Exercise entrainment of musculoskeletal connective tissue clocks.

The musculoskeletal system, crucial for movement and support, relies on the delicate balance of connective tissue homeostasis. Maintaining this equilibrium is essential for tissue health and function. There has been increasing evidence in the past decade that shows the circadian clock as a master regulator of extracellular matrix (ECM) homeostasis in several connective tissue clocks. Very recently, exercise has emerged as a significant entrainment factor for cartilage and intervertebral disk circadian rhythms. Understanding the implications of exercise on connective tissue peripheral clocks holds promise for enhancing tissue health and disease prevention. Exercise-induced factors such as heat, glucocorticoid release, mechanical loading, and inter-tissue cross talk may play pivotal roles in entraining the circadian rhythm of connective tissues. This mini review underscores the importance of elucidating the mechanisms through which exercise influences circadian rhythms in connective tissues to optimize ECM homeostasis. Leveraging exercise as a modulator of circadian rhythms in connective tissues may offer novel therapeutic approaches to physical training for preventing musculoskeletal disorders and enhancing recovery.

The role of mechanosensor Piezo1 in bone homeostasis and mechanobiology.

Bones and articular cartilage are important load-bearing tissues. The fluid flow inside the bone cells and cell interaction with the extracellular matrix serve as the mechanical cues for bones and joints. Piezo1 is an ion channel found on the cell surface of many cell types, including osteocytes and chondrocytes. It is activated in response to mechanical stimulation, which subsequently mediates a variety of signaling pathways in osteoblasts, osteocytes, and chondrocytes. Piezo1 activation in osteoblastic cells positively regulates osteogenesis, while its activation in joints mediates cartilage degradation. This review focuses on the most recent research on Piezo1 in bone development and regeneration.

Nitric oxide contributes to rapid sclerostin protein loss following mechanical load.

In response to mechanical loading of bone, osteocytes produce nitric oxide (NO•) and decrease sclerostin protein expression, leading to an increase in bone mass. However, it is unclear whether NO• production and sclerostin protein loss are mechanistically linked, and, if so, the nature of their hierarchical relationship within an established mechano-transduction pathway. Prior work showed that following fluid-shear stress (FSS), osteocytes produce NOX2-derived reactive oxygen species, inducing calcium (Ca2+) influx. Increased intracellular Ca2+ results in calcium-calmodulin dependent protein kinase II (CaMKII) activation, which regulates the lysosomal degradation of sclerostin protein. Here, we extend our discoveries, identifying NO• as a regulator of sclerostin degradation downstream of mechano-activated CaMKII. Pharmacological inhibition of nitric oxide synthase (NOS) activity in Ocy454 osteocyte-like cells prevented FSS-induced sclerostin protein loss. Conversely, short-term treatment with a NO• donor in Ocy454 cells or isolated murine long bones was sufficient to induce the rapid decrease in sclerostin protein abundance, independent of changes in Sost gene expression. Ocy454 cells express all three NOS genes, and transfection with siRNAs targeting eNOS/Nos3 was sufficient to prevent FSS-induced loss of sclerostin protein, while siRNAs targeting iNOS/Nos2 mildly blunted the loss of sclerostin but did not reach statistical significance. Similarly, siRNAs targeting both eNOS/Nos3 and iNOS/Nos2 prevented FSS-induced NO• production. Together, these data show iNOS/Nos2 and eNOS/Nos3 are the primary producers of FSS-dependent NO•, and that NO• is necessary and sufficient for sclerostin protein control. Further, selective inhibition of elements within this sclerostin-controlling mechano-transduction pathway indicated that NO• production occurs downstream of CaMKII activation. Targeting Camk2d and Camk2g with siRNA in Ocy454 cells prevented NO• production following FSS, indicating that CaMKII is needed for NO• production. However, NO• donation (1min) resulted in a significant increase in CaMKII activation, suggesting that NO• may have the ability to tune CaMKII response. Together, these data support that CaMKII is necessary for, and may be modulated by NO•, and that the interaction of these two signals is involved in the control of sclerostin protein abundance, consistent with a role in bone anabolic responses.

Mechanical loading rescues mechanoresponsiveness in a human osteoarthritis explant model despite Wnt activation.

OBJECTIVES: Optimizing rehabilitation strategies for osteoarthritis necessitates a comprehensive understanding of chondrocytes' mechanoresponse in both health and disease, especially in the context of the interplay between loading and key pathways involved in osteoarthritis (OA) development, like canonical Wnt signaling. This study aims to elucidate the role of Wnt signaling in the mechanoresponsiveness of healthy and osteoarthritic human cartilage. METHODS: We used an ex-vivo model involving short-term physiological mechanical loading of human cartilage explants. First, the loading protocol for subsequent experiments was determined. Next, loading was applied to non-OA-explants with or without Wnt activation with CHIR99021. Molecular read-outs of anabolic, pericellular matrix and matrix remodeling markers were used to assess the effect of Wnt on cartilage mechanoresponse. Finally, the same set-up was used to study the effect of loading in cartilage from patients with established OA. RESULTS: Our results confirm that physiological loading maintains expression of anabolic genes in non-OA cartilage, and indicate a deleterious effect of Wnt activation in the chondrocyte mechanoresponsiveness. This suggests that loading-induced regulation of chondrocyte markers occurs downstream of canonical Wnt signaling. Interestingly, our study highlighted contrasting mechanoresponsiveness in the model of Wnt activation and the established OA samples, with established OA cartilage maintaining its mechanoresponsiveness, and mechanical loading rescuing the chondrogenic phenotype. CONCLUSION: This study provides insights into the mechanoresponsiveness of human cartilage in both non-OA and OA conditions. These findings hold the potential to contribute to the development of strategies that optimize the effect of dynamic compression by correcting OA pathological cell signaling.

Transient receptor potential vanilloid 4 regulates extracellular matrix composition and mediates load-induced intervertebral disc degeneration in a mouse model.

OBJECTIVE: Transient receptor potential vanilloid 4 (TRPV4) is a multi-modally activated cation channel that mediates mechanotransduction pathways by which musculoskeletal tissues respond to mechanical load and regulate tissue health. Using conditional Trpv4 knockout mice, we investigated the role of Trpv4 in regulating intervertebral disc (IVD) health and injury-induced IVD degeneration. METHODS: Col2-Cre;Trpv4fl/f (Trpv4 KO) mice were used to knockout Trpv4 in all type 2 collagen-expressing cells. Effects of gene targeting alone was assessed in lumbar spines, using vertebral bone length measurement, histological, immunohistochemistry and gene expression analyses, and mechanical testing. Disc puncture was performed on caudal IVDs of wild-type (WT) and Trpv4 KO mice at 2.5- and 6.5-months-of-age. Six weeks after puncture (4- and 8-months-of-age at sacrifice), caudal spines were assessed using histological analyses. RESULTS: While loss of Trpv4 did not significantly alter vertebral bone length and tissue histomorphology compared to age-matched WT mice, Trpv4 KO mice showed decreased proteoglycan and PRG4 staining in the annulus fibrosus compared to WT. At the gene level, Trpv4 KO mice showed significantly increased expression of Acan, Bgn, and Prg4 compared to WT. Functionally, loss of Trpv4 was associated with significantly increased neutral zone length in lumbar IVDs. Following puncture, both Trpv4 KO and WT mice showed similar signs of degeneration at the site of injury. Interestingly, loss of Trpv4 prevented mechanically-induced degeneration in IVDs adjacent to sites of injury. CONCLUSION: These studies suggest a role for Trpv4 in regulating extracellular matrix synthesis and mediating the response of IVD tissues to mechanical stress.

Secretome from myoblasts statically loaded at low intensity promotes tenocyte proliferation via the IGF-1 receptor pathway.

Exercise is widely recognized as beneficial for tendon healing. Recently, it has been described that muscle-derived molecules secreted in response to static exercise influence tendon healing. In this study, the optimal static loading intensity for tendon healing and the composition of secretome released by myoblasts in response to different intensities of static strain were investigated. In an in vitro coculture model, myoblasts were mechanically loaded using a Flexcell Tension System. Tenocytes were seeded on transwell inserts that allowed communication between the tenocytes and myoblasts without direct contact. Proliferation and migration assays, together with RNA sequencing, were used to determine potential cellular signaling pathways. The secretome from myoblasts exposed to 2% static loading increased the proliferation and migration of the cocultured tenocytes. RNA-seq analysis revealed that this loading condition upregulated the expression of numerous genes encoding secretory proteins, including insulin-like growth factor-1 (IGF-1). Confirmation of IGF-1 expression and secretion was carried out using qPCR and enzyme-linked immunosorbt assay (ELISA), revealing a statistically significant upregulation in response to 2% static loading in comparison to both control conditions and higher loading intensities of 5% and 10%. Addition of an inhibitor of the IGF-1 receptor (PQ401) to the tenocytes significantly reduced myoblast secretome-induced tenocyte proliferation. In conclusion, IGF-1 may be an important molecule in the statically loaded myoblast secretome, which is responsible for influencing tenocytes during exercise-induced healing.

Enhancing piezoelectric performance of CNTs through B and N substitution under combined mechanical loads: insights from MD simulations.

The piezoelectric properties of carbon nanotubes (CNTs) doped with boron (B) and nitrogen (N) were studied using the classical molecular dynamics (MD) simulation software package large scale atomic/molecular massively parallel simulator. The interactions among the nanotube atoms C, N, and B were calculated using the Tersoff potential. MD simulations were performed to observe the changes in the piezoelectric coefficient of the doped CNTs under loading conditions like tension, torsion, and a combination of both. We considered a wide range of chirality to determine the influence of structural variation on the piezoelectric effect. The study revealed that B-CNTs exhibit superior piezoelectric coefficients compared to N-CNTs, indicating the significant role of dopant type. Moreover, under tensile loading, zigzag-oriented B-CNTs showed higher piezoelectric coefficients with a maximume33= 0.2441 C m-2, whereas under torsional loading, armchair-oriented B-CNTs showed enhanced response with a maximume36= 0.0564 C m-2. A notable observation was that under combined loading conditions (tensile and torsional), the piezoelectric behavior of the B-CNTs was dependent on the nanotube's chirality and did not yield a linear additive response. The polarization induced under combined loading in most of the doped CNTs is significantly higher than the sum of polarization generated under tensile and torsional loading conditions. This behavior suggests that the overall piezoelectric effect under combined loading can be enhanced, which emphasizes the need for an approach to optimize the mechanical loading condition. The results showcase the potential of B-/N-CNTs to be engineered for efficient performance by demonstrating that tailored mechanical loading can enhance the piezoelectric responses in doped CNTs, opening a pathway for highly functional and efficient nanoscale piezoelectric devices.

Effects of mechanical loading on matrix homeostasis and differentiation potential of periodontal ligament cells: A scoping review.

Various mechanical loadings, including mechanical stress, orthodontics forces, and masticatory force, affect the functions of periodontal ligament cells. Regulation of periodontal tissue destruction, formation, and differentiation functions are crucial processes for periodontal regeneration therapy. Numerous studies have reported that different types of mechanical loading play a role in maintaining periodontal tissue matrix homeostasis, and osteogenic differentiation of the periodontal ligament cells. This scoping review aims to evaluate the studies regarding the effects of various mechanical loadings on the secretion of extracellular matrix (ECM) components, regulation of the balance between formation and destruction of periodontal tissue matrix, osteogenic differentiation, and multiple differentiation functions of the periodontal ligament. An electronic search for this review has been conducted on two databases; MEDLINE via PubMed and SCOPUS. Study selection criteria included original research written in English that reported the effects of different mechanical loadings on matrix homeostasis and differentiation potential of periodontal ligament cells. The final 204 articles were mainly included in the present scoping review. Mechanical forces of the appropriate magnitude, duration, and pattern have a positive influence on the secretion of ECM components such as collagen, as well as regulate the secretion of matrix metalloproteinases and tissue inhibitors of matrix metalloproteinases. Additionally, these forces regulate a balance between osteoblastic and osteoclast differentiation. Conversely, incorrect mechanical loadings can lead to abnormal formation and destruction of both soft and hard tissue. This review provides additional insight into how mechanical loadings impact ECM homeostasis and multiple differentiation functions of periodontal ligament cells (PDLCs), thus making it valuable for regenerative periodontal treatment. In combination with advancing technologies, the utilization of ECM components, application of different aspects of mechanical force, and differentiation potential of PDLCs could bring potential benefits to future periodontal regeneration therapy.

Effects of different combinations of mechanical loading intensity, duration, and frequency on the articular cartilage in mice.

BACKGROUND: Understanding how healthy articular cartilage responds to mechanical loading is critical. Moderate mechanical loading has positive effects on the cartilage, such as maintaining cartilage homeostasis. The degree of mechanical loading is determined by a combination of intensity, frequency, and duration; however, the best combination of these parameters for knee cartilage remains unclear. This study aimed to determine which combination of intensity, frequency, and duration provides the best mechanical loading on healthy knee articular cartilage in vitro and in vivo. METHODS AND RESULTS: In this study, 33 male mice were used. Chondrocytes isolated from mouse knee joints were subjected to different cyclic tensile strains (CTSs) and assessed by measuring the expression of cartilage matrix-related genes. Furthermore, the histological characteristics of mouse tibial cartilages were quantified using different treadmill exercises. Chondrocytes and mice were divided into the control group and eight intervention groups: high-intensity, high-frequency, and long-duration; high-intensity, high-frequency, and short-duration; high-intensity, low-frequency, and long-duration; high-intensity, low-frequency, and short-duration; low-intensity, high-frequency, and long-duration; low-intensity, high-frequency, and short-duration; low-intensity, low-frequency, and long-duration; low-intensity, low-frequency, and short-duration. In low-intensity CTSs, chondrocytes showed anabolic responses by altering the mRNA expression of COL2A1 in short durations and SOX9 in long durations. Furthermore, low-intensity, low-frequency, and long-duration treadmill exercises minimized chondrocyte hypertrophy and enhanced aggrecan synthesis in tibial cartilages. CONCLUSION: Low-intensity, low-frequency, and long-duration mechanical loading is the best combination for healthy knee cartilage to maintain homeostasis and activate anabolic responses. Our findings provide a significant scientific basis for exercise and lifestyle instructions.

Interfacial Tissue Regeneration with Bone.

PURPOSE OF REVIEW: Interfacial tissue exists throughout the body at cartilage-to-bone (osteochondral interface) and tendon-to-bone (enthesis) interfaces. Healing of interfacial tissues is a current challenge in regenerative approaches because the interface plays a critical role in stabilizing and distributing the mechanical stress between soft tissues (e.g., cartilage and tendon) and bone. The purpose of this review is to identify new directions in the field of interfacial tissue development and physiology that can guide future regenerative strategies for improving post-injury healing. RECENT FINDINGS: Cues from interfacial tissue development may guide regeneration including biological cues such as cell phenotype and growth factor signaling; structural cues such as extracellular matrix (ECM) deposition, ECM, and cell alignment; and mechanical cues such as compression, tension, shear, and the stiffness of the cellular microenvironment. In this review, we explore new discoveries in the field of interfacial biology related to ECM remodeling, cellular metabolism, and fate. Based on emergent findings across multiple disciplines, we lay out a framework for future innovations in the design of engineered strategies for interface regeneration. Many of the key mechanisms essential for interfacial tissue development and adaptation have high potential for improving outcomes in the clinic.

The Role of Osteocytes in Pre-metastatic Niche Formation.

PURPOSE OF REVIEW: The formation of a pre-metastatic niche (PMN), in which primary cancer cells prime the distant site to be favorable to their engraftment and survival, may help explain the strong osteotropism observed in multiple cancers, such as breast and prostate. PMN formation, which includes extracellular matrix remodeling, increased angiogenesis and vascular permeability, enhanced bone marrow-derived cell recruitment and immune suppression, has mostly been described in soft tissues. In this review, we summarize current literature of PMN formation in bone. We also present evidence of a potential role for osteocytes to be the primary mediators of PMN development. RECENT FINDINGS: Osteocytes regulate the bone microenvironment in myriad ways beyond canonical bone tissue remodeling, including changes that contribute to PMN formation. Perilacunar tissue remodeling, which has been observed in both bone and non-bone metastatic cancers, is a potential mechanism by which osteocyte-cancer cell signaling stimulates changes to the bone microenvironment. Osteocytes also protect against endothelial permeability, including that induced by cancer cells, in a loading-mediated process. Finally, osteocytes are potent regulators of cells within the bone marrow, including progenitors and immune cells, and might be involved in this aspect of PMN formation. Osteocytes should be examined for their role in PMN formation.

Effects of the Type of Exercise Training on Bone Health Parameters in Adolescent Girls: A Systematic Review.

Interventional studies offer strong evidence for exercise's osteogenic impact on bone particularly during growth. With rising osteoporosis rates in older women, enhancing bone strength early in life is crucial. Thus, investigating the osteogenic effects of different types of physical activities in young females is crucial. Despite varied findings, only two systematic reviews tried to explore this topic without examining how different types of exercise may affect bone health in adolescent girls. The first aim of this systematic review was to assess the impact of exercise training on bone health parameters in adolescent girls, and the second aim was to investigate whether the type of exercise training can modulate this effect. A systematic literature search was conducted using common electronic databases from inception - January 2023. Seven studies (355 participants) were eligible for inclusion in this systematic review. Two studies dealt with resistance training, 3 studies applied plyometric training, 1 study used team sports, and 1 study used dancing. Results indicate that plyometric training increases lumbar spine bone mass in adolescent girls. Well-designed randomized controlled trials with a proper training period (> 12 weeks) are needed to advocate a specific type of training which has the highest osteogenic effect.

Team sports practice and bone health: A systematic review and meta- analysis.

The primary aim of this study was to explore the effects of team sports practice on bone health indices in adults engaged in team sports. The secondary aim was to investigate the osteogenic effects of each type of team sport. This systematic literature search was conducted using common electronic databases from inception in June 2023, using key terms (and synonyms searched for by the MeSH database) that were combined using the operators "AND", "OR", "NOT": (``men'' OR ``man'' OR ``women'' OR ``woman'') AND (``bone mineral density'' OR ``BMD'' OR ``bone mineral content'' OR ``BMC'' OR ``peak bone mass'' OR ``mechanical loading'' OR ``osteoporosis'' OR ``bone geometry'' OR ``bone resistance'') AND (``team sport'' OR ``sport'' OR rugby OR basketball OR volleyball OR handball OR soccer OR football OR ``players''). After screening, 16 studies were included in the final analysis (5 continents, 2740 participants). The training duration lasted 1 to 13 years. Team sport training had a moderate impact on whole body bone mineral density (WB BMD) (1.07 SMD; 95 % [0.77, 1.37], p < 0.00) but a more significant impact on whole body bone mineral content (WB BMC) (1.3 SMD; 95 % [0.81, 1.79], p < 0.00). Subgroup analyses indicated that rugby training had a moderate but non-significant impact on WB BMD (1.19 SMD; 95 % [-0.13, 2.52], p = 0.08) but a greater impact on WB BMC (2.12 SMD; 95 % [0.84, 3.39], p < 0.00); basketball training had a moderate but significant impact on WB BMD (1 SMD; 95 % [0.35, 1.64], p < 0.00) and a trivial non-significant impact on WB BMC (0.18 SMD; 95 % [-1.09, 1.46], p = 0.78); volleyball training had a moderate but non-significant impact on WB BMD (0.63 SMD; 95 % [-0.22, 1.49], p = 0.15) and a significant impact on WB BMC (2.39 SMD; 95 % [1.45, 3.33], p < 0.00). Handball training produced a moderate significant impact on WB BMD (1.02 SMD; 95 % [0.33, 1.71], p < 0.00) and WB BMC (0.97 SMD; 95 % [0.47, 1.48], p < 0.00), and soccer training led to moderate but significant effects on WB BMD (1.16 SMD; 95 % [0.88, 1.44], p < 0.00) and a large effect on WB BMC (1.34 SMD; 95 % [0.92, 1.77], p < 0.00). Rugby training was associated with a higher WB BMC compared to basketball training (p = 0.03). Our systematic review and meta-analysis suggests that team sports, such as rugby, basketball, volleyball, handball and soccer have moderate to large effects on WB BMD and WB BMC. Specifically, our findings indicate that handball and soccer enhance WB BMD and WB BMC, whereas rugby only increases WB BMC. There is currently insufficient evidence indicating the superiority of any type of sport training that improves bone health in adults.

The mechanical response to hockey-specific running gait: implications for the objective monitoring of (p)rehabilitation.

Field hockey is characterized by a unique dribbling position which has been associated with a prevalence of low back pain and lumbar injury. To quantify the biomechanical response of the hockey-specific running gait, twelve field hockey players completed treadmill running at speeds of 6, 9 and 12 km·hr-1 with a normal running gait and whilst manipulating the hockey stick to replicate dribbling. Mechanical loading was quantified using tri-axial accelerometery at L4, and muscular activity was measured for biceps femoris (BF), gluteus medius (GM) and quadratus lumborum (QL) of each limb. Running with the stick elicited greater mechanical loading in the medio-lateral (p = 0.001) and antero-posterior (p = 0.003) planes, and increased peak (p = 0.004) and mean (p = 0.002) EMG response of QL (p = 0.004). The greater planar mechanical loading and QL activation in response to hockey-specific running technique support epidemiological observations of lower back pain prevalence. The sensitivity of uni-axial mechanical loading to the hockey-specific running posture provides an efficacious means of objectively monitoring mechanical loading in-vivo, whilst the QL activation response has implications for (p)rehabilitative interventions. Running posture and speed can be considered as discrete progressions when considering training load.

Flies adaptively control flight to compensate for added inertia.

Animal locomotion is highly adaptive, displaying a large degree of flexibility, yet how this flexibility arises from the integration of mechanics and neural control remains elusive. For instance, animals require flexible strategies to maintain performance as changes in mass or inertia impact stability. Compensatory strategies to mechanical loading are especially critical for animals that rely on flight for survival. To shed light on the capacity and flexibility of flight neuromechanics to mechanical loading, we pushed the performance of fruit flies (Drosophila) near its limit and implemented a control theoretic framework. Flies with added inertia were placed inside a virtual reality arena which permitted free rotation about the vertical (yaw) axis. Adding inertia increased the fly's response time yet had little influence on overall gaze stabilization performance. Flies maintained stability following the addition of inertia by adaptively modulating both visuomotor gain and damping. By contrast, mathematical modelling predicted a significant decrease in gaze stabilization performance. Adding inertia altered saccades, however, flies compensated for the added inertia by increasing saccade torque. Taken together, in response to added inertia flies increase reaction time but maintain flight performance through adaptive neural control. Overall, adding inertia decreases closed-loop flight robustness. Our work highlights the flexibility and capacity of motor control in flight.

Osteocytes autophagy mediated by mTORC2 activation controls osteoblasts differentiation and osteoclasts activities under mechanical loading.

Autophagy is an important mechanosensitive response for cellular homeostasis and survival in osteocytes. However, the mechanism and its effect on bone metabolism have not yet clarified. The objective of this study was to evaluate how compressive cyclic force (CCF) induced autophagic response in osteocytes and to determine the effect of mechanically induced-autophagy on bone cells including osteocytes, osteoblasts, and osteoclasts. Autophagic puncta observed in MLO-Y4 cells increased after exposure to CCF. The upregulated levels of the LC3-II isoform and the degradation of p62 further confirmed the increased autophagic flux. Additionally, ATP synthesis and release, osteocalcin (OCN) expression, and cell survival increased in osteocytes as well. The Murine osteoblasts MC3T3-E1 cells and RAW 264.7 macrophage cells were cultured in conditioned medium collected from MLO-Y4 cells subjected to CCF. The concentration of FGF23 increased and the concentrations of SOST and M-CSF and RANKL/OPG ratio decreased significantly in the conditioned medium. Moreover, the promotion of osteogenic differentiation in MC3T3-E1 cells and inhibition of osteoclastogenesis and function in RAW 264.7 cells were significantly attenuated when osteocytes autophagy was inhibited by siAtg7. Our findings suggested that CCF induced protective autophagy in osteocytes and subsequently enhanced osteocytes survival and osteoblasts differentiation and downregulated osteoclasts activities. Further study revealed that CCF induced autophagic response in osteocytes through mechanistic target of rapamycin complex 2 (mTORC2) activation. In conclusion, CCF-induced osteocytes autophagy upon mTORC2 activation promoted osteocytes survival and osteogenic response and decreased osteoclastic function. Thus, osteocytes autophagy will provide a promising target for better understanding of bone physiology and treatment of bone diseases.

Mechanical loading and autophagy: A study on the BoNT-A injection-induced condylar cartilage degeneration.

Botulinum toxin A (BoNT-A) has emerged as a treatment option for temporomandibular disorder (TMD). By injecting BoNT-A into the masseter muscle, it is possible to reduce mechanical loading on the temporomandibular joint (TMJ). However, numerous prior studies have indicated excessive reduction in mechanical loading can have detrimental effects on TMJ cartilage. This study proposes that autophagy, a process influenced by mechanical loading, could play a role in BoNT-A-induced mandibular condyle cartilage degeneration. To explore this hypothesis, we employed both BoNT-A injection and an excessive biting model to induce variations in mechanical loading on the condyle cartilage of C57BL/6 mice, thereby simulating an increase and decrease in mechanical loading, respectively. Results showed a significant reduction in cartilage thickness and downregulation of Runt-related transcription factor 2 (Runx2) expression in chondrocytes following BoNT-A injection. In vitro experiments demonstrated that the reduction of Runx2 expression in chondrocytes is associated with autophagy, possibly dependent on decreased YAP expression induced by low mechanical loading. This study reveals the potential involvement of the YAP/LC3/Runx2 signaling pathway in BoNT-A mediated mandibular condylar cartilage degeneration.

Mechanical Loading Promotes the Migration of Endogenous Stem Cells and Chondrogenic Differentiation in a Mouse Model of Osteoarthritis.

Osteoarthritis (OA) is a major health problem, characterized by progressive cartilage degeneration. Previous works have shown that mechanical loading can alleviate OA symptoms by suppressing catabolic activities. This study evaluated whether mechanical loading can enhance anabolic activities by facilitating the recruitment of stem cells for chondrogenesis. We evaluated cartilage degradation in a mouse model of OA through histology with H&E and safranin O staining. We also evaluated the migration and chondrogenic ability of stem cells using in vitro assays, including immunohistochemistry, immunofluorescence, and Western blot analysis. The result showed that the OA mice that received mechanical loading exhibited resilience to cartilage damage. Compared to the OA group, mechanical loading promoted the expression of Piezo1 and the migration of stem cells was promoted via the SDF-1/CXCR4 axis. Also, the chondrogenic differentiation was enhanced by the upregulation of SOX9, a transcription factor important for chondrogenesis. Collectively, the results revealed that mechanical loading facilitated cartilage repair by promoting the migration and chondrogenic differentiation of endogenous stem cells. This study provided new insights into the loading-driven engagement of endogenous stem cells and the enhancement of anabolic responses for the treatment of OA.

Comparing the Effect of Mechanical Loading on Deep and Superficial Cartilage Using Quantitative UTE MRI.

BACKGROUND: The biomechanical properties of deep and superficial cartilage may be different, yet in vivo MRI validation is required. PURPOSE: To compare the effect of mechanical loading on deep and superficial cartilage in young healthy adults using ultrashort echo time (UTE)-T2* mapping. STUDY TYPE: Prospective, intervention. SUBJECTS: Thirty-one healthy adults (54.8% females, median age = 23 years). FIELD STRENGTH/SEQUENCE: 3-T, PD-FS, and UTE sequences with four echo times (TEs = 0.1, 0.5, 2.8, and 4.0 msec; 0.6 mm isotropic spatial resolution) of the left knee, acquired before and after loading exercise. ASSESSMENT: Quantitative UTE-T2* maps of the entire knee were generated using UTE images of four TEs. In deep and superficial cartilage of patella, medial and lateral femur, medial and lateral tibia cartilage (PC, MFC, LFC, MTC, and LTC), which were segmented manually, cartilage thickness and T2* values before and after loading were measured, extracted, taken averages of, and compared. Scan-rescan repeatability was evaluated. Body weight and body mass index (BMI) data were collected. Physical activity levels were evaluated using International Physical Activity Questionnaire. STATISTICAL TESTS: Paired sample t-tests, paired Wilcoxon Mann-Whitney tests, Pearson and Spearman correlation analyses, Kruskal-Wallis tests with post-hoc Bonferroni correction. A P-value <0.05 was considered statistically significant. RESULTS: The scan-rescan repeatability was good (RMSA-CV < 10%). After exercise, deep cartilage exhibited no significant differences in cartilage thickness (PPC = 0.576, PMTC = 0.991, PMFC = 0.899, PLTC = 0.861, PLFC = 0.290) and T2* values (PPC = 0.914, PMTC = 0.780, PMFC = 0.754, PLTC = 0.327, PLFC = 0.811), which both significantly decreased in superficial PC, MFC, LFC, and MTC. The T2* values of superficial MTC and deep MFC were moderately correlated with higher body weight (ρ = 0.431) and lower BMI (ρ = -0.499), respectively. DATA CONCLUSION: Deep and superficial cartilage may respond differently to mechanical loading as assessed by UTE-T2*. EVIDENCE LEVEL: 2 TECHNICAL EFFICACY: Stage 3.

Skeletal muscle fiber type-specific expressions of mechanosensors integrin-linked kinase, talin, and vinculin and their modulation by loading and environmental conditions in humans.

Mechanosensors control muscle integrity as demonstrated in mice. However, no information is available in human muscle about the distribution of mechanosensors and their adaptations to mechanical loading and environmental conditions (hypoxia). Here, we hypothesized that mechanosensors show fiber-type-specific distributions and that loading and environmental conditions specifically regulate mechanosensors. We randomly subjected 28 healthy males to one of the following groups (n = 7 each) consisting of nine loading sessions within 3 weeks: normoxia moderate (NM), normoxia intensive (NI), hypoxia moderate (HM), and hypoxia intensive (HI). We took six biopsies: pre (T0), 4 h (T1), and 24 h (T2) after the third as well as 4 h (T3), 24 h (T4), and 72 h (T5) after the ninth training session. We analyzed subjects' maximal oxygen consumption (V̇O2 max), maximal power output (Pmax), muscle fiber types and cross-sectional areas (CSA), fiber-type-specific integrin-linked kinase (ILK) localizations as well as ILK, vinculin and talin protein and gene expressions in dependence on loading and environmental conditions. V̇O2 max increased upon NM and HM, Pmax upon all interventions. Fiber types did not change, whereas CSA increased upon NI and HI, but decreased upon HM. ILK showed a type 2-specific fiber type localization. ILK, vinculin, and talin protein and gene expressions differed depending on loading and environmental conditions. Our data demonstrate that mechanosensors show fiber type-specific distributions and that exercise intensities rather than environmental variables influence their profiles in human muscles. These data are the first of their kind in human muscle and indicate that mechanosensors manage the mechanosensing at a fiber-type-specific resolution and that the intensity of mechanical stimulation has a major impact.