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 MC-specific 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 cepbα, identifying these early MCs as a distinct myeloid population arising from a common granulocyte/monocyte progenitor. Morpholino “knock-down” 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.

Primary familial brain calcification (PFBC) is characterized by calcium deposition in the brain, causing progressive movement disorders, psychiatric symptoms, and cognitive decline. PFBC is a heterogeneous disorder currently linked to variants in six different genes, but most patients remain genetically undiagnosed. Here, we identify biallelic NAA60 variants in ten individuals from seven families with autosomal recessive PFBC. The NAA60 variants lead to loss-of-function with lack of protein N-terminal (Nt)-acetylation activity. We show that the phosphate importer SLC20A2 is a substrate of NAA60 in vitro. In cells, loss of NAA60 caused reduced surface levels of SLC20A2 and a reduction in extracellular phosphate uptake. This study establishes NAA60 as a causal gene for PFBC, provides a possible biochemical explanation of its disease-causing mechanisms and underscores NAA60-mediated Nt-acetylation of transmembrane proteins as a fundamental process for healthy neurobiological functioning.

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 γ-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 hematopoietic stem cells revealed that definitive mast cells initially delineate from erythromyeloid progenitors. These studies identify a master role for Notch signaling in vertebrate mast cell development and establish developmental origins of this lineage. Moreover, these findings postulate targeting the Notch pathway as a therapeutic strategy in mast cell diseases.