Water homeostasis in the fish oocyte: new insights into the role and molecular regulation of a teleost-specific aquaporin.

The discovery of the role of a teleost-specific aquaporin (Aqp1ab) during the process of oocyte hydration in marine fish producing pelagic (floating) eggs, recently confirmed by molecular approaches, has revealed that this mechanism is more sophisticated than initially thought. Recent phylogenetic and genomic studies suggest that Aqp1ab likely evolved by tandem duplication from a common ancestor and further neofunctionalized in oocytes for water transport. Investigations into the regulation of Aqp1ab during oogenesis indicate that the mRNA and protein product are highly accumulated during early oocyte growth, possibly through the transcriptional activation of the aqp1ab promoter by the classical nuclear progesterone receptor and perhaps by Sry-related high mobility group [HMG]-box (Sox) transcription factors. During oocyte growth and maturation, Aqp1ab intracellular trafficking may be regulated by phosphorylation and/or dephosphorylation of specific C-terminal residues in Aqp1ab, as well as by signal-mediated sorting processes. These mechanisms possibly regulate the temporal insertion of Aqp1ab into the oocyte plasma membrane during oocyte hydration, although the intracellular signaling pathways involved are yet unknown. Interestingly, in some freshwater species that spawn partially hydrated eggs, high accumulation of transcripts encoding functional Aqp1ab channels have also been found in the ovary. These findings suggest that the Aqp1ab-mediated mechanism for oocyte hydration is likely conserved in teleosts. The tight regulation of Aqp1ab during oogenesis, at both the transcriptional and posttranslational levels, highlights the essential physiological role of this water channel and opens new research avenues for understanding the molecular basis of egg formation in fish.

Kinetics and fates of ammonia, urea, and uric acid during oocyte maturation and ontogeny of the Atlantic halibut (Hippoglossus hippoglossus L.).

Considering that amino acids constitute an important energy fuel during early life of the Atlantic halibut (Hippoglossus hippoglossus L.), it is of interest to understand how the nitrogenous end products are handled. In this study we focused on the kinetics and fates of ammonia, urea and uric acid. The results showed that ammonia (T(Amm): NH(3)+NH(4)(+)), and urea-N contents increased during final oocyte maturation. Urea-N excretion dominated the total nitrogenous end product formation in early embryos. Later, yolk T(Amm) levels increased in embryos and ammonia excretion was low. In the last part of the embryonic stage T(Amm) accumulation dominated, and was apparently due to yolk storage. Around hatching, the larval body tissues (larva with yolk-sac removed) accounted for 68% of whole animal urea-N accumulation, while T(Amm) levels increased predominately by yolk accumulation. Afterwards, ammonia excretion dominated and uric acid accumulation accounted for less than 1%. Urea, synthesised either through the ornithine-urea cycle, argininolysis or uricolysis, accounted for approximately 8% of total nitrogenous end product formation in yolk-sac larvae. The results suggested that a sequence occurred regarding which nitrogenous end products dominated and how they were handled. Urea excretion dominated in early embryos (<7 dPF), followed by yolk ammonia accumulation (7-12 dPF), and finally, ammonia excretion dominated in later embryonic and yolk-sac larval stages (>12 dPF).

Lipovitellins derived from two forms of vitellogenin are differentially processed during oocyte maturation in haddock (Melanogrammus aeglefinus).

In the process of cloning vitellogenin (Vtg) cDNAs from haddock (Melanogrammus aeglefinus), two related, but distinct, mRNAs were identified. Full-length cDNA sequences were determined for both Vtg types (Had1 and Had2), and the deduced amino acid sequences were found to be 54% identical to each other and 48-58% identical to other teleost Vtgs. To investigate the expression of the two Vtg mRNAs, proteins from prehydrated oocytes and fertilized eggs were separated on SDS-polyacrylamide gels. Only a single lipovitellin I band was detected in each sample, and the egg lipovitellin I was smaller (97 vs. 110 kDa) than the oocyte protein, indicative of proteolytic processing during oocyte hydration. Mass spectrometric (MALDI-TOFMS and tandem mass spectrometry) analyses of tryptic fragments from the haddock oocyte and egg lipovitellin I revealed that the lipovitellin I from prehydrated oocytes contained tryptic fragments that matched the sequences of both types of Vtg, suggesting that there were two proteins in this band, while the egg lipovitellin I contained tryptic fragments that only matched the Had1 cDNA sequence, indicating that the Had2 lipovitellin had been degraded during hydration. Physiological data from haddock oocytes and eggs demonstrate that, as in other marine fish that spawn pelagic eggs, the free amino acid content increases during oocyte hydration and apparently contributes to hydration by driving the osmotic uptake of water. The correlation of the disappearance of one lipovitellin I with the increase of free amino acids in the oocyte suggests that this protein is a major source of the free amino acids for oocyte hydration.