Targeted mutation of the murine arylhydrocarbon receptor nuclear translocator 2 (Arnt2) gene reveals partial redundancy with Arnt

The ubiquitously expressed basic helix–loop–helix (bHLH)-PAS protein ARNT (arylhydrocarbon receptor nuclear transporter) forms transcriptionally active heterodimers with a variety of other bHLH-PAS proteins, including HIF-1α (hypoxia-inducible factor-1α) and AHR (arylhydrocarbon receptor). These complexes regulate gene expression in response to hypoxia and xenobiotics, respectively, and mutation of the murine Arnt locus results in embryonic death by day 10.5 associated with placental, vascular, and hematopoietic defects. The closely related protein ARNT2 is highly expressed in the central nervous system and kidney and also forms complexes with HIF-1α and AHR. To assess unique roles for ARNT2 in development, and reveal potential functional overlap with ARNT, we generated a targeted null mutation of the murine Arnt2 locus. Arnt2−/− embryos die perinatally and exhibit impaired hypothalamic development, phenotypes previously observed for a targeted mutation in the murine bHLH-PAS gene Sim1 (Single-minded 1), and consistent with the recent proposal that ARNT2 and SIM1 form an essential heterodimer in vivo [Michaud, J. L., DeRossi, C., May, N. R., Holdener, B. C. & Fan, C. (2000) Mech. Dev. 90, 253–261]. In addition, cultured Arnt2−/− neurons display decreased hypoxic induction of HIF-1 target genes, demonstrating formally that ARNT2/HIF-1α complexes regulate oxygen-responsive genes. Finally, a strong genetic interaction between Arnt and Arnt2 mutations was observed, indicating that either gene can fulfill essential functions in a dose-dependent manner before embryonic day 8.5. These results demonstrate that Arnt and Arnt2 have both unique and overlapping essential functions in embryonic development.

Onset of natural killer cell lymphomas in transgenic mice carrying a truncated HMGI-C gene by the chronic stimulation of the IL-2 and IL-15 pathway

Rearrangements of the high mobility group protein I-C (HMGI-C) gene, consisting in the loss of the carboxyl-terminal tail, have been frequently detected in benign human tumors of mesenchymal origin. We have previously demonstrated that transgenic (TG) mice carrying a truncated HMGI-C construct (HMGI-C/T) exhibit a giant phenotype together with a predominantly abdominal/pelvic lipomatosis. Here, we report that HMGI-C/T TG mice develop natural killer (NK)-T/NK cell lymphomas starting from 12 months of age. We found an increased expression of IL-2 and IL-15 proteins and their receptors in these lymphomas, and we demonstrate that HMGI-C/T protein positively regulates their expression in vitro. Therefore, the HMGI-C/T-mediated chronic stimulation of the IL-2/IL-15 pathway could be responsible for the onset of NK-T/NK cell lymphomas in HMGI-C/T TG mice.

Complex regulation of human inducible nitric oxide synthase gene transcription by Stat 1 and NF-κB

The human inducible nitric oxide synthase (hiNOS) gene is expressed in several disease states and is also important in the normal immune response. Previously, we described a cytokine-responsive enhancer between −5.2 and −6.1 kb in the 5′-flanking hiNOS promoter DNA, which contains multiple nuclear factor κβ (NF-κB) elements. Here, we describe the role of the IFN-Jak kinase-Stat (signal transducer and activator of transcription) 1 pathway for regulation of hiNOS gene transcription. In A549 human lung epithelial cells, a combination of cytokines tumor necrosis factor-α, interleukin-1β, and IFN-γ (TNF-α, IL-1β, and IFN-γ) function synergistically for induction of hiNOS transcription. Pharmacological inhibitors of Jak2 kinase inhibit cytokine-induced Stat 1 DNA-binding and hiNOS gene expression. Expression of a dominant-negative mutant Stat 1 inhibits cytokine-induced hiNOS reporter expression. Site-directed mutagenesis of a cis-acting DNA element at −5.8 kb in the hiNOS promoter identifies a bifunctional NF-κB/Stat 1 motif. In contrast, gel shift assays indicate that only Stat 1 binds to the DNA element at −5.2 kb in the hiNOS promoter. Interestingly, Stat 1 is repressive to basal and stimulated iNOS mRNA expression in 2fTGH human fibroblasts, which are refractory to iNOS induction. Overexpression of NF-κB activates hiNOS promoter–reporter expression in Stat 1 mutant fibroblasts, but not in the wild type, suggesting that Stat 1 inhibits NF-κB function in these cells. These results indicate that both Stat 1 and NF-κB are important in the regulation of hiNOS transcription by cytokines in a complex and cell type-specific manner.

Lack of hepcidin gene expression and severe tissue iron overload in upstream stimulatory factor 2 (USF2) knockout mice

We previously reported the disruption of the murine gene encoding the transcription factor USF2 and its consequences on glucose-dependent gene regulation in the liver. We report here a peculiar phenotype of Usf2−/− mice that progressively develop multivisceral iron overload; plasma iron overcomes transferrin binding capacity, and nontransferrin-bound iron accumulates in various tissues including pancreas and heart. In contrast, the splenic iron content is strikingly lower in knockout animals than in controls. To identify genes that may account for the abnormalities of iron homeostasis in Usf2−/− mice, we used suppressive subtractive hybridization between livers from Usf2−/− and wild-type mice. We isolated a cDNA encoding a peptide, hepcidin (also referred to as LEAP-1, for liver-expressed antimicrobial peptide), that was very recently purified from human blood ultrafiltrate and from urine as a disulfide-bonded peptide exhibiting antimicrobial activity. Accumulation of iron in the liver has been recently reported to up-regulate hepcidin expression, whereas our data clearly show that a complete defect in hepcidin expression is responsible for progressive tissue iron overload. The striking similarity of the alterations in iron metabolism between HFE knockout mice, a murine model of hereditary hemochromatosis, and the Usf2−/− hepcidin-deficient mice suggests that hepcidin may function in the same regulatory pathway as HFE. We propose that hepcidin acts as a signaling molecule that is required in conjunction with HFE to regulate both intestinal iron absorption and iron storage in macrophages.

Functions of interleukin 1 receptor antagonist in gene knockout and overproducing mice.

Interleukin 1 receptor antagonist (IL-1ra) is a cytokine whose only known action is competitive inhibition of the binding of interleukin 1 (IL-1) to its receptor. To investigate the physiological roles of endogenously produced IL-1ra, we generated mice that either lack IL-1ra or overproduce it under control of the endogenous promoter. Mice lacking IL-1ra have decreased body mass compared with wild-type controls. They are more susceptible than controls to lethal endotoxemia but are less susceptible to infection with Listeria monocytogenes. Conversely, IL-1ra overproducers are protected from the lethal effects of endotoxin but are more susceptible to listeriosis. Serum levels of IL-1 following an endotoxin challenge are decreased in IL-1ra nulls and increased in IL-1ra overproducers in comparison to controls. These data demonstrate critical roles for endogenously produced IL-1ra in growth, responses to infection and inflammation, and regulation of cytokine expression.

Local production of the p40 subunit of interleukin 12 suppresses T-helper 1-mediated immune responses and prevents allogeneic myoblast rejection.

The p40 subunit of interleukin 12 (IL-12p40) has been known to act as an IL-12 antagonist in vitro. We here describe the immunosuppressive effect of IL-12p40 in vivo. A murine myoblast cell line, C2C12, was transduced with retro-virus vectors carrying the lacZ gene as a marker and the IL-12p40 gene. IL-12p40 secreted from the transfectant inhibited the IL-12-induced interferon gamma (IFN-gamma) production by splenocytes in vitro. Survival of C2C12 transplanted into allogeneic recipients was substantially prolonged when transduced with IL-12p40. Cytokine (IL-2 and IFN-gamma) production and cytotoxic T lymphocyte induction against allogeneic C2C12 were impaired in the recipients transplanted with the IL-12p40 transfectant. Delayed-type hypersensitivity response against C2C12 was also diminished in the IL-12p40 recipients. Furthermore, serum antibodies against beta-galactosidase of the T-helper 1-dependent isotypes (IgG2 and IgG3) were decreased in the IL-12p40 recipients. These results indicate that locally produced IL-12p40 exerts a potent immunosuppressive effect on T-helper 1-mediated immune responses that lead to allograft rejection. Therefore, IL-12p40 gene transduction would be useful for preventing the rejection of allografts and genetically modified own cells that are transduced with potentially antigenic molecules in gene therapy.

Suppression of tumorigenicity by the wild-type tuberous sclerosis 2 (Tsc2) gene and its C-terminal region.

The Tsc2 gene, which is mutationally inactivated in the germ line of some families with tuberous sclerosis, encodes a large, membrane-associated GTPase activating protein (GAP) designated tuberin. Studies of the Eker rat model of hereditary cancer strongly support the role of Tsc2 as a tumor suppressor gene. In this study, the biological activity of tuberin was assessed by expressing the wild-type Tsc2 gene in tumor cell lines lacking functional tuberin and also in rat fibroblasts with normal levels of endogenous tuberin. The colony forming efficiency of Eker rat-derived renal carcinoma cells was significantly reduced following reintroduction of wild-type Tsc2. Tumor cells expressing the transfected Tsc2 gene became more anchorage-dependent and lost their ability to form tumors in severe combined immunodeficient mice. At the cellular level, restoration of tuberin expression caused morphological changes characterized by enlargement of the cells and increased contact inhibition. As with the full-length Tsc2 gene, a clone encoding only the C terminus of tuberin (amino acids 1049-1809, including the GAP domain) was capable of reducing both colony formation and in vivo tumorigenicity when transfected into the Eker rat tumor cells. In normal Rat1 fibroblasts, conditional overexpression of tuberin also suppressed colony formation and cell growth in vitro. These results provide direct experimental evidence for the tumor suppressor function of Tsc2 and suggest that the tuberin C terminus plays an important role in this activity.

Interleukin 7 receptor-deficient mice lack gammadelta T cells.

The interleukin 7 receptor (IL-7R) plays a crucial role in early B- and T-cell development. It consists of a unique a chain and a common gamma chain [IL-2 receptor gamma chain (IL-2Rgamma)]. Gene inactivation of IL-7, IL-7R, and IL-2Rgamma resulted in severe impairment of B and T lymphopoiesis in mice. In addition, IL-2Rgamma-deficient mice lack gammadelta T cells in the skin and have the impaired development of natural killer (NK) cells and intraepithelial lymphocytes. To explore the role of IL-7/IL-7R system in gammadelta T- and NK-cell development, we have generated and analyzed IL-7R-deficient mice. gammadelta T cells were absent from skin, gut, liver, and spleen in the deficient mice. In contrast, alphabeta T and B cells were detected in reduced, but certain, numbers, and NK cells developed normally. The gammadelta T-cell development in fetal and adult thymus was also completely blocked. These results clearly demonstrate that the signal from IL-7R is indispensable for gammadelta T-cell development in both thymic and extrathymic pathways. On the contrary, it is suggested that NK-cell development requires cytokine(s) other than IL-7.

Cell-type and promoter-context dependent retinoic acid receptor (RAR) redundancies for RAR beta 2 and Hoxa-1 activation in F9 and P19 cells can be artefactually generated by gene knockouts.

By using RAR type (alpha, beta, or gamma)-specific synthetic retinoids and a pan-retinoic X receptor (RXR)-specific ligand, we have investigated the contribution of RARs and RXRs in the activation of RA target genes and the differentiation of embryonal carcinoma cells. We demonstrate cell-type- and promoter context-dependent functional redundancies that differ between the three RAR types for mediating the induction of RARbeta2 and Hoxa-1 in wild-type, RARgamma-/- and RARalpha-/- F9 cells and in P19 cells. The extent of redundancy between RARs is further modulated by the synergistic activation of RXRs with a pan-RXR agonist. We also demonstrate that the expression of RARbeta2 is auto-inducible in RARgamma-/- but not in wild-type F9 cells, indicating that the functional redundancies observed between RARs in gene disruption studies can be artefactually generated. Thus, even though all three RARs can functionally substitute each other for inducing the expression of RA target genes and cell differentiation, one RAR can cell-specifically override the activity of the other RARs. Interestingly, only RARgamma can mediate the retinoic acid-induced differentiation of wild-type F9 cells, whereas the differentiation of P19 cells can be mediated by either RARalpha or RARgamma.

Cloning and expression in Escherichia coli of the OGG1 gene of Saccharomyces cerevisiae, which codes for a DNA glycosylase that excises 7,8-dihydro-8-oxoguanine and 2,6-diamino-4-hydroxy-5-N-methylformamidopyrimidine.

A spontaneous mutator strain of Escherichia coli (fpg mutY) was used to clone the OGG1 gene of Saccharomyces cerevisiae, which encodes a DNA glycosylase activity that excises 7,8-dihydro-8-oxoguanine (8-OxoG). E. coli (fpg mutY) was transformed by a yeast DNA library, and clones that showed a reduced spontaneous mutagenesis were selected. The antimutator activity was associated with pYSB10, an 11-kbp recombinant plasmid. Cell-free extracts of E. coli (fpg mutY) harboring pYSB10 possess an enzymatic activity that cleaves a 34-mer oligonucleotide containing a single 8-oxoG opposite a cytosine (8-OxoG/C). The yeast DNA fragment of 1.7 kbp that suppresses spontaneous mutagenesis and overproduces the 8-OxoG/C cleavage activity was sequenced and mapped to chromosome XIII. DNA sequencing identified an open reading frame, designated OGG1, which encodes a protein of 376 amino acids with a molecular mass of 43 kDa. The OGG1 gene was inserted in plasmid pUC19, yielding pYSB110. E. coli (fpg) harboring pYSB110 was used to purify the Ogg1 protein of S. cerevisiae to apparent homogeneity. The Ogg1 protein possesses a DNA glycosylase activity that releases 8-OxoG and 2,6-diamino-4-hydroxy-5-N-methylformamidopyrimidine. The Ogg1 protein preferentially incises DNA that contains 8-OxoG opposite cytosine (8-OxoG/C) or thymine (8-OxoG/T). In contrast, Ogg1 protein does not incise the duplex where an adenine is placed opposite 8-OxoG (8-OxoG/A). The mechanism of strand cleavage by Ogg1 protein is probably due to the excision of 8-OxoG followed by a beta-elimination at the resulting apurinic/apyrimidinic site.

Myocyte-specific enhancer factor 2 acts cooperatively with a muscle activator region to regulate Drosophila tropomyosin gene muscle expression.

MEF2 (myocyte-specific enhancer factor 2) is a MADS box transcription factor that is thought to be a key regulator of myogenesis in vertebrates. Mutations in the Drosophila homologue of the mef2 gene indicate that it plays a key role in regulating myogenesis in Drosophila. We show here that the Drosophila tropomyosin I (TmI) gene is a target gene for mef2 regulation. The TmI gene contains a proximal and a distal muscle enhancer within the first intron of the gene. We show that both enhancers contain a MEF2 binding site and that a mutation in the MEF2 binding site of either enhancer significantly reduces reporter gene expression in embryonic, larval, and adult somatic body wall muscles of transgenic flies. We also show that a high level of proximal enhancer-directed reporter gene expression in somatic muscles requires the cooperative activity of MEF2 and a cis-acting muscle activator region located within the enhancer. Thus, mef2 null mutant embryos show a significant reduction but not an elimination of TmI expression in the body wall myoblasts and muscle fibers that are present. Surprisingly, there is little effect in these mutants on TmI expression in developing visceral muscles and dorsal vessel (heart), despite the fact that MEF2 is expressed in these muscles in wild-type embryos, indicating that TmI expression is regulated differently in these muscles. Taken together, our results show that mef2 is a positive regulator of tropomyosin gene transcription that is necessary but not sufficient for high level expression in somatic muscle of the embryo, larva, and adult.

Cell cycle-regulated binding of nuclear proteins to elements within a mouse H3.2 histone gene.

The histone gene family in mammals consists of 15-20 genes for each class of nucleosomal histone protein. These genes are classified as either replication-dependent or -independent in regard to their expression in the cell cycle. The expression of the replication-dependent histone genes increases dramatically as the cell prepares to enter S phase. Using mouse histone genes, we previously identified a coding region activating sequence (CRAS) involved in the upregulation of at least two (H2a and H3) and possibly all nucleosomal replication-dependent histone genes. Mutation of two seven-nucleotide elements, alpha and omega, within the H3 CRAS causes a decrease in expression in stably transfected Chinese hamster ovary cells comparable with the effect seen upon deletion of the entire CRAS. Further, nuclear proteins interact in a highly specific manner with nucleotides within these sequences. Mutation of these elements abolishes DNA/protein interactions in vitro. Here we report that the interactions of nuclear factors with these elements are differentially regulated in the cell cycle and that protein interactions with these elements are dependent on the phosphorylation/dephosphorylation state of the nuclear factors.

Targeted gene disruption of Hsp70-2 results in failed meiosis, germ cell apoptosis, and male infertility.

In addition to the five 70-kDa heat shock proteins (HSP70) common to germ cells and somatic tissues of mammals, spermatogenic cells synthesize HSP70-2 during meiosis. To determine if this unique stress protein has a critical role in meiosis, we used gene-targeting techniques to disrupt Hsp70-2 in mice. Male mice homozygous for the mutant allele (Hsp70-2 -/-) did not synthesize HSP70-2, lacked postmeiotic spermatids and mature sperm, and were infertile. However, neither meiosis nor fertility was affected in female Hsp70-2 -/- mice. We previously found that HSP70-2 is associated with synaptonemal complexes in the nucleus of meiotic spermatocytes from mice and hamsters. While synaptonemal complexes assembled in Hsp70-2 -/- spermatocytes, structural abnormalities became apparent in these cells by late prophase, and development rarely progressed to the meiotic divisions. Furthermore, analysis of nuclei and genomic DNA indicated that the failure of meiosis in Hsp70-2 -/- mice was coincident with a dramatic increase in spermatocyte apoptosis. These results suggest that HSP70-2 participates in synaptonemal complex function during meiosis in male germ cells and is linked to mechanisms that inhibit apoptosis.