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

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.

Identification and Characterization of Metastatic Factors by Gene Transfer into the Novel RIP-Tag; RIP-tva Murine Model

Metastatic cancer accounts for 90% of deaths in patients with solid tumors. There is an urgent need to better understand the drivers of cancer metastasis and to identify novel therapeutic targets. To investigate molecular events that drive the progression from primary cancer to metastasis, we have developed a bitransgenic mouse model, RIP-Tag; RIP-tva. In this mouse model, the rat insulin promoter (RIP) drives the expression of the SV40 T antigen (Tag) and the receptor for subgroup A avian leukosis virus (tva) in pancreatic β cells. The mice develop pancreatic neuroendocrine tumors with 100% penetrance through well-defined stages that are similar to human tumorigenesis, with stages including hyperplasia, angiogenesis, adenoma, and invasive carcinoma. Because RIP-Tag; RIP-tva mice do not develop metastatic disease, genetic alterations that promote metastasis can be identified easily. Somatic gene transfer into tva-expressing, proliferating pancreatic β premalignant lesions is achieved through intracardiac injection of avian retroviruses harboring the desired genetic alteration. A titer of >1 x 10⁸ infectious units per ml is considered appropriate for in vivo infection. In addition, avian retroviruses can infect cell lines derived from tumors in RIP-Tag; RIP-tva mice with high efficiency. The cell lines can also be used to characterize the metastatic factors. Here we demonstrate how to utilize this mouse model and cell lines to assess the functions of candidate genes in tumor metastasis.

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.

Cdc42 and the guanine nucleotide exchange factors Ect2 and trio mediate Fn14-induced migration and invasion of glioblastoma cells

Malignant glioblastomas are characterized by their ability to infiltrate into normal brain. We previously reported that binding of the multifunctional cytokine TNF-like weak inducer of apoptosis (TWEAK) to its receptor fibroblast growth factor-inducible 14 (Fn14) induces glioblastoma cell invasion via Rac1 activation. Here, we show that Cdc42 plays an essential role in Fn14-mediated activation of Rac1. TWEAK-treated glioma cells display an increased activation of Cdc42, and depletion of Cdc42 using siRNA abolishes TWEAK-induced Rac1 activation and abrogates glioma cell migration and invasion. In contrast, Rac1 depletion does not affect Cdc42 activation by Fn14, showing that Cdc42 mediates TWEAK-stimulated Rac1 activation. Furthermore, we identified two guanine nucleotide exchange factors (GEF), Ect2 and Trio, involved in TWEAK-induced activation of Cdc42 and Rac1, respectively. Depletion of Ect2 abrogates both TWEAK-induced Cdc42 and Rac1 activation, as well as subsequent TWEAK-Fn14-directed glioma cell migration and invasion. In contrast, Trio depletion inhibits TWEAK-induced Rac1 activation but not TWEAK-induced Cdc42 activation. Finally, inappropriate expression of Fn14 or Ect2 in mouse astrocytes in vivo using an RCAS vector system for glial-specific gene transfer in G-tva transgenic mice induces astrocyte migration within the brain, corroborating the in vitro importance of the TWEAK-Fn14 signaling cascade in glioblastoma invasion. Our results suggest that the TWEAK-Fn14 signaling axis stimulates glioma cell migration and invasion through two GEF-GTPase signaling units, Ect2-Cdc42 and Trio-Rac1. Components of the Fn14-Rho GEF-Rho GTPase signaling pathway present innovative drug targets for glioma therapy.

Histidine-rich glycoprotein can prevent development of mouse experimental glioblastoma

Extensive angiogenesis, formation of new capillaries from pre-existing blood vessels, is an important feature of malignant glioma. Several antiangiogenic drugs targeting vascular endothelial growth factor (VEGF) or its receptors are currently in clinical trials as therapy for high-grade glioma and bevacizumab was recently approved by the FDA for treatment of recurrent glioblastoma. However, the modest efficacy of these drugs and emerging problems with anti-VEGF treatment resistance welcome the development of alternative antiangiogenic therapies. One potential candidate is histidine-rich glycoprotein (HRG), a plasma protein with antiangiogenic properties that can inhibit endothelial cell adhesion and migration. We have used the RCAS/TV-A mouse model for gliomas to investigate the effect of HRG on brain tumor development. Tumors were induced with platelet-derived growth factor-B (PDGF-B), in the presence or absence of HRG. We found that HRG had little effect on tumor incidence but could significantly inhibit the development of malignant glioma and completely prevent the occurrence of grade IV tumors (glioblastoma).

Molecular analysis of fiber type-specific expression of murine myostatin promoter

Myostatin is a negative regulator of muscle growth, and absence of the functional myostatin protein leads to the heavy muscle phenotype in both mouse and cattle. Although the role of myostatin in controlling muscle mass is established, little is known of the mechanisms regulating the expression of the myostatin gene. In this study, we have characterized the murine myostatin promoter in vivo. Various constructs of the murine myostatin promoter were injected into the quadriceps muscle of mice, and the reporter luciferase activity was analyzed. The results indicate that of the seven E-boxes present in the 2.5-kb fragment of the murine myostatin promoter, the E5 E-box plays an important role in the regulation of promoter activity in vivo. Furthermore, the in vitro studies demonstrated that MyoD preferentially binds and upregulates the murine myostatin promoter activity. We also analyzed the activity of the bovine and murine promoters in murine skeletal muscle and showed that, despite displaying comparable levels of activity in murine myoblast cultures, bovine myostatin promoter activity is much weaker than murine myostatin promoter in mice. Finally, we demonstrate that in vivo, the 2.5-kb region of the murine myostatin promoter is sufficient to drive the activity of the reporter gene in a fiber type-specific manner.

GENE DELIVERY INTO THE CHICKEN EMBRYO BY USING REPLICATION-COMPETENT RETROVIRAL VECTORS

Rous sarcoma virus (RSV)-derived retroviral vectors have allowed for efficient gene transfer into the chicken embryo which is a classical model for studying vertebrate development. Current evidence reveals that this method can be used for regionally restricted expression, inducible expression, and for interfering with endogenous gene function, suggesting that gain-of-function and loss-of-function strategies for specific genes can be achieved spatially and temporally in the avian embryo. Thus, retroviral-mediated gene transfer into the chicken embryo coupled with a wide variety of strategies is now an important tool to address specific biological questions in the vertebrate.

Replicative retroviral vectors for cancer gene therapy

Poor efficiency of gene transfer into cancer cells constitutes the major bottleneck of current cancer gene therapy. We reasoned that because tumors are masses of rapidly dividing cells, they would be most efficiently transduced with vector systems allowing transgene propagation. We thus designed two replicative retrovirus-derived vector systems: one inherently replicative vector, and one defective vector propagated by a helper retrovirus. In vitro, both systems achieved very efficient transgene propagation. In immunocompetent mice, replicative vectors transduced >85% tumor cells, whereas defective vectors transduced <1% under similar conditions. It is noteworthy that viral propagation could be efficiently blocked by azido-thymidine, in vitro and in vivo. In a model of established brain tumors treated with suicide genes, replicative retroviral vectors (RRVs) were approximately 1000 times more efficient than defective adenoviral vectors. These results demonstrate the advantage and potential of RRVs and strongly support their development for cancer gene therapy.

Expression of exogenous protein in the egg white of transgenic chickens

Using a replication-deficient retroviral vector based on the avian leukosis virus (ALV), we inserted into the chicken genome a transgene encoding a secreted protein, beta-lactamase, under the control of the ubiquitous cytomegalovirus (CMV) promoter. Biologically active beta-lactamase was secreted into the serum and egg white of four generations of transgenic chickens. The expression levels were similar in successive generations, and expression levels in the magnum of the oviduct were constant over at least 16 months in transgenic hens, indicating that the transgene was stable and not subject to silencing. These results support the potential of the hen as a bioreactor for the production of commercially valuable, biologically active proteins in egg white.

Consistent production of transgenic chickens using replication-deficient retroviral vectors and high-throughput screening procedures

We have developed a novel method of DNA extraction combined with a high-throughput method of gene detection allowing thousands of potentially transgenic chicks to be screened quickly and reliably. By using this method and a replication-deficient retroviral vector based on avian leukosis virus (ALV), we have demonstrated germline transmission of three different transgenes. Several generations of chickens carrying intact transgenes were produced, validating the use of the ALV retroviral vectors for large-scale production of transgenic flocks. Fourth-generation chicks that were nontransgenic, hemizygous, or homozygous for the transgene were identified with the combined genetic screening methods.

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.

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.

Improvement of hatchability of chicken eggs injected by blastoderm cells

In our first experiment, we studied the effect of injection method of blastoderm cells (BC) into the subgerminal cavity of White Leghorn embryos on hatchability of chicken chimeras. Freshly laid eggs were injected through a hole made in the equatorial plane of the eggshell (Method A). In Method B, eggs were stored pointed end down for 5 to 7 d prior to injection, and a hole was cut in the blunt end of the eggshell. An advantage of Method B was that the early embryonic mortality was reduced (P < or = 0.01) and resulted in higher hatchability (41.0%; 43/105) than Method A (9.8%; 14/143). In the second experiment, we studied chicken hatchabililty as influenced by windowing (no hole, Group 1; hole in the equatorial plane, Group 2; hole in the blunt end of egg, Groups 3 and 4) and egg turning (Groups 1 and 4) or not (Groups 2 and 3) during incubation. The hatchability percentages were as follows: 67.9 (Group 1) 0.0, (Group 2) 23.3, (Group 3), and 56.8 (Group 4). A statistically significant difference (P < or = 0.05) was noted between Group 1 or 4 and the other groups. We found no statistically significant differences in the weight changes (g) but did note certain differences in the egg weight loss (%) among different egg treatments. In the third experiment, we investigated the influence of origins of BC donors: Rhode Island Red (RIR), Barred Plymouth Rock (BPR), and Green-legged Partridgelike (GP) on hatchability of putative and somatic chimera chickens. The hatchability of chimeras was dependent on the adequate assortment of BC of the donor and ranged from 7.4% (RIR) to 56.1% (GP). In the case of BC injection of the GP breed, good hatchability was accompanied by very high percentage (86.9; 20/23) of somatic chimeras.

Neural crest-directed gene transfer demonstrates Wnt1 role in melanocyte expansion and differentiation during mouse development

Wnt1 signaling has been implicated as one factor involved in neural crest-derived melanocyte (NC-M) development. Mice deficient for both Wnt1 and Wnt3a have a marked deficiency in trunk neural crest derivatives including NC-Ms. We have used cell lineage-directed gene targeting of Wnt signaling genes to examine the effects of Wnt signaling in mouse neural crest development. Gene expression was directed to cell lineages by infection with subgroup A avian leukosis virus vectors in lines of transgenic mice that express the retrovirus receptor tv-a. Transgenic mice with tva in either nestin-expressing neural precursor cells (line Ntva) or dopachrome tautomerase (DCT)-expressing melanoblasts (line DCTtva) were analyzed. We overstimulated Wnt signaling in two ways: directed gene transfer of Wnt1 to Ntva(+) cells and transfer of beta-catenin to DCTtva(+) NC-M precursor cells. In both methods, NC-M expansion and differentiation were effected. Significant increases were observed in the number of NC-Ms [melanin(+) and tyrosinase-related protein 1 (TYRP1)(+) cells], the differentiation of melanin(-) TYRP1(+) cells to melanin(+) TYRP1(+) NC-Ms, and the intensity of pigmentation per NC-M. These data are consistent with Wnt1 signaling being involved in both expansion and differentiation of migrating NC-Ms in the developing mouse embryo. The use of lineage-directed gene targeting will allow the dissection of signaling molecules involved in NC development and is adaptable to other mammalian developmental systems.

Gene transfer into chicken embryos as an effective system of analysis in developmental biology

Chicken embryos have been used as a model animal in developmental biology since the time of comparative and experimental embryology. Recent application of gene transfer techniques to the chicken embryo increases their value as an experimental animal. Today, gene transfer into chicken cells is performed by three major systems, lipofection, electroporation and the virus-mediated method. Each system has its own features and applicability. In this overview and the associated four minireviews, the methods and application of each system will be presented.

Perinatal deletion of B cells expressing surface Ig molecules that lack V(D)J-encoded determinants in the bursa of Fabricius is not due to intrafollicular competition

During embryonic development, the avian bursa of Fabricius selects B cell precursors that have undergone productive V(D)J recombination for expansion in oligoclonal follicles. During this expansion, Ig diversity is generated by gene conversion. We have used retroviral gene transfer in vivo to introduce surface Ig molecules that lack V(D)J-encoded determinants into B cell precursors. This truncated mu heavy chain supports both B cell expansion within embryo bursal lymphoid follicles and gene conversion. We show that individual follicles can be colonized exclusively by cells expressing the truncated mu chain and lacking endogenous surface IgM, ruling out a requirement for V(D)J-encoded determinants in the establishment of bursal lymphoid follicles. In striking contrast to their normal development in the embryo, bursal cells expressing the truncated mu-chain exhibit reduced rates of cell division and increased levels of apoptosis after hatching. The level of apoptosis in individual follicles reflects the proportion of cells within the follicle that express the truncated mu-chain. In particular, high levels of apoptosis are associated with follicles containing exclusively cells expressing the truncated micro receptor. Thus, apoptotic elimination of such cells is not due to competition within the follicle by cells expressing endogenous surface IgM receptors. This provides the first direct demonstration that the regulation of B cell development in the avian bursa after hatching differs fundamentally from that seen in the embryo. The requirement for intact IgM expression when the bursa is exposed to exogenous Ag implicates a role for Ag in avian B cell development after hatching.

Avian B cell development: lessons from transgenic models

The bursa of Fabricius is critical for the development of B lymphocytes in avian species. Despite considerable advances in our understanding of the molecular mechanisms by which avian antibody diversity is generated, many stages of B-cell development in the bursa and the means by which they are regulated remain unclear. Here we discuss the use of productive chicken retroviral vectors which allow gene transfer in vitro or in vivo as tools to probe the requirements for bursal B-cell development. Expression of a truncated form of bursal cell surface IgM, lacking variable region encoded determinants, is sufficient to promote the initial colonization and clonal expansion of B-cells within the bursa. Expression of this truncated IgM does not, however, protect developing bursal cells against the apoptosis that occurs within the bursa after hatch. Conversely, over-expression of the proto-oncogene bcl-2, following retroviral gene transfer, protects cells against apoptotic cell death but is not sufficient to allow B lineage progression in the absence of sIgM expression. Finally we discuss the use of regulated promoters within the retroviral gene transfer system to show that while bursal cells are susceptible to transformation by the v-rel oncogene in vitro, this oncogene preferentially targets mature peripheral cells in vivo.

Soluble forms of the subgroup A avian leukosis virus [ALV(A)] receptor Tva significantly inhibit ALV(A) infection in vitro and in vivo

The interactions between the subgroup A avian leukosis virus [ALV(A)] envelope glycoproteins and soluble forms of the ALV(A) receptor Tva were analyzed both in vitro and in vivo by quantitating the ability of the soluble Tva proteins to inhibit ALV(A) entry into susceptible cells. Two soluble Tva proteins were tested: the 83-amino-acid Tva extracellular region fused to two epitope tags (sTva) or fused to the constant region of the mouse immunoglobulin G heavy chain (sTva-mIgG). Replication-competent ALV-based retroviral vectors with subgroup B or C env were used to deliver and express the two soluble tv-a (stva) genes in avian cells. In vitro, chicken embryo fibroblasts or DF-1 cells expressing sTva or sTva-mIgG proteins were much more resistant to infection by ALV(A) ( approximately 200-fold) than were control cells infected by only the vector. The antiviral effect was specific for ALV(A), which is consistent with a receptor interference mechanism. The antiviral effect of sTva-mIgG was positively correlated with the amount of sTva-mIgG protein. In vivo, the stva genes were delivered and expressed in line 0 chicken embryos by the ALV(B)-based vector RCASBP(B). Viremic chickens expressed relatively high levels of stva and stva-mIgG RNA in a broad range of tissues. High levels of sTva-mIgG protein were detected in the sera of chickens infected with RCASBP(B)stva-mIgG. Viremic chickens infected with RCASBP(B) alone, RCASBP(B)stva, or RCASBP(B)stva-mIgG were challenged separately with ALV(A) and ALV(C). Both sTva and sTva-mIgG significantly inhibited infection by ALV(A) (95 and 100% respectively) but had no measurable effect on ALV(C) infection. The results of this study indicate that a soluble receptor can effectively block infection of at least some retroviruses and demonstrates the utility of the ALV experimental system in characterizing the mechanism(s) of viral entry.

'Shocking' developments in chick embryology: electroporation and in ovo gene expression

Efficient gene transfer by electroporation of chick embryos in ovo has allowed the development of new approaches to the analysis of gene regulation, function and expression, creating an exciting opportunity to build upon the classical manipulative advantages of the chick embryonic system. This method is applicable to other vertebrate embryos and is an important tool with which to address cell and developmental biology questions. Here we describe the technical aspects of in ovo electroporation, its different applications and future perspectives.

Salpalpha and Salpbeta, growth-arresting homologs of Sam68

Sam68, a nuclear RNA-binding protein, is a major substrate of the Src tyrosine kinase in mitotic cells. In addition to a tyrosine-rich C-terminal region, Sam68 also has six poly-proline (SH3-binding) sites, many of which are located in an amino-terminal region. Sam68 appears to act as an adaptor protein, associating with many SH2- and SH3-containing signal-transducing proteins (Richard et al., Mol. Cell. Biol. 15:186-197, 1995). Here we describe a novel 55kDa protein, Salpalpha, which has sequence similarity to Sam68 throughout its length. Salpalpha lacks the amino-terminal region found in Sam68, and has only a single poly-proline site, which binds the SH3 domain of the p85 subunit of PI 3-kinase. Salpalpha is tyrosine-phosphorylated when expressed in Rous sarcoma virus-infected chicken embryo fibroblasts (RSV-CEF); unlike Sam68, however, Salpalpha does not co-precipitate with v-Src. Salpbeta, an alternatively spliced isoform lacking the C-terminal tyrosine-rich region, is also tyrosine-phosphorylated in RSV-CEF, and also binds the SH3 domain of p85. We further show that expression of either Salpalpha or Salpbeta down-regulates the expression of Sam68 in CEF, and arrests the growth of these cells. Our results suggest that Salp may function as a negative regulator of cell growth.

Nspl1, a new Z-band-associated protein

Molecular characterization of a novel gene designated Neuroendocrine-Specific Protein-Like-1 (Nspl1) had revealed that this gene is expressed as two transcripts, a 1.2 kb transcript found predominantly in skeletal muscle and a 2.1 kb transcript expressed in the brain. The exceptionally high level of skeletal muscle expression prompted us to determine where the protein is localized to skeletal muscle. In vitro studies were performed using two plasmid constructs that generate full-length Nspl1 muscle-specific protein fused to the green fluorescent protein (GFP). In one construct, the GFP cDNA was fused to the N-terminus of the Nspl1 cDNA while in the second construct, the GFP cDNA was fused to the C-terminus of the Nspl1 cDNA. Transfection of either plasmid into mononucleated myoblasts showed that the Nspl1-GFP chimeric protein was associated with intermediate filaments. This was confirmed by using an antibody to stain desmin and finding that GFP-Nspl1 colocalizes with desmin. Chick primary myoblasts were transfected with the chimeric cDNAs and allowed to differentiate into mature myotubes. Results from this analysis and the use of monoclonal antibody to stain alpha-actinin, further localized the Nspl1 protein to the Z-band of mature myotubes. Confocal microscopy of the myotubes containing Nspl1-GFP demonstrates that Nspl1 is distributed continuously throughout the Z-disks.

Development of B cells expressing surface immunoglobulin molecules that lack V(D)J-encoded determinants in the avian embryo bursa of fabricius

Immunoglobulin gene rearrangement in avian B cell precursors generates surface Ig receptors of limited diversity. It has been proposed that specificities encoded by these receptors play a critical role in B lineage development by recognizing endogenous ligands within the bursa of Fabricius. To address this issue directly we have introduced a truncated surface IgM, lacking variable region domains, into developing B precursors by retroviral gene transfer in vivo. Cells expressing this truncated receptor lack endogenous surface IgM, and the low level of endogenous Ig rearrangements that have occurred within this population of cells has not been selected for having a productive reading frame. Such cells proliferate rapidly within bursal epithelial buds of normal morphology. In addition, despite reduced levels of endogenous light chain rearrangement, those light chain rearrangements that have occurred have undergone variable region diversification by gene conversion. Therefore, although surface expression of an Ig receptor is required for bursal colonization and the induction of gene conversion, the specificity encoded by the prediversified receptor is irrelevant and, consequently, there is no obligate ligand for V(D)J-encoded determinants of prediversified avian cell surface IgM receptor.

C/EBPbeta (NF-M) is essential for activation of the p20K lipocalin gene in growth-arrested chicken embryo fibroblasts

The p20K gene is induced in conditions of reversible growth arrest in chicken embryo fibroblasts (CEF). This expression is dependent on transcriptional activation and on a region of the promoter designated the quiescence-responsive unit (QRU). In this report, we describe the regulatory elements of the QRU responsible for activation in resting cells and characterize the trans-acting proteins interacting with these elements. We show that the QRU consists of functionally distinct domains including quiescence-specific and weak proliferation-responsive elements. The quiescence responsiveness of the QRU was mapped to two C/EBP binding sites, and the activity of the p20K promoter and its QRU was inhibited by the expression of a dominant negative mutant of C/EBPbeta in nondividing cells. The activation of QRU in response to serum starvation and contact inhibition correlated with the presence of a growth arrest-specific complex in electrophoretic mobility shift assays. This complex was supershifted by antibody for C/EBPbeta. C/EBPbeta accumulated in conditions of contact inhibition as a result of transcriptional activation. Therefore, C/EBPbeta was itself regulated as a growth arrest-specific gene in CEF. Finally, we show that the expression of p20K is regulated by linoleic acid, an essential fatty acid binding to p20K. The addition of linoleic acid to contact-inhibited CEF markedly repressed the synthesis of p20K without inducing mitogenesis. The activity of the QRU was inhibited by linoleic acid or the peroxisome proliferator-activated receptor PPARgamma2 in transient expression assays. Therefore, we have identified C/EBPbeta as a key activator of a growth arrest-specific gene in CEF and implicated an essential fatty acid, linoleic acid, in regulation of the QRU and the p20K lipocalin gene.

Mass determination of rous sarcoma virus virions by scanning transmission electron microscopy

The internal structural protein of retroviruses, Gag, comprises most of the mass of the virion, and Gag itself can give rise to virus-like particles when expressed in appropriate cells. Previously the stoichiometry of Gag in virions was inferred from indirect measurements carried out 2 decades ago. We now have directly determined the masses of individual particles of the prototypic avian retrovirus, Rous sarcoma virus (RSV), by using scanning transmission electron microscopy. In this technique, the number of scattered electrons in the dark-field image integrated over an individual freeze-dried virus particle on a grid is directly proportional to its mass. The RSV virions had a mean mass of 2.5 x 10(8) Da, corresponding to about 1,500 Gag molecules per virion. The population of virions was not homogeneous, with about one-third to two-thirds of the virions deviating from the mean by more than 10% of the mass in two respective preparations. The mean masses for virions carrying genomes of 7.4 or 9.3 kb were indistinguishable, suggesting that mass variability is not due to differences in RNA incorporation.

HiPER1, a phosphatase of the endoplasmic reticulum with a role in chondrocyte maturation

We have previously identified and partially cloned Band 17, a gene expressed in growth plate chondrocytes transiting from proliferation to hypertrophy. We now rename this gene HiPER1, Histidine Phosphatase of the Endoplasmic Reticulum-1, based on the results reported here. HiPER1 encodes two proteins of 318 (HiPER1(318)) and 449 (HiPER1(449)) amino acids, which are 20–21% identical to a group of yeast acid phosphatases that are in the histidine phosphatase family. HiPER1(449) is significantly more abundant than HiPER1(318), correlating with the abundance of the alternatively spliced messages encoding HiPER449 and HiPER318. Anti-HiPER1 antibodies detect two proteins of 53 and 55 kDa in growth plate chondrocytes that are absent in articular chondrocytes. We confirm that the 53 and 55 kDa proteins are HiPER1(449) by heterologous expression of the HiPER1(449) coding sequence in chick embryo fibroblasts. The 53 and 55 kDa proteins are glycosylated forms of HiPER1(449), as N-glycosidase F digestion reduces these proteins to 48 kDa, the predicted size of HiPER1(449) without the N-terminal signal sequence. Immunocytochemistry demonstrates that HiPER1(449) is found in chondrocytes maturing from proliferation to hypertrophy, but is not detectable in resting zone, deep hypertrophic zone or articular chondrocytes, a distribution that is consistent with the message distribution. HiPER1(449) was predicted to localize to the lumen of endoplasmic reticulum by an N-terminal signal sequence and by the C-terminal sequence Ala-Asp-Glu-Leu, which closely matches the consensus signal for ER retention, Lys-Asp-Glu-Leu. We confirm this prediction by demonstrating colocalization of HiPER1(449) with the ER protein HSP47 using dual-label immunofluorescence. PTHrP, a peptide that prevents hypertrophy in chondrocytes, suppressed HiPER1 and HiPER1(449) expression in vitro, an observation that further supports a role for HiPER1 in chondrocyte maturation. The yeast phosphatase homology, localization to the endoplasmic reticulum and pattern of expression suggest that HiPER1 represents a previously unrecognized intracellular pathway, involved in differentiation of chondrocytes.

Replication-competent retroviral vectors for expressing genes in avian cells in vitro and in vivo

Replication-competent retroviral vectors based on Rous sarcoma virus (RSV) are becoming increasingly popular for expressing genes in both primary cell cultures and embryonic chick tissues in ovo. In this article, we review the features of RSV and its life cycle that make it suitable for use as a vector. We describe the design and use of the RCAS and RCAS (BP) series of vectors, which are currently the most widely used RSV-based vectors, illustrating both their strengths and weakness. Finally, we outline laboratory protocols suitable for the banding of these retroviral vectors.

Expression of a murine leukemia virus Gag-Escherichia coli RNase HI fusion polyprotein significantly inhibits virus spread

The antiviral strategy of capsid-targeted viral inactivation (CTVI) was designed to disable newly produced virions by fusing a Gag or Gag-Pol polyprotein to a degradative enzyme (e.g., a nuclease or protease) that would cause the degradative enzyme to be inserted into virions during assembly. Several new experimental approaches have been developed that increase the antiviral effect of the CTVI strategy on retroviral replication in vitro. A Moloney murine leukemia virus (Mo-MLV) Gag-Escherichia coli RNase HI fusion has a strong antiviral effect when used prophylactically, inhibiting the spread of Mo-MLV and reducing virus titers 1,500- to 2,500-fold. A significant (approximately 100-fold) overall improvement of the CTVI prophylactic antiviral effect was produced by a modification in the culture conditions which presumably increases the efficiency of delivery and expression of the Mo-MLV Gag fusion polyproteins. The therapeutic effect of Mo-MLV Gag-RNase HI polyproteins is to reduce the production of infectious Mo-MLV up to 18-fold. An Mo-MLV Gag-degradative enzyme fusion junction was designed that can be cleaved by the Mo-MLV protease to release the degradative enzyme.

Avian models in developmental biology

The avian embryo is uniquely amenable to experimental manipulation. The most widely used models are chimeras resulting from heterotopic or orthotopic exchanges of rudiments between chick and quail embryos, according to Le Douarin's technique (1969). Cell migrations and fates are traced in these chimeras either through the identification of quail cell nuclei stained for DNA or by means of monoclonal antibodies that recognize a particular lineage in only one of the two species. The ontogeny of the hemopoietic and endothelial lineages, as enlightened through appropriately designed chimeras, is reviewed in the present article. Homologies recently disclosed in mouse and human embryo are emphasized. Finally, the possibilities afforded by retroviral somatic transgenesis in the avian embryo will be envisaged.

Atrial chamber-specific expression of the slow myosin heavy chain 3 gene in the embryonic heart

The quail slow myosin heavy chain 3 (slow MyHC 3) gene is expressed in the developing heart and in slow muscles of the developing limb. It is first expressed in the pulsatile cardiac tube in the embryo, and as the heart chamberizes its expression becomes restricted to the atria. To identify regulatory elements responsible for atrial-specific expression, the 5' upstream region of slow MyHC 3 gene was investigated. An atrial regulatory domain (ARD1) between -840 and -680 acts as an atrial cell-specific enhancer in primary cardiocyte cultures. ARD1 also specifies atrial-specific expression in vivo when the ARD1/heterologous promoter was introduced into developing chick embryos by a replication-competent retroviral vector. ARD1 is the first atrial cell-specific enhancer to be identified. Fine deletion and mutation analysis within ARD1 defined a 40-base pair vitamin D3 receptor-like element that controls atrial cell-specific expression of the slow MyHC 3 gene by inhibiting its expression in ventricular cardiocytes.

Expression of transduced genes in mice generated by infecting blastocysts with avian leukosis virus-based retroviral vectors

Transgenic mouse lines have been developed that express the tv-a receptor under the control of the chicken beta-actin promoter. These mice express the tv-a receptor in most or all tissues and in the early embryo. An avian leukosis virus (ALV)-based retroviral vector system was used for the efficient delivery of genes into preimplantation mouse embryos from these transgenic lines. Experimental animals could be generated quickly and easily by infecting susceptible blastocysts with ALV-based retroviral vectors. Expression of the delivered genes was controlled by either the constitutive viral promoter contained in the long terminal repeat or an internal nonviral tissue-specific promoter. Mating the infected founder chimeric animals produced animals that carry the ALV provirus as a transgene. A subset of the integrated proviruses expressed the chloramphenicol acetyltransferase reporter gene from either the promoter in the long terminal repeat or an internal promoter, which we believe indicates that many of the sites that are accessible to viral DNA insertion in preimplantation embryos are incompatible with expression in older animals. This approach should prove useful for studies on murine cell lineage and development, providing models for studying oncogenesis, and testing gene therapy strategies.

High efficiency gene transfer into the embryonic chicken CNS using B-subgroup retroviruses

Attempts to use replication-competent retroviruses to target genes to the chick CNS have met with limited success for injections performed prior to stage 14 using A- or E-subgroup viruses. This study was aimed at improving CNS infection by varying the stage of injection, viral envelope subgroup, viral titer, and the presence or absence of a transgene and/or the polycation polybrene in the inoculum. RCASBP vectors were injected into the neural tube of stages 3-13 embryos and protein expression was determined 9-48 hr later for forebrain, hindbrain, retina, and inner ear. Optimal injection parameters were defined which balanced good survival rates with high levels of transgene expression at early stages. The results demonstrate nearly complete expression of virus-mediated transgenes in neural tissues at stages 15-21 following injection of B-envelope RCASBP with polybrene at stages 7.5-12. This technique can now be applied to study the roles of genes in cell-autonomous events such as cell connectivity, physiology, and differentiation, as well as neural patterning and regional identity.

Preferential binding of MyoD-E12 versus myogenin-E12 to the murine sarcoma virus enhancer in vitro

The MyoD family of transcription factors regulates muscle-specific gene expression in vertebrates. In the adult rat, MyoD mRNA accumulates predominately in fast-twitch muscle, in particular type IIb and/or IIx fibers, whereas Myogenin mRNA is restricted to slow-twitch type I muscle fibers. Transgenic mice expressing the avian v-ski oncogene from the murine sarcoma virus (MSV) promoter-enhancer display preferential hypertrophy of type IIb fast-twitch muscle apparently because of the restricted expression of the transgene. We tested the hypothesis that preferential interactions of MyoD, as a heterodimer with E12, with the MSV enhancer, which has six E-box targets for MyoD family proteins, could contribute to v-ski gene expression in IIb muscle fibers. A series of quantitative binding studies was performed using an electrophoretic mobility shift assay to test MyoD-E12 versus Myogenin-E12 binding to the MSV enhancer. Our results indicate that MyoD-E12 binds the MSV enhancer with higher affinity and higher cooperativity than Myogenin-E12. Interestingly, MyoD-E12 bound all of the individual E-boxes tested with positive cooperativity indicating DNA-mediated dimerization of the protein subunits.

Retroviral vectors. From laboratory tools to molecular medicine

The majority of clinical trials for gene therapy currently employ retroviral-mediated gene delivery. This is because the life cycle of the retrovirus is well understood and can be effectively manipulated to generate vectors that can be efficiently and safely packaged. Here, we review the molecular technology behind the generation of recombinant retroviral vectors. We also highlight the problems associated with the use of these viruses as gene therapy vehicles and discuss future developments that will be necessary to maintain retroviral vectors at the forefront of gene transfer technology.

A system for tissue-specific gene targeting: transgenic mice susceptible to subgroup A avian leukosis virus-based retroviral vectors

Avian leukosis viruses (ALVs) have been used extensively as genetic vectors in avian systems, but their utility in mammals or mammalian cell lines is compromised by inefficient viral entry. We have overcome this limitation by generating transgenic mice that express the receptor for the subgroup A ALV under the control of the chicken alpha sk-actin promoter. The skeletal muscles of these transgenic animals are susceptible to efficient infection by subgroup A ALV. Because infection is restricted to cell lineages that express the transgene, the method has utility for studies of development and oncogenesis and will provide models for tissue-specific gene therapy.

Ectopic expression of genes during chicken limb pattern formation using replication defective retroviral vectors

A gene transfer method to ectopically express genes during chicken limb pattern formation using replication defective retroviral vectors has been established. Spherical non-proliferating (mitomycin C treated) aggregates of clonal retrovirus producing cells were grafted directly into developing chicken wing buds. The cell aggregates had to be placed in direct contact with the highly proliferative cells of the wing bud to promote efficient in vivo infection of embryonic cells by the released retroviral particles. The majority of grafts resulted in widespread expression of a reporter gene (encoding bacterial beta-galactosidase) during limb pattern formation and early limb bud outgrowth without affecting morphogenesis. This method provides a novel approach to study the effects of ectopic gene expression on limb pattern formation. Possible future applications to study other developmental processes are discussed.

Chimeric chickens and their use in manipulation of the chicken genome

Germline chimeric chickens can be made by injecting dispersed cells from Stage X blastoderms into recipient embryos at an equivalent stage of development. Colonization of the chimera by donor-derived cells is facilitated when the recipient embryo is compromised by exposure to irradiation prior to injection of the donor cells. Donor cells can be genetically manipulated by lipofection-mediated gene transfer before they are introduced into the recipient. The genetic modification is expressed in the ectoderm, mesoderm, and endoderm of the chimera after incubation for 96 h. Donor cells can also be cultured as dispersed cells in a monolayer or as whole-embryo explants for at least 48 h before transfer into recipients and retain the ability to enter both somatic and germline tissues in the resulting chimera. A strategy is proposed for the production of transgenic chickens using lipofection-mediated gene transfer to blastoderm cells isolated from Stage X embryos, which are subsequently injected into compromised recipients to yield a germline chimera.

Transgenesis in chickens

The application of transgenic technology to domestic poultry offers an alternative means to conventional practice for improvement of this highly productive agricultural species. The hen's reproductive system has unique characteristics which have imposed limitations on the use of established methods for artificial gene transfer. In this article, we review the various strategies that have been adopted to overcome the problem. Target sites for gene insertion include the fertilized ovum, the blastodermal embryo in the unincubated egg, and the primordial germ cells. Notable success in obtaining somatic and germline transformation has been achieved with the use of retroviral vectors to infect the blastodermal embryo. Current attempts to introduce DNA directly into the genome, without resort to pathogen-derived vectors, are discussed.

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.

Liver-specific expression of a phosphoenolpyruvate carboxykinase-neo gene in genetically modified chickens

In order to investigate the potential of the avian liver for the expression of recombinant proteins in vivo, replication-competent retroviral vectors were used to introduce a recombinant rat phosphoenolpyruvate carboxykinase promoter-driven neomycin resistance gene (PEPCKneo) into early Line 11 Leghorn embryos. After hatching, these birds possessed apparently intact PEPCKneo sequences in most tissues examined, however, the neo protein was expressed preferentially in the liver (up to .45% of total cellular protein). Therefore, the tissue specificity of the PEPCK promoter from the rat was retained in the chicken, although hormone responsiveness was not observed. Retroviral vectors used to transmit the genes were more stable during passage in either fibroblast cells or in the animal if the inserted genes were oriented in the same (sense) direction as the viral genome. After Geneticin drug selection in cultured cells, PEPCKneo mRNA was the predominant recombinant species observed on Northern blots, whereas embryos expressed mostly the RNA species originating in the retroviral long terminal repeats. The results demonstrate the potential usefulness of liver-specific gene expression in chickens, as well as the transcriptional effects observed when a foreign promoter is introduced into the replication-competent vector.

Recent advances in retrovirus vector technology

Retroviral vectors are widely used for the study of retroviral replication and to introduce DNA into somatic cells. An exciting new approach in retroviral vector technology is the use of internal ribosome entry sites from picornaviruses to provide stable expression of multiple genes. In addition, strategies are being developed that target the expression of retroviral vectors to specific cell populations.