We used the opportunities afforded by the zebrafish to determine upstream pathways regulating mast cell development in vivo and identify their cellular origin. Colocalization studies demonstrated zebrafish notch receptor expression in cells expressing carboxypeptidase A5 (cpa5), a zebrafish mast cell-specific marker. Inhibition of the Notch pathway resulted in decreased cpa5 expression in mindbomb mutants and wild-type embryos treated with the ␥-secretase inhibitor, Compound E. A series of morpholino knockdown studies specifically identified notch1b and gata2 as the critical factors regulating mast cell fate. Moreover, hsp70::GAL4;UAS::nicd1a transgenic embryos overexpressing an activated form of notch1, nicd1a, displayed increased cpa5, gata2, and pu.1 expression. This increase in cpa5 expression could be reversed and reduced below baseline levels in a dose-dependent manner using Compound E. Finally, evidence that cpa5 expression colocalizes with lmo2 in the absence of
IntroductionMast cells are best known for their role in acute and chronic allergic reactions; however, they also function as a crucial component in both innate and acquired immune responses 1 as well as solid tumor and leukemia progression. 2,3 Mast cells delineate from hematopoietic stem cells (HSCs) in the bone marrow but, unlike other blood cells, enter circulation as progenitors. They only complete maturation in resident tissues, which greatly hinders accurate lineage tracing studies in traditional mammalian models. 1 We have been using the zebrafish model to study mast cell development and, specifically, the transcriptional regulation of mast cell lineage commitment. The zebrafish is a highly efficient model system for studying blood cell development. 4-6 All of the major hematopoietic cellular lineages studied to date have zebrafish counterparts, and the fundamental genetic mechanisms that control hematopoiesis are well conserved. 4,7,8 We first described a mast cell counterpart in the zebrafish 9 and subsequently showed conserved roles of these cells in adaptive and innate responses to inflammatory stimuli. 10 Zebrafish carboxypeptidase a5 (cpa5) was identified as a zebrafish mast cell-specific marker, and gata2 and pu.1 were found to be the key transcription factors required for early mast cell lineage commitment in keeping with studies in mammalian systems. 9,11,12 The Notch signaling pathway is a critical regulator of cell fate determination conserved through evolution. Aberrant Notch signaling is associated with a wide range of human disorders from developmental syndromes to cancer. 13 Notch signaling is involved in the fate determination of a variety of cell types, including hematopoietic cells where it participates in differentiation, proliferation, and apoptosis. 14 In mammals, the Notch pathway consists of 4 Notch genes (Notch1-4), which encode transmembrane receptor proteins. These receptors are activated by 5 ligands encoded by the Delta and Serrate/Jagged gene families: Delta-like1, (Dll1), Dll3, Dll4, Jagged 1 (Jag1), and Jag2...