Two messenger RNA isoforms of the gonadotrophin-releasing hormone receptor, generated by alternative splicing and/or promoter usage, are differentially expressed in rainbow trout gonads during gametogenesis

The late-evolving salmon and trout join the GnRH1 club

Although it is known that the whitefish, an ancient salmonid, expresses three distinct gonadotropin-releasing hormone (GnRH) forms in the brain, it has been thought that the later-evolving salmonids (salmon and trout) had only two types of GnRH: GnRH2 and GnRH3. We now provide evidence for the expression of GnRH1 in the gonads of Atlantic salmon by rapid amplification of cDNA ends, real-time quantitative PCR and immunohistochemistry. We examined six different salmonid genomes and found that each assembly has one gene that likely encodes a viable GnRH1 prepropeptide. In contrast to both functional GnRH2 and GnRH3 paralogs, the GnRH1 homeolog can no longer express the hormone. Furthermore, the viable salmonid GnRH1 mRNA is composed of only three exons, rather than the four exons that build the GnRH2 and GnRH3 mRNAs. Transcribed gnrh1 is broadly expressed (in 17/18 tissues examined), with relative abundance highest in the ovaries. Expression of the gnrh2 and gnrh3 mRNAs is more restricted, primarily to the brain, and not in the gonads. The GnRH1 proximal promoter presents composite binding elements that predict interactions with complexes that contain diverse cell fate and differentiation transcription factors. We provide immunological evidence for GnRH1 peptide in the nucleus of 1-year-old type A spermatogonia and cortical alveoli oocytes. GnRH1 peptide was not detected during other germ cell or reproductive stages. GnRH1 activity in the salmonid gonad may occur only during early stages of development and play a key role in a regulatory network that controls mitotic and/or meiotic processes within the germ cell.

Gnrh receptor gnrhr2bbα is expressed exclusively in lhb-expressing cells in Atlantic salmon male parr

Gonadotropin-releasing hormone (Gnrh) plays a major role in the regulation of physiological and behavioural processes related to reproduction. In the pituitary, it stimulates gonadotropin synthesis and release via activation of Gnrh receptors (Gnrhr), belonging to the G protein-coupled receptor superfamily. Evidence suggests that differential regulation of the two gonadotropins (Fsh and Lh) is achieved through activation of distinct intracellular pathways and, probably, through the action of distinct receptors. However, the roles of the different Gnrhr isoforms in teleosts are still not well understood. This study investigates the gene expression of Gnrhr in the pituitary gland of precociously maturing Atlantic salmon (Salmo salar) male parr. A total of six Gnrhr paralogs were identified in the Atlantic salmon genome and named according to phylogenetic relationship; gnrhr1caα, gnrhr1caβ, gnrhr1cbα, gnrhr1cbβ, gnrhr2bbα, gnrhr2bbβ. All paralogs, except gnrhr1caα, were expressed in male parr pituitary during gonadal maturation as evidenced by qPCR analysis. Only one gene, gnrhr2bbα, was differentially expressed depending on maturational stage (yearly cycle), with high expression levels in maturing fish, increasing in parallel with gonadotropin subunit gene expression. Additionally, a correlation in daily expression levels was detected between gnrhr2bbα and lhb (daily cycle) in immature fish in mid-April. Double fluorescence in situ hybridization showed that gnrhr2bbα was expressed exclusively in lhb gonadotropes in the pituitary, with no expression detected in fshb cells. These results suggest the involvement of receptor paralog gnrhr2bbα in the regulation of lhb cells, and not fshb cells, in sexually maturing Atlantic salmon male parr.

Effect of gonadotropin-releasing hormone on phagocytic leucocytes of rainbow trout

To clarify the role of gonadotropin-releasing hormone (GnRH) in the fish immune system, in vitro effect of GnRH was examined in phagocytic leucocytes of rainbow trout (Oncorhynchus mykiss). Gene expression of GnRH-receptor was detected by RT-PCR in leucocytes from head kidney. Administration of sGnRH increased proliferation and mRNA levels of a proinflammatory cytokine, tumor necrosis factor (TNF)-α, in trout leucocytes. Superoxide production in zymosan-stimulated phagocytic leucocytes was also increased by sGnRH in a dose-related manner from 0.01 to 100 nM. There was no significant effect of sGnRH on mRNA levels of growth hormone (GH) expressed in trout phagocytic leucocytes. Immunoneutralization of GH by addition of anti-salmon GH serum into the medium could not block the stimulatory effect of sGnRH on superoxide production. These results indicate that GnRH stimulates phagocytosis in fish leucocytes through a GnRH-receptor-dependent pathway, and that the effect of GnRH is not mediated through paracrine GH in leucocytes.

Neuropeptides in the gonads: from evolution to pharmacology

Vertebrate gonads are the sites of synthesis and binding of many peptides that were initially classified as neuropeptides. These gonadal neuropeptide systems are neither well understood in isolation, nor in their interactions with other neuropeptide systems. Further, our knowledge of the control of these gonadal neuropeptides by peripheral hormones that bind to the gonads, and which themselves are under regulation by true neuropeptide systems from the hypothalamus, is relatively meager. This review discusses the existence of a variety of neuropeptides and their receptors which have been discovered in vertebrate gonads, and the possible way in which such systems could have evolved. We then focus on two key neuropeptides for regulation of the hypothalamo-pituitary-gonadal axis: gonadotropin-releasing hormone (GnRH) and gonadotropin-inhibitory hormone (GnIH). Comparative studies have provided us with a degree of understanding as to how a gonadal GnRH system might have evolved, and they have been responsible for the discovery of GnIH and its gonadal counterpart. We attempt to highlight what is known about these two key gonadal neuropeptides, how their actions differ from their hypothalamic counterparts, and how we might learn from comparative studies of them and other gonadal neuropeptides in terms of pharmacology, reproductive physiology and evolutionary biology.

Regulation of two forms of gonadotropin-releasing hormone receptor gene expression in the protandrous black porgy fish, Acanthopagrus schlegeli

Two GnRH receptors (GnRH-R I and GnRH-R II) were obtained in protandrous black porgy (Acanthopagrus schlegeli). We investigated their tissue distribution, developmental/seasonal changes and regulation of expression using in vivo and in vitro (primary cultures of dispersed pituitary cells) approaches. The relative expressions of GnRH-Rs in the pituitary and gonad were as follows: pituitary: GnRH-R I > GnRH-R II; testicular tissue: GnRH-R I > GnRH-R II; ovarian tissue: GnRH-R I = GnRH-R II. GnRH-R I but not GnRH-R II expression was higher in the pituitary during the spawning period as compared to the prespawning. The expression profiles of both forms of GnRH-R were variable in the gonads according to the gonadal stage and season. In vivo, hCG stimulated GnRH-R I and GnRH-R II expression in testis and ovary. The LHRH analog also up-regulated both receptors in testis and but increased only GnRH-R II in the ovary. Sex steroids (estradiol, E2 and testosterone, T) increased the expression of both receptors in the testis and ovary. In the pituitary, sex steroids (E2 and T) increased the expression of GnRH-R I, but not GnRH-II, both in vivo and in vitro. The LHRH analog also specifically up-regulated the expression of GnRH-R I, but not GnRH-R II, by pituitary cells in vitro. All these data suggest that GnRH-R I rather than GnRH-R II may play a major physiological role in the pituitary. In contrast, both GnRH-R I and GnRH-R II may participate in the regulation of gonadal functions, including a possible role during sex change.

Testes and brain gene expression in precocious male and adult maturing Atlantic salmon (Salmo salar)

BACKGROUND

The male Atlantic salmon generally matures in fresh water upon returning after one or several years at sea. Some fast-growing male parr develop an alternative life strategy where they sexually mature before migrating to the oceans. These so called 'precocious' parr or 'sneakers' can successfully fertilise adult female eggs and so perpetuate their line. We have used a custom-built cDNA microarray to investigate gene expression changes occurring in the salmon gonad and brain associated with precocious maturation. The microarray has been populated with genes selected specifically for involvement in sexual maturation (precocious and adult) and in the parr-smolt transformation.

RESULTS

Immature and mature parr collected from a hatchery-reared stock in January were significantly different in weight, length and condition factor. Changes in brain expression were small - never more than 2-fold on the microarray, and down-regulation of genes was much more pronounced than up-regulation. Significantly changing genes included isotocin, vasotocin, cathepsin D, anamorsin and apolipoprotein E. Much greater changes in expression were seen in the testes. Among those genes in the testis with the most significant changes in expression were anti-Mullerian hormone, collagen 1A, and zinc finger protein (Zic1), which were down-regulated in precocity and apolipoproteins E and C-1, lipoprotein lipase and anti-leukoproteinase precursor which were up-regulated in precocity. Expression changes of several genes were confirmed in individual fish by quantitative PCR and several genes (anti-Mullerian hormone, collagen 1A, beta-globin and guanine nucleotide binding protein (G protein) beta polypeptide 2-like 1 (GNB2L1) were also examined in adult maturing testes. Down-regulation of anti-Mullerian hormone was judged to be greater than 160-fold for precocious males and greater than 230-fold for November adult testes in comparison to July testes by this method. For anti-Mullerian hormone and guanine nucleotide binding protein beta polypeptide 2-like 1 expression changes in precocious males mirrored mature adults (November) but for collagen 1A and beta-globin the pattern was more complex.

CONCLUSIONS

Expression changes in the fish brain during the process of precocious sexual maturation were small compared to those in the testes. Microarray analysis suggested down-regulation of housekeeping functions and up-regulation of a small number of specific processes. Transcriptional changes in the testes were much more pronounced with anti-Mullerian hormone playing a major role. Expression profiles for mature parr and maturing adult testes indicate subtle differences in gene expression between these two related groups.

Perspectives on fish gonadotropins and their receptors

Teleosts lack a hypophyseal portal system and hence neurohormones are carried by nerve fibers from the preoptic region to the pituitary. The various cell types in the teleost pituitary are organized in discrete domains. Fish possess two gonadotropins (GtH) similar to FSH and LH in other vertebrates; they are heterodimeric hormones that consist of a common alpha subunit non-covalently associated with a hormone-specific beta subunit. In recent years the availability of molecular cloning techniques allowed the isolation of the genes coding for the GtH subunits in 56 fish species representing at least 14 teleost orders. Advanced molecular engineering provides the technology to produce recombinant GtHs from isolated cDNAs. Various expression systems have been used for the production of recombinant proteins. Recombinant fish GtHs were produced for carp, seabream, channel and African catfish, goldfish, eel, tilapia, zebrafish, Manchurian trout and Orange-spotted grouper. The hypothalamus in fishes exerts its regulation on the release of the GtHs via several neurohormones such as GnRH, dopamine, GABA, PACAP, IGF-I, norepinephrine, NPY, kisspeptin, leptin and ghrelin. In addition, gonadal steroids and peptides exert their effects on the gonadotropins either directly or via the hypothalamus. All these are discussed in detail in this review. In mammals, the biological activities of FSH and LH are directed to different gonadal target cells through the cell-specific expression of the FSH receptor (FSHR) and LH receptor (LHR), respectively, and the interaction between each gonadotropin-receptor couple is highly selective. In contrast, the bioactivity of fish gonadotropins seems to be less specific as a result of promiscuous hormone-receptor interactions, while FSHR expression in Leydig cells explains the strong steroidogenic activity of FSH in certain fish species.

The adipokinetic hormone system in Culicinae (Diptera: Culicidae): molecular identification and characterization of two adipokinetic hormone (AKH) precursors from Aedes aegypti and Culex pipiens and two putative AKH receptor variants from A. aegypti

Insect neuropeptides of the adipokinetic hormone (AKH) family induce the mobilization of energy stores to fuel flight, but also affect the nutritional balance during diapause and oogenesis. They are therefore important regulators for flight, hibernation, and reproduction in mosquitoes including those that transmit human pathogens. In this study, we identified and analyzed the genes encoding two AKH preprohormones in the Yellow fever mosquito, Aedes aegypti: Aedae-AKH-I encodes the octapeptide pELFTPSWa and Aedae-AKH-II the decapeptide pEVTFSRDWNAa. Identical AKHs were identified in the West Nile virus vector, Culex pipiens, whose genes were characterized in this study as Culpi-AKH-I and Culpi-AKH-II. Using Northern blot, transcript expression was shown in A. aegypti, for Aedae-AKH-I in the head/thorax tissues of pupae and females, as well as in the abdomen of adult males; Aedae-AKH-II was only expressed in adults. In an immunocytological study using an AKH-antibody, the corpus cardiacum (CC), the intrinsic CC-cells (X-cells), the nervi corporis cardiaci, cells in the brain and thoracic ganglia were stained. In addition, two splice variants of the AKH-receptor gene were characterized in A. aegypti, (Aedae-AKHR-I and -II). RT-PCR revealed that both variants of these typical G-protein-coupled receptors were expressed in all life stages. Aedae-AKHR-I expression was also detected in the ovaries, indicating once more the influence of the AKH/AKHR system during the insect's oogenesis. Based on phylogenetic data, we postulate two closely related types of AKH-receptors that could bind selectively the two AKH peptides found in A. aegypti.

mRNA expression of GnRH variants and receptors in the brain, pituitary and ovaries of pejerrey (Odontesthes bonariensis) in relation to the reproductive status

The present study examined the differential mRNA expression levels of three forms of GnRH (sGnRH, pjGnRH and cGnRH-II) and two forms of GnRH receptor (pjGnRH-R I and pjGnRH-R II) in the brain, pituitary, and ovaries of pejerrey in relation to the reproductive status. The analysis revealed the presence of significant amounts of mRNA of the three GnRH forms while the ovaries showed only two (sGnRH and pjGnRH). The GnRH receptor II was found ubiquitously in the brain, pituitary, and ovaries while the form I was detected only in the brain. The levels of pjGnRH mRNA in the brain and pjGnRH-R II in the pituitary gland varied in correlation with the ovarian condition. However, brain sGnRH and pjGnRH-R I mRNA levels reached a maximum during early stages of ovarian development. In contrast, the brain levels of cGnRH-II mRNA showed no variation. The present study also shows a good correlation of ovarian sGnRH and pjGnRH-R II mRNA levels with the reproductive condition, suggesting that these molecules are may be involved in the regulation of pejerrey ovarian function.

Molecular cloning of a molluscan gonadotropin-releasing hormone receptor orthologue specifically expressed in the gonad

Despite their economic importance, only very little information is available regarding (neuro)endocrine mechanisms of reproduction in bivalve molluscs. To gain insights into the molecular control of gonadic development of these animals, G protein-coupled receptors (GPCR) expressed in the gonad of the pacific oyster Crassostrea gigas were investigated. One such receptor was cloned by RT-PCR using oligonucleotide primers derived from consensus sequences of various vertebrate (neuro)peptide receptors. This receptor named Cg-GnRH-related receptor (Cg GnRH-R) exhibits a high degree of amino acid sequence identity with both vertebrate GnRH receptors and insect AKH receptors. Quantitative RT-PCR shows a specific expression of Cg-GnRH-R in both male and female gonads during the reproductive cycle. This demonstrates for the first time the plausible involvement of a GnRH receptor orthologue in the control of reproduction in a protostomian invertebrate.

Fish and chips: various methodologies demonstrate utility of a 16,006-gene salmonid microarray

Background

We have developed and fabricated a salmonid microarray containing cDNAs representing 16,006 genes. The genes spotted on the array have been stringently selected from Atlantic salmon and rainbow trout expressed sequence tag (EST) databases. The EST databases presently contain over 300,000 sequences from over 175 salmonid cDNA libraries derived from a wide variety of tissues and different developmental stages. In order to evaluate the utility of the microarray, a number of hybridization techniques and screening methods have been developed and tested.

Results

We have analyzed and evaluated the utility of a microarray containing 16,006 (16K) salmonid cDNAs in a variety of potential experimental settings. We quantified the amount of transcriptome binding that occurred in cross-species, organ complexity and intraspecific variation hybridization studies. We also developed a methodology to rapidly identify and confirm the contents of a bacterial artificial chromosome (BAC) library containing Atlantic salmon genomic DNA.

Conclusion

We validate and demonstrate the usefulness of the 16K microarray over a wide range of teleosts, even for transcriptome targets from species distantly related to salmonids. We show the potential of the use of the microarray in a variety of experimental settings through hybridization studies that examine the binding of targets derived from different organs and tissues. Intraspecific variation in transcriptome expression is evaluated and discussed. Finally, BAC hybridizations are demonstrated as a rapid and accurate means to identify gene content.

Molecular regulation of gonadotropin secretion by gonadotropin-releasing hormone in salmonid fishes

Gonadotropin-releasing hormone (GnRH) plays a central role in the control of reproductive function in vertebrates. In salmonids, salmon GnRH (sGnRH) secreted by preoptic GnRH neurons regulates gonadal maturation through stimulation of synthesis and release of pituitary gonadotropins (GTHs). In addition, several lines of our evidence indicate that sGnRH is involved in spawning behavior, and serves to integrate the gonadal maturation with the reproductive behavior. A growing number of studies show that the effects of GnRH are mediated by multiple subtypes of GnRH receptors, successive multiple signaling pathways, and finally multiple transcription factors which act cooperatively to stimulate transcription of GTH subunit genes. This complex regulatory system of the action of GnRH may serve as a molecular basis of divergent physiological strategies of reproductive success in various vertebrate species. In this article, recent data on the molecular mechanisms of action of GnRH are reviewed with special reference to the regulation of synthesis and release of GTHs in the pituitary of salmonids to elucidate the multifunctional action of GnRH.

Cloning and Expression of Gonadotropin-Releasing Hormone Receptor in the Brain and Pituitary of the European Sea Bass: An In Situ Hybridization Study1

A full-length cDNA encoding a GnRH receptor (GnRH-R) has been obtained from the pituitary of the European sea bass, Dicentrarchus labrax. The complete cDNA is 1814 base pairs (bp) in length and encodes a protein of 416 amino acids. The 5' UTR and 3' UTR are 239 bp and 324 bp in size, respectively. The expression sites of this GnRH-R were studied in the brain and pituitary of sea bass by means of in situ hybridization. A quantitative analysis of the expression of the GnRH-R gene along the reproductive cycle was also performed. The GnRH-R brain expression was especially relevant in the ventral telencephalon and rostral preoptic area. Some GnRH-R messenger-expressing cells were also evident in the dorsal telencephalon, caudal preoptic area, ventral thalamus, and periventricular hypothalamus. A conspicuous and specific GnRH-R expression was detected in the pineal gland. The highest expression of the GnRH-R gene was observed in the proximal pars distalis of the pituitary. This expression was evident in all LH cells and some FSH cells but not in somatotrophs. In the pituitary, the quantitative analysis revealed a higher expression of GnRH-R gene during late vitellogenesis in comparison with maturation, spawning, and postspawning/resting periods. However, in the brain, the highest GnRH-R expression was evident at spawning or postspawning/ resting periods. These results suggest that the expression of this GnRH-R is regulated in a different manner in the brain and the pituitary of sea bass.

Development and application of a salmonid EST database and cDNA microarray: data mining and interspecific hybridization characteristics

We report 80,388 ESTs from 23 Atlantic salmon (Salmo salar) cDNA libraries (61,819 ESTs), 6 rainbow trout (Oncorhynchus mykiss) cDNA libraries (14,544 ESTs), 2 chinook salmon (Oncorhynchus tshawytscha) cDNA libraries (1317 ESTs), 2 sockeye salmon (Oncorhynchus nerka) cDNA libraries (1243 ESTs), and 2 lake whitefish (Coregonus clupeaformis) cDNA libraries (1465 ESTs). The majority of these are 3' sequences, allowing discrimination between paralogs arising from a recent genome duplication in the salmonid lineage. Sequence assembly reveals 28,710 different S. salar, 8981 O. mykiss, 1085 O. tshawytscha, 520 O. nerka, and 1176 C. clupeaformis putative transcripts. We annotate the submitted portion of our EST database by molecular function. Higher- and lower-molecular-weight fractions of libraries are shown to contain distinct gene sets, and higher rates of gene discovery are associated with higher-molecular weight libraries. Pyloric caecum library group annotations indicate this organ may function in redox control and as a barrier against systemic uptake of xenobiotics. A microarray is described, containing 7356 salmonid elements representing 3557 different cDNAs. Analyses of cross-species hybridizations to this cDNA microarray indicate that this resource may be used for studies involving all salmonids.

Identification and characterization of a reptilian GnRH receptor from the leopard gecko

Gonadotropin-releasing hormone (GnRH) plays a pivotal role in the regulation of reproductive functions through interactions with its specific receptor. We describe the first molecular cloning and characterization of a full-length GnRH receptor (GnRHR) from the leopard gecko Eublepharis macularius. It has a distinct genomic structure consisting of five exons and four introns, compared with all the other reported GnRHR genes. A native GnRH form, cGnRH-II, stimulated inositol phosphate (IP) production in COS-7 cells transiently transfected with the GnRHR, in a dose dependent manner. The mRNA was expressed in all the tissues and organs examined. Molecular phylogenetic analysis revealed that the cloned GnRHR belongs to the type 2/nonmammalian I GnRHR. Low-expression levels were observed from the pituitary glands of reproductively active leopard geckos, indicating the possibility that there is at least one more type of GnRHR highly expressed in the pituitary gland for the gonadotropin secretion in this reptile.

Evolutionary aspects of GnRHs, GnRH neuronal systems and GnRH receptors in teleost fish

Gonadotrophin-releasing hormone (GnRH) was originally believed to be released by a unique set of hypophysiotrophic neurons to stimulate the release of gonadotrophins from the pituitary, therefore acting as a major initiator of the hormonal cascade controlling the reproductive axis. However, it now appears that each vertebrate species expresses two or three GnRH forms in multiple tissues and that GnRHs exert pleiotropic actions via several classes of receptors. This new vision of the GnRH systems arose progressively from numerous comparative studies in all vertebrate classes, but fish in general, and teleosts in particular, have often plaid a leading part in changing established concepts. To date fish still appear as attractive models to decipher the evolutionary mechanisms that led to the diversification of GnRH functions. Not only do teleosts exhibit the highest variety of GnRH variants, but recent data and whole genome analyses indicate that they may also possess multiple GnRH receptors. This paper intends to summarize the current situation with special emphasis on interspecies comparisons which provide insights into the possible evolutionary mechanisms leading to the diversification of GnRH functions.

Regulation of protein diversity by alternative pre-mRNA splicing with specific focus on chondrogenesis

Analysis of the human genome has dramatically demonstrated that the majority of protein diversity is generated by alternative splicing of pre-mRNA. This powerful and versatile mechanism controls the synthesis of functionally different protein isoforms that may be required during specific stages of development from a single gene. Consequently, ubiquitous and/or tissue-specific RNA splicing factors that regulate this splicing mechanism provide the basis for defining phenotypic characteristics of cells during differentiation. In this review, we will introduce the basic mechanisms of pre-mRNA alternative splicing, describe how this process is regulated by specific RNA splicing factors, and relate this to various systems of cell differentiation. Chondrogenesis, a well-defined differentiation pathway necessary for skeletogenesis, will be discussed in detail, with focus on some of the alternatively-spliced proteins known to be expressed during cartilage development. We propose a heuristic view that, ultimately, it is the regulation of these RNA splicing factors that determines the differentiation status of a cell. Studying regulation at the level of pre-mRNA alternative splicing will provide invaluable insights into how many developmental mechanisms are controlled, thus enabling us to manipulate a system to select for a specific differentiation pathway.