Initiation of transcription from the minute virus of mice P4 promoter is stimulated in rat cells expressing a c-Ha-ras oncogene

Best of most possible worlds: Hybrid gene therapy vectors based on parvoviruses and heterologous viruses

Parvoviruses and especially the adeno-associated virus (AAV) species provide an exciting and versatile platform for the rational design or molecular evolution of human gene-therapy vectors, documented by literature from over half a century, hundreds of clinical trials, and the recent commercialization of multiple AAV gene therapeutics. For the last three decades, the power of these vectors has been further potentiated through various types of hybrid vectors created by intra- or inter-genus juxtaposition of viral DNA and protein cis elements or by synergistic complementation of parvoviral features with those of heterologous, prokaryotic, or eukaryotic viruses. Here, we provide an overview of the history and promise of this rapidly expanding field of hybrid parvoviral gene-therapy vectors, starting with early generations of chimeric particles composed of a recombinant AAV genome encapsidated in shells of synthetic AAVs or of adeno-, herpes-, baculo-, or protoparvoviruses. We then dedicate our attention to two newer, highly promising types of hybrid vectors created via (1) pseudotyping of AAV genomes with bocaviral serotypes and capsid mutants or (2) packaging of AAV DNA into, or tethering of entire vector particles to, bacteriophages. Finally, we conclude with an outlook summarizing critical requirements and improvements toward clinical translation of these original concepts.

The MVMp P4 promoter is a host cell-type range determinant in vivo

The protoparvovirus early promoters, e.g. P4 of Minute Virus of Mice (MVM), play a critical role during infection. Initial P4 activity depends on the host transcription machinery only. Since this is cell-type dependent, it is hypothesized that P4 is a host cell-type range determinant. Yet host range determinants have mapped mostly to capsid, never P4. Here we test the hypothesis using the mouse embryo as a model system. Disruption of the CRE element of P4 drastically decreased infection levels without altering range. However, when we swapped promoter elements of MVM P4 with those from equivalent regions of the closely related H1 virus, we observed elimination of infection in fibroblasts and chondrocytes and the acquisition of infection in skeletal muscle. We conclude that P4 is a host range determinant and a target for modifying the productive infection potential of the virus - an important consideration in adapting these viruses for oncotherapy.

Parvoviruses: structure and infection

Parvoviruses package a ssDNA genome. Both nonpathogenic and pathogenic members exist, including those that cause fetal infections, encompassing the entire spectrum of virus phenotypes. Their small genomes and simple coding strategy has enabled functional annotation of many steps in the infectious life cycle. They assemble a multifunctional capsid responsible for cell recognition and the transport of the packaged genome to the nucleus for replication and progeny virus production. It is also the target of the host immune response. Understanding how the capsid structure relates to the function of parvoviruses provides a platform for recombinant engineering of viral gene delivery vectors for the treatment of clinical diseases, and is fundamental for dissecting the viral determinants of pathogenicity. This review focuses on our current understanding of parvovirus capsid structure and function with respect to the infectious life cycle.

Parvovirus B19 infection in Hashimoto's thyroiditis, papillary thyroid carcinoma, and anaplastic thyroid carcinoma

BACKGROUND

The human pathogenic parvovirus B19 (B19) has recently been detected in papillary thyroid carcinoma (PTC) and Hashimoto's thyroiditis (HT) tissues at a high frequency in two studies of a Chinese cohort. We wanted to extend these data to include another cohort and expand the thyroid tumor tissue types assessed. In particular, we were interested to find whether B19 also infects anaplastic thyroid carcinoma (ATC), one of the most aggressive human cancers.

METHODS

Commercially available thyroid tumor tissue arrays were used to detect B19 capsid protein by immunohistochemistry in various types of thyroid tumors and disorders. The arrays were representative of the four main types of thyroid tumors, as well as other thyroid autoimmune disorders such as HT and Graves' disease, and adenomas, goiters, lymphomas, and normal thyroid tissue. In total, at least 12 different types of thyroid conditions as well as normal tissue were represented, many with multiple subjects.

RESULTS

Twenty-one of the 24 (88%) PTC tumors, 3 of the 3 ATC/undifferentiated tumors, and 3 of the 3 HT tissue samples were positive for B19 capsid protein by immunohistochemistry. The localization of the protein differed based on pathological disease type, with a nuclear to cytoplasmic shift seen from unaffected to tumor tissue.

CONCLUSIONS

We extend the data available on B19 detection in the thyroid to show a high correlation of virus in another cohort of PTC and HT at the protein level. We also show, for the first time, B19 infection of much more highly aggressive ATC/undifferentiated tumors. Nuclear to cytoplasmic shift in B19 protein in cancer tissue suggests a possible link between B19 and thyroid cancer pathogenesis/progression.

The Peroxisomal 3-keto-acyl-CoA thiolase B Gene Expression Is under the Dual Control of PPARα and HNF4α in the Liver

PPARα and HNF4α are nuclear receptors that control gene transcription by direct binding to specific nucleotide sequences. Using transgenic mice deficient for either PPARα or HNF4α, we show that the expression of the peroxisomal 3-keto-acyl-CoA thiolase B (Thb) is under the dependence of these two transcription factors. Transactivation and gel shift experiments identified a novel PPAR response element within intron 3 of the Thb gene, by which PPARα but not HNF4α transactivates. Intriguingly, we found that HNF4α enhanced PPARα/RXRα transactivation from TB PPRE3 in a DNA-binding independent manner. Coimmunoprecipitation assays supported the hypothesis that HNF4α was physically interacting with RXRα. RT-PCR performed with RNA from liver-specific HNF4α-null mice confirmed the involvement of HNF4α in the PPARα-regulated induction of Thb by Wy14,643. Overall, we conclude that HNF4α enhances the PPARα-mediated activation of Thb gene expression in part through interaction with the obligate PPARα partner, RXRα.

Viral oncolysis that targets Raf-1 signaling control of nuclear transport

The central role of Raf protein kinase isoforms in human cancer demands specific anti-Raf therapeutic inhibitors. Parvoviruses are currently used in experimental cancer therapy due to their natural oncotropism and lytic life cycle. In searching for mechanisms underlying parvovirus oncolysis, we found that trimers of the major structural protein (VP) of the parvovirus minute virus of mice (MVM), which have to be imported into the nucleus for capsid assembly, undergo phosphorylation by the Raf-1 kinase. Purified Raf-1 phosphorylated the capsid subunits in vitro to the two-dimensional pattern found in natural MVM infections. VP trimers isolated from mammalian cells translocated into the nucleus of digitonin-permeabilized human cells. In contrast, VP trimers isolated from insect cells, which are devoid of Raf-1, were neither phosphorylated nor imported into the mammalian nucleus. However, the coexpression of a constitutively active Raf-1 kinase in insect cells restored VP trimer phosphorylation and nuclear transport competence. In MVM-infected normal and transformed cells, Raf-1 inhibition resulted in cytoplasmic retention of capsid proteins, preventing their nuclear assembly and progeny virus maturation. The level of Raf-1 activity in cancer cells was consistent with the extent of VP specific phosphorylation and with the permissiveness to MVM infection. Thus, Raf-1 control of nuclear translocation of MVM capsid assembly intermediates provides a novel target for viral oncolysis. MVM may reinforce specific therapies against frequent human cancers with deregulated Raf signaling.

Tumor targeting using canine parvovirus nanoparticles

Advances in genetics, proteomics and cellular and molecular biology are being integrated and translated to develop effective methods for the prevention and control of cancer. One such combined effort is to create multifunctional nanodevices that will specifically recognize tumors and thus enable early diagnosis and provide targeted treatment of this disease. Viral particles are being considered for this purpose since they are inherently nanostructures with well-defined geometry and uniformity, ideal for displaying molecules in a precise spatial distribution at the nanoscale level and subject to greater structural control. Viruses are presumably the most efficient nanocontainer for cellular delivery as they have naturally evolved mechanisms for binding to and entering cells. Virus-based systems typically require genetic or chemical modification of their surfaces to achieve tumor-specific interactions. Interestingly, canine parvovirus (CPV) has a natural affinity for transferrin receptors (TfRs) (both of canine and human origin) and this property could be harnessed as TfRs are overexpressed by a variety of human tumor cells. Since TfR recognition relies on the CPV capsid protein, we envisioned the use of virus or its shells as tumor targeting agents. We observed that derivatization of CPV virus-like particles (VLPs) with dye molecules did not impair particle binding to TfRs or internalization into human tumor cells. Thus CPV-based VLPs with a natural tropism for TfRs hold great promise in the development of novel nanomaterial for delivery of a therapeutic and/or genetic cargo.

Exploring the contribution of distal P4 promoter elements to the oncoselectivity of Minute Virus of Mice

Minute Virus of Mice (MVM) shares inherent oncotropic properties with other members of the genus Parvovirus. Two elements responsible, at least in part, for this oncoselectivity have been mapped to an Ets1 binding site adjacent to the P4 TATA box of the initiating promoter, P4, and to a more distal cyclic AMP responsive element (CRE), located within the telomeric hairpin stem. Here the CRE overlaps one half-site for the binding of parvoviral initiation factor (PIF), which is essential for viral DNA replication. We used a degenerate oligonucleotide selection approach to show that CRE binding protein (CREB) selects the sequence ACGTCAC within this context, rather than its more generally accepted palindromic TGACGTCA recognition site. We have developed strategies for manipulating these sequences directly within the left-end palindrome of the MVM infectious clone and used them to clone mutants whose CRE either matches the symmetric consensus sequence or is scrambled, or in which the PIF binding site is incrementally weakened with respect to the CRE. The panel of mutants were tested for fitness relative to wildtype in normal murine fibroblasts A9 or transformed human fibroblasts 324 K, through multiple rounds of growth in co-infected cultures, using a differential real-time quantitative PCR assay. We confirmed that inactivating the CRE substantially abrogates oncoselectivity, but found that improving its fit to the palindromic consensus is somewhat debilitating in either cell type. We also confirmed that reducing the PIF half-site spacing by one basepair enhances oncoselectivity, but found that a further basepair deletion significantly reduces this effect.

Encapsidation of minute virus of mice DNA: aspects of the translocation mechanism revealed by the structure of partially packaged genomes

Minute virus of mice (MVM) packages a single, negative-sense copy of its linear single-stranded DNA genome, but a chimeric virus, MML, in which >95% MVM sequence was fused to the right-hand terminus of LuIII, packages >40% positive-sense DNA. While encapsidation of both MML strands begins efficiently, genome translocation frequently stalls at specific sites in positive-sense DNA. Internalized sequences, derived from the 3' end of the strand, ranged from 1 to 5 kb in length, with species of around 2 kb predominating. When nuclease activity during isolation was minimized, these truncated species were found to be part of pre-excised 5 kb single-strands. Similarly, some partially encapsidated negative-sense DNAs were observed, forming a continuum of protected 3' sequences between 1 and 3 kb in length, but these were less abundant and more uniformly distributed than their positive-sense counterparts, indicating that the negative strand has evolved for efficient internalization. The paucity of protected DNAs shorter than 1-2 kb suggests that translocation is biphasic, proceeding efficiently through the first (3') third of the genome, but prone to stall thereafter. Sequences with conspicuous secondary structure, including stem-loop and guanidine rich regions, were found to interrupt packaging, especially when positioned near the 5' end of the strand. Since VP2 amino-terminal peptides were exposed at the particle surface in all packaging intermediates, extrusion of this peptide precedes translocation of the full-length strand.

The P4 promoter of the parvovirus minute virus of mice is developmentally regulated in transgenic P4-LacZ mice

Activation of the minute virus of mice (MVM) P4 promoter is a key step in the life cycle of the virus and is completely dependent on host transcription factors. Since transcription-factor composition varies widely in different cell types, there is the possibility that only some cell types in the host organism have the capacity to initiate expression from the P4 promoter and therefore that the promoter may be a factor in determining the tropism of MVM. In this study, the ability of various cell types to activate P4, independent of the other virus-host interactions, was examined in transgenic mouse lines bearing a beta-galactosidase reporter sequence driven by the P4 promoter. It was found that lacZ was expressed during embryogenesis and in the adult in a cell-type-specific and differentiation-dependent pattern. The data are consistent with cell-type and stage-specific activation of the P4 promoter having a role in determining the host cell-type range of MVM. The ability of some parvoviruses to replicate in, and kill oncogenically transformed cells, and to destroy induced tumors in laboratory animals is the basis of recent approaches to use MVM-based vectors in cancer gene therapy. Since these vectors rely on the activation of the P4 promoter by the target tissues, understanding the promoter dependence on cell-type and differentiation status is important for their design and potential use.

Tumor-selective gene transduction and cell killing with an oncotropic autonomous parvovirus-based vector

A recombinant MVMp of the fibrotropic strain of minute virus of mice (MVMp) expressing the chloramphenicol acetyltransferase reporter gene was used to infect a series of biologically relevant cultured cells, normal or tumor-derived, including normal melanocytes versus melanoma cells, normal mammary epithelial cells versus breast adenocarcinoma cells, and normal neurons or astrocytes versus glioma cells. As a reference cell system we used normal human fibroblasts versus the SV40-transformed fibroblast cell line NB324K. After infection, we observed good expression of the reporter gene in the different tumor cell types, but only poor expression if any in the corresponding normal cells. We also constructed a recombinant MVMp expressing the green fluorescent protein reporter gene and assessed by flow cytometry the efficiency of gene transduction into the different target cells. At a multiplicity of infection of 30, we observed substantial transduction of the gene into most of the tumor cell types tested, but only marginal transduction into normal cells under the same experimental conditions. Finally, we demonstrated that a recombinant MVMp expressing the herpes simplex virus thymidine kinase gene can, in vitro, cause efficient killing of most tumor cell types in the presence of ganciclovir, whilst affecting normal proliferating cells only marginally if at all. However, in the same experimental condition, breast tumor cells appeared to be resistant to GCV-mediated cytotoxicity, possibly because these cells are not susceptible to the bystander effect. Our data suggest that MVMp-based vectors could prove useful as selective vehicles for anticancer gene therapy, particularly for in vivo delivery of cytotoxic effector genes into tumor cells.

Parvovirus H-1-induced cell death: influence of intracellular NAD consumption on the regulation of necrosis and apoptosis

The autonomous parvovirus H-1 exerts tumor-suppressive effects in living organisms and has been shown to specifically interfere with the survival of transformed cells in culture. The mechanism(s) by which H-1 virus induces death of transformed cells is not yet well understood. It has recently been reported that H-1 virus induces apoptotic cell death in the human monocytic U937 cell line, as assessed by biochemical and morphological changes of infected cells (Rayet, B., Lopez-Guerrero, J.-A., Rommelaere, J., Dinsart, C., 1998. Induction of programmed cell death by parvovirus H-1 in U937 cells: connection with the TNFalpha signalling pathway. J. Virol. 72, 8893-8903). Here we show that parvovirus H-1 infection induced early biochemical changes pointing to apoptotic events also in the transformed human keratinocyte cell line, HeLa, and the transformed rat fibroblast cell line, P1. Morphologic changes, however, and in particular the early breakdown of plasma membrane integrity, suggested that apoptosis did not go to completion, leading to necrotic cell death as the major result of parvovirus infection of HeLa and P1 cells. Parvovirus infection of these, and to a significantly lesser extent of U937 cells, was accompanied by rapid depletion of intracellular NAD stores. Inhibition of NAD-consuming enzymes interfered with parvovirus-induced NAD depletion and increased the proportion of H-1 virus-infected cells displaying apoptotic features of cell death. In contrast, a similar prevention of NAD depletion through stimulation of NAD production had little influence on the cell death pathway, suggesting that NAD-consuming enzymes may promote necrosis in a direct way rather than through inducing the overall drop of intracellular NAD.

Atypical nucleoprotein complexes mediate CRE-dependent regulation of the early promoter of minute virus of mice

The P4 promoter of the parvovirus minute virus of mice (MVMp) directs transcription of the genes encoding non-structural proteins. We have previously shown that functional upstream CRE elements contribute to both the ras oncogene-dependent activation of promoter P4 and its down-modulation by known activators of cyclic AMP-dependent protein kinase A (PKA). In the present work, the nucleoprotein complexes formed with the P4 CRE elements were characterized with regard to their polypeptide constituents and the nucleotides taking part in the interaction. Atypical interactions, both at the protein-protein and protein-DNA level, were observed, which may be a reflection of the divergence of the parvoviral CREs from the usual consensus. The CRE-mediated regulation of promoter P4 by PKA and Ras is discussed in light of these findings.

Oncogenic transformation-dependent expression of a transcription factor NF-Y subunit

As a result of differential splicing, one subunit of the nuclear factor Y (NF-Y) consists of two major isoforms designated short (NF-YaS) and long (NF-YaL). In proliferating normal human fibroblasts, NF-YaL is by far the more expressed isoform. Surprisingly, NF-YaS was found by immunoblotting to be as prominent as NF-YaL in simian virus 40 (SV40)-transformed cell derivatives. As a consequence, two NF-Y/DNA complexes, one containing the long and the other the short isoform, were formed with extracts from transformed cells and a target promoter element in electrophoretic mobility-shift assays. Only the complex containing NF-YaL was detected with extracts from normal fibroblasts. Furthermore, the NF-Y recognition motif contributed to promoter activation in SV40-transformed cells but not in normal, cells. Our finding links transcription stimulation in transformed cells to quantitative changes in the expression of an NF-Ya subunit.

Dose-dependent regression of HeLa cell-derived tumours in SCID mice after parvovirus H-1 infection

Parvoviruses of rodents are endowed with oncosuppressive properties. In particular, parvoviral infections protect host animals from spontaneous and chemical- or virus-induced tumour initiation in laboratory animals. The present study was undertaken to substantiate the capacity of parvovirus H-1 to inhibit therapeutically the growth of established tumours originating from human carcinoma cells implanted in recipient mice. To this end, quickly growing s.c. carcinomas were established by injection of human cervical carcinoma cells (HeLa) into immunodeficient (SCID) mice. Tumour-bearing mice subsequently were inoculated with H-1 at various multiplicities of infection. H-1 virus infection led to regression of tumours, the onset and efficiency of which were dose-dependent.

A missense mutation in hepatocyte nuclear factor-4 alpha, resulting in a reduced transactivation activity, in human late-onset non-insulin-dependent diabetes mellitus

Non-insulin-dependent diabetes mellitus (NIDDM) is a heterogeneous disorder characterized by hyperglycemia resulting from defects in insulin secretion and action. Recent studies have found mutations in the hepatocyte nuclear factor-4 alpha gene (HNF-4alpha) in families with maturity-onset diabetes of the young (MODY), an autosomal dominant form of diabetes characterized by early age at onset and a defect in glucose-stimulated insulin secretion. During the course of our search for susceptibility genes contributing to the more common late-onset NIDDM forms, we observed nominal evidence for linkage between NIDDM and markers in the region of the HNF-4alpha/MODY1 locus in a subset of French families with NIDDM diagnosed before 45 yr of age. Thus, we screened these families for mutations in the HNF-4alpha gene. We found a missense mutation, resulting in a valine-to-isoleucine substitution at codon 393 in a single family. This mutation cosegregated with diabetes and impaired insulin secretion, and was not present in 119 control subjects. Expression studies showed that this conservative substitution is associated with a marked reduction of transactivation activity, a result consistent with this mutation contributing to the insulin secretory defect observed in this family.

Opposite transcriptional effects of cyclic AMP-responsive elements in confluent or p27KIP-overexpressing cells versus serum-starved or growing cells

The minute virus of mice, an autonomous parvovirus, requires entry of host cells into the S phase of the cell cycle for its DNA to be amplified and its genes expressed. This work focuses on the P4 promoter of this parvovirus, which directs expression of the transcription unit encoding the parvoviral nonstructural polypeptides. These notably include protein NS1, necessary for the S-phase-dependent burst of parvoviral DNA amplification and gene expression. The activity of the P4 promoter is shown to be regulated in a cell cycle-dependent manner. At the G1/S-phase transition, the promoter is activated via a cis-acting DNA element which interacts with phase-specific complexes containing the cellular transcription factor E2F. It is inhibited, on the other hand, in cells arrested in G1 due to contact inhibition. This inhibitory effect is not observed in serum-starved cells. It is mediated in cis by cyclic AMP response elements (CREs). Unlike serum-starved cells, confluent cells accumulate the cyclin-dependent kinase inhibitor p27, suggesting that the switch from CRE-mediated activation to CRE-mediated repression involves the p27 protein. Accordingly, plasmid-driven overexpression of p27 causes down-modulation of promoter P4 in growing cells, depending on the presence of at least two functional CREs. No such effect is observed with two other cyclin-dependent kinase inhibitors, p16 and p21. Given the importance of P4-driven synthesis of protein NS1 in parvoviral DNA amplification and gene expression, the stringent S-phase dependency of promoter P4 is likely a major determinant of the absolute requirement of the minute virus of mice for host cell proliferation.

Two regulatory elements of similar structure and placed in tandem account for the repressive activity of the first intron of the human apolipoprotein A-II gene

Recent reports indicate that apolipoprotein (apo) A-II, the second most abundant protein of high-density lipoproteins, plays a crucial role in counteracting the beneficial effect of apo A-I against atherogenesis. Transcription of the human apo A-II gene is controlled by an enhancer comprising 14 regulatory elements located upstream of its promoter whereas the first intron of this gene behaves as a silencer. Here we show that two sequence elements account for the repressive activity of this intron and correspond to negative regulatory elements termed NRE I and NRE II. The activity of intron I and the nuclear proteins binding to NRE I and II are encountered in hepatic cells but not in non-hepatic cells studied here. Both NREs form nucleoprotein complexes of very similar physicochemical characteristics and bind the same or closely related proteins. Site-directed mutagenesis, transient transfection and gel-shift analysis experiments indicate that both NREs exhibit similar structures, being composed of two sites required for maximal activity and optimal binding of transcription factors. Therefore two negative regulatory elements of similar structure and function, placed in tandem, account for the repressive activity of the first intron of the human apo A-II gene. These NREs do not exhibit structural similarity with known NREs of other genes.

ras oncogene-dependent activation of the P4 promoter of minute virus of mice through a proximal P4 element interacting with the Ets family of transcription factors

The P4 promoter of parvovirus minute virus of mice (MVMp) directs transcription of the genes coding for nonstructural proteins. The activity of promoter P4 is regulated by several cis-acting DNA elements. Among these, a promoter-proximal GC box was shown to be essential for P4 activity (J.K. Ahn, B.J. Gavin, G. Kumar, and D.C. Ward, J. Virol. 63:5425-5439, 1989). In this study, a motif homologous to an Ets transcription factor-binding site (EBS), located immediately upstream from the GC box, was found to be required for the full activity of promoter P4 in the ras-transformed rat fibroblast cell line FREJ4. In normal parental FR3T3 cells, the transcriptional function of P4 EBS was insignificant but could be restored by transient cell transfection with the c-Ha-ras oncogene. P4 EBS may thus contribute to the stimulation of promoter P4 in ras-transformed cells. Electrophoretic mobility shift assays using crude extracts from FREJ4 cells revealed the binding of a member(s) of the Ets family of transcription factors to the P4 EBS, as well as the interaction of two members of the Sp1 family, Sp1 and Sp3, with the adjacent GC box. When produced in Drosophila melanogaster SL2 cells, Ets-1 and Sp1 proteins acted synergistically to transactivate promoter P4 through their respective cognate sites.

Upstream CREs participate in the basal activity of minute virus of mice promoter P4 and in its stimulation in ras-transformed cells

The activity of the P4 promoter of the parvovirus minute virus of mice (prototype strain MVMp) is stimulated in ras-transformed FREJ4 cells compared with the parental FR3T3 line. This activation may participate in the oncolytic effect of parvoviruses, given that P4 drives a transcriptional unit encoding cytotoxic nonstructural proteins. Our results suggest that the higher transcriptional activity of promoter P4 in FREJ4 cells is mediated at least in part by upstream CRE elements. Accordingly, mutations in the CRE motifs impair P4 function more strongly in the FREJ4 derivative than in its FR3T3 parent. Further evidence that these elements contribute to hyperactivity of the P4 promoter in the ras transformant is the fact that they form distinct complexes with proteins from FREJ4 and FR3T3 cell extracts. This difference can be abolished by treating the FREJ4 cell extracts with cyclic AMP-dependent protein kinase (PKA) or treating original cultures with a PKA activator. These findings can be linked with two previously reported features of ras-transformed cells: the activation of a PKA-inhibited protein kinase cascade and the reduction of PKA-induced protein phosphorylation. In keeping with these facts, P4-directed gene expression can be up- or downmodulated in vivo by exposing cells to known inhibitors or activators of PKA, respectively.

Mesodermal expression of the chicken erg gene associated with precartilaginous condensation and cartilage differentiation

The ets gene superfamily encodes a class of transcription factors that bind to a purine rich sequence through a 85 amino-acid ETS domain. Among them, the human erg gene has been found to be involved in Ewing's sarcoma, primitive neurectodermal tumour of childhood and acute myeloid leukaemia. Nevertheless, little is known about human erg expression. Northern blot analyses have shown a human erg expression restricted to few cell lines and thymus, but the status concerning expression during development remains unknown probably because no homologue of this gene has yet been isolated and studied in other vertebrates. We thus choose to clone the chicken erg gene (ck-erg) and to study its expression during chicken development. We obtained a bona fide clone of ck-erg and defined the transcriptional modulating properties of its product. The ck-Erg protein acts as a transcriptional activator through a conventional consensus ETS binding site. Northern blot studies on various chicken tissues, in situ analyses and comparison with the well-characterised c-ets-1 expression show that ck-erg is expressed in mesoderm- and, to a lesser extent, in ectoderm-derived tissues. During chicken development, two salient features could be observed. From stage E1 to E3.5, ck-erg expression was widely distributed in mesodermal derivatives and neural crest, resembling c-ets-1 expression. However, by E6, the expression of ck-erg exhibited, unlike c-ets-1, a drastically new and strong signal in precartilaginous condensation zones and cartilaginous skeletal primordia. These stages are the first steps of bone formation during skeletal elaboration. Our results show for the first time a possible specific involvement of ck-erg in cartilage morphogenesis.

Identification and characterization of a neuroretina-specific enhancer element in the quail Pax-6 (Pax-QNR) gene

Using nuclear run-on assays, we showed that the tissue-specific expression of quail Pax-6 (Pax-QNR) P0-initiated mRNAs is due in part to regulation of the gene at the transcriptional level. Regulatory sequences governing neuroretina-specific expression of the P0-initiated mRNAs were investigated. By using reporter-based expression assays, we characterized a region within the Pax-QNR gene, located 7.5 kbp downstream from the P0 promoter, that functions as an enhancer in neuroretina cells but not in nonexpressing P0-initiated mRNA cells (quail embryo cells and quail retinal pigment epithelial cells). This enhancer element functioned in a position- and orientation-independent manner both on the Pax-QNR P0 promoter and the heterologous thymidine kinase promoter. Moreover, this enhancer element exhibited a developmental stage-specific activity during embryonic neuroretina development: in contrast to activity at day E7, the enhancer activity was very weak at day E5. This paralleled the level of expression of P0-initiated mRNAs observed at the same stages. Using footprinting, gel retardation, and Southwestern (DNA-protein) analysis, we demonstrated the existence of four neuroretina-specific nuclear protein-binding sites, involving multiple unknown factors. In addition we showed that the quail enhancer element is structurally and functionally conserved in mice. All of these results strongly suggest that this enhancer element may contribute to the neuroretina-specific transcriptional regulation of the Pax-6 gene in vivo.

Transcriptional activation by the parvoviral nonstructural protein NS-1 is mediated via a direct interaction with Sp1

The nonstructural protein NS-1, encoded by the parvovirus minute virus of mice, is a potent regulator of viral gene expression. NS-1 does not bind DNA in a sequence-specific manner, and the mechanism by which it modulates viral promoter function is unclear. We have used Gal4-NS-1 fusion protein constructs to identify and characterize an activating domain encoded within the C-terminal 88 amino acids of NS-1 which competes effectively with the acidic activator domain of the herpes simplex virus VP16 protein. DNA affinity chromatography and immunoprecipitation experiments demonstrate that protein-protein interactions between the transcription factor Sp1 and NS-1 are required to bind NS-1 to promoter DNA in vitro. Cotransfection of Gal4-NS-1 and Sp1-VP16 acidic activator constructs into Drosophila melanogaster Schneider cells, which lack endogenous Sp1, stimulates transcription from a minimal promoter containing five Gal4 binding sites, while single-construct transfections do not. Cotransfection of Schneider cells with wild-type NS-1 and Sp1 constructs activates transcription from a simian virus 40 promoter 10- to 30-fold over that of either construct alone. Thus, Sp1-NS-1 interactions in vivo can stimulate transcription from a heterologous promoter containing Sp1 binding sites.

NF-Y controls transcription of the minute virus of mice P4 promoter through interaction with an unusual binding site

Electrophoretic mobility shift assays performed with nuclear extracts from human fibroblasts revealed the formation of two major protein complexes with an oligonucleotide (nucleotides 78 to 107) from the palindromic region located upstream from the minute virus of mice (MVM) P4 promoter. It was shown that this oligonucleotide bound USF at the enhancer E box CACATG. The second complex contained the transcription factor NF-Y, whose association was surprising because its target sequence lacks the canonical CCAAT motif present in all mammalian NF-Y binding sites identified so far. The MVM NF-Y recognition element instead contains the CCAAC sequence. USF and NF-Y had distinct but overlapping sequence requirements for binding, suggesting that their associations with MVM DNA were mutually exclusive. Because of the palindromic nature of MVM DNA terminal sequences, NF-Y associated with the three nucleotide configurations corresponding to the hairpin structure and to the external and internal arms of the extended duplex replication form, respectively. However, owing to the imperfection of the palindrome, the binding of USF was restricted to the internal arm. Point mutations that suppressed the in vitro binding of NF-Y to the internal palindromic arm reduced the activity of the resident P4 promoter, while those preventing complex formation with USF did not, as determined by transient expression assays using the luciferase reporter gene. The data led to the identification of a novel P4 upstream regulatory region capable of interacting with two transcription factors, from which one (NF-Y) appeared to upmodulate the activity of the promoter.

Use of an autonomous parvovirus vector for selective transfer of a foreign gene into transformed human cells of different tissue origins and its expression therein

In this work, we report the transduction of a chloramphenicol acetyltransferase (CAT) reporter gene into a variety of normal and transformed human cells of various tissue origins. The vector used was MVM/P38cat, a recombinant of the prototype strain of the autonomous parvovirus minute virus of mice (MVMp). The CAT gene was inserted into the capsid-encoding region of the infectious molecular clone of MVMp genome, under the control of the MVM P38 promoter. When used to transfect permissive cells, the MVM/P38cat DNA was efficiently replicated and expressed the foreign CAT gene at high levels. By cotransfecting with a helper plasmid expressing the capsid proteins, it was possible to produce mixed virus stocks containing MVM/P38cat infectious particles and variable amounts of recombinant MVM. MVM/P38cat viral particles were successfully used to transfer the CAT gene and to express it in a variety of human cells. Both viral DNA replication and P38-driven CAT expression were achieved in fibroblasts, epithelial cells, T lymphocytes, and macrophages in a transformation-dependent way, but with an efficiency depending on the cell type. In transformed B lymphocytes, however, the vector was not replicated, nor did it express the CAT gene.

Lack of a detectable effect of capsid proteins on the cell-dependent activity of parvovirus MVMp promoters

Deletion of upstream elements from the early P4 promoter of parvovirus MVM (minute virus of mice, prototype strain MVMp) has a differential effect on its activity, depending on the host cell considered. This indicates that upstream motifs participate in the control of promoter P4 functioning and are responsive to factors which are, at least in part, cell-type specific. In contrast with other viral systems, the capsid proteins of MVMp had no detectable effect on gene expression driven by either the early P4 or late P38 promoter of MVMp in permissive and non-permissive cells.

Protein species of the parvovirus minute virus of mice strain MVMp: involvement of phosphorylated VP-2 subtypes in viral morphogenesis

The pattern of induced protein species of the prototype strain of the parvovirus minute virus of mice was determined in permissive A9 mouse fibroblast cells by high-resolution two-dimensional gel electrophoresis. Identities of the viral proteins in the gels were assigned by probing two-dimensional blots with antisera raised against either purified capsids (recognizing VP-1 and VP-2) or specific coding regions of the nonstructural proteins (NS-1 and NS-2) expressed as beta-galactosidase fusion products in bacteria. All viral proteins showed posttranslational modifications, phosphate being a common substituent. The NS-1 protein migrated as a basic polypeptide in the pI range of 7.4 to 7.8 with multiple stages of modification and as a likely minor but hyperphosphorylated component in the neutral region of the gel. The NS-2 isoforms were resolved at a pI value close to 5.5 as three groups of unevenly phosphorylated polypeptides, each composed of at least two protein species. Both VP-1 and VP-2 structural polypeptides were induced as heterogeneous phosphoproteins. The major VP-2 protein could be resolved in the form of a consistent pattern of three abundant (a to c), two intermediate (d and e), and one meager (f) neutral isoelectric focusing species or subtypes. This posttranslational modification precedes and is uncoupled from viral assembly, and all of the VP-2 subtypes are packaged into empty capsids at the induced stoichiometry. However, intracellular full virions harbored additional phosphorylated subtypes (g to l) and a subtle rearrangement in the whole VP-2 composition, while mature virions purified from lysed cultures lacked these subtypes, coordinately with the emergence of six neutral VP-3 subtypes. Thus, the virion coat undergoes a chemical transition entailed by genome encapsidation, in which phosphates seem to play a major role, triggering the preferential proteolytic cleavage of the more acidic VP-2 subtypes to VP-3. Parvoviruses, with small coding capacity, may regulate some morphogenetic steps, such as assembly, genome encapsidation, and maturation, by posttranslational modifications of their structural proteins.