The rainbow trout skeletal muscle beta-adrenergic system: characterization and signaling

Evolution and divergence of teleost adrenergic receptors: why sometimes 'the drugs don't work' in fish

Adrenaline and noradrenaline, released as hormones and/or neurotransmitters, exert diverse physiological functions in vertebrates, and teleost fishes are widely used as model organisms to study adrenergic regulation; however, such investigations often rely on receptor subtype-specific pharmacological agents (agonists and antagonists; see Glossary) developed and validated in mammals. Meanwhile, evolutionary (phylogenetic and comparative genomic) studies have begun to unravel the diversification of adrenergic receptors (ARs) and reveal that whole-genome duplications and pseudogenization events in fishes results in notable distinctions from mammals in their genomic repertoire of ARs, while lineage-specific gene losses within teleosts have generated significant interspecific variability. In this Review, we visit the evolutionary history of ARs (including α1-, α2- and β-ARs) to highlight the prominent interspecific differences in teleosts, as well as between teleosts and other vertebrates. We also show that structural modelling of teleost ARs predicts differences in ligand binding affinity compared with mammalian orthologs. To emphasize the difficulty of studying the roles of different AR subtypes in fish, we collate examples from the literature of fish ARs behaving atypically compared with standard mammalian pharmacology. Thereafter, we focus on specific case studies of the liver, heart and red blood cells, where our understanding of AR expression has benefited from combining pharmacological approaches with molecular genetics. Finally, we briefly discuss the ongoing advances in 'omics' technologies that, alongside classical pharmacology, will provide abundant opportunities to further explore adrenergic signalling in teleosts.

Evolutionary loss of the ß1-adrenergic receptor in salmonids

Whole-genome duplications (WGDs) have been at the heart of the diversification of ß-adrenergic receptors (ß-ARs) in vertebrates. Non-teleost jawed vertebrates typically possess three ß-AR genes: adrb1 (ß1-AR), adrb2 (ß2-AR), and adrb3 (ß3-AR), originating from the ancient 2R (two rounds) WGDs. Teleost fishes, owing to the teleost-specific WGD, have five ancestral adrb paralogs (adrb1, adrb2a, adrb2b, adrb3a and adrb3b). Salmonids are particularly intriguing from an evolutionary perspective as they experienced an additional WGD after separating from other teleosts. Moreover, adrenergic regulation in salmonids, especially rainbow trout, has been intensively studied for decades. However, the repertoire of adrb genes in salmonids has not been yet characterized. An exhaustive genome survey of diverse salmonids, spanning five genera, complemented by phylogenetic sequence analysis, revealed each species has seven adrb paralogs: two adrb2a, two adrb2b, two adrb3a and one adrb3b. Surprisingly, salmonids emerge as the first known jawed vertebrate lineage to lack adrb1. adrb1 is nevertheless highly expressed in the hearts of non-salmonid teleosts, indicating that the wealth of data on adrenergic regulation in salmonids should be generalised to other teleost fishes with caution. It is hypothesised that the loss of adrb1 could have been viable because of the evolutionary radiation of adrb2 and adrb3 genes attributable to the salmonid WGD.

Adrenergic signaling in teleost fish liver, a challenging path

Adrenergic receptors or adrenoceptors (ARs) belong to the huge family of G-protein coupled receptors (GPCRs) that have been well characterized in mammals primarily because of their importance as therapeutic drug targets. ARs are found across vertebrates and this review examines the path to identify and characterize these receptors in fish with emphasis on hepatic metabolism. The absence of reliable and specific pharmacological agents led investigators to define the fish hepatic AR system as relying solely on a β2-AR, cAMP-dependent signaling transduction pathway. The use of calcium-radiometric imaging, purified membranes for ligand-binding studies, and perifused rather than static cultured fish hepatocytes, unequivocally demonstrated that both α1- and β2-AR signaling systems existed in the fish liver consistent with studies in mammals. Additionally, the use of molecular tools and phylogenetic analysis clearly demonstrated the existence of multiple AR-types and -subtypes in hepatic and other tissues of a number of fish species. This review also examines the use of β-blockers as pharmaceuticals and how these drugs that are now in the aquatic environment may be impacting aquatic species including fish and some invertebrates. Clearly there is a large conservation of structure and function within the AR system of vertebrates but there remain a number of key questions that need to be addressed before a clear understanding of these systems can be resolved.

Silver nanoparticles stimulate glycogenolysis in rainbow trout (Oncorhynchus mykiss) hepatocytes

Silver nanoparticles (AgNPs) are found in many consumer products yet their biological effects on non-target aquatic organisms are yet to be fully understood. This research aimed to investigate the effects of AgNPs on cell signaling in rainbow trout (Oncorhynchus mykiss) hepatocytes. We focused on the β-adrenoreceptor (AR), which mediates glycogenolysis, and the glucocorticoid receptor (GCR), which mediates gluconeogenesis. These two receptors have been extensively studied in trout hepatocytes due to their key roles during the stress response to increase glucose availability (among other things), allowing the organisms to cope with the stressor. We show for the first time that AgNPs at a concentration of 1 μg/mL did not interfere with the function of either the β-AR or the GCR systems in rainbow trout hepatocytes, but at the concentration of 10 μg/mL AgNPs stimulated glycogenolysis which was apparently receptor-independent. This study suggests that AgNPs could affect hormone-regulated cell signaling pathways at a concentration of 10 μg/mL.

The modulation of catecholamines on immune response of scallop Chlamys farreri under heat stress

Catecholamines (CAs) play key roles in mediating the physiological responses to various stresses. In the present study, the expression of CA-related genes were examined in the hemocytes of scallop Chlamys farreri under heat stress, and several immune or metabolism-related parameters were investigated after heat stress and adrenoceptor antagonist stimulation. After the scallops were cultured at 28°C, the mRNA expression level of dopa decarboxylase (CfDDC) and α-adrenoceptor (CfαAR) increased significantly (P<0.01), whereas that of monoamine oxidase (CfMAO) was down-regulated in the first 6h (P<0.05), and then up-regulated to the maximum level at 24h (P<0.01). In the hemocytes of scallops injected with adrenoceptor antagonist, the expression levels of peptidoglycan-recognition protein (CfPGRP-S1) and C-type lectin (CfLec-1) began to increase significantly at 2 and 3h post propranolol and high temperature treatment, respectively (P<0.01). While the up-regulation of heat shock protein 70 (CfHSP70) post heat stress was significantly inhibited by prazosin injection (P<0.01), and that of hexokinase (CfHK) was inhibited by both prazosin and propranolol injection (P<0.01). Moreover, the remarkable increase of relative specific activity of SOD in the hemolymph post heat stress (P<0.01) was further up-regulated early after prazosin or propranolol injection (P<0.01), while that of the relative anti-bacterial ability was down-regulated by prazosin or propranolol treatment (P<0.01). These results collectively indicated that the catecholaminergic neuroendocrine system in scallop could be activated by heat stress to release CAs, which subsequently modulated the immune response and energy metabolism via α- and β-adrenoceptors.

The toxicity potential of pharmaceuticals found in the Douro River estuary (Portugal)--experimental assessment using a zebrafish embryo test

Fish embryos are a particularly vulnerable stage of development, so they represent optimal targets for screening toxicological effects of waterborne xenobiotics. Herein, the toxicity potential of two mixtures of pharmaceuticals was evaluated using a zebrafish embryo test. One of the mixtures corresponds to an environmentally realistic scenario and both have carbamazepine, fenofibric acid, propranolol, trimethoprim and sulfamethoxazole. The results evidenced morphological alterations, such as spinal deformities and yolk-sac oedemas. Moreover, heart rates decreased after both mixture exposures, e.g., at 48hpf, highest mixture versus blank control (47.8±4.9 and 55.8±3.7 beats/30s, respectively). The tail lengths also diminished significantly from 3208±145μm in blank control to 3130±126μm in highest mixture. The toxicological effects were concentration dependent. Mortality, hatching rate and the number of spontaneous movements were not affected. However, the low levels of pharmaceuticals did interfere with the normal development of zebrafish, which indicates risks for wild organisms.

Endocrine control of oleic acid and glucose metabolism in rainbow trout (Oncorhynchus mykiss) muscle cells in culture

The effects of insulin and IGF-I on fatty acid (FA) and glucose metabolism were examined using oleic acid or glucose as tracers in differentiated rainbow trout (Oncorhynchus mykiss) myotubes. Insulin and IGF-I significantly reduced the production of CO(2) from oleic acid with respect to the control values. IGF-I also significantly reduced the production of acid-soluble products (ASP) and the concentration of FA in the medium, while cellular triacylglycerols (TAG) tended to increase. Only insulin produced a significant accumulation of glycogen inside the cells in glucose distribution experiments. Incubation with catecholamines did not affect oleic acid metabolism. Cells treated with rapamycin [a target of rapamycin (TOR) inhibitor] significantly increased the oxidation of oleic acid to CO(2) and ASP, while the accumulation of TAG diminished. Rosiglitazone (a peroxisome proliferator-activated receptor gamma agonist) and etomoxir (a CPT-1 inhibitor) produced a severe and significant reduction in the production of CO(2) and ASP. Rosiglitazone and etomoxir also produced a significant accumulation of FA outside and inside the cells, respectively. No significant effects of these drugs on glucose distribution were observed. These data indicate that insulin and IGF-I act as anabolic hormones in trout myotubes in both oleic acid and glucose metabolism, although glucose oxidation appears to be less sensitive than FA oxidation to insulin and IGF-I. The use of rapamycin, etomoxir, and rosiglitazone may help us to understand the mechanisms of regulation of lipid metabolism in fish.

Anabolic effects of feeding beta2-adrenergic agonists on rainbow trout muscle proteases and proteins

beta2-Adrenergic agonists (BAAs) act as repartitioning agents in domestic animals by redistributing nutrients away from adipose tissue and towards muscle accretion. The mechanism involves altering the rates of protein degradation and synthesis. The aim of this study was to test the effects of chronic feeding of the BAAs clenbuterol (CLEN) and ractopamine (RACT) on rainbow trout (RBT) muscle. Specifically, we examined the activities and mRNA levels of genes in the major proteolytic pathways including calpains, the multi-catalytic proteasome and cathepsins, and the mRNA levels of genes encoding the myofibrillar proteins, fast-twitch and slow-twitch myosin heavy chains (f-MHC and s-MHC, respectively), and the cytoskeletal protein, beta-actin. RACT feeding significantly increased mRNA transcripts of the calpain catalytic subunit (Capn1), the regulatory subunit (cpns), and the calpastatin large isoform (CAST-L), without affecting the calpain enzyme activity. CLEN feeding significantly increased mRNA levels of the proteasome alpha subunit without a corresponding change in 20S enzyme activity. RACT significantly decreased cathepsin D activity without affecting mRNA levels suggesting that the action of RACT may be at the post-transcriptional level. In addition, both CLEN and RACT significantly increased mRNA transcripts of f-MHC and beta-actin genes suggesting an anabolic role of BAAs on myofibrillar and cytoskeletal proteins. Neither CLEN nor RACT altered mRNA expression of the s-MHC gene indicating no transformation of muscle fiber-types. This study supports a role for BAAs in inducing RBT muscle accretion by altering both protein synthesis and degradation.

Hormonal regulation of glycogen metabolism in white muscle slices from rainbow trout (Oncorhynchus mykissWalbaum)

To test the hypothesis that cortisol and epinephrine have direct regulatory roles in muscle glycogen metabolism and to determine what those roles might be, we developed an in vitro white muscle slice preparation from rainbow trout ( Oncorhynchus mykiss Walbaum). In the absence of hormones, glycogen-depleted muscle slices obtained from exercised trout were capable of significant glycogen synthesis, and the amount of glycogen synthesized was inversely correlated with the initial postexercise glycogen content. When postexercise glycogen levels were <5 μmol/g, about 4.3 μmol/g of glycogen were synthesized, but when postexercise glycogen levels were >5 μmol/g, only about 1.7 μmol/g of glycogen was synthesized. This difference in the amount of glycogen synthesized was reflected in the degree of activation of glycogen synthase. Postexercise glycogen content also influenced the response of the muscle to 10−8M epinephrine and 10−8M dexamethasone (a glucocorticoid analog). At high glycogen levels (>5 μmol/g), epinephrine and dexamethasone stimulated glycogen phosphorylase activity and net glycogenolysis, whereas at low (<5 μmol/g) glycogen levels, glycogenesis and activation of glycogen synthase activity prevailed. These data clearly indicate not only is trout muscle capable of in situ glycogenesis, but the amount of glycogen synthesized is a function of initial glycogen content. Furthermore, whereas dexamethasone and epinephrine directly stimulate muscle glycogen metabolism, the net effect is dependent on initial glycogen content.

Activity of the unique beta-adrenergic Na+/H+ exchanger in trout erythrocytes is controlled by a novel beta3-AR subtype

beta-Adrenoceptors (beta-ARs) are seven-transmembrane domain, G protein-coupled receptors that transduce the cellular effects of epinephrine and norepinephrine and play a pivotal role in the vertebrate stress response. This study reports the cloning and characterization of two previously unreported beta-ARs from the rainbow trout (Oncorhynchus mykiss). Phylogenetic analysis of amino acid sequences indicates that both beta-ARs are homologs of the mammalian beta3-AR. Analysis of tissue expression patterns indicates that one of these trout beta3-adrenoceptors (beta3a-AR) is highly expressed in gill and heart, whereas the second (beta3b-AR) is highly expressed by red blood cells (RBC). Expression of the beta3b-AR in the RBC coupled with the finding of a single category of beta-AR binding sites on RBC membranes provides strong evidence for the control of the trout RBC beta-AR Na+/H+ exchanger (beta-NHE) activity by signaling through this beta3b-subtype and not through a beta1-subtype as previously proposed. The RBC-specific trout beta3b-AR exhibits binding characteristics that distinguish this receptor from each of the three pharmacologically defined categories of mammalian beta-ARs (beta1-, beta2-, and beta3-AR). This study is the first to report the presence of a beta3-AR subtype in a fish species, and the proposal that the beta3b-AR controls RBC beta-NHE activity represents a novel role for the beta3-AR subtype in vertebrates.