Differential regulation of the N-myc gene in transfected cells and transgenic mice

Cytogenetic Assessment of the Rat Cell Line CLS-ACI-1: An in vitro Cell Model for Mycn Overexpression

Breast cancer is a complex and heterogeneous disease, and the establishment of cell models in order to properly study the disease at the molecular and cellular level is of utmost importance. Here, we present the cytogenetic characterization and gene expression analysis of the tumoral mammary rat cell line CLS-ACI-1. The use of banding and molecular cytogenetic techniques allowed the description of the complex CLS-ACI-1 karyotype and the identification of breakpoints in clonal chromosome rearrangements. Moreover, a Mycn and Erbb2 comparative expression analysis by RT-qPCR was performed, revealing a high expression level of Mycn in CLS-ACI-1 cells. Moreover, a considerable number of putative mutated genes and chromosome alterations detected through cytogenetic analysis seem to be in the MYCN biological network. Therefore, the CLS-ACI-1 cell line is presented as a promising cell model for the study of the role of MYCN in breast cancer and also as a tool for developing appropriate cancer therapies, namely for Mycn targeting.

Mammalian MYC Proteins and Cancer

The MYC family of proteins is a group of basic-helix-loop-helix-leucine zipper transcription factors that feature prominently in cancer. Overexpression of MYC is observed in the vast majority of human malignancies and promotes an extraordinary set of changes that impact cell proliferation, growth, metabolism, DNA replication, cell cycle progression, cell adhesion, differentiation, and metastasis. The purpose of this review is to introduce the reader to the mammalian family of MYC proteins, highlight important functional properties that endow them with their potent oncogenic potential, describe their mechanisms of action and of deregulation in cancer cells, and discuss efforts to target the unique properties of MYC, and of MYC-driven tumors, to treat cancer.

YY1 negatively regulates mouse myelin proteolipid protein (Plp1) gene expression in oligodendroglial cells

YY1 (Yin and Yang 1) is a multifunctional, ubiquitously expressed, zinc finger protein that can act as a transcriptional activator, repressor, or initiator element binding protein. Previous studies have shown that YY1 modulates the activity of reporter genes driven by the myelin PLP (proteolipid protein) (PLP1/Plp1) promoter. However, it is known that Plp1 intron 1 DNA contains regulatory elements that are required for the dramatic increase in gene activity, coincident with the active myelination period of CNS (central nervous system) development. The intron in mouse contains multiple prospective YY1 target sites including one within a positive regulatory module called the ASE (anti-silencer/enhancer) element. Results presented here demonstrate that YY1 has a negative effect on the activity of a Plp1-lacZ fusion gene [PLP(+)Z] in an immature oligodendroglial cell line (Oli-neu) that is mediated through sequences present in Plp1 intron 1 DNA. Yet YY1 does not bind to its alleged site in the ASE (even though the protein is capable of recognizing a target site in the promoter), indicating that the down-regulation of PLP(+)Z activity by YY1 in Oli-neu cells does not occur through a direct interaction of YY1 with the ASE sequence. Previous studies with Yy1 conditional knockout mice have demonstrated that YY1 is essential for the differentiation of oligodendrocyte progenitors. Nevertheless, the current study suggests that YY1 functions as a repressor (not an activator) of Plp1 gene expression in immature oligodendrocytes. Perhaps YY1 functions to keep the levels of PLP in check in immature cells before vast quantities of the protein are needed in mature myelinating oligodendrocytes.

Positive auto-regulation of MYCN in human neuroblastoma

MYCN oncogene is one of the most important regulators affecting the prognosis of neuroblastoma and is frequently amplified in the high-risk subsets. Despite its clinical significance, it remains unclear how the MYCN expression is regulated in human neuroblastomas. Here, we found the presence of a positive auto-regulatory mechanism of MYCN. Enforced expression of MYCN induced endogenous MYCN mRNA expression in SK-N-AS neuroblastoma cells with a single copy of MYCN gene. Luciferase reporter assay revealed that MYCN protein activates its own promoter activity in a dose-dependent manner and the downstream region relative to the transcription start sites is responsible for the activation. Furthermore, ChIP analysis showed that MYCN is directly recruited onto the intron 1 region of MYCN gene which contains two putative E-box sites. Intriguingly, in response to all-trans-retinoic acid (ATRA), MYCN was down-regulated in MYCN-amplified SK-N-BE neuroblastoma cells, and the recruitment of MYCN protein onto its own intron 1 region was reduced in association with an induction of neuronal differentiation. Collectively, our present results suggest that MYCN contributes to its own expression by forming a positive auto-regulatory loop in neuroblastoma cells.

MYCN as a target for cancer immunotherapy

MYCN is a potential target for cancer immunotherapy by virtue of its overexpression in numerous human malignancies and its functional role in tumour progression. Here we show limited expression of MYCN in normal human tissues indicating that anti-MYCN immune responses are unlikely to cross react with self tissues. An HLA-A2 restricted ten amino acid peptide epitope from MYCN, VILKKATEYV, was used to stimulate cytotoxic T cell lines from the peripheral blood of normal blood donors, and from a patient with MYCN amplified neuroblastoma. Strong and specific activity was seen against each MYCN overexpressing cell line and against autologous tumour cells. We generated two CTL clones capable of killing cells pulsed with as low as 0.5 nM of VIL peptide. Therefore strong and specific immune responses against MYCN expressing tumours are possible in patients with the most common HLA class 1 type in the Caucasian population.

Retinoic acid induced downregulation of MYCN is not mediated through changes in Sp1/Sp3

BACKGROUND

Use of retinoic acid (RA) has become the standard of care in the treatment of high risk neuroblastoma (NB). In vitro, RA induces growth arrest and differentiation, an effect that likely underlies its activity in the clinical setting. An important event in differentiation is the transcriptional downregulation of the MYCN oncogene, which is frequently activated in aggressive tumors. While it is known that Sp1/Sp3 and E2F are necessary to drive basal MYCN expression, the mechanism for its downregulation by RA remains enigmatic. Changes in E2F binding have been reported, however these occurred after the actual transcriptional response. Here, post-translational modifications of Sp proteins were examined as an alternate mechanism of RA-mediated promoter regulation.

PROCEDURE

Western blot was used to evaluate steady state levels of nuclear/cytoplasmic Sp1/Sp3. Promoter binding and DNA conformation were determined by gel shift, circular permutation, and chromatin immunoprecipitation assays. Immunoprecipitation/western and (32)P-phosphoamino analyses were used to detect glycosylation, acetylation, sumoylation, and phosphorylation.

RESULTS

RA did not affect the cellular level of Sp1/Sp3 proteins, their nuclear/cytoplasmic distribution, ability to bind the MYCN promoter, degree of Sp-induced DNA bending, or post-translational modifications.

CONCLUSIONS

MYCN RA response is not mediated solely though the region controlling basal activity. RA may be exerting its effects via multiple non-adjacent regulatory regions, potentially including basal motifs, either within the MYCN promoter or distally, on the same or even different chromosomes. Such cooperative trans-type DNA-protein interactions could explain the inaccessibility of this mechanism to the locus-specific approaches employed up to this point.

T-box genes coordinate regional rates of proliferation and regional specification during cardiogenesis

Mutations in T-box genes are the cause of several congenital diseases and are implicated in cancer. Tbx20-null mice exhibit severely hypoplastic hearts and express Tbx2, which is normally restricted to outflow tract and atrioventricular canal, throughout the heart. Tbx20 mutant hearts closely resemble those seen in mice overexpressing Tbx2 in myocardium, suggesting that upregulation of Tbx2 can largely account for the cardiac phenotype in Tbx20-null mice. We provide evidence that Tbx2 is a direct target for repression by Tbx20 in developing heart. We have also found that Tbx2 directly binds to the Nmyc1 promoter in developing heart, and can repress expression of the Nmyc1 promoter in transient transfection studies. Repression of Nmyc1 (N-myc) by aberrantly regulated Tbx2 can account in part for the observed cardiac hypoplasia in Tbx20 mutants. Nmyc1 is required for growth and development of multiple organs, including the heart, and overexpression of Nmyc1 is associated with childhood tumors. Despite its clinical relevance, the factors that regulate Nmyc1 expression during development are unknown. Our data present a paradigm by which T-box proteins regulate regional differences in Nmyc1 expression and proliferation to effect organ morphogenesis. We present a model whereby Tbx2 directly represses Nmyc1 in outflow tract and atrioventricular canal of the developing heart, resulting in relatively low proliferation. In chamber myocardium, Tbx20 represses Tbx2, preventing repression of Nmyc1 and resulting in relatively high proliferation. In addition to its role in regulating regional proliferation, we have found that Tbx20 regulates expression of a number of genes that specify regional identity within the heart, thereby coordinating these two important aspects of organ development.

E2F and Sp1/Sp3 Synergize but Are Not Sufficient to Activate the MYCN Gene in Neuroblastomas

Amplification of the MYCN gene, resulting in overexpression of MYCN, distinguishes a subset of neuroblastomas with poor prognosis. We recently identified MYCN as a target gene of the E2F transcription factors. Here we show that Sp1 and Sp3 cooperate with E2F-1 to activate the MYCN promoter. However, in a neuroblastoma cell line that does not express MYCN, overexpression of E2F-1 was not sufficient to activate the MYCN promoter even in the presence of trichostatin A and 5-aza-cytidine. This was because of a failure of E2F-1 to bind to the MYCN promoter in these cells, although access of E2F-1 to the inactive MYCN promoter was not blocked by a nucleosome. Differences in nucleosomal organization of the MYCN promoter in different cell lines did not correlate with gene activation per se but with the switch from basal to activated transcription. Binding of E2F and Sp1/Sp3 to the MYCN promoter in vivo correlated with acetylation of histones H3 and H4 and recruitment of RNA polymerase II and the protein acetyltransferase Tip60 but not with nucleosome remodeling. Our results define distinct chromatin states of the MYCN promoter, indicate that factors in addition to E2F and Sp1/Sp3 are required to activate MYCN in neuroblastomas, and provide evidence for a novel mechanism of controlling access of E2F to selected target genes.

N-myc oncogene expression in neuroblastoma is driven by Sp1 and Sp3

Regulation of N-myc oncogene expression is an important determinant of the biological behavior of neuroblastoma. The N-myc promoter contains several potential binding sites for transcription factors of the Sp1 family. Mutation of a CT-box motif contained within a 26 bp region required for N-myc downregulation by retinoic acid decreased basal transcriptional activity and altered DNA-protein interactions of the promoter, while mutations flanking this motif did neither. On super-shift, this region was shown to recruit Sp1 and Sp3 transcription factor proteins, while a functionally significant CT-box mutation resulted in their replacement by NF-1 transcription factor. Lysates from Drosophila S2 cells expressing exogenous Sp1, Sp3, and NF-1 proteins were able to partially mimic gel shift complexes seen with neuroblastoma nuclear extract and either wild type or mutant probes. Transient transfections of S2 cells showed that both individually and together, Sp1 and Sp3 were able to trans-activate a wild type CT-box-driven luciferase reporter construct in a dose-dependent manner. Transfection of the wild type but not mutant CT-box oligonucleotide was able to decrease endogenous N-myc expression in neuroblastoma cells. Together these results suggest that the CT-box element serves a critically functional role, and in the basal state, allows for N-myc trans-activation by Sp1 and Sp3. Moreover when mutated, the CT-box may still function as a binding motif for alternate transcription factors such as NF-1 that can allow persistent N-myc expression.

Effect of the Emu IgH enhancer on expression of a GFP reporter gene in transfected B cells and transgenic mice

Transgenic mice were generated to identify the first B cell maturation stage showing expression of an immunoglobulin transcriptional enhancer element (Emu)-green fluorescent protein (GFP) transgene, and to check the ability of the Emu element to behave as a locus control region. Flow cytometry experiments indicated that stably transfected 18-81 cells (a murine pre-B cell line) and A20 cells (a murine IgM(+) B cell line) maintained a constant GFP expression for several months in culture. Contrasting with in vitro results, flow cytometry experiments did not highlight GFP(+) B cells in spleen and bone marrow of Emu-GFP transgenic mice and no GFP transcripts were detected by Northern blot and reverse transcriptase polymerase chain reaction analysis. In transgenic mice, the lack of GFP expression seemed related to transgene DNA methylation occurring within all organs. Our results show dramatic differences for expression of the Emu-GFP transgene in vitro and in vivo. Despite that Emu was reported to efficiently control the in vivo expression of other associated transgenes, it is not sufficient to sustain GFP expression in transgenic mice and to counteract developmental silencing programs that occur in the embryo.

An alternative promoter of the human neuronal nitric oxide synthase gene is expressed specifically in Leydig cells

Neuronal nitric oxide synthase (nNOS) plays a modulatory role in the biology of a variety of neuroendocrine tissues and is especially relevant to gonadal function. We have previously reported the cloning and characterization of a variant of the nNOS protein, termed testis nNOS (TnNOS), the mRNA for which was restricted in expression to male gonadal tissues. To examine the cell-specificity of the testis-specific NOS regulatory regions we defined patterns of beta-galactosidase expression of an insertional transgene in which the reporter gene lacZ was under the transcriptional control of the human TnNOS promoter. beta-galactosidase activity was detected exclusively in the interstitial cells of the testis in transgenic mice. These cells also evidenced positive staining for nNOS protein and were identified as androgen-producing Leydig cells by staining with the Leydig cell marker, P(450)scc. Expression of the promoter was absent in cells of the seminiferous tubules, specifically germline cells of different stages and Sertoli cells. In contrast to the male gonad, beta-galactosidase activity was not detected in ovaries of adult female mice. Activity was also not evident in organs known to express full-length nNOS, such as skeletal muscle, kidney, or cerebellum. The same pattern of beta-galactosidase staining was observed in independent transgenic founders and was distinct from that observed for an endothelial NOS promoter/reporter transgene. In the testis of male adult eNOS promoter-reporter transgenic mice, beta-galactosidase activity was expressed only in endothelial cells of large- and medium-sized arterial blood vessels. Transcriptional activity of the human TnNOS promoter could not be detected in a variety of cell types, including Leydig cells, using episomal promoter-reporter constructs suggesting that a nuclear environment and higher order genomic complexity are required for appropriate promoter function. The restricted expression pattern of an nNOS variant in Leydig cells of the male gonad suggests an important role in the regulation of testosterone release and represents an intriguing model with which to dissect the molecular basis of Leydig cell-specific gene expression.

Identification of N-myc regulatory regions involved in embryonic expression

Our knowledge on the regulation of the N-myc proto-oncogene expression comes mostly from in vitro studies. Very few in vivo analyses have been performed to identify the regulatory elements involved in N-myc developmental expression. In the present study, we defined DNA regions required for the regulated expression of N-myc during early embryogenesis. We showed that the expression of N-myc driven by the human N-myc sequences previously described to control N-myc expression in appropriate cell types in vitro cannot rescue the mouse N-myc mutant phenotype, suggesting that regulatory elements necessary for N-myc embryonic expression were missing. To identify the regulatory DNA regions involved in N-myc expression, transgenic mouse lines carrying N-myc/lacZ reporter constructs were generated. Beta-galactosidase staining analysis at different stages of gestation revealed that >16 kb of mouse N-myc genomic sequences are required to recapitulate the entire spatiotemporal expression pattern of the endogenous N-myc gene between embryonic d 8.5 and 11.5. This observation supported the notion that the sequences previously identified by in vitro assays were not sufficient to reproduce the N-myc embryonic expression pattern. However, regulatory elements that can direct specific expression in the visceral arches, the limb buds, the CNS, and the dorsal root ganglia are included into the mouse N-myc genomic sequences tested. Altogether, these findings indicated that the regulation of the spatiotemporal expression pattern of N-myc during development necessitates multiple regulatory DNA elements.

The upstream regulatory regions of the hepatocyte growth factor gene promoter are essential for its expression in transgenic mice

To understand the molecular mechanisms of hepatocyte growth factor (HGF) gene transcription in vivo, we report the generation and characterization of transgenic mice harboring various lengths of the mouse HGF promoter linked to the chloramphenicol acetyltransferase reporter gene. Analysis of different tissues of the transgenic mouse lines having the 2.7-kilobase (kb) promoter construct revealed a pattern of reporter gene expression in embryonic and adult tissues that paralleled that of endogenous HGF gene expression. A similar expression pattern was observed in the 0.7-kb transgenic lines. However, in contrast to in vitro data, no promoter activity was detected in four independent transgenic lines harboring the 0.1-kb construct. Akin to the activity of the endogenous HGF gene, which is induced in the liver, lung, and spleen in response to 70% partial hepatectomy, the reporter gene driven by the 2.7-kb promoter construct was strongly induced, whereas that driven by the 0.7-kb promoter construct was modestly induced in these organs after partial hepatectomy. Together, these data suggest that the region between -0.1 and -0.7 kb of the HGF gene promoter is essential to drive its expression in vivo and that additional upstream sequences located between -0.7 and -2.7 kb are also necessary for its maximum inducibility in response to cues that stimulate tissue growth and regeneration.

Regulation of cell-type specific expression of lacZ by the 5'-flanking region of mouse GAD67 gene in the central nervous system of transgenic mice

The transcriptional regulation of the murine gene encoding the 67-kDa form of glutamic acid decarboxylase (GAD67) was studied by beta-galactosidase histochemistry in transgenic mice carrying fusion genes between progressively longer portions of the 5'-upstream regulatory region of GAD67 and E. coli lacZ. No expression was detected in brains of mice carrying 1.3 kb of upstream sequences including a housekeeping and two conventional promoters, and two negative regulatory elements with homology to known silencers. In mice carrying the same portion of the promoter region plus the first intron, lacZ expression in the adult central nervous system was found in few, exclusively neuronal sites. The number of correctly stained GABAergic centres increased dramatically with increasing the length of the 5'-upstream region included in the construct which suggests that multiple putative spatial enhancers are located in this region. Their action is influenced by epigenetic mechanisms that may be due to site-of-integration and transgene copy-number effects. Additional cis-acting elements are needed to obtain fully correct expression in all GABAergic neurons of the adult central nervous system.

Ret 4, a positive acting rhodopsin regulatory element identified using a bovine retina in vitro transcription system

Previous transgenic mouse studies demonstrated that the bovine rhodopsin sequence between -222 and +70 base pairs (bp) contains a minimal promoter, which is sufficient to direct photoreceptor cell-specific expression of a lacZ reporter gene. To more fully define the DNA regulatory elements and protein factors involved in regulating rhodopsin transcription, we have developed an in vitro transcription system derived from bovine retinal nuclear extracts. Retinal extracts, as compared to liver, HeLa, and Drosophila embryonic cell extracts, demonstrated preferential activity for the rhodopsin promoter. A template spanning the bovine rhodopsin upstream region from -590 to +15 bp showed significant activation relative to the basal activity seen with a TATA box containing -38 to +15 bp template. Deletion analysis indicated that the region between -85 and -38 bp contained significant positive regulatory activity. This activity was not observed with HeLa extracts, suggesting that it might be retina-specific. Systematic site-directed mutagenesis of the subregion from -64 to -38 bp indicated that sequences between -60 and -58 bp and between -48 and -40 bp harbor critical elements. The former sequence is part of the binding site for the retina-specific transcription factor Nrl, which has been implicated in rhodopsin regulation. Electrophoretic mobility shift assays showed that the latter sequence (-48 to -40 bp), and flanking DNA, designated Ret 4, is bound by both retina-specific and ubiquitously expressed protein factors. Shift assays with mutant oligomers further defined the putative recognition sequences for these protein factors. Together, our results suggest that multiple promoter elements and transcriptional factors are involved in regulating photoreceptor-specific rhodopsin transcription.

Adrenocortical-specific transgene expression directed by steroid hydroxylase gene promoters

The 5'-flanking regions of genes for three mouse adrenal steroid hydroxylases were analyzed for their ability to direct adrenal cortex-specific beta-galactosidase (beta-gal) reporter expression both in cell culture and transgenic mice. The 5'-flanking regions chosen were from the genes for steroid 21-hydroxylase (21-OHase), expressed throughout the adrenal cortex and mediating both glucocorticoid and mineralocorticoid synthesis, and aldosterone synthetase (AS) and steroid 11 beta-hydroxylase (11 beta-OHase), which catalyze respectively the terminal steps of mineralocorticoid synthesis in the zona glomerulosa and glucocorticoid synthesis in the zona fasciculata/reticularis. While 5.0 kb of 11 beta-OHase gene 5'-flanking region and 5.4 kb of the AS gene 5'-flanking region mediated respectively moderate and low levels of beta-gal reporter expression in Y1 adrenocortical tumor cells, neither of these 5'-flanking regions was able to direct reporter expression to the appropriate adrenocortical zone of transgenic mice. This suggests that additional regulatory elements, lying outside these 5'-flanking regions, are required for 11 beta-OHase and AS gene expression in the intact mouse. In contrast, 6.4 kb of the mouse 21-OHase A gene 5' flanking region was able to direct specific beta-galactosidase reporter expression, in both Y1 cells and transgenic mice, indicating that elements directing adrenal cortex-specific gene expression in vivo are located not more than 6.4 kb 5' of the 21-OHase gene transcription start site.

The beta-globin locus control region enhances transcription of but does not confer position-independent expression onto the lacZ gene in transgenic mice

The beta-globin locus control region (LCR) confers high levels of position-independent, copy number-dependent expression onto globin transgenes. Here > 40 independent transgenic mouse lines and founders that carried the LCR in cis with the beta-globin gene promoter driving a lacZ reporter gene were studied. Expression of the lacZ transgene was assayed by measuring beta-galactosidase enzyme activity in fetal liver extracts, the levels of which correlated with the quantity of lacZ mRNA determined using RNase protection assays. Unexpectedly, expression of the lacZ transgene was found to show strong position effects, varying as much as 700-fold per transgene copy. These position effects occurred even if the whole beta-globin gene was incorporated as part of the lacZ reporter gene. Moreover, DNase I-hypersensitive sites appeared in the transgene LCR in high expressing but not in low expressing lines, suggesting that the LCR itself was position dependent. In contrast, MEL cell clones, in which transcriptionally active integration sites were selected for, gave < 13-fold variation in expression per copy of an LCR-lacZ construct. These results show that the lacZ reporter affects the ability of the LCR to activate chromatin in mice and that culture cells are not an adequate model for position-independent gene expression studies.

Accessibility control of antigen-receptor variable-region gene assembly: role of cis-acting elements

Antigen receptor variable region genes are assembled from germline variable (V), diversity (D), and joining (J) gene segments. This process requires expression of V(D)J recombinase activity, and "accessibility" of variable gene segments to this recombinase. The exact mechanism by which variable gene segments become accessible during development is not known. However, several studies have shown that cis-acting elements that regulate transcription may also function to regulate accessibility. Here we review the evidence that transcriptional promoters, enhancers, and silencers are involved in regulation of accessibility. The manner in which these elements may combine to regulate accessibility is addressed. In addition, current and potential strategies for identifying and analyzing cis-acting elements that mediate locus accessibility are discussed.

pRb is necessary for inhibition of N-myc expression by TGF-beta 1 in embryonic lung organ cultures

The beta type transforming growth factors (TGF-beta) are potent inhibitors of epithelial cell proliferation, and data suggest that growth inhibition by TGF-beta 1 is mediated through suppression of Myc family genes in certain cell types. Indirect evidence has indicated that the product of the retinoblastoma gene (pRb) may also be involved in this pathway. Previously, we have shown that TGF-beta 1 inhibits branching morphogenesis and N-myc expression in mouse embryonic lung cultures. The purpose of this study was to determine the role of pRb in the inhibition of branching morphogenesis and N-myc expression by TGF-beta 1. Treatment with TGF-beta 1 was shown to inhibit development of lungs from homozygous Rb null (Rb−/−) and heterozygous null (Rb+/−) mouse embryos to the same extent as lungs from wild-type (Rb+/+) embryos. However, TGF-beta 1 treatment did not suppress N-myc expression in Rb−/− as it did in Rb+/+ embryonic lung explants as determined by in situ hybridization and quantitative RT-PCR. The effect of TGF-beta 1 treatment on N-myc expression in lungs from Rb+/− embryos was intermediate between that seen in Rb+/+ and Rb−/− embryos. Embryonic lungs derived from transgenic mice expressing the SV40 large T-antigen in lung epithelium under the control of the surfactant protein C promoter also showed inhibition of development in response to TGF-beta 1 treatment. The data demonstrate that pRb is necessary for TGF-beta 1 suppression of N-myc expression but not for TGF-beta 1 inhibition of branching morphogenesis; therefore, suppression of N-myc is not necessary for inhibition of branching morphogenesis by TGF-beta 1.

Indicator expression directed by regulatory sequences of the glial fibrillary acidic protein (GFAP) gene: in vivo comparison of distinct GFAP-lacZ transgenes

An increase in the expression of the glial fibrillary acidic protein (GFAP) gene by astrocytes appears to constitute a crucial component of the brain's response to injury because it is seen in many different species and features prominently in diverse neurological diseases. Previously, we have used a modified GFAP gene (C-339) to target the expression of beta-galactosidase (beta-gal) to astrocytes in transgenic mice (Mucke et al.; New Biol 3:465-474 1991). To determine to what extent the in vivo expression of GFAP-driven fusion genes is influenced by intragenic GFAP sequences, the E. coli lacZ reporter gene was either placed downstream of approximately 2 kb of murine GFAP 5' flanking region (C-259) or ligated into exon 1 of the entire murine GFAP gene (C-445). Transgenic mice expressing C-259 versus C-445 showed similar levels and distributions of beta-gal activity in their brains. Exclusion of intragenic GFAP sequences from the GFAP-lacZ fusion gene did not diminish injury-induced upmodulation of astroglial beta-gal expression or increase beta-gal expression in non-astrocytic brain cells. These results demonstrate that 2 kb of murine GFAP 5' flanking region is sufficient to restrict transgene expression primarily to astrocytes and to mediate injury-responsiveness in vivo. This sequence therefore constitutes a critical target for mediators of reactive astrocytosis. While acute penetrating brain injuries induced focal increases in beta-gal expression around the lesion sites in C-259, C-445, and C-339 transgenic mice, infection of C-339 transgenic mice with scrapie led to a widespread upmodulation of astroglial beta-gal expression. Hence, GFAP-lacZ transgenic mice can be used to monitor differential patterns of astroglial activation in vivo. These and related models should facilitate the assessment of strategies aimed at the in vivo manipulation of GFAP expression and astroglial activation.

TGF beta 1 inhibits branching morphogenesis and N-myc expression in lung bud organ cultures

Lung buds isolated from 11.5 days post coitum mouse embryos survive and undergo branching morphogenesis in culture. This organ culture system was used to examine the role of TGF beta 1 and N-myc expression in lung branching morphogenesis. By 24 hours, TGF beta 1 reversibly inhibited branching morphogenesis in a concentration-dependent manner. N-myc is known to be expressed during embryonic development in epithelial cells involved in branching morphogenesis and homozygous null N-myc mice have defects in lung development. In the present study, TGF beta 1 was shown to inhibit the steady-state level of N-myc RNA 3- to 4-fold at 14 and 48 hours of treatment as measured by northern blot and RNase protection analysis. Suppression of N-myc expression in epithelium was confirmed by in situ hybridization. Since inhibition of N-myc occurred prior to the observed changes in morphology and previous genetic studies have demonstrated and important role for N-myc in lung development, a model is proposed in which TGF beta 1 inhibits tracheobronchial development by inhibiting expression of N-myc.

Unusual topography of bovine rhodopsin promoter-lacZ fusion gene expression in transgenic mouse retinas

To define the cis-acting DNA elements required for rhodopsin expression, we generated lines of transgenic mice carrying sequences upstream of the bovine rhodopsin gene fused to the E. coli beta-galactosidase gene (lacZ). Upstream sequences extending from -2174 to +70 bp, from -734 to +70 bp, and from -222 to +70 bp direct photoreceptor-specific expression. All three -2174 lines demonstrate a superior-temporal to inferior-nasal gradient of expression across the retina, whereas lines carrying the shorter constructs demonstrate either spatially continuous expression across the retina, discrete clusters of expression, or both. As a complementary approach to defining regulatory elements, we compared DNA sequences 5' of the murine, bovine, and human rhodopsin genes. Significant homology between all three species was found just upstream of the transcription start site and at approximately 1.5 kb upstream.