Transcriptional regulation of endochondral ossification by HIF-2alpha during skeletal growth and osteoarthritis development

Nuclear factor (NF)-κB and its associated pathways are major molecular regulators of blood-induced joint damage in a murine model of hemophilia

BACKGROUND

The present study was designed to investigate the molecular signaling events from onset of bleeding through the development of arthropathy in a murine model of hemophilia A.

METHODS AND RESULTS

A sharp-injury model of hemarthrosis was used. A global gene expression array on joint-specific RNA isolated 3 h post-injury revealed nuclear factor-kappa B (NF-κB) as the major transcription factor triggering inflammation. As a number of genes encoding the cytokines, growth factors and hypoxia regulating factors are known to be activated by NF-κB and many of these are part of the pathogenesis of various joint diseases, we reasoned that NF-κB-associated pathways may play a crucial role in blood-induced joint damage. To further understand its role, we screened NF-κB-associated pathways between 1 h to 90 days after injury. After a single articular bleed, distinct members of the NF-κB family (NF-κB1/NF-κB2/RelA/RelB) and their responsive pro-inflammatory cytokines (IL-1β/IL-6/IFNγ/TNFα) were significantly up-regulated (> 2 fold, P < 0.05) in injured vs. control joints at the various time-points analyzed (1 h/3 h/7 h/24 h). After multiple bleeds (days 30/60/75/90), there was increased expression of NF-κB-associated factors that contribute to hypoxia (HIF-1α, 3.3-6.5 fold), angiogenesis (VEGF-α, 2.5-4.4 fold) and chondrocyte damage (matrix metalloproteinase-13, 2.8-3.8 fold) in the injured joints. Micro RNAs (miR) that are known to regulate NF-κB activation (miRs-9 and 155), inflammation (miRs-16, 155 and 182) and apoptosis (miRs-19a, 155 and 186) were also differentially expressed (-4 to +13-fold) after joint bleeding, indicating that the small RNAs could modulate the arthropathy phenotype.

CONCLUSIONS

These data suggest that NF-κB-associated signaling pathways are involved in the development of hemophilic arthropathy.

New findings in osteoarthritis pathogenesis: therapeutic implications

This review focuses on the new perspectives which can provide insight into the crucial pathways that drive cartilage-bone physiopathology. In particular, we discuss the critical signaling and effector molecules that can activate cellular and molecular processes in both cartilage and bone cells and which may be relevant in cross talk among joint compartments: growth factors (bone morphogenetic proteins and transforming growth factor), hypoxia-related factors, cell-matrix interactions [discoidin domain receptor 2 (DDR2) and syndecan 4], signaling molecules [WNT, Hedgehog (Hh)]. With the continuous progression of our knowledge on the molecular pathways involved in cartilage and bone changes in osteoarthritis (OA), an increasing number of potentially effective candidates for OA therapy are already under scrutiny in clinical trials to ascertain their possible safe use in an attempt to identify molecules active in slowing or halting OA progression and reducing joint pain. We then review the principal molecules currently under clinical investigation.

Notch signaling in chondrocytes modulates endochondral ossification and osteoarthritis development

Here we examined the involvement of Notch signaling in the endochondral ossification process, which is crucial for osteoarthritis (OA) development. Intracellular domains of Notch1 and -2 were translocated into the nucleus of chondrocytes with their differentiation in mouse limb cartilage and in mouse and human OA articular cartilage. A tissue-specific inactivation of the Notch transcriptional effector recombination signal binding protein for Ig kappa J (RBPjκ) in chondroprogenitor cells of SRY-box containing gene 9 (Sox9)-Cre;Rbpj(fl/fl) mouse embryos caused an impaired terminal stage of endochondral ossification in the limb cartilage. The RBPjκ inactivation in adult articular cartilage after normal skeletal growth using type II collagen (Col2a1)-Cre(ERT);Rbpj(fl/fl) mice by tamoxifen injection caused resistance to OA development in the knee joint. Notch intracellular domain with the effector RBPjκ stimulated endochondral ossification through induction of the target gene Hes1 in chondrocytes. Among the Notch ligands, Jagged1 was strongly induced during OA development. Finally, intraarticular injection of N-[N-(3,5-diflurophenylacetate)-L-alanyl]-(S)-phenylglycine t-butyl ester (DAPT), a small compound Notch inhibitor, to the mouse knee joint prevented OA development. The RBPjκ-dependent Notch signaling in chondrocytes modulates the terminal stage of endochondral ossification and OA development, representing an extracellular therapeutic target of OA.

Prolyl hydroxylase 3 (PHD3) modulates catabolic effects of tumor necrosis factor-α (TNF-α) on cells of the nucleus pulposus through co-activation of nuclear factor κB (NF-κB)/p65 signaling

Recent studies suggest a differential role of prolyl hydroxylase (PHD) isoforms in controlling hypoxia-inducible factor (HIF)-α degradation and activity in nucleus pulposus (NP) cells. However, the regulation and function of PHDs under inflammatory conditions that characterize disc disease are not yet known. Here, we show that in NP cells, TNF-α and IL-1β induce PHD3 expression through NF-κB. Lentiviral delivery of Sh-p65 and Sh-IKKβ confirms that cytokine-mediated PHD3 expression is NF-κB-dependent. It is noteworthy that although both cytokines induce HIF activity, mechanistic studies using Sh-HIF-1α and PHD3 promoter/enhancer constructs harboring well characterized hypoxia response element (HRE) show lack of HIF involvement in cytokine-mediated PHD3 expression. Loss-of-function studies clearly indicate that PHD3 serves as a co-activator of NF-κB signaling activity in NP cells; PHD3 interacts with, and co-localizes with, p65. We observed that when PHD3 is silenced, there is a significant decrease in TNF-α-induced expression of catabolic markers that include ADAMTS5, syndecan4, MMP13, and COX2, and at the same time, there is restoration of aggrecan and collagen type II expression. It is noteworthy that hydroxylase function of PHDs is not required for mediating cytokine-dependent gene expression. These findings show that by enhancing the activity of inflammatory cytokines, PHD3 may serve a critical role in degenerative disc disease.

Hypoxia-inducible factor-1 (HIF-1) but not HIF-2 is essential for hypoxic induction of collagen prolyl 4-hydroxylases in primary newborn mouse epiphyseal growth plate chondrocytes

Hypoxia-inducible factors (HIFs) are the master regulators of hypoxia-responsive genes. They play a critical role in the survival, development, and differentiation of chondrocytes in the avascular hypoxic fetal growth plate, which is rich in extracellular matrix (ECM) and in its main component, collagens. Several genes involved in the synthesis, maintenance, and degradation of ECM are regulated by HIFs. Collagen prolyl 4-hydroxylases (C-P4Hs) are key enzymes in collagen synthesis because the resulting 4-hydroxyprolines are necessary for the stability of all collagen molecules. The vertebrate C-P4Hs are α(2)β(2) tetramers with three isoforms of the catalytic α subunit, yielding C-P4Hs of types I-III. C-P4H-I is the main form in most cells, but C-P4H-II is the major form in chondrocytes. We postulated here that post-translational modification of collagens, particularly 4-hydroxylation of proline residues, could be one of the modalities by which HIF regulates the adaptive responses of chondrocytes in fetal growth plates. To address this hypothesis, we used primary epiphyseal growth plate chondrocytes isolated from newborn mice with conditionally inactivated genes for HIF-1α, HIF-2α, or the von Hippel-Lindau protein. The data obtained showed that C-P4H α(I) and α(II) mRNA levels were increased in hypoxic chondrocytes in a manner dependent on HIF-1 but not on HIF-2. Furthermore, the increases in the C-P4H mRNA levels were associated with both increased amounts of the C-P4H tetramers and augmented C-P4H activity in hypoxia. The hypoxia inducibility of the C-P4H isoenzymes is thus likely to ensure sufficient C-P4H activity for collagen synthesis occurring in chondrocytes in a hypoxic environment.

Macro view of microRNA function in osteoarthritis

Osteoarthritis (OA), the most common musculoskeletal disorder, is complex, multifaceted, and characterized by degradation of articular cartilage and alterations in other joint tissues. Although some pathogenic pathways have been characterized, current knowledge is incomplete and effective approaches to the prevention or treatment of OA are lacking. Understanding novel molecular mechanisms that are involved in the maintenance and destruction of articular cartilage, including extracellular regulators and intracellular signalling mechanisms in joint cells that control cartilage homeostasis, has the potential to identify new therapeutic targets in OA. MicroRNAs control tissue development and homeostasis by fine-tuning gene expression, with expression patterns specific to tissues and developmental stages, and are increasingly implicated in the pathogenesis of complex diseases such as cancer and cardiovascular disorders. The emergent roles of microRNAs in cartilage homeostasis and OA pathogenesis are summarized in this Review, alongside potential clinical applications.

Chondrogenesis, chondrocyte differentiation, and articular cartilage metabolism in health and osteoarthritis

Chondrogenesis occurs as a result of mesenchymal cell condensation and chondroprogenitor cell differentiation. Following chondrogenesis, the chondrocytes remain as resting cells to form the articular cartilage or undergo proliferation, terminal differentiation to chondrocyte hypertrophy, and apoptosis in a process termed endochondral ossification, whereby the hypertrophic cartilage is replaced by bone. Human adult articular cartilage is a complex tissue of matrix proteins that varies from superficial to deep layers and from loaded to unloaded zones. A major challenge to efforts to repair cartilage by stem cell-based and other tissue-engineering strategies is the inability of the resident chondrocytes to lay down a new matrix with the same properties as it had when it was formed during development. Thus, understanding and comparing the mechanisms of cartilage remodeling during development, osteoarthritis (OA), and aging may lead to more effective strategies for preventing cartilage damage and promoting repair. The pivotal proteinase that marks OA progression is matrix metalloproteinase 13 (MMP-13), the major type II collagen-degrading collagenase, which is regulated by both stress and inflammatory signals. We and other investigators have found that there are common mediators of these processes in human OA cartilage. We also observe temporal and spatial expression of these mediators in early through late stages of OA in mouse models and are analyzing the consequences of knockout or transgenic overexpression of critical genes. Since the chondrocytes in adult human cartilage are normally quiescent and maintain the matrix in a low turnover state, understanding how they undergo phenotypic modulation and promote matrix destruction and abnormal repair in OA may to lead to identification of critical targets for therapy to block cartilage damage and promote effective cartilage repair.

A large-scale replication study for the association of rs17039192 in HIF-2α with knee osteoarthritis

Osteoarthritis (OA) is a common disease with a genetic component for its etiology. Recently, a genetic association of a single nucleotide polymorphism (SNP), rs17039192 in HIF-2α with knee OA has been reported in a Japanese population; however, controversy exits for its replication and a role of HIF-2α in OA. This study aimed to evaluate the association of the SNP by a large-scale replication study. A total of 8,457 subjects (3,129 OA cases and 5,328 controls) from seven independent cohorts from six countries (Japan, China, Taiwan, Korea, Greece, and Australia) were recruited and genotyped. The association of rs17039192 with knee OA was evaluated by meta-analyses. The association of the HIF-2α SNP was not replicated in any of the populations. Contrary to the previous report, the odds ratios (ORs) of the risk allele frequency were all less than 1. A combined analysis for the seven populations also showed no replication of the association (OR = 0.91, 95% confidence interval = 0.81-1.03). Our large-scale meta-analysis showed that the association of rs17039192 in HIF-2α with knee OA is negative. The significance of HIF-2α in human OA (idiopathic OA as a common disease) should be further evaluated carefully.

Passing the baton: the HIF switch

Hypoxia is an inadequate oxygen supply to tissues and cells, which can restrict their function. The hypoxic response is primarily mediated by the hypoxia-inducible transcription factors, HIF-1 and HIF-2, which have both overlapping and unique target genes. HIF target gene activation is highly context specific and is not a reliable indicator of which HIF-α isoform is active. For example, in some cell lines, the individual HIFs have specific temporal and functional roles: HIF-1 drives the initial response to hypoxia (<24h) and HIF-2 drives the chronic response (>24h). Here, we review the significance of the HIF switch and the relation between HIF-1 and HIF-2 under both physiological and pathophysiological conditions.

GSK-3α and GSK-3β proteins are involved in early stages of chondrocyte differentiation with functional redundancy through RelA protein phosphorylation

Here we examine the roles of two isoforms of glycogen synthase kinase-3 (GSK-3), GSK-3α and GSK-3β, in skeletal development. Both isoforms were unphosphorylated and active in chondrocyte differentiation stages during SOX9 and type II collagen (COL2A1) expression. Although knock-out of both alleles of Gsk3a (Gsk3a(-/-)) or a single allele of Gsk3b (Gsk3b(+/-)) in mice did not significantly affect skeletal development, compound knock-out (Gsk3a(-/-);Gsk3b(+/-)) caused dwarfism with impairment of chondrocyte differentiation. GSK-3α and GSK-3β induced differentiation of cultured chondrocytes with functional redundancy in a cell-autonomous fashion, independently of the Wnt/β-catenin signal. Computational predictions followed by SOX9 and COL2A1 transcriptional assays identified RelA (NF-κB p65) as a key phosphorylation target of GSK-3. Among several phosphorylation residues in RelA, Thr-254 was identified as the critical phosphorylation site for GSK-3 that modulated chondrocyte differentiation. In conclusion, redundant functions of GSK-3α and GSK-3β through phosphorylation of RelA at Thr-254 play a crucial role in early stages of chondrocyte differentiation.

Selenium promotes proliferation of chondrogenic cell ATDC5 by increment of intracellular ATP content under serum deprivation

Selenium (Se) is an essential micronutrient, and low Se intake in Se-deficient areas plays roles in an endemic osteochondropathy characterized by chondronecrosis in growth plate and articular cartilage. However, the biological activities of Se on cartilage are largely unknown. In this study, we examined the effects of Se on chondrogenic cell ATDC5 and the possible mechanisms involved. We demonstrated that Se stimulated ATDC5 cell proliferation under serum deprivation but not routine culture. Furthermore, Se promoted G1-phase cell cycle progression along with induction of cyclin D1 expression at the mRNA and protein level. Moreover, Se increased intracellular ATP content and decreased intracellular superoxide anion concentration without affecting intracellular redox status as estimated by ratio of the reduced and oxidized glutathione. In addition, suppression of intracellular ATP synthesis by glycolysis inhibitor or mitochondrial uncoupler both abrogated Se-mediated cyclin D1 induction. These findings suggest Se stimulates proliferation of chondrogenic cell ATDC5 through acceleration of cell cycle progression accompanied with cyclin D1 induction by enhancement of intracellular ATP content. This novel finding provides evidence for a role of Se in cartilage formation and degenerative processes and further supports the relationship between Se status and cartilage function that may lead to better utilization of Se for cartilage homeostasis.

Role of S100A12 in the pathogenesis of osteoarthritis

S100A12 is a member of the S100 protein family, which are intracellular calcium-binding proteins. Although there are many reports on the involvement of S100A12 in inflammatory diseases, its presence in osteoarthritic cartilage has not been reported. The purpose of this study was to investigate the expression of S100A12 in human articular cartilage in osteoarthritis (OA) and to evaluate the role of S100A12 in human OA chondrocytes. We analyzed S100A12 expression by immunohistochemical staining of cartilage samples obtained from OA and non-OA patients. In addition, chondrocytes were isolated from knee cartilage of OA patients and treated with recombinant human S100A12. Real-time RT-PCR was performed to analyze mRNA expression. Protein production of matrix metalloproteinase 13 (MMP-13) and vascular endothelial growth factor (VEGF) in the culture medium were measured by ELISA. Immunohistochemical analyses revealed that S100A12 expression was markedly increased in OA cartilages. Protein production and mRNA expression of MMP-13 and VEGF in cultured OA chondrocytes were significantly increased by treatment with exogenous S100A12. These increases in mRNA expression and protein production were suppressed by administration of soluble receptor for advanced glycation end products (RAGE). Both p38 mitogen-activated protein kinase (MAPK) and nuclear factor-κB (NF-κB) inhibitors also suppressed the increases in mRNA expression and protein production of MMP-13 and VEGF. We demonstrated marked up-regulation of S100A12 expression in human OA cartilages. Exogenous S100A12 increased the production of MMP-13 and VEGF in human OA chondrocytes. Our data indicate the possible involvement of S100A12 in the development of OA by up-regulating MMP-13 and VEGF via p38 MAPK and NF-κB pathways.

Genome-wide association and functional studies identify the DOT1L gene to be involved in cartilage thickness and hip osteoarthritis

Hip osteoarthritis (HOA) is one of the most disabling and common joint disorders with a large genetic component that is, however, still ill-defined. To date, genome-wide association studies (GWAS) in osteoarthritis (OA) and specifically in HOA have yielded only few loci, which is partly explained by heterogeneity in the OA definition. Therefore, we here focused on radiographically measured joint-space width (JSW), a proxy for cartilage thickness and an important underlying intermediate trait for HOA. In a GWAS of 6,523 individuals on hip-JSW, we identified the G allele of rs12982744 on chromosome 19p13.3 to be associated with a 5% larger JSW (P = 4.8 × 10(-10)). The association was replicated in 4,442 individuals from three United Kingdom cohorts with an overall meta-analysis P value of 1.1 × 10(-11). The SNP was also strongly associated with a 12% reduced risk for HOA (P = 1 × 10(-4)). The SNP is located in the DOT1L gene, which is an evolutionarily conserved histone methyltransferase, recently identified as a potentially dedicated enzyme for Wnt target-gene activation in leukemia. Immunohistochemical staining of the DOT1L protein in mouse limbs supports a role for DOT1L in chondrogenic differentiation and adult articular cartilage. DOT1L is also expressed in OA articular chondrocytes. Silencing of Dot1l inhibited chondrogenesis in vitro. Dot1l knockdown reduces proteoglycan and collagen content, and mineralization during chondrogenesis. In the ATDC5 chondrogenesis model system, DOT1L interacts with TCF and Wnt signaling. These data are a further step to better understand the role of Wnt-signaling during chondrogenesis and cartilage homeostasis. DOT1L may represent a therapeutic target for OA.

Gli1 protein participates in Hedgehog-mediated specification of osteoblast lineage during endochondral ossification

With regard to Hedgehog signaling in mammalian development, the majority of research has focused on Gli2 and Gli3 rather than Gli1. This is because Gli1(-/-) mice do not show any gross abnormalities in adulthood, and no detailed analyses of fetal Gli1(-/-) mice are available. In this study, we investigated the physiological role of Gli1 in osteogenesis. Histological analyses revealed that bone formation was impaired in Gli1(-/-) fetuses compared with WT fetuses. Gli1(-/-) perichondrial cells expressed neither runt-related transcription factor 2 (Runx2) nor osterix, master regulators of osteogenesis, in contrast to WT cells. In vitro analyses showed that overexpression of Gli1 up-regulated early osteogenesis-related genes in both WT and Runx2(-/-) perichondrial cells, and Gli1 activated transcription of those genes via its association with their 5'-regulatory regions, underlying the function of Gli1 in the perichondrium. Moreover, Gli1(-/-);Gli2(-/-) mice showed more severe phenotypes of impaired bone formation than either Gli1(-/-) or Gli2(-/-) mice, and osteoblast differentiation was impaired in Gli1(-/-);Gli3(-/-) perichondrial cells compared with Gli3(-/-) cells in vitro. These data suggest that Gli1 itself can induce early osteoblast differentiation, at least to some extent, in a Runx2-independent manner. It also plays a redundant role with Gli2 and is involved in the repressor function of Gli3 in osteogenesis. On the basis of these findings, we propose that upon Hedgehog input, Gli1 functions collectively with Gli2 and Gli3 in osteogenesis.

Hypoxia-driven pathways in bone development, regeneration and disease

Adaptation to hypoxia is a critical cellular event both in pathological settings, such as cancer and ischaemia, and in normal development and differentiation. Oxygen is thought to be not only an indispensable metabolic substrate for a variety of in vivo enzymatic reactions, including mitochondrial respiration, but also a key regulatory signal in tissue development and homeostasis by controlling a specific genetic program. Hypoxia-inducible transcription factors (HIFs) HIF-1 and HIF-2 are central mediators of the homeostatic response that enables cells to survive and differentiate in low-oxygen conditions. Genetically altered mice have been used to identify important roles for HIF-1 and HIF-2 as well as vascular endothelial growth factor (VEGF)-a potent angiogenic factor and a downstream target of the HIF pathway-in the regulation of skeletal development, bone homeostasis and haematopoiesis. In this Review, we summarize the current knowledge of HIF signalling in cartilage, bone and blood, and pay particular attention to the complex relationship between HIF and VEGF in these tissues revealed by data from research using animal models. The study of these models expands our understanding of the cell autonomous, paracrine and autocrine effects that mediate the homeostatic responses downstream of HIFs and VEGF.

Mapping pathogenesis of arthritis through small animal models

Animal models have been used for a number of decades to study arthritis and have contributed greatly to unravelling mechanisms of pathogenesis and validating new targets for treatment. All animal models have sets of limitations and over the years there has been natural refinement of existing models as well as creation of new ones. The success of genetic modification in mice has fuelled an exponential increase in the use of murine models for arthritis research and has significantly increased our understanding of disease processes. This review focuses on those rodent models of RA and OA that have current utility and are widely used by the research community. We highlight the subtle but important differences in existing models by positioning them on a pathogenesis map whereby model selection is determined by the specific aspect of disease to be studied. We discuss the evolving challenges in in vivo arthritis studies and our perceived gaps for future new model development. The document includes technical and cost implications of performing the described models, and the ethical considerations of such approaches.

Chondrocyte hypertrophy and osteoarthritis: role in initiation and progression of cartilage degeneration?

OBJECTIVE

To review the literature on the role and regulation of chondrocyte terminal differentiation (hypertrophy-like changes) in osteoarthritis (OA) and to integrate this in a conceptual model of primary OA development.

METHODS

Papers investigating chondrocyte terminal differentiation in human OA cartilage and experimental models of OA were recapitulated and discussed. Focus has been on the occurrence of hypertrophy-like changes in chondrocytes and the factors described to play a role in regulation of chondrocyte hypertrophy-like changes in OA.

RESULTS

Chondrocyte hypertrophy-like changes are reported in both human OA and experimental OA models by most investigators. These changes play a crucial part in the OA disease process by protease-mediated cartilage degradation. We propose that altered chondrocyte behavior and concomitant cartilage degradation result in a disease-amplifying loop, leading to a mixture of disease stages and cellular responses within an OA joint.

CONCLUSION

Chondrocyte hypertrophy-like changes play a role in early and late stage OA. Since not all cells in an OA joint are synchronized, inhibition of hypertrophy-like changes might be a therapeutic target to slow down further OA progression.

Biological aspects of early osteoarthritis

PURPOSE

Early OA primarily affects articular cartilage and involves the entire joint, including the subchondral bone, synovial membrane, menisci and periarticular structures. The aim of this review is to highlight the molecular basis and histopathological features of early OA.

METHODS

Selective review of literature.

RESULTS

Risk factors for developing early OA include, but are not limited to, a genetic predisposition, mechanical factors such as axial malalignment, and aging. In early OA, the articular cartilage surface is progressively becoming discontinuous, showing fibrillation and vertical fissures that extend not deeper than into the mid-zone of the articular cartilage, reflective of OARSI grades 1.0-3.0. Early changes in the subchondral bone comprise a progressive increase in subchondral plate and subarticular spongiosa thickness. Early OA affects not only the articular cartilage and the subchondral bone but also other structures of the joint, such as the menisci, the synovial membrane, the joint capsule, ligaments, muscles and the infrapatellar fat pad. Genetic markers or marker combinations may become useful in the future to identify early OA and patients at risk.

CONCLUSION

The high socioeconomic impact of OA suggests that a better insight into the mechanisms of early OA may be a key to develop more targeted reconstructive therapies at this first stage of the disease.

LEVEL OF EVIDENCE

Systematic review, Level II.

Teriparatide as a chondroregenerative therapy for injury-induced osteoarthritis

There is no disease-modifying therapy for osteoarthritis, a degenerative joint disease that is projected to afflict more than 67 million individuals in the United States alone by 2030. Because disease pathogenesis is associated with inappropriate articular chondrocyte maturation resembling that seen during normal endochondral ossification, pathways that govern the maturation of articular chondrocytes are candidate therapeutic targets. It is well established that parathyroid hormone (PTH) acting via the type 1 PTH receptor induces matrix synthesis and suppresses maturation of chondrocytes. We report that the PTH receptor is up-regulated in articular chondrocytes after meniscal injury and in osteoarthritis in humans and in a mouse model of injury-induced knee osteoarthritis. To test whether recombinant human PTH(1-34) (teriparatide) would inhibit aberrant chondrocyte maturation and associated articular cartilage degeneration, we administered systemic teriparatide (Forteo), a Food and Drug Administration-approved treatment for osteoporosis, either immediately after or 8 weeks after meniscal/ligamentous injury in mice. Knee joints were harvested at 4, 8, or 12 weeks after injury to examine the effects of teriparatide on cartilage degeneration and articular chondrocyte maturation. Microcomputed tomography revealed increased bone volume within joints from teriparatide-treated mice compared to saline-treated control animals. Immediate systemic administration of teriparatide increased proteoglycan content and inhibited articular cartilage degeneration, whereas delayed treatment beginning 8 weeks after injury induced a regenerative effect. The chondroprotective and chondroregenerative effects of teriparatide correlated with decreased expression of type X collagen, RUNX2 (runt-related transcription factor 2), matrix metalloproteinase 13, and the carboxyl-terminal aggrecan cleavage product NITEGE. These preclinical findings provide proof of concept that Forteo may be useful for decelerating cartilage degeneration and inducing matrix regeneration in patients with osteoarthritis.

Recent progress in understanding molecular mechanisms of cartilage degeneration during osteoarthritis

Osteoarthritis (OA) is a highly prevalent disease affecting more than 20% of American adults. Predispositions include joint injury, heredity, obesity, and aging. Biomechanical alterations are commonly involved. However, the molecular mechanisms of this disease are complex, and there is currently no effective disease-modifying treatment. The initiation and progression of OA subtypes is a complex process that at the molecular level probably involves many cell types, signaling pathways, and changes in extracellular matrix. Ex vivo studies with tissue derived from OA patients and in vivo studies with mutant mice have suggested that pathways involving receptor ligands such as TGF-β1, WNT3a, and Indian hedgehog; signaling molecules such as Smads, β-catenin, and HIF-2a; and peptidases such as MMP13 and ADAMTS4/5 are probably involved to some degree. This review focuses on molecular mechanisms of OA development related to recent findings.

HIF-1α and HIF-2α degradation is differentially regulated in nucleus pulposus cells of the intervertebral disc

Studies of many cell types show that levels of hypoxia inducible factor (HIF)-1α and HIF-2α are primarily controlled by oxygen-dependent proteasomal degradation, catalyzed by HIF prolyl-hydroxylases (PHDs). However, in the hypoxic niche of the intervertebral disc, the mechanism of HIF-α turnover in nucleus pulposus cells is not yet known. We show that in nucleus pulposus cells HIF-1α and HIF-2α, degradation was mediated through 26S proteasome irrespective of oxygen tension. It is noteworthy that HIF-2α degradation through 26S proteasome was more pronounced in hypoxia. Surprisingly, treatment with DMOG, a PHD inhibitor, shows the accumulation of only HIF-1α and induction in activity of its target genes, but not of HIF-2α. Loss and gain of function analyses using lentiviral knockdown of PHDs and overexpression of individual PHDs show that in nucleus pulposus cells only PHD2 played a limited role in HIF-1α degradation; again HIF-2α degradation was unaffected. We also show that the treatment with inhibitors of lysosomal proteolysis results in a strong accumulation of HIF-1α and to a much smaller extent of HIF-2α levels. It is thus evident that in addition to PHD2 catalyzed degradation, the HIF-1α turnover in nucleus pulposus cells is primarily regulated by oxygen-independent pathways. Importantly, our data clearly suggests that proteasomal degradation of HIF-2α is not mediated by a classical oxygen-dependent PHD pathway. These results for the first time provide a rationale for the normoxic stabilization as well as the maintenance of steady-state levels of HIF-1α and HIF-2α in nucleus pulposus cells.

Aging and osteoarthritis

PURPOSE OF REVIEW

Osteoarthritis is strongly linked to aging but the mechanisms for this link are incompletely understood. The recent literature was reviewed to find studies providing new insights into the connection between aging and osteoarthritis.

RECENT FINDINGS

Basic aging studies in nonarticular cells suggest that a cell stress or cell damage response contributes to chronic inflammation that promotes age-related diseases. This cellular response results in the senescence-associated secretory phenotype which has many of the characteristics of an osteoarthritic chondrocyte in terms of the cytokines, chemokines, and proteases produced. Oxidative stress can promote cell senescence and studies have shown a role for oxidative stress in altering cell signaling pathways in chondrocytes that can disrupt the response to growth factors. Mitochondria are an important source of reactive oxygen species and studies continue to support a role for the mitochondria in osteoarthritis, including work suggesting changes in energy production. Cell death occurs in osteoarthritic cartilage and recent studies suggest autophagy may play a role in determining if a cell lives or dies when stressed.

SUMMARY

Continued progress is being made on characterizing aging-related changes in cartilage. Additional studies are needed that focus on the tissues outside of the articular cartilage that play a role in osteoarthritis. Because osteoarthritis occurs in older adults who also have age-related changes in muscle, bone, fat, and the nervous system, it is likely that a more general and systemic approach will be needed to better understand the link between aging and osteoarthritis.

Runx2 contributes to murine Col10a1 gene regulation through direct interaction with its cis-enhancer

We have recently shown that a 150-bp Col10a1 distal promoter (-4296 to -4147 bp) is sufficient to direct hypertrophic chondrocyte-specific reporter (LacZ) expression in vivo. More recently, through detailed sequence analysis we identified two putative tandem-repeat Runx2 binding sites within the 3'-end of this 150-bp region (TGTGGG-TGTGGC, -4187 to -4176 bp). Candidate electrophoretic mobility shift assay (EMSA), chromatin immunoprecipitation, and transfection studies demonstrate that these putative Runx2 sites bind Runx2 and mediate upregulated Col10a1/reporter activity in vitro. Transgenic studies using the 5'-sequence without Runx2 sites were not able to drive the cell-specific LacZ reporter activity, suggesting the in vivo requirement of the Runx2 sites located in the 3'-end in mediating Col10a1/reporter expression. Indeed, mutating the Runx2 sites in the context of the 150-bp promoter abolishes its capacity to drive hypertrophic chondrocyte-specific reporter expression in transgenic mice. We have also generated multiple transgenic mouse lines using only the 3'-sequence containing the Runx2 sites to drive the LacZ gene. Interestingly, no hypertrophic chondrocyte-specific blue staining was observed in these transgenic mice. Together, our data support that Runx2 directly interacts with murine Col10a1 cis-enhancer. This interaction is required but not sufficient for cell-specific Col10a1 promoter activity in vivo. Additional cooperative/repressive elements within the 5'- or 3'-sequences of this 150-bp promoter are needed to work with Runx2 together to mediate cell-specific Col10a1 expression. Further delineation of these elements/factors has the potential to identify novel therapeutic targets for multiple skeletal disorders, including osteoarthritis, that show abnormal Col10a1 expression and altered chondrocyte maturation.

C/EBPβ and RUNX2 cooperate to degrade cartilage with MMP-13 as the target and HIF-2α as the inducer in chondrocytes

To elucidate the molecular mechanism underlying the endochondral ossification process during the skeletal growth and osteoarthritis (OA) development, we examined the signal network around CCAAT/enhancer-binding protein-β (C/EBPβ, encoded by CEBPB), a potent regulator of this process. Computational predictions and a C/EBP motif-reporter assay identified RUNX2 as the most potent transcriptional partner of C/EBPβ in chondrocytes. C/EBPβ and RUNX2 were induced and co-localized in highly differentiated chondrocytes during the skeletal growth and OA development of mice and humans. The compound knockout of Cebpb and Runx2 in mice caused growth retardation and resistance to OA with decreases in cartilage degradation and matrix metalloproteinase-13 (Mmp-13) expression. C/EBPβ and RUNX2 cooperatively enhanced promoter activity of MMP13 through specific binding to a C/EBP-binding motif and an osteoblast-specific cis-acting element 2 motif as a protein complex. Human genetic studies failed to show the association of human CEBPB gene polymorphisms with knee OA, nor was there a genetic variation around the identified responsive region in the human MMP13 promoter. However, hypoxia-inducible factor-2α (HIF-2α), a functional and genetic regulator of knee OA through promoting endochondral ossification, was identified as a potent and functional inducer of C/EBPβ expression in chondrocytes by the CEBPB promoter assay. Hence, C/EBPβ and RUNX2, with MMP-13 as the target and HIF-2α as the inducer, control cartilage degradation. This molecular network in chondrocytes may represent a therapeutic target for OA.

Deletion of the oxygen-dependent degradation domain results in impaired transcriptional activity of hypoxia-inducible factors

Hypoxia-inducible factors (HIF1α/HIF2α) are key transcription factors that promote angiogenesis. The overexpression of degradation-resistant HIF mutants is considered a promising pro-angiogenic therapeutic tool. We compared the transcriptional activity of HIF1α/HIF2α mutants that obtained their resistance to oxygen-dependent degradation either by deletion of their entire oxygen-dependent degradation (ODD) domain or by replacement of prolyl residues that are crucial for oxygen-dependent degradation. Although all HIF mutants translocated into the nucleus, HIF1α and HIF2α mutants inclosing the point mutations were significantly more effective in trans-activating the target gene VEGF and in inducing tube formation of endothelial cells than mutants lacking the complete ODD domain.

Genetic evidence of the regulatory role of parathyroid hormone-related protein in articular chondrocyte maintenance in an experimental mouse model

OBJECTIVE

Parathyroid hormone-related protein (PTHrP) regulates the rate of differentiation of growth chondrocytes and is also expressed in articular chondrocytes. This study tested the hypothesis that PTHrP might have a regulatory role in articular chondrocyte maintenance.

METHODS

Control sequences of growth differentiation factor 5 were used to delete PTHrP from articular chondrocytes in the mid-region of mouse articular cartilage. Mice with conditional deletion of PTHrP (knockout [KO]) and littermate control mice were evaluated for degenerative changes using both a time-course design and destabilization of the medial meniscus (DMM) technique. A total histologic score of degenerative changes was determined for the femoral and tibial articular surfaces (total maximum score of 60).

RESULTS

The time-course study revealed degenerative changes in only a minority of the KO mice. In the DMM model, male KO mice were highly susceptible to DMM-induced degenerative changes (mean ± SEM total histologic score 45 ± 2.7 in KO mice versus 23 ± 1.4 in controls; P < 0.0001 by Mann-Whitney U test), with virtually no overlap between groups. PTHrP normally functions in a feedback loop with Indian hedgehog (IHH), in which a reduction in one signaling partner induces a compensatory increase in the other. A number of phenotypic and functional markers were documented in KO mice to suggest that the IHH-PTHrP axis is capable of compensating in response to a partial Cre-driven PTHrP deletion, a finding that underscores the need to subject the mouse articular cartilage to a destabilizing challenge in order to elicit frankly degenerative findings.

CONCLUSION

PTHrP may regulate articular chondrocyte maintenance in mice.

Cartilage biology in osteoarthritis--lessons from developmental biology

The cellular and molecular mechanisms responsible for the initiation and progression of osteoarthritis (OA), and in particular cartilage degeneration in OA, are not completely understood. Increasing evidence implicates developmental processes in OA etiology and pathogenesis. Herein, we review this evidence. We first examine subtle changes in cartilage development and the specification and formation of joints, which predispose to OA development, and second, we review the switch from an articular to a hypertrophic chondrocyte phenotype that is thought to be part of the OA pathological process ultimately resulting in cartilage degeneration. The latest studies are summarized and we discuss the concepts emerging from these findings in cartilage biology, in the light of our understanding of the developmental processes involved.

Non-canonical HIF-2α function drives autonomous breast cancer cell growth via an AREG-EGFR/ErbB4 autocrine loop

Tumor progression is intrinsically tied to the clonal selection of tumor cells with acquired phenotypes allowing to cope with a hostile microenvironment. Hypoxia-inducible factors (HIFs) master the transcriptional response to local tissue hypoxia, a hallmark of solid tumors. Here, we report significantly longer patient survival in breast cancer with high levels of HIF-2α. Amphiregulin (AREG) and WNT1-inducible signaling pathway protein-2 (WISP2) expression was strongly HIF-2α-dependent and their promoters were particularly responsive to HIF-2α. The endogenous AREG promoter recruited HIF-2α in the absence of a classical HIF-DNA interaction motif, revealing a novel mechanism of gene regulation. Loss of AREG expression in HIF-2α-depleted cells was accompanied by reduced activation of epidermal growth factor (EGF) receptor family members. Apparently opposing results from patient and in vitro data point to an HIF-2α-dependent auto-stimulatory tumor phenotype that, while promoting EGF signaling in cellular models, increased the survival of diagnosed and treated human patients. Our findings suggest a model where HIF-2α-mediated autocrine growth signaling in breast cancer sustains a state of cellular self-sufficiency, thereby masking unfavorable microenvironmental growth conditions, limiting adverse selection and improving therapy efficacy. Importantly, HIF-2α/AREG/WISP2-expressing tumors were associated with luminal tumor differentiation, indicative of a better response to classical treatments. Shifting the HIF-1/2α balance toward an HIF-2-dominated phenotype could thus offer a novel approach in breast cancer therapy.

Hypoxia inhibits osteogenesis in human mesenchymal stem cells through direct regulation of RUNX2 by TWIST

Background

Bone loss induced by hypoxia is associated with various pathophysiological conditions, however, little is known about the effects of hypoxia and related signaling pathways on osteoblast differentiation and bone formation. Because bone marrow-derived mesenchymal stem cells (MSCs) survive under hypoxic conditions and readily differentiate into osteoblasts by standard induction protocols, they are a good in vitro model to study the effects of hypoxia on osteoblast differentiation.

Methodology/Principle Findings

Using human MSCs, we discovered TWIST, a downstream target of HIF-1α, was induced under hypoxia and acted as a transcription repressor of RUNX2 through binding to the E-box located on the promoter of type 1 RUNX2. Suppression of type 1 RUNX2 by TWIST under hypoxia further inhibited the expression of BMP2, type 2 RUNX2 and downstream targets of RUNX2 in MSCs.

Conclusions/Significance

Our findings point to the important role of hypoxia-mediated signalling in osteogenic differentiation in MSCs through direct regulation of RUNX2 by TWIST, and provide a method for modifying MSC osteogenesis upon application of these cells in fracture healing and bone reconstruction.

Expression patterns and function of chromatin protein HMGB2 during mesenchymal stem cell differentiation

The superficial zone (SZ) of articular cartilage is critical in maintaining tissue function and homeostasis and represents the site of the earliest changes in osteoarthritis (OA). The expression of chromatin protein HMGB2 is restricted to the SZ, which contains cells expressing mesenchymal stem cell (MSC) markers. Age-related loss of HMGB2 and gene deletion are associated with reduced SZ cellularity and early onset OA. This study addressed HMGB2 expression patterns in MSC and its role during differentiation. HMGB2 was detected at higher levels in human MSC as compared with human articular chondrocytes, and its expression declined during chondrogenic differentiation of MSC. Lentiviral HMGB2 transduction of MSC suppressed chondrogenesis as reflected by an inhibition of Col2a1 and Col10a1 expression. Conversely, in bone marrow MSC from Hmgb2(-/-) mice, Col10a1 was more strongly expressed than in wild-type MSC. This is consistent with in vivo results from mouse growth plates showing that Hmgb2 is expressed in proliferating and prehypertrophic zones but not in hypertrophic cartilage where Col10a1 is strongly expressed. Osteogenesis was also accelerated in Hmgb2(-/-) MSC. The expression of Runx2, which plays a major role in late stage chondrocyte differentiation, was enhanced in Hmgb2(-/-) MSC, and HMGB2 negatively regulated the stimulatory effect of Wnt/β-catenin signaling on the Runx2 proximal promoter. These results demonstrate that HMGB2 expression is inversely correlated with the differentiation status of MSC and that HMGB2 suppresses chondrogenic differentiation. The age-related loss of HMGB2 in articular cartilage may represent a mechanism responsible for the decline in adult cartilage stem cell populations.

Hypoxic induction of vascular endothelial growth factor regulates murine hematopoietic stem cell function in the low-oxygenic niche

Hypoxia is emerging as an important characteristic of the hematopoietic stem cell (HSC) niche, but the molecular mechanisms contributing to quiescence, self-renewal, and survival remain elusive. Vascular endothelial growth factor A (VEGFA) is a key regulator of angiogenesis and hematopoiesis. Its expression is commonly regulated by hypoxia-inducible factors (HIF) that are functionally induced in low-oxygen conditions and that activate transcription by binding to hypoxia-response elements (HRE). Vegfa is indispensable for HSC survival, mediated by a cell-intrinsic, autocrine mechanism. We hypothesized that a hypoxic HSC microenvironment is required for maintenance or up-regulation of Vegfa expression in HSCs and therefore crucial for HSC survival. We have tested this hypothesis in the mouse model Vegfaδ/δ, where the HRE in the Vegfa promoter is mutated, preventing HIF binding. Vegfa expression was reduced in highly purified HSCs from Vegfaδ/δ mice, showing that HSCs reside in hypoxic areas. Loss of hypoxia-regulated Vegfa expression increases the numbers of phenotypically defined hematopoietic stem and progenitor cells. However, HSC function was clearly impaired when assessed in competitive transplantation assays. Our data provide further evidence that HSCs reside in a hypoxic microenvironment and demonstrate a novel way in which the hypoxic niche affects HSC fate, via the hypoxia-VEGFA axis.

In vitro study of enhanced osteogenesis induced by HIF-1α-transduced bone marrow stem cells

OBJECTIVES

Hypoxia-inducible factor 1α (HIF-1α) is a pivotal regulator of hypoxic and ischaemic vascular responses that drives transcriptional activation of hundreds of genes involved in vascular reactivity, angiogenesis and arteriogenesis. Previous reports based on gene knockout technology have demonstrated that HIF-1α can promote osteogenesis. However, this protein is easily degraded in a normoxic state, which makes in vitro studies of HIF-1α-induced mesenchymal stem cell (MSC) osteogenesis difficult. For better understanding of HIF-1α promoting osteogenesis, the role of HIF-1α-induced MSC osteogenesis in the normoxic state has been investigated here.

MATERIALS AND METHODS

HIF-1α was made to overexpress using a lentiviral vector, and its effects on bone marrow-derived mesenchymal stem cell (BMSC) osteogenesis were investigated. Real-time quantitative and western blotting (to assess expression levels of angiogenic and osteogenic related genes regulated by Lenti-HIF-1α), alkaline phosphatase (ALP) and alizarin red-S staining analyses, were performed.

RESULTS

In HIF-1α gene-transfected BMSCs, expression levels of angiogenic, cartilaginous and osteogenic genes were all increased significantly compared to Lenti LacZ-transfected cells, at both mRNA and protein levels. ALP activity and alizarin red-S staining were significantly enhanced in HIF-1α transduced cells compared to control cells, on day 21.

CONCLUSIONS

These results indicate that Lenti-HIF-1α can induce BMSC overexpression levels of angiogenic and osteogenic genes in vitro in the normoxic state. Further study will be focused on whether HIF-1α can also improve bone repair in vivo.

Proteases involved in cartilage matrix degradation in osteoarthritis

Osteoarthritis is a common joint disease for which there are currently no disease-modifying drugs available. Degradation of the cartilage extracellular matrix is a central feature of the disease and is widely thought to be mediated by proteinases that degrade structural components of the matrix, primarily aggrecan and collagen. Studies on transgenic mice have confirmed the central role of Adamalysin with Thrombospondin Motifs 5 (ADAMTS-5) in aggrecan degradation, and the collagenolytic matrix metalloproteinase MMP-13 in collagen degradation. This review discusses recent advances in current understanding of the mechanisms regulating expression of these key enzymes, as well as reviewing the roles of other proteinases in cartilage destruction. This article is part of a Special Issue entitled: Proteolysis 50 years after the discovery of lysosome.

A central role for hypoxic signaling in cartilage, bone, and hematopoiesis

Hypoxic signaling plays an essential role in maintaining oxygen homeostasis and cell survival. Hypoxia-inducible transcription factors HIF-1 and HIF-2 are central mediators of the cellular response to hypoxia by regulating the expression of genes controlling metabolic adaptation, oxygen delivery, and survival in response to oxygen deprivation. Recent studies have identified an important role for HIF-1 and HIF-2 in the regulation of skeletal development, bone formation, and regeneration, as well as joint formation and homeostasis. In addition, overexpression of HIF-1 and HIF-2 is clinically associated with osteosarcoma and osteoarthritis. Together, these findings implicate hypoxic signaling as a central regulator of bone biology and disease.

Osteoarthritis year 2010 in review: pathomechanisms

Osteoarthritis is characterized by progressive breakdown of articular cartilage. This review summarizes findings of the last year, which shed new light on mechanisms and factors involved in cartilage loss. Evidence is accumulating that the transcription factor hypoxia-inducible factor-2α (HIF-2α) is highly enhanced in OA cartilage and drives catabolic metalloproteinases, including the pivotal MMP-13. In addition, HIF-2α suppresses chondrocyte autophagy, herein promoting chondrocyte apoptosis. The crucial role of MMP-13 is further underlined by reduced OA pathology in MMP-13 deficient mice. An intriguing mechanism to drive MMP-13 production is activation of the chondrocyte discoidin domain receptor (DDR-2) receptor through interaction with denuded collagen type II. The latter might occur in a proteoglycan depleted peri-cellular matrix, where DDR-2 expression is enhanced in OA cartilage and transgenic suppression attenuates experimental OA. The initiating role of ADAMTS-5 in proteoglycan loss appears dependent on interaction with the transmembrane proteoglycan syndecan-4, since syndecan-4 deficient mice are less prone to experimental OA and display reduced ADAMTS-5 activity. Both aging and the osteoarthritis (OA) process itself induce deranged transforming growth factor-β (TGFβ)-receptor expression, causing a shift to dominant usage of the receptor ALK-1, in stead of ALK5 and resulting in a TGFβ mediated catabolic pathway. ALK-1 rather than TGFβ is a promising therapeutic target. Finally, the alarmins S100A8 and 9 have long been considered as markers of inflammatory joint destruction, but now appear to be catabolic mediators.

Roles of inflammatory and anabolic cytokines in cartilage metabolism: signals and multiple effectors converge upon MMP-13 regulation in osteoarthritis

Human cartilage is a complex tissue of matrix proteins that vary in amount and orientation from superficial to deep layers and from loaded to unloaded zones. A major challenge to efforts to repair cartilage by stem cell-based and other tissue engineering strategies is the inability of the resident chondrocytes to lay down new matrix with the same structural and resilient properties that it had upon its original formation. This is particularly true of the collagen network, which is susceptible to cleavage once proteoglycans are depleted. Thus, a thorough understanding of the similarities and particularly the marked differences in mechanisms of cartilage remodeling during development, osteoarthritis, and aging may lead to more effective strategies for preventing cartilage damage and promoting repair. To identify and characterize effectors or regulators of cartilage remodeling in these processes, we are using culture models of primary human and mouse chondrocytes and cell lines and mouse genetic models to manipulate gene expression programs leading to matrix remodeling and subsequent chondrocyte hypertrophic differentiation, pivotal processes which both go astray in OA disease. Matrix metalloproteinases (MMP)-13, the major type II collagen-degrading collagenase, is regulated by stress-, inflammation-, and differentiation-induced signals that not only contribute to irreversible joint damage (progression) in OA, but importantly, also to the initiation/onset phase, wherein chondrocytes in articular cartilage leave their natural growth- and differentiation-arrested state. Our work points to common mediators of these processes in human OA cartilage and in early through late stages of OA in surgical and genetic mouse models.

HIF-2α as a possible therapeutic target of osteoarthritis

OBJECTIVE

Endochondral ossification, a conversion process from nonvascularized and hypoxic cartilage to highly vascularized bone, plays a crucial role in osteoarthritis (OA) development as well as in physiological skeletal growth. We have shown that hypoxia-inducible factor-2α (HIF-2α, encoded by EPAS1) is an extensive regulator of the endochondal ossification process. Here we review the possible signaling network regulating OA development on the axis of HIF-2α.

METHODS

Peer reviewed publications published prior to August 2010 were searched in the Pubmed database. Articles that were relevant to HIF and molecular mechanisms of the endochondral ossification and OA were selected.

RESULTS

As a trigger of OA, mechanical stress may induce the upstream NF-κB signal and HIF-2α expression in joint cartilage of mice and humans, which causes transactivation of endochondral ossification-related molecules with the most potent β-subunit partner aryl hydrocarbon nuclear translocator-like (ARNTL). In contrast to HIF-2α, HIF-1α functions to maintain cartilage via a distinct mechanism, so that the shifting of the HIFs might possibly be involved in an OA pathogenesis.

CONCLUSION

Signals on the HIF-2α axis from NF-κB signaling to the endochondral ossification-related molecules, possibly in combination with HIF-2α and ARNTL, may represent a rational therapeutic target for OA with minimal effects on physiological skeletal homeostasis.

Cartilage biology, pathology, and repair

Osteoarthritis is one of the most common forms of musculoskeletal disease and the most prominent type of arthritis encountered in all countries. Although great efforts have been made to investigate cartilage biology and osteoarthritis pathology, the treatment has lagged behind that of other arthritides, as there is a lack of effective disease-modifying therapies. Numerous approaches for dealing with cartilage degradation have been tried, but enjoyed very little success to develop approved OA treatments with not only symptomatic improvement but also structure-modifying effect. In this review we discuss the most recent findings regarding the regulation of cartilage biology and pathology and highlight their potential therapeutic values.

Combined mRNA and microRNA profiling reveals that miR-148a and miR-20b control human mesenchymal stem cell phenotype via EPAS1

Mesenchymal stem cells (MSCs) are present in a wide variety of tissues during development of the human embryo starting as early as the first trimester. Gene expression profiling of these cells has focused primarily on the molecular signs characterizing their potential heterogeneity and their differentiation potential. In contrast, molecular mechanisms participating in the emergence of MSC identity in embryo are still poorly understood. In this study, human embryonic stem cells (hESs) were differentiated toward MSCs (ES-MSCs) to compare the genetic patterns between pluripotent hESs and multipotent MSCs by a large genomewide expression profiling of mRNAs and microRNAs (miRNAs). After whole genome differential transcriptomic analysis, a stringent protocol was used to search for genes differentially expressed between hESs and ES-MSCs, followed by several validation steps to identify the genes most specifically linked to the MSC phenotype. A network was obtained that encompassed 74 genes in 13 interconnected transcriptional systems that are likely to contribute to MSC identity. Pairs of negatively correlated miRNAs and mRNAs, which suggest miRNA-target relationships, were then extracted and validation was sought with the use of Pre-miRs. We report here that underexpression of miR-148a and miR-20b in ES-MSCs, compared with ESs, allows an increase in expression of the EPAS1 (Endothelial PAS domain 1) transcription factor that results in the expression of markers of the MSC phenotype specification.