Somatolactin stimulates in vitro gonadal steroidogenesis in coho salmon, Oncorhynchus kisutch

Anguillid Eels as a Model Species for Understanding Endocrinological Influences on the Onset of Spawning Migration of Fishes

Simple Summary

Endocrine regulation has been thought to play a major role in the onset of migration. Anguillid eels provide a good model for studying the onset mechanisms of migrations to breeding areas, because the process of the onset of migration occurs in inland waters. In this review, we summarize information about the silvering process in anguillid eels and the dynamics of mRNA expression of neurohormones and pituitary hormones, thyroid hormones, and sex steroids associated with the onset of the spawning migration. We also provide new results. Because 11-KT drastically increases during silvering, the role of 11-KT in the onset of spawning migration was discussed in detail.

Abstract

Anguillid eels are the iconic example of catadromous fishes, because of their long-distance offshore spawning migrations. They are also a good model for research on the onset mechanisms of migrations to breeding areas, because the migrations begin in inland waters. When eels transform from yellow eels to silver eels, it is called silvering. Silver eels show various synchronous external and internal changes during silvering, that include coloration changes, eye-size increases, and gonadal development, which appear to be pre-adaptations to the oceanic environment and for reproductive maturation. A strong gonadotropic axis activation occurs during silvering, whereas somatotropic and thyrotropic axes are not activated. Among various hormones, 11-ketotestosterone (11-KT) drastically increases during spawning migration onset. Gradual water temperature decreases simulating the autumn migratory season, inducing 11-KT increases. Administration of 11-KT appeared to cause changes related to silvering, such as early-stage oocyte growth and eye enlargement. Moreover, 11-KT may be an endogenous factor that elevates the migratory drive needed for the spawning migration onset. These findings suggested that water temperature decreases cause 11-KT to increase in autumn and this induces silvering and increases migratory drive. In addition, we newly report that 11-KT is associated with a corticotropin-releasing hormone that influences migratory behavior of salmonids. This evidence that 11-KT might be among the most important factors in the spawning migration onset of anguillid eels can help provide useful knowledge for understanding endocrinological mechanisms of the initiation of spawning migrations.

Transcriptomes of testis and pituitary from male Nile tilapia (O. niloticus L.) in the context of social status

African cichlids are well established models for studying social hierarchies in teleosts and elucidating the effects social dominance has on gene expression. Ascension in the social hierarchy has been found to increase plasma levels of steroid hormones, follicle stimulating hormone (Fsh) and luteinizing hormone (Lh) as well as gonadosomatic index (GSI). Furthermore, the expression of genes related to gonadotropins and steroidogenesis and signaling along the brain-pituitary-gonad axis (BPG-axis) is affected by changes of an animal’s social status. In this study, we use RNA-sequencing to obtain an in-depth look at the transcriptomes of testes and pituitaries from dominant and subordinate male Nile tilapia living in long-term stable social hierarchies. This allows us to draw conclusions about factors along the brain-pituitary-gonad axis that are involved in maintaining dominance over weeks or even months. We identify a number of genes that are differentially regulated between dominant and subordinate males and show that in high-ranking fish this subset of genes is generally upregulated. Genes differentially expressed between the two social groups comprise growth factors, related binding proteins and receptors, components of Wnt-, Tgfβ- and retinoic acid-signaling pathway, gonadotropin signaling and steroidogenesis pathways. The latter is backed up by elevated levels of 11-ketotestosterone, testosterone and estradiol in dominant males. Luteinizing hormone (Lh) is found in higher concentration in the plasma of long-term dominant males than in subordinate animals. Our results both strengthen the existing models and propose new candidates for functional studies to expand our understanding of social phenomena in teleost fish.

Characterization of CYP11A1 and its potential role in sex asynchronous gonadal development of viviparous black rockfish Sebastes schlegelii (Sebastidae)

Mitochondrial cytochrome P450 side-chain cleavage (P450scc), encoded by the cyp11a1 gene, initiates the first step of steroid biosynthesis. In this study, a 1554-bp open reading frame (ORF) of black rockfish (Sebastes schlegelii) cyp11a1 was cloned. The cyp11a1 gene is located on chromosome 5 and has 9 exons. The ORF encodes a putative precursor protein of 517 amino acids, and the predicted cleavable mitochondrial targeting peptide is located at amino acids 1-39. P450scc shares homology with other teleosts and tetrapods, which have relatively conserved binding regions with heme, cholesterol and adrenodoxin. Tissue distribution analysis revealed that the highest expression levels of cyp11a1 were detected in mature gonads and head kidney but that low levels were detected in gestational/regressed ovaries, regressed testes and other tissues. Immunostaining of P450scc was observed in testicular Leydig cells, ovarian theca cells, interrenal glands of head kidney, pituitary and multiple regions of brain. Particularly, two kinds of fish-specific P450scc-positive cells, including coronet cells of brain saccus vasculosus and hypophyseal somatolactin cells, were identified in black rockfish. Our results provide novel evidence for the potential role played by P450scc in reproduction behavior by mediating steroidogenesis in viviparous teleost.

Morphological colour adaptation during development in fish: involvement of growth hormone receptor 1

Morphological background adaptation is both an endocrine and a nervous response, involving changes in the amount of chromatophores and pigment concentration. However, whether this adaptation takes place at early developmental stages is largely unknown. Somatolactin (Sl) is a pituitary hormone present in fish, which has been associated to skin pigmentation. Moreover, growth hormone receptor type 1 (Ghr1) has been suggested to be the Sl receptor and was associated with background adaptation in adults. In this context, the aim of this work was to evaluate the ontogeny of morphological adaptation to background and the participation of ghr1 in this process. We found in larval stages of the cichlid Cichlasoma dimerus that the number of head melanophores and pituitary cells immunoreactive to Sl was increased in individuals reared with black backgrounds compared with that in fish grown in white tanks. In larval stages of the medaka Oryzias latipes, a similar response was observed, which was altered by ghr1 biallelic mutations using CRISPR/Cas9. Interestingly, melanophore and leucophore numbers were highly associated. Furthermore, we found that somatic growth was reduced in ghr1 biallelic mutant O. latipes, establishing the dual function of this growth hormone receptor. Taken together, these results show that morphological background adaptation is present at early stages during development and that is dependent upon ghr1 during this period.

17α-ethynylestradiol prevents the natural male-to-female sex change in gilthead seabream (Sparus aurata L.)

Exposure to 17α-ethynylestradiol (EE2, 5 μg/g food) impairs some reproductive events in the protandrous gilthead seabream and a short recovery period does not allow full recovery. In this study, spermiating seabream males in the second reproductive cycle (RC) were fed a diet containing 5 or 2.5 μg EE2/g food for 28 days and then a commercial diet without EE2 for the remaining RC. Individuals were sampled at the end of the EE2 treatment and then at the end of the RC and at the beginning of the third RC, 146 and 333 days after the cessation of treatment, respectively. Increased hepatic transcript levels of the gene coding for vitellogenin (vtg) and plasma levels of Vtg indicated both concentrations of EE2 caused endocrine disruption. Modifications in the histological organization of the testis, germ cell proliferation, plasma levels of the sex steroids and pituitary expression levels of the genes coding for the gonadotropin β-subunits, fshβ and lhβ were detected. The plasma levels of Vtg and most of the reproductive parameters were restored 146 days after treatments. However, although 50% of the control fish underwent sex reversal as expected at the third RC, male-to female sex change was prevented by both EE2 concentrations.

Effects of dietary microencapsulated Cymbopogon flexuosus essential oil on reproductive-related parameters in male Rhamdia quelen

In aquaculture, nutrition and supplemented diets have been shown to affect broodstock reproductive performance. In this study, we investigated the effects of dietary supplementation with Cymbopogon flexuosus essential oil (CFEO) microcapsules on reproductive-related parameters in silver catfish (Rhamdia quelen) male broodfish. Adult male broodstocks were separated into three groups according to the concentrations of supplemented CFEO (0.0 = control; 1.0 or 3.0 mL per kg of diet). After 20 days under experimental conditions, the animals were euthanized and the gonads were harvested for gonadosomatic index, sperm analysis, oxidative stress, and histopathology; testosterone levels were measured in the plasma; gene expression of prl, smtl, pomca, and pomcb was assessed in the pituitary gland by real-time PCR. The results showed no alterations on reproductive parameters in R. quelen males treated with Cymbopogon flexuosus essential oil compared to the control-diet animals. In conclusion, CFEO microcapsules supplied for 20 days in the concentrations of 1.00 or 3.00 mL per kilogram of diet did not affect the reproduction criteria evaluated in this study in male silver catfish.

Comparative aspects of GnRH-Stimulated signal transduction in the vertebrate pituitary - Contributions from teleost model systems

Gonadotropin-releasing hormone (GnRH) is a major regulator of reproduction through actions on pituitary gonadotropin release and synthesis. Although it is often thought that pituitary cells are exposed to only one GnRH, multiple GnRH forms are delivered to the pituitary of teleost fishes; interestingly this can include the cGnRH-II form usually thought to be non-hypophysiotropic. GnRHs can regulate other pituitary cell-types, both directly as well as indirectly, and multiple GnRH receptors (GnRHRs) may also be expressed in the pituitary, and even within a single pituitary cell-type. Literature on the differential actions of native GnRH isoforms in primary pituitary cells is largely derived from teleost fishes. This review will outline the diversity and complexity of GnRH-GnRHR signal transduction found within vertebrate gonadotropes as well as extra-gonadotropic sites with special emphasis on comparative studies from fish models. The implications that GnRHR transduction mechanisms are GnRH isoform-, function-, and cell-specific are also discussed.

Role of Growth Hormone in Breast Cancer

Breast cancer is one of the most common cancers diagnosed in women. Approximately two-thirds of all breast cancers diagnosed are classified as hormone dependent, which indicates that hormones are the key factors that drive the growth of these breast cancers. Ovarian and pituitary hormones play a major role in the growth and development of normal mammary glands and breast cancer. In particular, the effect of the ovarian hormone estrogen has received much attention in regard to breast cancer. Pituitary hormones prolactin and growth hormone have also been associated with breast cancer. Although the role of these pituitary hormones in breast cancers has been studied, it has not been investigated extensively. In this review, we attempt to compile basic information from most of the currently available literature to understand and demonstrate the significance of growth hormone in breast cancer. Based on the available literature, it is clear that growth hormone plays a significant role in the development, progression, and metastasis of breast cancer by influencing tumor angiogenesis, stemness, and chemoresistance.

Dopamine inhibits somatolactin gene expression in tilapia pituitary cells through the dopamine D2 receptors

Dopamine (DA) is an important neurotransmitter in the central nervous system of vertebrates and possesses key hypophysiotropic functions. Early studies have shown that DA has a potent inhibitory effect on somatolactin (SL) release in fish. However, the mechanisms responsible for DA inhibition of SL gene expression are largely unknown. To this end, tilapia DA type-1 (D1) and type-2 (D2) receptor transcripts were examined in the neurointermediate lobe (NIL) of the tilapia pituitary by real-time PCR. In tilapia, DA not only was effective in inhibiting SL mRNA levels in vivo and in vitro, but also could abolish pituitary adenylate cyclase-activating polypeptide (PACAP)- and salmon gonadotropin-releasing hormone (sGnRH)-stimulated SL gene expression at the pituitary level. In parallel studies, the specific D2 receptor agonists quinpirole and bromocriptine could mimic the DA-inhibited SL gene expression. Furthermore, the D2 receptor antagonists domperidone and (-)-sulpiride could abolish the SL response to DA or the D2 agonist quinpirole, whereas D1 receptor antagonists SCH23390 and SKF83566 were not effective in this respect. In primary cultures of tilapia NIL cells, D2 agonist quinpirole-inhibited cAMP production could be blocked by co-treatment with the D2 antagonist domperidone and the ability of forskolin to increase cAMP production was also inhibited by quinpirole. Using a pharmacological approach, the AC/cAMP pathway was shown to be involved in quinpirole-inhibited SL mRNA expression. These results provide evidence that DA can directly inhibit SL gene expression at the tilapia pituitary level via D2 receptor through the AC/cAMP-dependent mechanism.

Kisspeptin induction of somatolactin-α release in goldfish pituitary cells: functional role of cAMP/PKA-, PLC/PKC-, and Ca(2+)/calmodulin-dependent cascades

Although the importance of kisspeptin in the pituitary is firmly established, the signaling mechanisms for the pituitary actions of kisspeptin are still largely unknown. Somatolactin (SL), a member of the growth hormone (GH)/prolactin (PRL) family, is a pituitary hormone with pleiotropic functions in fish, but its regulation by kisspeptin has not been examined. To investigate the functional role of kisspeptin in SL regulation, expression of two paralogues of goldfish Kiss1 receptors (Kiss1ra and Kiss1rb) were confirmed in immunoidentified SLα but not SLβ cells isolated by RT-PCR coupled with laser capture microdissection. In goldfish pituitary cells prepared from neurointermediate lobe (NIL), synthetic goldfish Kiss decapeptides (gKiss1-10 and gKiss2-10) could increase SLα release. Consistent with the lack of Kiss1r expression in SLβ cells, SLβ release was not altered by kisspeptin stimulation. In parallel experiments, goldfish gKiss1-10 could elevate cyclic adenosine monophosphate (cAMP) production, upregulate protein kinase A (PKA) and protein kinase C (PKC) activities, and trigger a rapid rise in intracellular Ca(2+) levels in goldfish NIL cells. Using a pharmacological approach, cAMP/PKA and phospholipase C (PLC)/PKC pathways and subsequent activation of Ca(2+)/calmodulin (CaM)-dependent cascades were shown to be involved in SLα release induced by gKiss1-10. Apparently, the Ca(2+)-dependent cascades were triggered by extracellular Ca(2+) entry via voltage-sensitive Ca(2+) channels and mobilization of inositol trisphosphate-sensitive intracellular Ca(2+) stores. Our results demonstrate that gKiss1-10 can act directly at the pituitary level to trigger SLα release via a complex network of post-receptor signaling mechanisms.

Somatostatin-28 inhibitory action on somatolactin-α and -β gene expression in goldfish

Somatostain (SS) is known to inhibit growth hormone (GH) and prolactin (PRL) secretion. Somatolactin (SL) is a member of the GH/PRL family, but its regulation by goldfish brain somatostatin-28 (gbSS-28) has not been examined. To this end, the structural identity of goldfish SLα was established by 5'/3'-rapid amplification of cDNA ends. As revealed by in situ hybridization and immunohistochemical staining, the expression of SL isoforms was detected in pituitary cells located in the neurointermediate lobe (NIL). The transcripts of goldfish SS receptor 5a (Sst5a) but not Sst1b, Sst2, or Sst3a were detected in the goldfish NIL cells by RT-PCR. In goldfish pituitary cells, gbSS-28 not only had an inhibitory effect on basal SLα and SLβ mRNA levels but also could abolish insulin-like growth factor-stimulated SL gene expression. In primary cultures of goldfish NIL cells, gbSS-28 reduced forskolin-stimulated total cAMP production. With the use of a pharmacological approach, the adenylate cyclase (AC)/cAMP and phospholipase C (PLC)/inositol trisphosphate (IP3)/protein kinase C (PKC) cascades were shown to be involved in gbSS-28-inhibited SLα mRNA expression. Similar postreceptor signaling cascades were also observed for gbSS-28-reduced SLβ mRNA expression, except that PKC coupling to PLC was not involved. These results provide evidence that gbSS-28 can inhibit SLα and SLβ gene expression at the goldfish pituitary level via Sst5 through differential coupling of AC/cAMP and PLC/IP3/PKC cascades.

Fsh controls gene expression in fish both independently of and through steroid mediation

The mechanisms and the mediators relaying Fsh action on testicular functions are poorly understood. Unlike in mammals, in fish both gonadotropins (Fsh and Lh) are able to efficiently stimulate steroidogenesis, likely through a direct interaction with their cognate receptors present on the Leydig cells. In this context, it is crucial to understand if Fsh effects are mediated through the production of steroids. To address this issue we performed transcriptome studies after in vitro incubations of rainbow trout testis explants in the presence of Fsh alone or in combination with trilostane, an inhibitor of Δ4- steroidogenesis. Trilostane significantly reduced or suppressed the response of many genes to Fsh (like wisp1, testis gapdhs, cldn11, inha, vt1 or dmrt1) showing that, in fish, important aspects of Fsh action follow indirect pathways and require the production of Δ4-steroids. What is more, most of the genes regulated by Fsh through steroid mediation were similarly regulated by Lh (and/or androgens). In contrast, the response to Fsh of other genes was not suppressed in the presence of trilostane. These latter included genes encoding for anti-mullerian hormone, midkine a (pleiotrophin related), angiopoietine-related protein, cyclins E1 and G1, hepatocyte growth factor activator, insulin-like growth factor 1b/3. A majority of those genes were preferentially regulated by Fsh, when compared to Lh, suggesting that specific regulatory effects of Fsh did not depend on steroid production. Finally, antagonistic effects between Fsh and steroids were found, in particular for genes encoding key factors of steroidogenesis (star, hsd3b1, cyp11b2-2) or for genes of the Igf system (igf1b/3). Our study provides the first clear evidence that, in fish, Fsh exerts Δ4-steroid-independent regulatory functions on many genes which are highly relevant for the onset of spermatogenesis.

Pituitary adenylate cyclase-activating polypeptide (PACAP) stimulates release of somatolactin (SL)-α and SL-β from cultured goldfish pituitary cells via the PAC₁ receptor-signaling pathway, and affects the expression of SL-α and SL-β mRNAs

Pituitary adenylate cyclase-activating polypeptide (PACAP) is a neuropeptide that stimulates the release of adenohypophyseal hormone from the pituitary in fish. In the goldfish, PACAP induces the release of somatolactin (SL), in particular, from cultured pituitary cells. SL belongs to the growth hormone and prolactin family, and comprises two molecular variants termed SL-α and SL-β in goldfish. However, there is no information about the involvement of PACAP in the regulation of SL-α and SL-β release and the expression of their mRNAs. Therefore, we examined the effect of PACAP on SL-α and SL-β release from cultured goldfish pituitary cells. Treatment with PACAP (10(-10)-10(-7)M) increased the release of both SL-α and SL-β. The stimulatory action of PACAP (10(-9)M) on SL-α and SL-β release was blocked by treatment with a PACAP-selective receptor (PAC1R) antagonist, PACAP(6-38) (10(-6)M). We also examined whether PACAP affects the expression of SL-α and SL-β mRNAs in cultured pituitary cells. Treatment with PACAP (10(-9) and 10(-8)M) for 6h decreased the expression level of SL-α mRNA but increased that of SL-β mRNA. The action of PACAP (10(-8)M) on SL-β mRNA expression was blocked by treatment with PACAP(6-38) (10(-6)M), whereas PACAP(6-38) elicited no change in the expression of SL-α mRNA. These results indicate that in cultured goldfish pituitary cells, PACAP stimulates the release of SL-α and SL-β, and expression of SL-β mRNA, via the PAC1R-signaling pathway. However, the mechanism whereby PACAP inhibits the expression of SL-α mRNA does not seem to be mediated by PAC1R signaling.

The octadecaneuropeptide stimulates somatolactin release from cultured goldfish pituitary cells

The present study aimed to investigate the distribution of the octadecaneuropeptide (ODN) in the goldfish brain and to look for a possible effect of ODN on somatolactin (SL) release from pituitary cells. A discrete population of ODN-immunoreactive neurones was localised in the lateral part of the nucleus lateralis tuberis. These neurones sent projections through the neurohypophyseal tract towards the neurohypophysis, and nerve fibres were seen in the close vicinity of SL-producing cells in the pars intermedia. Incubation of cultured goldfish pituitary cells with graded concentrations of ODN (10(-9) -10(-5 ) m) induced a dose-dependent stimulation of SL-β, but not SL-α, release. ODN-evoked SL release was blocked by the metabotrophic endozepine receptor antagonist cyclo(1-8) [DLeu(5) ]OP but was not affected by the central-type benzodiazepine receptor antagonist flumazenil. ODN-induced SL release was suppressed by treatment with the phospholipase C (PLC) inhibitor U-73122 but not with the protein kinase A (PKA) inhibitor H-89. These results indicate that, in fish, ODN produced by hypothalamic neurones acts as a hypophysiotrophic neuropeptide stimulating SL release. The effect of ODN is mediated through a metabotrophic endozepine receptor positively coupled to the PLC/inositol 1,4,5-trisphosphate/protein kinase C-signalling pathway.

Signal transduction mechanisms for autocrine/paracrine regulation of somatolactin-α secretion and synthesis in carp pituitary cells by somatolactin-α and -β

Pituitary hormones can act locally via autocrine/paracrine mechanisms to modulate pituitary functions, which represents an interesting aspect of pituitary regulation other than the traditional hypothalamic input and feedback signals from the periphery. Somatolactin, a member of the growth hormone (GH)/prolactin (PL) family, is a pleiotropic hormone with diverse functions, but its pituitary actions are still unknown. Recently, two SL isoforms, SLα and SLβ, have been cloned in grass carp. Based on the sequences obtained, recombinant proteins of carp SLα and SLβ with similar bioactivity in inducing pigment aggregation in carp melanophores were produced. In carp pituitary cells, SLα secretion and cell content were elevated by static incubation with recombinant carp SLα and SLβ, respectively. These stimulatory actions occurred with a parallel rise in SLα mRNA level with no changes in SLβ secretion, cell content, and gene expression. In contrast, SLα mRNA expression could be reduced by removing endogenous SLα and SLβ with immunoneutralization. At the pituitary cell level, SLα release, cell content, and mRNA expression induced by carp SLα and SLβ could be blocked by inhibiting JAK2/STAT5, PI3K/Akt, MEK1/2, and p38 MAPK, respectively. Furthermore, SLα and SLβ induction also triggered rapid phosphorylation of STAT5, Akt, MEK1/2, ERK1/2, MKK3/6, and p38 MAPK. These results suggest that 1) SLα and SLβ produced locally in the carp pituitary can serve as novel autocrine/paracrine stimulators for SLα secretion and synthesis and 2) SLα production induced by local release of SLα and SLβ probably are mediated by the JAK2/STAT5, PI3K/Akt, and MAPK signaling pathways.

Profiles of mRNA expression for prolactin, growth hormone, and somatolactin in Japanese eels, Anguilla japonica: The effect of salinity, silvering and seasonal change

For understanding the functions of the growth hormone (GH)/prolactin (PRL)/somatolactin (SL) family of hormones, we examined pituitary mRNA expression of these hormones in anguillid eels in relation to salinity difference, silvering, and seasonal change. Female Japanese eels (Anguilla japonica) were collected in the brackish Hamana Lake and its freshwater rivers from July to December. To clarify the effect of salinity, the habitat use history of the eels were determined using otolith microchemistry. Expression levels of mRNA of each hormone were determined using real time PCR. Although GH and PRL have been known to be osmoregulatory hormones, there were no consistent differences in expression levels of these hormones between different salinity habitats. In contrast, SL mRNA expression was higher in eels from freshwater rivers than from the brackish lake. GH mRNA expression clearly decreased during silvering, whereas PRL and SL mRNA expression did not change. We also showed that PRL mRNA and SL mRNA decreased in the brackish lake and PRL mRNA increased in freshwater rivers from autumn to early winter. These findings provide basic knowledge for a further understanding of the role of these hormones.

Effect of stress during handling, seawater acclimation, confinement, and induced spawning on plasma ion levels and somatolactin-expressing cells in mature female Liza ramada

The present experiments were designed to determine the effect of different stress factors; handling, seawater acclimation, confinement, and induced spawning on plasma cortisol, hydro mineral balance as well as changes in size, number and integrated intensity of somatolactin (SL)-expressing cells in Liza ramada mature females confined to fresh water ponds. The plasma levels of cortisol, PO(4)(3-), Na(+), and K(+) were higher, while Ca(2+) and Mg(2+) were lower than controls during transportation without anesthesia. By using clove oil (5 mg L(-1)) as an anesthetic during transportation, the plasma cortisol, PO(4) (3-), Na(+), and K(+) were similar to controls, while Ca(2+) and Mg(2+) were higher. During seawater acclimation, the plasma cortisol and minerals were significantly higher except Na(+) which was lower than controls. In addition, during induction of spawning, the plasma levels of cortisol, PO(4)(3-), Na(+), K(+), and Mg(2+) were significantly higher than controls. The SL-producing cells are located in the pars intermedia (PI) bordering the neurohypophysis. The stress affected the number, size, and immunostaining of SL-expressing cells. During seawater acclimation, the size and the integrated intensity of SL immunoreactivity were lower, but the number of these cells was higher than controls. Furthermore, the number, size, and the integrated intensity of SL immunoreactivity were significantly lower than controls during handling and after spawning, which was opposite to confinement. The response of SL-expressing cells in PI in parallel with changes in cortisol and hydro mineral balance induced by stress support the possible role of SL in the adaptive response of fish to stress.

Characteristics of ovarian follicle steroidogenesis during vitellogenesis in an asynchronously ovulating stock of rainbow trout Oncorhynchus mykiss

This study explored several physiological criteria that could be used to assess the steroidogenic condition of the ovarian follicles of individual fish of an asynchronously ovulating captive rainbow trout Oncorhynchus mykiss stock. In these fish, the date of sampling, morphological variables such as gonado-somatic index or ovarian follicle mass and visual assessment of the ovary provided accurate indications of the maturational condition of an individual. The physiological variables measured included the in vitro basal and cyclic adenosine monophosphate (cAMP)-stimulated synthesis by ovarian follicles of 17β-oestradiol (E(2)) and testosterone (T); in addition, quantitative reverse-transcription (RT)-PCR was used to measure the relative expression of star and p450scc genes by ovarian follicles. The ratios of cAMP-stimulated E(2) and T synthesis to basal E(2) and T synthesis provided a reliable indication of differences in the steroidogenic status of the follicles of individual animals. On the basis of these criteria, together with the use of gene expression profiles, it was possible to classify individual fish as being at an early, mid or late-vitellogenic stage.

Cloning, phylogenetic analysis and expression of somatolactin and its receptor in Cichlasoma dimerus: their role in long-term background color acclimation

Somatolactin (SL) and SL receptor (SLR) belong to the growth hormone and cytokine type I receptor superfamilies, respectively. However, further research is required to define the duplications and functions of SL and its receptors in basal vertebrates including environmental background color adaptation in fish. In the present study, we cloned and sequenced SL and its putative receptor (SLR), classified and compared the sequences phylogenetically, and determined SL and SLR mRNA expression levels during long-term background color exposure in Cichlasoma dimerus, a freshwater South American cichlid. Our results show that C. dimerus SL and SLR share high sequence similarity with homologous from other perciform fish. Phylogenetic analysis indicates that C. dimerus SL belongs to the SLα clade sub-group. C. dimerus SLR is clearly a member of the GHR1 receptor subgroup, which includes the experimentally validated SLR from salmonids. Higher transcript levels of SLα in the pituitary and SLR in the epidermis and dermis cells of fish scales were observed in fish following long-term black background color exposure compared to those exposed to a white background. A higher number of melanophores was also observed in fish exposed for 10days to a black background compared to those exposed to a white background. These changes were concomitant to differences in SL or SLR transcript levels found in fish exposed to these two different background colors. Our results suggest, for the first time, that SLR is expressed in fish scales, and that there is an increase in SL in the pituitary and the putative SLR in likely target cells, i.e., melanophores, in long-term black background exposure in C. dimerus.

EST dataset of pituitary and identification of somatolactin and novel genes in Chinese sturgeon, Acipenser sinensis

Chinese sturgeon (Acipenser sinensis) is a rare and endangered species and also an important resource for the sturgeon aquaculture industry, however, a few genes have been identified in this species. We report here construction of a pituitary cDNA library from a 24 years old female Chinese sturgeon just after its spawning, and obtained 2,025 ESTs from the library. 885 unique sequences were identified, which were categorized into 12 functional groups. More than half of the unique sequences (57%) do not match with annotated sequences in the public databases. Three of these novel genes were further identified. Notably, a full-length of cDNA (1,143 bp) encoding somatolactin of 232 amino acids was identified. Phylogenetic analysis showed 97% amino acid identity with White sturgeon somatolactin. RT-PCR analysis indicated that the somatolactin mRNA was only detected in pituitary. Pituitary-specific expression of the somatolactin suggested that the protein may play important physiological functions in pituitary-endocrine system of the Chinese sturgeon.

Control of puberty in farmed fish

Puberty comprises the transition from an immature juvenile to a mature adult state of the reproductive system, i.e. the individual becomes capable of reproducing sexually for the first time, which implies functional competence of the brain-pituitary-gonad (BPG) axis. Early puberty is a major problem in many farmed fish species due to negative effects on growth performance, flesh composition, external appearance, behaviour, health, welfare and survival, as well as possible genetic impact on wild populations. Late puberty can also be a problem for broodstock management in some species, while some species completely fail to enter puberty under farming conditions. Age and size at puberty varies between and within species and strains, and are modulated by genetic and environmental factors. Puberty onset is controlled by activation of the BPG axis, and a range of internal and external factors are hypothesised to stimulate and/or modulate this activation such as growth, adiposity, feed intake, photoperiod, temperature and social factors. For example, there is a positive correlation between rapid growth and early puberty in fish. Age at puberty can be controlled by selective breeding or control of photoperiod, feeding or temperature. Monosex stocks can exploit sex dimorphic growth patterns and sterility can be achieved by triploidisation. However, all these techniques have limitations under commercial farming conditions. Further knowledge is needed on both basic and applied aspects of puberty control to refine existing methods and to develop new methods that are efficient in terms of production and acceptable in terms of fish welfare and sustainability.

Pituitary adenylate cyclase-activating polypeptide induces somatolactin release from cultured goldfish pituitary cells

In the goldfish pituitary, nerve fibers containing pituitary adenylate cyclase-activating polypeptide (PACAP) are located in close proximity to somatolactin (SL)-producing cells, and PACAP enhances SL release from cultured pituitary cells. However, there is little information about the mechanism of PACAP-induced SL release. In order to elucidate this issue, we used the cell immunoblot method. Treatment with PACAP at 10(-8) and 10(-7)M, but not with vasoactive intestinal polypeptide (VIP) at the same concentrations, increased the immunoblot area for SL-like immunoreactivity from dispersed pituitary cells, and PACAP-induced SL release was blocked by treatment with the PACAP selective receptor (PAC(1)R) antagonist, PACAP(6-38), at 10(-6)M, but not with the PACAP/VIP receptor antagonist, VIP(6-28). PACAP-induced SL release was also attenuated by treatment with the calmodulin inhibitor, calmidazolium at 10(-6)M. This led us to explore the signal transduction mechanism up to SL release, and we examined whether PACAP-induced SL release is mediated by the adenylate cyclase (AC)/cAMP/protein kinase A (PKA)- or the phospholipase C (PLC)/inositol 1,4,5-trisphosphate (IP(3))/protein kinase C (PKC)-signaling pathway. PACAP-induced SL release was attenuated by treatment with the AC inhibitor, MDL-12330A, at 10(-5)M or with the PKA inhibitor, H-89, at 10(-5)M. PACAP-induced SL release was suppressed by treatment with the PLC inhibitor, U-73122, at 3 x 10(-6)M or with the PKC inhibitor, GF109203X, at 10(-6)M. These results suggest that PACAP can potentially function as a hypophysiotropic factor mediating SL release via the PAC(1)R and subsequently through perhaps the AC/cAMP/PKA- and the PLC/IP(3)/PKC-signaling pathways in goldfish pituitary cells.

Cloning of somatolactin alpha, beta forms and the somatolactin receptor in Atlantic salmon: seasonal expression profile in pituitary and ovary of maturing female broodstock

BACKGROUND

Somatolactin (Sl) is a fish specific adenohypophyseal peptide hormone related to growth hormone (Gh). Some species, including salmonids, possess two forms: Sl alpha and Sl beta. The somatolactin receptor (slr) is closely related to the growth hormone receptor (ghr). Sl has been ascribed many physiological functions, including a role in sexual maturation. In order to clarify the role of Sl in the sexual maturation of female Atlantic salmon (Salmo salar), the full length cDNAs of slr, Sl alpha and Sl beta were cloned and their expression was studied throughout a seasonal reproductive cycle using real-time quantitative PCR (RTqPCR).

METHODS

Atlantic salmon Sl alpha, Sl beta and slr cDNAs were cloned using a PCR approach. Gene expression of Sl alpha, SL beta and slr was studied using RTqPCR over a 17 month period encompassing pre-vitellogenesis, vitellogenesis, ovulation and post ovulation in salmon females. Histological examination of ovarian samples allowed for the classification according to the degree of follicle maturation into oil drop, primary, secondary or tertiary yolk stage.

RESULTS

The mature peptide sequences of Sl alpha, Sl beta and slr are highly similar to previously cloned salmonid forms and contained the typical motifs. Phylogenetic analysis of Atlantic salmon Sl alpha and Sl beta shows that these peptides group into the two Sl clades present in some fish species. The Atlantic salmon slr grouped with salmonid slr amongst so-called type I ghr. An increase in pituitary Sl alpha and Sl beta transcripts before and during spawning, with a decrease post-ovulation, and a constant expression level of ovarian slr were observed. There was also a transient increase in Sl alpha and Sl beta in May prior to transfer from seawater to fresh water and ensuing fasting.

CONCLUSION

The up-regulation of Sl alpha and Sl beta during vitellogenesis and spawning, with a subsequent decrease post-ovulation, supports a role for Sl during gonadal growth and spawning. Sl could also be involved in calcium/phosphate mobilization associated with vitellogenesis or have a role in energy homeostasis associated with lipolysis during fasting. The up-regulation of both Sl alpha and Sl beta prior to fasting and freshwater transfer, suggests a role for Sl linked to reproduction that may be independent of the maturation induced fasting.

Grass carp somatolactin: I. Evidence for PACAP induction of somatolactin-alpha and -beta gene expression via activation of pituitary PAC-I receptors

Somatolactin (SL), the latest member of the growth hormone/prolactin family, is a novel pituitary hormone with diverse functions. At present, SL can be identified only in fish but not in tetrapods and its regulation at the pituitary level has not been fully characterized. Using grass carp as a model, we examined the direct effects of pituitary adenylate cyclase-activating polypeptide (PACAP) on SL secretion and synthesis at the pituitary cell level. As a first step, the structural identity of grass carp SL, SLalpha and SLbeta, was established by 5'/3'-rapid amplification of cDNA ends. These two SL isoforms are single-copy genes and are expressed in two separate populations of pituitary cells located in the pars intermedia. In the carp pituitary, PACAP nerve fibers were detected in the nerve tracts of the neurohypophysis and extended into the vicinity of pituitary cells forming the pars intermedia. In primary cultures of grass carp pituitary cells, PACAP was effective in stimulating SL release, cellular SL content, and total SL production. The increase in SL production also occurred with parallel rises in SLalpha and SLbeta mRNA levels. With the use of a combination of molecular and pharmacological approaches, PACAP-induced SL release and SL gene expression were shown to be mediated by pituitary PAC-I receptors. These findings, as a whole, suggest that PACAP may serve as a hypophysiotropic factor in fish stimulating SL secretion and synthesis at the pituitary level. Apparently, PACAP-induced SL production is mediated by upregulation of SLalpha and SLbeta gene expression through activation of PAC-I receptors.

Effect of salmon melanin-concentrating hormone and mammalian gonadotrophin-releasing hormone on somatolactin release in pituitary culture of Cichlasoma dimerus

We detected a close morphological association between melanin-concentrating hormone (MCH)-immunoreactive (ir) fibers and somatolactin (SL)-ir cells in the pars intermedia of the cichlid fish Cichlasoma dimerus by double-label immunofluorescence. Male pituitaries obtained from adult C. dimerus were incubated with 0.1-10 microM salmon MCH, and the amount of SL released into the culture medium was semi-quantified by Western blot. This assay showed an increase of SL release in a dose-dependent manner (linear regression: P<0.05). A close association of GnRH-ir fibers with SL-ir cells was also detected at the pars intermedia level. Male pituitaries were also incubated with 0.1-10 microM of mammalian GnRH, and SL release was semi-quantified by Western blot, showing an increase of released SL levels in a dose-dependent manner (linear regression: P<0.05). In contrast, SL release was unaffected from female pituitaries incubated with salmon MCH; however, an increasing tendency was observed when mammalian GnRH was used. Hypothalamic close association of MCH-ir perikarya and GnRH-ir fibers was found by double-label immunofluorescence indicating a possible relationship between them. These results suggest that SL, like other pituitary hormones, is under hypothalamic control and is involved in diverse physiological processes including background adaptation and reproduction. This study has also shown that the in vitro culture of a single C. dimerus pituitary is a feasible method for studying the control of SL release and other pituitary hormones.

Sexual maturation modulates expression of nuclear receptor types in laser-captured single cells of the cichlid (Oreochromis niloticus) pituitary

The role of steroid/thyroid hormones in the regulation of endocrine cells at the level of the pituitary has remained unclear. Therefore, using single-cell quantitative real-time PCR, we examined absolute amounts of transcripts for nuclear receptors [estrogen receptors (ERs) alpha, beta, and gamma; androgen receptors (ARs) a and b; glucocorticoid receptors (GRs) 1, 2a, and 2b; and thyroid hormone receptors (TRs) alpha1, alpha2, and beta] in pituitary cells of immature (IM) and mature (M) male tilapia, Oreochromis niloticus. In the two reproductive stages, ACTH cells expressed only ERbeta, whereas all other pituitary cell types expressed ERalpha + beta, and a subpopulation coexpressed ARa, ARb, GR1, GR2b, and TRbeta but lacked ERgamma, GR2a, TRalpha1, and TRalpha2. IM males had high percentages of LH cells (IM 46.0% vs. M 10.0%), GH cells (IM 23.3% vs. M 7.9%), and prolactin cells (IM 68.8% vs. M 6.0%) with ERbeta, and TSH cells (IM 19.2% vs. M 0.0%) and MSH cells (IM 25.6% vs. M 0.0%) with ERalpha + TRbeta. A high percentage of FSH cells in IM males expressed ERbeta (IM 46.9% vs. M 18.8%), and FSH cells in M males showed significantly high GR1 transcripts (IM 76.0 +/- 5.0 vs. M 195.0 +/- 10.7 copies per cell; P < 0.05), suggesting that FSH cells are regulated differently in the two reproductive stages. Coexpression of ERalpha + beta in high percentages of cells of the GH family (GH, IM 43.8% vs. M 14.3%; prolactin, IM 8.3% vs. M 59.7%; somatolactin, IM 22.2% vs. M 42.2%) suggests that the expression of both ERs is important for functionality. Thus, differential coexpression of genes for nuclear receptors in subpopulations of pituitary cell types suggests multiple steroid/thyroid hormone regulatory pathways at the level of the pituitary during the two reproductive stages.

Molecular characterization and sex-specific tissue expression of prolactin, somatolactin and insulin-like growth factor-I in yellow perch (Perca flavescens)

The cDNA sequences encoding prolactin (PRL), somatolactin (SL) and insulin-like growth factor-I (IGF-I) genes of the yellow perch were obtained. Brain, pituitary, gill, heart, liver, stomach, kidney, spleen, muscle and gonad tissues were analyzed from both male and female adult yellow perch for sex-specific tissue expression. The full length cDNA of yellow perch PRL consists of 2306 bp and PRL expression was highest in the yellow perch pituitary with low to moderate expression in other tissues including brain, gill and post-vitellogenic oocytes. The full length cDNA of yellow perch SL consists of 1589 bp and SL expression was highest in the yellow perch pituitary with low to moderate expression in other tissues including brain, gill, liver, stomach, spleen and kidney. The full length cDNA of yellow perch IGF-Ib consists of 814 bp and tissue expression analysis of yellow perch IGF-I revealed a second yellow perch transcript (IGF-Ia) that is 81 nucleotides smaller. Both IGF-Ib and IGF-Ia had the greatest expression in liver tissue with moderate expression in brain, spleen and kidney tissues of both sexes. These sequences are valuable molecular tools which can be used in future studies investigating the basis for sexually dimorphic growth in yellow perch.

The dawn and evolution of hormones in the adenohypophysis

The adenohypophysial hormones have been believed to have evolved from several ancestral genes by duplication followed by evolutionary divergence. To understand the origin and evolution of the endocrine systems in vertebrates, we have characterized adenohypophysial hormones in an agnathan, the sea lamprey Petromyzon marinus. In gnathostomes, adrenocorticotropin (ACTH) and melanotropin (MSH) together with beta-endorphins (beta-END) are encoded in a single gene, designated as proopiomelanocortin (POMC), however in sea lamprey, ACTH and MSH are encoded in two distinct genes, proopoicortin (POC) gene and proopiomelanotropin (POM) gene, respectively. The POC and POM genes are expressed specifically in the rostral pars distalis (RPD) and the pars intermedia (PI), respectively. Consequently, the final products from both tissues are the same in all vertebrates, i.e., ACTH from the PD and MSH from the PI. The POMC gene might have been established in the early stages of invertebrate evolution by internal gene duplication of the MSH domains. The ancestral gene might be then inherited in lobe-finned fish and tetrapods, while internal duplication and deletion of MSH domains as well as duplication of whole POMC gene took place in lamprey and gnathostome fish. Sea lamprey growth hormone (GH) is expressed in the cells of the dorsal half of the proximal pars distalis (PPD) and stimulates the expression of an insulin-like growth factor (IGF) gene in the liver as in other vertebrates. Its gene consists of 5 exons and 4 introns spanning 13.6 kb, which is the largest gene among known GH genes. GH appears to be the only member of the GH family in the sea lamprey, which suggests that GH is the ancestral hormone of the GH family that originated first in the molecular evolution of the GH family in vertebrates and later, probably during the early evolution of gnathostomes. The other member of the gene family, PRL and SL, appeared by gene duplication. A beta-chain cDNA belonging to the gonadotropin (GTH) and thyrotropin (TSH) family was cloned. It is expressed in cells of the ventral half of PPD. Since the expression of this gene is stimulated by lamprey gonadotropin-releasing hormone, it was assigned to be a GTHbeta. This GTHbeta is far removed from beta-subunits of LH, FSH, and TSH in an unrooted tree derived from phylogenetic analysis, and takes a position as an out group, suggesting that lampreys have a single GTH gene, which duplicated after the agnathans and prior to the evolution of gnathostomes to give rise to LH and FSH.

Involvement of somatolactin in background adaptation of the cichlid fish Cichlasoma dimerus

Somatolactin (SL) is a pituitary hormone present exclusively in fish that is involved in different physiological processes. The role of SL was evaluated in Cichlasoma dimerus (Teleostei, Perciformes) exposed for 10 days to a black and white background (BB and WB). Changes in alpha-melanophore stimulating hormone (alphaMSH) and melanin concentrating hormone (MCH) cells were also analyzed for comparison with SL. A melanin dispersing effect was observed in fish exposed to a BB, while a concentrating one was observed in those exposed to a WB. By Western blot, three SL-immunoreactive (ir) bands (32, 28 and 23.5 kD) were evidenced. Pituitary SL-ir levels were 2.66- and 2.67-fold greater in the 32 Kd and 28 kD bands, respectively, in BB fish compared with those of WB fish. The SL-ir 23.5 Kd band was not included in the analysis because of its unknown identity. In addition, SL-ir cell number and area were significantly higher in the BB condition (BB 22.73+/-1.46, WB 7.37+/-0.54 and BB 27.39+/-1.00 microm2; WB: 16.61+/-0.65 microm2). No significant differences were observed in the number of the hypothalamic MCH-ir neurons. However, a significant difference was observed in their nuclear area (BB 11.61+/-0.42 microm2, WB 17.80+/-0.84 microm2). alphaMSH-ir cells showed a marked increased in number (BB 35.96+/-1.22, WB 24.36+/-1.04), but no significant differences were observed in the cell area. In conclusion, this study presented clear evidence towards a possible involvement of SL in the adaptation to background colors in teleost together with alphaMSH and MCH.

Previtellogenic oocyte growth in salmon: relationships among body growth, plasma insulin-like growth factor-1, estradiol-17beta, follicle-stimulating hormone and expression of ovarian genes for insulin-like growth factors, steroidogenic-acute regulatory protein and receptors for gonadotropins, growth hormone, and somatolactin

Body growth during critical periods is known to be an important factor in determining the age of maturity and fecundity in fish. However, the endocrine mechanisms controlling oogenesis in fish and the effects of growth on this process are poorly understood. In this study interactions between the growth and reproductive systems were examined by monitoring changes in various components of the FSH-ovary axis, plasma insulin-like growth factor 1 (Igf1), and ovarian gene expression in relation to body and previtellogenic oocyte growth in coho salmon. Samples were collected from females during two hypothesized critical periods when growth influences maturation in this species. Body growth during the fall-spring months was strongly related to the degree of oocyte development, with larger fish possessing more advanced oocytes than smaller, slower growing fish. The accumulation of cortical alveoli in the oocytes was associated with increases in plasma and pituitary FSH, plasma estradiol-17beta, and ovarian steroidogenic acute regulatory protein (star) gene expression, whereas ovarian transcripts for growth hormone receptor and somatolactin receptor decreased. As oocytes accumulated lipid droplets, a general increase occurred in plasma Igf1 and components of the FSH-ovary axis, including plasma FSH, estradiol-17beta, and ovarian mRNAs for gonadotropin receptors, star, igf1, and igf2. A consistent positive relationship between plasma Igf1, estradiol-17beta, and pituitary FSH during growth in the spring suggests that these factors are important links in the mechanism by which body growth influences the rate of oocyte development.

Effects of endocrine disrupters on the expression of growth hormone and prolactin mRNA in the rainbow trout pituitary

It is now widely accepted that chemical pollutants in the environment can interfere with the endocrine system of animals, thus affecting development and reproduction. Some of these endocrine disrupters (EDs) can have estrogenic or antiestrogenic effects. Most studies to date have focused on the effects of EDs on the reproductive system and sex hormones and only limited information exists on how EDs may affect pituitary gland function. A rainbow trout (Oncorhynchus mykiss) pituitary gland culture system was used for studying the effects of EDs on growth hormone (GH) and prolactin (PRL) mRNA expression. We determined that the pituitary glands actively synthesized and secreted GH and PRL over the experimental time-course. In addition, we found that treatment with 17beta-estradiol (positive control) increased levels of GH and PRL mRNA, in a concentration-dependent manner. Treatment of pituitary glands with 500 and 1000 nM of a xenoestrogen, o,p'-DDT (o,p'-dichlorodiphenyltrichloroethane), resulted in a significant induction of GH and PRL mRNA, with a 20-fold increase for PRL and 3-fold increase for GH following treatment with 1000 nM o,p'-DDT. Co-incubation of pituitary glands with ICI 182 780 (a selective estrogen receptor antagonist) and o,p'-DDT resulted in inhibition of PRL mRNA levels; however, the stimulatory effect of DDT on GH mRNA was not seen in this experiment, nor was the inhibitory effect of ICI 182 780 observed with GH mRNA. To the contrary, ICI 182 780 (2.5 nM) had a stimulatory effect on GH mRNA levels. TCDD (2,3,7,8-tetrachlorodibenzo-p-dioxin), which is known to exert antiestrogenic effects, had an estrogenic-like effect that resulted in a concentration-dependent increase in the levels of GH and PRL mRNA. Co-incubation of pituitaries with TCDD and alpha-napthoflavone (ANF), which is an inhibitor of the aryl hydrocarbon receptor (AhR), caused an inhibition of TCDD-induced PRL mRNA at the higher and lower concentrations, but these effects were less consistent on GH mRNA levels. However, the responses of PRL and GH mRNA to co-incubation with TCDD and ANF, at the various concentrations, were bi-phasic wherein stimulation was seen at the low concentrations and inhibition at the high concentrations. Combined, these results suggest that o,p'-DDT and TCDD are xenoestrogens and that their effects on the expression of GH and PRL genes in the rainbow trout pituitary are modulated, in part, through the ER and AhR, respectively.

Production and purification of recombinant somatolactin beta and its effects on melanosome aggregation in zebrafish

A second form of somatolactin, somatolactin beta (SLbeta), was recently discovered in zebrafish (Danio rerio). This novel subtype of somatolactin is distantly related to somatolactin alpha (SLalpha) found in teleost species and is produced in a different region of the pituitary. To date, no physiological study of SLbeta has been reported. In order to study the physiological functions of SLbeta, recombinant SLbeta protein has been produced and purified. The cDNA of zebrafish SLbeta was cloned into a pET100 bacteria expression vector and His-tagged fusion proteins were produced in BL21 (DE3) Escherichia coli cells. The majority of recombinant somatolactins produced by E. coli were isolated in inclusion bodies although a small percentage of recombinant proteins (<1%) were also found in soluble form. Fusion proteins were solubilized from inclusion bodies using 6M guanidine hydrochloride. Pure recombinant somatolactins were obtained by affinity purification. The estimated molecular weight of 28 kDa by sodium dodecyl sulfate-polyacrylamide gel electrophoresis correlates with the molecular mass calculated from the deduced amino acid sequence of SLbeta. Thereafter, specific polyclonal antibodies against the recombinant SLbeta were developed. These antibodies recognized specifically a group of cells located in the anterior pars intermedia of the pituitary. The antibodies did not react with SLalpha, growth hormone or prolactin cells in the zebrafish pituitary glands. Furthermore, recombinant SLbeta induced melanosome aggregation in a concentration-dependent manner in skin of zebrafish scales. Significant melanosome aggregation was observed in zebrafish melanophores at a concentration of 1 microg/ml. These results, combined with previous reports demonstrate that the recombinant SLbeta proteins produced here are bioactive. The function of inducing melanosome aggregation is conserved among the somatolactin functions.

Identification of the salmon somatolactin receptor, a new member of the cytokine receptor family

Somatolactin (SL) is a pituitary hormone of the GH/prolactin (PRL) family that so far has been found only in fish. Compared with GH and PRL, the primary structure of SL is highly conserved among divergent fish species, suggesting it has an important function and a discriminating receptor that constrains structural change. However, SL functions are poorly understood, and receptors for SL have not yet been identified. During cloning of GH receptor cDNA from salmon, we found a variant with relatively high (38-58%) sequence identity to vertebrate GH receptors and low (28-33%) identity to PRL receptors; however, the recombinant protein encoding the extracellular domain showed only weak binding of GH. Ligand binding of the recombinant extracellular domain for this receptor confirmed that the cDNA encoded a specific receptor for SL. The SL receptor (SLR) has common features of a GH receptor including FGEFS motif, six cysteine residues in the extracellular domain, a single transmembrane region, and Box 1 and 2 regions in the intracellular domain. These structural characteristics place the SLR in the cytokine receptor type I homodimeric group, which includes receptors for GH, PRL, erythropoietin, thrombopoietin, granulocyte-colony stimulating factor, and leptin. Transcripts for SLR were found in 11 tissues with highest levels in liver and fat, supporting the notion that a major function of SL is regulation of lipid metabolism. Cloning SLR cDNA opens the way for discovery of new SL functions and target tissues in fish, and perhaps novel members of this receptor family in other vertebrates.

Three GnRH receptor types in laser-captured single cells of the cichlid pituitary display cellular and functional heterogeneity

The role of multiple gonadotropin-releasing hormone receptor (GnRH-R) types in the regulation of gonadotropic and nongonadotropic cells remains speculative. To address this issue, we developed a technology integrating laser-captured microdissection of single digoxigenin-labeled pituitary cells coupled with real-time quantitative PCR to examine the expression profiles of three endogenous GnRH-R types (R1, R2, and R3) in immature and mature males of tilapia Oreochromis niloticus. Here, in addition to gonadotropes (luteinizing and folicle-stimulating hormone, FSH), we show GnRH-Rs are also present in lactotropes, somatotropes, thyrotropes, melanotropes (melanocyte-stimulating hormone, MSH), corticotropes and somatolactin cells. Subpopulations of pituitary cells express single (42.9%), multiple (32.4%) or lack (24.7%) GnRH-Rs. In immature males, the percentage of FSH cells containing combinations of GnRH-Rs was significantly higher (R1+R2: 24%, P <0.05; R1+R2+R3: 25%, P <0.01) than in mature males, whereas the percentage showing only R1 and R1 and R3 transcripts (P <0.05) was higher in mature males. Significantly greater copies of R1 and R3 transcripts were found in MSH cells of immature and mature males, respectively (P <0.05). GnRH-R transcripts in other pituitary cells (lactotropes, R1 and R2; somatolactin cells/thyrotropes/corticotropes, R1, R2, and R3) were significantly higher in mature males (P <0.05) but were unaltered in somatotropes and luteinizing hormone cells. Thus, FSH and MSH cells are required for both reproductive states, whereas other pituitary cells are recruited only during testicular maturation. The differential expression of GnRH-Rs in gonadotropic and nongonadotropic cells demonstrates cellular and functional heterogeneity of mechanisms controlling normal sexual development.

In vitro effect of leptin on somatolactin release in the European sea bass (Dicentrarchus labrax): dependence on the reproductive status and interaction with NPY and GnRH

The aim of the present work was to investigate the neuroendocrine control of pituitary somatolactin (SL) release using dispersed pituitary cell culture obtained from male European sea bass (Dicentrarchus labrax) at different stages of sexual development. The effect of mouse recombinant leptin, sea bream gonadotropin releasing-hormone (sbGnRH) and porcine neuropeptide Y (pNPY) and their potential interaction on the SL release were investigated. High doses of leptin (10(-8)-10(-6)M) were differentially effective in inducing SL release depending on the sexual developmental stage. Porcine NPY alone was not effective on basal SL release, but it dose-dependently (0.1 and 1 nM) enhanced SL release induced by leptin (10(-6) and 10(-8)M) in late pre-pubertal but not in post-pubertal stages. No effect of sbGnRH in association or not with leptin was observed on SL release. These findings are the first evidences that leptin and pNPY can play an important role in the neuroendocrine control of pars intermedia function and SL release in fish. In addition, the sensitivity of SL producing cells to leptin and NPY only in prepubertal and pubertal stages, provides the potential role of SL in the nutritional control of the onset of puberty.

Differential expression and cellular localization of somatolactin-1 and -2 during early development in the gilthead sea bream

The patterns of expression of the somatolactin 1 and 2 (SL1 and SL2) transcripts were studied during the early development of the gilthead sea bream (Sparus aurata). Gene expression of SL1 and SL2 were detected in embryos and in larvae, although both transcripts presented different levels of expression. The SL1 transcripts in contrast to the SL2 transcripts presented high expression levels in embryos and younger larvae. Moreover, the SL2 transcripts were slightly present or absence in embryonic stage and the newly hatched larvae, respectively. The differences in the expression levels of SL1 and SL2 in embryos and larvae may be due to the fact that two distinct genes express both isoforms of the protein. Thus, both SLs may play different physiological roles throughout development. Moreover, the hybridization signals for SL1- and SL2-mRNAs were detected in 4-day-old larvae. Both in larvae and adults the somatolactotroph cells co-expressed both transcripts of SL and were located bordering the neurohypophysis in the pars intermedia.

Relation between the reproductive status and somatolactin cell activity in the pituitary of pejerrey, Odontesthes bonariensis (Atheriniformes)

The aim of this study was to analyze the relationship between somatolactin (SL) expressing cells and the reproductive status in a multiple spawning fish, the pejerrey Odontesthes bonariensis. Somatolactin cells were identified in adults of both sexes by immunocytochemistry using a heterologous piscine antiserum. The area of the cells that showed immunoreactivity to SL (ir-SL) was compared in specimens with different degrees of reproductive activity as inferred from histological examination of the gonads and calculation of the gonadosomatic index (GSI %). The results showed a significant difference between the area of ir-SL cells of resting/regressing (62.9 +/- 2.1 micron 2) and sexually active/vitellogenic (76.8 +/- 2.3 micron 2) females and a significant positive correlation between the ir-SL cellular area and the GSI % (P < 0.01 in both cases). In males, the correlation between the area of ir-SL cells and the GSI % was not statistically significant. However, in those animals with the highest GSI % values, the ir-SL cells appeared more numerous and showed an increase in the immunostained area when compared to individuals with lower GSI % values. The present in morphological observations are in accordance with biochemical data obtained from other species and support the assumption that SL might be involved in the regulation of reproduction in fish.

A homolog of mammalian PRL-releasing peptide (fish arginyl-phenylalanyl-amide peptide) is a major hypothalamic peptide of PRL release in teleost fish

Two PRL-releasing peptides (PrRP20 and PrRP31) were recently identified from mammalian hypothalamus by an orphan receptor strategy, and a C-terminal RF (arginyl-phenylalamyl-) amide peptide (RFa), structurally related to mammalian PrRP, was also identified from the brain of the Japanese crucian carp (C-RFa) by an intestine-contracting assay. However, to date there have been no reported studies that have examined the PRL-releasing effects of RFa in fish. In the present study we determined the cDNA, primary structure, and function of a homolog of the mammalian PrRP20 in the chum salmon, Oncorhynchus keta. An RFa cDNA encoding a preprohormone of 155 amino acids was cloned from the hypothalamus of chum salmon by 3'- and 5'-rapid amplification of cDNA ends. A native RFa was purified from an acid extract of salmon hypothalami by a Sep-Pak C(18) cartridge, affinity chromatography using anti-synthetic C-RFa, and reverse phase HPLC on an ODS-120T column. The salmon RFa proved to be identical with C-RFa on the basis of elution position on reverse phase HPLC. Immunocytochemical staining in rainbow trout, Oncorhynchus mykiss, revealed that C-RFa-immunoreactive cell bodies were located in the posterior part of hypothalamus and C-RFa-immunoreactive fibers were abundant from the hypothalamus to the ventral telencephalon. A small number of immunoreactive fibers were projected to the pituitary and terminated close to the PRL cells in the rostral pars distalis and to the somatolactin (SL) cells in the pars intermedia. The hypophysiotropic effects of the fish homolog were determined on the release of PRL, SL, and GH from the pituitary of the rainbow trout. Plasma PRL and SL levels were increased at 3 and 9 h, respectively, after ip injection of the synthetic C-RFa into the rainbow trout at doses of 50 and 500 ng/g body weight. In contrast, plasma GH levels were decreased after 1 h at 500 ng/g body weight. Perifusion of the trout pituitaries with synthetic C-RFa at concentrations of 10 pM to 100 nM demonstrated maximum PRL release at 100 pM and maximum SL release at 10 and 100 nM. However, GH release was not affected. These data are the first to demonstrate that a homolog of mammalian PrRP (fish RFa) is a major hypothalamic peptide of PRL release in teleost fish.

Update: brain and pituitary hormones of lampreys

Lampreys and hagfish of the class Agnatha are of particular importance in understanding endocrinological relationships since they represent the oldest lineages of extant vertebrates which evolved over 550 million years ago. This review briefly summarizes the latest findings on the reproductive endocrinology of the sea lampreys. Since the First International Symposium of Fish Endocrinology in 1988, when virtually little was known of the hypothalamic-pituitary-gonadal axis, substantial new biochemical, molecular, physiological and immunological evidence has now clearly shown that lamprey reproduction is controlled by the neuroendocrine axis. In addition, five brain and six pituitary hormones of lampreys have been identified mainly by Sower and Kawauchi and colleagues between 1986 and 2000. We now hypothesize that lamprey reproduction is a highly synchronized process that is initiated or mediated by a coordination of complex integration of environmental cues and hormonal mechanisms which is broadly similar to that exhibited by gnathostome vertebrates.

CDNA cloning and sequence of European sea bass (Dicentrarchus labrax) somatolactin

Three cDNA clones encoding for European sea bass somatolactin (SL) were obtained by RT-PCR and 3' RACE of RNA of pituitary origin. Clone 1 was 582 bp in length, and included a part of the signal peptide and the 5' end of the mature protein. Clone 2 (1075 bp) included a fragment of the coding sequence and the 3' untranslated region, which was 888 bp in length and contained two putative polyadenylation signals (AATAAA) at 12-17, and 202-207 nucleotides upstream of the poly (A) tail. Clone 3 was 624 bp in length and its nucleotide sequence encoding for the entire mature protein confirmed the sequence already determined from the first two clones. The size of SL mRNA transcripts was estimated by Northern blot analysis and a single band of approximately 1.6 kb was observed with pituitary RNAs. No band was found with RNAs of brain and liver origin. Alignment of the deduced amino acid sequence revealed that European sea bass SL shared 90-84% identity with perciform, pleuronectiform and scorpaeniform fish SLs, and 77-57% with other SLs of more distant fish orders, with a strict conservation of Cys residues and the N-glycosylation site (Asn-Lys-Thr) at 121-123 amino acid positions. The reconstruction of the phylogenetic tree based on SL nucleotide sequences. and analyzed by maximum likelihood distances, showed the same clustering as the present hierarchy of fish. When comparisons were made among SL, prolactin and growth hormone of European sea bass, the overall amino identity was relatively low (22-23%). However, a high degree of amino acid homology was found at the C-terminus, which contains three of the four Cys residues strictly conserved in all the members of GH/PRL family.

Implication of somatolactin in the regulation of sexual maturation and spawning of Mugil cephalus

Specific antibody for chum salmon somatolactin (SL) was used for immunocytochemical investigation of SL cell activity of Mugil cephalus during the gonadal cycle in both natural habitat and captivity. The SL-immunoreactive cells showed strong and specific immunoreactivity to antichum salmon SL. The number of SL-immunoreactive cells increased, as did the secretory and synthetic activity during sexual maturation and spawning in the natural habitat. The SL cells were rather small and moderately immunoreactive in immature fish; they were enlarged, their numbers increased, and they frequently showed more SL immunoreactivity during gonadal development. In addition, during late stages of maturation, small cell size with more or less SL immunoreactive cells were noted, indicating an active release of SL granules. Prespawning females tended to have more enlarged SL cells with stronger immunoreactivity than equivalent males. The SL cells showed an increase in the secretory activity during spawning as indicated by small size and weak immunoreactivity. The SL cells of M. cephalus reared in captivity showed high activity. This may be due to the low concentration of calcium in fresh water. The gradual stimulation of SL synthesis and release during sexual maturation and spawning of M. cephalus suggest that SL may be involved in the control of some steps of reproductive processes, such as steroidogenesis, calcium metabolism, and energy mobilization.

Colocalization of GnRH binding sites with gonadotropin-, somatotropin-, somatolactin-, and prolactin-expressing pituitary cells of the pejerrey, Odontesthes bonariensis, in vitro

Previous studies in the pejerrey, Odontesthes bonariensis, have demonstrated that fibers with immunoreactivity to gonadotropin-releasing hormone (ir-GnRH) reach all areas of the pituitary gland, the rostral pars distalis (RPD), the proximal pars distalis (PPD), and the pars intemedia (PI). A close association was shown between ir-GnRH fibers and gonadotropin (GtH)-, growth hormone (GH)-, somatolactin (SL)-, and prolactin (PRL)-expressing cells. The presence of only one GnRH variant, suspected to be a novel form, has been shown in pituitary extracts of this fish. In addition, GnRH may stimulate GtHs, GH, SL, and PRL levels in different fish species. The objective of the present study was to seek GnRH receptors and therefore colocalization with GtHs, GH, SL, and PRL cells in O. bonariensis using a pituitary primary cell culture system. GnRH binding sites were revealed by autoradiography of an iodinated superactive GnRH agonist ([(125)I]GnRH-A) and pituitary cells were identified by immunocytochemistry using piscine antisera. Following autoradiography, silver grains representing specific [(125)I]GnRH-A binding were associated with anti GtH, GH, SL, and PRL positive cells. These results demonstrate the presence of GnRH binding sites on these cells. It is suggested that GnRH may play a wide role in the neuroendocrine control of different pituitary hormones in addition to the GtHs.

Immunocytochemical and histological studies on the hypophyseal-gonadal system in the freshwater nile tilapia, oreochromis niloticus (L.), during sexual maturation and spawning in different habitats

The activities of the hypophyseal-gonadal system of O. niloticus in different habitats were investigated using immunocytochemical and histological techniques. The observed physico-chemical results indicated that Lake Nasser water characteristics are within the allowed and desired safety baseline levels. In contrast, Lake Manzalah exhibited high levels of Ca(2+), Mg(2+), SO(4)(2-) and heavy metals (Zn, Pb, and Cd). These water conditions affected the activity of the hypophyseal-gonadal axis of tilapia. The secretory and the synthetic activities of GTH and SL cells in the pituitary gland, in general, underwent obvious normal changes during gonad maturation and spawning of O. niloticus in Lake Nasser. In Lake Manzalah, the secretory activity of GTH and SL cells declined. This may be due to the effect of the high levels of heavy metals which interfere with the release of hormones and disturb the feedback mechanisms. In addition, the activity of both PRL and GH cells in Lake Nasser was higher than that of Lake Manzalah, but the synthetic activity of the ACTH and MSH cells was higher in Lake Manzalah. This may be related to the increased stress on fishes and the dark polluted water in Lake Manzalah. Both the environmental and the endocrine impacts in Lake Manzalah caused a decline in the gonadal activity as reflected by the decrease of sperm amount in the ripe testis, the appearance of oocytes in the testis and the degeneration of ripe oocytes (atresia) during the spawning season. J. Exp. Zool. 284:343-354, 1999. Copyright 1999 Wiley-Liss, Inc.

Isolation and cDNA cloning of somatolactin in rabbitfish (Siganus guttatus)

We report the isolation and cDNA cloning of somatolactin (SL) from rabbitfish, Siganus guttatus. Rabbitfish SL was isolated from an alkaline extract of the pituitary glands by gel filtration chromatography on Sephadex G-100 and reversed-phase high-performance liquid chromatography. SL was monitored by immunoblotting with flounder SL antiserum. The preparation (yield: 0.86 mg/g wet tissues) contained two immunoreactive bands of 24 and 28 kDa on SDS-PAGE. Overlapping partial cDNA clones corresponding to teleost SLs were amplified by PCR from single-strand cDNA from pituitary glands. Excluding the poly(A) tail, rabbitfish SL cDNA is 1605 bp long. It contains a 693-bp open reading frame encoding a signal peptide of 24 amino acids (aa) and a mature protein of 207 aa. Rabbitfish SL has two possible N-glycosylation sites at positions 11 and 121 and seven half Cys residues. The deduced amino acid sequence shows over 80% identity with those of advanced teleosts like sea bream, red drum, and flounder, 76% with the salmonids, 57% with the eel, and 46% with the goldfish SL.