Molecular characterization and functional regulation of melanocortin 2 receptor (MC2R) in the sea bass. A putative role in the adaptation to stress

Dietary tryptophan intervention counteracts stress-induced transcriptional changes in a teleost fish HPI axis during inflammation

Immune nutrition is currently used to enhance fish health by incorporating functional ingredients into aquafeeds. This study aimed to investigate the connections between tryptophan nutrition and the network that regulates the communication pathways between neuroendocrine and immune systems in European seabass (Dicentrarchus labrax). When tryptophan was supplemented in the diet of unstressed fish, it induced changes in the hypothalamic-pituitary-interrenal axis response to stress. Tryptophan-mediated effects were observed in the expression of anti-inflammatory cytokines and glucocorticoid receptors. Tryptophan supplementation decreased pro-opiomelanocortin b-like levels, that are related with adrenocorticotropic hormone and cortisol secretion. When stressed fish fed a tryptophan-supplemented diet were subjected to an inflammatory stimulus, plasma cortisol levels decreased and the expression of genes involved in the neuroendocrine response was altered. Modulatory effects of tryptophan dietary intervention on molecular patterns seem to be mediated by altered patterns in serotonergic activity.

PPARG dysregulation as a potential molecular target in adrenal Cushing's syndrome

Background

We performed a transcriptomic analysis of adrenal signaling pathways in various forms of endogenous Cushing's syndrome (CS) to define areas of dysregulated and druggable targets.

Methodology

Next-generation sequencing was performed on adrenal samples of patients with primary bilateral macronodular adrenal hyperplasia (PBMAH, n=10) and control adrenal samples (n=8). The validation groups included cortisol-producing adenoma (CPA, n=9) and samples from patients undergoing bilateral adrenalectomy for Cushing's disease (BADX-CD, n=8). In vivo findings were further characterized using three adrenocortical cell-lines (NCI-H295R, CU-ACC2, MUC1).

Results

Pathway mapping based on significant expression patterns identified PPARG (peroxisome proliferator-activated receptor gamma) pathway as the top hit. Quantitative PCR (QPCR) confirmed that PPARG (l2fc<-1.5) and related genes - FABP4 (l2fc<-5.5), PLIN1 (l2fc<-4.1) and ADIPOQ (l2fc<-3.3) - were significantly downregulated (p<0.005) in PBMAH. Significant downregulation of PPARG was also found in BADX-CD (l2fc<-1.9, p<0.0001) and CPA (l2fc<-1.4, p<0.0001). In vitro studies demonstrated that the PPARG activator rosiglitazone resulted in decreased cell viability in MUC1 and NCI-H295R (p<0.0001). There was also a significant reduction in the production of aldosterone, cortisol, and cortisone in NCI-H295R and in Dihydrotestosterone (DHT) in MUC1 (p<0.05), respectively.

Outcome

This therapeutic effect was independent of the actions of ACTH, postulating a promising application of PPARG activation in endogenous hypercortisolism.

Functional characterization of melanocortin 2 receptor (Mc2r) from a lobe-finned fish (Protopterus annectens) and insights into the molecular evolution of melanocortin receptors

Recent studies from our group on melanocortin 2 receptors (Mc2r) from basal families of actinopterygians have served to resolve that Mrap1 dependence and ACTH selectivity are features of even the most basal ray-finned fishes. However, there have been no studies on Mc2r function of the basal sarcopterygians, the lobe-finned fishes, represented by the extant members coelacanths and lungfishes. Here, we offer the first molecular and functional characterization of an Mc2r from a lobe-finned fish, the West African lungfish (Protopterus annectens). Plasmids containing cDNA constructs of lungfish (lf) Mc2r and Mrap1 were expressed in mammalian and zebrafish cell lines. Cells were then stimulated by human ACTH(1-24) and melanocyte stimulating hormone (α-MSH), as well as alanine-substituted analogs of hACTH(1-24) targeting residues within the H6F7R8W9 and K15K16R17R18P19 motifs. Activation of lfMc2r was assessed using a cAMP-responsive luciferase reporter gene assay. In these assays, lfMc2r required co-expression with lfMrap1, was selective for ACTH over α-MSH at physiological concentrations of the ligands, and was completely inhibited by multiple-alanine substitutions of the HFRW (A6-9) and KKRRP (A15-19) motifs. Single- and partial-alanine substitutions of the HFRW and KKRRP motifs varied in their impacts on receptor-ligand affinity from having no effect to completely inhibiting lfMc2r activation. This characterization of the Mc2r of a lobe-finned fish fulfills the last major extant vertebrate group for which Mc2r function had yet to be characterized. In doing so, we resolve that all basal bony vertebrate groups exhibit Mc2r function that substantially differs from that of the cartilaginous fishes, indicating that rapid and dramatic shift in Mc2r function occurred between the radiation of cartilaginous fishes and the emergence of bony fishes. We support this interpretation with a molecular clock analysis of the melanocortin receptors, which demonstrates the uniquely high rate of sequence divergence in Mc2r. Much remains to be understood regarding the molecular evolution of Mc2r during the early radiation of vertebrates that resulted in the derived functional characteristics of Mrap1 dependence and exclusive selectivity for ACTH.

Hypothalamus-pituitary-interrenal (HPI) axis signaling in Atlantic sturgeon (Acipenser oxyrinchus) and sterlet (Acipenser ruthenus)

In vertebrates, the hypothalamic-pituitary-adrenal/interrenal (HPA/HPI) axis is a highly conserved endocrine axis that regulates glucocorticoid production via signaling by corticotropin releasing hormone (CRH) and adrenocorticotropic hormone (ACTH). Once activated by ACTH, G_s protein-coupled melanocortin 2 receptors (Mc2r) present in corticosteroidogenic cells stimulate expression of steroidogenic acute regulatory protein (Star), which initiates steroid biosynthesis. In the present study, we examined the tissue distribution of genes involved in HPI axis signaling and steroidogenesis in the Atlantic sturgeon (Acipenser oxyrinchus) and provided the first functional characterization of Mc2r in sturgeon. Mc2r of A. oxyrinchus and the sterlet sturgeon (Acipenser ruthenus) are co-dependent on interaction with the melanocortin receptor accessory protein 1 (Mrap1) and highly selective for human (h) ACTH over other melanocortin ligands. A. oxyrinchus expresses key genes involved in HPI axis signaling in a tissue-specific manner that is indicative of the presence of a complete HPI axis in sturgeon. Importantly, we co-localized mc2r, mrap1, and star mRNA expression to the head kidney, indicating that this is possibly a site of ACTH-mediated corticosteroidogenesis in sturgeon. Our results are discussed in the context of other studies on the HPI axis of basal bony vertebrates, which, when taken together, demonstrate a need to better resolve the evolution of HPI axis signaling in vertebrates.

Fish Scales Produce Cortisol upon Stimulation with ACTH

Cortisol concentration in fish scales is a novel and reliable indicator of chronic stress. However, until now cortisol in scales has been considered to be accumulated through the circulation and it has not yet been studied whether it can be de novo produced from cells found in the scales. In the current study, scales of European sea bass, Dicentrarchus labrax, were stimulated in-vitro with a range of concentrations of adrenocorticotropic hormone (ACTH) to investigate if they can produce and release cortisol. Moreover, scales were exposed to a combination of ACTH and metyrapone, an inhibitor of cortisol production, to examine whether cortisol was actually produced in the scales. Results from ACTH administration showed that scales increased their cortisol release in a dose-dependent manner. This effect was reversed when scales were co-incubated with ACTH and metyrapone, indicating that cortisol was produced de novo and not released only upon stimulation with ACTH.

α-MSH is partially involved in the immunomodulation of Nile tilapia (Oreochromis niloticus) antibacterial immunity

α-Melanocyte-stimulating hormone (α-MSH) is a well-studied neuropeptide controlling skin and hair color. Besides, numerous immunomodulation roles of α-MSH were recorded in humans and mice. However, the regulatory effects of α-MSH in teleost immunity haven't been well elucidated. In this study, several precursor molecules of α-MSH (POMCs) and its receptors (MCRs) in Nile tilapia (Oreochromis niloticus) were characterized, and their expression characteristics and specific functions on antibacterial immunity were determined. Overall, POMCs and MCRs were principally detected in the brain, skin, and liver, and were remarkably promoted post Streptococcus agalactiae infection. However, tiny POMCs and MCRs were observed in tilapia immune organs (head kidney and spleen) or lymphocytes, and no evident immunomodulation effect was detected in vitro. Moreover, the in vivo challenge experiments revealed that α-MSH protects tilapia from bacterial infection by regulating responses in the brain and intestine. This study lays theoretical data for a deeper comprehension of the immunomodulation mechanisms of teleost α-MSH and the evolutional process of the vertebrate melanocortin system.

Structure/Function Studies on the Activation Motif of Two Non-Mammalian Mrap1 Orthologs, and Observations on the Phylogeny of Mrap1, Including a Novel Characterization of an Mrap1 from the Chondrostean Fish, Polyodon spathula

In derived bony vertebrates, activation of the melanocortin-2 receptor (Mc2r) by its ACTH ligand requires chaperoning by the Mc2r accessory protein (Mrap1). The N-terminal domain of the non-mammalian tetrapod MRAP1 from chicken (c; Gallus gallus) has the putative activation motif, W18D19Y20I21, and the N-terminal domain in the neopterygian ray-finned fish Mrap1 from bowfin (bf; Amia calva) has the putative activation motif, Y18D19Y20I21. The current study used an alanine-substitution paradigm to test the hypothesis that only the Y20 position in the Mrap1 ortholog of these non-mammalian vertebrates is required for activation of the respective Mc2r ortholog. Instead, we found that for cMRAP1, single alanine-substitution resulted in a gradient of inhibition of activation (Y20 >> D19 = W18 > I21). For bfMrap1, single alanine-substitution also resulted in a gradient of inhibition of activation (Y20 >> D19 > I21 > Y18). This study also included an analysis of Mc2r activation in an older lineage of ray-finned fish, the paddlefish (p), Polyodon spathula (subclass Chondronstei). Currently no mrap1 gene has been found in the paddlefish genome. When pmc2r was expressed alone in our CHO cell/cAMP reporter gene assay, no activation was observed following stimulation with ACTH. However, when pmc2r was co-expressed with either cmrap1 or bfmrap1 robust dose response curves were generated. These results indicate that the formation of an Mc2r/Mrap1 heterodimer emerged early in the radiation of the bony vertebrates.

New View on the Impact of the Low-Frequency Electromagnetic Field (50 Hz) on Stress Responses: Hormesis Effect

INTRODUCTION

Low-frequency electromagnetic field (50 Hz) (EMF) can modify crucial neuronal processes. Existing data indicate that exposure to EMF may represent a mild stressor and contribute to disturbances of the hypothalamic-pituitary-adrenal (HPA) axis. The important regulatory pathways controlling HPA axis activity include two types of corticosteroid receptors: mineralocorticoid receptors (MRs) and glucocorticoid receptors. They are particularly abundant in the hippocampus, a key locus of HPA axis feedback control. The research aimed at determining whether (1) EMF exhibits hormesis, it means bidirectional action depending on EMF intensity (1 or 7 mT) and (2) repeated EMF exposure changes stress response to subsequent stress factors.

METHODS

The exposure (7 days, 1 h/day) of adult rats to EMF (1 mT and 7 mT) was repeated 3 times. HPA axis hormones and their receptors were analysed after each following exposure. Moreover, the impact of EMF exposure on hormonal and behavioural responses to subsequent stress factor - open-field test was evaluated.

RESULTS

Our data suggest that exposure to EMF can establish a new "set-point" for HPA axis activity. The direction and dynamics of this process depend on the intensity of EMF and the number of exposures. EMF of 1 mT induced an adaptive stress response, but 7 mT EMF caused sensitization. Consequently, EMF changed the vulnerability of the organism to a subsequent stress factor. We have also shown the increase in MR mRNA abundance in the hippocampus of 1 mT EMF-exposed rats, which can represent the possible neuroprotective response and suggest therapeutic properties of EMFs.

Tryptophan Modulatory Role in European Seabass (Dicentrarchus labrax) Immune Response to Acute Inflammation under Stressful Conditions

The present work aimed to study the role of dietary tryptophan supplementation in modulating the European seabass (Dicentrarchus labrax) immune condition during stressful rearing conditions (i.e., 15 days exposure to high density), as well as the immune response to acute inflammation after intraperitoneal injection of a bacterial pathogen. Stress alone did not compromise seabass health indicators. In contrast, a clear peripheral and local inflammatory response was observed in response to the inoculated bacteria. Moreover, exposure to a high stocking density seemed to exacerbate the inflammatory response at early sampling points, compared to fish stocked at a lower density. In contrast, stressed fish presented some immune-suppressing effects on the T-cell surface glycoprotein receptor expressions at a late sampling point following inflammation. Regarding the effects of dietary tryptophan, no changes were observed on seabass immune indicators prior to inflammation, while a small number of immunosuppressive effects were observed in response to inflammation, supporting tryptophan’s role in the promotion of immune-tolerance signals during inflammation. Nonetheless, tryptophan dietary supplementation improved the inflammatory response against a bacterial pathogen during stressful conditions, supported by a reduction of plasma cortisol levels, an up-regulation of several immune-related genes at 48 h, and an inversion of the previously observed, stress-induced T-cell suppression. Finally, the involvement of tryptophan catabolism in macrophages was confirmed by the up-regulation of genes involved in the kynurenine pathway. The present study brings new insights regarding the immune modulatory role of tryptophan during stressful conditions in fish, thus allowing for the development of novel prophylactic protocols during vaccination by intraperitoneal injection in the European seabass.

Role of the Melanocortin System in Gonadal Steroidogenesis of Zebrafish

In teleost, as in other vertebrates, stress affects reproduction. A key component of the stress response is the pituitary secretion of the adrenocorticotropic hormone (ACTH), which binds to the melanocortin 2 receptor (MC2R) in the adrenal glands and activates cortisol biosynthesis. In zebrafish, Mc2r was identified in male and female gonads, while ACTH has been shown to have a physiological role in modulating reproductive activity. In this study, the hypothesis that other melanocortins may also affect how the zebrafish gonadal function is explored, specifically steroid biosynthesis, given the presence of members of the melanocortin signaling system in zebrafish gonads. Using cell culture, expression analysis, and cellular localization of gene expression, our new observations demonstrated that melanocortin receptors, accessory proteins, antagonists, and agonists are expressed in both the ovary and testis of zebrafish (n = 4 each sex). Moreover, melanocortin peptides modulate both basal and gonadotropin-stimulated steroid release from zebrafish gonads (n = 15 for males and n = 50 for females). In situ hybridization in ovaries (n = 3) of zebrafish showed mc1r and mc4r in follicular cells and adjacent to cortical alveoli in the ooplasm of previtellogenic and vitellogenic oocytes. In zebrafish testes (n = 3), mc4r and mc1r were detected exclusively in germ cells, specifically in spermatogonia and spermatocytes. Our results suggest that melanocortins are, directly or indirectly, involved in the endocrine control of vitellogenesis in females, through modulation of estradiol synthesis via autocrine or paracrine actions in zebrafish ovaries. Adult zebrafish testes were sensitive to low doses of ACTH, eliciting testosterone production, which indicates a potential role of this peptide as a paracrine regulator of testicular function.

Immuno-modulatory effects of nanoplastics and humic acids in the European seabass (Dicentrarchus labrax)

Pernicious effects of plastic particles, emergent contaminants worldwide, have been described in different species. In teleost species, alterations of immune function after exposure to nanoplastics (NPs) have been reported, but the interaction with cortisol - hypothalamic-pituitary-adrenal (HPI) axis has not yet been explored. Furthermore, the role of dissolved organic matter on the effects of NPs is poorly known. Thus, the aims of this research were to assess if polystyrene NPs (PSNPs) acted as a stressor on juvenile European seabass (Dicentrarchus labrax), interfering with the immune response, as well as to elucidate if humic acids (HA) modulated the potential effects of PSNPs. A short-term exposure to PSNPs and HA elicited an immuno-modulatory response, with an activation of steroidogenic stress-related pathways. An upregulation of anti-inflammatory cytokine (il10, tgfb) and stress-related (mc2r, gr1) transcripts were observed after exposure to HA and PSNPs both individually and in co-exposure. No notable alteration of inflammatory markers was consistently found, which may reflect a protective anti-inflammatory effect of HA in the presence of PSNPs. Nevertheless, there seems to be a more complex interaction between both components. Overall, data show that understanding the interaction of NPs with dissolved organic substances is key to deciphering their environmental risks.

Special features of neuroendocrine interactions between stress and reproduction in teleosts

Stress and reproduction are both essential functions for vertebrate survival, ensuring on one side adaptative responses to environmental changes and potential life threats, and on the other side production of progeny. With more than 25,000 species, teleosts constitute the largest group of extant vertebrates, and exhibit a large diversity of life cycles, environmental conditions and regulatory processes. Interactions between stress and reproduction are a growing concern both for conservation of fish biodiversity in the frame of global changes and for the development of sustainability of aquaculture including fish welfare. In teleosts, as in other vertebrates, adverse effects of stress on reproduction have been largely documented and will be shortly overviewed. Unexpectedly, stress notably via cortisol, may also facilitate reproductive function in some teleost species in relation to their peculiar life cyles and this review will provide some examples. Our review will then mainly address the neuroendocrine axes involved in the control of stress and reproduction, namely the corticotropic and gonadotropic axes, as well as their interactions. After reporting some anatomo-functional specificities of the neuroendocrine systems in teleosts, we will describe the major actors of the corticotropic and gonadotropic axes at the brain-pituitary-peripheral glands (interrenals and gonads) levels, with a special focus on the impact of teleost-specific whole genome duplication (3R) on the number of paralogs and their potential differential functions. We will finally review the current knowledge on the neuroendocrine mechanisms of the various interactions between stress and reproduction at different levels of the two axes in teleosts in a comparative and evolutionary perspective.

Hypothesis and Theory: Evaluating the Co-Evolution of the Melanocortin-2 Receptor and the Accessory Protein MRAP1

The melanocortin receptors (MCRs) and the MRAP accessory proteins belong to distinct gene families that are unique to the chordates. During the radiation of the chordates, the melancortin-2 receptor paralog (MC2R) and the MRAP1 paralog (melanocortin-2 receptor accessory protein 1) have co-evolved to form a heterodimer interaction that can influence the ligand selectivity and trafficking properties of MC2R. This apparently spontaneous interaction may have begun with the ancestral gnathostomes and has persisted in both the cartilaginous fishes and the bony vertebrates. The ramifications of this interaction had profound effects on the hypothalamus/anterior pituitary/adrenal-interrenal axis of bony vertebrates resulting in MC2R orthologs that are exclusively selective for the anterior pituitary hormone, ACTH, and that are dependent on MRAP1 for trafficking to the plasma membrane. The functional motifs within the MRAP1 sequence and their potential contact sites with MC2R are discussed. The ramifications of the MC2R/MRAP1 interaction for cartilaginous fishes are also discussed, but currently the effects of this interaction on the hypothalamus/pituitary/interrenal axis is less clear. The cartilaginous fish MC2R orthologs have apparently retained the ability to be activated by either ACTH or MSH-sized ligands, and the effect of MRAP1 on trafficking varies by species. In this regard, the possible origin of the dichotomy between cartilaginous fish and bony vertebrate MC2R orthologs with respect to ligand selectivity and trafficking properties is discussed in light of the evolution of functional amino acid motifs within MRAP1.

Analyzing the signaling properties of gar (Lepisosteus oculatus) melanocortin receptors: Evaluating interactions with MRAP1 and MRAP2

RT-PCR analysis of gar pituitary and brain indicated that different combinations of gar melanocortin receptor mRNAs are present in the same tissues with mRNAs for gar mrap1 and gar mrap2. Against this background, an objective of this study was to determine whether the ligand sensitivity for either ACTH or α-MSH was affected when gar (g) melanocortin receptors (Mcrs) were co-expressed with either of the accessory proteins gMrap1 or gMrap2 in Chinese Hamster Ovary cells. The results indicated that gMc2r has an obligatory requirement for co-expression with gMrap1 in order for the receptor to be activated by hACTH(1-24). In addition, activation of gMc2r did not occur when the receptor was expressed alone or co-expressed with gMrap2. Furthermore, co-expression of gMc2r with gMrap1 followed by stimulation with NDP-MSH resulted in a low level of activation (only at 10-7 M and 10-6 M). However, gMc1r, gMc3r, gMc4r, and gMc5r responded to stimulation by NDP-MSH in a more robust manner. Co-expression of gMc1r, gMc3r, gMc4r, and gMc5r with gMRAP1 had no effect on sensitivity to stimulation by NDP-MSH or hACTH(1-24). Co-expression with gMRAP2 had no negative or positive effect on ligand sensitivity for gMc1r, gMc3r, and gMc5r, however this treatment did increase the activation of CHO cells transfected with gMc4r following stimulation with both hACTH(1-24) (p < 0.001), and NDP-MSH (p < 0.001). Co-expression of gMC5R with either gMRAP1 or gMRAP2 increased trafficking of gMC5R to the plasma membrane. These pharmacological observations are compared to the response of melanocortin receptors from other neopterygian fishes, cartilaginous fishes, and tetrapods to stimulation by ACTH(1-24) and forms of α-MSH.

Novel zebrafish behavioral assay to identify modifiers of the rapid, nongenomic stress response

When vertebrates face acute stressors, their bodies rapidly undergo a repertoire of physiological and behavioral adaptations, which is termed the stress response. Rapid changes in heart rate and blood glucose levels occur via the interaction of glucocorticoids and their cognate receptors following hypothalamic-pituitary-adrenal axis activation. These physiological changes are observed within minutes of encountering a stressor and the rapid time domain rules out genomic responses that require gene expression changes. Although behavioral changes corresponding to physiological changes are commonly observed, it is not clearly understood to what extent hypothalamic-pituitary-adrenal axis activation dictates adaptive behavior. We hypothesized that rapid locomotor response to acute stressors in zebrafish requires hypothalamic-pituitary-interrenal (HPI) axis activation. In teleost fish, interrenal cells are functionally homologous to the adrenocortical layer. We derived eight frameshift mutants in genes involved in HPI axis function: two mutants in exon 2 of mc2r (adrenocorticotropic hormone receptor), five in exon 2 or 5 of nr3c1 (glucocorticoid receptor [GR]) and two in exon 2 of nr3c2 (mineralocorticoid receptor [MR]). Exposing larval zebrafish to mild environmental stressors, acute changes in salinity or light illumination, results in a rapid locomotor response. We show that this locomotor response requires a functioning HPI axis via the action of mc2r and the canonical GR encoded by nr3c1 gene, but not MR (nr3c2). Our rapid behavioral assay paradigm based on HPI axis biology can be used to screen for genetic and environmental modifiers of the hypothalamic-pituitary-adrenal axis and to investigate the effects of corticosteroids and their cognate receptor interactions on behavior.

Signal Transduction and Pathogenic Modifications at the Melanocortin-4 Receptor: A Structural Perspective

The melanocortin-4 receptor (MC4R) can be endogenously activated by binding of melanocyte-stimulating hormones (MSH), which mediates anorexigenic effects. In contrast, the agouti-related peptide (AgRP) acts as an endogenous inverse agonist and suppresses ligand-independent basal signaling activity (orexigenic effects). Binding of ligands to MC4R leads to the activation of different G-protein subtypes or arrestin and concomitant signaling pathways. This receptor is a key protein in the hypothalamic regulation of food intake and energy expenditure and naturally-occurring inactivating MC4R variants are the most frequent cause of monogenic obesity. In general, obesity is a growing problem on a global scale and is of social, medical, and economic relevance. A significant goal is to develop optimized pharmacological tools targeting MC4R without adverse effects. To date, this has not been achieved because of inter alia non-selective ligands across the five functionally different MCR subtypes (MC1-5R). This motivates further investigation of (i) the three-dimensional MC4R structure, (ii) binding mechanisms of various ligands, and (iii) the molecular transfer process of signal transduction, with the aim of understanding how structural features are linked with functional-physiological aspects. Unfortunately, experimentally elucidated structural information is not yet available for the MC receptors, a group of class A G-protein coupled receptors (GPCRs). We, therefore, generated MC4R homology models and complexes with interacting partners to describe approximate structural properties associated with signaling mechanisms. In addition, molecular insights from pathogenic mutations were incorporated to discriminate more precisely their individual malfunction of the signal transfer mechanism.

Regulation of divergent cortisol responsiveness in European sea bass, Dicentrarchus labrax L

Mechanisms regulating differences in cortisol responsiveness between low (LR) and high response (HR) individuals have been poorly studied. In this context, we aimed to study key regulatory processes in cortisol dynamics at the head kidneys of LR and HR European sea bass. To do so, resting plasma cortisol and ACTH concentrations were quantified in these fish. Additionally, the head kidneys of these individuals were superfused through an in vitro superfusion system and stimulated with the same amount of ACTH to assess their cortisol biosynthetic capacity. Moreover, the expression of important genes in cortisol regulation was assessed. Results showed that LR fish had lower resting cortisol concentrations than HR, although no differences existed in the circulating levels of ACTH. Additionally, the biosynthetic capacity of HR was higher than that of LR fish when in vitro stimulated with ACTH. At the molecular level, a statistically significant 3.4-fold higher expression of the ACTH receptor, mc2r, and a 2.3-fold, though not significant, higher expression of 11β-hydroxylase (cyp11b1), an enzyme involved in cortisol biosynthesis, was observed in the HR fish. Finally, a statistically significant 1.3-fold lower expression of 11β-hydroxysteroid dehydrogenase 2 (hsd11b2), an enzyme involved in cortisol inactivation, was observed in HR when compared to LR fish. Therefore, it was for the first time indicated that cortisol dynamics can also be regulated at the post-production level in the head kidney. Collectively, our results highlight the crucial role of the interrenal tissue in the regulation of differences in cortisol response between LR and HR sea bass individuals.

Central regulation of food intake in fish: an evolutionary perspective

Evidence indicates that central regulation of food intake is well conserved along the vertebrate lineage, at least between teleost fish and mammals. However, several differences arise in the comparison between both groups. In this review, we describe similarities and differences between teleost fish and mammals on an evolutionary perspective. We focussed on the existing knowledge of specific fish features conditioning food intake, anatomical homologies and analogies between both groups as well as the main signalling pathways of neuroendocrine and metabolic nature involved in the homeostatic and hedonic central regulation of food intake.

Effects of acute handling stress on short-term central expression of orexigenic/anorexigenic genes in zebrafish

Physiological mechanisms driving stress response in vertebrates are evolutionarily conserved. These mechanisms involve the activation of both the hypothalamic-sympathetic-chromaffin cell (HSC) and the hypothalamic-pituitary-adrenal (HPA) axes. In fish, the reduction of food intake levels is a common feature of the behavioral response to stress but the central mechanisms coordinating the energetic response are not well understood yet. In this work, we explore the effects of acute stress on key central systems regulating food intake in fish as well as on total body cortisol and glucose levels. We show that acute stress induced a rapid increase in total body cortisol with no changes in body glucose, at the same time promoting a prompt central response by activating neuronal pathways. All three orexigenic peptides examined, i.e., neuropeptide y (npy), agouti-related protein (agrp), and ghrelin, increased their central expression level suggesting that these neuronal systems are not involved in the short-term feeding inhibitory effects of acute stress. By contrast, the anorexigenic precursors tested, i.e., cart peptides and pomc, exhibited increased expression after acute stress, suggesting their involvement in the anorexigenic effects.

Allostatic Load and Stress Physiology in European Seabass (Dicentrarchus labrax L.) and Gilthead Seabream (Sparus aurata L.)

The present study aimed to compare effects of increasing chronic stress load on the stress response of European seabass (Dicentrarchus labrax) and gilthead seabream (Sparus aurata) to identify neuroendocrine functions that regulate this response. Fish were left undisturbed (controls) or exposed to three levels of chronic stress for 3 weeks and then subjected to an acute stress test (ACT). Chronic stress impeded growth and decreased feed consumption in seabass, not in seabream. In seabass basal cortisol levels are high and increase with stress load; the response to a subsequent ACT decreases with increasing (earlier) load. Basal cortisol levels in seabream increase with the stress load, whereas the ACT induced a similar response in all groups. In seabass and seabream plasma α-MSH levels and brain stem serotonergic activity and turnover were similar and not affected by chronic stress. Species-specific molecular neuro-regional differences were seen. In-situ hybridization analysis of the early immediate gene cfos in the preoptic area showed ACT-activation in seabream; in seabass the expression level was not affected by ACT and seems constitutively high. In seabream, expression levels of telencephalic crf, crfbp, gr1, and mr were downregulated; the seabass hypothalamic preoptic area showed increased expression of crf and gr1, and decreased expression of mr, and this increased the gr1/mr ratio considerably. We substantiate species-specific physiological differences to stress coping between seabream and seabass at an endocrine and neuroendocrine molecular level. Seabass appear less resilient to stress, which we conclude from high basal activities of stress-related parameters and poor, or absent, responses to ACT. This comparative study reveals important aquaculture, husbandry, and welfare implications for the rearing of these species.

Effects of subchronic exposure to waterborne cadmium on H-P-I axis hormones and related genes in rare minnows (Gobiocypris rarus)

The H (hypothalamic)-P (pituitary)-I (interrenal) axis is critical in the stress response and other activities of fish. To further investigate cadmium (Cd) toxicity on the H-P-I axis and to identify its potential regulatory genes in fish, the adult female rare minnows (Gobiocypris rarus) were exposed to subchronic (5weeks) levels of waterborne Cd in the present study. This kind of treatment caused dose-dependent decline in fish growth, with significance in the high dose group (100μg/L). Correspondingly, low dose (5-50μg/L) waterborne Cd disrupted the endocrine system of H-P-I axis just at the secretion level, while high dose Cd disrupted both the secretion and synthesis of cortisol and its downstream signals in rare minnows, revealed by the significantly upregulation and positive correlation of corticosteroidogenic genes including MC2R, StAR, CYP11A1, and CYP11B1 in the kidney (including the interrenal tissue) (P<0.05), and the significant alteration of Glcci1, Hsp90AA and Hsp90AB in the hepatopancreas, gill and intestine as well (P<0.05). The expression of Glcci1 was significantly decreased in hepatopancreas, gill and intestine of tested fish following treatment, and its positive correlation with GR (Glucocorticoid receptor) suggested its potential regulation on the cortisol and/or H-P-I axis in fish. The expression of FKBP5 in the intestine was positively and significantly correlated with that of Hsp90AA (P<0.05), and the Hsp90AB transcript in the hepatopancreas was positively correlated with that of Hsp90AA (P<0.05), which indicated that Hsp90AA and Hsp90AB were more likely to serve as cofactors of GR and FKBP5 in response to Cd exposure.

Hypothalamic-Pituitary-Adrenal Axis Modulation of Glucocorticoids in the Cardiovascular System

The collective of endocrine organs acting in homeostatic regulation—known as the hypothalamic-pituitary-adrenal (HPA) axis—comprises an integration of the central nervous system as well as peripheral tissues. These organs respond to imminent or perceived threats that elicit a stress response, primarily culminating in the release of glucocorticoids into the systemic circulation by the adrenal glands. Although the secretion of glucocorticoids serves to protect and maintain homeostasis in the typical operation at baseline levels, inadequate regulation can lead to physiologic and psychologic pathologies. The cardiovascular system is especially susceptible to prolonged dysregulation of the HPA axis and glucocorticoid production. There is debate about whether cardiovascular health risks arise from the direct detrimental effects of stress axis activation or whether pathologies develop secondary to the accompanying metabolic strain of excess glucocorticoids. In this review, we will explore the emerging research that indicates stress does have direct effects on the cardiovascular system via the HPA axis activation, with emphasis on the latest research on the impact of glucocorticoids signaling in the vasculature and the heart.

Whole genome detection of signature of positive selection in African cattle reveals selection for thermotolerance

As African indigenous cattle evolved in a hot tropical climate, they have developed an inherent thermotolerance; survival mechanisms include a light-colored and shiny coat, increased sweating, and cellular and molecular mechanisms to cope with high environmental temperature. Here, we report the positive selection signature of genes in African cattle breeds which contribute for their heat tolerance mechanisms. We compared the genomes of five indigenous African cattle breeds with the genomes of four commercial cattle breeds using cross-population composite likelihood ratio (XP-CLR) and cross-population extended haplotype homozygosity (XP-EHH) statistical methods. We identified 296 (XP-EHH) and 327 (XP-CLR) positively selected genes. Gene ontology analysis resulted in 41 biological process terms and six Kyoto Encyclopedia of Genes and Genomes pathways. Several genes and pathways were found to be involved in oxidative stress response, osmotic stress response, heat shock response, hair and skin properties, sweat gland development and sweating, feed intake and metabolism, and reproduction functions. The genes and pathways identified directly or indirectly contribute to the superior heat tolerance mechanisms in African cattle populations. The result will improve our understanding of the biological mechanisms of heat tolerance in African cattle breeds and opens an avenue for further study.

α-MSH and melanocortin receptors at early ontogeny in European sea bass (Dicentrarchus labrax, L.)

Temporal patterns of whole-body α-MSH concentrations and of transcripts of melanocortin receptors during early development as well as the endocrine response (α-MSH, cortisol, MCR mRNAs) to stress at the end of the larval period were characterized in Dicentrarchus labrax. Immunohistochemistry showed α-MSH positive cells in the pituitary pars intermedia in all stages examined. As development proceeds, α-MSH content gradually increases; mRNA levels of mc2r and mc4r remain low until first feeding where peak values are observed. Mc1r expression was constant during development, pomc mRNA levels remain low until the stage of flexion after which a significant increase is observed. At the stage of the formation of all fins, whole-body cortisol and α-MSH concentrations responded with peak values at 2 h post stress. Additionally, the stress challenge resulted in elevated transcript levels of pomc, mc2r and mc4r but not in mc1r, with a pattern characterized by peak values at 1 h post stress and a strong correlation with whole body α-MSH concentrations was found. Our data provide for the first time a view on the importance of the α-MSH stress response in early development of European sea bass, an additional and relatively poorly understood signal involved in the stress response in teleosts.

Estrogen Suppresses Interaction of Melanocortin 2 Receptor and Its Accessory Protein in the Primate Fetal Adrenal Cortex

We have shown that fetal adrenal fetal zone (FZ) volume and serum dehydroepiandrosterone sulfate (DHAS) levels were increased, whereas definitive and transitional zone (DZ/TZ) volume was unaltered, in baboons in which estrogen levels were suppressed by the administration of the aromatase inhibitor letrozole. The interaction of the melanocortin 2 receptor (MC2R) with its accessory protein (MRAP) is essential for trafficking MC2R to the adrenal cell surface for binding to ACTH. The present study determined whether the estrogen-dependent regulation of fetal adrenocortical development is mediated by ACTH and/or expression/interaction of MC2R and MRAP. Fetal pituitary proopiomelanocortin mRNA and plasma ACTH levels and fetal adrenal MC2R-MRAP interaction were assessed in baboons in which estrogen was suppressed/restored by letrozole/letrozole plus estradiol administration during the second half of gestation. Although fetal pituitary proopiomelanocortin and plasma ACTH levels and fetal adrenal MC2R and MRAP protein levels were unaltered, MC2R-MRAP interaction was 2-fold greater (P < .05) in the DZ/TZ in letrozole-treated baboons than in untreated animals and restored by letrozole plus estradiol treatment. We propose that the increasing levels of estradiol with advancing pregnancy suppress interaction of MC2R with MRAP, thereby diminishing MC2R movement to the cell membrane in the DZ/TZ. This would be expected to reduce progenitor cell proliferation in the DZ and migration to the FZ, thereby restraining FZ growth and DHAS production to maintain fetal adrenal DHAS and placental estradiol levels in a physiological range late in gestation.

Identifying the activation motif in the N-terminal of rainbow trout and zebrafish melanocortin-2 receptor accessory protein 1 (MRAP1) orthologs

The activation of mammalian melanocortin-2 receptor (MC2R) orthologs is dependent on a four-amino acid activation motif (LDYL/I) located in the N-terminal of mammalian MRAP1 (melanocortin-2 receptor accessory protein). Previous alanine substitution analysis had shown that the Y residue in this motif appears to be the most important for mediating the activation of mammalian MC2R orthologs. Similar, but not identical amino acid motifs were detected in rainbow trout MRAP1 (YDYL) and zebrafish MRAP1 (YDYV). To determine the importance of these residues in the putative activation motifs, rainbow trout and zebrafish MRAP1 orthologs were individually co-expressed in CHO cells with rainbow trout MC2R, and the activation of this receptor with either the wild-type MRAP1 ortholog or alanine-substituted analogs of the two teleost MRAP1s was analyzed. Alanine substitutions at all four amino acid positions in rainbow trout MRAP1 blocked activation of the rainbow trout MC2R. Single alanine substitutions of the D and Y residues in rainbow trout and zebrafish MRAP1 indicate that these two residues play a significant role in the activation of rainbow trout MC2R. These observations indicate that there are subtle differences in the way that teleost and mammalian MRAPs are involved in the activation of their corresponding MC2R orthologs.

Characterization of melanocortin receptors from stingray Dasyatis akajei, a cartilaginous fish

Melanocortin (MC) systems are composed of MC peptides such as adrenocorticotropic hormone (ACTH), several molecular forms of melanocyte-stimulating hormones (MSHs) and MC receptors (MCRs). Here we demonstrated that the cartilaginous fish, Dasyatis akajei (stingray) expresses five subtypes of MCR genes-mc1r to mc5r-as in the case of teleost and tetrapod species. This is the first evidence showing the presence of the full repertoire of melanocortin receptors in a single of cartilaginous fish. Expression of respective stingray mcr cDNAs in Chinese hamster ovary cells revealed that Des-acetyl-α-MSH exhibited cAMP-producing activity indistinguishable to ACTH(1-24) on MC1R and MC2R, while the activity of Des-acetyl-α-MSH on MC3R, MC4R, and MC5R were similar to or slightly greater than that of ACTH(1-24). Notably, in contrast to the other vertebrates, MC2R did not require coexpression with a melanocortin receptor-2 accessory protein 1 (mrap1) cDNA for functional expression. One of the roles of MC system resides in regulation of the pituitary-interrenal (PI) axis-a homologue of tetrapod pituitary-adrenal axis. In stingray, interrenal tissues were shown to express mc2r and mc5r as major MCR genes. These results established the presence of functional PI axis in stingray at the level of receptor molecule. While MC2R participates in adrenal functions together with MRAP1 in tetrapod species, the fact that sensitivity of MC5R to Des-acetyl-α-MSH and ACTH(1-24) were two order of magnitude higher than MC2R without coexpression with MRAP1 suggested that MC5R could play a more important role than MC2R to transmit signals conveyed by ACTH and MSHs if MRAP1 is really absent in the stingray.

Neuroendocrine regulation of the stress response in adult zebrafish, Danio rerio

The main objectives of this study were to investigate the dynamics of the cortisol stress response and the underlying molecular regulation in adult zebrafish exposed to acute and long-term stressors that differed in nature, duration and relative intensity. Fish showed a very rapid and prolonged increase in trunk cortisol concentrations, starting at around 15min and returning to basal levels at around 2h following exposure to acute stressors. In addition, acute stress affected significantly brain mRNA expression levels of several genes (corticotropin-releasing factor, crf; pro-opiomelanocortin, pomc; glucocorticoid receptor, gr; MR/GR ratio; prolactin, prl; hypocretin/orexin, hcrt; brain-derived neurotrophic factor, bdnf; c-fos). Exposure of fish to unpredictable relatively low-grade environmental and husbandry stressors (SP-1) did not affect the overall behaviour of fish, as well as trunk cortisol concentrations. Fish exposed to relatively higher-grade long-term stressors (SP-2) showed elevated cortisol levels as well as significant changes in most of gene transcripts. In particular, fish exposed to SP-2 showed statistically significant upregulation in brain gr, mr, prl and hcrt compared to SP-1 and control individuals. The highest mean values of bdnf transcripts were found in SP-2 exposed zebrafish and the lowest in control fish, while an approximately 5 to 6-fold upregulation was observed in c-fos mean relative mRNA levels of long-term stress-exposed fish, regardless of stressor intensity, compared to control zebrafish. In conclusion, we developed realistic acute and unpredictable long-term stress protocols, based on husbandry and environmental stressors and physical, chemical, mechanical and social stimuli that fish may experience either in nature or under intensive rearing conditions.

Views on the co-evolution of the melanocortin-2 receptor, MRAPs, and the hypothalamus/pituitary/adrenal-interrenal axis

A critical regulatory component of the hypothalamus/pituitary/adrenal axis (HPA) in mammals, reptiles and birds, and in the hypothalamus/pituitary/interrenal (HPI) axis of amphibians and teleosts (modern bony fishes) is the strict ligand selectivity of the melanocortin-2 receptor (MC2R). Tetrapod and teleost MC2R orthologs can only be activated by the anterior pituitary hormone, ACTH, but not by any of the MSH-sized ligands coded in POMC. In addition, both tetrapod and teleost MC2R orthologs require co-expression with the accessory protein, MRAP. However, the MC2R ortholog of the elephant shark, a cartilaginous fish, can be activated by either ACTH or the MSH-sized ligands, and the elephant shark MC2R ortholog does not require co-expression with an MRAP for activation. Given these observations, this review will provide a scenario for the co-evolution of MC2R and MRAP, based on the assumption that the obligate interaction between MC2R and MRAP evolved during the early radiation of the ancestral bony fishes.

Structure/function studies on the activation of the rainbow trout melanocortin-2 receptor

Functional expression of the rainbow trout (rt) melanocortin-2 receptor (MC2R) in CHO cells requires co-expression with a teleost melanocortin-2 receptor accessory protein (MRAP) such as zebrafish (zf) MRAP. Transiently transfected rtMC2R/zfMRAP1 CHO cells were used to evaluate the efficacy of alanine substituted analogs of hACTH(1-24) in three motifs in the ligand: H(6)F(7)R(8)W(9), G(10)K(11)P(12)V(13)G(14), and K(15)K(16)R(17)R(18)P(19). Alanine substitution at all positions in each motif either completely blocked activation of the receptor (H(6)F(7)R(8)W(9) and K(15)K(16)R(17)R(18)P(19)) or resulted in just over 400 fold increase in EC50 value (G(10)K(11)P(12)V(13)G(14)). Single alanine substitutions in the H(6)F(7)R(8)W(9) motif indicated that substitution at either W(9) or R(8) resulted in a much larger increase in EC50 values as compared to substitutions at either F(7) or W(9). Alanine substitution at either K(15)K(16) or R(17)R(18)P(19) in the K(15)K(16)R(17)R(18)P(19) motif resulted in a statistically equivalent increase in EC50 value of at least 600 fold. Finally, alanine substitutions in the G(10)K(11)P(12)V(13)G(14) motif resulted in increases in EC50 values presumably as a result of altering the secondary structure of the ligand. However, truncated analogs of hACTH(1-24) in which either G(10)G(14) (ACTH(1-22), or K(11)P(12)V(13) (ACTH(1-21) were removed had no stimulatory activity. Finally, some of the hACTH(1-24) analogs were tested using rainbow trout head kidney pieces in vitro to confirm whether the response to analogs seen with the transient transfected rtMC2R CHO cells was similar to that of trout interrenal cells. The results of these alanine substitution analog studies are used to construct a multistep hypothetical model for the activation of teleost and tetrapod MC2Rs to account for the unique ligand selectivity of this receptor.

Analysis of the pharmacological properties of chicken melanocortin-2 receptor (cMC2R) and chicken melanocortin-2 accessory protein 1 (cMRAP1)

The chicken (Gallus gallus) melanocortin-2 receptor (cMC2R) can be functionally expressed in CHO cells when chicken melanocortin-2 receptor accessory protein 1 (cMRAP1) is co-expressed. The transiently transfected CHO cells responded in a robust manner to stimulation by hACTH(1-24) (EC50 value=2.7 × 10(-12)M +/- 1.3 × 10(-12)), but the transfected CHO cells could not be stimulated by NDP-MSH at concentrations as high as 10(-7)M. Incubation of cMC2R/cMRAP1 transfected cells with alanine substituted analogs of hACTH(1-24) at amino acid positions F(7) or W(9) completely blocked stimulation of the transfected cells. Similarly, incubation of cMC2R/cMRAP1 transfected cells with an analog of hACTH(1-24) with alanine substitutions at amino acid positions R(17)R(18)P(19) resulted in a 276 fold shift in EC50 value relative to the positive control (p<0.004). Collectively these observations suggest that cMC2R has binding sites for the HFRW motif and KKRRP motif of hACTH(1-24), and both motifs are required for full activation of the receptor. While previous studies had shown that Anolis carolinensis MC2R and Xenopus tropicalis MC2R could be functionally expressed in CHO cells that co-expressed mouse MRAP1, co-expression of these non-mammalian tetrapod MC2Rs with cMRAP1 resulted in a significant increase in sensitivity to hACTH(1-24), as measured by EC50 value, for A. carolinensis MC2R (p<0.005) and X. tropicalis MC2R (p<0.007). The implications of these observations are discussed.