Notch Signaling Is Required for Mast Cell Development In the Zebrafish and May Represent a Novel Therapeutic Strategy In Systemic Mastocytosis

Abstract 930

We have been exploiting the advantages provided by the zebrafish system to elucidate the molecular pathways regulating mast cell (MC) development in vertebrates and to model human MC diseases, such as systemic mastocytosis (SM). SM is a pre-leukemic myeloproliferative disease that results from perturbed MC development and proliferation. We have previously described zebrafish MC equivalents; demonstrated the significance of carboxypeptidase A5 (cpa5) as a zebrafish MC-specific marker; and established pu.1 and gata2 as essential transcription factors for early MC development (Dobson et al., Blood 2008). More recent co-localization studies suggest that definitive MCs originate from erythroid myeloid progenitors (EMPs), but unlike other myeloid lineages are uniquely dependent on Notch pathway signaling. Notch receptors and their ligands are expressed on a number of hematopoietic cells, including MCs. In zebrafish embryos, cpa5 co-localizes with the EMP marker, lmo2 at 28 hours post-fertilization (hpf) and with notch1a, notch1b and notch3 at both 28 hpf and 7 days post-fertilization (dpf). Morpholino knockdown studies specifically implicate notch1b as the key notch gene involved in MC fate determination and gata2 as an intermediary. Notch1b “morphants” exclusively display absent cpa5 expression with a concomitant decrease in gata2 expression by whole mount in situ hybridization (WISH). Furthermore, the zebrafish Notch signaling mutant, mindbomb, displays profound delay in the onset of cpa5 expression anteriorly and absent MCs in the posterior blood island at 48 hpf. Wild type embryos treated with Compound E, (CpdE), a γ-secretase inhibitor that inhibits Notch signaling, show a similar phenotype at 50 μM and complete absence of cpa5 expression at 75 μM. Embryos treated with 50 μM CpdE show decreased gata2 expression, but wild type pu.1 and gata1 expression, suggesting a particular sensitivity of the MC lineage to Notch pathway inhibition potentially mediated through gata2. By extension, these findings suggest the Notch pathway may serve as a prospective therapeutic target in MC diseases like SM. We have established a transgenic zebrafish model of SM that ubiquitously expresses the human c-KIT D816V mutation under the zebrafish β-actin promoter. Beginning at 9 months of age, adult fish develop a number of skin and visceral lesions, some of which have been found to contain an abundance of MCs. Transgenic embryos lack a developmental phenotype but demonstrate evidence of increased caspase-3 mediated apoptosis as well as increased cell proliferation by 5-bromo-2-deoxyuridine (BrdU) assay, suggesting additional mutations are required to progress to SM. These studies have provided new insights into the role of Notch signaling in MC development and the opportunity to use the zebrafish as an in vivo model to identify and evaluate novel therapeutic strategies in MC diseases.