Molecular cloning and partial characterization of an ovarian receptor with seven ligand binding repeats, an orthologue of low-density lipoprotein receptor, in the cutthroat trout (Oncorhynchus clarki)

Lovastatin, an Up-Regulator of Low-Density Lipoprotein Receptor, Enhances Follicular Development in Mouse Ovaries

Ovarian aging hampers in vitro fertilization in assisted reproductive medicine and has no cure. Lipoprotein metabolism is associated with ovarian aging. It remains unclear how to overcome poor follicular development with aging. Upregulation of the low-density lipoprotein receptor (LDLR) enhances oogenesis and follicular development in mouse ovaries. This study investigated whether upregulation of LDLR expression using lovastatin enhances ovarian activity in mice. We performed superovulation using a hormone and used lovastatin to upregulate LDLR. We histologically analyzed the functional activity of lovastatin-treated ovaries and investigated gene and protein expression of follicular development markers, using RT-qPCR and Western blotting. Histological analysis showed that lovastatin significantly increased the numbers of antral follicles and ovulated oocytes per ovary. The in vitro maturation rate was 10% higher for lovastatin-treated ovaries than for control ovaries. Relative LDLR expression was 40% higher in lovastatin-treated ovaries than in control ovaries. Lovastatin significantly increased steroidogenesis in ovaries and promoted the expression of follicular development marker genes such as anti-Mullerian hormone, Oct3/4, Nanog, and Sox2. In conclusion, lovastatin enhanced ovarian activity throughout follicular development. Therefore, we suggest that upregulation of LDLR may help to improve follicular development in clinical settings. Modulation of lipoprotein metabolism can be used with assisted reproductive technologies to overcome ovarian aging.

Hepatic estrogen-responsive genes relating to oogenesis in cutthroat trout (Oncorhynchus clarki): The transcriptional induction in primary cultured hepatocytes and the in vitro promoter transactivation in responses to estradiol-17β

Estradiol-17β (E2) regulates transcription of estrogen-responsive genes via estrogen receptors (Esr). In many teleost species, choriogenin (chg), vitellogenin (vtg) and esr genes are transactivated by E2 in the liver. This study aimed i) to compare expression properties of all subtypes of these genes (chg: chgHα, chgHβ, chgL; vtg: vtgAs, vtgC; esr: esr1a, esr1b, esr2a, esr2b) in response to estrogen stimulation, and ii) to confirm how each of four Esr subtypes is involved in the transcriptional regulation of these estrogen-responsive genes in cutthroat trout hepatocytes. In hepatocytes in primary culture, all chg and vtg subtype mRNA levels, and those of esr1a, were increased by E2 treatment (10^-6 M) at 24 and 72 h post initiation (hpi), but esr1b, esr2a and esr2b mRNA levels were not. Treatment of hepatocytes with various concentrations of E2 (10^-11-10^-6 M) induced dose-dependent increases in the levels of all chg and vtg subtype mRNAs at 24 and 72 hpi. At both time points, the lowest dose that induced a significant increase in the expression levels of mRNAs (LOEC) for E2 differed among the genes; LOECs were estimated as 10^-11 M for chgHα at 24 hpi, as 10^-9 M for vtgC at 72 hpi, and as 10^-10 M for other mRNAs at both 24 and 72 hpi. Meanwhile, the levels of esr1a mRNA exhibited a dose-dependent increase at 24 and 72 hpi, but the LOEC shifted from 10^-9 M at 24 hpi to 10^-7 M at 72 hpi because of a decrease in mRNA levels at treatment groups exposed to high concentrations of E2. All Esr subtypes transactivated chg, vtg and esr1a promoters in the presence of E2 in vitro. The activation levels indicated that promoter activity of chgHα ≥ vtgAs > chgHβ > chgL ≥ vtgC ≥ esr1a when mediated by Esr1a, chgHβ > chgHα > chgHL > vtgAs ≥ vtgC ≥ esr1a by Esr1b, chgHβ ≥ chgL > chgHα ≥ vtgAs > vtgC > esr1a by Esr2a, and chgHβ ≥ chgHα ≥ vtgAs > chgL ≥ vtgC > esr1a by Esr2b. Collectively, different Esr subtypes were distinctly different in their ability to transactivate estrogen-responsive target genes, resulting in differential expression of chg, vtg and esr1a genes in the estrogen-exposed hepatocytes.

Knock out of a major vitellogenin receptor gene with eight ligand binding repeats in medaka (Oryzias latipes) using the CRISPR/Cas9 system

Recent studies of vitellogenesis engendered a novel model of teleost yolk formation in which multiple yolk precursors, vitellogenins (Vtgs), and their receptors (Vtgrs) interact to ensure proper yolk composition for embryonic development and larval growth. As a step toward verification of this concept, we examined the role of one candidate Vtgr, termed low-density lipoprotein receptor relative with eight ligand-binding repeat (Lr8), in the medaka, a representative teleost and established laboratory model. A homozygous lr8 knock out (lr8-KO) medaka was produced to perform reverse-genetic functional analyses. In ovaries of wild type (WT) medaka, Western blotting detected a putative Lr8 protein band at ~130 kDa, while immunohistochemistry detected the putative Lr8 signal at the periphery of the oocyte underneath the zona radiata. These signals disappeared in ovaries of the lr8-KO group. Offspring of lr8-KO medaka exhibited decreased survival rate compared to WT fish, but KO of lr8 was not 100% lethal. There was no significant difference in total yolk protein content or size of eggs between WT and lr8-KO fish. However, LC-MS/MS analyses revealed a remarkable decrease in the relative abundance of yolk proteins derived from VtgAb in lr8-KO eggs, in conjunction with a compensatory increase in proteins derived from VtgAa1. These findings strongly support the conclusion that Lr8 is an important receptor for VtgAb in medaka. The disruption of proper yolk composition by lr8-KO is possibly one cause of the low offspring survival.

Molecular characterization and expression profiles of six genes involved in vitellogenic deposition and hydrolysis of Chinese sturgeon (Acipenser sinensis) suggesting their transcriptional regulation on ovarian development

Ovary development of Chinese sturgeon (Acipenser sinensis) in controlled breeding has been reported to respond to dietary lipid levels. However, the corresponding molecular regulatory mechanism about ovary development of Chinese sturgeon is still unclear. To elucidate the molecular mechanism of vitellogenic deposition and hydrolysis, six key genes, namely, vtgr (vitellogenin receptor), atp6v1c1 (Vacuolar H⁺-ATPase subunit c1), atp6v1h (Vacuolar H⁺-ATPase subunit h), ctsb (cathepsin B), ctsd (cathepsin D) and ctsl (cathepsin L) involved in vitellogenic deposition and hydrolysis of Chinese sturgeon were cloned and characterized, and their spatio-temporal mRNA expression profiles as well as transcriptional responses to dietary lipid level were investigated. The full-length cDNA sequences of these six genes showed similar domain structure to their respective orthologous genes from other vertebrates. Tissue-specific expression patterns of these genes were observed in ovary, liver, muscle, spleen, brain, gill, intestine, heart, stomach and kidney. Ovarian expression level of vtgr was the highest in stage II, and ctsl expression was the highest in stage IV, while the mRNA expressions of other 4 genes were the highest in stage III. The increase of dietary lipid level promoted ovary development and elevated the expressions of vtgr, atp6v1c1, atp6v1h, ctsb and ctsd in the ovary. The results of the present study indicated that these genes are crucial for vitellogenic deposition, and provided a preliminary understanding on the molecular regulation of vitellogenic deposition and hydrolysis during ovary development of Chinese sturgeon.

Identification and structural characterization of the factors involved in vitellogenesis and its regulation in the African Osteoglossiforme of aquacultural interest Heterotis niloticus (Cuvier, 1829)

The African bonytongue (Heterotis niloticus) is an excellent candidate for fish farming because it has outstanding biological characteristics and zootechnical performances. However, the absence of sexual dimorphism does not favor its reproduction in captivity or the understanding of its reproductive behavior. Moreover, no molecular data related to its reproduction is yet available. This study therefore focuses on the structural identification of the different molecular actors of vitellogenesis expressed in the pituitary gland, the liver and the ovary of H. niloticus. A transcriptomic approach based on de novo RNA sequencing of the pituitary gland, ovary and liver of females in vitellogenesis led to the creation of three transcriptomes. In silico analysis of these transcriptomes identified the sequences of pituitary hormones such as prolactin (PRL), luteinizing hormone (LH) and follicle-stimulating hormone (FSH) and their ovarian receptors (PRLR, FSHR, LHR). In the liver and ovary, estrogen receptors (ER) beta and gamma, liver vitellogenins (VtgB and VtgC) and their ovarian receptors (VLDLR) were identified. Finally, the partial transcript of an ovarian Vtg weakly expressed compared to hepatic Vtg was identified based on structural criteria. Moreover, a proteomic approach carried out from mucus revealed the presence of one Vtg exclusively in females in vitellogenesis. In this teleost fish that does not exhibit sexual dimorphism, mucus Vtg could be used as a sexing biomarker based on a non-invasive technique compatible with the implementation of experimental protocols in vivo.

A de novo transcriptome assembly approach elucidates the dynamics of ovarian maturation in the swordfish (Xiphias gladius)

The Mediterranean swordfish (Xiphias gladius) has been recently classified as overfished and in 2016, the International Commission for the Conservation of the Atlantic Tunas (ICCAT) established a multi-annual management plan to recover this stock. To successfully achieve this goal, knowledge about swordfish biology is needed. To date, few studies on swordfish have been performed and none of them has provided useful insights into the reproductive biology at molecular level. Here we set to characterise the molecular dynamics underlying ovarian maturation by employing a de novo transcriptome assembly approach. Differential gene expression analysis in mature and immature ovaries identified a number of differentially expressed genes associated with biological processes driving ovarian maturation. Focusing on ovarian steroidogenesis and vitellogenin uptake, we depict the molecular dynamics characterizing these processes while a phylogenetic analysis let us identify a candidate vitellogenin receptor. This is the first swordfish transcriptome assembly and these findings provide in-depth understanding of molecular processes describing ovarian maturation. Moreover, the establishment of a publicly available database containing information on the swordfish transcriptome aims to boost research on this species with the long-term of developing more comprehensive and successful stock management plans.

Molecular cloning of vitellogenin gene promoters and in vitro and in vivo transcription profiles following estradiol-17β administration in the cutthroat trout

Transcription of vitellogenin (vtg) genes are initiated when estradiol-17β (E₂)-estrogen receptor (ER) complexes bind estrogen response elements (ERE) located in the gene promoter region. Transcriptional regulation of dual vtg subtypes (major salmonid A-type vtg: vtgAs; minor C-type vtg: vtgC) by E₂ was investigated under co-expression of a potential major transcriptional factor, erα1, in cutthroat trout. Two forms of trout vtgAs promoters (1 and 2) and one vtgC promoter were sequenced. These promoters structurally differ based on the number of EREs present. The vtgAs promoter 1 exhibited the highest maximal transcriptional activity by in vitro gene reporter assays. The concentration of E₂ that induces 50% of gene reporter activity (half-maximal effective concentrations, EC₅₀) was similar among all vtg promoters and also to the EC₅₀ of E₂ administered to induce vtg transcription in vivo. This study revealed a difference in transcriptional properties of multiple vtg promoters for the first time in a salmonid species, providing the basis to understand mechanisms underlying regulation of vitellogenesis via dual vtg gene expression.

Ovarian expression and localization of clathrin (Cltc) components in cutthroat trout, Oncorhynchus clarki: Evidence for Cltc involvement in endocytosis of vitellogenin during oocyte growth

To evaluate potential involvement of clathrin in endocytosis of vitellogenin (Vtg) by teleost oocytes, cDNAs encoding clathrin heavy chain (cltc) were cloned from ovaries of cutthroat trout. Quantitative PCR revealed three types of cltc (cltc-a1, cltc-a2, cltc-b) to be expressed in 10 different tissues including the ovary. The cltc-a1 alone exhibited a significant decrease in ovarian expression during vitellogenesis; this was correlated with a corresponding decrease in transcripts encoding the major Vtg receptor (Vtgr). No development-related changes in ovarian cltc-a2 or cltc-b transcript levels were observed. In situ hybridization revealed a strong ctlc signal in pre-vitellogenic oocytes, but not in vitellogenic oocytes. Western blotting using a rabbit antiserum (a-Cltc) raised against a recombinant Cltc preparation detected a polypeptide band with an apparent mass of ~170kDa in vitellogenic ovary extracts. Immunohistochemistry using a-Cltc revealed Cltc to be uniformly distributed throughout the ooplasm of perinucleolus stage oocytes, translocated to the periphery of lipid droplet stage oocytes, and localized to the oolemma during vitellogenesis. These patterns of cltc/Cltc distribution and abundance during oogenesis, which are identical to those previously reported for vtgr/Vtgr in this species, constitute the first empirical evidence that cltc-a1/Cltc-a1 is involved in Vtg endocytosis via the Vtgr in teleost fish.

Ovarian yolk formation in fishes: Molecular mechanisms underlying formation of lipid droplets and vitellogenin-derived yolk proteins

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

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

During certain stages in an animal's life cycle, energy requirements may exceed energy intake from the diet. The spawning migration of temperate eels is a textbook example of negative energy balance, forcing these fish to rely on stored fats (triacylglycerides) to provide their muscles with energy for swimming and their growing oocytes with the nutrients needed to develop and support healthy offspring. We predicted broad implications of this great need for endogenous triacylglycerides in terms of their packaging, transport, and ovarian uptake. To test this, serum lipid concentrations and transcript abundances of intestinal and hepatic triacylglyceride packagers and ovarian triacylglyceride modifiers and receivers were investigated throughout previtellogenesis (feeding phase) and into early vitellogenesis (fasting phase) in short-finned eels. A switch from exogenous to endogenous triacylglyceride packaging was seen as the liver upregulated transcript levels of apolipoprotein B and microsomal triacylglyceride transport protein and downregulated those of apolipoprotein E and lipoprotein lipase. In the intestine, the reverse response was observed. Furthermore, ovarian transcript abundances of triacylglyceride modifiers and receivers increased (apolipoprotein E, lipoprotein lipase, and vitellogenin receptor), indicative of increased triacylglyceride uptake during previtellogenesis. We propose that increased hepatic apolipoprotein B production is a conserved vertebrate response to prolonged periods of negative energy balance.

Low-density lipoprotein receptor affects the fertility of female mice

Low-density lipoprotein receptor (LDLR) has been demonstrated to play a central role in lipoprotein metabolism, with Ldlr-deficient (Ldlr–/–) mice developing severe dyslipidemia. In the present study we investigated whether Ldlr knockout could harm female reproduction and explored the mechanisms involved. The results indicate that although the number of litters born to Ldlr–/– mice did not differ significantly from that born to controls, the number of pups per litter was significantly lower in the former group. Interestingly, although Ldlr–/– mice were obese, the weight of their ovaries was lower than that in control mice. Serum cholesterol levels was significantly higher in Ldlr–/– mice than in their wild-type counterparts. In contrast, there were significant decreases in cholesterol, triglyceride and total lipid levels in ovaries of Ldlr–/– mice. Both ovarian lipid deposition, as detected by Oil red O staining, and lipid droplets, as evaluated by transmission electron microscopy, supported decreased lipid levels in ovaries from Ldlr–/– mice. In addition, Ldlr–/– mice had fewer ovarian follicles, more atretic follicles, lower oestrogen levels and spent significantly less time in oestrus than did the controls. Superovulation assays indicated immature Ldlr–/– mice ovulated fewer ova than controls. These results indicate that lack of Ldlr results in dyslipidaemia and poor fertility.

Lrp13 is a novel vertebrate lipoprotein receptor that binds vitellogenins in teleost fishes

Transcripts encoding a novel member of the lipoprotein receptor superfamily, termed LDL receptor-related protein (Lrp)13, were sequenced from striped bass (Morone saxatilis) and white perch (Morone americana) ovaries. Receptor proteins were purified from perch ovary membranes by protein-affinity chromatography employing an immobilized mixture of vitellogenins Aa and Ab. RT-PCR revealed lrp13 to be predominantly expressed in striped bass ovary, and in situ hybridization detected lrp13 transcripts in the ooplasm of early secondary growth oocytes. Quantitative RT-PCR confirmed peak lrp13 expression in the ovary during early secondary growth. Quantitative mass spectrometry revealed peak Lrp13 protein levels in striped bass ovary during late-vitellogenesis, and immunohistochemistry localized Lrp13 to the oolemma and zona radiata of vitellogenic oocytes. Previously unreported orthologs of lrp13 were identified in genome sequences of fishes, chicken (Gallus gallus), mouse (Mus musculus), and dog (Canis lupus familiaris). Zebrafish (Danio rerio) and Nile tilapia (Oreochromis niloticus) lrp13 loci are discrete and share genomic synteny. The Lrp13 appears to function as a vitellogenin receptor and may be an important mediator of yolk formation in fishes and other oviparous vertebrates. The presence of lrp13 orthologs in mammals suggests that this lipoprotein receptor is widely distributed among vertebrates, where it may generally play a role in lipoprotein metabolism.

Ovarian expression and localization of a vitellogenin receptor with eight ligand binding repeats in the cutthroat trout (Oncorhynchus clarki)

A cDNA encoding a vitellogenin receptor with 8 ligand binding repeats (vtgr) was cloned from ovaries of the cutthroat trout, Oncorhynchus clarki. In situ hybridization and quantitative PCR analyses revealed that the main site of vtgr mRNA expression was the oocytes. Expression was strongly detected in perinucleous stage oocytes, gradually decreased as oocytes grew, and became hardly detectable in vitellogenic oocytes. A rabbit antibody (a-Vtgr) was raised against a recombinant Vtgr protein in order to immunologically detect and localize Vtgr within the ovarian follicles. Western blotting using a-Vtgr detected a bold band with an apparent mass of ~95-105kDa in an ovarian preparation that also bound Sakhalin taimen, Hucho perryi, vitellogenin in ligand blots. Immunohistochemistry using a-Vtgr revealed that the Vtgr was uniformly distributed throughout the ooplasm of perinucleolus stage oocytes, subsequently translocated to the periphery of lipid droplet stage oocytes, and became localized to the oolemma during vitellogenesis. We provide the first characterization of Vtgr at both the transcriptional and the translational levels in the cutthroat trout, and our results suggest that this receptor is involved in uptake of Vtg by oocytes of this species.