Serum sclerostin increases in healthy adult men during bed rest

Muscle-bone crosstalk via endocrine signals and potential targets for osteosarcopenia-related fracture

Background

Osteosarcopenia is a syndrome coexisting sarcopenia and osteopenia/osteoporosis, with a high fracture risk. Recently, skeletal muscle and bone have been recognized as endocrine organs capable of communication through secreting myokines and osteokines, respectively. With a deeper understanding of the muscle-bone crosstalk, these endocrine signals exhibit an important role in osteosarcopenia development and fracture healing.

Methods

This review summarizes the role of myokines and osteokines in the development and treatment of osteosarcopenia and fracture, and discusses their potential for osteosarcopenia-related fracture treatment.

Results

Several well-defined myokines (myostatin and irisin) and osteokines (RANKL and SOST) are found to not only regulate skeletal muscle and bone metabolism but also influence fracture healing processes. Systemic interventions targeting these biochemical signals has shown promising results in improving the mass and functions of skeletal muscle and bone, as well as accelerating fracture healing processes.

Conclusion

The regulation of muscle-bone crosstalk via biochemical signals presents a novel and promising strategy for treating osteosarcopenia and fracture by simultaneously enhancing bone and muscle anabolism. We propose that myostatin, irisin, RANKL, and SOST may serve as potential targets to treat fracture patients with osteosarcopenia.

The translational potential of this article

Osteosarcopenia is an emerging geriatric syndrome where sarcopenia and osteoporosis coexist, with high fracture risk, delayed fracture healing, and increased mortality. However, no pharmacological agent is available to treat fracture patients with osteosarcopenia. This review summarizes the role of several myokines and osteokines in the development and treatment of osteosacropenia and fracture, as well as discusses their potential as intervention targets for osteosarcopenia-related fracture, which provides a novel and promising strategy for future osteosarcopenia-related fracture treatment.

Role of the Osteocyte in Musculoskeletal Disease

PURPOSE OF THE REVIEW

The purpose of this review is to summarize the role of the osteocyte in muscle atrophy in cancer patients, sarcopenia, spinal cord injury, Duchenne's muscular dystrophy, and other conditions associated with muscle deterioration.

RECENT FINDINGS

One type of bone cell, the osteocyte, appears to play a major role in muscle and bone crosstalk, whether physiological or pathological. Osteocytes are cells living within the bone-mineralized matrix. These cells are connected to each other by means of dendrites to create an intricately connected network. The osteocyte network has been shown to respond to different types of stimuli such as mechanical unloading, immobilization, aging, and cancer by producing osteocytes-derived factors. It is now becoming clear that some of these factors including sclerostin, RANKL, TGF-β, and TNF-α have detrimental effects on skeletal muscle. Bone and muscle not only communicate mechanically but also biochemically. Osteocyte-derived factors appear to contribute to the pathogenesis of muscle disease and could be used as a cellular target for new therapeutic approaches.

Polymorphism of LRP4 Gene (rs9667108) among Post Menopause Women with Osteoporosis

Background

Many studies have been done to identify the factors that influence the development and progression of osteoporosis. One genetic factor is polymorphisms of LRP4 gene. Regarding the lack of comprehensive study on polymorphisms of LRP4 gene in the north of Iran, mainly Mazandaran Province, we decided to investigate the polymorphism of this gene in postmenopausal women with osteoporosis.

Methods

This case-control study has been conducted at GhaemShahr Valiasr Hospital on 100 female patients with osteoporosis (average age of 58.1) and 90 healthy females without osteoporosis (average age of 55.2). After sampling and extraction of genomic DNA via of the salt deposition method, the genotype and SNP (rs9667108) polymorphism of LRP4 gene were evaluated with the PCR-RFLP method. Restriction enzymes cut the PCR products. In order to identify patients, their bone mineral density was tested by the DEXA method. The results of digestion (digestion enzyme) were analyzed by MedCalc, SPSS software, Hardy-Weinberg equilibrium, and Chi2.

Results

The statistical analysis has shown the significant relationship between SNP (rs9667108) polymorphism and the risk of osteoporosis disease in patients and control groups (P<0.05). In SNP (rs9667108), the GC genotype, compared to GG, increased the risk of disease significantly (1.556 time). Similarly, CC genotype, compared to GG genotype, increased the risk of this disease by 2.091 time.

Conclusion

The existence of mutation in the LRP4 gene could increase susceptibility to osteoporosis disease. Moreover, determining this patient's genotype in SNP (rs9667108) can be used to identify individuals who are in endanger osteoporosis.

Pharmaceutical treatment of bone loss: From animal models and drug development to future treatment strategies

Animal models are fundamental to advance our knowledge of the underlying pathophysiology of bone loss and to study pharmaceutical countermeasures against it. The animal model of post-menopausal osteoporosis from ovariectomy is the most widely used preclinical approach to study skeletal deterioration. However, several other animal models exist, each with unique characteristics such as bone loss from disuse, lactation, glucocorticoid excess, or exposure to hypobaric hypoxia. The present review aimed to provide a comprehensive overview of these animal models to emphasize the importance and significance of investigating bone loss and pharmaceutical countermeasures from perspectives other than post-menopausal osteoporosis only. Hence, the pathophysiology and underlying cellular mechanisms involved in the various types of bone loss are different, and this might influence which prevention and treatment strategies are the most effective. In addition, the review sought to map the current landscape of pharmaceutical countermeasures against osteoporosis with an emphasis on how drug development has changed from being driven by clinical observations and enhancement or repurposing of existing drugs to today's use of targeted anti-bodies that are the result of advanced insights into the underlying molecular mechanisms of bone formation and resorption. Moreover, new treatment combinations or repurposing opportunities of already approved drugs with a focus on dabigatran, parathyroid hormone and abaloparatide, growth hormone, inhibitors of the activin signaling pathway, acetazolamide, zoledronate, and romosozumab are discussed. Despite the considerable progress in drug development, there is still a clear need to improve treatment strategies and develop new pharmaceuticals against various types of osteoporosis. The review also highlights that new treatment indications should be explored using multiple animal models of bone loss in order to ensure a broad representation of different types of skeletal deterioration instead of mainly focusing on primary osteoporosis from post-menopausal estrogen deficiency.

Bone metabolism during strict head-down tilt bed rest and exposure to elevated levels of ambient CO2

Astronauts on the International Space Station are exposed to levels of atmospheric carbon dioxide (CO₂) above typical terrestrial levels. We explored the possibility that increased levels of ambient CO₂ further stimulate bone resorption during bed rest. We report here data from 2 ground-based spaceflight analog studies in which 12 male and 7 female subjects were placed in a strict 6° head-down tilt (HDT) position for either 30 days at 0.5% ambient CO₂ or 60 days with nominal environmental exposure to CO₂. Bone mineral density (BMD) and bone mineral content (BMC) were determined using dual-energy X-ray absorptiometry (DXA). Blood and urine were collected before and after HDT for biochemical analysis. No change was detected in either BMD or BMC, as expected given the study duration. Bone resorption markers increased after bed rest as expected; however, elevated CO₂ had no additive effect. Elevated CO₂ did not affect concentrations of minerals in serum and urine. Serum parathyroid hormone and 1,25-dihydroxyvitamin D were both reduced after bed rest, likely secondary to calcium efflux from bone. In summary, exposure to 0.5% CO₂ for 30 days did not exacerbate the typical bone resorption response observed after HDT bed rest. Furthermore, results from these strict HDT studies were similar to data from previous bed rest studies, confirming that strict 30-60 days of HDT can be used to evaluate changes in bone metabolism. This is valuable in the continuing effort to develop and refine efficacious countermeasure protocols to mitigate bone loss during spaceflight in low-Earth orbit and beyond.

Bone Tissue Responsiveness To Mechanical Loading-Possible Long-Term Implications of Swimming on Bone Health and Bone Development

PURPOSE OF REVIEW

To revisit the bone tissue mechanotransduction mechanisms behind the bone tissue response to mechanical loading and, within this context, explore the possible negative influence of regular swimming practice on bone health, particularly during the growth and development period.

RECENT FINDINGS

Bone is a dynamic tissue, responsive to mechanical loading and unloading, being these adaptative responses more intense during the growth and development period. Cross-sectional studies usually report a lower bone mass in swimmers compared to athletes engaged in weigh-bearing sports. However, studies with animal models show contradictory findings about the effect of swimming on bone health, highlighting the need for longitudinal studies. Due to its microgravity characteristics, swimming seems to impair bone mass, but mostly at the lower limbs. It is unkown if there is a causal relationship between swimming and low BMD or if other confounding factors, such as a natural selection whithin the sport, are the cause.

Denosumab for Prevention of Acute Onset Immobilization-Induced Alterations of Bone Turnover: A Randomized Controlled Trial

Metabolic bone disease is a devastating condition in critically ill patients admitted to an intensive care unit (ICU). We investigated the effects of early administration of the antiresorptive drug denosumab on bone metabolism in previously healthy patients. Fourteen patients with severe intracerebral or subarachnoid hemorrhage were included in a phase 2 trial. Within 72 hours after ICU admission, they were randomized in a 1:1 ratio to receive denosumab 60 mg or placebo subcutaneously. The primary endpoint was group differences in the percentage change of C-terminal telopeptide of type 1 collagen (CTX-1) levels in serum from denosumab/placebo application to 4 weeks thereafter. Changes in serum levels of bone formation markers and urinary calcium excretion were secondary outcome parameters. Regarding serum levels of CTX-1, changes over time averaged -0.45 ng/mL (95% confidence interval [CI] -0.72, -0.18) for the denosumab group and 0.29 ng/mL (95% CI -0.01, 0.58) for the placebo group. The primary endpoint, the group difference in changes between baseline and secondary measurement, adjusted for baseline serum levels and baseline neurological status, averaged -0.74 ng/mL (95% CI -1.14, -0.34; p = 0.002). The group difference in changes between baseline and secondary osteocalcin measurement averaged -5.60 ng/mL (95% CI -11.2, -0.04; p = 0.049). The group difference in averaged change between baseline and secondary measurement of 24-hour urine calcium excretion was significant (-1.77 mmol/L [95% CI -3.48, -0.06; p = 0.044]). No adverse events could be attributed to the study medication. The investigation proved that a single application of denosumab early after admission to an ICU prevents acute immobilization-associated increase in bone resorption among previously healthy individuals. © 2022 The Authors. Journal of Bone and Mineral Research published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research (ASBMR).

Mechanical loading attenuated negative effects of nucleotide analogue reverse-transcriptase inhibitor TDF on bone repair via Wnt/β-catenin pathway

The nucleotide analog reverse-transcriptase inhibitor, tenofovir disoproxil fumarate (TDF), is widely used to treat hepatitis B virus (HBV) and human immunodeficiency virus infection (HIV). However, long-term TDF usage is associated with an increased incidence of bone loss, osteoporosis, fractures, and other adverse reactions. We investigated the effect of chronic TDF use on bone homeostasis and defect repair in mice. In vitro, TDF inhibited osteogenic differentiation and mineralization in MC3T3-E1 cells. In vivo, 8-week-old C57BL/6 female mice were treated with TDF for 38 days to simulate chronic medication. Four-point bending test and μCT showed reduced bone biomechanical properties and microarchitecture in long bones. To investigate the effects of TDF on bone defect repair, we utilized a bilateral tibial monocortical defect model. μCT showed that TDF reduced new bone mineral tissue and bone mineral density (BMD) in the defect. To verify whether mechanical stimulation may be a useful treatment to counteract the negative bone effects of TDF, controlled dynamic mechanical loading was applied to the whole tibia during the matrix deposition phase on post-surgery days (PSDs) 5 to 8. Second harmonic generation (SHG) of collagen fibers and μCT showed that the reduction of new bone volume and bone mineral density caused by TDF was reversed by mechanical loading in the defect. Immunofluorescent deep tissue imaging showed that chronic TDF treatment reduced the number of osteogenic cells and the volume of new vessels. In addition, chronic TDF treatment inhibited the expressions of periostin and β-catenin, but increased the expression of sclerostin. Both negative effects were reversed by mechanical loading. Our study provides strong evidence that chronic use of TDF exerts direct and inhibitory impacts on bone repair, but appropriate mechanical loading could reverse these adverse effects.

Similarities Between Disuse and Age-Induced Bone Loss

Disuse and aging are known risk factors associated with low bone mass and quality deterioration, resulting in increased fracture risk. Indeed, current and emerging evidence implicate a large number of shared skeletal manifestations between disuse and aging scenarios. This review provides a detailed overview of current preclinical models of musculoskeletal disuse and the clinical scenarios they seek to recapitulate. We also explore and summarize the major similarities between bone loss after extreme disuse and advanced aging at multiple length scales, including at the organ/tissue, cellular, and molecular level. Specifically, shared structural and material alterations of bone loss are presented between disuse and aging, including preferential loss of bone at cancellous sites, cortical thinning, and loss of bone strength due to enhanced fragility. At the cellular level bone loss is accompanied, during disuse and aging, by increased bone resorption, decreased formation, and enhanced adipogenesis due to altered gap junction intercellular communication, WNT/β-catenin and RANKL/OPG signaling. Major differences between extreme short-term disuse and aging are discussed, including anatomical specificity, differences in bone turnover rates, periosteal modeling, and the influence of subject sex and genetic variability. The examination also identifies potential shared mechanisms underlying bone loss in aging and disuse that warrant further study such as collagen cross-linking, advanced glycation end products/receptor for advanced glycation end products (AGE-RAGE) signaling, reactive oxygen species (ROS) and nuclear factor κB (NF-κB) signaling, cellular senescence, and altered lacunar-canalicular connectivity (mechanosensation). Understanding the shared structural alterations, changes in bone cell function, and molecular mechanisms common to both extreme disuse and aging are paramount to discovering therapies to combat both age-related and disuse-induced osteoporosis. © 2022 American Society for Bone and Mineral Research (ASBMR).

Crosstalk between bone and other organs

Bone has long been considered as a silent organ that provides a reservoir of calcium and phosphorus, traditionally. Recently, further study of bone has revealed additional functions as an endocrine organ connecting systemic organs of the whole body. Communication between bone and other organs participates in most physiological and pathological events and is responsible for the maintenance of homeostasis. Here, we present an overview of the crosstalk between bone and other organs. Furthermore, we describe the factors mediating the crosstalk and review the mechanisms in the development of potential associated diseases. These connections shed new light on the pathogenesis of systemic diseases and provide novel potential targets for the treatment of systemic diseases.

Disuse Osteoporosis: Clinical and Mechanistic Insights

Disuse osteoporosis describes a state of bone loss due to local skeletal unloading or systemic immobilization. This review will discuss advances in the field that have shed light on clinical observations, mechanistic insights and options for the treatment of disuse osteoporosis. Clinical settings of disuse osteoporosis include spinal cord injury, other neurological and neuromuscular disorders, immobilization after fractures and bed rest (real or modeled). Furthermore, spaceflight-induced bone loss represents a well-known adaptive process to microgravity. Clinical studies have outlined that immobilization leads to immediate bone loss in both the trabecular and cortical compartments accompanied by relatively increased bone resorption and decreased bone formation. The fact that the low bone formation state has been linked to high levels of the osteocyte-secreted protein sclerostin is one of the many findings that has brought matrix-embedded, mechanosensitive osteocytes into focus in the search for mechanistic principles. Previous basic research has primarily involved rodent models based on tail suspension, spaceflight and other immobilization methods, which have underlined the importance of osteocytes in the pathogenesis of disuse osteoporosis. Furthermore, molecular-based in vitro and in vivo approaches have revealed that osteocytes sense mechanical loading through mechanosensors that translate extracellular mechanical signals to intracellular biochemical signals and regulate gene expression. Osteocytic mechanosensors include the osteocyte cytoskeleton and dendritic processes within the lacuno-canalicular system (LCS), ion channels (e.g., Piezo1), extracellular matrix, primary cilia, focal adhesions (integrin-based) and hemichannels and gap junctions (connexin-based). Overall, disuse represents one of the major factors contributing to immediate bone loss and osteoporosis, and alterations in osteocytic pathways appear crucial to the bone loss associated with unloading.

Sclerostin and Dickkopf-1 Characteristics According to Age and Physical Activity Levels in Premenopausal Women

We aimed to compare serum concentrations of sclerostin and DKK-1 in young (20-30 yrs, n = 25) and middle-aged (35-45 yrs, n = 25) premenopausal women and based on physical activity (PA) status. PA status was assessed by the International Physical Activity Questionnaire (low-moderate (≤ 2999 MET-min/week) and high (≥ 3000 MET-min/week). Serum sclerostin and DKK-1 levels were measured in fasting morning blood samples by ELISA. Areal bone mineral density (aBMD) was measured by DXA, and non-dominant tibia bone characteristics were assessed by pQCT. After adjusting for total body aBMD, middle-aged women had significantly (p < 0.001) higher (0.54 ± 0.01 ng/mL) serum sclerostin than young women (0.41 ± 0.01 ng/mL), and sclerostin was positively correlated with age (r_s = 0.065, p ≤ 0.001) and total PA score (r_s = 0.33, p = 0.021). Young women had higher left trochanter aBMD (p = 0.036) than middle-aged women and aBMD variables were higher (all p ≤ 0.043) in the high active group. Middle-aged women had higher 38% cortical vBMD than young women (p = 0.021), otherwise young women had higher values for pQCT variables (all p ≤ 0.036). Sclerostin showed significant correlations (r = 0.32 to 0.58, all p ≤ 0.026) with spine aBMD for the entire cohort and for each age group. Middle-aged women had significant correlations between sclerostin and hip aBMD sites (r = 0.043 to 0.56, all p ≤ 0.031). Sclerostin and cortical vBMD were positively correlated in the entire cohort (r = 0.35 to 0.50; both p < 0.013); split by age group, middle-aged women had positive correlations (r = 0.45 to 0.61 age and, all p ≤ 0.021) between sclerostin and pQCT variables. No significant differences for physical activity were observed for serum DKK-1 concentrations. Serum sclerostin concentrations were positively associated with age and bone characteristics in premenopausal women; however, these findings were not evident for circulating DKK-1. Further research is needed to elucidate the mechanisms for the discordant results in these Wnt inhibitors.

Bone permeability and mechanotransduction: Some current insights into the function of the lacunar-canalicular network

Lacunar-canalicular (LC) permeability involves the passage of fluids, nutrients, oxygen, ions, and signalling molecules through bone tissue, facilitating the maintenance of bone vitality and function and responses to various physiological conditions and diseases. LC permeability and fluid flow-shear stress/drag force play important roles in mechanotransduction in bone tissue by inducing mechanical stimuli in osteocytes, modulating cellular functions, and determining bone adaptation. Alterations in LC structure may therefore influence the fluid flow pattern through the LC network, thereby affecting the ability of osteocytes to sense and translate mechanical signals and possibly contributing to bone remodelling. Several bone-health conditions are associated with changes in LC structure and function and may affect mechanotransduction and responses, although the mechanisms underlying these associations are still not fully understood. In this review, recent studies of LC networks, their formation and transfer mechanical stimuli, and changes in structure, functional permeability, and mechanotransduction that result from age, pathology, and mechanical loading are discussed. Additionally, applications of vibration and low-intensity pulsed ultrasound in bone healthcare and regeneration fields are also presented.

Sclerostin and bone turnover markers response to cycling and running at the same moderate-to-vigorous exercise intensity in healthy men

BACKGROUND

Recreational cycling is a popular activity which stimulates and improves cardiovascular fitness. The corresponding benefits for bone are unclear.

PURPOSE

This study examined the effect of running (high-impact) vs. cycling (low-impact), at the same moderate-to-vigorous exercise intensity, on markers of bone formation (N-terminal propeptide of type I collagen, PINP) and bone resorption (C-telopeptide of type I collagen, CTX-1), a non-collagenous bone remodeling marker (osteocalcin), as well as bone-modulating factors, including parathyroid hormone (PTH), irisin (myokine) and sclerostin (osteokine).

METHODS

Thirteen healthy men (23.7 ± 1.0 y) performed two progressive exercise tests to exhaustion (peak VO₂) on a cycle ergometer (CE) and on a treadmill (TM). On subsequent separate days, in randomized order, participants performed 30-min continuous running or cycling at 70% heart rate reserve (HRR). Blood was drawn before, immediately post- and 1 h into recovery.

RESULTS

PTH transiently increased (CE, 51.7%; TM, 50.6%) immediately after exercise in both exercise modes. Sclerostin levels increased following running only (27.7%). Irisin increased following both running and cycling. In both exercise modes, CTX-1 decreased immediately after exercise, with no significant change in PINP and osteocalcin.

CONCLUSION

At the same moderate-to-vigorous exercise intensity, running appears to result in a greater transient sclerostin response compared with cycling, while the responses of bone markers, PTH and irisin are similar. The longer-term implications of this differential bone response need to be further examined.

Dissociation of Bone Resorption and Formation in Spaceflight and Simulated Microgravity: Potential Role of Myokines and Osteokines?

The dissociation of bone formation and resorption is an important physiological process during spaceflight. It also occurs during local skeletal unloading or immobilization, such as in people with neuromuscular disorders or those who are on bed rest. Under these conditions, the physiological systems of the human body are perturbed down to the cellular level. Through the absence of mechanical stimuli, the musculoskeletal system and, predominantly, the postural skeletal muscles are largely affected. Despite in-flight exercise countermeasures, muscle wasting and bone loss occur, which are associated with spaceflight duration. Nevertheless, countermeasures can be effective, especially by preventing muscle wasting to rescue both postural and dynamic as well as muscle performance. Thus far, it is largely unknown how changes in bone microarchitecture evolve over the long term in the absence of a gravity vector and whether bone loss incurred in space or following the return to the Earth fully recovers or partly persists. In this review, we highlight the different mechanisms and factors that regulate the humoral crosstalk between the muscle and the bone. Further we focus on the interplay between currently known myokines and osteokines and their mutual regulation.

Aging, Osteo-Sarcopenia, and Musculoskeletal Mechano-Transduction

The decline in the mass and function of bone and muscle is an inevitable consequence of healthy aging with early onset and accelerated decline in those with chronic disease. Termed osteo-sarcopenia, this condition predisposes the decreased activity, falls, low-energy fractures, and increased risk of co-morbid disease that leads to musculoskeletal frailty. The biology of osteo-sarcopenia is most understood in the context of systemic neuro-endocrine and immune/inflammatory alterations that drive inflammation, oxidative stress, reduced autophagy, and cellular senescence in the bone and muscle. Here we integrate these concepts to our growing understanding of how bone and muscle senses, responds and adapts to mechanical load. We propose that age-related alterations in cytoskeletal mechanics alter load-sensing and mechano-transduction in bone osteocytes and muscle fibers which underscores osteo-sarcopenia. Lastly, we examine the evidence for exercise as an effective countermeasure to osteo-sarcopenia.

Osteocytes and Weightlessness

PURPOSE OF REVIEW

Osteocytes are considered to be the cells responsible for mastering the remodeling process that follows the exposure to unloading conditions. Given the invasiveness of bone biopsies in humans, both rodents and in vitro culture systems are largely adopted as models for studies in space missions or in simulated microgravity conditions models on Earth.

RECENT FINDINGS

After a brief recall of the main changes in bone mass and osteoclastic and osteoblastic activities in space-related models, this review focuses on the potential role of osteocytes in directing these changes. The role of the best-known signalling molecules is questioned, in particular in relation to osteocyte apoptosis. The mechanotransduction actors identified in spatial conditions and the problems related to fluid flow and shear stress changes, probably enhanced by the alteration in fluid flow and lack of convection during spaceflight, are recalled and discussed.

Update on the effects of microgravity on the musculoskeletal system

With the reignited push for manned spaceflight and the development of companies focused on commercializing spaceflight, increased human ventures into space are inevitable. However, this venture would not be without risk. The lower gravitational force, known as microgravity, that would be experienced during spaceflight significantly disrupts many physiological systems. One of the most notably affected systems is the musculoskeletal system, where exposure to microgravity causes both bone and skeletal muscle loss, both of which have significant clinical implications. In this review, we focus on recent advancements in our understanding of how exposure to microgravity affects the musculoskeletal system. We will focus on the catabolic effects microgravity exposure has on both bone and skeletal muscle cells, as well as their respective progenitor stem cells. Additionally, we report on the mechanisms that underlie bone and muscle tissue loss resulting from exposure to microgravity and then discuss current countermeasures being evaluated. We reveal the gaps in the current knowledge and expound upon how current research is filling these gaps while also identifying new avenues of study as we continue to pursue manned spaceflight.

Molecular enhancement of fracture healing - Is there a role for Bone Morphogenetic Protein-2, parathyroid hormone, statins, or sclerostin-antibodies?

Enhancement of fracture healing has been a hot topic over the last two decades. This narrative review article is aimed to provide an update on current clinical use and evidence on four clinically available agents in the treatment of fracture healing: bone morphogenetic proteins-2 (BMP-2), parathyroid hormone, statins and sclerostin-antibodies. After first promising results from animal and clinical studies in the early 2000s, BMP-2 was studied mainly in open tibia shaft fractures treated with intramedullary nailing. There are conflicting results from different randomized clinical trials (RCTs) regarding fracture healing time and complications compared to BMP-2 free control treatment in open tibia fractures, as BMP-2 could not show significant differences in patients treated with reamed nails compared to BMP-2 free control treatment with reamed nailing only. Given that fact, its official use was limited in Europe to open tibia shaft fractures treated with unreamed tibial nailing by the European Medical Agency (EMA). Another more recent RCT failed to show equivalence of BMP- 2 together with allograft versus autograft for the treatment of tibia fractures with critical size defects. Recombinant human parathyroid hormone has proven anabolic effects on bone metabolism and is commonly used in treatment of severe osteoporosis. Different animal trials suggested an enhancement effect in fracture healing by PTH. In several clinical trials, PTH seems to have a stimulative effect for lower limb fractures. Statins, commonly used in treatment of dyslipidemia, could also enhance fracture healing in animal trials, especially when they were applied locally at the fracture site. For statins, there is only one RCT that failed to show significant effects for the oral administration of statins in undisplaced distal radius fractures. The role of sclerostin in fracture healing has more and more been understood. Application of sclerostin antibodies has been shown to be beneficial for fracture healing in animal trials. However, no RCTs on the effect of sclerostin antibodies on fracture healing have been performed yet. In conclusion, the "magic bullet" for molecular enhancement of fracture healing has not been identified yet, at least not with its optimal dosage and delivery method.

Global transcriptomic analysis of a murine osteocytic cell line subjected to spaceflight

Bone loss is a major health concern for astronauts during long-term spaceflight and for patients during prolonged bed rest or paralysis. Growing evidence suggests that osteocytes, the most abundant cells in the mineralized bone matrix, play a key role in sensing mechanical forces applied to the skeleton and integrating the orchestrated response into subcellular biochemical signals to modulate bone homeostasis. However, the precise molecular mechanisms underlying both mechanosensation and mechanotransduction in late-osteoblast-to-osteocyte cells under microgravity (µG) have yet to be elucidated. To unravel the mechanisms by which late osteoblasts and osteocytes sense and respond to mechanical unloading, we exposed the osteocytic cell line, Ocy454, to 2, 4, or 6 days of µG on the SpaceX Dragon-6 resupply mission to the International Space Station. Our results showed that µG impairs the differentiation of osteocytes, consistent with prior osteoblast spaceflight experiments, which resulted in the downregulation of key osteocytic genes. Importantly, we demonstrate the modulation of critical glycolysis pathways in osteocytes subjected to microgravity and discovered a set of mechanical sensitive genes that are consistently regulated in multiple cell types exposed to microgravity suggesting a common, yet to be fully elucidated, genome-wide response to microgravity. Ground-based simulated microgravity experiments utilizing the NASA rotating-wall-vessel were unable to adequately replicate the changes in microgravity exposure highlighting the importance of spaceflight missions to understand the unique environmental stress that microgravity presents to diverse cell types. In summary, our findings demonstrate that osteocytes respond to µG with an increase in glucose metabolism and oxygen consumption.

Kindlin-2 mediates mechanotransduction in bone by regulating expression of Sclerostin in osteocytes

Osteocytes act as mechanosensors in bone; however, the underlying mechanism remains poorly understood. Here we report that deleting Kindlin-2 in osteocytes causes severe osteopenia and mechanical property defects in weight-bearing long bones, but not in non-weight-bearing calvariae. Kindlin-2 loss in osteocytes impairs skeletal responses to mechanical stimulation in long bones. Control and cKO mice display similar bone loss induced by unloading. However, unlike control mice, cKO mice fail to restore lost bone after reloading. Osteocyte Kindlin-2 deletion impairs focal adhesion (FA) formation, cytoskeleton organization and cell orientation in vitro and in bone. Fluid shear stress dose-dependently increases Kindlin-2 expression and decreases that of Sclerostin by downregulating Smad2/3 in osteocytes; this latter response is abolished by Kindlin-2 ablation. Kindlin-2-deficient osteocytes express abundant Sclerostin, contributing to bone loss in cKO mice. Collectively, we demonstrate an indispensable novel role of Kindlin-2 in maintaining skeletal responses to mechanical stimulation by inhibiting Sclerostin expression during osteocyte mechanotransduction.

The effects of acute exercise on bone turnover markers in middle-aged and older adults: A systematic review

BACKGROUND

Bone turnover is the cellular machinery responsible for bone integrity and strength and, in the clinical setting, it is assessed using bone turnover markers (BTMs). Acute exercise can induce mechanical stress on bone which is needed for bone remodelling, but to date, there are conflicting results in regards to the effects of varying mechanical stimuli on BTMs.

OBJECTIVES

This systematic review examines the effects of acute aerobic, resistance and impact exercises on BTMs in middle and older-aged adults and examines whether the responses are determined by the exercise mode, intensity, age and sex.

METHODS

We searched PubMed, SCOPUS, Web of Science and EMBASE up to 22nd April 2020. Eligibility criteria included randomised controlled trials (RCTs) and single-arm studies that included middle-aged (50 to 65 years) and older adults (>65 years) and, a single-bout, acute-exercise (aerobic, resistance, impact) intervention with measurement of BTMs. PROSPERO registration number CRD42020145359.

RESULTS

Thirteen studies were included; 8 in middle-aged (n = 275, 212 women/63 men, mean age = 57.9 ± 1.5 years) and 5 in older adults (n = 93, 50 women/43 men, mean age = 68.2 ± 2.2 years). Eleven studies included aerobic exercise (AE, 7 middle-aged/4 older adults), and two included resistance exercise (RE, both middle-aged). AE significantly increased C-terminal telopeptide (CTX), alkaline phosphatase (ALP) and bone-ALP in middle-aged and older adults. AE also significantly increased total osteocalcin (tOC) in middle-aged men and Procollagen I Carboxyterminal Propeptide and Cross-Linked Carboxyterminal Telopeptide of Type I Collagen in older women. RE alone decreased ALP in older adults. In middle-aged adults, RE with impact had no effect on tOC or BALP, but significantly decreased CTX. Impact (jumping) exercise alone increased Procollagen Type 1 N Propeptide and tOC in middle-aged women.

CONCLUSION

Acute exercise is an effective tool to modify BTMs, however, the response appears to be exercise modality-, intensity-, age- and sex-specific. There is further need for higher quality and larger RCTs in this area.

Implications of Altered Endosome and Lysosome Biology in Space Environments

Space exploration poses multiple challenges for mankind, not only on a technical level but also to the entire physiology of the space traveller. The human system must adapt to several environmental stressors, microgravity being one of them. Lysosomes are ubiquitous to every cell and essential for their homeostasis, playing significant roles in the regulation of autophagy, immunity, and adaptation of the organism to changes in their environment, to name a few. Dysfunction of the lysosomal system leads to age-related diseases, for example bone loss, reduced immune response or cancer. As these conditions have been shown to be accelerated following exposure to microgravity, this review elucidates the lysosomal response to real and simulated microgravity. Microgravity activates the endo-lysosomal system, with resulting impacts on bone loss, muscle atrophy and stem cell differentiation. The investigation of lysosomal adaptation to microgravity can be beneficial in the search for new biomarkers or therapeutic approaches to several disease pathologies on earth as well as the potential to mitigate pathophysiology during spaceflight.

Molecular insights into therapeutic promise of targeting of Wnt/β-catenin signaling pathway in obesity

Obesity is a curable disorder which is a global health concern, linked to an excess amount of fat. It is caused by inherited and environmental factors and can be grim to maintain through dieting only. The importance of peculiar Wnt/β-catenin signaling has directed considerable efforts in the future production of therapeutic approaches in metabolic complications, including obesity. The article aims to examine the prospects of Wnt/β-catenin signaling cascade in obesity via directing effects of Wnt/β-catenin cascade in regulating appetite. A deep research on the literature available to date, for Wnt/β-catenin cascade in obesity is conducted using various medical databases like PubMed, MEDLINE from the internet. The articles published in English language were mainly preferred. Obesity has developed endemic worldwide, which initiates various obesity-related comorbidities. Obesity is implied by excessive deposition of fat primarily in the adipose tissue. Numerous studies have shown the vital impact of the Wnt/β-catenin signaling pathway in the growth of body part and biological homeostasis, while latent data illustrate the inherited variations in the Wnt/β-catenin cascade, correlating to several complications. The current article enlightens the stimulation of the Wnt/β-catenin cascade in obesity, mainly depot-explicit impact among adipose tissue during high caloric intake regulation and WAT browning event. Taken all together these data illustrate Wnt/β-catenin signaling cascade subsidizes to obesity promoted insulin resistance independent proliferation of adipose tissue.

Long-Term Immobilization in Elderly Females Causes a Specific Pattern of Cortical Bone and Osteocyte Deterioration Different From Postmenopausal Osteoporosis

Immobilization as a result of long-term bed rest can lead to gradual bone loss. Because of their distribution throughout the bone matrix and remarkable interconnectivity, osteocytes represent the major mechanosensors in bone and translate mechanical into biochemical signals controlling bone remodeling. To test whether immobilization affects the characteristics of the osteocyte network in human cortical bone, femoral diaphyseal bone specimens were analyzed in immobilized female individuals and compared with age-matched postmenopausal individuals with primary osteoporosis. Premenopausal and postmenopausal healthy individuals served as control groups. Cortical porosity, osteocyte number and lacunar area, the frequency of hypermineralized lacunae, as well as cortical bone calcium content (CaMean) were assessed using bone histomorphometry and quantitative backscattered electron imaging (qBEI). Bone matrix properties were further analyzed by Fourier transform infrared spectroscopy (FTIR). In the immobilization group, cortical porosity was significantly higher, and qBEI revealed a trend toward higher matrix mineralization compared with osteoporotic individuals. Osteocyte density and canalicular density showed a declining rate from premenopausal toward healthy postmenopausal and osteoporotic individuals with peculiar reductions in the immobilization group, whereas the number of hypermineralized lacunae accumulated inversely. In conclusion, reduced osteocyte density and impaired connectivity during immobilization are associated with a specific bone loss pattern, reflecting a phenotype clearly distinguishable from postmenopausal osteoporosis. Immobilization periods may lead to a loss of survival signals for osteocytes, provoking bone loss that is even higher than in osteoporosis states, whereas osteocytic osteolysis remains absent. © 2020 The Authors. Journal of Bone and Mineral Research published by American Society for Bone and Mineral Research.

Glucocorticoids Influencing Wnt/β-Catenin Pathway; Multiple Sites, Heterogeneous Effects

Glucocorticoid hormones are vital; their accurate operation is a necessity at all ages and in all life situations. Glucocorticoids regulate diverse physiological processes and they use many signaling pathways to fulfill their effect. As the operation of these hormones affects many organs, the excess of glucocorticoids is actually detrimental to the whole human body. The endogenous glucocorticoid excess is a relatively rare condition, but a significant proportion of adult people uses glucocorticoid medication for the treatment of chronic illnesses, therefore they are exposed to the side effects of long-term glucocorticoid treatment. Our review summarizes the adverse effects of glucocorticoid excess affecting bones, adipose tissue, brain and skin, focusing on those effects which involve the Wnt/β-catenin pathway.

Spaceflight-Induced Bone Tissue Changes that Affect Bone Quality and Increase Fracture Risk

PURPOSE OF REVIEW

Bone mineral density and systemic factors are used to assess skeletal health in astronauts. Yet, even in a general population, these measures fail to accurately predict when any individual will fracture. This review considers how long-duration human spaceflight requires evaluation of additional bone structural and material quality measures that contribute to microgravity-induced skeletal fragility.

RECENT FINDINGS

In both humans and small animal models following spaceflight, bone mass is compromised via reduced bone formation and elevated resorption levels. Concurrently, bone structural quality (e.g., trabecular microarchitecture) is diminished and the quality of bone material is reduced via impaired tissue mineralization, maturation, and maintenance (e.g., mediated by osteocytes). Bone structural and material quality are both affected by microgravity and may, together, jeopardize astronaut operational readiness and lead to increased fracture risk upon return to gravitational loading. Future studies need to directly evaluate how bone quality combines with diminished bone mass to influence bone strength and toughness (e.g., resistance to fracture). Bone quality assessment promises to identify novel biomarkers and therapeutic targets.

LRP receptors in chondrocytes are modulated by simulated microgravity and cyclic hydrostatic pressure

Mechanical loading is essential for the maintenance of musculoskeletal homeostasis. Cartilage has been demonstrated to be highly mechanoresponsive, but the mechanisms by which chondrocytes respond to mechanical stimuli are not clearly understood. The goal of the study was to determine how LRP4, LRP5, and LRP6 within canonical Wnt-signaling are regulated in simulated microgravity and cyclic hydrostatic pressure, and to investigate the potential role of LRP 4/5/6 in cartilage degeneration. Rat chondrosacroma cell (RCS) pellets were stimulated using either cyclic hydrostatic pressure (1Hz, 7.5 MPa, 4hr/day) or simulated microgravity in a rotating wall vessel (RWV) bioreactor (11RPM, 24hr/day). LRP4/5/6 mRNA expression was assessed by RT-qPCR and LRP5 protein expression was determined by fluorescent immunostaining. To further evaluate our in vitro findings in vivo, mice were subjected to hindlimb suspension for 14 days and the femoral heads stained for LRP5 expression. We found that, in vitro, LRP4/5/6 mRNA expression is modulated in a time-dependent manner by mechanical stimulation. Additionally, LRP5 protein expression is upregulated in response to both simulated microgravity and cyclic hydrostatic pressure. LRP5 is also upregulated in vivo in the articular cartilage of hindlimb suspended mice. This is the first study to examine how LRP4/5/6, critical receptors within musculoskeletal biology, respond to mechanical stimulation. Further elucidation of this mechanism could provide significant clinical benefit for the identification of pharmaceutical targets for the maintenance of cartilage health.

Periostin and sclerostin levels in individuals with spinal cord injury and their relationship with bone mass, bone turnover, fracture and osteoporosis status

BACKGROUND

Spinal cord injury (SCI) induces an acute alteration in bone metabolism. Although the aetiology of the bone disturbances is not precisely known, immobilisation reduces mechanical loading and the morphology of osteocytes, which are the primary mechanosensors. Periostin and sclerostin are secreted mostly by osteocytes and are involved in bone's mechanical response.

OBJECTIVE

The present study was conducted to determine whether individuals with SCI present alterations in serum periostin and sclerostin and to assess their relationships with bone mineral density, bone turnover markers, fracture status, time since injury, densitometric osteoporosis and paraplegic vs. tetraplegic status.

SUBJECTS AND METHODS

One hundred and thirty-one individuals with SCI (96 males and 35 females; 42.8 ± 13.7 yr old) with a mean 14.2 ± 12.1 years since the time of injury were evaluated and compared with 40 able-bodied controls in a cross-sectional study. Periostin and sclerostin were assayed by ELISA from Biomedica® (Vienna, Austria), and bone turnover markers and areal bone mineral density (aBMD) were concomitantly analysed.

RESULTS

Compared with controls, individuals with SCI presented higher periostin (p < 0.01), lower sclerostin (p < 0.001), similar markers of bone turnover levels and lower aBMD at the hip. Compared with chronic individuals, bone turnover markers, sclerostin excepted, values were higher as well as aBMD at hip in individuals with acute SCI. Moreover, the aBMD differences were more marked in tetraplegic than paraplegic individuals. Bone mineral density, fracture status, densitometric osteoporosis and paraplegia vs. tetraplegia did not seem to substantially influence the values of biological markers, sclerostin excepted.

CONCLUSION

This study showed for the first time that individuals with SCI presented higher periostin levels than healthy controls only during the acute phase. Conversely, sclerostin levels are lower whatever the post-injury time. Fractures and densitometric osteoporosis were not associated with differences in these two biological markers, whereas paraplegia vs. tetraplegia and fragility fracture status seemed to influence sclerostin levels only.

Exercise Mitigates Bone Loss in Women With Severe Obesity After Roux-en-Y Gastric Bypass: A Randomized Controlled Trial

CONTEXT

Bone loss after bariatric surgery potentially could be mitigated by exercise.

OBJECTIVE

To investigate the role of exercise training (ET) in attenuating bariatric surgery-induced bone loss.

DESIGN

Randomized, controlled trial.

SETTING

Referral center for bariatric surgery.

PATIENTS

Seventy women with severe obesity, aged 25 to 55 years, who underwent Roux-en-Y gastric bypass (RYGB).

INTERVENTION

Supervised, 6-month, ET program after RYGB vs. standard of care (RYGB only).

OUTCOMES

Areal bone mineral density (aBMD) was the primary outcome. Bone microarchitecture, bone turnover, and biochemical markers were secondary outcomes.

RESULTS

Surgery significantly decreased femoral neck, total hip, distal radius, and whole body aBMD (P < 0.001); and increased bone turnover markers, including collagen type I C-telopeptide (CTX), procollagen type I N-propeptide (P1NP), sclerostin, and osteopontin (P < 0.05). Compared with RYGB only, exercise mitigated the percent loss of aBMD at femoral neck [estimated mean difference (EMD), -2.91%; P = 0.007;], total hip (EMD, -2.26%; P = 0.009), distal radius (EMD, -1.87%; P = 0.038), and cortical volumetric bone mineral density at distal radius (EMD, -2.09%; P = 0.024). Exercise also attenuated CTX (EMD, -0.20 ng/mL; P = 0.002), P1NP (EMD, -17.59 ng/mL; P = 0.024), and sclerostin levels (EMD, -610 pg/mL; P = 0.046) in comparison with RYGB. Exercise did not affect biochemical markers (e.g., 25(OH)D, calcium, intact PTH, phosphorus, and magnesium).

CONCLUSION

Exercise mitigated bariatric surgery-induced bone loss, possibly through mechanisms involving suppression in bone turnover and sclerostin. Exercise should be incorporated in postsurgery care to preserve bone mass.

Serum sclerostin levels are positively related to bone mineral density in peritoneal dialysis patients: a cross-sectional study

Background

Sclerostin, an antagonist of the Wingless-type mouse mammary tumor virus integration site (Wnt) pathway that regulates bone metabolism, is a potential contributor of chronic kidney disease (CKD)–mineral and bone disorder (MBD), which has various forms of presentation, from osteoporosis to vascular calcification. The positive association of sclerostin with bone mineral density (BMD) has been demonstrated in CKD and hemodialysis (HD) patients but not in peritoneal dialysis (PD) patients. This study assessed the association between sclerostin and BMD in PD patients.

Methods

Eighty-nine PD patients were enrolled; their sera were collected for measurement of sclerostin and other CKD–MBD-related markers. BMD was also assessed simultaneously. We examined the relationship between sclerostin and each parameter through Spearman correlation analysis and by comparing group data between patients with above- and below-median sclerostin levels. Univariate and multiple logistic regression models were employed to define the most predictive of sclerostin levels in the above-median category.

Results

Bivariate analysis revealed that sclerostin was correlated with spine BMD (r = 0.271, P = 0.011), spine BMD T-score (r = 0.274, P = 0.010), spine BMD Z-score (r = 0.237, P = 0.027), and intact parathyroid hormone (PTH; r = − 0.357, P < 0.001) after adjustments for age and sex. High BMD, old age, male sex, increased weight and height, diabetes, and high osteocalcin and uric acid levels were observed in patients with high serum sclerostin levels and an inverse relation was noticed between PTH and sclerostin. Univariate logistic regression analysis demonstrated that BMD is positively correlated with above-median sclerostin levels (odds ratio [OR] = 65.61, P = 0.002); the correlation was retained even after multivariate adjustment (OR = 121.5, P = 0.007).

Conclusions

For the first time, this study demonstrated a positive association between serum sclerostin levels and BMD in the PD population.

Bed rest and accelerated aging in relation to the musculoskeletal and cardiovascular systems and frailty biomarkers: A review

Prolonged bed rest and lifelong physical inactivity cause deleterious effects to multiple physiological systems that appear to hasten aging processes. Many such changes are similar to those seen with microgravity in space, but at a much faster rate. Head down tilt bed rest models are used to study whole-body changes that occur with spaceflight. We propose that bed rest can be used to quantify accelerated human aging in relation to frailty. In particular, frailty as a measure of the accumulation of deficits estimates the variability in aging across systems, and moves away from the traditional single-system approach. Here, we provide an overview of the impact of bed rest on the musculoskeletal and cardiovascular systems as well as frailty-related biological markers and inflammatory cytokines. We also propose future inquiries to study the accumulation of deficits with head down bed rest and bed rest in the clinical setting, specifically to understand how unrepaired and unremoved subclinical and subcellular damage give rise to clinically observable health problems.

Reptiles in Space Missions: Results and Perspectives

Reptiles are a rare model object for space research. However, some reptile species demonstrate effective adaptation to spaceflight conditions. The main scope of this review is a comparative analysis of reptile experimental exposure in weightlessness, demonstrating the advantages and shortcomings of this model. The description of the known reptile experiments using turtles and geckos in the space and parabolic flight experiments is provided. Behavior, skeletal bones (morphology, histology, and X-ray microtomography), internal organs, and the nervous system (morphology, histology, and immunohistochemistry) are studied in the spaceflight experiments to date, while molecular and physiological results are restricted. Therefore, the results are discussed in the scope of molecular data collected from mammalian (mainly rodents) specimens and cell cultures in the parabolic and orbital flights and simulated microgravity. The published data are compared with the results of the gecko model studies after the 12–44.5-day spaceflights with special reference to the unique peculiarities of the gecko model for the orbital experiments. The complex study of thick-toed geckos after three spaceflights, in which all geckos survived and demonstrated effective adaptation to spaceflight conditions, was performed. However, future investigations are needed to study molecular mechanisms of gecko adaptation in space.

Muscle-bone crosstalk and potential therapies for sarco-osteoporosis

The nature of muscle-bone crosstalk has been historically considered to be only mechanical, where the muscle is the load applier while bone provides the attachment sites. However, this dogma has been challenged with the emerging notion that bone and muscle act as secretory endocrine organs affect the function of each other. Biochemical crosstalk occurs through myokines such as myostatin, irisin, interleukin (IL)-6, IL-7, IL-15, insulin-like growth factor-1, fibroblast growth factor (FGF)-2, and β-aminoisobutyric acid and through bone-derived factors including FGF23, prostaglandin E₂ , transforming growth factor β, osteocalcin, and sclerostin. Aside from the biochemical and mechanical interaction, additional factors including aging, circadian rhythm, nervous system network, nutrition intake, and exosomes also have effects on bone-muscle crosstalk. Here, we summarize the current research progress in the area, which may be conductive to identify potential novel therapies for the osteoporosis and sarcopenia, especially when they develop in parallel.

Osteocyte regulation of bone and blood

This past decade has witnessed a renewed interest in the function and biology of matrix-embedded osteocytes and these cells have emerged as master regulators of bone homeostasis. They secrete two very powerful proteins, sclerostin, a Wnt-inhibitor, that suppresses bone formation, and receptor-activator of NF-kB ligand (RANKL), a cytokine required for osteoclastogenesis. Neutralizing antibodies against these proteins are currently used for the treatment of osteoporosis. Recent studies however, ascribed yet another function to osteocytes: the control of hematopoiesis and the HSPC niche, directly and through secreted factors. In the absence of osteocytes there is an increase in HSC mobilization and abnormal lymphopoiesis whereas in the absence of G_sα signaling in these cells there is an increase of myeloid cells. How exactly osteocytes control hematopoiesis or the HSPC niche is still not completely understood. In this review we summarize the actions of osteocytes in bone and then analyze the effects of these cells on hematopoiesis. Future directions and gaps in current knowledge are further discussed.

Retention of osteocytic micromorphology by sclerostin antibody in a concurrent ovariectomy and functional disuse model

Prolonged mechanical unloading in bedridden patients and concurrent hormonal dysregulation represents the cause of one of the severest forms of osteoporosis, a condition for which there are very few efficacious interventions available to date. Sclerostin, a Wnt antagonist, acts as a negative regulator of bone formation. Sclerostin antibody (Scl-Ab)-mediated blockade of sclerostin can dramatically enhance bone formation and reduce bone resorption. This study was designed to investigate the therapeutic effect of the Scl-Ab on severe bone loss induced by concurrent mechanical unloading and estrogen deficiency in a hindlimb-suspended and ovariectomized rat model, and to study the cellular mechanisms underlying severe osteoporosis and Scl-Ab action. Unloading and ovariectomy resulted in severe loss of trabecular and cortical bone mass and strength; Scl-Ab can significantly counteract the deterioration of bone in unloaded and/or ovariectomized rats, with noticeably increased cortical bone formation. Scanning electron microscopy analysis revealed that unloading and ovariectomy lead to multiple morphological and structural abnormalities of osteocytes in cortical bone and the abnormalities were abolished by Scl-Ab administration. This study extends our previous conclusion that Scl-Ab represents a promising therapeutic approach for severe bone loss that occurs after being exposed to estrogen deficiency and prolonged mechanical unloading.

Systemic Bone Loss After Fracture

A history of prior fracture is the most reliable indicator of prospective fracture risk. Increased fracture risk is not confined to the region of the prior fracture, but is operant at all skeletal sites, providing strong evidence of systemic bone loss after fracture. Animal and human studies suggest that systemic bone loss begins shortly after fracture and persists for several years in humans. In fact, bone quantity and bone quality may never fully return to their pre-fracture levels, especially in older subjects, demonstrating a need for improved understanding of the mechanisms leading to systemic bone loss after fracture in order to reduce subsequent fracture risk. Although the process remains incompletely understood, mechanical unloading (disuse), systemic inflammation, and hormones that control calcium homeostasis may all contribute to systemic bone loss. Additionally, individual factors can potentially affect the magnitude and time course of systemic bone loss and recovery. The magnitude of systemic bone loss correlates positively with injury severity and age. Men may also experience greater bone loss or less recovery than women after fracture. This review details the current understanding of systemic bone loss following fracture, including possible underlying mechanisms and individual factors that may affect this injury response.

IGF-1 signaling mediated cell-specific skeletal mechano-transduction

Mechanical loading preserves bone mass and stimulates bone formation, whereas skeletal unloading leads to bone loss. In addition to osteocytes, which are considered the primary sensor of mechanical load, osteoblasts, and bone specific mesenchymal stem cells also are involved. The skeletal response to mechanical signals is a complex process regulated by multiple signaling pathways including that of insulin-like growth factor-1 (IGF-1). Conditional osteocyte deletion of IGF-1 ablates the osteogenic response to mechanical loading. Similarly, osteocyte IGF-1 receptor (IGF-1R) expression is necessary for reloading-induced periosteal bone formation. Transgenic overexpression of IGF-1 in osteoblasts results in enhanced responsiveness to in vivo mechanical loading in mice, a response which is eliminated by osteoblastic conditional disruption of IGF-1 in vivo. Bone marrow derived stem cells (BMSC) from unloaded bone fail to respond to IGF-1 in vitro. IGF-1R is required for the transduction of a mechanical stimulus to downstream effectors, transduction which is lost when the IGF-1R is deleted. Although the molecular mechanisms are not yet fully elucidated, the IGF signaling pathway and its interactions with potentially interlinked signaling cascades involving integrins, the estrogen receptor, and wnt/β-catenin play an important role in regulating adaptive response of cancer bone cells to mechanical stimuli. In this review, we discuss recent advances investigating how IGF-1 and other interlinked molecules and signaling pathways regulate skeletal mechano-transduction involving different bone cells, providing an overview of the IGF-1 signaling mediated cell-specific response to mechanical stimuli. © 2017 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 36:576-583, 2018.

Patients with non-ambulatory cerebral palsy have higher sclerostin levels and lower bone mineral density than patients with ambulatory cerebral palsy

Bone loss is a serious clinical issue in patients with cerebral palsy (CP). Sclerostin has garnered interest as a key mechanosensor in osteocytes, leading to considerations of the therapeutic utilization of anti-sclerostin medications. This study was undertaken to determine associations among mechanical unloading, sclerostin levels, and bone imbalance in patients with CP. A total of 28 patients with CP participated in this cross-sectional study. The following measurements were taken: anthropometrics, clinical diagnosis of CP subtype and ambulatory status, bone mineral density (BMD) z-scores at the lumbar spine and hip, and blood biochemical markers, including sclerostin, parathyroid hormone (PTH), osteocalcin, C-terminal telopeptide, 25-hydroxyvitamin D, 1,25-dihydroxyvitamin D, creatinine, calcium, and phosphorus. In analysis according to CP subtype, patients with spastic CP showed significantly lower BMD z-scores at the lumbar spine and femur neck regions than patients with dyskinetic CP. In analysis according to ambulatory status, patients with non-ambulatory CP showed significantly lower BMD z-scores at all lumbar spine and femoral sites, lower PTH and creatinine levels, and higher plasma sclerostin levels than patients with ambulatory CP. In regression analysis, ambulatory status was a significant determinant of plasma sclerostin levels. This study is the first to report on sclerostin levels and BMD in patients with CP, based on the hypothesis that patients who lack sufficient weight-bearing activities would show increased sclerostin levels and decreased BMD scores, compared with patients who sustain relatively sufficient physical activity. Therefore, this report may provide clinical insights for clinicians considering ambulatory status, sclerostin levels, and bone loss in patients with CP.

Role of Inactivity in Chronic Diseases: Evolutionary Insight and Pathophysiological Mechanisms

This review proposes that physical inactivity could be considered a behavior selected by evolution for resting, and also selected to be reinforcing in life-threatening situations in which exercise would be dangerous. Underlying the notion are human twin studies and animal selective breeding studies, both of which provide indirect evidence for the existence of genes for physical inactivity. Approximately 86% of the 325 million in the United States (U.S.) population achieve less than the U.S. Government and World Health Organization guidelines for daily physical activity for health. Although underappreciated, physical inactivity is an actual contributing cause to at least 35 unhealthy conditions, including the majority of the 10 leading causes of death in the U.S. First, we introduce nine physical inactivity-related themes. Next, characteristics and models of physical inactivity are presented. Following next are individual examples of phenotypes, organ systems, and diseases that are impacted by physical inactivity, including behavior, central nervous system, cardiorespiratory fitness, metabolism, adipose tissue, skeletal muscle, bone, immunity, digestion, and cancer. Importantly, physical inactivity, itself, often plays an independent role as a direct cause of speeding the losses of cardiovascular and strength fitness, shortening of healthspan, and lowering of the age for the onset of the first chronic disease, which in turn decreases quality of life, increases health care costs, and accelerates mortality risk.

Cortical and Trabecular Bone Microstructure Did Not Recover at Weight-Bearing Skeletal Sites and Progressively Deteriorated at Non-Weight-Bearing Sites During the Year Following International Space Station Missions

Risk for premature osteoporosis is a major health concern in astronauts and cosmonauts; the reversibility of the bone lost at the weight-bearing bone sites is not established, although it is suspected to take longer than the mission length. The bone three-dimensional structure and strength that could be uniquely affected by weightlessness is currently unknown. Our objective is to evaluate bone mass, microarchitecture, and strength of weight-bearing and non-weight-bearing bone in 13 cosmonauts before and for 12 months after a 4-month to 6-month sojourn in the International Space Station (ISS). Standard and advanced evaluations of trabecular and cortical parameters were performed using high-resolution peripheral quantitative computed tomography. In particular, cortical analyses involved determination of the largest common volume of each successive individual scan to improve the precision of cortical porosity and density measurements. Bone resorption and formation serum markers, and markers reflecting osteocyte activity or periosteal metabolism (sclerostin, periostin) were evaluated. At the tibia, in addition to decreased bone mineral densities at cortical and trabecular compartments, a 4% decrease in cortical thickness and a 15% increase in cortical porosity were observed at landing. Cortical size and density subsequently recovered and serum periostin changes were associated with cortical recovery during the year after landing. However, tibial cortical porosity or trabecular bone failed to recover, resulting in compromised strength. The radius, preserved at landing, unexpectedly developed postflight fragility, from 3 months post-landing onward, particularly in its cortical structure. Remodeling markers, uncoupled in favor of bone resorption at landing, returned to preflight values within 6 months, then declined farther to lower than preflight values. Our findings highlight the need for specific protective measures not only during, but also after spaceflight, because of continuing uncertainties regarding skeletal recovery long after landing. © 2017 American Society for Bone and Mineral Research.

Carbonic anhydrase III protects osteocytes from oxidative stress

Osteocytes are master orchestrators of bone remodeling; they control osteoblast and osteoclast activities both directly via cell-to-cell communication and indirectly via secreted factors, and they are the main postnatal source of sclerostin and RANKL (receptor activator of NF-kB ligand), two regulators of osteoblast and osteoclast function. Despite progress in understanding osteocyte biology and function, much remains to be elucidated. Recently developed osteocytic cell lines-together with new genome editing tools-has allowed a closer look at the biology and molecular makeup of these cells. By using single-cell cloning, we identified genes that are associated with high Sost/sclerostin expression and analyzed their regulation and function. Unbiased transcriptome analysis of high- vs. low-Sost/sclerostin-expressing cells identified known and novel genes. Dmp1 (dentin matrix protein 1), Dkk1 (Dickkopf WNT signaling pathway inhibitor 1), and Phex were among the most up-regulated known genes, whereas Srpx2, Cd200, and carbonic anhydrase III (CAIII) were identified as novel markers of differentiated osteocytes. Aspn, Enpp2, Robo2, Nov, and Serpina3g were among the transcripts that were most significantly suppressed in high-Sost cells. Considering that CAII was recently identified as being regulated by Sost/sclerostin and capable of controlling mineral homeostasis, we focused our attention on CAIII. Here, we report that CAIII is highly expressed in osteocytes, is regulated by parathyroid hormone both in vitro and in vivo, and protects osteocytes from oxidative stress.-Shi, C., Uda, Y., Dedic, C., Azab, E., Sun, N., Hussein, A. I., Petty, C. A., Fulzele, K., Mitterberger-Vogt, M. C., Zwerschke, W., Pereira, R., Wang, K., Divieti Pajevic, P. Carbonic anhydrase III protects osteocytes from oxidative stress.

Osteocyte Mechanobiology

PURPOSE OF REVIEW

Over the past decades, osteocytes have emerged as mechano-sensors of bone and master regulators of bone homeostasis. This article summarizes latest research and progress made in understanding osteocyte mechanobiology and critically reviews tools currently available to study these cells.

RECENT FINDINGS

Whereas increased mechanical forces promote bone formation, decrease loading is always associated with bone loss and skeletal fragility. Recent studies identified cilia, integrins, calcium channels, and G-protein coupled receptors as important sensors of mechanical forces and Ca²⁺ and cAMP signaling as key effectors. Among transcripts regulated by mechanical forces, sclerostin and RANKL have emerged as potential therapeutic targets for disuse-induced bone loss. In this paper, we review the mechanisms by which osteocytes perceive and transduce mechanical cues and the models available to study mechano-transduction. Future directions of the field are also discussed.

Serum Sclerostin Increases After Acute Physical Activity

Physical activity has a major impact on bone density and on osteoporosis prevention. Sclerostin is produced by osteocytes and inhibits bone formation. The impact of exercise on sclerostin secretion has not been studied so far. This pilot study aimed to explore circulating sclerostin levels immediately after acute exercise. Healthy young women practicing physical activity less than 120 min per week were enrolled. The exercise was a 45-min, low-speed, treadmill running test. Blood samples were taken at rest before exercise and within 5 min after the end of exercise. We assessed serum creatinine, 25-OH vitamin D, alkaline phosphatase, C-telopeptide of type I collagen, bone-specific alkaline phosphatase, and sclerostin. Sclerostin stability at rest was also validated over the same period of time among women fulfilling the same inclusion criteria. The study included 23 participants (mean ± SD age: 22.9 ± 1.5 years) for the exercise test and 9 participants for the resting test (26.1 ± 3.1 years). There was no difference in body mass index between the two groups. Sclerostin increased after exercise in comparison to baseline (mean ± SEM: 410 ± 27 vs. 290 ± 19 pg/mL; p < 0.001) corresponding to an increase of +44.3 ±5.5%. In the resting test, sclerostin remained stable (303 ± 20 vs. 294 ± 20 pg/mL, p = 0.76). There was a substantial increase in serum sclerostin in untrained healthy young women immediately after physical activity. These results suggest the existence of an acute release of systemic sclerostin in response to physical activity.