Cloning and characterization of a zebrafish homologue of human AQP1: a bifunctional water and gas channel

Abstract

The mammalian aquaporins AQP1, AQP4, and AQP5 have been shown to function not only as water channels but also as gas channels. Zebrafish have two genes encoding an AQP1 homologue, aqp1a and aqp1b. In the present study, we cloned the cDNA that encodes the zebrafish protein Aqp1a from the 72-h postfertilization (hpf) embryo of Danio rerio, as well as from the swim bladder of the adult. The deduced amino-acid sequence of aqp1a consists of 260 amino acids and is 59% identical to human AQP1. By analyzing the genomic DNA sequence, we identified four exons in the aqp1a gene. By in situ hybridization, aqp1a is expressed transiently in the developing vasculature and in erythrocytes from 16 to 48 h of development. Later, at 72 hpf, aqp1a is expressed in dermal ionocytes and in the swim bladder. Western blot analysis of adult tissues reveals that Aqp1a is most highly expressed in the eye and swim bladder. Xenopus oocytes expressing aqp1a have a channel-dependent (*) osmotic water permeability (Pf*) that is indistinguishable from that of human AQP1. On the basis of the magnitude of the transient change in surface pH (ΔpHS) that were recorded as the oocytes were exposed to either CO2 or NH3, we conclude that zebrafish Aqp1a is permeable to both CO2 and NH3. The ratio (ΔpHS*)CO2/Pf* is about half that of human AQP1, and the ratio (ΔpHS*)NH3/Pf* is about one-quarter that of human AQP1. Thus, compared with human AQP1, zebrafish Aqp1a has about twice the selectivity for CO2 over NH3