Fish egg yolk is largely derived from vitellogenins, which are synthesized in the liver, taken up from the maternal circulation by growing oocytes via receptor-mediated endocytosis and enzymatically processed into yolk proteins that are stored in the ooplasm. Lipid droplets are another major component of fish egg yolk, and these are mainly composed of neutral lipids that may originate from maternal plasma lipoproteins. This review aims to briefly summarize our current understanding of the molecular mechanisms underlying yolk formation in fishes. A hypothetical model of oocyte growth is proposed based on recent advances in our knowledge of fish yolk formation.

Egg quality is a complex biological trait and a major determinant of reproductive fitness in all animals. This study delivered the first proteomic portraits of egg quality in zebrafish, a leading biomedical model for early development. Egg batches of good and poor quality, evidenced by embryo survival for 24 h, were sampled immediately after spawning and used to create pooled or replicated sample sets whose protein extracts were subjected to different levels of fractionation before liquid chromatography and tandem mass spectrometry. Obtained spectra were searched against a zebrafish proteome database and detected proteins were annotated, categorized and quantified based on normalized spectral counts. Manually curated and automated enrichment analyses revealed poor quality eggs to be deficient of proteins involved in protein synthesis and energy and lipid metabolism, and of some vitellogenin products and lectins, and to have a surfeit of proteins involved in endo-lysosomal activities, autophagy, and apoptosis, and of some oncogene products, lectins and egg envelope proteins. Results of pathway and network analyses suggest that this aberrant proteomic profile results from failure of oocytes giving rise to poor quality eggs to properly transit through final maturation, and implicated Wnt signaling in the etiology of this defect. Quantitative comparisons of abundant proteins in good versus poor quality eggs revealed 17 candidate egg quality markers. Thus, the zebrafish egg proteome is clearly linked to embryo developmental potential, a phenomenon that begs further investigation to elucidate the root causes of poor egg quality, presently a serious and intractable problem in livestock and human reproductive medicine.

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Follicle-stimulating hormone (Fsh) is thought to act early in the process of spermatogenesis; however, its action in fish has not yet been clearly established. In the present work, we analyzed the effects of recombinant Fsh in sea bass (Dicentrarchus labrax) spermatogenesis according to two different approaches: direct injection of recombinant single-chain Fsh hormone (scFSH) and injection of scFSH coding sequence. Both approaches were efficient in increasing plasma Fsh at 7 and 15 days, respectively, after injection. The Fsh increment caused a significant increase in plasma 11-ketotestosterone levels and induced dramatic changes at the testicular level. Fsh-treated groups showed an increase in germ cell proliferation at Day 7, and cysts of spermatocytes and spermatids were observed at the end of the experiment. After treatment with Fsh, a suppression in amh transcripts and an increase of lhr transcripts were detected at Day 7 and Day 15, respectively, and an increment in fshr expression became evident at Day 23. These results show that Fsh initiates germ cell proliferation, triggering spermatogenesis in sea bass via androgen production and regulation of spermatogenesis-related genes.

Scrutiny of the zebrafish (Danio rerio) genomic database confirmed eight functional vitellogenin (vtg) genes, each with one or two transcript variants, and the encoded Vtg polypeptides were structurally and functionally characterized in detail by in silico and experimental analyses. There were five type I (vtgs1, 4, 5, 6, and 7), two type II (vtg2 and vtg8), and one type III (vtg3) vtg gene(s) encoding three major types of Vtg protein based on subdomain structure (Vtg-I, Vtg-II, and Vtg-III, respectively). Among various tissues of mature zebrafish, transcripts of the eight vtg genes were detected by RNA-Seq only in liver and intestine, with liver being the main site of vtg expression. All vtg transcripts except vtg8 were also detected in mature female liver by RT-qPCR. The relative abundances of Vtg proteins and their variants were quantified by LC-MS/MS in the liver of mature females and in eggs. The Vtgs were generally several fold more abundant in eggs, but profiles of abundance of the 19 different forms of Vtg evaluated were otherwise similar in liver and eggs, suggesting that yolk protein composition is determined largely by hepatic Vtg synthesis and secretion. Based on transcript and protein levels, Vtg-I is, by far, the dominant type of Vtg in zebrafish, followed by Vtg-II and then Vtg-III. When relative abundances of the different forms of Vtg were evaluated by LC-MS/MS in egg batches of good versus poor quality, no differences in the proportional abundance of individual forms of Vtg, or of different Vtg types, attributable to egg quality were observed.

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Water channels (aquaporins) were originally discovered in mammals with fourteen subfamilies now identified (AQP0-13). Here we show that a functional Aqp14 subfamily phylogenetically related to AQP4-type channels exists in all vertebrate lineages except hagfishes and eutherian mammals. In contrast to the water-selective classical aquaporins, which have four aromatic-arginine constriction residues, Aqp14 proteins present five non-aromatic constriction residues and facilitate the permeation of water, urea, ammonia, H2O2 and glycerol. Immunocytochemical assays suggest that Aqp14 channels play important osmoregulatory roles in piscine seawater adaptation. Our data indicate that Aqp14 intracellular trafficking is tightly regulated by the vasotocinergic/isotocinergic neuropeptide and receptor systems, whereby protein kinase C and A transduction pathways phosphorylate highly conserved C-terminal residues to control channel plasma membrane insertion. The neuropeptide regulation of Aqp14 channels thus predates the vasotocin/vasopressin regulation of AQP2-5-6 orthologs observed in tetrapods. These findings demonstrate that vertebrate Aqp14 channels represent an ancient subfamily of neuropeptide-regulated polytransporters.