Wnt Influence on Chondrocyte Differentiation and Cartilage Function

3D biofabrication and space: A 'far-fetched dream' or a 'forthcoming reality'?

The long duration space missions across the Low Earth Orbit (LEO) often expose the voyagers to an abrupt zero gravity influence. The severe extraterrestrial cosmic radiation directly causes a plethora of moderate to chronic healthcare crises. The only feasible solution to manage critical injuries on board is surgical interventions or immediate return to Earth. This led the group of space medicine practitioners to adopt principles from tissue engineering and develop human tissue equivalents as an immediate regenerative therapy on board. The current review explicitly demonstrates the constructive application of different tissue-engineered equivalents matured under the available ground-based microgravity simulation facilities. Further, it elucidates how augmenting the superiority of biomaterial-based 3D bioprinting technology can enhance their clinical applicability. Additionally, the regulatory role of weightlessness condition on the underlying cellular signaling pathways governing tissue morphogenesis has been critically discussed. This information will provide future directions on how 3D biofabrication can be used as a plausible tool for healing on-flight chronic health emergencies. Thus, in our review, we aimed to precisely debate whether 3D biofabrication is deployed to cater to on-flight healthcare anomalies or space-like conditions are being utilized for generating 3D bioprinted human tissue constructs for efficient drug screening and regenerative therapy.

The role of WNT and IL-1 signaling in osteoarthritis: therapeutic implications for platelet-rich plasma therapy

Different from inflammatory arthritis, where biologicals and targeted synthetic molecules have revolutionized the disease course, no drug has demonstrated a disease modifying activity in osteoarthritis, which remains one of the most common causes of disability and chronic pain worldwide. The pharmacological therapy of osteoarthritis is mainly directed towards symptom and pain relief, and joint replacement is still the only curative strategy. Elucidating the disease pathophysiology is essential to understand which mechanisms can be targeted by innovative therapies. It has extensively been demonstrated that aberrant WNT and IL-1 signaling pathways are responsible for cartilage degeneration, impaired chondrocyte metabolism and differentiation, increased extracellular matrix degradation, and altered subchondral bone homeostasis. Platelet-rich plasma is an autologous blood derivative containing a concentration of platelets that is much higher than the whole blood counterpart and has shown promising results in the treatment of early knee osteoarthritis. Among the proposed mechanisms, the modulation of WNT and IL-1 pathways is of paramount importance and is herein reviewed in light of the proposed regenerative approaches.

Primary Cilia: A Cellular Regulator of Articular Cartilage Degeneration

Osteoarthritis (OA) is the most common joint disease that can cause pain and disability in adults. The main pathological characteristic of OA is cartilage degeneration, which is caused by chondrocyte apoptosis, cartilage matrix degradation, and inflammatory factor destruction. The current treatment for patients with OA focuses on delaying its progression, such as oral anti-inflammatory analgesics or injection of sodium gluconate into the joint cavity. Primary cilia are an important structure involved in cellular signal transduction. Thus, they are very sensitive to mechanical and physicochemical stimuli. It is reported that the primary cilia may play an important role in the development of OA. Here, we review the correlation between the morphology (location, length, incidence, and orientation) of chondrocyte primary cilia and OA and summarize the relevant signaling pathways in chondrocytes that could regulate the OA process through primary cilia, including Hedgehog, Wnt, and inflammation-related signaling pathways. These data provide new ideas for OA treatment.

miR-335-5P contributes to human osteoarthritis by targeting HBP1

MicroRNA (miR)-335-5P has the ability to regulate chondrogenic differentiation and promote chondrogenesis in mouse mesenchymal stem cells. It is also abnormally elevated in human osteoarthritic chondrocytes. However, the biological function of miR-335-5P in osteoarthritis (OA) is not well understood. The present study investigated the mechanism of miR-335-5P in the pathogenesis of OA. To investigate the effect of miR-335-5P on the pathogenesis of OA in vitro, a miR-335-5P mimic and inhibitor were transfected into chondrocytes. Cell Counting kit-8 assay and flow cytometry were used to observe the effects of miR-335-5P on chondrocyte apoptosis and the expression of cartilage-specific genes, such as aggrecan, collagen II, matrix metalloproteinase 13 and collagen X, were detected by reverse transcription-quantitative PCR and western blot analysis. Moreover, the current study assessed whether HMG-box transcription factor 1 (HBP1) is a novel target of miR-335-5P with dual luciferase reporter assays. Finally, a rescue experiment was used to prove the regulation between miR-335-5P and HBP1. The results revealed that HBP1 was a novel target of miR-335-5P, and that miR-335-5P mediated the apoptosis of chondrocytes and changes in cartilage-specific genes via targeting HBP1. Overall, the present study revealed that miR-335-5P mediated the development of OA by targeting the HBP1 gene and promoting chondrocyte apoptosis. These data suggested that miR-335-5P may be used to develop novel early-stage diagnostic and therapeutic strategies for OA.

Berberine and musculoskeletal disorders: The therapeutic potential and underlying molecular mechanisms

BACKGROUND

Musculoskeletal disorders are a group of disorders that affect the joints, bones, and muscles, causing long-term disability. Berberine, an isoquinoline alkaloid, has been previously established to exhibit beneficial properties in preventing various diseases, including musculoskeletal disorders.

PURPOSE

This review article aims to recapitulate the therapeutic potential of berberine and its mechanism of action in treating musculoskeletal disorders.

METHODS

A wide range of literature illustrating the effects of berberine in ameliorating musculoskeletal disorders was retrieved from online electronic databases (PubMed and Medline) and reviewed.

RESULTS

Berberine may potentially retard the progression of osteoporosis, osteoarthritis and rheumatoid arthritis. Limited studies reported the effects of berberine in suppressing the proliferation of osteosarcoma cells. These beneficial properties of berberine are mediated in part through its ability to target multiple signaling pathways, including PKA, p38 MAPK, Wnt/β-catenin, AMPK, RANK/RANKL/OPG, PI3K/Akt, NFAT, NF-κB, Hedgehog, and oxidative stress signaling. In addition, berberine exhibited anti-apoptotic, anti-inflammatory, and immunosuppressive properties.

CONCLUSION

The current evidence indicates that berberine may be effective in preventing musculoskeletal disorders. However, findings from in vitro and in vivo investigations await further validation from human clinical trial.

Therapies for osteoarthritis today and tomorrow: Review

Osteoarthritis is a common human disease with well understood pathophysiology, signs and symptoms, prevalence, risk factors, pain, and suffering with great understanding of personal, economic and social effects around the world. There are no drugs or treatments considered "disease modifying", with symptomatic control aiming to stave off the final solution of total joint replacement. Regenerative medicine and use of mesenchymal stem cells (MSC) promised hope to change that but have so far fallen short. This review focuses on current knowledge and use of MSC in clinic, completed research, and future directions for development of this once so promising biological treatment. Powerful treatment for pain in form of monoclonal antibodies against Nerve Growth Factor (NGF) are getting close to FDA approval in the US. Wnt signaling pathway modulators that decrease inflammation, increase function and potential to regenerate cartilage should be presented to the FDA early next year.

The osteoarthritis-associated gene PAPSS2 promotes differentiation and matrix formation in ATDC5 chondrogenic cells

3'-Phosphoadenosine 5'-phosphosulfate synthetase 2 (PAPSS2) has been shown to be important in the development of normal skeletal structure. The aim of the present study was to evaluate the role of PAPSS2 in the differentiation of chondrocytes as well as their mechanisms. Using RNA interference-mediated via a lentivirus and a retrovirus, PAPSS2 gene silence and overexpression in ATDC5 chondrogenic cells were performed. Chondrocyte differentiation and chondrogenic-related gene markers associated with extracellular matrix formation were noted. The mRNA and protein expression for Wnt4, β-catenin and SOX9 genes were observed. The PAPSS2 transcript expression levels progressively decline in ATDC5-induced chondrocyte-like cells during differentiation. Silencing of PAPSS2 expression had a significantly attenuating effect on cell differentiation and decreased expression of collagen II and X. In contrast, over-expression of PAPSS2 promoted the differentiation of ATDC5 chondrogenic cells. The mRNA expression levels of Wnt4 and SOX9 decreased significantly in PAPSS2 knock down cells vs. control cells. However, this expression was increased in the cells over-expressing PAPSS2. These data indicate that PAPSS2 regulates aggrecan activity as well as cell differentiation. The findings favor a mechanism by which PAPSS2 induces differentiation in ATDC5 cells via direct regulation of early signaling events that promote formation of collagenous matrix components. This control is probably mediated via extracellular matrix formation Wnt/β-catenin signaling pathways.

A novel Wnt pathway inhibitor, SM04690, for the treatment of moderate to severe osteoarthritis of the knee: results of a 24-week, randomized, controlled, phase 1 study

OBJECTIVE

To assess the safety, pharmacokinetics, and exploratory efficacy of SM04690, a novel Wnt pathway inhibitor, as a potential disease modifying treatment for knee osteoarthritis (OA).

DESIGN

Subjects with Kellgren-Lawrence grade 2-3 knee OA were randomized in successive dose-escalation cohorts to receive a knee intra-articular (IA) injection with 0.03, 0.07, or 0.23 mg SM04690, or placebo (PBO) (4:1 ratio). Safety, pharmacokinetics, efficacy (WOMAC Total/Function/Pain, Pain VAS, Physician Global Assessment [MDGA], and OMERACT-OARSI Response), OA-related biomarker (P1NP, ß-CTX, and cartilage oligomeric matrix protein [COMP]), and radiographic/imaging data were collected at baseline and during 24-week follow-up.

RESULTS

61 subjects (SM04690 n = 50; PBO n = 11) enrolled. Two dose limiting toxicities (DLTs), increased pain following injection and paroxysmal tachycardia (also the single serious AE), were reported in the 0.07 mg cohort. A total of 72 AEs were reported; Sixteen (occurring in eight subjects) were considered related to study medication. There were three discontinuations; one due to an AE (0.03 mg cohort). Bone marrow edema (BME) remained constant for most subjects. No doses were excluded from further study due to DLT criteria. Plasma levels of SM04690 were below the limit of detection at all time points. At Week 24, improvements from baseline were seen in all cohorts for the exploratory measures WOMAC Total, WOMAC Function, WOMAC Pain, MDGA, Pain VAS, and OMERACT-OARSI response. Joint space width (JSW) improvement was observed in the 0.07 mg cohort (P = 0.02 vs PBO).

CONCLUSION

SM04690 appeared safe and well tolerated, with no evidence of systemic exposure. Exploratory efficacy analyses suggested positive trends for measurements of OA pain, function and disease-modifying osteoarthritis drug (DMOAD) properties. CLINICALTRIALS.

GOV REGISTRATION

NCT02095548.

Wnt7a Inhibits IL-1β Induced Catabolic Gene Expression and Prevents Articular Cartilage Damage in Experimental Osteoarthritis

Wnt7a is a protein that plays a critical role in skeletal development. However, its effect on cartilage homeostasis under pathological conditions is not known. In this study, we found a unique inverse correlation between Wnt7a gene expression and that of MMP and IL-1β in individual human OA cartilage specimens. Upon ectopic expression in primary human articular chondrocytes, Wnt7a inhibited IL-1β-induced MMP and iNOS gene expression. Western blot analysis indicated that Wnt7a induced both canonical Wnt signaling and NFAT and Akt non-canonical signaling. Interestingly, inhibiting the canonical and Akt pathway did not affect Wnt7a activity. However, inhibiting the NFAT pathway impaired Wnt7a’s ability to inhibit MMP expression, suggesting that Wnt7a requires NFAT signaling to exert this function. In vivo, intraarticular injection of lentiviral Wnt7a strongly attenuated articular cartilage damage induced by destabilization of the medial meniscus (DMM) OA-inducing surgery in mice. Consistently, Wnt7a also inhibited the progressive increase of joint MMP activity in DMM animals. These results indicate that Wnt7a signaling inhibits inflammatory stimuli-induced catabolic gene expression in human articular chondrocytes and is sufficient to attenuate MMP activities and promote joint cartilage integrity in mouse experimental OA, demonstrating a novel effect of Wnt7a on regulating OA pathogenesis.

Wnt/β-catenin signaling of cartilage canal and osteochondral junction chondrocytes and full thickness cartilage in early equine osteochondrosis

The objective of this study was to elucidate gene and protein expression of Wnt signaling molecules in chondrocytes of foals having early osteochondrosis (OC) versus normal controls. The hypothesis was that increased expression of components of Wnt signaling pathway in osteochondral junction (OCJ) and cartilage canal (CC) chondrocytes would be found in early OC when compared to controls. Paraffin-embedded osteochondral samples (7 OC, 8 normal) and cDNA from whole cartilage (7 OC, 10 normal) and chondrocytes surrounding cartilage canals and osteochondral junctions captured with laser capture microdissection (4 OC, 6 normal) were obtained from femoropatellar joints of 17 immature horses. Equine-specific Wnt signaling molecule mRNA expression levels were evaluated by two-step real-time qPCR. Spatial tissue protein expression of β-catenin, Wnt-11, Wnt-4, and Dkk-1 was determined by immunohistochemistry. There was significantly decreased Wnt-11 and increased β-catenin, Wnt-5b, Dkk-1, Lrp6, Wif-1, Axin1, and SC-PEP gene expression in early OC cartilage canal chondrocytes compared to controls. There was also significantly increased β-catenin gene expression in early OC osteochondral junction chondrocytes compared to controls. Based on this study, abundant gene expression differences in OC chondrocytes surrounding cartilage canals suggest pathways associated with catabolism and inhibition of chondrocyte maturation are targeted in early OC pathogenesis.

A review of 1α,25(OH)2D3 dependent Pdia3 receptor complex components in Wnt5a non-canonical pathway signaling

Wnt5a and 1α,25-dihydroxyvitamin D3 [1α,25(OH)2D3] regulate endochondral ossification. 1α,25(OH)2D3 initiates its calcium-dependent effects via its membrane-associated receptor, protein disulfide isomerase A3 (Pdia3). 1α,25(OH)2D3 binding to Pdia3 triggers the interaction between Pdia3 and phospholipase A2 (PLA2)-activating protein (PLAA), resulting in downstream activation of calcium/calmodulin-dependent protein kinase II (CaMKII), PLA2, and protein kinase C (PKC). Wnt5a initiates its calcium-dependent effects via binding its receptors Frizzled2 (FZD2) and Frizzled5 (FZD5) and receptor tyrosine kinase-like orphan receptor 2 (ROR2), activating intracellular calcium release and stimulating PKC and CaMKII. Recent efforts to determine the inter-relation between Wnt5a and 1α,25(OH)2D3 signaling pathways have demonstrated that Wnt5a signals through a CaMKII/PLA2/PGE2/PKC cascade in chondrocytes and osteoblasts in which the components of the Pdia3 receptor complex were required. Furthermore, ROR2, but not FZD2 or FZD5, was required to mediate the calcium-dependent actions of 1α,25(OH)2D3. This review provides evidence that 1α,25(OH)2D3 and Wnt5a mediate their calcium-dependent pathways via similar receptor components and proposes that these pathways may interact since they are competing for the same receptor complex components.

Human monoclonal antibody fragments targeting matrilin-3 in growth plate cartilage

PURPOSE

Many genetic disorders, including chondrodysplasias, and acquired disorders impair growth plate function, resulting in short and sometimes malformed bones. There are multiple endocrine and paracrine factors that promote chondrogenesis at the growth plate, which could potentially be used to treat these disorders. Targeting these growth factors specifically to the growth plate might augment the therapeutic skeletal effect while diminishing undesirable effects on non-target tissues.

METHODS

Using yeast display technology, we selected single-chain variable antibody fragments that bound to human and mouse matrilin-3, an extracellular matrix protein specifically expressed in cartilage tissue. The ability of the selected antibody fragments to bind matrilin-3 and to bind cartilage tissue in vitro and in vivo was assessed by ELISA and immunohistochemistry.

RESULTS

We identified antibody fragments that bound matrilin-3 with high affinity and also bound with high tissue specificity to cartilage homogenates and to cartilage structures in mouse embryo sections. When injected intravenously in mice, the antibody fragments specifically homed to cartilage.

CONCLUSIONS

Yeast display successfully selected antibody fragments that are able to target cartilage tissue in vivo. Coupling these antibodies to chondrogenic endocrine and paracrine signaling molecules has the potential to open up new pharmacological approaches to treat childhood skeletal growth disorders.

Age-related differential gene and protein expression in postnatal cartilage canal and osteochondral junction chondrocytes

Wnt/β-catenin, Indian hedgehog (Ihh)/Parathyroid-related peptide (PTHrP) and retinoid signaling pathways regulate cartilage differentiation, growth, and function during development and play a key role in endochondral ossification. The objective of this study was to elucidate the gene and protein expression of signaling molecules of these regulatory pathways in chondrocytes surrounding cartilage canals and the osteochondral junction during neonatal and pre-adolescent development. This study revealed cell-specific and age-related differences in gene and protein expression of signaling molecules of these regulatory pathways. A trend for higher gene expression of PTHrP along the cartilage canals and Ihh along the osteochondral junction suggests the presence of paracrine feedback in articular-epiphyseal cartilage. Differential expression of canonical (β-catenin, Wnt-4, Lrp4, Lrp6) and noncanonical Wnt signaling (Wnt-5b, Wnt-11) and their inhibitors (Dkk1, Axin1, sFRP3, sFRP5, Wif-1) surrounding the cartilage canals and osteochondral junction provides evidence of the complex interactions occurring during endochondral ossification.

Signaling components of the 1α,25(OH)2D3-dependent Pdia3 receptor complex are required for Wnt5a calcium-dependent signaling

Wnt5a and 1α,25(OH)2D3 are important regulators of endochondral ossification. In osteoblasts and growth plate chondrocytes, 1α,25(OH)2D3 initiates rapid effects via its membrane-associated receptor protein disulfide isomerase A3 (Pdia3) in caveolae, activating phospholipase A2 (PLA2)-activating protein (PLAA), calcium/calmodulin-dependent protein kinase II (CaMKII), and PLA2, resulting in protein kinase C (PKC) activation. Wnt5a initiates its calcium-dependent effects via intracellular calcium release, activating PKC and CaMKII. We investigated the requirement for components of the Pdia3 receptor complex in Wnt5a calcium-dependent signaling. We determined that Wnt5a signals through a CaMKII/PLA2/PGE2/PKC cascade. Silencing or blocking Pdia3, PLAA, or vitamin D receptor (VDR), and inhibition of calmodulin (CaM), CaMKII, or PLA2 inhibited Wnt5a-induced PKC activity. Wnt5a activated PKC in caveolin-1-silenced cells, but methyl-beta-cyclodextrin reduced its stimulatory effect. 1α,25(OH)2D3 reduced stimulatory effects of Wnt5a on PKC in a dose-dependent manner. In contrast, Wnt5a had a biphasic effect on 1α,25(OH)2D3-stimulated PKC activation; 50ng/ml Wnt5a caused a 2-fold increase in 1α,25(OH)2D3-stimulated PKC but higher Wnt5a concentrations reduced 1α,25(OH)2D3-stimulated PKC activation. Western blots showed that Wnt receptors Frizzled2 (FZD2) and Frizzled5 (FZD5), and receptor tyrosine kinase-like orphan receptor 2 (ROR2) were localized to caveolae. Blocking ROR2, but not FZD2 or FZD5, abolished the stimulatory effects of 1α,25(OH)2D3 on PKC and CaMKII. 1α,25(OH)2D3 membrane receptor complex components (Pdia3, PLAA, caveolin-1, CaM) interacted with Wnt5a receptors/co-receptors (ROR2, FZD2, FZD5) in immunoprecipitation studies, interactions that changed with either 1α,25(OH)2D3 or Wnt5a treatment. This study demonstrates that 1α,25(OH)2D3 and Wnt5a mediate their effects via similar receptor components and suggests that these pathways may interact.

Expansion on a matrix deposited by nonchondrogenic urine stem cells strengthens the chondrogenic capacity of repeated-passage bone marrow stromal cells

Human urine-derived stem cells (hUSCs) are a newly found type of stem cell with a potential for therapeutic application in urology. The aim of this study is to investigate whether hUSCs contribute to cartilage regeneration. Despite their characterization with multi-lineage differentiation capacities, in terms of osteogenesis, adipogenesis and myogenesis, hUSCs do not show the ability to differentiate into chondrocytes. Human bone marrow stromal cells (hBMSCs) are a tissue-specific stem cell for endochondral bone formation; however, repeated-passage hBMSCs have a lower capacity for chondrogenic differentiation. We found that the extracellular matrix (ECM) deposited by hUSCs (UECM) can greatly recharge repeated-passage hBMSCs toward chondrogenic differentiation, a result that might be explained by trophic factors released from hUSCs being immobilized in UECM. We also found that ECM from repeated-passage hBMSCs (BECM) have a limited rejuvenation effect. The Wnt11-mediated noncanonical signaling pathway might be responsible for UECM-mediated hBMSC rejuvenation and subsequent chondrogenic differentiation. Our data indicate that commercially available UECM from young healthy donors might represent a simple and promising approach for autologous hBMSC rejuvenation. This study also provides an excellent model for investigating the effect of trophic factors released by stem cells on tissue regeneration without interference by stem cell differentiation.

The Wnt serpentine receptor Frizzled-9 regulates new bone formation in fracture healing

Wnt signaling is a key regulator of bone metabolism and fracture healing. The canonical Wnt/β-catenin pathway is regarded as the dominant mechanism, and targeting this pathway has emerged as a promising strategy for the treatment of osteoporosis and poorly healing fractures. In contrast, little is known about the role of non-canonical Wnt signaling in bone. Recently, it was demonstrated that the serpentine receptor Fzd9, a Wnt receptor of the Frizzled family, is essential for osteoblast function and positively regulates bone remodeling via the non-canonical Wnt pathway without involving β-catenin-dependent signaling. Here we investigated whether the Fzd9 receptor is essential for fracture healing using a femur osteotomy model in Fzd9^−/− mice. After 10, 24 and 32 days the fracture calli were analyzed using biomechanical testing, histomorphometry, immunohistochemistry, and micro-computed tomography. Our results demonstrated significantly reduced amounts of newly formed bone at all investigated healing time points in the absence of Fzd9 and, accordingly, a decreased mechanical competence of the callus tissue in the late phase of fracture healing. In contrast, cartilage formation and numbers of osteoclasts degrading mineralized matrix were unaltered. β-Catenin immunolocalization showed that canonical Wnt-signaling was not affected in the absence of Fzd9 in osteoblasts as well as in proliferating and mature chondrocytes within the fracture callus. The expression of established differentiation markers was not altered in the absence of Fzd9, whereas chemokines Ccl2 and Cxcl5 seemed to be reduced. Collectively, our results suggest that non-canonical signaling via the Fzd9 receptor positively regulates intramembranous and endochondral bone formation during fracture healing, whereas it does not participate in the formation of cartilage or in the osteoclastic degradation of mineralized matrix. The finding that Fzd9, in addition to its role in physiological bone remodeling, regulates bone repair may have implications for the development of treatments for poorly or non-healing fractures.

Transcriptomic signatures in cartilage ageing

Introduction

Age is an important factor in the development of osteoarthritis. Microarray studies provide insight into cartilage aging but do not reveal the full transcriptomic phenotype of chondrocytes such as small noncoding RNAs, pseudogenes, and microRNAs. RNA-Seq is a powerful technique for the interrogation of large numbers of transcripts including nonprotein coding RNAs. The aim of the study was to characterise molecular mechanisms associated with age-related changes in gene signatures.

Methods

RNA for gene expression analysis using RNA-Seq and real-time PCR analysis was isolated from macroscopically normal cartilage of the metacarpophalangeal joints of eight horses; four young donors (4 years old) and four old donors (>15 years old). RNA sequence libraries were prepared following ribosomal RNA depletion and sequencing was undertaken using the Illumina HiSeq 2000 platform. Differentially expressed genes were defined using Benjamini-Hochberg false discovery rate correction with a generalised linear model likelihood ratio test (P < 0.05, expression ratios ± 1.4 log₂ fold-change). Ingenuity pathway analysis enabled networks, functional analyses and canonical pathways from differentially expressed genes to be determined.

Results

In total, the expression of 396 transcribed elements including mRNAs, small noncoding RNAs, pseudogenes, and a single microRNA was significantly different in old compared with young cartilage (± 1.4 log₂ fold-change, P < 0.05). Of these, 93 were at higher levels in the older cartilage and 303 were at lower levels in the older cartilage. There was an over-representation of genes with reduced expression relating to extracellular matrix, degradative proteases, matrix synthetic enzymes, cytokines and growth factors in cartilage derived from older donors compared with young donors. In addition, there was a reduction in Wnt signalling in ageing cartilage.

Conclusion

There was an age-related dysregulation of matrix, anabolic and catabolic cartilage factors. This study has increased our knowledge of transcriptional networks in cartilage ageing by providing a global view of the transcriptome.

Stimulation of superficial zone protein accumulation by hedgehog and Wnt signaling in surface zone bovine articular chondrocytes

OBJECTIVE

To determine the roles of the hedgehog and Wnt signaling pathways in accumulation of superficial zone protein (SZP) in surface zone articular chondrocytes.

METHODS

Explant cultures of disks of surface zone cartilage or isolated chondrocytes from the surface zone of articular cartilage of bovine stifle joints were cultured in serum-free chemically defined medium. Accumulation of SZP in the culture medium, in response to hedgehog proteins (sonic hedgehog [SHH] and Indian hedgehog [IHH]), Wnt proteins (Wnt-3a, Wnt-5a, and Wnt-11), agonists of the Wnt/β-catenin pathway (glycogen synthase kinase 3β [GSK-3β] inhibitors), and antagonists of the Wnt/β-catenin pathway, was investigated. The interaction between transforming growth factor β1 (TGFβ1) and hedgehog proteins or antagonists of the Wnt/β-catenin pathway was also investigated.

RESULTS

Hedgehog proteins stimulated SZP accumulation. Activation of the Wnt/β-catenin pathway by Wnt-3a and GSK-3β inhibitors led to inhibition of SZP accumulation; however, Wnt-5a and Wnt-11 had no influence on SZP accumulation. Conversely, antagonists of the Wnt/β-catenin pathway stimulated SZP accumulation. In addition, there were combinatorial effects of TGFβ1 and hedgehog proteins or antagonists of the Wnt/β-catenin pathway on SZP accumulation.

CONCLUSION

SHH and IHH signaling has a stimulatory effect on SZP accumulation in surface zone cartilage and isolated articular chondrocytes. These findings provide insight into the regulatory mechanisms of articular cartilage homeostasis and maintenance by morphogens.

Activation of Wnt Planar Cell Polarity (PCP) signaling promotes growth plate column formation in vitro

Disrupting the Wnt Planar Cell Polarity (PCP) signaling pathway in vivo results in loss of columnar growth plate architecture, but it is unknown whether activation of this pathway in vitro is sufficient to promote column formation. We hypothesized that activation of the Wnt PCP pathway in growth plate chondrocyte cell pellets would promote columnar organization in these cells that are normally oriented randomly in culture. Rat growth plate chondrocytes were transfected with plasmids encoding the Fzd7 cell-surface Wnt receptor, a Fzd7 deletion mutant lacking the Wnt-binding domain, or Wnt receptor-associated proteins Ror2 or Vangl2, and then cultured as three-dimensional cell pellets in the presence of recombinant Wnt5a or Wnt5b for 21 days. Cellular morphology was evaluated using histomorphometric measurements. Activation of Wnt PCP signaling components promoted the initiation of columnar morphogenesis in the chondrocyte pellet culture model, as measured by histomorphometric analysis of the column index (ANOVA p = 0.01). Activation of noncanonical Wnt signaling through overexpression of both the cell-surface Wnt receptor Fzd7 and receptor-associated protein Ror2 with addition of recombinant Wnt5a promotes the initiation of columnar architecture of growth plate chondrocytes in vitro, representing an important step toward growth plate regeneration.

The skeleton: a multi-functional complex organ: the growth plate chondrocyte and endochondral ossification

Endochondral ossification is the process that results in both the replacement of the embryonic cartilaginous skeleton during organogenesis and the growth of long bones until adult height is achieved. Chondrocytes play a central role in this process, contributing to longitudinal growth through a combination of proliferation, extracellular matrix (ECM) secretion and hypertrophy. Terminally differentiated hypertrophic chondrocytes then die, allowing the invasion of a mixture of cells that collectively replace the cartilage tissue with bone tissue. The behaviour of growth plate chondrocytes is tightly regulated at all stages of endochondral ossification by a complex network of interactions between circulating hormones (including GH and thyroid hormone), locally produced growth factors (including Indian hedgehog, WNTs, bone morphogenetic proteins and fibroblast growth factors) and the components of the ECM secreted by the chondrocytes (including collagens, proteoglycans, thrombospondins and matrilins). In turn, chondrocytes secrete factors that regulate the behaviour of the invading bone cells, including vascular endothelial growth factor and receptor activator of NFκB ligand. This review discusses how the growth plate chondrocyte contributes to endochondral ossification, with some emphasis on recent advances.

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.

R-spondin 1 protects against inflammatory bone damage during murine arthritis by modulating the Wnt pathway

OBJECTIVE

During the course of different musculoskeletal diseases, joints are progressively damaged by inflammatory, infectious, or mechanical stressors, leading to joint destruction and disability. While effective strategies to inhibit joint inflammation, such as targeted cytokine-blocking therapy, have been developed during the last decade, the molecular mechanisms of joint damage are still poorly understood. This study was undertaken to investigate the role of the Wnt pathway modulator R-Spondin 1 (RSpo1) in protecting bone and cartilage in a mouse model of arthritis.

METHODS

Tumor necrosis factor alpha (TNFalpha)-transgenic mice were treated with vehicle or Rspo1. Mice were evaluated for signs of arthritis, and histologic analysis of the hind paws was performed. Moreover, we determined the effect of Rspo1 on Wnt signaling activity and osteoprotegerin (OPG) expression in murine primary osteoblasts.

RESULTS

The secreted Wnt pathway modulator RSpo1 was highly effective in preserving the structural integrity of joints in a TNFalpha-transgenic mouse model of arthritis by protecting bone and cartilage from inflammation-related damage. RSpo1 antagonized the Wnt inhibitor Dkk-1 and modulated Wnt signaling in mouse mesenchymal cells. In osteoblasts, RSpo1 induced differentiation and expression of OPG, thereby inhibiting osteoclastogenesis in vitro. In vivo, RSpo1 promoted osteoblast differentiation and bone formation while blocking osteoclast development, thereby contributing to the integrity of joints during inflammatory arthritis.

CONCLUSION

Our results demonstrate the therapeutic potential of RSpo1 as an anabolic agent for the preservation of joint architecture.

Low-density lipoprotein receptor-related protein 5 (LRP5) expression in human osteoarthritic chondrocytes

The aim of this study was to investigate the activation of the Wnt/beta-catenin pathway in osteoarthritis and the role of low-density lipoprotein receptor-related protein 5 (LRP5) in human osteoarthritic chondrocytes. The influence of 1,25(OH)2D3 on the expression of the LRP5 gene in human chondrocytes was also assessed. Human cartilage was obtained from 11 patients with primary osteoarthritis (OA) undergoing total knee replacement surgery. Normal cartilage was obtained from five healthy individuals. Beta-catenin and LRP5 mRNA levels were investigated using real-time PCR and LRP5 protein expression using Western blot analysis. Furthermore, we evaluated the effect of 1,25(OH)2D3 on LRP5 mRNA expression levels in osteoarthritic chondrocytes. Blocking LRP5 expression was performed using small interfering RNA (siRNA) against LRP5, and subsequent MMP-13 mRNA and protein levels were evaluated by real-time PCR and Western blot analysis, respectively. We confirmed the activation of the Wnt/beta-catenin pathway in OA, as we observed significant up-regulation of beta-catenin mRNA expression in osteoarthritic chondrocytes. We also observed that LRP5 mRNA and protein expression were significantly up-regulated in osteoarthritic cartilage compared to normal cartilage, and LRP5 mRNA expression was further increased by vitamin D. Also, blocking LRP5 expression using siRNA against LRP5 resulted in a significant decrease in MMP-13 mRNA and protein expressions. Our findings suggest the catabolic role of LRP5 is mediated by the Wnt/beta-catenin pathway in human osteoarthritis.

Fibulin-3 negatively regulates chondrocyte differentiation

Fibulin-3 is a member of the fibulin family that has been newly recognized as extracellular matrix proteins. We assessed the effects of fibulin-3 overexpression on chondrocyte differentiation using the clonal murine cell line ATDC5. The ATDC5-FBLN3 stably expressing fibulin-3 protein was spindle-shaped cell compared to the ATDC5-mock with plump cell. The cell growth in the ATDC5-FBLN3 was accelerated in comparison to that in the ATDC5-mock. The ATDC5-FBLN3 was not stained by Alcian blue, nor was there any cartilage aggregate formed after the induction of chondrogenic differentiation. The expression of type II collagen, aggrecan, and type X collagen was completely suppressed in ATDC5-FBLN3 even after the induction of differentiation. The overexpression of fibulin-3 reduced the expression of Sox5 and Sox6, while it maintained the expression of Sox9. These findings suggest that fibulin-3 may play an important role as a negative regulator of chondrocyte differentiation.

Wif-1 is expressed at cartilage-mesenchyme interfaces and impedes Wnt3a-mediated inhibition of chondrogenesis

Wnt factors are involved in the regulation of all steps of cartilage development. The activity of Wnt factors is generally regulated at the extracellular level by factors like the Dkk family, sFRPs, Cerberus and Wnt inhibitory factor 1 (Wif-1). Here we report that Wif-1 is highly expressed at cartilage-mesenchyme interfaces of the early developing skeleton. In fetal and postnatal skeletal development, Wif-1 is expressed in a sharply restricted zone in the upper hyaline layer of epiphyseal and articular cartilage and in trabecular bone. Coimmunoprecipitation and pull-down assays using recombinant Wif-1 and Wnt factors show specific binding of Wif-1 to Wnt3a, Wnt4, Wnt5a, Wnt7a, Wnt9a and Wnt11. Moreover, Wif-1 was able to block Wnt3a-mediated activation of the canonical Wnt signalling pathway. Consequently, Wif-1 impaired growth of mesenchymal precursor cells and neutralised Wnt3a-mediated inhibition of chondrogenesis in micromass cultures of embryonic chick limb-bud cells. These results identify Wif-1 as a novel extracellular Wnt modulator in cartilage biology.

[Roles of TGF-b superfamily in the genesis, development and maintenance of cartilage]

The transforming growth factor beta (TGF-beta) superfamily is composed of TGF-beta subfamily and bone morphogenetic protein (BMP) subfamily. The ligands, ligand antagonists, receptors and intracellular transductors that engage in the TGF-beta superfamily signaling pathway play their unique roles during endochondral ossification via regulating the lineage differentiation, proliferation, maturation, apoptosis and mineralization of chondrocytes. BMP signaling dominates chondro-genesis through initiating the chondrocytic commitment of mesenchymal cells and maintaining the chondrocytic phenotype. During the development of growth plate, BMP signaling promotes the maturation of chondrocytes to facilitate ossification, whereas TGF-beta signaling inhibits the hypertrophic differentiation to preserve adequate chondrocytes within the growth plate. Both TGF-beta signaling and BMP signaling are indispensable for the maintenance and repair of articular cartilage. Therefore, it indicates that TGF-beta superfamily may function essentially all throughout the development of skeletons.

Large-scale association study for structural soundness and leg locomotion traits in the pig

BACKGROUND

Identification and culling of replacement gilts with poor skeletal conformation and feet and leg (FL) unsoundness is an approach used to reduce sow culling and mortality rates in breeding stock. Few candidate genes related to soundness traits have been identified in the pig.

METHODS

In this study, 2066 commercial females were scored for 17 traits describing body conformation and FL structure, and were used for association analyses. Genotyping of 121 SNPs derived from 95 genes was implemented using Sequenom's MassARRAY system.

RESULTS

Based on the association results from single trait and principal components using mixed linear model analyses and false discovery rate testing, it was observed that APOE, BMP8, CALCR, COL1A2, COL9A1, DKFZ, FBN1 and VDBP were very highly significantly (P < 0.001) associated with body conformation traits. The genes ALOX5, BMP8, CALCR, OPG, OXTR and WNT16 were very highly significantly (P < 0.001) associated with FL structures, and APOE, CALCR, COL1A2, GNRHR, IHH, MTHFR and WNT16 were highly significantly (P < 0.01) associated with overall leg action. Strong linkage disequilibrium between CALCR and COL1A2 on SSC9 was detected, and haplotype -ACGACC- was highly significantly (P < 0.01) associated with overall leg action and several important FL soundness traits.

CONCLUSION

The present findings provide a comprehensive list of candidate genes for further use in fine mapping and biological functional analyses.

Wnt signaling in cartilage development and degeneration

The Wnt signaling network, which is composed of Wnt ligands, receptors, antagonists, and intracellular signaling molecules, has emerged as a powerful regulator of cell fate, proliferation, and function in multicellular organisms. Over the past two decades, the critical role of Wnt signaling in embryonic cartilage and bone development has been well established, and much has been learnt regarding the role of Wnt signaling in chondrogenesis and cartilage development. However, relatively little is known about the role of Wnt signaling in adult articular cartilage and degenerative cartilage tissue. This review will briefly summarize recent advances in Wnt regulation of chondrogenesis and hypertrophic maturation of chondrocytes, and review data concerning the role of Wnt signaling in the maintenance and degeneration of articular chondrocytes and cartilage.

Radiographic osteoarthritis at three joint sites and FRZB, LRP5, and LRP6 polymorphisms in two population-based cohorts

OBJECTIVE

To examine the association of genetic variation in key players in the Wnt signaling pathway with aspects of osteoarthritis (OA) in two population-based cohort studies: the Rotterdam Study and the Chingford Study.

METHODS

Radiographic OA (ROA) was defined as a Kellgren/Lawrence score (K/L) score > or = 2 for the knee and hip. Total hip replacement (THR) was scored. Hand OA was defined as presence of ROA (K/L > or = 2) in two out of three hand joint groups [distal interphalangeal (DIPs), proximal interphalangeal (PIPs), first carpometacarpal (CMC1)/trapezio-scaphoid joint (TS)] of each hand. The concentration of urinary C-terminal cross-linked telopeptide of type II collagen (CTX-II) was standardized to the total urine creatinine. Genotypes for the amino acid variants, Arg200Trp and Arg324Gly of Frizzled-Related protein gene (FRZB), Ala1330Val of Low-density lipoprotein receptor-related protein 5 (LRP5) and Ile1062Val of Low-density lipoprotein receptor-related protein 6 (LRP6), were obtained using the Taqman allelic discrimination assay. A meta-analysis was performed for the FRZB Arg324Gly polymorphism and hip- and knee-OA using RevMan version 4.3.

RESULTS

No consistent associations were observed between the FRZB, LRP5 and LRP6 amino acid variants and radiographic hip-, knee-, or hand-OA or THR, in either study population. While power was limited for most studies to date, a meta-analysis of all published studies regarding the FRZB Arg324Gly polymorphism was performed for hip- and knee-OA separately. This showed no significant associations between the Gly324 allele and risk for hip- or knee OA, although there was large heterogeneity between studies for hip OA in females.

CONCLUSION

No association was seen between FRZB, LRP5 and LRP6 variants with radiographic osteoarthritic outcomes in two population-based cohorts. In future studies, increased power and standardization of OA-phenotypes are highly recommended for replication studies and to allow meta-analysis.

Brachy-syndactyly caused by loss of Sfrp2 function

Wnt signaling pathways are regulated both at the intracellular and extracellular levels. During embryogenesis, the in vivo effects of the secreted frizzled-related protein (Sfrp) family of Wnt inhibitors are poorly understood. Here, we show that inactivation of Sfrp2 results in subtle limb defects in mice with mesomelic shortening and consistent shortening of all autopodal elements that is clinically manifested as brachydactyly. In addition, there is soft-tissue syndactyly of the hindlimb. The brachydactyly is caused by decreased chondrocyte proliferation and delayed differentiation in distal limb chondrogenic elements. These data suggest that Sfrp2 can regulate both chondrogenesis and regression of interdigital mesenchyme in distal limb. Sfrp2 can also repress canonical Wnt signaling by Wnt1, Wnt9a, and Wnt4 in vitro. Sfrp2-/- and TOPGAL/Sfrp2-/- mice have a mild increase in beta-catenin and beta-galactosidase staining, respectively, in some phalangeal elements. This however does not exclude a potential concurrent effect on non-canonical Wnt signaling in the growth plate. In combination with what is known about BMP and Wnt signaling in human brachydactylies, our data establish a critical role for Sfrp2 in proper distal limb formation and suggest SFPR2 could be a novel candidate gene for human brachy-syndactyly defects.

Expression of novel extracellular sulfatases Sulf-1 and Sulf-2 in normal and osteoarthritic articular cartilage

INTRODUCTION

Changes in sulfation of cartilage glycosaminoglycans as mediated by sulfatases can regulate growth factor signaling. The aim of this study was to analyze expression patterns of recently identified extracellular sulfatases Sulf-1 and Sulf-2 in articular cartilage and chondrocytes.

METHODS

Sulf-1 and Sulf-2 expressions in human articular cartilage from normal donors and patients with osteoarthritis (OA) and in normal and aged mouse joints were analyzed by real-time polymerase chain reaction, immunohistochemistry, and Western blotting.

RESULTS

In normal articular cartilage, Sulf-1 and Sulf-2 mRNAs and proteins were expressed predominantly in the superficial zone. OA cartilage showed significantly higher Sulf-1 and Sulf-2 mRNA expression as compared with normal human articular cartilage. Sulf protein expression in OA cartilage was prominent in the cell clusters. Western blotting revealed a profound increase in Sulf protein levels in human OA cartilage. In normal mouse joints, Sulf expression was similar to human cartilage, and with increasing age, there was a marked upregulation of Sulf.

CONCLUSION

The results show low levels of Sulf expression, restricted to the superficial zone in normal articular cartilage. Sulf mRNA and protein levels are increased in aging and OA cartilage. This increased Sulf expression may change the sulfation patterns of heparan sulfate proteoglycans and growth factor activities and thus contribute to abnormal chondrocyte activation and cartilage degradation in OA.

Transforming growth factor-beta and Wnt signals regulate chondrocyte differentiation through Twist1 in a stage-specific manner

We investigated the molecular mechanisms underlying the transition between immature and mature chondrocytes downstream of TGF-beta and canonical Wnt signals. We used two developmentally distinct chondrocyte models isolated from the caudal portion of embryonic chick sternum or chick growth plates. Lower sternal chondrocytes exhibited immature phenotypic features, whereas growth plate-extracted cells displayed a hypertrophic phenotype. TGF-beta significantly induced beta-catenin in immature chondrocytes, whereas it repressed it in mature chondrocytes. TGF-beta further enhanced canonical Wnt-mediated transactivation of the Topflash reporter expression in lower sternal chondrocytes. However, it inhibited Topflash activity in a time-dependent manner in growth plate chondrocytes. Our immunoprecipitation experiments showed that TGF-beta induced Sma- and Mad-related protein 3 interaction with T-cell factor 4 in immature chondrocytes, whereas it inhibited this interaction in mature chondrocytes. Similar results were observed by chromatin immunoprecipitation showing that TGF-beta differentially shifts T-cell factor 4 occupancy on the Runx2 promoter in lower sternal chondrocytes vs. growth plate chondrocytes. To further determine the molecular switch between immature and hypertrophic chondrocytes, we assessed the expression and regulation of Twist1 and Runx2 in both cell models upon treatment with TGF-beta and Wnt3a. We show that Runx2 and Twist1 are differentially regulated during chondrocyte maturation. Furthermore, whereas TGF-beta induced Twist1 in mature chondrocytes, it inhibited Runx2 expression in these cells. Opposite effects were observed upon Wnt3a treatment, which predominates over TGF-beta effects on these cells. Finally, overexpression of chick Twist1 in mature chondrocytes dramatically inhibited their hypertrophy. Together, our findings show that Twist1 may be an important regulator of chondrocyte progression toward terminal maturation in response to TGF-beta and canonical Wnt signaling.

Wnt gene expression in the post-natal growth plate: regulation with chondrocyte differentiation

Longitudinal growth of long bones occurs at the growth plate by endochondral ossification. In the embryonic mouse, this process is regulated by Wnt signaling. Little is known about which members of the Wnt family of secreted signaling proteins might be involved in the regulation of the postnatal growth plate. We used microdissection and real-time PCR to study mRNA expression of Wnt genes in the mouse growth plate. Of the 19 known members of the Wnt family, only six were expressed in postnatal growth plate. Of these, Wnts -2b, -4, and -10b signal through the canonical beta-catenin pathway and Wnts -5a, -5b, and -11 signal through the noncanonical calcium pathway. The spatial expression for these six Wnts was remarkably similar, showing low mRNA expression in the resting zone, increasing expression as the chondrocytes differentiated into the proliferative and prehypertrophic state and then (except Wnt-2b) decreasing expression as the chondrocytes underwent hypertrophic differentiation. This overall pattern is broadly consistent with previous studies of embryonic mouse growth cartilage suggesting that Wnt signaling modulates chondrocyte proliferation and hypertrophic differentiation. We also found that mRNA expression of these Wnt genes persisted at similar levels at 4 weeks, when longitudinal bone growth is waning. In conclusion, we have identified for the first time the specific Wnt genes that are expressed in the postnatal mammalian growth plate. The six identified Wnt genes showed a similar pattern of expression during chondrocyte differentiation, suggesting overlapping or interacting roles in postnatal endochondral bone formation.

Alteration of matrix glycosaminoglycans diminishes articular chondrocytes' response to a canonical Wnt signal

OBJECTIVE

Although Wnt signaling is a key regulator of the chondrocyte life cycle during embryonic development, little is known about Wnt activity in articular cartilage. Recent studies have suggested an association between excess signaling through the canonical Wnt pathway and osteoarthritis (OA). Genetic and in vitro studies with Drosophila have shown that signaling by the orthologous protein, Wingless (Wg), is regulated by glycosaminoglycans (GAGs) found at the cell surface. The objective of this study was to determine whether alteration in GAG sulfation or matrix content, such as that occurs in OA cartilage, would affect articular chondrocytes' response to a canonical Wnt stimulus.

METHODS

Cells were isolated from shoulder joints of young calves (bovine articular chondrocytes, bACs) and from human cartilage (human articular chondrocytes, hACs) discarded during total knee replacement for OA. Conditioned media from a cell line that is stably transfected with Wnt3a was used as a source of Wnt protein that activates the canonical signaling pathway. Conditioned media from the parental cell line was used as a control. beta-catenin levels were measured by immunoblot. In some experiments, chondrocyte cultures were treated with sodium chlorate (NaClO3) to inhibit GAG sulfation, or with chondroitinase ABC (ChABC) to digest chondroitin sulfate (CS) in the matrix.

RESULTS

Cultured bACs showed low steady-state levels of beta-catenin that increased upon stimulation with Wnt3a. A decrease in either GAG sulfation or CS content diminished bACs' response to Wnt3a (approximately 40% and 37% of control, respectively). Similar effects on the response to Wnt3a via beta-catenin were observed for cultured hACs with undersulfation of GAGs (16% of control) and decreased CS content (20% of control).

CONCLUSION

This study demonstrates that articular chondrocytes respond to canonical Wnt stimulation, and that reduced sulfation or CS content diminishes that response.

Wnt signaling antagonists are potential prognostic biomarkers for the progression of radiographic hip osteoarthritis in elderly Caucasian women

OBJECTIVE

To determine whether serum levels of 2 Wnt signaling antagonists, Frizzled-related protein (FRP) and Dkk-1, are associated with the development and progression of radiographic hip osteoarthritis (RHOA).

METHODS

Pelvic radiographs were obtained a mean of 8.3 years apart in 5,928 Caucasian women >or=65 years of age who were enrolled in the Study of Osteoporotic Fractures. Random sampling of this cohort was performed, with approximately 180 subjects per group assigned to 2 nested case-control studies on RHOA incidence and progression. Baseline serum levels of FRP and Dkk-1 were measured by capture enzyme-linked immunosorbent assay. Odds ratios (ORs) and 95% confidence intervals (95% CIs) were calculated using logistic regression analyses with adjustment for potential covariates.

RESULTS

There were no differences in serum levels of FRP and Dkk-1 between case subjects with incidence or progression of RHOA and their respective control subjects. There was a trend for higher baseline serum levels of FRP to be associated with a reduced risk of incident RHOA (age-adjusted OR 0.59 [95% CI 0.32-1.09], P = 0.09 for women in the highest quartile versus women in the lowest quartile). There was no association of serum levels of FRP with progression of RHOA. Serum levels of Dkk-1 did not correlate with incident RHOA. However, higher serum levels of Dkk-1 were associated with diminished risk of RHOA progression (age-adjusted OR 0.43 [95% CI 0.23-0.79], P = 0.007 for women in the highest quartile compared with women in the lowest quartile).

CONCLUSION

Elevated circulating levels of Dkk-1 appeared to be associated with reduced progression of RHOA in elderly women, whereas the highest quartile of serum FRP levels tended to be associated with a modest reduction in risk of incident RHOA.

Evidence that the canonical Wnt signalling pathway regulates deer antler regeneration

Wnt signalling regulates many developmental processes, including the fate specification, polarity, migration, and proliferation of cranial neural crest. The canonical Wnt pathway has also been shown to play an important role in bone physiology and there is evidence for its recapitulation during organ regeneration in lower vertebrates. This study explores the role of the Wnt signalling pathway in deer antlers, frontal bone appendages that are the only mammalian organs capable of regeneration. Immunocytochemistry was used to map the distribution of the activated form of beta-catenin ((a)betaCAT). A low level of (a)betaCAT staining was detected in chondrocytes and in osteoblasts at sites of endochondral bone formation. However, (a)betaCAT was localised in cellular periosteum and in osteoblasts in intramembranous bone, where it co-localised with osteocalcin. The most intense (a)betaCAT staining was in dividing undifferentiated cells in the mesenchymal growth zone. Antler progenitor cells (APCs) were cultured from this region and when the canonical Wnt pathway was inhibited at the level of Lef/TCF by epigallocatechin gallate (EGCG), the cell number decreased. TUNEL staining revealed that this was as a result of increased apoptosis. Activation of the pathway by lithium chloride (LiCl) had no effect on cell number but inhibited alkaline phosphate activity (ALP), a marker of APC differentiation, whereas EGCG increased ALP activity. This study demonstrates that beta-catenin plays an important role in the regulation of antler progenitor cell survival and cell fate. It also provides evidence that beta-catenin's function in regulating bone formation by osteoblasts may be site-specific.

Secreted frizzled related protein 1 regulates Wnt signaling for BMP2 induced chondrocyte differentiation

Canonical Wnt signaling (beta-catenin/TCF) has emerged as a key regulator of skeletogenesis. In this study, chondrogenesis is examined in a mouse model in which the Wnt antagonist secreted frizzled related protein 1 (sFRP1) is non-functional and results in a high bone mass phenotype and activation through the canonical pathway of the Runx2 transcription factor that is essential for bone formation. We find during the period of rapid post-natal growth, shortened height of the growth plate and increased calcification of the hypertrophic zone (HZ) in the sFRP1-/- mouse, indicating accelerated endochondral ossification. Using mouse embryo fibroblasts (MEFs) induced into the chondrogenic lineage, increased chondrogenesis and accelerating differentiation of hypertrophic chondrocytes in the sFRP1-/- MEFs was observed compared to WT cells. The induced maturation of hypertrophic chondrocytes in sFRP1(-/-) MEFs was inversely correlated to phospho-beta-catenin levels, indicating involvement of activated canonical Wnt signaling characterized by an increased expression of collagen type 2a1 and Sox 9. However, an absence of Indian hedgehog expression which occurs in WT cells was found. SFRP1-/- cells also exhibited an early induction of collagen type 10a1. Thus, these modifications in gene expression are contributing mechanism(s) for increased chondrocyte differentiation in SFRP1-/- cells. These studies have identified sFRP1 as a critical negative regulator of Wnt signaling for the normal progression of chondrocyte differentiation. Microarray gene profiling provided additional novel insights into the regulatory factors for appropriate Wnt signaling necessary for the control of chondrocyte maturation.