Foxi1 regulates multipotent mucociliary progenitors and ionocyte specification through transcriptional and epigenetic mechanisms

Foxi1 is a master regulator of ionocytes (ISCs/INCs) across species and organs. Two subtypes of ISCs exist, and both α- and β-ISCs regulate pH- and ion-homeostasis in epithelia. Gain and loss of FOXI1 function are associated with human diseases, including Pendred syndrome, male infertility, renal acidosis, and cancers. Foxi1 was predominantly studied in the context of ISC specification, however, reports indicate additional functions in early and ectodermal development. Here, we re-investigated the functions of Foxi1 in Xenopus laevis embryonic mucociliary epidermis developpment and found a novel function for Foxi1 in the generation of Notch-ligand expressing mucociliary multipotent progenitors (MPPs). We demonstrate that MPPs are a distinct sub-population of epidermal cells in which Foxi1 has two concentration-dependent functions: At low levels, Foxi1 maintains ectodermal competence in MPPs through transcriptional and epigenetic mechanisms, while at high levels, Foxi1 induces a multi-step process of ISC specification and differentiation in cooperation with Ubp1 and Dmrt2. We further describe how foxi1 expression is affected through auto- and Notch-regulation, and how this developmental program affects mucociliary patterning. Together, we reveal novel functions for MPPs and Foxi1 in Xenopus mucociliary epidermis formation, relevant to our understanding of vertebrate development and human disease.