Corticosteroids in plasma of elasmobranchs

Hormonal effects on glucose and ketone metabolism in a perfused liver of an elasmobranch, the North Pacific spiny dogfish, Squalus suckleyi

Hormonal influence on hepatic function is a critical aspect of whole-body energy balance in vertebrates. Catecholamines and corticosteroids both influence hepatic energy balance via metabolite mobilization through glycogenolysis and gluconeogenesis. Elasmobranchs have a metabolic organization that appears to prioritize the mobilization of hepatic lipid as ketone bodies (e.g. 3-hydroxybutyrate [3-HB]), which adds complexity in determining the hormonal impact on hepatic energy balance in this taxon. Here, a liver perfusion was used to investigate catecholamine (epinephrine [E]) and corticosteroid (corticosterone [B] and 11-deoxycorticosterone [DOC]) effects on the regulation of hepatic glucose and 3-HB balance in the North Pacific Spiny dogfish, Squalus suckleyi. Further, hepatic enzyme activity involved in ketogenesis (3-hydroxybutyrate dehydrogenase), glycogenolysis (glycogen phosphorylase), and gluconeogenesis (phosphoenolpyruvate carboxykinase) were assessed in perfused liver tissue following hormonal application to discern effects on hepatic energy flux. mRNA transcript abundance key transporters of glucose (glut1 and glut4) and ketones (mct1 and mct2) and glucocorticoid function (gr, pepck, fkbp5, and 11βhsd2) were also measured to investigate putative cellular components involved in hepatic responses. There were no changes in the arterial-venous difference of either metabolite in all hormone perfusions. However, perfusion with DOC increased gr transcript abundance and decreased flow rate of perfusions, suggesting a regulatory role for this corticosteroid. Phosphoenolpyruvate carboxykinase activity increased following all hormone treatments, which may suggest gluconeogenic function; E also increased 3-hydroxybutyrate dehydrogenase activity, suggesting a function in ketogenesis, and decreased pepck and fkbp5 transcript abundance, potentially showing some metabolic regulation. Overall, we demonstrate hormonal control of hepatic energy balance using liver perfusions at various levels of biological organization in an elasmobranch.

Energy and corticosteroid mobilization following an induced stress response in an elasmobranch fish, the North Pacific spiny dogfish (Squalus acanthias suckleyi)

The dominant corticosteroid in elasmobranchs, 1α-hydroxycorticosterone (1α-OHB), has a described role in mineral regulation but a presumptive role in energy balance. Energy demand in vertebrates following exposure to a stressor typically involves an immediate but transient release of glucocorticoids as a means of mobilizing available energy stores, usually in the form of glucose. Although a glucocorticoid role for 1α-OHB would be expected, direct glucocorticoid function of this steroid has yet to be reported in any elasmobranch. In addition, elasmobranchs also utilize the metabolite β-hydroxybutyrate (β-HB), which is thought to replace the role fatty acids play in most vertebrates as a predominant fuel source in extrahepatic tissues. To determine the mobilization of metabolites and corticosteroids during a stress event, North Pacific spiny dogfish, Squalus acanthias suckleyi, were cannulated and held in a darkened isolation box to recover (24-48 h) before being subjected to an acute air exposure or corticosterone injection. Dogfish were then serially blood sampled at nine timepoints over 48 h. Glucose, β-HB, 1α-OHB, corticosterone, as well as lactate, pH, and osmolality were quantified in plasma samples. All measured variables increased in control and treatment groups within 48 h from the start of experimentation, and β-HB and 1α-OHB remained elevated for the duration of the experiment. There was no linear correlation between glucose and 1α-OHB, but there was a weak (R² = 0.230) although significant (p = 0.001), positive correlation between β-HB and 1α-OHB. Interestingly, there were also significant correlations between increasing circulating glucose and corticosterone (R² = 0.349; p < 0.001), and decreasing β-HB and corticosterone concentrations (R² = 0.180; p = 0.008). Our data suggest that following successive stressors of capture, surgery, and confinement, 1α-OHB was not correlated with circulating glucose, only weakly correlated with circulating β-HB concentrations (R² = 0.230; p = 0.001), and that corticosterone may also serve a role in energy mobilization in this species.

Challenges, pitfalls and surprises: development and validation of a monoclonal antibody for enzyme immunoassay of the steroid 1α-hydroxycorticosterone in elasmobranch species

Sharks and rays are popular species used in wildlife ecotourism and aquariums to educate the public on the behavior, ecology and conservation challenges of elasmobranchs. To understand long-term physiological health and welfare under varying social and husbandry conditions, we developed and validated an enzyme immunoassay (EIA) to measure stress/ionoregulatory hormones in managed and semi-free range southern rays (Hypanus americanus). Banked serum and interrenal samples from 27 female rays managed at Disney's The Seas with Nemo and Friends® and Castaway Cay were used to evaluate measurement of 1α-hydroxycorticosterone (1αOHB) relative to corticosterone (B). Although commercial EIAs are available for B, those tested exhibit only low relative cross-reactivity to 1αOHB (3-5%). To improve measurement of 1αOHB, we developed a monoclonal antibody using a synthesized 1αOHB-derivative for evaluation using high-performance liquid chromatography (HPLC) and EIA. Relative displacements of cross-reactant compounds showed that the antibody had good sensitivity for the target antigen 1αOHB, and low sensitivity to related steroids (desoxycorticosterone and B), but greater sensitivity to 11-dehydrocorticosterone. Tests of competitive vs. noncompetitive EIA formats, reagent titration, and incubation times of the antibody and conjugate were used to optimize sensitivity, repeatability and precision of measured 1αOHB in standards and samples (4 ng/ml, 90% binding). Tests of sample pre-treatment (pH adjustment) and extraction with varying solvent polarity were used to optimize measurement of 1αOHB in <1 ml (serum) or 1 g (interrenal) samples. HPLC analysis revealed the 1αOHB EIA to be superior for measurement of 1αOHB compared to use of a B EIA with or without HPLC fractioning. Results may prove useful for extrapolation to guide best practices for 1αOHB measurement in other elasmobranch species. Improved measurement of stress/ionoregulatory hormones in sharks and rays will be important for many aspects of collection, transport, medical treatment in aquaria and conservation management of these charismatic and ecologically important species.

Mechanisms for the evolution of a derived function in the ancestral glucocorticoid receptor

Understanding the genetic, structural, and biophysical mechanisms that caused protein functions to evolve is a central goal of molecular evolutionary studies. Ancestral sequence reconstruction (ASR) offers an experimental approach to these questions. Here we use ASR to shed light on the earliest functions and evolution of the glucocorticoid receptor (GR), a steroid-activated transcription factor that plays a key role in the regulation of vertebrate physiology. Prior work showed that GR and its paralog, the mineralocorticoid receptor (MR), duplicated from a common ancestor roughly 450 million years ago; the ancestral functions were largely conserved in the MR lineage, but the functions of GRs-reduced sensitivity to all hormones and increased selectivity for glucocorticoids-are derived. Although the mechanisms for the evolution of glucocorticoid specificity have been identified, how reduced sensitivity evolved has not yet been studied. Here we report on the reconstruction of the deepest ancestor in the GR lineage (AncGR1) and demonstrate that GR's reduced sensitivity evolved before the acquisition of restricted hormone specificity, shortly after the GR-MR split. Using site-directed mutagenesis, X-ray crystallography, and computational analyses of protein stability to recapitulate and determine the effects of historical mutations, we show that AncGR1's reduced ligand sensitivity evolved primarily due to three key substitutions. Two large-effect mutations weakened hydrogen bonds and van der Waals interactions within the ancestral protein, reducing its stability. The degenerative effect of these two mutations is extremely strong, but a third permissive substitution, which has no apparent effect on function in the ancestral background and is likely to have occurred first, buffered the effects of the destabilizing mutations. Taken together, our results highlight the potentially creative role of substitutions that partially degrade protein structure and function and reinforce the importance of permissive mutations in protein evolution.

Adaptation of a corticosterone ELISA to demonstrate sequence-specific effects of angiotensin II peptides and C-type natriuretic peptide on 1alpha-hydroxycorticosterone synthesis and steroidogenic mRNAs in the elasmobranch interrenal gland

It is thought that a single corticosteroid, 1alpha-hydroxycorticosterone (1alpha-B), is both a glucocorticoid and mineralocorticoid in the elasmobranch fishes. We investigated the putative mineralocorticoid role of 1alpha-B by examining regulation of interrenal 1alpha-B synthesis by osmoregulatory hormones in the euryhaline stingray Dasyatis sabina. Using synthesized steroid, a commercial enzyme-linked immunoassay was validated for the quantification of 1alpha-B. In interrenal cultures, the antinatriuretic peptide angiotensin II (ANG II) was potently steroidogenic, whereas C-type natriuretic peptide had no effect on 1alpha-B titers. However, both peptides significantly decreased abundance of rate-limiting steroidogenic mRNAs (steroidogenic acute regulatory protein, StAR; cholesterol side-chain cleavage, P450scc). We also isolated cDNAs encoding ANG II from three species of elasmobranch, verifying heterogeneity among elasmobranch peptides at the first amino acid position. Potential implications of this heterogeneity were investigated by examining the effects of homologous and heterologous ANG II on interrenal steroid production and steroidogenic mRNAs. Changes at amino acid position three, but not position one, of ANG II significantly affected steroidogenic potency. Conversely, changes at position one, but not position three, significantly affected the potency of ANG II to alter levels of steroidogenic mRNAs. This study is the first to demonstrate regulation of elasmobranch steroidogenic mRNAs by osmoregulatory peptides.

Evolution of hormone signaling in elasmobranchs by exploitation of promiscuous receptors

Specific interactions among proteins, nucleic acids, and metabolites drive virtually all cellular functions and underlie phenotypic complexity and diversity. Despite the fundamental importance of interactions, the mechanisms and dynamics by which they evolve are poorly understood. Here we describe novel interactions between a lineage-specific hormone and its receptors in elasmobranchs, a subclass of cartilaginous fishes, and infer how these associations evolved using phylogenetic and protein structural analyses. The hormone 1alpha-hydroxycorticosterone (1alpha-B) is a physiologically important steroid synthesized only in elasmobranchs. We show that 1alpha-B modulates gene expression in vitro by activating two paralogous intracellular transcription factors, the mineralocorticoid receptor (MR) and glucocorticoid receptor (GR), in the little skate Leucoraja erinacea; MR serves as a high-sensitivity and GR as a low-sensitivity receptor. Using functional analysis of extant and resurrected ancestral proteins, we show that receptor sensitivity to 1alpha-B evolved millions of years before the hormone itself evolved. The 1alpha-B differs from more ancient corticosteroids only by the addition of a hydroxyl group; the three-dimensional structure of the ancestral receptor shows that the ligand pocket contained ample unoccupied space to accommodate this moiety. Our findings indicate that the interactions between 1alpha-B and elasmobranch GR and MR proteins evolved by molecular exploitation: a novel hormone recruited into new functional partnerships two ancient receptors that had previously interacted with other ligands. The ancestral receptor's promiscuous capacity to fortuitously bind compounds that are slight structural variants of its original ligands set the stage for the evolution of this new interaction.

Evolution of the corticosteroid receptor signalling pathway in fish

The corticosteroid receptors (CR) control a vast array of physiological processes acting primarily as ligand-dependent transcription factors. The origins of the gnathostomata CRs can be traced back to an ancestral steroid receptor present in a primitive agnathan vertebrate. A genome duplication event in the early gnathostomes is believed to have produced a set of two CRs still present today in Sarcopterygii (lobe-finned fish and tetrapods), i.e. a glucocorticoid (GR) and mineralocorticoid receptor (MR), with divergent function and different ligands, cortisol and aldosterone, respectively. A further genome duplication occurred in the early evolutionary history of the teleosts, and the teleost CR system seems to have diversified, consisting now of 2 GRs and a MR. Teleosts lack aldosterone and the main corticosteroid is believed to be cortisol. However, the mineralocorticoid, 11-deoxycorticosterone (DOC), has been identified as an agonist for the rainbow trout MR, suggesting it may be the ancestral ligand for the MR. The retention of two GRs in teleosts suggests neofunctionalisation of one of the duplicated genes, but this hypothesis requires further work. In rainbow trout, transactivation and transrepression activities of the two GRs show marked differences in their sensitivity to glucocorticoids, suggesting a mechanism that may allow the two GRs to control different physiological pathways. Whether a similar mechanism is seen throughout the actinopterygii or whether this is specific to the salmonid lineage remains to be verified.

Effects of insulin-like growth factor-I, corticosterone, and 3,3', 5-tri-iodo-L-thyronine on glycosaminoglycan synthesis in vertebral cartilage of the clearnose skate, Raja eglanteria

Effects of insulin-like growth factor-I (IGF-I), corticosterone, and triiodothyronine (T(3)) on in vitro growth of vertebral cartilage of the clearnose skate, Raja eglanteria, were investigated. Uptake of [(35)S]sulfate in cultured vertebrae was used to characterize glycosaminoglycan (GAG) synthesis and cartilage growth. IGF-I significantly enhanced cartilage growth when concentrations of 1.28 and 12.8 nM were present in the culture system. Corticosterone significantly inhibited vertebral GAG synthesis at concentrations of 1, 10, and 100 nM. This effect was markedly pronounced in cartilage exposed to 1 and 10 nM corticosterone, in which GAG synthesis was virtually ceased. In contrast, T(3) (0.75, 7.5, and 75.0 nM) had no significant effect on sulfate uptake. These data suggest that IGF-I and corticosteroids may play important roles in regulating skeletal growth of elasmobranchs, as they appear to do in other vertebrates. While T(3) does not appear to exert an immediate, direct effect on vertebral growth, it may still influence elasmobranch chondrogenesis over longer culture periods or indirectly through other regulatory pathways. Thus, further information is necessary to characterize the role of thyroid hormones in the skeletal growth of these fishes. The present study is the first in vitro investigation on the hormonal regulation of elasmobranch cartilage growth. As such, the methods described herein provide a useful technique for examining these physiological processes. J. Exp. Zool. 284:549-556, 1999.

Short-term effects of 3,5,3'-triiodothyronine on the intermediary metabolism of the dogfish shark Squalus acanthias: evidence from enzyme activities

Plasma 3,5,3'-triiodothyronine (T3) concentration decreased significantly (P < 0.05), during 1-5 days of captivity, from levels in the freshly caught dogfish shark Squalus acanthias. The short-term effects of T3 treatment on the intermediary metabolism of S. acanthias were measured in the gill, kidney, liver, and white muscle. Animals were kept for 1-5 days before experimentation. Three hours after an intraperitoneal injection with either a low T3 dose (8.3 pmol T3/kg fish) or a high T3 dose (830 pmol T3/kg fish), selected enzymes of amino acid metabolism, lipid catabolism, ketone body metabolism, glycolysis, and oxidative metabolism were measured. Activity of enzymes of amino acid metabolism and lipid catabolism increased significantly (P < 0.05) in the liver of fish treated with a low T3 dose. The low dose of T3 apparently influences glycolysis as pyruvate kinase activity significantly increase (P < 0.05) in the kidney and white muscle.

Stage-related production of 21-hydroxylated progestins by the dogfish (Squalus acanthias) testis

Using shark (Squalus acanthias) testicular microsomes and [3H]progesterone ([3H]P) and [3H]17 alpha-hydroxyprogesterone ([3H]17 alpha-P) as substrates, two major products of 21-hydroxylase action were identified; respectively, 21-hydroxy-4-pregnen-3-one (11-deoxycorticosterone, DOC) and 17 alpha,21-dihydroxy-4-pregnen-3-one (11-deoxycortisol,S). Additional products of 17 alpha-hydroxylase/C-17,20-lyase action were detected: 17 alpha-hydroxyprogesterone (17 alpha P), testosterone (T), and androstenedione (AE). When microsomes derived from tissues in premeiotic (PrM), meiotic (M), and postmeiotic (PoM) stages of spermatogenesis were compared, maturation-related increases were observed with both 21-hydroxylase (3- to 20-fold) and 17 alpha-hydroxylase (2- to 6-fold). With [3H]P as tracer, the half-maximal substrate concentrations (Km = 0.2-1.0 microM) and maximal reaction velocities (Vmax = 9-25 pmol/mg protein/min) were similar for both enzymes when assayed in the same preparation, suggesting they compete for available substrate. Also, the presence of 1- or 10-fold molar excess radioinert DOC reduced conversion of [3H]P to 17 alpha-hydroxylated products. [3H]DOC itself was a substrate of 17 alpha-hydroxylation but not C-17,20-lyase action. Expression of 21-hydroxylase and 17 alpha-hydroxylase activities in cultured spermatocysts (intact germ cell/Sertoli cell units) was confirmed by detection of immunoreactive 17 alpha,20 beta,21-trihydroxy-4-pregnen-3-one (20 beta-S),S,17 alpha,20 beta-dihydroxy-4-pregnen-3-one (17 alpha,20 beta-P), DOC, P and T in spent media. 20 beta-S and S secretion increased and 17 alpha,20 beta-P decreased progressively with stage of maturation, but DOC was similar in all stages. P secretion was maximal and T lowest in M-stage spermatocysts, but when DOC (0.1 microM) was added to PrM or PoM spermatocysts, T output decreased. Taken together, the data suggest that 21-hydroxylating pathways, via direct and indirect mechanisms, affect accumulation of bioactive steroids (P and T) differentially by stage of spermatogenic development. Whether 21-hydroxylated progestins produced in shark testis have paracrine or endocrine actions in their own right remains to be investigated.

Aldosterone response to renin, angiotensin, ACTH, hemorrhage and sodium depletion in a freshwater teleost, Catostomus macrocheilus

1. Plasma aldosterone activity (PAA) was measured in individual large-scale suckers (Catostomus macrocheilus) by radioimmunoassay. 2. PAA in control groups was 12.0 +/- 6.0 (mean +/- SEM) pg/ml; one control group showed a sixfold elevation of PAA, possibly reflecting a seasonal effect. 3. Injections of sucker renin preparation, synthetic angiotensin II and synthetic adrenocorticotrophin had no effect upon PAA. PAA was also unaffected by sodium depletion, sodium loading and moderate hemorrhage. 4. Prolonged restraint stress coupled with severe hemorrhage significantly increased PAA to 47.1 +/- 5.1 pg/ml. 5. This study indicates that aldosterone secretion in teleost fish has either rudimentary or yet to be discovered controls and functions.

21-Hydroxypregna-1,4-diene-3,11,20-trione: a biliary metabolite of a cartilaginous fish, Raja sp

21-Hydroxypregna-1,4-diene-3,11,20-trione was isolated from skate bile and as an in vivo metabolite of 3H-1alpha-hydroxycorticosterone. Identity was established by chromatography and derivatization to constant 3H/14C ratio and mass spectrometry of the 20,21-acetonide. The new steroid was present in the free form and as the glucuronoside.

Renal tubule ion transport and collecting duct function in the elasmobranch little skate, Raja erinacea

Renal micropuncture and microdissection techniques with ultramicro fluid analysis have been applied to evaluate single nephron function in the skate, Raja erinacea. We have divided the skate nephron into three proximal tubular segments (PTS I-III), three distal coilings (DC I-III), and a countercurrent loop system located between the proximal segments and the distal coilings. The collecting duct is the principal site of urinary dilution. Following exposure of the fish to 75% seawater for about 24 hours, the sodium concentration difference between the end collecting duct lumen and plasma is decreased sufficiently to account for the urinary dilution. The principal site for magnesium, phosphate and sulphate secretion appears to be PTS II. This segment is located on the ventral surface of the kidney. PTS II is also the main nephron site for reabsorption of sodium and chloride in excess of water.