ROLE OF THE NOTCH LIGAND JAGGED1 IN ZEBRAFISH THYROID DEVELOPMENT

Notch signaling is the result of the complex interaction of various Delta and Jagged ligands and their Notch receptors. This signal is known to have a relevant and expanding role in cell fate determination, survival and proliferation during embryonic development. Though Notch pathway is known to have a fundamental role in different endocrine systems (i.e. anterior pituitary and pancreatic cell specification), its possible involvement in thyroid development has never been studied. Our previous gene expression analysis in a thyroid cell line revealed the presence of several transcripts belonging to the Jagged1-Notch pathway, thus suggesting its potential role in thyroid development. Then, we designed a series of experiments aimed at confirming these preliminary data in vitro and understand the role of Notch signaling in thyroid development. For this purpose, we took advantage of zebrafish, a versatile experimental model that has been shown to largely recapitulate the early events of mammalian thyroid gland organogenesis. Gene expression analysis and experimental manipulations of zebrafish embryos were performed to identify the possible contribution of Jagged1 ligands to the thyroid development. Loss of Notch receptors activation in mind bomb (mib) mutants or DAPT-treated embryos led to an increased number of thyroid primordium cells. Conversely, Notch gain of function resulted in an impaired expression of the early thyroid marker genes (nkx2.1a; tg) in vivo. Given that the entire pharyngeal architecture was seriously affected by these genetic conditions, the observed thyroid phenotypes could come from either cell-autonomous or non cell-autonomous activities of Notch signaling. In order to understand if Notch signaling plays a direct role in thyroid development, we then analyzed the co-expression pattern of the zebrafish jag1a and jag1b, orthologues of the mammalian Jagged1 ligand, and thyroid markers by double whole-mount in situ hybridization. These experiments show the co-expression of these ligands in the thyroid region, at different times. Next, to understand the biological role of these ligands, we took advantage of both morpholino knock-down strategy and specific zebrafish mutants. Thyroid development was impaired in several of these conditions, as shown by the altered expression of thyroid differentiation markers tg and slc5a5 and T4 production at different time points. As a consequence, a defective function of Jagged1 ligand might constitute a novel mechanism involved in the pathogenesis of congenital hypothyroidism