Mitoprotective therapy preserves chondrocyte viability and prevents cartilage degeneration in an ex vivo model of posttraumatic osteoarthritis

Mild exercise expedites joint clearance and slows joint degradation in a joint instability model of osteoarthritis in male rats

OBJECTIVE

Exercise remains a hallmark treatment for post-traumatic osteoarthritis (PTOA) and may maintain joint homeostasis in part by clearing inflammatory cytokines, cells, and particles. It remains largely unknown whether exercise-induced joint clearance can provide therapeutic relief of PTOA. In this study, we hypothesized that exercise could slow the progression of preclinical PTOA in part by enhancing knee joint clearance.

DESIGN

Surgical medial meniscal transection was used to induce PTOA in 3-month-old male Lewis rats. A sham surgery was used as a control. Mild treadmill walking was introduced 3 weeks post-surgery and maintained to 6 weeks post-surgery. Gait and isometric muscle torque were measured at the study endpoint. Near-infrared imaging tracked how exercise altered lymphatic and venous knee joint clearance during discrete time points of PTOA progression.

RESULTS

Exercise mitigated joint degradation associated with PTOA by preserving glycosaminoglycan content and reducing osteophyte volume (effect size (95% Confidence Interval (CI)); 1.74 (0.71-2.26)). PTOA increased hind step widths (0.57 (0.18-0.95) cm), but exercise corrected this gait dysfunction (0.54 (0.16-0.93) cm), potentially indicating pain relief. Venous, but not lymphatic, clearance was quicker 1-, 3-, and 6-weeks post-surgery compared to baseline. The mild treadmill walking protocol expedited lymphatic clearance rate in moderate PTOA (3.39 (0.20-6.59) hrs), suggesting exercise may play a critical role in restoring joint homeostasis.

CONCLUSIONS

We conclude that mild exercise has the potential to slow disease progression in part by expediting joint clearance in moderate PTOA.

The role and intervention of mitochondrial metabolism in osteoarthritis

Osteoarthritis (OA), a prevalent degenerative joint disease, affects a substantial global population. Despite the elusive etiology of OA, recent investigations have implicated mitochondrial dysfunction as a significant factor in disease pathogenesis. Mitochondria, pivotal cellular organelles accountable for energy production, exert essential roles in cellular metabolism. Hence, mitochondrial dysfunction can exert broad-ranging effects on various cellular processes implicated in OA development. This comprehensive review aims to provide an overview of the metabolic alterations occurring in OA and elucidate the diverse mechanisms through which mitochondrial dysfunction can contribute to OA pathogenesis. These mechanisms encompass heightened oxidative stress and inflammation, perturbed chondrocyte metabolism, and compromised autophagy. Furthermore, this review will explore potential interventions targeting mitochondrial metabolism as means to impede or decelerate the progression of OA. In summary, this review offers a comprehensive understanding of the involvement of mitochondrial metabolism in OA and underscores prospective intervention strategies.

Connexin 43 Regulates Intercellular Mitochondrial Transfer from Human Mesenchymal Stromal Cells to Chondrocytes

Background

The phenomenon of intercellular mitochondrial transfer from mesenchymal stromal cells (MSCs) has shown promise for improving tissue healing after injury and has potential for treating degenerative diseases like osteoarthritis (OA). Recently MSC to chondrocyte mitochondrial transfer has been documented, but the mechanism of transfer is unknown. Full-length connexin43 (Cx43, encoded by GJA1 ) and the truncated internally translated isoform GJA1-20k have been implicated in mitochondrial transfer between highly oxidative cells, but have not been explored in orthopaedic tissues. Here, our goal was to investigate the role of Cx43 in MSC to chondrocyte mitochondrial transfer. In this study, we tested the hypotheses that (a) mitochondrial transfer from MSCs to chondrocytes is increased when chondrocytes are under oxidative stress and (b) MSC Cx43 expression mediates mitochondrial transfer to chondrocytes.

Methods

Oxidative stress was induced in immortalized human chondrocytes using tert-Butyl hydroperoxide (t-BHP) and cells were evaluated for mitochondrial membrane depolarization and reactive oxygen species (ROS) production. Human bone-marrow derived MSCs were transduced for mitochondrial fluorescence using lentiviral vectors. MSC Cx43 expression was knocked down using siRNA or overexpressed (GJA1+ and GJA1-20k+) using lentiviral transduction. Chondrocytes and MSCs were co-cultured for 24 hrs in direct contact or separated using transwells. Mitochondrial transfer was quantified using flow cytometry. Co-cultures were fixed and stained for actin and Cx43 to visualize cell-cell interactions during transfer.

Results

Mitochondrial transfer was significantly higher in t-BHP-stressed chondrocytes. Contact co-cultures had significantly higher mitochondrial transfer compared to transwell co-cultures. Confocal images showed direct cell contacts between MSCs and chondrocytes where Cx43 staining was enriched at the terminal ends of actin cellular extensions containing mitochondria in MSCs. MSC Cx43 expression was associated with the magnitude of mitochondrial transfer to chondrocytes; knocking down Cx43 significantly decreased transfer while Cx43 overexpression significantly increased transfer. Interestingly, GJA1-20k expression was highly correlated with incidence of mitochondrial transfer from MSCs to chondrocytes.

Conclusions

Overexpression of GJA1-20k in MSCs increases mitochondrial transfer to chondrocytes, highlighting GJA1-20k as a potential target for promoting mitochondrial transfer from MSCs as a regenerative therapy for cartilage tissue repair in OA.

Oxidative stress as a key modulator of cell fate decision in osteoarthritis and osteoporosis: a narrative review

During aging and after traumatic injuries, cartilage and bone cells are exposed to various pathophysiologic mediators, including reactive oxygen species (ROS), damage-associated molecular patterns, and proinflammatory cytokines. This detrimental environment triggers cellular stress and subsequent dysfunction, which not only contributes to the development of associated diseases, that is, osteoporosis and osteoarthritis, but also impairs regenerative processes. To counter ROS-mediated stress and reduce the overall tissue damage, cells possess diverse defense mechanisms. However, cellular antioxidative capacities are limited and thus ROS accumulation can lead to aberrant cell fate decisions, which have adverse effects on cartilage and bone homeostasis. In this narrative review, we address oxidative stress as a major driver of pathophysiologic processes in cartilage and bone, including senescence, misdirected differentiation, cell death, mitochondrial dysfunction, and impaired mitophagy by illustrating the consequences on tissue homeostasis and regeneration. Moreover, we elaborate cellular defense mechanisms, with a particular focus on oxidative stress response and mitophagy, and briefly discuss respective therapeutic strategies to improve cell and tissue protection.

Synovial fluid mitochondrial DNA concentration reflects the degree of cartilage damage after naturally occurring articular injury

OBJECTIVE

To evaluate mitochondrial DNA (mtDNA) release from injured chondrocytes and investigate the utility of synovial fluid mtDNA concentration in early detection of posttraumatic osteoarthritis.

METHOD

We measured mtDNA release using four models of osteoarthritis: in vitro interleukin-1β stimulation of cultured equine chondrocytes, ex vivo mechanical impact of bovine cartilage explants, in vivo mechanical impact of equine articular cartilage, and naturally occurring equine intraarticular fracture. In our in vivo model, one group was treated with an intraarticular injection of the mitoprotective peptide SS-31 following cartilage injury. mtDNA content was quantified using qPCR. For naturally occurring cases of joint injury, clinical data (radiographs, arthroscopic video footage) were scored for criteria associated with degenerative joint disease.

RESULTS

Chondrocytes released mtDNA in the acute time frame following inflammatory and mechanical cellular stress in vitro. mtDNA was increased in equine synovial fluid following experimental and naturally occurring injury to the joint surface. In naturally occurring posttraumatic osteoarthritis, we found a strong positive correlation between the degree of cartilage damage and mtDNA concentration (r = 0.80, P = 0.0001). Finally, impact-induced mtDNA release was mitigated by mitoprotective treatment.

CONCLUSION

Changes in synovial fluid mtDNA occur following joint injury and correlate with the severity of cartilage damage. Mitoprotection mitigates increases in synovial fluid mtDNA suggesting that mtDNA release may reflect mitochondrial dysfunction. Further investigation of mtDNA as a potentially sensitive marker of early articular injury and response to mitoprotective therapy is warranted.

Osteoarthritis: a narrative review of molecular approaches to disease management

Osteoarthritis (OA) is a chronic, progressive degenerative whole joint disease that affects the articular cartilage, subchondral bone, ligaments, capsule, and synovium. While it is still believed to be a mechanically driven disease, the role of underlying co-existing inflammatory processes and mediators in the onset of OA and its progression is now more appreciated. Post-traumatic osteoarthritis (PTOA) is a subtype of OA that occurs secondary to traumatic joint insults and is widely used in pre-clinical models to help understand OA in general. There is an urgent need to develop new treatments as the global burden is considerable and expanding. In this review, we focus on the recent pharmacological advances in the treatment of OA and summarize the most significant promising agents based on their molecular effects. Those are classified here into broad categories: anti-inflammatory, modulation of the activity of matrix metalloproteases, anabolic, and unconventional pleiotropic agents. We provide a comprehensive analysis of the pharmacological advances in each of these areas and highlight future insights and directions in the OA field.

Understanding the Influence of Local Physical Stimuli on Chondrocyte Behavior

Investigating the mechanobiology of chondrocytes is challenging due to the complex micromechanical environment of cartilage tissue. The innate zonal differences and poroelastic properties of the tissue combined with its heterogeneous composition create spatial- and temporal-dependent cell behavior, which further complicates the investigation. Despite the numerous challenges, understanding the mechanobiology of chondrocytes is crucial for developing strategies for treating cartilage related diseases as chondrocytes are the only cell type within the tissue. The effort to understand chondrocyte behavior under various mechanical stimuli has been ongoing over the last 50 years. Early studies examined global biosynthetic behavior under unidirectional mechanical stimulus. With the technological development in high-speed confocal imaging techniques, recent studies have focused on investigating real-time individual and collective cell responses to multiple / combined modes of mechanical stimuli. Such efforts have led to tremendous advances in understanding the influence of local physical stimuli on chondrocyte behavior. In addition, we highlight the wide variety of experimental techniques, spanning from static to impact loading, and analysis techniques, from biochemical assays to machine learning, that have been utilized to study chondrocyte behavior. Finally, we review the progression of hypotheses about chondrocyte mechanobiology and provide a perspective on the future outlook of chondrocyte mechanobiology.

Osteoarthritis models: From animals to tissue engineering

Osteoarthritis (OA) is a chronic degenerative osteoarthropathy. Although it has been revealed that a variety of factors can cause or aggravate the symptoms of OA, the pathogenic mechanisms of OA remain unknown. Reliable OA models that accurately reflect human OA disease are crucial for studies on the pathogenic mechanism of OA and therapeutic drug evaluation. This review first demonstrated the importance of OA models by briefly introducing the OA pathological features and the current limitations in the pathogenesis and treatment of OA. Then, it mainly discusses the development of different OA models, including animal and engineered models, highlighting their advantages and disadvantages from the perspective of pathogenesis and pathology analysis. In particular, the state-of-the-art engineered models and their potential were emphasized, as they may represent the future direction in the development of OA models. Finally, the challenges in obtaining reliable OA models are also discussed, and possible future directions are outlined to shed some light on this area.

Mesenchymal stromal cells donate mitochondria to articular chondrocytes exposed to mitochondrial, environmental, and mechanical stress

Articular cartilage has limited healing capacity and no drugs are available that can prevent or slow the development of osteoarthritis (OA) after joint injury. Mesenchymal stromal cell (MSC)-based regenerative therapies for OA are increasingly common, but questions regarding their mechanisms of action remain. Our group recently reported that although cartilage is avascular and relatively metabolically quiescent, injury induces chondrocyte mitochondrial dysfunction, driving cartilage degradation and OA. MSCs are known to rescue injured cells and improve healing by donating healthy mitochondria in highly metabolic tissues, but mitochondrial transfer has not been investigated in cartilage. Here, we demonstrate that MSCs transfer mitochondria to stressed chondrocytes in cell culture and in injured cartilage tissue. Conditions known to induce chondrocyte mitochondrial dysfunction, including stimulation with rotenone/antimycin and hyperoxia, increased transfer. MSC-chondrocyte mitochondrial transfer was blocked by non-specific and specific (connexin-43) gap-junction inhibition. When exposed to mechanically injured cartilage, MSCs localized to areas of matrix damage and extended cellular processes deep into microcracks, delivering mitochondria to chondrocytes. This work provides insights into the chemical, environmental, and mechanical conditions that can elicit MSC-chondrocyte mitochondrial transfer in vitro and in situ, and our findings suggest a new potential role for MSC-based therapeutics after cartilage injury.

The current state of the osteoarthritis drug development pipeline: a comprehensive narrative review of the present challenges and future opportunities

In this narrative review article, we critically assess the current state of the osteoarthritis (OA) drug development pipeline. We discuss the current state-of-the-art in relation to the development and evaluation of candidate disease-modifying OA drugs (DMOADs) and the limitations associated with the tools and methodologies that are used to assess outcomes in OA clinical trials. We focus on the definition of DMOADs, highlight the need for an updated definition in the form of a consensus statement from all the major stakeholders, including academia, industry, regulatory agencies, and patient organizations, and provide a summary of the results of recent clinical trials of novel DMOAD candidates. We propose that DMOADs should be more appropriately targeted and investigated according to the emerging clinical phenotypes and molecular endotypes of OA. Based on the findings from recent clinical trials, we propose key topics and directions for the development of future DMOADs.

Human mesenchymal stromal cells release functional mitochondria in extracellular vesicles

Cartilage and other skeletal soft tissues heal poorly after injury, in part due to their lack of vascularity and low metabolic rate. No pharmacologic approaches have proven effective in preventing chronic degenerative disease after joint injury. Mesenchymal stromal cells (MSCs) have been investigated for their ability to treat pain associated with osteoarthritis (OA) and preserve articular cartilage. Limitations of MSCs include variability in cell phenotype, low engraftment and retention rates, and inconsistent clinical outcomes. Therefore, acellular biologic therapies such as extracellular vesicles (EVs) are currently being investigated. MSC-derived EVs have been found to replicate many of the therapeutic effects of their cells of origin, but the mechanisms driving this remain unclear. Recent evidence in non-orthopedic tissues suggests MSCs can rescue injured cells by donating mitochondria, restoring mitochondrial function in recipient cells, preserving cell viability, and promoting tissue repair. Our group hypothesized that MSCs package mitochondria for export into EVs, and that these so-called "mitoEVs" could provide a delivery strategy for cell-free mitochondria-targeted therapy. Therefore, the goals of this study were to: 1) characterize the vesicle fractions of the MSCs secretome with respect to mitochondrial cargoes, 2) determine if MSC-EVs contain functional mitochondria, and 3) determine if chondrocytes can take up MSC-derived mitoEVs. We isolated exosome, microvesicle, and vesicle-free fractions from MSC-conditioned media. Using a combination of dynamic light scattering and nanoparticle tracking, we determined that MSC-EV populations fall within the three size categories typically used to classify EVs (exosomes, microvesicles, apoptotic bodies). Fluorescent nanoparticle tracking, immunoblotting, and flow cytometry revealed that mitochondrial cargoes are abundant across all EV size populations, and mitoEVs are nearly ubiquitous among the largest EVs. Polarization staining indicated a subset of mitoEVs contain functional mitochondria. Finally, flow cytometry and fluorescent imaging confirmed uptake of mitoEVs by chondrocytes undergoing rotenone/antimycin-induced mitochondrial dysfunction. These data indicate that MSCs package intact, functional mitochondria into EVs, which can be transferred to chondrocytes in the absence of direct cell-cell interactions. This work suggests intercellular transfer of healthy MT to chondrocytes could represent a new, acellular approach to augment mitochondrial content and function in poorly-healing avascular skeletal soft tissues.

Evaluation of SS-31 as a Potential Strategy for Tendinopathy Treatment: An In Vitro Model

BACKGROUND

Studies in our laboratory have demonstrated mitochondrial dysfunction in human and animal models of supraspinatus tendinopathy. SS-31 (elamipretide) has been reported to improve mitochondrial function and to be effective in clinical trials for several diseases. The potential of SS-31 in treating tendinopathy has not been explored.

HYPOTHESIS

SS-31 would improve mitochondrial function in human tenocytes sampled from patients with tendinopathy.

STUDY DESIGN

Controlled laboratory study.

METHODS

Healthy tenocytes were obtained from normal hamstring tendon biopsy specimens in 9 patients undergoing anterior cruciate ligament reconstruction, and tenocytes were collected from degenerative supraspinatus tendon biopsy specimens in 9 patients undergoing rotator cuff repair. Tenocytes were cultured, used at passage 1, and assigned to 4 groups: healthy tenocytes, healthy tenocytes with 1μM SS-31 treatment for 72 hours, degenerative tenocytes, and degenerative tenocytes with 1μM SS-31 treatment for 72 hours. The outcomes included measurements of mitochondrial potential, mitochondrial morphology by transmission electron microscopy imaging, reactive oxygen species and superoxidative dismutase activity, gene expression, and cell viability.

RESULTS

An increase in the cell fraction with depolarized mitochondria was found in degenerative tenocytes (P = .014), followed by a decrease after SS-31 treatment (P = .018). Transmission electron microscopy images demonstrated morphological changes with a decreased number and size of mitochondria per cell in the degenerative tenocytes (P = .018) and with improvement after SS-31 treatment. There was no significant difference in the level of reactive oxygen species between healthy and degenerative tenocytes in culture, but superoxidative dismutase activity was significantly decreased in the degenerative group (P = .006), which then increased after SS-31 treatment (P = .012). These findings suggested that mitochondrial dysfunction may be reversed by SS-31 treatment. The gene expression of matrix metalloproteinase-1 (matrix remodeling, P = .029) and fatty acid-binding protein 4 (fatty infiltration, P = .046) was significantly upregulated in the degenerative tenocytes and reduced by SS-31 treatment (P = .048; P = .007). Gene expression for hypoxia-inducible factor1 α and the proapoptotic regulator Bcl-2-associated X protein was increased in the degenerative tenocytes. There was a significant decrease in cell viability in degenerative tenocytes as compared with the healthy tenocytes, with small improvement after treatment with SS-31.

CONCLUSION

There are changes in mitochondrial structure and function in tenocytes derived from degenerative tendons, and SS-31, as a mitochondrial protectant, could improve mitochondrial function and promote the healing of tendinopathy.

CLINICAL RELEVANCE

Mitochondrial dysfunction appears to play a role in the development of tendinopathy, and SS-31, as a mitochondrial protective agent, may be a therapeutic agent in the treatment of tendinopathy.

Urocortin-1 Is Chondroprotective in Response to Acute Cartilage Injury via Modulation of Piezo1

Post-traumatic OA (PTOA) is often triggered by injurious, high-impact loading events which result in rapid, excessive chondrocyte cell death and a phenotypic shift in residual cells toward a more catabolic state. As such, the identification of a disease-modifying OA drug (DMOAD) that can protect chondrocytes from death following impact injury, and thereby prevent cartilage degradation and progression to PTOA, would offer a novel intervention. We have previously shown that urocortin-1 (Ucn) is an essential endogenous pro-survival factor that protects chondrocytes from OA-associated pro-apoptotic stimuli. Here, using a drop tower PTOA-induction model, we demonstrate the extent of Ucn's chondroprotective role in cartilage explants exposed to excessive impact load. Using pathway-specific agonists and antagonists, we show that Ucn acts to block load-induced intracellular calcium accumulation through blockade of the non-selective cation channel Piezo1 rather than TRPV4. This protective effect is mediated primarily through the Ucn receptor CRF-R1 rather than CRF-R2. Crucially, we demonstrate that the chondroprotective effect of Ucn is maintained whether it is applied pre-impact or post-impact, highlighting the potential of Ucn as a novel DMOAD for the prevention of injurious impact overload-induced PTOA.

Fundamentals of OA. An initiative of Osteoarthritis and Cartilage. Obesity and metabolic factors in OA

OBJECTIVE

Obesity was once considered a risk factor for knee osteoarthritis (OA) primarily for biomechanical reasons. Here we provide an additional perspective by discussing how obesity also increases OA risk by altering metabolism and inflammation.

DESIGN

This narrative review is presented in four sections: 1) metabolic syndrome and OA, 2) metabolic biomarkers of OA, 3) evidence for dysregulated chondrocyte metabolism in OA, and 4) metabolic inflammation: joint tissue mediators and mechanisms.

RESULTS

Metabolic syndrome and its components are strongly associated with OA. However, evidence for a causal relationship is context dependent, varying by joint, gender, diagnostic criteria, and demographics, with additional environmental and genetic interactions yet to be fully defined. Importantly, some aspects of the etiology of obesity-induced OA appear to be distinct between men and women, especially regarding the role of adipose tissue. Metabolomic analyses of serum and synovial fluid have identified potential diagnostic biomarkers of knee OA and prognostic biomarkers of disease progression. Connecting these biomarkers to cellular pathophysiology will require future in vivo studies of joint tissue metabolism. Such studies will help reveal when a metabolic process or a metabolite itself is a causal factor in disease progression. Current evidence points towards impaired chondrocyte metabolic homeostasis and metabolic-immune dysregulation as likely factors connecting obesity to the increased risk of OA.

CONCLUSIONS

A deeper understanding of how obesity alters metabolic and inflammatory pathways in synovial joint tissues is expected to provide new therapeutic targets and an improved definition of "metabolic" and "obesity" OA phenotypes.

NSAIDs Reduce Therapeutic Efficacy of Mesenchymal Stromal Cell Therapy in a Rodent Model of Posttraumatic Osteoarthritis

BACKGROUND

Intra-articular injections of human mesenchymal stromal cells (hMSCs) have shown promise in slowing cartilage degradation in posttraumatic osteoarthritis (PTOA). Clinical use of cell therapies for osteoarthritis has accelerated in recent years without sufficient scientific evidence defining best-use practices. Common recommendations advise patients to avoid nonsteroidal anti-inflammatory drug (NSAID) use before and after cell injection over concerns that NSAIDs may affect therapeutic efficacy. Recommendations to restrict NSAID use are challenging for patients, and it is unclear if patients are compliant.

HYPOTHESIS

NSAIDs will reduce the efficacy of hMSC therapy in treating a preclinical model of PTOA.

STUDY DESIGN

Controlled laboratory study.

METHODS

Lewis rats underwent medial meniscal transection (MMT) surgery to induce PTOA or a sham (sham group) surgery that did not progress to PTOA. Rats received naproxen solution orally daily before (Pre-NSAID group) or after (Post-NSAID group) hMSC treatment, throughout the course of the experiment (Full-NSAID group), or received hMSCs without NSAIDs (No NSAID). Cartilage morphology and composition were quantified using contrast-enhanced micro-computed tomography and histology. Pain (secondary allodynia) was measured using a von Frey filament.

RESULTS

Injection of hMSCs attenuated cartilage degeneration associated with MMT. hMSCs prevented proteoglycan loss, maintained smooth cartilage surfaces, reduced cartilage lesions, reduced mineralized osteophyte formation, and reduced pain by week 7. The Pre-NSAID group had decreased proteoglycan levels compared with the hMSC group, although there were no other significant differences. Thus, pretreatment with NSAIDs had minimal effects on the therapeutic benefits of hMSC injections. The Post-NSAID and Full-NSAID groups, however, exhibited significantly worse osteoarthritis than the hMSC-only group, with greater proteoglycan loss, surface roughness, osteophyte volume, and pain.

CONCLUSION

Use of NSAIDs before hMSC injection minimally reduced the therapeutic benefits for PTOA, which included preservation of cartilage surface integrity as well as a reduction in osteophytes. Use of NSAIDs after injections, however, substantially reduced the therapeutic efficacy of cellular treatment.

CLINICAL RELEVANCE

Our data support the clinical recommendation of avoiding NSAID use after hMSC injection but suggest that using NSAIDs before treatment may not substantially diminish the therapeutic efficacy of cell treatment.

Strategies to protect against age-related mitochondrial decay: Do natural products and their derivatives help?

Mitochondria serve vital roles critical for overall cellular function outside of energy transduction. Thus, mitochondrial decay is postulated to be a key factor in aging and in age-related diseases. Mitochondria may be targets of their own decay through oxidative damage. However, treating animals with antioxidants has been met with only limited success in rejuvenating mitochondrial function or in increasing lifespan. A host of nutritional strategies outside of using traditional antioxidants have been devised to promote mitochondrial function. Dietary compounds are under study that induce gene expression, enhance mitochondrial biogenesis, mitophagy, or replenish key metabolites that decline with age. Moreover, redox-active compounds may now be targeted to mitochondria which improve their effectiveness. Herein we review the evidence that representative dietary effectors modulate mitochondrial function by stimulating their renewal or reversing the age-related loss of key metabolites. While in vitro evidence continues to accumulate that many of these compounds benefit mitochondrial function and/or prevent their decay, the results using animal models and, in some instances human clinical trials, are more mixed and sometimes even contraindicated. Thus, further research on optimal dosage and age of intervention are warranted before recommending potential mitochondrial rejuvenating compounds for human use.

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.

Proteomic Analysis and Cell Viability of Nine Amnion, Chorion, Umbilical Cord, and Amniotic Fluid-Derived Products

OBJECTIVE

Amnion products are used in various musculoskeletal surgeries and as injections for joint pain with conflicting reports of cell viability and protein contents. The objective of this study was to determine the full proteome and examine cell viability in 9 commercial amnion products using an unbiased bottom-up shotgun proteomics approach and confocal microscopy.

DESIGN

Products were subjected to liquid chromatography-tandem mass spectrometry (LC-MS/MS) analysis and searched against a UniProt Homo sapiens database. Relative protein abundance was determined for each sample. Based on proteomics results, lumican was measured by enzyme-linked immunosorbent assay (ELISA) and Western blot analysis was performed for interleukin-1 receptor antagonist (IL-1Ra) and tissue inhibitor of matrix metalloproteinase-2 (TIMP-2). Cell viability was determined by calcein AM (live) and ethidium homodimer (dead) staining and confocal microscopy.

RESULTS

Proteomic analysis revealed 919 proteins in the nine products. Proteins were primarily collagens, keratin, and albumin. Lumican, a small leucine-rich proteoglycan (SLRP) was found in all samples. Western blot analysis for IL-1Ra and TIMP-2 indicated presence of both proteins, with nonspecific antibody binding also present in all samples. No live cells were identified in any product.

CONCLUSIONS

Several novel proteins were identified through proteomics that might impart the beneficial effects of amnion products, including SLRPs, collagens, and regulators of fibroblast activity. IL-1Ra and TIMP-2 were identified, but concentrations measured by ELISA may be falsely increased due to nonspecific antibody binding. The concept that the amnion tissues provide live cells to aid in tissue regeneration cannot be supported by the findings of this study.

Cartilage articulation exacerbates chondrocyte damage and death after impact injury

Posttraumatic osteoarthritis (PTOA) is typically initiated by momentary supraphysiologic shear and compressive forces delivered to articular cartilage during acute joint injury and develops through subsequent degradation of cartilage matrix components and tissue remodeling. PTOA affects 12% of the population who experience osteoarthritis and is attributed to over $3 billion dollars annually in healthcare costs. It is currently unknown whether articulation of the joint post-injury helps tissue healing or exacerbates cellular dysfunction and eventual death. We hypothesize that post-injury cartilage articulation will lead to increased cartilage damage. Our objective was to test this hypothesis by mimicking the mechanical environment of the joint during and post-injury and determining if subsequent joint articulation exacerbates damage produced by initial injury. We use a model of PTOA that combines impact injury and repetitive sliding with confocal microscopy to quantify and track chondrocyte viability, apoptosis, and mitochondrial depolarization in a depth-dependent manner. Cartilage explants were harvested from neonatal bovine knee joints and subjected to either rapid impact injury (17.34 ± 0.99 MPa, 21.6 ± 2.45 GPa/s), sliding (60 min at 1 mm/s, under 15% axial compression), or rapid impact injury followed by sliding. Explants were then bisected and fluorescently stained for cell viability, caspase activity (apoptosis), and mitochondria polarization. Results show that compared to either impact or sliding alone, explants that were both impacted and slid experienced higher magnitudes of damage spanning greater tissue depths.

Mitochondria in Injury, Inflammation and Disease of Articular Skeletal Joints

Inflammation is an important biological response to tissue damage caused by injury, with a crucial role in initiating and controlling the healing process. However, dysregulation of the process can also be a major contributor to tissue damage. Related to this, although mitochondria are typically thought of in terms of energy production, it has recently become clear that these important organelles also orchestrate the inflammatory response via multiple mechanisms. Dysregulated inflammation is a well-recognised problem in skeletal joint diseases, such as rheumatoid arthritis. Interestingly osteoarthritis (OA), despite traditionally being known as a ‘non-inflammatory arthritis’, now appears to involve an element of chronic inflammation. OA is considered an umbrella term for a family of diseases stemming from a range of aetiologies (age, obesity etc.), but all with a common presentation. One particular OA sub-set called Post-Traumatic OA (PTOA) results from acute mechanical injury to the joint. Whether the initial mechanical tissue damage, or the subsequent inflammatory response drives disease, is currently unclear. In the former case; mechanobiological properties of cells/tissues in the joint are a crucial consideration. Many such cell-types have been shown to be exquisitely sensitive to their mechanical environment, which can alter their mitochondrial and cellular function. For example, in bone and cartilage cells fluid-flow induced shear stresses can modulate cytoskeletal dynamics and gene expression profiles. More recently, immune cells were shown to be highly sensitive to hydrostatic pressure. In each of these cases mitochondria were central to these responses. In terms of acute inflammation, mitochondria may have a pivotal role in linking joint tissue injury with chronic disease. These processes could involve the immune cells recruited to the joint, native/resident joint cells that have been damaged, or both. Taken together, these observations suggest that mitochondria are likely to play an important role in linking acute joint tissue injury, inflammation, and long-term chronic joint degeneration - and that the process involves mechanobiological factors. In this review, we will explore the links between mechanobiology, mitochondrial function, inflammation/tissue-damage in joint injury and disease. We will also explore some emerging mitochondrial therapeutics and their potential for application in PTOA.

Urolithin A Protects Chondrocytes From Mechanical Overloading-Induced Injuries

Physiological mechanical stimulation has been shown to promote chondrogenesis, but excessive mechanical loading results in cartilage degradation. Currently, the underlying mechanotransduction pathways in the context of physiological and injurious loading are not fully understood. In this study, we aim to identify the critical factors that dictate chondrocyte response to mechanical overloading, as well as to develop therapeutics that protect chondrocytes from mechanical injuries. Specifically, human chondrocytes were loaded in hyaluronic hydrogel and then subjected to dynamic compressive loading under 5% (DL-5% group) or 25% strain (DL-25% group). Compared to static culture and DL-5%, DL-25% reduced cartilage matrix formation from chondrocytes, which was accompanied by the increased senescence level, as revealed by higher expression of p21, p53, and senescence-associated beta-galactosidase (SA-β-Gal). Interestingly, mitophagy was suppressed by DL-25%, suggesting a possible role for the restoration mitophagy in reducing cartilage degeneration with mechanical overloading. Next, we treated the mechanically overloaded samples (DL-25%) with Urolithin A (UA), a natural metabolite previously shown to enhance mitophagy in other cell types. qRT-PCR, histology, and immunostaining results confirmed that UA treatment significantly increased the quantity and quality of cartilage matrix deposition. Interestingly, UA also suppressed the senescence level induced by mechanical overloading, demonstrating its senomorphic potential. Mechanistic analysis confirmed that UA functioned partially by enhancing mitophagy. In summary, our results show that mechanical overloading results in cartilage degradation partially through the impairment of mitophagy. This study also identifies UA's novel use as a compound that can protect chondrocytes from mechanical injuries, supporting high-quality cartilage formation/maintenance.

Mechanisms linking mitochondrial mechanotransduction and chondrocyte biology in the pathogenesis of osteoarthritis

Mechanical loading is essential for chondrocyte health. Chondrocytes can sense and respond to various extracellular mechanical signals through an integrated set of mechanisms. Recently, it has been found that mitochondria, acting as critical mechanotransducers, are at the intersection between extracellular mechanical signals and chondrocyte biology. Much attention has been focused on identifying how mechanical loading-induced mitochondrial dysfunction contributes to the pathogenesis of osteoarthritis. In contrast, little is known regarding the mechanisms underlying functional alterations in mitochondria induced by mechanical stimulation. In this review, we describe how chondrocytes perceive environmental mechanical signals. We discuss how mechanical load induces mitochondrial functional alterations and highlight the major unanswered questions in this field. We speculate that AMP-activated protein kinase (AMPK), a master regulator of energy homeostasis, may play an important role in coupling force transmission to mitochondrial health and intracellular biological responses.

Intersections Between Mitochondrial Metabolism and Redox Biology Mediate Posttraumatic Osteoarthritis

PURPOSE OF REVIEW

This review will cover foundational studies and recent findings that established key concepts for understanding the importance of redox biology to chondrocyte mitochondrial function and osteoarthritis pathophysiology after injury.

RECENT FINDINGS

Articular chondrocyte mitochondria can be protected with a wide variety of antioxidants that will be discussed within a framework suggested by classic studies. These agents not only underscore the importance of thiol metabolism and associated redox function for chondrocyte mitochondria but also suggest complex interactions with signal transduction pathways and other molecular features of osteoarthritis that require more thorough investigation. Emerging evidence also indicates that reductive stress could occur alongside oxidative stress. Recent studies have shed new light on historic paradoxes in chondrocyte redox and mitochondrial physiology, leading to the development of promising disease-modifying therapies for posttraumatic osteoarthritis.

Subchondral bone microenvironment in osteoarthritis and pain

Osteoarthritis comprises several joint disorders characterized by articular cartilage degeneration and persistent pain, causing disability and economic burden. The incidence of osteoarthritis is rapidly increasing worldwide due to aging and obesity trends. Basic and clinical research on osteoarthritis has been carried out for decades, but many questions remain unanswered. The exact role of subchondral bone during the initiation and progression osteoarthritis remains unclear. Accumulating evidence shows that subchondral bone lesions, including bone marrow edema and angiogenesis, develop earlier than cartilage degeneration. Clinical interventions targeting subchondral bone have shown therapeutic potential, while others targeting cartilage have yielded disappointing results. Abnormal subchondral bone remodeling, angiogenesis and sensory nerve innervation contribute directly or indirectly to cartilage destruction and pain. This review is about bone-cartilage crosstalk, the subchondral microenvironment and the critical role of both in osteoarthritis progression. It also provides an update on the pathogenesis of and interventions for osteoarthritis and future research targeting subchondral bone.

Intercellular mitochondrial transfer as a means of tissue revitalization

As the crucial powerhouse for cell metabolism and tissue survival, the mitochondrion frequently undergoes morphological or positional changes when responding to various stresses and energy demands. In addition to intracellular changes, mitochondria can also be transferred intercellularly. Besides restoring stressed cells and damaged tissues due to mitochondrial dysfunction, the intercellular mitochondrial transfer also occurs under physiological conditions. In this review, the phenomenon of mitochondrial transfer is described according to its function under both physiological and pathological conditions, including tissue homeostasis, damaged tissue repair, tumor progression, and immunoregulation. Then, the mechanisms that contribute to this process are summarized, such as the trigger factors and transfer routes. Furthermore, various perspectives are explored to better understand the mysteries of cell-cell mitochondrial trafficking. In addition, potential therapeutic strategies for mitochondria-targeted application to rescue tissue damage and degeneration, as well as the inhibition of tumor progression, are discussed.

Extracellular Vesicles in Musculoskeletal Pathologies and Regeneration

The incidence of musculoskeletal diseases is steadily increasing with aging of the population. In the past years, extracellular vesicles (EVs) have gained attention in musculoskeletal research. EVs have been associated with various musculoskeletal pathologies as well as suggested as treatment option. EVs play a pivotal role in communication between cells and their environment. Thereby, the EV cargo is highly dependent on their cellular origin. In this review, we summarize putative mechanisms by which EVs can contribute to musculoskeletal tissue homeostasis, regeneration and disease, in particular matrix remodeling and mineralization, pro-angiogenic effects and immunomodulatory activities. Mesenchymal stromal cells (MSCs) present the most frequently used cell source for EV generation for musculoskeletal applications, and herein we discuss how the MSC phenotype can influence the cargo and thus the regenerative potential of EVs. Induced pluripotent stem cell-derived mesenchymal progenitor cells (iMPs) may overcome current limitations of MSCs, and iMP-derived EVs are discussed as an alternative strategy. In the last part of the article, we focus on therapeutic applications of EVs and discuss both practical considerations for EV production and the current state of EV-based therapies.

Riding the tiger - physiological and pathological effects of superoxide and hydrogen peroxide generated in the mitochondrial matrix

Elevated mitochondrial matrix superoxide and/or hydrogen peroxide concentrations drive a wide range of physiological responses and pathologies. Concentrations of superoxide and hydrogen peroxide in the mitochondrial matrix are set mainly by rates of production, the activities of superoxide dismutase-2 (SOD2) and peroxiredoxin-3 (PRDX3), and by diffusion of hydrogen peroxide to the cytosol. These considerations can be used to generate criteria for assessing whether changes in matrix superoxide or hydrogen peroxide are both necessary and sufficient to drive redox signaling and pathology: is a phenotype affected by suppressing superoxide and hydrogen peroxide production; by manipulating the levels of SOD2, PRDX3 or mitochondria-targeted catalase; and by adding mitochondria-targeted SOD/catalase mimetics or mitochondria-targeted antioxidants? Is the pathology associated with variants in SOD2 and PRDX3 genes? Filtering the large literature on mitochondrial redox signaling using these criteria highlights considerable evidence that mitochondrial superoxide and hydrogen peroxide drive physiological responses involved in cellular stress management, including apoptosis, autophagy, propagation of endoplasmic reticulum stress, cellular senescence, HIF1α signaling, and immune responses. They also affect cell proliferation, migration, differentiation, and the cell cycle. Filtering the huge literature on pathologies highlights strong experimental evidence that 30-40 pathologies may be driven by mitochondrial matrix superoxide or hydrogen peroxide. These can be grouped into overlapping and interacting categories: metabolic, cardiovascular, inflammatory, and neurological diseases; cancer; ischemia/reperfusion injury; aging and its diseases; external insults, and genetic diseases. Understanding the involvement of mitochondrial matrix superoxide and hydrogen peroxide concentrations in these diseases can facilitate the rational development of appropriate therapies.

Mitoprotective therapy prevents rapid, strain-dependent mitochondrial dysfunction after articular cartilage injury

Posttraumatic osteoarthritis (PTOA) involves the mechanical and biological deterioration of articular cartilage that occurs following joint injury. PTOA is a growing problem in health care due to the lack of effective therapies combined with an aging population with high activity levels. Recently, acute mitochondrial dysfunction and altered cellular respiration have been associated with cartilage degeneration after injury. This finding is particularly important because recently developed mitoprotective drugs, including SS peptides, can preserve mitochondrial structure and function after acute injury in other tissues. It is not known, however, if cartilage injury induces rapid structural changes in mitochondria, to what degree mitochondrial dysfunction in cartilage depends on the mechanics of injury or the time frame over which such dysfunction develops. Similarly, it is unknown if SS-peptide treatment can preserve mitochondrial structure and function after cartilage injury. Here, we combined fast camera elastography, longitudinal fluorescence assays, and computer vision techniques to track the fates of thousands of individual cells. Our results show that impact induces mechanically dependent mitochondrial depolarization within a few minutes after injury. Electron microscopy revealed that impact causes rapid structural changes in mitochondria that are related to reduced mitochondrial function, namely, fission and loss of cristae structure. We found that SS-peptide treatment prior to impact protects the mitochondrial structure and preserves mitochondrial function at levels comparable with that of unimpacted control samples. Overall, this study reveals the vital role of mitochondria in mediating cartilage's peracute (within minutes) response to traumatic injury and demonstrates mitoprotection as a promising therapeutic strategy for injury-induced cartilage damage.

Pathomechanisms of Posttraumatic Osteoarthritis: Chondrocyte Behavior and Fate in a Precarious Environment

Traumatic injuries of the knee joint result in a wide variety of pathomechanisms, which contribute to the development of so-called posttraumatic osteoarthritis (PTOA). These pathogenetic processes include oxidative stress, excessive expression of catabolic enzymes, release of damage-associated molecular patterns (DAMPs), and synovial inflammation. The present review focuses on the underlying pathomechanisms of PTOA and in particular the behavior and fate of the surviving chondrocytes, comprising chondrocyte metabolism, regulated cell death, and phenotypical changes comprising hypertrophy and senescence. Moreover, possible therapeutic strategies, such as chondroanabolic stimulation, anti-oxidative and anti-inflammatory treatment, as well as novel therapeutic targets are discussed.

Adenosine A2A receptor (A2AR) stimulation enhances mitochondrial metabolism and mitigates reactive oxygen species-mediated mitochondrial injury

In OA chondrocytes, there is diminished mitochondrial production of ATP and diminished extracellular adenosine resulting in diminished adenosine A2A receptor (A2AR) stimulation and altered chondrocyte homeostasis which contributes to the pathogenesis of OA. We tested the hypothesis that A2AR stimulation maintains or enhances mitochondrial function in chondrocytes. The effect of A2AR signaling on mitochondrial health and function was determined in primary murine chondrocytes, a human chondrocytic cell line (T/C-28a2), primary human chondrocytes, and a murine model of OA by transmission electron microscopy analysis, mitochondrial stress testing, confocal live imaging for mitochondrial inner membrane polarity, and immunohistochemistry. In primary murine chondrocytes from A2AR^-/- null mice, which develop spontaneous OA by 16 weeks, there is mitochondrial swelling, dysfunction, and reduced mitochondrial content with increased reactive oxygen species (ROS) burden and diminished mitophagy, as compared to chondrocytes from WT animals. IL-1-stimulated T/C-28a2 cells treated with an A2AR agonist had reduced ROS burden with increased mitochondrial dynamic stability and function, findings which were recapitulated in primary human chondrocytes. In an obesity-induced OA mouse model, there was a marked increase in mitochondrial oxidized material which was markedly improved after intraarticular injections of liposomal A2AR agonist. These results are consistent with the hypothesis that A2AR ligation is mitoprotective in OA.

Stimulation of tolerogenic dendritic cells using dexamethasone and 1,25-dihydroxyvitamin D3 represses autologous T cell activation and chondrocyte inflammation

Human osteoarthritis (OA) has been reclassified as a systemic musculoskeletal disorder involving activation of the innate and adaptive immune systems. Elevated pro-inflammatory cytokines may serve a key function in the development of the disease. 1,25-Dihydroxyvitamin D3 and dexamethasone (vitD3/Dex) may inhibit inflammation by acting on tolerogenic dendritic cells (tolDCs) in chronic inflammatory conditions. In the present study, DCs were isolated from peripheral blood mononuclear cells of patients with OA. DCs expressing high levels of co-stimulatory molecules maintain a tolerogenic phenotype under stimulation with LPS, which promotes DC maturation to generate tolDCs. When vitD3/Dex were added in the current study, the tolDCs produced pro-inflammatory cytokines, including low levels of tumor necrosis factor-α, interleukin (IL)-1β, IL-6 and IL-10, and high levels of transforming growth factor-β. However, when vitD3/Dex were added to DCs without LPS stimulation, the levels of IL-10 were high. DCs with LPS stimulation increased the percentage of T-cells that produced IFN-γ and IL-17 and DCs with vitD3/Dex treatment alone increased the percentage of T-cells that produced IL-10 and FoxP3. However, those cytokines decrease in DCs co-processed with LPS and vitD3/Dex. The IL-10 release by the stimulated T cells was indicated to repress autologous T cell proliferation via soluble IL-10 and cell-cell contact. Furthermore, tolDCs and regulatory T cells suppressed matrix metalloproteinase (MMP)-1 and MMP-13 secretion by chondrocytes. Additionally, Akt and p38 mitogen-activated protein kinase signaling were demonstrated to be involved in the regulatory effects of Dec and vitD3 in DCs. The present findings suggest a novel mechanism underlying the beneficial effects of tolDCs, particularly in association with the pathogenesis of OA.

Microscale frictional strains determine chondrocyte fate in loaded cartilage

Mounting evidence suggests that altered lubricant levels within synovial fluid have acute biological consequences on chondrocyte homeostasis. While these responses have been connected to increased friction, the mechanisms behind this response remain unknown. Here, we combine a frictional bioreactor with confocal elastography and image-based cellular assays to establish the link between cartilage friction, microscale shear strain, and acute, adverse cellular responses. Our incorporation of cell-scale strain measurements reveals that elevated friction generates high shear strains localized near the tissue surface, and that these elevated strains are closely associated with mitochondrial dysfunction, apoptosis, and cell death. Collectively, our data establish two pathways by which chondrocytes negatively respond to friction: an immediate necrotic response and a longer term pathway involving mitochondrial dysfunction and apoptosis. Specifically, in the surface region, where shear strains can exceed 0.07, cells are predisposed to acute death; however, below this surface region, cells exhibit a pathway consistent with apoptosis in a manner predicted by local shear strains. These data reveal a mechanism through which cellular damage in cartilage arises from compromised lubrication and show that in addition to boundary lubricants, there are opportunities upstream of apoptosis to preserve chondrocyte health in arthritis therapy.