Differential Expression and Localization of Branchial AQP1 and AQP3 in Japanese Medaka (Oryzias latipes).

Laura V Ellis,Hannah M Weber,Steffen S Madsen,Rebecca J Bollinger,Christian K Tipsmark

doi:10.3390/cells8050422

Laura V Ellis, Hannah M Weber + Show 3 more

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https://doi.org/10.3390/cells8050422

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Abstract

Aquaporins (AQPs) facilitate transmembrane water and solute transport, and in addition to contributing to transepithelial water transport, they safeguard cell volume homeostasis. This study examined the expression and localization of AQP1 and AQP3 in the gills of Japanese medaka (Oryzias latipes) in response to osmotic challenges and osmoregulatory hormones, cortisol, and prolactin (PRL). AQP3 mRNA was inversely regulated in response to salinity with high levels in ion-poor water (IPW), intermediate levels in freshwater (FW), and low levels in seawater (SW). AQP3 protein levels decreased upon SW acclimation. By comparison, AQP1 expression was unaffected by salinity. In ex vivo gill incubation experiments, AQP3 mRNA was stimulated by PRL in a time- and dose-dependent manner but was unaffected by cortisol. In contrast, AQP1 was unaffected by both PRL and cortisol. Confocal microscopy revealed that AQP3 was abundant in the periphery of gill filament epithelial cells and co-localized at low intensity with Na+,K+-ATPase in ionocytes. AQP1 was present at a very low intensity in most filament epithelial cells and red blood cells. No epithelial cells in the gill lamellae showed immunoreactivity to AQP3 or AQP1. We suggest that both AQPs contribute to cellular volume regulation in the gill epithelium and that AQP3 is particularly important under hypo-osmotic conditions, while expression of AQP1 is constitutive.

Highlights

Euryhaline fish, such as the Japanese medaka (Oryzias latipes), are capable of acclimating to both freshwater (FW) and seawater (SW) environments
The current study aims to answer three main questions: (i) in which cell types do we find the two aquaporin paralogs, AQP1 and AQP3, expressed in the gill of medaka?; (ii) how are branchial AQP1 and AQP3 mRNA and protein expression and localization affected by environmental salinity?; and (iii) are PRL and cortisol involved in endocrine control of AQP expression? Time-course salinity transfer experiments from FW
The present study investigated events related to water balance in the gill and focused on the response of two aquaporin paralogs expressed in whole gill: AQP1 and AQP3

Summary

Introduction

Euryhaline fish, such as the Japanese medaka (Oryzias latipes), are capable of acclimating to both freshwater (FW) and seawater (SW) environments. In FW, they are challenged by osmotic water inflow and passive ion loss and must combat this by actively absorbing ions from the environment and getting rid of excess water [1,2]. In SW, integumental ion and water fluxes are basically reversed, meaning that the passive influx of ions and loss of water must be compensated for by the active secretion of monovalent ions in the gill and solute-linked water absorption in the gastrointestinal tract. Ionocytes in the gill are the cells responsible for the active transport of ions in both directions depending on the salinity. In addition to a high density of basolateral Na+ ,K+ -ATPase (Nka), ionocytes are equipped with specific apical and basolateral ion transport proteins that work together in ion absorption and excretion.

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