Molecular cloning of cDNA of mammalian and chicken II gonadotropin-releasing hormones (mGnRHs and cGnRH-II) in the beluga (Huso huso) and the disruptive effect of methylmercury on gene expression

Presence of GnRH3 in sturgeon and the roles of GnRH1 and GnRH2 on the regulation of LH/FSH in mature female sterlet Acipenser ruthenus in vitro

Three forms of gonadotropin-releasing hormones (GnRHs), ArGnRH1, ArGnRH2, and ArGnRH3, were identified in sterlet. Compared with their orthologue, ArGnRH1 and ArGnRH2 have conserved core decapeptide but show low identity in the signal peptide and the rest of the sequences. The existence of the GnRH3 paralogue of sturgeon was predicted for the first time with TBLASTN by using the amino acid sequences of catshark and whale shark GnRH3 precursor as queries against the whole genome and transcript data of sterlet. The predicted ArGnRH3 cDNA sequence was composed of three exons containing all the elements of the GnRH family. The successful molecular cloning of GnRH3 from sterlets verified its expression in the brain of sturgeons. The analysis of the ArGnRH3 amino acid sequence revealed a completely conserved decapeptide sequence that shows 100% identity with the sequence of teleosts and differs in one amino acid with that of the cartilaginous fish (catshark and whale shark) at the 5th position. The structure of the phylogenetic tree showed that a total of 52 vertebrate GnRH sequences were clustered into three main clades corresponding to GnRH1, GnRH2, and GnRH3. The ArGnRH3 sequence is the oldest GnRH3 identified in teleosts. The tissue distribution analysis showed that ArGnRH1 was expressed in all the 13 examined tissues of females and in most of the tested tissues of male fish, with the highest expression in the pituitary and hypothalamus. ArGnRH2 is only expressed in the pituitary, hypothalamus, and gonads of both female and male sterlets. ArGnRH3 mRNA could be detected in the pituitary, hypothalamus, and gonad in both female and male fish. It is also present in the spleen, head kidney, and gill in female fish and in kidney and heart in male fish. However, the ArGnRH3 only showed weak expression in all the positive tissues. ArGnRH1 and ArGnRH2 active decapeptides were synthesized to investigate their roles on the regulation of LH/FSH using a mixed brain cell line from a sexually mature female sterlet. The results showed that ArGnRH1 and ArGnRH2 exerted different effects on the gene expression and release of gonadotropins. ArGnRH1 promoted the expression of fshβ significantly around 48 h, and the expression was suppressed when the treatment time was extended to 72 h. ArGnRH1 had no significant effects on the level of either mRNA or secreted lh in any of the tested treatment length or concentrations. Moreover, ArGnRH1 did not stimulate the activity of gonadotropins in the maturation stage of female sturgeons. ArGnRH2 promoted the expression of fshβ at 24 h and 48 h and increased mRNA level of lhβ at 6 h and 48 h, accompanied by the significant secretion of LH at 72 h, although the high mRNA level of fsh did not correlate with the secretion of FSH in ArGnRH2-treated groups. In conclusion, ArGnRH2 plays an important role in the maturation stage of female sterlets. Therefore, ArGnRH2 has the potential to induce ovulation and spermiation in sturgeons.

Inorganic mercury effects on biomarker gene expressions of a freshwater amphipod at two temperatures

Mercury (Hg) is a global contaminant resulting of both natural processes and human activities. In aquatic environments, studies conducted on vertebrates highlighted changes of gene expression or activity of antitoxic and oxidative enzymes. However, although Hg is a highly toxic compound in aquatic environments, only a few studies have evaluated the lethal and sublethal effects of inorganic Hg on Gammarus sp. Therefore, this study aimed at evaluating the effects of inorganic Hg (HgCl₂) on the expression of 17 genes involved in crucial biological functions or mechanisms for organisms, namely respiration, osmoregulation, apoptosis, immune and endocrine system, and antioxidative and antitoxic defence systems. The study was performed in males of the freshwater amphipod Gammarus pulex exposed to two environmentally relevant concentrations (50 and 500 ng/L) at two temperature regime fluctuations (16 °C and 20 °C +/-2 °C) for 7 and 21 days. Results showed that G. pulex mortality was dependent on Hg concentration and temperature; the higher the concentration and temperature, the higher the mortality rate. In addition, the Integrated Biomarker Response emphasized that HgCl₂ toxicity was dependent on the concentration, time and temperature of exposure. Overall, antioxidant and antitoxic defences, as well as the endocrine and immune systems, were the biological functions most impacted by Hg exposure (based on the concentration, duration, and temperature tested). Conversely, osmoregulation was the least affected biological function. The results also demonstrated a possible adaptation of G. pulex after 21 days at 500 ng/L, regardless of the exposure temperature. This study allowed us to show that Hg deregulates many crucial biological functions after a short exposure, but that during a long exposure, an adaptation phenomenon could occur, regardless of temperature.

Toxicity of mercury: Molecular evidence

Minamata disease in Japan and the large-scale poisoning by methylmercury (MeHg) in Iraq caused wide public concerns about the risk emanating from mercury for human health. Nowadays, it is widely known that all forms of mercury induce toxic effects in mammals, and increasing evidence supports the concern that environmentally relevant levels of MeHg could impact normal biological functions in wildlife. The information of mechanism involved in mercurial toxicity is growing but knowledge gaps still exist between the adverse effects and mechanisms of action, especially at the molecular level. A body of data obtained from experimental studies on mechanisms of mercurial toxicity in vivo and in vitro points to that disruption of the antioxidant system may play an important role in the mercurial toxic effects. Moreover, the accumulating evidence indicates that signaling transduction, protein or/and enzyme activity, and gene regulation are involving in mediating toxic and adaptive response to mercury exposure. We conducted here a comprehensive review of mercurial toxic effects on wildlife and human, in particular synthesized key findings of molecular pathways involved in mercurial toxicity from the cells to human. We discuss the molecular evidence related mercurial toxicity to the adverse effects, with particular emphasis on the gene regulation. The further studies relying on Omic analysis connected to adverse effects and modes of action of mercury will aid in the evaluation and validation of causative relationship between health outcomes and gene expression.

Origin and Evolution of the Neuroendocrine Control of Reproduction in Vertebrates, With Special Focus on Genome and Gene Duplications

In humans, as in the other mammals, the neuroendocrine control of reproduction is ensured by the brain-pituitary gonadotropic axis. Multiple internal and environmental cues are integrated via brain neuronal networks, ultimately leading to the modulation of the activity of gonadotropin-releasing hormone (GnRH) neurons. The decapeptide GnRH is released into the hypothalamic-hypophysial portal blood system and stimulates the production of pituitary glycoprotein hormones, the two gonadotropins luteinizing hormone and follicle-stimulating hormone. A novel actor, the neuropeptide kisspeptin, acting upstream of GnRH, has attracted increasing attention in recent years. Other neuropeptides, such as gonadotropin-inhibiting hormone/RF-amide related peptide, and other members of the RF-amide peptide superfamily, as well as various nonpeptidic neuromediators such as dopamine and serotonin also provide a large panel of stimulatory or inhibitory regulators. This paper addresses the origin and evolution of the vertebrate gonadotropic axis. Brain-pituitary neuroendocrine axes are typical of vertebrates, the pituitary gland, mediator and amplifier of brain control on peripheral organs, being a vertebrate innovation. The paper reviews, from molecular and functional perspectives, the evolution across vertebrate radiation of some key actors of the vertebrate neuroendocrine control of reproduction and traces back their origin along the vertebrate lineage and in other metazoa before the emergence of vertebrates. A focus is given on how gene duplications, resulting from either local events or from whole genome duplication events, and followed by paralogous gene loss or conservation, might have shaped the evolutionary scenarios of current families of key actors of the gonadotropic axis.

A multidimensional concept for mercury neuronal and sensory toxicity in fish - From toxicokinetics and biochemistry to morphometry and behavior

BACKGROUND

Neuronal and sensory toxicity of mercury (Hg) compounds has been largely investigated in humans/mammals with a focus on public health, while research in fish is less prolific and dispersed by different species. Well-established premises for mammals have been governing fish research, but some contradictory findings suggest that knowledge translation between these animal groups needs prudence [e.g. the relative higher neurotoxicity of methylmercury (MeHg) vs. inorganic Hg (iHg)]. Biochemical/physiological differences between the groups (e.g. higher brain regeneration in fish) may determine distinct patterns. This review undertakes the challenge of identifying sensitive cellular targets, Hg-driven biochemical/physiological vulnerabilities in fish, while discriminating specificities for Hg forms.

SCOPE OF REVIEW

A functional neuroanatomical perspective was conceived, comprising: (i) Hg occurrence in the aquatic environment; (ii) toxicokinetics on central nervous system (CNS)/sensory organs; (iii) effects on neurotransmission; (iv) biochemical/physiological effects on CNS/sensory organs; (v) morpho-structural changes on CNS/sensory organs; (vi) behavioral effects. The literature was also analyzed to generate a multidimensional conceptualization translated into a Rubik's Cube where key factors/processes were proposed.

MAJOR CONCLUSIONS

Hg neurosensory toxicity was unequivocally demonstrated. Some correspondence with toxicity mechanisms described for mammals (mainly at biochemical level) was identified. Although the research has been dispersed by numerous fish species, 29 key factors/processes were pinpointed.

GENERAL SIGNIFICANCE

Future trends were identified and translated into 25 factors/processes to be addressed. Unveiling the neurosensory toxicity of Hg in fish has a major motivation of protecting ichtyopopulations and ecosystems, but can also provide fundamental knowledge to the field of human neurodevelopment.

Methylmercury-induced changes in gene transcription associated with neuroendocrine disruption in largemouth bass (Micropterus salmoides)

Methyl-mercury (MeHg) is a potent neuroendocrine disruptor that impairs reproductive processes in fish. The objectives of this study were to (1) characterize transcriptomic changes induced by MeHg exposure in the female largemouth bass (LMB) hypothalamus under controlled laboratory conditions, (2) investigate the health and reproductive impacts of MeHg exposure on male and female largemouth bass (LMB) in the natural environment, and (3) identify MeHg-associated gene expression patterns in whole brain of female LMB from MeHg-contaminated habitats. The laboratory experiment was a single injection of 2.5 μg MeHg/g body weight for 96 h exposure. The field survey compared river systems in Florida, USA with comparably lower concentrations of MeHg (Wekiva, Santa Fe, and St. Johns Rivers) in fish and one river system with LMB that contained elevated concentrations of MeHg (St. Marys River). Microarray analysis was used to quantify transcriptomic responses to MeHg exposure. Although fish at the high-MeHg site did not show overt health or reproductive impairment, there were MeHg-responsive genes and pathways identified in the laboratory study that were also altered in fish from the high-MeHg site relative to fish at the low-MeHg sites. Gene network analysis suggested that MeHg regulated the expression targets of neuropeptide receptor and steroid signaling, as well as structural components of the cell. Disease-associated gene networks related to MeHg exposure, based upon expression data, included cerebellum ataxia, movement disorders, and hypercalcemia. Gene responses in the CNS are consistent with the documented neurotoxicological and neuroendocrine disrupting effects of MeHg in vertebrates.

Changes in metabolic enzymes, cortisol and glucose concentrations of Beluga (Huso huso) exposed to dietary methylmercury

In this paper, effects of dietary methylmercury (MeHg) on several blood biochemical parameters including GLU (glucose), LDH (lactate dehydrogenase), AST (aspartate aminotransferase), ALT (alanine aminotransferase), ALP (alkaline phosphatase) and cortisol were investigated in the Beluga sturgeon (Huso huso). Beluga juveniles were fed for 32 days on four diets containing MeHg (control: 0.04 mg kg⁻¹; low: 0.76 mg kg⁻¹; medium: 7.88 mg kg⁻¹; and high 16.22 mg kg⁻¹ treatment). Significant increases (P < 0.05) were observed in all biochemical parameters, except ALP levels, which decreased significantly (P < 0.05) compared to the control group with either dose- or time-dependent effects. These results suggest that long-term dietary MeHg exposure may affect metabolic enzyme activity and glucose levels in Belugas. These findings provide useful information for environmental and fishery officials to apply in future decisions for managing fish resources in Caspian Sea.

Gene expression changes in female zebrafish (Danio rerio) brain in response to acute exposure to methylmercury

Methylmercury (MeHg) is a potent neurotoxicant and endocrine disruptor that accumulates in aquatic systems. Previous studies have shown suppression of hormone levels in both male and female fish, suggesting effects on gonadotropin regulation in the brain. The gene expression profile in adult female zebrafish whole brain induced by acute (96 h) MeHg exposure was investigated. Fish were exposed by injection to 0 or 0.5 µg MeHg/g. Gene expression changes in the brain were examined using a 22,000-feature zebrafish microarray. At a significance level of p < 0.01, 79 genes were up-regulated and 76 genes were down-regulated in response to MeHg exposure. Individual genes exhibiting altered expression in response to MeHg exposure implicate effects on glutathione metabolism in the mechanism of MeHg neurotoxicity. Gene ontology (GO) terms significantly enriched among altered genes included protein folding, cell redox homeostasis, and steroid biosynthetic process. The most affected biological functions were related to nervous system development and function, as well as lipid metabolism and molecular transport. These results support the involvement of oxidative stress and effects on protein structure in the mechanism of action of MeHg in the female brain. Future studies will compare the gene expression profile induced in response to MeHg with that induced by other toxicants and will investigate responsive genes as potential biomarkers of MeHg exposure.