David Traver

Transplantation and in vivo imaging of multilineage engraftment in zebrafish bloodless mutants

The zebrafish is firmly established as a genetic model for the study of vertebrate blood development. Here we have characterized the blood-forming system of adult zebrafish. Each major blood lineage can be isolated by flow cytometry, and with these lineal profiles, defects in zebrafish blood mutants can be quantified. We developed hematopoietic cell transplantation to study cell autonomy of mutant gene function and to establish a hematopoietic stem cell assay. Hematopoietic cell transplantation can rescue multilineage hematopoiesis in embryonic lethal gata1−/− mutants for over 6 months. Direct visualization of fluorescent donor cells in embryonic recipients allows engraftment and homing events to be imaged in real time. These results provide a cellular context in which to study the genetics of hematopoiesis.

BRAF Mutations Are Sufficient to Promote Nevi Formation and Cooperate with p53 in the Genesis of Melanoma

Melanoma is the most lethal form of skin cancer, and the incidence and mortality rates are rapidly rising. Epidemiologically, high numbers of nevi (moles) are associated with higher risk of melanoma . The majority of melanomas exhibit activating mutations in the serine/threonine kinase BRAF . BRAF mutations may be critical for the initiation of melanoma ; however, the direct role of BRAF in nevi and melanoma has not been tested in an animal model. To directly test the role of activated BRAF in nevus and melanoma development, we have generated transgenic zebrafish expressing the most common BRAF mutant form (V600E) under the control of the melanocyte mitfa promoter. Expression of mutant, but not wild-type, BRAF led to dramatic patches of ectopic melanocytes, which we have termed fish (f)-nevi. Remarkably, in p53-deficient fish, activated BRAF induced formation of melanocyte lesions that rapidly developed into invasive melanomas, which resembled human melanomas and could be serially transplanted. These data provide direct evidence that BRAF activation is sufficient for f-nevus formation, that BRAF activation is among the primary events in melanoma development, and that the p53 and BRAF pathways interact genetically to produce melanoma.

Intrinsic requirement for zinc finger transcription factor Gfi-1 in neutrophil differentiation.

We report essential roles of zinc finger transcription factor Gfi-1 in myeloid development. Gene-targeted Gfi-1(-/-) mice lack normal neutrophils and are highly susceptible to abscess formation by gram-positive bacteria. Arrested, morphologically atypical, Gr1(+)Mac1(+) myeloid cells expand with age in the bone marrow. RNAs encoding primary but not secondary or tertiary neutrophil (granulocyte) granule proteins are expressed. The atypical Gr1(+)Mac1(+) cell population shares characteristics of both the neutrophil and macrophage lineages and exhibits phagocytosis and respiratory burst activity. Reexpression of Gfi-1 in sorted Gfi-1(-/-) progenitors ex vivo rescues neutrophil differentiation in response to G-CSF. Thus, Gfi-1 not only promotes differentiation of neutrophils but also antagonizes traits of the alternate monocyte/macrophage program.

The zebrafish as a model organism to study development of the immune system.

Publisher Summary Early events in the development of the primitive and definitive blood forming system are still poorly understood. Additionally, the specification of both B and T cells occurs during embryogenesis and, given the completion of this process before birth, are difficult to study in mammals by forward genetics. Historically, the major strength of the zebrafish has been the opportunity it offered to carry forward genetic screens in a vertebrate organism in a relatively restricted space. Establishing the zebrafish as a model system for the study of the immune system will provide an alternative and complementary tool to the use of forward genetic screens in mice. Rapid advances in a variety of fields have allowed the zebrafish to become a more versatile tool for immunology.

Mice Defective in Two Apoptosis Pathways in the Myeloid Lineage Develop Acute Myeloblastic Leukemia

Fas-deficient (Fas(lpr/lpr)) mice constitutively expressing Bcl-2 in myeloid cells by the hMRP8 promoter often develop a fatal disease analogous to human acute myeloblastic leukemia (AML-M2). Hematopoietic cells from leukemic Fas(lpr/lpr)hMRP8bcl-2 animals form clonogenic blast colonies in vitro and can transfer disease to wild-type mice. In vitro ligation of Fas on Fas+/+ hMRP8bcl-2 marrow cells depletes approximately 50% of myeloid progenitor activity, demonstrating that Bcl-2 can only partially block Fas-mediated death signals in myelomonocytic progenitors. In addition, Fas(lpr/lpr) marrow contains greatly increased numbers of myeloid colony-forming cells as compared to Fas+/+ controls. Taken together, these data suggest that Fas has a novel role in the regulation of myelopoiesis and that Fas may act as a tumor suppressor to control leukemogenic transformation in myeloid progenitor cells.

Walking the Walk: Migration and Other Common Themes in Blood and Vascular Development

In this issue of Cell, a study by N. Cho and coworkers provides insight into the role of vascular endothelial growth factor (VEGF) signaling in Drosophila hematopoiesis. Their work suggests that an ancestral function of VEGF was to guide blood cell migration and highlights the conservation of at least one aspect of VEGF signaling during evolution.

The Zebrafish moonshine Gene Encodes Transcriptional Intermediary Factor 1γ, an Essential Regulator of Hematopoiesis

Hematopoiesis is precisely orchestrated by lineage-specific DNA-binding proteins that regulate transcription in concert with coactivators and corepressors. Mutations in the zebrafish moonshine (mon) gene specifically disrupt both embryonic and adult hematopoiesis, resulting in severe red blood cell aplasia. We report that mon encodes the zebrafish ortholog of mammalian transcriptional intermediary factor 1γ (TIF1γ) (or TRIM33), a member of the TIF1 family of coactivators and corepressors. During development, hematopoietic progenitor cells in mon mutants fail to express normal levels of hematopoietic transcription factors, including gata1, and undergo apoptosis. Three different mon mutant alleles each encode premature stop codons, and enforced expression of wild-type tif1γ mRNA rescues embryonic hematopoiesis in homozygous mon mutants. Surprisingly, a high level of zygotic tif1γ mRNA expression delineates ventral mesoderm during hematopoietic stem cell and progenitor formation prior to gata1 expression. Transplantation studies reveal that tif1γ functions in a cell-autonomous manner during the differentiation of erythroid precursors. Studies in murine erythroid cell lines demonstrate that Tif1γ protein is localized within novel nuclear foci, and expression decreases during erythroid cell maturation. Our results establish a major role for this transcriptional intermediary factor in the differentiation of hematopoietic cells in vertebrates.

T-lymphoblastic lymphoma cells express high levels of BCL2, S1P1, and ICAM1, leading to a blockade of tumor cell intravasation.

The molecular events underlying the progression of T-lymphoblastic lymphoma (T-LBL) to acute T-lymphoblastic leukemia (T-ALL) remain elusive. In our zebrafish model, concomitant overexpression of bcl-2 with Myc accelerated T-LBL onset while inhibiting progression to T-ALL. The T-LBL cells failed to invade the vasculature and showed evidence of increased homotypic cell-cell adhesion and autophagy. Further analysis using clinical biopsy specimens revealed autophagy and increased levels of BCL2, S1P1, and ICAM1 in human T-LBL compared with T-ALL. Inhibition of S1P1 signaling in T-LBL cells led to decreased homotypic adhesion inxa0vitro and increased tumor cell intravasation inxa0vivo. Thus, blockade of intravasation and hematologic dissemination in T-LBL is due to elevated S1P1 signaling, increased expression of ICAM1, and augmented homotypic cell-cell adhesion.

Dendritic Cell Development from Common Myeloid Progenitors

Abstract: Dendritic cells (DCs) are professional antigen‐presenting cells which both initiate adaptive immune responses and control tolerance to self‐antigens. It has been suggested that these different effects on responder cells depend on subsets of DCs arising from either myeloid or lymphoid hematopoietic origins. In this model, CD8α+ Mac‐1− DCs are supposed to be of lymphoid while CD8α− Mac‐1+ DCs are supposed to be of myeloid origin. Here we summarize our findings that both CD8α+ and CD8α− DCs can arise from clonogenic common myeloid progenitors (CMPs) in both thymus and spleen. Therefore CD8a expression on DCs does not indicate a lymphoid origin and differences among CD8α+ and CD8α− DCs might rather reflect maturation status than ontogeny. On the basis of transplantation studies, it seems likely that most of the DCs in secondary lymphoid organs and a substantial fraction of thymic DCs are myeloid‐derived.

Zebrafish kidney stromal cell lines support multilineage hematopoiesis

Studies of zebrafish hematopoiesis have been largely performed using mutagenesis approaches and retrospective analyses based upon gene expression patterns in whole embryos. We previously developed transplantation assays to test the repopulation potentials of candidate hematopoietic progenitor cells. We have been impaired, however, in determining cellular differentiation potentials by a lack of short-term functional assays. To enable more precise analyses of hematopoietic progenitor cells, we have created zebrafish kidney stromal (ZKS) cell lines. Culture of adult whole kidney marrow with ZKS cells results in the maintenance and expansion of hematopoietic precursor cells. Hematopoietic growth is dependent upon ZKS cells, and we show that ZKS cells express many growth factors and ligands previously demonstrated to be important in maintaining mammalian hematopoietic cells. In the absence of exogenous growth factors, ZKS cells maintain early hematopoietic precursors and support differentiation of lymphoid and myeloid cells. With the addition of zebrafish erythropoietin, ZKS cells also support the differentiation of erythroid precursors. These conditions have enabled the ability to ascertain more precisely the points at which hematopoietic mutants are defective. The development of robust in vitro assays now provide the means to track defined, functional outcomes for prospectively isolated blood cell subsets in the zebrafish.