Parathyroid hormone may maintain bone formation in hibernating black bears (Ursus americanus) to prevent disuse osteoporosis

Bone adaptation and osteoporosis prevention in hibernating mammals

Hibernating bears and rodents have evolved mechanisms to prevent disuse osteoporosis during the prolonged physical inactivity that occurs during hibernation. Serum markers and histological indices of bone remodeling in bears indicate reduced bone turnover during hibernation, which is consistent with organismal energy conservation. Calcium homeostasis is maintained by balanced bone resorption and formation since hibernating bears do not eat, drink, urinate, or defecate. Reduced and balanced bone remodeling protect bear bone structure and strength during hibernation, unlike the disuse osteoporosis that occurs in humans and other animals during prolonged physical inactivity. Conversely, some hibernating rodents show varying degrees of bone loss such as osteocytic osteolysis, trabecular loss, and cortical thinning. However, no negative effects of hibernation on bone strength in rodents have been found. More than 5000 genes in bear bone tissue are differentially expressed during hibernation, highlighting the complexity of hibernation induced changes in bone. A complete picture of the mechanisms that regulate bone metabolism in hibernators still alludes us, but existing data suggest a role for endocrine and paracrine factors such as cocaine- and amphetamine-regulated transcript (CART) and endocannabinoid ligands like 2-arachidonoyl glycerol (2-AG) in decreasing bone remodeling during hibernation. Hibernating bears and rodents evolved the capacity to preserve bone strength during long periods of physical inactivity, which contributes to their survival and propagation by allowing physically activity (foraging, escaping predators, and mating) without risk of bone fracture following hibernation. Understanding the biological mechanisms regulating bone metabolism in hibernators may inform novel treatment strategies for osteoporosis in humans.

Circulating insulin-like growth factor system adaptations in hibernating brown bears indicate increased tissue IGF availability

Brown bears conserve muscle and bone mass during 6 mo of inactive hibernation. The molecular mechanisms underlying hibernation physiology may have translational relevance for human therapeutics. We hypothesize that protective mechanisms involve increased tissue availability of insulin-like growth factors (IGFs). In subadult Scandinavian brown bears, we observed that mean plasma IGF-1 and IGF-2 levels during hibernation were reduced to 36 ± 10% and 56 ± 15%, respectively, compared with the active state (n = 12). Western ligand blotting identified IGF-binding protein (IGFBP)-3 as the major IGFBP in the active state, whereas IGFBP-2 was codominant during hibernation. Acid labile subunit (ALS) levels in hibernation were reduced to 41±16% compared with the active state (n = 6). Analysis of available grizzly bear RNA sequencing data revealed unaltered liver mRNA IGF-1, IGFBP-2, and IGFBP-3 levels, whereas ALS levels were significantly reduced during hibernation (n = 6). Reduced ALS synthesis and circulating levels during hibernation should prompt a shift from ternary IGF/IGFBP/ALS to smaller binary IGF/IGFBP complexes, thereby increasing IGF tissue availability. Indeed, size-exclusion chromatography of bear plasma demonstrated a shift to lower molecular weight IGF-containing complexes in the hibernating versus the active state. Furthermore, we note that the major IGF-2 mRNA isoform expressed in livers in both Scandinavian brown bears and grizzly bears was an alternative splice variant in which Ser29 is replaced with a tetrapeptide possessing a positively charged Arg residue. Homology modeling of the bear IGF-2/IGFBP-2 complex showed the tetrapeptide in proximity to the heparin-binding domain involved in bone-specific targeting of this complex. In conclusion, this study provides data which suggest that increased IGF tissue availability combined with tissue-specific targeting contribute to tissue preservation in hibernating bears.NEW & NOTEWORTHY Brown bears shift from circulating ternary IGF/IGFBP/ALS complexes in the active state to binary IGF/IGFBP complexes during hibernation, indicating increased tissue IGF-bioactivity. Furthermore, brown bears use a splice variant of IGF-2, suggesting increased bone-specific targeting of IGF anabolic signaling.

Resistance to disuse-induced iron overload in Daurian ground squirrels (Spermophilus dauricus) during extended hibernation inactivity

Iron overload occurs in disuse-induced osteoporosis. Hibernators are a natural animal model of resistance to disuse osteoporosis. We hypothesized that hibernators avoid iron overload to resist disuse-induced osteoporosis. Here, the role of iron metabolism in resistance to disuse osteoporosis was investigated by studying differences in iron content and iron metabolism in the femurs and livers of Daurian ground squirrels (Spermophilus dauricus) between the summer active and torpid states. Results showed that the femurs were generally well-maintained during torpor, with no significant differences observed in most bone microstructural parameters, except for a significantly lower (by 40%) trabecular bone connection density. Femur and liver iron concentrations were significantly lower during torpor (by 59% and 49%, respectively). Based on histological staining, livers were iron-negative and femurs showed a reduction in iron-positive area (by 83%) during torpor; The number of osteoblasts and osteoclasts showed no significant differences between the two groups. Most iron metabolism/homeostasis proteins expression levels in the femur and liver showed no significant differences between the two groups, with their stable expression likely preventing iron overload during inactivity. Higher femoral transferrin receptor 1 (TfR1) expression (by 108%) and lower liver ferritin expression (by 45%) were found in torpid squirrels. Lower liver ferritin may be related to the lower iron content, with the elevation in femoral TfR1 potentially related to restoration of bone iron levels. In conclusion, despite long periods of inactivity, iron levels in the femur and liver of squirrels were lower, bone formation and resorption were balanced and no iron overload was observed, as is found under disuse conditions in non-hibernators. Therefore, avoiding iron overload may be a potential mechanism for hibernators to avoid disuse-induced bone loss.

Transcriptional changes and preservation of bone mass in hibernating black bears

Physical inactivity leads to losses of bone mass and strength in most mammalian species. In contrast, hibernating bears show no bone loss over the prolonged periods (4–6 months) of immobility during winter, which suggests that they have adaptive mechanisms to preserve bone mass. To identify transcriptional changes that underlie molecular mechanisms preventing disuse osteoporosis, we conducted a large-scale gene expression screening in the trabecular bone and bone marrow, comparing hibernating and summer active bears through sequencing of the transcriptome. Gene set enrichment analysis showed a coordinated down-regulation of genes involved in bone resorption, osteoclast differentiation and signaling, and apoptosis during hibernation. These findings are consistent with previous histological findings and likely contribute to the preservation of bone during the immobility of hibernation. In contrast, no significant enrichment indicating directional changes in gene expression was detected in the gene sets of bone formation and osteoblast signaling in hibernating bears. Additionally, we revealed significant and coordinated transcriptional induction of gene sets involved in aerobic energy production including fatty acid beta oxidation, tricarboxylic acid cycle, oxidative phosphorylation, and mitochondrial metabolism. Mitochondrial oxidation was likely up-regulated by transcriptionally induced AMPK/PGC1α pathway, an upstream stimulator of mitochondrial function.

Body Protein Sparing in Hibernators: A Source for Biomedical Innovation

Proteins are not only the major structural components of living cells but also ensure essential physiological functions within the organism. Any change in protein abundance and/or structure is at risk for the proper body functioning and/or survival of organisms. Death following starvation is attributed to a loss of about half of total body proteins, and body protein loss induced by muscle disuse is responsible for major metabolic disorders in immobilized patients, and sedentary or elderly people. Basic knowledge of the molecular and cellular mechanisms that control proteostasis is continuously growing. Yet, finding and developing efficient treatments to limit body/muscle protein loss in humans remain a medical challenge, physical exercise and nutritional programs managing to only partially compensate for it. This is notably a major challenge for the treatment of obesity, where therapies should promote fat loss while preserving body proteins. In this context, hibernating species preserve their lean body mass, including muscles, despite total physical inactivity and low energy consumption during torpor, a state of drastic reduction in metabolic rate associated with a more or less pronounced hypothermia. The present review introduces metabolic, physiological, and behavioral adaptations, e.g., energetics, body temperature, and nutrition, of the torpor or hibernation phenotype from small to large mammals. Hibernating strategies could be linked to allometry aspects, the need for periodic rewarming from torpor, and/or the ability of animals to fast for more or less time, thus determining the capacity of individuals to save proteins. Both fat- and food-storing hibernators rely mostly on their body fat reserves during the torpid state, while minimizing body protein utilization. A number of them may also replenish lost proteins during arousals by consuming food. The review takes stock of the physiological, molecular, and cellular mechanisms that promote body protein and muscle sparing during the inactive state of hibernation. Finally, the review outlines how the detailed understanding of these mechanisms at play in various hibernators is expected to provide innovative solutions to fight human muscle atrophy, to better help the management of obese patients, or to improve the ex vivo preservation of organs.

Impact of Release Kinetics on Efficacy of Locally Delivered Parathyroid Hormone for Bone Regeneration Applications

Characterizing the release profile for materials-directed local delivery of bioactive molecules and its effect on bone regeneration is an important step to improve our understanding of, and ability to optimize, the bone healing response. This study examined the local delivery of parathyroid hormone (PTH) using a thiol-ene hydrogel embedded in a porous poly(propylene fumarate) (PPF) scaffold for bone regeneration applications. The aim of this study was to characterize the degradation-controlled in vitro release kinetics of PTH from the thiol-ene hydrogels, in vivo hydrogel degradation in a subcutaneous implant model, and bone healing in a rat critical size bone defect. Tethering PTH to the hydrogel matrix eliminated the early timepoint burst release that was observed in previous in vitro work where PTH was free to diffuse out of the matrix. Only 8% of the tethered PTH was released from the hydrogel during the first 2 weeks, but by day 21, 80% of the PTH was released, and complete release was achieved by day 28. In vivo implantation revealed that complete degradation of the hydrogel alone occurred by day 21; however, when incorporated in a three-dimensional printed osteoconductive PPF scaffold, the hydrogel persisted for >56 days. Treatment of bone defects with the composite thiol-ene hydrogel-PPF scaffold, delivering either 3 or 10 μg of tethered PTH 1-84, was found to increase bridging of critical size bone defects, whereas treatment with 30 μg of tethered PTH resulted in less bone ingrowth into the defect area. Continued development of this biomaterial delivery system for PTH could lead to improved therapies for treatment of nonunion fractures and critical size bone defects.

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.

Thiol-ene Hydrogels for Local Delivery of PTH for Bone Regeneration in Critical Size defects

Neither allograft nor commercially available bone graft substitutes provide the same quality of bone healing as autograft. Incorporation of bioactive molecules like parathyroid hormone (PTH) within bone graft substitute materials may provide similar, if not better treatment options to grafting. The goal of this work was to develop a biomaterial system for the local delivery of PTH to large bone defects for promoting bone regeneration. PTH was loaded in a thiol-ene hydrogel at several concentrations and polymerized in and around an osteoconductive poly(propylene fumarate) (PPF) scaffold. PTH was shown to be bioactive when released from the hydrogel for up to 21 days. Eighty percent of the PTH was released by day 3 with the remaining 20% released by day 14. Bone healing was quantified in rat critical size femoral defects that were treated with hydrogel/PPF and 0, 1, 3, 10, or 30 µg of PTH. Although complete osseous healing was not observed in all samples in any one treatment group, all samples in the 10 µg PTH group were bridged fully by bone or a combination of bone and cartilage containing hypertrophic chondrocytes and endochondral ossification. Outcome measures indicated improved defect bridging by a combination of bony and cartilaginous tissue in the 10 μg treatment group compared with empty bone defects and defects treated with only hydrogel/PPF (i.e., without PTH). Given the tailorability of the hydrogel, future studies will investigate the effects of prolonged gradual PTH release on bone healing. © 2019 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 38:536-544, 2020.

Cardiac adaptation in hibernating, free-ranging Scandinavian Brown Bears (Ursus arctos)

During six months of annual hibernation, the brown bear undergoes unique physiological changes to adapt to decreased metabolic rate. We compared cardiac structural and functional measures of hibernating and active bears using comprehensive echocardiography. We performed echocardiography on 13 subadult free-ranging, anaesthetised Scandinavian brown bears (Ursus arctos) during late hibernation and in early summer. Mean heart rate was 26 beats per minute (standard deviation (SD): 8) during hibernation vs 71 (SD: 14) during active state. All left ventricular (LV) systolic and diastolic measures were decreased during hibernation: mean ejection fraction: 44.2% (SD: 6.0) active state vs 34.0 (SD: 8.1) hibernation, P = 0.001; global longitudinal strain: −11.2% (SD: 2.0) vs −8.8 (SD: 3.3), P = 0.03; global longitudinal strain rate: −0.82 (SD: 0.15) vs −0.41 (SD: 0.18), P < 0.001; septal e’: 9.8 cm/s (SD: 1.8) vs 5.2 (SD: 2.7), P < 0.001. In general, measures of total myocardial motion (ejection fraction and global longitudinal strain) were decreased to a lesser extent than measures of myocardial velocities. In the hibernating brown bear, cardiac adaptation included decreased functional measures, primarily measures of myocardial velocities, but was not associated with cardiac atrophy. Understanding the mechanisms of these adaptations could provide pathophysiological insight of human pathological conditions such as heart failure.

Assessment of the LeadCare® Plus for Use on Scandinavian Brown Bears (Ursus arctos)

Lead (Pb) exposure is associated with adverse health effects in both humans and wildlife. Blood lead levels (BLL) of sentinel wildlife species can be used to monitor environmental lead exposure and ecosystem health. BLL analyzers, such as the LeadCare®, are validated for use in humans, assessed for use in some avian species and cattle, and are increasingly being used on wildlife to monitor lead exposure. The LeadCare® analyzers use a technique called anodic stripping voltammetry (ASV). Species-specific conversion equations have been proposed to approximate the levels found with gold standard measuring methods such as inductively coupled plasma mass spectrometry (ICP-MS) because the ASV method has been shown to underestimate BLL in some species. In this study we assessed the LeadCare® Plus (LCP) for use on Scandinavian brown bears (Ursus arctos). LCP measurements were correlated with ICP-MS with a Bland-Altman analyzed bias of 16.3-22.5%, showing a consistent overestimation of BLL analyzed with LCP. Based on this analysis we provide conversion equations for calculating ICP-MS BLL based on the LCP results in Scandinavian brown bears. Our study shows that the LeadCare® Plus can be used for monitoring of lead exposure by approximating gold standard levels using conversion equations. This enables comparison with other gold standard measured BLL within the observed range of this study (38.20-174.00 μg/L). Our study also found that Scandinavian brown bears are highly exposed to environmental lead.

Countering the Modern Metabolic Disease Rampage With Ancestral Endocannabinoid System Alignment

When primitive vertebrates evolved from ancestral members of the animal kingdom and acquired complex locomotive and neurological toolsets, a constant supply of energy became necessary for their continued survival. To help fulfill this need, the endocannabinoid (eCB) system transformed drastically with the addition of the cannabinoid-1 receptor (CB1R) to its gene repertoire. This established an eCB/CB1R signaling mechanism responsible for governing the whole organism's energy balance, with its activation triggering a shift toward energy intake and storage in the brain and the peripheral organs (i.e., liver and adipose). Although this function was of primal importance for humans during their pre-historic existence as hunter-gatherers, it became expendable following the successive lifestyle shifts of the Agricultural and Industrial Revolutions. Modernization of the world has further increased food availability and decreased energy expenditure, thus shifting the eCB/CB1R system into a state of hyperactive deregulated signaling that contributes to the 21st century metabolic disease pandemic. Studies from the literature supporting this perspective come from a variety of disciplines, including biochemistry, human medicine, evolutionary/comparative biology, anthropology, and developmental biology. Consideration of both biological and cultural evolution justifies the design of improved pharmacological treatments for obesity and Type 2 diabetes (T2D) that focus on peripheral CB1R antagonism. Blockade of peripheral CB1Rs, which universally promote energy conservation across the vertebrate lineage, represents an evolutionary medicine strategy for clinical management of present-day metabolic disorders.

PTH (1-34) and growth hormone in prevention of disuse osteopenia and sarcopenia in rats

Osteopenia and sarcopenia develops rapidly during disuse. The study investigated whether intermittent parathyroid hormone (1-34) (PTH) and growth hormone (GH) administered alone or in combination could prevent or mitigate disuse osteopenia and sarcopenia in rats. Disuse was achieved by injecting 4IU botulinum toxin A (BTX) into the right hindlimb musculature of 12-14-week-old female Wistar rats. Seventy-two rats were divided into six groups: 1. Baseline; 2. Ctrl; 3. BTX; 4. BTX+GH; 5. BTX+PTH; 6. BTX+PTH+GH. PTH (1-34) (60μg/kg/day) and GH (5mg/kg/day). The animals were sacrificed after 6weeks of treatment. Sarcopenia was established by histomorphometry, while the skeletal properties were determined using DXA, μCT, mechanical testing, and dynamic bone histomorphometry. Disuse resulted in lower muscle mass (-63%, p<0.05), trabecular BV/TV (-28%, p<0.05), Tb.Th (-11%, p<0.05), lower diaphyseal cortical thickness (-10%, p<0.001), and lower bone strength at the distal femoral metaphysis (-27%, p<0.001) compared to Ctrl animals. PTH fully counteracted the immobilization-induced lower BV/TV, Tb.Th, and distal femoral metaphyseal strength. GH increased muscle mass (+17%, p<0.05) compared to BTX, but did not prevent the immobilization-induced loss of bone strength, BV/TV, and cortical trabecular thickness. Combination of PTH and GH increased distal femoral metaphyseal bone strength (+45%, p<0.001), BV/TV (+50%, p<0.05), Tb.Th (+40%, p<0.05), and whole femoral aBMD (+15%, p<0.001) compared to BTX and muscle mass (+21%, p<0.05) compared to BTX+PTH. In conclusion, PTH and GH in combination is more efficient at preventing the disuse-related deterioration of bone strength, density, and micro-architecture than either PTH or GH given as monotherapy. Furthermore, GH, either alone or in combination with PTH, attenuated disuse-induced loss of muscle mass. The combination of PTH and GH resulted in a more effective treatment than PTH and GH as monotherapy.

Novel treatment strategies for chronic kidney disease: insights from the animal kingdom

Many of the >2 million animal species that inhabit Earth have developed survival mechanisms that aid in the prevention of obesity, kidney disease, starvation, dehydration and vascular ageing; however, some animals remain susceptible to these complications. Domestic and captive wild felids, for example, show susceptibility to chronic kidney disease (CKD), potentially linked to the high protein intake of these animals. By contrast, naked mole rats are a model of longevity and are protected from extreme environmental conditions through mechanisms that provide resistance to oxidative stress. Biomimetic studies suggest that the transcription factor nuclear factor erythroid 2-related factor 2 (NRF2) offers protection in extreme environmental conditions and promotes longevity in the animal kingdom. Similarly, during months of fasting, immobilization and anuria, hibernating bears are protected from muscle wasting, azotaemia, thrombotic complications, organ damage and osteoporosis - features that are often associated with CKD. Improved understanding of the susceptibility and protective mechanisms of these animals and others could provide insights into novel strategies to prevent and treat several human diseases, such as CKD and ageing-associated complications. An integrated collaboration between nephrologists and experts from other fields, such as veterinarians, zoologists, biologists, anthropologists and ecologists, could introduce a novel approach for improving human health and help nephrologists to find novel treatment strategies for CKD.

Raccoon dog model shows preservation of bone during prolonged catabolism and reduced physical activity

The raccoon dog (Nyctereutes procyonoides) is a promising animal model capable of preventing disuse-induced osteoporosis. Previous data suggest that this species resembles bears in the preservation of bone mass and biomechanical properties during prolonged passivity and catabolism. This longitudinal study examined the osteological properties of tibiae in farm-bred raccoon dogs that were either fed or fasted (n=6 per group) for a 10 week period. Peripheral quantitative computed tomography was utilized and plasma markers of bone turnover measured before fasting and at 9 weeks followed by mechanical testing (three-point bending), micro-computed tomography and Fourier transform infrared imaging at 10 weeks. Passive wintering with prolonged catabolism (body mass loss 32%) had no significant effects on bone mineralization, porosity or strength. The concentration of C-terminal telopeptide of type I collagen, indicative of bone resorption, increased in the plasma of the fasted raccoon dogs, while the bone formation markers were unchanged. The levels of 25-hydroxyvitamin D were reduced in the fasted animals. Based on these data, the preservation of bone in wintering raccoon dogs shares characteristics with that of bears with no apparent decrease in the formation of bone but increased resorption. To conclude, raccoon dogs were able to minimize bone loss during a 10 week period of catabolism and passivity.

My Scientific Journey: From an Agrarian Start to an Academic Setting

This article is a combination of an autobiography and a review of outstanding research done by over 70 graduate students, postdoctoral fellows, and visiting scientists along with excellent collaborators during my over-40-year career as a professor of reproductive physiology at the University of Illinois, Urbana-Champaign. I have also shared thoughts on mentoring, how research has changed over the years, and the future of reproductive physiology. I provide the reader with a snapshot of the challenges faced by a woman eager to obtain a PhD under the guidance of renowned professors in the early 1970s and to be hired as the first woman, and the only permanent female faculty member, for more than 20 years on a faculty of 40 men. As a comparative reproductive physiologist, I describe the various animal models used because they were the best models to answer specific questions in reproduction. Also, my graduate students and postdoctoral fellows were given the freedom to identify their research topics, articulate hypotheses to be tested, and select appropriate animal models. This approach caused students to take ownership of their research, resulting in the development of independent and creative scientists and over 170 publications, excluding chapters in top-tier journals. Finally, I am so grateful for a truly rich life mentoring graduate students and postdoctoral fellows who have become my lifelong friends.

Muscle-bone interactions: From experimental models to the clinic? A critical update

Bone is a biomechanical tissue shaped by forces from muscles and gravitation. Simultaneous bone and muscle decay and dysfunction (osteosarcopenia or sarco-osteoporosis) is seen in ageing, numerous clinical situations including after stroke or paralysis, in neuromuscular dystrophies, glucocorticoid excess, or in association with vitamin D, growth hormone/insulin like growth factor or sex steroid deficiency, as well as in spaceflight. Physical exercise may be beneficial in these situations, but further work is still needed to translate acceptable and effective biomechanical interventions like vibration therapy from animal models to humans. Novel antiresorptive and anabolic therapies are emerging for osteoporosis as well as drugs for sarcopenia, cancer cachexia or muscle wasting disorders, including antibodies against myostatin or activin receptor type IIA and IIB (e.g. bimagrumab). Ideally, increasing muscle mass would increase muscle strength and restore bone loss from disuse. However, the classical view that muscle is unidirectionally dominant over bone via mechanical loading is overly simplistic. Indeed, recent studies indicate a role for neuronal regulation of not only muscle but also bone metabolism, bone signaling pathways like receptor activator of nuclear factor kappa-B ligand (RANKL) implicated in muscle biology, myokines affecting bone and possible bone-to-muscle communication. Moreover, pharmacological strategies inducing isolated myocyte hypertrophy may not translate into increased muscle power because tendons, connective tissue, neurons and energy metabolism need to adapt as well. We aim here to critically review key musculoskeletal molecular pathways involved in mechanoregulation and their effect on the bone-muscle unit as a whole, as well as preclinical and emerging clinical evidence regarding the effects of sarcopenia therapies on osteoporosis and vice versa.

Prevention of muscle wasting and osteoporosis: the value of examining novel animal models

Bone mass and skeletal muscle mass are controlled by factors such as genetics, diet and nutrition, growth factors and mechanical stimuli. Whereas increased mechanical loading of the musculoskeletal system stimulates an increase in the mass and strength of skeletal muscle and bone, reduced mechanical loading and disuse rapidly promote a decrease in musculoskeletal mass, strength and ultimately performance (i.e. muscle atrophy and osteoporosis). In stark contrast to artificially immobilised laboratory mammals, animals that experience natural, prolonged bouts of disuse and reduced mechanical loading, such as hibernating mammals and aestivating frogs, consistently exhibit limited or no change in musculoskeletal performance. What factors modulate skeletal muscle and bone mass, and what physiological and molecular mechanisms protect against losses of muscle and bone during dormancy and following arousal? Understanding the events that occur in different organisms that undergo natural periods of prolonged disuse and suffer negligible musculoskeletal deterioration could not only reveal novel regulatory factors but also might lead to new therapeutic options. Here, we review recent work from a diverse array of species that has revealed novel information regarding physiological and molecular mechanisms that dormant animals may use to conserve musculoskeletal mass despite prolonged inactivity. By highlighting some of the differences and similarities in musculoskeletal biology between vertebrates that experience disparate modes of dormancy, it is hoped that this Review will stimulate new insights and ideas for future studies regarding the regulation of atrophy and osteoporosis in both natural and clinical models of muscle and bone disuse.

Suppressed bone remodeling in black bears conserves energy and bone mass during hibernation

Decreased physical activity in mammals increases bone turnover and uncouples bone formation from bone resorption, leading to hypercalcemia, hypercalcuria, bone loss and increased fracture risk. Black bears, however, are physically inactive for up to 6 months annually during hibernation without losing cortical or trabecular bone mass. Bears have been shown to preserve trabecular bone volume and architectural parameters and cortical bone strength, porosity and geometrical properties during hibernation. The mechanisms that prevent disuse osteoporosis in bears are unclear as previous studies using histological and serum markers of bone remodeling show conflicting results. However, previous studies used serum markers of bone remodeling that are known to accumulate with decreased renal function, which bears have during hibernation. Therefore, we measured serum bone remodeling markers (BSALP and TRACP) that do not accumulate with decreased renal function, in addition to the concentrations of serum calcium and hormones involved in regulating bone remodeling in hibernating and active bears. Bone resorption and formation markers were decreased during hibernation compared with when bears were physically active, and these findings were supported by histomorphometric analyses of bone biopsies. The serum concentration of cocaine and amphetamine regulated transcript (CART), a hormone known to reduce bone resorption, was 15-fold higher during hibernation. Serum calcium concentration was unchanged between hibernation and non-hibernation seasons. Suppressed and balanced bone resorption and formation in hibernating bears contributes to energy conservation, eucalcemia and the preservation of bone mass and strength, allowing bears to survive prolonged periods of extreme environmental conditions, nutritional deprivation and anuria.

Combination Therapy with Zoledronic Acid and Parathyroid Hormone Improves Bone Architecture and Strength following a Clinically-Relevant Dose of Stereotactic Radiation Therapy for the Local Treatment of Canine Osteosarcoma in Athymic Rats

Clinical studies using definitive-intent stereotactic radiation therapy (SRT) for the local treatment of canine osteosarcoma (OSA) have shown canine patients achieving similar median survival times as the current standard of care (amputation and adjuvant chemotherapy). Despite this, there remains an unacceptable high risk of pathologic fracture following radiation treatment. Zoledronic acid (ZA) and parathyroid hormone (PTH) are therapeutic candidates for decreasing this fracture risk post-irradiation. Due to differing mechanisms, we hypothesized that the combined treatment with ZA and PTH would significantly improve bone healing more than ZA or PTH treatment alone. Using an orthotopic model of canine osteosarcoma in athymic rats, we evaluated bone healing following clinically-relevant doses of radiation therapy (12 Gy x 3 fractions, 36 Gy total). Groups included 36 Gy SRT only, 36 Gy SRT plus ZA, 36 Gy SRT plus ZA and PTH, 36 Gy SRT plus PTH, and 36 Gy SRT plus localized PTH treatment. Our study showed significant increases in bone volume and increased polar moments of inertia (in the distal femoral metaphysis) 8 weeks after radiation in the combined (ZA/PTH) treatment group as compared to radiation treatment alone. Histomorphometric analysis revealed evidence of active mineralization at the study endpoint as well as successful tumor-cell kill across all treatment groups. This work provides further evidence for the expanding potential indications for ZA and PTH therapy, including post-irradiated bone disease due to osteosarcoma.

Arctic Ground Squirrels Limit Bone Loss during the Prolonged Physical Inactivity Associated with Hibernation

Prolonged disuse (e.g., physical inactivity) typically results in increased bone porosity, decreased mineral density, and decreased bone strength, leading to increased fracture risk in many mammals. However, bears, marmots, and two species of ground squirrels have been shown to preserve macrostructural bone properties and bone strength during long seasons of hibernation while they remain mostly inactive. Some small hibernators (e.g., 13-lined ground squirrels) show microstructural bone loss (i.e., osteocytic osteolysis) during hibernation, which is not seen in larger hibernators (e.g., bears and marmots). Arctic ground squirrels (Urocitellus parryii) are intermediate in size between 13-lined ground squirrels and marmots and are perhaps the most extreme rodent hibernator, hibernating for up to 8 mo annually with body temperatures below freezing. The goal of this study was to quantify the effects of hibernation and inactivity on cortical and trabecular bone properties in arctic ground squirrels. Cortical bone geometrical properties (i.e., thickness, cross-sectional area, and moment of inertia) at the midshaft of the femur were not different in animals sampled over the hibernation and active seasons. Femoral ultimate stress tended to be lower in hibernators than in summer animals, but toughness was not affected by hibernation. The area of osteocyte lacunae was not different between active and hibernating animals. There was an increase in osteocytic lacunar porosity in the hibernation group due to increased lacunar density. Trabecular bone volume fraction in the proximal tibia was unexpectedly greater in the hibernation group than in the active group. This study shows that, similar to other hibernators, arctic ground squirrels are able to preserve many bone properties during hibernation despite being physically inactive for up to 8 mo.

Insights from the Den: How Hibernating Bears May Help Us Understand and Treat Human Disease

Hibernating brown bears (Ursus arctos) and black bears (Ursus americanus) spend half of the year in a physically inactive state inside their winter dens without food intake and defecating and no or little urination. Under similar extreme conditions, humans would suffer from loss of lean body mass, heart failure, thrombosis, azotemia, osteoporosis, and more. However, bears exit the den in the spring strong without organ injuries. Translational animal models are used in human medicine but traditional experimental animals have several shortcomings; thus, we believe that it is time to systematically explore new models. In this review paper, we describe physiological adaptations of hibernating bears and how similar adaptations in humans could theoretically alleviate medical conditions. The bear has solved most of the health challenges faced by humans, including heart and kidney disease, atherosclerosis and thrombosis, and muscle wasting and osteoporosis. Understanding and applying this library of information could lead to a number of major discoveries that could have implications for the understanding and treatment of human disease.

Evidence for the adverse effect of starvation on bone quality: a review of the literature

Malnutrition and starvation's possible adverse impacts on bone health and bone quality first came into the spotlight after the horrors of the Holocaust and the ghettos of World War II. Famine and food restrictions led to a mean caloric intake of 200-800 calories a day in the ghettos and concentration camps, resulting in catabolysis and starvation of the inhabitants and prisoners. Severely increased risks of fracture, poor bone mineral density, and decreased cortical strength were noted in several case series and descriptive reports addressing the medical issues of these individuals. A severe effect of severely diminished food intake and frequently concomitant calcium- and Vitamin D deficiencies was subsequently proven in both animal models and the most common cause of starvation in developed countries is anorexia nervosa. This review attempts to summarize the literature available on the impact of the metabolic response to Starvation on overall bone health and bone quality.

A role for prostaglandins in rapid cycling suggested by episode-specific gene expression shifts in peripheral blood mononuclear cells: a preliminary report

OBJECTIVES

Over 12% of patients with bipolar disorder exhibit rapid cycling. The underlying biological mechanisms of this extreme form of bipolar disease are still unknown. This study aimed at replicating and extending findings of our previously published case report, where an involvement of prostaglandin synthesis-related genes in rapid cycling was first proposed.

METHODS

Psychopathological follow-up of the reported case was performed under cessation of celecoxib treatment. In a prospective observational study, patients with bipolar disorder (n = 47; of these, four had rapid cycling) or with monopolar depression (n = 97) were recruited over a period of three years. Repeated psychopathology measurements were conducted using standard instruments. Peripheral blood mononuclear cells (PBMC) were obtained during as many consecutive episodes as possible and processed for mRNA isolation and quantitative real-time reverse transcriptase polymerase chain reaction for prostaglandin D2 synthase (PTGDS), aldo-ketoreductase family 1, member C3 (AKR1C3), cyclooxygenase-2 (PAN means all splice variants) (COX2PAN ), prostaglandin-endoperoxide synthase 2 (PTGS2), and purinergic receptor P2X, ligand-gated ion channel 7 (P2RX7).

RESULTS

The follow-up of our original case of a patient with rapid cycling who had shown impressive psychopathological improvement under celecoxib revealed complete loss of this effect upon discontinuation of the COX2 inhibitor. Episode-specific gene expression measurements in PBMC of four newly recruited rapid cycling patients confirmed the higher expression of PTGDS in depressive compared to manic phases. Additionally, higher relative expression of PTGS2/COX2PAN was found. No comparable alterations were observable in samples available from the remaining 43 patients with bipolar disorder and the 97 monopolar depressed patients, emphasizing the advantages of the rapid cycling condition with its rapid and frequent shifts for identification of gene expression changes.

CONCLUSIONS

This study supports a role for prostaglandins in rapid cycling and advocates the cyclooxygenase cascade as a treatment target in this condition.

Low cardiac output as physiological phenomenon in hibernating, free-ranging Scandinavian brown bears (Ursus arctos) - an observational study

BACKGROUND

Despite 5-7 months of physical inactivity during hibernation, brown bears (Ursus arctos) are able to cope with physiological conditions that would be detrimental to humans. During hibernation, the tissue metabolic demands fall to 25% of the active state. Our objective was to assess cardiac function associated with metabolic depression in the hibernating vs. active states in free-ranging Scandinavian brown bears.

METHODS

We performed echocardiography on seven free-ranging brown bears in Dalarna, Sweden, anesthetized with medetomidine-zolazepam-tiletamine-ketamine during winter hibernation in February 2013 and with medetomidine-zolazepam-tiletamine during active state in June 2013. We measured cardiac output noninvasively using estimates of hemodynamics obtained by pulsed wave Doppler echocardiography and 2D imaging. Comparisons were made using paired T-tests.

RESULTS

During hibernation, all hemodynamic indices were significantly decreased (hibernating vs. active state): mean heart rate was 26.0 (standard deviation (SD): 5.6) beats per min vs. 75.0 (SD: 17.1) per min (P=0.002), mean stroke volume 32.3 (SD: 5.2) ml vs. 47.1 (SD: 7.9) ml (P=0.008), mean cardiac output 0.86 (SD: 0.31) l/min vs. 3.54 (SD: 1.04) l/min (P=0.003), and mean cardiac index 0.63 (SD: 0.21) l/min/kg vs. 2.45 (SD: 0.52) l/min/ m2 (P<0.001). Spontaneous echo contrast was present in all cardiac chambers in all seven bears during hibernation, despite the absence of atrial arrhythmias and valvular disease.

CONCLUSION

Free-ranging brown bears demonstrate hemodynamics comparable to humans during active state, whereas during hibernation, we documented extremely low-flow hemodynamics. Understanding these physiological changes in bears may help to gain insight into the mechanisms of cardiogenic shock and heart failure in humans.

Endocrine regulation of bone and energy metabolism in hibernating mammals

Precise coordination among organs is required to maintain homeostasis throughout hibernation. This is particularly true in balancing bone remodeling processes (bone formation and resorption) in hibernators experiencing nutritional deprivation and extreme physical inactivity, two factors normally leading to pronounced bone loss in non-hibernating mammals. In recent years, important relationships between bone, fat, reproductive, and brain tissues have come to light. These systems share interconnected regulatory mechanisms of energy metabolism that potentially protect the skeleton during hibernation. This review focuses on the endocrine and neuroendocrine regulation of bone/fat/energy metabolism in hibernators. Hibernators appear to have unique mechanisms that protect musculoskeletal tissues while catabolizing their abundant stores of fat. Furthermore, the bone remodeling processes that normally cause disuse-induced bone loss in non-hibernators are compared to bone remodeling processes in hibernators, and possible adaptations of the bone signaling pathways that protect the skeleton during hibernation are discussed. Understanding the biological mechanisms that allow hibernators to survive the prolonged disuse and fasting associated with extreme environmental challenges will provide critical information regarding the limit of convergence in mammalian systems and of skeletal plasticity, and may contribute valuable insight into the etiology and treatment of human diseases.

Serum immune-related proteins are differentially expressed during hibernation in the American black bear

Hibernation is an adaptation to conserve energy in the face of extreme environmental conditions and low food availability that has risen in several animal phyla. This phenomenon is characterized by reduced metabolic rate (∼25% of the active basal metabolic rate in hibernating bears) and energy demand, while other physiological adjustments are far from clear. The profiling of the serum proteome of the American black bear (Ursus americanus) may reveal specific proteins that are differentially modulated by hibernation, and provide insight into the remarkable physiological adaptations that characterize ursid hibernation. In this study, we used differential gel electrophoresis (DIGE) analysis, liquid chromatography coupled to tandem mass spectrometry, and subsequent MASCOT analysis of the mass spectra to identify candidate proteins that are differentially expressed during hibernation in captive black bears. Seventy serum proteins were identified as changing by ±1.5 fold or more, out of which 34 proteins increased expression during hibernation. The majority of identified proteins are involved in immune system processes. These included α₂-macroglobulin, complement components C1s and C4, immunoglobulin μ and J chains, clusterin, haptoglobin, C4b binding protein, kininogen 1, α₂-HS-glycoprotein, and apoplipoproteins A-I and A-IV. Differential expression of a subset of these proteins identified by proteomic analysis was also confirmed by immunodetection. We propose that the observed serum protein changes contribute to the maintenance of the hibernation phenotype and health, including increased capacities for bone maintenance and wound healing during hibernation in bears.

Hibernating bears (Ursidae): metabolic magicians of definite interest for the nephrologist

Muscle loss, osteoporosis, and vascular disease are common in subjects with reduced renal function. Despite intensive research of the underlying risk factors and mechanisms driving these phenotypes, we still lack effective treatment strategies for this underserved patient group. Thus, new approaches are needed to identify effective treatments. We believe that nephrologists could learn much from biomimicry; i.e., studies of nature's models to solve complicated physiological problems and then imitate these fascinating solutions to develop novel interventions. The hibernating bear (Ursidae) should be of specific interest to the nephrologist as they ingest no food or water for months, remaining anuric and immobile, only to awaken with low blood urea nitrogen levels, healthy lean body mass, strong bones, and without evidence for thrombotic complications. Identifying the mechanisms by which bears prevent the development of azotemia, sarcopenia, osteoporosis, and atherosclerosis despite being inactive and anuric could lead to novel interventions for both prevention and treatment of patients with chronic kidney disease.

Black bear parathyroid hormone has greater anabolic effects on trabecular bone in dystrophin-deficient mice than in wild type mice

Duchenne muscular dystrophy (DMD) is an X-linked neuromuscular disease that has deleterious consequences in muscle and bone, leading to decreased mobility, progressive osteoporosis, and premature death. Patients with DMD experience a higher-than-average fracture rate, particularly in the proximal and distal femur and proximal tibia. The dystrophin-deficient mdx mouse is a model of DMD that demonstrates muscle degeneration and fibrosis and osteoporosis. Parathyroid hormone, an effective anabolic agent for post-menopausal and glucocorticoid-induced osteoporosis, has not been explored for DMD. Black bear parathyroid hormone (bbPTH) has been implicated in the maintenance of bone properties during extended periods of disuse (hibernation). We cloned bbPTH and found 9 amino acid residue differences from human PTH. Apoptosis was mitigated and cAMP was activated by bbPTH in osteoblast cultures. We administered 28nmol/kg of bbPTH 1-84 to 4-week old male mdx and wild type mice via daily (5×/week) subcutaneous injection for 6 weeks. Vehicle-treated mdx mice had 44% lower trabecular bone volume fraction than wild type mice. No changes were found in femoral cortical bone geometry or mechanical properties with bbPTH treatment in wild type mice, and only medio-lateral moment of inertia changed with bbPTH treatment in mdx femurs. However, μCT analyses of the trabecular regions of the distal femur and proximal tibia showed marked increases in bone volume fraction with bbPTH treatment, with a greater anabolic response (7-fold increase) in mdx mice than wild type mice (2-fold increase). Trabecular number increased in mdx long bone, but not wild type bone. Additionally, greater osteoblast area and decreased osteoclast area were observed with bbPTH treatment in mdx mice. The heightened response to PTH in mdx bone compared to wild type suggests a link between dystrophin deficiency, altered calcium signaling, and bone. These findings support further investigation of PTH as an anabolic treatment for DMD-induced osteoporosis.

Wound healing during hibernation by black bears (Ursus americanus) in the wild: elicitation of reduced scar formation

Even mildly hypothermic body or limb temperatures can retard healing processes in mammals. Despite this, we observed that hibernating American black bears (Ursus americanus Pallas, 1780) elicit profound abilities in mounting inflammatory responses to infection and/or foreign bodies. In addition, they resolve injuries during hibernation while maintaining mildly hypothermic states (30-35 °C) and without eating, drinking, urinating or defecating. We describe experimental studies on free-ranging bears that document their abilities to completely resolve cutaneous cuts and punctures incurred during or prior to hibernation. We induced small, full-thickness cutaneous wounds (biopsies or incisions) during early denning, and re-biopsied sites 2-3 months later (near the end of denning). Routine histological methods were used to characterize these skin samples. All biopsied sites with respect to secondary intention (open circular biopsies) and primary intention (sutured sites) healed, with evidence of initial eschar (scab) formation, completeness of healed epidermis and dermal layers, dyskeratosis (inclusion cysts), and abilities to produce hair follicles. These healing abilities of hibernating black bears are a clear survival advantage to animals injured before or during denning. Bears are known to have elevated levels of hibernation induction trigger (delta-opioid receptor agonist) and ursodeoxycholic acid (major bile acid within plasma, mostly conjugated with taurine) during hibernation, which may relate to these wound-healing abilities. Further research as to the underlying mechanisms of wound healing during hibernation could have applications in human medicine. Unique approaches may be found to improve healing for malnourished, hypothermic, diabetic and elderly patients or to reduce scarring associated with burns and traumatic injuries.

Preservation of bone mass and structure in hibernating black bears (Ursus americanus) through elevated expression of anabolic genes

Physical inactivity reduces mechanical load on the skeleton, which leads to losses of bone mass and strength in non-hibernating mammalian species. Although bears are largely inactive during hibernation, they show no loss in bone mass and strength. To obtain insight into molecular mechanisms preventing disuse bone loss, we conducted a large-scale screen of transcriptional changes in trabecular bone comparing winter hibernating and summer non-hibernating black bears using a custom 12,800 probe cDNA microarray. A total of 241 genes were differentially expressed (P < 0.01 and fold change >1.4) in the ilium bone of bears between winter and summer. The Gene Ontology and Gene Set Enrichment Analysis showed an elevated proportion in hibernating bears of overexpressed genes in six functional sets of genes involved in anabolic processes of tissue morphogenesis and development including skeletal development, cartilage development, and bone biosynthesis. Apoptosis genes demonstrated a tendency for downregulation during hibernation. No coordinated directional changes were detected for genes involved in bone resorption, although some genes responsible for osteoclast formation and differentiation (Ostf1, Rab9a, and c-Fos) were significantly underexpressed in bone of hibernating bears. Elevated expression of multiple anabolic genes without induction of bone resorption genes, and the down regulation of apoptosis-related genes, likely contribute to the adaptive mechanism that preserves bone mass and structure through prolonged periods of immobility during 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.

Seasonal regulation of the growth hormone-insulin-like growth factor-I axis in the American black bear (Ursus americanus)

The American black bear maintains lean body mass for months without food during winter denning. We asked whether changes in the growth hormone/insulin-like growth factor-I (GH-IGF-I) axis may contribute to this remarkable adaptation to starvation. Serum IGF-I levels were measured by radioimmunoassay, and IGF-binding proteins (IGFBPs) were analyzed by ligand blotting. Initial studies in bears living in the wild showed that IGF-I levels are highest in summer and lowest in early winter denning. Detailed studies in captive bears showed that IGF-I levels decline in autumn when bears are hyperphagic, continue to decline in early denning, and later rise above predenning levels despite continued starvation in the den. IGFBP-2 increased and IGFBP-3 decreased in early denning, and these changes were also reversed in later denning. Treatment with GH (0.1 mg·kg(-1)·day(-1) × 6 days) during early denning increased serum levels of IGF-I and IGFBP-3 and lowered levels of IGFBP-2, indicating that denning bears remain responsive to GH. GH treatment lowered blood urea nitrogen levels, reflecting effects on protein metabolism. GH also accelerated weight loss and markedly increased serum levels of free fatty acids and β-hydroxybutyrate, resulting in a ketoacidosis (bicarbonate decreased to 15 meq/l), which was reversed when GH was withdrawn. These results demonstrate seasonal regulation of GH/IGF-I axis activity in black bears. Diminished GH activity may promote fat storage in autumn in preparation for denning and prevent excessive mobilization and premature exhaustion of fat stores in early denning, whereas restoration of GH/IGF activity in later denning may prepare the bear for normal activity outside the den.

Thirteen-lined ground squirrels (Ictidomys tridecemlineatus) show microstructural bone loss during hibernation but preserve bone macrostructural geometry and strength

Lack of activity causes bone loss In most animals. Hibernating bears have physiological processes to prevent cortical and trabecular bone loss associated with reduced physical activity, but different mechanisms of torpor among hibernating species may lead to differences in skeletal responses to hibernation. There are conflicting reports regarding whether small mammals experience bone loss during hibernation. To investigate this phenomenon, we measured cortical and trabecular bone properties in physically active and hibernating juvenile and adult 13-lined ground squirrels (Ictidomys tridecemlineatus, previous genus name Spermophilus). Cortical bone geometry, strength and mineral content were similar in hibernating compared with active squirrels, suggesting that hibernation did not cause macrostructural cortical bone loss. Osteocyte lacunar size increased (linear regression, P=0.001) over the course of hibernation in juvenile squirrels, which may indicate an osteocytic role in mineral homeostasis during hibernation. Osteocyte lacunar density and porosity were greater (+44 and +59%, respectively; P<0.0001) in hibernating compared with active squirrels, which may reflect a decrease in osteoblastic activity (per cell) during hibernation. Trabecular bone volume fraction in the proximal tibia was decreased (-20%; P=0.028) in hibernating compared with physically active adult squirrels, but was not different between hibernating and active juvenile squirrels. Taken together, these data suggest that 13-lined ground squirrels may be unable to prevent microstructural losses of cortical and trabecular bone during hibernation, but importantly may possess a biological mechanism to preserve cortical bone macrostructure and strength during hibernation, thus preventing an increased risk of bone fracture during remobilization in the spring.

Vitamin D status and bone and connective tissue turnover in brown bears (Ursus arctos) during hibernation and the active state

BACKGROUND

Extended physical inactivity causes disuse osteoporosis in humans. In contrast, brown bears (Ursus arctos) are highly immobilised for half of the year during hibernation without signs of bone loss and therefore may serve as a model for prevention of osteoporosis.

AIM

To study 25-hydroxy-vitamin D (25OHD) levels and bone turnover markers in brown bears during the hibernating state in winter and during the active state in summer. We measured vitamin D subtypes (D₂ and D₃), calcitropic hormones (parathyroid hormone [PTH], 1,25-dihydroxy-vitamin D [1,25(OH)₂D]) and bone turnover parameters (osteocalcin, ICTP, CTX-I), PTH, serum calcium and PIIINP.

MATERIAL AND METHODS

We drew blood from seven immobilised wild brown bears during hibernation in February and in the same bears while active in June.

RESULTS

Serum 25-hydroxy-cholecalciferol (25OHD₃) was significantly higher in the summer than in the winter (22.8±4.6 vs. 8.8±2.1 nmol/l, two tailed p-2p = 0.02), whereas 25-hydroxy-ergocalciferol (25OHD₂) was higher in winter (54.2±8.3 vs. 18.7±1.7 nmol/l, 2p<0.01). Total serum calcium and PTH levels did not differ between winter and summer. Activated 1,25(OH)₂D demonstrated a statistically insignificant trend towards higher summer levels. Osteocalcin levels were higher in summer than winter, whereas other markers of bone turnover (ICTP and CTX-I) were unchanged. Serum PIIINP, which is a marker of connective tissue and to some degree muscle turnover, was significantly higher during summer than during winter.

CONCLUSIONS

Dramatic changes were documented in the vitamin D₃/D₂ ratio and in markers of bone and connective tissue turnover in brown bears between hibernation and the active state. Because hibernating brown bears do not develop disuse osteoporosis, despite extensive physical inactivity we suggest that they may serve as a model for the prevention of this disease.

Preservation of bone mass and biomechanical properties during winter sleep--the raccoon dog (Nyctereutes procyonoides) as a novel model species

It has been established that almost 40% of postmenopausal women in the United States have osteopenia and models to study its prevention are thus urgently needed. Bears (Ursus spp.) displaying winter sleep were previously introduced as promising models to study the treatment of disuse-induced bone loss. The present study examined the potential of another analogous model species, the raccoon dog (Nyctereutes procyonoides), in bone research. Similar to bears, raccoon dogs display prolonged passivity and catabolism in winter, but they are of a moderate body mass, easy to handle and reared on farms. Wild specimens (n=51) were hunted in winter 2007-2008. The bone mineral density of femoral diaphysis and neck was examined with peripheral quantitative computed tomography, after which their mechanical properties were tested with the three-point bending and femoral neck loading tests. A subsample of the specimens was analyzed histologically. While the body mass of the raccoon dogs decreased from 7.0±0.3 to 4.5±0.2 kg (-36%) during winter, the bone mass and biomechanical properties remained unchanged despite of heavy wintertime catabolism similar to bears. Thus, the cortical mineral density remained at approximately 1400 mg/cm(3), the trabecular mineral density at 450 mg/cm(3) and the maximum load of the femoral neck at 700 N. However, in histological samples, the proportion of osteoid perimeter vs. mineralized bone perimeter decreased during wintering. A possible mechanism of bone mass preservation is the endocrine status of overwintering raccoon dogs, which could participate in preventing bone loss.

Endocannabinoid signalling: has it got rhythm?

Endogenous cannabinoid signalling is widespread throughout the body, and considerable evidence supports its modulatory role in many fundamental physiological processes. The daily and seasonal cycles of the relationship of the earth and sun profoundly affect the terrestrial environment. Terrestrial species have adapted to these cycles in many ways, most well studied are circadian rhythms and hibernation. The purpose of this review was to examine literature support for three hypotheses: (i) endocannabinoid signalling exhibits brain region-specific circadian rhythms; (ii) endocannabinoid signalling modulates the rhythm of circadian processes in mammals; and (iii) changes in endocannabinoid signalling contribute to the state of hibernation. The results of two novel studies are presented. First, we report the results of a study of healthy humans demonstrating that plasma concentrations of the endocannabinoid, N-arachidonylethanolamine (anandamide), exhibit a circadian rhythm. Concentrations of anandamide are threefold higher at wakening than immediately before sleep, a relationship that is dysregulated by sleep deprivation. Second, we investigated differences in endocannabinoids and congeners in plasma from Marmota monax obtained in the summer and during the torpor state of hibernation. We report that 2-arachidonoylglycerol is below detection in M. monax plasma and that concentrations of anandamide are not different. However, plasma concentrations of the anorexigenic lipid oleoylethanolamide were significantly lower in hibernation, while the concentrations of palmitoylethanolamide and 2-oleoylglycerol were significantly greater in hibernation. We conclude that available data support a bidirectional relationship between endocannabinoid signalling and circadian processes, and investigation of the contribution of endocannabinoid signalling to the dramatic physiological changes that occur during hibernation is warranted.

Platelet function in brown bear (Ursus arctos) compared to man

Background

Information on hemostasis and platelet function in brown bear (Ursus arctos) is of importance for understanding the physiological, protective changes during hibernation.

Objective

The study objective was to document platelet activity values in brown bears shortly after leaving the den and compare them to platelet function in healthy humans.

Methods

Blood was drawn from immobilized wild brown bears 7-10 days after leaving the den in mid April. Blood samples from healthy human adults before and after clopidogrel and acetylsalicylic acid administration served as control. We analyzed blood samples by standard blood testing and platelet aggregation was quantified after stimulation with various agonists using multiple electrode aggregometry within 3 hours of sampling.

Results

Blood samples were collected from 6 bears (3 females) between 1 and 16 years old and from 10 healthy humans. Results of adenosine diphosphate, aspirin, and thrombin receptor activating peptide tests in bears were all half or less of those in humans. Platelet and white blood cell counts did not differ between species but brown bears had more and smaller red blood cells compared with humans.

Conclusion

Using three different tests, we conclude that platelet function is lower in brown bears compared to humans. Our findings represent the first descriptive study on platelet function in brown bears and may contribute to explain how bears can endure denning without obvious thrombus building. However, the possibility that our findings reflect test-dependent and not true biological variations in platelet reactivity needs further studies.

Bone strength is maintained after 8 months of inactivity in hibernating golden-mantled ground squirrels, Spermophilus lateralis

Prolonged inactivity leads to disuse atrophy, a loss of muscle and bone mass. Hibernating mammals are inactive for 6-9 months per year but must return to full activity immediately after completing hibernation. This necessity for immediate recovery presents an intriguing conundrum, as many mammals require two to three times the period of inactivity to recover full bone strength. Therefore, if hibernators experience typical levels of bone disuse atrophy during hibernation, there would be inadequate time available to recover during the summer active season. We examined whether there were mechanical consequences as a result of the extended inactivity of hibernation. We dissected femur and tibia bones from squirrels in various stages of the annual hibernation cycle and measured the amount of force required to fracture these bones. Three groups were investigated; summer active animals were captured during the summer and immediately killed, animals in the 1 month detraining group were captured in the summer and killed following a 1-month period of restricted mobility, hibernating animals were killed after 8 months of inactivity. A three-point bend test was employed to measure the force required to break the bones. Apparent flexural strength and apparent flexural modulus (material stiffness) were calculated for femurs. There were no differences between groups for femur fracture force, tibia fracture force, or femur flexural strength. Femur flexural modulus was significantly less for the 1 month detraining group than for the hibernation and summer active groups. Thus, hibernators seem resistant to the deleterious effects of prolonged inactivity during the winter. However, they may be susceptible to immobilization-induced bone loss during the summer.

Grizzly bears (Ursus arctos horribilis) and black bears (Ursus americanus) prevent trabecular bone loss during disuse (hibernation)

Disuse typically causes an imbalance in bone formation and bone resorption, leading to losses of cortical and trabecular bone. In contrast, bears maintain balanced intracortical remodeling and prevent cortical bone loss during disuse (hibernation). Trabecular bone, however, is more detrimentally affected than cortical bone in other animal models of disuse. Here we investigated the effects of hibernation on bone remodeling, architectural properties, and mineral density of grizzly bear (Ursus arctos horribilis) and black bear (Ursus americanus) trabecular bone in several skeletal locations. There were no differences in bone volume fraction or tissue mineral density between hibernating and active bears or between pre- and post-hibernation bears in the ilium, distal femur, or calcaneus. Though indices of cellular activity level (mineral apposition rate, osteoid thickness) decreased, trabecular bone resorption and formation indices remained balanced in hibernating grizzly bears. These data suggest that bears prevent bone loss during disuse by maintaining a balance between bone formation and bone resorption, which consequently preserves bone structure and strength. Further investigation of bone metabolism in hibernating bears may lead to the translation of mechanisms preventing disuse-induced bone loss in bears into novel treatments for osteoporosis.

Association of leptin, 25-hydroxyvitamin D, and parathyroid hormone in women

Vitamin D deficiency and adipocytokines have been implicated in the etiology of aging-related diseases such as cancer, osteoporosis, and diseases of the cardiovascular system. The association between elevated parathyroid hormone (PTH) and low 25-hydroxyvitamin D (25-OH-VitD) in plasma is used to define vitamin D deficiency, yet their associated mechanistic pathways are unclear. Utilizing plasma samples from women in a previous intervention study, we measured plasma 25-OH-VitD, leptin, adiponectin, PTH, and lipid levels. We observed strong positive associations for leptin with PTH, gamma -tocopherol, and body mass index (BMI) and inverse associations with 25-OH-VitD and adiponectin. Although commonly accepted that vitamin D deficiency causes hyperparathyroidism, we observed this association primarily in individuals with elevated leptin levels, suggesting that leptin may be an important modifier of this effect consistent with 25-OH-VitD-mediated inhibition of leptin. Leptin was highly correlated with the BMI/25-OH-VitD ratio (r = 0.80; P < 0.0001), consistent with a model in which BMI (adiposity) and 25-OH-VitD are the primary determinants of circulating leptin and PTH levels. This model may explain the failure of some studies to observe elevated PTH in vitamin D deficient adolescents and provides important insight into epidemiological studies exploring the associations of these individual biomarkers with chronic disease risk and mortality.

Six months of disuse during hibernation does not increase intracortical porosity or decrease cortical bone geometry, strength, or mineralization in black bear (Ursus americanus) femurs

Disuse typically uncouples bone formation from resorption, leading to bone loss which compromises bone mechanical properties and increases the risk of bone fracture. Previous studies suggest that bears can prevent bone loss during long periods of disuse (hibernation), but small sample sizes have limited the conclusions that can be drawn regarding the effects of hibernation on bone structure and strength in bears. Here we quantified the effects of hibernation on structural, mineral, and mechanical properties of black bear (Ursus americanus) cortical bone by studying femurs from large groups of male and female bears (with wide age ranges) killed during pre-hibernation (fall) and post-hibernation (spring) periods. Bone properties that are affected by body mass (e.g. bone geometrical properties) tended to be larger in male compared to female bears. There were no differences (p>0.226) in bone structure, mineral content, or mechanical properties between fall and spring bears. Bone geometrical properties differed by less than 5% and bone mechanical properties differed by less than 10% between fall and spring bears. Porosity (fall: 5.5+/-2.2%; spring: 4.8+/-1.6%) and ash fraction (fall: 0.694+/-0.011; spring: 0.696+/-0.010) also showed no change (p>0.304) between seasons. Statistical power was high (>72%) for these analyses. Furthermore, bone geometrical properties and ash fraction (a measure of mineral content) increased with age and porosity decreased with age. These results support the idea that bears possess a biological mechanism to prevent disuse and age-related osteoporoses.

Mechanical stimulus alters conformation of type 1 parathyroid hormone receptor in bone cells

The molecular mechanisms by which bone cells transduce mechanical stimuli into intracellular biochemical responses have yet to be established. There is evidence that mechanical stimulation acts synergistically with parathyroid hormone PTH(1-34) in mediating bone growth. Using picosecond time-resolved fluorescence microscopy and G protein-coupled receptor conformation-sensitive fluorescence resonance energy transfer (FRET), we investigated conformational transitions in parathyroid hormone type 1 receptor (PTH1R). 1) A genetically engineered PTH1R sensor containing an intramolecular FRET pair was constructed that enabled detection of conformational activity of PTH1R in single cells. 2) The nature of ligand-dependent conformational change of PTH1R depends on the type of ligand: stimulation with the PTH(1-34) leads to conformational transitions characterized by decrease in FRET efficiency while NH(2)-terminal truncated ligand PTH(3-34) stimulates conformational transitions characterized by higher FRET efficiencies. 3) Stimulation of murine preosteoblastic cells (MC3T3-E1) with fluid shear stress (FSS) leads to significant changes in conformational equilibrium of the PTH1R in MC3T3-E1 cells, suggesting that mechanical perturbation of the plasma membrane leads to ligand-independent response of the PTH1R. Conformational transitions induced by mechanical stress were characterized by an increase in FRET efficiency, similar to those induced by the NH(2)-terminal truncated ligand PTH(3-34). The response to the FSS stimulation was inhibited in the presence of PTH(1-34) in the flow medium. These results indicate that the FSS can modulate the action of the PTH(1-34) ligand. 4) Plasma membrane fluidization using benzyl alcohol or cholesterol extraction also leads to conformational transitions characterized by increased FRET levels. We therefore suggest that PTH1R is involved in mediating primary mechanochemical signal transduction in MC3T3-E1 cells.

Mammalian hibernation as a model of disuse osteoporosis: the effects of physical inactivity on bone metabolism, structure, and strength

Reduced skeletal loading typically leads to bone loss because bone formation and bone resorption become unbalanced. Hibernation is a natural model of musculoskeletal disuse because hibernating animals greatly reduce weight-bearing activity, and therefore, they would be expected to lose bone. Some evidence suggests that small mammals like ground squirrels, bats, and hamsters do lose bone during hibernation, but the mechanism of bone loss is unclear. In contrast, hibernating bears maintain balanced bone remodeling and preserve bone structure and strength. Differences in the skeletal responses of bears and smaller mammals to hibernation may be due to differences in their hibernation patterns; smaller mammals may excrete calcium liberated from bone during periodic arousals throughout hibernation, leading to progressive bone loss over time, whereas bears may have evolved more sophisticated physiological processes to recycle calcium, prevent hypercalcemia, and maintain bone integrity. Investigating the roles of neural and hormonal control of bear bone metabolism could give valuable insight into translating the mechanisms that prevent disuse-induced bone loss in bears into novel therapies for treating osteoporosis.