Mechanism of development of ionocytes rich in vacuolar-type H+-ATPase in the skin of zebrafish larvae

Abstract

Mitochondrion-rich cells (MRCs), or ionocytes, play a central role in aquatic species, maintaining body fluid ionic homeostasis by actively taking up or excreting ions. Since their first description in 1932 in eel gills, extensive morphological and physiological analyses have yielded important insights into ionocyte structure and function, but understanding the developmental pathway specifying these cells remains an ongoing challenge. We previously succeeded in identifying a key transcription factor, Foxi3a, in zebrafish larvae by database mining. In the present study, we analyzed a zebrafish mutant, quadro (quo), deficient in foxi1 gene expression and found that foxi1 is essential for development of an MRC subpopulation rich in vacuolar-type H+-ATPase (vH-MRC). foxi1 acts upstream of Delta-Notch signaling that determines sporadic distribution of vH-MRC and regulates foxi3a expression. Through gain- and loss-of-function assays and cell transplantation experiments, we further clarified that (1) the expression level of foxi3a is maintained by a positive feedback loop between foxi3a and its downstream gene gcm2 and (2) Foxi3a functions cell-autonomously in the specification of vH-MRC. These observations provide a better understanding of the differentiation and distribution of the vH-MRC subtype.

Development of zebrafish epidermis

“…Hsiao et al (2007) first proposed a regulatory pathway for ionocyte differentiation and specification in zebrafish, and this model was mostly supported and further explored in subsequent studies (Fig. 2) (Janicke et al, 2007;Esaki et al, 2009;Hwang and Perry, 2010;Hwang et al, 2011). Janicke et al (2007) proved that ionocytes are derived from nonneural ectoderm via lineage tracing experiments.…”

The physiology of fish at low pH: the zebrafish as a model system

“…HRCs and NaRCs). Cruz et al (Cruz et al, 2013) demonstrated that cortisol treatment increased the proliferation of NaRCs and HRCs in zebrafish larval skin and adult gill through its stimulatory effects on foxi3a/3b, transcription factors involved in the regulation of ionocyte differentiation Esaki et al, 2009;Hsiao et al, 2007;Jänicke et al, 2007). Because of the increase in wholebody cortisol levels in acid-exposed larvae, the role of cortisol in ionocyte proliferation/epidermal development offers an attractive mechanism to explain the observed increase in HRCs in response to acid exposure.…”

Intoxication of zebrafish and mammalian cells by cholera toxin depends on the flotillin/reggie proteins but not Derlin-1 or -2

“…Thus, zebrafish embryos likely express the toxin's receptor GM1, and it can bind CT. We also performed confocal microscopy on live embryos exposed to fluorophore-labeled CTB and localized the bulk of toxin uptake to surface epithelial mitochondrion-rich cells (MRCs), the depletion of which correlated with toxin resistance (Supplemental Figure 2 and Supplemental Video 1). It is of note that these cells regulate salt and water homeostasis in the embryo (34); however, intoxication of other cell types might contribute to the overall phenotype observed (see below).…”

“…Embryos (40-hpf) were incubated in embryo water containing 20 nm CTB-Alexa Fluor 488 for 50 minutes, and then a further 10 minutes with 1 μM MitoTracker (labels MRCs), thought to regulate salt and water homeostasis in the zebrafish embryo (34). Embryos were washed and anesthetized (50 μg/ml MS-222) prior to imaging.…”