Mechanical load inhibits IL-1 induced matrix degradation in articular cartilage

Hippo-PKCζ-NFκB signaling axis: A druggable modulator of chondrocyte responses to mechanical stress

Recent studies have implicated a crucial role of Hippo signaling in cell fate determination by biomechanical signals. Here we show that mechanical loading triggers the activation of a Hippo-PKCζ-NFκB pathway in chondrocytes, resulting in the expression of NFκB target genes associated with inflammation and matrix degradation. Mechanistically, mechanical loading activates an atypical PKC, PKCζ, which phosphorylates NFκB p65 at Serine 536, stimulating its transcriptional activation. This mechanosensitive activation of PKCζ and NFκB p65 is impeded in cells with gene deletion or chemical inhibition of Hippo core kinases LATS1/2, signifying an essential role of Hippo signaling in this mechanotransduction. A PKC inhibitor AEB-071 or PKCζ knockdown prevents p65 Serine 536 phosphorylation. Our study uncovers that the interplay of the Hippo signaling, PKCζ, and NFκB in response to mechanical loading serves as a therapeutic target for knee osteoarthritis and other conditions resulting from mechanical overloading or Hippo signaling deficiencies.

Charge-Reversed Exosomes for Targeted Gene Delivery to Cartilage for Osteoarthritis Treatment

Gene therapy has the potential to facilitate targeted expression of therapeutic proteins to promote cartilage regeneration in osteoarthritis (OA). The dense, avascular, aggrecan-glycosaminoglycan (GAG) rich negatively charged cartilage, however, hinders their transport to reach chondrocytes in effective doses. While viral vector mediated gene delivery has shown promise, concerns over immunogenicity and tumorigenic side-effects persist. To address these issues, this study develops surface-modified cartilage-targeting exosomes as non-viral carriers for gene therapy. Charge-reversed cationic exosomes are engineered for mRNA delivery by anchoring cartilage targeting optimally charged arginine-rich cationic motifs into the anionic exosome bilayer by using buffer pH as a charge-reversal switch. Cationic exosomes penetrated through the full-thickness of early-stage arthritic human cartilage owing to weak-reversible ionic binding with GAGs and efficiently delivered the encapsulated eGFP mRNA to chondrocytes residing in tissue deep layers, while unmodified anionic exosomes do not. When intra-articularly injected into destabilized medial meniscus mice knees with early-stage OA, mRNA loaded charge-reversed exosomes overcame joint clearance and rapidly penetrated into cartilage, creating an intra-tissue depot and efficiently expressing eGFP; native exosomes remained unsuccessful. Cationic exosomes thus hold strong translational potential as a platform technology for cartilage-targeted non-viral delivery of any relevant mRNA targets for OA treatment.

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.

The p65-LOC727924-miR-26a/KPNA3-p65 regulatory loop mediates vasoactive intestinal peptide effects on osteoarthritis chondrocytes

Loss and dysfunction of articular chondrocytes, which disrupt the homeostasis of extracellular matrix formation and breakdown, promote the onset of osteoarthritis (OA). Targeting inflammatory pathways is an important therapeutic strategy for OA. Vasoactive intestinal peptide (VIP) is an immunosuppressive neuropeptide with potent anti-inflammatory effects; however, its role and mechanism in OA remain unclear. In this study, microarray expression profiling from the Gene Expression Omnibus database and integrative bioinformatics analyses were performed to identify differentially expressed lncRNAs in OA samples. qRT-PCR validation of the top ten different expressed lncRNAs indicated that the expression level of intergenic non-protein coding RNA 2203 (LINC02203, also named LOC727924) was the highest in OA cartilage compared to normal cartilage. Hence, the LOC727924 function was further investigated. LOC727924 was upregulated in OA chondrocytes, with a dominant sub-localization in the cytoplasm. In OA chondrocytes, LOC727924 knockdown boosted cell viability, suppressed cell apoptosis, reactive oxygen species (ROS) accumulation, increased aggrecan and collagen II, decreased matrix metallopeptidase (MMP)-3/13 and ADAM metallopeptidase with thrombospondin type 1 motif (ADAMTS)-4/5 levels, and reduced the levels of tumor necrosis factor alpha (TNF-α), interleukin 1 beta (IL-1β), and interleukin 6 (IL-6). LOC727924 could interact with the microRNA 26a (miR-26a)/ karyopherin subunit alpha 3 (KPNA3) axis by competitively targeting miR-26a for KPNA3 binding, therefore down-regulating miR-26a and upregulating KPNA3; in OA chondrocytes, miR-26a inhibition partially abolished LOC727924 knockdown effects on chondrocytes. miR-26a inhibited the nuclear translocation of p65 through targeting KPNA3 and p65 transcriptionally activated LOC727924, forming a p65-LOC727924-miR-26a/KPNA3-p65 regulatory loop to modulate OA chondrocyte phenotypes. In vitro, VIP improved OA chondrocyte proliferation and functions, down-regulated LOC727924, KPNA3, and p65 expression, and upregulated miR-26a expression; in vivo, VIP ameliorated destabilization of the medial meniscus (DMM)-induced damages on the mouse knee joint, down-regulated KPNA3, inhibited the nuclear translocation of p65. In conclusion, the p65-LOC727924-miR-26a/KPNA3-p65 regulatory loop modulates OA chondrocyte apoptosis, ROS accumulation, extracellular matrix (ECM) deposition, and inflammatory response in vitro and OA development in vivo, being one of the mechanisms mediating VIP ameliorating OA.

Hippo Signaling Modulates the Inflammatory Response of Chondrocytes to Mechanical Compressive Loading

Knee osteoarthritis (KOA) is a degenerative disease resulting from mechanical overload, where direct physical impacts on chondrocytes play a crucial role in disease development by inducing inflammation and extracellular matrix degradation. However, the signaling cascades that sense these physical impacts and induce the pathogenic transcriptional programs of KOA remain to be defined, which hinders the identification of novel therapeutic approaches. Recent studies have implicated a crucial role of Hippo signaling in osteoarthritis. Since Hippo signaling senses mechanical cues, we aimed to determine its role in chondrocyte responses to mechanical overload. Here we show that mechanical loading induces the expression of inflammatory and matrix-degrading genes by activating the nuclear factor-kappaB (NFκB) pathway in a Hippo-dependent manner. Applying mechanical compressional force to 3-dimensional cultured chondrocytes activated NFκB and induced the expression of NFκB target genes for inflammation and matrix degradation (i.e., IL1β and ADAMTS4). Interestingly, deleting the Hippo pathway effector YAP or activating YAP by deleting core Hippo kinases LATS1/2 blocked the NFκB pathway activation induced by mechanical loading. Consistently, treatment with a LATS1/2 kinase inhibitor abolished the upregulation of IL1β and ADAMTS4 caused by mechanical loading. Mechanistically, mechanical loading activates Protein Kinase C (PKC), which activates NFκB p65 by phosphorylating its Serine 536. Furthermore, the mechano-activation of both PKC and NFκB p65 is blocked in LATS1/2 or YAP knockout cells, indicating that the Hippo pathway is required by this mechanoregulation. Additionally, the mechanical loading-induced phosphorylation of NFκB p65 at Ser536 is blocked by the LATS1/2 inhibitor Lats-In-1 or the PKC inhibitor AEB-071. Our study suggests that the interplay of the Hippo signaling and PKC controls NFκB-mediated inflammation and matrix degradation in response to mechanical loading. Chemical inhibitors targeting Hippo signaling or PKC can prevent the mechanoresponses of chondrocytes associated with inflammation and matrix degradation, providing a novel therapeutic strategy for KOA.

Injury-related cell death and proteoglycan loss in articular cartilage: Numerical model combining necrosis, reactive oxygen species, and inflammatory cytokines

Osteoarthritis (OA) is a common musculoskeletal disease that leads to deterioration of articular cartilage, joint pain, and decreased quality of life. When OA develops after a joint injury, it is designated as post-traumatic OA (PTOA). The etiology of PTOA remains poorly understood, but it is known that proteoglycan (PG) loss, cell dysfunction, and cell death in cartilage are among the first signs of the disease. These processes, influenced by biomechanical and inflammatory stimuli, disturb the normal cell-regulated balance between tissue synthesis and degeneration. Previous computational mechanobiological models have not explicitly incorporated the cell-mediated degradation mechanisms triggered by an injury that eventually can lead to tissue-level compositional changes. Here, we developed a 2-D mechanobiological finite element model to predict necrosis, apoptosis following excessive production of reactive oxygen species (ROS), and inflammatory cytokine (interleukin-1)-driven apoptosis in cartilage explant. The resulting PG loss over 30 days was simulated. Biomechanically triggered PG degeneration, associated with cell necrosis, excessive ROS production, and cell apoptosis, was predicted to be localized near a lesion, while interleukin-1 diffusion-driven PG degeneration was manifested more globally. Interestingly, the model also showed proteolytic activity and PG biosynthesis closer to the levels of healthy tissue when pro-inflammatory cytokines were rapidly inhibited or cleared from the culture medium, leading to partial recovery of PG content. The numerical predictions of cell death and PG loss were supported by previous experimental findings. Furthermore, the simulated ROS and inflammation mechanisms had longer-lasting effects (over 3 days) on the PG content than localized necrosis. The mechanobiological model presented here may serve as a numerical tool for assessing early cartilage degeneration mechanisms and the efficacy of interventions to mitigate PTOA progression.

Homo sapiens May Incorporate Daily Acute Cycles of “Conditioning–Deconditioning” to Maintain Musculoskeletal Integrity: Need to Integrate with Biological Clocks and Circadian Rhythm Mediators

Human evolution required adaptation to the boundary conditions of Earth, including 1 g gravity. The bipedal mobility of Homo sapiens in that gravitational field causes ground reaction force (GRF) loading of their lower extremities, influencing the integrity of the tissues of those extremities. However, humans usually experience such loading during the day and then a period of relative unloading at night. Many studies have indicated that loading of tissues and cells of the musculoskeletal (MSK) system can inhibit their responses to biological mediators such as cytokines and growth factors. Such findings raise the possibility that humans use such cycles of acute conditioning and deconditioning of the cells and tissues of the MSK system to elaborate critical mediators and responsiveness in parallel with these cycles, particularly involving GRF loading. However, humans also experience circadian rhythms with the levels of a number of mediators influenced by day/night cycles, as well as various levels of biological clocks. Thus, if responsiveness to MSK-generated mediators also occurs during the unloaded part of the daily cycle, that response must be integrated with circadian variations as well. Furthermore, it is also possible that responsiveness to circadian rhythm mediators may be regulated by MSK tissue loading. This review will examine evidence for the above scenario and postulate how interactions could be both regulated and studied, and how extension of the acute cycles biased towards deconditioning could lead to loss of tissue integrity.

Delivering Mechanical Stimulation to Cells: State of the Art in Materials and Devices Design

Biochemical signals, such as growth factors, cytokines, and transcription factors are known to play a crucial role in regulating a variety of cellular activities as well as maintaining the normal function of different tissues and organs. If the biochemical signals are assumed to be one side of the coin, the other side comprises biophysical cues. There is growing evidence showing that biophysical signals, and in particular mechanical cues, also play an important role in different stages of human life ranging from morphogenesis during embryonic development to maturation and maintenance of tissue and organ function throughout life. In order to investigate how mechanical signals influence cell and tissue function, tremendous efforts have been devoted to fabricating various materials and devices for delivering mechanical stimuli to cells and tissues. Here, an overview of the current state of the art in the design and development of such materials and devices is provided, with a focus on their design principles, and challenges and perspectives for future research directions are highlighted.

Experimental-analytical approach to assessing mechanosensitive cartilage blood marker kinetics in healthy adults: dose-response relationship and interrelationship of nine candidate markers

Purpose: To determine the suitability of selected blood biomarkers of articular cartilage as mechanosensitive markers and to investigate the dose-response relationship between ambulatory load magnitude and marker kinetics in response to load. 

Methods: Serum samples were collected from 24 healthy volunteers before and at three time points after a 30-minute walking stress test performed on three test days. In each experimental session, one of three ambulatory loads was applied: 100% body weight (BW); 80%BW; 120%BW. Serum concentrations of COMP, MMP-3, MMP-9, ADAMTS-4, PRG-4, CPII, C2C and IL-6 were assessed using commercial enzyme-linked immunosorbent assays. A two-stage analytical approach was used to determine the suitability of a biomarker by testing the response to the stress test (criterion I) and the dose-response relationship between ambulatory load magnitude and biomarker kinetics (criterion II). 

Results. COMP, MMP-3 and IL-6 at all three time points after, MMP-9 at 30 and 60 minutes after, and ADAMTS-4 and CPII at immediately after the stress test showed an average response to load or an inter-individual variation in response to load of up to 25% of pre-test levels. The relation to load magnitude on average or an inter-individual variation in this relationship was up to 8% from load level to load level. There was a positive correlation for the slopes of the change-load relationship between COMP and MMP-3, and a negative correlation for the slopes between COMP, MMP-3 and IL-6 with MMP-9, and COMP with IL6. 

Conclusions: COMP, MMP-3, IL-6, MMP-9, and ADAMTS-4 warrant further investigation in the context of articular cartilage mechanosensitivity and its role in joint degeneration and OA. While COMP seems to be able to reflect a rapid response, MMP-3 seems to reflect a slightly longer lasting, but probably also more distinct response. MMP-3 showed also the strongest association with the magnitude of load.

State of Knowledge on Molecular Adaptations to Exercise in Humans: Historical Perspectives and Future Directions

For centuries, regular exercise has been acknowledged as a potent stimulus to promote, maintain, and restore healthy functioning of nearly every physiological system of the human body. With advancing understanding of the complexity of human physiology, continually evolving methodological possibilities, and an increasingly dire public health situation, the study of exercise as a preventative or therapeutic treatment has never been more interdisciplinary, or more impactful. During the early stages of the NIH Common Fund Molecular Transducers of Physical Activity Consortium (MoTrPAC) Initiative, the field is well-positioned to build substantially upon the existing understanding of the mechanisms underlying benefits associated with exercise. Thus, we present a comprehensive body of the knowledge detailing the current literature basis surrounding the molecular adaptations to exercise in humans to provide a view of the state of the field at this critical juncture, as well as a resource for scientists bringing external expertise to the field of exercise physiology. In reviewing current literature related to molecular and cellular processes underlying exercise-induced benefits and adaptations, we also draw attention to existing knowledge gaps warranting continued research effort. © 2021 American Physiological Society. Compr Physiol 12:3193-3279, 2022.

The Relationship Between Proteoglycan Loss, Overloading-Induced Collagen Damage, and Cyclic Loading in Articular Cartilage

OBJECTIVE

The interaction between proteoglycan loss and collagen damage in articular cartilage and the effect of mechanical loading on this interaction remain unknown. The aim of this study was to answer the following questions: (1) Is proteoglycan loss dependent on the amount of collagen damage and does it depend on whether this collagen damage is superficial or internal? (2) Does repeated loading further increase the already enhanced proteoglycan loss in cartilage with collagen damage?

DESIGN

Fifty-six bovine osteochondral plugs were equilibrated in phosphate-buffered saline for 24 hours, mechanically tested in compression for 8 hours, and kept in phosphate-buffered saline for another 48 hours. The mechanical tests included an overloading step to induce collagen damage, creep steps to determine tissue stiffness, and cyclic loading to induce convection. Proteoglycan release was measured before and after mechanical loading, as well as 48 hours post-loading. Collagen damage was scored histologically.

RESULTS

Histology revealed different collagen damage grades after the application of mechanical overloading. After 48 hours in phosphate-buffered saline postloading, proteoglycan loss increased linearly with the amount of total collagen damage and was dependent on the presence but not the amount of internal collagen damage. In samples without collagen damage, repeated loading also resulted in increased proteoglycan loss. However, repeated loading did not further enhance the proteoglycan loss induced by damaged collagen.

CONCLUSION

Proteoglycan loss is enhanced by collagen damage and it depends on the presence of internal collagen damage. Cyclic loading stimulates proteoglycan loss in healthy cartilage but does not lead to additional loss in cartilage with damaged collagen.

Anti-inflammatory effects of tibial axial loading on knee articular cartilage post traumatic injury

Early therapeutic intervention to mitigate inflammatory responses following joint injury may offer a potential strategy to prevent post-traumatic osteoarthritis (PTOA). In-vitro studies have demonstrated uniaxial dynamic compression mitigates the catabolic and apoptotic responses of articular cartilage (AC) in response to mechanical injury. The objectives of this study were (1) to develop a custom device that can apply dynamic tibial axial loading (TAL) to knee AC by mimicking therapeutic, in-vitro loading conditions and (2) to investigate the potential of TAL to reduce the inflammatory response of AC post traumatic acute joint injury using an ex-vivo porcine model. A TAL device was fabricated to apply dynamic compressive loading to knee AC by combining tibial axial compressive loading with continuous passive motion. Computational analyses demonstrated that the loading condition applied to the knee by the TAL device closely simulate uniaxial dynamic compression reported in previous in-vitro studies. Following single impact injury, injured porcine knees were subjected to TAL with a magnitude of 1/4 body weight at a frequency of 1 Hz for 30 min. AC samples were harvested 8 h post injury for analysis of pro-inflammatory cytokine expression (IL-1β and TNF-α). Expression of both cytokines was upregulated following injury; however, the change was notably mitigated in the specimens subjected to TAL. Thus, TAL may be an effective and potentially, practical-to-administer early intervention strategy to mitigate rapidly occurring detrimental events following acute AC injury, potentially slowing down progression to PTOA.

Development of a Porcine Model to Assess the Effect of In-Situ Knee Joint Loading On Site-Specific Cartilage Gene Expression

Cyclic mechanical loading of cartilage induces stresses and fluid flow which are thought to modulate chondrocyte metabolism. The uneven surface, plus the heterogeneity of cartilage within a joint, makes stress and fluid pressure distribution in the tissue non-uniform, and gene expression may vary at different sites as a function of load magnitude, frequency and time. In previous studies, cartilage explants were used for loading tests to investigate biological responses of the cartilage to mechanical loading. In contrast, we used loading tests on intact knee joints, to better reflect the loading conditions in a joint, and thus provide a more physiologically relevant mechanical environment. Gene expression levels in loaded samples for a selection of relevant genes were compared with those of the corresponding unloaded control samples to characterize potential differences. Furthermore, the effect of load magnitude and duration on gene expression levels were investigated. We observed differences in gene expression levels between samples from different sites in the same joint and between corresponding samples from the same site in loaded and unloaded joints. Consistent with previous findings, our results indicate that there is a critical upper and lower threshold of loading for triggering the expression of certain genes. Variations in gene expression levels may reflect the effect of local loading, topography and structure of the cartilage in an intact joint on the metabolic activity of the associated cells.

Experimental-analytical approach to assessing mechanosensitive cartilage blood marker kinetics in healthy adults: dose-response relationship and interrelationship of nine candidate markers

Purpose: To determine the suitability of selected blood biomarkers of articular cartilage as mechanosensitive markers and to investigate the dose-response relationship between ambulatory load magnitude and marker kinetics in response to load.  Methods: Serum samples were collected from 24 healthy volunteers before and at three time points after a 30-minute walking stress test performed on three test days. In each experimental session, one of three ambulatory loads was applied: 100% body weight (BW); 80%BW; 120%BW. Serum concentrations of COMP, MMP-3, MMP-9, ADAMTS-4, PRG-4, CPII, C2C and IL-6 were assessed using commercial enzyme-linked immunosorbent assays. A two-stage analytical approach was used to determine the suitability of a biomarker by testing the response to the stress test (criterion I) and the dose-response relationship between ambulatory load magnitude and biomarker kinetics (criterion II).  Results. COMP, MMP-3 and IL-6 at all three time points after, MMP-9 at 30 and 60 minutes after, and ADAMTS-4 and CPII at immediately after the stress test showed an average response to load or an inter-individual variation in response to load of up to 25% of pre-test levels. The relation to load magnitude on average or an inter-individual variation in this relationship was up to 8% from load level to load level. There was a positive correlation for the slopes of the change-load relationship between COMP and MMP-3, and a negative correlation for the slopes between COMP, MMP-3 and IL-6 with MMP-9, and COMP with IL6.  Conclusions: COMP, MMP-3, IL-6, MMP-9, and ADAMTS-4 warrant further investigation in the context of articular cartilage mechanosensitivity and its role in joint degeneration and OA. While COMP seems to be able to reflect a rapid response, MMP-3 seems to reflect a slightly longer lasting, but probably also more distinct response. MMP-3 showed also the strongest association with the magnitude of load.

Compressive mechanical stress enhances susceptibility to interleukin-1 by increasing interleukin-1 receptor expression in 3D-cultured ATDC5 cells

Background

Mechanical overload applied on the articular cartilage may play an important role in the pathogenesis of osteoarthritis. However, the mechanism of chondrocyte mechanotransduction is not fully understood. The purpose of this study was to assess the effects of compressive mechanical stress on interleukin-1 receptor (IL-1R) and matrix-degrading enzyme expression by three-dimensional (3D) cultured ATDC5 cells. In addition, the implications of transient receptor potential vanilloid 4 (TRPV4) channel regulation in promoting effects of compressive mechanical loading were elucidated.

Methods

ATDC5 cells were cultured in alginate beads with the growth medium containing insulin-transferrin-selenium and BMP-2 for 6 days. The cultured cell pellet was seeded in collagen scaffolds to produce 3D-cultured constructs. Cyclic compressive loading was applied on the 3D-cultured constructs at 0.5 Hz for 3 h. The mRNA expressions of a disintegrin and metalloproteinases with thrombospondin motifs 4 (ADAMTS4) and IL-1R were determined with or without compressive loading, and effects of TRPV4 agonist/antagonist on mRNA expressions were examined. Immunoreactivities of reactive oxygen species (ROS), TRPV4 and IL-1R were assessed in 3D-cultured ATDC5 cells.

Results

In 3D-cultured ATDC5 cells, ROS was induced by cyclic compressive loading stress. The mRNA expression levels of ADAMTS4 and IL-1R were increased by cyclic compressive loading, which was mostly prevented by pyrollidine dithiocarbamate. Small amounts of IL-1β upregulated ADAMTS4 and IL-1R mRNA expressions only when combined with compressive loading. TRPV4 agonist suppressed ADAMTS4 and IL-1R mRNA levels induced by the compressive loading, whereas TRPV4 antagonist enhanced these levels. Immunoreactivities to TRPV4 and IL-1R significantly increased in constructs with cyclic compressive loading.

Conclusion

Cyclic compressive loading induced mRNA expressions of ADAMTS4 and IL-1R through reactive oxygen species. TRPV4 regulated these mRNA expressions, but excessive compressive loading may impair TRPV4 regulation. These findings suggested that TRPV4 regulates the expression level of IL-1R and subsequent IL-1 signaling induced by cyclic compressive loading and participates in cartilage homeostasis.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12891-021-04095-x.

Methylprednisolone acetate mitigates IL1β induced changes in matrix metalloproteinase gene expression in skeletally immature ovine explant knee tissues

OBJECTIVE AND DESIGN

This study aimed at evaluating the effect of methylprednisolone (MPA) on messenger ribonucleic acid (mRNA) expression levels in immature ovine knee joint tissue explants following interleukin (IL)1β induction and to assess responsiveness of the explants.

MATERIAL OR SUBJECTS

Explants were harvested from the articular cartilage, synovium, and infrapatellar fat pad (IPFP) from immature female sheep.

TREATMENT

Methylprednisolone.

METHODS

The samples were allocated into six groups: (1) control, (2) MPA (10^-3 M), (3) MPA (10^-4 M), (4) IL1β, (5) IL1β + 10^-3 M MPA, or (6) IL1β + 10^-4 M MPA. mRNA expression levels for molecules relevant to inflammation, cartilage degradation/anabolism, activation of innate immunity, and adipose tissue/hormones were quantified. Fold changes with MPA treatment were compared via the comparative C_T method.

RESULTS

Methylprednisolone treatment significantly suppressed MMPs consistently across the cartilage (MMP1, MMP3, and MMP13), synovium (MMP1 and MMP3), and IPFP (MMP13) (all p < 0.05). Other genes that were less consistently suppressed include endogenous IL1β (cartilage) and IL6 (IPFP) (all p < 0.05), and others not affected either by IL-1 exposure or subsequent MPA include TGFβ1, TLR4, and adipose-related molecules.

CONCLUSIONS

Methylprednisolone significantly mitigated IL1β induced mRNA expression for MMPs in the immature cartilage, synovium, and IPFP, but the extent of the responsiveness was tissue-, location-, and gene-specific.

Mesenchymal stem cell therapy combined with arthroscopic abrasion arthroplasty regenerates cartilage in patients with severe knee osteoarthritis: a case series

Aim: To evaluate the safety and efficacy of adipose-derived mesenchymal stem cell (ADMSC) therapy in combination with arthroscopic abrasion arthroplasty (AAA) in advanced knee osteoarthritis (OA). Materials & methods: 27 patients with Grade IV OA of the knee underwent AAA and ADMSC therapy (50 × 10⁶ ADMSCs at baseline and 6 months). Clinical outcome was assessed over 36 months. Structural change was determined using MRI. Results: Treatment was well tolerated with no serious adverse events. Clinically significant improvements in pain and function were observed. Reproducible hyaline-like cartilage regeneration was seen in all participants. Conclusion: ADMSC therapy combined with AAA in Grade IV OA results in reproducible pain, functional and structural improvements. This represents a joint preservation technique for patients with advanced OA of the knee. Trial registration number: ACTRN12617000638336.

Ex vivo physiological compression of human osteoarthritis cartilage modulates cellular and matrix components

Mechanical stimulation appears to play a key role in cartilage homeostasis maintenance, but it can also contribute to osteoarthritis (OA) pathogenesis. Accumulating evidence suggests that cartilage loading in the physiological range contributes to tissue integrity maintenance, whereas excessive or reduced loading have catabolic effects. However, how mechanical stimuli can regulate joint homeostasis is still not completely elucidated and few data are available on human cartilage. We aimed at investigating human OA cartilage response to ex vivo loading at physiological intensity. Cartilage explants from ten OA patients were subjected to ex vivo controlled compression, then recovered and used for gene and protein expression analysis of cartilage homeostasis markers. Compressed samples were compared to uncompressed ones in presence or without interleukin 1β (IL-1β) or interleukin 4 (IL-4). Cartilage explants compressed in combination with IL-4 treatment showed the best histological scores. Mechanical stimulation was able to significantly modify the expression of collagen type II (collagen 2), aggrecan, SOX9 transcription factor, cartilage oligomeric matrix protein (COMP), collagen degradation marker C2C and vascular endothelial growth factor (VEGF). Conversely, ADAMTS4 metallopeptidase, interleukin 4 receptor alpha (IL4Rα), chondroitin sulfate 846 epitope (CS846), procollagen type 2 C-propeptide (CPII) and glycosaminoglycans (GAG) appeared not modulated. Our data suggest that physiological compression of OA human cartilage modulates the inflammatory milieu by differently affecting the expression of components and homeostasis regulators of the cartilage extracellular matrix.

Mechanical loading inhibits cartilage inflammatory signalling via an HDAC6 and IFT-dependent mechanism regulating primary cilia elongation

OBJECTIVE

Physiological mechanical loading reduces inflammatory signalling in numerous cell types including articular chondrocytes however the mechanism responsible remains unclear. This study investigates the role of chondrocyte primary cilia and associated intraflagellar transport (IFT) in the mechanical regulation of interleukin-1β (IL-1β) signalling.

DESIGN

Isolated chondrocytes and cartilage explants were subjected to cyclic mechanical loading in the presence and absence of the cytokine IL-1β. Nitric oxide (NO) and prostaglandin E₂ (PGE₂) release were used to monitor IL-1β signalling whilst Sulphated glycosaminoglycan (sGAG) release provided measurement of cartilage degradation. Measurements were made of HDAC6 activity and tubulin polymerisation and acetylation. Effects on primary cilia were monitored by confocal and super resolution microscopy. Involvement of IFT was analysed using ORPK cells with hypomorphic mutation of IFT88.

RESULTS

Mechanical loading suppressed NO and PGE₂ release and prevented cartilage degradation. Loading activated HDAC6 and disrupted tubulin acetylation and cilia elongation induced by IL-1β. HDAC6 inhibition with tubacin blocked the anti-inflammatory effects of loading and restored tubulin acetylation and cilia elongation. Hypomorphic mutation of IFT88 reduced IL-1β signalling and abolished the anti-inflammatory effects of loading indicating the mechanism is IFT-dependent. Loading reduced the pool of non-polymerised tubulin which was replicated by taxol which also mimicked the anti-inflammatory effects of mechanical loading and prevented cilia elongation.

CONCLUSIONS

This study reveals that mechanical loading suppresses inflammatory signalling, partially dependent on IFT, by activation of HDAC6 and post transcriptional modulation of tubulin.

Microstructured hydrogel scaffolds containing differential density interfaces promote rapid cellular invasion and vascularization

INTRODUCTION

Insufficient vascularization of currently available clinical biomaterials has limited their application to optimal wound beds. We designed a hydrogel scaffold with a unique internal microstructure of differential collagen densities to induce cellular invasion and neovascularization.

METHODS

Microsphere scaffolds (MSS) were fabricated by encasing 1% (w/v) type 1 collagen microspheres 50-150 μm in diameter in 0.3% collagen bulk. 1% and 0.3% monophase collagen scaffolds and Integra® disks served as controls. Mechanical characterization as well as in vitro and in vivo invasion assays were performed. Cell number and depth of invasion were analyzed using Imaris™. Cell identity was assessed immunohistochemically.

RESULTS

In vitro, MSS exhibited significantly greater average depth of cellular invasion than Integra® and monophase collagen controls. MSS also demonstrated significantly higher cell counts than controls. In vivo, MSS revealed significantly more cellular invasion spanning the entire scaffold depth at 14 days than Integra®. CD31+ expressing luminal structures suggestive of neovasculature were seen within MSS at 7 days and were more prevalent after 14 days. Multiphoton microscopy of MSS demonstrated erythrocytes within luminal structures after 14 days.

CONCLUSION

By harnessing simple architectural cues to induce cellular migration, MSS holds great potential for clinical translation as the next generation dermal replacement product.

STATEMENT OF SIGNIFICANCE

Large skin wounds require tissue engineered dermal substitutes in order to promote healing. Currently available dermal replacement products do not always adequately incorporate into the body, especially in complex wounds, due to poor neovascularization. In this paper, we present a hydrogel with an innovative microarchitecture that is composed of dense type I collagen microspheres suspended in a less-dense collagen bulk. We show that cell invasion into the scaffold is driven solely by mechanical cues inherent within this differential density interface, and that this induces robust vascular cell invasion both in vitro and in a rodent model. Our hydrogel performs favorably compared to the current clinical gold standard, Integra®. We believe this hydrogel scaffold may be the first of the next generation of dermal replacement products.

Effect of interface mechanical discontinuities on scaffold-cartilage integration

A consistent lack of lateral integration between scaffolds and adjacent articular cartilage has been exhibited in vitro and in vivo. Given the mismatch in mechanical properties between scaffolds and articular cartilage, the mechanical discontinuity that occurs at the interface has been implicated as a key factor, but remains inadequately studied. Our objective was to investigate how the mechanical environment within a mechanically loaded scaffold-cartilage construct might affect integration. We hypothesized that the magnitude of the mechanical discontinuity at the scaffold-cartilage interface would be related to decreased integration. To test this hypothesis, chondrocyte seeded scaffolds were embedded into cartilage explants, pre-cultured for 14 days, and then mechanically loaded for 28 days at either 1N or 6N of applied load. Constructs were kept either peripherally confined or unconfined throughout the duration of the experiment. Stress, strain, fluid flow, and relative displacements at the cartilage-scaffold interface and within the scaffold were quantified using biphasic, inhomogeneous finite element models (bFEMs). The bFEMs indicated compressive and shear stress discontinuities occurred at the scaffold-cartilage interface for the confined and unconfined groups. The mechanical strength of the scaffold-cartilage interface and scaffold GAG content were higher in the radially confined 1N loaded groups. Multivariate regression analyses identified the strength of the interface prior to the commencement of loading and fluid flow within the scaffold as the main factors associated with scaffold-cartilage integration. Our study suggests a minimum level of scaffold-cartilage integration is needed prior to the commencement of loading, although the exact threshold has yet to be identified. © 2019 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res.

Chondrocyte and mesenchymal stem cell derived engineered cartilage exhibits differential sensitivity to pro-inflammatory cytokines

Tissue engineering is a promising approach for the repair of articular cartilage defects, with engineered constructs emerging that match native tissue properties. However, the inflammatory environment of the damaged joint might compromise outcomes, and this may be impacted by the choice of cell source in terms of their ability to operate anabolically in an inflamed environment. Here, we compared the response of engineered cartilage derived from native chondrocytes and mesenchymal stem cells (MSCs) to challenge by TNFα and IL-1β in order to determine if either cell type possessed an inherent advantage. Compositional (extracellular matrix) and functional (mechanical) characteristics, as well as the release of catabolic mediators (matrix metalloproteinases [MMPs], nitric oxide [NO]) were assessed to determine cell- and tissue-level changes following exposure to IL-1β or TNF-α. Results demonstrated that MSC-derived constructs were more sensitive to inflammatory mediators than chondrocyte-derived constructs, exhibiting a greater loss of proteoglycans and functional properties at lower cytokine concentrations. While MSCs and chondrocytes both have the capacity to form functional engineered cartilage in vitro, this study suggests that the presence of an inflammatory environment is more likely to impair the in vivo success of MSC-derived cartilage repair. © 2018 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 36:2901-2910, 2018.

The Role of Wnt Pathway in the Pathogenesis of OA and Its Potential Therapeutic Implications in the Field of Regenerative Medicine

Introduction

Osteoarthritis (OA) is a degenerative joint disease characterized by articular cartilage degradation, subchondral damage, and bone remodelling, affecting most commonly weight-bearing joints, such as the knee and hip. The loss of cartilage leads to joint space narrowing, pain, and loss of function which could ultimately require total joint replacement. The Wnt/β catenin pathway is involved in the pathophysiology of OA and has been proposed as a therapeutic target. Endogenous and pharmacological inhibitors of this pathway were recently investigated within innovative therapies including the use of platelet-rich plasma (PRP) and mesenchymal stem cells (MSCs).

Methods

A review of the literature was performed on the PubMed database based on the following inclusion criteria: article written in English language in the last 20 years and dealing with (1) the role of Wnt-β catenin pathway in the pathogenesis of osteoarthritis and (2) pharmacologic or biologic strategies modulating the Wnt-β catenin pathway in the OA setting.

Results

Evidences support that Wnt signalling pathway is likely linked to OA progression and severity. Its inhibition through natural antagonists and new synthetic or biological drugs shares the potential to improve the clinical condition of the patients by affecting the pathological activity of Wnt/β-catenin signalling.

Conclusions

While further research is needed to better understand the mechanisms regulating the molecular interaction between OA regenerative therapies and Wnt, it seems that biologic therapies for OA exert modulation on Wnt/β catenin pathway that might be relevant in achieving the beneficial clinical effect of those therapeutic strategies.

Effect of biomechanical stress on endogenous antioxidant networks in bovine articular cartilage

Mechanosensitve pathways in chondrocytes are essential for maintaining articular cartilage homeostasis. Traumatic loading increases cartilage oxidation and causes cell death and osteoarthritis. However, sub-lethal doses of the pro-oxidant molecule tert-Butyl hydroperoxide (tBHP) protects against loading-induced chondrocyte death. We hypothesized that compressive cyclic loading at moderate strains (<20%) causes sub-lethal cartilage oxidation that induces an adaptive increase in the endogenous antioxidant defense network. We tested this hypothesis by subjecting healthy bovine articular cartilage explants to in vitro static or cyclic (1 Hz) compressive loading at 50 kPa (15% strain, "physiologic") versus 300 kPa (40% strain, "hyper-physiologic") for 12 h per day for 2 days. We also treated unloaded explants with 100 μM tBHP for 12 h per day for 2 days to differentiate between biomechanical and chemical pro-oxidant stimulation. All loading conditions induced glutathione oxidation relative to unloaded controls, but only the 50 kPa cyclic loading condition increased total glutathione content (twofold). This increase was associated with a greater expression of glutamate-cysteine ligase, the rate-limiting step in glutathione synthesis, compared to 300 kPa cyclic loading. 50 kPa cyclic loading also increased the expression of superoxide dismutase-1 and peroxiredoxin-3. Like 50 kPa loading, tBHP treatment also increased total glutathione content. However, tBHP treatment and 50 kPa cyclic loading differed in their effect on the expression of genes regulating antioxidant defense and cartilage matrix synthesis and degradation. These findings suggest that glutathione metabolism is a mechanosensitive antioxidant defense pathway in chondrocytes and that intermittent pro-oxidant treatment alone is insufficient to account for all changes in mediators of cartilage homeostasis associated with cyclic loading. © 2017 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 36:760-769, 2018.

Autologous Chondrocyte Implantation in Osteoarthritic Surroundings: TNFα and Its Inhibition by Adalimumab in a Knee-Specific Bioreactor

BACKGROUND

Autologous chondrocyte implantation (ACI) fails in up to 20% of cases. Advanced intra-articular degeneration paired with an inflammatory environment may be closely related to implantation failure. Certain cytokines have been identified to play a major role during early osteoarthritis.

PURPOSE

To investigate the effects of tumor necrosis factor α (TNFα) and its potential inhibition by adalimumab on cartilage regeneration in an in vitro model of ACI.

STUDY DESIGN

Controlled laboratory study.

METHODS

Bovine articular chondrocytes were cultivated and transferred at passage 3 to fibrin-polyurethane scaffolds. Constructs were loaded by compression (10%-20% scaffold height) and shear (±25°) in a fully characterized multiaxial load (L) bioreactor to simulate clinical ACI or were subjected to free swelling (FS) conditions for a duration of 2 weeks. TNFα (20 ng/mL), adalimumab (10 µg/mL), or both were added to the medium. To assess the outcome, DNA, GAG (glycosaminoglycan), and total collagen were quantified, and gene expression of anabolic (collagen 2, aggrecan, cartilage oligomeric protein, proteoglycan 4), catabolic (matrix metalloproteinases [MMP] 3 and 13), dedifferentiation (collagen 1), and hypertrophy (collagen 10) markers and proinflammatory cytokines (TNFα, IL-1β) was analyzed. Histological evaluation was performed with safranin O/fast green, toluidine blue, and immunohistochemistry of collagen 1 and 2. Apoptosis was analyzed by immunolabeling of anti-active caspase 3. For statistical evaluation, nonparametric tests were chosen with a significance level of P < .05.

RESULTS

A general downregulation of anabolic and upregulation of catabolic markers was detected in the TNFα groups. Collagen 2 was suppressed by TNFα (FS, P = .029; L, P = .006), while MMP 3 was significantly upregulated (FS, P = .035; L, P = .001). Dynamic loading induced a chondrogenic response, which could not fully antagonize the effect of the cytokine. Adalimumab antagonized all effects of TNFα. The histological and immunohistochemical assessments demonstrated less matrix formation in the cytokine-only groups. TNFα induced apoptosis, and this effect was increased by loading.

CONCLUSION

TNFα does negatively affect chondrogenesis under simulated ACI conditions. Both dynamic load and, more potentially, adalimumab showed the capability of antagonizing the negative effects.

CLINICAL RELEVANCE

Catabolic cytokine suppression (ie, TNFα inhibition) combined with compression and shear load may best meet the conditions for chondrogenesis in an osteoarthritic environment.

The initial repair response of articular cartilage after mechanically induced damage

The regenerative potential of articular cartilage (AC) defects is limited and depends on defect size, biomechanical conditions, and age. Early events after overloading might be predictive for cartilage degeneration in the long term. Therefore, the present aim is to investigate the temporal response of cartilage to overloading at cell, matrix, and tissue level during the first period after mechanical overloading. In the present study, the effect of high loading (∼8 MPa) at a high rate (∼14 MPa/s) at day 0 during a 9 day period on collagen damage, gene expression, cell death, and biochemical composition in AC was investigated. A model system was developed which enabled culturing osteochondral explants after loading. Proteoglycan content was repeatedly monitored over time using μCT, whereas other evaluations required destructive measurements. Changes in matrix related gene expressions indicated a degenerative response during the first 6 h after loading. After 24 h, this was restored and data suggested an initial repair response. Cell death and microscopic damage increased after 24 h following loading. These degradative changes were not restored within the 9 day culture period, and were accompanied by a slight loss of proteoglycans at the articular surface that extended into the middle zones. The combined findings indicate that high magnitude loading of articular cartilage at a high rate induces an initial damage that later initiates a healing response that can probably not be retained due to loss of cell viability. Consequently, the matrix cannot be restored in the short term. © 2016 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 35:1265-1273, 2017.

Effect of Static Load on the Nucleus Pulposus of Rabbit Intervertebral Disc Motion Segment in Ex vivo Organ Culture

Background:

The development of mechanically active culture systems helps increase the understanding of the role of mechanical stress in intervertebral disc (IVD) degeneration. Motion segment cultures allow for preservation of the native IVD structure, and adjacent vertebral bodies facilitate the application and control of mechanical loads. The purpose of this study was to establish loading and organ culture methods for rabbit IVD motion segments to study the effect of static load on the whole disc organ.

Methods:

IVD motion segments were harvested from rabbit lumbar spines and cultured in no-loading 6-well plates (control conditions) or custom-made apparatuses under a constant, compressive load (3 kg, 0.5 MPa) for up to 14 days. Tissue integrity, matrix synthesis, and the matrix gene expression profile were assessed after 3, 7, and 14 days of culturing and compared with those of fresh tissues.

Results:

The results showed that ex vivo culturing of motion segments preserved tissue integrity under no-loading conditions for 14 days whereas the static load gradually destroyed the morphology after 3 days. Proteoglycan contents were decreased under both conditions, with a more obvious decrease under static load, and proteoglycan gene expression was also downregulated. However, under static load, immunohistochemical staining intensity and collagen Type II alpha 1 (COL2A1) gene expression were significantly enhanced (61.54 ± 5.91, P = 0.035) and upregulated (1.195 ± 0.040, P = 0.000), respectively, compared with those in the controls (P < 0.05). In contrast, under constant compression, these trends were reversed. Our initial results indicated that short-term static load stimulated the synthesis of collagen Type II alpha 1; however, sustained constant compression led to progressive degeneration and specifically to a decreased proteoglycan content.

Conclusions:

A loading and organ culture system for ex vivo rabbit IVD motion segments was developed. Using this system, we were able to study the effects of mechanical stimulation on the biology of IVDs, as well as the pathomechanics of IVD degeneration.

Effects of Inflammation on Multiscale Biomechanical Properties of Cartilaginous Cells and Tissues

Cells within cartilaginous tissues are mechanosensitive and thus require mechanical loading for regulation of tissue homeostasis and metabolism. Mechanical loading plays critical roles in cell differentiation, proliferation, biosynthesis, and homeostasis. Inflammation is an important event occurring during multiple processes, such as aging, injury, and disease. Inflammation has significant effects on biological processes as well as mechanical function of cells and tissues. These effects are highly dependent on cell/tissue type, timing, and magnitude. In this review, we summarize key findings pertaining to effects of inflammation on multiscale mechanical properties at subcellular, cellular, and tissue level in cartilaginous tissues, including alterations in mechanotransduction and mechanosensitivity. The emphasis is on articular cartilage and the intervertebral disc, which are impacted by inflammatory insults during degenerative conditions such as osteoarthritis, joint pain, and back pain. To recapitulate the pro-inflammatory cascades that occur in vivo, different inflammatory stimuli have been used for in vitro and in situ studies, including tumor necrosis factor (TNF), various interleukins (IL), and lipopolysaccharide (LPS). Therefore, this review will focus on the effects of these stimuli because they are the best studied pro-inflammatory cytokines in cartilaginous tissues. Understanding the current state of the field of inflammation and cell/tissue biomechanics may potentially identify future directions for novel and translational therapeutics with multiscale biomechanical considerations.

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.

Effect of muscle contractions on cartilage: morphological and functional magnetic resonance imaging evaluation of the knee after spinal cord injury

Objective

To evaluate the effect of complete absence of muscle contractions on normal human cartilage in the presence of joint motion.

Methods

Patients with complete acute spinal cord injuries were enrolled. All patients underwent magnetic resonance imaging (MRI) on both knees as soon as their medical condition was stable and at six months after the primary lesion. All patients received rehabilitation treatment that included lower-limb passive motion exercises twice a day. The MRIs were analyzed by two radiologists with expertise in musculoskeletal disorders. A region of interest was established at the patellar facets and trochlea, and T2 relaxation times were calculated. The area under the cartilage T2 relaxation time curve was calculated and standardized.

Results

Fourteen patients with complete spinal cord injuries were enrolled, but only eight patients agreed to participate in the study and signed the informed consent statement. Two patients could not undergo knee MRI due to their clinical conditions. Initial knee MRIs were performed on six patients. After six months, only two patients underwent the second bilateral knee MRI. Both patients were neurologically classified as Frankel A. An increase in T2 values on the six-month MRI was observed for both knees, especially in the patellofemoral joint.

Conclusion

The absence of muscle contractions seems to be deleterious to normal human knee cartilage even in the presence of a normal range of motion. Further studies with a larger number of patients, despite their high logistical complexity, must be performed to confirm this hypothesis.

Effect of Static Load on the Nucleus Pulposus of Rabbit Intervertebral Disc Motion Segment in an Organ Culture

The development of mechanically active culture systems helps in understanding of the role of mechanical stress in intervertebral disc (IVD) degeneration. Motion segment cultures facilitate the application and control of mechanical loads. The purpose of this study was to establish a culturing method for rabbit IVD motion segments to observe the effect of static load on the whole disc organ. Segments were cultured in custom-made apparatuses under a constant, compressive load (3 kg) for 2 weeks. Tissue integrity, matrix synthesis, and matrix gene expression profile were assessed and compared with fresh one. The results showed ex vivo culturing of samples gradually destroyed the morphology. Proteoglycan contents and gene expression were decreased and downregulated obviously. However, immunohistochemical staining intensity and collagen type II gene expression were significantly enhanced and upregulated. In contrast, these trends were reversed under constant compression. These results indicated short-term static load stimulated the synthesis of type II collagen; however, constant compression led to progressive degeneration and specifically to proteoglycan. Through this study a loading and organ-culturing system for ex vivo rabbit IVD motion segments was developed, which can be used to study the effects of mechanical stimulation on the biology of IVDs and the pathomechanics of IVD degeneration.

Reduced tissue osmolarity increases TRPV4 expression and pro-inflammatory cytokines in intervertebral disc cells

The mechanical behaviour and cellular metabolism of intervertebral discs (IVDs) and articular cartilage are strongly influenced by their proteoglycan content and associated osmotic properties. This osmotic environment is a biophysical signal that changes with disease and may contribute to the elevated matrix breakdown and altered biologic response to loading observed in IVD degeneration and osteoarthritis. This study tested the hypothesis that changes in osmo-sensation by the transient receptor potential vallinoid-4 (TRPV4) ion channel occur with disease and contribute to the inflammatory environment found during degeneration. Immunohistochemistry on bovine IVDs from an inflammatory organ culture model were used to investigate if TRPV4 is expressed in the IVD and how expression changes with degeneration. Western blot, live-cell calcium imaging, and qRT-PCR were used to investigate whether osmolarity changes or tumour necrosis factor α (TNFα) regulate TRPV4 expression, and how altered TRPV4 expression influences calcium signalling and pro-inflammatory cytokine expression. TRPV4 expression correlated with TNFα expression, and was increased when cultured in reduced medium osmolarity and unaltered with TNFα-stimulation. Increased TRPV4 expression increased the calcium flux following TRPV4 activation and increased interleukin-1β (IL-1β) and IL-6 gene expression in IVD cells. TRPV4 expression was qualitatively elevated in regions of aggrecan depletion in degenerated human IVDs. Collectively, results suggest that reduced tissue osmolarity, likely following proteoglycan degradation, can increase TRPV4 signalling and enhance pro-inflammatory cytokine production, suggesting changes in TRPV4 mediated osmo-sensation may contribute to the progressive matrix breakdown in disease.

The mechanobiology of articular cartilage: bearing the burden of osteoarthritis

Articular cartilage injuries and degenerative joint diseases are responsible for progressive pain and disability in millions of people worldwide, yet there is currently no treatment available to restore full joint functionality. As the tissue functions under mechanical load, an understanding of the physiologic or pathologic effects of biomechanical factors on cartilage physiology is of particular interest. Here, we highlight studies that have measured cartilage deformation at scales ranging from the macroscale to the microscale, as well as the responses of the resident cartilage cells, chondrocytes, to mechanical loading using in vitro and in vivo approaches. From these studies, it is clear that there exists a complex interplay among mechanical, inflammatory, and biochemical factors that can either support or inhibit cartilage matrix homeostasis under normal or pathologic conditions. Understanding these interactions is an important step toward developing tissue engineering approaches and therapeutic interventions for cartilage pathologies, such as osteoarthritis.

The regional sensitivity of chondrocyte gene expression to coactive mechanical load and exogenous TNF-α stimuli

Both mechanical load and elevated levels of proinflammatory cytokines have been associated with the risk for developing osteoarthritis (OA), yet the potential interaction of these mechanical and biological factors is not well understood. The purpose of this study was to evaluate the response of chondrocytes to the effects of dynamic unconfined compression, TNF-α, and the simultaneous effects of dynamic unconfined compression and TNF-α. The response to these three treatments was markedly different and, taken together, the response in the gene expression of chondrocytes to the different treatment conditions suggest a complex interaction between structure, biology, and mechanical loading.

Distinctive subcellular inhibition of cytokine-induced SRC by salubrinal and fluid flow

A non-receptor protein kinase Src plays a crucial role in fundamental cell functions such as proliferation, migration, and differentiation. While inhibition of Src is reported to contribute to chondrocyte homeostasis, its regulation at a subcellular level by chemical inhibitors and mechanical stimulation has not been fully understood. In response to inflammatory cytokines and stress to the endoplasmic reticulum (ER) that increase proteolytic activities in chondrocytes, we addressed two questions: Do cytokines such as interleukin 1 beta (IL1β) and tumor necrosis factor alpha (TNFα) induce location-dependent Src activation? Can cytokine-induced Src activation be suppressed by chemically alleviating ER stress or by applying fluid flow? Using live cell imaging with two Src biosensors (i.e., cytosolic, and plasma membrane-bound biosensors) for a fluorescence resonance energy transfer (FRET) technique, we determined cytosolic Src activity as well as membrane-bound Src activity in C28/I2 human chondrocytes. In response to TNFα and IL1β, both cytosolic and plasma membrane-bound Src proteins were activated, but activation in the cytosol occurred earlier than that in the plasma membrane. Treatment with salubrinal or guanabenz, two chemical agents that attenuate ER stress, significantly decreased cytokine-induced Src activities in the cytosol, but not in the plasma membrane. In contrast, fluid flow reduced Src activities in the plasma membrane, but not in the cytosol. Collectively, the results demonstrate that Src activity is differentially regulated by salubrinal/guanabenz and fluid flow in the cytosol and plasma membrane.

Protective effects of interleukin-4 on tissue destruction and morphological changes of bovine nasal chondrocytes in vitro

BACKGROUND

Previous studies have shown that some cytokines have protective effects on cartilage in joint diseases. In the current study, effects of IL-4 against morphological changes and tissue degradation induced by IL-1α on bovine nasal cartilage (BNC) explants were investigated.

METHODS

Fresh BNC samples were prepared from a slaughterhouse under sterile conditions. BNC explants culture was treated with both IL-lα (10 ng/ml) and IL-4 (50 ng/ml) at the same time for 28 days. The morphological characteristics of explants were assessed by using histology techniques and invert microscopy. Matrix metalloproteinase-1 (MMP-1) production was assessed within different days by using Western blotting.

RESULTS

IL-lα induced prominent cartilage morphology degradation. The pro and active form of MMP-1 band substantially increased at day 21 of culture. In the presence of both IL-lα and IL-4, chondrocytes preserved their ordinary normal phenotype with intact extracellular matrix. In addition, a significant reduction in pro-MMP-1and inhibition of active MMP-1 was seen.

CONCLUSION

In conclusion, IL-4 could be regarded as a potential candidate in cartilage protecting against the degradation changes of IL-lα. It seems that the preservation effect of IL-4 is associated with significant reduction of MMP-1.

The disease modifying osteoarthritis drug diacerein is able to antagonize pro inflammatory state of chondrocytes under mild mechanical stimuli

OBJECTIVE

To investigate the combination of mild mechanical stimuli and a disease modifying osteoarthritis drug (DMOAD) in inflammatory activated chondrocytes and to study the combination of drug and mechanical tension on the cellular level as a model for an integrated biophysical approach for osteoarthritis (OA) treatments.

METHODS

Interleukin-1beta (IL-1β) stimulated C28/I2 cells underwent mild mechanically treatment while cultured in the presence of the DMOAD diacerein. The pharmacological input of diacerein was evaluated by cell viability and cell proliferation measurements. Inflammation and treatment induced changes in key regulatory proteins and components of the extracellular matrix (ECM) were characterized by quantitative real-time PCR (qPCR). The effects on metalloproteinase-1 (MMP-1) activity and glycosaminoglycan (GAG) concentration in cell supernatants of treated cells were investigated.

RESULTS

C28/I2 cells demonstrated significant changes in expression of inflammatory and cartilage destructive proteins in response to IL-1β stimulation. The chondroprotective action of diacerein in mechanically stimulated cells was mediated by a decrease in interleukin-8 (IL-8), fibronectin-1 (FN-1), collagen type I (Col 1) and MMP-1 expression levels, respectively. Augmented expression of interleukin-6 receptor (IL-6R) and the fibroblast growth factor receptors (FGFRs) by diacerein was not abolished by mechanical treatment. The observed effects were accompanied by a reduced cell proliferation rate, attenuated cell viability and extenuated MMP-1 activity.

CONCLUSION

Diacerein diversely regulates the expression of main regulatory proteins as well as components important to regenerate and set up ECM. Mechanical stimulation does not negatively influence the chondroprotective effect induced by diacerein treatment in immortalized human C28/I2 chondrocytes.

Stem Cell-assisted Approaches for Cartilage Tissue Engineering

The regeneration of damaged articular cartilage remains challenging due to its poor intrinsic capacity for repair. Tissue engineering of articular cartilage is believed to overcome the current limitations of surgical treatment by offering functional regeneration in the defect region. Selection of proper cell sources and ECM-based scaffolds, and incorporation of growth factors or mechanical stimuli are of primary importance to successfully produce artificial cartilage for tissue repair. When designing materials for cartilage tissue engineering, biodegradability and biocompatibility are the key factors in selecting material candidates, for either synthetic or natural polymers. The unique environment of cartilage makes it suitable to use a hydrogel with high water content in the cross-linked or thermosensitive (injectable) form. Moreover, design of composite scaffolds from two polymers with complementary physicochemical and biological properties has been explored to provide residing chondrocytes with a combination of the merits that each component contributes.

Effects of low-intensity pulsed ultrasound and hyperbaric oxygen on human osteoarthritic chondrocytes

Background

Although the individual effects of hyperbaric oxygen (HBO) and low-intensity pulsed ultrasound (LIPUS) on osteoarthritic (OA) chondrocytes have been reported, the effects of HBO combined with LIPUS treatment are unknown.

Methods

OA chondrocytes were obtained from patients undergoing knee replacement surgery. RNA was isolated for real-time polymerase chain reaction (PCR) analysis of inducible nitric oxide synthase (iNOS), type-II collagen, and aggrecan gene expression. The protein levels of MMP-3 and TIMP-1 were quantified by enzyme-linked immunosorbent assay (ELISA) after LIPUS or HBO treatment. The data are given as mean ± standard deviation (SD) of the results from three independent experiments. A p value less than 0.05 was defined as statistically significant.

Results

Our data suggested that ultrasound and HBO treatment increased cell bioactivity of OA chondrocytes. Real-time PCR analysis showed that HBO treatment increased the mRNA of type-II collagen, aggrecan, and TIMP-1 but suppressed the iNOS expression of OA chondrocytes. LIPUS treatment increased the type-II collagen and iNOS expression of OA chondrocytes. ELISA data showed that HBO or LIPUS treatment increased TIMP-1 production of OA chondrocyte. MMP-3 production was suppressed by HBO treatment. HBO combined with LIPUS treatments resulted in additive effect in TIMP-1 production and compensatory effect in iNOS expression.

Conclusion

HBO combined with LIPUS treatment-induced increase of the anabolic factor (TIMP-1)/catabolic factor (MMP-3) ratio may provide an additive therapeutic approach to slow the course of OA degeneration.

Subphysiological compressive loading reduces apoptosis following acute impact injury in a porcine cartilage model

BACKGROUND

Acute cartilage injuries induce cell death and are associated with an increased incidence of osteoarthritis development later in life. The objective of this study was to investigate the effect of posttraumatic cyclic compressive loading on chondrocyte viability and apoptosis in porcine articular cartilage plugs.

HYPOTHESIS

Compressive loading of acutely injured cartilage can maintain chondrocyte viability by reducing apoptosis after a traumatic impact injury.

STUDY DESIGN

In vitro controlled laboratory study.

LEVEL OF EVIDENCE

Level 5.

METHODS

Each experiment compared 4 test groups: control, impact, impact with compressive loading (either 0.5 or 0.8 MPa), and no impact but compressive loading (n = 15 per group). Flat, full-thickness articular cartilage plugs were harvested from the trochlear region of porcine knees. A drop tower was utilized to introduce an impact injury. The articular plugs were subjected to two 30-minute cycles of either 0.5 or 0.8 MPa of dynamic loading. Cell viability, apoptosis, and gene expression of samples were evaluated 24 hours postimpaction.

RESULTS

Cell viability staining showed that 0.5 MPa of dynamic compressive loading increased cell viability compared with the impact group. Apoptotic analysis revealed a decrease in apoptotic expression in the group with 0.5 MPa of dynamic compressive loading compared with the impact group. Significantly higher caspase 3 and lower collagen II expressions were observed in impacted samples without compressive loading, compared with those with. Compressive loading of nonimpacted samples significantly increased collagen II and decreased caspase 3 expressions.

CONCLUSION

In this porcine in vitro model, dynamic compressive loading at subphysiological levels immediately following impact injury decreases apoptotic expression, thereby maintaining chondrocyte viability.

CLINICAL RELEVANCE

Therapeutic exercises could be designed to deliver subphysiological loading to the injured cartilage, thereby minimizing injury.

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.

Low-intensity pulsed ultrasound inhibits messenger RNA expression of matrix metalloproteinase-13 induced by interleukin-1β in chondrocytes in an intensity-dependent manner

The effect of low-intensity pulsed ultrasound (LIPUS) on articular cartilage metabolism has been characterized. However, the effect of LIPUS intensity on articular cartilage degradation factors remains unknown. This study aimed to investigate the immediate effect of LIPUS at several intensities on cultured chondrocytes treated with interleukin-1β (IL-1β) to induce an inflammatory response and on articular cartilage explants. Cultured chondrocytes and articular cartilage explants were treated by LIPUS at intensities of 0, 7.5, 30 and 120 mW/cm(2) or 0, 27 and 67 mW/cm(2), respectively. mRNA analysis revealed that LIPUS inhibited induction of MMP13 mRNA expression by 100 pg/mL IL-1β in cultured chondrocytes in an intensity-dependent manner. LIPUS also inhibited MMP13 and MMP1 mRNA expression in articular cartilage explants. Our results indicate that LIPUS may potentially protect articular cartilage by inhibiting MMP mRNA expression in an intensity-dependent manner and should thus be considered a useful candidate for daily treatment of OA.

Multiscale mechanics of articular cartilage: potentials and challenges of coupling musculoskeletal, joint, and microscale computational models

Articular cartilage experiences significant mechanical loads during daily activities. Healthy cartilage provides the capacity for load bearing and regulates the mechanobiological processes for tissue development, maintenance, and repair. Experimental studies at multiple scales have provided a fundamental understanding of macroscopic mechanical function, evaluation of the micromechanical environment of chondrocytes, and the foundations for mechanobiological response. In addition, computational models of cartilage have offered a concise description of experimental data at many spatial levels under healthy and diseased conditions, and have served to generate hypotheses for the mechanical and biological function. Further, modeling and simulation provides a platform for predictive risk assessment, management of dysfunction, as well as a means to relate multiple spatial scales. Simulation-based investigation of cartilage comes with many challenges including both the computational burden and often insufficient availability of data for model development and validation. This review outlines recent modeling and simulation approaches to understand cartilage function from a mechanical systems perspective, and illustrates pathways to associate mechanics with biological function. Computational representations at single scales are provided from the body down to the microstructure, along with attempts to explore multiscale mechanisms of load sharing that dictate the mechanical environment of the cartilage and chondrocytes.