Growth factors produced by inflammatory cells and mesenchymal progenitors are required for proper bone regeneration. Signaling pathways activated downstream of these proteins work in concert and synergistically to drive osteoblast and/or chondrocyte differentiation. While dysregulation can result in abnormal healing, activating these pathways in the correct spatiotemporal context can enhance healing. Bone morphogenetic protein (BMP) signaling is well-recognized as being required for bone regeneration, and BMP is used clinically to enhance bone healing. However, it is imperative to develop new therapeutics that can be used alone or in conjunction with BMP to drive even more robust healing. Notch signaling is another highly conserved signaling pathway involved in tissue development and regeneration. Our work has explored Notch signaling during osteoblastogenesis and bone healing using both in vitro studies with human primary mesenchymal progenitor cells and in vivo studies with genetically modified mouse models. Notch signaling is required and sufficient for osteoblast differentiation, and is required for proper bone regeneration. Indeed, intact Notch signaling through the Jagged-1 ligand is required for BMP induced bone formation. On-going work continues to explore the intersection between BMP and Notch signaling, and determining cell types that express Notch receptors and Notch ligands during bone healing. Our long-term objective is to develop Notch signaling as a clinical therapy to repair bone

Bone healing outcome is highly dependent on the initial mechanical fracture environment [1]. In vivo, direct bone healing requires absolute stability and an interfragmentary strain (IFS) below 2% [2]. In the majority of cases, however, endochondral ossification is engaged where frequency and amplitude of IFS are key factors. Still, at the cellular level, the influence of those parameters remains unknown. Understanding the regulation of naïve hMSC differentiation is essential for developing effective bone healing strategies. Human bone-marrow-derived MSC (KEK-ZH-NR: 2010–0444/0) were embedded in 8% gelatin methacryol. Samples (5mm Ø x 4mm) were subjected to 0, 10 and 30% compressive strain (5sec compression, 2hrs pause sequence for 14 days) using a multi-well uniaxial bioreactor (RISystem) and in presence of chondro-permissive medium (CP, DMEM HG, 1% NEAA, 10 µM ITS, 50 µg/mL ascorbic acid, and 100 mM Dex). Cell differentiation was assessed by qRT-PCR and histo-/immunohistology staining. Experiments were repeated 5 times with cells from 5 donors in duplicate. ANOVA with Tukey post-hoc correction or Kurskal-Wallis test with Dunn's correction was used. Data showed a strong upregulation of hypertrophic related genes COMP, MMP13 and Type 10 collagen upon stimulation when compared to chondrogenic SOX9, ACAN, Type 2 collagen or to osteoblastic related genes Type 1 Collagen, Runx2. When compared to chondrogenic control medium, cells in CP with or without stimulation showed low proteoglycan synthesis as shown by Safranine-O-green staining. In addition, the cells were significantly larger in 10% and 30% strain compared to control medium with 0% strain. Type 1 and 10 collagens immunostaining showed stronger Coll 10 expression in the samples subjected to strain compared to control. Uniaxial deformation seems to mainly promote hypertrophic-like chondrocyte differentiation of MSC. Osteogenic or potentially late hypertrophic related genes are also induced by strain. Acknowledgments: Funded by the AO Foundation, StrainBot sponsored by RISystemAG & PERRENS 101 GmbH

Introduction. Cartilage homeoprotein 1 (CART-1) is a homeoprotein which has been suggested to play a role in chondrocyte differentiation and in skeletal development. It is expressed mainly in prechondrocytic mesenchymal condensations. Patients with mutations in the CART-1 gene display several craniofacial abnormalities, suggesting that CART-1 has a functional role in craniofacial skeletal development. However, its target genes and position in the established chondrogenic pathways is poorly documented. Given the fact that CART-1 is expressed predominantly in the chondrocyte lineage and its role in skeletal development, we hypothesized that CART-1 regulates expression of several pivotal genes involved in chondrogenic differentiation. Methods. The coding sequence of human CART-1 was custom synthesized with optimized codon usage and cloned into a p3XFLAG-CMV-7.1 expression vector. FLAG-CART-1 was transiently overexpressed in SW1353 cells by polyethyleneimine-mediated transfection (1,000 ng of plasmid/well in 12-well plates). FLAG-Empty vector was used as a negative control. FLAG-CART-1 overexpression was confirmed by means of anti-FLAG immunoblotting. To investigate a potential connection between CART-1 and established key chondrogenic pathways, TGFβ3 (10 ng/mL) was added to SW1353 cells in CART-1 overexpression cultures or their appropriate controls. Cells were harvested 48 hours after transfection and mRNA expression of several genes involved in chondrogenic differentiation was determined by qRT-PCR. Data represent three separate experiments performed in technical triplicate. Results. Overexpression of CART-1 was confirmed on protein level. CART-1 significantly upregulated the expression of hypertrophic markers MMP13 and COLX, while the expression of RUNX2, ALP and COL1 was significantly downregulated. The expression of COL2A1 and SOX9 was not altered in the presence of CART1. TGFβ3 significantly decreased MMP13 expression in SW1353 cultures, but induced the expression of COLX, RUNX2 and COL1. This TGFβ3 dependent behaviour was reversed when CART-1 was overexpressed in these cultures. Conclusion. Our results implicate a functional role for the homeodomain protein CART-1 in controlling the expression of several markers involved in chondrocyte differentiation and show important interactions with other signaling pathways involved in chondrogenic differentiation. Current efforts focus on further elucidating the connection between CART-1 and other chondrogenic pathways

Introduction. In vitro expansion of human articular chondrocytes (HACs) is required for cell-based strategies to treat cartilage defects. We have earlier shown that culturing HACs at increased osmolarity (i.e., 380 mOsm), as compared to plasma osmolarity (i.e., 280 mOsm), increases collagen type II (COL2A1) expression in vitro. Our earlier results showed that knockdown of TGF-β2, a prototypic member of the TGF-β superfamily and an accepted key regulator of chondrocyte differentiation, resulted in increased COL2A1 production. BMPs are members of the TGF-β superfamily which are known to be involved in the regulation of COL2A1 expression. In this study, we aimed to elucidate the role of BMP signaling, in the upregulation of COL2 production upon TGF-β2 knockdown (KD) under hyperosmotic culture conditions. Methods. HACs from five OA patients (passage 1) were cultured in cytokine-free medium, under 280 or 380 mOsm respectively, under standard 2D in vitro conditions. TGF-β2 knockdown (KD) by siRNA was performed in the presence or absence of the established bone morphogenetic protein (BMP) type I receptor (BMPRI) inhibitor dorsomorphin (10 μM). Expression of COL2A1 was evaluated by qRT-PCR. Results. Culturing HACs at 380 mOsm increased COL2A1 mRNA expression. Addition of dorsomorphin decreased COL2A1 mRNA expression at both 280 and 380 mOsm, but its expression was still significantly higher at 380 mOsm. In hyperosmotic 380 mOsm culture conditions, TGF-β2 KD further increased COL2A1 mRNA expression, while addition of dorsomorphin under these conditions abrogated this effect. Still, expression of COL2A1 mRNA levels remained higher as compared to 280 mOsm. Conclusion. This study confirms that BMP signalling is involved in the expression of the single best accepted key chondrocyte marker, COL2A1, in osteoarthritic HACs. However, inhibition of BMP signalling could not abrogate the increase in COL2A1 expression under hyperosmotic culture conditions. Our data suggest an inverse regulation of TGF-β2 and COL2A1, under these conditions, which may largely be dependent on increased BMPRI-mediated cell signaling. Our findings further suggest that hyperosmotic culture improves COL2A1 expression by means that are independent of TGF-β- and BMPRI-signaling. Further elucidation of the molecular network underlying this observation is ongoing

Adult chondrocytes experience a hypoxic environment in vivo. Culturing chondrocytes under oxygen tension that more closely resembles the in vivo situation, i.e. hypoxic conditions, has been shown to have positive effects on matrix synthesis. During redifferentiation of expanded chondrocytes, hypoxia increased collagen type II expression. However, the mechanism by which hypoxia enhances redifferentiation is still incompletely elucidated. We employed micro-bioreactor technology to elucidate the contribution of TGF-β superfamily ligands to the chondrocyte differentiation process under hypoxic conditions in vitro. Dedifferentiated chondrocytes in alginate were cultured for 48 hours under hypoxic (1% pO2) or normoxic (20%) conditions, using specialized bioreactor technology. Gene expression of chondrocyte-specific markers (SOX9, COL2A1, COL1A1, AGC1 and MMP13) as well as established hypoxia-controlled genes (GDF1-, PHD3, HAS2, VEGF, COX2) and components of the TGF-β superfamily signaling pathways were analyzed by qPCR and protein expression after 48 hours in combination with TGF-β superfamily ligand-specific siRNA as well as selected TGF-β superfamily receptor inhibitors. Hypoxic culture showed robust upregulation of the selected hypoxia-specific marker genes. In addition, well-established chondrocyte-specific markers like SOX9 and collagen type II were upregulated. TGF-β isoforms were selectively upregulated under hypoxia on both mRNA and protein level. In addition, both Activin receptor-like kinases, ALK1 and ALK5, were upregulated under hypoxia, while respective type II and III receptors were unresponsive. The hypoxia-induced COL2 expression was abrogated by TGF-β2 siRNA, as was ALK5 inhibition. Our data strongly indicates that TGF-β superfamily signaling pathways are involved in chondrocyte redifferentiation under low oxygen tension in vitro

Background. Transcription factor nuclear factor E2p45-related factor 2 (Nrf2) is crucial for controlling the antioxidant response and maintaining cellular redox homeostasis. Binding of Nrf2 to antioxidant response elements (ARE) promotes the expression of anti-oxidative stress enzymes. In osteoblasts, Nrf2 directly interacts with Runx2, a strong transcriptional activator of osteoblast-specific genes. Sox9, a key regulator of chondrocyte differentiation is dominant over Runx2 in mesenchymal chondrogenic precursors. We therefore aimed to elucidate the role of Nrf2, and its regulation of Sox9, in chondrocytes. Methods. ARE sites in SOX9 promoter fragments were inactivated and cloned into pGL3 prior to co-transfection with phRL-TK into C-28/I2 cells for dual luciferase assay (n=4). Analyses of Nrf2 and Sox9 expression (n=3), following Nrf2 RNA interference (RNAi) (Sigma-Mission shRNAs library), was performed by qPCR (Applied Biosystems) as well as by Nrf2 and Sox9 immunohistochemistry in femoral condyle cartilage of wild type (WT) and Nrf2-knockout (KO) mice with ethical approval. Results. The Sox9 promoter region contains several putative antioxidant response elements. Mutagenesis of the ARE2 site reduced SOX9 promoter activity by 50%. Successful knock-down of Nrf2 using Nrf2-specific shRNAs in C-28/I2 chondrocytes also revealed parallel suppression of Sox9 mRNA. Furthermore, Nrf2-KO mice have fewer Sox9-positive-chondrocytes in their articular cartilage compared to WT littermates. Conclusions. Successive deletion of two putative ARE sites in the SOX9 promoter region suggests that ARE2 positively regulates SOX9 transcription and is in line with Sox9 mRNA suppression upon Nrf-2-RNAi. Nrf2 binding may thus directly stimulate Sox9 expression. Nrf2-KO mice reveal reduced numbers of Sox9-positive hyaline chondrocytes, which may have important consequences for the extracellular matrix production in these animals. Our findings reveal a novel mechanism regulating extracellular matrix synthesis in chondrocytes and may improve cartilage regenerative medicine. Level of evidence. Preclinical

Background. Treatment of cartilage defects requires in vitro expansion of human articular chondrocytes (HACs) for autologous chondrocyte implantation (ACI). During standard expansion culture (i.e. plasma osmolarity, 280 mOsm) chondrocytes inevitably lose their specific phenotype (i.e. collagen type II (COL2) expression). This de-differentiation makes them inappropriate for ACI. Physiological osmolarity (i.e. 380 mOsm) improves COL2 expression in vitro, but the underlying reason is unknown. However, an accepted key regulator of chondrocyte differentiation, transforming growth factor beta (TGFβ), is known to stimulate COL2 production. In this study we aimed to elucidate if TGFβ signaling could potentially be driving the COL2 expression under physiological culture conditions. Material and methods. After informed consent was obtained, HACs were isolated from five osteoarthritis (OA) patients and cultured in cytokine-free medium of 280 or 380 mOsm, respectively, under standard 2D in vitro conditions with or without lentiviral TGFβ2 knockdown (RNAi). Expression of TGFβ isoforms, superfamily receptors and chondrocyte marker genes was evaluated by qRT-PCR, TGFβ2 protein secretion by ELISA and TGFβ bioactivity using luciferase reporter assays. Statistical significance was assessed by a student's t-test. Results. TGFβ isoform expression was differentially altered by physiological osmolarity. Specifically, 380 mOsm increased TGFβ2 expression and protein secretion, as well as TGFβ activity. Upon TGFβ2 isoform-specific knockdown COL2 expression was induced. Physiological osmolarity and TGFβ2 RNAi also induced TGFβ1, TGFβ3 and their type I receptor ALK5. Conclusions. We showed that TGFβ2 knockdown increases COL2 expression in human osteoarthritic chondrocytes in vitro, possibly through a regulatory feedback loop involving TGFβ1, TGFβ3 induction and an increased ALK5/ALK1 ratio. This study indicates that TGFβ signalling is involved in osmolarity-induced chondrocyte marker gene expression. Pharmacological targeting of this pathway holds potential to further improve future osmolarity-mediated phenotypic stabilisation in advanced cell-based cartilage repair strategies. Level of Evidence. preclinical. Disclosure. We have nothing to disclose

As cartilage has poor intrinsic repair capacity, in vitroexpansion of human articular chondrocytes (HACs) is required for cell-based therapies to treat cartilage pathologies. During standard expansion culture (i.e. plasma osmolarity, 280 mOsm) chondrocytes inevitably lose their specific phenotype and de-differentiate, which makes them inappropriate for autologous chondrocyte implantation. It has been shown that physiological osmolarity (i.e. 380 mOsm) increases collagen type II (COL2) expression in vitro, but the underlying molecular mechanism is unknown. Transforming growth factor beta (TGFβ) super family members are accepted key regulators of chondrocyte differentiation and known to stimulate COL2 production. In this study we aimed to elucidate the role of TGFβ superfamily member signalling as a molecular mechanism potentially driving the COL2 expression under physiological (380 mOsm) culture conditions. HACs from OA patients (p1) were cultured in cytokine-free medium of 280 or 380 mOsm, under standard 2D in vitroconditions, with or without lentiviral TGFβ2 knockdown (RNAi). Expression of TGFβ isoforms, BMPs and chondrocyte marker genes was evaluated by QPCR. TGFβ2 protein secretion was evaluated using ELISA and bioactivity was determined using an established reporter cell line. Involvement of BMP signaling was investigated by culturing OA HACs (p1) in the presence or absence of dorsomorphin (10 µM). Physiological osmolarity increased TGFβ2 and TGFβ3 mRNA expression, TGFβ2 protein secretion as well as general TGFβ activity by 380 mOsm. Upon TGFβ2 isoform-specific knockdown COL2 mRNA expression was induced. TGFβ2 RNAi induced expression of several BMPs (e.g. BMP2,-4,-6) and this induction was enhanced in culture conditions with physiological osmolarity. Dorsomorphin inhibited physiological osmolarity induced COL2 mRNA expression. TGFβ2 knockdown under 380 mOsm increases COL2 expression in human osteoarthritic chondrocytes in vitromost likely through a regulatory feedback loop via BMP signaling, which is involved in osmolarity-induced COL2 expression. Future studies will further elucidate the BMP-mediated regulatory feedback loop after TGF β2 knockdown and its influence on COL2 expression

Our aim was to evaluate the expression of transcription factors CCAAT/enhancer-binding protein-beta (C/EBP. β. ) and C/EBP-homologous protein (CHOP) in the growth plate. Proximal tibial epiphyseal growth plates from ten 15-day-old Wistar rats were used. Additionally, anti-proliferating cell nuclear antigen (PCNA), anti-5-bromo-2’-deoxyuridine (BrdU) immunostaining, terminal transferase dUTP nick end-labelling (TUNEL) and nucleolar organiser region-associated proteins (AgNOR) techniques were peformed. The histological morphology of the growth plate from C/EBP. β. knock-out mice was also analysed. The normal growth plate showed that C/EBP. β. and CHOP factors are expressed both in the germinative/ upper proliferative and in the lower proliferative zones. Furthermore, BdrU+ and PCNA+ cells were present exclusively in the germinative and proliferative zones, while TUNEL+ and AgNOR+ cells were seen in all three zones of the growth plate. Acellular areas, hypocellularity, the increase in cell death and anomalies in the architecture of the cell columns were observed in the growth plates of C/EBP. β. (−/ −) knockout mice. We suggest that C/EBP. β. and CHOP transcription factors may be key modulators participating in the chondrocyte differentiation process in the growth plate

Bone morphogenetic protein (BMP) has a crucial role in osteochondrogenesis of bone formation as well as in the repair of fractures. The interaction between hedgehog protein and BMPs is inferred from recent molecular studies. Hedgehog genes encode secreted proteins which mediate patterning and growth during skeletal development. We have shown that Indian hedgehog gene (Ihh) is expressed in cartilage anlage and later in mature and hypertrophic chondrocytes. This finding suggests that Ihh may regulate the development of chondrocytes. Our results in this study have shown that Ihh transcripts were expressed in hypertrophic chondrocytes in mice at three days but not at three weeks, although a similar expression pattern of α1 (X) collagen could be observed in both types of cartilage. To investigate the possibility that there are direct and age-dependent functions of Ihh in chondrocytes, cultured chondrocytes were treated with the amino-terminal fragment of Sonic hedgehog protein (Shh-N) which can functionally substitute for Ihh protein. Shh-N did not affect the proliferation and differentiation of chondrocytes from three-week-old mice but had a significant effect on three-day-old mice. It enhanced proliferation up to 128% of the control culture in a dose-dependent manner. Although there was no effect in Shh-N-treated cultures, Shh-N enhanced the stimulatory effect of parathyroid hormone (PTH) on the synthesis of proteoglycans. Because the effects of Shh-N on chondrocyte differentiation in this culture system differed from those of bone morphogenetic protein-2 (BMP2) and PTH, in terms of proteoglycan synthesis and ALPase activity, it is unlikely that BMP2 or PTH/PTH-related protein mediates the direct effects of Ihh in chondrocytes. Our study shows that Ihh can function in chondrocytes in a direct and age-dependent fashion

Summary Statement. Wnt/β-catenin gene expression is altered in early osteochondrosis, particularly in chondrocytes surrounding cartilage canals, and may be associated with disease initiation and/or pathogenesis. Introduction. Osteochondrosis (OC) is a disease of articular cartilage development involving abnormal endochondral ossification along the osteochondral junction. Associated etiological factors of OC have included rapid growth rate, biomechanical trauma, abnormal collagen turnover, aberrant paracrine signaling, and altered blood supply involving cartilage canals. Wnt signaling regulates chondrocyte differentiation/maturation during pre-/post-natal cartilage development. Gene expression profiling of leukocytes has revealed aberrant expression of Wnt/β-catenin pathway in early OC. The objective of this study was to elucidate the expression of molecules associated with Wnt/β-catenin signaling in early OC using an equine model. Our hypothesis was that there would be increased expression of Wnt signaling molecules in chondrocytes adjacent to cartilage canals and the osteochondral junction in early OC lesions compared to normal controls. Patients & Methods. Osteochondral samples were obtained (IACUC-approved) from femoropatellar joints of 15 euthanised immature horses (1–6 months old). Disease status was determined based on histology of osteochondral junctions (7 early OC, 8 normal controls). Osteochondral sections were frozen in OCT for laser capture microdissection (LCM) or fixed in 4% paraformaldehyde and paraffin-embedded for immunohistochemistry. Chondrocytes surrounding cartilage canals and osteochondral junctions were captured using LCM. RNA isolation and reverse transcription were performed. Equine-specific β-catenin, Wnt-4, Wnt-5b, Wnt-11, Dickkopf-1(Dkk-1), Lrp-4 and -6, Axin1, Wnt inhibitory factor(WIF)-1, secreted Frizzled-related protein-1, -3, and -5(Sfrp), retinoic acid receptor-gamma(RARG), RAR-inducible serine carboxypeptidase(SC-PEP) and 18S mRNA expression was evaluated by two-step real-time qPCR. Spatial protein expression was determined by immunohistochemistry using rabbit polyclonal (β-catenin, Wnt-11) or mouse monoclonal (Wnt-4, Dkk1) primary antibodies (confirmed by Western blot). Statistical analysis of early OC vs. normal controls was performed using Wilcoxon rank sum test (p <0.05). Results. Chondrocytes adjacent to cartilage canals had significantly increased gene expression of β-catenin (p=0.026), Wnt-5b (p=0.04), Lrp6 (p=0.026), WIF-1 (p=0.026), Dkk-1 (p=0.015), Axin1 (0.041), and SC-PEP (p=0.026), and decreased expression of Wnt-11 (p=0.04), in OC vs. normal controls. OC chondrocytes along osteochondral junctions had significantly increased gene expression of β-catenin (p=0.004) and SC-PEP (p=0.026), with a trend for increased Wnt-4 (p= 0.06) and Wnt-5b (p=0.06) compared to normal controls. Immunostaining for β-catenin was moderate in deep cartilage layers, including osteochondral junction chondrocytes. Wnt-4 immunostaining was moderate along the osteochondral junction and minimal along cartilage canals. Strong Wnt-11 protein expression was apparent in superficial cartilage layers and vascular cells lining cartilage canals and osteochondral junction. Mild to moderate Dkk1 immunostaining was found along osteochondral junction. Discussion/Conclusion. Wnt/β-catenin signaling regulates cartilage differentiation during development and is important in endochondral ossification. Increased gene expression of β-catenin in OC chondrocytes may affect chondrocyte hypertrophy or induce cartilage degeneration, depending on the stage of cartilage development, as β-catenin has been shown to play a dual role in cartilage growth and degeneration. In cells surrounding cartilage canals, increased gene expression of Lrp6, a co-receptor for Wnt proteins, provides further evidence of upregulation of canonical signaling in OC. However, increased Wnt inhibitor gene expression in OC chondrocytes, including Dkk1, WIF-1, and Axin1, may be an attempt to control activation of the canonical pathway

Summary Statement. Differential expression of canonical and noncanonical Wnt signalling along cartilage canals and osteochondral junctions is dependent on age. Increased gene expression of PTHrP along cartilage canals and Ihh along osteochondral junctions suggests paracrine feedback in articular-epiphyseal cartilage. Introduction. Wnt signaling has been shown to regulate chondrocyte differentiation during pre-/post-natal cartilage development. In addition, parathyroid-related peptide(PTHrP) and Indian hedgehog(Ihh) create a negative feedback loop in growth cartilage, but less is known in articular cartilage. The objective of this study was to elucidate expression of regulatory molecules in chondrocytes surrounding cartilage canals and osteochondral junctions during neonatal and pre-adolescent development. We hypothesised there would be increased expression of canonical Wnt signalling molecules and Ihh in osteochondral junction chondrocytes compared to cartilage canal chondrocytes. In addition, we hypothesised that Wnt signaling and PTHrP expression would be greater in neonates than pre-adolescents. Patients & Methods. Osteochondral samples were obtained(IACUC-approved) from normal femoropatellar joints of 14 euthanised immature horses(6 neonates, 8 pre-adolescents). Samples were frozen in OCT for laser capture microdissection(LCM) or fixed in 4% paraformaldehyde and paraffin-embedded for immunohistochemistry. Chondrocytes surrounding cartilage canals and osteochondral junctions were captured using LCM. Following RNA isolation, equine-specific β-catenin, Wnt-4, Wnt-5b, Wnt-11, Dickkopf-1(Dkk-1), low-density lipoprotein receptor-related protein-4,-6(Lrp4, Lrp6), Axin1, Wnt inhibitory factor-1(WIF)-1, secreted Frizzled-related protein-1,-3,-5(sFRP), retinoic acid receptor gamma(RARG), RAR-inducible serine carboxypeptidase(SC-PEP), Ihh, PTHrP, VEGF, PDGF, MMP-13, and 18S mRNA expression levels were evaluated by two-step real-time qPCR. Following immunohistochemistry using rabbit polyclonal or mouse monoclonal primary antibodies (confirmed by Western blot), spatial tissue protein expression was scored (0–3). Statistical analysis included Wilcoxon signed rank test(paired samples) or rank sum test(unpaired samples)(P<0.05). Results. Gene expression in chondrocytes along cartilage canals was significantly higher for PTHrP, β-catenin, Lrp6, Axin1, sFRP5, RARgamma, and SC-PEP than osteochondral junctions. Conversely, gene expression of Ihh, Wnt4, Wnt11, sFRP3, and VEGF were higher in osteochondral junction chondrocytes than cartilage canals. There was higher protein expression of β-catenin, PDGF, VEGF, and MMP-13 along osteochondral junctions than cartilage canals of pre-adolescents. Neonates had higher gene expression of PTHrP, Wnt-5b, sFRP3, Lrp6, and RARG in cartilage canal chondrocytes than pre-adolescents, while Ihh, Wnt-11, Lrp4, and Dkk1 were significantly higher in pre-adolescents. Immunostaining was higher for β-catenin and Wnt-11 in pre-adolescent osteochondral junction cartilage than neonates. No differences were found between age groups for Wnt-4 immunostaining. Dkk1 protein expression was significantly higher in the middle cartilage layer of pre-adolescents than neonates. Immunostaining was greater for Ihh and PTHrP in the deep cartilage layer of pre-adolescents than neonates. PDGF, VEGF, and MMP13 protein expression was higher in the superficial cartilage layer of pre-adolescents than neonates. Discussion. Wnt/β-catenin and Ihh/PTHrP signaling regulate cartilage differentiation during development and are important in endochondral ossification. This study revealed cell-specific, age-related differences in gene/protein expression of both regulatory pathways. Cells surrounding cartilage canals typically appeared small/rounded compared to larger chondrocytes along osteochondral junctions, likely due to different developmental stages. Higher PTHrP gene expression along cartilage canals and Ihh expression along osteochondral junctions may reflect these stages, suggesting paracrine feedback in articular-epiphyseal cartilage. β-catenin signaling may induce chondrocyte hypertrophy, potentially by enhancing Ihh and MMP-13 expression. Differential expression of canonical(β-catenin, Wnt-4, Lrp4, Lrp6) and noncanonical Wnt signalling(Wnt-5b, Wnt-11) and Wnt inhibitors (Dkk1, Axin1, sFRP3, sFRP5, Wif-1) surrounding cartilage canals and osteochondral junctions provides evidence of age-related interactions during postnatal development

Chondrogenic differentiation and cartilage homeostasis requires a high cellular translational capacity to meet the demands for cartilaginous extracellular matrix production. Box C/D and H/ACA snoRNAs guide post-transcriptional 2′-O ribose methylation and pseudouridylation of specific ribosomal RNA (rRNA) nucleotides, respectively. How specific rRNA modifications influence rRNA function is poorly documented, but modifications are thought to tune rRNA folding and interaction with ribosomal proteins, which is critical for ribosome function. We hypothesise that chondrocyte translational capacity is supported by snoRNA-mediated post-transcriptional fine-tuning of rRNAs. ATDC5 progenitor cells were differentiated into the chondrogenic lineage, resembling mature and mineralising chondrocytes after 7 or 14 days, respectively. UBF-1 (rRNA transcription factor), fibrillarin (box C/D methyltransferase) and dyskerin (box H/ACA pseudouridylase) expression displayed highest fold induction at day 5/6 in differentiation. Ribosomal RNA content per cell was increased at day 7, but not at day 14 in differentiation. These data suggest that ribosome biogenesis adapts to the chondrocyte's differentiation status. RNA-Seq of RNA species <200 nt revealed expression of at least 224 individual snoRNAs. Due to initiation of chondrogenic differentiation (Δt0-t7), 21 snoRNAs were differentially expressed (DE; FDRadj-p<0.05, logFC>1or<−1). Mineralization (Δt7-t14) induced DE of 23 snoRNAs. Comparing t0 with t14 resulted in DE of 43 snoRNAs. To anticipate on the biological relevance of DE snoRNAs, their rRNA target nucleotides were plotted in 18S, 5.8S and 28S rRNA secondary structures. This revealed that DE snoRNAs, amongst others, target nucleotide modifications in the 28S peptidyl transferase center and the 18S decoding center (DC). Snora40 was DE, targeting helix 27/18S rRNA. Helix 27 controls DC function. Helix 68 of 28S rRNA is part of the ribosome's E-site, therefore, DE snord36c and snora31 (targeting helix 68) could potentially fine-tune the translation mechanism. As a final example we found snord46 to be DE (target: helix 69/28S rRNA). Mutations in helix 69 have been shown to severely affect cell viability. Our data show that increased demand for translational capacity during chondrogenic differentiation is associated with differential expression of snoRNAs, potentially controlling ribosome fidelity via site-specific rRNA-modifications. These data enable us to determine the role of individual snoRNAs in tuning the chondrocyte's translational properties and current efforts focus on confirming site-specific rRNA-modifications and determine their biological relevance

Heterotopic ossi?cation is the abnormal formation of bone in soft tissues and is a frequent complication of hip replacement surgery. Heterotopic ossi?cations are described to develop via endochondral ossification and standard treatment is administration of indomethacin. It is currently unknown how indomethacin influences heterotopic ossi?cation on a molecular level, therefore we aimed to determine whether indomethacin might influence heterotopic ossi?cation via impairing the chondrogenic phase of endochondral ossification. ATDC5, human bone marrow stem cells (hBMSCs) and rabbit periosteal agarose cultures were employed as progenitor cell models; SW1353, human articular chondrocytes and differentiated ATDC5 cells were used as matured chondrocyte cell models. All cells were cultured in the presence of (increasing) concentrations of indomethacin. The action of indomethacin was confirmed by decreased PGE2 levels in all experiments, and was determined by specific PGE2 ELISA. Gene- and protein expression analyses were employed to determine chondrogenic outcome. Progenitor cell models differentiating in the chondrogenic lineage (ATDC5, primary human bone marrow stem cells and ex vivo periosteal agarose cultures) were treated with increasing concentrations of indomethacin and a dose-dependent decrease in gene- and protein expression of chondrogenic and hypertrophic markers as well as decreased glycosaminoglycan content was observed. Even when hypertrophic differentiation was provoked the addition of indomethacin resulted in decreased hypertrophic marker expression. Interestingly, when mature chondrocytes were treated with indomethacin, a clear increase in collagen type 2 expression was observed. Similarly, when ATDC5 cells and bone marrow stem cells were pre-differentiated to obtain a chondrocyte phenotype and indomethacin was added from this time point onwards, low concentrations of indomethacin also resulted in increased chondrogenic differentiation. Indomethacin induces differential effects on in vitro endochondral ossification, depending on the chondrocyte's differentiation stage, with complete inhibition of chondrogenic differentiation as the most pronounced action. This observation may provide a rationale behind the elusive mode of action of indomethacin in the treatment of heterotopic ossifications

Objectives

The role of mechanical stress and transforming growth factor beta 1 (TGF-β1) is important in the initiation and progression of osteoarthritis (OA). However, the underlying molecular mechanisms are not clearly known.

Objectives

This study aimed to investigate the functional effects of microRNA (miR)-214-5p on osteoblastic cells, which might provide a potential role of miR-214-5p in bone fracture healing.

Objectives

After an injury, the biological reattachment of tendon to bone is a challenge because healing takes place between a soft (tendon) and a hard (bone) tissue. Even after healing, the transition zone in the enthesis is not completely regenerated, making it susceptible to re-injury. In this study, we aimed to regenerate Achilles tendon entheses (ATEs) in wounded rats using a combination of kartogenin (KGN) and platelet-rich plasma (PRP).