Positive regulation of c-Myc by cohesin is direct, and evolutionarily conserved

Contact between sister chromatids from S phase to anaphase depends on cohesin, a large multi-subunit protein complex. Mutations in sister chromatid cohesion proteins underlie the human developmental condition, Cornelia de Lange syndrome. Roles for cohesin in regulating gene expression, sometimes in combination with CCCTC-binding factor (CTCF), have emerged. We analyzed zebrafish embryos null for cohesin subunit rad21 using microarrays to determine global effects of cohesin on gene expression during embryogenesis. This identified Rad21-associated gene networks that included myca (zebrafish c-myc), p53 and mdm2. In zebrafish, cohesin binds to the transcription start sites of p53 and mdm2, and depletion of either Rad21 or CTCF increased their transcription. In contrast, myca expression was strongly downregulated upon loss of Rad21 while depletion of CTCF had little effect. Depletion of Rad21 or the cohesin-loading factor Nipped-B in Drosophila cells also reduced expression of myc and Myc target genes. Cohesin bound the transcription start site plus an upstream predicted CTCF binding site at zebrafish myca. Binding and positive regulation of the c-Myc gene by cohesin is conserved through evolution, indicating that this regulation is likely to be direct. The exact mechanism of regulation is unknown, but local changes in histone modification associated with transcription repression at the myca gene were observed in rad21 mutants.

Expression of zebrafish cxcl8 (interleukin-8) and its receptors during development and in response to immune stimulation

Cxcl8 is a pro-inflammatory chemokine, best known for its role in neutrophil chemotaxis. Signalling through its receptors, Cxcr1 and Cxcr2, is induced by inflammatory stimuli evoked by microbial, chemical or environmental stress, and hormonal signals. While it is recognised that Cxcl8 signalling is active in the gut mucosa, this is not as well understood as its role in leukocyte trafficking. Here, we report the characterisation of genes encoding the zebrafish Cxcl8, Cxcr1 and Cxcr2. By a combination of genomic, expression and functional analyses, we show that the Cxcl8 signalling pathway is conserved in zebrafish. As in humans, cxcl8 is expressed in zebrafish leukocytes. Transcripts were also detected in intestinal epithelial cells, and this expression is upregulated under inflammatory conditions caused by bacterial or chemical insult. Expression of cxcr1 and cxcr2 is robust within the developing gut. This work provides a model for the study of Cxcl8 signalling during gut inflammation.

Intestinal differentiation in zebrafish requires Cdx1b, a functional equivalent of mammalian Cdx2

BACKGROUND & AIMS

The ParaHox transcription factor Cdx2 is an essential determinant of intestinal phenotype in mammals throughout development, influencing gut function, homeostasis, and epithelial barrier integrity. Cdx2 expression demarcates the zones of intestinal stem cell proliferation in the adult gut, with deregulated expression implicated in intestinal metaplasia and cancer. However, in vivo analysis of these prospective roles has been limited because inactivation of Cdx2 in mice leads to preimplantation embryonic lethality. We used the zebrafish, a valuable model for studying gut development, to generate a system to further understanding of the role of Cdx2 in normal intestinal function and in disease states.

METHODS

We isolated and characterized the zebrafish cdx1b ortholog and analyzed its function by antisense morpholino gene knockdown.

RESULTS

We showed that zebrafish Cdx1b replaces the role of Cdx2 in gut development. Evolutionary studies have indicated that the zebrafish cdx2 loci were lost following the genome-wide duplication event that occurred in teleosts. Zebrafish Cdx1b is expressed exclusively in the developing intestine during late embryogenesis and regulates intestinal cell proliferation and terminal differentiation.

CONCLUSIONS

This work established an in vivo system to explore further the activity of Cdx2 in the gut and its impact on processes such as inflammation and cancer.

Cohesin-dependent regulation of Runx genes

Runx transcription factors determine cell fate in many lineages. Maintaining balanced levels of Runx proteins is crucial, as deregulated expression leads to cancers and developmental disorders. We conducted a forward genetic screen in zebrafish for positive regulators of runx1 that yielded the cohesin subunit rad21. Zebrafish embryos lacking Rad21, or cohesin subunit Smc3, fail to express runx3 and lose hematopoietic runx1 expression in early embryonic development. Failure to develop differentiated blood cells in rad21 mutants is partially rescued by microinjection of runx1 mRNA. Significantly, monoallelic loss of rad21 caused a reduction in the transcription of runx1 and of the proneural genes ascl1a and ascl1b, indicating that downstream genes are sensitive to Rad21 dose. Changes in gene expression were observed in a reduced cohesin background in which cell division was able to proceed, indicating that cohesin might have a function in transcription that is separable from its mitotic role. Cohesin is a protein complex essential for sister chromatid cohesion and DNA repair that also appears to be essential for normal development through as yet unknown mechanisms. Our findings provide evidence for a novel role for cohesin in development, and indicate potential for monoallelic loss of cohesin subunits to alter gene expression.

Runx3 is required for hematopoietic development in zebrafish

We cloned zebrafish runx3/aml2/cbfa3 and examined its expression and function during embryogenesis. In the developing embryo, runx3 is dynamically expressed in hematopoietic, neuronal, and cartilaginous tissues. Hematopoietic expression of runx3 commences late in embryogenesis in the ventral tail intermediate cell mass and later colocalizes with spi1 and lyz in circulating blood cells. In the cloche mutant, hematopoietic expression was absent, suggesting that Runx3 functions downstream of cloche in a hematopoietic pathway. Neuronal tissues expressing runx3 include the trigeminal ganglia and Rohon-Beard neurons. Runx3 appears to contribute to normal development of primitive and definitive hematopoietic cells. When Runx3 function was compromised using morpholino oligonucleotides, a reduction in the number of mature blood cells was observed. Furthermore, Runx3 depletion decreased runx1 expression in the ventral wall of the dorsal aorta and reduced the number of spi1- and lyz-containing blood cells. Conversely, ubiquitous overexpression of runx3 led to an increase in primitive blood cell numbers, together with an increase in runx1-expressing cells in the ventral wall of the dorsal aorta. We propose a role for Runx3 in the regulation of blood cell numbers.

Pathways in blood and vessel development revealed through zebrafish genetics

Studies in zebrafish have potential to contribute to understanding of the vertebrate hematopoietic and vasculogenic systems. Our research has examined the roles of several molecules in pathways that lead to the development of blood and vessels in zebrafish, and has provided insights into the regulation of these processes. Gdf6a/radar, a member of the bone morphogenetic protein (BMP) family, is expressed in the zebrafish hypochord and primitive gut endoderm; structures that flank the developing dorsal aorta and posterior cardinal vein. This pattern of expression positions Gdf6a/radar as a candidate regulator of vasculogenesis. Support for such a role has come from experiments where Gdf6a/radar function was depleted with antisense morpholino oligonucleotides. This resulted in vascular leakiness, suggesting that Gdf6a/radar is involved in maintenance of vascular integrity. The transcription factor Runx1 is known to play a critical role in mammalian definitive hematopoiesis. When Runx1 expression domains and function were analyzed in zebrafish, the importance of this gene in definitive hematopoiesis was confirmed. However there was also evidence for a wider role, including involvement in vascular development and neuropoiesis. This work has laid the foundation for an ethylnitrosourea (ENU) mutagenesis screen based on runx1 whole-mount in situ hybridzation, that aims to identify genes operative in the runx1 pathway. An additional member of the Runx family, Runx3, is also involved in developmental hematopoiesis, with a function distinct from that of Runx1. We hypothesize that Runx1 and Runx3 form a continuum of transcriptional control within the hematopoietic system. An added attraction of zebrafish is that models of human disease can be generated, and we have shown that this system has potential for the study of Runx1-mediated leukemogenesis.

Radar is required for the establishment of vascular integrity in the zebrafish

The precise assembly of an integrated network of blood vessels is essential for the survival of vertebrate embryos. However, the processes by which primitive endothelial cells form mature vessels capable of supplying oxygen and nutrients to developing tissues remain incompletely understood. Here, we propose a role for Radar, one of the zebrafish orthologues of gdf6, in establishing integrity of the trunk vasculature in zebrafish embryos. We show that radar expression is appropriately placed, both spatially and temporally, to perform such a role. Transcripts for radar are detected in the hypochord and the primitive gut endoderm. These tissues intimately flank developing axial vessels in the trunk and have been previously implicated in the regulation of vascular development. Morpholino-based targeted gene knock-down has generated a Radar-specific loss-of-function zebrafish model. These embryos display normal initiation of vascular patterning and commencement of circulation. However, by day 2 of development, the integrity of the axial vasculature is compromised with hemorrhages and circulation short-circuits throughout the developing trunk. We show that this aberrant vascular development is specific to a reduction of the radar gene product. These results suggest that Radar is involved in a signaling pathway required for establishing the integrity of the axial vessels during zebrafish development.

Runx1 is required for zebrafish blood and vessel development and expression of a human RUNX1-CBF2T1 transgene advances a model for studies of leukemogenesis

RUNX1/AML1/CBFA2 is essential for definitive hematopoiesis, and chromosomal translocations affecting RUNX1 are frequently involved in human leukemias. Consequently, the normal function of RUNX1 and its involvement in leukemogenesis remain subject to intensive research. To further elucidate the role of RUNX1 in hematopoiesis, we cloned the zebrafish ortholog (runx1) and analyzed its function using this model system. Zebrafish runx1 is expressed in hematopoietic and neuronal cells during early embryogenesis. runx1 expression in the lateral plate mesoderm co-localizes with the hematopoietic transcription factor scl, and expression of runx1 is markedly reduced in the zebrafish mutants spadetail and cloche. Transient expression of runx1 in cloche embryos resulted in partial rescue of the hematopoietic defect. Depletion of Runx1 with antisense morpholino oligonucleotides abrogated the development of both blood and vessels, as demonstrated by loss of circulation, incomplete development of vasculature and the accumulation of immature hematopoietic precursors. The block in definitive hematopoiesis is similar to that observed in Runx1 knockout mice, implying that zebrafish Runx1 has a function equivalent to that in mammals. Our data suggest that zebrafish Runx1 functions in both blood and vessel development at the hemangioblast level, and contributes to both primitive and definitive hematopoiesis. Depletion of Runx1 also caused aberrant axonogenesis and abnormal distribution of Rohon-Beard cells, providing the first functional evidence of a role for vertebrate Runx1 in neuropoiesis.

To provide a base for examining the role of Runx1 in leukemogenesis, we investigated the effects of transient expression of a human RUNX1-CBF2T1 transgene [product of the t(8;21) translocation in acute myeloid leukemia] in zebrafish embryos. Expression of RUNX1-CBF2T1 caused disruption of normal hematopoiesis, aberrant circulation, internal hemorrhages and cellular dysplasia. These defects reproduce those observed in Runx1-depleted zebrafish embryos and RUNX1-CBF2T1 knock-in mice. The phenotype obtained with transient expression of RUNX1-CBF2T1 validates the zebrafish as a model system to study t(8;21)-mediated leukemogenesis.

In situ hybridization screen in zebrafish for the selection of genes encoding secreted proteins

An in situ hybridization expression screen using a signal sequence trap system has been conducted in zebrafish to isolate cDNAs that encode secreted proteins. Random clones (secreted expressed sequence tags; sESTs) were sequenced from zebrafish embryonic (18-24 hr postfertilization) and adult kidney libraries. From the two RNA sources, 627 random sEST cDNAs were identified as being homologous or identical to known genes and 166 clones encode currently unidentified genes. The sESTs represent a broad range of enzymes and other regulatory molecules. Whole-mount in situ hybridization analysis was carried out by using antisense probes generated from 244 selected sESTs, and a range of expression patterns was obtained. Genetic mapping undertaken with sEST sequences demonstrated that assignment of map position was attainable by using 5' primers. The signal sequence trap system used in this work has yielded a range of cDNAs that encode secreted proteins and, together with analysis of patterns of expression and genetic mapping, has the potential to facilitate analysis of signaling pathways central to development and physiology.

The smad proteins and TGFbeta signalling: uncovering a pathway critical in cancer

The critical role of TGFbeta in development and growth control is well established but the signalling pathway has only recently been elucidated. Identification of the Smads as TGFbeta's intracellular signalling mediators has led to an explosion of information on a novel signalling network that links the cell surface to the nucleus. Many cancers develop resistance to the growth-inhibitory effects of TGFbeta and mutations in signalling pathway components have been discovered that may underly tumour progression.

The TGF-beta type II receptor in chronic myeloid leukemia: analysis of microsatellite regions and gene expression

Genomic instability is one mechanism proposed to play a role in the disease progression of chronic myeloid leukemia (CML). Microsatellite regions in the type II transforming growth factor-beta receptor (TGF-beta RII) gene appear to be targets for mutation in some cancers displaying microsatellite instability (replication error phenotype, RER+). Furthermore, TGF-beta RII mutations in RER+ tumors have been associated with decreased TGF-beta RII mRNA levels. As TGF-beta is a potent negative growth regulator of hematopoietic cells, investigations were undertaken to determine whether inactivation of the receptor by microsatellite alteration might be involved in the progression of CML. Analysis of TGF-beta RII mRNA expression by RNase protection, with comparison of cells from the chronic, accelerated and blast phases of CML, showed no change in TGF-beta RII transcript levels during disease progression. However, during each phase of the disease, low levels of TGF-beta RII were detected when compared with the hematopoietic cells of normal donors. Furthermore, this decreased expression was also observed in the other myeloproliferative disorders, polycythemia rubra vera (PRV) and essential thrombocythemia (ET). The leukemia cell lines K562 and HL-60 had no detectable TGF-beta RII mRNA. Two microsatellite regions found altered in RER+ colon cancers were analyzed to establish if these sequences were aberrant in CML. No alteration was detected in either of these regions in any phase of the disease. These results suggest that alterations of the microsatellite regions in the TGF-beta RII gene are not involved in the progression of CML. Decreased expression of TGF-beta RII in CML cells and leukemia cell lines raises the possibility that altered expression of the receptor may play a role in the initiation and/or maintenance of the disease state.

Isolation of zebrafish gdf7 and comparative genetic mapping of genes belonging to the growth/differentiation factor 5, 6, 7 subgroup of the TGF-beta superfamily

The Growth/differentiation factor (Gdf) 5, 6, 7 genes form a closely related subgroup belonging to the TGF-beta superfamily. In zebrafish, there are three genes that belong to the Gdf5, 6, 7 subgroup that have been named radar, dynamo, and contact. The genes radar and dynamo both encode proteins most similar to mouse GDF6. The orthologous identity of these genes on the basis of amino acid similarities has not been clear. We have identified gdf7, a fourth zebrafish gene belonging to the Gdf5, 6, 7 subgroup. To assign correct orthologies and to investigate the evolutionary relationships of the human, mouse, and zebrafish Gdf5, 6, 7 subgroup, we have compared genetic map positions of the zebrafish and mammalian genes. We have mapped zebrafish gdf7 to linkage group (LG) 17, contact to LG9, GDF6 to human chromosome (Hsa) 8 and GDF7 to Hsa2p. The radar and dynamo genes have been localized previously to LG16 and LG19, respectively. A comparison of syntenies shared among human, mouse, and zebrafish genomes indicates that gdf7 is the ortholog of mammalian GDF7/Gdf7. LG16 shares syntenic relationships with mouse chromosome (Mmu) 4, including Gdf6. Portions of LG16 and LG19 appear to be duplicate chromosomes, thus suggesting that radar and dynamo are both orthologs of Gdf6. Finally, the mapping data is consistent with contact being the zebrafish ortholog of mammalian GDF5/Gdf5.

Expression and functional analysis of two isoforms of the human GM-CSF receptor alpha chain in myeloid development and leukaemia

The human granulocyte-macrophage colony-stimulating factor receptor (GM-CSFR) alpha chain RNA is alternatively spliced to yield receptor isoforms. Two of these, alpha 1 and alpha 2, differ in their cytoplasmic domains. Because the GM-CSFR beta chain (beta c) is shared with the receptors for interleukins 3 and 5 it is possible that the alpha chain confers specificity on the GM-CSF response and that the different isoforms might refine this response further. Studies have been directed at determination of the respective biological roles of the alpha 1 and alpha 2 isoforms. Expression of the isoforms was examined by RNase protection analysis in normal granulocytes and a variety of cell lines of haemopoietic origin, at different stages of differentiation and activation. Expression was also analysed in cells from patients with a variety of leukaemic subtypes. Results demonstrated that the relative abundance of the isoforms was similar in all cell populations examined. The human GM-CSFR alpha 1 or alpha 2 receptors were independently expressed in the murine factor-dependent cell line FDC-P1, so that the properties of the receptors could be compared. Cell lines that expressed either receptor could be converted to growth in response to human GM-CSF and assumed a more differentiated phenotype when compared with the parental cell line. However, the morphology, expression of cell surface antigens and dose-growth response characteristics did not differ significantly between cells that expressed either the alpha 1 or alpha 2 receptor. These studies demonstrate that the alpha 1 and alpha 2 subunits of the GM-CSF receptor are co-ordinately regulated in both normal and malignant haemopoiesis. Furthermore, each receptor is able to deliver both proliferative and differentiative signals to myeloid cells.

New insights into the control of cell growth; the role of the AxI family

AxI, Dtk and Mer are recently described receptors that constitute a new receptor tyrosine kinase subfamily. They bind the vitamin K-dependent protein growth-arrest-specific gene 6 (Gas6) that is structurally related to the anticoagulation factor protein S. Studies suggest a role for these receptors in developmental processes. In the function of the hematopoietic and nervous systems and in tumorigenesis.

Expression of murine interleukin 11 and its receptor alpha-chain in adult and embryonic tissues

Interleukin 11 (IL-11) is a multifunctional cytokine that has diverse effects on blood cells and their precursors and on a number of cell types outside of the hematopoietic system. The cDNAs encoding murine IL-11 and its receptor alpha-chain (IL-11R alpha) have recently been isolated. We have used the RNase protection assay to examine the expression of murine IL-11 and IL-11R alpha in a range of adult mouse tissues, in embryos, and during development of embryonic stem (ES) cells into cystic embryoid bodies in vitro. The testis showed a high level of IL-11 gene expression while a much lower level of expression was detected in the lung, stomach, small intestine, and large intestine. Expression of IL-11 was not detected between day 10.5 and day 18.5 post coitum of embryonic development or in differentiating ES cells in vitro. In contrast, the IL-11R alpha was found to be expressed in all adult tissues examined, during embryonic development, and in totipotent and differentiating ES cells.

Production of tumour necrosis factor-alpha by cultured human peripheral blood leucocytes in response to the anti-tumour agent 5,6-dimethylxanthenone-4-acetic acid (NSC 640488)

The investigative anti-tumour agent 5,6-dimethylxanthenonone-4-acetic acid (DMXAA, NSC 640488), developed in this laboratory as an improved analogue of flavone acetic acid (FAA, NSC 347512), is currently in clinical trial. The ability of DMXAA to up-regulate tumour necrosis factor (TNF) mRNA and protein synthesis in cultured human peripheral blood leucocytes (HPBLs) has been investigated and compared with that of flavone acetic acid (FAA) and of bacterial lipopolysaccharide (LPS). Human peripheral blood leucocytes were isolated from buffy coats obtained from a blood transfusion centre and also from blood samples from laboratory volunteers. At a concentration of 400 microg ml(-1) and an incubation time of 2 h, DMXAA up-regulated mRNA synthesis in six of eight individuals tested, as measured by Northern blotting. The degree of up-regulation varied in different individuals from one to nine times that of control levels. In contrast, FAA caused no induction above that of control levels and in some cases suppressed expression relative to controls, extending previous data that DMXAA but not FAA up-regulates TNF mRNA in the human HL-60 tumour cell line. At the same concentration but with longer incubation times (6-12 h), DMXAA induced increases in TNF protein in 11 of 15 samples of HPBLs from buffy coats and also in 11 of 15 samples of HPBLs from volunteers, as measured by cytotoxicity assays with L929 cells. FAA caused no increase in TNF protein, while LPS induced TNF to approximately 20-fold higher levels than did DMXAA. Considerable heterogeneity of response was observed with both sources of HPBLs, and there was little or no correlation between the extent of TNF induction by DMXAA and LPS in individual samples. In vitro analysis of the response of human peripheral blood leucocytes to DMXAA may be a useful test in clinical trials of agents such as DMXAA.

The Dtk receptor tyrosine kinase, which binds protein S, is expressed during hematopoiesis

Dtk (Tyro 3/Sky/Rse/Brt/Tif) belongs to a recently recognized subfamily of receptor tyrosine kinases that also includes Ufo (Axl/Ark) and Mer (Eyk). Ligands for Dtk and Ufo have been identified as protein S and the related molecule Gas6, respectively. This study examined expression of Dtk during ontogeny of the hematopoietic system and compared the pattern of expression with that of Ufo. Both receptors were abundantly expressed in differentiating embryonic stem cells, yolk sac blood islands, para-aortic splanchnopleural mesoderm, fractionated AA4+ fetal liver cells, and fetal thymus from day 14 until birth. Although Ufo was expressed at moderate levels in adult bone marrow, expression of Dtk in this tissue was barely detectable. In adult bone marrow subpopulations fractionated using counterflow centrifugal elutriation, immunomagnetic bead selection for lineage-depletion and FACS sorting for c-kit expression, very low levels of Dtk and/or Ufo were detected in some cell fractions. These results suggest that Dtk and Ufo are likely to be involved in the regulation of hematopoiesis, particularly during the embryonic stages of blood cell development.

Analysis of the murine Dtk gene identifies conservation of genomic structure within a new receptor tyrosine kinase subfamily

The receptor tyrosine kinase Dtk/Tyro 3/Sky/rse/brt/tif is a member of a new subfamily of receptors that also includes Axl/Ufo/Ark and Eyk/Mer. These receptors are characterized by the presence of two immunoglobulin-like loops and two fibronectin type III repeats in their extracellular domains. The structure of the murine Dtk gene has been determined. The gene consists of 21 exons that are distributed over 21 kb of genomic DNA. An isoform of Dtk is generated by differential splicing of exons from the 5' region of the gene. The overall genomic structure of Dtk is virtually identical to that determined for the human UFO gene. This particular genomic organization is likely to have been duplicated and closely maintained throughout evolution.

Identification of a novel receptor tyrosine kinase expressed in acute myeloid leukemic blasts

Using the polymerase chain reaction with degenerate oligonucleotides derived from conserved motifs within the catalytic kinase domain of protein tyrosine kinases, and RNA extracted from embryonic stem cells, sequences that encode a segment of the kinase domain of several potentially novel receptor tyrosine kinases (RTKs) have been identified. One of these was selected for further study because in Northern analysis it hybridized to RNA from multipotential hematopoietic cell lines, but not from lines representative of lineage-committed cells. A cDNA for this receptor, designated developmental tyrosine kinase (DTK), was isolated and encodes a protein with structural similarities to AXL. Together these receptors form a new class of RTK. DTK is expressed in a number of human leukemic cell lines, and in the blasts of 6 of 11 patients with acute myeloid leukemia (AML) analyzed. The structure of DTK suggests that it may function as a cell adhesion molecule, and mediate cell-to-cell or cell-matrix interactions between hematopoietic cells and their respective microenvironments.