Role of Aquaporins during Teleost Gametogenesis and Early Embryogenesis

Differential expression of aquaporin genes and the influence of environmental hypertonicity on their expression in juveniles of air-breathing stinging catfish (Heteropneustes fossilis)

Aquaporins (AQPs) are a superfamily of transmembrane channel proteins that are responsible for the transport of water and some other molecules to and from the cell, mainly for osmoregulation under anisotonicity. We investigated here the expression patterns of different AQP isoforms and also during exposure to hypertonicity (300 mOsmol/L) for 48 h in juvenile stages of air-breathing stinging catfish (Heteropneustes fossilis). A total of 8 mRNA transcripts for different isoforms of AQPs and their translated proteins could be detected in the anterior and posterior regions of S1, S2, and S3 stages of juveniles of stinging catfish at variable levels. In general, more expression of mRNAs for different aqp genes was seen in the S2 and S3 juveniles than in the S1 juveniles. Most interestingly, exposure to hypertonicity of S2 juveniles for a period of 48 h led to increased expression of most of the aqp genes both at transcriptional and translational levels, except for aqp3 in the anterior and posterior regions and aqp1 in the anterior region, showing maximum expression at later stages of hypertonic exposure. Thus, it is evident that AQPs play crucial roles in maintaining the water and ionic balances under anisotonic conditions even at the early developmental stages of stinging catfish as a biochemical adaptational strategy to survive and grow in anisotonic environment.

Characterization of aquaporin-1ab (Aqp1ab) mRNA in mud loach (Misgurnus mizolepis) exposed to heavy metal and immunostimulant stimuli

Aquaporins (AQPs) are key proteins that regulate fluid homeostasis in cells via modulating osmotic water transport. In the present study, we identified three variants of Aqp1ab transcript (mmAQP1ab x1, mmAQP1ab x2, and mmAQP1ab x3) in mud loaches (Misgurnus mizolepis), and their expression patterns were examined in response to heavy metal and immunostimulant exposure. Mud loach Aqp1ab gene has a somewhat different organizational structure (i.e. five exons interrupted by four introns) compared to most other teleostean Aqp1ab orthologues, which have four exons. The 5'-flanking regulatory region of Aqp gene showed diverse transcription factor binding motifs, particularly those associated with stress/immune responses. Developmental expression patterns indicated that Aqp1ab mRNA was maternally inherited, presumably important for fine-tuning gene expression during embryonic and early larval developments. Expression of mud loach Aqp1ab mRNA was significantly and differentially modulated in several tissues (intestine, kidneys, spleen, and liver) in response to various heavy metal treatments. In addition, Aqp1ab gene expression was highly induced in response to immune challenge (LPS and polyI:C injections). Collectively, our results suggested that AQPs are multifunctional effectors playing diverse roles in cellular pathways relevant to immune and/or stress adaptation responses, in addition to their involvement in osmoregulation.

CO2 induced seawater acidification impacts survival and development of European eel embryos

Fish embryos may be vulnerable to seawater acidification resulting from anthropogenic carbon dioxide (CO₂) emissions or from excessive biological CO₂ production in aquaculture systems. This study investigated CO₂ effects on embryos of the European eel (Anguilla anguilla), a catadromous fish that is considered at risk from climate change and that is targeted for hatchery production to sustain aquaculture of the species. Eel embryos were reared in three independent recirculation systems with different pH/CO₂ levels representing “control” (pH 8.1, 300 μatm CO₂), end-of-century climate change (“intermediate”, pH 7.6, 900 μatm CO₂) and “extreme” aquaculture conditions (pH 7.1, 3000 μatm CO₂). Sensitivity analyses were conducted at 4, 24, and 48 hours post-fertilization (hpf) by focusing on development, survival, and expression of genes related to acute stress response (crhr1, crfr2), stress/repair response (hsp70, hsp90), water and solute transport (aqp1, aqp3), acid-base regulation (nkcc1a, ncc, car15), and inhibitory neurotransmission (GABAAα6b, Gabra1). Results revealed that embryos developing at intermediate pH showed similar survival rates to the control, but egg swelling was impaired, resulting in a reduction in egg size with decreasing pH. Embryos exposed to extreme pH had 0.6-fold decrease in survival at 24 hpf and a 0.3-fold change at 48 compared to the control. These observed effects of acidification were not reflected by changes in expression of any of the here studied genes. On the contrary, differential expression was observed along embryonic development independent of treatment, indicating that the underlying regulating systems are under development and that embryos are limited in their ability to regulate molecular responses to acidification. In conclusion, exposure to predicted end-of-century ocean pCO₂ conditions may affect normal development of this species in nature during sensitive early life history stages with limited physiological response capacities, while extreme acidification will negatively influence embryonic survival and development under hatchery conditions.

Intestinal osmoregulatory mechanisms differ in Mediterranean and Atlantic European sea bass: A focus on hypersalinity

European sea bass (Dicentrarchus labrax) migrate towards habitats where salinity can reach levels over 60‰, notably in Mediterranean lagoons. D. labrax are genetically subdivided in Atlantic and Mediterranean lineages and have evolved in slightly different salinities. We compared Atlantic and West-Mediterranean populations regarding their capacity to tolerate hypersalinity with a focus on the involvement of the intestine in solute-driven water reabsorption. Fish were analyzed following a two-week transfer from seawater (SW, 36‰) to either SW or hypersaline water (HW, 55‰). Differences among lineages were observed in posterior intestines of fish maintained in SW regarding NKA activities and mRNA expressions of nkaα1a, aqp8b, aqp1a and aqp1b with systematic higher levels in Mediterranean sea bass. High salinity transfer triggered similar responses in both lineages but at different magnitudes which may indicate slight different physiological strategies between lineages. High salinity transfer did not significantly affect the phenotypic traits measured in the anterior intestine. In the posterior intestine however, the size of enterocytes and NKA activity were higher in HW compared to SW. In this tissue, nka-α1a, nkcc2, aqp8ab and aqp8aa mRNA levels were higher in HW compared to SW as well as relative protein expression of AQP8ab. For aqp1a, 1b, 8aa and 8b, an opposite trend was observed. The sub-apical localization of AQP8ab in enterocytes suggests its role in transepithelial water reabsorption. Strong apical NKCC2/NCC staining indicates an increased Na⁺ and Cl^- reuptake by enterocytes which could contribute to solute-coupled water reuptake in cells where AQP8ab is expressed.

Benzo[a]pyrene exposure disrupts steroidogenesis and impairs spermatogenesis in diverse reproductive stages of male scallop (Chlamys farreri)

Benzo[a]pyrene (BaP), a model compound of polycyclic aromatic hydrocarbon known to impair reproductive functions of vertebrates, while the data is scarce in marine invertebrates. To investigate the toxic effects of BaP on invertebrates reproduction, we exposed male scallop (Chlamys farreri) to BaP (0, 0.38 and 3.8 μg/L) throughout three stages of reproductive cycle (early gametogenesis stage, late gametogenesis stage and ripe stage). The results demonstrated that BaP decreased the gonadosomatic index and mature sperms counts in a dose-dependent manner. Significant changes in sex hormones contents and increased 17β-estradiol/testosterone ratio suggested that BaP produced the estrogenic endocrine effects in male scallops. In support of this view, we confirmed that BaP significantly altered transcripts of genes along the upstream PKA and PKC mediated signaling pathway like fshr, lhcgr, adcy, PKA, PKC, PLC and NR5A2. Subsequently, the expressions of genes encoding downstream steroidogenic enzymes (e.g., 3β-HSD, CYP17 and 17β-HSD) were impacted, which corresponded well with hormonal alterations. In addition, BaP suppressed transcriptions of spermatogenesis-related genes, including ccnd2, SCP3, NRF1 and AQP9. Due to different functional demands, these transcript profiles involved in spermatogenesis exhibited a stage-specific expression pattern. Furthermore, histopathological analysis determined that BaP significantly inhibited testicular development and maturation in male scallops. Overall, the present findings indicated that, playing as an estrogenic-like chemical, BaP could disrupt the steroidogenesis pathway, impair spermatogenesis and caused histological damages, thereby inducing reproductive toxicities with dose- and stage-specific effects in male scallops. And the adverse outcomes might threaten the stability of bivalve populations and destroy the function of marine ecosystems in the long term.

Deep Transcriptomic Analysis Reveals the Dynamic Developmental Progression during Early Development of Channel Catfish (Ictalurus punctatus)

The transition from fertilized egg to larva in fish is accompanied with various biological processes. We selected seven early developmental stages in channel catfish, Ictalurus punctatus, for transcriptome analysis, and covered 22,635 genes with 590 million high-quality RNA-sequencing (seq) reads. Differential expression analysis between neighboring developmental timepoints revealed significantly enriched biological categories associated with growth, development and morphogenesis, which was most evident at 2 vs. 5 days post fertilization (dpf) and 5 vs. 6 dpf. A gene co-expression network was constructed using the Weighted Gene Co-expression Network Analysis (WGCNA) approach and four critical modules were identified. Among candidate hub genes, GDF10, FOXA2, HCEA and SYCE3 were involved in head formation, egg development and the transverse central element of synaptonemal complexes. CK1, OAZ2, DARS1 and UBE2V2 were mainly associated with regulation of cell cycle, growth, brain development, differentiation and proliferation of enterocytes. IFI44L and ZIP10 were critical for the regulation of immune activity and ion transport. Additionally, TCK1 and TGFB1 were related to phosphate transport and regulating cell proliferation. All these genes play vital roles in embryogenesis and regulation of early development. These results serve as a rich dataset for functional genomic studies. Our work reveals new insights of the underlying mechanisms in channel catfish early development.

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

Egg quality in fishes has been a topic of research in aquaculture and fisheries for decades as it represents an important life history trait and is critical for captive propagation and successful recruitment. A major factor influencing egg quality is proper yolk formation, as most fishes are oviparous and the developing offspring are entirely dependent on stored egg yolk for nutritional sustenance. These maternally derived nutrients consist of proteins, carbohydrates, lipids, vitamins, minerals, and ions that are transported from the liver to the ovary by lipoprotein particles including vitellogenins. The yolk composition may be influenced by broodstock diet, husbandry, and other intrinsic and extrinsic conditions. In addition, a number of other maternal factors that may influence egg quality also are stored in eggs, such as gene transcripts, that direct early embryonic development. Dysfunctional regulation of gene or protein expression may lead to poor quality eggs and failure to thrive within hours of fertilization. These gene transcripts may provide important markers as their expression levels may be used to screen broodstock for potential spawning success. In addition to such intrinsic factors, stress may lead to ovarian atresia or reproductive failure and can impact fish behavior, fecundity, and ovulation rate. Finally, postovulatory aging may occur when eggs become overripe and the fish fails to spawn in a timely fashion, leading to low fertility, often encountered during manual strip spawning of fish.

Effects of the anti-androgen cyproterone acetate (CPA) on oocyte meiotic maturation in rainbow trout (Oncorhynchus mykiss)

In the present study, we aimed at characterizing the effect of cyproterone acetate (CPA), an anti-androgenic compound, on oocyte meiotic maturation in a freshwater teleost fish species, the rainbow trout (Oncorhynchus mykiss). Fully-grown post-vitellogenic ovarian follicles were incubated in vitro with CPA, luteinizing hormone (Lh) or a combination of CPA and Lh. Incubations were also performed using a combination of Lh and testosterone (T). The occurrence of oocyte maturation (i.e., resumption of the meiotic process) was assessed by monitoring germinal vesicle breakdown (GVBD) after a 72h in vitro incubation. The effect of CPA on the production of 17,20β-dihydroxy-4-pregnen-3-one (17,20βP), the natural maturation-inducing steroid (MIS), was quantified by radioimmunoassay. Our results show that CPA dramatically inhibits Lh-induced oocyte maturation and MIS synthesis. We also observed a synergistic effect of Lh and T on oocyte maturation in highly competent oocytes (i.e., able to resume meiosis after stimulation by low doses of Lh). Our results also show that a combination of CPA and Lh inhibits phosphorylation of extracellular signal-regulated kinase (Erk), kinases that are associated with oocyte maturation in many species. As a whole, our results indicate that CPA has a potential to alter meiotic maturation in rainbow trout. Further analyses are, however, needed to determine the mechanisms by which this anti-androgen interferes with the meiotic process. Furthermore, the present study provides a framework for better understanding of the ecological consequences of exposure to anti-androgens and resulting meiotic maturation abnormalities observed in trout.

Discovery of sex-related genes through high-throughput transcriptome sequencing from the salmon louse Caligus rogercresseyi

Understanding the molecular underpinnings involved in the reproduction of the salmon louse is critical for designing novel strategies of pest management for this ectoparasite. However, genomic information on sex-related genes is still limited. In the present work, sex-specific gene transcription was revealed in the salmon louse Caligus rogercresseyi using high-throughput Illumina sequencing. A total of 30,191,914 and 32,292,250 high quality reads were generated for females and males, and these were de novo assembled into 32,173 and 38,177 contigs, respectively. Gene ontology analysis showed a pattern of higher expression in the female as compared to the male transcriptome. Based on our sequence analysis and known sex-related proteins, several genes putatively involved in sex differentiation, including Dmrt3, FOXL2, VASA, and FEM1, and other potentially significant candidate genes in C. rogercresseyi, were identified for the first time. In addition, the occurrence of SNPs in several differentially expressed contigs annotating for sex-related genes was found. This transcriptome dataset provides a useful resource for future functional analyses, opening new opportunities for sea lice pest control.

A rapid transcriptome response is associated with desiccation resistance in aerially-exposed killifish embryos

Delayed hatching is a form of dormancy evolved in some amphibian and fish embryos to cope with environmental conditions transiently hostile to the survival of hatchlings or larvae. While diapause and cryptobiosis have been extensively studied in several animals, very little is known concerning the molecular mechanisms involved in the sensing and response of fish embryos to environmental cues. Embryos of the euryhaline killifish Fundulus heteroclitus advance dvelopment when exposed to air but hatching is suspended until flooding with seawater. Here, we investigated how transcriptome regulation underpins this adaptive response by examining changes in gene expression profiles of aerially incubated killifish embryos at ∼100% relative humidity, compared to embryos continuously flooded in water. The results confirm that mid-gastrula embryos are able to stimulate development in response to aerial incubation, which is accompanied by the differential expression of at least 806 distinct genes during a 24 h period. Most of these genes (∼70%) appear to be differentially expressed within 3 h of aerial exposure, suggesting a broad and rapid transcriptomic response. This response seems to include an early sensing phase, which overlaps with a tissue remodeling and activation of embryonic development phase involving many regulatory and metabolic pathways. Interestingly, we found fast (0.5–1 h) transcriptional differences in representatives of classical “stress” proteins, such as some molecular chaperones, members of signalling pathways typically involved in the transduction of sensor signals to stress response genes, and oxidative stress-related proteins, similar to that described in other animals undergoing dormancy, diapause or desiccation. To our knowledge, these data represent the first transcriptional profiling of molecular processes associated with desiccation resistance during delayed hatching in non-mammalian vertebrates. The exceptional transcriptomic plasticity observed in killifish embryos provides an important insight as to how the embryos are able to rapidly adapt to non-lethal desiccation conditions.

Identification of aquaporins in eggs and early embryogenesis of the sea urchin Paracentrotus lividus

Sea urchins are echinoderms, marine invertebrates found at the base of the deutorostome lineage, which show separate sexes and are external spawners. In the sea urchin, efficient regulation of water homeostasis is essential for many biological processes such as cellular respiration, normal fertilization and correct embryo growth. In order to clarify some of these processes, the present study reports on the identification and function of aquaporin proteins in the sea urchin. Our results show, by immunoblot, immunoelectron microscopy and immunofluorescence analysis, the presence of aquaporin1- and aquaporin3-like proteins in virgin eggs and in early embryogenesis of Paracentrotus lividus and, by using known inhibitors of aquaporin functions, the functional and relevant role of aquaporin-3 in the fertilization process. AQP3 in particular seems to play a crucial role in high velocity water flux formations involved in the detachment of the vitelline layer during the slow block of polyspermy, while the presence of AQP1 and the increase of AQP3 in the first phase of the P. lividus developmental cycle, suggest their involvement in the appropriate homeostasis for embryo development.