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1 Department of Biological Sciences, Tokyo Institute of Technology, Yokohama, Japan
2 Division of Molecular and Developmental Biology, National Institute of Genetics, Mishima, Shizuoka, Japan
* To whom correspondence should be addressed. E-mail: shirose{at}bio.titech.ac.jp.
Uptake of Na+ from the environment is an indispensable strategy for the survival of freshwater fish, as they easily lose Na+ from the plasma to a diluted environment. Nevertheless, the location of, and molecules involved in Na+ uptake remain poorly understood. In this study, we utilized Sodium Green, a Na+-dependent fluorescent reagent, to provide direct evidence that Na+ absorption takes place in a subset of the mitochondrion-rich (MR) cells on the yolk sac surface of zebrafish larvae. Combined with immunohistochemistry, we revealed that the Na+-absorbing MR cells were exceptionally rich in vacuolar-type H+-ATPase (H+-ATPase) but moderately rich in Na+/K+-ATPase. We also addressed the function of foxi3
transcription factor that is specifically expressed in the H+-ATPase-rich MR cells. When foxi3 was depleted from zebrafish embryos by antisense morpholino oligonucleotide injection, differentiation of the MR cells was completely blocked and Na+ influx was severely reduced, indicating that MR cells are the primary sites for Na+ absorption. Additionally, foxi3 expression is initiated at gastrula stage in the presumptive ectoderm; thus we propose that foxi3 is a key gene in the control of MR cell differentiation. We also utilized a set of ion transport inhibitors to assess the molecules involved in the process and discuss the observations.
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