Proteomic Identification of Seasonally Expressed Proteins Contributing to Heart Function and the Avoidance of Skeletal Muscle Disuse Atrophy in a Hibernating Mammal

Hibernation in the thirteen-lined ground squirrel (Ictidomys tridecemlineatus) takes place over 4-6 months and is characterized by multiday bouts of hypothermic torpor (5-7 °C core body temperature) that are regularly interrupted every 1-2 weeks by brief (12-24 h) normothermic active periods called interbout arousals. Our goal was to gain insight into the molecular mechanisms that underlie the hibernator's ability to preserve heart function and avoid the deleterious effects of skeletal muscle disuse atrophy over prolonged periods of inactivity, starvation, and near-freezing body temperatures. To achieve this goal, we performed organelle enrichment of heart and skeletal muscle at five seasonal time points followed by LC-MS-based label-free quantitative proteomics. In both organs, we saw an increase in the levels of many proteins as ground squirrels transition from an active state to a prehibernation state in the fall. Interestingly, seasonal abundance patterns identified DHRS7C, SRL, TRIM72, RTN2, and MPZ as potential protein candidates for mitigating disuse atrophy in skeletal muscle, and ex vivo contractile mechanics analysis revealed no deleterious effects in the ground squirrel's muscles despite prolonged sedentary activity. Overall, an increased understanding of protein abundance in hibernators may enable novel therapeutic strategies to treat muscle disuse atrophy and heart disease in humans.

Differential bone remodeling mechanism in hindlimb unloaded and hibernating Daurian ground squirrels: a comparison between artificial and natural disuse within the same species

Loss of bone mass can occur in mammals after prolonged disuse but the situation for hibernators that are in a state of torpor for many months of the year is not yet fully understood. The present study assesses the bone remodeling mechanisms present in Daurian ground squirrels (Spermophilus dauricus) during hibernation as compared with a model of hindlimb disuse. Differences in microstructure, mechanical properties, bone remodeling-related proteins (Runx2, OCN, ALP, RANKL, CTK and MMP-9) and key proteins of Wnt/β-catenin signaling pathway (GSK-3β and phospho-β-catenin) were evaluated in ground squirrels under 3 conditions: summer active (SA) vs. hibernation (HIB) vs. hindlimb unloaded (HLU). The results indicated that the body weight in HLU ground squirrels was lower than the SA group, and the middle tibia diameter in the HLU group was lower than that in SA and HIB groups. The thickness of cortical and trabecular bone in femurs from HLU ground squirrels was lower than in SA and HIB groups. Most parameters of the tibia in the HLU group were lower than those in SA and HIB groups, which indicated cortical bone loss in ground squirrels. Moreover, our data showed that the changes in microscopic parameters in the femur were more obvious than those in the tibia in HLU and HIB ground squirrels. The levels of Runx2 and ALP were lower in HLU ground squirrels than SA and HIB groups. The protein levels of OCN were unchanged in the three groups, but the protein levels of ALP were lower in the HLU group than in SA and HIB groups. RANKL, CTK and MMP-9 protein levels were significantly decreased in tibia of HLU ground squirrels as compared with SA and HIB groups. In addition, the protein expression levels of RANKL, CTK and MMP-9 showed no statistical difference between SA and HIB ground squirrels. Thus, the mechanisms involved in the balance between bone formation and resorption in hibernating and hindlimb unloading ground squirrels may be different. The present study showed that in femur, the Wnt signaling pathway was inhibited, the protein level of GSK-3β was increased, and the protein expression of phospho-β-catenin was decreased in the HIB group as compared with the SA group, which indicates that the Wnt signaling pathway has a great influence on the femur of the HIB group. In conclusion, the natural anti-osteoporosis properties of Daurian ground squirrels are seasonal. The squirrels do not experience bone loss when they are inactive for a long time during hibernation, but the mechanisms of anti-osteoporosis did not work in HLU summer active squirrels.

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.

Remarkable Plasticity of Bone Iron Homeostasis in Hibernating Daurian Ground Squirrels (Spermophilus dauricus) May Be Involved in Bone Maintenance

Iron overload is an independent risk factor for disuse osteoporosis. Hibernating animals are natural models of anti-disuse osteoporosis; however, whether iron metabolism is involved in bone adaptation and maintenance during hibernation is unclear. To investigate this question, Daurian ground squirrels (Spermophilus dauricus) (n = 5-6/group) were used to study changes in bone iron metabolism and its possible role in anti-disuse osteoporosis during hibernation. Iron content in the femur and liver first decreased in the torpor group (vs. summer group, -66.8% and -25.8%, respectively), then recovered in the post-hibernation group, suggesting remarkable plasticity of bone iron content. The expression of ferritin in the femur and hepcidin in the liver also initially decreased in the torpor group (vs. summer group, -28.5% and -38.8%, respectively), then increased in the inter-bout arousal (vs. torpor group, 126.2% and 58.4%, respectively) and post-hibernation groups (vs. torpor group, 153.1% and 27.1%, respectively). In conclusion, bone iron metabolism in hibernating Daurian ground squirrels showed remarkable plasticity, which may be a potential mechanism to avoid disuse bone loss during extended periods of inactivity. However, the specific location of iron during low-iron hibernation and the source of iron in post-hibernation recovery need to be further explored.

Paleoproteomics

Paleoproteomics, the study of ancient proteins, is a rapidly growing field at the intersection of molecular biology, paleontology, archaeology, paleoecology, and history. Paleoproteomics research leverages the longevity and diversity of proteins to explore fundamental questions about the past. While its origins predate the characterization of DNA, it was only with the advent of soft ionization mass spectrometry that the study of ancient proteins became truly feasible. Technological gains over the past 20 years have allowed increasing opportunities to better understand preservation, degradation, and recovery of the rich bioarchive of ancient proteins found in the archaeological and paleontological records. Growing from a handful of studies in the 1990s on individual highly abundant ancient proteins, paleoproteomics today is an expanding field with diverse applications ranging from the taxonomic identification of highly fragmented bones and shells and the phylogenetic resolution of extinct species to the exploration of past cuisines from dental calculus and pottery food crusts and the characterization of past diseases. More broadly, these studies have opened new doors in understanding past human-animal interactions, the reconstruction of past environments and environmental changes, the expansion of the hominin fossil record through large scale screening of nondiagnostic bone fragments, and the phylogenetic resolution of the vertebrate fossil record. Even with these advances, much of the ancient proteomic record still remains unexplored. Here we provide an overview of the history of the field, a summary of the major methods and applications currently in use, and a critical evaluation of current challenges. We conclude by looking to the future, for which innovative solutions and emerging technology will play an important role in enabling us to access the still unexplored "dark" proteome, allowing for a fuller understanding of the role ancient proteins can play in the interpretation of the past.

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.

Be cool to be far: Exploiting hibernation for space exploration

In mammals, torpor/hibernation is a state that is characterized by an active reduction in metabolic rate followed by a progressive decrease in body temperature. Torpor was successfully mimicked in non-hibernators by inhibiting the activity of neurons within the brainstem region of the Raphe Pallidus, or by activating the adenosine A1 receptors in the brain. This state, called synthetic torpor, may be exploited for many medical applications, and for space exploration, providing many benefits for biological adaptation to the space environment, among which an enhanced protection from cosmic rays. As regards the use of synthetic torpor in space, to fully evaluate the degree of physiological advantage provided by this state, it is strongly advisable to move from Earth-based experiments to 'in the field' tests, possibly on board the International Space Station.

Differential bone remodeling mechanism in hindlimb unloaded rats and hibernating Daurian ground squirrels: a comparison between artificial and natural disuse

To determine that differential bone remodeling mechanism (especially Wnt signaling) in hindlimb unloaded rats and hibernating Daurian ground squirrels, the bone microstructure, mechanical properties, and expression levels of bone remodeling related proteins and key proteins of Wnt/β-catenin signaling were analyzed in this study. The thickness of cortical and trabecular bone was decreased in femur of hindlimb unloaded rats, while it was maintained in femur of hibernating ground squirrels. Interestingly, the ultimate bending energy and ultimate normalized displacement were reduced and the bending rigidity was increased in tibia of hibernating ground squirrels. Besides, the protein level of Runx2 was decreased in femur and tibia of unloaded rats, while it was maintained in tibia and even increased in femur of hibernating ground squirrels. The protein levels of RANKL and MMP-9 were increased in femur and tibia in unloaded rats, while they were maintained in both femur and tibia of hibernating ground squirrels. The protein level of GSK-3β was increased in femur and tibia of unloaded rats, while it was maintained in both femur and tibia of hibernating ground squirrels. The phospho-β-catenin expression was increased in both femur and tibia of unloaded rats, while it was only decreased in femur, but maintained in tibia of hibernating ground squirrels. In conclusion, the femur and tibia in hindlimb unloaded rats showed obvious bone loss, while they mitigated disuse-induced bone loss in hibernating ground squirrels, involving differential protein expression of key molecules in bone remodeling. In comparison with hindlimb unloaded rats, promoting osteoblast differentiation through activating canonical GSK-3β/β-catenin signaling involving Runx2 might be an adaptation to natural disuse in femur of hibernating Daurian ground squirrels. However, there was no statistical change in the protein levels of bone formation related proteins, GSK-3β and phospho-β-catenin in tibia of hibernating Daurian ground squirrels.

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.

Hyperplasia and accelerated hypertrophy of marrow adipocytes with knee immobilization were sustained despite remobilization

Skeletal disuse can cause an accumulation of bone marrow adipose tissue (MAT) characterized by a combination of marrow adipocyte hyperplasia and/or hypertrophy. The malleability of MAT accumulation and of the hyperplasia and hypertrophy upon remobilization is unknown. In this study, we showed extensive hyperplasia and accelerated hypertrophy of bone marrow adipocytes in the proximal tibia epiphysis of rat knees immobilized for durations between 1 and 32 wk. Similar histomorphometric measures of adipocytes carried out in unoperated controls allowed distinguishing the effects of immobilization from the effects of aging. Although both knee immobilization and aging led to adipocyte hypertrophy, adipocyte hyperplasia was the hallmark signature effect of immobilization on MAT. Both bone marrow adipocyte hyperplasia and hypertrophy were sustained despite knee remobilization for durations up to four times the duration of immobilization. These results suggest that adipocyte hyperplasia is the predominant mechanism explaining MAT accumulation in skeletal disuse. In this model, the changes were unremitting for the investigated time points. Investigating the cellular and molecular mechanisms of marrow adipocyte mechanoregulation will be important to better understand how adipocytes adapt to changes in mechanical environments.NEW & NOTEWORTHY This longitudinal study elucidates the response of marrow adipose tissue adipocytes in weight-bearing joints to changes in different mechanical environments, and we provide insight on the malleability of the changes over time. In a rat animal model, knee immobilization induced hyperplasia and accelerated the age-dependent hypertrophy of adipocytes. Changes in adipocyte number and size were sustained despite unassisted remobilization. Multimodal distributions of cell size were characteristic of bone marrow adipocytes.

The effects of neurectomy and hibernation on bone properties and the endocannabinoid system in marmots (Marmota flaviventris)

Hibernators have adapted a physiological mechanism allowing them to undergo long periods of inactivity without experiencing bone loss. However, the biological mechanisms that prevent bone loss are unknown. Previous studies found meaningful changes, between active and hibernating marmots, in the endocannabinoid system of many tissues, including bone. Cannabinoid receptors (CB1 and CB2) have divergent localization in bone. CB1 is predominately found on sympathetic nerve terminals, while CB2 is more abundant on bone cells and their progenitors. This study aimed to determine the contribution of innervation on endocannabinoid regulation of bone properties in hibernating (during torpor) and non-hibernating yellow-bellied marmots. Neurectomy, a model for disuse osteoporosis, was performed unilaterally in both hibernating and active marmots. Endocannabinoid concentrations were measured in bone marrow, cortical, and trabecular regions from fourth metatarsals of both hindlimbs using microflow chromatography-tandem quadrupole mass spectrometry. Trabecular bone architectural properties of fifth metatarsals were evaluated using micro-computed tomography. There were ligand-specific increases with neurectomy in active, but not hibernating, marmots. Trabecular bone architectural properties were not affected by neurectomy during hibernation, but did show some minor negative changes in active marmots. These findings suggest protection from bone loss in hibernating rodents is peripherally rather than centrally regulated. Furthermore, findings suggest even active marmots with normal metabolism are partially protected from disuse induced bone loss compared to laboratory rodents. Understanding the mechanism hibernators use to maintain bone density may guide development for novel bone loss prevention therapies.

Seasonal Changes in Endocannabinoid Concentrations between Active and Hibernating Marmots (Marmota flaviventris)

Hibernation is a naturally occurring model for studying diseases such as obesity and osteoporosis. Hibernators, marmots (Marmota flaviventris) among them, are able to nearly double their body mass by increasing fat stores prior to hibernation without the negative consequences of obesity. They are also physically inactive for extended periods of time without experiencing negative effects on the skeleton. The endocannabinoid system is involved in modulating neural signaling, circannual rhythms, behavior, appetite, thermogenesis, and bone and energy metabolism. These systems are also altered to maintain homeostasis during hibernation. This study aims to better understand the involvement of the endocannabinoid system in the regulation of physiological processes during hibernation by quantifying the seasonal variation of endocannabinoids and endocannabinoid-like ligands in both active and hibernating marmots. We hypothesized that there would be significant changes in endocannabinoid concentrations at the tissue level in marmots between active and hibernating states. Concentrations were measured in brain, serum, brown adipose tissue, white adipose tissue, bone marrow, cortical bone, and trabecular bone using microflow chromatography coupled with tandem quadrupole mass spectrometry. Significant changes were found, such as a 30-fold decrease in 2-arachidonoyl glycerol (2-AG) in cortical bone during hibernation. Many endocannabinoid and endocannabinoid-like ligands decreased in brown adipose tissue, white adipose tissue, and cortical bone, while several ligands increased in bone marrow. This result supports our hypothesis and suggests the possibility of a peripherally controlled shift in energy metabolism, reduction in bone metabolism, and suppression of the immune system during hibernation.

Hibernating Little Pocket Mice Show Few Seasonal Changes in Bone Properties

Periods of disuse or physical inactivity increases bone porosity and decreases bone mineral density, resulting in a loss of bone mechanical competence in many animals. Although large hibernators like bears and marmots prevent bone loss during hibernation, despite long periods of physical inactivity, some small hibernators do lose bone during hibernation. Little pocket mice (Perognathus longimembris) remain underground during winter hibernation and undergo bouts of torpor and interbout arousals, but the torpor bout duration is shorter than other rodent hibernators. Additionally, little pocket mice may enter torpor during summer estivation. In this study, cortical and trabecular bone architectural, mineral, and mechanical properties were analyzed for femurs from little pocket mice captured during 8 different months (March to October) to determine seasonal effects on bone. There were no differences in any bone properties between the pre-hibernation month of October and the post-hibernation month of March, suggesting winter hibernation did not adversely affect bone properties. However, cortical area was higher in March than April, May, and June. Bone mechanical and osteocyte lacunar properties were not different between any months. Trabecular bone in the distal femoral epiphysis showed no changes between months. The distal femoral metaphyseal region showed higher trabecular spacing and lower trabecular number in May than August, otherwise, there were no differences in trabecular parameters. The few monthly differences in bone properties may be due to physical inactivity from periodic summer estivation or from the timing of birth and growth in spring and summer months. Anat Rec, 300:2175-2183, 2017. © 2017 Wiley Periodicals, Inc.