, our laboratory described selection from a subgroup B avian sarcoma-leukosis virus of an extended-host-range variant (LT/SI) with two adjacent amino acid substitutions in hr1. This virus retains its ability to use the subgroup BD receptor but can also infect QT6/BD cells, which bear a related subgroup E receptor (R. A. Taplitz and J. M. Coffin, J. Virol 71:7814-7819, 1997). Here, we report further analysis of this unusual variant. First, one (L154S) of the two substitutions is sufficient for host range extension, while the other (T155I) does not alter host range. Second, these mutations extend host range to non-avian cell types, including human, dog, cat, mouse, rat, and hamster. Third, interference experiments imply that the mutants interact efficiently with the subgroup BD receptor and possibly the related subgroup E receptor, but they have another means of entry that is not dependent on these interactions. Fourth, binding studies indicate that the mutant SU proteins retain the ability to interact as monomers with subgroup BD and BDE receptors but only bind the subgroup E receptor in the context of an Env trimer. Further, the mutant SU proteins bind well to chicken cells but do not bind any better than wild-type subgroup B to QT6 or human cells, even though the corresponding viruses are capable of infecting these cells.

Retrovirus-mediated gene expression during chick visual system development

The chick embryo is an excellent model for studying eye morphogenesis, retinal cell fate determination, and retinotectal projections due to its accessibility and the available molecular tools. Avian replication-competent retroviruses allow efficient infection of proliferating cells and stable integration of the viral genome, including up to 2.3kb of foreign cDNA, into the host chromosome. High-titer retroviruses are produced by transient transfection of avian DF-1 cells followed by centrifugation of the culture medium. Targeted infection of the optic vesicle, the lens vesicle, the retina and pigmented epithelium, the periocular mesenchyme, and the tectum can be performed at different developmental stages in ovo. In addition, retroviruses can be used to transduce genes of interest into various ocular tissue explants or cells in vitro. Virus-mediated gene expression can be detected within 12h of infection. Therefore, avian replication-competent retroviruses serve as powerful tools to misexpress wild-type and mutant gene products and to study molecular mechanisms underlying vertebrate visual system development.

Novel monoclonal antibody directed at the receptor binding site on the avian sarcoma and leukosis virus Env complex

We report here on the generation of a mouse monoclonal antibody directed against Rous sarcoma virus (RSV) subgroup A Env that will be useful in functional and structural analysis of RSV Env, as well as in approaches employing the RCAS/Tva system for gene targeting. BALB/c mice were primed and given boosters twice with EnvA-expressing NIH 3T3 cells. Resulting hybridomas were tested by enzyme-linked immunosorbent assay against RCANBP virions and SU-A-immunoglobulin G immunoadhesin. One highly reactive hybridoma clone, mc8C5, was subcloned and tested in immunofluorescence, immunoprecipitation (IP), and Western blotting assays. In all three assays, mc8C5-4 subgroup-specifically recognizes SR-A Env, through the SU domain, expressed from different vectors in both avian and mammalian cells. This multifunctionality is notable for a mouse monoclonal. We furthermore observed a preference for binding to terminally glycosylated Env over core-glycosylated Env precursor in IPs, suggesting that the epitope is at least partially conformational and dependent on glycosylation. Most importantly, we found mc8C5-4 inhibited Env function: in vitro, the monoclonal not only interferes with binding of the EnvA receptor, Tva, but it also blocks the Tva-induced conformational change required for activation of the fusion peptide, without inducing that change itself. Infection of Tva-expressing avian or mammalian cells by avian sarcoma and leukosis virus (ASLV) or EnvA-pseudotyped murine leukemia virus, respectively, is efficiently inhibited by mc8C5-4. The apparent interference of the monoclonal with the EnvA-Tva complex formation suggests that the epitope seen by mc8C5 overlaps with the receptor binding site. This is supported by the observation that mutations of basic residues in hr2 or of the downstream glycosylation site, which both impair Tva-binding to EnvA, have similar effects on the binding of mc8C5. Thus, anti-ASLV-SU-A mc8C5-4 proves to be a unique new immunoreagent that targets the receptor-binding site on a prototypical retroviral envelope.

Overexpression of Dlx5 in chicken calvarial cells accelerates osteoblastic differentiation

Our laboratory and others have shown that a homeodomain protein binding site plays an important role in transcription of the Collal gene in osteoblasts. This suggests that homeodomain proteins have an important role in osteoblast differentiation. We have investigated the role of Dlx5 in osteoblastic differentiation. In situ hybridization studies indicated that Dlx5 is expressed in chick calvarial osteoblasts (cCOB) in vivo. Northern blot analysis indicated that Dlx5 expression in cultured cCOBs is induced concurrently with osteoblastic markers. To study the effect of overexpression of Dlx5 on osteoblast differentiation, we infected primary osteoblast cultures from 15-day-old embryonal chicken calvaria with replication competent retroviral vectors [RCASBP(A)] expressing Dlx5 or control replication competent avian splice acceptor brianhightiter polymerase subtype A [RCASBP(A)]. Expression of Collal, osteopontin, alkaline phosphatase, and osteocalcin messenger RNA (mRNA) occurred sooner and at higher levels in cultures infected with RCASBP(A)DLX5 than in RCASBP(A)-infected cultures. Mineralization of Dlx5-expressing cultures was evident by days 12-14, and RCAS-infected control osteoblasts did not begin to mineralize until day 17. Dlx5 also stimulated osteoblastic differentiation of calvarial cells that do not normally undergo osteoblastic differentiation in vitro. Our results suggest that Dlx5 plays an important role in inducing calvarial osteoblast differentiation.

Avian bic, a gene isolated from a common retroviral site in avian leukosis virus-induced lymphomas that encodes a noncoding RNA, cooperates with c-myc in lymphomagenesis and erythroleukemogenesis

bic is a novel gene identified at a common retroviral integration site in avian leukosis virus-induced lymphomas and has been implicated as a collaborator with c-myc in B lymphomagenesis. It lacks an extensive open reading frame and is believed to function as an untranslated RNA (W. Tam, Gene 274:157-167, 2001; W. Tam, D. Ben-Yehuda, and W. S. Hayward, Mol. Cell. Biol. 17:1490-1502, 1997). The oncogenic potential of bic, particularly its ability to cooperate with c-myc in oncogenesis, was tested directly by expressing c-myc and bic, either singly or in pairwise combination, in cultured chicken embryo fibroblasts (CEFs) and in chickens using replication-competent retrovirus vectors. Coexpression of c-myc and bic in CEFs caused growth enhancement of cells. Most importantly, chick oncogenicity assays demonstrated that bic can cooperate with c-myc in lymphomagenesis and erythroleukemogenesis. The present study provides direct evidence for the involvement of untranslated RNAs in oncogenesis and provides further support for the role of noncoding RNAs as riboregulators.

Construction and characterization of a replication-competent retroviral shuttle vector plasmid

We constructed two versions of an RCASBP-based retroviral shuttle vector, RSVP (RCASBP shuttle vector plasmid), containing either the zeocin or blasticidin resistance gene. In this vector, the drug resistance gene is expressed in avian cells from the long terminal repeat (LTR) promoter, whereas in bacteria the resistance gene is expressed from a bacterial promoter. The vector contains a bacterial origin of replication (ColE1) to allow circular viral DNA to replicate as a plasmid in bacteria. The vector also contains the lac operator sequence, which binds to the lac repressor protein, providing a simple and rapid way to purify the vector DNA. The RSVP plasmid contains the following sequence starting with the 5" end: LTR, gag, pol, env, drug resistance gene, lac operator, ColE1, LTR. After this plasmid was transfected into DF-1 cells, we were able to rescue the circularized unintegrated viral DNA from RSVP simply by transforming the Hirt DNA into Escherichia coli. Furthermore, we were able to rescue the integrated provirus. DNA from infected cells was digested with an appropriate restriction enzyme (ClaI) and the vector-containing segments were enriched using lac repressor protein and then self-ligated. These enriched fractions were used to transform E. coli. The transformation was successful and we did recover integration sites, but higher-efficiency rescue was obtained with electroporation. The vector is relatively stable upon passage in avian cells. Southern blot analyses of genomic DNAs derived from successive viral passages under nonselective conditions showed that the cassette (drug resistance gene-lac operator-ColE1) insert was present in the vector up to the third viral passage for both resistance genes, which suggests that the RSVP vectors are stable for approximately three viral passages. Together, these results showed that RSVP vectors are useful tools for cloning unintegrated or integrated viral DNAs.

Regulated gene expression in the chicken embryo by using replication-competent retroviral vectors

Rous sarcoma virus (RSV)-derived retroviral vector could efficiently deliver the green fluorescent protein (GFP), which is driven by the internal cytomegalovirus enhancer/promoter, into restricted cell populations in the chicken embryo. RSV-derived vectors coupled with the tet regulatory elements also revealed doxycycline-dependent inducible GFP expression in the chicken embryo in ovo.

Participation of Cbfa1 in regulation of chondrocyte maturation

OBJECTIVE

Cbfa1 is a transcription factor, which is classified into the runt family. The mice lacking this gene display complete loss of bone formation, indicating that Cbfa1 is an essential factor for osteoblast differentiation. The Cbfa1-deficient mice also show an abnormality in cartilage development. Although cartilage anlagens are well formed in these mice, endochondral ossification is blocked, and most of chondrocytes fail to differentiate into their maturation form as characterized by the absence of type X collagen and low levels of alkaline phosphatase activity. It is suggested that Cbfa1 may participate in chondrocyte differentiation. In this study, we have investigated the role of Cbfa1 in chondrocytes during their cytodifferentiation in vitro.

DESIGN

To investigate the role of Cbfa1 in regulation of chondrocyte differentiation, we over-expressed Cbfa1 or its dominant negative form in cultured chick chondrocytes using a retrovirus (RCAS)system and examined changes in chondrocyte behaviour induced by the introduced genes.

RESULTS

Mature chondrocytes isolated form the cephalic portion of sterna seemed to express Cbfa1 more prominently than immature chondrocytes isolated from the one-third caudal portion of sterna. Over-expression of Cbfa1 in immature chondrocytes strongly stimulated alkaline phosphatase activity and matrix calcification. In contrast, expression of a dominant negative form of Cbfa1, which lacks the C-terminal PST domain, severely inhibited alkaline phosphatase activity and matrix calcification in mature chondrocytes.

CONCLUSION

Taken together with the observation that Cbfa1 transcripts dominantly localized in hypertrophic chondrocytes as well as in osteoblasts, it is suggested that Cbfa1 plays an important role in the progression of chondrocyte maturation.

Use of type I collagen green fluorescent protein transgenes to identify subpopulations of cells at different stages of the osteoblast lineage

Green fluorescent protein (GFP)-expressing transgenic mice were produced containing a 3.6-kilobase (kb; pOBCol3.6GFPtpz) and a 2.3-kb (pOBCol2.3GFPemd) rat type I collagen (Col1a1) promoter fragment. The 3.6-kb promoter directed strong expression of GFP messenger RNA (mRNA) to bone and isolated tail tendon and lower expression in nonosseous tissues. The 2.3-kb promoter expressed the GFP mRNA in the bone and tail tendon with no detectable mRNA elsewhere. The pattern of fluorescence was evaluated in differentiating calvarial cell (mouse calvarial osteoblast cell [mCOB]) and in marrow stromal cell (MSC) cultures derived from the transgenic mice. The pOBCol3.6GFPtpz-positive cells first appeared in spindle-shaped cells before nodule formation and continued to show a strong signal in cells associated with bone nodules. pOBCol2.3GFPemd fluorescence first appeared in nodules undergoing mineralization. Histological analysis showed weaker pOBCol3.6GFPtpz-positive fibroblastic cells in the periosteal layer and strongly positive osteoblastic cells lining endosteal and trabecular surfaces. In contrast, a pOBCol2.3GFPemd signal was limited to osteoblasts and osteocytes without detectable signal in periosteal fibroblasts. These findings suggest that Col1a1GFP transgenes are marking different subpopulations of cells during differentiation of skeletal osteoprogenitors. With the use of other promoters and color isomers of GFP, it should be possible to develop experimental protocols that can reflect the heterogeneity of cell differentiation in intact bone. In primary culture, this approach will afford isolation of subpopulations of these cells for molecular and cellular analysis.

Conditional ablation of the osteoblast lineage in Col2.3deltatk transgenic mice

Two transgenic mouse lines were generated with a DNA construct bearing a 2.3-kilobase (kb) fragment of the rat alpha1 type I collagen promoter driving a truncated form of the herpes thymidine kinase gene (Col2.3Atk). Expression of the transgene was found in osteoblasts coincident with other genetic markers of early osteoblast differentiation. Mice treated with ganciclovir (GCV) for 16 days displayed extensive destruction of the bone lining cells and decreased osteoclast number. In addition, a dramatic decrease in bone marrow elements was observed, which was more severe in the primary spongiosum and marrow adjacent to the diaphyseal endosteal bone. Immunostaining for transgene expression within the bone marrow was negative and marrow stromal cell cultures developed normally in the presence of GCV until the point of early osteoblast differentiation. Our findings suggest that the early differentiating osteoblasts are necessary for the maintenance of osteoclasts and hematopoiesis. Termination of GCV treatment produced an exaggerated response of new bone formation in cortical and trabecular bone. The Col2.3deltatk mouse should be a useful model to define the interrelation between bone and marrow elements as well as a model to analyze the molecular and cellular events associated with a defined wave of osteogenesis on termination of GCV treatment.

JAC, a direct target of oncogenic transcription factor Jun, is involved in cell transformation and tumorigenesis

Using subtractive hybridization techniques, we have isolated a gene termed JAC that is strongly and specifically activated in avian fibroblasts transformed by the v-jun oncogene of avian sarcoma virus 17 (ASV17), but not in cells transformed by other oncogenic agents. Furthermore, JAC is highly expressed in cell lines derived from jun-induced avian fibrosarcomas. Kinetic analysis using a doxycycline-controlled conditional cell transformation system showed that expression of the 0.8-kb JAC mRNA is induced rapidly upon activation of the oncogenic v-jun allele. Nucleotide sequence analysis and transcriptional mapping revealed that the JAC gene contains two exons, with the longest ORF confined to exon 2. The deduced 68-amino acid chicken JAC protein is rich in cysteine residues and displays 37% sequence identity to mammalian high-sulfur keratin-associated proteins. The promoter region of JAC contains a consensus (5'-TGACTCA-3') and a nonconsensus (5'-TGAGTAA-3') AP-1 binding site in tandem, which are both specifically bound by the Gag-Jun hybrid protein encoded by ASV17. Mutational analysis revealed that the two AP-1 sites confer strong transcriptional activation by Gag-Jun in a synergistic manner. Ectopic expression of JAC in avian fibroblasts leads to anchorage-independent growth, strongly suggesting that deregulation of JAC is an essential event in jun-induced cell transformation and tumorigenesis.

The organization of mature Rous sarcoma virus as studied by cryoelectron microscopy

We have studied the organization of mature infectious Rous sarcoma virus (RSV), suspended in vitreous ice, using transmission electron microscopy. The enveloped virions are spherical in shape, have a mean diameter of 127 nm, and vary significantly in size. Image processing reveals the presence of the viral matrix protein underlying the lipid bilayer and the viral envelope proteins external to the lipid bilayer. In the interior of the virus, the characteristic mature retroviral core is clearly imaged. In contrast to lentiviruses, such as human immunodeficiency virus, the core of RSV is essentially isometric. The capsid, or external shell of the core, has a faceted, almost polygonal appearance in electron micrographs, but many capsids also exhibit continuous surface curvature. Cores are not uniform in size or shape. Serrations observed along the projected faces of the core suggest a repetitive molecular structure. Some isolated cores were observed in the sample, confirming that cores are at least transiently stable in the absence of the viral envelope. Using an approach grounded in geometric probability, we estimate the size of the viral core from the projection data. We show that the size of the core is not tightly controlled and that core size and virion size are positively correlated. From estimates of RNA packing density we conclude that either the RNA within the core is loosely packed or, more probably, that it does not fill the core.