Molecular pathways during marine fish egg hydration: the role of aquaporins

The pre-ovulatory hydration of the oocyte of marine teleosts, a unique process among vertebrates that occurs concomitantly with meiosis resumption (oocyte maturation), is a critical process for the correct development and survival of the embryo. Increasing information is available on the molecular mechanisms that control oocyte maturation in fish, but the identification of the cellular processes involved in oocyte hydration has remained long ignored. During the past few years, a number of studies have identified the major inorganic and organic osmolytes that create a transient intra-oocytic osmotic potential for hydrating the oocytes, whereas water influx was believed to occur passively. Recent work, however, has uncovered the role of a novel molecular water channel (aquaporin), designated aquaporin-1b (Aqp1b), which facilitates water permeation and resultant swelling of the oocyte. The Aqp1b belongs to a teleost-specific subfamily of water-selective aquaporins, similar to mammalian aquaporin-1 (AQP1) that has possibly evolved by duplication of a common ancestor and further neofunctionalization in oocytes of marine teleosts for water uptake. Strikingly, Aqp1b shows specific regulatory domains at the cytoplasmic tail, which are key to the vesicular trafficking and temporal insertion of Aqp1b in the oocyte plasma membrane during the phase of maximal hydration. These findings are revealing that the mechanism of oocyte hydration in marine teleosts is a highly regulated process based on the interplay between the generation of inorganic and organic osmolytes and the controlled insertion of Aqp1b in the oocyte surface. The discovery of Aqp1b in teleosts provides an important insight into the molecular basis of the production of viable eggs in marine fish.