Hibernating bear serum hinders osteoclastogenesis in-vitro

Bears do not suffer from osteoporosis during hibernation, which is associated with long-term inactivity, lack of food intake, and cold exposure. However, the mechanisms involved in bone loss prevention have scarcely been elucidated in bears. We investigated the effect of serum from hibernating Japanese black bears (Ursus thibetanus japonicus) on differentiation of peripheral blood mononuclear cells (PBMCs) to osteoclasts (OCs). PBMCs collected from 3 bears were separately cultured with 10% serum of 4 active and 4 hibernating bears (each individual serum type was assessed separately by a bear PBMCs), and differentiation were induced by treatment with macrophage colony stimulating factor (M-CSF) and receptor activator of NF-kB ligand (RANKL). PBMCs that were cultured with the active bear serum containing medium (ABSM) differentiated to multi-nucleated OCs, and were positive for TRAP stain. However, cells supplemented with hibernating bear serum containing medium (HBSM) failed to form OCs, and showed significantly lower TRAP stain (p < 0.001). On the other hand, HBSM induced proliferation of adipose derived mesenchymal stem cells (ADSCs) similarly to ABSM (p > 0.05), indicating no difference on cell growth. It was revealed that osteoclastogenesis of PBMCs is hindered by HBSM, implying an underlying mechanism for the suppressed bone resorption during hibernation in bears. In addition, this study for the first time showed the formation of bears’ OCs in-vitro.

Life in the fat lane: seasonal regulation of insulin sensitivity, food intake, and adipose biology in brown bears

Grizzly bears (Ursus arctos horribilis) have evolved remarkable metabolic adaptations including enormous fat accumulation during the active season followed by fasting during hibernation. However, these fluctuations in body mass do not cause the same harmful effects associated with obesity in humans. To better understand these seasonal transitions, we performed insulin and glucose tolerance tests in captive grizzly bears, characterized the annual profiles of circulating adipokines, and tested the anorectic effects of centrally administered leptin at different times of the year. We also used bear gluteal adipocyte cultures to test insulin and beta-adrenergic sensitivity in vitro. Bears were insulin resistant during hibernation but were sensitive during the spring and fall active periods. Hibernating bears remained euglycemic, possibly due to hyperinsulinemia and hyperglucagonemia. Adipokine concentrations were relatively low throughout the active season but peaked in mid-October prior to hibernation when fat content was greatest. Serum glycerol was highest during hibernation, indicating ongoing lipolysis. Centrally administered leptin reduced food intake in October, but not in August, revealing seasonal variation in the brain's sensitivity to its anorectic effects. This was supported by strong phosphorylated signal transducer and activator of transcription 3 labeling within the hypothalamus of hibernating bears; labeling virtually disappeared in active bears. Adipocytes collected during hibernation were insulin resistant when cultured with hibernation serum but became sensitive when cultured with active season serum. Heat treatment of active serum blocked much of this action. Clarifying the cellular mechanisms responsible for the physiology of hibernating bears may inform new treatments for metabolic disorders.

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.

Changes in expression of hepatic genes involved in lipid metabolism during prehibernation period in captive adult female Japanese black bears (Ursus thibetanus japonicus)

Body fat accumulation in the prehibernation period is crucial for survival and reproduction during hibernation for bears. Bear body mass increases rapidly during their autumnal hyperphagia phase, which is attributed not only to an increase in food availability, but also to physiological changes in lipid metabolism. To test this hypothesis, we investigated changes in blood biochemical values and mRNA expression levels of hepatic genes involved in lipid metabolism during the active period (June, August, October, and November) in Japanese black bears ( Ursus thibetanus japonicus Schlegel, 1857), which were fed a constant ration throughout this period. Blood biochemical analysis revealed that plasma triglyceride concentrations decreased in October and November, implying that peripheral triglyceride uptake was accelerated in autumn. The liver was sampled by needle biopsy. Real-time polymerase chain reaction (PCR) analysis revealed that mRNA expressions of enzymes involved in glycolysis (glucokinase), as well as fatty acid and triglyceride synthesis (ATP-citrate lyase, acetyl-CoA carboxykinase 1, fatty acid synthase, and diacylglycerol O-acyltransferase 2), increased in November, which suggests that hepatic lipogenesis becomes accelerated during the hyperphagia phase. These results suggest that lipid metabolism is seasonally controlled even without changes in food intake. These physiological changes seen in the prehibernation period would contribute to the rapid mass gain necessary for hibernation.

Investigating the mechanism for maintaining eucalcemia despite immobility and anuria in the hibernating American black bear (Ursus americanus)

Ursine hibernation uniquely combines prolonged skeletal unloading, anuria, pregnancy, lactation, protein recycling, and lipolysis. This study presents a radiographic and biochemical picture of bone metabolism in free-ranging, female American black bears (Ursus americanus) that were active (spring bears and autumn bears) or hibernating (hibernating bears). Hibernating bears included lactating and non-lactating individuals. We measured serum calcium, albumin, inorganic phosphate, creatinine, bone specific alkaline phosphatase (BSALP), CTX, parathyroid hormone, insulin-like growth factor-I (IGF-l), leptin, 25-hydroxyvitamin D [25(OH)D], 1,25-dihydroxyvitamin D [1,25(OH)(2)D] and sclerostin from 35 to 50 tranquilized hibernating bears and 14 to 35 tranquilized spring bears. We compared metacarpal cortical indices (MCI), measured by digital X-ray radiogrammetry, from 60 hunter-killed autumn bears and 79 tranquilized, hibernating bears. MCI was greater in autumn than winter in younger bears, but showed no seasonal difference in older bears. During hibernation eucalcemia was maintained, BSALP was suppressed, and CTX was in the range expected for anuria. During hibernation 1,25(OH)(2)D was produced despite anuria. 1,25(OH)(2)D and IGF-I were less in hibernating than spring bears. In a quarter of hibernating bears, sclerostin was elevated. Leptin was greater in hibernating than spring bears. In hibernating bears, leptin correlated positively with BSALP in non-lactating bears and with CTX in lactating bears. Taken together the biochemical and radiographic findings indicate that during hibernation, bone turnover was persistent, balanced, and suppressed; bone resorption was lower than expected for an unloaded skeleton; and there was no unloading-induced bone loss. The skeleton appears to perceive that it was loaded when it was actually unloaded during hibernation. However, at the level of sclerostin, the skeleton recognized that it was unloaded. During hibernation leptin appeared anabolic in non-lactating bears and catabolic in lactating bears. We hypothesize that ursine hibernation may represent a natural model in which suppression of the sympathetic nervous system prevents unloading-induced bone loss by influencing leptin's skeletal effects and preventing transmission of loading information.