Mechanically loaded myotubes affect osteoclast formation

The 40-min HIIT acutely induced bone formation which was likely through the increases in muscle derived interleukin 6 and adiponectin activation: The 16 weeks of HIIT intervention, longitudinal randomized controlled trial

PURPOSE

There is some controversy regarding cytokines released from adipocytes, particularly adiponectin, leptin, and IL6 that regulate bone remodeling. In addition, IL6 is released from muscle contraction, which might have a distinct role in bone remodeling. Hence, this study investigated whether muscle contraction during a session of 40 min of high intensity interval training (40-min HIIT) and after 16 weeks of HIIT (16-wk HIIT) altered the release of those cytokines and bone remodeling in overweight women.

METHODS

In total, 22 overweight, premenopausal women were randomly assigned to either the exercise or the control group. The exercise participants engaged in the 40-min HIIT session at 80-90 % of their heart rate reserve (HRR) three times weekly for 16 weeks, while the control participants performed their routine daily activities. Blood was drawn after overnight fasting and immediately after completing the 40-min HIIT sessions to investigate the association of adiponectin, leptin, IL6, CTX, and P1NP through the acute effect of the 40-min HIIT sessions. This process was repeated after the 16-wk intervention program to observe the training effect of HIIT on cytokines linkage. The bone mineral density (BMD) levels of the distal tibia, femur, and lumbar spine were determined prior to and after the 16-wk intervention using dual-energy X-ray absorptiometry.

RESULTS

The P1NP level increased by 8.29-20.52 % (95 % CI) and by 2.91-15.54 % after completing the first and last bouts of the 40-min HIIT sessions, respectively. In addition, IL6 increased by 13.39-28.03 % (95 % CI), while serum CTX and adiponectin were unaltered from the acute effect of the 40-min HIIT sessions. There was an association between the increases in P1NP and adiponectin (r = 0.682, p = 0.015); however, the increase in P1NP was mostly associated with the increase in IL6 (r = 0.572, p = 0.054) after completing a 40-min HIIT session. After the 16-wk HIIT program, the resting adiponectin level of the exercise participants increased; however, this was associated with neither bone biomarkers nor BMD. The BMDs of the exercise participants were maintained; however, the tibial BMD of the control participants decreased with an increase in the resting CTX level after 16 weeks.

CONCLUSION

Muscle contraction during the 40-min HIIT session elevated the IL6 level, which might have subsequently enhanced bone formation. Furthermore, the association between acute changes in adiponectin and P1NP suggested the possibility of an increase in the sensitivity of the adiponectin receptor in osteoblasts.

Advances in the research on myokine-driven regulation of bone metabolism

The traditional view posits that bones and muscles interact primarily through mechanical coupling. However, recent studies have revealed that myokines, proteins secreted by skeletal muscle cells, play a crucial role in the regulation of bone metabolism. Myokines are widely involved in bone metabolism, influencing bone resorption and formation by interacting with factors related to bone cell secretion or influencing bone metabolic pathways. Here, we review the research progress on the myokine regulation of bone metabolism, discuss the mechanism of myokine regulation of bone metabolism, explore the pathophysiological relationship between sarcopenia and osteoporosis, and provide future perspectives on myokine research, with the aim of identify potential specific diagnostic markers and therapeutic entry points.

Estrogen regulation of myokines that enhance osteoclast differentiation and activity

Osteoporosis and sarcopenia are maladies of aging that negatively affect more women than men. In recent years, it has become apparent that bone and muscle are coupled not only mechanically as muscle pulls on bone, but also at a higher level with myokines, biochemical and molecular signaling occurring between cells of the two tissues. However, how estrogen deficiency in females impacts the chemical crosstalk between bone and muscle cells is not understood. We hypothesize that changes in estrogen signaling alters myokine expression and intensifies bone loss in women. In our present study, we demonstrate that conditioned media from ovariectomized or skeletal muscle deficient in estrogen receptor α (ERα) expression enhances osteoclast differentiation and activity. Using a cytokine array, we identified myokines that have altered expressions in response to loss of estrogen signaling in muscle. Lastly, we demonstrate that conditional deletion of ERα in skeletal muscle results in osteopenia due to an increase in the osteoclast surface per bone surface. Our results suggest that estrogen signaling modulates expression of myokines that regulate osteoclast differentiation and activity.

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.

Applying exercise-mimetic engineered skeletal muscle model to interrogate the adaptive response of irisin to mechanical force

Physical exercise induces the secretion of irisin from contractile muscle into circulation; however, the adaptive response of irisin to mechanical stimulus in skeletal muscle in vitro remains numerously unknown. In an effort to investigate whether irisin is inducible in vitro, we developed a bioreactor consisting of a retractable mechanical force controller and a conditional tissue culture system. Upon this model, a distinguished surge of irisin was detected in stretched myotubes as cyclic strain initiated, and the surge was able to be stalled by knocking out FNDC5. Intriguingly, increased irisin secretory is associated with the shifts of MyHC isoforms from anaerobic type to aerobic type in myotubes. We further revealed that PGC-1α1 and PGC-1α4 mRNAs expression, rather than PGC-1α2 and PGC-1α3, contributed to the generation of irisin in myotubes during cyclic strain. Lastly, combined with co-culturing MC3T3 osteoblasts, we demonstrated the bioactivity of generated irisin, promoting the osteogenic differentiation.

•

Irisin is producible in an exercise-mimetic engineered skeletal muscle model

•

Enhanced irisin production in response to a long-term cyclic stretch

•

PGC-1α1 and PGC-1α4 mRNAs expression contributed to the generation of irisin

•

Demonstration that induced irisin in our model regulating osteoblasts as native ways

Molecular and Biomechanical Adaptations to Mechanical Stretch in Cultured Myotubes

Myotubes are mature muscle cells that form the basic structural element of skeletal muscle. When stretching skeletal muscles, myotubes are subjected to passive tension as well. This lead to alterations in myotube cytophysiology, which could be related with muscular biomechanics. During the past decades, much progresses have been made in exploring biomechanical properties of myotubes in vitro. In this review, we integrated the studies focusing on cultured myotubes being mechanically stretched, and classified these studies into several categories: amino acid and glucose uptake, protein turnover, myotube hypertrophy and atrophy, maturation, alignment, secretion of cytokines, cytoskeleton adaption, myotube damage, ion channel activation, and oxidative stress in myotubes. These biomechanical adaptions do not occur independently, but interconnect with each other as part of the systematic mechanoresponse of myotubes. The purpose of this review is to broaden our comprehensions of stretch-induced muscular alterations in cellular and molecular scales, and to point out future challenges and directions in investigating myotube biomechanical manifestations.

Impact of Intrinsic Muscle Weakness on Muscle-Bone Crosstalk in Osteogenesis Imperfecta

Bone and muscle are highly synergistic tissues that communicate extensively via mechanotransduction and biochemical signaling. Osteogenesis imperfecta (OI) is a heritable connective tissue disorder of severe bone fragility and recently recognized skeletal muscle weakness. The presence of impaired bone and muscle in OI leads to a continuous cycle of altered muscle-bone crosstalk with weak muscles further compromising bone and vice versa. Currently, there is no cure for OI and understanding the pathogenesis of the skeletal muscle weakness in relation to the bone pathogenesis of OI in light of the critical role of muscle-bone crosstalk is essential to developing and identifying novel therapeutic targets and strategies for OI. This review will highlight how impaired skeletal muscle function contributes to the pathophysiology of OI and how this phenomenon further perpetuates bone fragility.

A Pound of Flesh: What Cachexia Is and What It Is Not

Body weight loss, mostly due to the wasting of skeletal muscle and adipose tissue, is the hallmark of the so-called cachexia syndrome. Cachexia is associated with several acute and chronic disease states such as cancer, chronic obstructive pulmonary disease (COPD), heart and kidney failure, and acquired and autoimmune diseases and also pharmacological treatments such as chemotherapy. The clinical relevance of cachexia and its impact on patients’ quality of life has been neglected for decades. Only recently did the international community agree upon a definition of the term cachexia, and we are still awaiting the standardization of markers and tests for the diagnosis and staging of cancer-related cachexia. In this review, we discuss cachexia, considering the evolving use of the term for diagnostic purposes and the implications it has for clinical biomarkers, to provide a comprehensive overview of its biology and clinical management. Advances and tools developed so far for the in vitro testing of cachexia and drug screening will be described. We will also evaluate the nomenclature of different forms of muscle wasting and degeneration and discuss features that distinguish cachexia from other forms of muscle wasting in the context of different conditions.

Bone-Muscle Mutual Interactions

PURPOSE OF REVIEW

The purpose of this review is to describe the current state of our thinking regarding bone-muscle interactions beyond the mechanical perspective.

RECENT FINDINGS

Recent and prior evidence has begun to dissect many of the molecular mechanisms that bone and muscle use to communicate with each other and to modify each other's function. Several signaling factors produced by muscle and bone have emerged as potential mediators of these biochemical/molecular interactions. These include muscle factors such as myostatin, Irisin, BAIBA, IL-6, and the IGF family and the bone factors FGF-23, Wnt1 and Wnt3a, PGE2, FGF9, RANKL, osteocalcin, and sclerostin. The identification of these signaling molecules and their underlying mechanisms offers the very real and exciting possibility that new pharmaceutical approaches can be developed that will permit the simultaneous treatments of diseases that often occur in combination, such as osteoporosis and sarcopenia.

AMP-activated protein kinase (AMPK) regulates autophagy, inflammation and immunity and contributes to osteoclast differentiation and functionabs

Osteoclasts are multinucleated giant cells, responsible for bone resorption. Osteoclast differentiation and function requires a series of cytokines to remove the old bone, which coordinates with the induction of bone remodelling by osteoblast-mediated bone formation. Studies have demonstrated that AMP-activated protein kinase (AMPK) play a negative regulatory role in osteoclast differentiation and function. Research involving AMPK, a nutrient and energy sensor, has primarily focused on osteoclast differentiation and function; thus, its role in autophagy, inflammation and immunity remains poorly understood. Autophagy is a conservative homoeostatic mechanism of eukaryotic cells, and response to osteoclast differentiation and function; however, how it interacts with inflammation remains unclear. Additionally, based on the regulatory function of different AMPK subunits for osteoclast differentiation and function, its activation is regulated by upstream factors to perform bone metabolism. This review summarises the critical role of AMPK-mediated autophagy, inflammation and immunity by upstream and downstream signalling during receptor activator of nuclear factor kappa-B ligand-induced osteoclast differentiation and function. This pathway may provide therapeutic targets for bone-related diseases, as well as function as a biomarker for bone homoeostasis.

Primary osteoporosis in men: an unmet medical need

Osteoporosis is a skeletal disease characterized by loss of bone strength and increased risk of fractures. Even though fracture prevalence is higher in women, fractures also constitute a significant public health issue in older men. Men are screened less and more frequently undertreated than female patients. It is the goal of this review, to summarize updated information about the current understanding of pathophysiology and clinical aspects of diagnosis and treatment of osteoporosis in men.

Closer to Nature Through Dynamic Culture Systems

Mechanics in the human body are required for normal cell function at a molecular level. It is now clear that mechanical stimulations play significant roles in cell growth, differentiation, and migration in normal and diseased cells. Recent studies have led to the discovery that normal and cancer cells have different mechanosensing properties. Here, we discuss the application and the physiological and pathological meaning of mechanical stimulations. To reveal the optimal conditions for mimicking an in vivo microenvironment, we must, therefore, discern the mechanotransduction occurring in cells.

The effects of myokines on osteoclasts and osteoblasts

Recently, muscle has received much attention as an endocrine organ regulating other biological targets, including the pancreas, liver, and adipose tissue. Although there is a possibility that muscle-secreting factors biochemically affect bone metabolism in a paracrine manner, the net effects of myokines on the biology of osteoclasts and osteoblasts, particularly on bone mass in vivo, have not yet been thoroughly investigated. Therefore, we performed in vitro as well as animal experiments using conditioned media (CM) collected from C2C12 myoblast and myotube cultures to better understand the interactions between muscle and bone. Compared with non-CM (i.e., control) and myoblast CM, myotube CM markedly inhibited in vitro bone resorption through the suppression of osteoclast differentiation and resorptive activity of individual osteoclasts. Consistently, the expressions of osteoclast differentiation markers, such as tartrate-resistant acid phosphatase (Trap) and calcitonin receptor (Ctr), decreased with myotube CM. Myotube CM significantly stimulated preosteoblast viability and migration and reduced apoptosis, thereby resulting in an increase in calvaria bone formation. Importantly, systemic treatment with myotube CM for 4 weeks increased bone per tissue volume by 30.7% and 19.6% compared with control and myoblast CM, respectively. These results support the hypothesis that muscle plays beneficial roles in bone health via secretion of anabolic factors, in addition to mechanical stimuli, and importantly indicate that muscle-derived factors can be potential therapeutic targets against metabolic bone diseases.

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.

Muscle-Bone Crosstalk: Emerging Opportunities for Novel Therapeutic Approaches to Treat Musculoskeletal Pathologies

Osteoporosis and sarcopenia are age-related musculoskeletal pathologies that often develop in parallel. Osteoporosis is characterized by a reduced bone mass and an increased fracture risk. Sarcopenia describes muscle wasting with an increasing risk of injuries due to falls. The medical treatment of both diseases costs billions in health care per year. With the impact on public health and economy, and considering the increasing life expectancy of populations, more efficient treatment regimens are sought. The biomechanical interaction between both tissues with muscle acting on bone is well established. Recently, both tissues were also determined as secretory endocrine organs affecting the function of one another. New exciting discoveries on this front are made each year, with novel signaling molecules being discovered and potential controversies being described. While this review does not claim completeness, it will summarize the current knowledge on both the biomechanical and the biochemical link between muscle and bone. The review will highlight the known secreted molecules by both tissues affecting the other and finish with an outlook on novel therapeutics that could emerge from these discoveries.

Osteocyte secreted factors inhibit skeletal muscle differentiation

It is generally accepted that bone and muscle possess the capacity to act in an autocrine, paracrine, or endocrine manner, with a growing body of evidence that suggests muscle can secrete muscle specific cytokines or "myokines", which influence bone metabolism. However, there has been little investigation into the identity of bone specific cytokines that modulate skeletal muscle differentiation and function. This study aimed to elucidate the influence of osteocytes on muscle progenitor cells in vitro and to identify potential bone specific cytokines or "osteokines". We treated C2C12 myoblasts with media collected from differentiated osteocytes (Ocy454 cells) grown in 3D, either under static or fluid flow culture conditions (2 dynes/cm²). C2C12 differentiation was significantly inhibited with a 75% reduction in the number of myofibers formed. mRNA analysis revealed a significant reduction in the expression of myogenic regulatory genes. Cytokine array analysis on the conditioned media demonstrated that osteocytes produce a significant number of cytokines "osteokines" capable of inhibiting myogenesis. Furthermore, we demonstrated that when osteocytes are mechanically activated they induce a greater inhibitory effect on myogenesis compared to a static state. Lastly, we identified the downregulation of numerous cytokines, including Il-6, Il-13, Il-1β, MIP-1α, and Cxcl9, involved in myogenesis, which may lead to future investigation of the role "osteokines" play in musculoskeletal health and pathology.

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

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

Effects of myokines on bone

The links between muscle and bone have been recently examined because of the increasing number of patients with osteoporosis and sarcopenia. Myokines are skeletal muscle-derived humoral cytokines and growth factors, which exert physiological and pathological functions in various distant organs, including the regulation of glucose, energy and bone metabolism. Myostatin is a crucial myokine, the expression of which is mainly limited to muscle tissues. The inhibition of myostatin signaling increases bone remodeling, bone mass and muscle mass, and it may provide a target for the treatment of both sarcopenia and osteoporosis. As myostatin is involved in osteoclast formation and bone destruction in rheumatoid arthritis, myostatin may be a target myokine for the treatment of accelerated bone resorption and joint destruction in rheumatoid arthritis. Numerous other myokines, including transforming growth factor-β, follistatin, insulin-like growth factor-I, fibroblast growth factor-2, osteoglycin, FAM5C, irisin, interleukin (IL)-6, leukemia inhibitory factor, IL-7, IL-15, monocyte chemoattractant protein-1, ciliary neurotrophic factor, osteonectin and matrix metalloproteinase 2, also affect bone cells in various manners. However, the effects of myokines on bone metabolism are largely unknown. Further research is expected to clarify the interaction between muscle and bone, which may lead to greater diagnosis and the development of the treatment for muscle and bone disorders, such as osteoporosis and sarcopenia.

Hydroxyapatite with High Carbonate Substitutions Promotes Osteoclast Resorption through Osteocyte-like Cells

The role of ceramic biomaterials in the repair of bone defects varies from materials that purely fill the physical defects of the injured bone to scaffolds that control cellular behaviors. In this study, we investigated the osteoclast formation related to the osteoconductivity of ceramic biomaterials. We performed in vitro cocultures using osteocyte-like cells and bone marrow cells and in vivo implantations of hydroxyapatite with different amounts of carbonate substitutions into rat femurs. The analyses of the cocultures revealed that bone marrow cells differentiated into osteoclasts and were activated to resorb the substratum when grown on hydroxyapatite with higher numbers of carbonate substitutions. This was indicated by the expression of macrophage colony-stimulating factor and receptor activator of the nuclear factor-kappa B ligand that induce osteoclast differentiation by osteocyte-like cells and characteristic resorption pits. The increased osteoclastogenesis in vivo was observed near the hydroxyapatite with more carbonate substitutions after implantation into the rat femurs. These results suggest that the content of carbonate ions in an apatite crystal lattice has an inductive effect on osteoclastogenesis in the vicinity of the implanted ceramic biomaterial. The results contribute to the design of biomaterials that would be resorbed by osteoclasts after fulfilling their primary function as scaffolds for cell growth and eventually bone regeneration.

IL-6 and IGF-1 Signaling Within and Between Muscle and Bone: How Important is the mTOR Pathway for Bone Metabolism?

Insulin-like growth factor 1 (IGF-1) and interleukin 6 (IL-6) play an important role in the adaptation of both muscle and bone to mechanical stimuli. Here, we provide an overview of the functions of IL-6 and IGF-1 in bone and muscle metabolism, and the intracellular signaling pathways that are well known to mediate these functions. In particular, we discuss the Akt/mammalian target of rapamycin (mTOR) pathway which in skeletal muscle is known for its key role in regulating the rate of mRNA translation (protein synthesis). Since the role of the mTOR pathway in bone is explored to a much lesser extent, we discuss what is known about this pathway in bone and the potential role of this pathway in bone remodeling. We will also discuss the possible ways of influencing IGF-1 or IL-6 signaling by osteocytes and the clinical implications of pharmacological or nutritional modulation of the Akt/mTOR pathway.