Evidence for two distinct waves of epidermal ionocyte differentiation during medaka embryonic development

Interspecific comparisons of anuran embryonic epidermal landscapes and energetic trade-offs in response to changes in salinity

BACKGROUND

Freshwater salinization is an emerging stressor in amphibian populations, and embryonic stages are most vulnerable. To better understand the variation in embryonic osmoregulation, we challenged embryos of two phylogenetically diverse anuran species, Xenopus laevis and Lithobates (Rana) sylvaticus, along a gradient of non-lethal salinities. We hypothesized embryos at higher salinities will display epidermal plasticity as a coping response and increase energy expenditure related to osmoregulation demands, thereby reducing energy for growth and development.

RESULTS

Scanning electron microscopy revealed an extra mucus-secreting cell type and higher ionocyte proportions in the X. laevis epidermis, suggesting more osmoregulatory machinery than L. sylvaticus. Under elevated salinity, X. laevis displayed greater increases in goblet cell proportions, mucus secretion, and reductions in ionocyte apical area compared with L. sylvaticus. Although both species increased oxygen consumption rates and reduced body length with elevated salinity, these effects were proportionally greater in L. sylvaticus at the highest salinity, and only this species slowed developmental rates.

CONCLUSION

These findings support the hypothesis that frog embryos respond to salinity by altering the cellular landscape of their epidermis. We show that epidermal cell types, as well as the magnitude of epidermal plasticity and energetic trade-offs in response to salinity, vary among amphibian species.

A mesenchymal to epithelial switch in Fgf10 expression specifies an evolutionary-conserved population of ionocytes in salivary glands

Fibroblast growth factor 10 (FGF10) is well established as a mesenchyme-derived growth factor and a critical regulator of fetal organ development in mice and humans. Using a single-cell RNA sequencing (RNA-seq) atlas of salivary gland (SG) and a tamoxifen inducible Fgf10CreERT2:R26-tdTomato mouse, we show that FGF10pos cells are exclusively mesenchymal until postnatal day 5 (P5) but, after P7, there is a switch in expression and only epithelial FGF10pos cells are observed after P15. Further RNA-seq analysis of sorted mesenchymal and epithelial FGF10pos cells shows that the epithelial FGF10pos population express the hallmarks of ancient ionocyte signature Forkhead box i1 and 2 (Foxi1, Foxi2), Achaete-scute homolog 3 (Ascl3), and the cystic fibrosis transmembrane conductance regulator (Cftr). We propose that epithelial FGF10pos cells are specialized SG ionocytes located in ducts and important for the ionic modification of saliva. In addition, they maintain FGF10-dependent gland homeostasis via communication with FGFR2bpos ductal and myoepithelial cells.

Insights into molecular and cellular mechanisms of hormonal actions on fish ion regulation derived from the zebrafish model

Fish have sophisticated mechanisms of ionic and acid-base regulation for maintaining body fluid homeostasis. Many hormones have been proposed to control the ionic and acid-base regulation mechanisms in fishes; however, lots of the proposed actions lack convincing cellular/molecular evidence. With the advantages of available genetic databases and molecular manipulation techniques, zebrafish has become an emerging model for research into ion transport physiology and functional regulation. Different types of ionocytes were found to transport ions through various sets of ion transporters, and the molecular mechanisms of ionocyte proliferation and differentiation have also been dissected, providing a competent platform with which to precisely study the ion transport pathways and ionocytes targeted by hormones, including isotocin, prolactin, cortisol, stanniocalcin-1, calcitonin, endothelin-1, vitamin D, parathyroid hormone 1, catecholamines, the renin-angiotensin-system, estrogen-related receptor α, and calcitonin gene-related peptide, which have been demonstrated to positively or negatively regulate ion transport through specific receptors at different molecular levels (transcriptional, translational, or posttranslational) or at different developmental stages of ionocytes (proliferation or differentiation). The knowledge obtained in zebrafish not only enhances our understanding of the hormonal control of fish ion regulation, but also informs studies on other animal species, thereby providing insights into related fields.