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

Abstract

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.

Algal MIPs, high diversity and conserved motifs

“…Regarding the free living aquatic algae of this study we note that in some species of fish, MIPs have an important physiological function, regulating the buoyancy of the egg by adjusting the water content and thereby controlling the depth and hence the milieu for the developing egg [41,42]. It seems possible that a similar regulation can occur in free living algae in order to find optimal conditions for photosynthesis, uptake of nutrients, transport by currents or possibly to escape predators.…”

Oogenesis and Egg Quality in Finfish: Yolk Formation and Other Factors Influencing Female Fertility

“…The small peptides and free amino acids generated during maturational yolk processing, along with inorganic ions, act as osmotic effectors that drive an influx of water into the oocyte [26,32,52]. Water diffuses through aquaporins in the oocyte membrane [40,41], and this influx of water results in oocyte hydration and proper egg buoyancy. This hydration also provides metabolic water for dilution of embryonic waste products and an increased egg volume to allow an adequate space for embryonic development.…”

Aquaporin Evolution in Fishes

“…Despite this degeneracy, Aqp1ab has been found to play a conserved role in the oocyte hydration of teleosts (reviewed by Cerdà, 2009; Finn and Fyhn, 2010). The novel role of Aqp1ab in oocyte hydration was first suggested for gilthead seabream (Fabra et al, 2005), and has recently been experimentally demonstrated in the acanthomorph teleost Atlantic halibut (Zapater et al, 2011).…”