Metabolic response to a glucagon challenge varies with adiposity and life-history stage in fasting northern elephant seals

Mercury Bioaccumulation and Cortisol Interact to Influence Endocrine and Immune Biomarkers in a Free-Ranging Marine Mammal

Mercury bioaccumulation from deep-ocean prey and the extreme life history strategies of adult female northern elephant seals (Mirounga angustirostris) provide a unique system to assess the interactive effects of mercury and stress on animal health by quantifying blood biomarkers in relation to mercury (skeletal muscle and blood mercury) and cortisol concentrations. The thyroid hormone thyroxine (tT4) and the antibody immunoglobulin E (IgE) were associated with mercury and cortisol concentrations interactively, where the magnitude and direction of the association of each biomarker with mercury or cortisol changed depending on the concentration of the other factor. For example, when cortisol concentrations were lowest, tT4 was positively related to muscle mercury, whereas tT4 had a negative relationship with muscle mercury in seals that had the highest cortisol concentrations. Additionally, we observed that two thyroid hormones, triiodothyronine (tT3) and reverse triiodothyronine (rT3), were negatively (tT3) and positively (rT3) associated with mercury concentrations and cortisol in an additive manner. As an example, tT3 concentrations in late breeding seals at the median cortisol concentration decreased by 14% across the range of observed muscle mercury concentrations. We also observed that immunoglobulin M (IgM), the pro-inflammatory cytokine IL-6 (IL-6), and a reproductive hormone, estradiol, were negatively related to muscle mercury concentrations but were not related to cortisol. Specifically, estradiol concentrations in late molting seals decreased by 50% across the range of muscle mercury concentrations. These results indicate important physiological effects of mercury on free-ranging apex marine predators and interactions between mercury bioaccumulation and extrinsic stressors. Deleterious effects on animals' abilities to maintain homeostasis (thyroid hormones), fight off pathogens and disease (innate and adaptive immune system), and successfully reproduce (endocrine system) can have significant individual- and population-level consequences.

Changes in apolipoprotein abundance dominate proteome responses to prolonged fasting in elephant seals

Unlike many animals that reduce activity during fasting, northern elephant seals (NES) undergo prolonged fasting during energy-intensive life-history stages such as reproduction and molting, fueling fasting energy needs by mobilizing fat stores accrued during foraging. NES display several unique metabolic features such as high fasting metabolic rates, elevated blood lipid and high-density lipoprotein (HDL) cholesterol levels, efficient protein sparing and resistance to oxidative stress during fasting. However, the cellular mechanisms that regulate these adaptations are still not fully understood. To examine how metabolic coordination is achieved during prolonged fasting, we profiled changes in blubber, skeletal muscle and plasma proteomes of adult female NES over a 5 week fast associated with molting. We found that while blubber and muscle proteomes were remarkably stable over fasting, over 50 proteins changed in abundance in plasma, including those associated with lipid storage, mobilization, oxidation and transport. Apolipoproteins dominated the blubber, plasma and muscle proteome responses to fasting. APOA4, APOE and APOC3, which are associated with lipogenesis and triglyceride accumulation, decreased, while APOA1, APOA2 and APOM, which are associated with lipid mobilization and HDL function, increased over fasting. Our findings suggest that changes in apolipoprotein composition may underlie the maintenance of high HDL levels and, together with adipokines and hepatokines that facilitate lipid catabolism, may mediate the metabolic transitions between feeding and fasting in NES. Many of these proteins have not been previously studied in this species and provide intriguing hypotheses about metabolic regulation during prolonged fasting in mammals.

Changes in serum adipokines during natural extended fasts in female northern elephant seals

Adipose tissue is essential to endotherms for thermoregulation and energy storage as well as functioning as an endocrine organ. Adipose derived hormones, or adipokines, regulate metabolism, energy expenditure, reproduction, and immune function in model systems but are less well studied in wildlife. Female northern elephant seals (NES) achieve high adiposity during foraging and then undergo natural fasts up to five weeks long during haul-outs associated with reproduction and molting, resulting in large changes in adipose reserves. We measured circulating levels of four adipokines: leptin, resistin, adiponectin, and kisspeptin-54, in 196 serum samples from female NES at the beginning and end of their breeding and molting fasts. We examined the relationships between these adipokines and life-history stage, adiposity, mass, cortisol, and an immune cytokine involved in the innate immune response interleukin 6 (IL-6). All four adipokines varied with life-history stage. Leptin concentrations were highest at the beginning of the breeding haul-out. Resistin concentrations were higher throughout the breeding haul-out compared to the molt haul-out. Adiponectin concentrations were highest at the beginning of both haul-outs. Kisspeptin-54 concentrations were highest at the end of the breeding haul-out. Leptin, resistin, and adiponectin were associated with measures of body condition, either adiposity, mass, or both. Resistin, adiponectin, and kisspeptin-54 were associated with circulating cortisol concentrations. Resistin was strongly associated with circulating IL-6, a multifunctional cytokine. Adiponectin was associated with glucose concentrations, suggesting a potential role in tissue-specific insulin sensitivity during life-history stages categorized by high adiposity. Increased cortisol concentrations late in lactation were associated with increased kisspeptin-54, suggesting a link to ovulation initiation in NES. This study suggests dramatic changes in circulating adipokines with life-history and body condition that may exert important regulatory roles in NES. The positive relationship between adiponectin and adiposity as well as the lack of a relationship between leptin and kisspeptin-54 differed from model systems. These differences from biomedical model systems suggest the potential for modifications of expression and function of adipose-derived hormones in species that undergo natural changes in adiposity as part of their life-history.

Metabolic response of dolphins to short-term fasting reveals physiological changes that differ from the traditional fasting model

Bottlenose dolphins (Tursiops truncatus) typically feed on prey that are high in lipid and protein content and nearly devoid of carbohydrate, a dietary feature shared with other marine mammals. However, unlike fasted-adapted marine mammals that predictably incorporate fasting into their life history, dolphins feed intermittently throughout the day and are not believed to be fasting-adapted. To assess whether the physiological response to fasting in the dolphin shares features with or distinguishes them from those of fasting-adapted marine mammals, the plasma metabolomes of eight bottlenose dolphins were compared between post-absorptive and 24-h fasted states. Increases in most identified free fatty acids and lipid metabolites and reductions in most amino acids and their metabolites were consistent with the upregulation of lipolysis and lipid oxidation and the downregulation of protein catabolism and synthesis. Consistent with a previously hypothesized diabetic-like fasting state, fasting was associated with elevated glucose and patterns of certain metabolites (e.g. citrate, cis-aconitate, myristoleic acid) indicative of lipid synthesis and glucose cycling to protect endogenous glucose from oxidative disposal. Pathway analysis predicted an upregulation of cytokines, decreased cell growth and increased apoptosis including apoptosis of insulin-secreting β-cells. Metabolomic conditional mutual information networks were estimated for the post-absorptive and fasted states and 'topological modules' were estimated for each using the eigenvector approach to modularity network division. A dynamic network marker indicative of a physiological shift toward a negative energy state was subsequently identified that has the potential conservation application of assessing energy state balance in at-risk wild dolphins.

Functional Studies with Primary Cells Provide a System for Genome-to-Phenome Investigations in Marine Mammals

Marine mammals exhibit some of the most dramatic physiological adaptations in their clade and offer unparalleled insights into the mechanisms driving convergent evolution on relatively short time scales. Some of these adaptations, such as extreme tolerance to hypoxia and prolonged food deprivation, are uncommon among most terrestrial mammals and challenge established metabolic principles of supply and demand balance. Non-targeted omics studies are starting to uncover the genetic foundations of such adaptations, but tools for testing functional significance in these animals are currently lacking. Cellular modeling with primary cells represents a powerful approach for elucidating the molecular etiology of physiological adaptation, a critical step in accelerating genome-to-phenome studies in organisms in which transgenesis is impossible (e.g., large-bodied, long-lived, fully aquatic, federally protected species). Gene perturbation studies in primary cells can directly evaluate whether specific mutations, gene loss, or duplication confer functional advantages such as hypoxia or stress tolerance in marine mammals. Here, we summarize how genetic and pharmacological manipulation approaches in primary cells have advanced mechanistic investigations in other non-traditional mammalian species, and highlight the need for such investigations in marine mammals. We also provide key considerations for isolating, culturing, and conducting experiments with marine mammal cells under conditions that mimic in vivo states. We propose that primary cell culture is a critical tool for conducting functional mechanistic studies (e.g., gene knockdown, over-expression, or editing) that can provide the missing link between genome- and organismal-level understanding of physiological adaptations in marine mammals.

Adiposity and fat metabolism during combined fasting and lactation in elephant seals

Animals that fast depend on mobilizing lipid stores to power metabolism. Northern elephant seals (Mirounga angustirostris) incorporate extended fasting into several life-history stages: development, molting, breeding and lactation. The physiological processes enabling fasting and lactation are important in the context of the ecology and life history of elephant seals. The rare combination of fasting and lactation depends on the efficient mobilization of lipid from adipose stores and its direction into milk production. The mother elephant seal must ration her finite body stores to power maintenance metabolism, as well as to produce large quantities of lipid and protein-rich milk. Lipid from body stores must first be mobilized; the action of lipolytic enzymes and hormones stimulate the release of fatty acids into the bloodstream. Biochemical processes affect the release of specific fatty acids in a predictable manner, and the pattern of release from lipid stores is closely reflected in the fatty acid content of the milk lipid. The content of the milk may have substantial developmental, thermoregulatory and metabolic consequences for the pup. The lactation and developmental patterns found in elephant seals are similar in some respects to those of other mammals; however, even within the limited number of mammals that simultaneously fast and lactate, there are important differences in the mechanisms that regulate lipid mobilization and milk lipid content. Although ungulates and humans do not fast during lactation, there are interesting comparisons to these groups regarding lipid mobilization and milk lipid content patterns.

The demands of lactation promote differential regulation of lipid stores in fasting elephant seals

Fasting animals must ration stored reserves appropriately for metabolic demands. Animals that experience fasting concomitant with other metabolically demanding activities are presented with conflicting demands of energy conservation and expenditure. Our objective was to understand how fasting northern elephant seals regulate the mobilization of lipid reserves and subsequently milk lipid content during lactation. We sampled 36 females early and 39 at the end of lactation. To determine the separate influences of lactation from fasting, we also sampled fasting but non-lactating females early and late (8 and 6 seals, respectively) in their molting fasting period. Mass and adiposity were measured, as well as circulating non-esterified fatty acid (NEFA), triacylglycerol (TAG), cortisol, insulin and growth hormone levels. Milk was collected from lactating females. Milk lipid content increased from 31% in early to 51% in late lactation. In lactating females plasma NEFA was positively related to cortisol and negatively related to insulin, but in molting seals, only variation in cortisol was related to NEFA. Milk lipid content varied with mass, adiposity, NEFA, TAG, cortisol and insulin. Surprisingly, growth hormone concentration was not related to lipid metabolites or milk lipid. Suppression of insulin release appears to be the differential regulator of lipolysis in lactating versus molting seals, facilitating mobilization of stored lipids and maintenance of high NEFA concentrations for milk synthesis. Milk lipid was strongly impacted by the supply of substrate to the mammary gland, indicating regulation at the level of mobilization of lipid reserves.

Adiposity and fat metabolism in lactating and fasting northern elephant seals

Several taxa of animals fast completely from food and water during energy-intensive periods such as lactation, breeding, and development. In elephant seals, these behaviors are sustained by high adiposity, high rates of fat mobilization, and reduced oxidation of carbohydrates and proteins. Adiposity and the regulation of lipolysis directly affect lactation energetics, milk composition, and mating success. Long-term fasting induces changes in regulation of lipolysis and lipid metabolism that influence fatty acid (FA) availability and the onset of insulin resistance. Hypoinsulinemia and elevated circulating FAs are also associated with several unique features of carbohydrate metabolism, including elevated plasma glucose, gluconeogenesis, and Cori cycle activity as well as high rates of pyruvate and tricarboxylic acid cycling. Glucose-lactate pools and triacylglycerol-FA cycles may be linked via glyceroneogenesis and this may be an important pathway influencing both fat and carbohydrate metabolism. Together, these features allow a sustained, high intensity, fat-based metabolism without substantial accumulation of ketoacids.