Serotonin and MucXS release by small secretory cells depend on Xpod, a SSC specific marker gene

Abstract

Summary Mucus secretion and ciliary motility are hallmarks for muco‐ciliary epithelia (MCE). Both, mammalian airways as well as the less complex epidermis of Xenopus embryos show cilia‐driven mucus flow to protect the organism against harmful effects by exogenous pathogens or pollutants. Four cell types set up the epidermal MCE in Xenopus. Multi‐ciliated cells (MCCs) generate an anterior to posterior flow of mucus. Ion secreting cells (ISCs) are characterized by the expression of ion transporters, presumably to maintain a favorable homeostasis. The largest cell type is represented by goblet cells, which cover most of the epidermis and exhibit secretory properties. Additionally, small secretory cells (SSCs) release mucus, antibiotic compounds, and the monoamine serotonin (5‐hydroxytryptamine; 5‐HT). We have recently shown that serotonin regulates flow velocity by acting on ciliary beat frequency. Here, we describe the identification and functional characterization of Xenopus polka‐dots (Xpod). No homologous genes or proteins were found in other vertebrates, including Xenopus tropicalis. We demonstrate that Xpod serves as an SSC‐specific marker, starting to be expressed shortly after SSC specification at neurula stages. Overexpression of a tagged Xpod protein resulted in the localization of secretory granules. Notch signaling induced SSC cell fate, in contrast to its repressing effect on MCC and ISC specification. Xpod loss‐of‐function revealed that mucus and 5‐HT release by SSCs was severely diminished, which impaired the ciliary beating of MCCs. In summary, Xpod specifically marked SSCs and was required for muco‐ciliary secretion in Xenopus laevis.

Crb3 stabilizes activated Ezrin-Radixin-Moesin to organize the apical domain of multiciliated cells

“…To date, very few SSC markers have been described and they are either transcription factors such as Foxa1, or allow the detection of secreted compounds (Otogelin-like, HNK1, IPTKb, serotonin, Xpod) (Dubaissi et al, 2014; Kurrle et al, 2020; Walentek et al, 2014). Thus Crb3.S is, so far, the sole transmembrane protein described to be highly expressed in SSCs.…”

Signaling Control of Mucociliary Epithelia: Stem Cells, Cell Fates, and the Plasticity of Cell Identity in Development and Disease

“…Consequently, inhibition of NICD transcriptional activity by a dominant-negative DNA-binding mutant of the CSF/RBPJ homolog SuH (SuH-DBM) or a dominantnegative form of Mastermind leads to excessive production of ionocytes and MCCs, while overexpression of a constitutive active NICD suppresses ionocyte and MCC specification [Deblandre et al, 1999;Stubbs et al, 2006;Quigley et al, 2011;Walentek, 2018]. It was further shown that overexpression of intermediate NICD levels in Xenopus increases the number of SSCs, that is, a secretory cell type that also produces mucins [Walentek, 2018;Kurrle et al, 2020]. Similarly, data from the mouse airway demonstrated that low Notch levels lead to specification of MCCs, while high Notch levels promote the production of club secretory cells [Guseh et al, 2009;Tsao et al, 2009].…”

“…More data are also needed regarding the Notch regulation of ionocytes in the mammalian lung as in contrast to the Xenopus epidermis, where this cell type was initially described, Notch inhibition was shown to negatively affect ionocyte abundance [Quigley et al, 2011;Plasschaert et al, 2018]. Conversely, SSCs that share the characteristic serotonin production and secretion with mammalian PNECs are dependent on high Notch signaling in the frog, and the opposite is true for mouse PNECs, which are increased in numbers upon developmental deletion of multiple Notch receptors in the epithelium [Morimoto et al, 2012;Walentek, 2018;Kurrle et al, 2020]. In Xenopus, Notch inhibition was also shown to lead to downregulated expression of ∆N-TP63, while in the mammalian airways, Notch seems largely dispensable for BCs, and elevated Notch is required for differentiation of luminal epithelial cell types [Rock et al, 2011;Sirour et al, 2011].…”

Xenopus epidermal and endodermal epithelia as models for mucociliary epithelial evolution, disease, and metaplasia

“…ISCs are specified by Foxi1, and are subdivided into α‐ and β‐ISCs by expression of Ubp1 in β‐ISCs, which is regulated by Notch signaling as well (Quigley et al, 2011). SSCs are specified by Foxa1 and require higher Notch levels as compared to ISCs and MCCs (Dubaissi et al, 2014; Kurrle, Kunesch, Bogusch, & Schweickert, 2020; Walentek, 2018; Walentek et al, 2014). After cell fate acquisition, cells insert apically into the epithelial layer.…”

“…A similar mechanism could regulate the apical expansion in ISCs as they also contain an F‐actin rich surface. SSCs, which are specified and emerge slightly later than ISCs and MCCs, do not seem to have such an active expansion mechanism, but rely on the production of secretory vesicles for expansion of their apical surface, which strongly increases until premetamorphosis stages (Kurrle et al, 2020; Walentek et al, 2014) (Figures 1 and 2).…”