Molecular and cellular basis of formation, hardening, and breakdown of the egg envelope in fish

Genome editing in East African cichlids and tilapias: state-of-the-art and future directions

African cichlid fishes of the Cichlidae family are a group of teleosts important for aquaculture and research. A thriving research community is particularly interested in the cichlid radiations of the East African Great Lakes. One key goal is to pinpoint genetic variation underlying phenotypic diversification, but the lack of genetic tools has precluded thorough dissection of the genetic basis of relevant traits in cichlids. Genome editing technologies are well established in teleost models like zebrafish and medaka. However, this is not the case for emerging model organisms, such as East African cichlids, where these technologies remain inaccessible to most laboratories, due in part to limited exchange of knowledge and expertise. The Cichlid Science 2022 meeting (Cambridge, UK) hosted for the first time a Genome Editing Workshop, where the community discussed recent advances in genome editing, with an emphasis on CRISPR/Cas9 technologies. Based on the workshop findings and discussions, in this review we define the state-of-the-art of cichlid genome editing, share resources and protocols, and propose new possible avenues to further expand the cichlid genome editing toolkit.

Critical Role of the Cortical Alveolus Protease Alveolin in Chorion Hardening In Vivo at Medaka Fertilization

Alveolin is a cortical alveolus proteinase that is secreted in the perivitelline space (PVS) at fertilization to act on the chorion. Purified alveolin is known to induce chorion hardening in vitro by processing zona pellucida B (ZPB), a major chorion component. However, in vivo function of alveolin remains unclear; thus, in this study, the effects of alveolin efficiency (Alv^-/-) at the organism level were investigated using the medaka, Oryzias latipes. The Alv^-/- fertilized eggs were mechanically fragile; however, they developed normally and left offspring as long as they were carefully handled before hatching. A mechanical press test showed that the Alv^-/- fertilized eggs were six times more fragile than the wild-type eggs. They were 35% larger owing to the enlarged PVS, 34% thinner, and permeable to even 10 kDa FITC-dextran. These results are consistent with the transmission electron microscopy observation that the periphery of the inner layers was highly porous in the Alv^-/- chorion. In chorion hardening, the alveolin-mediated processing of ZPB and the transglutaminase (TGase)-mediated crosslinking of chorion components are the key steps. This study was the first to show that alveolin also processed TGase concomitantly with ZPB, which greatly facilitated the crosslinking. Thus, alveolin was concluded to be the primary trigger for chorion hardening in vivo. Furthermore, fertilization in a balanced salt solution could partially improve the impaired chorion hardening of the Alv^-/- eggs fertilized in water, probably through an alveolin-independent mechanism.

Molecular Mechanisms of Spawning Habits for the Adaptive Radiation of Endemic East Asian Cyprinid Fishes

Despite the widespread recognition of adaptive radiation as a driver of speciation, the mechanisms by which natural selection generates new species are incompletely understood. The evolutionary radiation of endemic East Asian cyprinids has been proposed as evolving through a change in spawning habits, involving a transition from semibuoyant eggs to adhesive eggs in response to crosslinked river-lake system formation. Here, we investigated the molecular mechanisms that underpin this radiation, associated with egg hydration and adhesiveness. We demonstrated that semibuoyant eggs enhance hydration by increasing the degradation of yolk protein and accumulation of Ca²⁺ and Mg²⁺ ions, while adhesive eggs improve adhesiveness and hardness of the egg envelope by producing an adhesive layer and a unique 4th layer to the egg envelope. Based on multiomics analyses and verification tests, we showed that during the process of adaptive radiation, adhesive eggs downregulated the “vitellogenin degradation pathway,” “zinc metalloprotease pathway,” and “ubiquitin-proteasome pathway” and the pathways of Ca²⁺ and Mg²⁺ active transport to reduce their hydration. At the same time, adhesive eggs upregulated the crosslinks of microfilament-associated proteins and adhesive-related proteins, the hardening-related proteins of the egg envelope, and the biosynthesis of glycosaminoglycan in the ovary to generate adhesiveness. These findings illustrate the novel molecular mechanisms associated with hydration and adhesiveness of freshwater fish eggs and identify critical molecular mechanisms involved in the adaptive radiation of endemic East Asian cyprinids. We propose that these key egg attributes may function as “magic traits” in this adaptive radiation.

Medaka, Oryzias latipes, egg envelopes are created by ovarian-expressed ZP proteins and liver-expressed choriogenins

The medaka (Oryzias latipes) egg envelope (chorion) is composed of three major glycoproteins, Zona Interna (ZI)-1, -2, and -3, that originate in the spawning female liver as the precursor proteins Choriogenin (Chg.)H, Chg.Hm, and Chg.L, respectively. These ZI and Chg. proteins contain a structural ZP protein domain that is conserved among the egg envelope proteins of all animals. While ovarian expression of ZP proteins (e.g., ZPCs and ZPB) has been reported in medakas, the functions of these proteins remain unknown. Thus, the present study aimed to determine whether the ovary-expressed medaka ZP protein, mZPC5, is involved in forming the chorion matrix.The mZPC5 gene (mzpc5) was expressed in the ovaries but not the livers of mature female medakas, as shown by reverse transcription-polymerase chain reaction assays with mzpc5-specific primers. In situ hybridization analysis revealed that ovarian mzpc5 expression was restricted to the ooplasm of early (stage I-III) previtellogenic oocytes, and its expression signal weakened with oocyte growth. Following sodium-dodecyl-sulfate polyacrylamide gel electrophoresis (SDS-PAGE) and Western blotting analysis with anti-mZPC5 antibodies, two immunoreactive proteins were detected in the ovary and chorion extracts. These proteins were approximately 50 and 74 kDa in size, like ZI-3 and ZI-2, respectively.Immunohistochemical assays using anti-mZPC5 and anti-Chg.H antibodies localized the mZPC5 protein in the ooplasm of early previtellogenic oocytes. With oocyte growth, mZPC5 tended to accumulate in the chorion, co-localizing with Chg.H.We previously showed that ovary-expressed ZP proteins could not compensate for Chg.L function loss in gene knock-out (chg.l -/-) medakas. As in our previous study, the chg.l-/- females produced oocytes with thin chorions, resulting in infertile soft eggs. However, in the present study, mZPC5 and Chg.H were co-localized in the chg.l-/- chorions. These results suggested that in the medaka previtellogenic oocyte, 1) mZPC5 is secreted from the ooplasm and deposited on the outer surface of its plasma membrane, creating the thin chorion layer; and 2) following the accumulation of liver-derived Chgs., the 3D structure of the chorion matrix is formed cooperatively with mZPC5 and Chgs. during oogenesis. More research is needed to confirm the functions of mZPC5 in chorion structure and physiology.

Morphological description of gametes in cave and surface populations of Astyanax mexicanus (De Filippi, 1853)

The Mexican tetra Astyanax mexicanus presents two contrasting morphs, a widely distributed surface morph and a cave-adapted morph. These cave-adapted morphs have evolved independently from two different lineages (i.e. 'old' and 'new' lineages); therefore, this model system gives a unique opportunity to explore parallel adaptive evolution in biological traits. The present study corresponds to the first morphological description of the Astyanax mexicanus maturation process of the spermatozoa and oocytes, using thermal and hormonal stimuli to promote spermatogenesis and oogenesis, considering surface and cave morphs from both lineages. We corroborate the relevance of thermal and hormonal stimuli to promote gamete maturation. The hormone Ovaprim (GnRHa + Domperidone) is an effective promoter of ovarian development, maturation end in oocytes and spawning in Astyanax mexicanus. The sperm morphology of Astyanax mexicanus includes the sperm head, the midpiece, and tail or flagellum. We found differences in the spermatozoan total length between environments (F = 9.929, P = 0.05) and linages (F = 49.86, P = 0.005). The oocytes showed a spherical conformation with a mean diameter of 822.4 ± 194.1 μm for the surface populations, and 604.6 ± 38.3 µm for the cave populations. The oocyte chorion presents ridges and grooves that are arranged radially towards the micropyle. A plug in the micropyle zone was observed after fertilization, confirmed by the outer membrane of the chorion, which provides some weak adhesiveness to the substrate. We observed differences in chorion thickness between the contrasting environmental conditions. This is the first morphological characterization of the Sótanos Vázquez, Escondido and Tigre, which previous to this study were only known from speleological expeditions, with no previous biological information available.

Embryotoxicity of Polystyrene Microspheres of Different Sizes to the Marine Medaka Oryzias melastigma (McClelland, 1839)

Polystyrene microplastics (PS-MPs) are potentially harmful to marine organisms, especially during the early developmental stages, although the underlying mechanism remains unclear. The present study evaluated the growth and morphological characteristics of marine medaka Oryzias melastigma (McClelland, 1839) embryos exposed to PS-MP. PS-MPs of three different sizes (0.05, 0.5, and 6.0 μm with a concentration of 106 particles/L) were subjected to waterborne exposure for 19 d. The hatching time and rate of embryos exposed to 0.5 and 6.0 μm PS-MPs were significantly lower than those of the control, while no significant difference was observed in the 0.05 μm treatment. No significant differences were observed in the mortality rate of the embryos, embryo diameter, and relevant gene expression levels, including il6, il8, il-1β, jak, stat-3, nf-κb, hif-1α, epo, cyp1a1, ahr, sod, cat, and gpx, but with the exception of vtg. Fluorescent PS-MPs were found on the embryo surfaces when the embryos were exposed to 0.5 and 6.0 μm PS-MPs, but no signals were detected inside embryos using confocal microscopy. Therefore, the results indicate that PS-MPs having a diameter of 6.0 μm can only attach to the surface or villus of embryos and not enter the embryos through the membrane pores, whereas PS-MPs with diameters of 0.05 and 0.5 μm cannot enter the embryos.

Targeted deletion of liver-expressed Choriogenin L results in the production of soft eggs and infertility in medaka, Oryzias latipes

Egg envelopes (chorions) in medaka, Oryzias latipes, are composed of three major glycoproteins: ZI-1, - 2, and - 3. These gene-encoded chorion glycoproteins are expressed in the liver and/or ovarian oocytes of sexually mature female fish. In medaka, the glycoproteins produced in the female liver are induced by estrogen as Choriogenin (Chg.) H and Chg. H minor (m), which correspond to the zona pellucida (ZP) B (ZPB) protein in mammals, and Chg. L, which corresponds to ZPC in mammals. Chg. H, Chg. Hm, and Chg. L, are then converted to ZI-1, - 2, and - 3, respectively, during oogenesis in medaka ovaries.In the present study, we established a medaka line in which the chg.l gene was inactivated using the transcription activator-like effector nuclease (TALEN) technique. Neither intact chg.l transcripts nor Chg. L proteins were detected in livers of sexually mature female homozygotes for the mutation (homozygous chg.l knockout: chg.l-/-). The chg.l-/- females spawned string-like materials containing "smashed eggs." Closer examination revealed the oocytes in the ovaries of chg.l-/- females had thin chorions, particularly at the inner layer, despite a normal growth rate. In comparing chorions from normal (chg.l+/+) and chg.l-/- oocytes, the latter exhibited abnormal architecture in the chorion pore canals through which the oocyte microvilli pass. These microvilli mediate the nutritional exchange between the oocyte and surrounding spaces and promote sperm-egg interactions during fertilization. Thus, following in vitro fertilization, no embryos developed in the artificially inseminated oocytes isolated from chg.l-/- ovaries. These results demonstrated that medaka ZI-3 (Chg.L) is the major component of the inner layer of the chorion, as it supports and maintains the oocyte's structural shape, enabling it to withstand the pressures exerted against the chorion during spawning, and is essential for successful fertilization. Therefore, gene products of oocyte-specific ZP genes that may be expressed in medaka oocytes cannot compensate for the loss Chg. L function to produce offspring for this species.

Chorion Alterations in Eyed-Stage Salmonid Eggs Farmed in La Araucanía, Chile: A Retrospective Study

Simple Summary

The chorion (also called egg envelope) is the primary envelope that protects the fish embryo against drying, mechanical actions, and abrupt changes in the water conditions. Alterations of the chorion during the embryo incubation are not unusual, but these are scarcely reported. Increased occurrence of chorion alterations can lead to decreased reproductive performance and important losses for fish farms. Here, we described several chorion alterations observed in samples of embryonated eggs collected from different salmon and trout farms located in southern Chile over a period of 14 years. We detected four types of chorion alterations and found soft chorion as the most prevalent alteration in the years analyzed, affecting mainly Atlantic and Coho Salmon. Eggs of Rainbow Trout displayed fewer chorion alterations among the three species analyzed. As the eggs analyzed here were produced under standard industrial conditions, we conclude that these alterations are possibly linked to changes in water conditions, which need to be further investigated.

Abstract

The chorion is the primary envelop that protects the fish embryo against mechanical actions, pathogens, and abrupt changes in physical and chemicals conditions of the incubation medium. During embryo development, chorion alterations are not rare, but the occurrence of these is scarcely reported. Increased frequency of chorion alterations can result in increased embryo mortality and thus decreased reproductive performance and losses for fish farms. In this study, we characterize different chorion alterations observed in samples collected over 14 years from 12 salmon and trout farms located in the region of La Araucanía in southern Chile, which sent live eyed-stage embryos (‘eyed-eggs’) for quality analysis to our laboratory. We found soft chorion as the most common alteration observed, being present in the whole 14-year series analyzed in Atlantic Salmon (Salmo salar) and affecting up to 35.0% of the samples examined in a year. This alteration also affected up to 20.0 and 5.7% of Coho Salmon (Oncorhynchus kisutch) and Rainbow Trout (Oncorhynchus mykiss) samples analyzed in a year, respectively. We also found an increase of other chorion alterations, including perforated and white-spotted chorion in Atlantic and Coho Salmon, in the last 8 years. Among the three species, Rainbow Trout exhibited fewer chorion alterations. As the embryonated eggs analyzed here were obtained from broodstocks maintained under standard industrial conditions, these alterations might be linked to changes in environmental conditions affecting the incubation water that need to be further investigated.

Proteomics support the threespine stickleback egg coat as a protective oocyte envelope

Ancestrally marine threespine stickleback fish (Gasterosteus aculeatus) have undergone an adaptive radiation into freshwater environments throughout the Northern Hemisphere, creating an excellent model system for studying molecular adaptation and speciation. Ecological and behavioral factors have been suggested to underlie stickleback reproductive isolation and incipient speciation, but reproductive proteins mediating gamete recognition during fertilization have so far remained unexplored. To begin to investigate the contribution of reproductive proteins to stickleback reproductive isolation, we have characterized the stickleback egg coat proteome. We find that stickleback egg coats are comprised of homologs to the zona pellucida (ZP) proteins ZP1 and ZP3, as in other teleost fish. Our molecular evolutionary analyses indicate that across teleosts, ZP3 but not ZP1 has experienced positive Darwinian selection. Mammalian ZP3 is also rapidly evolving, and surprisingly some residues under selection in stickleback and mammalian ZP3 directly align. Despite broad homology, however, we find differences between mammalian and stickleback ZP proteins with respect to glycosylation, disulfide bonding, and sites of synthesis. Taken together, the changes we observe in stickleback ZP protein architecture suggest that the egg coats of stickleback fish, and perhaps fish more generally, have evolved to fulfill a more protective functional role than their mammalian counterparts.

Embryonic development of the fire-eye-tetra Moenkhausia oligolepis (Characiformes: Characidae)

This study describes the embryonic development of Moenkhausia oligolepis in laboratory conditions. After fertilization, the embryos were collected every 10 min up to 2 h, then every 20 min up to 4 h, and afterwards every 30 min until hatching. The fertilized eggs of M. oligolepis measured approximately 0.85 ± 0.5 mm and had an adhesive surface. Embryonic development lasted 14 h at 25ºC through the zygote, cleavage, blastula, gastrula, neurula, and segmentation phases. Hatching occurred in embryos around the 30-somites stage. The present results contribute only the second description of embryonic development to a species from the Moenkhausia genus, being also the first for this species. Such data are of paramount importance considering the current conflicting state of this genus phylogenetic classification and may help taxonomic studies. Understanding the biology of a species that is easily managed in laboratory conditions and has an ornamental appeal may assist studies in its reproduction to both supply the aquarium market and help the species conservation in nature. Moreover, these data enable the use of M. oligolepis as a model species in biotechnological applications, such as the germ cell transplantation approach.

Study of mastoparan analog peptides against Candida albicans and safety in zebrafish embryos (Danio rerio)

Aim: In this work, mastoparan analog peptides from wasp venom were tested against Candida albicans and safety assays were performed using cell culture and model zebrafish. Materials & methods: Minimal inhibitory concentration was determined and toxicity was performed using human skin keratinocyte and embryo zebrafish. Also, permeation of peptides through embryo chorion was performed. Results: The peptides demonstrated anti-C. albicans activity, with low cytotoxicity and nonteratogenicity in Danio rerio. The compounds had different permeation through chorion, suggesting that this occurs due to modifications in their amino acid sequence. Conclusion: The results showed that the studied peptides can be used as structural study models for novel potential antifungal agents.

Microinjection of Marine Fish Eggs

Microinjection is a powerful tool for studying embryonic development and analyzing gene functions in fish. This technique was first applied to model species of fish such as zebrafish and medaka whose egg chorions could be removed or softened before microinjection. Recent progress in genome editing using TALEN and CRISPR has opened the opportunity to analyze gene functions in a much wider range of fish including those important to marine aquaculture. Therefore, application of the microinjection technique is also required in these species. However, the characteristics of fish eggs vary widely between species and several technical difficulties need to be overcome in order to use microinjection in a wider range of species. To obtain consistent results with microinjection, an optimal method has to be developed for each target species. In this chapter, we describe the physical characteristics of the eggs of fish species that have been used in microinjection experiments in our laboratory and detail the microinjection system we developed for fish eggs with a hard chorion, such as those of marine species.

Egg Coat Proteins Across Metazoan Evolution

All animal oocytes are surrounded by a glycoproteinaceous egg coat, a specialized extracellular matrix that serves both structural and species-specific roles during fertilization. Egg coat glycoproteins polymerize into the extracellular matrix of the egg coat using a conserved protein-protein interaction module-the zona pellucida (ZP) domain-common to both vertebrates and invertebrates, suggesting that the basic structural features of egg coats have been conserved across hundreds of millions of years of evolution. Egg coat proteins, as with other proteins involved in reproduction, are frequently found to be rapidly evolving. Given that gamete compatibility must be maintained for the fitness of sexually reproducing organisms, this finding is somewhat paradoxical and suggests a role for adaptive diversification in reproductive protein evolution. Here we review the structure and function of metazoan egg coat proteins, with an emphasis on the potential role their evolution has played in the creation and maintenance of species boundaries.

Stickleback embryos use ATP-binding cassette transporters as a buffer against exposure to maternally derived cortisol

Offspring from females that experience stressful conditions during reproduction often exhibit altered phenotypes and many of these effects are thought to arise owing to increased exposure to maternal glucocorticoids. While embryos of placental vertebrates are known to regulate exposure to maternal glucocorticoids via placental steroid metabolism, much less is known about how and whether egg-laying vertebrates can control their steroid environment during embryonic development. We tested the hypothesis that threespine stickleback (Gasterosteus aculeatus) embryos can regulate exposure to maternal steroids via active efflux of maternal steroids from the egg. Embryos rapidly (within 72 h) cleared intact steroids, but blocking ATP-binding cassette (ABC) transporters inhibited cortisol clearance. Remarkably, this efflux of cortisol was sufficient to prevent a transcriptional response of embryos to exogenous cortisol. Taken together, these findings suggest that, much like their placental counterparts, developing fish embryos can actively regulate their exposure to maternal cortisol. These findings highlight the fact that even in egg-laying vertebrates, the realized exposure to maternal steroids is mediated by both maternal and embryonic processes and this has important implications for understanding how maternal stress influences offspring development.

Stage susceptibility of Japanese medaka (Oryzias latipes) to selenomethionine and hypersaline developmental toxicity

Anthropogenic disturbance of seleniferous soils can lead to selenium contamination of waterways. Although selenium is an essential micronutrient, bioaccumulation and maternal transfer of proteinaceous selenomethionine (SeMet) can result in embryo toxicity. Furthermore, as the climate changes, the salinity of spawning grounds in water-restrained estuaries is increasing. Although a small increase in salinity may not directly impact adult fish, it may alter the detoxification strategies of developing organisms. Previous research indicates that hypersalinity may potentiate SeMet embryo toxicity at an early developmental stage. However, embryonic development is a complex, spatiotemporal process with a constantly shifting cellular microenvironment. To generate thresholds and an adverse outcome pathway for the interactions between selenium and salinity, we sought to identify windows of susceptibility for lethality and deformities in the Japanese medaka (Oryzias latipes). Embryos were treated in freshwater or saltwater for 24 h with 0.5 µM, 5 µM, and 50 µM SeMet at 6 different developmental stages (9, 17, 25, 29, 34, and 38). Survival, hatch, deformities (total, type, and severity), and days to hatch were quantified. Selenium embryo tissue measurements were performed. Selenomethionine exposures of 5 µM and 50 µM significantly decreased survival and hatch at all stages. However, SeMet uptake was stage-dependent and increased with stage. Stage 17 (early neurulation) was identified as the most susceptible stage for lethality and deformities. Selenomethionine in saltwater caused significantly greater toxicity than freshwater at stage 25 (early organogenesis), suggesting a role for liver and osmoregulatory organogenesis in toxicity.

Disparate developmental patterns of immune responses to bacterial and viral infections in fish

During early stages of development vertebrates rely on an immature immune system to fight pathogens, but in non mammalian species few studies have taken an in-depth analysis of the transition from reliance on innate immune mechanisms to the appearance of adaptive immunity. Using rainbow trout as a model we characterized responses to two natural pathogens of this species, the Gram negative bacterium Aeromonas salmonicida and the virus VHSV, using microarray analysis at four early life history stages; eyed egg, post hatch, first feeding and three weeks post first feeding when adaptive immunity starts to be effective. All stages responded to both infections, but the complexity of the response increased with developmental stage. The response to virus showed a clear interferon response only from first feeding. In contrast, bacterial infection induced a marked response from early stages, with modulation of inflammatory, antimicrobial peptide and complement genes across all developmental stages. Whilst the viral and bacterial responses were distinct, there were modulated genes in common, mainly of general inflammatory molecules. This work provides a first platform to explore the development of fish immunity to infection, and to compare the age-dependent changes (from embryo to adults) across vertebrates.

Protein-Carbohydrate Interaction between Sperm and the Egg-Coating Envelope and Its Regulation by Dicalcin, a Xenopus laevis Zona Pellucida Protein-Associated Protein

Protein-carbohydrate interaction regulates multiple important processes during fertilization, an essential biological event where individual gametes undergo intercellular recognition to fuse and generate a zygote. In the mammalian female reproductive tract, sperm temporarily adhere to the oviductal epithelium via the complementary interaction between carbohydrate-binding proteins on the sperm membrane and carbohydrates on the oviductal cells. After detachment from the oviductal epithelium at the appropriate time point following ovulation, sperm migrate and occasionally bind to the extracellular matrix, called the zona pellucida (ZP), which surrounds the egg, thereafter undergoing the exocytotic acrosomal reaction to penetrate the envelope and to reach the egg plasma membrane. This sperm-ZP interaction also involves the direct interaction between sperm carbohydrate-binding proteins and carbohydrates within the ZP, most of which have been conserved across divergent species from mammals to amphibians and echinoderms. This review focuses on the carbohydrate-mediated interaction of sperm with the female reproductive tract, mainly the interaction between sperm and the ZP, and introduces the fertilization-suppressive action of dicalcin, a Xenopus laevis ZP protein-associated protein. The action of dicalcin correlates significantly with a dicalcin-dependent change in the lectin-staining pattern within the ZP, suggesting a unique role of dicalcin as an inherent protein that is capable of regulating the affinity between the lectin and oligosaccharides attached on its target glycoprotein.

Identification of the origin and localization of chorion (egg envelope) proteins in an ancient fish, the white sturgeon, Acipenser transmontanus

In many modern teleost fish, chorion (egg envelope) glycoproteins are synthesized in the liver of females, and the expression of those genes is controlled by endogenous estrogen released from the ovary during maturation. However, among the classical teleosts, such as salmonid, carp, and zebrafish, the chorion glycoproteins are synthesized in the oocyte, as in higher vertebrates. Sturgeon, which are members of the subclass Chondrostei, represent an ancient lineage of ray-finned fishes that differ from other teleosts in that their sperm possess acrosomes, their eggs have numerous micropyles, and early embryo development is similar to that of amphibians. In order to understand the molecular mechanisms of chorion formation and the phylogenetic relationship between sturgeon and other teleosts, we used specific antibodies directed against the primary components of sturgeon chorion glycoproteins, using immunoblotting and immunocytochemistry approaches. The origin of each chorion glycoprotein was determined through analyses of both liver and ovary, and their localization during ovarian development was investigated. Our data indicate that the origin of the major chorion glycoproteins of sturgeon, ChG1, ChG2, and ChG4, derive not only from the oocyte itself but also from follicle cells in the ovary, as well as from hepatocytes. In the follicle cell layer, granulosa cells were found to be the primary source of ChGs during oogenesis in white sturgeon. The unique origins of chorion glycoproteins in sturgeon suggest that sturgeons are an intermediate form in the evolution of the teleost lineage.

Adhesion mechanisms in European whitefish Coregonus lavaretus eggs: is this a survival mechanism for high-energy spawning grounds?

This study examined the potential biochemical and mechanical structures that may contribute to egg adhesion in European whitefish Coregonus lavaretus. Experiments showed that eggs from a population of C. lavaretus from Loch Eck remained non-adhesive in a solution chemically similar to ovarian fluid but became adhesive seconds after contact with water. Examination of the ultrastructure of the chorion showed that the morphology changed significantly after contact with water, with nodule-like protuberances attached to connective filaments on the surface present in water-hardened but not non-water hardened eggs. Biochemical analysis showed the presence of Chain A, RNase ZF-3e proteins in the chorion of water-hardened but not non-water hardened eggs. Histochemical staining of the chorion of C. lavaretus eggs showed that the externa, but not the interna, stained positively for the presence of glycoproteins. From these results, it was concluded that C. lavaretus from Loch Eck possess both anatomical and biochemical adhesive mechanisms that have been undocumented in this species so far.

Molecular co-evolution of a protease and its substrate elucidated by analysis of the activity of predicted ancestral hatching enzyme

Background

Hatching enzyme is a protease that digests the egg envelope, enabling hatching of the embryo. We have comprehensively studied the molecular mechanisms of the enzyme action to its substrate egg envelope, and determined the gene/protein structure and phylogenetic relationships. Because the hatching enzyme must have evolved while maintaining its ability to digest the egg envelope, the hatching enzyme-egg envelope protein pair is a good model for studying molecular co-evolution of a protease and its substrate.

Results

Hatching enzymes from medaka (Oryzias latipes) and killifish (Fundulus heteroclitus) showed species-specific egg envelope digestion. We found that by introducing four medaka-type residue amino acid substitutions into recombinant killifish hatching enzyme, the mutant killifish hatching enzyme could digest medaka egg envelope. Further, we studied the participation of the cleavage site of the substrate in the species-specificity of hatching enzyme. A P2-site single amino acid substitution was responsible for the species-specificity. Estimation of the activity of the predicted ancestral enzymes towards various types of cleavage sites along with prediction of the evolutionary timing of substitutions allowed prediction of a possible evolutionary pathway, as follows: ancestral hatching enzyme, which had relatively strict substrate specificity, developed broader specificity as a result of four amino acid substitutions in the active site cleft of the enzyme. Subsequently, a single substitution occurred within the cleavage site of the substrate, and the recent feature of species-specificity was established in the hatching enzyme-egg envelope system.

Conclusions

The present study clearly provides an ideal model for protease-substrate co-evolution. The evolutionary process giving rise to species-specific egg envelope digestion of hatching enzyme was initiated by amino acid substitutions in the enzyme, resulting in altered substrate specificity, which later allowed an amino acid substitution in the substrate.

Diffusion of small molecules into medaka embryos improved by electroporation

BACKGROUND

Diffusion of small molecules into fish embryos is essential for many experimental procedures in developmental biology and toxicology. Since we observed a weak uptake of lithium into medaka eggs we started a detailed analysis of its diffusion properties using small fluorescent molecules.

RESULTS

Contrary to our expectations, not the rigid outer chorion but instead membrane systems surrounding the embryo/yolk turned out to be the limiting factor for diffusion into medaka eggs. The consequence is a bi-phasic uptake of small molecules first reaching the pervitelline space with a diffusion half-time in the range of a few minutes. This is followed by a slow second phase (half-time in the range of several hours) during which accumulation in the embryo/yolk takes place. Treatment with detergents improved the uptake, but strongly affected the internal distribution of the molecules. Testing electroporation we could establish conditions to overcome the diffusion barrier. Applying this method to lithium chloride we observed anterior truncations in medaka embryos in agreement with its proposed activation of Wnt signalling.

CONCLUSIONS

The diffusion of small molecules into medaka embryos is slow, caused by membrane systems underneath the chorion. These results have important implications for pharmacologic/toxicologic techniques like the fish embryo test, which therefore require extended incubation times in order to reach sufficient concentrations in the embryos.

PFOS induced precocious hatching of Oryzias melastigma--from molecular level to individual level

Perfluorooctane sulfonate (PFOS) showed a risk of causing adverse effects including hatching of animals. However, there are no systematical studies about the effects of PFOS on fish embryo hatching. In this study, a precocious hatching was detected in Oryzias melastigma embryos upon PFOS exposure. The average hatching time was shortened and the hatching rate increased after PFOS exposure. The subsequent survival rate decreased significantly in the larvae hatching from the PFOS-treated embryos compared to the control. The influence occurred in a PFOS concentration-dependent manner. To further elucidate the inner molecular change, full length cDNAs of hatching enzymes HCE and LCE were first cloned in O. melastigma by degenerate RT-PCR and RACE. PFOS elicited a transcriptional response of both HCE and LCE. The expressions of both genes were significantly up-regulated earlier in the PFOS-treated group than in the control group. The activity of hatching enzyme also significantly increased upon exposure. In all, our results showed that exposure to PFOS could induce the hatching enzyme both at transcriptional and enzymatic activity level and further lead to the precocious hatching of O. melastigma embryos, which induced the decrease of the subsequent larvae survival.

Taking the plunge: California Grunion embryos emerge rapidly with environmentally cued hatching

The process of hatching has been well studied in some model species of teleosts: the medaka Oryzias latipes, the mummichog Fundulus heteroclitus, and the zebrafish Danio rerio. These models are compared to the California Grunion, Leuresthes tenuis that has some unique features of reproduction related to tidal synchrony of spawning and environmentally cued hatching (ECH). During oviposition at spring tides, this marine teleost spawns out of water to bury its clutches on sandy beaches in the high intertidal zone. After embryos of L. tenuis reach hatching competence, hatching can be triggered at any time. Incubation above the water line inhibits hatching until ECH is triggered by rising tides during the following lunar phase, and hatching occurs within a few seconds. We review the embryo's response to environmental cues at hatching and the effects of the surrounding medium on the chorionase and chorion for this form of ECH. Leuresthes tenuis shares some similarities as well as some important differences with the model species. Comparison of hatching across teleostean taxa indicates great variability in stage at hatching and in duration of incubation that suggest hatching plasticity in response to environmental cues may be more widespread than currently appreciated.

Early expression of zona pellucida proteins under octylphenol exposure in Cichlasoma dimerus (Perciformes, Cichlidae)

An increasing number of widely used industrial and agricultural chemicals are being found to cause endocrine disruption. In fishes, xenoestrogens can induce female proteins, and in some cases, the development of testis-ova, demonstrating feminization of males. In this study we analyzed the effect of an acute exposure of adult male Cichlasoma dimerus fish to estradiol (E(2)) and octylphenol (OP). E(2) and OP were injected at 10 and 50 μg/g body weight doses, respectively. After a single OP dose, liver was processed for RNA extraction at 1, 3, 12, 24, and 72 h. PCR was performed using cDNA and primers for egg coat or zona pellucida proteins (ZP). Genes encoding ZPB and ZPC isoforms were sequenced. E(2)-induced fish were sacrificed at 72 h. Using multiple OP or E(2) injections, blood and surface mucus were sampled on days 0, 3, 6, 9, and 13. On day 13 fish were sacrificed for liver and testis dissection. Histological examination of E(2) and OP-treated fish livers showed cellular disarray and intense cytoplasmatic basophilia within hepatocytes, probably due to increased mRNA synthesis, as well as hypertrophied euchromatic nuclei, and conspicuous nucleoli, indicative of augmented cell activity. An abnormal amount of sperm and immature germ cells within the testis lumen were seen in treated fish, suggesting reproductive impairment. Both plasma and mucus revealed the presence of ZP (and vitellogenin) at day 3 and thereafter with E(2) treatment, using Western and Dot blot techniques; OP effects were delayed in time. These results validate the analysis of mucus by Dot blot as an easy and rapid technique to address endocrine disruption caused by OP. Quantitative gene expression showed induction of liver ZPB and ZPC upon OP injection; muscle, brain, and intestine did not express any ZP. Both ZPs were induced at 1h post injection, but only ZPB expression was statistically significant. At 12h, both ZPs increased significantly, reaching the same levels of E(2)-challenged males after 72 h. Therefore, OP mimicked the action of E(2) with a prompt and strong xenoestrogenic effect, evidenced by the early response through mRNA and protein expression of ZP and the concomitant histological liver and testis alterations.

Conservation of the egg envelope digestion mechanism of hatching enzyme in euteleostean fishes

We purified two hatching enzymes, namely high choriolytic enzyme (HCE; EC 3.4.24.67) and low choriolytic enzyme (LCE; EC 3.4.24.66), from the hatching liquid of Fundulus heteroclitus, which were named Fundulus HCE (FHCE) and Fundulus LCE (FLCE). FHCE swelled the inner layer of egg envelope, and FLCE completely digested the FHCE-swollen envelope. In addition, we cloned three Fundulus cDNAs orthologous to cDNAs for the medaka precursors of egg envelope subunit proteins (i.e. choriogenins H, H minor and L) from the female liver. Cleavage sites of FHCE and FLCE on egg envelope subunit proteins were determined by comparing the N-terminal amino acid sequences of digests with the sequences deduced from the cDNAs for egg envelope subunit proteins. FHCE and FLCE cleaved different sites of the subunit proteins. FHCE efficiently cleaved the Pro-X-Y repeat regions into tripeptides to dodecapeptides to swell the envelope, whereas FLCE cleaved the inside of the zona pellucida domain, the core structure of egg envelope subunit protein, to completely digest the FHCE-swollen envelope. A comparison showed that the positions of hatching enzyme cleavage sites on egg envelope subunit proteins were strictly conserved between Fundulus and medaka. Finally, we extended such a comparison to three other euteleosts (i.e. three-spined stickleback, spotted halibut and rainbow trout) and found that the egg envelope digestion mechanism was well conserved among them. During evolution, the egg envelope digestion by HCE and LCE orthologs was established in the lineage of euteleosts, and the mechanism is suggested to be conserved.

Vertebrate maternal-effect genes: Insights into fertilization, early cleavage divisions, and germ cell determinant localization from studies in the zebrafish

In the earliest stages of animal development prior to the commencement of zygotic transcription, all critical cellular processes are carried out by maternally-provided molecular products accumulated in the egg during oogenesis. Disruption of these maternal products can lead to defective embryogenesis. In this review, we focus on maternal genes with roles in the fundamental processes of fertilization, cell division, centrosome regulation, and germ cell development with emphasis on findings from the zebrafish, as this is a unique and valuable model system for vertebrate reproduction.

Hatching mechanism of the Chinese soft-shelled turtle Pelodiscus sinensis

The mechanism by which the embryo hatches out of the egg envelope, the vitelline membrane and egg white, was studied in the Chinese soft-shelled turtle Pelodiscus sinensis. The cDNA of the turtle hatching enzyme (HE) was 1555bp-long and a mature enzyme of 321 amino acids. The mature HE was composed of an astacin protease domain of 200 amino acids and a CUB domain of 121 amino acids, and the estimated molecular size was 35,311. The protease domain contained two active site consensus sequences, HExxHxxGFxHExxRxDR and MHY. An immunoblotting test of an extract of allanto-chorions revealed a 40-kDa band by cross-reaction with the anti-Xenopus HE antiserum. The first change in the envelopes was the appearance of a hole, 1mm in diameter, at the location around the animal pole of day 8 incubation eggs. A cluster of tall cells, forming a circle in the avascular chorion of day 8 embryos and facing the edge of the hole, had various sizes of inclusion bodies and secretory granules that were labeled by immuno-electron microscopic staining with the antiserum. The egg envelopes were degraded gradually from the animal pole side towards the vegetal pole side in accordance with translocation of the avascular site of the chorion in the same direction. Labeled cells degenerated, presumably when the chorion was underlain by allantois in succeeding developmental stages. The vitelline membrane and egg white were totally digested, presumably by secreted HE, during the hatching period and were consumed for embryonic growth.

Extracellular matrix modifications at fertilization: regulation of dityrosine crosslinking by transamidation

Fertilization is accompanied by the construction of an extracellular matrix that protects the new zygote. In sea urchins, this structure is built from glycoproteins residing at the egg surface and in secretory vesicles at the egg cortex. Four enzymatic activities are required for the transformation of these proteins into the mechanically and chemically resilient fertilization envelope: proteolysis, transamidation, NADPH-dependent oxidation and peroxidation. Here, we identify the Strongylocentrotus purpuratus enzymes responsible for the formation of epsilon(gamma-glutamyl)lysine crosslinks (transamidation). We find that these two transglutaminases are activated by local acidification and act on specific substrates within the fertilization envelope (including ovoperoxidase, rendezvin and SFE9). Surprisingly, these enzymes also regulate dityrosine crosslinking both by direct conjugation of ovoperoxidase and by modulating hydrogen peroxide production. Together, these results emphasize how transglutaminases can coordinate the activities of other enzymes during extracellular matrix transmogrifications.

Purification and characterization of zebrafish hatching enzyme - an evolutionary aspect of the mechanism of egg envelope digestion

There are two hatching enzyme homologues in the zebrafish genome: zebrafish hatching enzyme ZHE1 and ZHE2. Northern blot and RT-PCR analysis revealed that ZHE1 was mainly expressed in pre-hatching embryos, whereas ZHE2 was rarely expressed. This was consistent with the results obtained in an experiment conducted at the protein level, which demonstrated that one kind of hatching enzyme, ZHE1, was able to be purified from the hatching liquid. Therefore, the hatching of zebrafish embryo is performed by a single enzyme, different from the finding that the medaka hatching enzyme is an enzyme system composed of two enzymes, medaka high choriolytic enzyme (MHCE) and medaka low choriolytic enzyme (MLCE), which cooperatively digest the egg envelope. The six ZHE1-cleaving sites were located in the N-terminal regions of egg envelope subunit proteins, ZP2 and ZP3, but not in the internal regions, such as the ZP domains. The digestion manner of ZHE1 appears to be highly analogous to that of MHCE, which partially digests the egg envelope and swells the envelope. The cross-species digestion using enzymes and substrates of zebrafish and medaka revealed that both ZHE1 and MHCE cleaved the same sites of the egg envelope proteins of two species, suggesting that the substrate specificity of ZHE1 is quite similar to that of MHCE. However, MLCE did not show such similarity. Because HCE and LCE are the result of gene duplication in the evolutionary pathway of Teleostei, the present study suggests that ZHE1 and MHCE maintain the character of an ancestral hatching enzyme, and that MLCE acquires a new function, such as promoting the complete digestion of the egg envelope swollen by MHCE.

Vitellogenin, zona radiata protein, cathepsin D and heat shock protein 70 as biomarkers of exposure to xenobiotics

The antagonistic and/or synergistic effects of different chemical compounds were examined in the marine teleost, Gobius niger, by testing a series of biomarkers involved in fish reproduction. Among the biomarkers analysed, vitellogenin (VTG) and zona radiata proteins (ZRP) are key molecules involved in reproduction, widely used to detect the presence of pollutants in the marine environment, while heat shock protein 70 (HSP70) and cathepsin D (CATD) have recently been introduced as bioindicators of endocrine disruption. The detection of VTG and ZRP in the plasma of wild male specimens is universally accepted as an early warning signal of environmental pollution. The evaluation of VTG, ZRP and CATD expression demonstrated the oestrogenic effect of nonylphenol on both male and female fish; on the contrary beta-naphthoflavone behaves mainly as an anti-oestrogen although, when co-injected with compounds with oestrogenic activity, it enhances ZRP gene expression. Regarding the chaperone, all treatments stressed the fish, inducing an increase in HSP70 gene transcription. The results obtained underlined the importance of testing the effects of compound mixtures: fish in the wild are subjected to a blend of chemicals and the effects observed derive from the synergic or antagonistic interactions of these compounds.

Free-radical crosslinking of specific proteins alters the function of the egg extracellular matrix at fertilization

All animal embryos begin development by modifying the egg extracellular matrix. This protein-rich matrix protects against polyspermy, microbes and mechanical stress via enzyme-dependent transformations that alter the organization of its constituents. Using the sea urchin fertilization envelope,a well-defined extracellular structure formed within minutes of fertilization,we examine the mechanisms whereby limited permeability is established within this matrix. We find that the fertilization envelope acquires a barrier filtration of 40,000 daltons within minutes of insemination via a peroxidase-dependent mechanism, with dynamics that parallel requisite production of hydrogen peroxide by the zygote. To identify the molecular targets of this free-radical modification, we developed an in vivo technique to label and isolate the modified matrix components for mass spectrometry. This method revealed that four of the six major extracellular matrix components are selectively crosslinked, discriminating even sibling proteins from the same gene. Thus, specific free-radical chemistry is essential for establishing the embryonic microenvironment of early development.

ZP genes in avian species illustrate the dynamic evolution of the vertebrate egg envelope

The vertebrate egg envelope is composed of a set of related proteins, encoded by the ZP genes. The apparent simplicity of the egg envelope is in contrast to the number of ZP genes identified by conventional cloning and data mining of genome sequences from a number of vertebrates. The vertebrate ZP genes fall into five classes, ZP1, ZP2, ZP3, ZPD and ZPAX. Analysis of chicken genome and EST sequence data has revealed the presence of seven distinct ZP genes, falling into these classes that are expressed in the female reproductive system. Comparison with the repertoire of ZP genes in other vertebrates suggests a major source of diversity in the composition of the egg envelope is a continual process of amplification, diversification and attrition of ZP gene sequences.

The distribution of transglutaminase in the rat oocytes and embryos

Transglutaminases (TGs) are calcium-dependent enzymes that catalyze the transamidation of glutamine residues of a protein substrate to form intermolecular isopeptide bonds. The zona pellucida (ZP) is an extracellular, glycoprotein matrix that surrounds the oocytes of all Eutherian mammals. We aimed to identify the immunoreactivity of tissue transglutaminase (tTG) and ultrastructural changes occuring in rat oocytes before and after fertilization. Female rats were stimulated to superovulate, then mated with males. Oocytes and embryos were collected and examined by immunohistochemistry and electron microscopy. Before fertilization, tTG was present only in the oolemma and the cortical cytoplasm. After fertilization, tTG reactivity increased in the ZP of the early zygote and the preimplantation embryos, but decreased in the cytoplasm and perivitelline space (PVS). After fertilization, the PVS ultrastructure became asymmetrical and large around the polar bodies with many cortical granule contents. In conclusion, tTG immunoreactivity was found to be spatially and temporarily heterogeneous in the rat oocytes and embryos, especially in the ZP.

Analysis of the exon–intron structures of fish, amphibian, bird and mammalian hatching enzyme genes, with special reference to the intron loss evolution of hatching enzyme genes in Teleostei

Using gene cloning and in silico cloning, we analyzed the structures of hatching enzyme gene orthologs of vertebrates. Comparison led to a hypothesis that hatching enzyme genes of Japanese eel conserve an ancestral structure of the genes of fishes, amphibians, birds and mammals. However, the exon-intron structure of the genes was different from species to species in Teleostei: Japanese eel hatching enzyme genes were 9-exon-8-intron genes, and zebrafish genes were 5-exon-4-intron genes. In the present study, we further analyzed the gene structures of fishes belonging to Acanthopterygii. In the species of Teleostei we examined, diversification of hatching enzyme gene into two paralogous genes for HCE (high choriolytic enzyme) and LCE (low choriolytic enzyme) was found only in the acanthopterygian fishes such as medaka Oryzias latipes, Fundulus heteroclitus, Takifugu rubripes and Tetraodon nigroviridis. In addition, the HCE gene had no intron, while the LCE gene consisted of 8 exons and 7 introns. Phylogenetic analysis revealed that HCE and LCE genes were paralogous to each other, and diverged during the evolutionary lineage to Acanthopterygii. Analysis of gene synteny and cluster structure showed that the syntenic genes around the HCE and LCE genes were highly conserved between medaka and Teraodon, but such synteny was not found around the zebrafish hatching enzyme genes. We hypothesize that the zebrafish hatching enzyme genes were translocated from chromosome to chromosome, and lost some of their introns during evolution.

Cloning of a quail homologue of hatching enzyme: its conserved function and additional function in egg envelope digestion

The aim of the present study was to reveal molecular entities participating in the digestion of the egg envelope in the Japanese quail, Coturnix japonica. We isolated a 1,510-bp cDNA from extraembryonic tissues of developing embryos and designated it quail hatching enzyme (QHE) cDNA. The QHE cDNA was found to code a protein molecule comprising an astacin protease domain in the N-terminal half and a complement subcomponents C1r/C1s, Uegf, Bmp1 (CUB) domain in the C-terminal half. A phylogenetic analysis showed that QHE belonged to the hatching enzyme group and was distinct from other proteases in the astacin family. Northern blotting and in situ hybridization demonstrated that expression of the QHE mRNA occurred twice during the development: first in ectodermal cells of the yolk sac on days 0-5, then in those of the albumen sac on days 8-13. Zymography revealed that proteolytic activity in extracts of days 3-4 and 9-12 embryos appeared at the position of 40 kDa. Immunoblotting tests showed that anti-QHE antiserum stained a 40-kDa molecule in extracts of day 3 area vitellina. Anti-QHE antibody stained the ectodermal cells of the area opaca on days 0-1, those of the area vitellina of the yolk sac on days 2-5, and those of the albumen sac on days 9-12. The temporal and spatial expression pattern of QHE mRNA was closely associated with digestion of the vitelline membrane occurring on days 1-4, and with that of the egg white on days 9-12.

Effects of water hardness on the physiological responses to chronic waterborne silver exposure in early life stages of rainbow trout (Oncorhynchus mykiss)

Early life stages of rainbow trout were exposed to 0, 0.1 and 1 microg/L Ag (as AgNO(3)) in very soft water (2mg/L CaCO(3)), moderately hard water (150 mg/L CaCO(3)) and hard water (400mg/L CaCO(3)) of low dissolved organic carbon concentration (0.5mg C/L) from fertilization to swim-up (64 days) under flow-through conditions, and monitored for whole embryo/larval silver accumulation, Na(+) and Cl(-) concentrations, Na(+) uptake and Na(+)K(+)-ATPase activity. The objective of the study was to investigate potential protective effects of water hardness on the physiological responses to chronic silver exposure. In the absence of silver, there was little effect of hardness on the ionoregulatory parameters studied, though higher hardness did improve survival post-hatch. At all three water hardness levels, whole embryo/larval Na(+) uptake was low and relatively constant prior to 50% hatch, but dramatically increased following 50% hatch, whereas Na(+)K(+)-ATPase activity steadily increased over development. Whole embryo/larval Na(+) and Cl(-) concentrations were low and constant prior to 50% hatch, but following 50% hatch Na(+) concentration increased, while Cl(-) concentration decreased. Following 50% hatch, exposure to 0.1 and 1 microg/L Ag resulted in a decrease in whole embryo/larval Na(+) concentration, Cl(-) concentration, Na(+) uptake and Na(+)K(+)-ATPase activity, indicating that the mechanism of chronic silver toxicity involves an ionoregulatory disturbance, and is similar to the mechanism of acute silver toxicity. An increase in water hardness reduced or eliminated the effect of silver on these parameters while enhancing survival, suggesting that the nature of the protective effect of hardness involves effects on the ionoregulatory disturbance associated with silver exposure. An increase in water hardness did not fully protect against the accumulation of silver associated with silver exposure. These results suggest that it may be possible to model chronic silver toxicity using a biotic ligand type model, and that a physiologically based model may be more appropriate because Na(+)K(+)-ATPase activity or Na(+) uptake is an endpoint for prediction rather than whole embryo or larval silver accumulation.