Aquaporin 1 Is Involved in Acid Secretion by Ionocytes of Zebrafish Embryos through Facilitating CO2 Transport

Jiun-Lin Horng,Pei-Lin Chao,Tin-Han Shih,Li-Yih Lin,Po-Yen Chen,Patrick Prunet

doi:10.1371/journal.pone.0136440

Jiun-Lin Horng, Pei-Lin Chao + Show 4 more

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https://doi.org/10.1371/journal.pone.0136440

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Abstract

Mammalian aquaporin 1 (AQP1) is well known to function as a membrane channel for H2O and CO2 transport. Zebrafish AQP1a.1 (the homologue of mammalian AQP1) was recently identified in ionocytes of embryos; however its role in ionocytes is still unclear. In this study, we hypothesized that zebrafish AQP1a.1 is involved in the acid secretion by ionocytes through facilitating H2O and CO2 diffusion. A real-time PCR showed that mRNA levels of AQP1a.1 in embryos were induced by exposure to 1% CO2 hypercapnia for 3 days. In situ hybridization and immunohistochemistry showed that the AQP1a.1 transcript was highly expressed by acid-secreting ionocytes, i.e., H+-ATPase-rich (HR) cells. A scanning ion-selective electrode technique (SIET) was applied to analyze CO2-induced H+ secretion by individual ionocytes in embryos. H+ secretion by HR cells remarkably increased after a transient loading of CO2 (1% for 10 min). AQP1a.1 knockdown with morpholino oligonucleotides decreased the H+ secretion of HR cells by about half and limited the CO2 stimulated increase. In addition, exposure to an AQP inhibitor (PCMB) for 10 min also suppressed CO2-induced H+ secretion. Results from this study support our hypothesis and provide in vivo evidence of the physiological role of AQP1 in CO2 transport.

Highlights

Aquaporins (AQPs) are expressed in a variety of water-transporting epithelia, such as the kidney, stomach, and small intestine and play an important role in facilitating water transport across cell membranes [1]
A previous study used an Reverse transcription (RT)-polymerase chain reaction (PCR) to show that aqp1a.1, aqp3a, aqp3b, aqp4, aqp7, aqp8a.1, aqp9a, aqp10a, and aqp12 are expressed by zebrafish gills [28]
The results showed that aqp1a.1, aqp3a, aqp8a.1, and aqp9a in gills had relatively higher expression levels among those aqps (Fig 1A); aqp1a.1, aqp3a, aqp4, aqp8a.1, and aqp12 had higher expression levels in whole embryos (Fig 1B)

Summary

Introduction

Aquaporins (AQPs) are expressed in a variety of water-transporting epithelia, such as the kidney, stomach, and small intestine and play an important role in facilitating water transport across cell membranes [1]. AQPs transport small molecules such as urea and glycerol [1, 2]. AQPs were shown to be permeable to gas molecules. AQP1 was the first channel protein identified to be a CO2 gas channel. It is highly expressed in tissues necessary for CO2 transport such as pulmonary capillaries, vascular smooth muscle, and red blood cells, supporting the hypothesis that AQP1 can function as a CO2 channel [2]

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