Load activated FGFR and beta1 integrins target distinct chondrocyte mechano-response genes

In response to mechanical stimuli, chondrocytes adapt their transcriptional activity, thereby shaping the cellular mechano-response; however, it remains unclear whether the activation of cell surface receptors during mechanical loading converge in the activation of the same mechano-response genes, or whether pathway-specific genes can be defined. We aimed to determine whether load-activated FGF/FGFR signalling and β1 integrins activate ERK and control the same or distinct subsets of mechano-regulated genes. To this end, tissue-engineered neocartilage was generated from murine costal chondrocytes or human articular chondrocytes and subjected to dynamic unconfined compression with or without FGFR inhibition. To assess the role of β1 integrins, neocartilage was generated from embryonic β1 integrin-deficient or wild type costal chondrocytes. Load-activated FGFR signalling drove ERK activation in murine chondrocytes, and partially also in human chondrocytes, and mechano-response genes could be classified according to their regulation: Fosl1, Itga5, Ngf and Timp1 were regulated by load-activated FGFR depending on the developmental stage, whereas β1 integrins controlled Inhba expression. In human chondrocytes, load-activated FGFR signalling controlled expression of BMP2, PTGS2 and DUSP5, but not FOSB. We show here that the chondrocyte loading response is coordinated by concurrent activation of multiple receptors, and identified for the first time distinct target genes of activated receptors. These insights open up the opportunity to pharmacologically shape the mechano-response of chondrocytes in future studies with promising implications for the management of osteoarthritis and the development of novel therapeutic strategies.

Implications of mechanosensitive ion channels in the pathogenesis of osteoarthritis: a comprehensive review

Osteoarthritis (OA) is the predominant cause of joint pain and limited mobility in older people, and its prevalence is increasing as the population ages. Given the lack of effective therapeutic interventions, the disability rate associated with OA is a staggering 53%, which significantly affects the wellbeing of those affected and represents a significant social and family financial burden. Consequently, OA has emerged as a pressing social and public health concern globally. Various forms of mechanical strain, such as dynamic compression, fluid shear, tissue shear, and hydrostatic pressure, serve as crucial physical stimuli perceived by chondrocytes. Recent studies indicate that aberrant mechanical loading represents a fundamental risk factor for OA. Upon exposure to mechanical loading, chondrocytes translate mechanical cues into chemical signals primarily via mechanosensitive ion channels, resulting in alterations in cartilage metabolism. Numerous studies have demonstrated the significance of mechanosensitive ion channels in the pathogenesis of OA, suggesting that therapeutic interventions targeting these channels on chondrocytes may offer potential benefits.

Cellular signaling pathways in the nervous system activated by various mechanical and electromagnetic stimuli

Mechanical stimuli, such as stretch, shear stress, or compression, activate a range of biomolecular responses through cellular mechanotransduction. In the nervous system, studies on mechanical stress have highlighted key pathophysiological mechanisms underlying traumatic injury and neurodegenerative diseases. However, the biomolecular pathways triggered by mechanical stimuli in the nervous system has not been fully explored, especially compared to other body systems. This gap in knowledge may be due to the wide variety of methods and definitions used in research. Additionally, as mechanical stimulation techniques such as ultrasound and electromagnetic stimulation are increasingly utilized in psychological and neurorehabilitation treatments, it is vital to understand the underlying biological mechanisms in order to develop accurate pathophysiological models and enhance therapeutic interventions. This review aims to summarize the cellular signaling pathways activated by various mechanical and electromagnetic stimuli with a particular focus on the mammalian nervous system. Furthermore, we briefly discuss potential cellular mechanosensors involved in these processes.

Enzymatic digestion does not compromise sliding-mediated cartilage lubrication

Articular cartilage's remarkable low-friction properties are essential to joint function. In osteoarthritis (OA), cartilage degeneration (e.g., proteoglycan loss and collagen damage) decreases tissue modulus and increases permeability. Although these changes impair lubrication in fully depressurized and slowly slid cartilage, new evidence suggests such relationships may not hold under biofidelic sliding conditions more representative of those encountered in vivo. Our recent studies using the convergent stationary contact area (cSCA) configuration demonstrate that articulation (i.e., sliding) generates interfacial hydrodynamic pressures capable of replenishing cartilage interstitial fluid/pressure lost to compressive loading through a mechanism termed tribological rehydration. This fluid recovery sustains in vivo-like kinetic friction coefficients (µk<0.02 in PBS and <0.005 in synovial fluid) with little sensitivity to mechanical properties in healthy tissue. However, the tribomechanical function of compromised cartilage under biofidelic sliding conditions remains unknown. Here, we investigated the effects of OA-like changes in cartilage mechanical properties, modeled via enzymatic digestion of mature bovine cartilage, on its tribomechanical function during cSCA sliding. We found no differences in sliding-driven tribological rehydration behaviors or µk between naïve and digested cSCA cartilage (in PBS or synovial fluid). This suggests that OA-like cartilage retains sufficient functional properties to support naïve-like fluid recovery and lubrication under biofidelic sliding conditions. However, OA-like cartilage accumulated greater total tissue strains due to elevated strain accrual during initial load application. Together, these results suggest that elevated total tissue strains-as opposed to activity-mediated strains or friction-driven wear-might be the key biomechanical mediator of OA pathology in cartilage. STATEMENT OF SIGNIFICANCE: Osteoarthritis (OA) decreases cartilage's modulus and increases its permeability. While these changes compromise frictional performance in benchtop testing under low fluid load support (FLS) conditions, whether such observations hold under sliding conditions that better represent the joints' dynamic FLS conditions in vivo is unclear. Here, we leveraged biofidelic benchtop sliding experiments-that is, those mimicking joints' native sliding environment-to examine how OA-like changes in mechanical properties effect cartilage's natural lubrication. We found no differences in sliding-mediated fluid recovery or kinetic friction behaviors between naïve and OA-like cartilage. However, OA-like cartilage experienced greater strain accumulation during load application, suggesting that elevated tissue strains (not friction-driven wear) may be the primary biomechanical mediator of OA pathology.

Transducing compressive forces into cellular outputs in cancer and beyond

In living organisms, cells sense mechanical forces (shearing, tensile, and compressive) and respond to those physical cues through a process called mechanotransduction. This process includes the simultaneous activation of biochemical signaling pathways. Recent studies mostly on human cells revealed that compressive forces selectively modulate a wide range of cell behavior, both in compressed and in neighboring less compressed cells. Besides participating in tissue homeostasis such as bone healing, compression is also involved in pathologies, including intervertebral disc degeneration or solid cancers. In this review, we will summarize the current scattered knowledge of compression-induced cell signaling pathways and their subsequent cellular outputs, both in physiological and pathological conditions, such as solid cancers.

A High-Throughput Mechanical Activator for Cartilage Engineering Enables Rapid Screening of in vitro Response of Tissue Models to Physiological and Supra-Physiological Loads

Articular cartilage is crucially influenced by loading during development, health, and disease. However, our knowledge of the mechanical conditions that promote engineered cartilage maturation or tissue repair is still incomplete. Current in vitro models that allow precise control of the local mechanical environment have been dramatically limited by very low throughput, usually just a few specimens per experiment. To overcome this constraint, we have developed a new device for the high throughput compressive loading of tissue constructs: the High Throughput Mechanical Activator for Cartilage Engineering (HiT-MACE), which allows the mechanoactivation of 6 times more samples than current technologies. With HiT-MACE we were able to apply cyclic loads in the physiological (e.g., equivalent to walking and normal daily activity) and supra-physiological range (e.g., injurious impacts or extensive overloading) to up to 24 samples in one single run. In this report, we compared the early response of cartilage to physiological and supra-physiological mechanical loading to the response to IL-1β exposure, a common but rudimentary in vitro model of cartilage osteoarthritis. Physiological loading rapidly upregulated gene expression of anabolic markers along the TGF-β1 pathway. Notably, TGF-β1 or serum was not included in the medium. Supra-physiological loading caused a mild catabolic response while IL-1β exposure drove a rapid anabolic shift. This aligns well with recent findings suggesting that overloading is a more realistic and biomimetic model of cartilage degeneration. Taken together, these findings showed that the application of HiT-MACE allowed the use of larger number of samples to generate higher volume of data to effectively explore cartilage mechanobiology, which will enable the design of more effective repair and rehabilitation strategies for degenerative cartilage pathologies.

Metformin use and the risk of total knee replacement among diabetic patients: a propensity-score-matched retrospective cohort study

Metformin has been shown to modulate meta-inflammation, an important pathogenesis in knee osteoarthritis (OA). The study aimed to test the association between regular metformin use with total knee replacement (TKR) in patients with diabetes. This is a retrospective study with electronic records retrieved in Hong Kong public primary care. Patients with diabetes aged ≥ 45 who visited during 2007 to 2010, were followed up for a four-year period from 2011 to 2014 to determine the incidence of TKR. Propensity score matching based on age, sex, co-medications and chronic conditions was conducted to adjust for confounding. Cox regression was implemented to examine the association between metformin use and TKR. In total, 196,930 patients were eligible and 93,330 regular metformin users (defined as ≥ 4 prescriptions over the previous year) and non-users were matched. Among 46,665 regular users, 184 TKRs were conducted, 17.1% fewer than that among non-users. Cox regression showed that regular metformin users had a 19%-lower hazard of TKR [hazard ratio (HR) = 0.81, 95% confidence interval: 0.67 to 0.98, P = 0.033], with a dose–response relationship. Findings suggest a potential protective effect of metformin on knee OA progression and later TKR incidence among diabetic patients.

Multiscale Strain Transfer in Cartilage

The transfer of stress and strain signals between the extracellular matrix (ECM) and cells is crucial for biochemical and biomechanical cues that are required for tissue morphogenesis, differentiation, growth, and homeostasis. In cartilage tissue, the heterogeneity in spatial variation of ECM molecules leads to a depth-dependent non-uniform strain transfer and alters the magnitude of forces sensed by cells in articular and fibrocartilage, influencing chondrocyte metabolism and biochemical response. It is not fully established how these nonuniform forces ultimately influence cartilage health, maintenance, and integrity. To comprehend tissue remodelling in health and disease, it is fundamental to investigate how these forces, the ECM, and cells interrelate. However, not much is known about the relationship between applied mechanical stimulus and resulting spatial variations in magnitude and sense of mechanical stimuli within the chondrocyte’s microenvironment. Investigating multiscale strain transfer and hierarchical structure-function relationships in cartilage is key to unravelling how cells receive signals and how they are transformed into biosynthetic responses. Therefore, this article first reviews different cartilage types and chondrocyte mechanosensing. Following this, multiscale strain transfer through cartilage tissue and the involvement of individual ECM components are discussed. Finally, insights to further understand multiscale strain transfer in cartilage are outlined.

Novel role of estrogen receptor-α on regulating chondrocyte phenotype and response to mechanical loading

OBJECTIVE

In knee cartilage from patients with osteoarthritis (OA), both preserved cartilage and damaged cartilage are observed. In this study, we aim to compare preserved with damaged cartilage to identify the molecule(s) that may be responsible for the mechanical loading-induced differences within cartilage degradation.

METHODS

Preserved and damaged cartilage were harvested from the same OA knee joint. RNA Sequencing was performed to examine the transcriptomic differences between preserved and damaged cartilage cells. Estrogen receptor-α (ERα) was identified, and its function of was tested through gene knockin and knockout. The role of ERα in mediating chondrocyte response to mechanical loading was examined via compression of chondrocyte-laded hydrogel in a strain-controlled manner. Findings from the studies on human samples were verified in animal models.

RESULTS

Level of estrogen receptor α (ERα) was significantly reduced in damaged cartilage compared to preserved cartilage, which were observed in both human and mice samples. Knockdown of ESR1, the gene encoding ERα, resulted in an upregulation of senescence- and OA-relevant markers in chondrocytes. Conversely, knockin of ESR1 partially reversed the osteoarthritic and senescent phenotype of OA chondrocytes. Using a three-dimensional (3D) culture model, we demonstrated that mechanical overload significantly suppressed ERα level in chondrocytes with concomitant upregulation of osteoarthritic phenotype. When ESR1 expression was suppressed, mechanical loading enhanced hypertrophic and osteogenic transition.

CONCLUSION

Our study demonstrates a new estrogen-independent role of ERα in mediating chondrocyte phenotype and its response to mechanical loading, and suggests that enhancing ERα level may represent a new method to treat osteoarthritis.

NG2/CSPG4 regulates cartilage degeneration during TMJ osteoarthritis

Changes in the mechanical homeostasis of the temporomandibular joint (TMJ) can lead to the initiation and progression of degenerative arthropathies such as osteoarthritis (OA). Cells sense and engage with their mechanical microenvironment through interactions with the extracellular matrix. In the mandibular condylar cartilage, the pericellular microenvironment is composed of type VI collagen. NG2/CSPG4 is a transmembrane proteoglycan that binds with type VI collagen, and has been implicated in the cell stress response through mechanical loading-sensitive signaling networks including ERK 1/2. The objective of this study is to define the role of NG2/CSPG4 in the initiation and progression of TMJ OA and to determine if NG2/CSPG4 engages ERK 1/2 in a mechanical loading dependent manner. In vivo, we induced TMJ OA in control and NG2/CSPG4 knockout mice using a surgical destabilization approach. In control mice, NG2/CSPG4 is depleted during the early stages of TMJ OA and NG2/CSPG4 knockout mice have more severe cartilage degeneration, elevated expression of key OA proteases, and suppression of OA matrix synthesis genes. In vitro, we characterized the transcriptome and protein from control and NG2/CSPG4 knockout cells and found significant dysregulation of the ERK 1/2 signaling axis. To characterize the mechanobiological response of NG2/CSPG4, we applied mechanical loads on cell-agarose-collagen scaffolds using a compression bioreactor and illustrate that NG2/CSPG4 knockout cells fail to mechanically activate ERK 1/2 and are associated with changes in the expression of the same key OA biomarkers measured in vivo. Together, these findings implicate NG2/CSPG4 in the mechanical homeostasis of TMJ cartilage and in the progression of degenerative arthropathies including OA.

Mechanosignalling in cartilage: an emerging target for the treatment of osteoarthritis

Mechanical stimuli have fundamental roles in articular cartilage during health and disease. Chondrocytes respond to the physical properties of the cartilage extracellular matrix (ECM) and the mechanical forces exerted on them during joint loading. In osteoarthritis (OA), catabolic processes degrade the functional ECM and the composition and viscoelastic properties of the ECM produced by chondrocytes are altered. The abnormal loading environment created by these alterations propagates cell dysfunction and inflammation. Chondrocytes sense their physical environment via an array of mechanosensitive receptors and channels that activate a complex network of downstream signalling pathways to regulate several cell processes central to OA pathology. Advances in understanding the complex roles of specific mechanosignalling mechanisms in healthy and OA cartilage have highlighted molecular processes that can be therapeutically targeted to interrupt pathological feedback loops. The potential for combining these mechanosignalling targets with the rapidly expanding field of smart mechanoresponsive biomaterials and delivery systems is an emerging paradigm in OA treatment. The continued advances in this field have the potential to enable restoration of healthy mechanical microenvironments and signalling through the development of precision therapeutics, mechanoregulated biomaterials and drug systems in the near future.

Mechanical Cues: Bidirectional Reciprocity in the Extracellular Matrix Drives Mechano-Signalling in Articular Cartilage

The composition and organisation of the extracellular matrix (ECM), particularly the pericellular matrix (PCM), in articular cartilage is critical to its biomechanical functionality; the presence of proteoglycans such as aggrecan, entrapped within a type II collagen fibrillar network, confers mechanical resilience underweight-bearing. Furthermore, components of the PCM including type VI collagen, perlecan, small leucine-rich proteoglycans—decorin and biglycan—and fibronectin facilitate the transduction of both biomechanical and biochemical signals to the residing chondrocytes, thereby regulating the process of mechanotransduction in cartilage. In this review, we summarise the literature reporting on the bidirectional reciprocity of the ECM in chondrocyte mechano-signalling and articular cartilage homeostasis. Specifically, we discuss studies that have characterised the response of articular cartilage to mechanical perturbations in the local tissue environment and how the magnitude or type of loading applied elicits cellular behaviours to effect change. In vivo, including transgenic approaches, and in vitro studies have illustrated how physiological loading maintains a homeostatic balance of anabolic and catabolic activities, involving the direct engagement of many PCM molecules in orchestrating this slow but consistent turnover of the cartilage matrix. Furthermore, we document studies characterising how abnormal, non-physiological loading including excessive loading or joint trauma negatively impacts matrix molecule biosynthesis and/or organisation, affecting PCM mechanical properties and reducing the tissue’s ability to withstand load. We present compelling evidence showing that reciprocal engagement of the cells with this altered ECM environment can thus impact tissue homeostasis and, if sustained, can result in cartilage degradation and onset of osteoarthritis pathology. Enhanced dysregulation of PCM/ECM turnover is partially driven by mechanically mediated proteolytic degradation of cartilage ECM components. This generates bioactive breakdown fragments such as fibronectin, biglycan and lumican fragments, which can subsequently activate or inhibit additional signalling pathways including those involved in inflammation. Finally, we discuss how bidirectionality within the ECM is critically important in enabling the chondrocytes to synthesise and release PCM/ECM molecules, growth factors, pro-inflammatory cytokines and proteolytic enzymes, under a specified load, to influence PCM/ECM composition and mechanical properties in cartilage health and disease.

A Novel In Vitro and In Silico System for Analyzing Complex Mechanobiological Behavior of Chondrocytes in Three-Dimensional Hydrogel Constructs

Physiological loading is essential for the maintenance of articular cartilage through the regulation of tissue remodeling. To correctly understand the behavior of chondrocytes in their native environment, cell stimulating devices and bioreactors have been developed to examine the effect of mechanical stimuli on chondrocytes. This study describes the design and validation of a novel system for analyzing chondrocyte deformation patterns. This involves an in vitro mechanical device for a controlled application of multi-axial-loading regimes to chondrocyte-seeded agarose constructs and in silico models for analyzing chondrocyte deformation patterns. The computer-controlled device precisely applies compressive, tensile, and shear strains to hydrogel constructs using a customizable macro-based program. The synchronization of the displacements is shown to be accurate with a 1.2% error and is highly reproducible. The device design allows housing for up to eight novel designed free-swelling three-dimensional hydrogel constructs. Constructs include mesh ends and are optimized to withstand the application of up to 7% mechanical tensile and 15% shear strains. Constructs were characterized through mapping the strain within as mechanical load was applied and was validated using light microscopy methods, chondrocyte viability using live/dead imaging, and cell deformation strains. Images were then analyzed to determine the complex deformation strain patterns of chondrocytes under a range of dynamic mechanical stimulations. This is one of the first systems that have characterized construct strains to cellular strains. The features in this device make the system ideally suited for a systematic approach for the investigation of the response of chondrocytes to a complex physiologically relevant deformation profile.

Examining the Association of Knee Pain with Modifiable Cardiometabolic Risk Factors

OBJECTIVE

There is a well-established link between obesity and knee osteoarthritis, and recent research has implicated diabetes as a potential cause of cartilage degeneration. This study uses the National Health and Nutrition Examination Survey (NHANES) database to examine the association between knee pain and various metabolic factors.

METHODS

A retrospective cross-sectional study of the NHANES database from 1999 to 2004 was performed. The main outcome was any knee pain and bilateral knee pain. Main effects of interest were body mass index (BMI), and hemoglobin A1c (HbA1c). We additionally assessed various patient factors including age, race, poverty, gender and smoking status. Multivariable logistic regression models and interaction terms were analyzed.

RESULTS

Data on 12,900 patients was included. In the main adjusted analysis, the modifiable risk factors associated with any knee pain were: overweight (OR 0.91; 95% CI 0.85, 0.97), obesity (OR 1.54; 95% CI 1.42, 1.66), glycemic control (OR 1.20; 95% CI 1.03, 1.38), and current smokers (OR 1.15; 95% CI 1.05, 1.27), all p<0.05. These same factors remain significant for bilateral knee pain. Subgroup analysis showed patients under 65 years old have a 5% increase in risk of any knee pain as their body mass index increases, but patients 65 years and older have a 10% increase in risk.

CONCLUSION

This study confirms the association of knee pain with increased weight, glycemic control, current smoking and age. Most of these risk factors can be modified in patients with knee pain and should be discussed when providing conservative treatment options.

Do Mast Cells Have a Role in Tendon Healing and Inflammation?

Understanding the links between the tendon healing process, inflammatory mechanisms, and tendon homeostasis/pain after tissue damage is crucial in developing novel therapeutics for human tendon disorders. The inflammatory mechanisms that are operative in response to tendon injury are not fully understood, but it has been suggested that inflammation occurring in response to nerve signaling, i.e., neurogenic inflammation, has a pathogenic role. The mechanisms driving such neurogenic inflammation are presently not clear. However, it has recently been demonstrated that mast cells present within the injured tendon can express glutamate receptors, raising the possibility that mast cells may be sensitive to glutamate signaling and thereby modulate neurogenic inflammation following tissue injury. In this review, we discuss the role of mast cells in the communication with peripheral nerves, and their emerging role in tendon healing and inflammation after injury.

Matrix metalloproteinase-13, NF-κB p65 and interleukin-1β are associated with the severity of knee osteoarthritis

Knee osteoarthritis (KOA) is a prevalent disease, especially in the elderly. The present study examined the expression of matrix metalloproteinase-13 (MMP-13), NF-κBp65 and interleukin (IL)-lβ in the synovial tissues of KOA patients and the role of MMP-13 and the NF-κBp65 signalling pathway in KOA pathogenesis. A total of 100 KOA patients were enrolled in our hospital from December 2015 to December 2017 and were classified into either a mild KOA group (Outerbridge grade 1 and 2) or a severe KOA group (Outerbridge grade 3 and 4). Non-OA patients were included as controls. Synovial tissues from patients in both groups were collected for detection of the mRNA and protein expression of MMP-13, NF-κBp65 and IL-lβ. Synovial tissue slices were subjected to haematoxylin and eosin staining and immunohistochemistry (SP method). Cartilage tissues were observed under a light microscope after Safranin O-fast green staining. Reverse transcription-quantitative PCR and western blot analyses demonstrated that the expression of MMP-13, NF-κBp65 and IL-lβ in the mild and severe groups were substantially upregulated compared with the control group (all P<0.05). A positive correlation between MMP-13 and NF-κBp65 expression in the KOA synovial tissues was identified (P<0.05). Immunohistochemistry revealed that the expression of MMP-13 and NF-κBp65 was related to the severity of KOA (MMP-13: severe, 92.54%; moderate, 76.52%; control: 32.14%; and NF-κBp65: severe, 85.56%; moderate, 48.12%; control: 28.32%). This evidence indicated that the severity of KOA was related to MMP-13 and NF-κBp65 expression. The NF-κB signalling pathway may be activated during OA progression, which could upregulate the expression of MMP-13 and IL-1β and accelerate the deterioration of articular cartilage.

Detrimental effects of long sedentary bouts on the biomechanical response of cartilage to sliding

Purpose/Aim: Epidemiological evidence suggests, contrary to popular mythos, that increased exercise/joint activity does not place articular cartilage at increased risk of disease, but instead promotes joint health. One explanation for this might be activity-induced cartilage rehydration; where joint articulation drives restoration of tissue hydration, thickness, and dependent tribomechanical outcomes (e.g., load support, stiffness, and lubricity) lost to joint loading. However, there have been no studies investigating how patterning of intermittent articulation influences the hydration and biomechanical functions of cartilage.Materials and Methods: Here we leveraged the convergent stationary contact area (cSCA) testing configuration and its unique ability to drive tribological rehydration, to elucidate how intermittency of activity affects the biomechanical functions of bovine stifle cartilage under well-controlled sliding conditions that have been designed to model a typical "day" of human joint activity.Results: For a fixed volume of "daily" activity (30 min) and sedentary time (60 min), breaking up intermittent activity into longer and less-frequent bouts (corresponding to longer continuous sedentary periods) resulted in the exposure of articular cartilage to markedly greater strains, losses of interstitial pressure, and friction coefficients.Conclusions: These results demonstrated that the regularity of ex vivo activity regimens, specifically the duration of sedentary bouts, had a dominant effect on the biomechanical functions of articular cartilage. In more practical terms, the results suggest that brief but regular movement patterns (e.g., every hour) may be biomechanically preferred to long and infrequent movement patterns (e.g., a long walk after a sedentary day) when controlling for daily activity volume (e.g., 30 min).

Extruded Bioreactor Perfusion Culture Supports the Chondrogenic Differentiation of Human Mesenchymal Stem/Stromal Cells in 3D Porous Poly(ɛ-Caprolactone) Scaffolds

Novel bioengineering strategies for the ex vivo fabrication of native-like tissue-engineered cartilage are crucial for the translation of these approaches to clinically manage highly prevalent and debilitating joint diseases. Bioreactors that provide different biophysical stimuli have been used in tissue engineering approaches aimed at enhancing the quality of the cartilage tissue generated. However, such systems are often highly complex, expensive, and not very versatile. In the current study, a novel, cost-effective, and customizable perfusion bioreactor totally fabricated by additive manufacturing (AM) is proposed for the study of the effect of fluid flow on the chondrogenic differentiation of human bone-marrow mesenchymal stem/stromal cells (hBMSCs) in 3D porous poly(ɛ-caprolactone) (PCL) scaffolds. hBMSCs are first seeded and grown on PCL scaffolds and hBMSC-PCL constructs are then transferred to 3D-extruded bioreactors for continuous perfusion culture under chondrogenic inductive conditions. Perfused constructs show similar cell metabolic activity and significantly higher sulfated glycosaminoglycan production (≈1.8-fold) in comparison to their non-perfused counterparts. Importantly, perfusion bioreactor culture significantly promoted the expression of chondrogenic marker genes while downregulating hypertrophy. This work highlights the potential of customizable AM platforms for the development of novel personalized repair strategies and more reliable in vitro models with a wide range of applications.

Physical Stimulations for Bone and Cartilage Regeneration

A wide range of techniques and methods are actively invented by clinicians and scientists who are dedicated to the field of musculoskeletal tissue regeneration. Biological, chemical, and physiological factors, which play key roles in musculoskeletal tissue development, have been extensively explored. However, physical stimulation is increasingly showing extreme importance in the processes of osteogenic and chondrogenic differentiation, proliferation and maturation through defined dose parameters including mode, frequency, magnitude, and duration of stimuli. Studies have shown manipulation of physical microenvironment is an indispensable strategy for the repair and regeneration of bone and cartilage, and biophysical cues could profoundly promote their regeneration. In this article, we review recent literature on utilization of physical stimulation, such as mechanical forces (cyclic strain, fluid shear stress, etc.), electrical and magnetic fields, ultrasound, shock waves, substrate stimuli, etc., to promote the repair and regeneration of bone and cartilage tissue. Emphasis is placed on the mechanism of cellular response and the potential clinical usage of these stimulations for bone and cartilage regeneration.

Fos Promotes Early Stage Teno-Lineage Differentiation of Tendon Stem/Progenitor Cells in Tendon

Stem cells have been widely used in tendon tissue engineering. The lack of refined and controlled differentiation strategy hampers the tendon repair and regeneration. This study aimed to find new effective differentiation factors for stepwise tenogenic differentiation. By microarray screening, the transcript factor Fos was found to be expressed in significantly higher amounts in postnatal Achilles tendon tissue derived from 1 day as compared with 7‐days‐old rats. It was further confirmed that expression of Fos decreased with time in postnatal rat Achilles tendon, which was accompanied with the decreased expression of multiply tendon markers. The expression of Fos also declined during regular in vitro cell culture, which corresponded to the loss of tendon phenotype. In a cell‐sheet and a three‐dimensional cell culture model, the expression of Fos was upregulated as compared with in regular cell culture, together with the recovery of tendon phenotype. In addition, significant higher expression of tendon markers was found in Fos‐overexpressed tendon stem/progenitor cells (TSPCs), and Fos knock‐down gave opposite results. In situ rat tendon repair experiments found more normal tendon‐like tissue formed and higher tendon markers expression at 4 weeks postimplantation of Fos‐overexpressed TSPCs derived nonscaffold engineering tendon (cell‐sheet), as compared with the control group. This study identifies Fos as a new marker and functional driver in the early stage teno‐lineage differentiation of tendon, which paves the way for effective stepwise tendon differentiation and future tendon regeneration. Stem Cells Translational Medicine2017;6:2009–2019

Subcellular domain-dependent molecular hierarchy of SFK and FAK in mechanotransduction and cytokine signaling

Focal adhesion kinase (FAK) and Src family kinases (SFK) are known to play critical roles in mechanotransduction and other crucial cell functions. Recent reports indicate that they reside in different microdomains of the plasma membrane. However, little is known about their subcellular domain-dependent roles and responses to extracellular stimuli. Here, we employed fluorescence resonance energy transfer (FRET)-based biosensors in conjunction with collagen-coupled agarose gels to detect subcellular activities of SFK and FAK in three-dimensional (3D) settings. We observed that SFK and FAK in the lipid rafts and nonrafts are differently regulated by fluid flow and pro-inflammatory cytokines. Inhibition of FAK in the lipid rafts blocked SFK response to fluid flow, while inhibition of SFK in the non-rafts blocked FAK activation by the cytokines. Ex-vivo FRET imaging of mouse cartilage explants showed that intermediate level of interstitial fluid flow selectively decreased cytokine-induced SFK/FAK activation. These findings suggest that SFK and FAK exert distinctive molecular hierarchy depending on their subcellular location and extracellular stimuli.

Friction-Induced Mitochondrial Dysregulation Contributes to Joint Deterioration in Prg4 Knockout Mice

Deficiency of PRG4 (lubricin), the boundary lubricant in mammalian joints, contributes to increased joint friction accompanied by superficial and upper intermediate zone chondrocyte caspase-3 activation, as shown in lubricin-null (Prg4^-/-) mice. Caspase-3 activity appears to be reversible upon the restitution of Prg4 either endogenously in vivo, in a gene trap mouse, or as an applied lubricant in vitro. In this study we show that intra-articular injection of human PRG4 in vivo in Prg4^-/- mice prevented caspase-3 activation in superficial zone chondrocytes and was associated with a modest decrease in whole joint friction measured ex vivo using a joint pendulum method. Non-lubricated Prg4^-/- mouse cartilage shows caspase cascade activation caused by mitochondrial dysregulation, and significantly higher levels of peroxynitrite (ONOO^- and ^-OH) and superoxide (O^-₂) compared to Prg4^+/+ and Prg4^+/- cartilage. Enzymatic activity levels of caspase 8 across Prg4 mutant mice were not significantly different, indicating no extrinsic apoptosis pathway activation. Western blots showed caspase-3 and 9 activation in Prg4^-/- tissue extracts, and the appearance of nitrosylated Cys163 in the active cleft of caspase-3 which inhibits its enzymatic activity. These findings are relevant to patients at risk for arthrosis, from camptodactyl-arthropathy-coxa vara-pericarditis (CACP) syndrome and transient lubricin insufficiency due to trauma and inflammation.

MicroRNA-29b Contributes to Collagens Imbalance in Human Osteoarthritic and Dedifferentiated Articular Chondrocytes

Objective

Decreased expression of collagen type II in favour of collagen type I or X is one hallmark of chondrocyte phenotype changes in osteoarthritic (OA) cartilage. MicroRNA- (miR-) 29b was previously shown to target collagens in several tissues. We studied whether it could contribute to collagen imbalance in chondrocytes with an impaired phenotype.

Methods

After preliminary microarrays screening, miR-29b levels were measured by RT- quantitative PCR in in vitro models of chondrocyte phenotype changes (IL-1β challenge or serial subculturing) and in chondrocytes from OA and non-OA patients. Potential miR-29b targets identified in silico in 3′-UTRs of collagens mRNAs were tested with luciferase reporter assays. The impact of premiR-29b overexpression in ATDC5 cells was studied on collagen mRNA levels and synthesis (Sirius red staining) during chondrogenesis.

Results

MiR-29b level increased significantly in IL-1β-stimulated and weakly in subcultured chondrocytes. A 5.8-fold increase was observed in chondrocytes from OA versus non-OA patients. Reporter assays showed that miR-29b targeted COL2A1 and COL1A2 3′-UTRs although with a variable recovery upon mutation. In ATDC5 cells overexpressing premiR-29b, collagen production was reduced while mRNA levels increased.

Conclusions

By acting probably as a posttranscriptional regulator with a different efficacy on COL2A1 and COL1A2 expression, miR-29b can contribute to the collagens imbalance associated with an abnormal chondrocyte phenotype.

Global chondrocyte gene expression after a single anabolic loading period: Time evolution and re-inducibility of mechano-responses

Aim of this study was a genome-wide identification of mechano-regulated genes and candidate pathways in human chondrocytes subjected to a single anabolic loading episode and characterization of time evolution and re-inducibility of the response. Osteochondral constructs consisting of a chondrocyte-seeded collagen-scaffold connected to β-tricalcium-phosphate were pre-cultured for 35 days and subjected to dynamic compression (25% strain, 1 Hz, 9 × 10 min over 3 hr) before microarray-profiling was performed. Proteoglycan synthesis was determined by ³⁵ S-sulfate-incorporation over 24 hr. Cell viability and hardness of constructs were unaltered by dynamic compression while proteoglycan synthesis was significantly stimulated (1.45-fold, p = 0.016). Among 115 significantly regulated genes, 114 were up-regulated, 48 of them ≥ twofold. AP-1-relevant transcription factors FOSB and FOS strongly increased in line with elevated ERK1/2-phosphorylation and rising MAP3K4 expression. Expression of proteoglycan-synthesizing enzymes CHSY1 and GALNT4 was load-responsive as were factors associated with the MAPK-, TGF-β-, calcium-, retinoic-acid-, Wnt-, and Notch-signaling pathway which were significantly upregulated SOX9, and BMP6 levels rose significantly also after multiple loading episodes at daily intervals even at the 14th cycle with no indication for desensitation. Canonical pSmad2/3 and pSmad1/5/9-signaling showed no consistent regulation. This study associates novel genes with mechanoregulation in chondrocytes, raising SOX9 protein levels with anabolic loading and suggests that more pathways than so far anticipated apparently work together in a complex network of stimulators and feedback-regulators. Upregulation of mechanosensitive indicators extending differentially into the resting time provides crucial knowledge to maximize cartilage matrix deposition for the generation of high-level cartilage replacement tissue.

ESTABLISHING A LIVE CARTILAGE-ON-CARTILAGE INTERFACE FOR TRIBOLOGICAL TESTING

Mechano-biochemical wear encompasses the tribological interplay between biological and mechanical mechanisms responsible for cartilage wear and degradation. The aim of this study was to develop and start validating a novel tribological testing system, which better resembles the natural joint environment through incorporating a live cartilage-on-cartilage articulating interface, joint specific kinematics, and the application of controlled mechanical stimuli for the measurement of biological responses in order to study the mechano-biochemical wear of cartilage. The study entailed two parts. In Part 1, the novel testing rig was used to compare two bearing systems: (a) cartilage articulating against cartilage (CoC) and (b) metal articulating against cartilage (MoC). The clinically relevant MoC, which is also a common tribological interface for evaluating cartilage wear, should produce more wear to agree with clinical observations. In Part II, the novel testing system was used to determine how wear is affected by tissue viability in live and dead CoC articulations. For both parts, bovine cartilage explants were harvested and tribologically tested for three consecutive days. Wear was defined as release of glycosaminoglycans into the media and as evaluation of the tissue structure. For Part I, we found that the live CoC articulation did not cause damage to the cartilage, to the extent of being comparable to the free swelling controls, whereas the MoC articulation caused decreased cell viability, extracellular matrix disruption, and increased wear when compared to CoC, and consistent with clinical data. These results provided confidence that this novel testing system will be adequate to screen new biomaterials for articulation against cartilage, such as in hemiarthroplasty. For Part II, the live and dead cartilage articulation yielded similar wear as determined by the release of proteoglycans and aggrecan fragments, suggesting that keeping the cartilage alive may not be essential for short term wear tests. However, the biosynthesis of glycosaminoglycans was significantly higher due to live CoC articulation than due to the corresponding live free swelling controls, indicating that articulation stimulated cell activity. Moving forward, the cell response to mechanical stimuli and the underlying mechano-biochemical wear mechanisms need to be further studied for a complete picture of tissue degradation.

Interstitial Perfusion Culture with Specific Soluble Factors Inhibits Type I Collagen Production from Human Osteoarthritic Chondrocytes in Clinical-Grade Collagen Sponges

Articular cartilage has poor healing ability and cartilage injuries often evolve to osteoarthritis. Cell-based strategies aiming to engineer cartilaginous tissue through the combination of biocompatible scaffolds and articular chondrocytes represent an alternative to standard surgical techniques. In this context, perfusion bioreactors have been introduced to enhance cellular access to oxygen and nutrients, hence overcoming the limitations of static culture and improving matrix deposition. Here, we combined an optimized cocktail of soluble factors, the BIT (BMP-2, Insulin, Thyroxin), and clinical-grade collagen sponges with a bidirectional perfusion bioreactor, namely the oscillating perfusion bioreactor (OPB), to engineer in vitro articular cartilage by human articular chondrocytes (HACs) obtained from osteoarthritic patients. After amplification, HACs were seeded and cultivated in collagen sponges either in static or dynamic conditions. Chondrocyte phenotype and the nature of the matrix synthesized by HACs were assessed using western blotting and immunohistochemistry analyses. Finally, the stability of the cartilaginous tissue produced by HACs was evaluated in vivo by subcutaneous implantation in nude mice. Our results showed that perfusion improved the distribution and quality of cartilaginous matrix deposited within the sponges, compared to static conditions. Specifically, dynamic culture in the OPB, in combination with the BIT cocktail, resulted in the homogeneous production of extracellular matrix rich in type II collagen. Remarkably, the production of type I collagen, a marker of fibrous tissues, was also inhibited, indicating that the association of the OPB with the BIT cocktail limits fibrocartilage formation, favoring the reconstruction of hyaline cartilage.

Strain-induced mechanotransduction through primary cilia, extracellular ATP, purinergic calcium signaling, and ERK1/2 transactivates CITED2 and downregulates MMP-1 and MMP-13 gene expression in chondrocytes

OBJECTIVE

To determine the strain-induced signaling pathways involved in regulating the transactivation of the transcription regulator Cbp/p300 Interacting Transactivator with ED-rich tail 2 (CITED2) and downstream targets in chondrocytes.

METHODS

Primary human chondrocytes or C28/I2 chondrocytic cells were subjected to various strain regimes. C57BL/6 mice were subjected to treadmill running. Loss-of-function was carried out using siRNA or inhibitors specific for targeted molecules. mRNA levels were assayed by RT-qPCR, and proteins by western blotting, immunofluorescence, and/or immunohistochemical staining. CITED2 promoter activity was assayed in chondrocytes using wild-type or mutant constructs.

RESULTS

Cyclic strain at 5%, 1 Hz induced CITED2 expression and suppressed expression of matrix metalloproteinase (MMP)-1 and -13 at the messenger RNA (mRNA) and protein levels in human chondrocytes. Abolishing primary cilia through knockdown of intraflagellar transport protein (IFT88) attenuated CITED2 gene expression and decreased protein levels. Similar effects were observed with inhibitors of extracellular adenosine triphosphate (ATP) or P2 purinergic receptors, or antagonists of Ca(2+) signaling. Knockdown of IFT88 in articular chondrocytes in vivo diminished treadmill induced-CITED2 expression and upregulated MMPs. Knockdown of hypoxia-inducible factor (HIF)1α, specificity protein 1 (Sp1), or deletion of the shear stress response element (SSRE) in the CITED2 promoter limited cyclic strain-induced transactivation of CITED2. However, the strain induced-transactivation of CITED2 was abolished only on knockdown of HIF1α, Sp1, and SSRE or by loss-of-function of IFT88 or extracellular-signal-regulated kinases (ERK)1/2.

CONCLUSIONS

CITED2 transactivation is a critical event in signaling generated by strain and transduced by primary cilia, extracellular ATP, P2 purinergic receptors, and Ca(2+) signaling. Strain-induced CITED2 transactivation requires HIF1α, Sp1, and an intact SSRE and leads to the downregulation of MMPs such as MMP-1 and MMP-13.

Cell death induced by mechanical compression on engineered muscle results from a gradual physiological mechanism

Deep tissue injury (DTI), a type of pressure ulcer, arises in the muscle layers adjacent to bony prominences due to sustained mechanical loading. DTI presents a serious problem in the clinic, as it is often not visible until reaching an advanced stage. One of the causes can be direct mechanical deformation of the muscle tissue and cell. The mechanism of cell death induced by mechanical compression was studied using bio-artificial skeletal muscle tissues. Compression was applied by placing weights on top of the constructs. The morphological changes of the cytoskeleton and the phosphorylation of mitogen-activated protein kinases (MAPK) under compression were investigated. Moreover, inhibitors for each of the three major MAPK groups, p38, ERK, and JNK, were applied separately to look at their roles in the compression caused apoptosis and necrosis. The present study for the first time showed that direct mechanical compression activates MAPK phosphorylation. Compression also leads to a gradual destruction of the cytoskeleton. The percentage apoptosis is strongly reduced by p38 and JNK inhibitors down to the level of the unloaded group. This phenomenon could be observed up to 24h after initiation of compression. Therefore, cell death in bio-artificial muscle tissue caused by mechanical compression is primarily caused by a physiological mechanism, rather than through a physical mechanism which kills the cell directly. These findings reveal insight of muscle cell death under mechanical compression. Moreover, the result indicates a potential clinical solution to prevent DTI by pre-treating with p38 or/and JNK inhibitors.

Mechanotransduction in primary human osteoarthritic chondrocytes is mediated by metabolism of energy, lipids, and amino acids

Chondrocytes are the sole cell type found in articular cartilage and are repeatedly subjected to mechanical loading in vivo. We hypothesized that physiological dynamic compression results in changes in energy metabolism to produce proteins for maintenance of the pericellular and extracellular matrices. The objective of this study was to develop an in-depth understanding for the short term (<30min) chondrocyte response to sub-injurious, physiological compression by analyzing metabolomic profiles for human chondrocytes harvested from femoral heads of osteoarthritic donors. Cell-seeded agarose constructs were randomly assigned to experimental groups, and dynamic compression was applied for 0, 15, or 30min. Following dynamic compression, metabolites were extracted and detected by HPLC-MS. Untargeted analyzes examined changes in global metabolomics profiles and targeted analysis examined the expression of specific metabolites related to central energy metabolism. We identified hundreds of metabolites that were regulated by applied compression, and we report the detection of 16 molecules not found in existing metabolite databases. We observed patient-specific mechanotransduction with aging dependence. Targeted studies found a transient increase in the ratio of NADP+ to NADPH and an initial decrease in the ratio of GDP to GTP, suggesting a flux of energy into the TCA cycle. By characterizing metabolomics profiles of primary chondrocytes in response to applied dynamic compression, this study provides insight into how OA chondrocytes respond to mechanical load. These results are consistent with increases in glycolytic energy utilization by mechanically induced signaling, and add substantial new data to a complex picture of how chondrocytes transduce mechanical loads.

Runx1 Activities in Superficial Zone Chondrocytes, Osteoarthritic Chondrocyte Clones and Response to Mechanical Loading

Runx1, the hematopoietic lineage determining transcription factor, is present in perichondrium and chondrocytes. Here we addressed Runx1 functions, by examining expression in cartilage during mouse and human osteoarthritis (OA) progression and in response to mechanical loading. Spared and diseased compartments in knees of OA patients and in mice with surgical destabilization of the medial meniscus were examined for changes in expression of Runx1 mRNA (Q-PCR) and protein (immunoblot, immunohistochemistry). Runx1 levels were quantified in response to static mechanical compression of bovine articular cartilage. Runx1 function was assessed by cell proliferation (Ki67, PCNA) and cell type phenotypic markers. Runx1 is enriched in superficial zone (SZ) chondrocytes of normal bovine, mouse, and human tissues. Increasing loading conditions in bovine cartilage revealed a positive correlation with a significant elevation of Runx1. Runx1 becomes highly expressed at the periphery of mouse OA lesions and in human OA chondrocyte 'clones' where Runx1 co-localizes with Vcam1, the mesenchymal stem cell (MSC) marker and lubricin (Prg4), a cartilage chondroprotective protein. These OA induced cells represent a proliferative cell population, Runx1 depletion in MPCs decreases cell growth, supporting Runx1 contribution to cell expansion. The highest Runx1 levels in SZC of normal cartilage suggest a function that supports the unique phenotype of articular chondrocytes, reflected by upregulation under conditions of compression. We propose Runx1 co-expression with Vcam1 and lubricin in murine cell clusters and human 'clones' of OA cartilage, participate in a cooperative mechanism for a compensatory anabolic function.

Mechanotransduction pathways in bone pathobiology

The skeleton is subject to dynamic changes throughout life and bone remodeling is essential for maintenance of bone functionality. The cell populations which predominantly participate in bone and cartilage remodeling, namely osteocytes, osteoblasts, osteoclasts and chondrocytes sense and respond to external mechanical signals and via a series of molecular cascades control bone metabolism and turnover rate. The aforementioned process, known as mechanotransduction, is the underlying mechanism that controls bone homeostasis and function. A wide array of cross-talking signaling pathways has been found to play an important role in the preservation of bone and cartilage tissue health. Moreover, alterations in bone mechanotransduction pathways, due to genetic, hormonal and biomechanical factors, are considered responsible for the pathogenesis of bone and cartilage diseases. Extensive research has been conducted and demonstrated that aberrations in mechanotransduction pathways result in disease-like effects, however only few signaling pathways have actually been engaged in the development of bone disease. The aim of the present review is to present these signaling molecules and cascades that have been found to be mechano-responsive and implicated in bone disease development, as revealed by research in the last five years. In addition, the role of these molecules as prognostic or diagnostic disease markers and their potential as therapeutic targets are also discussed.

Quantitative tracking of passage and 3D culture effects on chondrocyte and fibrochondrocyte gene expression

Success in cartilage and fibrocartilage tissue engineering relies heavily on using an appropriate cell source. Many different cell sources have been identified, including primary and stem cells, along with experimental strategies to obtain the required number of cells or to induce chondrogenesis. However, no definitive method exists to quantitatively evaluate the similarity of the resulting cell phenotypes to those of the native cells between candidate strategies. In this study, we develop an integrative approach to enable such evaluations by deriving, from gene expression profiles, two quantitative metrics representing the nearest location within the range of native cell phenotypes and the deviation from it. As an example application to evaluating potential cell sources for cartilage or meniscus tissue engineering, we examine phenotypic changes of juvenile and adult articular chondrocytes and fibrochondrocytes across multiple passages and subsequent 3D culture. A substantial change was observed in cell phenotype due to the isolation process itself, followed by a clear progression toward the outer meniscal cell phenotype with passage. The new metrics also indicated that 3D culture moderately reduced the passage-induced deviation from the native meniscal phenotypes for juvenile chondrocytes and adult fibrochondrocytes, which was not obvious through examination of individual gene expressions. However, brief 3D culture alone did not move any of the cells towards an inner meniscal phenotype, the most relevant target for meniscal tissue engineering. This integrative approach of examining and combining multiple gene expressions can be used to evaluate various other tissue-engineering strategies to direct cells toward the desired phenotype. Copyright © 2015 John Wiley & Sons, Ltd.

Compression induces Ephrin-A2 in PDL fibroblasts via c-fos

Ephrin-A2-EphA2 and ephrin-B2-EphB4 interactions have been implicated in the regulation of bone remodeling. We previously demonstrated a potential role for members of the Eph-ephrin family of receptor tyrosine kinases for bone remodeling during orthodontic tooth movement: compression-dependent upregulation of ephrin-A2 in fibroblasts of the periodontal ligament (PDL) attenuated osteogenesis in osteoblasts of the alveolar bone. However, factors affecting the regulation of ephrin-A2 expression upon the application of compressive forces remained unclear. Here, we report a mechano-dependent pathway of ephrin-A2 induction in PDL fibroblasts (PDLFs) involving extracellular signal-regulated kinases (ERK) 1/2 and c-fos. PDLF subjected to compressive forces (30.3 g/cm(2)) upregulated c-fos and ephrin-A2 mRNA and protein expression and displayed increased ERK1/2 phosphorylation. Inhibition of the MAP kinase kinase (MEK)/ERK1/2 pathway using the specific MEK inhibitor U0126 significantly reduced ephrin-A2 messenger RNA upregulation upon compression. Silencing of c-fos using a small interfering RNA approach led to a significant inhibition of ephrin-A2 induction upon the application of compressive forces. Interestingly, ephrin-A2 stimulation of PDLF induced c-fos expression and led also to the induction of ephrin-A2 expression. Using a reporter gene construct in murine 3T3 cells, we found that ephrin-A2 was able to stimulate serum response element (SRE)-dependent luciferase activity. As the regulation of c-fos is SRE dependent, ephrin-A2 might induce c-fos via SRE activation. Taken together, we provide evidence for an ERK1/2- and c-fos-dependent regulation of ephrin-A2 in compressed PDLF and suggest a novel pathway for ephrin-A2 induction emanating from ephrin-A2 itself. We showed previously that ephrin-A2 at compression sites might contribute to tooth movement by inhibiting osteogenic differentiation. The regulatory pathway of ephrin-A2 induction during tooth movement identified in this study might be accessible for pharmacological interventions.

Mechanostimulation changes the catabolic phenotype of human dedifferentiated osteoarthritic chondrocytes

PURPOSE

The treatment of cartilage defects with matrix-embedded autologous chondrocytes is a promising method to support the repair process and to foster reconstitution of full functionality of the joint.

METHODS

Human osteoarthritic chondrocytes were harvest from nine different patients (mean ± SD age 68 ± 8 years) who underwent total knee replacement. The chondrocytes were embedded after a precultivation phase into a collagen I hydrogel. Mid-term intermitted mechanostimulation on matrix-embedded dedifferentiated human osteoarthritic chondrocytes was performed by intermittently applying a cyclic sinusoid compression regime for 4 days (cycles of 1 h of sinusoidal stimulation (1 Hz) and 4 h of break; maximum compression 2.5%). Stimulated (Flex) and non-stimulated (No Flex) cell matrix constructs were analysed concerning the expression of genes involved in tissue metabolism, the content of sulphated glycosaminoglycans (sGAG) and the morphology of the chondrocytes.

RESULTS

Gene expression analysis showed a high significant increase in collagen type II expression (p < 0.001), a significant increase in aggrecan expression (p < 0.04) and a high significant decrease in MMP-13 expression (p < 0.001) under stimulation condition compared with unstimulated controls. No significant changes were found in the gene expression rate of MMP-3. This positive effect of the mechanostimulation was confirmed by the analyses of sGAG. Mechanically stimulated cell-matrix constructs had nearly tripled sGAG content than the non-stimulated control (p < 0.002). In addition, histological examination showed that morphology of chondrocytes was altered from a spindle-shaped to a chondrocyte-characteristic rounded phenotype.

CONCLUSION

Mid-term intermitted mechanical stimulation in vitro has the potential to improve the cell quality of cell matrix constructs prepared from dedifferentiated osteoarthritic chondrocytes. This observation may extend the inclusion criteria for matrix-assisted autologous chondrocyte implantation (MACI) and confirms the importance of moderate dynamic compression in clinical rehabilitation after MACI.

Extracellular and intracellular mechanisms of mechanotransduction in three-dimensionally embedded rat chondrocytes

Purpose

Articular cartilage homeostasis involves modulation of chondrocyte matrix synthesis in response to mechanical stress (MS). We studied extracellular and intracellular mechanotransduction pathways mediating this response.

Methods

We first confirmed rapid up-regulation of the putative chondro-protective cytokine, interleukin (IL)-4, as an immediate response to MS. We then studied the role of IL-4 by investigating responses to exogenous IL-4 or a specific IL-4 inhibitor, combined with MS. Next we investigated the intracellular second messengers. Since chondrocyte phenotype alters according to the extracellular environment, we characterized the response to mechanotransduction in 3-dimensionally embedded chondrocytes.

Results

Expression of aggrecan and type II collagen was significantly up-regulated by exogenous IL-4 whereas MS-induced matrix synthesis was inhibited by an IL-4 blocker. Further, MS-induced matrix synthesis was completely blocked by a p38 MAPK inhibitor, while it was only partially blocked by inhibitors of other putative second messengers.

Conclusion

IL-4 mediates an extracellular pathway of mechanotransduction, perhaps via an autocrine/paracrine loop, while p38 mediates an intracellular pathway prevalent only in a 3-dimensional environment.

In search of the pivot point of mechanotransduction: mechanosensing of stem cells

Stem cells are undifferentiated cells with the ability to self-renew and to differentiate into diverse specialized cell types; hence, they have great potential in tissue engineering and cell therapies. In addition to biochemical regulation, the physical properties of the microenvironments, such as scaffold topography, substrate stiffness, and mechanical forces, including fluid shear stress, compression, and tensile strain, can also regulate the proliferation and differentiation of stem cells. Upon physical stimuli, cytoskeleton rearrangements are expected to counterbalance the extracellular mechanical forces, trigger signaling cascades, and eventually cause epigenetic modifications. This article mainly focuses on the mechanosensing, which is the upstream event of stem cell mechanotransduction and the downstream one of physical stimuli. Putative mechanosensors such as ion channels, integrins, and cell membrane as well as primary cilia are discussed. Because mechanical environment is an important stem cell niche, identification of mechanosensors not only can elucidate the mechanisms of mechanotransduction and fate commitments but also bring new prospects of the mechanical control as well as drug development for clinical application.

Type X collagen levels are elevated in serum from human osteoarthritis patients and associated with biomarkers of cartilage degradation and inflammation

Background

Osteoarthritis (OA) is the most common degenerative joint disease, of which the pathogenesis is inadequately understood. Hypertrophy-like changes have been observed as part of the progression of OA. The aim of the study was to develop and characterize a novel biomarker of chondrocytes hypertrophy and investigate how this marker was associated with cartilage degradation and inflammation in patients with various degrees of OA.

Methods

A competitive ELISA, C-Col10, applying a well-characterized monoclonal antibody was developed as a biomarker of chondrocyte hypertrophy through measurement of type X collagen (ColX). The levels of C-Col10, C2M (matrix metalloproteinase-derived fragments of type II collagen) and hsCRP (high sensitive C-reactive protein) were quantified by ELISAs in serum of 271 OA patients stratified by Kellgren-Lawrence (KL) score 0–4. Associations between serum levels of the three biomarkers (log transformed) were analyzed by Pearson’s correlation and differences in C-Col10 levels between patients with high and low levels of inflammation measured by hsCRP were analyzed by ANOVA.

Results

We developed a C-Col10 assay measuring the C-terminus of ColX. We found significantly higher levels of ColX in patients with KL score 2 compared to patients with no radiographic evidence of OA (KL0) (p = 0.04). Levels of ColX were significantly elevated in OA patients with above normal hsCRP levels (p < 0.0001), as well as significantly correlated with levels of C2M (r = 0.55, p < 0.0001), which suggested that chondrocyte hypertrophy was associated with inflammation and cartilage degradation. There was no correlation between C2M and hsCRP. Age and BMI adjustment didn’t change the results. Immuno-staining revealed that ColX was predominately located around the hypertrophic chondrocytes and the clustered chondrocytes indicating that C-Col10 measures may be linked to cartilage hypertrophic changes.

Conclusions

We developed a novel assay, C-Col10, for measurement of chondrocyte hypertrophy and found its levels significantly elevated in OA patients with KL score of 2, and also in OA patients with above normal hsCRP levels. Concentration of C-Col10 strongly correlated with levels of C2M, a marker of cartilage destruction. The data suggest that chondrocyte hypertrophy and subsequent collagen X fragmentation seem to be increased in a subset of patients with inflammatory OA.

Electronic supplementary material

The online version of this article (doi:10.1186/1471-2474-15-309) contains supplementary material, which is available to authorized users.

Connexin43 hemichannels mediate small molecule exchange between chondrocytes and matrix in biomechanically-stimulated temporomandibular joint cartilage

OBJECTIVE

Connexin (Cx) 43 hemichannels play a role in mechanotransduction. This study was undertaken in order to determine if Cx43 hemichannels were activated in rat temporomandibular joint (TMJ) chondrocytes under mechanical stimulation.

METHODS

Sprague-Dawley rats were stimulated dental-mechanically. Cx43 expression in rat TMJ cartilage was determined with immunohistochemistry and real-time PCR, and Cx43 hemichannel opening was evaluated by the extra- and intracellular levels of prostaglandin E2 (PGE2). Both primary rat chondrocytes and ATDC5 cells were treated with fluid flow shear stress (FFSS) to induce hemichannel opening. The Cx43 expression level was then determined by real-time PCR or Western blotting, and the extent of Cx43 hemichannel opening was evaluated by measuring both PGE2 release and cellular dye uptake.

RESULTS

Cx43 expression and intra- and extracellular PGE2 levels were increased in mechanically-stimulated rat TMJ cartilage compared to the unstimulated control. The FFSS treatment increased Cx43 expression and induced Cx43 hemichannel opening in primary rat chondrocytes and ATDC5 cells indicated by enhanced PGE2 release and dye uptake. Furthermore, the Cx43 hemichannel opening could be blocked by the addition of 18β-glycyrrhetinic acid, a Cx channel inhibitor, Cx43-targeting siRNA, or by withdrawal of FFSS stimulation. The migration of cytosolic Cx43 protein to the plasma membrane in ATDC5 cells was still significant after 8 h post 2-h FFSS treatment, and the Cx43 protein level was still high at 48 h, which returned to control levels at 72 h after treatment.

CONCLUSION

Cx43 hemichannels are activated and mediate small molecule exchange between TMJ chondrocytes and matrix under mechanical stimulation.

Off-loading of cyclic hydrostatic pressure promotes production of extracellular matrix by chondrocytes

The addition of cyclic hydrostatic pressure (cHP) to cell culture medium has been used to promote extracellular matrix (ECM) production by articular chondrocytes. Though a combination of cHP followed by atmospheric pressure (AP) has been examined previously, the rationale of such a combination was unclear. We compared the effects of loading once versus twice (combinations of cHP followed by AP) regarding both gene expression and biochemical and histological phenotypes of chondrocytes. Isolated bovine articular chondrocytes were embedded in a collagen gel and incubated for 14 days under conditions combining cHP and AP. The gene expression of aggrecan core protein and collagen type II were upregulated in response to cHP, and those levels were maintained for at least 4 days after cHP treatment. Accumulation of cartilage-specific sulfated glycosaminoglycans following cHP for 7 days and subsequent AP for 7 days was significantly greater than that of the AP control (p < 0.05). Therefore, incubation at AP after loading with cHP was found to beneficially affect ECM accumulation. Manipulating algorithms of cHP combined with AP will be useful in producing autologous chondrocyte-based cell constructs for implantation.

Homeostatic mechanisms in articular cartilage and role of inflammation in osteoarthritis

Osteoarthritis (OA) is a whole joint disease, in which thinning and disappearance of cartilage is a critical determinant in OA progression. The rupture of cartilage homeostasis whatever its cause (aging, genetic predisposition, trauma or metabolic disorder) induces profound phenotypic modifications of chondrocytes, which then promote the synthesis of a subset of factors that induce cartilage damage and target other joint tissues. Interestingly, among these factors are numerous components of the inflammatory pathways. Chondrocytes produce cytokines, chemokines, alarmins, prostanoids, and adipokines and express numerous cell surface receptors for cytokines and chemokines, as well as Toll-like receptors. These receptors activate intracellular signaling pathways involved in inflammatory and stress responses of chondrocytes in OA joints. This review focuses on mechanisms responsible for the maintenance of cartilage homeostasis and highlights the role of inflammatory processes in OA progression.

Regulation of chondrocyte proliferation through GIT1-Rac1-mediated ERK1/2 pathway by PDGF

There are many growth factors contributing to fracture healing after bone fractures. Platelet-derived growth factor (PDGF) released from platelets is a factor promoting cell division and proliferation, and first appears around the sites of fractures. Culture of chondrocytes in vitro are stimulated by PDGF to proliferation, its presence being upregulated in the extracellular matrix of cartilage; the main components include aggrecan and type II collagen. PDGF induces the expression of G the protein-coupled receptor kinase interacting protein 1 (GIT1), promoting Rac1 and ERK1/2 phosphorylation. Both knocking down GIT1 expression by siRNA and blocking phosphorylation of Rac1 inhibit this induced proliferation of chondrocyte. GIT1 and Rac1 control each other, having a synergistic effect on activation of the ERK1/2 pathway. The results suggest that PDGF regulates chondrocyte proliferation through activation of ERK1/2 pathway by upregulation of GIT1 expression and Rac1 phosphorylation.

Moderate dynamic compression inhibits pro-catabolic response of cartilage to mechanical injury, tumor necrosis factor-α and interleukin-6, but accentuates degradation above a strain threshold

OBJECTIVE

Traumatic joint injury can initiate early cartilage degeneration in the presence of elevated inflammatory cytokines (e.g., tumor necrosis factor (TNF)-α and interleukin (IL)-6). The positive/negative effects of post-injury dynamic loading on cartilage degradation and repair in vivo are not well-understood. This study examined the effects of dynamic strain on immature bovine cartilage in vitro challenged with TNF-α + IL-6 and its soluble receptor (sIL-6R) with/without initial mechanical injury.

METHODS

Groups of mechanically injured or non-injured explants were cultured in TNF-α + IL-6/sIL-6R for 8 days. Intermittent dynamic compression was applied concurrently at 10%, 20%, or 30% strain amplitude. Outcome measures included sulfated glycosaminoglycan (sGAG) loss (dimethylmethylene blue (DMMB)), aggrecan biosynthesis ((35)S-incorporation), aggrecanase activity (Western blot), chondrocyte viability (fluorescence staining) and apoptosis (nuclear blebbing via light microscopy), and gene expression (qPCR).

RESULTS

In bovine explants, cytokine alone and injury-plus-cytokine treatments markedly increased sGAG loss and aggrecanase activity, and induced chondrocyte apoptosis. These effects were abolished by moderate 10% and 20% strains. However, 30% strain amplitude greatly increased apoptosis and had no inhibitory effect on aggrecanase activity. TNF + IL-6/sIL-6R downregulated matrix gene expression and upregulated expression of inflammatory genes, effects that were rescued by moderate dynamic strains but not by 30% strain.

CONCLUSIONS

Moderate dynamic compression inhibits the pro-catabolic response of cartilage to mechanical injury and cytokine challenge, but there is a threshold strain amplitude above which loading becomes detrimental to cartilage. Our findings support the concept of appropriate loading for post-injury rehabilitation.

The mechanical microenvironment of high concentration agarose for applying deformation to primary chondrocytes

Cartilage and chondrocytes experience loading that causes alterations in chondrocyte biological activity. In vivo chondrocytes are surrounded by a pericellular matrix with a stiffness of ~25-200kPa. Understanding the mechanical loading environment of the chondrocyte is of substantial interest for understanding chondrocyte mechanotransduction. The first objective of this study was to analyze the spatial variability of applied mechanical deformations in physiologically stiff agarose on cellular and sub-cellular length scales. Fluorescent microspheres were embedded in physiologically stiff agarose hydrogels. Microsphere positions were measured via confocal microscopy and used to calculate displacement and strain fields as a function of spatial position. The second objective was to assess the feasibility of encapsulating primary human chondrocytes in physiologically stiff agarose. The third objective was to determine if primary human chondrocytes could deform in high-stiffness agarose gels. Primary human chondrocyte viability was assessed using live-dead imaging following 24 and 72h in tissue culture. Chondrocyte shape was measured before and after application of 10% compression. These data indicate that (1) displacement and strain precision are ~1% and 6.5% respectively, (2) high-stiffness agarose gels can maintain primary human chondrocyte viability of >95%, and (3) compression of chondrocytes in 4.5% agarose can induce shape changes indicative of cellular compression. Overall, these results demonstrate the feasibility of using high-concentration agarose for applying in vitro compression to chondrocytes as a model for understanding how chondrocytes respond to in vivo loading.