Mast cells (MCs) play critical roles in allergy and inflammation, yet their development remains controversial due to limitations posed by traditional animal models. The zebrafish provides a highly efficient system for studying vertebrate hematopoiesis. We have identified zebrafish MCs in the gill and intestine, which resemble their mammalian counterparts both structurally and functionally. Carboxypeptidase A5 (cpa5), a MCspecific enzyme, is expressed in zebrafish blood cells beginning at 24 hours post fertilization (hpf). At 28 hpf, colocalization is observed with pu.1, mpo, l-plastin, and lysozyme C, but not fms or cepb␣, identifying these early MCs as a distinct myeloid population arising from a common granulocyte/ monocyte progenitor. Morpholino "knockdown" studies demonstrate that transcription factors gata-2 and pu.1, but not gata-1 or fog-1, are necessary for early MC development. These studies validate the zebrafish as an in vivo tool for studying MC ontogeny and function with future capacity for modeling human MC diseases. (Blood. 2008;112:2969-2972) IntroductionMast cells (MCs) play central roles in allergic and inflammatory reactions. 1,2 Stimulation of cell-surface receptors, such as C-KIT and the high-affinity IgE receptor, 1,2 results in the release of mediators from cytoplasmic granules, including tryptase and histamine. 2 MC number and function are regulated by their development, proliferation, migration, and survival. 1 Barriers to understanding these processes include accessibility and imaging limitations posed by traditional animal models. The zebrafish has proven itself to be a robust and highly conserved model for studying vertebrate hematopoiesis. 3 Here, we provide the first evidence that the zebrafish possesses MC equivalents that share structural and functional characteristics with their mammalian counterparts. Furthermore, we demonstrate the utility of the zebrafish as an in vivo tool in dissecting the contribution of transcription factors to MC development. MethodsZebrafish were maintained, bred, and developmentally staged according to Westerfield. 4 Use of zebrafish in this study was approved by the Dalhousie University Animal Care Committee. Zebrafish gills and intestine were fixed in 10% neutral buffered formalin, and standard staining procedures were applied ( Figure 1A-F). Immunohistochemistry was facilitated by antigen retrieval ( Figure 1I,J). For electron microscopy, tissues were fixed overnight in 2% glutaraldehyde in 0.1 M caccodylate and postfixed in 1% osmium tetroxide. Thin sections (90 nm) were stained in 25% uranyl acetate in methanol and lead citrate.Bromophenol blue and 10 g compound 48/80 or saline were injected intraperitoneally, and blood was extracted by cardiac puncture after 2.5 minutes. Tryptase activity was measured in plasma spectrophotometrically at 415 nm by the release of p-nitroanilide from N-benzoyl-DL-argininep-nitroanilide (BAPNA), a tryptase substrate.Digoxogenin-or fluorescein isothiocyanate (FITC)-labeled antisense mRNA probes for zebrafish carboxypep...
Mast cells (MCs) are well-known for their role in allergic reactions and inflammation, but their developmental origin is controversial. In addition, abnormal clonal proliferation of MCs, referred to as systemic mastocytosis (SM), may be associated with acute myeloid leukemia, portending a poor outcome. Mutations in the C-KIT tyrosine kinase have been identified in SM, that can be potentially targeted by small molecule tyrosine kinase inhibitors. The zebrafish is a robust model organism for studying hematopoiesis and leukemogenesis, and has an inherent capacity to accommodate genetic and chemical modifier screens. Thus, this system holds potential for elucidating MC lineage and for use in high-throughput screening of targeted therapeutics in MC diseases. MCs have not been previously described in zebrafish. We have identified putative MCs in adult zebrafish gill and intestine containing eosinophilic granules that stain with peroxidase acid shift (PAS) and toluidine blue, as well as with carboxypeptidase A5 (cpa5), a zebrafish homologue of the mammalian MC specific enzyme, CPA1. Electron microscopic analysis demonstrates a striking morphologic resemblance to mammalian MCs with abundant homogeneous dense granules. Classical functional studies reveal degranulation of these cells and increased histamine production upon stimulation with compound 48/80 and stimulated IgE. Whole mount in situ hybridization experiments on zebrafish embryos demonstrate cpa5 expression in a population of blood cells at 28 hours post-fertilization co-localizing with the early myeloid marker, pu.1, the granulocytic marker, mpo, and monocytic markers, l-plastin and lysozyme C. These data point to the existence of a zebrafish MC equivalent and suggests that this lineage arises at the level of the granulocyte/monocyte progenitor. Ongoing characterization of these cells will provide further insight into the mechanisms underlying MC development. Furthermore, a zebrafish cpa5 promoter element has been cloned and is being used to generate transgenic zebrafish lines. These transgenic zebrafish will provide a valuable tool in identifying new effective therapeutic agents targeting MCs in allergic and immune responses, as well as in their contribution to leukemic progression.
Mast cells (MCs), tissue counterparts to mammalian basophils, are known for their role in allergic reactions, inflammation and cancer progression, yet their developmental origin remains controversial due to limitations studying these cells in traditional animal models. The zebrafish provides a highly efficient system for studying vertebrate MC development. All other major hematopoietic lineages have zebrafish counterparts and the fundamental genetic mechanisms controlling hematopoiesis are well conserved. We are the first to identify zebrafish MCs in the gill and intestine. These cells demonstrate classic MC phenotypes including prominent metachromatic granules following staining with toluidine blue and positive immunohistochemical reactions to antibodies against human tryptase and C-KIT. Electron microscopy demonstrates a striking morphologic resemblance to mammalian MCs. Functional studies using the stimulating agent, Compound 48/80 or formalin-killed Aeromonas result in MC degranulation and increased blood levels of key mediators, such as tryptase. These cells also express carboxypeptidase A5 (cpa5), a zebrafish homologue of the human mast cell-specific CPA enzymes. Cpa5 expression in zebrafish embryonic blood cells begins at 24 hours post fertilization and co-localizes with a number of established granulocytic and monocytic markers suggesting that MCs arise from a common granulocyte/monocyte progenitor. Morpholino knockdown studies have demonstrated that the transcription factors gata-2 and pu.1, but not gata-1 are necessary for early MC development. Interestingly, friend of gata-1 (fog-1) may also be required, but in a gata-1 dependent manner. Ongoing morpholino and mutant rescue studies will further establish in vivo the contribution that these transcription factors make to vertebrate MC development. Finally, we have cloned a cpa5 promoter element and shown it can drive expression of the green fluorescent protein in early zebrafish MCs providing a means for generating transgenic zebrafish lines to model human MC diseases for use in high throughput small molecule modifier screens.
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