Triacylglyceride physiology in the short-finned eel, Anguilla australis—changes throughout early oogenesis

Lipoprotein receptors in ovary of eel, Anguilla australis: molecular characterisation of putative vitellogenin receptors

Lipoprotein receptors, including low-density lipoprotein receptor (LDLr) relatives (Lrs) and LDLr-related proteins (Lrps), belong to the LDLr supergene family and participate in diverse physiological functions. In this study, novel sequences of lr and lrp genes expressed in the ovary of the short-finned eel, Anguilla australis, during early gonadal development are presented. The genes encoding the LDLr-like, Lrp1-like, Lrp1b-like, Lrp3, Lrp4-like, Lrp5-like, Lrp6, Lrp10, Lrp11, Lrp12-like, and Lr11-like proteins were found and identified by sequence and structure analysis, in addition to phylogenetic analysis. Genes encoding proteins previously implicated in follicle development and vitellogenin (Vtg) uptake in oviparous vertebrates were also identified, i.e. lr8 (including lr8 + and lr8- variants) and lrp13; their identification was reinforced by conserved synteny with orthologues in other teleost fish. Compared to other lr/lrp genes, the genes encoding Lr8 + , Lr8-, and Lrp13 were highly expressed in ovary during early development, decreasing as oocyte development advanced when induced by hypophysation. Furthermore, lr8 + , lr8-, and lrp13 were dominantly expressed in the ovary when compared with 17 other tissues. Finally, this study successfully detected the expression of both lr8 variants, which showed different expression patterns to those reported in other oviparous vertebrates and provided the first characterisation of Lrp13 in Anguilla sp. We propose that lr8 + , lr8-, and lrp13 encode putative Vtg receptors in anguillid eels.

Steroid implants for the induction of vitellogenesis in feminized European silver eels (Anguilla anguilla L.)

Assisted propagation of the European eel will lead to a closed production cycle supplying the aquaculture industry with juvenile glass eels. Females require long-term weekly treatment with pituitary extract (PE), which is stressful and causes abnormalities in oogenesis. We tested the effects of 17α-methyltestosterone (17 MT), as potent androgen activating the androgen receptor, and 17β-estradiol (E2), as an inducer of vitellogenesis, to shorten the duration of PE treatment.Four groups of feminized eels were subjected to a simulated migration and subsequent injection with implants containing 17 MT (17 MT-group), E2 (E2-group) or 17 MT plus E2 (17 MT + E2-group) to test for synergistic effects, or without any steroids as controls (C-group). The effects of a 2-months treatment were investigated by determining the eye index (EI), hepatosomatic and gonadosomatic index (HSI and GSI, respectively), plasma steroid concentrations by liquid chromatography mass spectrometry (LCMS), gonadal histology, expression of androgen receptors a and b (ara, arb); estrogen receptor 1 (esr1); FSH receptor (fshr); vitellogenin receptor (vtgr) and aromatase (cyp19), and the required number of weekly PE injections to fully mature. For many parameters, both the 17 MT and E2 groups showed an increase vs. controls, with the 17 MT + E2 group showing a synergistic effect, as seen for EI, GSI (3.4 for 17 MT and for E2, 6.6 for 17 MT + E2), oocyte diameter and ara, arb and esr1 expression. Concentrations of almost all focal steroids decreased with simulated migration and steroid treatment. Only eels of the 17 MT-group showed increased expression of cyp19 and of fshr, while fshr expression increased 44-fold in the 17 MT + E2 group, highlighting that co-implantation is most effective in raising fshr mRNA levels. Specific for eels of the E2 groups were vitellogenesis-associated changes such as an increase of HSI, plasma E2, and presence of yolk in the oocytes. Steroid treatments reduced the duration of PE treatment, again synergistically for co-implantation. In conclusion, E2 is necessary to start vitellogenesis, but 17 MT has specific effects on cyp19 and fshr expression. The combination is necessary for synergistic effects and as such, steroid implants could be applied in assisted reproduction protocols for European eel to improve oocyte quality leading to the production of more vital larvae.

Are Cell Junctions Implicated in the Regulation of Vitellogenin Uptake? Insights from an RNAseq-Based Study in Eel, Anguilla australis

At the onset of puberty, ovarian follicles become competent to incorporate large amounts of vitellogenin (Vtg). Using an RNAseq-based approach, transcriptomes from pre-vitellogenic (PV) and early vitellogenic (EV) ovaries from wild-caught eel, Anguilla australis, were compared to investigate the expression of specific genes encoding cell junction proteins that could be involved in regulating Vtg uptake. Partial support was found for the mechanical barrier hypothesis proposing that the access of Vtg to the oolemma is restricted by a tight junction (TJ) network within the granulosa cell layer, which changes between the PV and EV stage. Among 25 genes encoding TJ-constituting proteins, five were down-regulated and two were up-regulated. A chemical barrier hypothesis stating that gap junctions (GJs) are involved in modulating Vtg uptake was not supported, as only five GJs were found to be expressed in the ovary with no significant changes in expression between stages. Furthermore, the endocytic pathway was found to be up-regulated during the PV-EV transition. Finally, the study showed that gene expression patterns may help identify suitable candidates involved in the regulation of Vtg uptake, and provided novel sequence data for A. australis, including putative Vtg receptors corresponding to Lr8 and Lrp13 members of the low-density lipoprotein receptor family.

Changes in lipid biology during ovarian development in farmed beluga sturgeon, Huso huso L

The present study was conducted to understand key biochemical, physiological, and molecular changes associated with ovarian growth and with lipid transfer and/or accumulation into the ovary during oogenesis in captive beluga sturgeon. Plasma levels of triacylglycerides, cholesterol, phospholipid, and sex steroid hormones were determined and all were found to increase notably throughout development from the perinucleolar to the tertiary yolk stage. Using fast protein liquid chromatography, we recognized three major lipoprotein peaks in chromatograms from all samples. These peaks were characterized as containing very low-density lipoprotein (Vldl), low-density lipoprotein/high-density lipoprotein (Ldl/Hdl), and plasma proteins. While Ldl/Hdl represented the most abundant lipoprotein fraction, the relative abundance of different lipoprotein classes did not change with the stage of oogenesis. Eluted lipoproteins were separated using sodium dodecyl-sulfate polyacrylamide gel electrophoresis and sequenced. The peptide sequence spectra for 66-kDa, 205-kDa, 29-kDa, and 70-kDa bands matched with albumin, vitellogenin (Vtg) AB2b, immunoglobulin light-chain precursor, and immunoglobulin heavy-chain, respectively. The large amount of albumin in the plasma protein peak and the confined presence of Vtg AB2b to within Ldl/Hdl reinforce the lipoprotein classification. Lastly, transcript levels of genes encoding ovarian lipoprotein lipase (lpl), apolipoprotein E (apoe), very low-density lipoprotein receptors (vldlr), and low-density lipoprotein receptor-related protein 8-like (lrp8) were estimated using quantitative RT-PCR. The high mRNA levels of lpl, apoe, and lipoprotein receptors vldlr and lrp8 in previtellogenic females suggest that sturgeon oocytes need to be prepared to accept and traffic Vtg and lipids internally, before the start of vitellogenesis.

Induction of oocyte development in previtellogenic eel, Anguilla australis

The role of gonadotropins during early ovarian development in fish remains little understood. Concentrations of gonadotropins were therefore experimentally elevated in vivo by administration of recombinant follicle-stimulating hormone (rec-Fsh) or human chorionic gonadotropin (hCG) and the effects on ovarian morphology, sex steroid levels and mRNA levels of genes expressed in pituitary and ovary examined. Hormones were injected thrice at weekly intervals in different doses (20, 100 or 500 µg/kg BW for rec-Fsh and 20, 100 or 500 IU/kg BW for hCG). All treatments, especially at the highest doses of either rec-Fsh or hCG, induced ovarian development, reflected in increased oocyte size and lipid uptake. Both gonadotropins up-regulated follicle-stimulating hormone receptor (fshr) mRNA levels and plasma levels of estradiol-17β (E2). Exogenous gonadotropins largely decreased the expression of follicle-stimulating hormone β-subunit (fshb) and had little effect on those of luteinizing hormone β-subunit (lhb) in the pituitary. It is proposed that the effects of hCG on ovarian development in previtellogenic eels could be indirect as a significant increase in plasma levels of 11-ketotestosterone (11-KT) was found in eels treated with hCG. Using rec-Fsh and hCG has potential for inducing puberty in eels in captivity, and indeed, in teleost fish at large.

Genetic Parameters for Yolk Cholesterol and Transcriptional Evidence Indicate a Role of Lipoprotein Lipase in the Cholesterol Metabolism of the Chinese Wenchang Chicken

The yolk cholesterol has been reported to affect egg quality and breeding performance in chickens. However, the genetic parameters and molecular mechanisms regulating yolk cholesterol remain largely unknown. Here, we used the Wenchang chicken, a Chinese indigenous breed with a complete pedigree, as an experimental model, and we examined 24 sire families (24 males and 240 females) and their 362 daughters. First, egg quality and yolk cholesterol content were determined in 40-week-old chickens of two consecutive generations, and the heritability of these parameters was analyzed using the half-sib correlation method. Among first-generation individuals, the egg weight, egg shape index, shell strength, shell thickness, yolk weight, egg white height, Haugh unit, and cholesterol content were 45.36 ± 4.44 g, 0.81 ± 0.12, 3.07 ± 0.92 kg/cm2, 0.340 ± 0.032 mm, 15.57 ± 1.64 g, 3.36 ± 1.15 mm, 58.70 ± 12.33, and 274.3 ± 36.73 mg/egg, respectively. When these indexes were compared to those of the following generation, no statistically significant difference was detected. Although yolk cholesterol content was not associated with egg quality in females, an increase in yolk cholesterol content was correlated with increased yolk weight and albumin height in sire families (p < 0.05). Moreover, the heritability estimates for the yolk cholesterol content were 0.328 and 0.530 in female and sire families, respectively. Therefore, the yolk cholesterol content was more strongly associated with the sire family. Next, chickens with low and high yolk cholesterol contents were selected for follicular membrane collection. Total RNA was extracted from these samples and used as a template for transcriptional sequencing. In total, 375 down- and 578 upregulated genes were identified by comparing the RNA sequencing data of chickens with high and low yolk cholesterol contents. Furthermore, Gene Ontology term and Kyoto Encyclopedia of Genes and Genomes pathway enrichment analyses indicated the involvement of energy metabolism and immune-related pathways in yolk cholesterol deposition. Several genes participating in the regulation of the yolk cholesterol content were located on the sex chromosome Z, among which lipoprotein lipase (LPL) was associated with the peroxisome proliferator-activated receptor signaling pathway and the Gene Ontology term cellular component. Collectively, our data suggested that the ovarian steroidogenesis pathway and the downregulation of LPL played critical roles in the regulation of yolk cholesterol content.

A mechanistic model for studying the initiation of anguillid vitellogenesis by comparing the European eel (Anguilla anguilla) and the shortfinned eel (A. australis)

An inverse relation exists between the maturation stage at the start of the oceanic reproductive migration and the migration distance to the spawning grounds for the various eel species. The European eel Anguilla anguilla migrates up to 5-6000 km and leaves in a previtellogenic state. The shortfinned eel A. australis migrates 2-4000 km and leaves in an early vitellogenic state. In this study, we compared the early pubertal events in European silver eels with those in silver shortfinned eels to gain insights into the initiation of vitellogenesis. Immediately after being caught, yellow and silver eels of both species were measured and sampled for blood and tissues. Eye index (EI), gonadosomatic index (GSI) and hepatosomatic index (HSI) were calculated. Plasma 11-ketotestosterone (11-KT) and 17β-estradiol (E2) levels were measured by radioimmunoassay. Pituitary, liver and ovaries were dissected for quantitative real-time PCR analyses (pituitary dopamine 2b receptor d2br, gonadotropin-releasing hormone receptors 1 and 2 gnrhr1 and gnrhr2, growth hormone gh and follicle-stimulating hormone-β fshb; liver estrogen receptor 1 esr1; gonad follicle-stimulating hormone receptor fshr, androgen receptors α and β ara and arb, vitellogenin receptor vtgr and P450 aromatase cyp19). Silver eels of both species showed a drop in pituitary gh expression, progressing gonadal development (GSI of ∼1.5 in European eels and ∼3.0 in shortfinned eels) and steroid level increases. In shortfinned eels, but not European eels, expression of fshb, gnrhr1 and gnrhr2, and d2br in the pituitary was up-regulated in the silver-stage as compared to yellow-stage females, as was expression of fshr, ara and arb in the ovaries. Expression of esr1 in European eels remained low while esr1 expression was up-regulated over 100-fold in silver shortfinned eels. The mechanistic model for anguillid vitellogenesis that we present suggests a first step that involves a drop in Gh and a second step that involves Fsh increase when switching in the life history trade-off from growth to reproduction. The drop in Gh is associated with gonadal development and plasma steroid increase but precedes brain-pituitary-gonad axis (BPG) activation. The Fsh increase marks BPG activation and increased sensitivity of the liver to estrogenic stimulation, but also an increase in D2br-mediated dopaminergic signaling to the pituitary.

11-Ketotestosterone induces oocyte growth, but does not affect oocyte cytology in pre-vitellogenic captive beluga, Huso huso L

An effect of 11-ketotestosterone (11-KT) on growth of previtellogenic (PV) ovaries of eel, salmon and Atlantic cod has been demonstrated. The purpose of this study was to investigate the effects of 11-KT treatment (in vivo) on ovarian growth, on hormonal and biochemical changes in blood, and on ovarian mRNA levels of lipidation-related genes in captive beluga with PV oocytes. In addition, the potential involvement of lipoprotein lipase (Lpl), an important enzyme for extracellular hydrolysis of lipoprotein-associated lipids, was evaluated. Twelve beluga (4-year olds) were treated with an intraperitoneal slow-release implant of either 11-KT (2.5 mg) or a compressed matrix (control). Ovarian biopsy was done to obtain pre- (day 0: T0) and post-treatment (day 21: T21) data on histology and target gene expression. Three weeks of exposure resulted in an increase in serum 11-KT levels from 2.2 ng/mL to 83 ng/mL but did not yield significant changes in serum levels of triacylglycerides and cholesterol. Furthermore, 11-KT implantation increased oocyte diameters from 259 μm (T0) to 309 μm by T21. Regardless of the increase in oocyte size, ovaries remained in the PV stage, mostly as late perinucleolar oocytes. Meanwhile, at the molecular level, the expression of lipidation-related transcripts [lpl, apolipoprotein E (apoe), very low density lipoprotein receptors (vldlr), low-density lipoprotein receptor-related protein 8-like (lrp8)] was significantly up-regulated after three weeks. Immunostaining for Lpl by Western blotting indicated three immunoreactive bands (70, 58 and 37 kDa) in ovarian homogenates from beluga, but signal intensity was not affected by treatment. Altogether, the administration of 11-KT increased 11-KT serum levels, oocyte size, and the expression of genes associated with lipid uptake. However, this treatment did not advance ovarian development beyond the PV stage.

Triacylglyceride physiology in the short-finned eel, Anguilla australis--the effects of androgen

The importance of androgens (especially 11-ketotestosterone) during previtellogenesis in eels is well established. In wild pubertal migrants, circulating 11-ketotestosterone levels correlate with a number of morphological and molecular changes. Here, we test the prediction that this correlation represents a causal relationship by artificially raising the levels of circulating 11-ketotestosterone in prepubertal nonmigratory female and pubertal, migratory male short-finned eels (Anguilla australis) using sustained-release hormone implants. In females, increases in hepatosomatic index and transcript copy numbers of hepatic apolipoprotein B and microsomal triacylglyceride transfer protein indicated increased repackaging of endogenously sourced triacylglycerides. These changes in liver measures were reflected in increased concentrations of serum triacylglycerides. However, despite a small increase in gonadosomatic index, ovarian lipoprotein receptor transcript abundances were not affected by 11-ketotestosterone. Interestingly, no such changes in hepatic gene expression were detected in a dose-response experiment using males. We propose that the androgens are inducing the observed changes in previtellogenic females, although it remains unclear to what extent these effects are direct or indirect.