An in vitro differentiation system for the examination of transgene activation in mouse macrophages

A crucial role for the ubiquitously expressed transcription factor Sp1 at early stages of hematopoietic specification

Mammalian development is regulated by the interplay of tissue-specific and ubiquitously expressed transcription factors, such as Sp1. Sp1 knockout mice die in utero with multiple phenotypic aberrations, but the underlying molecular mechanism of this differentiation failure has been elusive. Here, we have used conditional knockout mice as well as the differentiation of mouse ES cells as a model with which to address this issue. To this end, we examined differentiation potential, global gene expression patterns and Sp1 target regions in Sp1 wild-type and Sp1-deficient cells representing different stages of hematopoiesis. Sp1(-/-) cells progress through most embryonic stages of blood cell development but cannot complete terminal differentiation. This failure to fully differentiate is not seen when Sp1 is knocked out at later developmental stages. For most Sp1 target and non-target genes, gene expression is unaffected by Sp1 inactivation. However, Cdx genes and multiple Hox genes are stage-specific targets of Sp1 and are downregulated at an early stage. As a consequence, expression of genes involved in hematopoietic specification is progressively deregulated. Our work demonstrates that the early absence of active Sp1 sets a cascade in motion that culminates in a failure of terminal hematopoietic differentiation and emphasizes the role of ubiquitously expressed transcription factors for tissue-specific gene regulation. In addition, our global side-by-side analysis of the response of the transcriptional network to perturbation sheds a new light on the regulatory hierarchy of hematopoietic specification.

Selection-independent generation of gene knockout mouse embryonic stem cells using zinc-finger nucleases

Gene knockout in murine embryonic stem cells (ESCs) has been an invaluable tool to study gene function in vitro or to generate animal models with altered phenotypes. Gene targeting using standard techniques, however, is rather inefficient and typically does not exceed frequencies of 10(-6). In consequence, the usage of complex positive/negative selection strategies to isolate targeted clones has been necessary. Here, we present a rapid single-step approach to generate a gene knockout in mouse ESCs using engineered zinc-finger nucleases (ZFNs). Upon transient expression of ZFNs, the target gene is cleaved by the designer nucleases and then repaired by non-homologous end-joining, an error-prone DNA repair process that introduces insertions/deletions at the break site and therefore leads to functional null mutations. To explore and quantify the potential of ZFNs to generate a gene knockout in pluripotent stem cells, we generated a mouse ESC line containing an X-chromosomally integrated EGFP marker gene. Applying optimized conditions, the EGFP locus was disrupted in up to 8% of ESCs after transfection of the ZFN expression vectors, thus obviating the need of selection markers to identify targeted cells, which may impede or complicate downstream applications. Both activity and ZFN-associated cytotoxicity was dependent on vector dose and the architecture of the nuclease domain. Importantly, teratoma formation assays of selected ESC clones confirmed that ZFN-treated ESCs maintained pluripotency. In conclusion, the described ZFN-based approach represents a fast strategy for generating gene knockouts in ESCs in a selection-independent fashion that should be easily transferrable to other pluripotent stem cells.

Common themes and cell type specific variations of higher order chromatin arrangements in the mouse

Background

Similarities as well as differences in higher order chromatin arrangements of human cell types were previously reported. For an evolutionary comparison, we now studied the arrangements of chromosome territories and centromere regions in six mouse cell types (lymphocytes, embryonic stem cells, macrophages, fibroblasts, myoblasts and myotubes) with fluorescence in situ hybridization and confocal laser scanning microscopy. Both species evolved pronounced differences in karyotypes after their last common ancestors lived about 87 million years ago and thus seem particularly suited to elucidate common and cell type specific themes of higher order chromatin arrangements in mammals.

Results

All mouse cell types showed non-random correlations of radial chromosome territory positions with gene density as well as with chromosome size. The distribution of chromosome territories and pericentromeric heterochromatin changed during differentiation, leading to distinct cell type specific distribution patterns. We exclude a strict dependence of these differences on nuclear shape. Positional differences in mouse cell nuclei were less pronounced compared to human cell nuclei in agreement with smaller differences in chromosome size and gene density. Notably, the position of chromosome territories relative to each other was very variable.

Conclusion

Chromosome territory arrangements according to chromosome size and gene density provide common, evolutionary conserved themes in both, human and mouse cell types. Our findings are incompatible with a previously reported model of parental genome separation.

Regulation of stromal cell cyclooxygenase-2 in the Apc Min/+ mouse model of intestinal tumorigenesis

Cyclooxygenase-2 (Cox-2) is expressed predominantly by stromal cells in intestinal adenomas from the Apc(Min/+) mouse model of familial adenomatous polyposis. We investigated the mechanistic basis of stromal cell Cox-2 expression in Apc(Min/+) mouse adenomas, as well as Cox-2 expression and activity in histologically normal (HN) Apc(Min/+) mouse intestine, in order to gain further insights into regulation of Cox-2 as a potential chemoprevention target. Upregulation of Cox-2 in intestinal tumours is not an intrinsic feature of Apc(Min/+) macrophages as bone marrow-derived Apc(Min/+) macrophages did not exhibit an abnormality in Cox-2 expression or activity. Intestinal permeability to lactulose or mannitol was similar in Apc(Min/+) mice and wild-type littermates, implying that macrophage activation by luminal antigen is unlikely to explain stromal cell Cox-2 induction. Moreover, stromal cells exhibited differential expression of Cox-2 and inducible nitric oxide synthase, suggesting 'alternative' (M2) rather than 'classical' (M1) macrophage activation. Flow cytometric sorting of isolated stromal mononuclear cells (SMNCs), on the basis of M-lysozyme and specific macrophage marker expression, demonstrated that macrophages, neutrophils and non-myelomonocytic cells all contributed to lamina propria prostaglandin (PG) E(2) synthesis. However, the majority of PGE(2) synthesis by macrophages was via a Cox-2-dependent pathway compared with predominant Cox-1-derived PGE(2) production by non-myelomonocytic cells. SMNCs from HN Apc(Min/+) intestinal mucosa exhibited similar levels of Cox-2 mRNA and protein, but produced more Cox-2-derived PGE(2) than wild-type cells at 70 days of age. There was an age-dependent decline in PGE(2) synthesis by Apc(Min/+) SMNCs, despite tumour progression. These data suggest that other Cox-2-independent factors also control PGE(2) levels during Apc(Min/+) mouse intestinal tumorigenesis. Regulation of macrophage Cox-2 expression and other steps in PGE(2) synthesis (e.g. PGE synthase) are valid targets for novel chemoprevention strategies that could minimize or avoid systemic COX-2 inhibition.

Targeted modification of the complete chicken lysozyme gene by poxvirus-mediated recombination

We have developed a novel ex vivo system for the rapid one-step targeted modification of large eucaryotic DNA sequences. The highly recombinant environment resulting from infection of rabbit cornea cells with the Shope fibroma virus was exploited to mediate precise modifications of the complete chicken lysozyme gene domain (21.5 kb). Homologous recombination was designed to occur between target DNA (containing the complete lysozyme gene domain) maintained in a lambda bacteriophage vector and modified targeting DNA maintained in a plasmid. The targeting plasmids were designed to transfer exogenous sequences (for example, beta-galactosidase alpha-complement, green fluorescent protein, and hydrophobic tail coding sequences) to specific sites within the lysozyme gene domain. Cotransfection of the target phage and a targeting plasmid into Shope fibroma virus infected cells resulted in the poxvirus-mediated transfer of the modified sequences from plasmid to phage. Phage DNA (recombinant and nonrecombinant) was then harvested from the total cellular DNA by packaging into lambda phage particles and correct recombinants were identified. Four different gene-targeting pairings were carried out, and from 3% to 11% of the recovered phages were recombinant. Using this poxvirus-mediated targeting system, four different regions of the chicken lysozyme gene domain have been modified precisely by our research group overall with a variety of inserts (6-971 bp), deletions (584-3000 bp), and replacements. We have never failed to obtain the desired recombinant. Poxvirus-mediated recombination thus constitutes a routine, rapid, and remarkably efficient genetic engineering system for the precise modification of large eucaryotic gene domains when compared with traditional practices.

Dynamic reorganization of chromatin structure and selective DNA demethylation prior to stable enhancer complex formation during differentiation of primary hematopoietic cells in vitro

In order to gain insights in the true molecular mechanisms involved in cell fate decisions, it is important to study the molecular details of gene activation where such decisions occur, which is at the level of the chromatin structure of individual genes. In the study presented here we addressed this issue and examined the dynamic development of an active chromatin structure at the chicken lysozyme locus during the differentiation of primary myeloid cells from transgenic mouse bone marrow. Using in vivo footprinting we found that stable enhancer complex assembly and high-level gene expression are late events in cell differentiation. However, even before the onset of gene expression and stable transcription factor binding, specific chromatin alterations are observed. This includes changes in DNA topology and the selective demethylation of CpG dinucleotides located in the cores of critical transcription factor binding sites, but not in flanking DNA. These results firmly support the idea that epigenetic programs guiding blood cell differentiation are engraved into the chromatin of lineage-specific genes and that such chromatin changes are implemented before cell lineage specification.

Lack of inducible nitric oxide synthase promotes intestinal tumorigenesis in the Apc(Min/+) mouse

BACKGROUND & AIMS

The role of the inducible isoform of nitric oxide synthase (Nos2 or iNOS) in intestinal tumorigenesis is unclear. Conflicting data also exist regarding the ability of Nos2 to modulate expression and/or activity of cyclooxygenase 2 (Cox-2), which promotes intestinal tumorigenesis. Therefore, we determined the effect of a null Nos2 genotype on intestinal tumorigenesis and Cox-2 expression/activity in the Apc(Min/+) mouse model of familial adenomatous polyposis.

METHODS

Apc(Min/+)Nos2(-/-) mice were generated by successive crosses between C57BL/6-Apc(Min/+) and C57BL/6-Nos2(tm1Lau) mice. Adenoma characteristics of age-matched Apc(Min/+)Nos2(+/+) and Apc(Min/+)Nos2(-/-) mice were compared. The level and cellular localization of Nos2 messenger RNA (mRNA) expression in Apc(Min/+)Nos2(+/+) mouse intestine was determined. Cox-2 expression and activity were measured in both intestinal tissue and bone marrow-derived macrophages in vitro.

RESULTS

Apc(Min/+)Nos2(-/-) mice developed significantly more intestinal adenomas than Apc(Min/+)Nos2(+/+) littermates. Epithelial cell Nos2 mRNA expression was decreased in adenomas compared with histologically normal Apc(Min/+)Nos2(+/+) intestine. There was no significant difference in Cox-2 expression or activity in either intestine or bone marrow-derived macrophages from Apc(Min/+)Nos2(+/+) and Apc(Min/+)Nos2(-/-) animals.

CONCLUSIONS

Nos2 plays an antineoplastic role in the Apc(Min/+) mouse model of familial adenomatous polyposis. Nos2 does not modulate Cox-2 expression or activity in the Apc(Min/+) mouse.

Ontogenesis of protein kinase C betaII and its anchoring protein RACK1 in the maturation of alveolar macrophage functional responses

Alveolar macrophages are the resident airway cells primarily responsible for the protection of the lungs against inhaled toxins and other biologically active materials. The purpose of this study was to investigate the maturation with age of alveolar macrophage functional responses. We characterised the ontogenesis of PKC betaII and its anchoring protein RACK1 in correlation with PKC-dependent immune functions, such as TNF-alpha, hydrogen peroxide production and lysozyme release in resident alveolar macrophages obtained from rats 2, 4 and 12 weeks old. Our results show an age-associated increase in the expression of PKC betaII and RACK1, which correlated with a maturation of alveolar macrophage functional responses.

In vitro differentiation of c-myb(-/-) ES cells reveals that the colony forming capacity of unilineage macrophage precursors and myeloid progenitor commitment are c-Myb independent

Mice homozygous for an inactivated c-myb allele exhibit embryonic (primitive) blood formation but die at about day 15 of gestation because of a failure to generate adult (definitive) haemopoiesis. Recently, it has been shown that commitment to definitive haemopoiesis does occur in vivo, but that some point in the subsequent development towards the differentiated lineages is compromised. Here we have asked whether it is possible to demonstrate this same distinction between the development of primitive and definitive haemopoiesis during the in vitro differentiation of c-myb null ES cells, and whether this can be used to define more precisely at which developmental stage the absence of c-Myb blocks the adult haemopoietic lineages. We investigated the kinetics of progenitor formation and commitment to differentiation using a combination of colony forming assays and analysis of RNA and surface antigen expression. Primitive unilineage macrophage and erythroid precursor commitment could develop in the absence of c-Myb. No precursors characteristic of definitive haemopoiesis were detected; nevertheless, we could show the expression of a programme of transcription and surface antigens which is consistent with the appearance of definitive progenitors blocked at an early multipotential stage.

Origins and functions of phagocytes in the embryo

To review the data on the origins, phenotype, and function of embryonic phagocytes that has accumulated over past decade. Most of the relevant articles were selected based on the PubMed database entries. In additional, the Interactive Fly database (http://sdb.bio. purdue.edu/fly/aimain/1aahome.htm), FlyBase (http://flybase.bio. indiana.edu:82/), and TBase (http://tbase.jax.org/) were used to search for relevant information and articles. Phagocytes in a vertebrate embryo develop in two sites (yolk sac and liver) and contribute to organogenesis in part through their ability to recognize and clear apoptotic cells. Yolk sac-derived phagocytes differ in differentiation pathway and marker gene expression from macrophages produced via classic hematopoietic progenitors in the liver. We argue that yolk sac-derived phagocytes constitute a separate cell lineage. This conclusion raises the question of whether primitive phagocytes persist into the adulthood.

Herpesvirus saimiri-based gene delivery vectors maintain heterologous expression throughout mouse embryonic stem cell differentiation in vitro

In order to achieve a high efficiency of gene delivery into rare cell types like stem cells the use of viral vectors is presently without alternative. An ideal stem cell gene therapy vector would be able to infect primitive progenitor cells and sustain or activate gene expression in differentiated progeny. However, many viral vectors are inactivated when introduced in developing systems where cell differentiation occurs. To this end, we have developed a mouse in vitro model for testing herpesvirus saimiri (HVS)-based gene therapy vectors. We demonstrate here for the first time that HVS is able to infect totipotent mouse embryonic stem (ES) cells with high efficiency. We have transduced ES cells with a recombinant virus carrying the enhanced green fluorescent protein (EGFP) gene and the neomycin resistance gene (NeoR) driven by a CMV promoter and the SV40 promoter, respectively. ES cells maintain the viral episomal genome and can be terminally differentiated into mature haematopoietic cells. Moreover, heterologous gene expression is maintained throughout in vitro differentiation. Besides its obvious use in gene therapy, this unique expression system has wide ranging applications in studies aimed at understanding gene function and expression in cell differentiation and development.

In vitro-differentiated embryonic stem cell macrophages: a model system for studying atherosclerosis-associated macrophage functions

Monocytes/macrophages (Mo) appear to play a critical role in the initiation and progression of atherosclerotic lesions. In this study, we characterized in vitro-differentiated embryonic stem (ES) cell macrophages as a model system for studying atherosclerosis-associated Mo functions. Using immunofluorescence staining and Western analysis, we demonstrate that ES Mo express typical macrophage cell surface markers, as well as the known receptors for modified forms of low density lipoprotein (LDL), including the Mo scavenger receptors (SR-A type I and type II), CD36, and CD68. Differentiated ES Mo specifically bind and degrade 125I-labeled acetylated LDL with high affinity, and their incubation with acetylated LDL (15 microg/mL) for 48 hours produces characteristic "foamy" Mo, as visualized by oil red O staining. ES Mo also express matrix-degrading metalloproteinases (MMP-3, MMP-9), which have been implicated in collagen breakdown in the fibrous cap of atherosclerotic plaques, and secrete cytokines (tumor necrosis factor-alpha, interleukin-6) in response to inflammatory stimuli. Transfection experiments, using a green fluorescent protein reporter gene, driven by the myeloid-specific promoter, CD11b, demonstrated that ES Mo can also be used to study macrophage-restricted gene expression in vitro. Taken together, these data demonstrate that ES Mo exhibit many properties typical of arterial lesion macrophages. Its ease of genetic manipulation makes it an attractive system for investigations of macrophage functions in vitro.

Structure and function analysis of the human myeloid cell nuclear differentiation antigen promoter: evidence for the role of Sp1 and not of c-Myb or PU.1 in myelomonocytic lineage-specific expression

The human myeloid nuclear differentiation antigen (MNDA) is expressed specifically in maturing cells of the myelomonocytic lineage and in monocytes and granulocytes. Epitope enhancement was used to confirm the strict lineage- and stage-specific expression of MNDA in bone marrow as well as in other paraffin-embedded fixed tissues. A 1-kb region of the gene that includes 5' flanking sequence was reported earlier to contain functional promoter activity and was specifically demethylated in expressing cells in contrast to null cells. Further analysis has revealed that this 1-kb fragment promotes higher reporter gene activity in MNDA-expressing cells than non-expressing cells, indicating cell-specific differences in transactivation. This sequence contains consensus elements consistent with myeloid-specific gene expression, including a PU.1 consensus site near the major transcription start site and a cluster of c-Myb sites located several hundred bases upstream of this region. However, analysis of deletion mutants localized nearly all of the promoter activity to a short region (-73 to -16) that did not include the cluster of c-Myb sites. A 4-bp mutation of the core Sp1 consensus element (GC box) (-20) reduced overall promoter activity of the 1-kb fragment. Mutation of the PU.1 site did not significantly affect promoter activity. Only a small region (-35 to +22) including the Sp1 element and transcription start site, but not the PU.1 site was footprinted. The 4-bp mutation of the core Sp1 consensus element abolished footprinting at the site and an antibody super-shift reaction showed that Sp1 is one of the factors binding the consensus site. The Sp1 site also co-localizes with a DNase I hypersensitive site. The results indicate that DNA methylation, chromatin structure, and transactivation at an Sp1 site contribute to the highly restricted expression of this myelomonocytic lineage specific gene.

Role of positive and negative cis-regulatory elements in the transcriptional activation of the lysozyme locus in developing macrophages of transgenic mice

Expression of the chicken lysozyme locus in macrophages is regulated by at least six different positive and negative cis-regulatory elements. Chromatin of the chicken lysozyme locus is gradually reorganized during macrophage differentiation, indicating that each cis-regulatory element is activated at a different developmental stage. Irrespective of their differential developmental activation, individual cis-regulatory regions are capable of driving transcription of the lysozyme gene in mature macrophages of transgenic mice. In order to examine the role of different cis-regulatory regions in lysozyme locus activation, we analyzed the time course of transcriptional up-regulation of deletion mutants of the lysozyme locus in a new in vitro differentiation system based on enriched primary macrophage precursor cells from the bone marrow of transgenic mice. We show that constructs carrying cis-regulatory elements which are structurally reorganized early in development are also transcriptionally active at an early stage. A construct in which the early enhancer has been deleted shows a delay in transcriptional activation. The presence or absence of a negative regulatory element has no influence on the time course of transcriptional activation of the lysozyme locus.

Evolution of gene regulation as revealed by differential regulation of the chicken lysozyme transgene and the endogenous mouse lysozyme gene in mouse macrophages

Lysozyme gene expression is a marker for macrophage differentiation in vertebrates. We have previously shown that expression of the complete chicken lysozyme gene domain in macrophages of transgenic mice is directly correlated to the copy number of integrated transgenes. Thus, the chicken lysozyme locus in the mouse acts as an independent regulatory unit irrespective of its random position in the host genome. This finding allowed a comparative analysis of the regulation of the endogenous mouse lysozyme M gene and the chicken lysozyme transgene in the same animal. We demonstrate by transcript analysis of total tissue RNA and by in situ hybridization, that both genes are expressed in macrophages. In macrophages of the same animal the regulation of both genes in response to external signals was distinctly different: the lysozyme transgene responded to various agents influencing macrophage activation, in contrast, mouse lysozyme RNA levels remained unchanged under these conditions. Thus, as in chicken macrophages, the chicken lysozyme expression level in mouse macrophages is coupled to the macrophage activation status, while the mouse lysozyme is not. Our results suggest, that the cis-regulatory elements of lysozyme genes have evolved more rapidly than the function and expression of the trans-acting factors involved in the regulation of macrophage-specific gene activation.