PTHrP regulates growth plate chondrocyte differentiation and proliferation in a Gli3 dependent manner utilizing hedgehog ligand dependent and independent mechanisms

Deletion of Fibroblast growth factor 9 globally and in skeletal muscle results in enlarged tuberosities at sites of deltoid tendon attachments

BACKGROUND

The growth of most bony tuberosities, like the deltoid tuberosity (DT), rely on the transmission of muscle forces at the tendon-bone attachment during skeletal growth. Tuberosities distribute muscle forces and provide mechanical leverage at attachment sites for joint stability and mobility. The genetic factors that regulate tuberosity growth remain largely unknown. In mouse embryos with global deletion of fibroblast growth factor 9 (Fgf9), the DT size is notably enlarged. In this study, we explored the tissue-specific regulation of DT size using both global and targeted deletion of Fgf9.

RESULTS

We showed that cell hypertrophy and mineralization dynamics of the DT, as well as transcriptional signatures from skeletal muscle but not bone, were influenced by the global loss of Fgf9. Loss of Fgf9 during embryonic growth led to increased chondrocyte hypertrophy and reduced cell proliferation at the DT attachment site. This endured hypertrophy and limited proliferation may explain the abnormal mineralization patterns and locally dysregulated expression of markers of endochondral development in Fgf9^null attachments. We then showed that targeted deletion of Fgf9 in skeletal muscle leads to postnatal enlargement of the DT.

CONCLUSION

Taken together, we discovered that Fgf9 may play an influential role in muscle-bone cross-talk during embryonic and postnatal development.

Deer antler extract potentially facilitates xiphoid cartilage growth and regeneration and prevents inflammatory susceptibility by regulating multiple functional genes

BACKGROUND

Deer antler is a zoological exception due to its fantastic characteristics, including amazing growth rate and repeatable regeneration. Deer antler has been used as a key ingredient in traditional Chinese medicine relating to kidney and bone health for centuries. The aim of this study was to dissect the molecular regulation of deer antler extract (DAE) on xiphoid cartilage (XC).

METHODS

The DAE used in this experiment was same as the one that was prepared as previously described. The specific pathogen-free (SPF) grade Sprague-Dawley (SD) rats were randomly divided into blank group (n =10) and DAE group (n =10) after 1-week adaptive feeding. The DAE used in this experiment was same as the one that was prepared as previously described. The rats in DAE group were fed with DAE for 3 weeks at a dose of 0.2 g/kg per day according to the body surface area normalization method, and the rats in blank group were fed with drinking water. Total RNA was extracted from XC located in the most distal edge of the sternum. Illumina RNA sequencing (RNA-seq) in combination with quantitative real-time polymerase chain reaction (qRT-PCR) validation assay was carried out to dissect the molecular regulation of DAE on XC.

RESULTS

We demonstrated that DAE significantly increased the expression levels of DEGs involved in cartilage growth and regeneration, but decreased the expression levels of DEGs involved in inflammation, and mildly increased the expression levels of DEGs involved in chondrogenesis and chondrocyte proliferation.

CONCLUSIONS

Our findings suggest that DAE might serve as a complementary therapeutic regent for cartilage growth and regeneration to treat cartilage degenerative disease, such as osteoarthritis.

Dmrt2 promotes transition of endochondral bone formation by linking Sox9 and Runx2

Endochondral bone formation is fundamental for skeletal development. During this process, chondrocytes undergo multiple steps of differentiation and coordinated transition from a proliferating to a hypertrophic stage, which is critical to advance skeletal development. Here, we identified the transcription factor Dmrt2 (double-sex and mab-3 related transcription factor 2) as a Sox9-inducible gene that promotes chondrocyte hypertrophy in pre-hypertrophic chondrocytes. Epigenetic analysis further demonstrated that Sox9 regulates Dmrt2 expression through an active enhancer located 18 kb upstream of the Dmrt2 gene and that this enhancer's chromatin status is progressively activated through chondrocyte differentiation. Dmrt2-knockout mice exhibited a dwarf phenotype with delayed initiation of chondrocyte hypertrophy. Dmrt2 augmented hypertrophic chondrocyte gene expression including Ihh through physical and functional interaction with Runx2. Furthermore, Dmrt2 deficiency reduced Runx2-dependent Ihh expression. Our findings suggest that Dmrt2 is critical for sequential chondrocyte differentiation during endochondral bone formation and coordinates the transcriptional network between Sox9 and Runx2.

Enchondromatosis and Growth Plate Development

PURPOSE OF REVIEW

Enchondroma is a common cartilage benign tumor that develops from dysregulation of chondrocyte terminal differentiation during growth plate development. Here we provide an overview of recent progress in understanding causative mutations for enchondroma, dysregulated signaling and metabolic pathways in enchondroma, and the progression from enchondroma to malignant chondrosarcoma.

RECENT FINDINGS

Several signaling pathways that regulate chondrocyte differentiation are dysregulated in enchondromas. Somatic mutations in the metabolic enzymes isocitrate dehydrogenase 1 and 2 (IDH1/2) are the most common findings in enchondromas. Mechanisms including metabolic regulation, epigenetic regulation, and altered signaling pathways play a role in enchondroma formation and progression. Multiple pathways regulate growth plate development in a coordinated manner. Deregulation of the process can result in chondrocytes failing to undergo differentiation and the development of enchondroma.

Murine Limb Explant Cultures to Assess Cartilage Development

To investigate chondrocyte biology in an organized structure, limb explant cultures have been established that allow for the cultivation of the entire cartilaginous skeletal elements. In these organ cultures, the arrangement of chondrocytes in the cartilage elements and their interaction with the surrounding perichondrium and joint tissue are maintained. Chondrocyte proliferation and differentiation can thus be studied under nearly in vivo conditions. Growth factors and other soluble agents can be administered to the explants and their effect on limb morphogenesis, gene expression and cell-matrix interactions can be studied. Cotreatment with distinct growth factors and their inhibitors as well as the use of transgenic mice will allow one to decipher the epistatic relationship between different signaling systems and other regulators of chondrocyte differentiation. Here we describe the protocol to culture cartilage explants ex vivo and discuss the advantages and disadvantages of the culture system.

Transcriptional, epigenetic and microRNA regulation of growth plate

Endochondral ossification is a critical event in bone formation, particularly in long shaft bones. Many cellular differentiation processes work in concert to facilitate the generation of cartilage primordium to formation of trabecular structures, all of which occur within the growth plate. Previous studies have revealed that the growth plate is tightly regulated by various transcription factors, epigenetic systems, and microRNAs. Hence, understanding these mechanisms that regulate the growth plate is crucial to furthering the current understanding on skeletal diseases, and in formulating effective treatment strategies. In this review, we focus on describing the function and mechanisms of the transcription factors, epigenetic systems, and microRNAs known to regulate the growth plate.

Effect of parathyroid hormone-related protein on odontogenic differentiation in human dental pulp cells

BACKGROUND

Parathyroid hormone-related protein (PTHrP) plays an important role in many physiological processes, including bone regeneration. The function of PTHrP is similar to PTH. It promotes osteogenic differentiation in MC3T3-E1 cells. The aim of this study was to investigate whether PTHrP might have odontogenic differentiation ability in human dental pulp cells (hDPCs).

METHODS

The viability of hDPCs after stimulation with PTHrP was measured. Real-time polymerase chain reaction and Western blot analysis were performed to evaluate the expression levels of odontogenic markers and activation of protein kinase B (PKB/AKT), extracellular signal-regulated kinase (ERK), c-Jun N-terminal kinase (JNK), and p38 mitogen-activated protein kinase (MAPK). To evaluate mineralized nodule formation, alkaline phosphatase (ALP) staining and alizarin red S staining were performed.

RESULTS

PTHrP promoted odontogenic differentiation as evidenced by the formation of mineralized nodules, the induction of ALP activity, and the upregulation of odontogenic markers (dentin sialophosphoprotein and dentin matrix protein-1). The phosphorylation of AKT, ERK, JNK, and p38 was increased by PTHrP. However, an AKT inhibitor (LY294002), an ERK inhibitor (U0126), a JNK inhibitor (SP600125), and a p38 inhibitor (SB203580) inhibited the increase of mineralization induced by PTHrP.

CONCLUSION

The present study revealed that PTHrP could promote odontogenic differentiation and mineralization through activating the AKT, ERK, JNK, and p38 signaling pathways. These results provide novel insights into the odontogenic action of PTHrP.

Extracellular matrix turnover and inflammation in chemically-induced TMJ arthritis mouse models

The temporomandibular joint (TMJ) is a fibrocartilaginous tissue critical for chewing and speaking. In patients with temporomandibular disorders (TMDs), permanent tissue loss can occur. Recapitulating the complexity of TMDs in animal models is difficult, yet critical for the advent of new therapies. Synovial fluid from diseased human samples revealed elevated levels of tumor necrosis factor alpha (TNF-alpha). Here, we propose to recapitulate these findings in mice by subjecting murine TMJs with TNF-alpha or CFA (Complete Freund’s Adjuvant) in mandibular condyle explant cultures and by local delivery in vivo using TMJ intra-articular injections. Both TNF-alpha and CFA delivery to whole mandibular explants and in vivo increased extracellular matrix deposition and increased cartilage thickness, while TNF-alpha treated explants had increased expression of inflammatory cytokines and degradative enzymes. Moreover, the application of TNF-alpha or CFA in both models reduced cell number. CFA delivery in vivo caused soft tissue inflammation, including pannus formation. Our work provides two methods of chemically induced TMJ inflammatory arthritis through a condyle explant model and intra-articular injection model that replicate findings seen in synovial fluid of human patients, which can be used for further studies delineating the mechanisms underlying TMJ pathology.

Effect of Recombinant Human Parathyroid Hormone on Rotator Cuff Healing After Arthroscopic Repair

PURPOSE

To assess the effect of teriparatide, a recombinant human parathyroid hormone, on rotator cuff healing after arthroscopic repair compared with patients who were not treated with teriparatide.

METHODS

This was a prospective propensity-matched study. Thirty-one patients who underwent arthroscopic rotator cuff repair for tears >2 cm in size between January 2015 and June 2016 were recruited (group I). Daily subcutaneous injections of teriparatide 20 μg were administered for 3 months following surgery. In the same period, propensity score matching (1-to-4) was performed to generate an untreated control group undergoing rotator cuff repair alone (group II) with the same tear size. Magnetic resonance imaging evaluation of tendon healing was performed at least 1 year postoperatively, as well as the range of shoulder motion, American Shoulder and Elbow Surgeons score, Constant score, and simple shoulder test.

RESULTS

There was no significant difference of the retraction size, the anterior to posterior dimension of torn rotator cuff, or the preoperative bone mineral density in groups I and II (P = .78, .87, and .96, respectively). The rate of retear was significantly lower in group I than in group II (16% vs 33.9%; P = .04). Range of motion and functional scores were not significantly different between the 2 groups (P > .05).

CONCLUSIONS

Teriparatide, a recombinant human parathyroid hormone, can be a systemic treatment option that significantly enhances the tendon-to-bone healing after arthroscopic rotator cuff repair for patients with rotator cuff tears >2 cm.

LEVEL OF EVIDENCE

Level III, case-control study.

Synergistic co-regulation and competition by a SOX9-GLI-FOXA phasic transcriptional network coordinate chondrocyte differentiation transitions

The growth plate mediates bone growth where SOX9 and GLI factors control chondrocyte proliferation, differentiation and entry into hypertrophy. FOXA factors regulate hypertrophic chondrocyte maturation. How these factors integrate into a Gene Regulatory Network (GRN) controlling these differentiation transitions is incompletely understood. We adopted a genome-wide whole tissue approach to establish a Growth Plate Differential Gene Expression Library (GP-DGEL) for fractionated proliferating, pre-hypertrophic, early and late hypertrophic chondrocytes, as an overarching resource for discovery of pathways and disease candidates. De novo motif discovery revealed the enrichment of SOX9 and GLI binding sites in the genes preferentially expressed in proliferating and prehypertrophic chondrocytes, suggesting the potential cooperation between SOX9 and GLI proteins. We integrated the analyses of the transcriptome, SOX9, GLI1 and GLI3 ChIP-seq datasets, with functional validation by transactivation assays and mouse mutants. We identified new SOX9 targets and showed SOX9-GLI directly and cooperatively regulate many genes such as Trps1, Sox9, Sox5, Sox6, Col2a1, Ptch1, Gli1 and Gli2. Further, FOXA2 competes with SOX9 for the transactivation of target genes. The data support a model of SOX9-GLI-FOXA phasic GRN in chondrocyte development. Together, SOX9-GLI auto-regulate and cooperate to activate and repress genes in proliferating chondrocytes. Upon hypertrophy, FOXA competes with SOX9, and control toward terminal differentiation passes to FOXA, RUNX, AP1 and MEF2 factors.

In the development of the mammalian growth plate, while several transcription factors are individually well known for their key roles in regulating phases of chondrocyte differentiation, there is little information on how they interact and cooperate with each other. We took an unbiased genome wide approach to identify the transcription factors and signaling pathways that play dominant roles in the chondrocyte differentiation cascade. We developed a searchable library of differentially expressed genes, GP-DGEL, which has fine spatial resolution and global transcriptomic coverage for discovery of processes, pathways and disease candidates. Our work identifies a novel regulatory mechanism that integrates the action of three transcription factors, SOX9, GLI and FOXA. SOX9-GLI auto-regulate and cooperate to activate and repress genes in proliferating chondrocytes. Upon entry into prehypertrophy, FOXA competes with SOX9, and control of hypertrophy passes to FOXA, RUNX, AP1 and MEF2 factors.

A Tunable, Three-Dimensional In Vitro Culture Model of Growth Plate Cartilage Using Alginate Hydrogel Scaffolds

Defining the final size and geometry of engineered tissues through precise control of the scalar and vector components of tissue growth is a necessary benchmark for regenerative medicine, but it has proved to be a significant challenge for tissue engineers. The growth plate cartilage that promotes elongation of the long bones is a good model system for studying morphogenetic mechanisms because cartilage is composed of a single cell type, the chondrocyte; chondrocytes are readily maintained in culture; and growth trajectory is predominately in a single vector. In this cartilage, growth is generated via a differentiation program that is spatially and temporally regulated by an interconnected network composed of long- and short-range signaling mechanisms that together result in the formation of functionally distinct cellular zones. To facilitate investigation of the mechanisms underlying anisotropic growth, we developed an in vitro model of the growth plate cartilage by using neonatal mouse growth plate chondrocytes encapsulated in alginate hydrogel beads. In bead cultures, encapsulated chondrocytes showed high viability, cartilage matrix deposition, low levels of chondrocyte hypertrophy, and a progressive increase in cell proliferation over 7 days in culture. Exogenous factors were used to test functionality of the parathyroid-related protein-Indian hedgehog (PTHrP-IHH) signaling interaction, which is a crucial feedback loop for regulation of growth. Consistent with in vivo observations, exogenous PTHrP stimulated cell proliferation and inhibited hypertrophy, whereas IHH signaling stimulated chondrocyte hypertrophy. Importantly, the treatment of alginate bead cultures with IHH or thyroxine resulted in formation of a discrete domain of hypertrophic cells that mimics tissue architecture of native growth plate cartilage. Together, these studies are the first demonstration of a tunable in vitro system to model the signaling network interactions that are required to induce zonal architecture in growth plate chondrocytes, which could also potentially be used to grow cartilage cultures of specific geometries to meet personalized patient needs.

Spop promotes skeletal development and homeostasis by positively regulating Ihh signaling

Indian Hedgehog (Ihh) regulates chondrocyte and osteoblast differentiation through the Glioma-associated oncogene homolog (Gli) transcription factors. Previous in vitro studies suggested that Speckle-type POZ protein (Spop), part of the Cullin-3 (Cul3) ubiquitin ligase complex, targets Gli2 and Gli3 for degradation and negatively regulates Hedgehog (Hh) signaling. In this study, we found defects in chondrocyte and osteoblast differentiation in Spop-null mutant mice. Strikingly, both the full-length and repressor forms of Gli3, but not Gli2, were up-regulated in Spop mutants, and Ihh target genes Patched 1 (Ptch1) and parathyroid hormone-like peptide (Pthlh) were down-regulated, indicating compromised Hh signaling. Consistent with this finding, reducing Gli3 dosage greatly rescued the Spop mutant skeletal defects. We further show that Spop directly targets the Gli3 repressor for ubiquitination and degradation. Finally, we demonstrate in a conditional mutant that loss of Spop results in brachydactyly and osteopenia, which can be rescued by reducing the dosage of Gli3. In summary, Spop is an important positive regulator of Ihh signaling and skeletal development.

The chondrocytic journey in endochondral bone growth and skeletal dysplasia

The endochondral bones of the skeleton develop from a cartilage template and grow via a process involving a cascade of chondrocyte differentiation steps culminating in formation of a growth plate and the replacement of cartilage by bone. This process of endochondral ossification, driven by the generation of chondrocytes and their subsequent proliferation, differentiation, and production of extracellular matrix constitute a journey, deviation from which inevitably disrupts bone growth and development, and is the basis of human skeletal dysplasias with a wide range of phenotypic severity, from perinatal lethality to progressively deforming. This highly coordinated journey of chondrocyte specification and fate determination is controlled by a myriad of intrinsic and extrinsic factors. SOX9 is the master transcription factor that, in concert with varying partners along the way, directs the different phases of the journey from mesenchymal condensation, chondrogenesis, differentiation, proliferation, and maturation. Extracellular signals, including bone morphogenetic proteins, wingless-related MMTV integration site (WNT), fibroblast growth factor, Indian hedgehog, and parathyroid hormone-related peptide, are all indispensable for growth plate chondrocytes to align and organize into the appropriate columnar architecture and controls their maturation and transition to hypertrophy. Chondrocyte hypertrophy, marked by dramatic volume increase in phases, is controlled by transcription factors SOX9, Runt-related transcription factor, and FOXA2. Hypertrophic chondrocytes mediate the cartilage to bone transition and concomitantly face a live-or-die situation, a subject of much debate. We review recent insights into the coordination of the phases of the chondrocyte journey, and highlight the need for a systems level understanding of the regulatory networks that will facilitate the development of therapeutic approaches for skeletal dysplasia.

TGF-β/Smad3 stimulates stem cell/developmental gene expression and vascular smooth muscle cell de-differentiation

Atherosclerotic-associated diseases are the leading cause of death in the United States. Despite recent progress, interventional treatments for atherosclerosis can be complicated by restenosis resulting from neo-intimal hyperplasia. We have previously demonstrated that TGF-β and its downstream signaling protein Smad3∶1) are up-regulated following vascular injury, 2) together drive smooth muscle cell (SMC) proliferation and migration and 3) enhance the development of intimal hyperplasia. In order to determine a mechanism through which TGF-β/Smad3 promote these effects, Affymetrix gene expression arrays were performed on primary rat SMCs infected with Smad3 and stimulated with TGF-β or infected with GFP alone. More than 200 genes were differentially expressed (>2.0 fold change, p<0.05) in TGF-β/Smad3 stimulated SMCs. We then performed GO term enrichment analysis using the DAVID bioinformatics database and found that TGF-β/Smad3 activated the expression of multiple genes related to either development or cell differentiation, several of which have been shown to be associated with multipotent stem or progenitor cells. Quantitative real-time PCR confirmed up-regulation of several developmental genes including FGF1, NGF, and Wnt11 (by 2.5, 6 and 7 fold, respectively) as well as stem/progenitor cell associated genes CD34 and CXCR4 (by 10 and 45 fold, respectively). In addition, up-regulation of these factors at protein levels were also confirmed by Western blotting, or by immunocytochemistry (performed for CXCR4 and NGF). Finally, TGF-β/Smad3 down regulated transcription of SMC contractile genes as well as protein production of smooth muscle alpha actin, calponin, and smooth muscle myosin heavy chain. These combined results suggest that TGF-β/Smad3 stimulation drives SMCs to a phenotypically altered state of de-differentiation through the up-regulation of developmental related genes.

Cartilage explant cultures

To investigate chondrocyte biology in an organized structure, limb explant cultures have been established that allow the cultivation of the entire cartilaginous skeletal elements. In these organ cultures, the arrangement of chondrocytes in the cartilage elements and their interaction with the surrounding perichondrium and joint tissue are maintained. Chondrocyte proliferation and differentiation can thus be studied under nearly in vivo conditions. Growth factors and other soluble agents can be administered to the explants, and their effect on limb morphogenesis, gene expression, and cell-matrix interactions can be studied. Co-treatment with distinct growth factors and their inhibitors as well as use of transgenic mice will allow one to decipher the epistatic relationship between different signaling systems and other regulators of chondrocyte differentiation. Here we describe the protocol to culture cartilage explants ex vivo and discuss the advantages and disadvantages of the culture system.

The Ihh signal is essential for regulating proliferation and hypertrophy of cultured chicken chondrocytes

The Indian hedgehog (Ihh) signal plays a vital role in regulating proliferation and hypertrophy of chondrocytes. To investigate its function in postnatal chicken (Gallus gallus) chondrocytes, cyclopamine was used to inhibit Ihh signaling. The MTT and ALP assays revealed the downgrade-proliferation and upgrade-differentiation of chondrocytes. To further elucidate the mechanism, the mRNA expression levels of Ihh, parathyroid hormone related protein (PTHrP), Gli-2, Bcl-2, Bone Morphogenetic Protein 6 (BMP-6), type X collagen (Col X) and type II collagen (Col II) were detected by quantitative real-time RT-PCR analysis, and the protein expressions of Ihh, Col X, and Col II were determined using Western blot analysis. After the Ihh signal was blocked, chondrocytes demonstrated high expression levels of PTHrP and Col X and low levels of Gli-2, BMP-6, Bcl-2 and Col II although Ihh expression was increased. Based on these results, the Ihh signal is essential for balancing chicken chondrocyte proliferation and hypertrophy, and the regulatory function of PTHrP acts in an Ihh-dependent manner. Furthermore, BMP-6 and Bcl-2 played roles in maintaining the development of chondrocytes and may be downstream regulatory factors of Ihh signaling.

Hedgehog signaling regulates MyoD expression and activity

The inhibition of MyoD expression is important for obtaining muscle progenitors that can replenish the satellite cell niche during muscle repair. Progenitors could be derived from either embryonic stem cells or satellite cells. Hedgehog (Hh) signaling is important for MyoD expression during embryogenesis and adult muscle regeneration. To date, the mechanistic understanding of MyoD regulation by Hh signaling is unclear. Here, we demonstrate that the Hh effector, Gli2, regulates MyoD expression and associates with MyoD gene elements. Gain- and loss-of-function experiments in pluripotent P19 cells show that Gli2 activity is sufficient and required for efficient MyoD expression during skeletal myogenesis. Inhibition of Hh signaling reduces MyoD expression during satellite cell activation in vitro. In addition to regulating MyoD expression, Hh signaling regulates MyoD transcriptional activity, and MyoD activates Hh signaling in myogenic conversion assays. Finally, Gli2, MyoD, and MEF2C form a protein complex, which enhances MyoD activity on skeletal muscle-related promoters. We therefore link Hh signaling to the function and expression of MyoD protein during myogenesis in stem cells.

Suppressor of fused (Sufu) mediates the effect of parathyroid hormone-like hormone (Pthlh) on chondrocyte differentiation in the growth plate

Growth plate chondrocytes undergo a coordinated process of differentiation, regulating long bone growth. Parathyroid hormone-like hormone (Pthlh) inhibits hypertrophic differentiation in the growth plate chondrocytes and reduces Hedgehog (Hh) signaling. In mice lacking the Hh mediator Suppressor of fused (Sufu), Pthlh treatment resulted in the up-regulation of Hh activity and an increased number of hypertrophic chondrocytes. Furthermore, Pthlh increased Sufu protein levels, and in chondrocytes lacking Sufu, it was unable to process Hh-regulated Gli transcription factors. Pthlh regulates chondrocyte differentiation and Gli activity in a Sufu-dependent manner, with Sufu acting as a molecular switch in its regulation of differentiation.

Relating the chondrocyte gene network to growth plate morphology: from genes to phenotype

During endochondral ossification, chondrocyte growth and differentiation is controlled by many local signalling pathways. Due to crosstalks and feedback mechanisms, these interwoven pathways display a network like structure. In this study, a large-scale literature based logical model of the growth plate network was developed. The network is able to capture the different states (resting, proliferating and hypertrophic) that chondrocytes go through as they progress within the growth plate. In a first corroboration step, the effect of mutations in various signalling pathways of the growth plate network was investigated.

SOX9 governs differentiation stage-specific gene expression in growth plate chondrocytes via direct concomitant transactivation and repression

Cartilage and endochondral bone development require SOX9 activity to regulate chondrogenesis, chondrocyte proliferation, and transition to a non-mitotic hypertrophic state. The restricted and reciprocal expression of the collagen X gene, Col10a1, in hypertrophic chondrocytes and Sox9 in immature chondrocytes epitomise the precise spatiotemporal control of gene expression as chondrocytes progress through phases of differentiation, but how this is achieved is not clear. Here, we have identified a regulatory element upstream of Col10a1 that enhances its expression in hypertrophic chondrocytes in vivo. In immature chondrocytes, where Col10a1 is not expressed, SOX9 interacts with a conserved sequence within this element that is analogous to that within the intronic enhancer of the collagen II gene Col2a1, the known transactivation target of SOX9. By analysing a series of Col10a1 reporter genes in transgenic mice, we show that the SOX9 binding consensus in this element is required to repress expression of the transgene in non-hypertrophic chondrocytes. Forced ectopic Sox9 expression in hypertrophic chondrocytes in vitro and in mice resulted in down-regulation of Col10a1. Mutation of a binding consensus motif for GLI transcription factors, which are the effectors of Indian hedgehog signaling, close to the SOX9 site in the Col10a1 regulatory element, also derepressed transgene expression in non-hypertrophic chondrocytes. GLI2 and GLI3 bound to the Col10a1 regulatory element but not to the enhancer of Col2a1. In addition to Col10a1, paired SOX9-GLI binding motifs are present in the conserved non-coding regions of several genes that are preferentially expressed in hypertrophic chondrocytes and the occurrence of pairing is unlikely to be by chance. We propose a regulatory paradigm whereby direct concomitant positive and negative transcriptional control by SOX9 ensures differentiation phase-specific gene expression in chondrocytes. Discrimination between these opposing modes of transcriptional control by SOX9 may be mediated by cooperation with different partners such as GLI factors.

Kif7 promotes hedgehog signaling in growth plate chondrocytes by restricting the inhibitory function of Sufu

Proper regulation of Indian hedgehog (Ihh) signaling is vital for chondrocyte proliferation and differentiation in the growth plate. Its dysregulation causes skeletal dysplasia, osteoarthritis or cartilaginous neoplasia. Here, we show that Suppressor of fused (Sufu) and Kif7 are essential regulators of Ihh signaling. While Sufu acts as a negative regulator of Gli transcription factors, Kif7 functions both positively and negatively in chondrocytes. Kif7 plays a role in the turnover of Sufu and the exclusion of Sufu-Gli complexes from the primary cilium. Importantly, halving the dose of Sufu restores normal hedgehog pathway activity and chondrocyte development in Kif7-null mice, demonstrating that the positive role of Kif7 is to restrict the inhibitory activity of Sufu. Furthermore, Kif7 also inhibits Gli transcriptional activity in the chondrocytes when Sufu function is absent. Therefore, Kif7 regulates the activity of Gli transcription factors through both Sufu-dependent and -independent mechanisms.

Don't hedge your bets: hedgehog signaling as a central mediator of endochondral bone development and cartilage diseases

Cell differentiation and patterning are vital processes in the development of the appendicular skeleton. The hedgehog (Hh) signaling pathway plays a central role in regulating the anterior-posterior axis of the distal limb as well as the length of endochondral bones. Ligand-induced Hh signaling inhibits the processing of the Gli transcription factors from activator to repressor isoforms. In the growth plate, Indian hedgehog inhibits Gli processing, resulting in accumulation of Gli activators that induce chondrocyte maturation and hypertrophic differentiation. Parathyroid hormone-like hormone promote and Gli processing to repressor forms, thus regulating the rate of hypertrophic differentiation. In cartilage diseases such as osteoarthritis and cartilage tumors, there is a recapitulation of developmental processes that involve increased Hh signaling. Studies have shown that pharmacological inhibitors of Hh signaling can attenuate the progression osteoarthritis and cartilage tumor growth. Thus, Hh blockade can serve as a potential therapy for the treatment of various cartilage diseases.

Transcriptional networks controlling chondrocyte proliferation and differentiation during endochondral ossification

During endochondral ossification bones are formed as cartilage templates in which chondrocytes proliferate, differentiate into hypertrophic chondrocytes and are gradually replaced by bone. Postnatally, remnants of embryonic chondrocytes remain in a restricted domain between the ossified regions of the bones forming the growth plate. The coordinated proliferation and differentiation of chondrocytes ensures the continuous elongation of the epiphyseal growth plates. The sequential changes between proliferation and differentiation are tightly regulated by secreted growth factors, which activate chondrocyte-specific transcription factors. Transcription factors that play critical roles in regulating cell-type-specific gene expression include Sox9, Gli2/3, and Runx2. The interaction of these transcription factors with general transcriptional regulators like histone-modifying enzymes provides an additional level of regulation to fine-tune the expression of target genes in different chondrocyte populations. This review will outline recent advances in the analysis of the complex transcriptional network that regulates the distinct steps of chondrocyte differentiation.

A cis-regulatory site downregulates PTHLH in translocation t(8;12)(q13;p11.2) and leads to Brachydactyly Type E

Parathyroid hormone-like hormone (PTHLH) is an important chondrogenic regulator; however, the gene has not been directly linked to human disease. We studied a family with autosomal-dominant Brachydactyly Type E (BDE) and identified a t(8;12)(q13;p11.2) translocation with breakpoints (BPs) upstream of PTHLH on chromosome 12p11.2 and a disrupted KCNB2 on 8q13. We sequenced the BPs and identified a highly conserved Activator protein 1 (AP-1) motif on 12p11.2, together with a C-ets-1 motif translocated from 8q13. AP-1 and C-ets-1 bound in vitro and in vivo at the derivative chromosome 8 breakpoint [der(8) BP], but were differently enriched between the wild-type and BP allele. We differentiated fibroblasts from BDE patients into chondrogenic cells and found that PTHLH and its targets, ADAMTS-7 and ADAMTS-12 were downregulated along with impaired chondrogenic differentiation. We next used human and murine chondrocytes and observed that the AP-1 motif stimulated, whereas der(8) BP or C-ets-1 decreased, PTHLH promoter activity. These results are the first to identify a cis-directed PTHLH downregulation as primary cause of human chondrodysplasia.

Atf4 regulates chondrocyte proliferation and differentiation during endochondral ossification by activating Ihh transcription

Activating transcription factor 4 (Atf4) is a leucine-zipper-containing protein of the cAMP response element-binding protein (CREB) family. Ablation of Atf4 (Atf4(-/-)) in mice leads to severe skeletal defects, including delayed ossification and low bone mass, short stature and short limbs. Atf4 is expressed in proliferative and prehypertrophic growth plate chondrocytes, suggesting an autonomous function of Atf4 in chondrocytes during endochondral ossification. In Atf4(-/-) growth plate, the typical columnar structure of proliferative chondrocytes is disturbed. The proliferative zone is shortened, whereas the hypertrophic zone is transiently expanded. The expression of Indian hedgehog (Ihh) is markedly decreased, whereas the expression of other chondrocyte marker genes, such as type II collagen (Col2a1), PTH/PTHrP receptor (Pth1r) and type X collagen (Col10a1), is normal. Furthermore, forced expression of Atf4 in chondrocytes induces endogenous Ihh mRNA, and Atf4 directly binds to the Ihh promoter and activates its transcription. Supporting these findings, reactivation of Hh signaling pharmacologically in mouse limb explants corrects the Atf4(-/-) chondrocyte proliferation and short limb phenotypes. This study thus identifies Atf4 as a novel transcriptional activator of Ihh in chondrocytes that paces longitudinal bone growth by controlling growth plate chondrocyte proliferation and differentiation.

SCF, BDNF, and Gas6 are regulators of growth plate chondrocyte proliferation and differentiation

We previously demonstrated that bovine epiphyseal chondrocytes separated by density gradient centrifugation differ in proliferative response to IGF-I and IGF-I receptor number. To identify novel modifiers of IGF-I action at the growth plate, we used microarray analyses to compare bovine hypertrophic and reserve zones and identified several receptors differentially expressed across the growth plate: NTRK2 [receptor for brain-derived neurotrophic factor (BDNF)], KIT [receptor for stem cell factor (SCF)], and MER and AXL [two receptors for growth arrest-specific 6 (Gas6)]. The corresponding ligands were tested for their ability to stimulate either proliferation of isolated chondrocytes or differentiation in ATDC5 cells. Each factor inhibited IGF-I-mediated proliferation in isolated chondrocytes by attenuating ERK1/2 activation. SCF, BDNF, Gas6, and C-type natriuretic peptide promoted differentiation in ATDC5 cells, each factor producing different expression patterns for collagen X, collagen 2, aggrecan, and lysyl oxidase. Whereas multiple factors stimulated ATDC5 differentiation, only IGF-I and high-dose insulin, out of several factors implicated in chondrocyte maturation, stimulated proliferation of isolated chondrocytes. IGF-I appears to be the primary proliferative signal in growth plate chondrocytes, whereas multiple factors including SCF, BDNF, and Gas6 regulate the pace of differentiation at the growth plate.

Gli2 and p53 cooperate to regulate IGFBP-3- mediated chondrocyte apoptosis in the progression from benign to malignant cartilage tumors

Clinical evidence suggests that benign cartilage lesions can progress to malignant chondrosarcoma, but the molecular events in this progression are unknown. Mice that develop benign cartilage lesions due to overexpression of Gli2 in chondrocytes developed lesions similar to chondrosarcomas when they were also deficient in p53. Gli2 overexpression and p53 deficiency had opposing effects on chondrocyte differentiation, but had additive effects negatively regulating apoptosis. Regulation of Igfbp3 expression and insulin-like growth factor (IGF) signaling by Gli and p53 integrated their effect on apoptosis. Treatment of human chondrosarcomas or fetal mouse limb explants with IGFBP3 or by blocking IGF increased the apoptosis rate, and mice expressing Gli2 developed substantially fewer tumors when they were also deficient for Igf2. IGF signaling-meditated apoptosis regulates the progression to malignant chondrosarcoma.

ADAMTS-7, a direct target of PTHrP, adversely regulates endochondral bone growth by associating with and inactivating GEP growth factor

ADAMTS-7, a metalloproteinase that belongs to ADAMTS family, is important for the degradation of cartilage extracellular matrix proteins in arthritis. Herein we report that ADAMTS-7 is upregulated during chondrocyte differentiation and demonstrates the temporal and spatial expression pattern during skeletal development. ADAMTS-7 potently inhibits chondrocyte differentiation and endochondral bone formation, and this inhibition depends on its proteolytic activity. The cysteine-rich domain of ADAMTS-7 is required for its interaction with the extracellular matrix, and the C-terminal four-thrombospondin motifs are necessary for its full proteolytic activity and inhibition of chondrocyte differentiation. ADAMTS-7 is an important target of canonical PTHrP signaling, since (i) PTHrP induces ADAMTS-7, (ii) ADAMTS-7 is downregulated in PTHrP null mutant (PTHrP-/-) growth plate chondrocytes, and (iii) blockage of ADAMTS-7 almost abolishes PTHrP-mediated inhibition of chondrocyte hypertrophy and endochondral bone growth. ADAMTS-7 associates with granulin-epithelin precursor (GEP), an autocrine growth factor that has been implicated in tissue regeneration, tumorigenesis, and inflammation. In addition, ADAMTS-7 acts as a new GEP convertase and neutralizes GEP-stimulated endochondral bone formation. Collectively, these findings demonstrate that ADAMTS-7, a direct target of PTHrP signaling, negatively regulates endochondral bone formation by associating with and inactivating GEP chondrogenic growth factor.

Parathyroid hormone-related protein regulates glioma-associated oncogene transcriptional activation: lessons learned from bone development and cartilage neoplasia

Hedgehog and parathyroid hormone-related protein (PTHrP) signaling play important roles regulating the differentiation of chondrocytes, which form the template for growing bone. By studying the interaction of the pathways in normal and neoplastic growth-plate chondrocytes (from enchondromas, a benign cartilage tumor), an unexpected direct regulation of hedgehog-mediated transcriptional activation by parathyroid hormone-related protein was uncovered. This regulation acts through the processing of the hedgehog-activated transcription factor, glioma-associated oncogene-three (Gli3). When PTHrP activates its receptor, Gli3 is processed to its repressor form though a protein kinase A (PKA) -dependent mechanism. Thus, activation of a G protein-coupled receptor can negatively regulate hedgehog-mediated transcription independent of hedgehog ligand activity, raising intriguing therapeutic possibilities.

Trps1, a regulator of chondrocyte proliferation and differentiation, interacts with the activator form of Gli3

Trps1, the gene mutated in human Tricho-Rhino-Phalangeal syndrome, represents an atypical member of the GATA-family of transcription factors. Here we show that Trps1 interacts with Indian hedgehog (Ihh)/Gli3 signaling and regulates chondrocyte differentiation and proliferation. We demonstrate that Trps1 specifically binds to the transactivation domain of Gli3 in vitro and in vivo, whereas the repressor form of Gli3 does not interact with Trps1. A domain of 185aa within Trps1, containing three predicted zinc fingers, is sufficient for interaction with Gli3. Using different mouse models we find that in distal chondrocytes Trps1 and the repressor activity of Gli3 are required to expand distal cells and locate the expression domain of Parathyroid hormone related peptide. In columnar proliferating chondrocytes Trps1 and Ihh/Gli3 have an activating function. The differentiation of columnar and hypertrophic chondrocytes is supported by Trps1 independent of Gli3. Trps1 seems thus to organize chondrocyte differentiation interacting with different subsets of co-factors in distinct cell types.

Peptide hormones as developmental growth and differentiation factors

Peptide hormones, usually considered to be endocrine factors responsible for communication between tissues remotely located from each other, are increasingly being found to be synthesized in developing tissues, where they act locally. Several hormones are now known to be produced in developing tissues that are unrelated to the endocrine gland of origin in the adult. These hormones are synthesized locally, and are active as differentiation and survival factors, before the developing adult endocrine tissue becomes functional. There is increasing evidence for paracrine and/or autocrine actions for these factors during development, thus, placing them among the conventional growth and differentiation factors. We review the evidence for the view that thyroid hormones, growth hormone, prolactin, insulin, and parathyroid hormone-related protein are developmental growth and differentiation factors.