A Positive Regulatory Loop between foxi3a and foxi3b Is Essential for Specification and Differentiation of Zebrafish Epidermal Ionocytes

Chung-Der Hsiao,Yun-Jin Jiang,Ying-Jey Guh,Ming Ma,Pung-Pung Hwang,May-Su You,Thomas Zwaka

doi:10.1371/journal.pone.0000302

Abstract

BackgroundEpidermal ionocytes play essential roles in the transepithelial transportation of ions, water, and acid-base balance in fish embryos before their branchial counterparts are fully functional. However, the mechanism controlling epidermal ionocyte specification and differentiation remains unknown.Methodology/Principal FindingsIn zebrafish, we demonstrated that Delta-Notch-mediated lateral inhibition plays a vital role in singling out epidermal ionocyte progenitors from epidermal stem cells. The entire epidermal ionocyte domain of genetic mutants and morphants, which failed to transmit the DeltaC-Notch1a/Notch3 signal from sending cells (epidermal ionocytes) to receiving cells (epidermal stem cells), differentiates into epidermal ionocytes. The low Notch activity in epidermal ionocyte progenitors is permissive for activating winged helix/forkhead box transcription factors of foxi3a and foxi3b. Through gain- and loss-of-function assays, we show that the foxi3a-foxi3b regulatory loop functions as a master regulator to mediate a dual role of specifying epidermal ionocyte progenitors as well as of subsequently promoting differentiation of Na+,K+-ATPase-rich cells and H+-ATPase-rich cells in a concentration-dependent manner.Conclusions/SignificanceThis study provides a framework to show the molecular mechanism controlling epidermal ionocyte specification and differentiation in a low vertebrate for the first time. We propose that the positive regulatory loop between foxi3a and foxi3b not only drives early ionocyte differentiation but also prevents the complete blockage of ionocyte differentiation when the master regulator of foxi3 function is unilaterally compromised.

Highlights

In terrestrial vertebrates, diverse types of ionocytes (ICs) that are distributed in the bladder, inner ear, and kidney have evolved to play essential roles in transepithelial ion, water, and acid-base transportation
According to the data obtained from studies of epidermal ICs and ciliated cells [48], such a salt-and-pepper distribution pattern is the outcome of D-N lateral inhibition
We demonstrated that foxi3a is the earliest marker expressed in epidermal IC progenitors. foxi3a and foxi3b are both sufficient to determine the epidermal IC identity by a novel positive regulatory loop

Summary

Introduction

Diverse types of ionocytes (ICs) that are distributed in the bladder, inner ear, and kidney have evolved to play essential roles in transepithelial ion, water, and acid-base transportation. Vital dye binding and immunocytochemistry, distinct types of epidermal or branchial ICs have been reported in diverse fish species, and have been suggested to mediate an equivalent function as their counterparts in the kidneys of terrestrial vertebrates [9,10]. Epidermal ionocytes play essential roles in the transepithelial transportation of ions, water, and acid-base balance in fish embryos before their branchial counterparts are fully functional. Through gain- and loss-of-function assays, we show that the foxi3a-foxi3b regulatory loop functions as a master regulator to mediate a dual role of specifying epidermal ionocyte progenitors as well as of subsequently promoting differentiation of Na+,K+-ATPase-rich cells and H+-ATPase-rich cells in a concentration-dependent manner. We propose that the positive regulatory loop between foxi3a and foxi3b drives early ionocyte differentiation and prevents the complete blockage of ionocyte differentiation when the master regulator of foxi function is unilaterally compromised

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Discussion

Conclusion