Proteomic and Transcriptomic Changes in Hibernating Grizzly Bears Reveal Metabolic and Signaling Pathways that Protect against Muscle Atrophy

Muscle atrophy is a physiological response to disuse and malnutrition, but hibernating bears are largely resistant to this phenomenon. Unlike other mammals, they efficiently reabsorb amino acids from urine, periodically activate muscle contraction, and their adipocytes differentially responds to insulin. The contribution of myocytes to the reduced atrophy remains largely unknown. Here we show how metabolism and atrophy signaling are regulated in skeletal muscle of hibernating grizzly bear. Metabolic modeling of proteomic changes suggests an autonomous increase of non-essential amino acids (NEAA) in muscle and treatment of differentiated myoblasts with NEAA is sufficient to induce hypertrophy. Our comparison of gene expression in hibernation versus muscle atrophy identified several genes differentially regulated during hibernation, including Pdk4 and Serpinf1. Their trophic effects extend to myoblasts from non-hibernating species (including C. elegans), as documented by a knockdown approach. Together, these changes reflect evolutionary favored adaptations that, once translated to the clinics, could help improve atrophy treatment.

A protocol for the isolation and cultivation of brown bear (Ursus arctos) adipocytes

Brown bears (Ursus arctos) exhibit hyperphagia each fall and can become obese in preparation for hibernation. They do this without displaying the physiological problems typically seen in obese humans, such as Type 2 diabetes and heart disease. The study of brown bear hibernation biology could therefore aid in the development of novel methods for combating metabolic diseases. To this end, we isolated mesenchymal stem cells from subcutaneous fat biopsies, and culture methods were developed to differentiate these into the adipogenic lineage. Biopsies were taken from 8 captive male (N = 6) and female (N = 2) brown bears, ages 2-12 years. Plastic adherent, fibroblast-like cells were proliferated and subsequently cryopreserved or differentiated. Differentiation conditions were optimized with respect to fetal bovine serum content and time spent in differentiation medium. Cultures were characterized through immunostaining, RT-qPCR, and Oil red O staining to quantify lipid accumulation. Adiponectin, leptin, and glycerol medium concentrations were also determined over the course of differentiation. The culturing protocol succeeded in generating hormone-sensitive lipase-expressing, lipid-producing white-type adipocytes (UCP1 negative). Serum concentration and time of exposure to differentiation medium were both positively related to lipid production. Cells cultured to low passage numbers retained similar lipid production and expression of lipid markers PLIN2 and FABP4. Ultimately, the protocols described here may be useful to biologists in the field investigating the health of wild bear populations and could potentially increase our understanding of metabolic disorders in humans.

Increased cardiac alpha-myosin heavy chain in left atria and decreased myocardial insulin-like growth factor (Igf-I) expression accompany low heart rate in hibernating grizzly bears

Grizzly bears (Ursus arctos horribilis) tolerate extended periods of extremely low heart rate during hibernation without developing congestive heart failure or cardiac chamber dilation. Left ventricular atrophy and decreased left ventricular compliance have been reported in this species during hibernation. We evaluated the myocardial response to significantly reduced heart rate during hibernation by measuring relative myosin heavy-chain (MyHC) isoform expression and expression of a set of genes important to muscle plasticity and mass regulation in the left atria and left ventricles of active and hibernating bears. We supplemented these data with measurements of systolic and diastolic function via echocardiography in unanesthetized grizzly bears. Atrial strain imaging revealed decreased atrial contractility, decreased expansion/reservoir function (increased atrial stiffness), and decreased passive-filling function (increased ventricular stiffness) in hibernating bears. Relative MyHC-α protein expression increased significantly in the atrium during hibernation. The left ventricle expressed 100% MyHC-β protein in both groups. Insulin-like growth factor (IGF-I) mRNA expression was reduced by ∼50% in both chambers during hibernation, consistent with the ventricular atrophy observed in these bears. Interestingly, mRNA expression of the atrophy-related ubiquitin ligases Muscle Atrophy F-box (MAFBx) and Muscle Ring Finger 1 did not increase, nor did expression of myostatin or hypoxia-inducible factor 1α (HIF-1α). We report atrium-specific decreases of 40% and 50%, respectively, in MAFBx and creatine kinase mRNA expression during hibernation. Decreased creatine kinase expression is consistent with lowered energy requirements and could relate to reduced atrial emptying function during hibernation. Taken together with our hemodynamic assessment, these data suggest a potential downregulation of atrial chamber function during hibernation to prevent fatigue and dilation due to excessive work against an optimally filled ventricle, a response unpredicted by the Frank-Starling mechanism.

390 ISOLATION AND ADIPOGENESIS OF GRIZZLY BEAR MESENCHYMAL STEM CELLS

During winter hibernation, grizzly bears (Ursus arctos horribilis) do not eat but instead rely on internal fat stores as a primary source of metabolic energy. The resulting seasonal fluctuations in appetite and body mass make the grizzly bear a naturally occurring animal model for human conditions such as obesity and anorexia. An in vitro model of hibernating bear stem cells might enhance our understanding of processes such as stem cell proliferation and differentiation. Mesenchymal stem cells, derived from bone marrow and adipose tissue among others, differentiate into adipocytes and might play important roles in energy metabolism. In the current study, we examined the in vitro viability and morphology of mesenchymal stem cells isolated from grizzly bear adipose tissue (ADSC) and bone marrow (BMSC); these ADSC and BMSCs underwent adipogenic differentiation for 0, 7, 14, 21, and 28 days. Bone marrow stem cells and ADSC were isolated using mechanical disaggregation, collagenase digestion, centrifugation, and plating onto tissue culture polystyrene. Cell viability and proliferation was quantified using the colony forming unit assay and a hemocytometer. Both stem cell types were differentiated into adipocytes using 10 μM insulin, 1 μM dexamethasone, and 0.5 mM isobutylmethylxanthine (all Sigma- Aldrich, St. Louis, MO, USA) with the addition of 10% fetal bovine (FBS) or bear serum from the active feeding period. Adipogenic differentiation was confirmed using Oil Red O and quantified using ImageJ. Statistical analysis was performed using an unpaired t-test between treatments of the same time point. All cells were isolated within 28 h of tissue harvest. Adipose-derived stem cells formed an average of 11 colonies (0.011%), whereas BMSC formed 1.5 colonies (0.0015%) per 100 000 cells. Doubling time forADSC was approximately 54 h in 10% FBS. BothADSC and BMSC had an initial spindle-shaped morphology, which gradually became more rounded during adipogenic differentiation. For bear serum at Day 28, ADSC had a significantly (P < 0.01) greater stained area per cell than did BMSC. In summary, both types of mesenchymal stem cells successfully differentiated into adipocytes and maintained viability. In conclusion, grizzly bear mesenchymal stem cells canbesuccessfully isolated, expanded, and differentiated in culture. These results allow for future studies using the bear as an in vitro model for fat metabolism during hibernation and active periods. This work was partially supported by the Carle Foundation Hospital, the Intel Scholar’s Research Program, USDA Multi-State Research Project W1171, and the Illinois Regenerative Medicine Institute (IDPH # 63080017). In addition, the authors would like to thank Agatha Luszpak for support with the analysis.

Digestibility, nitrogen excretion, and mean retention time by North American porcupines (Erethizon dorsatum) consuming natural forages

North American porcupines (Erethizon dorsatum) subsist predominantly on low-protein, high-fiber, high-tannin diets. Therefore, we measured the porcupine's ability to digest dry matter, fiber, and protein by conducting digestion trials on eight natural forages and one pelleted ration varying in concentration of fiber, nitrogen, and tannins. On these diets, dry matter intake ranged from 5 to 234 g/kg(0.75)/d and dry matter digestibility ranged from 62% to 96%. Porcupines digested highly lignified fiber better than many large hindgut fermenters and ruminants. The porcupine's ability to digest fiber may be explained, in part, by their lengthy mean retention time of particles (38.43+/-0.56 h). True nitrogen digestibility was 92% for nontannin forages and pellets. Endogenous urinary nitrogen was 205 mg N/kg(0.75)/d, and metabolic fecal nitrogen was 2.8 g N/kg dry matter intake. Porcupines achieved nitrogen balance at relatively low levels of nitrogen intake (346 mg N/kg(0.75)/d). Tannins reduced the porcupines' ability to digest protein. However, the reduction in protein digestion was not predictable from the amount of bovine serum albumin precipitated. Like many herbivores, porcupines may ameliorate the effects of certain tannins in natural forages on protein digestibility through physiological and behavioral adaptations.

Why bears consume mixed diets during fruit abundance

In many ecosystems, grizzly bears (Ursus arctos) and black bears (Ursus americanus) feed heavily on berries and fruits in the fall. While a few birds and mammals are exclusively frugivorous, bears and most other animals consume fruit as part of a mixed diet. The mixed-diet strategy avoids potential calcium, protein, amino acid, or other nutrient deficiencies that can occur on fruit diets. However, we hypothesized that the high carbohydrate - low protein content of fruit would increase energy metabolism and force bears to use dietary mixing to meet protein requirements and, thereby, reduce energy metabolism. We examined the effects of six plant-based diets containing from 2.3 to 35% crude protein on intake, maintenance costs, and efficiency of gain of captive grizzly and black bears. In addition, the food habits of six populations of wild grizzly and black bears were analyzed, to determine the crude protein and digestible dry matter content of their diets. Efficiency of gain (0.53 ± 0.02 (±SD) g gain/g digestible dry matter intake) did not differ across diets. However, maintenance costs differed, ranging from 24 g·(kg0.75·day)-1 (120 kcal (1 cal = 4.1868 J) digestible energy (DE)·(kg0.75·day)-1) on the 35% protein pelleted diet to 80 g·(kg0.75·day)-1 (340 kcal DE·(kg0.75·day)-1) on fruit diets containing 2.3-5.6% protein (P = 0.0001). Supplementation of the fruit diet with additional protein increased mass gain but did not completely reverse the growth-depressing effect of the fruit-only diet. Protein limitations or other characteristics of fruit diets that increase energy metabolism and intake may be strategies that also directly benefit plants, by increasing either seed dispersal or propagation.

Effect of seasonal differences in dietary meat intake on changes in body mass and composition in wild and captive brown bears

The influence of seasonal dietary meat intake on changes in body mass and composition in wild and captive brown bears (Ursus arctos) was investigated because the importance and availability of meat to brown bear populations is currently an important management consideration in several North American ecosystems. Adult female brown bears on the Kenai Peninsula, Alaska, utilized meat heavily in both spring and fall. Meat accounted for 76.2 ± 26.0% (mean ± 1 SD; primarily moose carrion and calves) of assimilated carbon and nitrogen in the spring and 80.4 ± 22.2% (primarily salmon) in the fall. Mass increases in the spring (71.8 ± 28.2%) were mostly lean body mass, but increases in the fall (81.0 ± 19.5%) were primarily fat. Daily intake by captive brown bears fed meat ad libitum during 12-day trials was positively related to body mass. Mass change was positively related to intake in both seasons, but the composition of the gain varied by season, with spring gains primarily lean body mass (64.2 ± 9.4%), while fall gains were 78.8 ± 19.6% lipid. Absolute rates of gain by wild bears occasionally equaled, but were usually much less than, those of captive bears. This was likely due to a combination of factors, which included the time required to locate and handle meat resources, the limited availability of or access to meat resources, and (or) the duration of meat resource availability. Estimated intake by bears not feeding selectively on high-energy components of moose and salmon were 8.5 ± 1.5 kg/day and 541 ± 156 kg/year and 10.8 ± 4.6 kg/day and 1003 ± 489 kg/year, respectively. Intake would drop by as much as 58% for bears feeding exclusively on salmon roe. Management strategies for areas with brown bears that consume significant amounts of meat should address the perpetuation and availability of these meat resources.

The importance of meat, particularly salmon, to body size, population productivity, and conservation of North American brown bears

We hypothesized that the relative availability of meat, indicated by contribution to the diet, would be positively related to body size and population productivity of North American brown, or grizzly, bears (Ursus arctos). Dietary contributions of plant matter and meat derived from both terrestrial and marine sources were quantified by stable-isotope analysis (δ13C and δ15N) of hair samples from 13 brown bear populations. Estimates of adult female body mass, mean litter size, and population density were obtained from two field studies of ours and from other published reports. The populations ranged from largely vegetarian to largely carnivorous, and food resources ranged from mostly terrestrial to mostly marine (salmon, Oncorhynchus spp.). The proportion of meat in the diet was significantly correlated with mean adult female body mass (r = 0.87, P < 0.01), mean litter size (r = 0.72, P < 0.01), and mean population density (r = 0.91, P < 0.01). Salmon was the most important source of meat for the largest, most carnivorous bears and most productive populations. We conclude that availability of meat, particularly salmon, greatly influences habitat quality for brown bears at both the individual level and the population level.

Use of stable isotopes to determine diets of living and extinct bears

The potential use of stable-isotope analyses (δ13C and δ15N) to estimate bear diets was assessed in 40-day feeding trials using American black bears (Ursus americanus). Bear plasma and red blood cells have half-lives of ~4 days and ~28 days, respectively. The isotopic signature of bear plasma is linearly related to that of the diet, and with the exception of adipose tissue, there is no isotopic fractionation across bear tissues. Isotopic analyses were used to estimate the diets of three bear populations: Pleistocene cave bears (U. speleaus) in Europe, grizzly bears (Ursus arctos horribilis) inhabiting the Columbia River drainage prior to 1931, and brown bears (U. arctos) of Chichagof and Admiralty islands, Alaska. Cave bears were omnivores with terrestrially produced meat contributing from 41 to 78% (58 ± 14%) of their metabolized carbon and nitrogen. Salmon contributed from 33 to 90% (58 ± 23%) of the metabolized carbon and nitrogen in grizzly bears from the Columbia River drainage. Finally, most brown bears on Chichagof and Admiralty islands feed upon salmon during the late summer and fall; however, a subpopulation of bears exists that does not utilize salmon.

Diet selection by a ruminant generalist browser in relation to plant chemistry

Captive mule deer (Odocoileus hemionus hemionus) and black-tailed deer (O. h. sitkensis) were used in cafeteria-type, two choice feeding trials to test the hypothesis that digestible dry matter (energy) and nontannin phenolics of tree, shrub, and forb leaves are major determinants of diet preference. Deer selected plants in relation to a trade-off between the benefit derived from digestible dry matter and the cost of nontannin phenolics presumably associated with toxicity when absorbed. When one of the forages contained both the highest digestible dry matter and lowest nontannin phenolics, the deer always preferred that plant. When one forage had the highest digestible dry matter but the other plant had the lowest nontannin phenolics, the deer selected the high-energy plant when the difference in nontannin phenolics was relatively small, but they preferred the low-energy plant when the other forage had much higher levels of nontannin phenolics. Tannins influenced diet choice only as one of the factors reducing digestible dry matter in these forages. Apparently total dry-matter intake was constrained by the nontannin phenolic fraction but not by tannins. Tannins and nontannin phenolics both contribute to defending plants against browsers.

Adaptive rumen function in elk (Cervus elaphus nelsoni) and mule deer (Odocoileus hemionus hemionus)

We tested the hypothesis that rumen function is adaptive to diet quality and intake rate using ruminally fistulated elk and mule deer. In experiment 1 we measured rumen particle-size distribution, rumen fill, and particle and liquid passage rates of animals fed three diets varying in quality (chopped pea, alfalfa, and wheat hays). In experiment 2, similar measurements were obtained on elk fed alfalfa hay ad libitum or at restricted intake levels. Rumen characteristics and passage rates of particles and liquids were similar for animals consuming alfalfa and pea hays. Intake, rumen dry-matter concentration and fill, and liquid passage rate were significantly lower when animals consumed wheat hay. Few significant differences in rumen characteristics or passage rates were found between animals fed alfalfa ad libitum or at restricted levels. Rumen liquid volume and dry-matter fill were related linearly to intake (r2 = 0.98 for both) in deer and elk fed alfalfa and pea hays. However, liquid volume and dry-matter fill of elk fed wheat hay and alfalfa at restricted levels were higher than the deer–elk interspecific regression, indicating an adaptive ruminal response. We concluded that rumen function was adaptive to both diet quality and availability, but that the response likely was subject to the limitations imposed by food characteristics and the inherent limitations of rumen structure and function.