The distinct role of the Runx proteins in chondrocyte differentiation and intervertebral disc degeneration: findings in murine models and in human disease

Cell signaling and transcriptional regulation of osteoblast lineage commitment, differentiation, bone formation, and homeostasis

The initiation of osteogenesis primarily occurs as mesenchymal stem cells undergo differentiation into osteoblasts. This differentiation process plays a crucial role in bone formation and homeostasis and is regulated by two intricate processes: cell signal transduction and transcriptional gene expression. Various essential cell signaling pathways, including Wnt, BMP, TGF-β, Hedgehog, PTH, FGF, Ephrin, Notch, Hippo, and Piezo1/2, play a critical role in facilitating osteoblast differentiation, bone formation, and bone homeostasis. Key transcriptional factors in this differentiation process include Runx2, Cbfβ, Runx1, Osterix, ATF4, SATB2, and TAZ/YAP. Furthermore, a diverse array of epigenetic factors also plays critical roles in osteoblast differentiation, bone formation, and homeostasis at the transcriptional level. This review provides an overview of the latest developments and current comprehension concerning the pathways of cell signaling, regulation of hormones, and transcriptional regulation of genes involved in the commitment and differentiation of osteoblast lineage, as well as in bone formation and maintenance of homeostasis. The paper also reviews epigenetic regulation of osteoblast differentiation via mechanisms, such as histone and DNA modifications. Additionally, we summarize the latest developments in osteoblast biology spurred by recent advancements in various modern technologies and bioinformatics. By synthesizing these insights into a comprehensive understanding of osteoblast differentiation, this review provides further clarification of the mechanisms underlying osteoblast lineage commitment, differentiation, and bone formation, and highlights potential new therapeutic applications for the treatment of bone diseases.

Runx2 deletion in hypertrophic chondrocytes impairs osteoclast mediated bone resorption

Deletion of Runx2 gene in proliferating chondrocytes results in complete failure of endochondral ossification and perinatal lethality. We reported recently that mice with Runx2 deletion specifically in hypertrophic chondrocytes (HCs) using the Col10a1-Cre transgene survive and exhibit enlarged growth plates due to decreased HC apoptosis and cartilage resorption. Bulk of chondrogenesis occurs postnatally, however, the role of Runx2 in HCs during postnatal chondrogenesis is unknown. Despite limb dwarfism, adult homozygous (Runx2HC/HC) mice showed a significant increase in length of growth plate and articular cartilage. Consistent with doubling of the hypertrophic zone, collagen type X expression was increased in Runx2HC/HC mice. In sharp contrast, expression of metalloproteinases and aggrecanases were markedly decreased. Impaired cartilage degradation was evident by the retention of significant amount of safranin-O positive cartilage. Histomorphometry and μCT uncovered increased trabecular bone mass with a significant increase in BV/TV ratio, trabecular number, thickness, and a decrease in trabecular space in Runx2HC/HC mice. To identify if this is due to increased bone synthesis, expression of osteoblast differentiation markers was evaluated and found to be comparable amongst littermates. Histomorphometry confirmed similar number of osteoblasts in the littermates. Furthermore, dynamic bone synthesis showed no differences in mineral apposition or bone formation rates. Surprisingly, three-point-bending test revealed Runx2HC/HC bones to be structurally less strong. Interestingly, both the number and surface of osteoclasts were markedly reduced in Runx2HC/HC littermates. Rankl and IL-17a ligands that promote osteoclast differentiation were markedly reduced in Runx2HC/HC mice. Bone marrow cultures were performed to independently establish Runx2 and hypertrophic chondrocytes role in osteoclast development. The culture from the Runx2HC/HC mice formed significantly fewer and smaller osteoclasts. The expression of mature osteoclast markers, Ctsk and Mmp9, were significantly reduced in the cultures from Runx2HC/HC mice. Thus, Runx2 functions extend beyond embryonic development and chondrocyte hypertrophy by regulating cartilage degradation, osteoclast differentiation, and bone resorption during postnatal endochondral ossification.

Hypertrophic chondrocytes at the junction of musculoskeletal structures

Hypertrophic chondrocytes are found at unique locations at the junction of skeletal tissues, cartilage growth plate, articular cartilage, enthesis and intervertebral discs. Their role in the skeleton is best understood in the process of endochondral ossification in development and bone fracture healing. Chondrocyte hypertrophy occurs in degenerative conditions such as osteoarthritis. Thus, the role of hypertrophic chondrocytes in skeletal biology and pathology is context dependent. This review will focus on hypertrophic chondrocytes in endochondral ossification, in which they exist in a transient state, but acting as a central regulator of differentiation, mineralization, vascularization and conversion to bone. The amazing journey of a chondrocyte from being entrapped in the extracellular matrix environment to becoming proliferative then hypertrophic will be discussed. Recent studies on the dynamic changes and plasticity of hypertrophic chondrocytes have provided new insights into how we view these cells, not as terminally differentiated but as cells that can dedifferentiate to more progenitor-like cells in a transition to osteoblasts and adipocytes, as well as a source of skeletal stem and progenitor cells residing in the bone marrow. This will provide a foundation for studies of hypertrophic chondrocytes at other skeletal sites in development, tissue maintenance, pathology and therapy.

Single-cell RNA-seq analysis reveals that immune cells induce human nucleus pulposus ossification and degeneration

Background and aims

Determining the transcriptomes and molecular mechanism underlying human degenerative nucleus pulposus (NP) is of critical importance for treating intervertebral disc degeneration (IDD). Here, we aimed to elucidate the detailed molecular mechanism of NP ossification and IDD using single-cell RNA sequencing.

Methods

Single-cell RNA-seq and bioinformatic analysis were performed to identify NP cell populations with gene signatures, biological processes and pathways, and subpopulation analysis, RNA velocity analysis, and cell-to-cell communication analysis were performed in four IDD patients. We also verified the effects of immune cells on NP ossification using cultured NP cells and a well-established rat IDD model.

Results

We identified five cell populations with gene expression profiles in degenerative NP at single-cell resolution. GO database analysis showed that degenerative NP-associated genes were mainly enriched in extracellular matrix organization, immune response, and ossification. Gene set enrichment analysis showed that rheumatoid arthritis signaling, antigen processing and presentation signaling were activated in the blood cell cluster. We revealed that stromal cells, which are progenitor cells, differentiated toward an ossification phenotype and delineated interactions between immune cells (macrophages and T cells) and stromal cells. Immune factors such as TNF-α, CD74 and CCL-3 promoted the differentiation of stromal cells toward an ossification phenotype in vitro. Blocking TNF-α with a specific inhibitor successfully reversed NP ossification and modified NP morphology in vivo.

Conclusion

Our study revealed an increase in macrophages and T cells in degenerative NP, which induced stromal cell differentiation toward an ossification phenotype, and contributed to the identification of a novel therapeutic target to delay IDD.

The roles of Runx1 in skeletal development and osteoarthritis: A concise review

Runt-related transcription factor-1 (Runx1) is well known for its functions in hematopoiesis and leukemia but recent research has focused on its role in skeletal development and osteoarthritis (OA). Deficiency of the Runx1 gene is fatal in early embryonic development, and specific knockout of Runx1 in cell lineages of cartilage and bone leads to delayed cartilage formation and impaired bone calcification. Runx1 can regulate genes including collagen type II (Col2a1) and X (Col10a1), SRY-box transcription factor 9 (Sox9), aggrecan (Acan) and matrix metalloproteinase 13 (MMP-13), and the up-regulation of Runx1 improves the homeostasis of the whole joint, even in the pathological state. Moreover, Runx1 is activated as a response to mechanical compression, but impaired in the joint with the pathological progress associated with osteoarthritis. Therefore, interpretation about the role of Runx1 could enlarge our understanding of key marker genes in the skeletal development and an increased understanding of Runx1 could be helpful to identify treatments for osteoarthritis. This review provides the most up-to-date advances in the roles and bio-mechanisms of Runx1 in healthy joints and osteoarthritis from all currently published articles and gives novel insights in therapeutic approaches to OA based on Runx1.

Exogenous Indian hedgehog antagonist damages intervertebral discs homeostasis in adult mice

Background

Vismodegib, as an exogenous Indian hedgehog (Ihh) antagonist, has been approved by the Food and Drug Administration (FDA) for the clinical treatment of patients with basal cell carcinoma, and previous observations implicate the potential therapeutic of vismodegib in osteoarthritis treatment. However, there is no direct evidence for the role of Ihh signaling in intervertebral discs (IVDs) homeostasis of adult mice. The aim of the present study is to assess the effect of systemic administration of Smoothened inhibitor (SMOi) - vismodegib on IVDs homeostasis during the adult stage.

Methods

The expression of glioma-associated oncogene homolog 1 (Gli1), the downstream targeting gene of Ihh signaling, in IVDs of adult mice after receiving systemic administration of SMOi was examined by immunohistochemistry. The pathological changes of vertebral bodies after SMOi treatment were evaluated by X-ray and micro-CT. The effects of SMOi on homeostasis of IVDs including cartilaginous endplates (CEP), growth plates (GP) and annulus fibrous (AF) were evaluated by histological analysis. The expressions of Aggrecan, Matrix metalloproteinase 13 (MMP13) and Runt-related transcription factor 2 (Runx2), in IVDs were also investigated by immunohistochemistry. Changes in chondrocyte apoptosis and proliferation in IVDs were evaluated by terminal deoxynucleotidyl transferase dUTP nick-end labeling (TUNEL) assay and analyzing the expression of the cell proliferation antigen Ki-67.

Results

Systemic administration of SMOi significantly decreased the expression of Gli1 in IVDs that indicating effective inhibition of Ihh signaling. Bone mass of vertebral bodies was diminished after SMOi treatment. Moreover, IVDs degeneration (IDD) like defects including CEP sclerosis, degenerative nucleus pulposus (NP) and fissure within AF, as well as narrowed or fused GP and loss bone mass of vertebral bodies was observed in SMOi-treated mice. The severity of IDD was time-dependent with the administration of SMOi treatment after 2-8 weeks. The expressions of Aggrecan, MMP13 and Runx2 in IVDs of mice receiving SMOi treatment were significantly decreased. In addition, chondrocyte apoptosis was significantly enhanced, while chondrocyte proliferation was significantly inhibited.

Conclusions

Our study propose that systemic administration of vismodegib damages IVDs homeostasis via inhibition of Ihh signaling in adult mice. The clinical application of Ihh signaling antagonists such as vismodegib should be careful considering these side adverse.

The Translational Potential of this Article

Vismodegib as an exogenous antagonist of Ihh signaling has been approved by the FDA for the clinical treatment of patients with basal cell carcinoma. However, it is still unknown whether vismodegib will has adverse effects on the patient or animal model of IVDs cartilage homeostasis. Based on our study, systemic administration of vismodegib damages IVDs homeostasis via inhibition of Ihh signaling in adult mice and special attention should be paid to the clinical application of vismodegib.

Genetic animal models of scoliosis: A systematical review

Scoliosis is a complex disease with undetermined pathogenesis and has a strong relationship with genetics. Models of scoliosis in animals have been established for better comprehending its pathogenesis and treatment. In this review, we searched all the genetic animal models with body curvature in databases, and reviewed the related genes and scoliosis types. Meanwhile, we also summarized the pathogenesis of scoliosis reported so far. Summarizing the positive phenotypic animal models contributes to a better understanding on the pathogenesis of scoliosis and facilitates the selection of experimental models when a possible pathogenic factor is concerned.

Spatially defined single-cell transcriptional profiling characterizes diverse chondrocyte subtypes and nucleus pulposus progenitors in human intervertebral discs

A comprehensive understanding of the cellular heterogeneity and molecular mechanisms underlying the development, homeostasis, and disease of human intervertebral disks (IVDs) remains challenging. Here, the transcriptomic landscape of 108 108 IVD cells was mapped using single-cell RNA sequencing of three main compartments from young and adult healthy IVDs, including the nucleus pulposus (NP), annulus fibrosus, and cartilage endplate (CEP). The chondrocyte subclusters were classified based on their potential regulatory, homeostatic, and effector functions in extracellular matrix (ECM) homeostasis. Notably, in the NP, a PROCR⁺ resident progenitor population showed enriched colony-forming unit-fibroblast (CFU-F) activity and trilineage differentiation capacity. Finally, intercellular crosstalk based on signaling network analysis uncovered that the PDGF and TGF-β cascades are important cues in the NP microenvironment. In conclusion, a single-cell transcriptomic atlas that resolves spatially regulated cellular heterogeneity together with the critical signaling that underlies homeostasis will help to establish new therapeutic strategies for IVD degeneration in the clinic.

Spatially defined single-cell transcriptional profiling characterizes diverse chondrocyte subtypes and nucleus pulposus progenitors in human intervertebral discs

A comprehensive understanding of the cellular heterogeneity and molecular mechanisms underlying the development, homeostasis, and disease of human intervertebral disks (IVDs) remains challenging. Here, the transcriptomic landscape of 108 108 IVD cells was mapped using single-cell RNA sequencing of three main compartments from young and adult healthy IVDs, including the nucleus pulposus (NP), annulus fibrosus, and cartilage endplate (CEP). The chondrocyte subclusters were classified based on their potential regulatory, homeostatic, and effector functions in extracellular matrix (ECM) homeostasis. Notably, in the NP, a PROCR+ resident progenitor population showed enriched colony-forming unit-fibroblast (CFU-F) activity and trilineage differentiation capacity. Finally, intercellular crosstalk based on signaling network analysis uncovered that the PDGF and TGF-β cascades are important cues in the NP microenvironment. In conclusion, a single-cell transcriptomic atlas that resolves spatially regulated cellular heterogeneity together with the critical signaling that underlies homeostasis will help to establish new therapeutic strategies for IVD degeneration in the clinic.

Spatially defined single-cell transcriptional profiling characterizes diverse chondrocyte subtypes and nucleus pulposus progenitors in human intervertebral discs

A comprehensive understanding of the cellular heterogeneity and molecular mechanisms underlying the development, homeostasis, and disease of human intervertebral disks (IVDs) remains challenging. Here, the transcriptomic landscape of 108 108 IVD cells was mapped using single-cell RNA sequencing of three main compartments from young and adult healthy IVDs, including the nucleus pulposus (NP), annulus fibrosus, and cartilage endplate (CEP). The chondrocyte subclusters were classified based on their potential regulatory, homeostatic, and effector functions in extracellular matrix (ECM) homeostasis. Notably, in the NP, a PROCR⁺ resident progenitor population showed enriched colony-forming unit-fibroblast (CFU-F) activity and trilineage differentiation capacity. Finally, intercellular crosstalk based on signaling network analysis uncovered that the PDGF and TGF-β cascades are important cues in the NP microenvironment. In conclusion, a single-cell transcriptomic atlas that resolves spatially regulated cellular heterogeneity together with the critical signaling that underlies homeostasis will help to establish new therapeutic strategies for IVD degeneration in the clinic.

Prenatal muscle forces are necessary for vertebral segmentation and disc structure, but not for notochord involution in mice

Embryonic muscle forces are necessary for normal vertebral development and spinal curvature, but their involvement in intervertebral disc (IVD) development remains unclear. The aim of the current study was to determine how muscle contractions affect (1) notochord involution and vertebral segmentation, and (2) IVD development including the mechanical properties and morphology, as well as collagen fibre alignment in the annulus fibrosus. Muscular dysgenesis (mdg) mice were harvested at three prenatal stages: at Theiler Stage (TS)22 when notochord involution starts, at TS24 when involution is complete, and at TS27 when the IVD is formed. Vertebral and IVD development were characterised using histology, immunofluorescence, and indentation testing. The results revealed that notochord involution and vertebral segmentation occurred independently of muscle contractions between TS22 and TS24. However, in the absence of muscle contractions, we found vertebral fusion in the cervical region at TS27, along with (i) a displacement of the nucleus pulposus towards the dorsal side, (ii) a disruption of the structural arrangement of collagen in the annulus fibrosus, and (iii) an increase in viscous behaviour of the annulus fibrosus. These findings emphasise the important role of mechanical forces during IVD development, and demonstrate a critical role of muscle loading during development to enable proper annulus fibrosus formation. They further suggest a need for mechanical loading in the creation of fibre-reinforced tissue engineering replacement IVDs as a therapy for IVD degeneration.

Activation of nuclear factor-kappa B by TNF promotes nucleus pulposus mineralization through inhibition of ANKH and ENPP1

Spontaneous mineralization of the nucleus pulposus (NP) has been observed in cases of intervertebral disc degeneration (IDD). Inflammatory cytokines have been implicated in mineralization of multiple tissues through their modulation of expression of factors that enable or inhibit mineralization, including TNAP, ANKH or ENPP1. This study examines the underlying factors leading to NP mineralization, focusing on the contribution of the inflammatory cytokine, TNF, to this pathologic event. We show that human and bovine primary NP cells express high levels of ANKH and ENPP1, and low or undetectable levels of TNAP. Bovine NPs transduced to express TNAP were capable of matrix mineralization, which was further enhanced by ANKH knockdown. TNF treatment or overexpression promoted a greater increase in mineralization of TNAP-expressing cells by downregulating the expression of ANKH and ENPP1 via NF-κB activation. The increased mineralization was accompanied by phenotypic changes that resemble chondrocyte hypertrophy, including increased RUNX2 and COL10A1 mRNA; mirroring the cellular alterations typical of samples from IDD patients. Disc organ explants injected with TNAP/TNF- or TNAP/shANKH-overexpressing cells showed increased mineral content inside the NP. Together, our results confirm interactions between TNF and downstream regulators of matrix mineralization in NP cells, providing evidence to suggest their participation in NP calcification during IDD.

Integrated transcriptome and proteome analyses identify novel regulatory network of nucleus pulposus cells in intervertebral disc degeneration

BACKGROUND

Degeneration of intervertebral disc is a major cause of lower back pain and neck pain. Studies have tried to unveil the regulatory network using either transcriptomic or proteomic analysis. However, neither have fully elucidated the exact mechanism of degeneration process. Since post-transcriptional regulation may affect gene expression by modulating the translational process of mRNA to protein product, a combined transcriptomic and proteomic study may provide more insight into the key regulatory network of Intervertebral disc degeneration.

METHODS

In order to obtain the proteomic and transcriptomic data, we performed label-free proteome analysis on freshly isolated nucleus pulposus cells and obtained transcriptome profiling data from the Gene Expression Omnibus repository. To identify the key regulatory network of intervertebral disc degeneration in nucleus pulposus cells, we performed bioinformatic analyses and established a protein-RNA interacting network. To validate the candidate genes, we performed in vitro experimentation and immunochemistry labeling to identify their potential function during nucleus pulposus degeneration.

RESULTS

The label-free proteome analysis identified altogether 656 proteins, and 503 of which were differentially expressed between nucleus pulposus cells from degenerated or normal disc cells. Using the existing nucleus pulposus transcriptomic profiling data, we integrated the proteomic and transcriptomic data of nucleus pulposus cells, and established a protein-RNA interacting network to show the combined regulatory network of intervertebral disc degeneration. In the network, we found 9 genes showed significant changes, and 6 of which (CHI3L1, KRT19, COL6A2, DPT, TNFAIP6 and COL11A2) showed concordant changes in both protein and mRNA level. Further functional analysis showed these candidates can significantly affect the degeneration of the nucleus pulposus cell when altering their expression.

CONCLUSIONS

This study is the first to use combined analysis of proteomic and transcriptomic profiling data to identify novel regulatory network of nucleus pulposus cells in intervertebral disc degeneration. Our established protein-RNA interacting network demonstrated novel regulatory mechanisms and key genes that may play vital roles in the pathogenesis of intervertebral disc degeneration.

The Role of Panx3 in Age-Associated and Injury-Induced Intervertebral Disc Degeneration

Pannexin 3 (Panx3) is a mechanosensitive, channel-forming glycoprotein implicated in the progression of post-traumatic osteoarthritis. Despite evidence for Panx3 expression in the intervertebral disc (IVD), its function in this cartilaginous joint structure remained unknown. Using Panx3 knockout mice, this study investigated the role of Panx3 in age-associated IVD degeneration and degeneration induced by annulus fibrosus (AF) needle puncture. Loss of Panx3 did not significantly impact the progression of age-associated histopathological IVD degeneration; however, loss of Panx3 was associated with decreased gene expression of Acan, Col1a1, Mmp13 and Runx2 and altered localization of COLX in the IVD at 19 months-of-age. Following IVD injury in the caudal spine, histological analysis of wild-type mice revealed clusters of hypertrophic cells in the AF associated with increased pericellular proteoglycan accumulation, disruptions in lamellar organization and increased lamellar thickness. In Panx3 knockout mice, hypertrophic AF cells were rarely detected and AF structure was largely preserved post-injury. Interestingly, uninjured IVDs adjacent to the site of injury more frequently showed evidence of early nucleus pulposus degeneration in Panx3 knockout mice but remained healthy in wild-type mice. These findings suggest a role for Panx3 in mediating the adaptive cellular responses to altered mechanical stress in the IVD, which may buffer aberrant loads transferred to adjacent motion segments.

Evolution and expansion of the RUNX2 QA repeat corresponds with the emergence of vertebrate complexity

Runt-related transcription factor 2 (RUNX2) is critical for the development of the vertebrate bony skeleton. Unlike other RUNX family members, RUNX2 possesses a variable poly-glutamine, poly-alanine (QA) repeat domain. Natural variation within this repeat is able to alter the transactivation potential of RUNX2, acting as an evolutionary 'tuning knob' suggested to influence mammalian skull shape. However, the broader role of the RUNX2 QA repeat throughout vertebrate evolution is unknown. In this perspective, we examine the role of the RUNX2 QA repeat during skeletal development and discuss how its emergence and expansion may have facilitated the evolution of morphological novelty in vertebrates.

Runx1 up-regulates chondrocyte to osteoblast lineage commitment and promotes bone formation by enhancing both chondrogenesis and osteogenesis

One of the fundamental questions in bone biology is where osteoblasts originate and how osteoblast differentiation is regulated. The mechanism underlying which factors regulate chondrocyte to osteoblast lineage commitment remains unknown. Our data showed that Runt-related transcription factor 1 (Runx1) is expressed at different stages of both chondrocyte and osteoblast differentiation. Runx1 chondrocyte-specific knockout (Runx1f/fCol2α1-cre) mice exhibited impaired cartilage formation, decreased bone density, and an osteoporotic phenotype. The expressions of chondrocyte differentiation regulation genes, including Sox9, Ihh, CyclinD1, PTH1R, and hypertrophic chondrocyte marker genes including Col2α1, Runx2, MMP13, Col10α1 in the growth plate were significantly decreased in Runx1f/fCol2α1-cre mice chondrocytes. Importantly, the expression of osteoblast differentiation regulation genes including Osx, Runx2, ATF4, and osteoblast marker genes including osteocalcin (OCN) and osteopontin (OPN) were significantly decreased in the osteoblasts of Runx1f/fCol2α1-cre mice. Notably, our data showed that osteoblast differentiation regulation genes and marker genes are also expressed in chondrocytes and the expressions of these marker genes were significantly decreased in the chondrocytes of Runx1f/fCol2α1-cre mice. Our data showed that chromatin immunoprecipitation (ChIP) and promoter mapping analysis revealed that Runx1 directly binds to the Indian hedgehog homolog (Ihh) promoter to regulate its expression, indicating that Runx1 directly regulates the transcriptional expression of chondrocyte genes. Collectively, we revealed that Runx1 signals chondrocyte to osteoblast lineage commitment and promotes endochondral bone formation through enhancing both chondrogenesis and osteogenesis genes expressions, indicating Runx1 may be a therapeutic target to enhance endochondral bone formation and prevent osteoporosis fractures.

MiR-520d-5p modulates chondrogenesis and chondrocyte metabolism through targeting HDAC1

MicroRNAs (miRNAs) play an essential role in the chondrogenesis and the progression of osteoarthritis (OA). This study aimed to determine miRNAs associated with chondrogenesis of human mesenchymal stem cells (hMSCs) and chondrocyte metabolism. MiRNAs were screened in hMSCs during chondrogenesis by RNA-seq and qRT-PCR. MiRNA expression was determined in primary human chondrocytes (PHCs), and degraded cartilage samples. MiRNA mimics and inhibitors were transfected to cells to determine the effect of miRNA. Bioinformatic analysis and luciferase reporter assays were applied to determine the target gene of miRNA. The results demonstrated that miR-520d-5p was increased in hMSCs chondrogenesis. The overexpression and knockdown of miR-520d-5p promoted and inhibited chondrogenesis, and regulated chondrocyte metabolism. Histone deacetylase 1 (HDAC1) was decreased in hMSCs chondrogenesis, and HDAC1 was a targeting gene of miR-520d-5p. CI994, HDAC1 inhibitor, elevated cartilage-specific gene expressions and promoted hMSCs chondrogenesis. In IL-1β-treated PHCs, CI994 promoted AGGRECAN expression and suppressed MMP-13 expression, abolishing the effect of IL-1β on PHCs. Taken together, these results suggest that miR-520d-5p promotes hMSCs chondrogenesis and regulates chondrocyte metabolism through targeting HDAC1. This study provides novel understanding of the molecular mechanism of OA progression.

Gain-of-function variants and overexpression of RUNX2 in patients with nonsyndromic midline craniosynostosis

Craniosynostosis (CS), the premature fusion of one or more cranial sutures, is a relatively common congenital anomaly, occurring in 3-5 per 10,000 live births. Nonsyndromic CS (NCS) accounts for up to 80% of all CS cases, yet the genetic factors contributing to the disorder remain largely unknown. The RUNX2 gene, encoding a transcription factor critical for bone and skull development, is a well known CS candidate gene, as copy number variations of this gene locus have been found in patients with syndromic craniosynostosis. In the present study, we aimed to characterize RUNX2 to better understand its role in the genetic etiology and in the molecular mechanisms underlying midline suture ossification in NCS. We report four nonsynonymous variants, one intronic variant and one 18 bp in-frame deletion in RUNX2 not found in our study control population. Significant difference in allele frequency (AF) for the deletion variant RUNX2 p.Ala84-Ala89del (ClinVar 257,095; dbSNP rs11498192) was observed in our sagittal NCS cohort when compared to the general population (P = 1.28 × 10^-6), suggesting a possible role in the etiology of NCS. Dual-luciferase assays showed that three of four tested RUNX2 variants conferred a gain-of-function effect on RUNX2, further suggesting their putative pathogenicity in the tested NCS cases. Downregulation of RUNX2 expression was observed in prematurely ossified midline sutures. Metopic sites showed significant downregulation of promoter 1-specific isoforms compared to sagittal sites. Suture-derived mesenchymal stromal cells showed an increased expression of RUNX2 over matched unfused suture derived cells. This demonstrates that RUNX2, and particularly the distal promoter 1-isoform group, are overexpressed in the osteogenic precursors within the pathological suture sites.

Network Analysis Identifies Gene Regulatory Network Indicating the Role of RUNX1 in Human Intervertebral Disc Degeneration

Intervertebral disc (IVD) degeneration (IDD) is a multifactorial physiological process which is often associated with lower back pain. Previous studies have identified some molecular markers associated with disc degeneration, which despite their significant contributions, have provided limited insight into the etiology of IDD. In this study, we utilized a network medicine approach to uncover potential molecular mediators of IDD. Our systematic analyses of IDD associated with 284 genes included functional annotation clustering, interaction networks, network cluster analysis and Transcription factors (TFs)-target gene network analysis. The functional enrichment and protein–protein interaction network analysis highlighted the role of inflammatory genes and cytokine/chemokine signaling in IDD. Moreover, sub-network analysis identified significant clusters possessing organized networks of 24 cytokine and chemokine genes, which may be considered as key modulators for IDD. The expression of these genes was validated in independent microarray datasets. In addition, the regulatory network analysis identified the role of multiple transcription factors, with RUNX1 being a master regulator in the pathogenesis of IDD. Our analyses highlighted the role of cytokine genes and interacting pathways in IDD and further improved our understanding of the genetic mechanisms underlying IDD.

Aging aggravates intervertebral disc degeneration by regulating transcription factors toward chondrogenesis

Osterix is a critical transcription factor of mesenchymal stem cell fate, where its loss or loss of Wnt signaling diverts differentiation to a chondrocytic lineage. Intervertebral disc (IVD) degeneration activates the differentiation of prehypertrophic chondrocyte-like cells and inactivates Wnt signaling, but its interactive role with osterix is unclear. First, compared to young-adult (5 mo), mechanical compression of old (18 mo) IVD induced greater IVD degeneration. Aging (5 vs 12 mo) and/or compression reduced the transcription of osterix and notochordal marker T by 40-75%. Compression elevated the transcription of hypertrophic chondrocyte marker MMP13 and pre-osterix transcription factor RUNX2, but less so in 12 mo IVD. Next, using an Ai9/td reporter and immunohistochemical staining, annulus fibrosus and nucleus pulposus cells of young-adult IVD expressed osterix, but aging and compression reduced its expression. Lastly, in vivo LRP5-deficiency in osterix-expressing cells inactivated Wnt signaling in the nucleus pulposus by 95%, degenerated the IVD to levels similar to aging and compression, reduced the biomechanical properties by 45-70%, and reduced the transcription of osterix, notochordal markers and chondrocytic markers by 60-80%. Overall, these data indicate that age-related inactivation of Wnt signaling in osterix-expressing cells may limit regeneration by depleting the progenitors and attenuating the expansion of chondrocyte-like cells.

Higenamine inhibits IL-1β-induced inflammation in human nucleus pulposus cells

Intervertebral disc degeneration (IDD) is a natural progression of the aging process associated with inflammation. Higenamine, a plant-based alkaloid, has been identified to possess various pharmacological properties, including anti-inflammatory activity. In the present study, we aimed to evaluate the role of higenamine in interleukin (IL)-1β-induced inflammation in human nucleus pulposus cells (NPCs). The results showed that higenamine improved cell viability in IL-1β-induced NPCs. The IL-1β-dependent up-regulation of inflammatory molecules including inducible nitric oxide synthase (iNOS), nitric oxide (NO), prostaglandin E2 (PGE2), cyclooxygenase-2 (COX-2), tumor necrosis factor alpha (TNF-α), and IL-6 was attenuated by higenamine in NPCs. The increased productions of matrix metalloproteinases (MMP-3 and MMP-13), as well as a disintegrin and metalloprotease with thrombospondin motifs (ADAMTS-4 and ADAMTS-5) were significantly mitigated by higenamine treatment. Furthermore, we also found that higenamine suppressed the IL-1β-induced activation of NF-κB signaling pathway in NPCs. In conclusion, the present study proved that higenamine exhibited anti-inflammatory activity against IL-1β-induced inflammation in NPCs via inhibiting NF-κB signaling pathway. These results suggested that higenamine might be a therapeutic agent for the treatment of IDD.

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.

Treatment of Intervertebral Disk Disease by the Administration of mRNA Encoding a Cartilage-Anabolic Transcription Factor

Intervertebral disk (IVD) degeneration is often associated with severity of lower back pain. IVD core is an avascular, highly hydrated tissue composed of type II collagen, glycosaminoglycans, and proteoglycans. The disk degeneration is not only a destruction of IVD structure but also is related to a disorder of the turnover of the disk matrix, leading the jelly-like IVD core to be replaced by fibrous components. Here we present a disease-modifying strategy for IVD degenerative diseases by direct regulation of the cells in the IVD using mRNA medicine, to alter the misbalanced homeostasis during disk degeneration. When mRNA encoding a cartilage-anabolic transcription factor, runt-related transcription factor-1, was administered to a rat model of coccygeal disk degeneration using a polyplex nanomicelle composed of polyethylene glycol-polyamino acid block copolymers and mRNA, the disk height was maintained to a significantly higher extent (≈81%) compared to saline control (69%), with prevention of fibrosis in the disk tissue. In addition, the use of nanomicelles effectively prevented inflammation, which was observed by injection of naked mRNA into the disk. This proof-of-concept study revealed that mRNA medicine has a potential for treating IVD degenerative diseases by introducing a cartilage-anabolic factor into the host cells, proposing a new therapeutic strategy using mRNA medicine.

Runx1 regulates osteogenic differentiation of BMSCs by inhibiting adipogenesis through Wnt/β-catenin pathway

OBJECTIVE

Bone marrow stem cells (BMSCs) can commit to both adipocyte and osteoblast lineages. However, the mechanism underlying how transcription factors regulate this process remains elusive. Our aims were to determine the role of runt-related transcription factor 1 (Runx1) in BMSCs lineage determination and the underlying mechanisms.

STUDY DESIGN

BMSCs from mouse femur bone marrow were harvested and cultured in osteogenic medium. Runx1 was knocked down in BMSCs using lentivirus. Alkaline phosphatase (ALP), Von Kossa and Oil Red O staining were performed on the Runx1-transduced BMSCs and control cells to see the differences of osteogenic and adipogenic differentiation in these groups. Real-time quantitative PCR and Western blot were performed to analyse the expression levels of osteogenic and adipogenic factors regulated by Runx1 at gene and protein levels.

RESULTS

In BMSCs with Runx1 knockdown, the expression levels of osteogenic-related genes decreased significantly while the adipogenic genes C/EBPα, PPARγ and Fabp4 increased by 12-fold, 10-fold, and 30-fold, respectively, compared with the control cells. ALP activity and Von kossa staining were greatly decreased in Runx1-transfected cells while the Oil Red O staining was comparable to that in the control groups. Canonical Wnt signaling was investigated in the Runx1-deficient BMSCs, and a 50% decrease in the expression of active β-catenin in these cells was found. Lef1 and Tcf1, which are regulated by β-catenin were also decreased in Runx1-deficient cells compared with the levels in controls. Moreover, although there was no difference in the expression of Wnt3a among the three groups of cells, the expression of Wnt10b decreased by 80% in Runx1-deficient BMSCs compared with the levels in the other two groups.

CONCLUSIONS

Our results show Runx1 promotes the capacity of osteogenesis in BMSCs while inhibits their adipogenesis through canonical Wnt/β-catenin pathway, which provides new insights into osteoblast development.

Runx2 is required for postnatal intervertebral disc tissue growth and development

Runx2 plays an essential role in embryonic disc tissue development in mice. However, the role of runt-related transcription factor 2 (Runx2) in postnatal disc tissue growth and development has not been defined. In the present studies, we generated Runx2 conditional knockout (KO) mice (Runx2^Agc1ER ), in which Runx2 was deleted in Aggrecan-expressing cells in disc tissue at postnatal 2-weeks of age. We then analyzed changes in disc tissue growth and development using histology and immunohistochemical methods in 3-month-old mice. We found that large vacuolated notochordal cells were accumulated in the nucleus pulposus (NP) in Runx2 KO mice. The growth plate cartilage tissue in the disc was thicker in Runx2 KO mice. We also found a significant upregulation of Indian hedgehog (Ihh) expression in the cells in NP cells and in annulus fibrosus cells of Runx2 KO mice. These results demonstrated that Runx2 may play an important role in postnatal disc tissue development through interacting with Ihh signaling.

DNA methylation of the RUNX2 P1 promoter mediates MMP13 transcription in chondrocytes

The Runt-related transcription factor 2 (RUNX2) is critical for bone formation as well as chondrocyte maturation. Matrix metalloproteinase (MMP)-13 is a major contributor to cartilage degradation in osteoarthritis (OA). We and others have shown that the abnormal MMP13 gene expression in OA chondrocytes is controlled by changes in the DNA methylation status of specific CpG sites of the proximal promoter, as well as by the actions of different transactivators, including RUNX2. The present study aimed to determine the influence of the methylation status of specific CpG sites in the RUNX2 promoter on RUNX2-driven MMP13 gene expression in OA chondrocytes. We observed a significant correlation between MMP13 mRNA levels and RUNX2 gene expression in human OA chondrocytes. RUNX2 overexpression enhanced MMP13 promoter activity, independent of the MMP13 promoter methylation status. A significant negative correlation was observed between RUNX2 mRNA levels in OA chondrocytes and the percentage methylation of the CpG sites in the RUNX2 P1 promoter. Accordingly, the activity of the wild type RUNX2 promoter was decreased upon methylation treatment in vitro. We conclude that RUNX2 gene transcription is regulated by the methylation status of specific CpG sites in the promoter and may determine RUNX2 availability in OA cartilage for transactivation of genes such as MMP13.

13cRA regulates the differentiation of antler chondrocytes through targeting Runx3

Although 13cRA is involved in the regulation of cellular proliferation and differentiation, its physiological roles in chondrocyte proliferation and differentiation still remain unknown. Here, we showed that 13cRA could induce the proliferation of sika deer antler chondrocytes and expression of Ccnd3 and Cdk6. Administration of 13cRA to antler chondrocytes resulted in an obvious increase in the expression of chondrocyte marker Col II and hypertrophic chondrocyte marker Col X. Silencing of Crabp2 expression by specific siRNA could prevent the 13cRA-induced up-regulation of Col X, whereas overexpression of Crabp2 showed the opposite effects. Further study found that Crabp2 mediated the regulation of 13cRA on the expression of Runx3 which was highly expressed in the antler cartilage and inhibited the differentiation of antler chondrocytes. Moreover, attenuation of Runx3 expression greatly raised 13cRA-induced chondrocyte differentiation. Simultaneously, 13cRA could stimulate the expression of Cyp26a1 and Cyp26b1 in the antler chondrocytes. Inhibition of Cyp26a1 and/or Cyp26b1 reinforced the effects of 13cRA on the expression of Col X and Runx3, while overexpression of Cyp26b1 rendered the antler chondrocytes hyposensitive to 13cRA. Collectively, 13cRA may play an important role in the differentiation of antler chondrocytes through targeting Runx3. Crabp2 enhances the effects of 13cRA on chondrocyte differentiation, while Cyp26a1 and Cyp26b1 weaken the sensitivity of antler chondrocytes to 13cRA.

Sequencing and bioinformatics analysis of the differentially expressed genes in herniated discs with or without calcification

The purpose of this study was to detect the differentially expressed genes between ossified herniated discs and herniated discs without ossification. In addition, we sought to identify a few candidate genes and pathways by using bioinformatics analysis. We analyzed 6 samples each of ossified herniated discs (experimental group) and herniated discs without ossification (control group). Purified mRNA and cDNA extracted from the samples were subjected to sequencing. The NOISeq method was used to statistically identify the differentially expressed genes (DEGs) between the 2 groups. An in-depth analysis using bioinformatics tools based on the DEGs was performed using Gene Ontology (GO) enrichment, Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway enrichment, and protein-protein interaction network analysis. The top 6 DEGs were verified using reverse transcription-quantitative polymerase chain reaction (RT-qPCR). A total of 132 DEGs was detected. A total of 129 genes in the ossified group were upregulated and 3 genes were found to be downregulated as compared to the control group. The top 3 cellular components in GO ontologies analysis were extracellular matrix components. GO functions were mainly related to the glycoprotein in the cell membrane and extracellular matrix. The GO process was related to completing response to stimulus, immune reflex and defense. The top 5 KEGG enrichment pathways were associated with infection and inflammation. Three of the top 20 DEGs [sclerostin (SOST), WNT inhibitory factor 1 (WIF1) and secreted frizzled related protein 4 (SFRP4)] were related to the inhibition of the Wnt pathway. The ossified discs exhibited a higher expression of the top 6 DEGs [SOST, joining chain of multimeric IgA and IgM (IGJ; also known as JCHAIN), defensin alpha 4 (DEFA4), SFRP4, proteinase 3 (PRTN3) and cathepsin G (CTSG)], with the associated P-values of 0.045, 0.000, 0.008, 0.010, 0.015 and 0.002, respectively, as calculated by the independent sample t-test. The gene expression profiling of the 2 groups revealed differential gene expression. Thus, our data suggest that Wnt pathway abnormality and local inflammation may be related to disc ossification.

Pamidronate Down-regulates Tumor Necrosis Factor-alpha Induced Matrix Metalloproteinases Expression in Human Intervertebral Disc Cells

Background

N-containing bisphosphonates (BPs), such as pamidronate and risedronate, can inhibit osteoclastic function and reduce osteoclast number by inducing apoptotic cell death in osteoclasts. The aim of this study is to demonstrate the effect of pamidronate, second generation nitrogen-containing BPs and to elucidate matrix metallo-proteinases (MMPs) mRNA expression under serum starvation and/or tumor necrosis factor alpha (TNF-α) stimulation on metabolism of intervertebral disc (IVD) cells in vitro.

Methods

Firstly, to test the effect of pamidronate on IVD cells in vitro, various concentrations (10^-12, 10^-10, 10^-8, and 10^-6 M) of pamidronate were administered to IVD cells. Then DNA and proteoglycan synthesis were measured and messenger RNA (mRNA) expressions of type I collagen, type II collagen, and aggrecan were analyzed. Secondly, to elucidate the expression of MMPs mRNA in human IVD cells under the lower serum status, IVD cells were cultivated in full serum or 1% serum. Thirdly, to elucidate the expression of MMPs mRNA in IVD cells under the stimulation of 1% serum and TNF-α (10 ng/mL) In this study, IVD cells were cultivated in three dimensional alginate bead.

Results

Under the lower serum culture, IVD cells in alginate beads showed upregulation of MMP 2, 3, 9, 13 mRNA. The cells in lower serum and TNF-α also demonstrated upregulation of MMP-2, 3, 9, and 13 mRNA. The cells with various doses of pamidronate and lower serum and TNF-α were reveled partial down-regulation of MMPs.

Conclusions

Pamidronate, N-containing second generation BPs, was safe in metabolism of IVD in vitro maintaining chondrogenic phenotype and matrix synthesis, and down-regulated TNF-α induced MMPs expression.

Differences in calcification and osteogenic potential of herniated discs according to the severity of degeneration based on Pfirrmann grade: a cross-sectional study

BACKGROUND

Herniated discs may exhibit calcification, and calcified discs may complicate surgical treatment. However, the osteogenic potential and expression of osteogenic markers in degenerative discs of different degenerative grades are still unclear. Our purposes are to study the differences in calcification rate and osteogenic potential of herniated discs according to different degenerative grades.

METHODS

Fifty-eight lumbar intervertebral discs were removed from 41 patients. After grading according to the Pfirrmann scale, calcification was analyzed by micro computed tomography (μ-CT), and expression of osteogenic markers was analyzed by immunohistochemistry and real-time quantitative polymerase chain reaction (qPCR). Data from μ-CT scans were compared with the Kruskal-Wallis test. The Mann-Whitney test was applied to compare data between any two groups. Differences in osteogenic mRNA expression in different regions of the removed discs (posterior vs. anterior) were analyzed by paired t tests. Differences in the posterior portion of removed discs of different Pfirrmann grades were analyzed by one-way analysis of variance (ANOVA), and comparisons of data between discs of any two grades were completed with least significant difference (LSD) tests.

RESULTS

Significant differences in calcification according to μ-CT scanning were observed between discs of different degenerative grades. Nearly half of the discs of Pfirrmann grade V showed the highest degree of calcification compared to Pfirrman grade II discs. Bone morphogenetic protein (BMP)-2, Osterix, and Osteocalcin were detected histologically in discs of Pfirrmann grades III-V. Alkaline phosphatase (ALP) expression was observed in discs showing evidence of calcification. The qPCR analysis showed that BMP-2, Osterix, and Osteocalcin were expressed in most degenerated discs. We also observed greater expression of these osteogenic markers in the posterior portion of removed discs than in the anterior portion.

CONCLUSIONS

The osteogenic potential of degenerated intervertebral discs appears to increase with the severity of degeneration and to be greater in the tissue near the spinal canal than in tissue in the inner portion of the disc.

Runx1 Activities in Superficial Zone Chondrocytes, Osteoarthritic Chondrocyte Clones and Response to Mechanical Loading

Runx1, the hematopoietic lineage determining transcription factor, is present in perichondrium and chondrocytes. Here we addressed Runx1 functions, by examining expression in cartilage during mouse and human osteoarthritis (OA) progression and in response to mechanical loading. Spared and diseased compartments in knees of OA patients and in mice with surgical destabilization of the medial meniscus were examined for changes in expression of Runx1 mRNA (Q-PCR) and protein (immunoblot, immunohistochemistry). Runx1 levels were quantified in response to static mechanical compression of bovine articular cartilage. Runx1 function was assessed by cell proliferation (Ki67, PCNA) and cell type phenotypic markers. Runx1 is enriched in superficial zone (SZ) chondrocytes of normal bovine, mouse, and human tissues. Increasing loading conditions in bovine cartilage revealed a positive correlation with a significant elevation of Runx1. Runx1 becomes highly expressed at the periphery of mouse OA lesions and in human OA chondrocyte 'clones' where Runx1 co-localizes with Vcam1, the mesenchymal stem cell (MSC) marker and lubricin (Prg4), a cartilage chondroprotective protein. These OA induced cells represent a proliferative cell population, Runx1 depletion in MPCs decreases cell growth, supporting Runx1 contribution to cell expansion. The highest Runx1 levels in SZC of normal cartilage suggest a function that supports the unique phenotype of articular chondrocytes, reflected by upregulation under conditions of compression. We propose Runx1 co-expression with Vcam1 and lubricin in murine cell clusters and human 'clones' of OA cartilage, participate in a cooperative mechanism for a compensatory anabolic function.

Loss of osteoblast Runx3 produces severe congenital osteopenia

Congenital osteopenia is a bone demineralization condition that is associated with elevated fracture risk in human infants. Here we show that Runx3, like Runx2, is expressed in precommitted embryonic osteoblasts and that Runx3-deficient mice develop severe congenital osteopenia. Runx3-deficient osteoblast-specific (Runx3(fl/fl)/Col1α1-cre), but not chondrocyte-specific (Runx3(fl/fl)/Col1α2-cre), mice are osteopenic. This demonstrates that an osteoblastic cell-autonomous function of Runx3 is required for proper osteogenesis. Bone histomorphometry revealed that decreased osteoblast numbers and reduced mineral deposition capacity in Runx3-deficient mice cause this bone formation deficiency. Neonatal bone and cultured primary osteoblast analyses revealed a Runx3-deficiency-associated decrease in the number of active osteoblasts resulting from diminished proliferation and not from enhanced osteoblast apoptosis. These findings are supported by Runx3-null culture transcriptome analyses showing significant decreases in the levels of osteoblastic markers and increases in the levels of Notch signaling components. Thus, while Runx2 is mandatory for the osteoblastic lineage commitment, Runx3 is nonredundantly required for the proliferation of these precommitted cells, to generate adequate numbers of active osteoblasts. Human RUNX3 resides on chromosome 1p36, a region that is associated with osteoporosis. Therefore, RUNX3 might also be involved in human bone mineralization.

Abnormal Differentiation of Dental Pulp Cells in Cleidocranial Dysplasia

Cleidocranial dysplasia (CCD) is a skeletal dysplasia caused by heterozygous mutations of RUNX2, a gene that is essential for the mineralization of bone and tooth. We isolated primary dental pulp cells from a 10-y-old patient and tested their proliferative capacity, alkaline phosphatase activity, and ability to form mineralized nodules, in comparison with those from 7 healthy children. All these measures were reduced in primary dental pulp cells from the CCD patient. The expression of the osteoblast/odontoblast-associated genes RUNX2, ALP, OCN, and DSPP was also found to be significantly decreased in the primary dental pulp cells of the CCD patient. The osteoclast-related markers TRAP, CTSK, CTR, and MMP9 were decreased in primary dental pulp cells cocultured with human peripheral blood mononuclear cells. Moreover, the expression of RANKL and the ratio of RANKL/OPG were both reduced in the cells from the CCD patient, indicating that the RUNX2 mutation interfered with the bone-remodeling pathway and decreased the capacity of primary dental pulp cells to support osteoclast differentiation. These effects may be partly responsible for the defects in tooth development and the retention of primary teeth that is typical of CCD.

Epigenetic differences in human cartilage between mild and severe OA

The development of osteoarthritis (OA) depends on genetic and environmental factors, which influence the biology of the chondrocyte via epigenetic regulation. Changes within the epigenome might lead the way to discovery of new pathogenetic pathways. We performed a genome-wide methylation screening to identify potential differences between paired mild and severe osteoarthritic human cartilage. Sixteen female patients suffering from OA underwent total knee joint replacement. Cartilage specimens collected from corresponding macroscopically undamaged and from damaged areas were processed for DNA extraction and histology to evaluate the histological grading of the disease. Paired specimens were analysed for the methylation status of the whole genome using human promoter microarrays (Agilent, Santa Clara, CA). Selected target genes were then validated via methylation-specific qPCR. One thousand two hundred and fourteen genetic targets were identified differentially methylated between mild and severe OA. One thousand and seventy of these targets were found hypermethylated and 144 hypomethylated. The descriptive analysis of these genes by Gene Ontology (GO), KEGG pathway and protein domain analyses points to pathways of development and differentiation. We identified a list of genes which are differently methylated in mild and severe OA cartilage. Within the pathways of growth and development new therapeutic targets might arise by improving our understanding of pathogenetic mechanisms in OA.

Constitutive nuclear expression of dentin matrix protein 1 fails to rescue the Dmp1-null phenotype

Dentin matrix protein 1 (DMP1) plays multiple roles in bone, tooth, phosphate homeostasis, kidney, salivary gland, reproductive cycles, and the development of cancer. In vitro studies have indicated two different biological mechanisms: 1) as a matrix protein, DMP1 interacts with αvβ3 integrin and activates MAP kinase signaling; and 2) DMP1 serves as a transcription co-factor. In vivo studies have demonstrated its key role in osteocytes. This study attempted to determine whether DMP1 functions as a transcription co-factor and regulates osteoblast functions. For gene expression comparisons using adenovirus constructs, we targeted the expression of DMP1 either to the nucleus only by replacing the endogenous signal peptide with a nuclear localization signal (NLS) sequence (referred to as (NLS)DMP1) or to the extracellular matrix as the WT type (referred to as (SP)DMP1) in MC3T3 osteoblasts. High levels of DMP1 in either form greatly increased osteogenic gene expression in an identical manner. However, the targeted (NLS)DMP1 transgene driven by a 3.6-kb rat Col 1α1 promoter in the nucleus of osteoblasts and osteocytes failed to rescue the phenotyope of Dmp1-null mice, whereas the (SP)DMP1 transgene rescued the rickets defect. These studies support the notion that DMP1 functions as an extracellular matrix protein, rather than as a transcription co-factor in vivo. We also show that DMP1 continues its expression in osteoblasts during postnatal development and that the deletion of Dmp1 leads to an increase in osteoblast proliferation. However, poor mineralization in the metaphysis indicates a critical role for DMP1 in both osteoblasts and osteocytes.

Leptin induces terminal differentiation of rat annulus fibrosus cells via activation of MAPK signaling

Both leptin and its receptor are expressed in rat annulus fibrosus (AF) cells. However, little is known about their role and mechanism during disc degeneration. The mitogen activating protein kinase (MAPK) pathway which mediates leptin-induced terminal differentiation of rat AF cells was analyzed using PCR, Western-blot and immunocytochemistry. It was found that leptin-induced AF cells terminal differentiation, which may be attributed to upregulated p38 and ERK1/2 phosphorylation, however, JNK phosphorylation was not observed. Specific inhibitors of p38 or ERK1/2, but not JNK, could inhibit the stimulative activity of leptin on collagen X and MMP-13 protein levels. This study, for the first time, shows that the MAPK pathway, especially p38 and ERK1/2 signaling, plays a distinct role in leptin-induced AF cells terminal differentiation.

Understanding developmental mechanisms in the context of osteoarthritis

Osteoarthritis (OA) is a joint disease that is highly related to aging. However, as OA development is the consequence of interplay between external stimuli, such as mechanical loading and the structure and physiology of the joint, it can be anticipated that variation in developmental processes early in life will affect OA development later in life. Genes involved in patterning processes, such as the Hox genes, but also genes that encode transcription factors, growth factors and cytokines and their respective receptors and those that encode molecules involved in formation of the extracellular matrix, will influence embryonic skeletal development and OA incidence and severity in the adult. The function of genes involved in patterning processes can be partly be understood by close analysis of inborn diseases that result in musculoskeletal syndromes, but a deeper understanding will be the result of specific gene knockouts or overexpression in transgenic mouse models.

Cartilage to bone transitions in health and disease

Aberrant redeployment of the 'transient' events responsible for bone development and postnatal longitudinal growth has been reported in some diseases in what is otherwise inherently 'stable' cartilage. Lessons may be learnt from the molecular mechanisms underpinning transient chondrocyte differentiation and function, and their application may better identify disease aetiology. Here, we review the current evidence supporting this possibility. We firstly outline endochondral ossification and the cellular and physiological mechanisms by which it is controlled in the postnatal growth plate. We then compare the biology of these transient cartilaginous structures to the inherently stable articular cartilage. Finally, we highlight specific scenarios in which the redeployment of these embryonic processes may contribute to disease development, with the foresight that deciphering those mechanisms regulating pathological changes and loss of cartilage stability will aid future research into effective disease-modifying therapies.

Evaluation of the association between runt-related transcription factor 2 expression and intervertebral disk aging in dogs

OBJECTIVE

To investigate the relationship between runt-related transcription factor 2 (RUNX2) expression in canine nucleus pulposus (NP) cells and intervertebral disk aging in chondrodystrophoid dogs.

ANIMALS

7 healthy Beagles (mean age, 35.6 months) and 11 Dachshunds with herniated disks (mean age, 61 months).

PROCEDURES

All dogs underwent MRI examination of the thoracic and lumbar vertebral column immediately before sample collection under general anesthesia. The disk center-to-CSF T2-weighted signal intensity ratio was determined for healthy Beagles. Samples of NP were obtained from nonherniated disks in healthy Beagles and from herniated disks during surgical treatment of hospitalized Dachshunds. Samples were evaluated for RUNX2 and matrix metalloproteinase 13 transcript expression via reverse transcriptase PCR assay; RUNX2 protein expression was evaluated via immunohistochemical analysis, and correlation between these variables and age of dogs was evaluated. A 3' and 5' rapid amplification of cDNA ends method was used to identify the RUNX2 coding region.

RESULTS

RUNX2 cDNA had > 97% conservation with the human cDNA sequence and approximately 95% conservation with the mouse cDNA sequence; RUNX2 and matrix metalloproteinase 13 mRNA expression and RUNX2 protein expression in NP cells were positively correlated with age. The disk center-to-CSF T2-weighted signal intensity ratio was negatively correlated with RUNX2 protein expression in the NP of healthy dogs.

CONCLUSIONS AND CLINICAL RELEVANCE

Results indicated that RUNX2 mRNA and protein expression in the NP are enhanced in aging intervertebral disks in dogs.

Runx2 haploinsufficiency ameliorates the development of ossification of the posterior longitudinal ligament

Ossification of the Posterior Longitudinal Ligament (OPLL) is a disease that is characterized by the ectopic calcification of the ligament; however, the pathogenesis of OPLL remains to be investigated. We attempted to identify the in vivo role of Runx2, a master regulator of osteoblast differentiation and skeletal mineralization, in the pathogenesis of OPLL. The expression of Runx2 in the ligament was examined using in situ hybridization and immunohistochemistry and by monitoring the activity of a LacZ gene that was inserted into the Runx2 gene locus. To investigate the functional role of Runx2, we studied ENPP1^ttw/ttw mice, a mouse model of OPLL, that were crossed with heterozygous Runx2 mice to decrease the expression of Runx2, and we performed histological and quantitative radiological analyses using 3D-micro CT. Runx2 was expressed in the ligament of wild-type mice. The induction of Runx2 expression preceded the development of ectopic calcification in the OPLL-like region of the ENPP1^ttw/ttw mice. Runx2 haploinsufficiency ameliorated the development of ectopic calcification in the ENPP1^ttw/ttw mice. Collectively, this study demonstrated that Runx2 is expressed in an OPLL-like region, and its elevation is a prerequisite for developing the complete OPLL-like phenotype in a mouse model of OPLL.

Articular cartilage development: a molecular perspective

In this article, development of articular cartilage and endochondral ossification is reviewed, from the perspective of both morphologic aspects of histogenesis and molecular biology, particularly with respect to key signaling molecules and extracellular matrix components most active in cartilage development. The current understanding of the roles of transforming growth factor β and associated signaling molecules, bone morphogenic proteins, and molecules of the Wnt-β catenin system in chondrogenesis are described. Articular cartilage development is a highly conserved complex biological process that is dynamic and robust in nature, which proceeds well without incident or failure in all joints of most young growing individuals.

Cutting edge issues in polymyositis

Skeletal muscle is the target tissue of immunoflogistic processes in patients affected with idiopathic inflammatory myopathies (IIM). IIM are classified into three major forms: polymyositis (PM), dermatomyositis (DM), and inclusion body myositis. Recent data suggest that, in the major subsets of myositis, antigens in muscles drive a B-cell antigen-specific immune response. Moreover, some non-immunological mechanisms have been advocated. In this regard, an increased expression of Jo-1 and Mi-2 in muscle biopsies from PM and DM patients compared to normal muscle has been demonstrated; these candidate autoantigens in myositis are expressed at high levels in regenerating muscle cells rather than in mature myotubes. Myositis autoantigen upregulation has also been observed in neoplastic tissues, thus representing a potential link between cancer and autoimmunity in myositis. Myositis-specific autoantibodies (MSA) are disease markers and target intracellular proteins involved in key processes such as translocation and nuclear transcription. Myositis target antigens encompass aminoacyl-tRNA synthetases, the Mi-2 helicase/histone deacetylase protein complex, the signal recognition particle ribonucleoprotein, together with novel target antigens including p155/140, CADM-140, and SAE. Despite their high specificity for autoimmune myositis, MSA target non-muscle restricted proteins ubiquitary to all cell types, making the specific muscle involvement difficult to explain. Non-immunological mechanisms also seem to contribute to the pathogenesis of IIM; activation of endoplasmic reticulum stress response due to muscle regeneration and inflammation but independent to MHC-1 up-regulation has been recently reported in patients with myositis.

NELL-1 promotes cartilage regeneration in an in vivo rabbit model

Repair of cartilage due to joint trauma remains challenging due to the poor healing capacity of cartilage and adverse effects related to current growth factor-based strategies. NELL-1 (Nel-like molecule-1; Nel [a protein strongly expressed in neural tissue encoding epidermal growth factor like domain]), a protein first characterized in the context of premature cranial suture fusion, is believed to accelerate differentiation along the osteochondral lineage. We previously demonstrated the ability of NELL-1 protein to maintain the cartilaginous phenotype of explanted rabbit chondrocytes in vitro. Our objective in the current study is to determine whether NELL-1 can affect endogenous chondrocytes in an in vivo cartilage defect model. To generate the implant, NELL-1 was incorporated into chitosan nanoparticles and embedded into alginate hydrogels. These implants were press fit into 3-mm circular osteochondral defects created in the femoral condylar cartilage of 3-month-old New Zealand White rabbits (n=10). Controls included unfilled defects (n=8) and defects filled with phosphate-buffered saline-loaded chitosan nanoparticles embedded in alginate hydrogels (n=8). Rabbits were sacrificed 3 months postimplantation for histological analysis. Defects filled with alginate containing NELL-1 demonstrated significantly improved cartilage regeneration. Remarkably, histology of NELL-1-treated defects closely resembled that of native cartilage, including stronger Alcian blue and Safranin-O staining and increased deposition of type II collagen and absence of the bone markers type I collagen and Runt-related transcription factor 2 (Runx2) as demonstrated by immunohistochemistry. Our results suggest that NELL-1 may produce functional cartilage with properties similar to native cartilage, and is an exciting candidate for tissue engineering-based approaches for treating diverse pathologies of cartilage defects and degeneration.

Calcium/calmodulin-dependent protein kinase II activity regulates the proliferative potential of growth plate chondrocytes

For tissues that develop throughout embryogenesis and into postnatal life, the generation of differentiated cells to promote tissue growth is at odds with the requirement to maintain the stem cell/progenitor cell population to preserve future growth potential. In the growth plate cartilage, this balance is achieved in part by establishing a proliferative phase that amplifies the number of progenitor cells prior to terminal differentiation into hypertrophic chondrocytes. Here, we show that endogenous calcium/calmodulin-dependent protein kinase II (CamkII, also known as Camk2) activity is upregulated prior to hypertrophy and that loss of CamkII function substantially blocks the transition from proliferation to hypertrophy. Wnt signaling and Pthrp-induced phosphatase activity negatively regulate CamkII activity. Release of this repression results in activation of multiple effector pathways, including Runx2- and β-catenin-dependent pathways. We present an integrated model for the regulation of proliferation potential by CamkII activity that has important implications for studies of growth control and adult progenitor/stem cell populations.

Hypertrophic differentiation and calcification during intervertebral disc degeneration

BACKGROUND

In degenerative intervertebral discs (IVDs) collagen type X expression and calcifications have been demonstrated, resembling advanced osteoarthritis (OA), which is associated with hypertrophic differentiation, characterized by the production of collagen type X, Runt-related transcription factor 2 (Runx2), osteoprotegerin (OPG), alkaline phosphatase (ALP) and calcifications.

OBJECTIVE

The aim of this study was to determine if hypertrophic differentiation occurs during IVD degeneration.

METHODS

IVDs from all Thompson degeneration grades were prepared for histology, extraction of nucleus pulposus (NP) and annulus fibrosis (AF) tissue (N=50) and micro-CT (N=27). The presence of collagen type X, OPG and Runx2 was determined by immunohistochemistry, with OPG levels also determined by Enzyme-linked immunosorbent assay (ELISA). The presence of calcification was determined by micro-CT, von Kossa and Alizarin Red staining.

RESULTS

Immunohistochemical staining for collagen type X, OPG, Runx2 appeared more intense in the NP of degenerative compared to healthy IVD samples. OPG levels correlated significantly with degeneration grade (NP: P<0.000; AF: P=0.002) and the number of microscopic calcifications (NP: P=0.002; AF: P=0.008). The extent of calcifications on micro-CT also correlated with degeneration grade (NP: P<0.001, AF: P=0.001) as did von Kossa staining (NP: P=0.015, AF: P=0.016). ALP staining was only incidentally seen in the transition zone of grades IV and V degenerated IVDs.

CONCLUSION

This study for the first time demonstrates that hypertrophic differentiation occurs during IVD degeneration, as shown by an increase in OPG levels, the presence of ALP activity, increased immunopositivity of Runx2 and collagen type X.

Runx1 and Runx2 cooperate during sternal morphogenesis

Chondrocyte differentiation is strictly regulated by various transcription factors, including Runx2 and Runx3; however, the physiological role of Runx1 in chondrocyte differentiation remains unknown. To examine the role of Runx1, we generated mesenchymal-cell-specific and chondrocyte-specific Runx1-deficient mice [Prx1 Runx1(f/f) mice and alpha1(II) Runx1(f/f) mice, respectively] to circumvent the embryonic lethality of Runx1-deficient mice. We then mated these mice with Runx2 mutant mice to obtain mesenchymal-cell-specific or chondrocyte-specific Runx1; Runx2 double-mutant mice [Prx1 DKO mice and alpha1(II) DKO mice, respectively]. Prx1 Runx1(f/f) mice displayed a delay in sternal development and Prx1 DKO mice completely lacked a sternum. By contrast, alpha1(II) Runx1(f/f) mice and alpha1(II) DKO mice did not show any abnormal sternal morphogenesis or chondrocyte differentiation. Notably, Runx1, Runx2 and the Prx1-Cre transgene were co-expressed specifically in the sternum, which explains the observation that the abnormalities were limited to the sternum. Histologically, mesenchymal cells condensed normally in the prospective sternum of Prx1 DKO mice; however, commitment to the chondrocyte lineage, which follows mesenchymal condensation, was significantly impaired. In situ hybridization analyses demonstrated that the expression of alpha1(II) collagen (Col2a1 - Mouse Genome Informatics), Sox5 and Sox6 in the prospective sternum of Prx1 DKO mice was severely attenuated, whereas Sox9 expression was unchanged. Molecular analyses revealed that Runx1 and Runx2 induce the expression of Sox5 and Sox6, which leads to the induction of alpha1(II) collagen expression via the direct regulation of promoter activity. Collectively, these results show that Runx1 and Runx2 cooperatively regulate sternal morphogenesis and the commitment of mesenchymal cells to become chondrocytes through the induction of Sox5 and Sox6.

The polycystic kidney disease 1 (Pkd1) gene is required for the responses of osteochondroprogenitor cells to midpalatal suture expansion in mice

Mechanical stress is known to modulate postnatal skeletal growth and development. However, the mechanisms underlying the mechanotransduction are not fully understood. Polycystin-1 (PC1) is a promising candidate among proteins that may play a role in the process as it has been shown to function as a flow sensor in renal epithelium and it is known to be important for skeletal development. To investigate whether PC1 is involved in mechanotransduction in skeletal tissues, mice with a conditional deficiency for PC1 in neural crest cells, osteoblasts or chondrocytes were subjected to midpalatal suture expansion. Dynamic bone labeling revealed that new bone formation in response to expansion was significantly reduced in Wnt1Cre;Pkd1 mice, as the suture area containing new bone was 14.0+/-3.4% in mutant mice versus 65.0+/-3.8% in control mice at 2 weeks (p<0.001). In contrast, stress-induced new bone formation was not affected in OsxCre;Pkd1 mice. The increase in cell proliferation and differentiation into osteoblasts, seen in wild-type mice 1 day after force delivery, was not observed until 14 days in Wnt1Cre;Pkd1 mice. TUNEL labeling showed a significant increase in apoptotic suture cells at days 1 and 3 (from 7.0+/-0.5% to 13.5+/-1.4% at day 1 and from 4.6+/-1.1% to 10.5+/-1.7% at day 3, p<0.05). Abnormal ossification of nasal cartilage of Wnt1Cre;Pkd1 mice was accelerated upon suture expansion. Such ossification was also observed, but to a lesser extent in Col2a1-ERCre;Pkd1 mice. Transcript levels of Runx2 and MMP13 were significantly increased in the nasal cartilage of Wnt1Cre;Pkd1 mice compared to controls (p<0.05 and p<0.001, respectively), and in mutant mice with expansion versus without expansion (p<0.05 and p<0.001, respectively). Lack of PC1 in chondroprogenitor cells also resulted in increased cell apoptosis and an altered arrangement of chondrocytes in nasal cartilage. These results indicate that PC1 plays a critical role in the response of osteochondroprogenitor cells to the mechanical tissue stress induced by midpalatal suture expansion. They also suggest that the combination of an in vivo mechanical model, such as midpalatal suture expansion, with conditional deficiency for proteins that play a role in mechanotransduction, represents a powerful experimental strategy to explore underlying mechanisms.