Mechanical stimulation controls osteoclast function through the regulation of Ca2+-activated Cl- channel Anoctamin 1

Preventive effects of transcutaneous CO2 application on disuse osteoporosis and muscle atrophy in a rat hindlimb suspension model

We previously demonstrated that transcutaneous CO2 application promotes muscle fiber-type switching, fracture healing, and osteogenesis by increasing blood flow and angiogenesis. Here, we aimed to investigate the preventive effects of transcutaneous CO2 application on disuse osteoporosis and muscle atrophy in a rat hindlimb suspension model. Eleven-week-old male Sprague-Dawley rats were divided into hindlimb suspension (HS), HS with transcutaneous CO2 application (HSCO2), and control groups. HSCO2 rats were administered transcutaneous 100 % CO2 gas in their bilateral hindlimbs, five times a week for 20 min. After 3 weeks, we harvested the gastrocnemius, femur, and tibia for assessment. Histological analysis revealed a significant decrease in the gastrocnemius myofiber cross-sectional area in HS rats compared to the control rats, whereas HSCO2 rats exhibited a significant increase compared to HS rats. Micro-computed tomography showed significant bone atrophy in the trabecular and cortical bones of the femur in HS rats compared to those of the control rats, whereas significant improvement was noted in HSCO2 rats. Histological analysis of the proximal tibia revealed more marrow adipose tissue in the HS rats than in the control rats. However, in the HSCO2 rats, fewer marrow adipose tissue and osteoclasts were observed. Moreover, HSCO2 rats had more osteoblasts and higher expression of peroxisome proliferator-activated receptor gamma coactivator 1-alpha (PGC-1α) and vascular endothelial growth factor (VEGF) than the HS rats. The gastrocnemius and distal femur of HSCO2 rats also exhibited elevated PGC-1α and VEGF expression and upregulation of the myogenesis markers and osteogenesis markers compared to those of HS rats. This treatment effectively prevented disuse osteoporosis and muscle atrophy by promoting local angiogenesis and blood flow. PGC-1α is crucial for promoting this angiogenic pathway. Transcutaneous CO2 application may be a novel preventive procedure for disuse osteoporosis and muscle atrophy, complementing medication and rehabilitation.

Synthetic nanointerfacial bioengineering of Ti implants: on-demand regulation of implant-bone interactions for enhancing osseointegration

Titanium and its alloys are the most commonly used biometals for developing orthopedic implants to treat various forms of bone fractures and defects, but their clinical performance is still challenged by the unfavorable mechanical and biological interactions at the implant-tissue interface, which substantially impede bone healing at the defects and reduce the quality of regenerated bones. Moreover, the impaired osteogenesis capacity of patients under certain pathological conditions such as diabetes and osteoporosis may further impair the osseointegration of Ti-based implants and increase the risk of treatment failure. To address these issues, various modification strategies have been developed to regulate the implant-bone interactions for improving bone growth and remodeling in situ. In this review, we provide a comprehensive analysis on the state-of-the-art synthetic nanointerfacial bioengineering strategies for designing Ti-based biofunctional orthopedic implants, with special emphasis on the contributions to (1) promotion of new bone formation and binding at the implant-bone interface, (2) bacterial elimination for preventing peri-implant infection and (3) overcoming osseointegration resistance induced by degenerative bone diseases. Furthermore, a perspective is included to discuss the challenges and potential opportunities for the interfacial engineering of Ti implants in a translational perspective. Overall, it is envisioned that the insights in this review may guide future research in the area of biometallic orthopedic implants for improving bone repair with enhanced efficacy and safety.

Efferocytosis and Bone Dynamics

PURPOSE OF REVIEW

This review summarizes the recently published scientific evidence regarding the role of efferocytosis in bone dynamics and skeletal health.

RECENT FINDINGS

Several types of efferocytes have been identified within the skeleton, with macrophages being the most extensively studied. Efferocytosis is not merely a 'clean-up' process vital for maintaining skeletal homeostasis; it also plays a crucial role in promoting resolution pathways and orchestrating bone dynamics, such as osteoblast-osteoclast coupling during bone remodeling. Impaired efferocytosis has been associated with aging-related bone loss and various skeletal pathologies, including osteoporosis, osteoarthritis, rheumatoid arthritis, and metastatic bone diseases. Accordingly, emerging evidence suggests that targeting efferocytic mechanisms has the potential to alleviate these conditions. While efferocytosis remains underexplored in the skeleton, recent discoveries have shed light on its pivotal role in bone dynamics, with important implications for skeletal health and pathology. However, there are several knowledge gaps and persisting technical limitations that must be addressed to fully unveil the contributions of efferocytosis in bone.

Insights into the function and regulation of the calcium-activated chloride channel TMEM16A

The TMEM16A channel, a member of the TMEM16 protein family comprising chloride (Cl-) channels and lipid scramblases, is activated by the free intracellular Ca2+ increments produced by inositol 1,4,5-trisphosphate (IP3)-induced Ca2+ release after GqPCRs or Ca2+ entry through cationic channels. It is a ubiquitous transmembrane protein that participates in multiple physiological functions essential to mammals' lives. TMEM16A structure contains two identical 10-segment monomers joined at their transmembrane segment 10. Each monomer harbours one independent hourglass-shaped pore gated by Ca2+ ligation to an orthosteric site adjacent to the pore and controlled by two gates. The orthosteric site is created by assembling negatively charged glutamate side chains near the pore´s cytosolic end. When empty, this site generates an electrostatic barrier that controls channel rectification. In addition, an isoleucine-triad forms a hydrophobic gate at the boundary of the cytosolic vestibule and the inner side of the neck. When the cytosolic Ca2+ rises, one or two Ca2+ ions bind to the orthosteric site in a voltage (V)-dependent manner, thus neutralising the electrostatic barrier and triggering an allosteric gating mechanism propagating via transmembrane segment 6 to the hydrophobic gate. These coordinated events lead to pore opening, allowing the Cl- flux to ensure the physiological response. The Ca2+-dependent function of TMEM16A is highly regulated. Anions with higher permeability than Cl- facilitate V dependence by increasing the Ca2+ sensitivity, intracellular protons can replace Ca2+ and induce channel opening, and phosphatidylinositol 4,5-bisphosphate bound to four cytosolic sites likely maintains Ca2+ sensitivity. Additional regulation is afforded by cytosolic proteins, most likely by phosphorylation and protein-protein interaction mechanisms.

Mechanical protein polycystin-1 directly regulates osteoclastogenesis and bone resorption

Mechanical loading is required for bone homeostasis, but the underlying mechanism is still unclear. Our previous studies revealed that the mechanical protein polycystin-1 (PC1, encoded by Pkd1) is critical for bone formation. However, the role of PC1 in bone resorption is unknown. Here, we found that PC1 directly regulates osteoclastogenesis and bone resorption. The conditional deletion of Pkd1 in the osteoclast lineage resulted in a reduced number of osteoclasts, decreased bone resorption, and increased bone mass. A cohort study of 32,500 patients further revealed that autosomal dominant polycystic kidney disease, which is mainly caused by loss-of-function mutation of the PKD1 gene, is associated with a lower risk of hip fracture than those with other chronic kidney diseases. Moreover, mice with osteoclast-specific knockout of Pkd1 showed complete resistance to unloading-induced bone loss. A mechanistic study revealed that PC1 facilitated TAZ nuclear translocation via the C-terminal tail-TAZ complex and that conditional deletion of Taz in the osteoclast lineage resulted in reduced osteoclastogenesis and increased bone mass. Pharmacological regulation of the PC1-TAZ axis alleviated unloading- and estrogen deficiency- induced bone loss. Thus, the PC1-TAZ axis may be a potential therapeutic target for osteoclast-related osteoporosis.

Mechanosensitivity and mechanotransductive properties of osteoclasts

Mechanical loading is essential for maintaining bone health. Here, we aimed to investigate the role of ATP and ADP in the mechanotransduction of bone-resorptive osteoclasts. Single osteoclast in primary cultures from 10 to 12-wk-old mice was mechanically stimulated by a gentle touch with a micropipette. Changes in cytosolic free calcium [Ca2+]i were analyzed in Fura-2 loaded osteoclasts. The cell injury was assessed by analyzing the cellular Fura-2 loss and classified as severe or mild using k-means. Osteoclasts responded to mechanical stimuli with transient calcium elevation (primary responders) and transduced these signals to neighboring cells, which responded with delayed calcium elevations (secondary responders). Severely injured osteoclasts had higher calcium transients than mildly injured cells. Fluid shear stress similarly induced reversible cell injury in osteoclasts. Secondary responses were abolished by treatment with A-804598, a specific inhibitor of P2X7, but not suramin, a broad P2 receptor blocker. Osteoclasts responded to ATP and ADP with concentration-dependent changes in [Ca2+]i. We performed osteoclast micropipette stimulation in the presence of phosphoenolpyruvate and pyruvate kinase which converted all ADP in solution to ATP, or with hexokinase converting all ATP to ADP. Osteoclasts with mild membrane injury demonstrated similar calcium responses in ATP and ADP-rich environments. However, when the mechanotransductive signal to severe osteoclast injury was converted to ADP, the fraction of secondary responders and their [Ca2+]i amplitude was higher. This study suggests the importance of osteoclast mechanobiology and the role of ADP-mediated signaling in conditions of altered mechanical loading associated with bone loss.NEW & NOTEWORTHY Osteoclasts are rarely considered as cells that participate in mechanical signaling in bone. We show that osteoclasts are capable of sensing and transmitting mechanical signals to neighboring cells. Mechanical stimulation commonly induces minor repairable membrane injury in osteoclasts. ATP and especially ADP were found to play important roles in the mechanoresponsiveness of osteoclasts. This study highlights the importance of osteoclast mechanobiology especially in conditions of altered mechanical loading associated with bone loss, such as in microgravity.

Daily low-intensity pulsed ultrasound stimulation mitigates joint degradation and pain in a post-traumatic osteoarthritis rat model

Objectives

The aim of this study was to investigate the effects of low-intensity pulsed ultrasound (LIPUS) in a post-traumatic osteoarthritis (OA) rat model and in vitro.

Methods

Thirty-eight male, four-month-old Sprague Dawley rats were randomly assigned to Sham, Sham ​+ ​US, OA, and OA ​+ ​US. Sham surgery was performed to serve as a negative control, and anterior cruciate ligament transection was used to induce OA. Three days after the surgical procedures, Sham ​+ ​US and OA ​+ ​US animals received daily LIPUS treatment, while the rest of the groups received sham ultrasound (US) signals. Behavioral pain tests were performed at baseline and every week thereafter. After 31 days, the tissues were collected, and histological analyses were performed on knees and innervated dorsal root ganglia (DRG) neurons traced by retrograde labeling. Furthermore, to assess the activation of osteoclasts by LIPUS treatment, RAW264.7 ​cells were differentiated into osteoclasts and treated with LIPUS.

Results

Joint degradation in cartilage and bone microarchitecture were mitigated in OA ​+ ​US compared to OA. OA ​+ ​US showed improvements in behavioral pain tests. A significant increase of large soma-sized DRG neurons was located in OA compared to Sham. In addition, a greater percentage of large soma-sized innervated neurons were calcitonin gene-related peptide-positive. Daily LIPUS treatment suppressed osteoclastogenesis in vitro, which was confirmed via histological analyses and mRNA expression. Finally, lower expression of netrin-1, a sensory innervation-related protein, was found in the LIPUS treated cells.

Conclusion

Our findings demonstrate that early intervention using LIPUS treatment has protective effects from the progression of knee OA, including reduced tissue degradation, mitigated pain characteristics, improved subchondral bone microarchitecture, and less sensory innervation. Furthermore, daily LIPUS treatment has a suppressive effect on osteoclastogenesis, which may be linked to the suppression of sensory innervation in OA.

The translational potential of this article

This study presents a new potential for early intervention in treating OA symptoms through the use of LIPUS, which involves the suppression of osteoclastogenesis and the alteration of DRG profiles. This intervention aims to delay joint degradation and reduce pain.