Regulation of energy metabolism during social interactions in rainbow trout: a role for AMP-activated protein kinase

Negative feedback regulation in the hypothalamic-pituitary-interrenal axis of rainbow trout (Oncorhynchus mykiss) subjected to chronic social stress

The formation of dominance hierarchies in pairs of juvenile rainbow trout (Oncorhynchus mykiss) results in subordinate individuals exhibiting chronically elevated plasma cortisol concentrations. Cortisol levels reflect a balance between cortisol production, which is coordinated by the hypothalamic-pituitary-interrenal (HPI) axis in teleost fish, and negative feedback regulation and hormone clearance, which act to lower cortisol levels. However, the mechanisms contributing to the longer-term elevation of cortisol levels during chronic stress are not well established in fishes. The current study aimed to determine how subordinate fish maintain elevated cortisol levels, by testing the prediction that negative feedback and clearance mechanisms are impaired by chronic social stress. Plasma cortisol clearance was unchanged by social stress based on a cortisol challenge trial, hepatic abundance of the cortisol-inactivating enzyme 11-beta hydroxysteroid dehydrogenase type 2 (11βHSD2), and tissue fate of labelled cortisol. The capacity for negative feedback regulation in terms of transcript and protein abundances of corticosteroid receptors in the preoptic area (POA) and pituitary appeared stable. However, changes in 11βHSD2 and mineralocorticoid receptor (MR) expression suggest subtle regulatory changes in the pituitary that may alter negative feedback. The chronic cortisol elevation observed during social subordination likely is driven by HPI axis activation and compounded by dysregulated negative feedback.

Social status-dependent regulation and function of the somatotropic axis in juvenile rainbow trout

Juvenile rainbow trout (Oncorhynchus mykiss) develop social hierarchies when competing for resources in a constrained environment. Among the physiological consequences of social status are changes in organismal energy metabolism, which generally favour anabolic pathways in dominant fish and catabolic pathways in subordinate fish. The somatotropic axis is an important regulator of metabolism and growth that could be involved in mediating metabolic changes in response to social status in juvenile rainbow trout. Here we used juvenile trout housed either in dyads or individually (sham controls) to determine whether social status changes indices of somatotropic axis function. Although pituitary growth hormone expression (gh1 and gh2) did not differ among groups, circulating growth hormone (GH) increased ∼12-fold in subordinate fish compared to sham and dominant fish. Social status caused consistent differential expression of GH receptor paralogues in liver and muscle, two principal target tissues of GH. Compared to dominant and/or sham fish, ghra paralogue expression (ghra1 and ghra2) was lower, while ghrb1 expression was higher in subordinate fish. Across tissues, ghra paralogue expression was generally positively correlated with expression of insulin growth factors (igf1, igf2), while ghrb1 expression was positively correlated with transcript abundance of hormone sensitive lipase (hsl1). Because igf and hsl expression are subject to context-dependent GH control in rainbow trout, these results suggest that increased circulating GH in conjunction with differential expression of ghr paralogues may translate into prioritization of downstream catabolic lipolytic pathways in subordinate rainbow trout. These findings support a social context-dependent role for GH signalling in mediating metabolic changes in juvenile rainbow trout.

Behavioural and physiological plasticity in social hierarchies

Individuals occupying dominant and subordinate positions in social hierarchies exhibit divergent behaviours, physiology and neural functioning. Dominant animals express higher levels of dominance behaviours such as aggression, territorial defence and mate-guarding. Dominants also signal their status via auditory, visual or chemical cues. Moreover, dominant animals typically increase reproductive behaviours and show enhanced spatial and social cognition as well as elevated arousal. These biobehavioural changes increase energetic demands that are met via shifting both energy intake and metabolism and are supported by coordinated changes in physiological systems including the hypothalamic-pituitary-adrenal and hypothalamic-pituitary-gonadal axes as well as altered gene expression and sensitivity of neural circuits that regulate these behaviours. Conversely, subordinate animals inhibit dominance and often reproductive behaviours and exhibit physiological changes adapted to socially stressful contexts. Phenotypic changes in both dominant and subordinate individuals may be beneficial in the short-term but lead to long-term challenges to health. Further, rapid changes in social ranks occur as dominant animals socially ascend or descend and are associated with dynamic modulations in the brain and periphery. In this paper, we provide a broad overview of how behavioural and phenotypic changes associated with social dominance and subordination are expressed in neural and physiological plasticity. This article is part of the theme issue 'The centennial of the pecking order: current state and future prospects for the study of dominance hierarchies'.

Alanine alters the carbohydrate metabolism of rainbow trout: glucose flux and cell signaling

In rainbow trout, dietary carbohydrates are poorly metabolized compared with other macronutrients. One prevalent hypothesis suggests that high dietary amino acid levels could contribute to the poor utilization of carbohydrates in trout. In mammals, alanine is considered an important gluconeogenic precursor, but has recently been found to stimulate AMP-activated protein kinase (AMPK) to reduce glucose levels. In trout, the effect of alanine on glucose flux is unknown. The goal of this study was to determine the effects of 4 h exogenous alanine infusion on glucose metabolism in rainbow trout. Glucose flux, and the rate of glucose appearance (Ra) and disposal (Rd) were measured in vivo. Key glycolytic and gluconeogenic enzyme expression and activity, and cell signaling molecules relevant to glucose metabolism were assessed in the liver and muscle. The results show that alanine inhibits glucose Ra (from 13.2±2.5 to 7.3±1.6 μmol kg-1 min-1) and Rd (from 13.2±2.5 to 7.4±1.5 μmol kg-1 min-1) and the slight mismatch between Ra and Rd caused a reduction in glycemia, similar to the effects of insulin in trout. The reduction in glucose Rd can be partially explained by a reduction in glut4b expression in red muscle. In contrast to mammals, trout alanine-dependent glucose-lowering effects did not involve hepatic AMPK activation, suggesting a different mechanistic basis. Interestingly, protein kinase B (AKT) activation increased only in muscle, similar to effects observed in insulin-infused trout. We speculate that alanine-dependent effects were probably mediated through stimulation of insulin secretion, which could indirectly promote alanine oxidation to provide the needed energy.

Behavioral and physiological evidence that increasing group size ameliorates the impacts of social disturbance

Intra-group social stability is important for the long-term productivity and health of social organisms. We evaluated the effect of group size on group stability in the face of repeated social perturbations using a cooperatively breeding fish, Neolamprologus pulcher In a laboratory study, we compared both the social and physiological responses of individuals from small versus large groups to the repeated removal and replacement of the most dominant group member (the breeder male), either with a new male (treatment condition) or with the same male (control condition). Individuals living in large groups were overall more resistant to instability but were seemingly slower to recover from perturbation. Members of small groups were more vulnerable to instability but recovered faster. Breeder females in smaller groups also showed greater physiological preparedness for instability following social perturbations. In sum, we discover both behavioral and physiological evidence that living in larger groups helps to dampen the impacts of social instability in this system.

Hypothalamic AMPKα2 regulates liver energy metabolism in rainbow trout through vagal innervation

Hypothalamic AMPK plays a major role in the regulation of whole body metabolism and energy balance. Present evidence has demonstrated that this canonical mechanism is evolutionarily conserved. Thus, recent data demonstrated that inhibition of AMPKα2 in fish hypothalamus led to decreased food intake and liver capacity to use and synthesize glucose, lipids, and amino acids. We hypothesize that a signal of abundance of nutrients from the hypothalamus controls hepatic metabolism. The vagus nerve is the most important link between the brain and the liver. We therefore examined in the present study whether surgical transection of the vagus nerve in rainbow trout is sufficient to alter the effect in liver of central inhibition of AMPKα2. Thus, we vagotomized (VGX) or not (Sham) rainbow trout and then intracerebroventricularly administered adenoviral vectors tagged with green fluorescent protein alone or linked to a dominant negative isoform of AMPKα2. The inhibition of AMPKα2 led to reduced food intake in parallel with changes in the mRNA abundance of hypothalamic neuropeptides [neuropeptide Y (npy), agouti-related protein 1 (agrp1), and cocaine- and amphetamine-related transcript (cartpt)] involved in food intake regulation. Central inhibition of AMPKα2 resulted in the liver having decreased capacity to use and synthesize glucose, lipids, and amino acids. Notably, these effects mostly disappeared in VGX fish. These results support the idea that autonomic nervous system actions mediate the actions of hypothalamic AMPKα2 on liver metabolism. Importantly, this evidence indicates that the well-established role of hypothalamic AMPK in energy balance is a canonical evolutionarily preserved mechanism that is also present in the fish lineage.

Pck-ing up steam: Widening the salmonid gluconeogenic gene duplication trail

Rainbow trout have, as salmonid fish species, undergone sequential genome duplication events in their evolutionary history. In addition to a teleost-specific whole genome duplication approximately 320-350 million years ago, rainbow trout and salmonids in general underwent an additional salmonid lineage-specific genome duplication event approximately 80 million years ago. Through the recent sequencing of salmonid genome sequences, including the rainbow trout, the identification and study of duplicated genes has become available. A particular focus of interest has been the evolution and regulation of rainbow trout gluconeogenic genes, as recent molecular and gene expression evidence points to a possible contribution of previously uncharacterized gluconeogenic gene paralogues to the rainbow trout long-studied glucose intolerant phenotype. Since the publication of the initial rainbow trout genome draft, resequencing and annotation have further improved genome coverage. Taking advantage of these recent improvements, we here identify a salmonid-specific genome duplication of ancestral mitochondrial phosphoenolpyruvate carboxykinase 2 isoenzyme, we termed pck2a and pck2b. Cytosolic phosphoenolpyruvate carboxykinase (Pck1) and, more recently mitochondrial Pck2, are considered to be the rate-limiting enzymes in de novo gluconeogenesis. Following in silico confirmation of salmonid pck2a and pck2b evolutionary history, we simultaneously profiled cytosolic pck1 and mitochondrial pck2a and pck2b expression in rainbow trout liver under several experimental conditions known to regulate hepatic gluconeogenesis. Cytosolic pck1 abundance was increased by nutritional (diets with a high protein to carbohydrate ratio compared to diets with a low carbohydrate to protein ratio) and glucoregulatory endocrine factors (glucagon and cortisol), revealing that the well-described transcriptional regulation of pck1 in mammals is present in rainbow trout. Conversely, and in contrast to mammals, we here describe endocrine regulation of pck2a (decrease in abundance in response to glucagon infusion), and nutritional, social-status-dependent and hypoxia-dependent regulation of pck2b. Specifically, pck2b transcript abundance increased in trout fed a diet with a low protein to carbohydrate ratio compared to a diet with a high protein to carbohydrate ratio, in dominant fish compared to subordinate fish as well as hypoxia. This specific and differential expression of rainbow trout pck2 ohnologues is indicative of functional diversification, and possible functional consequences are discussed in light of the recently highlighted gluconeogenic roles of mitochondrial pck2 in mammalian models.

Differential Role of Hypothalamic AMPKα Isoforms in Fish: an Evolutive Perspective

In mammals, hypothalamic AMP-activated protein kinase (AMPK) α1 and α2 isoforms mainly relate to regulation of thermogenesis/liver metabolism and food intake, respectively. Since both isoforms are present in fish, which do not thermoregulate, we assessed their role(s) in hypothalamus regarding control of food intake and energy homeostasis. Since many fish species are carnivorous and mostly mammals are omnivorous, assessing if the role of hypothalamic AMPK is different is also an open question. Using the rainbow trout as a fish model, we first observed that food deprivation for 5 days did not significantly increase phosphorylation status of AMPKα in hypothalamus. Then, we administered adenoviral vectors that express dominant negative (DN) AMPKα1 or AMPKα2 isoforms. The inhibition of AMPKα2 (but not AMPKα1) led to decreased food intake. The central inhibition of AMPKα2 resulted in liver with decreased capacity of use and synthesis of glucose, lipids, and amino acids suggesting that a signal of nutrient abundance flows from hypothalamus to the liver, thus suggesting a role for central AMPKα2 in the regulation of peripheral metabolism in fishes. The central inhibition of AMPKα1 induced comparable changes in liver metabolism though at a lower extent. From an evolutionary point of view, it is of interest that the function of central AMPKα2 remained similar throughout the vertebrate lineage. In contrast, the function of central AMPKα1 in fish relates to modulation of liver metabolism whereas in mammals modulates not only liver metabolism but also brown adipose tissue and thermogenesis.

Social status affects lipid metabolism in rainbow trout, Oncorhynchus mykiss

Juvenile rainbow trout ( Oncorhynchus mykiss) confined in pairs form social hierarchies in which socially subordinate fish display characteristic traits, including reduced growth rates and altered glucose metabolism. These effects are, in part, mediated by chronically elevated cortisol levels and/or reduced feeding. To determine the effects of social status on lipid metabolism, trout were held in pairs for 4 days, following which organismal and liver-specific indexes of lipid metabolism were measured. At the organismal level, circulating triglycerides were elevated in dominant trout, whereas subordinate trout exhibited elevated concentrations of circulating free fatty acids (FFAs) and lowered plasma total cholesterol levels. At the molecular level, increased expression of lipogenic genes in dominant trout and cpt1a in subordinate trout was identified, suggesting a contribution of increased de novo lipogenesis to circulating triglycerides in dominant trout and reliance on circulating FFAs for β-oxidation in the liver of subordinates. Given the emerging importance of microRNAs (miRNA) in the regulation of hepatic lipid metabolism, candidate miRNAs were profiled, revealing increased expression of the lipogenic miRNA-33 in dominant fish. Because the Akt-TOR-S6-signaling pathway is an important upstream regulator of hepatic lipid metabolism, its signaling activity was quantified. However, the only difference detected among groups was a strong increase in S6 phosphorylation in subordinate trout. In general, the changes observed in lipid metabolism of subordinates were not mimicked by either cortisol treatment or fasting alone, indicating the existence of specific, emergent effects of subordinate social status itself on this fuel.