Molecular markers predictive of the capacity of expanded human articular chondrocytes to form stable cartilage in vivo

Epigenetic Priming Enhances Chondrogenic Potential of Expanded Chondrocytes

Expansion of chondrocytes presents a major obstacle in the cartilage regeneration procedure, such as matrix-induced autologous chondrocyte implantation. Dedifferentiation of chondrocytes during the expansion process leads to the emergence of a fibrotic (chondrofibrotic) phenotype that decreases the chondrogenic potential of the implanted cells. We aim to (1) determine the extent that chromatin architecture of H3K27me3 and H3K9me3 remodels during dedifferentiation and persists after the transfer to a three-dimensional (3D) culture; and (2) to prevent this persistent remodeling to enhance the chondrogenic potential of expanded bovine chondrocytes, used as a model system. Chromatin architecture remodeling of H3K27me3 and H3K9me3 was observed at 0 population doublings, 8 population doublings, and 16 population doublings (PD16) in a two-dimensional (2D) culture and after encapsulation of the expanded chondrocytes in a 3D hydrogel culture. Chondrocytes were treated with inhibitors of epigenetic modifiers (epigenetic priming) for PD16 and then encapsulated in 3D hydrogels. Chromatin architecture of chondrocytes and gene expression were evaluated before and after encapsulation. We observed a change in chromatin architecture of epigenetic modifications H3K27me3 and H3K9me3 during chondrocyte dedifferentiation. Although inhibiting enzymes that modify H3K27me3 and H3K9me3 did not alter the dedifferentiation process in 2D culture, applying these treatments during the 2D expansion did increase the expression of select chondrogenic genes and protein deposition of type II collagen when transferred to a 3D environment. Overall, we found that epigenetic priming of expanded bovine chondrocytes alters the cell fate when chondrocytes are later encapsulated into a 3D environment, providing a potential method to enhance the success of cartilage regeneration procedures.

A Protein-Adsorbent Hydrogel with Tunable Stiffness for Tissue Culture Demonstrates Matrix-Dependent Stiffness Responses

Although tissue culture plastic has been widely employed for cell culture, the rigidity of plastic is not physiologic. Softer hydrogels used to culture cells have not been widely adopted in part because coupling chemistries are required to covalently capture extracellular matrix (ECM) proteins and support cell adhesion. To create an in vitro system with tunable stiffnesses that readily adsorbs ECM proteins for cell culture, we present a novel hydrophobic hydrogel system via chemically converting hydroxyl residues on the dextran backbone to methacrylate groups, thereby transforming non-protein adhesive, hydrophilic dextran to highly protein adsorbent substrates. Increasing methacrylate functionality increases the hydrophobicity in the resulting hydrogels and enhances ECM protein adsorption without additional chemical reactions. These hydrophobic hydrogels permit facile and tunable modulation of substrate stiffness independent of hydrophobicity or ECM coatings. Using this approach, we show that substrate stiffness and ECM adsorption work together to affect cell morphology and proliferation, but the strengths of these effects vary in different cell types. Furthermore, we reveal that stiffness mediated differentiation of dermal fibroblasts into myofibroblasts is modulated by the substrate ECM. Our material system demonstrates remarkable simplicity and flexibility to tune ECM coatings and substrate stiffness and study their effects on cell function.

WTAP-mediated m6A modification of FRZB triggers the inflammatory response via the Wnt signaling pathway in osteoarthritis

Osteoarthritis (OA) is the most common form of arthritis. However, the exact pathogenesis remains unclear. Emerging evidence shows that N6-methyladenosine (m6A) modification may have an important role in OA pathogenesis. This study aimed to investigate the role of m6A writers and the underlying mechanisms in osteoarthritic cartilage. Among m6A methyltransferases, Wilms tumor 1-associated protein (WTAP) expression most significantly differed in clinical osteoarthritic cartilage. WTAP regulated extracellular matrix (ECM) degradation, inflammation and antioxidation in human chondrocytes. Mechanistically, the m6A modification and relative downstream targets in osteoarthritic cartilage were assessed by methylated RNA immunoprecipitation sequencing (MeRIP-seq) and RNA sequencing, which indicated that the expression of frizzled-related protein (FRZB), a secreted Wnt antagonist, was abnormally decreased and accompanied by high m6A modification in osteoarthritic cartilage. In vitro dysregulated WTAP had positive effects on β-catenin expression by targeting FRZB, which finally contributed to the cartilage injury phenotype in chondrocytes. Intra-articular injection of adeno-associated virus-WTAP alleviated OA progression in a mouse model, while this protective effect could be reversed by the application of a Wnt/β-catenin activator. In summary, this study revealed that WTAP-dependent RNA m6A modification contributed to Wnt/β-catenin pathway activation and OA progression through post-transcriptional regulation of FRZB mRNA, thus providing a potentially effective therapeutic strategy for OA treatment.

Hydroquinone, an Environmental Pollutant, Affects Cartilage Homeostasis through the Activation of the Aryl Hydrocarbon Receptor Pathway

Exposure to environmental pollutants has a proven detrimental impact on different aspects of human health. Increasing evidence has linked pollution to the degeneration of tissues in the joints, although through vastly uncharacterised mechanisms. We have previously shown that exposure to hydroquinone (HQ), a benzene metabolite that can be found in motor fuels and cigarette smoke, exacerbates synovial hypertrophy and oxidative stress in the synovium. To further understand the impact of the pollutant on joint health, here we investigated the effect of HQ on the articular cartilage. HQ exposure aggravated cartilage damage in rats in which inflammatory arthritis was induced by injection of Collagen type II. Cell viability, cell phenotypic changes and oxidative stress were quantified in primary bovine articular chondrocytes exposed to HQ in the presence or absence of IL-1β. HQ stimulation downregulated phenotypic markers genes SOX-9 and Col2a1, whereas it upregulated the expression of the catabolic enzymes MMP-3 and ADAMTS5 at the mRNA level. HQ also reduced proteoglycan content and promoted oxidative stress alone and in synergy with IL-1β. Finally, we showed that HQ-degenerative effects were mediated by the activation of the Aryl Hydrocarbon Receptor. Together, our findings describe the harmful effects of HQ on articular cartilage health, providing novel evidence surrounding the toxic mechanisms of environmental pollutants underlying the onset of articular diseases.

Articular Cartilage Chondroprogenitors: Isolation and Directed Differentiation

Experimental data suggests that tissue-specific progenitors are present within hyaline articular cartilage with the potential to contribute to growth, maintenance, and repair. In this chapter, we show how colony-forming progenitor-like cells can be isolated from bovine articular cartilage using differential adhesion to fibronectin. Furthermore, we describe the optimal conditions and factors required to differentiate these progenitor cells to produce hyaline articular cartilage.

Disease modification and symptom relief in osteoarthritis using a mutated GCP-2/CXCL6 chemokine

We showed that the chemokine receptor C-X-C Motif Chemokine Receptor 2 (CXCR2) is essential for cartilage homeostasis. Here, we reveal that the CXCR2 ligand granulocyte chemotactic protein 2 (GCP-2) was expressed, during embryonic development, within the prospective permanent articular cartilage, but not in the epiphyseal cartilage destined to be replaced by bone. GCP-2 expression was retained in adult articular cartilage. GCP-2 loss-of-function inhibited extracellular matrix production. GCP-2 treatment promoted chondrogenesis in vitro and in human cartilage organoids implanted in nude mice in vivo. To exploit the chondrogenic activity of GCP-2, we disrupted its chemotactic activity, by mutagenizing a glycosaminoglycan binding sequence, which we hypothesized to be required for the formation of a GCP-2 haptotactic gradient on endothelia. This mutated version (GCP-2-T) had reduced capacity to induce transendothelial migration in vitro and in vivo, without affecting downstream receptor signaling through AKT, and chondrogenic activity. Intra-articular adenoviral overexpression of GCP-2-T, but not wild-type GCP-2, reduced pain and cartilage loss in instability-induced osteoarthritis in mice. We suggest that GCP-2-T may be used for disease modification in osteoarthritis.

Cellular therapy and tissue engineering for cartilage repair

Articular cartilage (AC) has limited capacity for repair. The first attempt to repair cartilage using tissue engineering was reported in 1977. Since then, cell-based interventions have entered clinical practice in orthopaedics, and several tissue engineering approaches to repair cartilage are in the translational pipeline towards clinical application. Classically, these involve a scaffold, substrate or matrix to provide structure, and cells such as chondrocytes or mesenchymal stromal cells to generate the tissue. We discuss the advantages and drawbacks of the use of various cell types, natural and synthetic scaffolds, multiphasic or gradient-based scaffolds, and self-organizing or self-assembling scaffold-free systems, for the engineering of cartilage constructs. Several challenges persist including achieving zonal tissue organization and integration with the surrounding tissue upon implantation. Approaches to improve cartilage thickness, organization and mechanical properties include mechanical stimulation, culture under hypoxic conditions, and stimulation with growth factors or other macromolecules. In addition, advanced technologies such as bioreactors, biosensors and 3D bioprinting are actively being explored. Understanding the underlying mechanisms of action of cell therapy and tissue engineering approaches will help improve and refine therapy development. Finally, we discuss recent studies of the intrinsic cellular and molecular mechanisms of cartilage repair that have identified novel signals and targets and are inspiring the development of molecular therapies to enhance the recruitment and cartilage reparative activity of joint-resident stem and progenitor cells. A one-fits-all solution is unrealistic, and identifying patients who will respond to a specific targeted treatment will be critical.

In vivo potency assay for the screening of bioactive molecules on cartilage formation

Cartilage regeneration is a priority in medicine for the treatment of osteoarthritis and isolated cartilage defects. Several molecules with potential for cartilage regeneration are under investigation. Unfortunately, in vitro chondrogenesis assays do not always predict the stability of the newly formed cartilage in vivo. Therefore, there is a need for a stringent, quantifiable assay to assess in vivo the capacity of molecules to promote the stable formation of cartilage that is resistant to calcification and endochondral bone formation. We developed an ectopic cartilage formation assay (ECFA) that enables one to assess the capacity of bioactive molecules to support cartilage formation in vivo using cartilage organoids. The ECFA predicted good clinical outcomes when used as a quality control for efficacy of chondrocyte preparations before implantation in patients with cartilage defects. In this assay, articular chondrocytes from human donors or animals are injected either intramuscularly or subcutaneously in nude mice. As early as 2 weeks later, cartilage organoids can be retrieved. The size of the implants and their degree of differentiation can be assessed by histomorphometry, immunostainings of molecular markers and real-time PCR. Mineralization can be assessed by micro-computed tomography or by staining. The effects of molecules on cartilage formation can be tested following the systemic administration of the molecule in mice previously injected with chondrocytes, or after co-injection of chondrocytes with cell lines overexpressing and secreting the protein of interest. Here we describe the ECFA procedure, including steps for harvesting human and bovine articular cartilage, isolating primary chondrocytes, preparing overexpression cell lines, injecting the cells intramuscularly and retrieving the implants. This assay can be performed by technicians and researchers with appropriate animal training within 3 weeks.

Cell Sources for Cartilage Repair-Biological and Clinical Perspective

Cell-based therapy represents a promising treatment strategy for cartilage defects. Alone or in combination with scaffolds/biological signals, these strategies open many new avenues for cartilage tissue engineering. However, the choice of the optimal cell source is not that straightforward. Currently, various types of differentiated cells (articular and nasal chondrocytes) and stem cells (mesenchymal stem cells, induced pluripotent stem cells) are being researched to objectively assess their merits and disadvantages with respect to the ability to repair damaged articular cartilage. In this paper, we focus on the different cell types used in cartilage treatment, first from a biological scientist’s perspective and then from a clinician’s standpoint. We compare and analyze the advantages and disadvantages of these cell types and offer a potential outlook for future research and clinical application.

Mesenchymal Stem Cells: Current Concepts in the Management of Inflammation in Osteoarthritis

Osteoarthritis (OA) has traditionally been known as a “wear and tear” disease, which is mainly characterized by the degradation of articular cartilage and changes in the subchondral bone. Despite the fact that OA is often thought of as a degenerative disease, the catabolic products of the cartilage matrix often promote inflammation by activating immune cells. Current OA treatment focuses on symptomatic treatment, with a primary focus on pain management, which does not promote cartilage regeneration or attenuate joint inflammation. Since articular cartilage have no ability to regenerate, thus regeneration of the tissue is one of the key targets of modern treatments for OA. Cell-based therapies are among the new therapeutic strategies for OA. Mesenchymal stem cells (MSCs) have been extensively researched as potential therapeutic agents in cell-based therapy of OA due to their ability to differentiate into chondrocytes and their immunomodulatory properties that can facilitate cartilage repair and regeneration. In this review, we emphasized current knowledge and future perspectives on the use of MSCs by targeting their regeneration potential and immunomodulatory effects in the treatment of OA.

Role of Matrix-Associated Autologous Chondrocyte Implantation with Spheroids in the Treatment of Large Chondral Defects in the Knee: A Systematic Review

Autologous chondrocyte implantation (ACI) is a cell therapy for the treatment of focal cartilage defects. The ACI product that is currently approved for use in the European Union (EU) consists of spheroids of autologous matrix-associated chondrocytes. These spheroids are spherical aggregates of ex vivo expanded human autologous chondrocytes and their self-synthesized extracellular matrix. The aim is to provide an overview of the preclinical and nonclinical studies that have been performed to ensure reproducible quality, safety, and efficacy of the cell therapy, and to evaluate the clinical data on ACI with spheroids. A systematic review was performed to include all English publications on self-aggregated spheroids of chondrocytes cultured in autologous serum without other supplements. A total of 20 publications were included, 7 pre- and nonclinical and 13 clinical research publications. The pre- and nonclinical research publications describe the development from concept to in vivo efficacy and quality- and safety-related aspects such as biodistribution, tumorigenicity, genetic stability, and potency. The evaluation of clinical research shows short- to mid-term safety and efficacy for the ACI with spheroid-based treatment of cartilage defects in both randomized clinical trials with selected patients, as well as in routine treatment providing real-world data in more complex patients.

ROR2 blockade as a therapy for osteoarthritis

Osteoarthritis is characterized by the loss of the articular cartilage, bone remodeling, pain, and disability. No pharmacological intervention can currently halt progression of osteoarthritis. Here, we show that blocking receptor tyrosine kinase-like orphan receptor 2 (ROR2) improves cartilage integrity and pain in osteoarthritis models by inhibiting yes-associated protein (YAP) signaling. ROR2 was up-regulated in the cartilage in response to inflammatory cytokines and mechanical stress. The main ligand for ROR2, WNT5A, and the targets YAP and connective tissue growth factor were up-regulated in osteoarthritis in humans. In vitro, ROR2 overexpression inhibited chondrocytic differentiation. Conversely, ROR2 blockade triggered chondrogenic differentiation of C3H10T¹/₂ cells and suppressed the expression of the cartilage-degrading enzymes a disintegrin and metalloproteinase with thrombospondin motifs (ADAMTS)-4 and ADAMTS-5. The chondrogenic effect of ROR2 blockade in the cartilage was independent of WNT signaling and was mediated by down-regulation of YAP signaling. ROR2 signaling induced G protein and Rho-dependent nuclear accumulation of YAP, and YAP inhibition was required but not sufficient for ROR2 blockade-induced chondrogenesis. ROR2 silencing protected mice from instability-induced osteoarthritis with improved structural outcomes, sustained pain relief, and without apparent side effects or organ toxicity. Last, ROR2 silencing in human articular chondrocytes transplanted in nude mice led to the formation of cartilage organoids with more and better differentiated extracellular matrix, suggesting that the anabolic effect of ROR2 blockade is conserved in humans. Thus, ROR2 blockade is efficacious and well tolerated in preclinical animal models of osteoarthritis.

Ectopic models recapitulating morphological and functional features of articular cartilage

BACKGROUND

Articular cartilage is an extremely specialized connective tissue which covers all diarthrodial joints. Implantation of chondrogenic cells without or with additional biomaterial scaffolds in ectopic locationsin vivo generates substitutes of cartilage with structural and functional characteristics that are used in fundamental investigations while also serving as a basis for translational studies.

METHODS

Literature search in Pubmed.

RESULTS AND DISCUSSION

This narrative review summarizes the most relevant ectopic models, among which subcutaneous, intramuscular, and kidney capsule transplantation and elaborates on implanted cells and biomaterial scaffolds and on their use to recapitulate morphological and functional features of articular cartilage. Although the absence of a physiological joint environment and biomechanical stimuli is the major limiting factor, ectopic models are an established component for articular cartilage research aiming to generate a bridge between in vitro data and the clinically more relevant translational orthotopic in vivo models when their limitations are considered.

WNT Signalling in Osteoarthritis and Its Pharmacological Targeting

Osteoarthritis (OA) is a highly disabling musculoskeletal condition affecting millions of people worldwide. OA is characterised by progressive destruction and irreversible morphological changes of joint tissues and architecture. At molecular level, de-regulation of several pathways contributes to the disruption of tissue homeostasis in the joint. Overactivation of the WNT/β-catenin signalling pathway has been associated with degenerative processes in OA. However, the multiple layers of complexity in the modulation of the signalling and the still insufficient knowledge of the specific molecular drivers of pathogenetic mechanisms have made difficult the pharmacological targeting of this pathway for therapeutic purposes. This review aims to provide an overview of the WNT/β-catenin signalling in OA with a particular focus on its role in the articular cartilage. Pathway components whose targeting showed therapeutic potential will be highlighted and described. A specific section will be dedicated to Lorecivivint, the first inhibitor of the β-catenin-dependent pathway currently in phase III clinical trial as OA-modifying agent.

Adult ovine chondrocytes in expansion culture adopt progenitor cell properties that are favorable for cartilage tissue engineering

Human chondrocytes in expansion culture can become progenitor-like in their ability to proliferate extensively and secrete neocartilage in chondrogenic culture. Sheep are used as a large animal model for cartilage tissue engineering, although for testing progenitor-like chondrocytes it is important that ovine chondrocytes resemble human in the ability to adopt progenitor properties. Here, we investigate whether ovine chondrocytes can adopt progenitor properties as indicated by rapid proliferation in a colony-forming fashion, and high levels of neocartilage secretion in chondrogenic culture. In conditions known to promote expansion of mesenchymal stromal cells, ovine chondrocytes proliferated through approximately 12 population doublings in 10 days. Time-lapse imaging indicated rapid proliferation in a colony-forming pattern. Expanded ovine chondrocytes that were seeded into agarose and cultured in chondrogenic medium accumulated neocartilage over 2 weeks, to a greater extent than primary chondrocytes. These data confirm that ovine chondrocytes resemble human chondrocytes in their ability to acquire progenitor properties that are important for cartilage tissue engineering. Given the broad interest in using progenitor cells to heal connective tissues, next we compared proliferation and trilineage differentiation of ovine chondrocytes, meniscus cells, and tenocytes. Meniscus cells and tenocytes experienced more than 13 population doublings in 10 days. In chondrogenic culture, cartilage matrix accumulation, and gene expression were largely similar among the cell types. All cell types resisted osteogenesis, while expanded tenocytes and meniscal cells were capable of adipogenesis. While ovine connective tissue cells demonstrated limited lineage plasticity, these data support the potential to promote certain progenitor properties with expansion.

Influence of Conditioned Media on the Re-Differentiation Capacity of Human Chondrocytes in 3D Spheroid Cultures

A major challenge of cell-based therapy for cartilage lesions is the preservation of the chondrogenic phenotype during ex vivo cell cultivation. In this in vitro study, the chondro-inductive capacity of two different hyaline cartilage-conditioned cell culture media on human chondrocytes in 3D spheroids was determined. Media were conditioned by incubation of 200 mg/mL vital or devitalized cartilage matrix in growth media over 35 days. The media were analyzed for the content of soluble procollagen type (Col) II and glycosaminoglycans (GAGs) as well as released TGF-β1, IGF-1 and IGFBP3. Unconditioned medium served as a negative control while the positive medium control was supplemented with TGF-β1 and IGF-1. Spheroid cultures prepared from human chondrocytes were cultivated at 37 °C, 5% CO₂ and 21% O₂ in the respective media and controls. After 14 and 35 days, the deposition of ECM components was evaluated by histological analysis. Vital cartilage-conditioned medium contained significantly higher levels of Col II and active TGF-β1 compared to medium conditioned with the devitalized cartilage matrix. Despite these differences, the incubation with vital as well as devitalized cartilage conditioned medium led to similar results in terms of deposition of proteoglycans and collagen type II, which was used as an indicator of re-differentiation of human chondrocytes in spheroid cultures. However, high density 3D cell cultivation showed a positive influence on re-differentiation.

Lessons from joint development for cartilage repair in the clinic

More than 250 years ago, William Hunter stated that when cartilage is destroyed it never recovers. In the last 20 years, the understanding of the mechanisms that lead to joint formation and the knowledge that some of these mechanisms are reactivated in the homeostatic responses of cartilage to injury has offered an unprecedented therapeutic opportunity to achieve cartilage regeneration. Very large investments in ambitious clinical trials are finally revealing that, although we do not have perfect medicines yet, disease modification is a feasible possibility for human osteoarthritis.

Human pluripotent stem cell-derived chondroprogenitors for cartilage tissue engineering

The cartilage of joints, such as meniscus and articular cartilage, is normally long lasting (i.e., permanent). However, once damaged, especially in large animals and humans, joint cartilage is not spontaneously repaired. Compensating the lack of repair activity by supplying cartilage-(re)forming cells, such as chondrocytes or mesenchymal stromal cells, or by transplanting a piece of normal cartilage, has been the basis of therapy for biological restoration of damaged joint cartilage. Unfortunately, current biological therapies face problems on a number of fronts. The joint cartilage is generated de novo from a specialized cell type, termed a 'joint progenitor' or 'interzone cell' during embryogenesis. Therefore, embryonic chondroprogenitors that mimic the property of joint progenitors might be the best type of cell for regenerating joint cartilage in the adult. Pluripotent stem cells (PSCs) are expected to differentiate in culture into any somatic cell type through processes that mimic embryogenesis, making human (h)PSCs a promising source of embryonic chondroprogenitors. The major research goals toward the clinical application of PSCs in joint cartilage regeneration are to (1) efficiently generate lineage-specific chondroprogenitors from hPSCs, (2) expand the chondroprogenitors to the number needed for therapy without loss of their chondrogenic activity, and (3) direct the in vivo or in vitro differentiation of the chondroprogenitors to articular or meniscal (i.e., permanent) chondrocytes rather than growth plate (i.e., transient) chondrocytes. This review is aimed at providing the current state of research toward meeting these goals. We also include our recent achievement of successful generation of "permanent-like" cartilage from long-term expandable, hPSC-derived ectomesenchymal chondroprogenitors.

Dual Network Hydrogels Incorporated with Bone Morphogenic Protein-7-Loaded Hyaluronic Acid Complex Nanoparticles for Inducing Chondrogenic Differentiation of Synovium-Derived Mesenchymal Stem Cells

Alginate-poloxamer (ALG-POL) copolymer with optimal POL content was synthesized, and it was combined with silk fibroin (SF) for building ALG-POL/SF dual network hydrogels. Hyaluronic acid(HA)/chitosan-poly(dioxanone)(CH-PDO) complex nanoparticles (NPs) with optimized composition and high encapsulation efficiency were employed as a vehicle for loading bone morphogenic protein-7 (BMP-7). BMP-7-loaded HA/CH-PDO NPs were incorporated into ALG-POL/SF hydrogel for constructing composite gels to achieve controlled release of BMP-7. These gels showed thermosensitive sol-gel transitions near physiological temperature and pH; and they were tested to be elastic, tough and strong. Some gels exhibited abilities to administer the BMP-7 release in nearly linear manners for a few weeks. Synovium-derived mesenchymal stem cells (SMSCs) were seeded into optimally fabricated gels for assessing their chondrogenic differentiation potency. Real-time PCR analyses showed that the blank ALG-POL/SF gels were not able to induce the chondrogenic differentiation of SMSCs, whereas SMSCs were detected to significantly express cartilage-related genes once they were seeded in the BMP-7-loaded ALG-POL/SF gel for two weeks. The synthesis of cartilaginous matrix components further confirmed that SMSCs seeded in the BMP-7-loaded ALG-POL/SF gel differentiated toward chondrogenesis. Results suggest that BMP-7-loaded ALG-POL/SF composite gels can function as a promising biomaterial for cartilage tissue engineering applications.

Collagen Type I and II Blend Hydrogel with Autologous Mesenchymal Stem Cells as a Scaffold for Articular Cartilage Defect Repair

Collagen type II is a promising material to repair cartilage defects since it is a major component of articular cartilage and plays a key role in chondrocyte function. This study investigated the chondrogenic differentiation of bone marrow-derived mesenchymal stem cells (MSCs) embedded within a 3:1 collagen type I to II blend (Col I/II) hydrogel or an all collagen type I (Col I) hydrogel. Glycosaminoglycan (GAG) production in Col I/II hydrogels was statistically higher than that in Col I hydrogels or pellet culture, and these results suggested that adding collagen type II promoted GAG production. Col I/II hydrogels had statistically lower alkaline phosphatase (AP) activity than pellets cultured in a chondrogenic medium. The ability of MSCs encapsulated in Col I/II hydrogels to repair cartilage defects was investigated by creating two cartilage defects in the femurs of rabbits. After 13 weeks, histochemical staining suggested that Col I/II blend hydrogels provided favorable conditions for cartilage repair. Histological scoring revealed a statistically higher cartilage repair score for the Col I/II hydrogels compared to either the Col I hydrogels or empty defect controls. Results from this study suggest that there is clinical value in the cartilage repair capabilities of our Col I/II hydrogel with encapsulated MSCs.

Thickness-wise growth technique for human articular chondrocytes to fabricate three-dimensional cartilage grafts

INTRODUCTION

Cutting the cost of manufacturing is important for extending the use of tissue-engineered therapeutic products. The present study aimed to develop a simple method for fabrication of cartilaginous tissues for regenerative therapy, utilizing the phenomenon where human articular chondrocytes grow thickness-wise and spontaneously form three-dimensionally thick tissues.

METHODS

Normal human articular chondrocytes (NHACs) were cultured with varying concentrations of transforming growth factor beta 1 (TGF-β1) and/or fibroblast growth factor-2 (FGF-2) to optimize the culture condition for thickness-wise growth of chondrocytes. Next, the tissues grown in the optimal condition were subjected to re-differentiation culture in attached and detached states to assess differentiation capacity by evaluating secreted factors, histological analysis, and a gene expression assay.

RESULTS

NHACs grew thickness-wise efficiently in the presence of 1 ng/mL TGF-β1 and 10 ng/mL FGF-2. After two weeks of culture, NHACs grew with 11-fold higher thickness and 16-fold higher cell number compared to cells which were neither treated with TGF-β1 nor with FGF-2. These thickness-wise-grown chondrocytes could be re-differentiated by a differentiation medium according to the increase in melanoma inhibitory activity (MIA) and positive safranin-O staining. Interestingly, the cartilaginous gene expression was considerably different between the attached and detached conditions even in the same culture medium, indicating the necessity of detachment and shrinkage to achieve further differentiation.

CONCLUSIONS

Spontaneous thickness-wise growth might provide a simple tissue-engineering method for manufacturing cartilaginous 3D tissues.

BCP crystals promote chondrocyte hypertrophic differentiation in OA cartilage by sequestering Wnt3a

OBJECTIVE

Calcification of cartilage with basic calcium phosphate (BCP) crystals is a common phenomenon during osteoarthritis (OA). It is directly linked to the severity of the disease and known to be associated to hypertrophic differentiation of chondrocytes. One morphogen regulating hypertrophic chondrocyte differentiation is Wnt3a.

METHODS

Calcification and sulfation of extracellular matrix of the cartilage was analysed over a time course from 6 to 22 weeks in mice and different OA grades of human cartilage. Wnt3a and ß-catenin was stained in human and murine cartilage. Expression of sulfation modulating enzymes (HS2St1, HS6St1) was analysed using quantitative reverse transcription PCR (RT-PCR). The influence of BCP crystals on the chondrocyte phenotype was investigated using quantitative RT-PCR for the marker genes Axin2, Sox9, Col2, MMP13, ColX and Aggrecan. Using western blot for β-catenin and pLRP6 we investigated the activation of Wnt signalling. The binding capacity of BCP for Wnt3a was analysed using immunohistochemical staining and western blot.

RESULTS

Here, we report that pericellular matrix sulfation is increased in human and murine OA. Wnt3a co-localised with heparan sulfate proteoglycans in the pericellular matrix of chondrocytes in OA cartilage, in which canonical Wnt signalling was activated. In vitro, BCP crystals physically bound to Wnt3a. Interestingly, BCP crystals were sufficient to induce canonical Wnt signalling as assessed by phosphorylation of LRP6 and stabilisation of β-catenin, and to induce a hypertrophic shift of the chondrocyte phenotype.

CONCLUSION

Consequently, our data identify BCP crystals as a concentrating factor for Wnt3a in the pericellular matrix and an inducer of chondrocyte hypertrophy.

Progenitor Cells Activated by Platelet Lysate in Human Articular Cartilage as a Tool for Future Cartilage Engineering and Reparative Strategies

Regenerative strategies for human articular cartilage are still challenging despite the presence of resident progenitor cell population. Today, many efforts in the field of regenerative medicine focus on the use of platelet derivatives due to their ability to reactivate endogenous mechanisms supporting tissue repair. While their use in orthopedics continues, mechanisms of action and efficacy need further characterization. We describe that the platelet lysate (PL) is able to activate chondro-progenitor cells in a terminally differentiated cartilage tissue. Primary cultures of human articular chondrocytes (ACs) and cartilage explants were set up from donor hip joint biopsies and were treated in vitro with PL. PL recruited a chondro-progenitors (CPCs)-enriched population from ex vivo cartilage culture, that showed high proliferation rate, clonogenicity and nestin expression. CPCs were positive for in vitro tri-lineage differentiation and formed hyaline cartilage-like tissue in vivo without hypertrophic fate. Moreover, the secretory profile of CPCs was analyzed, together with their migratory capabilities. Some CPC-features were also induced in PL-treated ACs compared to fetal bovine serum (FBS)-control ACs. PL treatment of human articular cartilage activates a stem cell niche responsive to injury. These facts can improve the PL therapeutic efficacy in cartilage applications.

Heterotopic ossification of tendon and ligament

Much of the similarities of the tissue characteristics, pathologies and mechanisms of heterotopic ossification (HO) formation are shared between HO of tendon and ligament (HOTL). Unmet need and no effective treatment has been developed for HOTL, primarily attributable to poor understanding of cellular and molecular mechanisms. HOTL forms via endochondral ossification, a common process of most kinds of HO. HOTL is a dynamic pathologic process that includes trauma/injury, inflammation, mesenchymal stromal cell (MSC) recruitment, chondrogenic differentiation and, finally, ossification. A variety of signal pathways involve HOTL with multiple roles in different stages of HO formation, and here in this review, we summarize the progress and provide an up‐to‐date understanding of HOTL.

Biomarker Signatures of Quality for Engineering Nasal Chondrocyte-Derived Cartilage

The definition of quality controls for cell therapy and engineered product manufacturing processes is critical for safe, effective, and standardized clinical implementation. Using the example context of cartilage grafts engineered from autologous nasal chondrocytes, currently used for articular cartilage repair in a phase II clinical trial, we outlined how gene expression patterns and generalized linear models can be introduced to define molecular signatures of identity, purity, and potency. We first verified that cells from the biopsied nasal cartilage can be contaminated by cells from a neighboring tissue, namely perichondrial cells, and discovered that they cannot deposit cartilaginous matrix. Differential analysis of gene expression enabled the definition of identity markers for the two cell populations, which were predictive of purity in mixed cultures. Specific patterns of expression of the same genes were significantly correlated with cell potency, defined as the capacity to generate tissues with histological and biochemical features of hyaline cartilage. The outlined approach can now be considered for implementation in a good manufacturing practice setting, and offers a paradigm for other regenerative cellular therapies.

Cartilage tissue engineering combining microspheroid building blocks and microneedle arrays

Purpose: Scaffold-free cartilage tissue engineering circumvents issues with scaffold seeding, potential toxicity response, and impaired host integration. However, precisely controlling and maintaining a scaffold-free construct shape have been challenging. We explored the feasibility of microneedle arrays to print tissue using cellular microspheroids as building blocks.Materials and Methods: Human embryonic-derived mesenchymal stem cells or infrapatellar fat pad mesenchymal stem cells were used to create microspheroids of 500 µm in diameter, which were assembled on microneedle arrays in a predefined arrangement using a robotic system under computer vision. Microspheroids on microneedles were cultured to permit fusion into a tissue construct. Infrapatellar fat pad mesenchymal stem cell constructs were either implanted into chondral defects created in human osteoarthritic cartilage explants or maintained on the microneedle array for 3 weeks. Embryonic-derived mesenchymal stem cell constructs were designed to be press-fit into 3 mm subchondral defects in New Zealand White rabbits and maintained for up to 8 weeks to assess retention, early tissue repair, and more mature cartilage regeneration.Results: Microspheroids of both cell types fused together in culture to form neotissues of predefined shape and size. Infrapatellar fat pad mesenchymal stem cell neotissues expressed high levels of chondrogenic genes and integrated with the surrounding osteoarthritic host cartilage. Embryonic-derived mesenchymal stem cell constructs generated chondrogenic neotissue in vivo as early as 2 weeks and more mature tissue by 8 weeks with increased glycosaminoglycan deposition.Conclusions: We constructed defined scaffold-free shapes by bioprinting and fusing microspheroids. Proof of concept was shown in the repair of ex vivo osteoarthritic human cartilage and in vivo rabbit osteochondral (OC) defects.

Preconditioning of mesenchymal stromal cells with low-intensity ultrasound: influence on chondrogenesis and directed SOX9 signaling pathways

Background

Continuous low-intensity ultrasound (cLIUS) facilitates the chondrogenic differentiation of human mesenchymal stromal cells (MSCs) in the absence of exogenously added transforming growth factor-beta (TGFβ) by upregulating the expression of transcription factor SOX9, a master regulator of chondrogenesis. The present study evaluated the molecular events associated with the signaling pathways impacting SOX9 gene and protein expression under cLIUS.

Methods

Human bone marrow-derived MSCs were exposed to cLIUS stimulation at 14 kPa (5 MHz, 2.5 Vpp) for 5 min. The gene and protein expression of SOX9 was evaluated. The specificity of SOX9 upregulation under cLIUS was determined by treating the MSCs with small molecule inhibitors of select signaling molecules, followed by cLIUS treatment. Signaling events regulating SOX9 expression under cLIUS were analyzed by gene expression, immunofluorescence staining, and western blotting.

Results

cLIUS upregulated the gene expression of SOX9 and enhanced the nuclear localization of SOX9 protein when compared to non-cLIUS-stimulated control. cLIUS was noted to enhance the phosphorylation of the signaling molecule ERK1/2. Inhibition of MEK/ERK1/2 by PD98059 resulted in the effective abrogation of cLIUS-induced SOX9 expression, indicating that cLIUS-induced SOX9 upregulation was dependent on the phosphorylation of ERK1/2. Inhibition of integrin and TRPV4, the upstream cell-surface effectors of ERK1/2, did not inhibit the phosphorylation of ERK1/2 and therefore did not abrogate cLIUS-induced SOX9 expression, thereby suggesting the involvement of other mechanoreceptors. Consequently, the effect of cLIUS on the actin cytoskeleton, a mechanosensitive receptor regulating SOX9, was evaluated. Diffused and disrupted actin fibers observed in MSCs under cLIUS closely resembled actin disruption by treatment with cytoskeletal drug Y27632, which is known to increase the gene expression of SOX9. The upregulation of SOX9 under cLIUS was, therefore, related to cLIUS-induced actin reorganization. SOX9 upregulation induced by actin reorganization was also found to be dependent on the phosphorylation of ERK1/2.

Conclusions

Collectively, preconditioning of MSCs by cLIUS resulted in the nuclear localization of SOX9, phosphorylation of ERK1/2 and disruption of actin filaments, and the expression of SOX9 was dependent on the phosphorylation of ERK1/2 under cLIUS.

Electronic supplementary material

The online version of this article (10.1186/s13287-019-1532-2) contains supplementary material, which is available to authorized users.

Human Articular Chondrocytes Retain Their Phenotype in Sustained Hypoxia While Normoxia Promotes Their Immunomodulatory Potential

OBJECTIVE

To assess the phenotype of human articular chondrocytes cultured in normoxia (21% O2) or continuous hypoxia (2% O2).

DESIGN

Chondrocytes were extracted from patients undergoing total knee replacement (n = 5) and cultured in ~21% (normoxic chondrocytes, NC) and 2% (hypoxic chondrocytes, HC) oxygen in both monolayer and 3-dimensional (3D) pellet culture and compared with freshly isolated chondrocytes (FC). Cells were assessed by flow cytometry for markers indicative of mesenchymal stromal cells (MSCs), chondrogenic-potency and dedifferentiation. Chondrogenic potency and immunomodulatory gene expression was assessed in NC and HC by reverse transcription quantitative polymerase chain reaction. Immunohistochemistry was used to assess collagen II production following 3D pellet culture.

RESULTS

NC were positive (>97%, n = 5) for MSC markers, CD73, CD90, and CD105, while HC demonstrated <90% positivity (n = 4) and FC (n = 5) less again (CD73 and CD90 <20%; CD105 <40%). The markers CD166 and CD151, indicative of chondrogenic de-differentiation, were significantly higher on NC compared with HC and lowest on FC. NC also produced the highest levels of CD106 and showed the greatest levels of IDO expression, following interferon-γ stimulation, indicating immunomodulatory potential. NC produced the highest levels of CD49c (>60%) compared with HC and FC in which production was <2%. Hypoxic conditions upregulated expression of SOX9, frizzled-related protein (FRZB), fibroblast growth factor receptor 3 (FGFR3), and collagen type II (COL2A1) and downregulated activin receptor-like kinase 1 (ALK1) in 3 out of 4 patients compared with normoxic conditions for monolayer cells.

CONCLUSIONS

Hypoxic conditions encourage retention of a chondrogenic phenotype with some immunomodulatory potential, whereas normoxia promotes dedifferentiation of chondrocytes toward an MSC phenotype with loss of chondrogenic potency but enhanced immunomodulatory capacity.

Tissue Specific Differentiation of Human Chondrocytes Depends on Cell Microenvironment and Serum Selection

Therapeutic options to cure osteoarthritis (OA) are not yet available, although cell-based therapies for the treatment of traumatic defects of cartilage have already been developed using, e.g., articular chondrocytes. In order to adapt cell-based therapies to treat OA, appropriate cell culture conditions are necessary. Chondrocytes require a 3-dimensional (3D) environment for redifferentiation after 2-dimensional (2D) expansion. Fetal bovine serum (FBS) is commonly used as a medium supplement, although the usage of a xenogeneic serum could mask the intrinsic behavior of human cells in vitro. The aim of this study was to compare human articular chondrocytes cultivated as monolayers (2D) and the development of microtissues (3D) in the presence of FBS with those cultivated with human serum (HS). Evaluation of the expression of various markers via immunocytochemistry on monolayer cells revealed a higher dedifferentiation degree of chondrocytes cultivated with HS. Scaffold-free microtissues were generated using the agar overlay technique, and their differentiation level was evaluated via histochemistry and immunohistochemistry. Microtissues cultivated in the medium with FBS showed a higher redifferentiation level. This was evidenced by bigger microtissues and a more cartilage-like composition of the matrix with not any/less positivity for cartilage-specific markers in HS versus moderate-to-high positivity in FBS-cultured microtissues. The present study showed that the differentiation degree of chondrocytes depends both on the microenvironment of the cells and the serum type with FBS achieving the best results. However, HS should be preferred for the engineering of cartilage-like microtissues, as it rather enables a "human-based" situation in vitro. Hence, cultivation conditions might be further optimized to gain an even more adequate and donor-independent redifferentiation of chondrocytes in microtissues, e.g., designing a suitable chemically-defined serum supplement.

Chondrogenic Progenitor Cells Exhibit Superiority Over Mesenchymal Stem Cells and Chondrocytes in Platelet-Rich Plasma Scaffold-Based Cartilage Regeneration

BACKGROUND

Platelet-rich plasma (PRP) has been considered a promising tool for cartilage regeneration. However, increasing evidence has demonstrated the controversial effects of PRP on tissue regeneration, partially due to the unsatisfactory cell source. Chondrogenic progenitor cells (CPCs) have gained increasing attention as a potential cell source due to their self-renewal and multipotency, especially toward the chondrogenic lineage, and, thus, may be an appropriate alternative for cartilage engineering.

PURPOSE

To compare the effects of PRP on CPC, mesenchymal stem cell (MSC), and chondrocyte proliferation, chondrogenesis, and cartilage regeneration.

STUDY DESIGN

Controlled laboratory study.

METHODS

Whole blood samples were obtained from 5 human donors to create PRPs (0, 1000 × 10⁹, and 2000 × 10⁹ platelets per liter). The proliferation and chondrogenesis of CPCs, bone marrow-derived MSCs (BMSCs), and chondrocytes were evaluated via growth kinetic and CCK-8 assays. Immunofluorescence, cytochemical staining, and gene expression analyses were performed to assess chondrogenic differentiation and cartilaginous matrix formation. The in vivo effects of CPCs, BMSCs, and chondrocytes on cartilage regeneration after PRP treatment were measured by use of histopathological, biochemical, and biomechanical techniques in a cartilage defect model involving mature male New Zealand White rabbits (critical size, 5 mm).

RESULTS

The CPCs possessed migration abilities and proliferative capacities superior to those of the chondrocytes, while exhibiting a chondrogenic predisposition stronger than that of the BMSCs. The growth kinetic, CCK-8, cytochemical staining, and biochemical analyses revealed that the CPCs simultaneously displayed a higher cell density than the chondrocytes and stronger chondrogenesis than the BMSCs after PRP stimulation. In addition, the in vivo study demonstrated that the PRP+CPC construct yielded better histological (International Cartilage Repair Society [ICRS] score, mean ± SEM, 1197.2 ± 163.2) and biomechanical (tensile modulus, 1.523 ± 0.194) results than the PRP+BMSC (701.1 ± 104.9, P < .05; 0.791 ± 0.151, P < .05) and PRP+chondrocyte (541.6 ± 98.3, P < .01; 0.587 ± 0.142, P < .01) constructs at 12 weeks after implantation.

CONCLUSION

CPCs exhibit superiority over MSCs and chondrocytes in PRP scaffold-based cartilage regeneration, and PRP+CPC treatment may be a favorable strategy for cartilage repair.

CLINICAL RELEVANCE

These findings provide evidence highlighting the preferable role of CPCs as a cell source in PRP-mediated cartilage regeneration and may help researchers address the problem of unsatisfactory cell sources in cartilage engineering.

A soft 3D polyacrylate hydrogel recapitulates the cartilage niche and allows growth-factor free tissue engineering of human articular cartilage

Cartilage degeneration or damage treatment is still a challenge, but, tissue engineering strategies, which combine cell therapy strategies, which combine cell therapy and scaffolds, and have emerged as a promising new approach. In this regard, polyurethanes and polyacrylates polymers have been shown to have clinical potential to treat osteochondral injuries. Here, we have used polymer microarrays technology to screen 380 different polyurethanes and polyacrylates polymers. The top polymers with potential to maintain chondrocyte viability were selected, with scale-up studies performed to evaluate their ability to support chondrocyte proliferation during long-term culture, while maintaining their characteristic phenotype. Among the selected polymers, poly (methylmethacrylate-co-methacrylic acid), showed the highest level of chondrogenic potential and was used to create a 3D hydrogel. Ultrastructural morphology, microstructure and mechanical testing of this novel hydrogel revealed robust characteristics to support chondrocyte growth. Furthermore, in vitro and in vivo biological assays demonstrated that chondrocytes cultured on the hydrogel had the capacity to produce extracellular matrix similar to hyaline cartilage, as shown by increased expression of collagen type II, aggrecan and Sox9, and the reduced expression of the fibrotic marker's collagen type I. In conclusion, hydrogels generated from poly (methylmethacrylate-co-methacrylic acid) created the appropriate niche for chondrocyte growth and phenotype maintenance and might be an optimal candidate for cartilage tissue-engineering applications. SIGNIFICANCE STATEMENT: Articular cartilage has limited self-repair ability due to its avascular nature, therefore tissue engineering strategies have emerged as a promising new approach. Synthetic polymers displaygreat potential and are widely used in the clinical setting. In our study, using the polymer microarray technique a novel type of synthetic polyacrylate was identified, that was converted into hydrogels for articular cartilage regeneration studies. The hydrogel based on poly (methylmethacrylate-co-methacrylic acid-co-PEG-diacrylate) had a controlable ultrastructural morphology, microstructure (porosity) and mechanical properties (stiffness) appropriate for cartilage engineering. Our hydrogel created the optimal niche for chondrocyte growth and phenotype maintenance for long-term culture, producing a hyaline-like cartilage extracellular matrix. We propose that this novel polyacrylate hydrogel could be an appropriate support to help in the treatment efficient cartilage regeneration.

Replacement of Rat Tracheas by Layered, Trachea-Like, Scaffold-Free Structures of Human Cells Using a Bio-3D Printing System

Current scaffold-based tissue engineering approaches are subject to several limitations, such as design inflexibility, poor cytocompatibility, toxicity, and post-transplant degradation. Thus, scaffold-free tissue-engineered structures can be a promising solution to overcome the issues associated with classical scaffold-based materials in clinical transplantation. The present study seeks to optimize the culture conditions and cell combinations used to generate scaffold-free structures using a Bio-3D printing system. Human cartilage cells, human fibroblasts, human umbilical vein endothelial cells, and human mesenchymal stem cells from bone marrow are aggregated into spheroids and placed into a Bio-3D printing system with dedicated needles positioned according to 3D configuration data, to develop scaffold-free trachea-like tubes. Culturing the Bio-3D-printed structures with proper flow of specific medium in a bioreactor facilitates the rearrangement and self-organization of cells, improving physical strength and tissue function. The Bio-3D-printed tissue forms small-diameter trachea-like tubes that are implanted into rats with the support of catheters. It is confirmed that the tubes are viable in vivo and that the tracheal epithelium and capillaries proliferate. This tissue-engineered, scaffold-free, tubular structure can represent a significant step toward clinical application of bioengineered organs.

Coculture of hWJMSCs and pACs in Oriented Scaffold Enhances Hyaline Cartilage Regeneration In Vitro

Seed cells of articular cartilage tissue engineering face many obstacles in their application because of the dedifferentiation of chondrocytes or unstable chondrogenic differentiation status of pluripotent stem cells. To overcome mentioned dilemmas, a simulation of the articular cartilage microenvironment was constructed by primary articular cartilage cells (pACs) and acellular cartilage extracellular matrix- (ACECM-) oriented scaffold cocultured with human umbilical cord Wharton's jelly-derived mesenchymal stem cells (hWJMSCs) in vitro. The coculture groups showed more affluent cartilage special matrix ingredients including collagen II and aggrecan based on the results of histological staining and western blotting and cut down as many pACs as possible. The RT-PCR and cell viability experiments also demonstrated that hWJMSCs were successfully induced to differentiate into chondrocytes when cultured in the simulated cartilage microenvironment, as confirmed by the significant upregulation of collagen II and aggrecan, while the cell proliferation activity of pACs was significantly improved by cell-cell interactions. Therefore, compared with monoculture and chondrogenic induction of inducers, coculture providing a simulated native articular microenvironment was a potential and temperate way to regulate the biological behaviors of pACs and hWJMSCs to regenerate the hyaline articular cartilage.

Extracellular matrix derived from chondrocytes promotes rapid expansion of human primary chondrocytes in vitro with reduced dedifferentiation

A significant expansion of autologous chondrocytes in vitro is required for cell-based cartilage repair. However, the in vitro expansion of chondrocytes under standard culture conditions inevitably leads to the dedifferentiation of chondrocytes and contributes to suboptimal clinical outcomes. To address this challenge, we focused our efforts on developing an improved in vitro expansion protocol, which shortens the expansion time with decreased dedifferentiation. It is known that the tissue microenvironment plays a critical role in regulating the cellular functions of resident cells and provides guidance in tissue-specific regeneration. We hypothesized that chondrocyte extracellular matrix (ECM) mimics a native microenvironment and that it may support chondrocyte expansion in vitro. To test this hypothesis, we prepared decellularized ECMs from allogeneic human articular chondrocytes (HAC) (AC-ECM) and bone marrow stromal cells (BM-ECM) and studied their effects on the in vitro expansion of primary HAC. The differential composition and physical properties of these two ECMs were revealed by mass spectrometry and atomic force microscopy. Compared with standard tissue culture polystyrene (TCP) or BM-ECM, HAC cultured on AC-ECM proliferated faster and maintained the highest ratio of COL2A1/COL1A1. Furthermore, a pellet culture study demonstrated that cells expanded on AC-ECM produced a more cartilage-like ECM than cells expanded on BM-ECM or TCP. This is the first report on modulating chondrocyte expansion and dedifferentiation using cell type-specific ECM and on identifying AC-ECM as a preferred substrate for in vitro expansion of HAC cell-based therapies. STATEMENT OF SIGNIFICANCE: To reduce the dedifferentiation of chondrocytes during in vitro expansion, cell type-specific extracellular matrix (ECM), which mimics a native microenvironment, was prepared from human articular chondrocytes (AC-ECM) or bone marrow stromal cells (BM-ECM). As demonstrated by mass spectrometry and atomic force microscopy, AC-ECM and BM-ECM have differential ECM compositions and physical characteristics. Human articular chondrocytes (HAC) expanded faster and maintained a better chondrocyte phenotype on AC-ECM than on BM-ECM or a standard culture surface. AC-ECM has potential to be developed for expanding HAC for cell-based therapies.

In vitro chondrogenic potency of surplus chondrocytes from autologous transplantation procedures does not predict short-term clinical outcomes

BACKGROUND

Autologous chondrocyte implantation (ACI) has been used over the last two decades to treat focal cartilage lesions aiming to delay or prevent the onset of osteoarthritis; however, some patients do not respond adequately to the procedure. A number of biomarkers that can forecast the clinical potency of the cells have been proposed, but evidence for the relationship between in vitro chondrogenic potential and clinical outcomes is missing. In this study, we explored if the ability of cells to make cartilage in vitro correlates with ACI clinical outcomes. Additionally, we evaluated previously proposed chondrogenic biomarkers and searched for new biomarkers in the chondrocyte proteome capable of predicting clinical success or failure after ACI.

METHODS

The chondrogenic capacity of chondrocytes derived from 14 different donors was defined based on proteoglycans staining and visual histological grading of tissues generated using the pellet culture system. A Lysholm score of 65 two years post-ACI was used as a cut-off to categorise "success" and "failure" clinical groups. A set of predefined biomarkers were investigated in the chondrogenic and clinical outcomes groups using flow cytometry and qPCR. High-throughput proteomics of cell lysates was used to search for putative biomarkers to predict chondrogenesis and clinical outcomes.

RESULTS

Visual histological grading of pellets categorised donors into "high" and "low" chondrogenic groups. Direct comparison between donor-matched in vitro chondrogenic potential and clinical outcomes revealed no significant associations. Comparative analyses of selected biomarkers revealed that expression of CD106 and TGF-β-receptor-3 was enhanced in the low chondrogenic group, while expression of integrin-α1 and integrin-β1 was significantly upregulated in the high chondrogenic group. Additionally, increased surface expression of CD166 was observed in the clinical success group, while the gene expression of cartilage oligomeric matrix protein was downregulated. High throughput proteomics revealed no differentially expressed proteins from success and failure clinical groups, whereas seven proteins including prolyl-4-hydroxylase 1 were differentially expressed when comparing chondrogenic groups.

CONCLUSION

In our limited material, we found no correlation between in vitro cartilage-forming capacity and clinical outcomes, and argue on the limitations of using the chondrogenic potential of cells or markers for chondrogenesis as predictors of clinical outcomes.

Preconditioning strategies for improving the survival rate and paracrine ability of mesenchymal stem cells in acute kidney injury

Acute kidney injury (AKI) is a common, severe emergency case in clinics, with high incidence, significant mortality and increased costs. Despite development in the understanding of its pathophysiology, the therapeutic choices are still confined to dialysis and renal transplantation. Considering their antiapoptotic, immunomodulatory, antioxidative and pro‐angiogenic effects, mesenchymal stem cells (MSCs) may be a promising candidate for AKI management. Based on these findings, some clinical trials have been performed, but the results are contradictory (NCT00733876, NCT01602328). The low engraftment, poor survival rate, impaired paracrine ability and delayed administration of MSCs are the four main reasons for the limited clinical efficacy. Investigators have developed a series of preconditioning strategies to improve MSC survival rates and paracrine ability. In this review, by summarizing these encouraging studies, we intend to provide a comprehensive understanding of various preconditioning strategies on AKI therapy and improve the prognosis of AKI patients by regenerative medicine.

Chasing Chimeras - The elusive stable chondrogenic phenotype

The choice of the best-suited cell population for the regeneration of damaged or diseased cartilage depends on the effectiveness of culture conditions (e.g. media supplements, three-dimensional scaffolds, mechanical stimulation, oxygen tension, co-culture systems) to induce stable chondrogenic phenotype. Herein, advances and shortfalls in in vitro, preclinical and clinical setting of various in vitro microenvironment modulators on maintaining chondrocyte phenotype or directing stem cells towards chondrogenic lineage are critically discussed. Chondrocytes possess low isolation efficiency, limited proliferative potential and rapid phenotypic drift in culture. Mesenchymal stem cells are relatively readily available, possess high proliferation potential, exhibit great chondrogenic differentiation capacity, but they tend to acquire a hypertrophic phenotype when exposed to chondrogenic stimuli. Embryonic and induced pluripotent stem cells, despite their promising in vitro and preclinical data, are still under-investigated. Although a stable chondrogenic phenotype remains elusive, recent advances in in vitro microenvironment modulators are likely to develop clinically- and commercially-relevant therapies in the years to come.

Novel preconditioning strategies for enhancing the migratory ability of mesenchymal stem cells in acute kidney injury

Acute kidney injury (AKI) remains a worldwide public health issue due to its increasing incidence, significant mortality, and lack of specific target-orientated therapy. Developments in mesenchymal stem cell (MSC) research make MSCs a promising candidate for AKI management but relevant clinical trials show confusing results (NCT00733876, NCT01602328). One primary cause of the limited therapeutic effect may result from poor engraftment of transplanted cells. To solve this problem, investigators have developed a series of preconditioning strategies to improve MSC engraftment in animal AKI models. In this review, we summarize these previous studies, providing an integrated and updated view of different preconditioning strategies aimed at promoting the therapeutic effect of MSCs in AKI patients.

Fully Dedifferentiated Chondrocytes Expanded in Specific Mesenchymal Stem Cell Growth Medium with FGF2 Obtains Mesenchymal Stem Cell Phenotype In Vitro but Retains Chondrocyte Phenotype In Vivo

Given recent progress in regenerative medicine, we need a means to expand chondrocytes in quantity without losing their regenerative capability. Although many reports have shown that growth factor supplementation can have beneficial effects, the use of growth factor-supplemented basal media has widespread effect on the characteristics of chondrocytes. Chondrocytes were in vitro cultured in the 2 most widely used chondrocyte growth media, conventional chondrocyte culture medium and mesenchymal stem cell (MSC) culture medium, both with and without fibroblast growth factor-2 (FGF2) supplementation. Their expansion rates, expressions of extracellular matrix-related factors, senescence, and differentiation potentials were examined in vitro and in vivo. Our results revealed that chondrocytes quickly dedifferentiated during expansion in all tested media, as assessed by the loss of type II collagen expression. The 2 basal media (chondrocyte culture medium vs. MSC culture medium) were associated with distinct differences in cell senescence. Consistent with the literature, FGF2 was associated with accelerated dedifferentiation during expansion culture and superior redifferentiation upon induction. However, chondrocytes expanded in FGF2-containing conventional chondrocyte culture medium showed MSC-like features, as indicated by their ability to direct ectopic bone formation and cartilage formation. In contrast, chondrocytes cultured in FGF2-supplemented MSC culture medium showed potent chondrogenesis and almost no bone formation. The present findings show that the chosen basal medium can exert profound effects on the characteristics and activity of in vitro-expanded chondrocytes and indicate that right growth factor/medium combination can help chondrocytes retain a high-level chondrogenic potential without undergoing hypertrophic transition.

Mesenchymal Stem/Progenitor Cells Derived from Articular Cartilage, Synovial Membrane and Synovial Fluid for Cartilage Regeneration: Current Status and Future Perspectives

Large articular cartilage defects remain an immense challenge in the field of regenerative medicine because of their poor intrinsic repair capacity. Currently, the available medical interventions can relieve clinical symptoms to some extent, but fail to repair the cartilaginous injuries with authentic hyaline cartilage. There has been a surge of interest in developing cell-based therapies, focused particularly on the use of mesenchymal stem/progenitor cells with or without scaffolds. Mesenchymal stem/progenitor cells are promising graft cells for tissue regeneration, but the most suitable source of cells for cartilage repair remains controversial. The tissue origin of mesenchymal stem/progenitor cells notably influences the biological properties and therapeutic potential. It is well known that mesenchymal stem/progenitor cells derived from synovial joint tissues exhibit superior chondrogenic ability compared with those derived from non-joint tissues; thus, these cell populations are considered ideal sources for cartilage regeneration. In addition to the progress in research and promising preclinical results, many important research questions must be answered before widespread success in cartilage regeneration is achieved. This review outlines the biology of stem/progenitor cells derived from the articular cartilage, the synovial membrane, and the synovial fluid, including their tissue distribution, function and biological characteristics. Furthermore, preclinical and clinical trials focusing on their applications for cartilage regeneration are summarized, and future research perspectives are discussed.

Stem cell-based therapeutic strategies for cartilage defects and osteoarthritis

The gold standard cell therapy for repair of articular cartilage defects is autologous chondrocyte implantation, with good outcomes long-term. Mesenchymal stromal/stem cells (MSCs) from bone marrow or connective tissues such as fat are being pursued as alternatives for cartilage repair, and are trialled via intra-articular administration in patients with knee osteoarthritis. Early-phase clinical studies concur on safety and provide some promising insight into efficacy, but the mechanism of action remains unclear. Recent studies implicate extracellular vesicles as important mediators of MSC action, offering exciting therapeutic prospects. Our increasing understanding of the mechanisms underlying intrinsic articular cartilage maintenance and repair fosters hope that novel/repurposed therapeutics could elicit repair through activation of endogenous stem/progenitor cells to maintain healthy joints and prevent osteoarthritis.

An Innovative Laboratory Procedure to Expand Chondrocytes with Reduced Dedifferentiation

Objective In vitro expansion of chondrocytes is required for cartilage tissue engineering and clinical cell-based cartilage repair practices. However, the dedifferentiation of chondrocytes during in vitro expansion continues to be a challenge. This study focuses on identifying a cell culture surface to support chondrocyte expansion with reduced dedifferentiation. Design A less adhesive culture surface, non-tissue culture treated surface (NTC), was tested for its suitability for culturing chondrocytes. The cell expansion and the expression of chondrocyte markers were monitored for at least 2 passages on NTC in comparison with conventional tissue culture treated polystyrene surface (TCP). The ability of expanded chondrocytes to form cartilage tissues was evaluated using pellet culturing and subcutaneous implantation in nude mice. Results NTC supported bovine chondrocyte proliferation to a clinically relevant expansion requirement within 2 passages. Chondrocyte phenotypes were better maintained when cultured on NTC than on TCP. In vitro pellet culture studies showed that chondrocytes expanded on NTC expressed a higher level of chondrocyte extracellular matrix. Furthermore, the cells expanded on NTC or TCP were implanted subcutaneously as pellets in nude mice for 6 weeks. The recovered pellets showed cartilage-like tissue formation from cells expanded on NTC but not from the cells expanded on TCP. Conclusions This study presents an innovative and easy culturing procedure to expand chondrocytes with reduced dedifferentiation. This procedure has potential to be developed to expand chondrocytes in vitro for basic research, tissue engineering, and possibly for clinical applications.

Ectopic implantation of juvenile osteochondral tissues recapitulates endochondral ossification

Subcutaneous implantation in a mouse can be used to investigate tissue maturation in vivo. Here we demonstrate that this simple model can recapitulate endochondral ossification associated with native skeletal development. By histological and micro-computed tomography analysis we investigated morphological changes of immature bovine osteochondral tissues over the course of subcutaneous implantation in immunocompromised mice for up to 10 weeks. We observed multiple similarities between the ectopic process and native endochondral ossification: (i) permanent cartilage retention in the upper zones; (ii) progressive loss of transient cartilage accompanied by bone formation at the interface; and (iii) remodelling of nascent endochondral bone into mature cancellous bone. Importantly, these processes were mediated by osteoclastogenesis and vascularization. Taken together, these findings advance our understanding of how the simple ectopic model can be used to study phenotypic changes associated with endochondral ossification of native and engineered osteochondral tissues in vivo.

Biomaterials for articular cartilage tissue engineering: Learning from biology

Articular cartilage is commonly described as a tissue that is made of up to 80% water, is devoid of blood vessels, nerves, and lymphatics, and is populated by only one cell type, the chondrocyte. At first glance, an easy tissue for clinicians to repair and for scientists to reproduce in a laboratory. Yet, chondral and osteochondral defects currently remain an open challenge in orthopedics and tissue engineering of the musculoskeletal system, without considering osteoarthritis. Why do we fail in repairing and regenerating articular cartilage? Behind its simple and homogenous appearance, articular cartilage hides a heterogeneous composition, a high level of organisation and specific biomechanical properties that, taken together, make articular cartilage a unique material that we are not yet able to repair or reproduce with high fidelity. This review highlights the available therapies for cartilage repair and retraces the research on different biomaterials developed for tissue engineering strategies. Their potential to recreate the structure, including composition and organisation, as well as the function of articular cartilage, intended as cell microenvironment and mechanically competent replacement, is described. A perspective of the limitations of the current research is given in the light of the emerging technologies supporting tissue engineering of articular cartilage.

STATEMENT OF SIGNIFICANCE

The mechanical properties of articular tissue reflect its functionally organised composition and the recreation of its structure challenges the success of in vitro and in vivo reproduction of the native cartilage. Tissue engineering and biomaterials science have revolutionised the way scientists approach the challenge of articular cartilage repair and regeneration by introducing the concept of the interdisciplinary approach. The clinical translation of the current approaches are not yet fully successful, but promising results are expected from the emerging and developing new generation technologies.

Vascular Endothelial Growth Factor Sequestration Enhances In Vivo Cartilage Formation

Autologous chondrocyte transplantation for cartilage repair still has unsatisfactory clinical outcomes because of inter-donor variability and poor cartilage quality formation. Re-differentiation of monolayer-expanded human chondrocytes is not easy in the absence of potent morphogens. The Vascular Endothelial Growth Factor (VEGF) plays a master role in angiogenesis and in negatively regulating cartilage growth by stimulating vascular invasion and ossification. Therefore, we hypothesized that its sole microenvironmental blockade by either VEGF sequestration by soluble VEGF receptor-2 (Flk-1) or by antiangiogenic hyperbranched peptides could improve chondrogenesis of expanded human nasal chondrocytes (NC) freshly seeded on collagen scaffolds. Chondrogenesis of several NC donors was assessed either in vitro or ectopically in nude mice. VEGF blockade appeared not to affect NC in vitro differentiation, whereas it efficiently inhibited blood vessel ingrowth in vivo. After 8 weeks, in vivo glycosaminoglycan deposition was approximately two-fold higher when antiangiogenic approaches were used, as compared to the control group. Our data indicates that the inhibition of VEGF signaling, independently of the specific implementation mode, has profound effects on in vivo NC chondrogenesis, even in the absence of chondroinductive signals during prior culture or at the implantation site.

Featured Article: In vitro development of personalized cartilage microtissues uncovers an individualized differentiation capacity of human chondrocytes

Personalized features in the treatment of knee injuries and articular replacement therapies play an important role in modern life with increasing demand. Therefore, cell-based therapeutic approaches for the regeneration of traumatic defects of cartilage tissue were developed. However, great variations in the quality of repair tissue or therapeutic outcome were observed. The aim of the study was to capture and visualize individual differentiation capacities of chondrocytes derived from different donors with regard to a possible personal regeneration capacity using a cell-based therapy. The redifferentiation potential of monolayer cultured cells was analyzed in a scaffold-free three-dimensional tissue model. Furthermore, stimulating options using cartilage maturation factors such as L-ascorbic acid and transforming growth factor beta 2 (TGF-β₂) on this process were of special interest. Cells and tissues were analyzed via histological and immunohistochemical methods. Gene expression was measured by quantitative real-time polymerase chain reaction. In monolayer culture, cells from all donors showed an almost identical differentiation profile. In contrast, the differentiation state of cartilage-like three-dimensional microtissues revealed clear differences with respect to individual donors. Analyses at the protein and mRNA levels showed high variations regarding cartilage-typical matrix components (e.g. proteoglycans, collagen type II) and intracellular proteins (e.g. S100). Interestingly, only donor chondrocytes with a basic tendency to re-differentiate in a three-dimensional environment were able to increase this tissue-specific maturation when exposed to L-ascorbic acid and/or TGF-β2. Our approach revealed clear-cut possibilities for classification of individual donors into responders or non-responders. On the basis of these results an in vitro platform could be designed to discriminate responders from non-responders. This in vitro three-dimensional test system may be a suitable basis to establish a "personalized diagnostic tool" with the opportunity to assess the capacity of expanded chondrocytes to respond to an autologous cell-based therapy. Impact statement A challenge in cell-based cartilage regeneration therapies is the identification of a "personalized diagnostic tool" to predict the chondrogenic potency of cells from patients who are going to be treated with autologous cells. Comparing the phenotype of isolated chondrocytes from different donors in vitro revealed an individual cartilage-specific differentiation capacity. These personalized features are not detectable in vitro until the monolayer cells have the possibility to rearrange in 3D tissues. Cells from articular cartilage in monolayer culture may not be a suitable basis to discriminate responders from non-responders with respect to a personalized cell-based therapy to treat cartilage defects. A more physiological 3D (micro-)environment enable the cells to present their individual differentiation capacity. The here described microtissue model might be the basis for an in vitro platform to predict the therapeutic outcome of autologous cell-based cartilage repair and/or a suitable tool to identify early biomarkers to classify the patients.

Strategies to Mitigate Variability in Engineering Human Nasal Cartilage

Skin cancer and its associated treitments can have devastating consequences for survivors; this is particularly true when cancer occurs on the nose. Recent work has applied cell-based tissue engineering (TE) strategies to develop nasal cartilage constructs for reconstruction of the nose. In this study, we have generated human nasal cartilage on a clinically approved collagen scaffold to investigate the donor-to-donor variability of TE cartilage and evaluated strategies to mitigate it. We also evaluated the gene expression of the family of fibroblast growth factor receptors (FGFR1-4) and their association with tissue quality. FGFR1 was significantly positively correlated with GAG/DNA; a measure of chondrogenic capacity. We implemented two strategies: hypoxic culture and co-culture with mesenchymal stromal cells (MSCs) to increase tissue quality. Total glycosaminoglycan (GAG) content varied significantly between donors initially, with >10–fold difference between the best and worst donor tissue. Our co-culture strategy was able to increase TE construct quality from poor quality donor tissue while supressing hypertrophy relative to MSCs alone. However, no differences were observed with the use of hypoxic culture. Tissues generated using co-culture with MSCs became vascularized and calcified in vivo, demonstrating a non-stable cartilage phenotype in co-culture and MSCs cartilage constructs.

Dedifferentiated Chondrocytes in Composite Microfibers As Tool for Cartilage Repair

Tissue engineering (TE) approaches using biomaterials have gain important roles in the regeneration of cartilage. This paper describes the production by microfluidics of alginate-based microfibers containing both extracellular matrix (ECM)-derived biomaterials and chondrocytes. As ECM components gelatin or decellularized urinary bladder matrix (UBM) were investigated. The effectiveness of the composite microfibers has been tested to modulate the behavior and redifferentiation of dedifferentiated chondrocytes. The complete redifferentiation, at the single-cell level, of the chondrocytes, without cell aggregate formation, was observed after 14 days of cell culture. Specific chondrogenic markers and high cellular secretory activity was observed in embedded cells. Notably, no sign of collagen type 10 deposition was determined. The obtained data suggest that dedifferentiated chondrocytes regain a functional chondrocyte phenotype when embedded in appropriate 3D scaffold based on alginate plus gelatin or UBM. The proposed scaffolds are indeed valuable to form a cellular microenvironment mimicking the in vivo ECM, opening the way to their use in cartilage TE.