Extrapituitary Expression of the Prolactin Gene in the Goldfish, African Clawed Frog and Mouse

Prolactin and the evolution of male pregnancy

Prolactin (PRL) is a multifunctional hormone of broad physiological importance, and is involved in many aspects of fish reproduction, including the regulation of live birth (viviparity) and both male and female parental care. Previous research suggests that PRL also plays an important reproductive role in syngnathid fishes (seahorses, pipefish and seadragons), a group with a highly derived reproductive strategy, male pregnancy - how the PRL axis has come to be co-opted for male pregnancy remains unclear. We investigated the molecular evolution and expression of the genes for prolactin and its receptor (PRLR) in an evolutionarily diverse sampling of syngnathid fishes to explore how the co-option of PRL for male pregnancy has impacted its evolution, and to clarify whether the PRL axis is also involved in regulating reproductive function in species with more rudimentary forms of male pregnancy. In contrast to the majority of teleost fishes, all syngnathid fishes tested carry single copies of PRL and PRLR that cluster genetically within the PRL1 and PRLRa lineages of teleosts, respectively. PRL1 gene expression in seahorses and pipefish is restricted to the pituitary, while PRLRa is expressed in all tissues, including the brood pouch of species with both rudimentary and complex brooding structures. Pituitary PRL1 expression remains stable throughout pregnancy, but PRLRa expression is specifically upregulated in the male brood pouch during pregnancy, consistent with the higher affinity of pouch tissues for PRL hormone during embryonic incubation. Finally, immunohistochemistry of brood pouch tissues reveals that both PRL1 protein and PRLRa and Na⁺/K⁺ ATPase-positive cells line the inner pouch epithelium, suggesting that pituitary-derived PRL1 may be involved in brood pouch osmoregulation during pregnancy. Our data provide a unique molecular perspective on the evolution and expression of prolactin and its receptor during male pregnancy, and provide the foundation for further manipulative experiments exploring the role of PRL in this unique form of reproduction.

Brain and Pituitary Transcriptome Analyses Reveal the Differential Regulation of Reproduction-Related LncRNAs and mRNAs in Cynoglossus semilaevis

Long noncoding RNAs (lncRNAs) have been identified to be involved in half-smooth tongue sole (Cynoglossus semilaevis) reproduction. However, studies of their roles in reproduction have focused mainly on the ovary, and their expression patterns and potential roles in the brain and pituitary are unclear. Thus, to explore the mRNAs and lncRNAs that are closely associated with reproduction in the brain and pituitary, we collected tongue sole brain and pituitary tissues at three stages for RNA sequencing (RNA-seq), the 5,135 and 5,630 differentially expressed (DE) mRNAs and 378 and 532 DE lncRNAs were identified in the brain and pituitary, respectively. The RNA-seq results were verified by RT-qPCR. Moreover, enrichment analyses were performed to analyze the functions of DE mRNAs and lncRNAs. Interestingly, their involvement in pathways related to metabolism, signal transduction and endocrine signaling was revealed. LncRNA-target gene interaction networks were constructed based on antisense, cis and trans regulatory mechanisms. Moreover, we constructed competing endogenous RNA (ceRNA) networks. In summary, this study provides mRNA and lncRNA expression profiles in the brain and pituitary to understand the molecular mechanisms regulating tongue sole reproduction.

Hormonal and neuroendocrine control of reproductive function in teleost fish

Reproduction is one of the important physiological events for the maintenance of the species. Hormonal and neuroendocrine regulation of teleost requires multiple and complex interactions along the hypothalamic-pituitary-gonad (HPG) axis. Within this axis, gonadotropin-releasing hormone (GnRH) regulates the synthesis and release of gonadotropins, follicle-stimulating hormone (FSH), and luteinizing hormone (LH). Steroidogenesis drives reproduction function in which the development and differentiation of gonads. In recent years, new neuropeptides have become the focus of reproductive physiology research as they are involved in the different regulatory mechanisms of these species' growth, metabolism, and reproduction. However, especially in fish, the role of these neuropeptides in the control of reproductive function is not well studied. The study of hormonal and neuroendocrine events that regulate reproduction is crucial for the development and success of aquaculture.

Characterization and expression analyses of somatolactin-α and -β genes in rare minnows (Gobiocypris rarus) following waterborne cadmium exposure

Using reverse transcription-polymerase chain reaction (RT-PCR) and RACE (rapid amplification of cDNA ends), somatolactin-α (rmSLα) and -β (rmSLβ) were identified from the pituitary gland of rare minnows (Gobiocypris rarus). The full-length cDNAs of these two genes were 1288 and 801 bp, encoding prepeptides of 250 and 228 amino acids residues, respectively. rmSLβ can be detected in the brain (including the pituitary), ovary, testis, and gill, while rmSLα was mainly expressed in the brain. On the other hand, rmSLα was expressed in all the fetal developmental stages; however, rmSLβ can just be detected in the stages since from 14 h post-fertilization (hpf). After exposure to acute waterborne cadmium (Cd), rmSLα was distinctly upregulated in juvenile rare minnows at all detected time points, from 24 to 96 h and 10 days, while rmSLβ was significantly altered only in 96 h or 10-day treatment groups. As for adults, acute Cd exposure caused alterations of both rmSLα and rmSLβ in the brain (containing the pituitary) at the 24 h; subchronic waterborne Cd treatment led to upregulation of rmSLα, while decrease of mSLβ in the brain. Alteration of rmSL transcripts following waterborne Cd exposure further confirmed the endocrine disruption of this heavy metal. Besides, exposure to as low as 5 μg/L Cd caused alteration of rmSLα, which suggested that rmSLα might be a potential biomarker for risk assessment of aquatic Cd.

Extra-pituitary prolactin (PRL) and prolactin-like protein (PRL-L) in chickens and zebrafish

It is generally believed that in vertebrates, prolactin (PRL) is predominantly synthesized and released by pituitary lactotrophs and plays important roles in many physiological processes via activation of PRL receptor (PRLR), including water and electrolyte balance, reproduction, growth and development, metabolism, immuno-modulation, and behavior. However, there is increasing evidence showing that PRL and the newly identified 'prolactin-like protein (PRL-L)', a novel ligand of PRL receptor, are also expressed in a variety of extra-pituitary tissues, such as the brain, skin, ovary, and testes in non-mammalian vertebrates. In this brief review, we summarize the recent research progress on the structure, biological activities, and extra-pituitary expression of PRL and PRL-L in chickens (Gallus gallus) and zebrafish (Danio rerio) from our and other laboratories and briefly discuss their potential paracrine/autocrine roles in non-mammalian vertebrates, which may promote us to rethink the broad spectrum of PRL actions previously attributed to pituitary PRL only.

Grass carp prolactin: molecular cloning, tissue expression, intrapituitary autoregulation by prolactin and paracrine regulation by growth hormone and luteinizing hormone

Prolactin (PRL), a pituitary hormone with diverse functions, is well-documented to be under the control of both hypothalamic and peripheral signals. Intrapituitary modulation of PRL expression via autocrine/paracrine mechanisms has also been reported, but similar information is still lacking in lower vertebrates. To shed light on autocrine/paracrine regulation of PRL in fish model, grass carp PRL was cloned and its expression in the carp pituitary has been confirmed. In grass carp pituitary cells, local secretion of PRL could suppress PRL release with concurrent rises in PRL production and mRNA levels. Paracrine stimulation by growth hormone (GH) was found to up- regulate PRL secretion, PRL production and PRL transcript expression, whereas the opposite was true for the local actions of luteinizing hormone (LH). Apparently, local interactions of PRL, GH and LH via autocrine/paracrine mechanisms could modify PRL production in carp pituitary cells through differential regulation of PRL mRNA stability and gene transcription.

Diverse mechanisms for body fluid regulation in teleost fishes

Teleost fishes are the major group of ray-finned fishes and represent more than one-half of the total number of vertebrate species. They have experienced in their evolution an additional third-round whole genome duplication just after the divergence of their lineage, which endowed them with an extra adaptability to invade various aquatic habitats. Thus their physiology is also extremely diverse compared with other vertebrate groups as exemplified by the many patterns of body fluid regulation or osmoregulation. The key osmoregulatory organ for teleosts, whose body fluid composition is similar to mammals, is the gill, where ions are absorbed from or excreted into surrounding waters of various salinities against concentration gradients. It has been shown that the underlying molecular physiology of gill ionocytes responsible for ion regulation is highly variable among species. This variability is also seen in the endocrine control of osmoregulation where some hormones have distinct effects on body fluid regulation in different teleost species. A typical example is atrial natriuretic peptide (ANP); ANP is secreted in response to increased blood volume and acts on various osmoregulatory organs to restore volume in rainbow trout as it does in mammals, but it is secreted in response to increased plasma osmolality, and specifically decreases NaCl, and not water, in the body of eels. The distinct actions of other osmoregulatory hormones such as growth hormone, prolactin, angiotensin II, and vasotocin among teleost species are also evident. We hypothesized that such diversity of ionocytes and hormone actions among species stems from their intrinsic differences in body fluid regulation that originated from their native habitats, either fresh water or seawater. In this review, we summarized remarkable differences in body fluid regulation and its endocrine control among teleost species, although the number of species is still limited to substantiate the hypothesis.

Endocrine regulation of prolactin cell function and modulation of osmoreception in the Mozambique tilapia

Prolactin (PRL) cells of the Mozambique tilapia, Oreochromis mossambicus, are osmoreceptors by virtue of their intrinsic osmosensitivity coupled with their ability to directly regulate hydromineral homeostasis through the actions of PRL. Layered upon this fundamental osmotic reflex is an array of endocrine control of PRL synthesis and secretion. Consistent with its role in fresh water (FW) osmoregulation, PRL release in tilapia increases as extracellular osmolality decreases. The hyposmotically-induced release of PRL can be enhanced or attenuated by a variety of hormones. Prolactin release has been shown to be stimulated by gonadotropin-releasing hormone (GnRH), 17-β-estradiol (E2), testosterone (T), thyrotropin-releasing hormone (TRH), atrial natriuretic peptide (ANP), brain-natriuretic peptide (BNP), C-type natriuretic peptide (CNP), ventricular natriuretic peptide (VNP), PRL-releasing peptide (PrRP), angiotensin II (ANG II), leptin, insulin-like growth factors (IGFs), ghrelin, and inhibited by somatostatin (SS), urotensin-II (U-II), dopamine, cortisol, ouabain and vasoactive intestinal peptide (VIP). This review is aimed at providing an overview of the hypothalamic and extra-hypothalamic hormones that regulate PRL release in euryhaline Mozambique tilapia, particularly in the context on how they may modulate osmoreception, and mediate the multifunctional actions of PRL. Also considered are the signal transduction pathways through which these secretagogues regulate PRL cell function.

Evolutionary origin and divergence of the growth hormone receptor family: insight from studies on sea lamprey

Sea lamprey, one of the oldest extant lineages of vertebrates, Agnatha, was used to clarify the evolutionary origin and divergence of the growth hormone receptor (GHR) family. A single full-length cDNA encoding a protein that shares amino acid identity with GHRs and prolactin receptors (PRLRs) previously characterized from teleost fish was identified. Expression of the GHR/PRLR-like transcript was widespread among tissues, including brain, pituitary, heart, liver, and skeletal muscle, which is consistent with the broad physiological roles of GH-family peptides. Phylogenetic analysis suggests that the lamprey possess an ancestral gene encoding a common GHR/PRLR that diverged to give rise to distinct GHRs and PRLRs later in the course of vertebrate evolution. After the divergence of the Actinopterygian and Sarcopterygian lineages, the GHR gene was duplicated in the Actinopterygian lineage during the fish-specific genome duplication event giving rise to two GHRs in teleosts, type 1 GHR and type 2 GHR. A single GHR gene orthologous to the teleost type 1 GHR persisted in the Sarcopterygian lineage, including the common ancestor of tetrapods. Within the teleosts, several subsequent independent duplication events occurred that led to several GHR subtypes. A revised nomenclature for vertebrate GHRs is proposed that represents the evolutionary history of the receptor family. Structural features of the receptor influence ligand binding, receptor dimerization, linkage to signal effector pathways, and, ultimately, hormone function.

The role of prolactin in fish reproduction

Prolactin (PRL) has one of the broadest ranges of functions of any vertebrate hormone, and plays a critical role in regulating aspects of reproduction in widely divergent lineages. However, while PRL structure, mode of action and functions have been well-characterised in mammals, studies of other vertebrate lineages remain incomplete. As the most diverse group of vertebrates, fish offer a particularly valuable model system for the study of the evolution of reproductive endocrine function. Here, we review the current state of knowledge on the role of prolactin in fish reproduction, which extends to migration, reproductive development and cycling, brood care behaviour, pregnancy, and nutrient provisioning to young. We also highlight significant gaps in knowledge and advocate a specific bidirectional research methodology including both observational and manipulative experiments. Focusing research efforts towards the thorough characterisation of a restricted number of reproductively diverse fish models will help to provide the foundation necessary for a more explicitly evolutionary analysis of PRL function.

Characterization of genes encoding prolactin and prolactin receptors in starry flounder Platichthys stellatus and their expression upon acclimation to freshwater

This study aims to investigate the genes encoding prolactin (PRL) and prolactin receptors (PRLR) and their tissue-specific expression in starry flounder Platichthys stellatus. Starry flounder PRL gene consisting of five exons encodes an ORF of 212 amino acid residue comprised of a putative signal peptide of 24 amino acids and a mature protein of 188 amino acids. It showed amino acid identities of 73 % with tuna Thunnus thynnus, 71 % with black porgy Acanthopagrus schlegelii, 69 % with Nile tilapia Oreochromis niloticus, 64 % with pufferfish Takifugu rubripes, 63 % with rainbow trout Oncorhynchus mykiss, and 60 % with mangrove rivulus Kryptolebias marmoratus. Phylogenetic analysis of piscine PRLs also demonstrated a similarity between starry flounder and other teleosts but with a broad distinction from non-teleost PRLs. PRLR gene consists of eight exons encoding a protein of 528 amino acid residues. It showed a similarity to the PRLR2 subtype as reflected by amino acid identities of 54 % with A. schlegelii, 48.1 % with K. marmoratus, 46.3 % with tilapia O. mossambicus, and 46.1 % with O. niloticus PRLR2 as compared to PRLR1 isoform having less than 30 % identities. While mRNA transcript corresponding to PRL was detected only from the pituitary, most of PRLR mRNA was detected in the gill, kidney, and intestine, with a small amount in the ovary. The level of PRL transcript progressively increased during 6 days of acclimation to freshwater and then decreased but stayed higher than that of seawater at 60 days of acclimation. An opposite pattern of changes including a decrease at the beginning of the acclimation but a slight increase in the level osmolality was found as adaptation continued. The results support the osmoregulatory role of PRL signaling in starry flounder.

Corticosteroids disrupt amphibian metamorphosis by complex modes of action including increased prolactin expression

Although thyroid hormones (TH) are the primary morphogens regulating amphibian metamorphosis, other hormones including corticosteroids are known to participate in this regulation. The present study investigated effects of corticosteroids on larval development of the amphibian Xenopus laevis. Premetamorphic tadpoles (stage 51) were treated with aldosterone (ALDO; 100 nM), corticosterone (B; 10, 100, 500 nM) and dexamethasone (DEX; 10, 100, 500 nM) for 21 days and organismal responses were assessed by gross morphology determining stage development, whole body length (WBL), and hind limb length (HLL). B and DEX reduced WBL and HLL and caused abnormal development including the lack of fore limb emergence while ALDO treatment showed no significant effect. Gene expression analyses using RT-PCR revealed up-regulation of prolactin (PRL) in brain, but down-regulation of type III deiodinase in tail tissue induced by the glucocorticoids B and DEX. Additionally, stromelysin-3 transcript in tail tissue was decreased by B. ALDO at 100 nM had no effect on mRNA expression, neither in brain nor in tail tissue. These findings indicate that corticosteroids modulate TH-dependent metamorphosis by complex mechanisms that even include indirect effects triggered by increased PRL mRNA expression.

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.

Molecular characterisation of prolactin and analysis of extrapituitary expression in the European sea bass Dicentrarchus labrax under various salinity conditions

Although prolactin has been demonstrated to be the main hormone involved in adaptation to dilute media in several freshwater teleosts, few studies have been conducted in marine teleosts. In the Mediterranean, the sea bass Dicentrarchus labrax inhabits environments ranging from the open sea to coastal lagoons, where salinity varies greatly. We characterised the prolactin (prl) gene and analysed its expression in two organs (gill and intestine) in D. labrax acclimated to either freshwater or seawater. A 2819 bp long sequence encompassing the prl gene and a part (282 bp) of the promoter were identified, and these comprised 5 coding exons separated by 4 introns. Prolactin was similarly expressed in fresh- and seawater adapted fish, although expression in gills was significantly greater than in the intestine. Nonetheless, individuals unable to successfully regulate osmotic balance in freshwater presented overall low expression rates. Results are discussed according to the mechanism of sea bass adaptation in the wild and to their life cycle between open sea and lagoons. Finally, a phylogenetic analysis indicated that teleosts are not branched according to their life-history features (e.g. seawater vs. freshwater habitats), and no signature of positive selection was detected across the phylogeny of the prl gene in teleosts.

Prolactin and growth hormone in fish osmoregulation

Prolactin is an important regulator of multiple biological functions in vertebrates, and has been viewed as essential to ion uptake as well as reduction in ion and water permeability of osmoregulatory surfaces in freshwater and euryhaline fish. Prolactin-releasing peptide seems to stimulate prolactin expression in the pituitary and peripheral organs during freshwater adaptation. Growth hormone, a member of the same family of hormones as prolactin, promotes acclimation to seawater in several teleost fish, at least in part through the action of insulin-like growth factor I. In branchial epithelia, development and differentiation of the seawater-type chloride cell (and their underlying biochemistry) is regulated by GH, IGF-I, and cortisol, whereas the freshwater-type chloride cell is regulated by prolactin and cortisol. In the epithelia of gastrointestinal tract, prolactin induces cell proliferation during freshwater adaptation, whereas cortisol stimulates both cell proliferation and apoptosis. We propose that control of salinity acclimation in teleosts by prolactin and growth hormone primarily involves regulation of cell proliferation, apoptosis, and differentiation (the latter including upregulation of specific ion transporters), and that there is an important interaction of these hormones with corticosteroids.

Identification of the reptilian prolactin and its receptor cDNAs in the leopard gecko, Eublepharis macularius

In spite of their physiological significance, there is no available information about the nucleotide sequences of prolactin (PRL) and its receptor in reptilian species. In order to fill this gap, PRL and its receptor cDNAs were identified in a reptilian species, the leopard gecko Eublepharis macularius. The deduced leopard gecko PRL polypeptide showed high identities with the corresponding polypeptides of other reptiles. The leopard gecko PRL receptor (PRLR) was estimated to have tandem repeated regions in its extracellular domain, which had been originally found in avian PRLR. Molecular phylogenetic analysis suggests that these tandem repeated regions were generated by the duplication of the extracellular region in the latest common ancestor among reptiles and birds. In addition, tissue distributions of PRL and PRLR in the leopard gecko were examined by the reverse transcription-polymerase chain reaction (RT-PCR). PRLR mRNA was detected in all tissues examined and highly expressed in the whole brain, pituitary, intestine, kidney, ovary, oviduct and testis. Whereas, PRL mRNA was expressed in the whole brain, pituitary, ovary and testis. The co-expressions of PRL and its receptor in some extrapituitary organs suggest that PRL acts as an autocrine/paracrine factor in such organs of the leopard gecko.

cDNA sequence and spatio-temporal expression of prolactin in the orange-spotted grouper, Epinephelus coioides

The cDNA sequence encoding the prolactin (PRL) gene of the orange-spotted grouper was obtained by randomly sequencing its pituitary cDNA library. The full-length cDNA of orange-spotted grouper PRL consists of 1342 bp. The open reading frame encodes a protein of 212 amino acids, which consists of a putative signal peptide of 24 residues and a mature protein of 188 amino acids. Northern blot analysis identified a single transcript of 1.4 kb in the pituitary gland, indicating perhaps the presence of only one form of PRL in the orange-spotted grouper. The expression of PRL gene in different tissues and during embryonic development of orange-spotted grouper was determined using one-step RT-PCR followed by Southern blot analysis. In addition to the pituitary gland, high levels of PRL gene expression were detected in the olfactory bulb, forebrain, midbrain, hypothalamus, hindbrain, medulla, gill, spleen, and adipose tissue. Very low levels of expression were detected in the kidney, liver, and blood cells. The PRL gene was also expressed in the gonads, with the higher level of expression observed in the stage 3 as compared to stage 2 ovary. During embryonic development, mRNA for PRL was detected at all time points examined except at 2h 15min post-fertilization, and its expression peaked at 9h post-fertilization, at the time of neurulation. Results from the current study suggest that PRL may have important paracrine or autocrine roles in diverse tissue types, including the brain and the gonads, and during embryonic development of the orange-spotted grouper.

Fishy tales of prolactin-releasing peptide

Prolactin (PRL) is an important regulator of multiple biological functions, but a specific PRL-releasing factor, PRL-releasing peptide (PrRP), was isolated only recently from mammals and teleosts. Although this peptide seems to be a strong candidate for being a physiologically relevant stimulator of PRL expression and secretion in teleost pituitary and peripheral organs, it may not be a typical or classic hypothalamic releasing factor in rats. We now know that its biological actions are not limited solely to PRL stimulation, because it is also a neuromodulator of several hypothalamus-pituitary axes and is involved in some brain circuits with the regulation of food intake and cardiovascular functions. Moreover, it plays a direct role in hypertension and retinal information processing. It is the purpose of this review to provide a comprehensive survey of our current knowledge of PrRP and to provide a comparative point of view.

The role of prolactin in fish osmoregulation: a review

The protein hormone prolactin (PRL) was first discovered as an anterior pituitary factor capable of stimulating milk production in mammals. We now know that PRL has over 300 different functions in vertebrates. In fish, PRL plays an important role in freshwater osmoregulation by preventing both the loss of ions and the uptake of water. This paper will review what is currently known about the structure and evolution of fish PRL and its mechanisms of action in relation to the maintenance of hydromineral balance. Historically, functional studies of fish PRL were carried out using heterologous PRLs and the results varied greatly between experiments and species. In some cases this variability was due to the ability of these PRLs to bind to both growth hormone and PRL receptors. In fact, a recurring theme in the literature is that the actions of PRL cannot be generalized to all fish due to marked differences between species. Many of the effects of PRL on hydromineral balance are specific to euryhaline fish, which is appropriate given that they frequently experience sudden changes in environmental salinity. Much of the recent work has focused on the isolation and characterization of fish PRLs and their receptors. These studies have provided the necessary tools to obtain a better understanding of the evolution of PRL and its role in osmoregulation.