Unilateral heat accelerates bone elongation and lengthens extremities of growing mice

Brown fat-specific mitoribosomal function is crucial for preventing cold exposure-induced bone loss

This study examines the interplay between ambient temperature, brown adipose tissue (BAT) function, and bone metabolism, emphasizing the effects of cold exposure and BAT mitochondrial activity on bone health. Utilizing ovariectomized (OVX) mice to model primary osteoporosis and BAT-specific mitochondrial dysfunction (BKO) mice, we evaluated the impact of housing temperature on bone density, immune modulation in bone marrow, and the protective role of BAT against bone loss. Cold exposure was found to universally reduce bone mass, enhance osteoclastogenesis, and alter bone marrow T-cell populations, implicating the immune system in bone remodeling under cold stress. The thermogenic function of BAT, driven by mitochondrial oxidative phosphorylation, was crucial in protecting against bone loss. Impaired BAT function, through surgical removal or mitochondrial dysfunction, exacerbated bone loss in cold environments, highlighting BAT's metabolic role in maintaining bone health. Furthermore, cold-induced changes in BAT function led to systemic metabolic shifts, including elevated long-chain fatty acids, which influenced osteoclast differentiation and activity. These findings suggest a systemic mechanism connecting environmental temperature and BAT metabolism with bone physiology, providing new insights into the metabolic and environmental determinants of bone health. Future research could lead to novel bone disease therapies targeting these pathways.

Enhancing Bone Healing through Localized Cold Therapy in a Murine Femoral Fracture Model

Fracture healing, a critical and complex biological process, often presents challenges in clinical practice with the current standards failing to fully address the medical needs for rapid and effective recovery. In this work, a localized cold therapy is investigated as an alternative approach to expedite bone healing. We hypothesized that optimized cold application can enhance bone healing within a fracture model by inducing hypoxia, leading to accelerated angiogenesis along with improved osteogenesis. A short, localized cold exposure is directly applied to the fracture site over a four-week period in a mouse fracture model, aiming to assess its impact on bone formation through mechanisms of angiogenesis and osteogenesis. Our results revealed a significantly greater volume of new bone tissue and enhanced vascularity at the fracture site in the cold-treated group compared to controls. Calcified tissue histology analysis showed that the accelerated callus maturation and development of the vascular network following cold exposure were associated with an activity increase of alkaline phosphatase (ALP) and transient receptor potential vanilloid 1 (TRPV1). These biological changes were accompanied by a hypoxic environment induced during cold therapy. The study provides compelling evidence supporting the efficacy of intermittent cold therapy in accelerating fracture healing. These promising results highlight the need for further research in larger-scale studies and diverse fracture models, underlining the potential of cold therapy as a novel, non-invasive treatment strategy in orthopedic care.

Cold exposure-induced plasma exosomes impair bone mass by inhibiting autophagy

Recently, environmental temperature has been shown to regulate bone homeostasis. However, the mechanisms by which cold exposure affects bone mass remain unclear. In our present study, we observed that exposure to cold temperature (CT) decreased bone mass and quality in mice. Furthermore, a transplant of exosomes derived from the plasma of mice exposed to cold temperature (CT-EXO) can also impair the osteogenic differentiation of BMSCs and decrease bone mass by inhibiting autophagic activity. Rapamycin, a potent inducer of autophagy, can reverse cold exposure or CT-EXO-induced bone loss. Microarray sequencing revealed that cold exposure increases the miR-25-3p level in CT-EXO. Mechanistic studies showed that miR-25-3p can inhibit the osteogenic differentiation and autophagic activity of BMSCs. It is shown that inhibition of exosomes release or downregulation of miR-25-3p level can suppress CT-induced bone loss. This study identifies that CT-EXO mediates CT-induced osteoporotic effects through miR-25-3p by inhibiting autophagy via targeting SATB2, presenting a novel mechanism underlying the effect of cold temperature on bone mass.

Climate change and its effects on body size and shape: the role of endocrine mechanisms

In many organisms, rapidly changing environmental conditions are inducing dramatic shifts in diverse phenotypic traits with consequences for fitness and population viability. However, the mechanisms that underlie these responses remain poorly understood. Endocrine signalling systems often influence suites of traits and are sensitive to changes in environmental conditions; they are thus ideal candidates for uncovering both plastic and evolved consequences of climate change. Here, we use body size and shape, a set of integrated traits predicted to shift in response to rising temperatures with effects on fitness, and insulin-like growth factor-1 as a case study to explore these ideas. We review what is known about changes in body size and shape in response to rising temperatures and then illustrate why endocrine signalling systems are likely to be critical in mediating these effects. Lastly, we discuss research approaches that will advance understanding of the processes that underlie rapid responses to climate change and the role endocrine systems will have. Knowledge of the mechanisms involved in phenotypic responses to climate change will be essential for predicting both the ecological and the long-term evolutionary consequences of a warming climate. This article is part of the theme issue 'Endocrine responses to environmental variation: conceptual approaches and recent developments'.

Temperature-dependent Developmental Plasticity and Its Effects on Allen's and Bergmann's Rules in Endotherms

Ecogeographical rules, describing common trends in animal form across space and time, have provided key insights into the primary factors driving species diversity on our planet. Among the most well-known ecogeographical rules are Bergmann's rule and Allen's rule, with each correlating ambient temperature to the size and shape of endotherms within a species. In recent years, these two rules have attracted renewed research attention, largely with the goal of understanding how they emerge (e.g., via natural selection or phenotypic plasticity) and, thus, whether they may emerge quickly enough to aid adaptations to a warming world. Yet despite this attention, the precise proximate and ultimate drivers of Bergmann's and Allen's rules remain unresolved. In this conceptual paper, we articulate novel and classic hypotheses for understanding whether and how plastic responses to developmental temperatures might contributed to each rule. Next, we compare over a century of empirical literature surrounding Bergmann's and Allen's rules against our hypotheses to uncover likely avenues by which developmental plasticity might drive temperature-phenotype correlations. Across birds and mammals, studies strongly support developmental plasticity as a driver of Bergmann's and Allen's rules, particularly with regards to Allen's rule. However, plastic contributions toward each rule appear largely non-linear and dependent upon: (1) efficiency of energy use (Bergmann's rule) and (2) thermal advantages (Allen's rule) at given ambient temperatures. These findings suggest that, among endotherms, rapid changes in body shape and size will continue to co-occur with our changing climate, but generalizing the direction of responses across populations is likely naive.

Photothermal extracellular matrix based nanocomposite films and their effect on the osteogenic differentiation of BMSCs

Mild thermal stimulation in vivo could induce osteogenic differentiation of bone marrow mesenchymal stem cells (BMSCs). In this study, nano-functionalized photothermal extracellular matrix (ECM) nanocomposite films were obtained through adding graphene during cell culture, so that graphene could directly integrate with the ECM secreted by cells. Owing to the similarity of the ECM to the in vivo microenvironment and the apparent photothermal effect of graphene nanoflakes, heat could be generated and transferred at the material-cell interface in a biomimetic way. It was demonstrated that such nanocomposite films achieved an interface temperature rise with light illumination. This could be easily sensed by BMSCs through the ECM. According to alkaline phosphatase, osteogenic related gene expression, mineral deposition, and upregulated expression of heat shock protein (HSP70) and p-ERK, composite films with proper illumination significantly promoted the differentiation of BMSCs into osteoblasts. This work endeavors to study the thermal regulation of BMSC differentiation and provide a new perspective on biocompatible osteo-implant materials which can be remotely controlled.

Photo-thermic mineralized collagen coatings and their modulation of macrophages polarization

Macrophages polarization in bone immune microenvironment is crucial in bone regeneration. In this work, mineralized collagen (MC) coatings with photo-thermal effect were prepared through incorporation of polydopamine (PDA). MC coatings with different thicknesses were deposited on titanium substrate through electrochemical deposition. PDA preformed on the substrate, acting as a photo-thermal agent. The effects of light illumination, i.e., different thermal effects, on the polarization of mouse bone marrow-derived macrophages were explored. It was found that heat can promote the M1 polarization of macrophages and inhibit the M2 polarization. Also, gene expression results revealed that such photo illumination based macrophage modulation is effective and safe. It provides a possible way for the design of functional materials to regulate the bone immune microenvironment.

Remodeling Effects of the Combination of GGT Scaffolds, Percutaneous Electrical Stimulation, and Acupuncture on Large Bone Defects in Rats

The regeneration defect of bone is a long-term physiological process after bone injuries. To accelerate the bone remodeling process, the combination of chemical and physical stimulations provides an efficient strategy to allow maturation and to functionalize osteoclasts and osteoblasts. This study aims to investigate the dual effects of a tricalcium phosphate (TCP)-based gelatin scaffold (GGT) in combination with electroacupuncture stimulation on the activation of osteoclasts and osteoblasts, as well as new bone regrowth in vitro and in vivo. We demonstrated that electrical stimulation changes the pH of a culture medium and activates osteoblasts and osteoclasts in an in vitro co-culture system. Furthermore, we showed that electroacupuncture stimulation can enhance osteogenesis and new bone regrowth in vivo and can upregulate the mechanism among parathyroid hormone intact (PTH-i), calcium, osteoclasts, and osteoblasts in the bone-defected rats. Those results showed the potential interest to combine the electroacupuncture technique with GGT scaffolds to improve bone remodeling after injury.

Temperature, heat shock proteins and growth regulation of the bone tissue

Ambient heat modulates the elongation of bones in mammals, and the mechanism of such a plasticity has not been studied completely. The influence of heat on growth and development of bone depends on its values. Five zones of temperature influence on the bone tissue with different biological effects have been distinguished : a) under-threshold thermal zone < 36.6 ºС, insufficient amount of heat is a limiting factor for osteogenesis; b) normal temperature zone 36.6‒37.5 ºС, the processes of breakdown and development of bone in this temperature range is balanced; b) zone of mild thermal shock 39‒41 ºС, the processes of functioning of osteoblasts, osteocytes and formation of the bone tissue intensify; d) the zone of sublethal thermal shock > 42 ºС, growth of bone slows; e) zone of non-critical shock > 50 ºС, bone tissue cells die. We propose a model of the mechanism of influence of heat shock on bone growth. Mild heat shock is a type of stress to which membrane enzymes adenylyl cyclase and cAMP-protein kinase react. Protein kinase A phosphorylates the gene factors of thermal shock proteins, stress proteins and enzymes of energy-generating processes – glycolysis and lipolysis. Heat shock protein HSP70 activates alkaline phosphatase and promotes the process of mineralization of the bone tissue. In the cells, there is intensification in syntheses of insulin-like growth factor-I, factors of mitogenic action, signals of intensification of blood circulation (NO) and synthesis of somatotropin. The affinity between insulin-like growth factor I and its acid-labile subunit decreases, leading to increased free and active insulin-like growth factor I. Against the background of acceleration of the capillarization process, energy generation and the level of stimulators of growth of bone tissue, mitotic and functional activities of producer cells of the bone – osteoblasts and osteocytes – activate. The generally known Allen’s rule has been developed and expanded: “Warm-blooded animals of different species have longer distal body parts (tails) if after birth the young have developed in the conditions of higher temperature”. The indicated tendency is realized through increased biosynthesis of heat shock proteins and other stimulators of growth processes in the bone tissue.

Effects of Twelve Sessions of High-Temperature Sauna Baths on Body Composition in Healthy Young Men

The health benefits of sauna baths are attracting ever-increasing interest. Therefore, the purpose of this study was to evaluate the effects of 12 high-temperature (100 °C) sauna baths on body composition of 23 healthy young men, divided into a control group (CG) and a sauna group (SG). Both groups were initially evaluated by dual-energy X-ray absorptiometry (DXA), after which the SG experienced 12 sessions of sauna baths at high temperatures (100 °C). Initial measurements were carried out after the sauna sessions and after two weeks of decay in both groups. The muscle mass of the right leg (pre vs. decay: 9.50 (5.59) vs. 10.52 (5.15); p < 0.05; Δ 1.07%), bone mineral density (pre vs. post: 1.221 (0.35) vs. 1.315 (0.45); p < 0.05; Δ 7.7%) and bone mineral content (pre vs. post: 0.470 (0.21) vs. 0.499 (0.22); p < 0.05; Δ 6.17%) of the left leg increased in the SG after the sauna baths. It seems that exposure to heat at high temperatures could produce improvements in bone and muscle mass.

Warmth Prevents Bone Loss Through the Gut Microbiota

Osteoporosis is the most prevalent metabolic bone disease, characterized by low bone mass and microarchitectural deterioration. Here, we show that warmth exposure (34°C) protects against ovariectomy-induced bone loss by increasing trabecular bone volume, connectivity density, and thickness, leading to improved biomechanical bone strength in adult female, as well as in young male mice. Transplantation of the warm-adapted microbiota phenocopies the warmth-induced bone effects. Both warmth and warm microbiota transplantation revert the ovariectomy-induced transcriptomics changes of the tibia and increase periosteal bone formation. Combinatorial metagenomics/metabolomics analysis shows that warmth enhances bacterial polyamine biosynthesis, resulting in higher total polyamine levels in vivo. Spermine and spermidine supplementation increases bone strength, while inhibiting polyamine biosynthesis in vivo limits the beneficial warmth effects on the bone. Our data suggest warmth exposure as a potential treatment option for osteoporosis while providing a mechanistic framework for its benefits in bone disease.

The Actions of IGF-1 in the Growth Plate and Its Role in Postnatal Bone Elongation

PURPOSE OF REVIEW

Bone elongation is a complex process driven by multiple intrinsic (hormones, growth factors) and extrinsic (nutrition, environment) variables. Bones grow in length by endochondral ossification in cartilaginous growth plates at ends of developing long bones. This review provides an updated overview of the important factors that influence this process.

RECENT FINDINGS

Insulin-like growth factor-1 (IGF-1) is the major hormone required for growth and a drug for treating pediatric skeletal disorders. Temperature is an underrecognized environmental variable that also impacts linear growth. This paper reviews the current state of knowledge regarding the interaction of IGF-1 and environmental factors on bone elongation. Understanding how internal and external variables regulate bone lengthening is essential for developing and improving treatments for an array of bone elongation disorders. Future studies may benefit from understanding how these unique relationships could offer realistic new approaches for increasing bone length in different growth-limiting conditions.

Climatic adaptation in human inferior nasal turbinate morphology: Evidence from Arctic and equatorial populations

OBJECTIVES

The nasal turbinates directly influence the overall size, shape, and surface area of the nasal passages, and thus contribute to intranasal heat and moisture exchange. However, unlike the encapsulating walls of the nasal cavity, ecogeographic variation in nasal turbinate morphology among humans has not yet been established. Here we investigate variation in inferior nasal turbinate morphology in two populations from climatically extreme environments.

MATERIALS AND METHODS

Twenty-three linear measurements of the inferior turbinate, nasal cavity walls, and airway passages were collected from CT scans of indigenous modern human crania from Equatorial Africa (n = 35) and the Arctic Circle (n = 35). MANOVA and ANCOVA were employed to test for predicted regional and sex differences in morphology between the samples.

RESULTS

Significant morphological differences were identified between the two regional samples, with no evidence of significant sexual dimorphism or region-sex interaction effect. Individuals from the Arctic Circle possessed superoinferiorly and mediolaterally larger inferior turbinates compared to Equatorial Africans. In conjunction with the surrounding nasal cavity walls, these differences in turbinate morphology produced airway dimensions that were both consistent with functional expectations and more regionally distinct than either skeletal component independently.

CONCLUSION

This study documents the existence of ecogeographic variation in human nasal turbinate morphology reflecting climate-mediated evolutionary demands on intranasal heat and moisture exchange. Humans adapted to cold-dry environments exhibit turbinate morphologies that enhance contact between respired air and nasal mucosa to facilitate respiratory air conditioning. Conversely, humans adapted to hot-humid environments exhibit turbinate morphologies that minimize air-to-mucosa contact, likely to minimize airflow resistance and/or facilitate expiratory heat-shedding.

Low temperature decreases bone mass in mice: Implications for humans

OBJECTIVES

Humans exhibit significant ecogeographic variation in bone size and shape. However, it is unclear how significantly environmental temperature influences cortical and trabecular bone, making it difficult to recognize adaptation versus acclimatization in past populations. There is some evidence that cold-induced bone loss results from sympathetic nervous system activation and can be reduced by nonshivering thermogenesis (NST) via uncoupling protein (UCP1) in brown adipose tissue (BAT). Here we test two hypotheses: (1) low temperature induces impaired cortical and trabecular bone acquisition and (2) UCP1, a marker of NST in BAT, increases in proportion to degree of low-temperature exposure.

METHODS

We housed wildtype C57BL/6J male mice in pairs at 26 °C (thermoneutrality), 22 °C (standard), and 20 °C (cool) from 3 weeks to 6 or 12 weeks of age with access to food and water ad libitum (N = 8/group).

RESULTS

Cool housed mice ate more but had lower body fat at 20 °C versus 26 °C. Mice at 20 °C had markedly lower distal femur trabecular bone volume fraction, thickness, and connectivity density and lower midshaft femur cortical bone area fraction versus mice at 26 °C (p < .05 for all). UCP1 expression in BAT was inversely related to temperature.

DISCUSSION

These results support the hypothesis that low temperature was detrimental to bone mass acquisition. Nonshivering thermogenesis in brown adipose tissue increased in proportion to low-temperature exposure but was insufficient to prevent bone loss. These data show that chronic exposure to low temperature impairs bone architecture, suggesting climate may contribute to phenotypic variation in humans and other hominins.

Heat-Induced Limb Length Asymmetry Has Functional Impact on Weight Bearing in Mouse Hindlimbs

Limb length inequality results from many types of musculoskeletal disorders. Asymmetric weight bearing from a limb length discrepancy of less than 2% can have debilitating consequences such as back problems and early-onset osteoarthritis. Existing treatments include invasive surgeries and/or drug regimens that are often only partially effective. As a noninvasive alternative, we previously developed a once daily limb-heating model using targeted heat on one side of the body for 2 weeks to unilaterally increase bone length by up to 1.5% in growing mice. In this study, we applied heat for 1 week to determine whether these small differences in limb length are functionally significant, assessed by changes in hindlimb weight bearing. We tested the hypothesis that heat-induced limb length asymmetry has a functional impact on weight bearing in mouse hindlimbs. Female 3-week-old C57BL/6 mice (N = 12 total) were treated with targeted intermittent heat for 7 days (40 C for 40 min/day). High-resolution x-ray (N = 6) and hindlimb weight bearing data (N = 8) were acquired at the start and end of the experiments. There were no significant left-right differences in starting tibial length or hindlimb weight bearing. After 1-week heat exposure, tibiae (t = 7.7, p < 0.001) and femora (t = 11.5, p < 0.001) were ~1 and 1.4% longer, respectively, on the heat-treated sides (40 C) compared to the non-treated contralateral sides (30 C). Tibial elongation rate was over 6% greater (t = 5.19, p < 0.001). Hindlimb weight bearing was nearly 20% greater (t = 11.9, p < 0.001) and significantly correlated with the increase in tibial elongation rate on the heat-treated side (R² = 0.82, p < 0.01). These results support the hypothesis that even a small limb length discrepancy can cause imbalanced weight distribution in healthy mice. The increase in bone elongation rate generated by localized heat could be a way to equalize limb length and weight bearing asymmetry caused by disease or trauma, leading to new approaches with better outcomes by using heat to lengthen limbs and reduce costly side effects of more invasive interventions.

Heat-stimuli-enhanced osteogenesis using clinically available biomaterials

A recent study reported that heat stress stimulates osteogenesis in an in vivo rat model using alginate gel and magnetite cationic liposomes. However, for clinical use, the efficacy for promoting osteogenesis needs to be investigated using clinically approved materials, and preferably with animals larger than rats. The aim of this study was to evaluate multiple heat stimuli-triggered osteogenesis in rat tibial defect models using already clinically applicable materials (Resovist® and REGENOS®) and determine the efficacy also in the rabbit. Fifty-eight rats and 10 rabbits were divided into two groups, respectively, with or without hyperthermia treatment at 45°C for 15 min. (hyperthermia; 20 rats once a week, 8 rats three times a week, 5 rabbits once a week, control; 30 rats and 5 rabbits). Micro-CT assessment at 4 weeks revealed that a significantly stimulated osteogenesis was observed in the once a week group of both rats and rabbits as compared to the control group (p = 0.018 and 0.036, respectively). In contrast, the three times a week group did not show enhanced osteogenesis. Histological examination and image analysis showed consistent results in which the area of mineralized bone formation in the once a week hyperthermia group was significantly increased compared with that in the control group at four weeks (rat; p = 0.026, rabbit; p = 0.031). Newly formed bone was observed in the grafted materials from the periphery toward the center, and more osteoclasts were found in the once a week group. Heat stress also induced enhanced alkaline phosphatase expression in cultured osteoblastic cells, MC3T3, in vitro (p = 0.03). On the other hand, heat stress had no obvious effects on chondrogenic differentiation using ATDC5 cells. Our study demonstrates that heat-stimuli with clinically applicable novel heating materials can promote significant osteogenesis, and may thus be a promising treatment option for diseases associated with bone defects.

Effects of imbalanced muscle loading on hip joint development and maturation

The mechanical loading environment influences the development and maturation of joints. In this study, the influence of imbalanced muscular loading on joint development was studied using localized chemical denervation of hip stabilizing muscle groups in neonatal mice. It was hypothesized that imbalanced muscle loading, targeting either gluteal muscles or quadriceps muscles, would lead to bilateral hip joint asymmetry, as measured by acetabular coverage, femoral head volume and bone morphometry, and femoral-acetabular shape. The contralateral hip joints as well as age-matched, uninjected mice were used as controls. Altered bone development was analyzed using micro-computed tomography, histology, and image registration techniques at postnatal days (P) 28, 56, and 120. This study found that unilateral muscle unloading led to reduced acetabular coverage of the femoral head, lower total volume, lower bone volume ratio, and lower mineral density, at all three time points. Histologically, the femoral head was smaller in unloaded hips, with thinner triradiate cartilage at P28 and thinner cortical bone at P120 compared to contralateral hips. Morphological shape changes were evident in unloaded hips at P56. Unloaded hips had lower trabecular thickness and increased trabecular spacing of the femoral head compared to contralateral hips. The present study suggests that decreased muscle loading of the hip leads to altered bone and joint shape and growth during postnatal maturation. Statement of Clinical Significance: Adaptations from altered muscle loading during postnatal growth investigated in this study have implications on developmental hip disorders that result from asymmetric loading, such as patients with limb-length inequality or dysplasia. © 2016 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 35:1128-1136, 2017.