Inhibitory function of parathyroid hormone-related protein on chondrocyte hypertrophy: the implication for articular cartilage repair

Effect of a retinoic acid analogue on BMP-driven pluripotent stem cell chondrogenesis

Osteoarthritis is the most common degenerative joint condition, leading to articular cartilage (AC) degradation, chronic pain and immobility. The lack of appropriate therapies that provide tissue restoration combined with the limited lifespan of joint-replacement implants indicate the need for alternative AC regeneration strategies. Differentiation of human pluripotent stem cells (hPSCs) into AC progenitors may provide a long-term regenerative solution but is still limited due to the continued reliance upon growth factors to recapitulate developmental signalling processes. Recently, TTNPB, a small molecule activator of retinoic acid receptors (RARs), has been shown to be sufficient to guide mesodermal specification and early chondrogenesis of hPSCs. Here, we modified our previous differentiation protocol, by supplementing cells with TTNPB and administering BMP2 at specific times to enhance early development (referred to as the RAPID-E protocol). Transcriptomic analyses indicated that activation of RAR signalling significantly upregulated genes related to limb and embryonic skeletal development in the early stages of the protocol and upregulated genes related to AC development in later stages. Chondroprogenitors obtained from RAPID-E could generate cartilaginous pellets that expressed AC-related matrix proteins such as Lubricin, Aggrecan, and Collagen II, but additionally expressed Collagen X, indicative of hypertrophy. This protocol could lay the foundations for cell therapy strategies for osteoarthritis and improve the understanding of AC development in humans.

Local injection of abaloparatide promotes mandibular condyle lengthening in adolescent rats via enhancing chondrogenesis and ossification

BACKGROUND

Mandibular condylar hypoplasia negatively affects patient's facial appearance and dentofacial function.

OBJECTIVE

To investigate the effect of local injection of the drug abaloparatide (ABL), an analogue of parathyroid hormone related protein (PTHrP), on promoting lengthening of the mandibular condyle.

METHODS

Thirty adolescent male Sprague-Dawley rats were randomly divided into two groups, which received the injection of ABL or normal saline (the control) every 3 days in the temporomandibular joint (TMJ) cavity. Cone-beam computed tomography and immunohistochemistry assays were performed at 2, 4 and 6 weeks since the injection. Mandibular condylar chondrocytes (MCC) and pre-osteoblasts were treated with ABL or PBS, followed by the CCK-8 detection, IC50, real-time PCR assay, Western Blot and immunofluorescence staining.

RESULTS

In vivo, compared with the control, the ABL group significantly increased the mandibular condylar process length (by 1.34 ± 0.59 mm at 6 weeks), the thickness of the cartilage layer, and enhanced the matrix synthesis. The ABL group had significant up-regulation of SOX 9, COL II, PTHrP and PTH1R, down-regulation of COL X in the cartilage, up-regulation of RUNX 2, and unchanged osteoclastogenesis in the subchondral bone. In vitro, the intra-TMJ injection of ABL promoted the MCC proliferation, with up-regulated expression of chondrogenic genes, and enhanced osteogenic differentiation of the pre-osteoblasts.

CONCLUSIONS

Intra-TMJ injection of abaloparatide promotes mandibular condyle lengthening in the adolescent rats via enhancing chondrogenesis in the mandibular condylar cartilage and ossification in the subchondral bone.

Advancing drug delivery to articular cartilage: From single to multiple strategies

Articular cartilage (AC) injuries often lead to cartilage degeneration and may ultimately result in osteoarthritis (OA) due to the limited self-repair ability. To date, numerous intra-articular delivery systems carrying various therapeutic agents have been developed to improve therapeutic localization and retention, optimize controlled drug release profiles and target different pathological processes. Due to the complex and multifactorial characteristics of cartilage injury pathology and heterogeneity of the cartilage structure deposited within a dense matrix, delivery systems loaded with a single therapeutic agent are hindered from reaching multiple targets in a spatiotemporal matched manner and thus fail to mimic the natural processes of biosynthesis, compromising the goal of full cartilage regeneration. Emerging evidence highlights the importance of sequential delivery strategies targeting multiple pathological processes. In this review, we first summarize the current status and progress achieved in single-drug delivery strategies for the treatment of AC diseases. Subsequently, we focus mainly on advances in multiple drug delivery applications, including sequential release formulations targeting various pathological processes, synergistic targeting of the same pathological process, the spatial distribution in multiple tissues, and heterogeneous regeneration. We hope that this review will inspire the rational design of intra-articular drug delivery systems (DDSs) in the future.

Temporomandibular Joint Fibrocartilage Contains CD105 Positive Mouse Mesenchymal Stem/Progenitor Cells with Increased Chondrogenic Potential

Objective

A specific type of mesenchymal stem/progenitor cells (MSPCs), CD105+ is reported to aid in cartilage regeneration through TGF-β/Smad2-signalling. The purpose of this study was to identify and characterize CD105+ MSPCs in temporomandibular joint (TMJ) cartilage.

Materials and Methods

MSPCs were isolated from mouse TMJ condyle explants and evaluated for their clonogenicity and pluripotential abilities. MSPC were examined for CD105 antigen using immunohistochemistry and flow cytometry.

Results

Immunohistochemistry revealed presence of CD105+ MSPCs in the proliferative zone of condyle's cartilage. Only 0.2% of isolated MSPCs exhibited CD105, along with the stem cell surface markers CD44 and Sca-1. In CD105+ MSPCs, intracellular immunostaining revealed significantly higher (p < 0.05) protein levels of collagen type 1, 2, proteoglycan 4. Ability for chondrogenic differentiation was found to be significantly higher (p < 0.05) after 4 weeks compared to CD105- cells, using alcian blue staining. CD105+ cells were found to resemble an early MSPC subgroup with significantly higher gene expression of biglycan, proteoglycan 4, collagen type 2, Gli2, Sox5 (p < 0.001) and Sox9 (p < 0.05). In contrast, significantly lower levels of Runx2 (p < 0.05), Osterix, Trps1, Col10a1 (p < 0.01), Ihh (p < 0.001) related to chondrocyte senescence and commitment to osteogenic lineage, were observed compared to CD105- cells.

Conclusion

The study showed the existence of a CD105+ MSPC subgroup within TMJ fibrocartilage that may be activated to aid in fibrocartilage repair.

Supplementation of articular cartilage-derived chondroprogenitors with bone morphogenic protein-9 enhances chondrogenesis without affecting hypertrophy

INTRODUCTION

Chondroprogenitors (CPCs) have emerged as a promising cellular therapy for cartilage-related pathologies due to their inherent primed chondrogenic potential. Studies report that the addition of growth factors such as parathyroid hormone (PTH) and Bone Morphogenic Protein (BMP) enhance the chondroinducive potential in chondrocytes and mesenchymal stem cells. This study evaluated if supplementation of the standard culture medium for cell expansion with 1-34 PTH and BMP-9 would enhance the chondrogenic potential of CPCs and reduce their hypertrophic tendency.

METHODS

Human chondrocytes were isolated from patients undergoing total knee replacement for osteoarthritis (n = 3). Following fibronectin adhesion assay, passage 1 CPCs were divided and further expanded under three culture conditions (a) control, i.e., cells continued under standard culture conditions, (b) 1-34 PTH group, additional intermittent 6 h exposure with 1-34 PTH and (c) BMP-9 group, additional BMP-9 during culture expansion. All the groups were evaluated for population-doubling, cell cycle analysis, surface marker and gene expression for chondrogenesis, hypertrophy, multilineage differentiation and GAG (glycosaminoglycan)/DNA following chondrogenic differentiation.

RESULTS

Concerning growth kinetics, the BMP-9 group exhibited a significantly lower S-phase and population-doubling when compared to the other two groups. Qualitative analysis for chondrogenic potential (Alcian blue, Safranin O staining and Toluidine blue for GAG) revealed that the BMP-9 group exhibited the highest uptake. The BMP-9 group also showed significantly higher COL2A1 expression than the control group, with no change in the hypertrophy marker expression.

CONCLUSION

BMP-9 can potentially be used as an additive for CPCs expansion, to enhance their chondrogenic potential without affecting their low hypertrophic tendency. The mitigating effects of 1-34PTH on hypertrophy would benefit further investigation when used in combination with BMP-9 to enhance chondrogenesis whilst reducing hypertrophy.

GDF5+ chondroprogenitors derived from human pluripotent stem cells preferentially form permanent chondrocytes

It has been established in the mouse model that during embryogenesis joint cartilage is generated from a specialized progenitor cell type, distinct from that responsible for the formation of growth plate cartilage. We recently found that mesodermal progeny of human pluripotent stem cells gave rise to two types of chondrogenic mesenchymal cells in culture: SOX9+ and GDF5+ cells. The fast-growing SOX9+ cells formed in vitro cartilage that expressed chondrocyte hypertrophy markers and readily underwent mineralization after ectopic transplantation. In contrast, the slowly growing GDF5+ cells derived from SOX9+ cells formed cartilage that tended to express low to undetectable levels of chondrocyte hypertrophy markers, but expressed PRG4, a marker of embryonic articular chondrocytes. The GDF5+-derived cartilage remained largely unmineralized in vivo. Interestingly, chondrocytes derived from the GDF5+ cells seemed to elicit these activities via non-cell-autonomous mechanisms. Genome-wide transcriptomic analyses suggested that GDF5+ cells might contain a teno/ligamento-genic potential, whereas SOX9+ cells resembled neural crest-like progeny-derived chondroprogenitors. Thus, human pluripotent stem cell-derived GDF5+ cells specified to generate permanent-like cartilage seem to emerge coincidentally with the commitment of the SOX9+ progeny to the tendon/ligament lineage.

BMP7 increases protein synthesis in SW1353 cells and determines rRNA levels in a NKX3-2-dependent manner

BMP7 is a morphogen capable of counteracting the OA chondrocyte hypertrophic phenotype via NKX3-2. NKX3-2 represses expression of RUNX2, an important transcription factor for chondrocyte hypertrophy. Since RUNX2 has previously been described as an inhibitor for 47S pre-rRNA transcription, we hypothesized that BMP7 positively influences 47S pre-rRNA transcription through NKX3-2, resulting in increased protein translational capacity. Therefor SW1353 cells and human primary chondrocytes were exposed to BMP7 and rRNA (18S, 5.8S, 28S) expression was determined by RT-qPCR. NKX3-2 knockdown was achieved via transfection of a NKX3-2-specific siRNA duplex. Translational capacity was assessed by the SUNsET assay, and 47S pre-rRNA transcription was determined by transfection of a 47S gene promoter-reporter plasmid. BMP7 treatment increased protein translational capacity. This was associated by increased 18S and 5.8S rRNA and NKX3-2 mRNA expression, as well as increased 47S gene promotor activity. Knockdown of NKX3-2 led to increased expression of RUNX2, accompanied by decreased 47S gene promotor activity and rRNA expression, an effect BMP7 was unable to restore. Our data demonstrate that BMP7 positively influences protein translation capacity of SW1353 cells and chondrocytes. This is likely caused by an NKX3-2-dependent activation of 47S gene promotor activity. This finding connects morphogen-mediated changes in cellular differentiation to an aspect of ribosome biogenesis via key transcription factors central to determining the chondrocyte phenotype.

Early Addition of Parathyroid Hormone-Related Peptide Regulates the Hypertrophic Differentiation of Mesenchymal Stem Cells

OBJECTIVE

Chondrogenic differentiation of mesenchymal stem cells (MSCs) into hyaline cartilage is complicated by terminal hypertrophic differentiation. In growth plate, parathyroid hormone-related peptide (1-34) (PTHrP) plays a crucial role in maintaining chondrocytes in their proliferation state by counteracting the hypertrophic differentiation. This study aims to test the effect of PTHrP supplementation at different time points on chondrogenic differentiation of MSCs and assess the final quality of differentiated chondrocytes.

METHODS

Human periosteum and bone marrow MSCs isolated from 3 patient samples (donor unmatched) were characterized by flow cytometry and multilineage differentiation. The cells were differentiated into chondrocytes in the presence of transforming growth factor-β (TGF-β) and the PTHrP (1-34) was added from 4th or 14th day of culture. The outcome was analyzed by histology, immunohistochemistry, and gene expression.

RESULTS

Flow cytometry and multilineage differentiation confirmed that the cells isolated from periosteum and bone marrow exhibited the phenotype of MSCs. During chondrogenic differentiation, pellets that received PTHrP from the 4th day of culture showed a significant reduction in hypertrophic markers (COL10A1 and RUNX) than the addition of PTHrP from the 14th day and TGF-β alone treated samples. Furthermore, 4th day supplementation of PTHrP significantly improved the expression of cartilage-specific markers (COL2A1, SOX9, ACAN) in both periosteum and bone marrow-derived MSCs. Histology and immunostaining with collagen type X data corroborated the gene expression outcomes.

CONCLUSION

The outcome showed that supplementing PTHrP from the 4th day of chondrogenic differentiation produced better chondrocytes with less hypertrophic markers in both bone marrow and periosteal-derived MSCs.

Cyclooxygenase-2 regulates PTHrP transcription in human articular chondrocytes and is involved in the pathophysiology of osteoarthritis in rats

Background

Cyclooxygenase-2 (COX-2) inhibitors are prescribed for the management of osteoarthritis (OA)-associated pain and inflammation. However, the role of COX-2 in normal and osteoarthritic articular chondrocytes has not been well investigated. We hypothesize that COX-2 plays a role in articular chondrocytes under normal conditions and during OA progression.

Methods

In vivo COX-2 levels in articular cartilage of normal and papain-induced osteoarthritic rats were compared. The role of COX-2 in human articular chondrocytes (HACs) was tested in vitro by COX-2 overexpression or activity inhibition. The levels of COX-2 and marker gene for normal function or articular cartilage degeneration were evaluated: mRNA by qRT-PCR; proteins by western blotting or immunohistochemistry; and glycosaminoglycan (GAG) by Safranin O-fast green staining. Parathyroid hormone-related protein (PTHrP) promoter activity was detected with luciferase reporter assays.

Results

In the OA rat study, COX-2 and PTHrP were simultaneously increased in osteoarthritic rat chondrocytes, while increased PTHrP levels were reduced by celecoxib, a COX-2 selective inhibitor. The levels of normal cartilage matrices, GAG and type II collagen decreased, while markers of degeneration, collagen type X and MMP13 were elevated in osteoarthritic articular chondrocytes. Celecoxib rescued the loss of GAG and the increased collagen type X and MMP13 levels. In vitro, COX-2 overexpression in HACs significantly increased Col2a1, Col10a1, PTHrP and MMP13 mRNA expression, which was decreased when COX-2 activity was suppressed. More importantly, COX-2 overexpression upregulated the PTHrP transcription, mRNA expression and protein levels.

Conclusion

COX-2 plays a pathophysiological role by preventing terminal differentiation of articular chondrocytes by upregulating PTHrP expression at the early stage of OA progression.

The Translational potential of this article

COX2 up-regulates PTHrP expression in normal and OA articular chondrocytes.

Comparison of Gene Expression Patterns in Articular Cartilage and Xiphoid Cartilage

Cartilage is a resilient and smooth connective tissue that is found throughout the body. Among the three major types of cartilage, namely hyaline cartilage, elastic cartilage, and fibrocartilage, hyaline cartilage is the most widespread type of cartilage predominantly located in the joint surfaces (articular cartilage, AC). It remains a huge challenge for orthopedic surgeons to deal with AC damage since it has limited capacity for self-repair. Xiphoid cartilage (XC) is a vestigial cartilage located in the distal end of the sternum. XC-derived chondrocytes exhibit strong chondrogenic differentiation capacity. Thus, XC could become a potential donor site of chondrocytes for cartilage repair and regeneration. However, the underlying gene expression patterns between AC and XC are still largely unknown. In the present study, we used state-of-the-art RNA-seq technology combined with validation method to investigate the gene expression patterns between AC and XC, and identified a series of differentially expressed genes (DEGs) involved in chondrocyte commitment and differentiation including growth factors, transcription factors, and extracellular matrices. We demonstrated that the majority of significantly up-regulated DEGs (XC vs. AC) in XC were involved in regulating cartilage regeneration and repair, whereas the majority of significantly up-regulated DEGs (XC vs. AC) in AC were involved in regulating chondrocyte differentiation and maturation. This study has increased our knowledge of transcriptional networks in hyaline cartilage and elastic cartilage. It also supports the use of XC-derived chondrocytes as a potential cell resource for cartilage regeneration and repair.

Functional Duality of Chondrocyte Hypertrophy and Biomedical Application Trends in Osteoarthritis

Chondrocyte hypertrophy is one of the key indicators in the progression of osteoarthritis (OA). However, compared with other OA indications, such as cartilage collapse, sclerosis, inflammation, and protease activation, the mechanisms by which chondrocyte hypertrophy contributes to OA remain elusive. As the pathological processes in the OA cartilage microenvironment, such as the alterations in the extracellular matrix, are initiated and dictated by the physiological state of the chondrocytes, in-depth knowledge of chondrocyte hypertrophy is necessary to enhance our understanding of the disease pathology and develop therapeutic agents. Chondrocyte hypertrophy is a factor that induces OA progression; it is also a crucial factor in the endochondral ossification. This review elaborates on this dual functionality of chondrocyte hypertrophy in OA progression and endochondral ossification through a description of the characteristics of various genes and signaling, their mechanism, and their distinguishable physiological effects. Chondrocyte hypertrophy in OA progression leads to a decrease in chondrogenic genes and destruction of cartilage tissue. However, in endochondral ossification, it represents an intermediate stage at the process of differentiation of chondrocytes into osteogenic cells. In addition, this review describes the current therapeutic strategies and their mechanisms, involving genes, proteins, cytokines, small molecules, three-dimensional environments, or exosomes, against the OA induced by chondrocyte hypertrophy. Finally, this review proposes that the contrasting roles of chondrocyte hypertrophy are essential for both OA progression and endochondral ossification, and that this cellular process may be targeted to develop OA therapeutics.

Rejuvenated Stem/Progenitor Cells for Cartilage Repair Using the Pluripotent Stem Cell Technology

It is widely accepted that chondral defects in articular cartilage of adult joints are never repaired spontaneously, which is considered to be one of the major causes of age-related degenerative joint disorders, such as osteoarthritis. Since mobilization of subchondral bone (marrow) cells and addition of chondrocytes or mesenchymal stromal cells into full-thickness defects show some degrees of repair, the lack of self-repair activity in adult articular cartilage can be attributed to lack of reparative cells in adult joints. In contrast, during a fetal or embryonic stage, joint articular cartilage has a scar-less repair activity, suggesting that embryonic joints may contain cells responsible for such activity, which can be chondrocytes, chondroprogenitors, or other cell types such as skeletal stem cells. In this respect, the tendency of pluripotent stem cells (PSCs) to give rise to cells of embryonic characteristics will provide opportunity, especially for humans, to obtain cells carrying similar cartilage self-repair activity. Making use of PSC-derived cells for cartilage repair is still in a basic or preclinical research phase. This review will provide brief overviews on how human PSCs have been used for cartilage repair studies.

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.

Histone deacetylase 4 deletion results in abnormal chondrocyte hypertrophy and premature ossification from collagen type 2α1‑expressing cells

Histone deacetylase 4 (HDAC4) plays a vital role in chondrocyte hypertrophy and bone formation. To investigate the function of HDAC4 in postnatal skeletal development, the present study developed lineage‑specific HDAC4‑knockout mice [collagen type 2α1 (Col2α1)‑Cre, HDAC4d/d mice] by crossing transgenic mice expressing Cre recombinase. Thus, a specific ablation of HDAC4 was performed in Col2α1‑expressing mice cells. The knee joints of HDAC4fl/fl and Col2α1‑Cre, HDAC4d/d mice were analyzed at postnatal day (P)2‑P21 using an in vivo bromodeoxyuridine (BrdU) assay, and Safranin O, Von Kossa and whole‑body staining were used to evaluate the developmental growth plate, hypertrophic differentiation, mineralization and skeletal mineralization patterns. The trabecular bone was analyzed using microcomputed tomography. The expressions of BrdU, proliferating cell nuclear antigen (PCNA), matrix metalloproteinase (MMP)‑13, runt‑related transcription factor (Runx)‑2, osteoprotegerin (OPG), CD34, type X collagen (ColX), osteocalcin and Wnt5a were determined using immunohistochemistry, in situ hybridization (ISH) and reverse transcription‑quantitative (RT‑q)PCR. The results demonstrated that HDAC4‑null mice (HDAC4d/d mice) were severely runted; these mice had a shortened hypertrophic zone (histopathological evaluation), accelerated vascular invasion and articular mineralization (Von Kossa staining), elevated expressions of MMP‑13, Runx2, OPG and CD34 (RT‑qPCR and immunohistochemistry), downregulated expression of the proliferative marker BrdU and PCNA (immunohistochemistry), increased expression of ColX and decreased expression of Wnt5a (ISH). In conclusion, chondrocyte‑derived HDAC4 was responsible for regulating chondrocyte proliferation and differentiation as well as endochondral bone formation.

Parathyroid hormone (1-34) promotes the effects of 3D printed scaffold-seeded bone marrow mesenchymal stem cells on meniscus regeneration

Background

Cell-based tissue engineering represents a promising management for meniscus repair and regeneration. The present study aimed to investigate whether the injection of parathyroid hormone (PTH) (1-34) could promote the regeneration and chondroprotection of 3D printed scaffold seeded with bone marrow mesenchymal stem cells (BMSCs) in a canine total meniscal meniscectomy model.

Methods

3D printed poly(e-caprolactone) scaffold seeded with BMSCs was cultured in vitro, and the effects of in vitro culture time on cell growth and matrix synthesis of the BMSCs–scaffold construct were evaluated by microscopic observation and cartilage matrix content detection at 7, 14, 21, and 28 days. After that, the tissue-engineered meniscus based on BMSCs–scaffold cultured for the appropriate culture time was selected for in vivo implantation. Sixteen dogs were randomly divided into four groups: PTH + BMSCs–scaffold, BMSCs–scaffold, total meniscectomy, and sham operation. The regeneration of the implanted tissue and the degeneration of articular cartilage were assessed by gross, histological, and immunohistochemical analysis at 12 weeks postoperatively.

Results

In vitro study showed that the glycosaminoglycan (GAG)/DNA ratio and the expression of collagen type II (Col2) were significantly higher on day 21 as compared to the other time points. In vivo study showed that, compared with the BMSCs–scaffold group, the PTH + BMSCs–scaffold group showed better regeneration of the implanted tissue and greater similarity to native meniscus concerning gross appearance, cell composition, and cartilage extracellular matrix deposition. This group also showed less expression of terminal differentiation markers of BMSC chondrogenesis as well as lower cartilage degeneration with less damage on the knee cartilage surface, higher expression of Col2, and lower expression of degeneration markers.

Conclusions

Our results demonstrated that PTH (1-34) promotes the regenerative and chondroprotective effects of the BMSCs–3D printed meniscal scaffold in a canine model, and thus, their combination could be a promising strategy for meniscus tissue engineering.

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.

Regulation of Hedgehog signaling Offers A Novel Perspective for Bone Homeostasis Disorder Treatment

The hedgehog (HH) signaling pathway is central to the regulation of bone development and homeostasis. HH signaling is not only involved in osteoblast differentiation from bone marrow mesenchymal stem cells (BM-MSCs), but also acts upstream within osteoblasts via the OPG/RANK/RANKL axis to control the expression of RANKL. HH signaling has been found to up-regulate parathyroid hormone related protein (PTHrP) expression in osteoblasts, which in turn activates its downstream targets nuclear factor of activated T cells (NFAT) and cAMP responsive element binding protein (CREB), and as a result CREB and NFAT cooperatively increase RANKL expression and osteoclastogenesis. Osteoblasts must remain in balance with osteoclasts in order to avoid excessive bone formation or resorption, thereby maintaining bone homeostasis. This review systemically summarizes the mechanisms whereby HH signaling induces osteoblast development and controls RANKL expression through PTHrP in osteoblasts. Proper targeting of HH signaling may offer a therapeutic option for treating bone homeostasis disorders.

Role of NGF-TrkA signaling in calcification of articular chondrocytes

Nerve growth factor (NGF) is a key regulator of chronic osteoarthritic pain, but the exact targets of NGF action on human articular cartilage is unknown. This study aimed to test the hypothesis that the NGF-tropomyosin receptor kinase A (TrkA) (high-affinity NGF receptor) pathway plays a role in the calcification process of human articular chondrocytes (hACs). A 14-aa small peptide of NGF (Nsp) previously shown to activate NGF signaling in rat PC12 cells was used as an NGF signaling agonist, and recombinant NGF and the pan-Trk inhibitor GNF-5837 were employed as signaling modulating agents. The functional consequences of NGF-TrkA signaling were examined in human healthy articular chondrocytes maintained under conditions supportive of osteogenesis in vitro. The NGF-mimetic bioactivity of Nsp was first confirmed on the basis of maintenance of neurite outgrowth in PC12 cells. Primary human chondrocytes responded to Nsp in vitro. Perturbation of NGF signaling with NGF, Nsp, and GNF-5837 resulted in a strong induction of chondrocyte calcification, and gene expression data suggested that the Indian Hedgehog-parathyroid hormone-related protein signaling axis was involved. These findings suggest functional involvement of NGF signaling in calcification of hACs and the importance of NGF signaling in articular cartilage homeostasis.-Jiang, Y., Tuan, R. S. Role of NGF-TrkA signaling in calcification of articular chondrocytes.

PTH decreases in vitro human cartilage regeneration without affecting hypertrophic differentiation

Regenerated cartilage formed after Autologous Chondrocyte Implantation may be of suboptimal quality due to postulated hypertrophic changes. Parathyroid hormone-related peptide, containing the parathyroid hormone sequence (PTHrP 1–34), enhances cartilage growth during development and inhibits hypertrophic differentiation of mesenchymal stromal cells (MSCs) and growth plate chondrocytes. This study aims to determine the possible anabolic and/or hypertrophic effect of PTH on human articular chondrocytes. Healthy human articular cartilage-derived chondrocytes (n = 6 donors) were cultured on type II collagen-coated transwells with/without 0.1 or 1.0 μM PTH from day 0, 9, or 21 until the end of culture (day 28). Extracellular matrix production, (pre)hypertrophy and PTH signaling were assessed by RT-qPCR and/or immunohistochemistry for collagen type I, II, X, RUNX2, MMP13, PTHR1 and IHH and by determining glycosaminoglycan production and DNA content. The Bern score assessed cartilage quality by histology. Regardless of the concentration and initiation of supplementation, PTH treatment significantly decreased DNA and glycosaminoglycan content and reduced the Bern score compared with controls. Type I collagen deposition was increased, whereas PTHR1 expression and type II collagen deposition were decreased by PTH supplementation. Expression of the (pre)hypertrophic markers MMP13, RUNX2, IHH and type X collagen were not affected by PTH. In conclusion, PTH supplementation to healthy human articular chondrocytes did not affect hypertrophic differentiation, but negatively influenced cartilage quality, the tissues’ extracellular matrix and cell content. Although PTH may be an effective inhibitor of hypertrophic differentiation in MSC-based cartilage repair, care may be warranted in applying accessory PTH treatment due to its effects on articular chondrocytes.

The Good the Bad and the Ugly of Glycosaminoglycans in Tissue Engineering Applications

High sulfation, low cost, and the status of heparin as an already FDA- and EMA- approved product, mean that its inclusion in tissue engineering (TE) strategies is becoming increasingly popular. However, the use of heparin may represent a naïve approach. This is because tissue formation is a highly orchestrated process, involving the temporal expression of numerous growth factors and complex signaling networks. While heparin may enhance the retention and activity of certain growth factors under particular conditions, its binding 'promiscuity' means that it may also inhibit other factors that, for example, play an important role in tissue maintenance and repair. Within this review we focus on articular cartilage, highlighting the complexities and highly regulated processes that are involved in its formation, and the challenges that exist in trying to effectively engineer this tissue. Here we discuss the opportunities that glycosaminoglycans (GAGs) may provide in advancing this important area of regenerative medicine, placing emphasis on the need to move away from the common use of heparin, and instead focus research towards the utility of specific GAG preparations that are able to modulate the activity of growth factors in a more controlled and defined manner, with less off-target effects.

Current research on pharmacologic and regenerative therapies for osteoarthritis

Osteoarthritis (OA) is a degenerative joint disorder commonly encountered in clinical practice, and is the leading cause of disability in elderly people. Due to the poor self-healing capacity of articular cartilage and lack of specific diagnostic biomarkers, OA is a challenging disease with limited treatment options. Traditional pharmacologic therapies such as acetaminophen, non-steroidal anti-inflammatory drugs, and opioids are effective in relieving pain but are incapable of reversing cartilage damage and are frequently associated with adverse events. Current research focuses on the development of new OA drugs (such as sprifermin/recombinant human fibroblast growth factor-18, tanezumab/monoclonal antibody against β-nerve growth factor), which aims for more effectiveness and less incidence of adverse effects than the traditional ones. Furthermore, regenerative therapies (such as autologous chondrocyte implantation (ACI), new generation of matrix-induced ACI, cell-free scaffolds, induced pluripotent stem cells (iPS cells or iPSCs), and endogenous cell homing) are also emerging as promising alternatives as they have potential to enhance cartilage repair, and ultimately restore healthy tissue. However, despite currently available therapies and research advances, there remain unmet medical needs in the treatment of OA. This review highlights current research progress on pharmacologic and regenerative therapies for OA including key advances and potential limitations.

The microwell-mesh: A novel device and protocol for the high throughput manufacturing of cartilage microtissues

Microwell platforms are frequently described for the efficient and uniform manufacture of 3-dimensional (3D) multicellular microtissues. Multiple partial or complete medium exchanges can displace microtissues from discrete microwells, and this can result in either the loss of microtissues from culture, or microtissue amalgamation when displaced microtissues fall into common microwells. Herein we describe the first microwell platform that incorporates a mesh to retain microtissues within discrete microwells; the microwell-mesh. We show that bonding a nylon mesh with an appropriate pore size over the microwell openings allows single cells to pass through the mesh into the microwells during the seeding process, but subsequently retains assembled microtissues within discrete microwells. To demonstrate the utility of this platform, we used the microwell-mesh to manufacture hundreds of cartilage microtissues, each formed from 5 × 10(3) bone marrow-derived mesenchymal stem/stromal cells (MSC). The microwell-mesh enabled reliable microtissue retention over 21-day cultures that included multiple full medium exchanges. Cartilage-like matrix formation was more rapid and homogeneous in microtissues than in conventional large diameter control cartilage pellets formed from 2 × 10(5) MSC each. The microwell-mesh platform offers an elegant mechanism to retain microtissues in microwells, and we believe that this improvement will make this platform useful in 3D culture protocols that require multiple medium exchanges, such as those that mimic specific developmental processes or complex sequential drug exposures.

Endogenous parathyroid hormone and knee osteoarthritis: a cross-sectional study

AIM

Although animal studies have shown that parathyroid hormone (PTH) analogs can have disease-modifying effect on experimental osteoarthritis, there is little human data. This study aimed to analyze the association between endogenous PTH and knee osteoarthritis in humans.

METHODS

This was a cross-sectional study including 5880 participants of The Fifth Korean National Health and Nutrition Examination Survey. Serum intact PTH was measured by chemiluminescence assay. Radiographic knee osteoarthritis was defined as a Kellgren-Lawrence grade of ≥ 2. The association between knee osteoarthritis and natural log-transformed PTH (lnPTH) was modeled using logistic regression analyses adjusting for age, body mass index categories, calcium intake and serum 25-hydroxyvitamin D in both sexes.

RESULTS

The prevalence of radiographic knee osteoarthritis tended to be higher in the highest quartile of PTH in women. In men, there was no clear trend across the quartiles of PTH. In multivariable analyses, there was no significant association between lnPTH and radiographic knee osteoarthritis in either sex (odds ratio [95% confidence interval]): 0.81 (0.64-1.02) in women and 0.88 (0.67-1.16) in men. Analyses on symptomatic radiographic knee osteoarthritis showed no significant association.

CONCLUSION

Although there was a trend for a negative association in women, no statistically significant association was found between endogenous PTH and knee osteoarthritis.

The paracrine feedback loop between vitamin D₃ (1,25(OH)₂D₃) and PTHrP in prehypertrophic chondrocytes

The endocrine feedback loop between vitamin D₃ (1,25(OH)₂D₃) and parathyroid hormone (PTH) plays a central role in skeletal development. PTH-related protein (PTHrP) shares homology and its receptor (PTHR1) with PTH. The aim of this study was to investigate whether there is a functional paracrine feedback loop between 1,25(OH)₂D₃ and PTHrP in the growth plate, in parallel with the endocrine feedback loop between 1,25(OH)₂D₃ and PTH. This was investigated in ATDC5 cells treated with 10⁻⁸ M 1,25(OH)₂D₃ or PTHrP, Col2-pd2EGFP transgenic mice, and primary Col2-pd2EGFP growth plate chondrocytes isolated by FACS, using RT-qPCR, Western blot, PTHrP ELISA, chromatin immunoprecipitation (ChIP) assay, silencing of the 1,25(OH)₂D₃ receptor (VDR), immunofluorescent staining, immunohistochemistry, and histomorphometric analysis of the growth plate. The ChIP assay confirmed functional binding of the VDR to the PTHrP promoter, but not to the PTHR1 promoter. Treatment with 1,25(OH)₂D₃ decreased PTHrP protein production, an effect which was prevented by silencing of the VDR. Treatment with PTHrP significantly induced VDR production, but did not affect 1α- and 24-hydroxylase expression. Hypertrophic differentiation was inhibited by PTHrP and 1,25(OH)₂D₃ treatment. Taken together, these findings indicate that there is a functional paracrine feedback loop between 1,25(OH)₂D₃ and PTHrP in the growth plate. 1,25(OH)₂D₃ decreases PTHrP production, while PTHrP increases chondrocyte sensitivity to 1,25(OH)₂D₃ by increasing VDR production. In light of the role of 1,25(OH)₂D₃ and PTHrP in modulating chondrocyte differentiation, 1,25(OH)₂D₃ in addition to PTHrP could potentially be used to prevent undesirable hypertrophic chondrocyte differentiation during cartilage repair or regeneration.

Total-body irradiation produces late degenerative joint damage in rats

PURPOSE

Premature musculoskeletal joint failure is a major source of morbidity among childhood cancer survivors. Radiation effects on synovial joint tissues of the skeleton are poorly understood. Our goal was to assess long-term changes in the knee joint from skeletally mature rats that received total-body irradiation while skeletal growth was ongoing.

MATERIALS AND METHODS

14 week-old rats were irradiated with 1, 3 or 7 Gy total-body doses of 18 MV X-rays. At 53 weeks of age, structural and compositional changes in knee joint tissues (articular cartilage, subchondral bone, and trabecular bone) were characterized using 7T MRI, nanocomputed tomography (nanoCT), microcomputed tomography (microCT), and histology.

RESULTS

T2 relaxation times of the articular cartilage were lower after exposure to all doses. Likewise, calcifications were observed in the articular cartilage. Trabecular bone microarchitecture was compromised in the tibial metaphysis at 7 Gy. Mild to moderate cartilage erosion was scored in the 3 and 7 Gy rats.

CONCLUSIONS

Late degenerative changes in articular cartilage and bone were observed after total-body irradiation in adult rats exposed prior to skeletal maturity. 7T MRI, microCT, nanoCT, and histology identified potential prognostic indicators of late radiation-induced joint damage.

Effect of parathyroid hormone on early chondrogenic differentiation from mesenchymal stem cells

Background

Treatment of articular cartilage injuries remains a difficult challenge due to the limited capacity for intrinsic repair. Mesenchymal stem cells (MSCs) can differentiate into chondrocytes under certain culture conditions. This study focused on the modulatory effects of parathyroid hormone (PTH) on chondrogenic differentiation from MSCs.

Methods

MSCs were treated with various concentrations of PTH under chondrogenic pellet culture condition. RNA was isolated for real-time polymerase chain reaction (PCR) and gene expressions of collagen type II α1 chain (Col2a1), collagen type X α1 chain, collagen type I α1 chain, SRY-box9 (Sox9), and type 1 PTH/PTHrP receptor (PTH1R) were examined. Chondrogenic differentiation was also evaluated by histological findings.

Results

PTH had opposite effects on chondrogenesis, depending on the concentration. A low to moderate concentration of PTH promoted chondrogenic differentiation of MSCs with increased expression of Sox9, Col2a1, and PTH1R, whereas chondrogenesis of MSCs was inhibited rather than stimulated with a higher concentration of PTH.

Conclusion

This study provides insights into the modulatory effect of PTH on chondrogenic differentiation from MSCs and the therapeutic potential for cartilage regeneration. Based on clinical experience regarding the efficacy and safety of PTH for bone metabolism, PTH may also be useful clinically for cartilage repair.

Subchondral chitosan/blood implant-guided bone plate resorption and woven bone repair is coupled to hyaline cartilage regeneration from microdrill holes in aged rabbit knees

OBJECTIVE

Little is known of how to routinely elicit hyaline cartilage repair tissue in middle-aged patients. We tested the hypothesis that in skeletally aged rabbit knees, microdrill holes can be stimulated to remodel the bone plate and induce a more integrated, voluminous and hyaline cartilage repair tissue when treated by subchondral chitosan/blood implants.

DESIGN

New Zealand White rabbits (13 or 32 months old, N = 7) received two 1.5 mm diameter, 2 mm depth drill holes in each knee, either left to bleed as surgical controls or press-fit with a 10 kDa (distal hole: 10K) or 40 kDa (proximal hole: 40K) chitosan/blood implant with fluorescent chitosan tracer. Post-operative knee effusion was documented. Repair tissues at day 0 (N = 1) and day 70 post-surgery (N = 6) were analyzed by micro-computed tomography, and by histological scoring and histomorphometry (SafO, Col-2, and Col-1) at day 70.

RESULTS

All chitosan implants were completely cleared after 70 days, without increasing transient post-operative knee effusion compared to controls. Proximal control holes had worse osteochondral repair than distal holes. Both implant formulations induced bone remodeling and improved lateral integration of the bone plate at the hole edge. The 40K implant inhibited further bone repair inside 50% of the proximal holes, while the 10K implant specifically induced a "wound bloom" reaction, characterized by decreased bone plate density in a limited zone beyond the initial hole edge, and increased woven bone (WB) plate repair inside the initial hole (P = 0.016), which was accompanied by a more voluminous and hyaline cartilage repair (P < 0.05 vs control defects).

CONCLUSION

In a challenging aged rabbit model, bone marrow-derived hyaline cartilage repair can be promoted by treating acute drill holes with a biodegradable subchondral implant that elicits bone plate resorption followed by anabolic WB repair within a 70-day repair period.

Flow-perfusion interferes with chondrogenic and hypertrophic matrix production by mesenchymal stem cells

Flow-perfusion is being promoted as a way to grow tissue-engineered cartilage in vitro. Yet, there is a concern that flow-perfusion may induce unwanted mechanical effects on chondrogenesis and terminal differentiation. Therefore, the aim of this study is to evaluate the effect of fluid flow on chondrogenesis and chondrocyte hypertrophy of MSCs in a well-established pellet culture model. Human MSC pellets were mounted into 3D-printed porous scaffolds in basic chondrogenic differentiation medium, containing TGF-β2. Constructs were then allowed to form cartilaginous matrix for 18 days, before they were transferred to a custom-built flow-perfusion system. A continuous flow of 1.22ml min(-1) was applied to the constructs for 10 days. Controls were maintained under static culture conditions. To evaluate chondrogenic and hypertrophic differentiation, RNA was isolated at day 20 and 28 and histology, immunohistochemistry and western blot analyses were performed after 28 days of culture. Abundant matrix was formed in the constructs, but production of chondrogenic and hypertrophic matrix components was affected by flow-perfusion. Although gene expression levels of the (late) hypertrophic and osteogenic marker osteocalcin increased by flow-perfusion, this did not result in more collagen type X protein deposition. Decreased GAG release, in combination with diminished collagen II staining, indicates reduced chondrogenesis in response to flow-perfusion. Caution should thus be taken when applying flow-perfusion to cultures to improve nutrient diffusion. Although we show that it is possible to influence the differentiation of chondrogenic differentiated MSCs by flow-perfusion, effects are inconsistent and strongly donor-dependent.

Advances in osteoarthritis genetics

Osteoarthritis (OA), the most common form of arthritis, is a highly debilitating disease of the joints and can lead to severe pain and disability. There is no cure for OA. Current treatments often fail to alleviate its symptoms leading to an increased demand for joint replacement surgery. Previous epidemiological and genetic research has established that OA is a multifactorial disease with both environmental and genetic components. Over the past 6 years, a candidate gene study and several genome-wide association scans (GWAS) in populations of Asian and European descent have collectively established 15 loci associated with knee or hip OA that have been replicated with genome-wide significance, shedding some light on the aetiogenesis of the disease. All OA associated variants to date are common in frequency and appear to confer moderate to small effect sizes. Some of the associated variants are found within or near genes with clear roles in OA pathogenesis, whereas others point to unsuspected, less characterised pathways. These studies have also provided further evidence in support of the existence of ethnic, sex, and joint specific effects in OA and have highlighted the importance of expanded and more homogeneous phenotype definitions in genetic studies of OA.

Cell-based approaches to joint surface repair: a research perspective

Repair of lesions of the articular cartilage lining the joints remains a major clinical challenge. Surgical interventions include osteochondral autograft transfer and microfracture. They can provide some relief of symptoms to patients, but generally fail to durably repair the cartilage. Autologous chondrocyte implantation has thus far shown the most promise for the durable repair of cartilage, with long-term follow-up studies indicating improved structural and functional outcomes. However, disadvantages of this technique include the need for additional surgery, availability of sufficient chondrocytes for implantation, and maintenance of their phenotype during culture-expansion. Mesenchymal stem cells offer an attractive alternative cell-source for cartilage repair, due to their ease of isolation and amenability to ex vivo expansion while retaining stem cell properties. Preclinical and clinical studies have demonstrated the potential of mesenchymal stem cells to promote articular cartilage repair, but have also highlighted several key challenges. Most notably, the quality and durability of the repair tissue, its resistance to endochondral ossification, and its effective integration with the surrounding host tissue. In addition, challenges exist related to the heterogeneity of mesenchymal stem cell preparations and their quality-control, as well as optimising the delivery method. Finally, as our knowledge of the cellular and molecular mechanisms underlying articular cartilage repair increases, promising studies are emerging employing bioactive scaffolds or therapeutics that elicit an effective tissue repair response through activation and mobilisation of endogenous stem and progenitor cells.

The promotion of osteochondral repair by combined intra-articular injection of parathyroid hormone-related protein and implantation of a bi-layer collagen-silk scaffold

The repair of osteochondral defects can be enhanced with scaffolds but is often accompanied with undesirable terminal differentiation of bone marrow-derived mesenchymal stem cells (BMSCs). Parathyroid hormone-related protein (PTHrP) has been shown to inhibit aberrant differentiation, but administration at inappropriate time points would have adverse effects on chondrogenesis. This study aims to develop an effective tissue engineering strategy by combining PTHrP and collagen-silk scaffold for osteochondral defect repair. The underlying mechanisms of the synergistic effect of combining PTHrP administration with collagen-silk scaffold implantation for rabbit knee joint osteochondral defect repair were investigated. In vitro studies showed that PTHrP treatment significantly reduced Alizarin Red staining and expression of terminal differentiation-related markers. This is achieved in part through blocking activation of the canonical Wnt/β-catenin signaling pathway. For the in vivo repair study, intra-articular injection of PTHrP was carried out at three different time windows (4-6, 7-9 and 10-12 weeks) together with implantation of a bi-layer collagen-silk scaffold. Defects treated with PTHrP at the 4-6 weeks time window exhibited better regeneration (reconstitution of cartilage and subchondral bone) with minimal terminal differentiation (hypertrophy, ossification and matrix degradation), as well as enhanced chondrogenesis (cell shape, Col2 and GAG accumulation) compared with treatment at other time windows. Furthermore, the timing of PTHrP administration also influenced PTHrP receptor expression, thus affecting the treatment outcome. Our results demonstrated that intra-articular injection of PTHrP at 4-6 weeks post-injury together with collagen-silk scaffold implantation is an effective strategy for inhibiting terminal differentiation and enhancing chondrogenesis, thus improving cartilage repair and regeneration in a rabbit model.