Shear-stress sensing by PIEZO1 regulates tendon stiffness in rodents and influences jumping performance in humans

Personalized Injury Risk Assessment and Exercise Prescription Based on Tendon Strain

Tendon strain determines the mechanical demand on a tendon and regulates the metabolic and structural response. Imbalances of muscle strength and tendon stiffness can substantially change tendon operating strain, affecting the individual consequences of loading. Such imbalances can be identified based on maximum tendon strain during fixed-end contractions, with implications for a personalized assessment of injury risk and exercise prescription.

Reactive oxygen species in tendon injury and repair

Reactive oxygen species (ROS) are chemical moieties that in physiological concentrations serve as fast-acting signaling molecules important for cellular homeostasis. However, their excess either due to overproduction or inability of the antioxidant system to inactivate them results in oxidative stress, contributing to cellular dysfunction and tissue damage. In tendons, which are hypovascular, hypocellular, and composed predominantly of extracellular matrix (ECM), particularly collagen I, ROS likely play a dual role: regulating cellular processes such as inflammation, proliferation, and ECM remodeling under physiological conditions, while contributing to tendinopathy and impaired healing when dysregulated. This review explores the sources of ROS in tendons, including NADPH oxidases and mitochondria, and their role in key processes such as tissue adaptation to mechanical load and injury repair, also in systemic conditions such as diabetes. In addition, we integrate the emerging perspective that calcium signaling-mediated by mechanically activated ion channels-plays a central role in tendon mechanotransduction under daily mechanical loads. We propose that mechanical overuse (overload) may lead to hyperactivation of calcium channels, resulting in chronically elevated intracellular calcium levels that amplify ROS production and oxidative stress. Although direct evidence linking calcium channel hyperactivity, intracellular calcium dysregulation, and ROS generation under overload conditions is currently circumstantial, this review aims to highlight these connections and identify them as critical avenues for future research. By framing ROS within the context of both adaptive and maladaptive responses to mechanical load, this review provides a comprehensive synthesis of redox biology in tendon injury and repair, paving the way for future work, including development of therapeutic strategies targeting ROS and calcium signaling to enhance tendon recovery and resilience.

Using load to improve tendon/ligament tissue engineering and develop novel treatments for tendinopathy

Tendon and ligament injuries are highly prevalent but heal poorly, even with proper care. Restoration of native tissue function is complicated by the fact that these tissues vary anatomically in terms of their mechanical properties, composition, and structure. These differences develop as adaptations to diverse mechanical demands; however, pathology may alter the loads placed on the tissue. Musculoskeletal loads can be generally categorized into tension, compression, and shear. Each of these regulate distinct molecular pathways that are involved in tissue remodeling, including many of the canonical tenogenic genes. In this review, we provide a perspective on the stage-specific regulation of mechanically sensitive pathways during development and maturation of tendon and ligament tissue, including scleraxis, mohawk, and others. Furthermore, we discuss structural features of healing and diseased tendon that may contribute to aberrant loading profiles, and how the associated disturbance in molecular signaling may contribute to incomplete healing or the formation of degenerative phenotypes. The perspectives provided here draw from studies spanning in vitro, animal, and human experiments of healthy and diseased tendon to propose a more targeted approach to advance rehabilitation, orthobiologics, and tissue engineering.

Subject-specific biomechanics influences tendon strains in patients with Achilles tendinopathy

The treatment of Achilles tendinopathy is challenging, as 40% of patients do not respond to existing rehabilitation protocols. These protocols neglect individual Achilles tendon (AT) characteristics, which are crucial for healing of the tendon tissue. Although prior studies suggest an optimal strain for AT regeneration (6% tendon strains), it is unclear if current protocols meet this condition. Our study aimed to analyse the impact of a selection of rehabilitation exercises on tendon strains in patients with Achilles tendinopathy, using subject-specific finite element (FE) models of the free AT. Second, this study aimed to explain the influence of muscle forces and material properties on AT strains. The 21 FE models of the AT included the following subject-specific features: geometry estimated from 3D freehand ultrasound images, Elastic modulus estimated from the experimental stress‒strain curve, and muscle forces estimated using a combination of 3D motion capture and musculoskeletal modelling. Exercises were ranked based on strain progression, starting from concentric and eccentric exercises, and going to more functional exercises, which impose a greater load on the AT. There was no significant difference between the unilateral heel drop and walking, and both exercises fell within the optimal strain range. However, when examining individual strains, it became evident that there was diversity in exercise rankings among participants, as well as exercises falling within the optimal strain range. Muscle forces notably affected strains more than material properties. Our findings indicate the importance of tailored rehabilitation protocols that account for individual morphological, material, and muscle characteristics.

Vascular Smooth Muscle Cells Transdifferentiate into Chondrocyte-Like Cells and Facilitate Meniscal Fibrocartilage Regeneration

The effective and translational strategy to regenerate knee meniscal fibrocartilage remained challenging. Herein, we first identified vascular smooth muscle cells (VSMCs) transdifferentiated into fibrochondrocytes and participated in spontaneous meniscal regeneration using smooth muscle cell lineage tracing transgenic mice meniscal defect model. Then, we identified low-intensity pulsed ultrasound (LIPUS) acoustic stimulus enhanced fibrochondrogenic transdifferentiation of VSMCs in vitro and in vivo. Mechanistically, LIPUS stimulus could up-regulate mechanosensitive ion channel Piezo1 expression and then activate the transforming growth factor β1 (TGFβ1) signal, following repression of the Notch signal, consequently enhancing fibrochondrogenic transdifferentiation of VSMCs. Finally, we demonstrated that the regular LIPUS stimulus enhanced anisotropic native-like meniscal fibrocartilage tissue regeneration in a beagle canine subtotal meniscectomy model at 6 months postoperatively. The single-cell RNA sequencing analysis confirmed the role of VSMC fibrochondrogenic transdifferentiation in meniscal regeneration.

The modulating role of uniaxial straining in the IL-1β and TGF-β mediated inflammatory response of human primary ligamentocytes

Biomechanical (over-)stimulation, in addition to inflammatory and fibrotic stimuli, severely impacts the anterior cruciate ligament (ACL) biology, contributing to the overall chronic nature of desmopathy. A major challenge has been the lack of representative two-dimensional (2D) in vitro models mimicking inflammatory processes in the presence of dynamic mechanical strain, both being crucial for ligament homeostasis. Physiological levels of strain exert anti-inflammatory effects, while excessive strain can facilitate inflammatory mechanisms. Adhering to the 3Rs (Replacement, Reduction and Refinement) principles of animal research, this study aims to investigate the role of a dynamic biomechanical in vitro environment on inflammatory mechanisms by combining a Flexcell culture system with primary human ligamentocytes for the study of ligament pathology. Primary ligamentocytes from OA patients were cultured under animal-free conditions with human platelet lysate, and exposed to either IL-1β or TGF-β3 to simulate different inflammatory microenvironments. Cells were subjected to different magnitudes of mechanical strain. Results showed that cells aligned along the force axis under strain. This study highlights the critical role of the mechanical microenvironment in modulating inflammatory and fibrotic cellular responses in ligamentocyte pathology, providing valuable insights into the complex interplay between biomechanical stimuli and cytokine signaling. These findings not only advance our understanding of ligament biology but also can pave the way for the development of more targeted therapeutic strategies for ligament injuries and diseases, potentially improving patient outcomes in orthopedic medicine.

A Tympanic Piezo-Bioreactor Modulates Ion Channel-Associated Mechanosignaling to Stabilize Phenotype and Promote Tenogenesis in Human Tendon-Derived Cells

Preserving the function of human tendon-derived cells (hTDCs) during cell expansion is a significant challenge in regenerative medicine. In this study, a non-genetic approach is introduced to control the differentiation of hTDCs using a newly developed tympanic bioreactor. The system mimics the functionality of the human tympanic membrane, employing a piezoelectrically tuned acoustic diaphragm made of polyvinylidene fluoride-co-trifluoroethylene and boron nitride nanotubes. The diaphragm is vibrationally actuated to deliver targeted electromechanical stimulation to hTDCs. The results demonstrate that the system effectively maintains the tendon-specific phenotype of hTDCs, even under conditions that typically induce nonspecific differentiation, such as osteogenesis. This stabilization is achieved by modulating integrin-mediated mechanosignaling via ion channel-regulated calcium activity, potentially by TREK-1 and PIEZO1, yet targeted studies are required for confirmation. Finally, the system sustains the activation of key differentiation pathways (bone morphogenetic protein, BMP) while downregulating osteogenesis-associated (mitogen-ctivated protein kinase, MAPK and wingless integrated, WNT) pathways and upregulating Focal Adhesion Kinase (FAK) signaling. This approach offers a finely tunable, dose-dependent control over hTDC differentiation, presenting significant potential for non-genetic approaches in cell therapy, tendon tissue engineering, and the regeneration of other mechanosensitive tissues.

Regional changes in the free Achilles tendon volume in response to repeated submaximal contractions

INTRODUCTION

The Achilles tendon (AT) may become smaller in volume following acute bouts of heavy and sustained loading likely because of transient fluid exudation to the periphery and this could augment cellular mechanotransduction and tendon adaptation. Given the structure of the AT is distinct across its length, regional changes in the free AT volume may occur in response to loading. This study aimed to investigate whether the change in tendon volume in response to repeated submaximal loading is distinct across the free AT length.

METHODS

Sixteen ATs of healthy males and females (age 24.4 ± 9.4 years, body mass 70.9 ± 16.1 kg, height 1.7 ± 0.1 m) were scanned at rest using freehand 3D ultrasound. Scanning was done before and immediately after submaximal (75 %) voluntary isometric plantarflexion contractions (8 s) involving four sets of ten repetitions. Regional volumetric changes were assessed across the free AT length by dividing the tendon into distal, mid, and proximal regions.

RESULTS

Significant reduction in the free AT volume occurred across all tendon regions in response to the intervention, however, the mid- region exhibited the greatest reduction in volume compared to the proximal region (P = 0.025).

DISCUSSION

The fact that volume reduction was greatest in the mid-region compared to the proximal region of the free AT may suggest greater tendon adaptation, via mechanotransduction pathways, in the mid-region and this may be important for tendon health and injury prevention.

Biological sensing of fluid flow-lessons from PIEZO1

The flow sensing endothelial cell lining of blood and lymphatic vessels is essential in vertebrates. While the mechanisms are still mysterious in many regards, several critical components became apparent through molecular biology studies. In this article, we focus on PIEZO1, which forms unusual force-sensing ion channels capable of rapid transduction of force into biological effect. We describe current knowledge and emerging challenges. We suggest the idea of using computation to construct the flow sensing mechanism of endothelium to advance understanding, develop testable hypotheses and potentially design novel therapeutic strategies and synthetic flow sensing devices.

Piezo activity levels need to be tightly regulated to maintain normal morphology and function in pericardial nephrocytes

Due to their position on glomerular capillaries, podocytes are continuously counteracting biomechanical filtration forces. Most therapeutic interventions known to generally slow or prevent the progression of chronic kidney disease appear to lower these biomechanical forces on podocytes, highlighting the critical need to better understand podocyte mechano-signalling pathways. Here we investigated whether the mechanotransducer Piezo is involved in a mechanosensation pathway in Drosophila nephrocytes, the podocyte homologue in the fly. Loss of function analysis in Piezo depleted nephrocytes reveal a severe morphological and functional phenotype. Further, pharmacological activation of endogenous Piezo with Yoda1 causes a significant increase of intracellular Ca++ upon exposure to a mechanical stimulus in nephrocytes, as well as filtration disturbances. Elevated Piezo expression levels also result in a severe nephrocyte phenotype. Interestingly, expression of Piezo which lacks mechanosensitive channel activity, does not result in a severe nephrocyte phenotype, suggesting the observed changes in Piezo wildtype overexpressing cells are caused by the mechanosensitive channel activity. Moreover, blocking Piezo activity using the tarantula toxin GsMTx4 reverses the phenotypes observed in nephrocytes overexpressing Piezo. Taken together, here we provide evidence that Piezo activity levels need to be tightly regulated to maintain normal pericardial nephrocyte morphology and function.

Exploring Metabolic Mechanisms in Calcific Tendinopathy and Shoulder Arthrofibrosis: Insights and Therapeutic Implications

Rotator cuff calcific tendinopathy and arthrofibrosis of the shoulder (adhesive capsulitis) are debilitating musculoskeletal disorders that significantly impact joint function and impair quality of life. Despite its high prevalence and common clinical presentation, the metabolic mechanisms underlying these conditions characterized by pain, and reduced mobility, remain poorly understood. This review aims to elucidate the role of metabolic processes implicated in the pathogenesis of calcific tendinopathy and shoulder arthrofibrosis. We will be focusing on the mechanistic role of how these processes contribute to disease progression and can direct potential therapeutic targets. Calcific tendinopathy is marked by aberrant calcium deposition within tendons, influenced by disrupted calcium and phosphate homeostasis, and altered cellular responses. Key molecular pathways, including bone morphogenetic proteins (BMPs), Wnt signaling, and transforming growth factor-beta (TGF-β), play crucial roles in the pathophysiology of calcification, calcium imbalance, and muscle fibrosis. In contrast, shoulder arthrofibrosis involves excessive collagen deposition and fibrosis within the shoulder joint capsule, driven by metabolic dysregulation and inflammation. The TGF-β signaling pathway and inflammatory cytokines, such as interleukin-6 (IL-6), are central to the fibrotic response. A comparative analysis reveals both shared and distinct metabolic pathways between these conditions, highlighting the interplay between inflammation, cellular metabolism, extracellular matrix remodeling, calcific deposition, and calcium migration to the glenohumeral joints, resulting in adhesive capsulitis, thereby providing insights into their pathophysiology. This review discusses current therapeutic approaches and their limitations, advocating for the development of targeted therapies that address specific metabolic dysregulations. Future therapeutic strategies focus on developing targeted interventions that address the underlying metabolic dysregulation, aiming to improve patient outcomes and advance clinical management. This review offers a comprehensive overview of the metabolic mechanisms involved in calcific tendinopathy and shoulder arthrofibrosis, providing a foundation for future research and therapeutic development.

Addressing muscle-tendon imbalances in adult male athletes with personalized exercise prescription based on tendon strain

PURPOSE

Imbalances of muscle strength and tendon stiffness can increase the operating strain of tendons and risk of injury. Here, we used a new approach to identify muscle-tendon imbalances and personalize exercise prescription based on tendon strain during maximum voluntary contractions (εmax) to mitigate musculotendinous imbalances in male adult volleyball athletes.

METHODS

Four times over a season, we measured knee extensor strength and patellar tendon mechanical properties using dynamometry and ultrasonography. Tendon micromorphology was evaluated through an ultrasound peak spatial frequency (PSF) analysis. While a control group (n = 12) continued their regular training, an intervention group (n = 10) performed exercises (3 × /week) with personalized loads to elicit tendon strains that promote tendon adaptation (i.e., 4.5-6.5%).

RESULTS

Based on a linear mixed model, εmax increased significantly in the control group over the 9 months of observation (pCon = 0.010), while there was no systematic change in the intervention group (pInt = 0.575). The model residuals of εmax, as a measure of imbalances in muscle-tendon adaptation, demonstrated a significant reduction over time exclusively in the intervention group (pInt = 0.007). While knee extensor muscle strength increased in both groups by ~ 8% (pCon < 0.001, pInt = 0.064), only the intervention group showed a trend toward increased normalized tendon stiffness (pCon = 0.824, pInt = 0.051). PSF values did not change significantly in either group (p > 0.05).

CONCLUSION

These results suggest that personalized exercise prescription can reduce muscle-tendon imbalances in athletes and could provide new opportunities for tendon injury prevention.

Effects of Moderate and Heavy Slow Resistance Training on Achilles and Patellar Tendons and Muscles Aponeuroses in Elderly Men

To investigate the effect of moderate or heavy slow resistance training on structural and mechanical properties of patellar and Achilles tendons in older men, in vivo. Healthy older men (n = 27) undertook a 12-week resistance training program (3 times/week) of triceps surae and quadriceps muscle-tendon complexes. Participants were randomly assigned to either a moderate load of 55% 1RM (O55, n = 13, age: 70.0 ± 4.6 years) or a progressively heavier load from 55% up to 80% 1RM (O80, n = 14, age: 69.8 ± 4.4 years). Tendon morphology was investigated using a 1.5 T MRI scanner. Mechanical properties of the free Achilles tendon (AT) and medial gastrocnemius (MG) aponeurosis, patellar tendon (PT), and deep aponeurosis of the vastus lateralis (VL) muscle were assessed using ultrasound imaging. Both training programs led to an increase in mean CSA of the PT (O55; +2.7%, O80; +1.7%, p = 0.002) and AT (O55; +2.8%, O80; +2.5%, p < 0.001). Force and stress were increased for all the four regions investigated, while stiffness and Young's modulus assessed at a common force level were only increased for the AT and VL aponeurosis. No interaction between group and time was demonstrated for any of the outcomes. No interaction between time and the four tendon sites was observed for any of the outcomes at common force. A supervised short-term heavy or moderate slow resistance training of equal volume yielded an increase in tendon size and altered mechanical properties of tendinous tissue.

Mechanisms of mechanotransduction and physiological roles of PIEZO channels

Mechanical force is an essential physical element that contributes to the formation and function of life. The discovery of the evolutionarily conserved PIEZO family, including PIEZO1 and PIEZO2 in mammals, as bona fide mechanically activated cation channels has transformed our understanding of how mechanical forces are sensed and transduced into biological activities. In this Review, I discuss recent structure-function studies that have illustrated how PIEZO1 and PIEZO2 adopt their unique structural design and curvature-based gating dynamics, enabling their function as dedicated mechanotransduction channels with high mechanosensitivity and selective cation conductivity. I also discuss our current understanding of the physiological and pathophysiological roles mediated by PIEZO channels, including PIEZO1-dependent regulation of development and functional homeostasis and PIEZO2-dominated mechanosensation of touch, tactile pain, proprioception and interoception of mechanical states of internal organs. Despite the remarkable progress in PIEZO research, this Review also highlights outstanding questions in the field.

Molecular mechanisms of mechanosensing and plasticity of tendons and ligaments

Tendons and ligaments, crucial components of the musculoskeletal system, connect muscles to bones. In the realm of sports, tendons and ligaments are vulnerable tissues, with injuries such as Achilles tendon rupture and anterior cruciate ligament tears directly impacting an athlete's career. Furthermore, repetitive trauma and tissue degeneration can lead to conditions like secondary osteoarthritis, ultimately affecting the overall quality of life. Recent research highlights the pivotal role of mechanical stress in maintaining homeostasis within tendons and ligaments. This review delves into the latest insights on the structure of tendons and ligaments and the plasticity of tendon tissue in response to mechanical loads.

Inflammatory Signaling via PEIZO1 Engages and Enhances the LPS-Mediated Apoptosis during Clinical Mastitis

Bovine clinical mastitis is characterized by inflammation and immune responses, with apoptosis of mammary epithelial cells as a cellular reaction to infection. PIEZO1, identified as a mechanotransduction effector channel in nonruminant animals and sensitive to both mechanical stimuli or inflammatory signals like lipopolysaccharide (LPS). However, its role in inflammatory processes in cattle has not been well-documented. The aim of this study was to elucidate the in situ expression of PIEZO1 in bovine mammary gland and its potential involvement in clinical mastitis. We observed widespread distribution and upregulation of PIEZO1 in mammary epithelial cells in clinical mastitis cows and LPS-induced mouse models, indicating a conserved role across species. In vitro studies using mammary epithelial cells (MAC-T) revealed that LPS upregulates PIEZO1. Notably, the effects of PIEZO1 artificial activator Yoda1 increased apoptosis and NLRP3 expression, effects mitigated by PIEZO1 silencing or NLRP3 inhibition. In conclusion, the activation of the PIEZO1-NLRP3 pathway induces abnormal apoptosis in mammary epithelial cells, potentially serving as a regulatory mechanism to combat inflammatory responses to abnormal stimuli.

Structure and function of Achilles and patellar tendons following moderate slow resistance training in young and old men

The aim of this study was to investigate the effect of aging and resistance training with a moderate load on the size and mechanical properties of the patellar (PT) and Achilles tendon (AT) and their associated aponeuroses; medial gastrocnemius (MG) and vastus lateralis (VL). Young (Y55; 24.8 ± 3.8 yrs, n = 11) and old men (O55; 70.0 ± 4.6 yrs, n = 13) were assigned to undergo a training program (12 weeks; 3 times/week) of moderate slow resistance training [55% of one repetition maximum (RM)] of the triceps surae and quadriceps muscles. Tendon dimensions were assessed using 1.5 T magnetic resonance imaging before and after 12 weeks. AT and PT cross sectional area (CSA) were determined every 10% of tendon length. Mechanical properties of the free AT, MG aponeurosis, PT, and VL aponeurosis were assessed using ultrasonography (deformation) and tendon force measurements. CSA of the AT but not PT was greater in O55 compared with Y55. At baseline, mechanical properties were generally lower in O55 than Y55 for AT, MG aponeurosis and VL aponeurosis (Young's modulus) but not for PT. CSA of the AT and PT increased equally in both groups following training. Further, for a given force, stiffness and Young's modulus also increased equally for VL aponeurosis and AT, for boths groups. The present study highlights that except for the PT, older men have lower tendon (AT, MG aponeurosis, and VL aponeurosis) mechanical properties than young men and 12-weeks of moderate slow resistance training appears sufficient to improve tendon size and mechanical adaptations in both young and older men. New and Noteworthy: These novel findings suggest that short-term moderate slow resistance training induces equal improvements in tendon size and mechanics regardless of age.

Intermittent cyclic stretch of engineered ligaments drives hierarchical collagen fiber maturation in a dose- and organizational-dependent manner

Hierarchical collagen fibers are the primary source of strength in tendons and ligaments; however, these fibers largely do not regenerate after injury or with repair, resulting in limited treatment options. We previously developed a static culture system that guides ACL fibroblasts to produce native-sized fibers and early fascicles by 6 weeks. These constructs are promising ligament replacements, but further maturation is needed. Mechanical cues are critical for development in vivo and in engineered tissues; however, the effect on larger fiber and fascicle formation is largely unknown. Our objective was to investigate whether intermittent cyclic stretch, mimicking rapid muscle activity, drives further maturation in our system to create stronger engineered replacements and to explore whether cyclic loading has differential effects on cells at different degrees of collagen organization to better inform engineered tissue maturation protocols. Constructs were loaded with an established intermittent cyclic loading regime at 5 or 10 % strain for up to 6 weeks and compared to static controls. Cyclic loading drove cells to increase hierarchical collagen organization, collagen crimp, and tissue tensile properties, ultimately producing constructs that matched or exceeded immature ACL properties. Further, the effect of loading on cells varied depending on degree of organization. Specifically, 10 % load drove early improvements in tensile properties and composition, while 5 % load was more beneficial later in culture, suggesting a shift in mechanotransduction. This study provides new insight into how cyclic loading affects cell-driven hierarchical fiber formation and maturation, which will help to develop better rehabilitation protocols and engineer stronger replacements. STATEMENT OF SIGNIFICANCE: Collagen fibers are the primary source of strength and function in tendons and ligaments throughout the body. These fibers have limited regenerate after injury, with repair, and in engineered replacements, reducing treatment options. Cyclic load has been shown to improve fibril level alignment, but its effect at the larger fiber and fascicle length-scale is largely unknown. Here, we demonstrate intermittent cyclic loading increases cell-driven hierarchical fiber formation and tissue mechanics, producing engineered replacements with similar organization and mechanics as immature ACLs. This study provides new insight into how cyclic loading affects cell-driven fiber maturation. A better understanding of how mechanical cues regulate fiber formation will help to develop better engineered replacements and rehabilitation protocols to drive repair after injury.

The role of mechanotransduction in tendon

Tendons play an important role in the maintenance of motor function by connecting muscles and bones and transmitting forces. Particularly, the role of mechanical stress has primarily focused on the key mechanism of tendon homeostasis, with much research on this topic. With the recent development of molecular biological techniques, the mechanisms of mechanical stress sensing and signal transduction have been gradually elucidated with the identification of mechanosensor in tendon cells and the master regulator in tendon development. This review provides a comprehensive overview of the structure and function of tendon tissue, including the role for physical performance and the detailed mechanism of mechanotransduction in its regulation. An important lesson is that the role of mechanotransduction in tendon tissue is only partially clarified, indicating the complexity of the mechanisms of motor function and fueling increasing interest in uncovering these mechanisms.

Changes in patellar tendon complaints and shear wave velocity patterns among competitive alpine skiers during a 4-year post-growth spurt follow-up

Patellar tendon (PT) complaints are frequent in competitive alpine skiers and such complaints are characterized by a long-lasting affection. Since PTs are subject to maturation up to 1-2 years after growth spurt, this early career stage may be decisive for the further course of complaints. The aim of this study was to investigate the evolution of PT complaints and shear wave velocity patterns among competitive alpine skiers during a 4-year post-growth spurt follow-up. The PT complaints and SWV patterns of forty-seven skiers were analysed at baseline (i.e., immediately after their peak height growth at 13-15 years of age) and were re-analysed at 4-year follow-up. The PTs were scanned via three-dimensional SWE. Symptomatic skiers were identified based on pain sensation under loading and pressure-induced pain around the PT. The prevalence of PT complaints decreased from 29.8% at baseline to 12.8% at follow-up (Pearson's χ2 = 9.429; p = 0.002). SWV decreased from the baseline assessment to the follow-up in the proximal and distal regions (p < 0.05). SWV coefficient of variation (CV) in the distal and mid-portion regions was greater at baseline than at follow-up (p < 0.05). At the follow-up assessment, compared to "healthy" skiers, "healed" skiers who recovered from PT complaints had lower SWVs in the proximal region (p = 0.020) and greater SWV CVs in the proximal region (p = 0.028). Moreover, symptomatic skiers had significantly greater SWV CVs in the mid-portion region than did "healthy" subjects with no history of PT complaints (p = 0.020). The average SWV was negatively correlated with the SWV (proximal: r = -0.74, p < 0.001; mid-portion: r = -0.37 p = 0.011; and distal: r = -0.58, p < 0.001). The occurrence of PT complaints decreased over a 4-year post-growth spurt follow-up. "Healed" skiers who were symptomatic at baseline had an even greater average decrease in the proximal and mid-portion SWV than "healthy" skiers with no history of PT complaints. This may lead to the hypothesis that PT complaints in adolescent skiers are not self-eliminating towards the end of adolescence, as at least structural irregularities appear to persist for several years after the onset of initial symptoms. Furthermore, "healed" and symptomatic tendons exhibited increased SWV variability, supporting the hypothesis that SWV CV may provide additional valuable information on the mechanical properties of PTs affected by overuse-related complaints.

The emerging role of Piezo1 in the musculoskeletal system and disease

Piezo1, a mechanosensitive ion channel, has emerged as a key player in translating mechanical stimuli into biological signaling. Its involvement extends beyond physiological and pathological processes such as lymphatic vessel development, axon growth, vascular development, immunoregulation, and blood pressure regulation. The musculoskeletal system, responsible for structural support, movement, and homeostasis, has recently attracted attention regarding the significance of Piezo1. This review aims to provide a comprehensive summary of the current research on Piezo1 in the musculoskeletal system, highlighting its impact on bone formation, myogenesis, chondrogenesis, intervertebral disc homeostasis, tendon matrix cross-linking, and physical activity. Additionally, we explore the potential of targeting Piezo1 as a therapeutic approach for musculoskeletal disorders, including osteoporosis, muscle atrophy, intervertebral disc degeneration, and osteoarthritis.

Synergistic Biofilter Tube for Promoting Scarless Tendon Regeneration

Inspired by the imbalance between extrinsic and intrinsic tendon healing, this study fabricated a new biofilter scaffold with a hierarchical structure based on a melt electrowriting technique. The outer multilayered fibrous structure with connected porous characteristics provides a novel passageway for vascularization and isolates the penetration of scar fibers, which can be referred to as a biofilter process. In vitro experiments found that the porous architecture in the outer layer can effectively prevent cell infiltration, whereas the aligned fibers in the inner layer can promote cell recruitment and growth, as well as the expression of tendon-associated proteins in a simulated friction condition. It was shown in vivo that the biofilter process could promote tendon healing and reduce scar invasion. Herein, this novel strategy indicates great potential to design new biomaterials for balancing extrinsic and intrinsic healing and realizing scarless tendon healing.

A high throughput cell stretch device for investigating mechanobiology in vitro

Mechanobiology is a rapidly advancing field, with growing evidence that mechanical signaling plays key roles in health and disease. To accelerate mechanobiology-based drug discovery, novel in vitro systems are needed that enable mechanical perturbation of cells in a format amenable to high throughput screening. Here, both a mechanical stretch device and 192-well silicone flexible linear stretch plate were designed and fabricated to meet high throughput technology needs for cell stretch-based applications. To demonstrate the utility of the stretch plate in automation and screening, cell dispensing, liquid handling, high content imaging, and high throughput sequencing platforms were employed. Using this system, an assay was developed as a biological validation and proof-of-concept readout for screening. A mechano-transcriptional stretch response was characterized using focused gene expression profiling measured by RNA-mediated oligonucleotide Annealing, Selection, and Ligation with Next-Gen sequencing. Using articular chondrocytes, a gene expression signature containing stretch responsive genes relevant to cartilage homeostasis and disease was identified. The possibility for integration of other stretch sensitive cell types (e.g., cardiovascular, airway, bladder, gut, and musculoskeletal), in combination with alternative phenotypic readouts (e.g., protein expression, proliferation, or spatial alignment), broadens the scope of high throughput stretch and allows for wider adoption by the research community. This high throughput mechanical stress device fills an unmet need in phenotypic screening technology to support drug discovery in mechanobiology-based disease areas.

Intermittent Cyclic Stretch of Engineered Ligaments Drives Hierarchical Collagen Fiber Maturation in a Dose- and Organizational-Dependent Manner

Hierarchical collagen fibers are the primary source of strength in tendons and ligaments, however these fibers do not regenerate after injury or with repair, resulting in limited treatment options. We previously developed a culture system that guides ACL fibroblasts to produce native-sized fibers and fascicles by 6 weeks. These constructs are promising ligament replacements, but further maturation is needed. Mechanical cues are critical for development in vivo and in engineered tissues; however, the effect on larger fiber and fascicle formation is largely unknown. Our objective was to investigate whether intermittent cyclic stretch, mimicking rapid muscle activity, drives further maturation in our system to create stronger engineered replacements and to explore whether cyclic loading has differential effects on cells at different degrees of collagen organization to better inform engineered tissue maturation protocols. Constructs were loaded with an established intermittent cyclic loading regime at 5 or 10% strain for up to 6 weeks and compared to static controls. Cyclic loading drove cells to increase hierarchical collagen organization, collagen crimp, and tissue mechanics, ultimately producing constructs that matched or exceeded immature ACL properties. Further, the effect of loading on cells varied depending on degree of organization. Specifically, 10% load drove early improvements in mechanics and composition, while 5% load was more beneficial later in culture, suggesting a cellular threshold response and a shift in mechanotransduction. This study provides new insight into how cyclic loading affects cell-driven hierarchical fiber formation and maturation, which will help to develop better rehabilitation protocols and engineer stronger replacements.

Effect of the temporal coordination and volume of cyclic mechanical loading on human Achilles tendon adaptation in men

Human tendons adapt to mechanical loading, yet there is little information on the effect of the temporal coordination of loading and recovery or the dose-response relationship. For this reason, we assigned adult men to either a control or intervention group. In the intervention group, the two legs were randomly assigned to one of five high-intensity Achilles tendon (AT) loading protocols (i.e., 90% maximum voluntary contraction and approximately 4.5 to 6.5% tendon strain) that were systematically modified in terms of loading frequency (i.e., sessions per week) and overall loading volume (i.e., total time under loading). Before, at mid-term (8 weeks) and after completion of the 16 weeks intervention, AT mechanical properties were determined using a combination of inverse dynamics and ultrasonography. The cross-sectional area (CSA) and length of the free AT were measured using magnetic resonance imaging pre- and post-intervention. The data analysis with a linear mixed model showed significant increases in muscle strength, rest length-normalized AT stiffness, and CSA of the free AT in the intervention group (p < 0.05), yet with no marked differences between protocols. No systematic effects were found considering the temporal coordination of loading and overall loading volume. In all protocols, the major changes in normalized AT stiffness occurred within the first 8 weeks and were mostly due to material rather than morphological changes. Our findings suggest that-in the range of 2.5-5 sessions per week and 180-300 s total high strain loading-the temporal coordination of loading and recovery and overall loading volume is rather secondary for tendon adaptation.

High-content method for mechanosignaling studies using IsoStretcher technology and quantitative Ca2+ imaging applied to Piezo1 in cardiac HL-1 cells

The importance of mechanosensory transduction pathways in cellular signalling has prominently come to focus in the last decade with the discovery of the Piezo ion channel family. Mechanosignaling involving Piezo1 ion channels in the function of the heart and cardiovascular system has only recently been identified to have implications for cardiovascular physiology and pathophysiology, in particular for heart failure (i.e., hypertrophy or dilative cardiomyopathy). These results have emphasized the need for higher throughput methods to study single-cell cardiovascular mechanobiology with the aim of identifying new targets for therapeutic interventions and stimulating the development of new pharmacological agents. Here, we present a novel method to assess mechanosignaling in adherent cardiac cells (murine HL-1 cell line) using a combination of isotropic cell stretch application and simultaneous Ca2+ fluorescence readout with quantitative analysis. The procedure implements our IsoStretcher technology in conjunction with a single-cell- and population-based analysis of Ca2+ signalling by means of automated image registration, cell segmentation and analysis, followed by automated classification of single-cell responses. The method is particularly valuable for assessing the heterogeneity of populations with distinct cellular responses to mechanical stimulation and provides more user-independent unbiased drug response classifications.

A Hierarchical Mechanotransduction System: From Macro to Micro

Mechanotransduction is a strictly regulated process whereby mechanical stimuli, including mechanical forces and properties, are sensed and translated into biochemical signals. Increasing data demonstrate that mechanotransduction is crucial for regulating macroscopic and microscopic dynamics and functionalities. However, the actions and mechanisms of mechanotransduction across multiple hierarchies, from molecules, subcellular structures, cells, tissues/organs, to the whole-body level, have not been yet comprehensively documented. Herein, the biological roles and operational mechanisms of mechanotransduction from macro to micro are revisited, with a focus on the orchestrations across diverse hierarchies. The implications, applications, and challenges of mechanotransduction in human diseases are also summarized and discussed. Together, this knowledge from a hierarchical perspective has the potential to refresh insights into mechanotransduction regulation and disease pathogenesis and therapy, and ultimately revolutionize the prevention, diagnosis, and treatment of human diseases.

The role of mechanically sensitive ion channel Piezo1 in bone remodeling

Piezo1 (2010) was identified as a mechanically activated cation channel capable of sensing various physical forces, such as tension, osmotic pressure, and shear force. Piezo1 mediates mechanosensory transduction in different organs and tissues, including its role in maintaining bone homeostasis. This review aimed to summarize the function and possible mechanism of Piezo1 in the mechanical receptor cells in bone tissue. We found that it is a potential therapeutic target for the treatment of bone diseases.

Personalized tendon loading reduces muscle-tendon imbalances in male adolescent elite athletes

An imbalanced adaptation of muscle strength and tendon stiffness in response to training may increase tendon strain (i.e., the mechanical demand on the tendon) and consequently tendon injury risk. This study investigated if personalized tendon loading inducing tendon strain within the effective range for adaptation (4.5%-6.5%) can reduce musculotendinous imbalances in male adolescent handball athletes (15-16 years). At four measurement time points during a competitive season, we assessed knee extensor muscle strength and patellar tendon mechanical properties using dynamometry and ultrasonography and estimated the tendon's structural integrity with a peak spatial frequency (PSF) analysis of proximal tendon ultrasound scans. A control group (n = 13) followed their usual training routine, an intervention group (n = 13) integrated tendon exercises into their training (3x/week for ~31 weeks) with a personalized intensity corresponding to an average of ~6.2% tendon strain. We found a significant time by group interaction (p < 0.005) for knee extensor muscle strength and normalized patellar tendon stiffness with significant increases over time only in the intervention group (p < 0.001). There were no group differences or time-dependent changes in patellar tendon strain during maximum voluntary contractions or PSF. At the individual level, the intervention group demonstrated lower fluctuations of maximum patellar tendon strain during the season (p = 0.005) and a descriptively lower frequency of athletes with high-level tendon strain (≥9%). The findings suggest that the personalized tendon loading program reduced muscle-tendon imbalances in male adolescent athletes, which may provide new opportunities for tendon injury prevention.

Piezo1 channel causes lens sclerosis via transglutaminase 2 activation

Presbyopia is caused by age-related lenticular hardening, resulting in near vision loss, and it occurs in almost every individual aged ≥50 years. The lens experiences mechanical pressure during for focal adjustment to change its thickness. As lenticular stiffening results in incomplete thickness changes, near vision is reduced, which is known as presbyopia. Piezo1 is a mechanosensitive channel that constantly senses pressure changes during the regulation of visual acuity, and changes in Piezo1 channel activity may contribute to presbyopia. However, no studies have reported on Piezo1 activation or the onset of presbyopia. To elucidate the relevance of Piezo1 activation and cross-linking in the development of presbyopia, we analysed the function of Piezo1 in the lens. The addition of Yoda1, a Piezo1 activator, induced an increase in transglutaminase 2 (TGM2) mRNA expression and activity through the extra-cellular signal-regulated kinase (ERK) 1/2 and c-Jun-NH2-terminal kinase1/2 pathways. In ex vivo lenses, Yoda1 treatment induced γ-crystallin cross-linking via TMG2 activation. Furthermore, Yoda1 eye-drops in mice led to lenticular hardening via TGM2 induction and activation in vivo, suggesting that Yoda1-treated animals could serve as a model for presbyopia. Our findings indicate that this presbyopia-animal model could be useful for screening drugs for lens-stiffening inhibition.

Light-Induced Nanoscale Deformation in Azobenzene Thin Film Triggers Rapid Intracellular Ca2+ Increase via Mechanosensitive Cation Channels

Epithelial cells are in continuous dynamic biochemical and physical interaction with their extracellular environment. Ultimately, this interplay guides fundamental physiological processes. In these interactions, cells generate fast local and global transients of Ca2+ ions, which act as key intracellular messengers. However, the mechanical triggers initiating these responses have remained unclear. Light-responsive materials offer intriguing possibilities to dynamically modify the physical niche of the cells. Here, a light-sensitive azobenzene-based glassy material that can be micropatterned with visible light to undergo spatiotemporally controlled deformations is used. Real-time monitoring of consequential rapid intracellular Ca2+ signals reveals that the mechanosensitive cation channel Piezo1 has a major role in generating the Ca2+ transients after nanoscale mechanical deformation of the cell culture substrate. Furthermore, the studies indicate that Piezo1 preferably responds to shear deformation at the cell-material interphase rather than to absolute topographical change of the substrate. Finally, the experimentally verified computational model suggests that Na+ entering alongside Ca2+ through the mechanosensitive cation channels modulates the duration of Ca2+ transients, influencing differently the directly stimulated cells and their neighbors. This highlights the complexity of mechanical signaling in multicellular systems. These results give mechanistic understanding on how cells respond to rapid nanoscale material dynamics and deformations.

On the contribution of solid and fluid behavior to the modeling of the time-dependent mechanics of tendons under semi-confined compression

The present work aims to investigate whether it is possible to identify and quantify the contributions of the interstitial fluid and the solid skeleton to the overall time-dependent behavior of tendons based on a single mechanical test. For this purpose, the capabilities of three different time-dependent models (a viscoelastic, a poroelastic and a poroviscoelastic) were investigated in the modeling of the experimental behavior obtained from semi-confined compression with stress relaxation tests transverse to collagen fibers. The main achieved result points out that the poroviscoelastic model was the only one capable to characterize both the experimental responses of the force and volume changes of the tissue samples. Moreover, further analysis of this model shows that while the kinematics of the sample are mainly governed by the fluid flow (pore pressure contribution of the model), the behavior intrinsically associated with the viscoelastic solid skeleton makes a significant contribution to the experimental force response. This study reinforces the importance of taking both the experimental kinematics and kinetics of tendon tissues into account during the constitutive characterization procedure.

The LINC complex regulates Achilles tendon elastic modulus, Achilles and tail tendon collagen crimp, and Achilles and tail tendon lateral expansion during early postnatal development

There is limited understanding of how mechanical signals regulate tendon development. The nucleus has emerged as a major regulator of cellular mechanosensation, via the linker of nucleoskeleton and cytoskeleton (LINC) protein complex. Specific roles of LINC in tenogenesis have not been explored. In this study, we investigate how LINC regulates tendon development by disabling LINC-mediated mechanosensing via dominant negative (dn) expression of the Klarsicht, ANC-1, and Syne Homology (KASH) domain, which is necessary for LINC to function. We hypothesized that LINC regulates mechanotransduction in developing tendon, and that disabling LINC would impact tendon mechanical properties and structure in a mouse model of dnKASH. We used Achilles (AT) and tail (TT) tendons as representative energy-storing and limb-positioning tendons, respectively. Mechanical testing at postnatal day 10 showed that disabling the LINC complex via dnKASH significantly impacted tendon mechanical properties and cross-sectional area, and that effects differed between ATs and TTs. Collagen crimp distance was also impacted in dnKASH tendons, and was significantly decreased in ATs, and increased in TTs. Overall, we show that disruption to the LINC complex specifically impacts tendon mechanics and collagen crimp structure, with unique responses between an energy-storing and limb-positioning tendon. This suggests that nuclear mechanotransduction through LINC plays a role in regulating tendon formation during neonatal development.

Meniscal fibrocartilage regeneration inspired by meniscal maturational and regenerative process

Meniscus is a complex and crucial fibrocartilaginous tissue within the knee joint. Meniscal regeneration remains to be a scientific and translational challenge. We clarified that mesenchymal stem cells (MSCs) participated in meniscal maturation and regeneration using MSC-tracing transgenic mice model. Here, inspired by meniscal natural maturational and regenerative process, we developed an effective and translational strategy to facilitate meniscal regeneration by three-dimensionally printing biomimetic meniscal scaffold combining autologous synovium transplant, which contained abundant intrinsic MSCs. We verified that this facilitated anisotropic meniscus-like tissue regeneration and protected cartilage from degeneration in large animal model. Mechanistically, the biomechanics and matrix stiffness up-regulated Piezo1 expression, facilitating concerted activation of calcineurin and NFATc1, further activated YAP-pSmad2/3-SOX9 axis, and consequently facilitated fibrochondrogenesis of MSCs during meniscal regeneration. In addition, Piezo1 induced by biomechanics and matrix stiffness up-regulated collagen cross-link enzyme expression, which catalyzed collagen cross-link and thereby enhanced mechanical properties of regenerated tissue.

Mechanosensitive GPCRs and ion channels in shear stress sensing

As a universal mechanical cue, shear stress plays essential roles in many physiological processes, ranging from vascular morphogenesis and remodeling to renal transport and airway barrier function. Disrupted shear stress is commonly regarded as a major contributor to various human diseases such as atherosclerosis, hypertension, and chronic kidney disease. Despite the importance of shear stress in physiology and pathophysiology, our current understanding of mechanosensors that sense shear stress is far from complete. An increasing number of candidate mechanosensors have been proposed to mediate shear stress sensing in distinct cell types, including G protein-coupled receptors (GPCRs), G proteins, receptor tyrosine kinases, ion channels, glycocalyx proteins, and junctional proteins. Although multiple types of mechanosensors might be able to convert shear stress into downstream biochemical signaling events, in this review, we will focus on discussing the mechanosensitive GPCRs (angiotensin II type 1 receptor, GPR68, histamine H1 receptor, adhesion GPCRs) and ion channels (Piezo, TRP) that have been reported to be directly activated by shear stress.

Tendon cell and tissue culture: Perspectives and recommendations

The wide variety of cell and tissue culture systems used to study and engineer tendons can make it difficult to choose the best approach and "optimal" culture conditions to test a given hypothesis. Therefore, a breakout session was organized at the 2022 ORS Tendon Section Meeting that focused on establishing a set of guidelines for conducting cell and tissue culture studies of tendon. This paper summarizes the outcomes of that discussion and presents recommendations for future studies. In the case of studying tendon cell behavior, cell and tissue culture systems are reductionist models in which the culture conditions should be strictly defined to approximate the in vivo condition as closely as possible. In contrast, for tissue engineering tendon replacements, the culture conditions do not need to replicate native tendon, but the outcome measures for success should be narrowly defined for the specific clinical application. Common recommendations for both applications are that researchers should perform a baseline phenotypic characterization of the cells that are ultimately used for experimentation. For models of tendon cell behavior, culture conditions should be well justified by existing literature and meticulously reported, tissue explant viability should be assessed, and comparisons to in vivo conditions should be made to determine baseline physiological relevance. For tissue engineering applications, the functional/structural/compositional outcome targets should be defined by the specific tendons they seek to replace, with key biologic and material properties prioritized for construct assessment. Lastly, when engineering tendon replacements, researchers should utilize clinically approved cGMP materials to facilitate clinical translation.

Longitudinal Evidence for High-Level Patellar Tendon Strain as a Risk Factor for Tendinopathy in Adolescent Athletes

BACKGROUND

High tendon strain leads to sub-rupture fatigue damage and net-catabolic signaling upon repetitive loading. While high levels of tendon strain occur in adolescent athletes at risk for tendinopathy, a direct association has not yet been established. Therefore, in this prospective longitudinal study, we examined the hypothesis that adolescent athletes who develop patellar tendon pain have shown increased levels of strain in advance.

METHODS

In 44 adolescent athletes (12-17 years old), patellar tendon mechanical properties were measured using ultrasonography and inverse dynamics at four time points during a season. Fourteen athletes developed clinically relevant tendon pain (SYM; i.e., reduction of the VISA-P score of at least 13 points), while 23 remained asymptomatic (ASYM; VISA-P score of > 87 points). Seven cases did not fall into one of these categories and were excluded. Tendon mechanical properties of SYM in the session before the development of symptoms were compared to a randomly selected session in ASYM.

RESULTS

Tendon strain was significantly higher in SYM compared to ASYM (p = 0.03). The risk ratio for developing symptoms was 2.3-fold higher in athletes with tendon strain ≥9% (p = 0.026). While there was no clear evidence for systematic differences of the force applied to the tendon or tendon stiffness between SYM and ASYM (p > 0.05), subgroup analysis indicated that tendon force increased prior to the development of symptoms only in SYM (p = 0.034).

DISCUSSIO

The study provides novel longitudinal evidence that high tendon strain could be an important risk factor for patellar tendinopathy in adolescent athletes. We suggest that inadequate adaptation of tendon stiffness to increases in muscle strength may occur if adolescent athletes are subject to mechanical loading which does not  provide effective tendon stimulation.

PIEZO1 is downregulated in glenohumeral chondrocytes in early cuff tear arthropathy following a massive rotator cuff tear in a mouse model

Introduction: A massive rotator cuff tear (RCT) leads to glenohumeral joint destabilization and characteristic degenerative changes, termed cuff tear arthropathy (CTA). Understanding the response of articular cartilage to a massive RCT will elucidate opportunities to promote homeostasis following restoration of joint biomechanics with rotator cuff repair. Mechanically activated calcium-permeating channels, in part, modulate the response of distal femoral chondrocytes in the knee against injurious loading and inflammation. The objective of this study was to investigate PIEZO1-mediated mechanotransduction of glenohumeral articular chondrocytes in the altered biomechanical environment following RCT to ultimately identify potential therapeutic targets to attenuate cartilage degeneration after rotator cuff repair. Methods: First, we quantified mechanical susceptibility of chondrocytes in mouse humeral head cartilage ex vivo with treatments of specific chemical agonists targeting PIEZO1 and TRPV4 channels. Second, using a massive RCT mouse model, chondrocytes were assessed for mechano-vulnerability, PIEZO1 expression, and calcium signaling activity 14-week post-injury, an early stage of CTA. Results: In native humeral head chondrocytes, chemical activation of PIEZO1 (Yoda1) significantly increased chondrocyte mechanical susceptibility against impact loads, while TRPV4 activation (GSK101) significantly decreased impact-induced chondrocyte death. A massive RCT caused morphologic and histologic changes to the glenohumeral joint with decreased sphericity and characteristic bone bruising of the posterior superior quadrant of the humeral head. At early CTA, chondrocytes in RCT limbs exhibit a significantly decreased functional expression of PIEZO1 compared with uninjured or sham controls. Discussion: In contrast to the hypothesis, PIEZO1 expression and activity is not increased, but rather downregulated, after massive RCT at the early stage of cuff tear arthropathy. These results may be secondary to the decreased axial loading after glenohumeral joint decoupling in RCT limbs.

Mechanosensitive Piezo1 channel in physiology and pathophysiology of the central nervous system

Since the discovery of the mechanosensitive Piezo1 channel in 2010, there has been a significant amount of research conducted to explore its regulatory role in the physiology and pathology of various organ systems. Recently, a growing body of compelling evidence has emerged linking the activity of the mechanosensitive Piezo1 channel to health and disease of the central nervous system. However, the exact mechanisms underlying these associations remain inadequately comprehended. This review systematically summarizes the current research on the mechanosensitive Piezo1 channel and its implications for central nervous system mechanobiology, retrospects the results demonstrating the regulatory role of the mechanosensitive Piezo1 channel on various cell types within the central nervous system, including neural stem cells, neurons, oligodendrocytes, microglia, astrocytes, and brain endothelial cells. Furthermore, the review discusses the current understanding of the involvement of the Piezo1 channel in central nervous system disorders, such as Alzheimer's disease, multiple sclerosis, glaucoma, stroke, and glioma.

Force-induced motions of the PIEZO1 blade probed with fluorimetry

Mechanical forces are thought to activate mechanosensitive PIEZO channels by changing the conformation of a large transmembrane blade domain. Yet, whether different stimuli induce identical conformational changes in this domain remains unclear. Here, we repurpose a cyclic permuted green fluorescent protein as a conformation-sensitive probe to track local rearrangements along the PIEZO1 blade. Two independent probes, one inserted in an extracellular site distal to the pore and the other in a distant intracellular proximal position, elicit sizable fluorescence signals when the tagged channels activate in response to fluid shear stress of low intensity. Neither cellular indentations nor osmotic swelling of the cell elicit detectable fluorescence signals from either probe, despite the ability of these stimuli to activate the tagged channels. High-intensity flow stimuli are ineffective at eliciting fluorescence signals from either probe. Together, these findings suggest that low-intensity fluid shear stress causes a distinct form of mechanical stress to the cell.

Increased Ca2+ signaling through CaV 1.2 induces tendon hypertrophy with increased collagen fibrillogenesis and biomechanical properties

Tendons are tension-bearing tissues transmitting force from muscle to bone for body movement. This mechanical loading is essential for tendon development, homeostasis, and healing after injury. While Ca2+ signaling has been studied extensively for its roles in mechanotransduction, regulating muscle, bone, and cartilage development and homeostasis, knowledge about Ca2+ signaling and the source of Ca2+ signals in tendon fibroblast biology are largely unknown. Here, we investigated the function of Ca2+ signaling through CaV 1.2 voltage-gated Ca2+ channel in tendon formation. Using a reporter mouse, we found that CaV 1.2 is highly expressed in tendon during development and downregulated in adult homeostasis. To assess its function, we generated ScxCre;CaV 1.2TS mice that express a gain-of-function mutant CaV 1.2 in tendon. We found that mutant tendons were hypertrophic, with more tendon fibroblasts but decreased cell density. TEM analyses demonstrated increased collagen fibrillogenesis in the hypertrophic tendons. Biomechanical testing revealed that the hypertrophic tendons display higher peak load and stiffness, with no changes in peak stress and elastic modulus. Proteomic analysis showed no significant difference in the abundance of type I and III collagens, but mutant tendons had about two-fold increase in other ECM proteins such as tenascin C, tenomodulin, periostin, type XIV and type VIII collagens, around 11-fold increase in the growth factor myostatin, and significant elevation of matrix remodeling proteins including Mmp14, Mmp2, and cathepsin K. Taken together, these data highlight roles for increased Ca2+ signaling through CaV 1.2 on regulating expression of myostatin growth factor and ECM proteins for tendon collagen fibrillogenesis during tendon formation.

Acoustic and Magnetic Stimuli-Based Three-Dimensional Cell Culture Platform for Tissue Engineering

In a conventional two-dimensional (2D) culture method, cells are attached to the bottom of the culture dish and grow into a monolayer. These 2D culture methods are easy to handle, cost-effective, reproducible, and adaptable to growing many different types of cells. However, monolayer 2D cell culture conditions are far from those of natural tissue, indicating the need for a three-dimensional (3D) culture system. Various methods, such as hanging drop, scaffolds, hydrogels, microfluid systems, and bioreactor systems, have been utilized for 3D cell culture. Recently, external physical stimulation-based 3D cell culture platforms, such as acoustic and magnetic forces, were introduced. Acoustic waves can establish acoustic radiation force, which can induce suspended objects to gather in the pressure node region and aggregate to form clusters. Magnetic targeting consists of two components, a magnetically responsive carrier and a magnetic field gradient source. In a magnetic-based 3D cell culture platform, cells are aggregated by changing the magnetic force. Magnetic fields can manipulate cells through two different methods: positive magnetophoresis and negative magnetophoresis. Positive magnetophoresis is a way of imparting magnetic properties to cells by labeling them with magnetic nanoparticles. Negative magnetophoresis is a label-free principle-based method. 3D cell structures, such as spheroids, 3D network structures, and cell sheets, have been successfully fabricated using this acoustic and magnetic stimuli-based 3D cell culture platform. Additionally, fabricated 3D cell structures showed enhanced cell behavior, such as differentiation potential and tissue regeneration. Therefore, physical stimuli-based 3D cell culture platforms could be promising tools for tissue engineering.

PIEZO1 gain-of-function gene variant is associated with elevated tendon stiffness in humans

Prolonged periods of increased physical demands can elicit anabolic tendon adaptations that increase stiffness and mechanical resilience or conversely can lead to pathological processes that deteriorate tendon structural quality with ensuing pain and potential rupture. Although the mechanisms by which tendon mechanical loads regulate tissue adaptation are largely unknown, the ion channel PIEZO1 has been implicated in tendon mechanotransduction, with human carriers of the PIEZO1 gain-of-function variant E756del displaying improved dynamic vertical jump performance compared with noncarriers. Here, we sought to examine whether increased tendon stiffness in humans could explain this increased performance. We assessed tendon morphological and mechanical properties with ultrasound-based techniques in 77 participants of Middle- and West-African descent, and we measured their vertical jumping performance to assess potential functional consequences in the context of high tendon strain-rate loading. Carrying the E756del gene variant (n = 30) was associated with 46.3 ± 68.3% (P = 0.002) and 45.6 ± 69.2% (P < 0.001) higher patellar tendon stiffness and Young's modulus compared with noncarrying controls, respectively. Although these tissue level measures strongly corroborate the initial postulate that PIEZO1 plays an integral part in regulating tendon material properties and stiffness in humans, we found no detectable correlation between tendon stiffness and jumping performance in the tested population that comprised individuals of highly diverse physical fitness level, dexterity, and jumping ability.NEW & NOTEWORTHY The E756del gene variant causes overactivity of the mechanosensitive membrane channel PIEZO1 and is suspected to upregulate tendon collagen cross linking. In human carriers of E756del, we found increased patellar tendon stiffness but similar tendon lengths and cross-sectional areas, directly supporting the premise that PIEZO1 regulates human tendon stiffness at the level of tissue material properties.

Mechanoepigenetic regulation of extracellular matrix homeostasis via Yap and Taz

Cells integrate mechanical cues to direct fate specification to maintain tissue function and homeostasis. While disruption of these cues is known to lead to aberrant cell behavior and chronic diseases, such as tendinopathies, the underlying mechanisms by which mechanical signals maintain cell function are not well understood. Here, we show using a model of tendon de-tensioning that loss of tensile cues in vivo acutely changes nuclear morphology, positioning, and expression of catabolic gene programs, resulting in subsequent weakening of the tendon. In vitro studies using paired ATAC/RNAseq demonstrate that the loss of cellular tension rapidly reduces chromatin accessibility in the vicinity of Yap/Taz genomic targets while also increasing expression of genes involved in matrix catabolism. Concordantly, the depletion of Yap/Taz elevates matrix catabolic expression. Conversely, overexpression of Yap results in a reduction of chromatin accessibility at matrix catabolic gene loci, while also reducing transcriptional levels. The overexpression of Yap not only prevents the induction of this broad catabolic program following a loss of cellular tension, but also preserves the underlying chromatin state from force-induced alterations. Taken together, these results provide novel mechanistic details by which mechanoepigenetic signals regulate tendon cell function through a Yap/Taz axis.

Quantification of patellar tendon strain and opportunities for personalized tendon loading during back squats

Tendon strain during exercise is a critical regulatory factor in tendon adaptive responses and there are indications for an optimal range of strain that promotes tendon adaptation. Back squats are used to improve patellar tendon properties in sport and clinical settings. To date, the operating patellar tendon strain during back squats is unknown and current recommendations for individual exercise loading are based on the one repetition maximum (1RM). Here, we quantified patellar tendon strain during loaded back squats at 40, 60 and 80% of the 1RM and during maximum isometric knee extension contractions (MVC) using ultrasonography. Kinematics, ground reaction forces and muscle electromyographic activity were also recorded. Additionally, maximum tendon strain during the MVC and the percentage of 1RM were used as explanatory variables to estimate the individual patellar tendon strain during the squats. Strain increased with increasing 1RM loading (4.7 to 8.2%), indicating that already medium-loading back squats may provide a sufficient stimulus for tendon adaptation. The individual variability was, however, too high to generalize these findings. Yet, there was a high agreement between the individually estimated and measured patellar tendon strain (R² = 0.858) during back squats. We argue that this approach may provide new opportunities for personalized tendon exercise.

Integrative single-cell RNA and ATAC sequencing reveals that the FOXO1-PRDX2-TNF axis regulates tendinopathy

Introduction

Tendinopathy, the most common form of chronic tendon disorder, leads to persistent tendon pain and loss of function. Profiling the heterogeneous cellular composition in the tendon microenvironment helps to elucidate rational molecular mechanisms of tendinopathy.

Methods and results

In this study, through a multi-modal analysis, a single-cell RNA- and ATAC-seq integrated tendinopathy landscape was generated for the first time. We found that a specific cell subpopulation with low PRDX2 expression exhibited a higher level of inflammation, lower proliferation and migration ability, which not only promoted tendon injury but also led to microenvironment deterioration. Mechanistically, a motif enrichment analysis of chromatin accessibility showed that FOXO1 was an upstream regulator of PRDX2 transcription, and we confirmed that functional blockade of FOXO1 activity induced PRDX2 silencing. The TNF signaling pathway was significantly activated in the PRDX2-low group, and TNF inhibition effectively restored diseased cell degradation.

Discussion

We revealed an essential role of diseased cells in tendinopathy and proposed the FOXO1-PRDX2-TNF axis is a potential regulatory mechanism for the treatment of tendinopathy.

Growth-related changes in the ultrastructure of the quadriceps tendon

BACKGROUND

The purpose of this study was to investigate the effect of growth on the ultrastructural characteristics of the quadriceps tendon (QT).

METHODS

Eighteen included patients were classified into three groups based on age and epiphyseal plate condition: the 'immature group' consisted of patients with open epiphyseal plates (11.5 ± 1.6 years old; mean ± standard deviation), the 'young group' consisted of patients aged <20 years with closed epiphyseal plates (15.8 ± 1.0 years), and the 'adult group' consisted of all patients aged >20 years (29.8 ± 11.3 years) irrespective of epiphyseal plate condition. Tendon tissue samples were used for ultrastructural analysis by transmission electron microscopy. Minimum collagen fibril diameters were measured from the cross sections of collagen fibril images using Image J software. The average number of collagen fibers per sample was 797 ± 109, and the average collagen fibril diameter of each sample was compared using one-way analysis of variance.

RESULTS

The mean collagen fibril diameter was 89.7 ± 14.4 nm in the immature group, 94.8 ± 16.4 nm in the young group, and 107.2 ± 12.1 nm in the adult group, with significant differences between the immature and adult groups, and between the young and adult groups (P = 0.001 and P = 0.021, respectively); however, no significant differences were observed between the immature and young adult groups (P = 0.49).

CONCLUSIONS

The collagen fibril diameter of the QT was found to have increased with growth. The study provided insights into graft selection.

Glycosaminoglycans modulate microscale mechanics and viscoelasticity in fatigue injured tendons

Tendinopathies are common injuries that typically occur from overuse and fatigue. Treatments target late-stage symptoms with limited success, leading to high rates of reinjury. Early intervention could halt tendinopathy progression to rupture but requires a better understanding of the biomechanical environment associated with early-stage disease. While fatigue injured tendons are further damaged by exercise that is initiated immediately after onset of injury, exercise that is initiated after a brief delay promotes repair. Similar macroscale mechanical properties and collagen damage throughout this delay period suggests that microscale, non-collagenous matrix changes after fatigue injury modulate tendon mechanotransduction and shifts the exercise response from detrimental to reparative. Glycosaminoglycans (GAGs) and proteoglycans (PGs) are increased during chronic tendinopathy, but their role in early-stage disease is unknown. We hypothesized that increased GAGs from fatigue injury modulate viscoelasticity and microscale strains to enable repair from exercise. Various GAG types were increased in the weeks after onset of fatigue injury in the extracellular and pericellular matrices of rat patellar tendons. Enzymatic removal of GAGs from these fatigued tendons increased microscale shear strain, suggesting that GAGs modulate the cell microenvironment after fatigue injury. GAG removal decreased dynamic modulus in the toe region and decreased loss tangent in the linear region of the stress-strain curve in fatigued tendons, suggesting the GAG increase modulates tendon multiscale mechanics and viscoelasticity during fiber uncrimping and fibril sliding and strain transfer. GAGs may influence repair in response to exercise and could serve as a therapeutic target for tendinopathy.

Is the fluid volume fraction equal to the water content in tendons? Insights on biphasic modeling

The mass density of highly hydrated soft tissues is generally assumed to be very close to that of the water, resulting that the fluid mass fraction (water content) being equal to the fluid volume fraction. Within this context, the present study aims to investigate whether such an assumption actually holds for tendon tissues and to what extent it may affect the constitutive characterizations based on biphasic (poroelastic) models. Once the water content was assessed by a classical drying assay, the fluid volume fraction was obtained based on an image segmentation approach. The main achieved results point out that the fluid volume fraction is ∼20% higher than the water content in the studied tendons (flexor digitorum profundus bovine tendons). Based on this, it is shown that the use of the water content instead of the fluid volume fraction may considerably bias the results drawn by biphasic modeling of tendons. Accordingly, a proper measurement of the fluid volume fraction is then required.

The Role of the Piezo1 Mechanosensitive Channel in the Musculoskeletal System

Since the recent discovery of the mechanosensitive Piezo1 channels, many studies have addressed the role of the channel in various physiological or even pathological processes of different organs. Although the number of studies on their effects on the musculoskeletal system is constantly increasing, we are still far from a precise understanding. In this review, the knowledge available so far regarding the musculoskeletal system is summarized, reviewing the results achieved in the field of skeletal muscles, bones, joints and cartilage, tendons and ligaments, as well as intervertebral discs.