Roles of TRPV4 and piezo channels in stretch-evoked Ca2+ response in chondrocytes

Heavy mechanical force decelerates orthodontic tooth movement via Piezo1-induced mitochondrial calcium down-regulation

Orthodontic tooth movement (OTM) depends on periodontal ligament cells (PDLCs), which sense biomechanical stimuli and initiate alveolar bone remodeling. Light (optimal) forces accelerate OTM, whereas heavy forces decelerate it. However, the mechanisms by which PDLCs sense biomechanical stimuli and affect osteoclastic activities under different mechanical forces (MFs) remain unclear. This study demonstrates that mechanosensitive ion channel Piezo1-mediated Ca2+ signal conversion is crucial for sensing and delivering biomechanical signals in PDLCs under heavy-force conditions. Heavy MF up-regulated Piezo1 in PDLCs, reducing mitochondrial Ca2+ influx by inhibiting ITPR3 expression in mitochondria-associated membranes. Decreased mitochondrial calcium uptake led to reduced cytoplasmic release of mitochondrial DNA and inhibited the activation of the cGAS‒STING signaling cascade, subsequently inhibiting monocyte-to-osteoclast differentiation. Inhibition of Piezo1 or up-regulation of STING expression under heavy MF conditions significantly increased osteoclast activity and accelerated OTM. These findings suggest that heavy MF-induced Piezo1 expression in PDLCs is closely related to the control of osteoclast activity during OTM and plays an essential role in alveolar bone remodeling. This mechanism may be a potential therapeutic target for accelerating OTM.

Developmental dynamics mimicking inversely engineered pericellular matrix for articular cartilage regeneration

The mechanical mismatch of scaffold matrix-mesenchymal stem cells (MSCs) has been a longstanding issue in the clinical application of MSC-based therapy for articular cartilage (AC) regeneration. Existing tissue-engineered scaffolds underestimate the importance of the natural chondrocyte pericellular matrix (PCM). Here, we reveal the temporal and spatial characteristics of collagen distribution around the chondrocytes. Next, we demonstrate a rationally designed layer-by-layer single-cell encapsulation system which can mimic PCM mechanical responses and enhance MSC chondrogenesis via reestablished the mechanical coupling of PCM-like primitive matrix and chondrocytes. This successfully simulates the temporal and spatial characteristics of collagen secretion. Through investigation of the micromechanical environment of the cells and full-atom simulation analysis of TRPV4, we determine the specific mechanisms by which cellular mechanical forces near the cell are converted into biological signals. The TRPV4-YAP/TAZ-PI3K-Akt signaling pathway is involved in MSC cartilage formation through a joint analysis of the mRNA sequencing and spatial transcriptome results. In a rat model of articular cartilage defects, our inversely engineered pericellular matrix-encapsulated MSC-loaded scaffolds show regenerative performance that are superior to those of scaffolds loaded with only MSCs. These results demonstrate the feasibility of using a PCM-mimicking system to improve MSC chondrogenesis and the efficacy of AC repair.

Biomechanical research using advanced micro-nano devices: In-Vitro cell Characterization focus

BACKGROUND

Cells in the body reside in a dynamic microenvironment subjected to various physical stimuli, where mechanical stimulation plays a crucial role in regulating cellular physiological behaviors and functions.

AIM OF REVIEW

Investigating the mechanisms and interactions of mechanical transmission is essential for understanding the physiological and functional interplay between cells and physical stimuli. Therefore, establishing an in vitro biomechanical stimulation cell culture system holds significant importance for research related to cellular biomechanics.

KEY SCIENTIFIC CONCEPTS OF REVIEW

In this review, we primarily focused on various biomechanically relevant cell culture systems and highlighted the advancements and prospects in their preparation processes. Firstly, we discussed the types and characteristics of biomechanics present in the microenvironment within the human body. Subsequently, we introduced the research progress, working principles, preparation processes, potential advantages, applications, and challenges of various biomechanically relevant in vitro cell culture systems. Additionally, we summarized and categorized currently commercialized biomechanically relevant cell culture systems, offering a comprehensive reference for researchers in related fields.

Roles for TRPV4 in disease: A discussion of possible mechanisms

The transient receptor potential vanilloid 4 (TRPV4) ion channel is a ubiquitously expressed Ca2+-permeable ion channel that controls intracellular calcium ([Ca2+]i) homeostasis in various types of cells. The physiological roles for TRPV4 are tissue specific and the mechanisms behind this specificity remain mostly unclarified. It is noteworthy that mutations in the TRPV4 channel have been associated to a broad spectrum of congenital diseases, with most of these mutations mainly resulting in gain-of-function. Mutations have been identified in human patients showing a variety of phenotypes and symptoms, mostly related to skeletal and neuromuscular disorders. Since TRPV4 is so widely expressed throughout the body, it comes as no surprise that the literature is growing in evidence linking this protein to malfunction in systems other than the skeletal and neuromuscular. In this review, we summarize the expression patterns of TRPV4 in several tissues and highlight findings of recent studies that address critical structural and functional features of this channel, particularly focusing on its interactions and signaling pathways related to Ca2+ entry. Moreover, we discuss the roles of TRPV4 mutations in some diseases and pinpoint some of the mechanisms underlying pathological states where TRPV4's malfunction is prominent.

The role of mechanosensitive ion channel Piezo1 in knee osteoarthritis inflammation

Mechanosensitive ion channel Piezo1 is known to mediate a variety of inflammatory pathways and is also involved in the occurrence and development of many orthopedic diseases. Although its role in the inflammatory mechanism of knee osteoarthritis (KOA) has been reported, a systematic explanation is yet to be seen. This article aims to summarize the role of inflammatory responses in the pathogenesis of KOA and elucidate the mechanism by which the Piezo1-mediated inflammatory response contributes to the pathogenesis of KOA, providing a theoretical basis for the prevention and treatment of knee osteoarthritis. The results indicate that in the mechanism leading to knee osteoarthritis, Piezo1 can mediate the inflammatory response through chondrocytes and synovial cells, participating in the pathological progression of KOA. Piezo1 has the potential to become a new target for the prevention and treatment of this disease. Additionally, as pain is one of the most severe manifestations in KOA patients, the inflammatory response mediated by Piezo1, which causes the release of inflammatory mediators and pro-inflammatory factors leading to pain, can be further explored.

Insufficient Mechanical Loading Downregulates Piezo1 in Chondrocytes and Impairs Fracture Healing Through ApoE-Induced Senescence

Insufficient mechanical loading impairs fracture healing; however, the underlying mechanisms remain unclear. Increasing evidence indicates that Piezo1 plays an important role in fracture healing, although the effect of Piezo1 on the endochondral ossification of chondrocytes has been overlooked. This study reports that mechanical unloading down-regulates the expression of Piezo1 in chondrocytes and leads to fracture nonunion. Single-cell sequencing of calluses revealed that specific deletion of Piezo1 in chondrocytes upregulated the expression of apolipoprotein E (ApoE) in hypertrophic chondrocytes, resulting in delayed cartilage-to-bone transition due to enhanced chondrocyte senescence. Based on these results, an injectable and thermosensitive hydrogel is developed, which released an ApoE antagonist in situ at the fracture site. This hydrogel effectively attenuated chondrocyte senescence and, thus, promoted cartilage-to-bone transition as well as the fracture healing process. Overall, this data provide a new perspective on the activity of chondrocytes in fracture healing and a new direction for the treatment of fracture nonunion caused by insufficient mechanical loading.

Microfabrication Technologies for Nanoinvasive and High-Resolution Magnetic Neuromodulation

The increasing demand for precise neuromodulation necessitates advancements in techniques to achieve higher spatial resolution. Magnetic stimulation, offering low signal attenuation and minimal tissue damage, plays a significant role in neuromodulation. Conventional transcranial magnetic stimulation (TMS), though noninvasive, lacks the spatial resolution and neuron selectivity required for spatially precise neuromodulation. To address these limitations, the next generation of magnetic neurostimulation technologies aims to achieve submillimeter-resolution and selective neuromodulation with high temporal resolution. Invasive and nanoinvasive magnetic neurostimulation are two next-generation approaches: invasive methods use implantable microcoils, while nanoinvasive methods use magnetic nanoparticles (MNPs) to achieve high spatial and temporal resolution of magnetic neuromodulation. This review will introduce the working principles, technical details, coil designs, and potential future developments of these approaches from an engineering perspective. Furthermore, the review will discuss state-of-the-art microfabrication in depth due to its irreplaceable role in realizing next-generation magnetic neuromodulation. In addition to reviewing magnetic neuromodulation, this review will cover through-silicon vias (TSV), surface micromachining, photolithography, direct writing, and other fabrication technologies, supported by case studies, providing a framework for the integration of magnetic neuromodulation and microelectronics technologies.

Self-generating electricity system driven by aqueous humor flow and trabecular meshwork contraction motion activated BCKa for glaucoma intraocular pressure treatment

Primary open-angle glaucoma (POAG) is the most common form of glaucoma and the leading cause of irreversible vision loss and blindness worldwide. Intraocular pressure (IOP) is the only modifiable risk factor, and prompt treatment to lower IOP can effectively slow the rate of vision loss due to glaucoma. Trabecular meshwork (TM) cells can maintain IOP homeostasis by correcting and adjusting the resistance to aqueous humor outflow in response to sustained pressure changes. TM cells' function is reduced, and membrane ion channels are impaired in POAG. The dysfunction of Large conductance Ca2+-activated K+ (BKCa) plays a central role in the pathogenesis of POAG. In this work, we targeted MXene nanoparticles (MXene-RGD) with piezoelectric response to TM cells in a 3D model of glaucoma in vitro as well as in the rabbit Transient Ocular Hypertension (OHT) Model in vivo. MXene-RGD gives the TM electromechanical transfer properties, while the self-enhancing and self-generated electricity properties of the TM are determined by the aqueous humor flow rate and the size of the deformation of the TM. MXene-RGD is nontoxic, as illustrated by a cell toxicity study and histological examination. In a 3D in vitro model of high-pressure glaucoma, whole-cell patch-clamp confirmed that piezoelectric stimulation turns on BKCa, which reduces the volume of the cell. MXene-RGD was injected into the anterior chamber with minimal trauma, i.e., anterior chamber injection, and specifically targeted to TM cells. The OHT model in vivo confirmed the potential IOP-lowering ability of MXene-RGD. We evaluated the ion channels involved in the reduction of IOP by MXene-RGD by pre-treatment with a BKCa channel blocker (iberiotoxin, IbTX) and a voltage-gated Ca2+channel blocker (nifedipine). Quantitative qPCR analysis showed that MXene-RGD inhibited the upregulation of mRNA expression levels of the myofibroblast marker α-smooth muscle actin (α-SMA) and the inflammatory response marker interleukin-6 (IL-6) induced by IOP. Histology confirmed that MXene-RGD attenuated IOP-induced proliferation and collagen production in the TM. Taken together, we present for the first time a minimally invasive surgical approach for targeting TM cells for POAG by utilizing piezoresponse nanomaterials to target BKCa to repair or awaken the ability of TM cells to regulate IOP homeostasis on their own.

Mechanical stimulation promotes fibrochondrocyte proliferation by activating the TRPV4 signaling pathway during tendon-bone insertion healing: CCN2 plays an important regulatory role

Background

We previously confirmed that mechanical stimulation is an important factor in the repair of tendon-bone insertion (TBI) injuries and that mechanoreceptors such as transient receptor potential ion-channel subfamily V member 4 (TRPV4; also known as transient receptor potential vanilloid 4) are key to transforming mechanical stimulation into intracellular biochemical signals. This study aims to elucidate the mechanism of mechanical stimulation regulating TRPV4.

Methods

Immunohistochemical staining and western blotting were used to evaluate cartilage repair at the TBI after injury. The RNA expression and protein expression of mechanoreceptors and key pathway molecules regulating cartilage proliferation were analyzed. TBI samples were collected for transcriptome sequencing to detect gene expression. Calcium-ion imaging and flow cytometry were used to evaluate the function of TPRV4 and cellular communication network factor 2 (CCN2) after the administration of siRNA, recombinant adenovirus and agonists.

Results

We found that treadmill training improved the quality of TBI healing and enhanced fibrochondrocyte proliferation. The transcriptome sequencing results suggested that the elevated expression of the mechanistically stimulated regulator CCN2 and the exogenous administration of recombinant human CCN2 significantly promoted TRPV4 protein expression and fibrochondrocyte proliferation. In vitro, under mechanical stimulation conditions, small interfering RNA (siRNA)-CCN2 not only inhibited the proliferation of primary fibrochondrocytes but also suppressed TRPV4 protein expression and activity. Subsequently, primary fibrochondrocytes were treated with the TRPV4 agonist GSK1016790A and the recombinant adenovirus TRPV4 (Ad-TRPV4), and GSK1016790A partially reversed the inhibitory effect of siRNA-CCN2. The phosphoinositide 3-kinase/ protein kinase B (PI3K/AKT) signaling pathway participated in the above process.

Conclusions

Mechanical stimulation promoted fibrochondrocyte proliferation and TBI healing by activating TRPV4 channels and the PI3K/AKT signaling pathway, and CCN2 may be a key regulatory protein in maintaining TRPV4 activation.

Identification and Properties of TRPV4 Mutant Channels Present in Polycystic Kidney Disease Patients

Polycystic kidney disease (PKD), a disease characterized by the enlargement of the kidney through cystic growth is the fourth leading cause of end-stage kidney disease world-wide. Transient receptor potential Vanilloid 4 (TRPV4), a calcium-permeable TRP, channel participates in kidney cell physiology and since TRPV4 forms complexes with another channel whose malfunction is associated to PKD, TRPP2 (or PKD2), we sought to determine whether patients with PKD, exhibit previously unknown mutations in TRPV4. Here, we report the presence of mutations in the TRPV4 gene in patients diagnosed with PKD and determine that they produce gain-of-function (GOF). Mutations in the sequence of the TRPV4 gene have been associated to a broad spectrum of neuropathies and skeletal dysplasias but not PKD, and their biophysical effects on channel function have not been elucidated. We identified and examined the functional behavior of a novel E6K mutant and of the previously known S94L and A217S mutant TRVP4 channels. The A217S mutation has been associated to mixed neuropathy and/or skeletal dysplasia phenotypes, however, the PKD carriers of these variants had not been diagnosed with these reported clinical manifestations. The presence of certain mutations in TRPV4 may influence the progression and severity of PKD through GOF mechanisms. PKD patients carrying TRVP4 mutations are putatively more likely to require dialysis or renal transplant as compared to those without these mutations.

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.

Effect of dynamic loading on calcium signaling in In-Situ chondrocytes

Chondrocytes respond to mechanical stimuli by increasing their intracellular calcium concentration. The response depends on the cellular environment. Previous studies have investigated chondrocytes under slow strain rates or cells embedded in hydrogels, but the response of chondrocytes in their native environment under physiologically relevant cyclic loads and dynamic hydrostatic pressure has not been studied. This study investigated the calcium signaling response of in-situ chondrocytes under physiological cyclic compressive loads and hydrostatic pressure with varying frequency and load rates. Bovine cartilage explants were stained with a fluorescent calcium indicator dye and subjected to physiologically relevant cyclic loads using a custom-built loading device secured on a confocal/multiphoton microscope. Calcium fluorescence intensities of the cells were tracked and analyzed. Loading groups were compared using one-way ANOVA followed by a post-hoc test with Tukey correction (α = 0.05). The percentage of cells signaling increased in all compressive loading conditions compared to the no-load baseline. The percentage of cells responding under 1 Hz load was significantly greater than the slow ramp and 0.1 Hz group (p < 0.05). The number of compression cycles had no effect on the calcium signaling response (p > 0.05). The width and time between consecutive peaks were not different between different loading conditions (p > 0.05). Calcium signaling of in-situ chondrocytes did not increase under dynamic hydrostatic pressure of magnitudes up to 0.2 MPa at frequencies of 0.5 Hz and 0.05 Hz (p > 0.05). In conclusion, in-situ chondrocytes respond to physiological compressive loads in a strain rate-dependent manner with an increased number of responsive cells and unaltered temporal characteristics.

Ion channels in osteoarthritis: emerging roles and potential targets

Osteoarthritis (OA) is a highly prevalent joint disease that causes substantial disability, yet effective approaches to disease prevention or to the delay of OA progression are lacking. Emerging evidence has pinpointed ion channels as pivotal mediators in OA pathogenesis and as promising targets for disease-modifying treatments. Preclinical studies have assessed the potential of a variety of ion channel modulators to modify disease pathways involved in cartilage degeneration, synovial inflammation, bone hyperplasia and pain, and to provide symptomatic relief in models of OA. Some of these modulators are currently being evaluated in clinical trials. This review explores the structures and functions of ion channels, including transient receptor potential channels, Piezo channels, voltage-gated sodium channels, voltage-dependent calcium channels, potassium channels, acid-sensing ion channels, chloride channels and the ATP-dependent P2XR channels in the osteoarthritic joint. The discussion spans channel-targeting drug discovery and potential clinical applications, emphasizing opportunities for further research, and underscoring the growing clinical impact of ion channel biology in OA.

Mechano-adaptation: Exercise-driven Piezo1 & Piezo2 augmentation and chondroprotection in articular cartilage

Chondrocytes in adult joints are mechanosensitive post-mitotic quiescent cells with robustly expressed both Piezo1 and Piezo2 ion channels. Here, we examined the mechano-adaptation and Piezo modulations in articular chondrocytes using a mouse exercise model. We first found differential expression patterns of PIEZO1 and PIEZO2 in articular chondrocytes of healthy knee joints; chondrocytes in tibial cartilage (T) exhibit significantly higher PIEZO1 and PIEZO2 than femoral chondrocytes (F). Interestingly, a few weeks of exercise caused both PIEZO1 and PIEZO2 augmentation in F and T compared to the sedentary control group. Despite the increased expression levels of these mechanosensors, chondrocytes in exercised cartilage exhibit significantly reduced mechanical susceptibility against 1mJ impact. PIEZO1 modulation was relatively more rapid than PIEZO2 channels post-exercise. We tested the exercise-induced effect using Piezo1-conditional knockout (Pz1-cKO; Agc1 CreERT2 ;Piezo1 fl/fl ). Pz1-cKO mice exhibit diminished exercise-driven chondroprotection against 1mJ impact, suggesting essential roles of Piezo1-mediated mechanotransduction for physiologic-induced cartilage matrix homeostasis. In addition, using a mouse OA model, we further found the modulated PIEZO1 in chondrocytes, consistent with reports in Ren et al., but without PIEZO2 modulations over OA progression. In summary, our data reveal the distinctly tuned Piezo1 and Piezo2 channels in chondrocytes post-exercise and post-injury, in turn modulating the mechanical susceptibility of chondrocytes. We postulate that Piezo1 is a tightly-regulated biphasic biomarker ; Piezo1 antagonism may increase cellular survival post-injury and Piezo1 (with Piezo2) agonism to promote cartilage ECM restoration.

Promoting Articular Cartilage Regeneration through Microenvironmental Regulation

In recent years, as the aging population continues to grow, osteoarthritis (OA) has emerged as a leading cause of disability, with its incidence rising annually. Current treatments of OA include exercise and medications in the early stages and total joint replacement in the late stages. These approaches only relieve pain and reduce inflammation; however, they have significant side effects and high costs. Therefore, there is an urgent need to identify effective treatment methods that can delay the pathological progression of this condition. The changes in the articular cartilage microenvironment, which are complex and diverse, can aggravate the pathological progression into a vicious cycle, inhibiting the repair and regeneration of articular cartilage. Understanding these intricate changes in the microenvironment is crucial for devising effective treatment modalities. By searching relevant research articles and clinical trials in PubMed according to the keywords of articular cartilage, microenvironment, OA, mechanical force, hypoxia, cytokine, and cell senescence. This study first summarizes the factors affecting articular cartilage regeneration, then proposes corresponding treatment strategies, and finally points out the future research direction. We find that regulating the opening of mechanosensitive ion channels, regulating the expression of HIF-1, delivering growth factors, and clearing senescent cells can promote the formation of articular cartilage regeneration microenvironment. This study provides a new idea for the treatment of OA in the future, which can promote the regeneration of articular cartilage through the regulation of the microenvironment so as to achieve the purpose of treating OA.

Critical signaling molecules in the temporomandibular joint osteoarthritis under different magnitudes of mechanical stimulation

The mechanical stress environment in the temporomandibular joint (TMJ) is constantly changing due to daily mandibular movements. Therefore, TMJ tissues, such as condylar cartilage, the synovial membrane and discs, are influenced by different magnitudes of mechanical stimulation. Moderate mechanical stimulation is beneficial for maintaining homeostasis, whereas abnormal mechanical stimulation leads to degeneration and ultimately contributes to the development of temporomandibular joint osteoarthritis (TMJOA), which involves changes in critical signaling molecules. Under abnormal mechanical stimulation, compensatory molecules may prevent degenerative changes while decompensatory molecules aggravate. In this review, we summarize the critical signaling molecules that are stimulated by moderate or abnormal mechanical loading in TMJ tissues, mainly in condylar cartilage. Furthermore, we classify abnormal mechanical stimulation-induced molecules into compensatory or decompensatory molecules. Our aim is to understand the pathophysiological mechanism of TMJ dysfunction more deeply in the ever-changing mechanical environment, and then provide new ideas for discovering effective diagnostic and therapeutic targets in TMJOA.

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.

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.

The PIEZO1/miR-155-5p/GDF6/SMAD2/3 signaling axis is involved in inducing the occurrence and progression of osteoarthritis under excessive mechanical stress

OBJECTIVE

To elucidate the molecular mechanism of overloading-induced osteoarthritis (OA) and to find a novel therapeutic target.

METHODS

We utilized human cartilage specimens, mouse chondrocytes, a destabilization of the medial meniscus (DMM) mouse model, and a mouse hindlimb weight-bearing model to validate the role of overloading on chondrocyte senescence and OA development. Then, we observed the effect of PIEZO1-miR-155-5p-GDF6-SMAD2/3 signaling axis on the preservation of joint metabolic homeostasis under overloading in vivo, in vitro and ex vivo by qPCR, Western blot, enzyme-linked immunosorbent assay (ELISA), immunohistochemistry, immunofluorescence, SA-β-gal staining, CCK8 assay, et al. Finally, we verified the therapeutic effects of intra-articular injection of miR-155-5p inhibitor or recombinant GDF6 on the murine overloading-induced OA models.

RESULTS

Chondrocytes sensesed the mechanical overloading through PIEZO1 and up-regulated miR-155-5p expression. MiR-155-5p mimics could copy the effects of overloading-induced chondrocyte senescence and OA. Additionally, miR-155-5p could suppress the mRNA expression of Gdf6-Smad2/3 in various tissues within the joint. Overloading could disrupt joint metabolic homeostasis by downregulating the expression of anabolism indicators and upregulating the expression of catabolism indicators in the chondrocytes and synoviocytes, while miR-155-5p inhibition or GDF6 supplementation could exert an antagonistic effect by preserving the joint homeostasis. Finally, in the in vivo overloading models, intra-articular injection of miR-155-5p inhibitor or recombinant GDF6 could significantly mitigate the severity of impending OA and lessened the progression of existing OA.

CONCLUSION

GDF6 overexpression or miR-155-5p inhibition could attenuate overloading-induced chondrocyte senescence and OA through the PIEZO1-miR-155-5p-GDF6-SMAD2/3 signaling pathway. Our study provides a new therapeutic target for the treatment of overloading-induced OA.

Recent advancements in cartilage tissue engineering innovation and translation

Articular cartilage was expected to be one of the first successfully engineered tissues, but today, cartilage repair products are few and they exhibit considerable limitations. For example, of the cell-based products that are available globally, only one is marketed for non-knee indications, none are indicated for severe osteoarthritis or rheumatoid arthritis, and only one is approved for marketing in the USA. However, advances in cartilage tissue engineering might now finally lead to the development of new cartilage repair products. To understand the potential in this field, it helps to consider the current landscape of tissue-engineered products for articular cartilage repair and particularly cell-based therapies. Advances relating to cell sources, bioactive stimuli and scaffold or scaffold-free approaches should now contribute to progress in therapeutic development. Engineering for an inflammatory environment is required because of the need for implants to withstand immune challenge within joints affected by osteoarthritis or rheumatoid arthritis. Bringing additional cartilage repair products to the market will require an understanding of the translational vector for their commercialization. Advances thus far can facilitate the future translation of engineered cartilage products to benefit the millions of patients who suffer from cartilage injuries and arthritides.

LIPUS regulates the progression of knee osteoarthritis in mice through primary cilia-mediated TRPV4 channels

Osteoarthritis (OA) is a common disease in middle-aged and elderly people. An imbalance in calcium ion homeostasis will contribute to chondrocyte apoptosis and ultimately lead to the progression of OA. Transient receptor potential channel 4 (TRPV4) is involved in the regulation of intracellular calcium homeostasis. TRPV4 is expressed in primary cilia, which can sense mechanical stimuli from outside the cell, and its abnormal expression is closely related to the development of OA. Low-intensity pulsed ultrasound (LIPUS) can alleviate chondrocyte apoptosis while the exact mechanism is unclear. In this project, with the aim of revealing the mechanism of action of LIPUS, we proposed to use OA chondrocytes and animal models, LIPUS intervention, inhibition of primary cilia, use TRPV4 inhibitors or TRPV4 agonist, and use Immunofluorescence (IF), Immunohistochemistry (IHC), Western Blot (WB), Quantitative Real-time PCR (QP) to detect the expression of cartilage synthetic matrix and endoplasmic reticulum stress markers. The results revealed that LIPUS altered primary cilia expression, promoted synthetic matrix metabolism in articular chondrocytes and was associated with primary cilia. In addition, LIPUS exerted a active effect on OA by activating TRPV4, inducing calcium inward flow, and facilitating the entry of NF-κB into the nucleus to regulate synthetic matrix gene transcription. Inhibition of TRPV4 altered primary cilia expression in response to LIPUS stimulation, and knockdown of primary cilia similarly inhibited TRPV4 function. These results suggest that LIPUS mediates TRPV4 channels through primary cilia to regulate the process of knee osteoarthritis in mice.

Pleiotropic physiological functions of Piezo1 in human body and its effect on malignant behavior of tumors

Mechanosensitive ion channel protein 1 (Piezo1) is a large homotrimeric membrane protein. Piezo1 has various effects and plays an important and irreplaceable role in the maintenance of human life activities and homeostasis of the internal environment. In addition, recent studies have shown that Piezo1 plays a vital role in tumorigenesis, progression, malignancy and clinical prognosis. Piezo1 is involved in regulating the malignant behaviors of a variety of tumors, including cellular metabolic reprogramming, unlimited proliferation, inhibition of apoptosis, maintenance of stemness, angiogenesis, invasion and metastasis. Moreover, Piezo1 regulates tumor progression by affecting the recruitment, activation, and differentiation of multiple immune cells. Therefore, Piezo1 has excellent potential as an anti-tumor target. The article reviews the diverse physiological functions of Piezo1 in the human body and its major cellular pathways during disease development, and describes in detail the specific mechanisms by which Piezo1 affects the malignant behavior of tumors and its recent progress as a new target for tumor therapy, providing new perspectives for exploring more potential effects on physiological functions and its application in tumor therapy.

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.

Mechanical overload induces TMJ disc degeneration via TRPV4 activation

OBJECTIVE

The temporomandibular joint (TMJ) disc cushions intraarticular stress during mandibular movements. While mechanical overloading is related to cartilage degeneration, the pathogenesis of TMJ disc degeneration is unclear. Here, we determined the regulatory role of mechanoinductive transient receptor potential vanilloid 4 (TRPV4) in mechanical overload-induced TMJ disc degeneration.

METHODS

We explored the effect of mechanical overload on the TMJ discs in a rat occlusal interference model in vivo, and by applying sustained compressive force in vitro. TRPV4 inhibition was delivered by small interfering RNA or GSK2193874; TRPV4 activation was delivered by GSK1016790A. The protective effect of TRPV4 inhibition was validated in the rat occlusal interference model.

RESULTS

Occlusal interference induced TMJ disc degeneration with enhanced extracellular matrix degradation in vivo and mechanical overload promoted inflammatory responses in the TMJ disc cells via Ca2+ influx with significantly upregulated TRPV4. TRPV4 inhibition reversed mechanical overload-induced inflammatory responses; TRPV4 activation simulated mechanical overload-induced inflammatory responses. Moreover, TRPV4 inhibition alleviated TMJ disc degeneration in the rat occlusal interference model.

CONCLUSION

Our findings suggest TRPV4 plays a pivotal role in the pathogenesis of mechanical overload-induced TMJ disc degeneration and may be a promising target for the treatment of degenerative changes of the TMJ disc.

The interplay between biochemical mediators and mechanotransduction in chondrocytes: Unravelling the differential responses in primary knee osteoarthritis

In primary or idiopathic osteoarthritis (OA), it is unclear which factors trigger the shift of articular chondrocyte activity from pro-anabolic to pro-catabolic. In fact, there is a controversy about the aetiology of primary OA, either mechanical or inflammatory. Chondrocytes are mechanosensitive cells, that integrate mechanical stimuli into cellular responses in a process known as mechanotransduction. Mechanotransduction occurs thanks to the activation of mechanosensors, a set of specialized proteins that convert physical cues into intracellular signalling cascades. Moderate levels of mechanical loads maintain normal tissue function and have anti-inflammatory effects. In contrast, mechanical over- or under-loading might lead to cartilage destruction and increased expression of pro-inflammatory cytokines. Simultaneously, mechanotransduction processes can regulate and be regulated by pro- and anti-inflammatory soluble mediators, both local (cells of the same joint, i.e., the chondrocytes themselves, infiltrating macrophages, fibroblasts or osteoclasts) and systemic (from other tissues, e.g., adipokines). Thus, the complex process of mechanotransduction might be altered in OA, so that cartilage-preserving chondrocytes adopt a different sensitivity to mechanical signals, and mechanic stimuli positively transduced in the healthy cartilage may become deleterious under OA conditions. This review aims to provide an overview of how the biochemical exposome of chondrocytes can alter important mechanotransduction processes in these cells. Four principal mechanosensors, i.e., integrins, Ca2+ channels, primary cilium and Wnt signalling (canonical and non-canonical) were targeted. For each of these mechanosensors, a brief summary of the response to mechanical loads under healthy or OA conditions is followed by a concise overview of published works that focus on the further regulation of the mechanotransduction pathways by biochemical factors. In conclusion, this paper discusses and explores how biological mediators influence the differential behaviour of chondrocytes under mechanical loads in healthy and primary OA.

Role of mechanically-sensitive cation channels Piezo1 and TRPV4 in trabecular meshwork cell mechanotransduction

Glaucoma is one of the leading causes of irreversible blindness in developed countries, and intraocular pressure (IOP) is primary and only treatable risk factor, suggesting that to a significant extent, glaucoma is a disease of IOP disorder and pathological mechanotransduction. IOP-lowering ways are limited to decreaseing aqueous humour (AH) production or increasing the uveoscleral outflow pathway. Still, therapeutic approaches have been lacking to control IOP by enhancing the trabecular meshwork (TM) pathway. Trabecular meshwork cells (TMCs) have endothelial and myofibroblast properties and are responsible for the renewal of the extracellular matrix (ECM). Mechanosensitive cation channels, including Piezo1 and TRPV4, are abundantly expressed in primary TMCs and trigger mechanostress-dependent ECM and cytoskeletal remodelling. However, prolonged mechanical stimulation severely affects cellular biosynthesis through TMC mechanotransduction, including signaling, gene expression, ECM remodelling, and cytoskeletal structural changes, involving outflow facilities and elevating IOP. As for the functional coupling relationship between Piezo1 and TRPV4 channels, inspired by VECs and osteoblasts, we hypothesized that Piezo1 may also act upstream of TRPV4 in glaucomatous TM tissue, mediating the activation of TRPV4 via Ca2+ inflow or Ca2+ binding to phospholipase A2(PLA2), and thus be involved in increasing TM outflow resistance and elevated IOP. Therefore, this review aims to help identify new potential targets for IOP stabilization in ocular hypertension and primary open-angle glaucoma by understanding the mechanical transduction mechanisms associated with the development of glaucoma and may provide ideas into novel treatments for preventing the progression of glaucoma by targeting mechanotransduction.

[Role of Piezo mechanosensitive ion channels in the osteoarticular system]

Objective

To summarize the role of Piezo mechanosensitive ion channels in the osteoarticular system, in order to provide reference for subsequent research.

Methods

Extensive literature review was conducted to summarize the structural characteristics, gating mechanisms, activators and blockers of Piezo ion channels, as well as their roles in the osteoarticular systems.

Results

The osteoarticular system is the main load-bearing and motor tissue of the body, and its ability to perceive and respond to mechanical stimuli is one of the guarantees for maintaining normal physiological functions of bones and joints. The occurrence and development of many osteoarticular diseases are closely related to abnormal mechanical loads. At present, research shows that Piezo mechanosensitive ion channels differentiate towards osteogenesis by responding to stretching stimuli and regulating cellular Ca 2+ influx signals; and it affects the proliferation and migration of osteoblasts, maintaining bone homeostasis through cellular communication between osteoblasts-osteoclasts. Meanwhile, Piezo1 protein can indirectly participate in regulating the formation and activity of osteoclasts through its host cells, thereby regulating the process of bone remodeling. During mechanical stimulation, the Piezo1 ion channel maintains bone homeostasis by regulating the expressions of Akt and Wnt1 signaling pathways. The sensitivity of Piezo1/2 ion channels to high strain mechanical signals, as well as the increased sensitivity of Piezo1 ion channels to mechanical transduction mediated by Ca 2+ influx and inflammatory signals in chondrocytes, is expected to become a new entry point for targeted prevention and treatment of osteoarthritis. But the specific way mechanical stimuli regulate the physiological/pathological processes of bones and joints still needs to be clarified.

Conclusion

Piezo mechanosensitive ion channels give the osteoarticular system with important abilities to perceive and respond to mechanical stress, playing a crucial mechanical sensing role in its cellular fate, bone development, and maintenance of bone and cartilage homeostasis.

[Piezo1 Mediates the Regulation of Substrate Stiffness on Primary Cilia in Chondrocytes]

Objective

To investigate how substrate stiffness regulates the morphology of primary cilia in chondrocytes and to illustrate how Piezo1 mediates the morphology regulation of primary cilia by substrate stiffness.

Methods

Polydimethylsiloxane (PDMS) curing agent and the main agent (Dow Corning, Beijing, China) were mixed at the ratio of 1∶10 (stiff), 1∶50 (medium stiffness), and 1∶70 (soft), respectively, to prepare substrate films with the thickness of 1 mm at different levels of stiffness, including stiff substrate of (2.21±0.12) MPa, medium-stiffness substrate of (54.47±6.06) kPa, and soft substrate of (2.13±0.10) kPa. Chondrocytes were cultured with the substrates of three different levels of stiffness. Then, the cells were treated with Tubastatin A (Tub A) to inhibit histone deacetylase 6 (HDAC6), Piezo1 activator Yoda1, and inhibitor GsMTx4, respectively. The effects of HDAC6, Yoda1, and GsMTx4 on chondrocyte morphology and the length of primary cilia were analyzed through immunofluorescence staining.

Results

The stiff substrate increased the spread area of the chondrocytes. Immunofluorescence assays showed that the cytoskeleton and the nuclear area of the cells on the stiff substrate were significantly increased (P<0.05) and the primary cilia were significantly extended (P<0.05) compared with those on the medium-stiffness and soft substrates. However, the presence rate of primary cilia was not affected. The HDAC6 activity of chondrocytes increased with the decrease in substrate stiffness. When the activity of HDAC6 was inhibited, the cytoskeletal area, the nuclei area, and the primary cilium length were increased more significantly on the stiff substrate (P<0.05). Further testing showed that Piezo1 activator and inhibitor could regulate the activity of HDAC6 in chondrocytes, and that the length of primary cilia was significantly increased after treatment with the activator Yoda1 (P<0.05). On the other hand, the length of primary cilia was significantly shortened on the stiff substrate after treatment with the inhibitor GsMTx4 (P<0.05).

Conclusion

Both substrate stiffness and Piezo1 may affect the morphology of chondrocyte primary cilia by regulating HDAC6 activity.

The Multifaceted Functions of TRPV4 and Calcium Oscillations in Tissue Repair

The transient receptor potential vanilloid 4 (TRPV4) specifically functions as a mechanosensitive ion channel and is responsible for conveying changes in physical stimuli such as mechanical stress, osmotic pressure, and temperature. TRPV4 enables the entry of cation ions, particularly calcium ions, into the cell. Activation of TRPV4 channels initiates calcium oscillations, which trigger intracellular signaling pathways involved in a plethora of cellular processes, including tissue repair. Widely expressed throughout the body, TRPV4 can be activated by a wide array of physicochemical stimuli, thus contributing to sensory and physiological functions in multiple organs. This review focuses on how TRPV4 senses environmental cues and thereby initiates and maintains calcium oscillations, critical for responses to organ injury, tissue repair, and fibrosis. We provide a summary of TRPV4-induced calcium oscillations in distinct organ systems, along with the upstream and downstream signaling pathways involved. In addition, we delineate current animal and disease models supporting TRPV4 research and shed light on potential therapeutic targets for modulating TRPV4-induced calcium oscillation to promote tissue repair while reducing tissue fibrosis.

A Novel Superparamagnetic Multifunctional Nerve Scaffold: A Remote Actuation Strategy to Boost In Situ Extracellular Vesicles Production for Enhanced Peripheral Nerve Repair

Extracellular vesicles (EVs) have inherent advantages over cell-based therapies in regenerative medicine because of their cargos of abundant bioactive cues. Several strategies are proposed to tune EVs production in vitro. However, it remains a challenge for manipulation of EVs production in vivo, which poses significant difficulties for EVs-based therapies that aim to promote tissue regeneration, particularly for long-term treatment of diseases like peripheral neuropathy. Herein, a superparamagnetic nanocomposite scaffold capable of controlling EVs production on-demand is constructed by incorporating polyethyleneglycol/polyethyleneimine modified superparamagnetic nanoparticles into a polyacrylamide/hyaluronic acid double-network hydrogel (Mag-gel). The Mag-gel is highly sensitive to a rotating magnetic field (RMF), and can act as mechano-stimulative platform to exert micro/nanoscale forces on encapsulated Schwann cells (SCs), an essential glial cell in supporting nerve regeneration. By switching the ON/OFF state of the RMF, the Mag-gel can scale up local production of SCs-derived EVs (SCs-EVs) both in vitro and in vivo. Further transcriptome sequencing indicates an enrichment of transcripts favorable in axon growth, angiogenesis, and inflammatory regulation of SCs-EVs in the Mag-gel with RMF, which ultimately results in optimized nerve repair in vivo. Overall, this research provides a noninvasive and remotely time-scheduled method for fine-tuning EVs-based therapies to accelerate tissue regeneration, including that of peripheral nerves.

Enthesis maturation in engineered ligaments is differentially driven by loads that mimic slow growth elongation and rapid cyclic muscle movement

Entheses are complex attachments that translate load between elastic-ligaments and stiff-bone via organizational and compositional gradients. Neither natural healing, repair, nor engineered replacements restore these gradients, contributing to high re-tear rates. Previously, we developed a culture system which guides ligament fibroblasts in high-density collagen gels to develop early postnatal-like entheses, however further maturation is needed. Mechanical cues, including slow growth elongation and cyclic muscle activity, are critical to enthesis development in vivo but these cues have not been widely explored in engineered entheses and their individual contribution to maturation is largely unknown. Our objective here was to investigate how slow stretch, mimicking ACL growth rates, and intermittent cyclic loading, mimicking muscle activity, individually drive enthesis maturation in our system so to shed light on the cues governing enthesis development, while further developing our tissue engineered replacements. Interestingly, we found these loads differentially drive organizational maturation, with slow stretch driving improvements in the interface/enthesis region, and cyclic load improving the ligament region. However, despite differentially affecting organization, both loads produced improvements to interface mechanics and zonal composition. This study provides insight into how mechanical cues differentially affect enthesis development, while producing some of the most organized engineered enthesis to date. STATEMENT OF SIGNIFICANCE: Entheses attach ligaments to bone and are critical to load transfer; however, entheses do not regenerate with repair or replacement, contributing to high re-tear rates. Mechanical cues are critical to enthesis development in vivo but their individual contribution to maturation is largely unknown and they have not been widely explored in engineered replacements. Here, using a novel culture system, we provide new insight into how slow stretch, mimicking ACL growth rates, and intermittent cyclic loading, mimicking muscle activity, differentially affect enthesis maturation in engineered ligament-to-bone tissues, ultimately producing some of the most organized entheses to date. This system is a promising platform to explore cues regulating enthesis formation so to produce functional engineered replacements and better drive regeneration following repair.

[Sevoflurane alleviates ventilator-induced lung injury in rats by down-regulating the TRPV4/C-PLA2 signaling pathway]

OBJECTIVE

To explore the mechanism underlying the protective effect of sevoflurane against ventilator-induced lung injury (VILI).

METHODS

Thirty-two SD rats were randomized into mechanical ventilation (MV) group, MV+sevoflurane group (MS group), MV+sevoflurane+transient receptor potential vanillate subtype 4 (TRPV4) agonist group (MST group) and MV+ sevoflurane + vehicle group (MSV group). Arachidonic acid (AA) in the lung tissues was quantified with ELISA. TRPV4, cytoplasmic phospholipase A2 (C-PLA2) and myosin light chain kinase (MLCK) protein expressions were detected by Western blotting. Lung injury in the rats was evaluated by assessing MLCK protein expression level, pulmonary permeability index, lung wet/dry ratio, leukocyte count in the bronchoalveolar lavage fluid (BALF), myeloperoxidase content in lung tissue, and histological score of the lungs.

RESULTS

The rats in MV group showed significantly increased TRPV4 and C-PLA2 expression levels in the lung tissues with increased lung permeability and obvious lung inflammation compared with those in the other 3 groups (P < 0.05). No significant differences were found in the parameters associated with lung injuries between MS group and MSV group. Compared with those in MST group, the rats in MS group and MSV group showed significantly reduced AA production and TRPV4 and C-PLA2 expressions in the lungs (P < 0.05) with alleviated lung hyper-permeability and inflammation (P < 0.05).

CONCLUSION

Sevoflurane protects against VILI in rats by down-regulating the TRPV4/C-PLA2 signaling pathway.

Double-edged role of mechanical stimuli and underlying mechanisms in cartilage tissue engineering

Mechanical stimuli regulate the chondrogenic differentiation of mesenchymal stem cells and the homeostasis of chondrocytes, thus affecting implant success in cartilage tissue engineering. The mechanical microenvironment plays fundamental roles in the maturation and maintenance of natural articular cartilage, and the progression of osteoarthritis Hence, cartilage tissue engineering attempts to mimic this environment in vivo to obtain implants that enable a superior regeneration process. However, the specific type of mechanical loading, its optimal regime, and the underlying molecular mechanisms are still under investigation. First, this review delineates the composition and structure of articular cartilage, indicating that the morphology of chondrocytes and components of the extracellular matrix differ from each other to resist forces in three top-to-bottom overlapping zones. Moreover, results from research experiments and clinical trials focusing on the effect of compression, fluid shear stress, hydrostatic pressure, and osmotic pressure are presented and critically evaluated. As a key direction, the latest advances in mechanisms involved in the transduction of external mechanical signals into biological signals are discussed. These mechanical signals are sensed by receptors in the cell membrane, such as primary cilia, integrins, and ion channels, which next activate downstream pathways. Finally, biomaterials with various modifications to mimic the mechanical properties of natural cartilage and the self-designed bioreactors for experiment in vitro are outlined. An improved understanding of biomechanically driven cartilage tissue engineering and the underlying mechanisms is expected to lead to efficient articular cartilage repair for cartilage degeneration and disease.

Mucopolysaccharidosis IVA: Current Disease Models and Drawbacks

Mucopolysaccharidosis IVA (MPS IVA) is a rare disorder caused by mutations in the N-acetylgalactosamine-6-sulfate-sulfatase (GALNS) encoding gene. GALNS leads to the lysosomal degradation of the glycosaminoglyccreasans keratan sulfate and chondroitin 6-sulfate. Impaired GALNS enzymes result in skeletal and non-skeletal complications in patients. For years, the MPS IVA pathogenesis and the assessment of promising drugs have been evaluated using in vitro (primarily fibroblasts) and in vivo (mainly mouse) models. Even though value information has been raised from those studies, these models have several limitations. For instance, chondrocytes have been well recognized as primary cells affected in MPS IVA and responsible for displaying bone development impairment in MPS IVA patients; nonetheless, only a few investigations have used those cells to evaluate basic and applied concepts. Likewise, current animal models are extensively represented by mice lacking GALNS expression; however, it is well known that MPS IVA mice do not recapitulate the skeletal dysplasia observed in humans, making some comparisons difficult. This manuscript reviews the current in vitro and in vivo MPS IVA models and their drawbacks.

Piezo2 Channel Upregulation is Involved in Mechanical Allodynia in CYP-Induced Cystitis Rats

Mechanical sensing Piezo2 channel in primary sensory neurons has been shown contribute to mechanical allodynia in somatic chronic pain conditions. Interstitial cystitis (IC)-associated pain is often triggered by bladder filling, a presentation that mimics the mechanical allodynia. In the present study, we aimed to examine the involvement of sensory Piezo2 channel in IC-associated mechanical allodynia using a commonly employed cyclophosphamide (CYP)-induced IC model rat. Piezo2 channels in dorsal root ganglia (DRGs) was knocked down by intrathecal injections of Piezo2 anti-sense oligodeoxynucleotides (ODNs) in CYP-induced cystitis rats, and mechanical stimulation-evoked referred bladder pain was measured in the lower abdomen overlying the bladder using von Frey filaments. Piezo2 expression at the mRNA, protein, and functional levels in DRG neurons innervating the bladder was detected by RNA-fluorescence in situ hybridization, western blotting, immunofluorescence, and Ca2+ imaging, respectively. We found that Piezo2 channels were expressed on most (> 90%) of the bladder primary afferents, including afferents that express CGRP, TRPV1 and stained with isolectin B4. CYP-induced cystitis was associated with Piezo2 upregulation in bladder afferent neurons at the mRNA, protein, and functional levels. Knockdown of Piezo2 expression in DRG neurons significantly suppressed mechanical stimulation-evoked referred bladder pain as well as bladder hyperactivity in CYP rats compared to CYP rats treated with mismatched ODNs. Our results suggest upregulation of Piezo2 channels is involved in the development of bladder mechanical allodynia and bladder hyperactivity in CYP-induced cystitis. Targeting Piezo2 might be an attractive therapeutic approach for IC-related bladder pain.

[Research progress of chondrocyte mechanotransduction mediated by TRPV4 and PIEZOs]

Mechanical signal transduction are crucial for chondrocyte in response to mechanical cues during the growth, development and osteoarthritis (OA) of articular cartilage. Extracellular matrix (ECM) turnover regulates the matrix mechanical microenvironment of chondrocytes. Thus, understanding the mechanotransduction mechanisms during chondrocyte sensing the matrix mechanical microenvironment can develop effective targeted therapy for OA. In recent decades, growing evidences are rapidly advancing our understanding of the mechanical force-dependent cartilage remodeling and injury responses mediated by TRPV4 and PIEZOs. In this review, we highlighted the mechanosensing mechanism mediated by TRPV4 and PIEZOs during chondrocytes sensing mechanical microenvironment of the ECM. Additionally, the latest progress in the regulation of OA by inflammatory signals mediated by TRPV4 and PIEZOs was also introduced. These recent insights provide the potential mechanotheraputic strategies to target these channels and prevent cartilage degeneration associated with OA. This review will shed light on the pathogenesis of articular cartilage, searching clinical targeted therapies, and designing cell-induced biomaterials.

Primary cilium-mediated mechanotransduction in cartilage chondrocytes

Osteoarthritis (OA) is one of the most prevalent joint disorders associated with the degradation of articular cartilage and an abnormal mechanical microenvironment. Mechanical stimuli, including compression, shear stress, stretching strain, osmotic challenge, and the physical properties of the matrix microenvironment, play pivotal roles in the tissue homeostasis of articular cartilage. The primary cilium, as a mechanosensory and chemosensory organelle, is important for detecting and transmitting both mechanical and biochemical signals in chondrocytes within the matrix microenvironment. Growing evidence indicates that primary cilia are critical for chondrocytes signaling transduction and the matrix homeostasis of articular cartilage. Furthermore, the ability of primary cilium to regulate cellular signaling is dynamic and dependent on the cellular matrix microenvironment. In the current review, we aim to elucidate the key mechanisms by which primary cilia mediate chondrocytes sensing and responding to the matrix mechanical microenvironment. This might have potential therapeutic applications in injuries and OA-associated degeneration of articular cartilage.

Cellular mechanotransduction in health and diseases: from molecular mechanism to therapeutic targets

Cellular mechanotransduction, a critical regulator of numerous biological processes, is the conversion from mechanical signals to biochemical signals regarding cell activities and metabolism. Typical mechanical cues in organisms include hydrostatic pressure, fluid shear stress, tensile force, extracellular matrix stiffness or tissue elasticity, and extracellular fluid viscosity. Mechanotransduction has been expected to trigger multiple biological processes, such as embryonic development, tissue repair and regeneration. However, prolonged excessive mechanical stimulation can result in pathological processes, such as multi-organ fibrosis, tumorigenesis, and cancer immunotherapy resistance. Although the associations between mechanical cues and normal tissue homeostasis or diseases have been identified, the regulatory mechanisms among different mechanical cues are not yet comprehensively illustrated, and no effective therapies are currently available targeting mechanical cue-related signaling. This review systematically summarizes the characteristics and regulatory mechanisms of typical mechanical cues in normal conditions and diseases with the updated evidence. The key effectors responding to mechanical stimulations are listed, such as Piezo channels, integrins, Yes-associated protein (YAP) /transcriptional coactivator with PDZ-binding motif (TAZ), and transient receptor potential vanilloid 4 (TRPV4). We also reviewed the key signaling pathways, therapeutic targets and cutting-edge clinical applications of diseases related to mechanical cues.

Biomechanics of Chondrocytes and Chondrons in Healthy Conditions and Osteoarthritis: A Review of the Mechanical Characterisations at the Microscale

Biomechanical studies are expanding across a variety of fields, from biomedicine to biomedical engineering. From the molecular to the system level, mechanical stimuli are crucial regulators of the development of organs and tissues, their growth and related processes such as remodelling, regeneration or disease. When dealing with cell mechanics, various experimental techniques have been developed to analyse the passive response of cells; however, cell variability and the extraction process, complex experimental procedures and different models and assumptions may affect the resulting mechanical properties. For these purposes, this review was aimed at collecting the available literature focused on experimental chondrocyte and chondron biomechanics with direct connection to their biochemical functions and activities, in order to point out important information regarding the planning of an experimental test or a comparison with the available results. In particular, this review highlighted (i) the most common experimental techniques used, (ii) the results and models adopted by different authors, (iii) a critical perspective on features that could affect the results and finally (iv) the quantification of structural and mechanical changes due to a degenerative pathology such as osteoarthritis.

TRPV4 regulates osteoblast differentiation and mitochondrial function that are relevant for channelopathy

Different ion channels present in the osteoblast regulate the cellular functions including bio-mineralization, a process that is a highly stochastic event. Cellular events and molecular signaling involved in such process is poorly understood. Here we demonstrate that TRPV4, a mechanosensitive ion channel is endogenously present in an osteoblast cell line (MC3T3-E1) and in primary osteoblasts. Pharmacological activation of TRPV4 enhanced intracellular Ca²⁺-level, expression of osteoblast-specific genes and caused increased bio-mineralization. TRPV4 activation also affects mitochondrial Ca²⁺-levels and mitochondrial metabolisms. We further demonstrate that different point mutants of TRPV4 induce different mitochondrial morphology and have different levels of mitochondrial translocation, collectively suggesting that TRPV4-mutation-induced bone disorders and other channelopathies are mostly due to mitochondrial abnormalities. These findings may have broad biomedical implications.

Integrating physicomechanical and biological strategies for BTE: biomaterials-induced osteogenic differentiation of MSCs

Large bone defects are a major global health concern. Bone tissue engineering (BTE) is the most promising alternative to avoid the drawbacks of autograft and allograft bone. Nevertheless, how to precisely control stem cell osteogenic differentiation has been a long-standing puzzle. Compared with biochemical cues, physicomechanical stimuli have been widely studied for their biosafety and stability. The mechanical properties of various biomaterials (polymers, bioceramics, metal and alloys) become the main source of physicomechanical stimuli. By altering the stiffness, viscoelasticity, and topography of materials, mechanical stimuli with different strengths transmit into precise signals that mediate osteogenic differentiation. In addition, externally mechanical forces also play a critical role in promoting osteogenesis, such as compression stress, tensile stress, fluid shear stress and vibration, etc. When exposed to mechanical forces, mesenchymal stem cells (MSCs) differentiate into osteogenic lineages by sensing mechanical stimuli through mechanical sensors, including integrin and focal adhesions (FAs), cytoskeleton, primary cilium, ions channels, gap junction, and activating osteogenic-related mechanotransduction pathways, such as yes associated proteins (YAP)/TAZ, MAPK, Rho-GTPases, Wnt/β-catenin, TGFβ superfamily, Notch signaling. This review summarizes various biomaterials that transmit mechanical signals, physicomechanical stimuli that directly regulate MSCs differentiation, and the mechanical transduction mechanisms of MSCs. This review provides a deep and broad understanding of mechanical transduction mechanisms and discusses the challenges that remained in clinical translocation as well as the outlook for the future improvements.

Activation of the Mechanosensitive Ion Channels Piezo1 and TRPV4 in Primary Human Healthy and Osteoarthritic Chondrocytes Exhibits Ion Channel Crosstalk and Modulates Gene Expression

Osteoarthritis (OA) is the most common degenerative joint disease causing pain and functional limitations. Physical activity as a clinically relevant, effective intervention alleviates pain and promotes joint function. In chondrocytes, perception and transmission of mechanical signals are controlled by mechanosensitive ion channels, whose dysfunction in OA chondrocytes is leading to disease progression. Signaling of mechanosensitive ion channels Piezo/TRPV4 was analyzed by Yoda1/GSK1016790A application and calcium-imaging of Fura-2-loaded chondrocytes. Expression analysis was determined by qPCR and immunofluorescence in healthy vs. OA chondrocytes. Chondrocytes were mechanically stimulated using the Flexcell™ technique. Yoda1 and GSK1016790A caused an increase in intracellular calcium [Ca²⁺]_i for Yoda1, depending on extracellularly available Ca²⁺. When used concomitantly, the agonist applied first inhibited the effect of subsequent agonist application, indicating mutual interference between Piezo/TRPV4. Yoda1 increased the expression of metalloproteinases, bone-morphogenic protein, and interleukins in healthy and OA chondrocytes to a different extent. Flexcell™-induced changes in the expression of MMPs and ILs differed from changes induced by Yoda1. We conclude that Piezo1/TRPV4 communicate with each other, an interference that may be impaired in OA chondrocytes. It is important to consider that mechanical stimulation may have different effects on OA depending on its intensity.

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

Mechanical force loading is essential for maintaining bone homeostasis, and unloading exposure can lead to bone loss. Osteoclasts are the only bone resorbing cells and play a crucial role in bone remodeling. The molecular mechanisms underlying mechanical stimulation-induced changes in osteoclast function remain to be fully elucidated. Our previous research found Ca²⁺-activated Cl^- channel Anoctamin 1 (Ano1) was an essential regulator for osteoclast function. Here, we report that Ano1 mediates osteoclast responses to mechanical stimulation. In vitro, osteoclast activities are obviously affected by mechanical stress, which is accompanied by the changes of Ano1 levels, intracellular Cl^- concentration and Ca²⁺ downstream signaling. Ano1 knockout or calcium binding mutants blunts the response of osteoclast to mechanical stimulation. In vivo, Ano1 knockout in osteoclast blunts loading induced osteoclast inhibition and unloading induced bone loss and. These results demonstrate that Ano1 plays an important role in mechanical stimulation induced osteoclast activity changes.

Antagonism of TRPV4 channels partially reduces mechanotransduction in rat skeletal muscle afferents

Mechanical distortion of working skeletal muscle induces sympathoexcitation via thin fibre afferents, a reflex response known as the skeletal muscle mechanoreflex. However, to date, the receptor ion channels responsible for mechanotransduction in skeletal muscle remain largely undetermined. Transient receptor potential vanilloid 4 (TRPV4) is known to sense mechanical stimuli such as shear stress or osmotic pressure in various organs. It is hypothesized that TRPV4 in thin-fibre primary afferents innervating skeletal muscle is involved in mechanotransduction. Fluorescence immunostaining revealed that 20.1 ± 10.1% of TRPV4 positive neurons were small dorsal root ganglion (DRG) neurons that were DiI-labelled, and among them 9.5 ± 6.1% of TRPV4 co-localized with the C-fibre marker peripherin. In vitro whole-cell patch clamp recordings from cultured rat DRG neurons demonstrated that mechanically activated current amplitude was significantly attenuated after the application of the TRPV4 antagonist HC067047 compared to control (P = 0.004). Such reductions were also observed in single-fibre recordings from a muscle-nerve ex vivo preparation where HC067047 significantly decreased afferent discharge to mechanical stimulation (P = 0.007). Likewise, in an in vivo decerebrate rat preparation, the renal sympathetic nerve activity (RSNA) and mean arterial pressure (MAP) responses to passive stretch of hindlimb muscle were significantly reduced by intra-arterial injection of HC067047 (ΔRSNA: P = 0.019, ΔMAP: P = 0.002). The findings suggest that TRPV4 plays an important role in mechanotransduction contributing to the cardiovascular responses evoked by the skeletal muscle mechanoreflex during exercise. KEY POINTS: Although a mechanical stimulus to skeletal muscle reflexively activates the sympathetic nervous system, the receptors responsible for mechanotransduction in skeletal muscle thin fibre afferents have not been fully identified. Evidence suggests that TRPV4 is a mechanosensitive channel that plays an important role in mechanotransduction within various organs. Immunocytochemical staining demonstrates that TRPV4 is expressed in group IV skeletal muscle afferents. In addition, we show that the TRPV4 antagonist HC067047 decreases the responsiveness of thin fibre afferents to mechanical stimulation at the muscle tissue level as well as at the level of dorsal root ganglion neurons. Moreover, we demonstrate that intra-arterial HC067047 injection attenuates the sympathetic and pressor responses to passive muscle stretch in decerebrate rats. These data suggest that antagonism of TRPV4 attenuates mechanotransduction in skeletal muscle afferents. The present study demonstrates a probable physiological role for TRPV4 in the regulation of mechanical sensation in somatosensory thin fibre muscle afferents.

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.

Trimethylamine-N-oxide sensitizes chondrocytes to mechanical loading through the upregulation of Piezo1

BACKGROUND

Mechanical strain plays a crucial role in chondrocyte apoptosis and osteoarthritis (OA) disease progression through Piezo1. Trimethylamine-N-oxide (TMAO) is a diet-derived metabolite that correlates positively with multiple chronic diseases. Herein, we explored the potential role of TMAO in sensitizing chondrocytes to Piezo1-mediated mechanotransduction.

METHODS

The cytotoxicity of TMAO on chondrocytes was assayed. Piezo1 expression was measured after TMAO intervention. Pathological mechanical loading or Yoda1 (a specific Piezo1 channel activator) was administered in chondrocytes. The calcium levels and cytoskeleton in chondrocytes were observed by fluorescence microscopy. Flow cytometry, western blotting, and mitochondrial membrane potential assays were utilized to evaluate apoptosis. A rat OA model was constructed by anterior cruciate ligament transection. Hematoxylin-eosin staining, Safranin-O/Fast Green staining, immunochemistry, and TUNEL were applied to estimate OA severity.

RESULTS

TMAO intervention alone did not affect chondrocyte viability up to 600 μM. TMAO significantly increased Piezo1 expression and up-regulated intracellular calcium levels, further leading to cytoskeletal damage. Mechanical strain or Yoda1 treatment significantly induced chondrocyte apoptosis. Notably, TMAO intervention further aggravated chondrocyte apoptosis and cartilage destruction under pathological mechanical loading.

CONCLUSION

TMAO significantly up-regulated Piezo1 expression and sensitized chondrocytes to mechanical loading, which may be closely related to the pathogenesis of OA.

The Impact of Plasma Membrane Ion Channels on Bone Remodeling in Response to Mechanical Stress, Oxidative Imbalance, and Acidosis

The extracellular milieu is a rich source of different stimuli and stressors. Some of them depend on the chemical–physical features of the matrix, while others may come from the ‘outer’ environment, as in the case of mechanical loading applied on the bones. In addition to these forces, a plethora of chemical signals drives cell physiology and fate, possibly leading to dysfunctions when the homeostasis is disrupted. This variety of stimuli triggers different responses among the tissues: bones represent a particular milieu in which a fragile balance between mechanical and metabolic demands should be tuned and maintained by the concerted activity of cell biomolecules located at the interface between external and internal environments. Plasma membrane ion channels can be viewed as multifunctional protein machines that act as rapid and selective dual-nature hubs, sensors, and transducers. Here we focus on some multisensory ion channels (belonging to Piezo, TRP, ASIC/EnaC, P2XR, Connexin, and Pannexin families) actually or potentially playing a significant role in bone adaptation to three main stressors, mechanical forces, oxidative stress, and acidosis, through their effects on bone cells including mesenchymal stem cells, osteoblasts, osteoclasts, and osteocytes. Ion channel-mediated bone remodeling occurs in physiological processes, aging, and human diseases such as osteoporosis, cancer, and traumatic events.

A new look at osteoarthritis: Threshold potentials and an analogy to hypocalcemia

Cartilage is a tissue that consist of very few cells embedded in a highly negatively charged extracellular matrix (ECM). This tissue is dealing with several electrical potentials which have been shown to control the production of ECM. Cartilage is present at joints and is constantly prone to degradation. Failing to repair the damage will result in the occurrence of osteoarthritis (OA). This perspective aims to link biophysical insights with biomolecular research in order to provide an alternative view on the possible causes of OA. Firstly, we hypothesize the existence of a threshold potential, which should be reached in order to initiate repair but if not met, unrepaired damage will evolve to OA. Measurements of the magnitude of this threshold electrical potential would be a helpful diagnostic tool. Secondly, since electrical potential alterations can induce chondrocytes to synthesize ECM, a cellular sensor must be present. We here propose an analogy to the hypocalcemia ‘unshielding’ situation to comprehend electrical potential generation and explore possible sensing mechanisms translating the electrical message into cellular responses. A better understanding of the cellular voltage sensors and down-stream signalling mechanisms may lead to the development of novel treatments for cartilage regeneration.

Mechanotransduction pathways in articular chondrocytes and the emerging role of estrogen receptor-α

In the synovial joint, mechanical force creates an important signal that influences chondrocyte behavior. The conversion of mechanical signals into biochemical cues relies on different elements in mechanotransduction pathways and culminates in changes in chondrocyte phenotype and extracellular matrix composition/structure. Recently, several mechanosensors, the first responders to mechanical force, have been discovered. However, we still have limited knowledge about the downstream molecules that enact alterations in the gene expression profile during mechanotransduction signaling. Recently, estrogen receptor α (ERα) has been shown to modulate the chondrocyte response to mechanical loading through a ligand-independent mechanism, in line with previous research showing that ERα exerts important mechanotransduction effects on other cell types, such as osteoblasts. In consideration of these recent discoveries, the goal of this review is to position ERα into the mechanotransduction pathways known to date. Specifically, we first summarize our most recent understanding of the mechanotransduction pathways in chondrocytes on the basis of three categories of actors, namely mechanosensors, mechanotransducers, and mechanoimpactors. Then, the specific roles played by ERα in mediating the chondrocyte response to mechanical loading are discussed, and the potential interactions of ERα with other molecules in mechanotransduction pathways are explored. Finally, we propose several future research directions that may advance our understanding of the roles played by ERα in mediating biomechanical cues under physiological and pathological conditions.

The role of PIEZO ion channels in the musculoskeletal system

PIEZO1 and PIEZO2 are mechanosensitive cation channels that are highly expressed in numerous tissues throughout the body and exhibit diverse, cell-specific functions in multiple organ systems. Within the musculoskeletal system, PIEZO1 functions to maintain muscle and bone mass, sense tendon stretch, and regulate senescence and apoptosis in response to mechanical stimuli within cartilage and the intervertebral disc. PIEZO2 is essential for transducing pain and touch sensations as well as proprioception in the nervous system, which can affect musculoskeletal health. PIEZO1 and PIEZO2 have been shown to act both independently as well as synergistically in different cell types. Conditions that alter PIEZO channel mechanosensitivity, such as inflammation or genetic mutations, can have drastic effects on these functions. For this reason, therapeutic approaches for PIEZO-related disease focus on altering PIEZO1 and/or PIEZO2 activity in a controlled manner, either through inhibition with small molecules, or through dietary control and supplementation to maintain a healthy cell membrane composition. Although many opportunities to better understand PIEZO1 and PIEZO2 remain, the studies summarized in this review highlight how crucial PIEZO channels are to musculoskeletal health and point to promising possible avenues for their modulation as a therapeutic target.