Inhibition of myostatin signal pathway may be involved in low-intensity pulsed ultrasound promoting bone healing

Mechanism and physical activities in bone-skeletal muscle crosstalk

Bone and skeletal muscle work in coordination to maintain the function of the musculoskeletal system, in which skeletal muscle contraction drives the movement of the bone lever system while bone provides insert sites for skeletal muscle through the bone-muscle junction. Existing evidence suggests that factors secreted by skeletal muscle and bone mediate the interaction between the two tissues. Herein, we focused on the relationship between skeletal muscle and bone and the underlying mechanism of the interaction. Exercise can promote bone strength and secrete osteocalcin and insulin-like growth factor I into the blood, thus improving muscle quality. In addition, exercise can also promote myostatin, interleukin-6, Irisin, and apelin in muscles to enter the blood so that they can act on bones to maintain the balance between bone absorption and bone formation. There is a special regulatory axis interleukin-6/osteocalcin between myokines and osteokines, which is mainly influenced by exercise. Therefore, we pay attention to the important factors in the bone-muscle intersection that are affected by exercise, which were found or their functions were expanded, which strengthened the connection between organs of the whole body, highlighting the importance of exercise and contributing to the diagnosis, prevention, and treatment of osteoporosis and sarcopenia in the clinic.

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.

Pulsed frequency modulated ultrasound promotes therapeutic effects of osteoporosis induced by ovarian failure in mice

Low-intensity pulsed ultrasound (LIPUS) has been proved to be an effective technique for the treatment of osteoporosis. To better activate the bone formation-related markers, promote the different stages of osteogenesis, and further enhance the therapeutic effects of ultrasound, this study employed pulsed frequency modulated ultrasound (pFMUS) to treat mice with osteoporosis, which was caused by ovarian failure due to 4-vinylcyclohexene dioxide (VCD) injection. Healthy 8-week-old female C57BL/6J mice were randomly divided into four groups: Sham (S), VCD-control (V), VCD + LIPUS (VU), and VCD + pFMUS (VFU). VU and VFU groups were treated by LIPUS and pFMUS, respectively. Serum analysis, micro-computed tomography (micro-CT), mechanical testing and hematoxylin and eosin (HE) staining were performed to evaluate the therapeutic effects of ultrasound. Quantitative reverse-transcription PCR (qRT-PCR) and western blot analysis were used to explore the mechanism of ultrasound on osteoporosis. Results showed that pFMUS might have better therapeutic effects than traditional LIPUS in terms of bone microstructure and bone strength. In addition, pFMUS could promote bone formation by activating phosphoinositide-3 kinase/protein kinase B (PI3K/Akt) pathway, and slow down bone resorption by increasing osteoprotegerin/receptor activator of nuclear factor κB ligand (OPG/RANKL) ratio. This study is of positive prognostic significance when understanding the mechanism of ultrasound regulation on osteoporosis and establishing novel treatment plan of osteoporosis by multi-frequency ultrasound.

A Numerical Study on Mechanical Effects of Low-Intensity Pulsed Ultrasound on Trabecular Bone and Osteoblasts

The lack of sufficient mechanical stimulation to the human bone, results in disuse osteoporosis. Low-intensity pulsed ultrasound (LIPUS) promotes fracture healing and the treatment of disuse osteoporosis, but its biomechanical mechanism remains unknown. Simulative research on the mechanical effects of LIPUS on disuse trabecular bone and osteoblasts have been performed. The von Mises stress of disuse trabecular bone and osteoblasts obviously increased under LIPUS irradiation. The average von Mises stress of osteoblasts were two orders of magnitude higher under the irradiation of simulant LIPUS than that without LIPUS irradiation, and the von Mises stress of osteoblasts was positively correlated with the amplitude of sound pressure excitation. The results showed that LIPUS irradiation could obviously improve the mechanical micro-environment of trabecular bone and osteoblasts to alleviate the lack of mechanical stimulation. The results of the research can reveal the biomechanical mechanism of LIPUS in the treatment of disuse osteoporosis to some extent and provide theoretical guidance for clinical treatment of disuse osteoporosis through physical methods.

Molecular basis and therapeutic potential of myostatin on bone formation and metabolism in orthopedic disease

Myostatin, a member of the transforming growth factor-β (TGF-β) superfamily, is a key autocrine/paracrine inhibitor of skeletal muscle growth. Recently, researchers have postulated that myostatin is a negative regulator of bone formation and metabolism. Reportedly, myostatin is highly expressed in the fracture area, affecting the endochondral ossification process during the early stages of fracture healing. Furthermore, myostatin is highly expressed in the synovium of patients with rheumatoid arthritis (RA) and is an effective therapeutic target for interfering with osteoclast formation and joint destruction in RA. Thus, myostatin is a potent anti-osteogenic factor and a direct modulator of osteoclast differentiation. Evaluation of the molecular pathway revealed that myostatin can activate SMAD and mitogen-activated protein kinase signaling pathways, inhibiting the Wnt/β-catenin pathway to synergistically regulate muscle and bone growth and metabolism. In summary, inhibition of myostatin or the myostatin signaling pathway has therapeutic potential in the treatment of orthopedic diseases. This review focused on the effects of myostatin on bone formation and metabolism and discussed the potential therapeutic effects of inhibiting myostatin and its pathways in related orthopedic diseases.

Genetic response to low‑intensity ultrasound on mouse ST2 bone marrow stromal cells

Although low‑intensity ultrasound (LIUS) is a clinically established procedure, the early cellular effect of LIUS on a genetic level has not yet been studied. The current study investigated the early response genes elicited by LIUS in bone marrow stromal cells (BMSCs) using global‑scale microarrays and computational gene expression analysis tools. Mouse ST2 BMSCs were treated with LIUS [I_SATA, 25 mW/cm² for 20 min with a frequency of 1.11 MHz in a pulsed‑wave mode (0.2‑s burst sine waves repeated at 1 kHz)], then cultured for 0.5, 1 and 3 h at 37˚C. The time course of changes in gene expression was evaluated using GeneChip^® high‑density oligonucleotide microarrays and Ingenuity^® Pathway Analysis tools. The results were verified by reverse transcription‑quantitative polymerase chain reaction (RT‑qPCR). A single exposure of LIUS did not affect cell morphology, cell growth or alkaline phosphatase activity. However, 61 upregulated and 103 downregulated genes were identified from 0.5 to 3 h after LIUS treatment. Two significant gene networks, labeled E and H, were identified from the upregulated genes, while a third network, labeled T, was identified from the downregulated genes. Gene network E or H containing the immediate‑early genes FBJ osteosarcoma oncogene and early growth response 1 or the heat shock proteins heat shock protein 1a/b was associated mainly with the biological functions of bone physiology and protein folding or apoptosis, respectively. Gene network T containing transcription factors fos‑like antigen 1 and serum response factor was also associated with the biological functions of the gene expression. RT‑qPCR indicated that the expression of several genes in the gene networks E and H were elevated in LIUS‑treated cells. LIUS was demonstrated to induce gene expression after short application in mouse ST2 BMSCs. The results of the present study provide a basis for the elucidation of the detailed molecular mechanisms underlying the cellular effects of LIUS.

Bone and Muscle Crosstalk in Aging

Osteoporosis and sarcopenia are two age-related diseases that affect the quality of life in the elderly. Initially, they were thought to be two independent diseases; however, recently, increasing basic and clinical data suggest that skeletal muscle and bone are both spatially and metabolically connected. The term “osteosarcopenia” is used to define a condition of synergy of low bone mineral density with muscle atrophy and hypofunction. Bone and muscle cells secrete several factors, such as cytokines, myokines, and osteokines, into the circulation to influence the biological and pathological activities in local and distant organs and cells. Recent studies reveal that extracellular vesicles containing microRNAs derived from senescent skeletal muscle and bone cells can also be transported and aid in regulating bone-muscle crosstalk. In this review, we summarize the age-related changes in the secretome and extracellular vesicle-microRNAs secreted by the muscle and bone, and discuss their interactions between muscle and bone cells during aging.

Muscle, Bone, and Fat Crosstalk: the Biological Role of Myokines, Osteokines, and Adipokines

PURPOSE OF REVIEW

Skeletal muscle and bone are connected anatomically and physiologically, and play a crucial role in human locomotion and metabolism. Historically, the coupling between muscle and bone has been viewed in light of mechanotransduction, which dictates that the mechanical forces applied to muscle are transmitted to the skeleton to initiate bone formation. However, these organs also communicate through the endocrine system, orchestrated by a family of cytokines namely myokines (derived from myocytes) and osteokines (derived from bone cells). A third player in this biochemical crosstalk is adipose tissue and the secretion of adipokines (derived from adipocytes). In this review, we discuss the bidirectional effects of myokines and osteokines on muscle and bone metabolism, and the impact of adipokines on both of these secretory organs.

RECENT FINDINGS

Several myokines, notably, IL6, irisin, IGF-1, BDNF, myostatin, and FGF2 exert anabolic/catabolic effects on bone, while the osteokines osteocalcin and sclerostin have shown to induce muscle anabolism and catabolism, respectively. Adipokines, such as leptin, resistin, adiponectin, and TNFα (released from adipose tissue), can also modulate muscle and bone metabolism. Contrarily, exercise-mediated release of lipolytic myokines (IL6, irisin, and LIF) stimulates thermogenesis by promoting the browning of adipocytes. Myokines, osteokines, and adipokines exert autocrine/paracrine effects locally as well as through the endocrine system, to regulate muscle, bone, and fat metabolism. Reductions in physical activity and increases in energy intake, both linked with aging, leads to adipocyte hypertrophy and the recruitment of immunological cells (macrophages). In turn, this releases pro-inflammatory adipokines which induces chronic low-grade inflammation (LGI), a key player in the pathology of several diseases. However, exercise-induced stimulation of bioactive cytokines, through muscle-bone-fat crosstalk, increases muscle anabolism, bone formation, mitochondrial biogenesis, glucose utilization, and fatty acid oxidation, and attenuates chronic LGI.

Low-intensity pulsed ultrasound promotes the expression of immediate-early genes in mouse ST2 bone marrow stromal cells

PURPOSE

The effects of low-intensity pulsed ultrasound (LIPUS) on the expression of immediate-early genes (IEGs) in bone marrow stromal cells (BMSCs) were evaluated to elucidate the early cellular response to LIPUS.

METHODS

Mouse ST2 BMSCs were treated with LIPUS (I_SATA, 12-34 mW/cm² for 20 min), then cultured at 37 °C. The expression levels of four IEGs (Fos, Egr1, Jun, and Ptgs2) and ERK1/2, a mitogen-activated protein kinase/extracellular signal-regulated kinase (MAPK/ERK), were assessed using real-time quantitative PCR and Western blot analyses, respectively.

RESULTS

A single exposure of LIPUS at an intensity of 25 mW/cm² significantly and transiently increased the expression levels of all four IEGs, and the peak expression was detected at 30-60 min after LIPUS stimulation. LIPUS exposure also significantly increased the phosphorylation level of ERK1/2. U0126, an inhibitor of MAPK/ERK, significantly prevented LIPUS-induced expression of Fos and Egr1, but not that of Jun and Ptgs2. On the other hand, treatment of the cells with LIPUS did not affect cell growth or alkaline phosphatase activity, a marker of osteoblast differentiation.

CONCLUSION

These results suggest that LIPUS exposure significantly induces expression of IEGs such as Fos and Egr1 via the MAPK/ERK pathway in ST2 BMSCs.