Impact of growth factors and PTHrP on early and late chondrogenic differentiation of human mesenchymal stem cells

Mechanoinduction of PTHrP/cAMP-signaling governs proteoglycan production in mesenchymal stromal cell-derived neocartilage

Abnormal mechanical loading is one of the major risk factors for articular cartilage degeneration. Engineered mesenchymal stromal cell (MSC)-derived cartilage holds great promise for cell-based cartilage repair. However, physiological loading protocols were shown to reduce matrix synthesis of MSC-derived neocartilage in vitro and the regulators of this undesired mechanoresponse remain poorly understood. Parathyroid hormone-related protein (PTHrP) is involved in cartilage development and can affect extracellular matrix (ECM) production during MSC chondrogenesis opposingly, depending on a continuous or transient exposure. PTHrP is induced by various mechanical cues in multiple tissues and species; but whether PTHrP is regulated in response to loading of human engineered neocartilage and may affect matrix synthesis in a positive or negative manner is unknown. The aim of this study was to investigate whether dynamic loading adjusts PTHrP-signaling in human MSC-derived neocartilage and whether it regulates matrix synthesis and other factors involved in the MSC mechanoresponse. Interestingly, MSC-derived chondrocytes significantly upregulated PTHrP mRNA (PTHLH) expression along with its second messenger cAMP in response to loading in our custom-built bioreactor. Exogenous PTHrP(1-34) induced the expression of known mechanoresponse genes (FOS, FOSB, BMP6) and significantly decreased glycosaminoglycan (GAG) and collagen synthesis similar to loading. The adenylate-cyclase inhibitor MDL-12,330A rescued the load-mediated decrease in GAG synthesis, indicating a direct involvement of cAMP-signaling in the reduction of ECM production. According to COL2A1-corrected hypertrophy-associated marker expression, load and PTHrP treatment shared the ability to reduce expression of MEF2C and PTH1R. In conclusion, the data demonstrate a significant mechanoinduction of PTHLH and a negative contribution of the PTHrP-cAMP signaling axis to GAG synthesis in MSC-derived chondrocytes after loading. To improve ECM synthesis and the mechanocompetence of load-exposed neocartilage, inhibition of PTHrP activity should be considered for MSC-based cartilage regeneration strategies.

Cell Proliferation, Chondrogenic Differentiation, and Cartilaginous Tissue Formation in Recombinant Silk Fibroin with Basic Fibroblast Growth Factor Binding Peptide

Regeneration of articular cartilage remains a challenge for patients who have undergone cartilage injury, osteochondritis dissecans and osteoarthritis. Here, we describe a new recombinant silk fibroin with basic fibroblast growth factor (bFGF) binding peptide, which has a genetically introduced sequence PLLQATLGGGS, named P7. In this study, we cultured a human mesenchymal cell line derived from bone marrow, UE6E7-16, in wild-type fibroin sponge (FS) and recombinant silk fibroin sponge with P7 peptide (P7 FS). We compared cell proliferation, chondrogenic differentiation and cartilaginous tissue formation between the two types of sponge. After stimulation with bFGF at 3 ng/mL, P7 FS showed significantly higher cell growth (1.2-fold) and higher cellular DNA content (5.6-fold) than did wild-type FS. To promote chondrogenic differentiation, cells were cultured in the presence of TGF-β at 10 ng/mL for 28 days. Immunostaining of P7 FS showed SOX9-positive cells comparable to wild-type FS. Alcian-Blue staining of P7 FS also showed cartilaginous tissue formation equivalent to wild-type FS. A significant increase in cell proliferation in P7 FS implies future clinical application of this transgenic fibroin for regeneration of articular cartilage. To produce cartilaginous tissue efficiently, transgenic fibroin sponges and culture conditions must be improved. Such changes should include the selection of growth factors involved in chondrogenic differentiation and cartilage formation.

Mesenchymal stromal cell chondrogenesis under ALK1/2/3-specific BMP inhibition: a revision of the prohypertrophic signalling network concept

BACKGROUND

In vitro chondrogenesis of mesenchymal stromal cells (MSCs) driven by the essential chondro-inducer transforming growth factor (TGF)-β is instable and yields undesired hypertrophic cartilage predisposed to bone formation in vivo. TGF-β can non-canonically activate bone morphogenetic protein-associated ALK1/2/3 receptors. These have been accused of driving hypertrophic MSC misdifferentiation, but data remained conflicting. We here tested the antihypertrophic capacity of two highly specific ALK1/2/3 inhibitors - compound A (CompA) and LDN-212854 (LDN21) - in order to reveal potential prohypertrophic contributions of these BMP/non-canonical TGF-β receptors during MSC in vitro chondrogenesis.

METHODS

Standard chondrogenic pellet cultures of human bone marrow-derived MSCs were treated with TGF-β and CompA (500 nM) or LDN21 (500 nM). Daily 6-hour pulses of parathyroid hormone-related peptide (PTHrP[1-34], 2.5 nM, from day 7) served as potent antihypertrophic control treatment. Day 28 samples were subcutaneously implanted into immunodeficient mice.

RESULTS

All groups underwent strong chondrogenesis, but GAG/DNA deposition and ACAN expression were slightly but significantly reduced by ALK inhibition compared to solvent controls along with a mild decrease of the hypertrophy markers IHH-, SPP1-mRNA, and Alkaline phosphatase (ALP) activity. When corrected for the degree of chondrogenesis (COL2A1 expression), only pulsed PTHrP but not ALK1/2/3 inhibition qualified as antihypertrophic treatment. In vivo, all subcutaneous cartilaginous implants mineralized within 8 weeks, but PTHrP pretreated samples formed less bone and attracted significantly less haematopoietic marrow than ALK1/2/3 inhibitor groups.

CONCLUSIONS

Overall, our data show that BMP-ALK1/2/3 inhibition cannot program mesenchymal stromal cells toward stable chondrogenesis. BMP-ALK1/2/3 signalling is no driver of hypertrophic MSC misdifferentiation and BMP receptor induction is not an adverse prohypertrophic side effect of TGF-β that leads to endochondral MSC misdifferentiation. Instead, the prohypertrophic network comprises misregulated PTHrP/hedgehog signalling and WNT activity, and a potential contribution of TGF-β-ALK4/5-mediated SMAD1/5/9 signalling should be further investigated to decide about its postulated prohypertrophic activity. This will help to successfully engineer cartilage replacement tissues from MSCs in vitro and translate these into clinical cartilage regenerative therapies.

Preclinical Studies and Clinical Trials on Cell-Based Treatments for Meniscus Regeneration

This study aims at performing a thorough review of cell-based treatment strategies for meniscus regeneration in preclinical and clinical studies. The PubMed, Embase, and Web of Science databases were searched for relevant studies (both preclinical and clinical) published from the time of database construction to December 2022. Data related to cell-based therapies for in situ regeneration of the meniscus were extracted independently by two researchers. Assessment of risk of bias was performed according to the Cochrane Handbook for Systematic Reviews of Interventions. Statistical analyses based on the classification of different treatment strategies were performed. A total of 5730 articles were retrieved, of which 72 preclinical studies and 6 clinical studies were included in this review. Mesenchymal stem cells (MSCs), especially bone marrow MSCs (BMSCs), were the most commonly used cell type. Among preclinical studies, rabbit was the most commonly used animal species, partial meniscectomy was the most commonly adopted injury pattern, and 12 weeks was the most frequently chosen final time point for assessing repair outcomes. A range of natural and synthetic materials were used to aid cell delivery as scaffolds, hydrogels, or other morphologies. In clinical trials, there was large variation in the dose of cells, ranging from 16 × 106 to 150 × 106 cells with an average of 41.52 × 106 cells. The selection of treatment strategy for meniscus repair should be based on the nature of the injury. Cell-based therapies incorporating various "combination" strategies such as co-culture, composite materials, and extra stimulation may offer greater promise than single strategies for effective meniscal tissue regeneration, restoring natural meniscal anisotropy, and eventually achieving clinical translation. Impact Statement This review provides an up-to-date and comprehensive overview of preclinical and clinical studies that tested cell-based treatments for meniscus regeneration. It presents novel perspectives on studies published in the past 30 years, giving consideration to the cell sources and dose selection, delivery methods, extra stimulation, animal models and injury patterns, timing of outcome assessment, and histological and biomechanical outcomes, as well as a summary of findings for individual studies. These unique insights will help to shape future research on the repair of meniscus lesions and inform the clinical translation of new cell-based tissue engineering strategies.

Research progress of functional motifs based on growth factors in cartilage tissue engineering: A review

Osteoarthritis is a chronic degenerative joint disease that exerts significant impacts on personal life quality, and cartilage tissue engineering is a practical treatment in clinical. Various growth factors are involved in cartilage regeneration and play important roles therein, which is the focus of current cartilage repair strategy. To compensate for the purification difficulty, high cost, poor metabolic stability, and circulating dilution of natural growth factors, the concept of functional motifs (also known as mimetic peptides) from original growth factor was introduced in recent studies. Here, we reviewed the selection mechanisms, biological functions, carrier scaffolds, and modification methods of growth factor-related functional motifs, and evaluated the repair performance in cartilage tissue engineering. Finally, the prospects of functional motifs in researches and clinical application were discussed.

Chondrogenic Differentiation of Human Mesenchymal Stem Cells via SOX9 Delivery in Cationic Niosomes

Gene transfer to mesenchymal stem cells constitutes a powerful approach to promote their differentiation into the appropriate cartilage phenotype. Although viral vectors represent gold standard vehicles, because of their high efficiency, their use is precluded by important concerns including an elevated immunogenicity and the possibility of insertional mutagenesis. Therefore, the development of new and efficient non-viral vectors is under active investigation. In the present study, we developed new non-viral carriers based on niosomes to promote the effective chondrogenesis of human MSCs. Two different niosome formulations were prepared by varying their composition on non-ionic surfactant, polysorbate 80 solely (P80), or combined with poloxamer 407 (P80PX). The best niosome formulation was proven to transfer a plasmid, encoding for the potent chondrogenic transcription factor SOX9 in hMSC aggregate cultures. Transfection of hMSC aggregates via nioplexes resulted in an increased chondrogenic differentiation with reduced hypertrophy. These results highlight the potential of niosome formulations for gene therapy approaches focused on cartilage repair.

Nestin-GFP transgene labels immunoprivileged bone marrow mesenchymal stem cells in the model of ectopic foci formation

Immune privileges are demonstrated for different types of quiescent stem cells of adult mammalian organisms. Mesenchymal stem cells (MSCs) are believed to have immune privileges; however, an accurate experimental confirmation hasn’t been presented. Here, we provide direct experimental evidence that MSCs of C57Black/6J murine bone marrow (BM) are immune privileged in vivo and retain their functionality after prolonged exposure to the uncompromised immune system. The BM of Nes-Gfp transgenic mice was implanted as a tissue fragment under the kidney capsule in isogenic C57Black/6J immunocompetent recipients. Nestin-Gfp strain provides a fluorescent immunogenic marker for a small fraction of BM cells, including GFP+CD45– MSCs. Despite the exposure of xenogenically marked MSCs to the fully-functional immune system, primary ectopic foci of hematopoiesis formed. Six weeks after implantation, multicolor fluorescence cytometry revealed both GFP+CD45– and GFP+CD45+ cells within the foci. GFP+CD45– cells proportion was 2.0 × 10–5 ×÷9 and it didn’t differ significantly from syngenic Nes-GFP transplantation control. According to current knowledge, the immune system of the recipients should eliminate GFP+ cells, including GFP+ MSCs. These results show that MSCs evade immunity. Primary foci were retransplanted into secondary Nes-GFP recipients. The secondary foci formed, in which CD45–GFP+ cells proportion was 6.7 × 10–5 ×÷2.2, and it didn’t differ from intact Nes-GFP BM. The results demonstrate that MSCs preserve self-renewal and retain their functionality after prolonged immune exposure. The success of this study relied on the implantation of BM fragments without prior dissociation of cells and the fact that the vast majority of implanted cells were immunologically equivalent to the recipients.

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.

circPhc3 sponging microRNA‑93‑3p is involved in the regulation of chondrocyte function by mechanical instability in osteoarthritis

Cartilage extracellular matrix (ECM) metabolism disorder caused by mechanical instability is a leading cause of osteoarthritis (OA), but the exact mechanisms have not been fully elucidated. Recent studies have suggested an important role of circular RNAs (circRNAs/circs) in OA. The present study aimed to investigate whether circRNAs might have a role in mechanical instability‑regulated chondrocyte matrix metabolism in OA. The expression levels of circPhc3 in human and mouse OA cartilage samples were measured using reverse transcription‑quantitative PCR and fluorescence in situ hybridization. The effects of circPhc3 on chondrocyte ECM metabolism were further investigated by overexpressing and knocking down circPhc3 in OA chondrocytes. The downstream target of circPhc3 was examined by performing a luciferase reporter assay. The results showed that the expression of circPhc3 was reduced in human and mouse OA cartilage. Moreover, circPhc3 was involved in mechanical loading‑regulated production of ECM and cartilage‑degrading enzymes. Further studies showed that circPhc3 regulated chondrocyte matrix metabolism primarily by binding to microRNA (miR)‑93‑3p, and mechanistic studies found that miR‑93‑3p targeting of FoxO1 was involved in chondrocyte matrix metabolism. Taken together, these results indicated that circPhc3 may serve an important role in the progression of OA and may be a good target for the treatment of OA.

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.

Vascularized adipocyte organoid model using isolated human microvessel fragments

Tissue organoids are proving valuable for modeling tissue health and disease in a variety of applications. This is due, in part, to the dynamic cell-cell interactions fostered within the 3D tissue-like space. To this end, the more that organoids recapitulate the different cell-cell interactions found in native tissue, such as that between parenchyma and the microvasculature, the better the fidelity of the model. The microvasculature, which is comprised of a spectrum of cell types, provides not only perfusion in its support of tissue health, but also important cellular interactions and biochemical dynamics important in tissue phenotype and function. Here, we incorporate whole, intact human microvessel fragments isolated from adipose tissue into organoids to form both MSC and adipocyte vascularized organoids. Isolated microvessels retain their native structure and cell composition, providing a more complete representation of the microvasculature within the organoids. Microvessels expanded via sprouting angiogenesis within organoids comprised of either MSCs or MSC-derived adipocytes and grew out of the organoids when placed in a 3D collagen matrix. In MSC organoids, a ratio of 50 MSCs to 1 microvessel fragment created the optimal vascularization response. We developed a new differentiation protocol that enabled the differentiation of MSCs into adipocytes while simultaneously promoting microvessel angiogenesis. The adipocyte organoids contained vascular networks, were responsive in a lipolysis assay, and expressed the functional adipocyte markers adiponectin and PPARγ. The presence of microvessels promoted insulin receptor expression by adipocytes and modified IL-6 secretion following a TNF-alpha challenge. Overall, we demonstrate a robust method for vascularizing high cell-density organoids with potential implications for other tissues as well.

Suppression of Hypertrophy During in vitro Chondrogenesis of Cocultures of Human Mesenchymal Stem Cells and Nasal Chondrocytes Correlates With Lack of in vivo Calcification and Vascular Invasion

Objective

Human nasal septal chondrocytes (NC) are a promising minimally invasive derivable chondrogenic cell source for cartilage repair. However, the quality of NC-derived cartilage is variable between donors. Coculture of NC with mesenchymal stem cells (MSCs) mitigates the variability but with undesirable markers of chondrocyte hypertrophy, such as type X collagen, and the formation of unstable calcifying cartilage at ectopic sites. In contrast, monoculture NC forms non-calcifying stable cartilage. Formation of a stable NC-MSC coculture cartilage is crucial for clinical application. The aim of this study was to explore the utility of parathyroid hormone-related peptide (PTHrP) hormone to suppress chondrocyte hypertrophy in NC-MSC cocultures and form stable non-calcifying cartilage at ectopic sites.

Methods

Human NC and bone marrow MSCs, and cocultures of NC and MSC (1:3 ratio) were aggregated in pellet form and subjected to in vitro chondrogenesis for 3 weeks in chondrogenic medium in the presence and absence of PTHrP. Following in vitro chondrogenesis, the resulting pellets were implanted in immunodeficient athymic nude mice for 3 weeks.

Results

Coculture of NC and MSC resulted in synergistic cartilage matrix production. PTHrP suppressed the expression of hypertrophy marker, type X collagen (COL10A1), in a dose-dependent fashion without affecting the synergism in cartilage matrix synthesis, and in vivo calcification was eradicated with PTHrP. In contrast, cocultured control (CC) pellets without PTHrP treatment expressed COL10A1, calcified, and became vascularized in vivo.

MiR-218 affects hypertrophic differentiation of human mesenchymal stromal cells during chondrogenesis via targeting RUNX2, MEF2C, and COL10A1

Background

Human mesenchymal stromal cells (MSC) hold hopes for cartilage regenerative therapy due to their chondrogenic differentiation potential. However, undesirable occurrence of calcification after ectopic transplantation, known as hypertrophic degeneration, remains the major obstacle limiting application of MSC in cartilage tissue regeneration approaches. There is growing evidence that microRNAs (miRs) play essential roles in post-transcriptional regulation of hypertrophic differentiation during chondrogenesis. Aim of the study was to identify new miR candidates involved in repression of hypertrophy-related targets.

Methods

The miR expression profile in human articular chondrocytes (AC) was compared to that in hypertrophic chondrocytes derived from human MSC by microarray analysis, and miR expression was validated by qPCR. Putative targets were searched by in silico analysis and validated by miR reporter assay in HEK293T, by functional assays (western blotting and ALP-activity) in transiently transfected SaOS-2 cells, and by a miR pulldown assay in human MSC. The expression profile of miR-218 was assessed by qPCR during in vitro chondrogenesis of MSC and re-differentiation of AC. MSC were transfected with miR-218 mimic, and differentiation outcome was assessed over 28 days. MiR-218 expression was quantified in healthy and osteoarthritic cartilage of patients.

Results

Within the top 15 miRs differentially expressed between chondral AC versus endochondral MSC differentiation, miR-218 was selected as a candidate miR predicted to target hypertrophy-related genes. MiR-218 was downregulated during chondrogenesis of MSC and showed a negative correlation to hypertrophic markers, such as COL10A1 and MEF2C. It was confirmed in SaOS-2 cells that miR-218 directly targets hypertrophy-related COL10A1, MEF2C, and RUNX2, as a gain of ectopic miR-218 mimic caused drop in MEF2C and RUNX2 protein accumulation, with attenuation of COL10A1 expression and significant concomitant reduction of ALP activity. A miR pulldown assay confirmed that miR-218 directly targets RUNX2, MEF2C in human MSC. Additionally, the gain of miR-218 in human MSC attenuated hypertrophic markers (MEF2C, RUNX2, COL10A1, ALPL), although with no boost of chondrogenic markers (GAG deposition, COL2A1) due to activation of WNT/β-catenin signaling. Moreover, no correlation between miR-218 expression and a pathologic phenotype in the cartilage of osteoarthritis (OA) patients was found.

Conclusions

Although miR-218 was shown to target pro-hypertrophic markers MEF2C, COL10A1, and RUNX2 in human MSC during chondrogenic differentiation, overall, it could not significantly reduce the hypertrophic phenotype or boost chondrogenesis. This could be explained by a concomitant activation of WNT/β-catenin signaling counteracting the anti-hypertrophic effects of miR-218. Therefore, to achieve a full inhibition of the endochondral pathway, a whole class of anti-hypertrophic miRs, including miR-218, needs to be taken into consideration.

The Role of the Microenvironment in Controlling the Fate of Bioprinted Stem Cells

The field of tissue engineering and regenerative medicine has made numerous advances in recent years in the arena of fabricating multifunctional, three-dimensional (3D) tissue constructs. This can be attributed to novel approaches in the bioprinting of stem cells. There are expansive options in bioprinting technology that have become more refined and specialized over the years, and stem cells address many limitations in cell source, expansion, and development of bioengineered tissue constructs. While bioprinted stem cells present an opportunity to replicate physiological microenvironments with precision, the future of this practice relies heavily on the optimization of the cellular microenvironment. To fabricate tissue constructs that are useful in replicating physiological conditions in laboratory settings, or in preparation for transplantation to a living host, the microenvironment must mimic conditions that allow bioprinted stem cells to proliferate, differentiate, and migrate. The advances of bioprinting stem cells and directing cell fate have the potential to provide feasible and translatable approach to creating complex tissues and organs. This review will examine the methods through which bioprinted stem cells are differentiated into desired cell lineages through biochemical, biological, and biomechanical techniques.

Concurrent multi-lineage differentiation of mesenchymal stem cells through spatial presentation of growth factors

Severe tendon and ligament injuries are estimated to affect between 300 000 and 400 000 people annually. Surgical repairs of these injuries often have poor long-term clinical outcomes because of resection of the interfacial tissue-the enthesis-and subsequent stress concentration at the attachment site. A healthy enthesis consists of distinct regions of bone, fibrocartilage, and tendon, each with distinct cell types, extracellular matrix components, and architecture, which are important for tissue function. Tissue engineering, which has been proposed as a potential strategy for replacing this tissue, is currently limited by its inability to differentiate multiple lineages of cells from a single stem cell population within a single engineered construct. In this study, we develop a multi-phasic gelatin methacrylate hydrogel construct system for spatial presentation of proteins, which is then validated for multi-lineage differentiation towards the cell types of the bone-tendon enthesis. This study determines growth factor concentrations for differentiation of mesenchymal stem cells towards osteoblasts, chondrocytes/fibrochondrocytes, and tenocytes, which maintain similar differentiation profiles in 3D hydrogel culture as assessed by qPCR and immunofluorescence staining. Finally, it is shown that this method is able to guide heterogeneous and spatially confined changes in mesenchymal stem cell genes and protein expressions with the tendency to result in osteoblast-, fibrochondrocyte-, and tenocyte-like expression profiles. Overall, we demonstrate the utility of the culture technique for engineering other musculoskeletal tissue interfaces and provide a biochemical approach for recapitulating the bone-tendon enthesis in vitro.

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.

Effect of autologous adipose-derived mesenchymal stem cell therapy in the treatment of an osteochondral lesion of the ankle

Osteochondral lesions (OCLs) of the talus are rare but can be associated with significant morbidity and may lead to the development of osteoarthritis. An improved understanding of the action of mesenchymal stem cells (MSCs) has seen renewed interest in their role in cartilage repair, with early preclinical and clinical research showing benefits in symptomatic and structural improvement. A 42-year-old man presented with an unstable OCL of the talus and onset of early osteoarthritis with a history of multiple previous ankle arthroscopies for ankle impingement. The patient underwent arthroscopic removal of the OCL in combination with adipose-derived MSC therapy. The patient reported progressive improvement as measured by the validated Foot and Ankle Disability Index. Repeat MRI with additional T2 mapping techniques showed successful regeneration of hyaline-like cartilage. This case is the first to show the successful use of MSC therapy in the management of an ankle OCL. Trial registration: Australian New Zealand Clinical Trials Registry - ACTRN12617000638336.

Significance of MEF2C and RUNX3 Regulation for Endochondral Differentiation of Human Mesenchymal Progenitor Cells

Guiding progenitor cell development between chondral versus endochondral pathways is still an unachieved task of cartilage neogenesis, and human mesenchymal progenitor cell (MPC) chondrogenesis is considered as a valuable model to better understand hypertrophic development of chondrocytes. Transcription factors Runx2, Runx3, and Mef2c play prominent roles for chondrocyte hypertrophy during mouse development, but little is known on the importance of these key fate-determining factors for endochondral development of human MPCs. The aim of this study was to unravel the regulation of RUNX2, RUNX3, and MEF2C during MPC chondrogenesis, the pathways driving their expression, and the downstream hypertrophic targets affected by their regulation. RUNX2, RUNX3, and MEF2C gene expression was differentially regulated during chondrogenesis of MPCs, but remained low and unregulated when non-hypertrophic articular chondrocytes were differentiated under the same conditions. RUNX3 and MEF2C mRNA and protein levels rose in parallel to hypertrophic marker upregulation, but surprisingly, RUNX2 gene expression changed only by trend and RUNX2 protein remained undetectable. While RUNX3 expression was driven by TGF-β and BMP signaling, MEF2C responded to WNT-, BMP-, and Hedgehog-pathway inhibition. MEF2C but not RUNX3 levels correlated significantly with COL10A1, IHH, and IBSP gene expression when hypertrophy was attenuated. IBSP was a downstream target of RUNX3 and MEF2C but not RUNX2 in SAOS-2 cells, underlining the capacity of RUNX3 and MEF2C to stimulate osteogenic marker expression in human cells. Conclusively, RUNX3 and MEF2C appeared more important than RUNX2 for human endochondral MPC chondrogenesis. Pathways altering the speed of chondrogenesis (FGF, TGF-β, BMP) affected RUNX2 or RUNX3, while pathways changing hypertrophy (WNT, PTHrP/HH) regulated mainly MEF2C. Taken together, reduction of MEF2C levels is a new goal to shift human cartilage neogenesis toward the chondral pathway.

Is a Local Administration of Parathyroid Hormone Effective to Tendon-to-Bone Healing in a Rat Rotator Cuff Repair Model?

To evaluate the effect of local parathyroid hormone (PTH) administration on rotator cuff tendon-to-bone healing in a rat model compared with systemic PTH injection and untreated controls. PTH-alginate scaffold was prepared and sustained release of PTH was confirmed. Bilateral supraspinatus tendon repairs were performed in 39 rats (group 1, supraspinatus repair only; group 2, supraspinatus repair with systemic PTH injection; group 3, supraspinatus repair with local PTH administration via an absorbable scaffold; n = 13 each). Biomechanical (cross-sectional area, mode of failure, load to failure, and ultimate stress: right side) and histological analyses (hematoxylin and eosin stain, Masson's Trichrome stain Picrosirius red stain, Immunohistochemistry for BMP2, PTH1R, ColI, and ColIII: Left side) were performed to evaluate tendon-to-bone healing quality at 8 weeks after repair, and blood test (osteocalcin and procollagen type I N-terminal pro-peptide [PINP] levels) was performed in all rats. There was no intergroup difference in the healing failure rate (p = 0.910) or failure mode (p = 0.585). Biomechanically, subjects in groups 2 and 3 exhibited significantly larger cross-sectional areas and higher ultimate failure loads and ultimate stress than those in group 1 (all p < 0.05); however, no differences were noted between groups 2 and 3 (all p > 0.05). Histologically, groups 2 and 3 exhibited more organized tendon-to-bone interface structures with higher density, parallel orientation, and collagen fiber continuity than group 1 (all p < 0.05 except collagen fiber continuity in group 1 vs. 2); however, no differences in histological parameters between groups 2 and 3 (all p > 0.05). The protein levels of bone morphogenic protein 2, PTH 1 receptor, and collagen I and III and the serum level of PINP were increased in groups 2 and 3 versus group 1 (all p < 0.05) without showing differences between groups 2 and 3 (all p > 0.05). Local PTH administration using an absorbable scaffold improved the biomechanical and histological outcomes of rotator cuff tendon-to-bone healing comparable with systemic PTH injection at 8 weeks after repair in a rat model. © 2019 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 38:82-91, 2020.

Regulation of WNT5A and WNT11 during MSC in vitro chondrogenesis: WNT inhibition lowers BMP and hedgehog activity, and reduces hypertrophy

Re-directing mesenchymal stromal cell (MSC) chondrogenesis towards a non-hypertrophic articular chondrocyte-(AC)-like phenotype is important for improving articular cartilage neogenesis to enhance clinical cartilage repair strategies. This study is the first to demonstrate that high levels of non-canonical WNT5A followed by WNT11 and LEF1 discriminated MSC chondrogenesis from AC re-differentiation. Moreover, β-catenin seemed incompletely silenced in differentiating MSCs, which altogether suggested a role for WNT signaling in hypertrophic MSC differentiation. WNT inhibition with the small molecule IWP-2 supported MSC chondrogenesis according to elevated proteoglycan deposition and reduced the characteristic upregulation of BMP4, BMP7 and their target ID1, as well as IHH and its target GLI1 observed during endochondral differentiation. Along with the pro-hypertrophic transcription factor MEF2C, multiple hypertrophic downstream targets including IBSP and alkaline phosphatase activity were reduced by IWP-2, demonstrating that WNT activity drives BMP and hedgehog upregulation, and MSC hypertrophy. WNT inhibition almost matched the strong anti-hypertrophic capacity of pulsed parathyroid hormone-related protein application, and both outperformed suppression of BMP signaling with dorsomorphin, which also reduced cartilage matrix deposition. Yet, hypertrophic marker expression under IWP-2 remained above AC level, and in vivo mineralization and ectopic bone formation were reduced but not eliminated. Overall, the strong anti-hypertrophic effects of IWP-2 involved inhibition but not silencing of pro-hypertrophic BMP and IHH pathways, and more advanced silencing of WNT activity as well as combined application of IHH or BMP antagonists should next be considered to install articular cartilage neogenesis from human MSCs.

Hypoxia Activates the PTHrP -MEF2C Pathway to Attenuate Hypertrophy in Mesenchymal Stem Cell Derived Cartilage

Articular cartilage lacks an intrinsic repair capacity and due to the ability of mesenchymal stem cells (MSCs) to differentiate into chondrocytes, MSCs have been touted as a cellular source to regenerate damaged cartilage. However, a number of prevailing concerns for such a treatment remain. Generally, administration of MSCs into a cartilage defect results in poor regeneration of the damaged cartilage with the repaired cartilage consisting primarily of fibro-cartilage rather than hyaline cartilage. Methods that improve the chondrogenic potential of transplanted MSCs in vivo may be advantageous. In addition, the proclivity of MSC-derived cartilage to undergo hypertrophic differentiation or form bone in vivo also remains a clinical concern. If MSC-derived cartilage was to undergo hypertrophic differentiation in vivo, this would be deleterious in a clinical setting. This study focuses on establishing a mechanism of action by which hypoxia or low oxygen tension can be used to both enhance chondrogenesis and attenuate hypertrophic differentiation of both MSC and ATDC5 derived chondrocytes. Having elucidated a novel mechanism of action, the subsequent goals of this study were to develop an in vitro culture regime to mimic the beneficial effects of physiological low oxygen tension in a normoxic environment.

Determinants of stem cell lineage differentiation toward chondrogenesis versus adipogenesis

Adult stem cells, also termed as somatic stem cells, are undifferentiated cells, detected among differentiated cells in a tissue or an organ. Adult stem cells can differentiate toward lineage specific cell types of the tissue or organ in which they reside. They also have the ability to differentiate into mature cells of mesenchymal tissues, such as cartilage, fat and bone. Despite the fact that the balance has been comprehensively scrutinized between adipogenesis and osteogenesis and between chondrogenesis and osteogenesis, few reviews discuss the relationship between chondrogenesis and adipogenesis. In this review, the developmental and transcriptional crosstalk of chondrogenic and adipogenic lineages are briefly explored, followed by elucidation of signaling pathways and external factors guiding lineage determination between chondrogenic and adipogenic differentiation. An in-depth understanding of overlap and discrepancy between these two mesenchymal tissues in lineage differentiation would benefit regeneration of high-quality cartilage tissues and adipose tissues for clinical applications.

Regulation of WNT5A and WNT11 during MSC in vitro chondrogenesis: WNT inhibition lowers BMP and hedgehog activity, and reduces hypertrophy

Re-directing mesenchymal stromal cell (MSC) chondrogenesis towards a non-hypertrophic articular chondrocyte-(AC)-like phenotype is important for improving articular cartilage neogenesis to enhance clinical cartilage repair strategies. This study is the first to demonstrate that high levels of non-canonical WNT5A followed by WNT11 and LEF1 discriminated MSC chondrogenesis from AC re-differentiation. Moreover, β-catenin seemed incompletely silenced in differentiating MSCs, which altogether suggested a role for WNT signaling in hypertrophic MSC differentiation. WNT inhibition with the small molecule IWP-2 supported MSC chondrogenesis according to elevated proteoglycan deposition and reduced the characteristic upregulation of BMP4, BMP7 and their target ID1, as well as IHH and its target GLI1 observed during endochondral differentiation. Along with the pro-hypertrophic transcription factor MEF2C, multiple hypertrophic downstream targets including IBSP and alkaline phosphatase activity were reduced by IWP-2, demonstrating that WNT activity drives BMP and hedgehog upregulation, and MSC hypertrophy. WNT inhibition almost matched the strong anti-hypertrophic capacity of pulsed parathyroid hormone-related protein application, and both outperformed suppression of BMP signaling with dorsomorphin, which also reduced cartilage matrix deposition. Yet, hypertrophic marker expression under IWP-2 remained above AC level, and in vivo mineralization and ectopic bone formation were reduced but not eliminated. Overall, the strong anti-hypertrophic effects of IWP-2 involved inhibition but not silencing of pro-hypertrophic BMP and IHH pathways, and more advanced silencing of WNT activity as well as combined application of IHH or BMP antagonists should next be considered to install articular cartilage neogenesis from human MSCs.

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.

High tibial osteotomy in combination with arthroscopic abrasion arthroplasty and autologous adipose-derived mesenchymal stem cell therapy in the treatment of advanced knee osteoarthritis

Osteoarthritis is a progressive and debilitating condition. An increasing number of total knee replacements are being performed under the age of 65. Improved understanding of the action of mesenchymal stem cells (MSC) has seen renewed interest in their role in cartilage repair. A 43-year-old man presented with grade IV medial compartment knee osteoarthritis. The patient underwent high tibial osteotomy (HTO) and arthroscopic abrasion arthroplasty in combination with adipose-derived MSC therapy. The patient reported improvement in pain and function as measured by validated outcome scores. Repeat MRI including T2 mapping techniques showed hyaline-like cartilage regeneration. This case highlights the potential benefit of surgical interventions including HTO in combination with MSC therapy in early-onset severe osteoarthritis. This technique may considerably delay or prevent the need for total knee replacement in young patients. Further controlled trials are needed to confirm the reproducibility of this outcome.

Targeting anti-chondrogenic factors for the stimulation of chondrogenesis: A new paradigm in cartilage repair

Trauma and age-related cartilage disorders represent a major global cause of morbidity, resulting in chronic pain and disability in patients. A lack of effective therapies, together with a rapidly aging population, creates an impressive clinical and economic burden on healthcare systems. In this scenario, experimental therapies based on transplantation or in situ stimulation of skeletal Mesenchymal Stem/progenitor Cells (MSCs) have raised great interest for cartilage repair. Nevertheless, the challenge of guiding MSC differentiation and preventing cartilage hypertrophy and calcification still needs to be overcome. While research has mostly focused on the stimulation of cartilage anabolism using growth factors, several issues remain unresolved prompting the field to search for novel solutions. Recently, inhibition of anti-chondrogenic regulators has emerged as an intriguing opportunity. Anti-chondrogenic regulators include extracellular proteins as well as intracellular transcription factors and microRNAs that act as potent inhibitors of pro-chondrogenic signals. Suppression of these inhibitors can enhance MSC chondrogenesis and production of cartilage matrix. We here review the current knowledge concerning different types of anti-chondrogenic regulators. We aim to highlight novel therapeutic targets for cartilage repair and discuss suitable tools for suppressing their anti-chondrogenic functions. Further effort is needed to unveil the therapeutic perspectives of this approach and pave the way for effective treatment of cartilage injuries in patients. © 2018 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res.

Independent chondrogenic potential of canine bone marrow-derived mesenchymal stem cells in monolayer expansion cultures decreases in a passage-dependent pattern

Although chondroinductive growth factors are considered necessary for chondrogenesis of bone marrow-derived mesenchymal stem cells (BMSC), independent and spontaneous chondrogenesis has been previously demonstrated in adult horses, bovine calves and adult human BMSC. Surprisingly, adult canine BMSC under similar culture conditions previously failed to demonstrate chondrogenesis. The present study evaluated independent chondrogenic potential of BMSC sourced from three young dogs in the absence of known chondroinductive factors. BMSC were culture expanded in 10% DMEM up to third passage (P3). At each passage, the phenotype of BMSC was evaluated by RT-PCR gel electrophoresis and qPCR. BMSC exhibited a chondrogenic phenotype in the absence of dexamethasone and TGF-β1 as verified by the expression of Sox-9, type II collagen and aggrecan. Sox-9 was significantly downregulated (P<0.05) from P1−P3 compared to P0 while type II and X collagen, and aggrecan were significantly downregulated at P3 compared to P0. There was a significant (P<0.01) negative correlation between passaging and Sox-9, type II collagen and aggrecan gene expression. These results indicate that independent chondrogenic potential and phenotype retention of BMSC decreases in a passage-dependent pattern. Therefore, caution should be exercised for future experiments evaluating the chondrogenic potential of BMSC after extensive expansion cultures in 10% DMEM.

Co-culture of bone marrow stromal cells and chondrocytes in vivo for the repair of the goat condylar cartilage defects

This study explored the feasibility of inducing the differentiation of BMSCs into chondrocytes through co-culture with chondrocytes in hydrogel constructs (Pluronic F-127 gel) in vivo for the repair of goat mandibular condylar cartilage defects. Chondrocytes and BMSCs were isolated from goat auricular cartilage and bone marrow, respectively, and were mixed at a ratio of 3:7. BMSCs were labelled with green fluorescence protein (GFP) using a retrovirus vector for tracing. Mixed cells were re-suspended in 30% Pluronic F-127 at a concentration of 5×10⁷ cells/ml to form a gel-cell complex. The gel-cell complex was implanted into the temporomandibular joint condylar articular cartilage defects. The whole temporomandibular joint and adjacent tissues were harvested at 4, 8, and 12 weeks after surgery, and gross observation, histology and collagen II expression were evaluated. In the co-culture group, cartilage-like tissues were formed, and abundant type II collagen could be detected by immunohistochemistry in the condylar cartilage defects. Confocal microscopy revealed that implanted GFP-labelled BMSCs were embedded in cartilage-like tissues. The co-culture system described herein provides a chondrogenic microenvironment to induce the chondrogenic differentiation of BMSCs in vivo without any additional cellular factors.

Pre-transplantational Control of the Post-transplantational Fate of Human Pluripotent Stem Cell-Derived Cartilage

Cartilage pellets generated from ectomesenchymal progeny of human pluripotent stem cells (hPSCs) in vitro eventually show signs of commitment of chondrocytes to hypertrophic differentiation. When transplanted subcutaneously, most of the surviving pellets were fully mineralized by 8 weeks. In contrast, treatment with the adenylyl cyclase activator, forskolin, in vitro resulted in slightly enlarged cartilage pellets containing an increased proportion of proliferating immature chondrocytes that expressed very low levels of hypertrophic/terminally matured chondrocyte-specific genes. Forskolin treatment also enhanced hyaline cartilage formation by reducing type I collagen gene expression and increasing sulfated glycosaminoglycan accumulation in the developed cartilage. Chondrogenic mesoderm from hPSCs and dedifferentiated nasal chondrocytes responded similarly to forskolin. Furthermore, forskolin treatment in vitro increased the frequency at which the cartilage pellets maintained unmineralized chondrocytes after subcutaneous transplantation. Thus, the post-transplantational fate of chondrocytes originating from hPSC-derived chondroprogenitors can be controlled during their genesis in vitro.

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Forskolin/cAMP suppresses/delays BMP-induced chondrocyte maturation in vitro

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Forskolin supports chondrocyte proliferation and hyaline chondrogenesis in vitro

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Forskolin suppresses osteogenesis and BMP signaling gene expression in cartilage

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In vitro forskolin treatment improves in vivo maintenance of uncalcified cartilage

Co-treatment of TGF-β3 and BMP7 is superior in stimulating chondrocyte redifferentiation in both hypoxia and normoxia compared to single treatments

Signaling by members of the transforming growth factor-β (TGF-β) superfamily, such as TGF-β3 and BMP7, and oxygen tension play a pivotal role in chondrocyte biology. The objective of this research was to investigate the endogenous BMP7 expression in human osteoarthritis (OA) cartilage and the effect of oxygen tension on the single or combined treatment with TGF-β3 and BMP7 on OA chondrocyte redifferentiation in three dimensional (3D) pellet cultures. The results showed the expression of BMP7 and its intracellular signaling target SMAD1/5/8 was decreased in early OA, while it was increased in later stages of OA. The combined treatment with TGF-β3 and BMP7, both in normoxia and hypoxia, was more effective than TGF-β3 or BMP7 alone in redifferentiating chondrocytes. This was reflected by Alcian blue/Safranin O staining and collagen type II protein expression, as well as by gene expression. Hypoxia elevated TGF-β3 and BMP7-induced matrix formation of OA chondrocytes and alleviated the catabolic gene expression. Interestingly, cells cultured under normoxia displayed mild signs of an inflammatory stress response, which was effectively counteracted by culturing the cells under low oxygen tension. Our data underscores the important modulatory role of oxygen tension on the chondrocyte's responsiveness to TGF-β3 and/or BMP7.

Functionality of decellularized matrix in cartilage regeneration: A comparison of tissue versus cell sources

Increasing evidence indicates that decellularized extracellular matrices (dECMs) derived from cartilage tissues (T-dECMs) or chondrocytes/stem cells (C-dECMs) can support proliferation and chondrogenic differentiation of cartilage-forming cells. However, few review papers compare the differences between these dECMs when they serve as substrates for cartilage regeneration. In this review, after an introduction of cartilage immunogenicity and decellularization methods to prepare T-dECMs and C-dECMs, a comprehensive comparison focuses on the effects of T-dECMs and C-dECMs on proliferation and chondrogenic differentiation of chondrocytes/stem cells in vitro and in vivo. Key factors within dECMs, consisting of microarchitecture characteristics and micromechanical properties as well as retained insoluble and soluble matrix components, are discussed in-depth for potential mechanisms underlying the functionality of these dECMs in regulating chondrogenesis. With this information, we hope to benefit dECM based cartilage engineering and tissue regeneration for future clinical application.

STATEMENT OF SIGNIFICANCE

The use of decellularized extracellular matrix (dECM) is becoming a promising approach for tissue engineering and regeneration. Compared to dECM derived from cartilage tissue, recently reported dECM from cell sources exhibits a distinct role in cell based cartilage regeneration. In this review paper, for the first time, tissue and cell based dECMs are comprehensively compared for their functionality in cartilage regeneration. This information is expected to provide an update for dECM based cartilage regeneration.

PPAR-δ Agonist With Mesenchymal Stem Cells Induces Type II Collagen-Producing Chondrocytes in Human Arthritic Synovial Fluid

Osteoarthritis (OA) is an inflammatory joint disease characterized by degeneration of articular cartilage within synovial joints. An estimated 27 million Americans suffer from OA, and the population is expected to reach 67 million in the United States by 2030. Thus, it is urgent to find an effective treatment for OA. Traditional OA treatments have no disease-modifying effect, while regenerative OA therapies such as autologous chondrocyte implantation show some promise. Nonetheless, current regenerative therapies do not overcome synovial inflammation that suppresses the differentiation of mesenchymal stem cells (MSCs) to chondrocytes and the expression of type II collagen, the major constituent of functional cartilage. We discovered a synergistic combination that overcame synovial inflammation to form type II collagen-producing chondrocytes. The combination consists of peroxisome proliferator–activated receptor (PPAR) δ agonist, human bone marrow (hBM)-derived MSCs, and hyaluronic acid (HA) gel. Interestingly, those individual components showed their own strong enhancing effects on chondrogenesis. GW0742, a PPAR-δ agonist, greatly enhanced MSC chondrogenesis and the expression of type II collagen and glycosaminoglycan (GAG) in hBM-MSC-derived chondrocytes. GW0742 also increased the expression of transforming growth factor β that enhances chondrogenesis and suppresses cartilage fibrillation, ossification, and inflammation. HA gel also increased MSC chondrogenesis and GAG production. However, neither GW0742 nor HA gel could enhance the formation of type II collagen-producing chondrocytes from hBM-MSCs within human OA synovial fluid. Our data demonstrated that the combination of hBM-MSCs, PPAR-δ agonist, and HA gel significantly enhanced the formation of type II collagen-producing chondrocytes within OA synovial fluid from 3 different donors. In other words, the novel combination of PPAR-δ agonist, hBM-MSCs, and HA gel can overcome synovial inflammation to form type II collagen cartilage within human OA synovial fluid. This novel articularly injectable formula could improve OA treatment in the future clinical application.

Developmentally inspired programming of adult human mesenchymal stromal cells toward stable chondrogenesis

It is generally accepted that adult human bone marrow-derived mesenchymal stromal cells (hMSCs) are default committed toward osteogenesis. Even when induced to chondrogenesis, hMSCs typically form hypertrophic cartilage that undergoes endochondral ossification. Because embryonic mesenchyme is obviously competent to generate phenotypically stable cartilage, it is questioned whether there is a correspondence between mesenchymal progenitor compartments during development and in adulthood. Here we tested whether forcing specific early events of articular cartilage development can program hMSC fate toward stable chondrogenesis. Inspired by recent findings that spatial restriction of bone morphogenetic protein (BMP) signaling guides embryonic progenitors toward articular cartilage formation, we hypothesized that selective inhibition of BMP drives the phenotypic stability of hMSC-derived chondrocytes. Two BMP type I receptor-biased kinase inhibitors were screened in a microfluidic platform for their time- and dose-dependent effect on hMSC chondrogenesis. The different receptor selectivity profile of tested compounds allowed demonstration that transient blockade of both ALK2 and ALK3 receptors, while permissive to hMSC cartilage formation, is necessary and sufficient to maintain a stable chondrocyte phenotype. Remarkably, even upon compound removal, hMSCs were no longer competent to undergo hypertrophy in vitro and endochondral ossification in vivo, indicating the onset of a constitutive change. Our findings demonstrate that adult hMSCs effectively share properties of embryonic mesenchyme in the formation of transient but also of stable cartilage. This opens potential pharmacological strategies to articular cartilage regeneration and more broadly indicates the relevance of developmentally inspired protocols to control the fate of adult progenitor cell systems.

Redifferentiation of aged human articular chondrocytes by combining bone morphogenetic protein-2 and melanoma inhibitory activity protein in 3D-culture

Melanoma inhibitory activity (MIA) affects the differentiation to hyaline cartilage and can inhibit the osteogenic potential of bone morphogenetic protein (BMP)-2 in mesenchymal stem cells (MSC). The aim of this study was to investigate if MIA also inhibits the osteogenic potential of BMP-2 in human articular chondrocytes during redifferentiation, which may lead to a higher grade of differentiation without calcification. HAC of four female patients (mean age: 73.75 ±6.98) were seeded into 3D culture for 28 days; after adding the recombinant proteins, four groups were formed (Control, BMP-2, MIA, BMP-2+MIA). Samples were analysed for gene expression, glycosaminoglycan (GAG) content and histology on day 0, 14 and 28. Collagen type 2 (COL2A1) was significantly increased in the BMP-2 containing groups on day 28; BMP-2 (100-fold, p = 0.001), BMP-2+MIA (65-fold, p = 0.009) and similar to the level of native cartilage. Higher aggrecan (Agg) levels were present in the BMP-2 (3-fold, p = 0.007) and BMP-2+MIA (4-fold, p = 0.002) group after 14 days and in the BMP-2 (9-fold, p = 0.001) group after 28 days. Collagen type 10 (COL10A1) was increased in the BMP-2 containing groups (6-fold, p = 0.006) but these levels were significantly below native cartilage. Alkaline phosphatase (ALP), collagen type 1 (COL1A1) and the glycosaminoglycan (GAG) content did not reveal any relevant differences between groups. BMP-2 is a potent inducer for differentiation of HAC. A significant enhancement of this effect in combination with MIA could not be observed. Furthermore no significant reduction of osteogenic markers during re-differentiation of chondrocytes was present combining BMP-2 and MIA.

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.

Enhanced chondrogenesis of bone marrow-derived stem cells by using a combinatory cell therapy strategy with BMP-2/TGF-β1, hypoxia, and COL1A1/HtrA1 siRNAs

Mesenchymal stem cells (MSCs) hold promise for cartilage engineering. Here, we aimed to determine the best culture conditions to induce chondrogenesis of MSCs isolated from bone marrow (BM) of aged osteoarthritis (OA) patients. We showed that these BM-MSCs proliferate slowly, are not uniformly positive for stem cell markers, and maintain their multilineage potential throughout multiple passages. The chondrogenic lineage of BM-MSCs was induced in collagen scaffolds, under normoxia or hypoxia, by BMP-2 and/or TGF-β1. The best chondrogenic induction, with the least hypertrophic induction, was obtained with the combination of BMP-2 and TGF-β1 under hypoxia. Differentiated BM-MSCs were then transfected with siRNAs targeting two markers overexpressed in OA chondrocytes, type I collagen and/or HtrA1 protease. siRNAs significantly decreased mRNA and protein levels of type I collagen and HtrA1, resulting in a more typical chondrocyte phenotype, but with frequent calcification of the subcutaneously implanted constructs in a nude mouse model. Our 3D culture model with BMP-2/TGF-β1 and COL1A1/HtrA1 siRNAs was not effective in producing a cartilage-like matrix in vivo. Further optimization is needed to stabilize the chondrocyte phenotype of differentiated BM-MSCs. Nevertheless, this study offers the opportunity to develop a combinatory cellular therapy strategy for cartilage tissue engineering.

Electrospun Microfiber Scaffolds with Anti-Inflammatory Tributanoylated N-Acetyl-d-Glucosamine Promote Cartilage Regeneration

Tissue-engineering strategies offer promising tools for repairing cartilage damage; however, these strategies suffer from limitations under pathological conditions. As a model disease for these types of nonideal systems, the inflammatory environment in an osteoarthritic (OA) joint limits the efficacy of engineered therapeutics by disrupting joint homeostasis and reducing its capacity for regeneration. In this work, we investigated a sugar-based drug candidate, a tributanoylated N-acetyl-d-glucosamine analogue, called 3,4,6-O-Bu3GlcNAc, that is known to reduce nuclear factor kappa-light-chain-enhancer of activated B cells (NF-κB) signaling in osteoarthritis. 3,4,6-O-Bu3GlcNAc not only inhibited NFκB signaling but also exerted chondrogenic and anti-inflammatory effects on chondrocytes isolated from patients with osteoarthritis. 3,4,6-O-Bu3GlcNAc also increased the expression of extracellular matrix proteins and induced cartilage tissue production in three-dimensional in vitro hydrogel culture systems. To translate these chondrogenic and anti-inflammatory properties to tissue regeneration in osteoarthritis, we implanted 3,4,6-O-Bu3GlcNAc-loaded poly(lactic-co-glycolic acid) microfiber scaffolds into rats. The drug-laden scaffolds were biocompatible, and when seeded with human OA chondrocytes, similarly promoted cartilage tissue formation. 3,4,6-O-Bu3GlcNAc combined with the appropriate structural environment could be a promising therapeutic approach for osteoarthritis.

Environmental preconditioning rejuvenates adult stem cells' proliferation and chondrogenic potential

Adult stem cells are a promising cell source for cartilage regeneration. Unfortunately, due to donor age and ex vivo expansion, stem cell senescence becomes a huge hurdle for these cells to be used clinically. Increasing evidence indicates that environmental preconditioning is a powerful approach in promoting stem cells' ability to resist a harsh environment post-engraftment, such as hypoxia and inflammation. However, few reports organize and evaluate the literature regarding the rejuvenation effect of environmental preconditioning on stem cell proliferation and chondrogenic differentiation capacity, which are important variables for stem cell based tissue regeneration. This report aims to identify several critical environmental factors such as oxygen concentration, growth factors, and extracellular matrix and to discuss their preconditioning influence on stem cells' rejuvenation including proliferation and chondrogenic potential as well as underlying molecular mechanisms. We believe that environmental preconditioning based rejuvenation is a simpler and safer strategy to program pre-engraftment stem cells for better survival and enhanced proliferation and differentiation capacity without the undesired effects of some treatments, such as genetic manipulation.

Differential expression of TGF-β superfamily members and role of Smad1/5/9-signalling in chondral versus endochondral chondrocyte differentiation

Proteins of the transforming-growth-factor-β (TGF-β)-superfamily have a remarkable ability to induce cartilage and bone and the crosstalk of TGF-β - and BMP-signalling pathways appears crucial during chondrocyte development. Aim was to assess the regulation of TGF-β-superfamily members and of Smad2/3- and Smad1/5/9-signalling during endochondral in vitro chondrogenesis of mesenchymal stromal cells (MSC) relative to chondral redifferentiation of articular chondrocytes (AC) to adjust chondrocyte development of MSC towards a less hypertrophic phenotype. While MSC increased BMP4 and BMP7 and reduced TGFBR2 and TGFBR3-expression during chondrogenesis, an opposite regulation was observed during AC-redifferentiation. Antagonists CHRD and CHL2 rose significantly only in AC-cultures. AC showed higher initial BMP4, pSmad1/5/9 and SOX9 protein levels, a faster (re-)differentiation but a similar decline of pSmad2/3- and pSmad1/5/9-signalling versus MSC-cultures. BMP-4/7-stimulation of MSC-pellets enhanced SOX9 and accelerated ALP-induction but did not shift differentiation towards osteogenesis. Inhibition of BMP-signalling by dorsomorphin significantly reduced SOX9, raised RUNX2, maintained collagen-type-II and collagen-type-X lower and kept ALP-activity at levels reached at initiation of treatment. Conclusively, ALK1,2,3,6-signalling was essential for MSC-chondrogenesis and its prochondrogenic rather than prohypertrophic role may explain why inhibition of canonical BMP-signalling could not uncouple cartilage matrix production from hypertrophy as this was achieved with pulsed PTHrP-application.

Regeneration of Articular Cartilage by Human ESC-Derived Mesenchymal Progenitors Treated Sequentially with BMP-2 and Wnt5a

The success of cell‐based therapies to restore joint cartilage requires an optimal source of reparative progenitor cells and tight control of their differentiation into a permanent cartilage phenotype. Bone morphogenetic protein 2 (BMP‐2) has been extensively shown to promote mesenchymal cell differentiation into chondrocytes in vitro and in vivo. Conversely, developmental studies have demonstrated decreased chondrocyte maturation by Wingless‐Type MMTV Integration Site Family, Member 5A (Wnt5a). Thus, we hypothesized that treatment of human embryonic stem cell (hESC)‐derived chondroprogenitors with BMP‐2 followed by Wnt5a may control the maturational progression of these cells into a hyaline‐like chondrocyte phenotype. We examined the effects of sustained exposure of hESC‐derived mesenchymal‐like progenitors to recombinant Wnt5a or BMP‐2 in vitro. Our data indicate that BMP‐2 promoted a strong chondrogenic response leading to terminal maturation, whereas recombinant Wnt5a induced a mild chondrogenic response without promoting hypertrophy. Moreover, Wnt5a suppressed BMP‐2‐mediated chondrocyte maturation, preventing the formation of fibrocartilaginous tissue in high‐density cultures treated sequentially with BMP‐2 and Wnt5a. Implantation of scaffoldless pellets of hESC‐derived chondroprogenitors pretreated with BMP‐2 followed by Wnt5a into rat chondral defects induced an articular‐like phenotype in vivo. Together, the data establish a novel role for Wnt5a in controlling the progression from multipotency into an articular‐like cartilage phenotype in vitro and in vivo. Stem Cells Translational Medicine2017;6:40–50

Mesenchymal stem cell therapy in the treatment of osteoarthritis: reparative pathways, safety and efficacy - a review

Osteoarthritis is a leading cause of pain and disability across the world. With an aging population its prevalence is likely to further increase. Current accepted medical treatment strategies are aimed at symptom control rather than disease modification. Surgical options including joint replacement are not without possible significant complications. A growing interest in the area of regenerative medicine, led by an improved understanding of the role of mesenchymal stem cells in tissue homeostasis and repair, has seen recent focused efforts to explore the potential of stem cell therapies in the active management of symptomatic osteoarthritis. Encouragingly, results of pre-clinical and clinical trials have provided initial evidence of efficacy and indicated safety in the therapeutic use of mesenchymal stem cell therapies for the treatment of knee osteoarthritis. This paper explores the pathogenesis of osteoarthritis and how mesenchymal stem cells may play a role in future management strategies of this disabling condition.

Therapeutic use of hormones on tendinopathies: a narrative review

BACKGROUND

Hormones can modify tendon homeostasis, some of them leading to tendon damage, while others are essentials for healing. This narrative review summarizes the current knowledge on the topic, focusing on the hormones normally secreted by endocrine glands.

METHODS

A search in PubMed, Web of Knowledge and EMBASE, using the terms tendinopathy or tendon, combined with estrogens, testosterone, thyroid and parathyroid hormones, glucocorticoids and growth hormone, independently, was performed. Relevant articles focusing on the correlation between hormones and tendons, and their therapeutic use in tendinopathies, were selected.

RESULTS

Tendon abnormalities observed in subjects with hyperparathyroidism, hypercortisolism and acromegaly are described. At present, experimental studies and preliminary observations in humans suggest that parathyroid and growth hormones, locally administered, are promising therapeutic tools in specific tendon disorders. Local injections of glucocorticoids are useful in several tendinopathies, exploiting their anti-inflammatory and anti-proliferative properties, but carry the risk of further tendon degeneration and ruptures, due to the detrimental direct effect of glucocorticoids on the tendon structure.

CONCLUSION

Because tendons injuries are frequent, often with long lasting sequels, it is important to improve our understanding concerning the therapeutic potential of hormones on healing.

LEVEL OF EVIDENCE

IV.

Effects of Dexamethasone Concentration and Timing of Exposure on Chondrogenesis of Equine Bone Marrow-Derived Mesenchymal Stem Cells

OBJECTIVE

Dexamethasone is known to support mesenchymal stem cell (MSC) chondrogenesis, although the effects of dose and timing of exposure are not well understood. The objective of this study was to investigate these variables using a laboratory model of MSC chondrogenesis.

DESIGN

Equine MSCs were encapsulated in agarose and cultured in chondrogenic medium with 1 or 100 nM dexamethasone, or without dexamethasone, for 15 days. Samples were analyzed for extracellular matrix (ECM) accumulation, prostaglandin E2 and alkaline phosphatase secretion, and gene expression of selected collagens and catabolic enzymes. Timing of exposure was evaluated by ECM accumulation after dexamethasone was withdrawn over the first 6 days, or withheld for up to 3 or 6 days of culture.

RESULTS

ECM accumulation was not significantly different between 1 and 100 nM dexamethasone, but was suppressed ~40% in dexamethasone-free cultures. Prostaglandin E2 secretion, and expression of catabolic enzymes, including matrix metalloproteinase 13, and type X collagen was generally lowest in 100 nM dexamethasone and not significantly different between 1 nM and dexamethasone-free cultures. Dexamethasone could be withheld for at least 2 days without affecting ECM accumulation, while withdrawal studies suggested that dexamethasone supports ECM accumulation beyond day 6.

CONCLUSION

One nanomolar dexamethasone supported robust cartilage-like ECM accumulation despite not having an effect on markers of inflammation, although higher concentrations of dexamethasone may be necessary to suppress undesirable hypertrophic differentiation. While early exposure to dexamethasone was not critical, sustained exposure of at least a week appears to be necessary to maximize ECM accumulation.

The Regulatory Role of Signaling Crosstalk in Hypertrophy of MSCs and Human Articular Chondrocytes

Hypertrophic differentiation of chondrocytes is a main barrier in application of mesenchymal stem cells (MSCs) for cartilage repair. In addition, hypertrophy occurs occasionally in osteoarthritis (OA). Here we provide a comprehensive review on recent literature describing signal pathways in the hypertrophy of MSCs-derived in vitro differentiated chondrocytes and chondrocytes, with an emphasis on the crosstalk between these pathways. Insight into the exact regulation of hypertrophy by the signaling network is necessary for the efficient application of MSCs for articular cartilage repair and for developing novel strategies for curing OA. We focus on articles describing the role of the main signaling pathways in regulating chondrocyte hypertrophy-like changes. Most studies report hypertrophic differentiation in chondrogenesis of MSCs, in both human OA and experimental OA. Chondrocyte hypertrophy is not under the strict control of a single pathway but appears to be regulated by an intricately regulated network of multiple signaling pathways, such as WNT, Bone morphogenetic protein (BMP)/Transforming growth factor-β (TGFβ), Parathyroid hormone-related peptide (PTHrP), Indian hedgehog (IHH), Fibroblast growth factor (FGF), Insulin like growth factor (IGF) and Hypoxia-inducible factor (HIF). This comprehensive review describes how this intricate signaling network influences tissue-engineering applications of MSCs in articular cartilage (AC) repair, and improves understanding of the disease stages and cellular responses within an OA articular joint.

Modeling and Validation of Multilayer Poly(Lactide-Co-Glycolide) Scaffolds for In Vitro Directed Differentiation of Juxtaposed Cartilage and Bone

Polymeric scaffolds, which release growth factors in a temporally controlled manner, have successfully directed the differentiation of stem cells into monolithic tissues of a single lineage. However, engineering precise boundaries in multilineage functional tissues, such as the juxtaposed cartilaginous and osseous tissue present in articulated joints, often remains a challenge. This work demonstrates a precise materials system for in vitro reconstruction of the three-dimensional architecture of these types of human tissues. Multilayer poly(lactide-co-glycolide) (PLG) scaffolds were used to produce spatiotemporal gradients to direct the differentiation of an initially uniform population of mesenchymal stem cells (MSCs) into juxtaposed cartilage and bone. Specifically, growth factors (chondrogenic transforming growth factor-β3 and osteogenic bone morphogenetic protein-4) and their neutralizing antibodies were incorporated within distinct layers of the PLG scaffolds to create spatially segregated morphogen fields within the scaffold volume. The multilayer PLG scaffold designs were optimized by mathematical modeling, and generation of spatially segregated morphogen gradients was validated by assessing activity of luciferase reporter cell lines responsive to each growth factor. Scaffolds seeded with MSCs demonstrated production of juxtaposed cartilage and bone, as evaluated by biochemical staining and western blotting for tissue-specific matrix proteins. This work demonstrates a significant advance for the engineering of implantable constructs comprising tissues of multiple lineages, with potential applications in orthopedic regenerative medicine.

Strategies to minimize hypertrophy in cartilage engineering and regeneration

Due to a blood supply shortage, articular cartilage has a limited capacity for self-healing once damaged. Articular chondrocytes, cartilage progenitor cells, embryonic stem cells, and mesenchymal stem cells are candidate cells for cartilage regeneration. Significant current attention is paid to improving chondrogenic differentiation capacity; unfortunately, the potential chondrogenic hypertrophy of differentiated cells is largely overlooked. Consequently, the engineered tissue is actually a transient cartilage rather than a permanent one. The development of hypertrophic cartilage ends with the onset of endochondral bone formation which has inferior mechanical properties. In this review, current strategies for inhibition of chondrogenic hypertrophy are comprehensively summarized; the impact of cell source options is discussed; and potential mechanisms underlying these strategies are also categorized. This paper aims to provide guidelines for the prevention of hypertrophy in the regeneration of cartilage tissue. This knowledge may also facilitate the retardation of osteophytes in the treatment of osteoarthritis.

Sequential exposure to fibroblast growth factors (FGF) 2, 9 and 18 enhances hMSC chondrogenic differentiation

OBJECTIVE

To test the effects of sequential exposure to FGF2, 9 and 18 on human Mesenchymal Stem Cells (hMSC) differentiation during in vitro chondrogenesis.

DESIGN

Control and FGF2-expanded hMSC were cultured in aggregates in the presence of rhFGF9, rhFGF18 or rhFGFR3-specific signaling FGF variants, starting at different times during the chondroinductive program. Quantitative real time polymerase chain reaction (qRT-PCR) and immunocytochemistry were performed at different stages. The aggregate cultures were switched to a hypertrophy-inducing medium along with rhFGFs and neutralizing antibodies against FGFR1 and FGFR3. Histological/immunohistochemical/biochemical analyses were performed.

RESULTS

FGF2-exposed hMSC during expansion up-regulated Sox9 suggesting an early activation of the chondrogenic machinery. FGF2, FGF9 and 18 modulated the expression profile of FGFR1 and FGFR3 in hMSC during expansion and chondrogenesis. In combination with transforming growth factor-beta (TGF-β), FGF9 and FGF18 inhibited chondrogenesis when added at the beginning of the program (≤ d7), while exhibiting an anabolic effect when added later (≥d14), an effect mediated by FGFR3. Finally, FGFR3 signaling induced by either FGF9 or FGF18 delayed the appearance of spontaneous and induced hypertrophy-related changes.

CONCLUSIONS

The stage of hMSC-dependent chondrogenesis at which the growth factors are added impacts the progression of the differentiation program: increased cell proliferation and priming (FGF2); stimulated early chondrogenic differentiation (TGF-β, FGF9/FGF18) by shifting the chondrogenic program earlier; augmented extracellular matrix (ECM) production (FGF9/FGF18); and delayed terminal hypertrophy (FGF9/FGF18). Collectively, these factors could be used to optimize pre-implantation conditions of hMSC when used to engineer cartilage grafts.

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.