The postnatal role of Sox9 in cartilage

Mechanisms of bone development and repair

Bone development occurs through a series of synchronous events that result in the formation of the body scaffold. The repair potential of bone and its surrounding microenvironment - including inflammatory, endothelial and Schwann cells - persists throughout adulthood, enabling restoration of tissue to its homeostatic functional state. The isolation of a single skeletal stem cell population through cell surface markers and the development of single-cell technologies are enabling precise elucidation of cellular activity and fate during bone repair by providing key insights into the mechanisms that maintain and regenerate bone during homeostasis and repair. Increased understanding of bone development, as well as normal and aberrant bone repair, has important therapeutic implications for the treatment of bone disease and ageing-related degeneration.

CXCL12-CXCR4 Interplay Facilitates Palatal Osteogenesis in Mice

Cranial neural crest cells (CNCCs), identified by expression of transcription factor Sox9, migrate to the first branchial arch and undergo proliferation and differentiation to form the cartilage and bone structures of the orofacial region, including the palatal bone. Sox9 promotes osteogenic differentiation and stimulates CXCL12-CXCR4 chemokine-receptor signaling, which elevates alkaline phosphatase (ALP)-activity in osteoblasts to initiate bone mineralization. Disintegration of the midline epithelial seam (MES) is crucial for palatal fusion. Since we earlier demonstrated chemokine-receptor mediated signaling by the MES, we hypothesized that chemokine CXCL12 is expressed by the disintegrating MES to promote the formation of an osteogenic center by CXCR4-positive osteoblasts. Disturbed migration of CNCCs by excess oxidative and inflammatory stress is associated with increased risk of cleft lip and palate (CLP). The cytoprotective heme oxygenase (HO) enzymes are powerful guardians harnessing injurious oxidative and inflammatory stressors and enhances osteogenic ALP-activity. By contrast, abrogation of HO-1 or HO-2 expression promotes pregnancy pathologies. We postulate that Sox9, CXCR4, and HO-1 are expressed in the ALP-activity positive osteogenic regions within the CNCCs-derived palatal mesenchyme. To investigate these hypotheses, we studied expression of Sox9, CXCL12, CXCR4, and HO-1 in relation to palatal osteogenesis between E15 and E16 using (immuno)histochemical staining of coronal palatal sections in wild-type (wt) mice. In addition, the effects of abrogated HO-2 expression in HO-2 KO mice and inhibited HO-1 and HO-2 activity by administrating HO-enzyme activity inhibitor SnMP at E11 in wt mice were investigated at E15 or E16 following palatal fusion. Overexpression of Sox9, CXCL12, CXCR4, and HO-1 was detected in the ALP-activity positive osteogenic regions within the palatal mesenchyme. Overexpression of Sox9 and CXCL12 by the disintegrating MES was detected. Neither palatal fusion nor MES disintegration seemed affected by either HO-2 abrogation or inhibition of HO-activity. Sox9 progenitors seem important to maintain the CXCR4-positive osteoblast pool to drive osteogenesis. Sox9 expression may facilitate MES disintegration and palatal fusion by promoting epithelial-to-mesenchymal transformation (EMT). CXCL12 expression by the MES and the palatal mesenchyme may promote osteogenic differentiation to create osteogenic centers. This study provides novel evidence that CXCL12-CXCR4 interplay facilitates palatal osteogenesis and palatal fusion in mice.

Positive Effects of a Young Systemic Environment and High Growth Differentiation Factor 11 Levels on Chondrocyte Proliferation and Cartilage Matrix Synthesis in Old Mice

OBJECTIVE

To investigate the effects of a young systemic environment and growth differentiation factor 11 (GDF-11) on aging cartilage.

METHODS

A heterochronic parabiosis model (2-month-old mouse and 12-month-old mouse [Y/O]), an isochronic parabiosis model (12-month-old mouse and 12-month-old mouse [O/O]), and 12-month-old mice alone (O) were evaluated. Knee joints and chondrocytes from old mice were examined by radiography, histology, cell proliferation assays, immunohistochemistry, Western blotting, and quantitative reverse transcriptase-polymerase chain reaction 16 weeks after parabiosis surgery. GDF-11 was injected into 12-month-old mouse joints daily for 16 weeks. Cartilage degeneration, cell proliferation, and osteoarthritis-related gene expression were evaluated.

RESULTS

Osteoarthritis Research Society International scores in old mice were significantly lower in the Y/O group than in the O/O and O groups (both P < 0.05). The percentage of 5-ethynyl-2'-deoxyuridine-positive chondrocytes in old mice was significantly higher in the Y/O group than in the other groups (P < 0.05). Type II collagen (CII) and SOX9 messenger RNA levels differed in cartilage from old mice in the Y/O group compared to the O/O and O groups (both P < 0.05). RUNX-2, CX, and matrix metalloproteinase 13 levels were significantly lower in cartilage from old mice in the Y/O group compared to the O/O and O groups (both P < 0.05). Similar results were obtained for protein expression levels and after GDF-11 treatment in vitro and in vivo. Phosphorylated Smad2/3 (pSmad2/3) levels were higher in the recombinant GDF-11-treated group than in the control group.

CONCLUSION

A young systemic environment promotes chondrocyte proliferation and cartilage matrix synthesis in old mice. GDF-11, a "young factor," contributes to these effects through the up-regulation of pSmad2/3.

Activation of the SOX-5, SOX-6, and SOX-9 Trio of Transcription Factors Using a Gene-Activated Scaffold Stimulates Mesenchymal Stromal Cell Chondrogenesis and Inhibits Endochondral Ossification

Current treatments for articular cartilage defects relieve symptoms but often only delay cartilage degeneration. Mesenchymal stem cells (MSCs) have shown chondrogenic potential but tend to undergo endochondral ossification when implanted in vivo. Harnessing factors governing joint development to functionalize biomaterial scaffolds, termed developmental engineering, might allow to prime host MSCs to regenerate mature articular cartilage in situ without requiring cell isolation or ex vivo expansion. Therefore, the aim of this study is to develop a gene-activated scaffold capable of delivering developmental cues to host MSCs, thus priming MSCs for articular cartilage differentiation and inhibiting endochondral ossification. It is shown that delivery of the SOX-Trio induced MSCs to over-express COL2A1 and ACAN and deposit a sulfated and collagen type II rich extracellular matrix while hypertrophic gene expression and collagen type X deposition is inhibited. When cell-free SOX-Trio-activated scaffolds are implanted ectopically in vivo, they induced spontaneous chondrogenesis without evidence of hypertrophy. MSCs pre-cultured on SOX-Trio-activated scaffolds prior to implantation differentiate into phenotypically stable chondrocytes as evidenced by a lack of collagen X expression or vascular invasion. This SOX-trio-activated scaffold represents a potent, single treatment, developmentally inspired strategy to prime MSCs in situ for articular cartilage defect repair.

Detection of a 4 bp Mutation in the 3'UTR Region of Goat Sox9 Gene and Its Effect on the Growth Traits

Simple Summary

The sex determining region Y (SRY)-type high mobility group (HMG) box 9 (Sox9) gene is critically important in the formation and development of cartilage and is considered the “main regulator” of chondrogenesis. Additionally, a large number of studies have shown that mutations in a single allele of human Sox9 can lead to campomelic dysplasia syndrome. Therefore, the mutations of Sox9 have been the subject of increasing interest among researchers. However, no studies to date have examined the association between Sox9 gene variants and growth traits in goats. Here, we detected a 4 bp indel in the 3′Untranslated Regions (3′UTR) region of Sox9 in Shaanbei white cashmere (SBWC) goats (n = 1109) and studied the association between this indel and growth traits. The 4 bp indel of Sox9 was significantly associated with body length, heart girth, hip width, and all body measurement indexes (p < 0.05) in SBWC goats. Thus, this deletion could be used as an effective molecular marker for maximizing the growth traits of goats in breeding programs.

Abstract

The SRY-type HMG box 9 (Sox9) gene plays an important role in chondrocyte development as well as changes in hypertrophic chondrocytes, indicating that Sox9 can regulate growth in animals. However, no studies to date have examined the correlation between variations in Sox9 and growth traits in goats. Here, we found a 4 bp indel in the 3′UTR of Sox9 and verified its association with growth traits in Shaanbei white cashmere goats (n = 1109). The frequencies of two genotypes (ID and II) were 0.397 and 0.603, respectively, and polymorphic information content (PIC) values showed that the indel had a medium PIC (PIC > 0.25). The 4 bp indel was significantly correlated with body length (p = 0.006), heart girth (p = 0.001), and hip width (p = 4.37 × 10 ⁻⁴). Notably, individuals with the ID genotype had significantly superior phenotypic traits compared with individuals bearing the II genotype. Hence, we speculated that the 4 bp indel is an important mutation affecting growth traits in goat, and may serve as an effective DNA molecular marker for marker-assisted selection in goat breeding programs.

Hypoxia Inducible Factor-1α in Osteochondral Tissue Engineering

Damage to osteochondral (OC) tissues can lead to pain, loss of motility, and progress to osteoarthritis. Tissue engineering approaches offer the possibility of replacing damaged tissues and restoring joint function; however, replicating the spatial and functional heterogeneity of native OC tissue remains a pressing challenge. Chondrocytes in healthy cartilage exist in relatively low-oxygen conditions, while osteoblasts in the underlying bone experience higher oxygen pressures. Such oxygen gradients also exist in the limb bud, where they influence OC tissue development. The cellular response to these spatial variations in oxygen pressure, which is mediated by the hypoxia inducible factor (HIF) pathway, plays a central role in regulating osteo- and chondrogenesis by directing progenitor cell differentiation and promoting and maintaining appropriate extracellular matrix production. Understanding the role of the HIF pathway in OC tissue development may enable new approaches to engineer OC tissue. In this review, we discuss strategies to spatially and temporarily regulate the HIF pathway in progenitor cells to create functional OC tissue for regenerative therapies. Impact statement Strategies to engineer osteochondral (OC) tissue are limited by the complex and varying microenvironmental conditions in native bone and cartilage. Indeed, native cartilage experiences low-oxygen conditions, while the underlying bone is relatively normoxic. The cellular response to these low-oxygen conditions, which is mediated through the hypoxia inducible factor (HIF) pathway, is known to promote and maintain the chondrocyte phenotype. By using tissue engineering scaffolds to spatially and temporally harness the HIF pathway, it may be possible to improve OC tissue engineering strategies for the regeneration of damaged cartilage and its underlying subchondral bone.

The cartilage matrisome in adolescent idiopathic scoliosis

The human spinal column is a dynamic, segmented, bony, and cartilaginous structure that protects the neurologic system and simultaneously provides balance and flexibility. Children with developmental disorders that affect the patterning or shape of the spine can be at risk of neurologic and other physiologic dysfunctions. The most common developmental disorder of the spine is scoliosis, a lateral deformity in the shape of the spinal column. Scoliosis may be part of the clinical spectrum that is observed in many developmental disorders, but typically presents as an isolated symptom in otherwise healthy adolescent children. Adolescent idiopathic scoliosis (AIS) has defied understanding in part due to its genetic complexity. Breakthroughs have come from recent genome-wide association studies (GWAS) and next generation sequencing (NGS) of human AIS cohorts, as well as investigations of animal models. These studies have identified genetic associations with determinants of cartilage biogenesis and development of the intervertebral disc (IVD). Current evidence suggests that a fraction of AIS cases may arise from variation in factors involved in the structural integrity and homeostasis of the cartilaginous extracellular matrix (ECM). Here, we review the development of the spine and spinal cartilages, the composition of the cartilage ECM, the so-called "matrisome" and its functions, and the players involved in the genetic architecture of AIS. We also propose a molecular model by which the cartilage matrisome of the IVD contributes to AIS susceptibility.

Regulation of terminal hypertrophic chondrocyte differentiation in Prmt5 mutant mice modeling infantile idiopathic scoliosis

Idiopathic scoliosis (IS) is the most common type of musculoskeletal defect affecting children worldwide, and is classified by age of onset, location and degree of spine curvature. Although rare, IS with onset during infancy is the more severe and rapidly progressive form of the disease, associated with increased mortality due to significant respiratory compromise. The pathophysiology of IS, in particular for infantile IS, remains elusive. Here, we demonstrate the role of PRMT5 in the infantile IS phenotype in mouse. Conditional genetic ablation of PRMT5 in osteochondral progenitors results in impaired terminal hypertrophic chondrocyte differentiation and asymmetric defects of endochondral bone formation in the perinatal spine. Analysis of these several markers of endochondral ossification revealed increased type X collagen (COLX) and Ihh expression, coupled with a dramatic reduction in Mmp13 and RUNX2 expression, in the vertebral growth plate and in regions of the intervertebral disc in the Prmt5 conditional mutant mice. We also demonstrate that PRMT5 has a continuous role in the intervertebral disc and vertebral growth plate in adult mice. Altogether, our results establish PRMT5 as a critical promoter of terminal hypertrophic chondrocyte differentiation and endochondral bone formation during spine development and homeostasis.

This article has an associated First Person interview with the first author of the paper.

Summary: Loss of Prmt5 in osteochondral progenitors impairs terminal hypertrophic chondrocyte differentiation, leading to defects in endochondral bone formation and models infantile idiopathic scoliosis in mouse.

Dysregulation of STAT3 signaling is associated with endplate-oriented herniations of the intervertebral disc in Adgrg6 mutant mice

Degenerative changes of the intervertebral disc (IVD) are a leading cause of disability affecting humans worldwide and has been attributed primarily to trauma and the accumulation of pathology during aging. While genetic defects have also been associated with disc degeneration, the precise mechanisms driving the initiation and progression of disease have remained elusive due to a paucity of genetic animal models. Here, we discuss a novel conditional mouse genetic model of endplate-oriented disc herniations in adult mice. Using conditional mouse genetics, we show increased mechanical stiffness and reveal dysregulation of typical gene expression profiles of the IVD in adhesion G-protein coupled receptor G6 (Adgrg6) mutant mice prior to the onset of endplate-oriented disc herniations in adult mice. We observed increased STAT3 activation prior to IVD defects and go on to demonstrate that treatment of Adgrg6 conditional mutant mice with a small molecule inhibitor of STAT3 activation ameliorates endplate-oriented herniations. These findings establish ADGRG6 and STAT3 as novel regulators of IVD endplate and growth plate integrity in the mouse, and implicate ADGRG6/STAT3 signaling as promising therapeutic targets for endplate-oriented disc degeneration.

Back pain is a leading cause of disability in humans worldwide and one of the most common culprits of these issues are the consequence of degenerative changes of the intervertebral disc. Here, we demonstrate that conditional loss of the Adgrg6 gene in cartilaginous tissues of the spine results in endplate-oriented disc herniations and degenerative changes of the intervertebral disc in mice. We further establish that these obvious degenerative changes of the disc are preceded by substantial alterations in normal gene expression profiles, including upregulation of pro-inflammatory STAT3 signaling, and increased mechanical stiffness of the intervertebral disc. Increased STAT3 activation is a signal observed in other models of degenerative musculoskeletal tissues. As such, we tested whether systemic treatment with a small-molecule STAT3 inhibitor would protect against the formation of endplate-oriented disc herniations in conditional Adgrg6 mutant mice, and report a significant positive improvement of histopathology in our treatment group. Taken together, we demonstrate a novel conditional model of endplate-oriented disc herniation in mouse. We establish ADGRG6 and STAT3 as novel regulators of endplate integrity of the intervertebral disc in mouse and suggest that modulation of ADGRG6/STAT3 signaling could provide robust disease-modifying targets for endplate-oriented disc degeneration in humans.

Anisotropic cytocompatible electrospun scaffold for tendon tissue engineering elicits limited inflammatory response in vitro

Tendon tears are a relevant concern for today's national health systems because of their social impact and high recurrence rate. The current gold standard for fixing tendon tears is surgical repair; however, this strategy is not able to fully re-establish tendon integrity and functionality. Tissue engineering approaches aim at promoting tissue regeneration by delivering the opportune signals to the injured site combining biomaterials, cells and biochemical cues. Electrospinning is currently one of the most versatile polymer processing techniques that allows manufacturing of nano- and micro-fibres substrates. Such fibrous morphology is deemed to be an ideal substrate to convey topographical cues to cells. Here we evaluated the potential of polycaprolactone processed by means of electrospinning technology for tendon tissue engineering. Fibrous free-of-defects substrate with random and aligned fibres were successfully fabricated. Rat tenocytes were used to assess the cytocompatibility of the substrates for application as tendon tissue engineered devices. Tenocytes were able to proliferate and adapt to the substrates topography acquiring an elongated morphology, which is the precondition for oriented collagen deposition, when seeded on aligned fibres. Real time Polymerase Chain Reaction (Rt-PCR) also revealed the overall maintenance of tenocyte phenotype over 7 days culture. To verify suitability for in vivo implantation, the level of inflammatory cytokine genes expressed by THP-1 cells cultured in presence of electrospun polycaprolactone substrates was evaluated. Inflammatory response was limited. The novel preliminary in vitro work presented herein showing tenocytes compatibility and limited inflammatory cytokines synthesis suggests that electrospun polycaprolactone may be taken into consideration as substrate for tendon healing applications.

Experimental Induction of Physeal Injuries by Fracture, Drill, and Ablation Techniques: Analyses of Immunohistochemical Findings

BACKGROUND

Although physeal fractures and physeal bars can result in significant clinical consequences to growth and development of the injured physis, little orthopaedic research has focused upon this topic. Our objective was to extend a previously developed rat model to examine the immunohistochemical features following surgical application of techniques disrupting the physis.

METHODS

Physes were surgically disrupted using fracture (control), epiphyseal scrape (ES), or epiphyseal drill (ED). After 1, 3, 6, 10, or 21 days, animals were euthanized, sites processed for histology and immunohistochemical localization of vascular endothelial growth factor (VEGF), Factor VIII, Sox-9, PTHrP (parathyroid hormone-related protein) and PTHrP-R (parathyroid hormone-related protein receptor) in resting, proliferative, and hypertrophic physeal zones. Incidence of physeal bars, vertical septa and islands within the metaphysis was quantified. Semiquantitative analysis of immunohistochemistry was performed.

RESULTS

Physeal bars, vertical septa, and displaced cartilage islands were present each of the surgical treatments. Fisher's exact test showed a statistically significant increase in the presence of physeal bars (P=0.002) and vertical septa (P=0.012) in the ED group at 10 and 21 days. Analysis of VEGF showed significant differences among the surgical treatments involving the resting zone, and the proliferative zone for days 1, 6, and 21 (P≤0.02) with greater mean scores present in the fracture (control) group, followed by the ED group; the lowest scores were present in the ES group. PTHrP-R immunolocalization showed significant differences among treatments in the hypertrophic zone at days 6 and 21 (P=0.022 and 0.044, respectively).

CONCLUSIONS

On the basis of the type of surgical treatment, results show significant differences in the presence of VEGF (reflecting the vascular bed) in the resting and proliferating zones at days 1, 6, and 21. VEGF localization was less abundant in the ED group (which had more physeal bars), suggesting that lack of vascular ingrowth plays a role in physeal bar formation.

CLINICAL RELEVANCE

Basic science data presented here provide insight into the importance of the various regions of the physis and its repair and continued growth after physeal fracture. We suggest that a better understanding of the cellular basis of physeal arrest following physeal fracture may have future relevance for the development of treatments to prevent or correct arrest.

Characterization of Krt19 CreERT allele for targeting the nucleus pulposus cells in the postnatal mouse intervertebral disc

Intervertebral disc degeneration and associated back pain are relatively common but sparsely understood conditions, affecting over 70% of the population during some point of life. Disc degeneration is often associated with a loss of nucleus pulposus (NP) cells. Genetic mouse models offer convenient avenues to understand the cellular and molecular regulation of the disc during its formation, growth, maintenance, and aging. However, due to the lack of inducible driver lines to precisely target NP cells in the postnatal mouse disc, progress in this area of research has been moderate. NP cells are known to express cytokeratin 19 (Krt19), and tamoxifen (Tam)‐inducible Krt19^CreERT allele is available. The current study describes the characterization of Krt19^CreERT allele to specifically and efficiently target NP cells in neonatal, skeletally mature, middle‐aged, and aged mice using two independent fluorescent reporter lines. The efficiency of recombination at all ages was validated by immunostaining for KRT19. Results show that following Tam induction, Krt19^CreERT specifically drives recombination of NP cells in the spine of neonatal and aged mice, while no recombination was detected in the surrounding tissues. Knee joints from skeletally mature Tam‐treated Krt19^CreERT/+; R26^tdTOM mouse show the absence of recombination in all tissues and cells of the knee joint. Thus, this study provides evidence for the use of Krt19^CreERT allele for genetic characterization of NP cells at different stages of the mouse life.

Genome-wide meta-analysis and replication studies in multiple ethnicities identify novel adolescent idiopathic scoliosis susceptibility loci

Adolescent idiopathic scoliosis (AIS) is the most common musculoskeletal disorder of childhood development. The genetic architecture of AIS is complex, and the great majority of risk factors are undiscovered. To identify new AIS susceptibility loci, we conducted the first genome-wide meta-analysis of AIS genome-wide association studies, including 7956 cases and 88 459 controls from 3 ancestral groups. Three novel loci that surpassed genome-wide significance were uncovered in intragenic regions of the CDH13 (P-value_rs4513093 = 1.7E-15), ABO (P-value_ rs687621 = 7.3E-10) and SOX6 (P-value_rs1455114 = 2.98E-08) genes. Restricting the analysis to females improved the associations at multiple loci, most notably with variants within CDH13 despite the reduction in sample size. Genome-wide gene-functional enrichment analysis identified significant perturbation of pathways involving cartilage and connective tissue development. Expression of both SOX6 and CDH13 was detected in cartilage chondrocytes and chromatin immunoprecipitation sequencing experiments in that tissue revealed multiple HeK27ac-positive peaks overlapping associated loci. Our results further define the genetic architecture of AIS and highlight the importance of vertebral cartilage development in its pathogenesis.

IL-1β Damages Fibrocartilage and Upregulates MMP-13 Expression in Fibrochondrocytes in the Condyle of the Temporomandibular Joint

The temporomandibular joint (TMJ), which differs anatomically and biochemically from hyaline cartilage-covered joints, is an under-recognized joint in arthritic disease, even though TMJ damage can have deleterious effects on physical appearance, pain and function. Here, we analyzed the effect of IL-1β, a cytokine highly expressed in arthritic joints, on TMJ fibrocartilage-derived cells, and we investigated the modulatory effect of mechanical loading on IL-1β-induced expression of catabolic enzymes. TMJ cartilage degradation was analyzed in 8–11-week-old mice deficient for IL-1 receptor antagonist (IL-1RA^−/−) and wild-type controls. Cells were isolated from the juvenile porcine condyle, fossa, and disc, grown in agarose gels, and subjected to IL-1β (0.1–10 ng/mL) for 6 or 24 h. Expression of catabolic enzymes (ADAMTS and MMPs) was quantified by RT-qPCR and immunohistochemistry. Porcine condylar cells were stimulated with IL-1β for 12 h with IL-1β, followed by 8 h of 6% dynamic mechanical (tensile) strain, and gene expression of MMPs was quantified. Early signs of condylar cartilage damage were apparent in IL-1RA^−/− mice. In porcine cells, IL-1β strongly increased expression of the aggrecanases ADAMTS4 and ADAMTS5 by fibrochondrocytes from the fossa (13-fold and 7-fold) and enhanced the number of MMP-13 protein-expressing condylar cells (8-fold). Mechanical loading significantly lowered (3-fold) IL-1β-induced MMP-13 gene expression by condylar fibrochondrocytes. IL-1β induces TMJ condylar cartilage damage, possibly by enhancing MMP-13 production. Mechanical loading reduces IL-1β-induced MMP-13 gene expression, suggesting that mechanical stimuli may prevent cartilage damage of the TMJ in arthritic patients.

Chondrogenesis of human mesenchymal stem cells by microRNA loaded triple polysaccharide nanoparticle system

Degenerative cartilage is the pathology of severe depletion of extracellular matrix components in articular cartilage. In diseases like osteoarthritis, misregulation of microRNAs contributes the pathology and collectively leads to disruption of the homeostasis. In this study chondroitin sulfate/hyaluronic acid/chitosan nanoparticles were prepared and successfully characterized chemically and morphologically. Results demonstrated higher chondroitin sulfate amounts led smaller nanoparticles, but lower surface zeta potential due to high electronegativity. After optimization of chondroitin sulfate amounts regarding size and charge, nanoparticles were loaded with microRNA-149-5p, a therapeutic miRNA downregulated in osteoarthritis, and evaluated focusing on their loading efficiency, release behaviour, cytotoxicity and gene transfection efficiency in vitro. Results showed all nanoparticle formulations were non-toxic and promising gene delivery agents, due to increased levels of microRNA-149-5p and decreased mRNA levels of microRNA's target, FUT-1. Highest gene transfection efficiency was obtained with the nanoparticle formulation which had the highest chondroitin sulfate load and smallest size. In addition, owing to their high chondroitin sulfate cargo, all nanoparticles were reported to enhance chondrogenesis, which was demonstrated by gene expression analysis and sulfated glycosaminoglycan (sGAG) staining. The obtained data suggest that the delivery of microRNA-149-5p via polysaccharide based carriers could achieve collaborative impact in cartilage regeneration and have a potential to enhance osteoarthritis treatment.

Possible Contribution of Wnt-Responsive Chondroprogenitors to the Postnatal Murine Growth Plate

Active cell proliferation and turnover in the growth plate is essential for embryonic and postnatal bone growth. We performed a lineage tracing of Wnt/β-catenin signaling responsive cells (Wnt-responsive cells) using Axin2^CreERT2 ;Rosa26ZsGreen mice and found a novel cell population that resides in the outermost layer of the growth plate facing the Ranvier's groove (RG; the perichondrium adjacent to growth plate). These Wnt-responsive cells rapidly expanded and contributed to formation of the outer growth plate from the neonatal to the growing stage but stopped expanding at the young adult stage when bone longitudinal growth ceases. In addition, a second Wnt-responsive sporadic cell population was localized within the resting zone of the central part of the growth plate during the postnatal growth phase. While it induced ectopic chondrogenesis in the RG, ablation of β-catenin in the Wnt-responsive cells strongly inhibited expansion of their descendants toward the growth plate. These findings indicate that the Wnt-responsive cell population in the outermost layer of the growth plate is a unique cell source of chondroprogenitors involving lateral growth of the growth plate and suggest that Wnt/β-catenin signaling regulates function of skeletal progenitors in a site- and stage-specific manner. © 2019 American Society for Bone and Mineral Research.

Epigenetic Mechanisms Underlying the Aging of Articular Cartilage and Osteoarthritis

Aging is a progressive and complicated bioprocess with overall decline in physiological function. Osteoarthritis (OA) is the most common joint disease in middle-aged and older populations. Since the prevalence of OA increases with age and breakdown of articular cartilage is its major hallmark, OA has long been thought of as "wear and tear" of joint cartilage. Nevertheless, recent studies have revealed that changes in the chondrocyte function and matrix components may reduce the material properties of articular cartilage and predispose the joint to OA. The aberrant gene expression in aging articular cartilage that is regulated by various epigenetic mechanisms plays an important role in age-related OA pathogenesis. This review begins with an introduction to the current understanding of epigenetic mechanisms, followed by mechanistic studies on the aging of joint tissues, epigenetic regulation of age-dependent gene expression in articular cartilage, and the significance of epigenetic mechanisms in OA pathogenesis. Our recent findings on age-dependent expression of 2 transcription factors, nuclear factor of activated T cell 1 (NFAT1) and SOX9, and their roles in the formation and aging of articular cartilage are summarized in the review. Chondrocyte dysfunction in aged mice, which is mediated by epigenetically regulated spontaneous reduction of NFAT1 expression in articular cartilage, is highlighted as an important advance in epigenetics and cartilage aging. Potential therapeutic strategies for age-related cartilage degeneration and OA using epigenetic molecular tools are discussed at the end.

Roles and regulation of SOX transcription factors in skeletogenesis

SOX transcription factors participate in the specification, differentiation and activities of many cell types in development and beyond. The 20 mammalian family members are distributed into eight groups based on sequence identity, and while co-expressed same-group proteins often have redundant functions, different-group proteins typically have distinct functions. More than a handful of SOX proteins have pivotal roles in skeletogenesis. Heterozygous mutations in their genes cause human diseases, in which skeletal dysmorphism is a major feature, such as campomelic dysplasia (SOX9), or a minor feature, such as LAMSHF syndrome (SOX5) and Coffin-Siris-like syndromes (SOX4 and SOX11). Loss- and gain-of-function experiments in animal models have revealed that SOX4 and SOX11 (SOXC group) promote skeletal progenitor survival and control skeleton patterning and growth; SOX8 (SOXE group) delays the differentiation of osteoblast progenitors; SOX9 (SOXE group) is essential for chondrocyte fate maintenance and differentiation, and works in cooperation with SOX5 and SOX6 (SOXD group) and other types of transcription factors. These and other SOX proteins have also been proposed, mainly through in vitro experiments, to have key roles in other aspects of skeletogenesis, such as SOX2 in osteoblast stem cell self-renewal. We here review current knowledge of well-established and proposed skeletogenic roles of SOX proteins, their transcriptional and non-transcriptional actions, and their modes of regulation at the gene, RNA and protein levels. We also discuss gaps in knowledge and directions for future research to further decipher mechanisms underlying skeletogenesis in health and diseases and identify treatment options for skeletal malformation and degeneration diseases.

Aggrecan Hypomorphism Compromises Articular Cartilage Biomechanical Properties and Is Associated with Increased Incidence of Spontaneous Osteoarthritis

The gene encoding the proteoglycan aggrecan (Agc1) is abundantly expressed in cartilage during development and adulthood, and the loss or diminished deposition of the protein results in a wide range of skeletal malformations. Furthermore, aggrecan degradation is a hallmark of cartilage degeneration occurring in osteoarthritis. In the present study, we investigated the consequences of a partial loss of aggrecan in the postnatal skeleton and in the articular cartilage of adult mice. We took advantage of the previously described Agc1^{tm(IRES-CreERT2)} mouse line, which allows for conditional and timely-regulated deletion of floxed, cartilage-expressed genes. As previously reported, the introduction of the CreER^T2 cassette in the 3’UTR causes a disruption of the normal expression of Agc1 resulting in a hypomorphic deposition of the protein. In homozygous mice, we observed a dwarf phenotype, which persisted throughout adulthood supporting the evidence that reduced aggrecan amount impairs skeletal growth. Homozygous mice exhibited reduced proteoglycan staining of the articular cartilage at 6 and 12 months of age, increased stiffening of the extracellular matrix at six months, and developed severe cartilage erosion by 12 months. The osteoarthritis in the hypomorph mice was not accompanied by increased expression of catabolic enzymes and matrix degradation neoepitopes. These findings suggest that the degeneration found in homozygous mice is likely due to the compromised mechanical properties of the cartilage tissue upon aggrecan reduction.

A Genetic Variant in GPR126 Causing a Decreased Inclusion of Exon 6 Is Associated with Cartilage Development in Adolescent Idiopathic Scoliosis Population

Adolescent idiopathic scoliosis (AIS) is the most common spinal deformity disease in adolescents but its etiology and pathogenesis are still unclear. The current study aims to identify the relationship between single nucleotide polymorphisms (SNPs) of G protein-coupled receptor 126 (GPR126) gene and AIS predisposition. GPR126 contains 26 exons and alternative splicing of exon 6 and exon 25 produces 4 protein-coding transcripts. We genotyped SNPs of GPR126 gene around exon 6 and exon 25 in 131 Chinese AIS patients and 132 healthy controls and provided evidence that SNP rs41289839 G>A is strongly associated with AIS susceptibility. Linkage disequilibrium analysis suggests that rs41289839 and other AIS-related SNPs were in strong LD. Next, we demonstrated that rs41289839 G>A inhibits the inclusion of exon 6 during alternative splicing, resulting in a decreased expression level of exon 6-included transcript (GPR126-exon6ⁱⁿ) relative to the exon 6 excluded transcript (GPR126-exon6^ex) by minigene assay. Chondrogenic differentiation experiment showed that GPR126-exon6ⁱⁿ has a high expression level relative to GPR126-exon6^ex during chondrogenic differentiation of hMSCs. Our findings indicate that newly discovered SNP is related to cartilage development and may provide valuable insights into the etiology and pathogenesis of adolescent idiopathic scoliosis.

SFMBT2 positively regulates SOX9 and chondrocyte proliferation

SRY‑box 9 (SOX9) is the master regulator of the chondrocyte phenotype, which is essential for differentiating chondrogenic mesenchymal condensations into chondrocytes, and is involved in regulating every stage of chondrocyte differentiation. SOX9 deletion in chondrocytes at the late stages of cartilage development results in decreased chondrocyte proliferation; inhibited expression of cartilage matrix genes, including Indian hedgehog and the downstream parathyroid hormone‑related protein; and premature conversion of proliferating chondrocytes into hypertrophic chondrocytes, which mineralize their matrix prematurely. Therefore, SOX9 is considered vital for the majority of phases of chondrocyte lineage, from early condensations to the differentiation of proliferating chondrocytes, leading to chondrocyte hypertrophy. It has been reported that SOX9 expression is decreased in osteoarthritis (OA) cartilage. Regeneration or repair of cartilage degradation in OA remains a challenge. Previous studies have indicated that overexpression of SOX9 can promote cartilage repair and can be used as a potential therapeutic agent at the early stages of human OA. The present study identified Scm‑like with four malignant brain tumor domains 2 (SFMBT2) as a novel regulator of SOX9 expression in human chondrocytes. Our previous study revealed that SFMBT2 is negatively regulated in OA cartilage, and decreased levels of SFMBT2 contribute to the catabolic phenotype of chondrocytes. The present study detected increased expression levels of SFMBT2 in early cartilage development and during the early phases of chondrogenesis. Overexpression of SFMBT2 in C28/I2 cells upregulated SOX9 expression in a dose‑dependent manner. Furthermore, SFMBT2 positively regulated C28/I2 cell proliferation and restored the decreased levels of SOX9 in chondrocytes following tumor necrosis factor‑α treatment. Additional studies may reveal novel insights into the molecular mechanism involved and the potential role of SFMBT2 in cartilage repair and OA management.

Therapy for cartilage defects: functional ectopic cartilage constructed by cartilage-simulating collagen, chondroitin sulfate and hyaluronic acid (CCH) hybrid hydrogel with allogeneic chondrocytes

OBJECTIVE

To regenerate functional cartilage-mimicking ectopic cartilage as a source for the restoration of cartilage defects, we used a previously synthesized three-phase collagen, chondroitin sulfate and hyaluronic acid (CCH) hydrogel for the encapsulation of allogeneic chondrocytes with a diffusion chamber system that was buried subcutaneously in the host for 4 weeks and then implanted into a cartilage defect.

METHODS

The CCH hydrogel was prepared and seeded with allogeneic chondrocytes from new-born rabbits, prior to being enveloped in a diffusion chamber that prevents cell ingrowth and vascular invasion of the host, as described previously. A collagen hydrogel (C) was used as the control. The diffusion chamber was embedded subcutaneously in an adult rabbit. 4 weeks later, the regenerated tissue was harvested from the diffusion chamber and then further used for cartilage repair in the same host. To evaluate the regenerated tissue, cell viability assay using calcein-acetoxymethyl (calcein-AM)/propidium iodide (PI) staining, biochemical analysis by examination of total DNA and GAG content, gene expression detection using RT-PCR for Col 1a1, Col 2a1, Acan, and Sox9, biomechanical detection and histological evaluation were implemented.

RESULTS

Analysis of the cell activity and biochemical evaluation in vitro showed that cell proliferation, GAG secretion and gene/protein expression of cartilage specific markers were much higher in the CCH group than those in the C group. The CCH constructed ectopic cartilage tissue in vivo showed the typical characteristics of hyaline cartilage with higher expression of cartilage matrix markers compared with the C groups, as evidenced by morphological and histological findings as well as RT-PCR analysis. Furthermore, ectopic cartilage from CCH successfully facilitated the cartilage restoration, with higher morphological and histological scores and greater mechanical strength than that from C.

CONCLUSION

The three-phase CCH hydrogel, which is closer to natural cartilage matrix and is stiffer than collagen, may replace collagen as the "gold standard" for cartilage tissue engineering. This study may provide a new insight for cartilage repair using ectopic cartilage reconstructed from functional materials and allogeneic cells.

Latest advances in intervertebral disc development and progenitor cells

This paper is a concise review aiming to assemble the most relevant topics presented by the authors at ORS-Philadelphia Spine Research Society Fourth International Spine Research Symposium. It centers on the latest advances in disc development, its main structural entities, and the populating cells, with emphasis on the advances in pivotal molecular pathways responsible for forming the intervertebral discs (IVD). The objective of finding and emphasizing pathways and mechanisms that function to control tissue formation is to identify and to explore modifications occurring during normal aging, disease, and tissue repair. Thus, to comprehend that the cellular and molecular basis of tissue degeneration are crucial in the study of the dynamic interplay that includes cell-cell communication, gene regulation, and growth factors required to form a healthy and functional tissue during normal development.

Sox9 is increased in arterial plaque and stenosis, associated with synthetic phenotype of vascular smooth muscle cells and causes alterations in extracellular matrix and calcification

Vascular smooth muscle cells (VSMC) exhibit a dual role in progression and maintenance of arteriosclerosis. They are fundamental for plaque stability but also can drive plaque progression. During pathogenic vascular remodeling, VSMC transdifferentiate into a phenotype with enhanced proliferation and migration. Moreover, they exert an increased capacity to generate extracellular matrix proteins. A special lineage of transdifferentiated VSMC expresses Sox9, a multi-functional transcription factor. The aim of the study was to examine the role of Sox9 in phenotypic alterations leading to arteriosclerosis. Using mouse models for arterial stenosis, Sox9 induction in diseased vessels was verified. The phenotypic switch of VSMC from contractile to proliferative nature caused a significant increase of Sox9 expression. Various factors known to be involved in the progression of arteriosclerosis were examined for their ability to modulate Sox9 expression in VSMC. While PDGF-BB resulted in a strong transient upregulation of Sox9, TGF-β1 appeared to be responsible for a moderate, but prolonged increase of Sox9 expression. Beside the regulation, functional studies focused on knockout and overexpression of Sox9. A Sox9-dependent alteration of extracellular matrix could be revealed and was associated with an upregulated calcium deposition. Taken together, Sox9 is identified as important factor of VSMC function by modulation the extracellular matrix composition and calcium deposition, which are important processes in plaque development.

Impairment of chondrocyte proliferation after exposure of young murine cartilage to an aged systemic environment in a heterochronic parabiosis model

AIM:

The aim of this study was to investigate whether an aged systemic environment could impair young cartilage tissue in mice.

METHODS:

Mice differing in age were randomly divided into three groups. Group 1 was the experimental group (Y/O group) consisting of the heterochronic parabiosis model (2-month-old/12-month-old, young/old). Group 2 was the surgical control group (Y/Y group) with the isochronic parabiosis model (2-month-old/2-month-old, young/young). Group 3 consisted of the ageing control mice (2-month-old alone, Y group). Young knee cartilages collected from all three groups at 4 months after surgery were compared. Fluorescence molecular tomography (FMT) was used to confirm whether the two mice in parabiosis shared a common blood circulation at 2 weeks after surgery. The knee joints of young mice were examined radiologically at 4 months after surgery. Histological scoring was assigned to grade the severity of osteoarthritis (OA). Immunohistochemistry and quantitative reverse transcription polymerase chain reaction were used to evaluate OA-related protein expression and gene expression, and chondrocyte proliferation was determined with EdU staining.

RESULTS:

FMT imaging confirmed cross-circulation in the parabiotic pairs. The percentage of EdU-positive chondrocytes in young mice from the Y/O group was significantly lower compared with those of the Y/Y and Y groups (p <0.05 for both). There was no statistically significant difference in the mRNA expression of collagen type II (Col2), collagen type X (Col10), and matrix metalloproteinase 13 (MMP13) among the three groups (P>0.05), but expression of sex-determining region Y box 9 (Sox9) mRNA in young cartilage from the Y/O group was markedly attenuated compared to those in the Y/Y and Y groups (p <0.05 for both). In the Y/O group, mRNA expression of runt-related transcription factor 2 (Runx2) in young cartilage was significantly increased compared to the Y/Y and Y groups (p <0.05 for both). The changes in Col2, Col10, MMP13, Runx2 and Sox9 at the protein level mimicked the alterations found at the mRNA level. Loss of cartilage proteoglycan in young mice from the Y/O group was significantly greater compared to the Y/Y and Y groups (p <0.05 for both), despite the lack of significant difference among the three groups in OARIS and osteophytosis scores.

CONCLUSION:

Heterochronic parabiosis exerts a negative effect on chondrocyte proliferation in the knee cartilage of young mice.

Histone hypo-acetylation of Sox9 mediates nicotine-induced weak cartilage repair by suppressing BMSC chondrogenic differentiation

BACKGROUND

Nicotine has negative effects on tissue repair, little research concerns its effect on the cartilage repair of tissue engineering stem cells. The present study aimed to investigate the effects of nicotine on the bone marrow-derived mesenchymal stem cells' (BMSCs) chondrogenic repair function of cartilage defects and explored the molecular mechanism.

METHODS

A cartilage defect model of rat was repaired by BMSC transplantation, and treated with nicotine or saline at 2.0 mg/kg/d in 12 weeks. Nicotine's effect on chondrogenic differentiation was studied by exposing BMSCs to nicotine at 0.1, 1, 10, and 100 μM, and methyllycaconitine (MLA), which is a selective α7-nicotinic acetylcholine receptor (nAChR) inhibitor and si-RNA of nuclear factor of activated T cells 2 (NFATc2), were used to verify the molecular mechanism of nicotine's effect.

RESULTS

Data showed that nicotine inhibited cartilage repair function by suppressing SRY-type high-mobility group box 9 (Sox9) in regenerated tissues. Further in vitro study demonstrated that nicotine enhanced intracellular Ca²⁺ and activity of calcineurin (CaN) through α7-nAChR, increased the nucleic expressions of NFATc2 and the bindings to SOX9 promoter, and thus reduced the acetylation of H3K9 and H3K14 in SOX9 promoter.

CONCLUSIONS

Findings from this study demonstrated that nicotine suppressed the chondrogenic differentiation of BMSCs in vivo and in vitro, which offers insight into the risk assessment of cartilage defect repair in a nicotine exposure population and its therapeutic target.

SIRT7 is an important regulator of cartilage homeostasis and osteoarthritis development

Sirtuins (SIRT1-7) are NAD+-dependent deacetylase/deacylases that regulate a wide variety of biological functions. Although the roles of sirtuins in cartilage homeostasis and cartilage diseases have been well studied, there is no information on the contribution of SIRT7 to cartilage homeostasis and osteoarthritis (OA) pathologies. Here, we demonstrate that Sirt7 knockout mice are resistant to the development of aging-associated OA and forced exercise-induced OA. Attenuation of Sirt7 in the murine chondrogenic cell line ATDC5 increased the deposition of a glycosaminoglycan-rich extracellular matrix and the mRNA expression of extracellular matrix components such as Col2a1 and Acan. Mechanistically, we found that SIRT7 suppressed the transcriptional activity of SOX9, which is an important transcription factor in chondrocytes, and that the enzymatic activity of SIRT7 was required for its function. Our results indicate that SIRT7 is a novel important regulator of cartilage homeostasis and OA development.

Excess maternal and postnatal thyroxine alters chondrocyte numbers and the composition of the extracellular matrix of growth cartilage in rats

Purpose/Aim: The aim of this study was to evaluate the effects of excess maternal and postnatal thyroxine on chondrocytes and the extracellular matrix (ECM) of growth cartilage.

MATERIALS AND METHODS

We used 16 adult female Wistar rats divided into two groups: thyroxine treatment and control. From weaning to 40 days of age, offspring of the treated group (n = 8) received L-thyroxine. Plasma free T4 was measured. Histomorphometric analysis was performed on thyroids and femurs of all offspring. Alcian blue histochemical staining and real-time reverse transcription polymerase chain reaction measurements of gene expression levels of Sox9, Runx2, Aggrecan, Col I, Col II, Alkaline phosphatase, Mmp2, Mmp9, and Bmp2 were performed. Data were analyzed for statistical significance by student's t-test.

RESULTS

Excess maternal and postnatal thyroxine reduced the intensity of Alcian blue staining, altered the number of chondrocytes in proliferative and hypertrophic zones in growth cartilage, and reduced the gene expression of Sox9, Mmp2, Mmp9, Col II, and Bmp2 in the growth cartilage of all offspring. Additionally, excess thyroxine altered the gene expression of Runx2, Aggrecan and Col I, and this effect was dependent on age.

CONCLUSIONS

Excess thyroxine in neonates suppresses chondrocyte proliferation, stimulates chondrocyte hypertrophy and changes the ECM composition by reducing the amount of proteoglycans and glycosaminoglycans (GAGs). Prolonged exposure to excess thyroxine suppresses chondrocyte activity in general, with a severe reduction in the proteoglycan content of cartilage and the expression of gene transcripts essential for endochondral growth and characteristics of the chondrocyte phenotype.

Exposure to reversine affects the chondrocyte morphology and phenotype in vitro

Articular chondrocytes derived from osteoarthritic tissues (OA HAC) show a severely reduced chondrogenic commitment. This impairment undermines their use for tissue-engineered cartilage repair, which relies on cell proliferation and growth to meet therapeutic needs, but also on efficient cell plasticity to recover the chondrogenic phenotype. Reversine (Rev), a 2,6-disubstituted purine inhibitor of spindle-assembly checkpoints, was described to convert differentiated mesenchymal cells to their undifferentiated precursors. We hypothesized that Rev exposure could divert OA HAC to more plastic cells, re-boosting their subsequent commitment. HAC were enzymatically released from OA cartilage specimens, expanded for 2 weeks and treated with 5 μm Rev in dimethylsulphoxide (DMSO) or with DMSO alone for 6 days. Cell growth was assessed using the AlamarBlue^TM assay. Cytoskeletal structure, endoproliferation and caspase-3-immunopositivity were assayed by epifluorescence microscopy. The OA HAC chondrogenic performance was evaluated by quantitative reverse transcription-polymerase chain reaction (RT-PCR) for glyceraldehyde-3-phosphate dehydrogenase, Sox9, Aggrecan (Agg), type II collagen (Col2), Ki67, cyclinD1, transforming growth factor-β1 (TGF-β1), -2 and -3, interleukin-1β (IL-1β) and -6 , SMAD3 and -7, and vascular endothelial growth factor. Rev-treated OA HAC recovered polygonal morphology and reduced Ki67 expression and proliferation. Cell-cycle impairment accounted for altered cytoskeletal organization, endoproliferation and apoptosis, whereas a compensatory mechanism sustained the increased cyclinD1 transcript levels. Sox9, Agg and TGFs were overexpressed, but not Col2. IL transcripts were massively downregulated. These events were dose-related and transient. Overall, in spite of a higher Rev-induced transcriptional activity for extracellular matrix components and in spite of a Rev-treated cell phenotype closer to that of the three-dimensional native articular chondrocyte, Rev effects seem unleashed from a full regained chondrogenic potential.

Dwarfism in homozygous Agc1CreERT mice is associated with decreased expression of aggrecan

Aggrecan (Acan), a large proteoglycan is abundantly expressed in cartilage tissue. Disruption of Acan gene causes dwarfism and perinatal lethality of homozygous mice. Because of sustained expression of Acan in the growth plate and articular cartilage, Agc^Cre model has been developed for the regulated ablation of target gene in chondrocytes. In this model, the IRES-CreERT-Neo-pgk transgene is knocked-in the 3'UTR of the Acan gene. We consistently noticed variable weight and size among the Agc^Cre littermates, prompting us to examine the cause of this phenotype. Wild-type, Cre-heterozygous (Agc^+/Cre ), and Cre-homozygous (Agc^Cre/Cre ) littermates were indistinguishable at birth. However, by 1-month, Agc^Cre/Cre mice showed a significant reduction in body weight (18-27%) and body length (19-22%). Low body weight and dwarfism was sustained through adulthood and occurred in both genders. Compared with wild-type and Agc^+/Cre littermates, long bones and vertebrae were shorter in Agc^Cre/Cre mice. Histological analysis of Agc^Cre/Cre mice revealed a significant reduction in the length of the growth plate and the thickness of articular cartilage. The amount of proteoglycan deposited in the cartilage of Agc^Cre/Cre mice was nearly half of the WT littermates. Analysis of gene expression indicates impaired differentiation of chondrocyte in hyaline cartilage of Agc^Cre/Cre mice. Notably, both Acan mRNA and protein was reduced by 50% in Agc^Cre/Cre mice. A strong correlation was noted between the level of Acan mRNA and the body length. Importantly, Agc^+/Cre mice showed no overt skeletal phenotype. Thus to avoid misinterpretation of data, only the Agc^+/Cre mice should be used for conditional deletion of a target gene in the cartilage tissue.

Hypoxia suppresses serum deprivation-induced degradation of the nucleus pulposus cell extracellular matrix through the JNK and NF-κB pathways

Intervertebral disc (IVD) degeneration is associated with the imbalance between anabolism and catabolism of the nucleus pulposus (NP) extracellular matrix (ECM). Serum deprivation (SD) has been reported to exacerbate IVD degeneration; however, the effect of SD on ECM metabolism is not fully understood. Hypoxia plays important roles in maintaining the physiological functions of IVD cells; however, whether hypoxia has any effect on NP ECM production under conditions of SD is still unclear. In the current study, we established an in vitro SD model by exposing NP cells to serum-free medium. SD decreased the expression of aggrecan and collagen II, as well as the production of sulfated glycosaminoglycan (sGAG) in a time-dependent manner. However, hypoxia abolished SD-mediated down-regulation of aggrecan and collagen II expression via JNK1/2 activation. Moreover, hypoxia abolished SD-induced MMP-3 and MMP-13 expression by inhibiting NF-κB activation, p65 translocation, and MMP-3 and MMP-13 promoter activity. These results indicated that, hypoxia maintained ECM production under conditions of SD. This effect was elicited in part through JNK1/2-mediated up-regulation of matrix gene expression and down-regulation of MMP expression, through the inhibition of NF-κB. © 2017 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 35:2059-2066, 2017.

An Integrative Developmental Genomics and Systems Biology Approach to Identify an In Vivo Sox Trio-Mediated Gene Regulatory Network in Murine Embryos

Embryogenesis is an intricate process involving multiple genes and pathways. Some of the key transcription factors controlling specific cell types are the Sox trio, namely, Sox5, Sox6, and Sox9, which play crucial roles in organogenesis working in a concerted manner. Much however still needs to be learned about their combinatorial roles during this process. A developmental genomics and systems biology approach offers to complement the reductionist methodology of current developmental biology and provide a more comprehensive and integrated view of the interrelationships of complex regulatory networks that occur during organogenesis. By combining cell type-specific transcriptome analysis and in vivo ChIP-Seq of the Sox trio using mouse embryos, we provide evidence for the direct control of Sox5 and Sox6 by the transcriptional trio in the murine model and by Morpholino knockdown in zebrafish and demonstrate the novel role of Tgfb2, Fbxl18, and Tle3 in formation of Sox5, Sox6, and Sox9 dependent tissues. Concurrently, a complete embryonic gene regulatory network has been generated, identifying a wide repertoire of genes involved and controlled by the Sox trio in the intricate process of normal embryogenesis.

Role of MicroRNA in Osteoarthritis

Although the potential effect of aberrant expression of catabolic and anabolic genes on the development of osteoarthritis (OA) is well-documented, the regulatory mechanism for the expression of these genes in articular chondrocytes remains to be elucidated. The recent advances in epigenetic studies have identified microRNA (miRNA) as one of the epigenetic mechanisms for the regulation of gene expression. This mini review highlights the role of miRNA in the regulation of gene expression in articular chondrocytes and its significance in the pathogenesis of OA, with a discussion on the potential of miRNA as a new biomarker and therapeutic target for OA. Further investigations are required to determine the specificity, sensitivity, and efficacy of miRNA for clinical applications.

Irx3 and Bmp2 regulate mouse mesenchymal cell chondrogenic differentiation in both a Sox9-dependent and -independent manner

Sox9, a master regulator of cartilage development, controls the cell fate decision to differentiate from mesenchymal to chondrogenic cells. In addition, Sox9 regulates the proliferation and differentiation of chondrocytes, as well as the production of cartilage-specific proteoglycans. The existence of Sox9-independent mechanisms in cartilage development remains to be determined. Here, we attempted to identify genes involved in such putative mechanisms via microarray analysis using a mouse chondrogenic cell line, N1511. We first focused on transcription factors that exhibited upregulated expression following Bmp2 treatment, which was not altered by subsequent treatment with Sox9 siRNA. Among these, we selected positive regulators for chondrogenesis and identified Iroquois-related homeobox 3 (Irx3) as one of the candidate genes. Irx3 expression gradually increased with chondrocyte terminal differentiation in a reciprocal manner to Sox9 expression, and promoted the chondrogenic differentiation of mesenchymal cells upon Bmp2 treatment. Furthermore, Irx3 partially rescued impaired chondrogenesis by upregulating the expression of epiphycan and lumican under reduced Sox9 expression. Finally, Irx3 was shown to act in concert with Bmp2 signaling to activate the p38 MAPK pathway, which in turn stimulated Sox9 expression, as well as the expression of epiphycan and lumican in a Sox9-independent manner. These results indicate that Irx3 represents a novel chondrogenic factor of mesenchymal cells, acts synergistically with Bmp2-mediated signaling, and regulates chondrogenesis independent of the transcriptional machinery associated with Sox9-mediated regulation.

Recapitulation of physiological spatiotemporal signals promotes in vitro formation of phenotypically stable human articular cartilage

Standard isotropic culture fails to recapitulate the spatiotemporal gradients present during native development. Cartilage grown from human mesenchymal stem cells (hMSCs) is poorly organized and unstable in vivo. We report that human cartilage with physiologic organization and in vivo stability can be grown in vitro from self-assembling hMSCs by implementing spatiotemporal regulation during induction. Self-assembling hMSCs formed cartilage discs in Transwell inserts following isotropic chondrogenic induction with transforming growth factor β to set up a dual-compartment culture. Following a switch in the basal compartment to a hypertrophic regimen with thyroxine, the cartilage discs underwent progressive deep-zone hypertrophy and mineralization. Concurrent chondrogenic induction in the apical compartment enabled the maintenance of functional and hyaline cartilage. Cartilage homeostasis, chondrocyte maturation, and terminal differentiation markers were all up-regulated versus isotropic control groups. We assessed the in vivo stability of the cartilage formed under different induction regimens. Cartilage formed under spatiotemporal regulation in vitro resisted endochondral ossification, retained the expression of cartilage markers, and remained organized following s.c. implantation in immunocompromised mice. In contrast, the isotropic control groups underwent endochondral ossification. Cartilage formed from hMSCs remained stable and organized in vivo. Spatiotemporal regulation during induction in vitro recapitulated some aspects of native cartilage development, and potentiated the maturation of self-assembling hMSCs into stable and organized cartilage resembling the native articular cartilage.

Mesoderm-specific Stat3 deletion affects expression of Sox9 yielding Sox9-dependent phenotypes

To date, mutations within the coding region and translocations around the SOX9 gene both constitute the majority of genetic lesions underpinning human campomelic dysplasia (CD). While pathological coding-region mutations typically result in a non-functional SOX9 protein, little is known about what mechanism(s) controls normal SOX9 expression, and subsequently, which signaling pathways may be interrupted by alterations occurring around the SOX9 gene. Here, we report the identification of Stat3 as a key modulator of Sox9 expression in nascent cartilage and developing chondrocytes. Stat3 expression is predominant in tissues of mesodermal origin, and its conditional ablation using mesoderm-specific TCre, in vivo, causes dwarfism and skeletal defects characteristic of CD. Specifically, Stat3 loss results in the expansion of growth plate hypertrophic chondrocytes and deregulation of normal endochondral ossification in all bones examined. Conditional deletion of Stat3 with a Sox9Cre driver produces palate and tracheal irregularities similar to those described in Sox9+/- mice. Furthermore, mesodermal deletion of Stat3 causes global embryonic down regulation of Sox9 expression and function in vivo. Mechanistic experiments ex vivo suggest Stat3 can directly activate the expression of Sox9 by binding to its proximal promoter following activation. These findings illuminate a novel role for Stat3 in chondrocytes during skeletal development through modulation of a critical factor, Sox9. Importantly, they further provide the first evidence for the modulation of a gene product other than Sox9 itself which is capable of modeling pathological aspects of CD and underscore a potentially valuable therapeutic target for patients with the disorder.

SOX9 protein is stabilized by TGF-β and regulates PAPSS2 mRNA expression in chondrocytes

OBJECTIVE

We previously identified 3'-phosphoadenosine 5'-phosphosulfate synthase 2 (PAPSS2) as a transcriptional target of transforming growth factor β (TGF-β) in chondrocytes. PAPSS2 is required for proper sulfation of proteoglycans in cartilage. Defective sulfation in the matrix results in alterations in mechanical properties of the cartilage that would be expected to result in degeneration. The objective of this study was to identify factors that regulate PAPSS2 expression and compare to a known TGF-β responsive gene, proteoglycan 4/lubricin (PRG4). In this study, TGF-β-mediated regulation of SOX9 was characterized, and the involvement of SOX9 in regulation of PAPSS2 mRNA was investigated.

DESIGN

Primary bovine articular chondrocytes grown in micromass culture and ATDC5 cells were used as the model system. Adenoviruses were used to express SOX9 and SMAD3. siRNA was used to knock-down Sox9 and Smad3. Western blot and real-time quantitative RT-PCR (qPCR) were used to measure changes in protein and mRNA levels in response to treatment.

RESULTS

Over-expression of SOX9 was sufficient to up-regulate PAPSS2 mRNA. TGF-β treatment of SOX9-expressing cells resulted in enhanced up-regulation of PAPSS2 mRNA, suggesting that SOX9 cooperates with TGF-β signaling. Furthermore, Sox9 was required for full TGF-β-mediated induction of Papss2. In contrast, PRG4 was regulated by SMAD3 but not SOX9. SOX9 protein levels were increased after treatment with TGF-β, although SOX9 mRNA was not. SOX9 protein was post-translationally stabilized after treatment with TGF-β.

CONCLUSIONS

TGF-β stabilizes SOX9 protein, and SOX9 is sufficient and necessary for TGF-β-mediated regulation of PAPSS2 mRNA, providing a novel mechanism for TGF-β-mediated gene regulation in chondrocytes.

Donor-Matched Comparison of Chondrogenic Potential of Equine Bone Marrow- and Synovial Fluid-Derived Mesenchymal Stem Cells: Implications for Cartilage Tissue Regeneration

Mesenchymal stem cells (MSCs) have been demonstrated to be useful for cartilage tissue regeneration. Bone marrow (BM) and synovial fluid (SF) are promising sources for MSCs to be used in cartilage regeneration. In order to improve the clinical outcomes, it is recommended that prior to clinical use, the cellular properties and, specifically, their chondrogenic potential must be investigated. The purpose of this study is to compare and better understand the in vitro chondrogenic potential of equine bone marrow-derived mesenchymal stem cells (BMMSCs) and synovial fluid-derived mesenchymal stem cells (SFMSCs) populated from the same equine donor. BM- and SF-derived MSCs cultures were generated from five equine donors, and the MSCs were evaluated in vitro for their morphology, proliferation, trilineage differentiation, and immunophenotyping. Differences in their chondrogenic potentials were further evaluated quantitatively using glycosaminoglycan (GAG) content and via immunofluorescence of chondrogenic differentiation protein markers, SRY-type HMG box9, Aggrecan, and collagen II. The BMMSCs and SFMSCs were similar in cellular morphology, viability, and immunophenotype, but, varied in their chondrogenic potential, and expression of the key chondrogenic proteins. The SFMSCs exhibited a significant increase in GAG content compared to the BMMSCs (P < 0.0001) in three donors, suggesting increased levels of chondrogenesis. The expression of the key chondrogenic proteins correlated positively with the GAG content, suggesting that the differentiation process is dependent on the expression of the target proteins in these three donors. Our findings suggest that even though SFMSCs were hypothesized to be more chondrogenic relative to BMMSCs, there was considerable donor-to-donor variation in the primary cultures of MSCs which can significantly affect their downstream application.

SOX9 and the many facets of its regulation in the chondrocyte lineage

SOX9 is a pivotal transcription factor in developing and adult cartilage. Its gene is expressed from the multipotent skeletal progenitor stage and is active throughout chondrocyte differentiation. While it is repressed in hypertrophic chondrocytes in cartilage growth plates, it remains expressed throughout life in permanent chondrocytes of healthy articular cartilage. SOX9 is required for chondrogenesis: it secures chondrocyte lineage commitment, promotes cell survival, and transcriptionally activates the genes for many cartilage-specific structural components and regulatory factors. Since heterozygous mutations within and around SOX9 were shown to cause the severe skeletal malformation syndrome called campomelic dysplasia, researchers around the world have worked assiduously to decipher the many facets of SOX9 actions and regulation in chondrogenesis. The more we learn, the more we realize the complexity of the molecular networks in which SOX9 fulfills its functions and is regulated at the levels of its gene, RNA, and protein, and the more we measure the many gaps remaining in knowledge. At the same time, new technologies keep giving us more means to push further the frontiers of knowledge. Research efforts must be pursued to fill these gaps and to better understand and treat many types of cartilage diseases in which SOX9 has or could have a critical role. These diseases include chondrodysplasias and cartilage degeneration diseases, namely osteoarthritis, a prevalent and still incurable joint disease. We here review the current state of knowledge of SOX9 actions and regulation in the chondrocyte lineage, and propose new directions for future fundamental and translational research projects.

TGF-β regulates phosphorylation and stabilization of Sox9 protein in chondrocytes through p38 and Smad dependent mechanisms

Members of the TGF-β superfamily are important regulators of chondrocyte function. Sox9, a key transcriptional regulator of chondrogenesis, is required for TGF-β-mediated regulation of specific cartilage genes. TGF-β can signal through a canonical, Smad-mediated pathway or non-conical pathways, including p38. Here we show that both pathways are activated in chondrocytes after treatment with TGF-β and that TGF-β stabilizes Sox9 protein and increases phosphorylation of Sox9. Mutagenesis of potential serine phosphorylation sites on Sox9 was used to demonstrate that serine 211 is required to maintain normal basal levels of Sox9 as well as mediate increased Sox9 levels in response to TGF-β. The serine 211 site is in a motif that is targeted by p38 kinase. We used siRNA and pharmacological agents to show that p38 and Smad3 independently regulate the phosphorylation and stability of Sox9. Previously, we demonstrated that Papss2 is a downstream transcriptional target of Sox9 and TGF-β. Here we show that p38 is required for TGF-β-mediated regulation of Papss2 mRNA. Together the results suggest a new mechanism for TGF-β-mediated gene regulation in chondrocytes via p38 and phosphorylation and stabilization of Sox9. Understanding how TGF-β regulates Sox9 may lead to identification of therapeutic targets for OA.

Transcriptional control of chondrocyte specification and differentiation

A milestone in the evolutionary emergence of vertebrates was the invention of cartilage, a tissue that has key roles in modeling, protecting and complementing the bony skeleton. Cartilage is elaborated and maintained by chondrocytes. These cells derive from multipotent skeletal progenitors and they perform highly specialized functions as they proceed through sequential lineage commitment and differentiation steps. They form cartilage primordia, the primary skeleton of the embryo. They then transform these primordia either into cartilage growth plates, temporary drivers of skeletal elongation and endochondral ossification, or into permanent tissues, namely articular cartilage. Chondrocyte fate decisions and differentiated activities are controlled by numerous extrinsic and intrinsic cues, and they are implemented at the gene expression level by transcription factors. The latter are the focus of this review. Meritorious efforts from many research groups have led over the last two decades to the identification of dozens of key chondrogenic transcription factors. These regulators belong to all types of transcription factor families. Some have master roles at one or several differentiation steps. They include SOX9 and RUNX2/3. Others decisively assist or antagonize the activities of these masters. They include TWIST1, SOX5/6, and MEF2C/D. Many more have tissue-patterning roles and regulate cell survival, proliferation and the pace of cell differentiation. They include, but are not limited to, homeodomain-containing proteins and growth factor signaling mediators. We here review current knowledge of all these factors, one superclass, class, and family at a time. We then compile all knowledge into transcriptional networks. We also identify remaining gaps in knowledge and directions for future research to fill these gaps and thereby provide novel insights into cartilage disease mechanisms and treatment options.

Discoidin Domain Receptor 2 as a Potential Therapeutic Target for Development of Disease-Modifying Osteoarthritis Drugs

Osteoarthritis (OA) is the most common form of arthritis disorders, but the identification of therapeutic targets to effectively prevent OA has been increasingly difficult. The goal of this investigation is to provide experimental evidence that discoidin domain receptor 2 (DDR2) may be an ideal target for the development of disease-modifying OA drugs. Ddr2 was conditionally deleted from articular cartilage of adult mouse knee joints. Aggrecan-CreERT2;floxed Ddr2 mice, which were generated by crossing Aggrecan-CreERT2 mice with floxed Ddr2 mice, then received tamoxifen injections at the age of 8 weeks. The mice were then subjected to destabilization of the medial meniscus (DMM) surgery. At 8 and 16 weeks after DMM, mice were euthanized for the collection of knee joints. In a separate experiment, Aggrecan-CreERT2;floxed Ddr2 mice were subjected to DMM at the age of 10 weeks. The mice then received tamoxifen injections at 8 weeks after DMM. The mice were euthanized for the collection of knee joints at 16 weeks after DMM. The progressive process of articular cartilage degeneration was significantly delayed in the knee joints of Ddr2-deficient mice in comparison to their control littermates. Articular cartilage damage in the knee joints of the mice was associated with increased expression profiles of both Ddr2 and matrix metalloproteinase 13. These findings suggest that DDR2 may be an ideal target for the development of disease-modifying OA drugs.

PKCε is a regulator of hypertrophic differentiation of chondrocytes in osteoarthritis

OBJECTIVE

Osteoarthritis (OA) is a common and highly debilitating degenerative disease whose complex pathogenesis and the multiplicity of the molecular processes involved, hinder its complete understanding. Protein Kinase C (PKC) novel isozyme PKCε recently proved to be an interesting molecule for further investigations as it can represent an intriguing, new actor in the acquisition of a OA phenotype by the chondrocyte.

DESIGN

PKCε was modulated in primary chondrocytes from human OA patient knee cartilage samples by means of short hairpin RNA (ShRNA) and the expression of cartilage specific markers observed at mRNA and protein level. The involvement of Histone deacetylases (HDACs) signaling pathway was also investigated through the use of specific inhibitors MS-275 and Inhibitor VIII.

RESULTS

PKCε loss induces up-regulation of Runt-domain transcription factor (RUNX2), Metalloproteinase 13 (MMP13) and Collagen X (COL10) as well as an enhanced calcium deposition in OA chondrocyte cultures. In parallel, PKCε knock-down also leads to SOX9 and Collagen II (COL2) down-modulation and to a lower deposition of glycosaminoglycans (GAGs) in the extracellular matrix (ECM). This novel regulatory role of PKCε over cartilage hypertrophic phenotype is exerted via an HDAC-mediated pathway, as HDAC2 and HDAC4 expression is modulated by PKCε. HDAC2 and HDAC4, in turn, are at least in part responsible for the modulation of the master transcription factors RUNX2 and SOX9, key regulators of chondrocyte phenotype.

CONCLUSIONS

PKCε prevents the phenotypic progression of the OA chondrocyte, acting on cartilage specific markers through the modulation of the transcription factors SOX9 and RUNX2. The loss of PKCε enhances, in fact, the OA hypertrophic phenotype, with clear implications in the pathophysiology of the disease.

SOX9 directly Regulates CTGF/CCN2 Transcription in Growth Plate Chondrocytes and in Nucleus Pulposus Cells of Intervertebral Disc

Several lines of evidence indicate that connective tissue growth factor (CTGF/CCN2) stimulates chondrocyte proliferation and maturation. Given the fact that SOX9 is essential for several steps of the chondrocyte differentiation pathway, we asked whether Ctgf (Ccn2) is the direct target gene of SOX9. We found that Ctgf mRNA was down-regulated in primary sternal chondrocytes from Sox9^flox/flox mice infected with Ad-CMV-Cre. We performed ChIP-on-chip assay using anti-SOX9 antibody, covering the Ctgf gene from 15 kb upstream of its 5′-end to 10 kb downstream of its 3′-end to determine SOX9 interaction site. One high-affinity interaction site was identified in the Ctgf proximal promoter by ChIP-on-chip assay. An important SOX9 regulatory element was found to be located in −70/−64 region of the Ctgf promoter. We found the same site for SOX9 binding to the Ctgf promoter in nucleus pulposus (NP) cells. The loss of Sox9 in growth plate chondrocytes in knee joint and in NP cells in intervertebral disc led to the decrease in CTGF expression. We suggest that Ctgf is the direct target gene of SOX9 in chondrocytes and NP cells. Our study establishes a strong link between two regulatory molecules that have a major role in cartilaginous tissues.

Induced superficial chondrocyte death reduces catabolic cartilage damage in murine posttraumatic osteoarthritis

Joints that have degenerated as a result of aging or injury contain dead chondrocytes and damaged cartilage. Some studies have suggested that chondrocyte death precedes cartilage damage, but how the loss of chondrocytes affects cartilage integrity is not clear. In this study, we examined whether chondrocyte death undermines cartilage integrity in aging and injury using a rapid 3D confocal cartilage imaging technique coupled with standard histology. We induced autonomous expression of diphtheria toxin to kill articular surface chondrocytes in mice and determined that chondrocyte death did not lead to cartilage damage. Moreover, cartilage damage after surgical destabilization of the medial meniscus of the knee was increased in mice with intact chondrocytes compared with animals whose chondrocytes had been killed, suggesting that chondrocyte death does not drive cartilage damage in response to injury. These data imply that chondrocyte catabolism, not death, contributes to articular cartilage damage following injury. Therefore, therapies targeted at reducing the catabolic phenotype may protect against degenerative joint disease.

Acetylation reduces SOX9 nuclear entry and ACAN gene transactivation in human chondrocytes

Changes in the content of aggrecan, an essential proteoglycan of articular cartilage, have been implicated in the pathophysiology of osteoarthritis (OA), a prevalent age-related, degenerative joint disease. Here, we examined the effect of SOX9 acetylation on ACAN transactivation in the context of osteoarthritis. Primary chondrocytes freshly isolated from degenerated OA cartilage displayed lower levels of ACAN mRNA and higher levels of acetylated SOX9 compared with cells from intact regions of OA cartilage. Degenerated OA cartilage presented chondrocyte clusters bearing diffused immunostaining for SOX9 compared with intact cartilage regions. Primary human chondrocytes freshly isolated from OA knee joints were cultured in monolayer or in three-dimensional alginate microbeads (3D). SOX9 was hypo-acetylated in 3D cultures and displayed enhanced binding to a -10 kb ACAN enhancer, a result consistent with higher ACAN mRNA levels than in monolayer cultures. It also co-immunoprecipitated with SIRT1, a major deacetylase responsible for SOX9 deacetylation. Finally, immunofluorescence assays revealed increased nuclear localization of SOX9 in primary chondrocytes treated with the NAD SIRT1 cofactor, than in cells treated with a SIRT1 inhibitor. Inhibition of importin β by importazole maintained SOX9 in the cytoplasm, even in the presence of NAD. Based on these data, we conclude that deacetylation promotes SOX9 nuclear translocation and hence its ability to activate ACAN.