Smad6 is induced by BMP-2 and modulates chondrocyte differentiation

Uncovering pathways regulating chondrogenic differentiation of CHH fibroblasts

Mutations in the non-coding snoRNA component of mitochondrial RNA processing endoribonuclease (RMRP) are the cause of cartilage-hair hypoplasia (CHH). CHH is a rare form of metaphyseal chondrodysplasia characterized by disproportionate short stature and abnormal growth plate development. The process of chondrogenic differentiation within growth plates of long bones is vital for longitudinal bone growth. However, molecular mechanisms behind impaired skeletal development in CHH patients remain unclear. We employed a transdifferentiation model (FDC) combined with whole transcriptome analysis to investigate the chondrogenic transdifferentiation capacity of CHH fibroblasts and to examine pathway regulation in CHH cells during chondrogenic differentiation. We established that the FDC transdifferentiation model is a relevant in vitro model of chondrogenic differentiation, with an emphasis on the terminal differentiation phase, which is crucial for longitudinal bone growth. We demonstrated that CHH fibroblasts are capable of transdifferentiating into chondrocyte-like cells, and show a reduced commitment to terminal differentiation. We also found a number of key factors of BMP, FGF, and IGF-1 signalling axes to be significantly upregulated in CHH cells during the chondrogenic transdifferentiation. Our results support postulated conclusions that RMRP has pleiotropic functions and profoundly affects multiple aspects of cell fate and signalling. Our findings shed light on the consequences of pathological CHH mutations in snoRNA RMRP during chondrogenic differentiation and the relevance and roles of non-coding RNAs in genetic diseases in general.

Exploring the Association Between Genetic Polymorphisms in Genes Involved in Craniofacial Development and Isolated Tooth Agenesis

Tooth agenesis is a common congenital anomaly in humans and is more common in oral cleft patients than in the general population. Many previous studies suggested that oral cleft and tooth agenesis share a similar genetic background. Therefore, this study explored the association between isolated tooth agenesis and genetic polymorphisms in genes that are crucial for craniofacial and tooth development. Panoramic radiographs, anamnesis, and genomic DNA from 273 patients were included. Patients were classified as tooth agenesis present, when at least one permanent tooth was congenitally missing. Patients with syndromes and oral cleft were excluded. Only unrelated patients were included. The genetic polymorphisms in BMP2 (rs235768 and rs1005464), BMP4 (rs17563), RUNX2 (rs59983488 and rs1200425), and SMAD6 (rs3934908 and rs2119261) were genotyped by real-time polymerase chain reaction. Genotype and allele distributions were compared between the tooth agenesis phenotypes and controls by Chi-square test. Haplotype and diplotype analysis were also performed, in addition to multivariate analysis (alpha of 0.05). A total of 86 tooth agenesis cases and 187 controls were evaluated. For the rs235768 in BMP2, patients carrying TT genotype have higher chance to present tooth agenesis [p < 0.001; prevalence ratio (PR) = 8.29; 95% confidence interval (CI) = 4.26–16.10]. The TT genotype in rs3934908 (SMAD6) was associated with higher chance to present third molar agenesis (p = 0.023; PR = 3.25; 95% CI = 1.17–8.99). BMP2 was also associated in haplotype and diplotype analysis with tooth agenesis. In conclusion, genetic polymorphisms in BMP2 and SMAD6 were associated with isolated tooth agenesis.

Signaling Pathways in Bone Development and Their Related Skeletal Dysplasia

Bone development is a tightly regulated process. Several integrated signaling pathways including HH, PTHrP, WNT, NOTCH, TGF-β, BMP, FGF and the transcription factors SOX9, RUNX2 and OSX are essential for proper skeletal development. Misregulation of these signaling pathways can cause a large spectrum of congenital conditions categorized as skeletal dysplasia. Since the signaling pathways involved in skeletal dysplasia interact at multiple levels and have a different role depending on the time of action (early or late in chondrogenesis and osteoblastogenesis), it is still difficult to precisely explain the physiopathological mechanisms of skeletal disorders. However, in recent years, significant progress has been made in elucidating the mechanisms of these signaling pathways and genotype–phenotype correlations have helped to elucidate their role in skeletogenesis. Here, we review the principal signaling pathways involved in bone development and their associated skeletal dysplasia.

Gremlin, Noggin, Chordin and follistatin differentially modulate BMP induced suppression of androgen secretion by bovine ovarian theca cells

Bone morphogenetic proteins (BMP) are firmly implicated as intra-ovarian regulators of follicle function and steroidogenesis but information is lacking regarding the regulation of BMP signalling by extracellular binding proteins co-expressed in the ovary. In this study we compared the abilities of four BMP binding proteins (gremlin, noggin, chordin, follistatin) to antagonize the action of four different BMPs (BMP2 BMP4, BMP6, BMP7) on LH-induced androstenedione secretion by bovine theca cells in primary culture. Expression of the four BMP binding proteins and BMPs investigated here has previously been documented in bovine follicles. All four BMPs suppressed androstenedione secretion by >85%. Co-treatment with gremlin antagonized BMP2- and, less potently, BMP4-induced suppression of androgen secretion but did not affect responses to BMP6 and BMP7. Noggin antagonized the effects of three BMPs (rank order: BMP4 > BMP2 > BMP7) but did not affect the response to BMP6. Follistatin partially reversed the suppressive effects of BMP6 on androgen secretion but did not affect BMP2, BMP4 and BMP7 action. Chordin had no effect on the response to any of the four BMPs. BMP6 treatment upregulated thecal expression of GREM1, NOG, CHRD and SMAD6 mRNA whilst inhibiting expression of the four BMPs. Taken together with previous work documenting the intra-ovarian expression of different BMPs, BMP binding proteins and signalling receptors, these observations reinforce the conclusion that extracellular binding proteins selectively modulate BMP-dependent alterations in thecal steroidogenesis. As such they likely constitute an important regulatory component of this, and other intra-ovarian actions of BMPs.

Sox9 augments BMP2-induced chondrogenic differentiation by downregulating Smad7 in mesenchymal stem cells (MSCs)

Cartilage injuries caused by arthritis or trauma pose formidable challenges for effective clinical management due to the limited intrinsic proliferative capability of chondrocytes. Autologous stem cell-based therapies and transgene-enhanced cartilage tissue engineering may open new avenues for the treatment of cartilage injuries. Bone morphogenetic protein 2 (BMP2) induces effective chondrogenesis of mesenchymal stem cells (MSCs) and can thus be explored as a potential therapeutic agent for cartilage defect repair. However, BMP2 also induces robust endochondral ossification. Although the precise mechanisms through which BMP2 governs the divergence of chondrogenesis and osteogenesis remain to be fully understood, blocking endochondral ossification during BMP2-induced cartilage formation may have practical significance for cartilage tissue engineering. Here, we investigate the role of Sox9-donwregulated Smad7 in BMP2-induced chondrogenic differentiation of MSCs. We find that overexpression of Sox9 leads to a decrease in BMP2-induced Smad7 expression in MSCs. Sox9 inhibits BMP2-induced expression of osteopontin while enhancing the expression of chondrogenic marker Col2a1 in MSCs. Forced expression of Sox9 in MSCs promotes BMP2-induced chondrogenesis and suppresses BMP2-induced endochondral ossification. Constitutive Smad7 expression inhibits BMP2-induced chondrogenesis in stem cell implantation assay. Mouse limb explant assay reveals that Sox9 expands BMP2-stimulated chondrocyte proliferating zone while Smad7 promotes BMP2-intitated hypertrophic zone of the growth plate. Cell cycle analysis indicates that Smad7 induces significant early apoptosis in BMP2-stimulated MSCs. Taken together, our results strongly suggest that Sox9 may facilitate BMP2-induced chondrogenesis by downregulating Smad7, which can be exploited for effective cartilage tissue engineering.

TGF-β Family Signaling in Connective Tissue and Skeletal Diseases

The transforming growth factor β (TGF-β) family of signaling molecules, which includes TGF-βs, activins, inhibins, and numerous bone morphogenetic proteins (BMPs) and growth and differentiation factors (GDFs), has important functions in all cells and tissues, including soft connective tissues and the skeleton. Specific TGF-β family members play different roles in these tissues, and their activities are often balanced with those of other TGF-β family members and by interactions with other signaling pathways. Perturbations in TGF-β family pathways are associated with numerous human diseases with prominent involvement of the skeletal and cardiovascular systems. This review focuses on the role of this family of signaling molecules in the pathologies of connective tissues that manifest in rare genetic syndromes (e.g., syndromic presentations of thoracic aortic aneurysm), as well as in more common disorders (e.g., osteoarthritis and osteoporosis). Many of these diseases are caused by or result in pathological alterations of the complex relationship between the TGF-β family of signaling mediators and the extracellular matrix in connective tissues.

Combinatorial Signal Perception in the BMP Pathway

The bone morphogenetic protein (BMP) signaling pathway comprises multiple ligands and receptors that interact promiscuously with one another and typically appear in combinations. This feature is often explained in terms of redundancy and regulatory flexibility, but it has remained unclear what signal-processing capabilities it provides. Here, we show that the BMP pathway processes multi-ligand inputs using a specific repertoire of computations, including ratiometric sensing, balance detection, and imbalance detection. These computations operate on the relative levels of different ligands and can arise directly from competitive receptor-ligand interactions. Furthermore, cells can select different computations to perform on the same ligand combination through expression of alternative sets of receptor variants. These results provide a direct signal-processing role for promiscuous receptor-ligand interactions and establish operational principles for quantitatively controlling cells with BMP ligands. Similar principles could apply to other promiscuous signaling pathways.

TGF-β and BMP signaling in osteoblast, skeletal development, and bone formation, homeostasis and disease

Transforming growth factor-beta (TGF-β) and bone morphogenic protein (BMP) signaling has fundamental roles in both embryonic skeletal development and postnatal bone homeostasis. TGF-βs and BMPs, acting on a tetrameric receptor complex, transduce signals to both the canonical Smad-dependent signaling pathway (that is, TGF-β/BMP ligands, receptors, and Smads) and the non-canonical-Smad-independent signaling pathway (that is, p38 mitogen-activated protein kinase/p38 MAPK) to regulate mesenchymal stem cell differentiation during skeletal development, bone formation and bone homeostasis. Both the Smad and p38 MAPK signaling pathways converge at transcription factors, for example, Runx2 to promote osteoblast differentiation and chondrocyte differentiation from mesenchymal precursor cells. TGF-β and BMP signaling is controlled by multiple factors, including the ubiquitin–proteasome system, epigenetic factors, and microRNA. Dysregulated TGF-β and BMP signaling result in a number of bone disorders in humans. Knockout or mutation of TGF-β and BMP signaling-related genes in mice leads to bone abnormalities of varying severity, which enable a better understanding of TGF-β/BMP signaling in bone and the signaling networks underlying osteoblast differentiation and bone formation. There is also crosstalk between TGF-β/BMP signaling and several critical cytokines’ signaling pathways (for example, Wnt, Hedgehog, Notch, PTHrP, and FGF) to coordinate osteogenesis, skeletal development, and bone homeostasis. This review summarizes the recent advances in our understanding of TGF-β/BMP signaling in osteoblast differentiation, chondrocyte differentiation, skeletal development, cartilage formation, bone formation, bone homeostasis, and related human bone diseases caused by the disruption of TGF-β/BMP signaling.

Canonical Wnt signaling skews TGF-β signaling in chondrocytes towards signaling via ALK1 and Smad 1/5/8

BACKGROUND

Both Wnt signaling and TGF-β signaling have been implicated in the regulation of the phenotype of many cell types including chondrocytes, the only cell type present in the articular cartilage. A changed chondrocyte phenotype, resulting in chondrocyte hypertrophy, is one of the main hallmarks of osteoarthritis. TGF-β signaling via activin-like kinase (ALK)5, resulting in Smad 2/3 phosphorylation, inhibits chondrocyte hypertrophy. In contrast, TGF-β signaling via ALK1, leading to Smad 1/5/8 phosphorylation, has been shown to induce chondrocyte hypertrophy. In this study, we investigated the capability of Wnt3a and WISP1, a protein downstream in canonical Wnt signaling, to skew TGF-β signaling in chondrocytes from the protective Smad 2/3 towards the Smad 1/5/8 pathway.

RESULTS

Stimulation with Wnt3a, either alone or in combination with its downstream protein WISP1, decreased TGF-β-induced C-terminal phosphorylation of Smad 2/3. In addition, both Wnt3a and WISP1 increased Smad 1/5/8 phosphorylation at the C-terminal domain in both murine and human chondrocytes. DKK-1, a selective inhibitor of canonical Wnt signaling, abolished these effects. TGF-β signaling via Smad 2/3, measured by the functional CAGA12-Luc reporter construct activity, was decreased by stimulation with Wnt3a in accordance with the decrease in Smad 2/3 phosphorylation found on Western blot. Furthermore, in vivo overexpression of the canonical Wnt8a decreased Smad 2/3 phosphorylation and increased Smad 1/5/8 phosphorylation.

CONCLUSIONS

Our data show that canonical Wnt signaling is able to skew TGF-β signaling towards dominant signaling via the ALK1/Smad 1/5/8 pathway, which reportedly leads to chondrocyte hypertrophy. In this way canonical Wnts and WISP1, which we found to be increased during experimental osteoarthritis, may contribute to osteoarthritis pathology.

Smad7 regulates terminal maturation of chondrocytes in the growth plate

Members of the bone morphogenetic protein (BMP) superfamily, including transforming growth factor-betas (TGFβ), regulate multiple aspects of chondrogenesis. Smad7 is an intracellular inhibitor of BMP and TGFβ signaling. Studies in which Smad7 was overexpressed in chondrocytes demonstrated that Smad7 can impact chondrogenesis by inhibiting BMP signaling. However, whether Smad7 is actually required for endochondral ossification in vivo is unclear. Moreover, whether Smad7 regulates TGFβ in addition to BMP signaling in developing cartilage is unknown. In this study, we found that Smad7 is required for both axial and appendicular skeletal development. Loss of Smad7 led to impairment of the cell cycle in chondrocytes and to defects in terminal maturation. This phenotype was attributed to upregulation of both BMP and TGFβ signaling in Smad7 mutant growth plates. Moreover, Smad7-/- mice develop hypocellular cores in the medial growth plates, associated with elevated HIF1α levels, cell death, and intracellular retention of types II and X collagen. Thus, Smad7 may be required to mediate cell stress responses in the growth plate during development.

Ski inhibits TGF-β/phospho-Smad3 signaling and accelerates hypertrophic differentiation in chondrocytes

Since transforming growing factor-β (TGF-β)/Smad signaling inhibits chondrocyte maturation, endogenous negative regulators of TGF-β signaling are likely also important regulators of the chondrocyte differentiation process. One such negative regulator, Ski, is an oncoprotein that is known to inhibit TGF-β/Smad3 signaling via its interaction with phospho-Smad3 and recruitment of histone deacetylases (HDACs) to the DNA binding complex. Based on this, we hypothesized that Ski inhibits TGF-β signaling and accelerates maturation in chondrocytes via recruitment of HDACs to transcriptional complexes containing Smads. We tested this hypothesis in chick upper sternal chondrocytes (USCs), where gain and loss of Ski expression experiments were performed. Over-expression of Ski not only reversed the inhibitory effect of TGF-β on the expression of hypertrophic marker genes such as type X collagen (colX) and osteocalcin, it induced these genes basally as well. Conversely, knockdown of Ski by RNA interference led to a reduction of colX and osteocalcin expression under basal conditions. Furthermore, Ski blocked TGF-β induction of cyclinD1 and caused a basal up-regulation of Runx2, consistent with the observed acceleration of hypertrophy. Regarding mechanism, not only does Ski associate with phospho-Smad2 and 3, but its association with phospho-Smad3 is required for recruitment of HDAC4 and 5. Implicating this recruitment of HDACs in the phenotypic effects of Ski in chondrocytes, the HDAC inhibitor SAHA reversed the up-regulation of colX and osteocalcin in Ski over-expressing cells. These results suggest that inhibition of TGF-β signaling by Ski, which involves its association with phospho-Smad3 and recruitment of HDAC4 and 5, leads to accelerated chondrocyte differentiation.

Expression of BMP-receptor type 1A correlates with progress of osteoarthritis in human knee joints with focal cartilage lesions

BACKGROUND AIMS

Bone morphogenetic protein-2 (BMP-2) and its receptor type 1A (BMPR-1A) play significant roles in cartilage metabolism. The aim of this study was to evaluate a possible correlation between intra-articular expression of these proteins and the degree of osteoarthritis (OA) in human knees.

METHODS

Biopsies of synovia and debrided cartilage were taken in 15 patients undergoing autologous chondrocyte implantation. Expression of BMP-2 and BMPR-1A was evaluated semi-quantitatively by immunohistologic staining. These data were complemented by grading of cartilage lesions according to International Cartilage Repair Society (ICRS), defect size, duration of complaints, knee osteoarthritis scoring system (KOSS) and Henderson and Kellgren-Lawrence scores. General histologic stainings were used to determine Mankin, Pritzker and Krenn scores.

RESULTS

The expression of BMPR-1A but not of BMP-2 was significantly higher in cartilage biopsies taken in type 3 lesions with intact subchondral layer compared with type 4 defects (P < 0.05). In cartilage areas of bordering sectors, the intensity of immunohistologic staining of BMPR-1A was statistically significantly higher in mature cartilage compared with repair zones (P < 0.05). Expression of BMP-2 and its receptor 1A correlated in the cartilage biopsies (P < 0.02) but not in the synovia. The degree of OA was scored in all biopsies according to Mankin and Pritzker, and these scores correlated statistically significantly with BMPR-1A expression in the synovia (P < 0.05). In patients with an osteochondritis dissecans, the degree of OA was higher compared with other causes of chondromalacia, as evaluated by defect size, ICRS score, duration of complaints, Pritzker score and expression of BMPR-1A in cartilage (P < 0.05).

CONCLUSIONS

These data support the role of BMPR-1A as an indicator of OA progression in human knees with circumscribed cartilage lesions.

Immunohistological localization of BMP-2, BMP-7, and their receptors in knee joints with focal cartilage lesions

Introduction. Although it is well known that BMP-2 and BMP-7 play significant roles in cartilage metabolism, data about intra-articular expression and localization of these proteins and their receptors in humans are rare. Methods. Biopsies of synovia and debrided cartilage were taken in patients undergoing autologous chondrocyte implantation. Expression of BMP-2, BMP-7, and their receptors BMPR-1A, BMPR-1B and BMPR-2 were semiquantitatively evaluated by immunohistological staining. Results. BMP-7 was equally highly expressed in all cartilage and synovial biopsies. Increased levels of BMPR-1A, but not of BMPR-1B, and BMPR-2, were found in all synovial and 47% of all cartilage samples (P = 0.002). BMP-2 was positively scored in 47% of all cartilage and 40% of all synovial specimens. Defect size, KOSS, Henderson or Kellgren-Lawrence score did not statistically significant correlate with the expression of the analyzed proteins or Mankin and Pritzker scores. Duration of symptoms and localization of lesions were associated with KOSS (P < 0.02), but there was no influence of these parameters on protein expression. Conclusions. BMP-2, BMP-7, and BMPR-1A were expressed in cartilage and synovia of knees with focal cartilage lesions. Although defect localization and duration of symptoms decisively influence KOSS, there was no associated alteration of protein expression observed.

Smad6 is essential to limit BMP signaling during cartilage development

Bone morphogenetic protein (BMP) signaling pathways regulate multiple aspects of endochondral bone formation. The importance of extracellular antagonists as regulators of BMP signaling has been defined. In vitro studies reveal that the intracellular regulators, inhibitory Smads 6 and 7, can regulate BMP-mediated effects on chondrocytes. Although in vivo studies in which inhibitory Smads were overexpressed in cartilage have shown that inhibitory Smads have the potential to limit BMP signaling in vivo, the physiological relevance of inhibitory Smad activity in skeletal tissues is unknown. In this study, we have determined the role of Smad6 in endochondral bone formation. Loss of Smad6 in mice leads to defects in both axial and appendicular skeletal development. Specifically, Smad6-/- mice exhibit a posterior transformation of the seventh cervical vertebra, bilateral ossification centers in lumbar vertebrae, and bifid sternebrae due to incomplete sternal band fusion. Histological analysis of appendicular bones revealed delayed onset of hypertrophic differentiation and mineralization at midgestation in Smad6-/- mice. By late gestation, however, an expanded hypertrophic zone, associated with an increased pool of proliferating cells undergoing hypertrophy, was evident in Smad6 mutant growth plates. The mutant phenotype is attributed, at least in part, to increased BMP responsiveness in Smad6-deficient chondrocytes. Overall, our results show that Smad6 is required to limit BMP signaling during endochondral bone formation.

Bone morphogenetic protein signalling activity distinguishes histological subsets of paediatric germ cell tumours

Germ cell tumours (GCTs) are cancers of the testis, ovary or extragonadal sites that occur in infants, children and adults. Testicular GCT is the most common cancer in young men aged 15-40 years. Abnormalities in developmental signalling pathways such as wnt/β-catenin, TGF-β/BMP and Hedgehog have been described in many childhood tumours. To date, however, the status of BMP signalling in GCTs has not been described. Herein, we examine BMP-SMAD signalling in a set of clinically-annotated paediatric GCTs. We find that BMP signalling activity is absent in undifferentiated tumours such as seminomas and dysgerminomas, but robustly present in most yolk sac tumours, a differentiated tumour type. Gene expression profiling of TGF-β/BMP pathway genes in germinomas and yolk sac tumours reveals a set of genes that distinguish the two tumour types. There is significant intertumoural heterogeneity between tumours of the same histological subclass, implying that the BMP pathway can be differentially regulated in individual tumours. Finally, through miRNA expression profiling, we identify differential regulation of a set of miRNAs predicted to target the TGF-β/BMP pathway at multiple sites. Taken together, these results suggest that the BMP signalling pathway may represent a new therapeutical target for childhood GCTs.

Bone morphogenetic proteins and articular cartilage: To serve and protect or a wolf in sheep clothing's?

OBJECTIVE

Alterations in chondrocyte differentiation and matrix remodeling play a central role in osteoarthritis (OA). Chondrocyte differentiation and remodeling are amongst others regulated by the so-called Bone Morphogenetic Proteins (BMPs). Although BMPs are considered protective for articular cartilage these factors can also be involved in chondrocyte hypertrophy and matrix degradation. This review is focused on these opposed roles of BMPs in OA development and progression.

METHODS

Peer reviewed publications published prior to August 2009 were searched in the Pubmed database. Articles that were relevant for the role of endogenous BMPs in OA were selected. Since good quality reviews on the application of BMP supplementation in cartilage tissue engineering have been described this subject has not been covered in this review.

RESULTS

BMPs can stimulate both chondrocyte matrix synthesis and chondrocyte terminal differentiation. The latter results in elevated matrix metalloproteinase-13 (MMP-13) production. Stimulation of matrix synthesis will be protective for cartilage while elevated MMP-13 activity will drive matrix degradation. What action of BMPs is dominant in OA is not yet elucidated and their role might be different in patient subgroups.

CONCLUSION

BMPs can be protective for articular cartilage but can, due to their effect on chondrocyte differentiation, have harmful effects on articular cartilage and contribute to OA progression.

Smad signaling in skeletal development and regeneration

Smad proteins are intracellular molecules that mediate the canonical signaling cascade of TGFbeta superfamily growth factors. The TGFbeta superfamily comprises two groups of growth factors, BMPs and TGFbetas. Both groups can be further divided into several sub-groups based on sequence homologies and functional similarities. Ligands of the TGFbeta superfamily bind to cell surface receptors to activate Smad proteins in the cytoplasm; then the activated Smad proteins translocate into the nucleus to activate or repress specific target gene transcription. Both groups of growth factors play important roles in skeletal development and regeneration. However, whether these effects reflect signaling through canonical Smad pathways, or other non-canonical signaling pathways in vivo remains a mystery. Moreover, the mechanisms utilized by Smad proteins to initiate nuclear events and their interactions with cytoplasmic proteins are still under intensive investigation. This review will discuss the most recent progress understanding Smad signaling in the context of skeletal development and regeneration.

A role for age-related changes in TGFbeta signaling in aberrant chondrocyte differentiation and osteoarthritis

Transforming growth factor beta (TGFβ) is a growth factor with many faces. In our osteoarthritis (OA) research we have found that TGFβ can be protective as well as deleterious for articular cartilage. We postulate that the dual effects of TGFβ on chondrocytes can be explained by the fact that TGFβ can signal via different receptors and related Smad signaling routes. On chondrocytes, TGFβ not only signals via the canonical type I receptor ALK5 but also via the ALK1 receptor. Notably, signaling via ALK5 (Smad2/3 route) results in markedly different chondrocyte responses than ALK1 signaling (Smad1/5/8), and we postulate that the balance between ALK5 and ALK1 expression on chondrocytes will determine the overall effect of TGFβ on these cells. Importantly, signaling via ALK1, but not ALK5, stimulates MMP-13 expression by chondrocytes. In cartilage of ageing mice and in experimental OA models we have found that the ALK1/ALK5 ratio is significantly increased, favoring TGFβ signaling via the Smad1/5/8 route, changes in chondrocyte differentiation and MMP-13 expression. Moreover, human OA cartilage showed a significant correlation between ALK1 and MMP-13 expression. In this paper we summarize concepts in OA, its link with ageing and disturbed growth factor responses, and a potential role of TGFβ signaling in OA development.

TGF-beta signaling in chondrocyte terminal differentiation and osteoarthritis: modulation and integration of signaling pathways through receptor-Smads

OBJECTIVE

Chondrocytes and alteration in chondrocyte differentiation play a central role in osteoarthritis. Chondrocyte differentiation is amongst others regulated by members of the transforming growth factor-beta (TGF-beta) superfamily. The major intracellular signaling routes of this family are via the receptor-Smads. This review is focused on the modulation of receptor-Smad signaling and how this modulation can affect chondrocyte differentiation and potentially osteoarthritis development.

METHODS

Peer reviewed publications published prior to April 2009 were searched in the Pubmed database. Articles that were relevant for the role of TGF-beta superfamily/Smad signaling in chondrocyte differentiation and for differential modulation of receptor-Smads were selected.

RESULTS

Chondrocyte terminal differentiation is stimulated by Smad1/5/8 activation and inhibited the by Smad2/3 pathway, most likely by modulation of Runx2 function. Several proteins and signaling pathways differentially affect Smad1/5/8 and Smad2/3 signaling. This will result in an altered Smad1/5/8 and Smad2/3 balance and subsequently have an effect on chondrocyte differentiation and osteoarthritis development.

CONCLUSION

Modulation of receptor-Smads signaling can be expect to play an essential role in both the regulation of chondrocyte differentiation and osteoarthritis development and progression.

[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.

Chondrogenesis, bone morphogenetic protein-4 and mesenchymal stem cells

OBJECTIVE

As adult cartilage has very limited potential to regenerate, cartilage repair is challenging. Available treatments have several disadvantages, including formation of fibrocartilage instead of hyaline-like cartilage, as well as eventual ossification of the newly formed tissue. The focus of this review is the application of bone morphogenetic protein-4 (BMP-4) and mesenchymal stem cells (MSCs) in cartilage repair, a combination that could potentially lead to the formation of permanent hyaline-like cartilage in the defect.

METHODS

This review is based on recent literature in the orthopaedic and tissue engineering fields, and is focused on MCSs and bone morphogenetic proteins (BMPs).

RESULTS

BMP-4, a stimulator of chondrogenesis, both in vitro and in vivo, is a potential therapeutic agent for cartilage regeneration. BMP-4 delivery can improve the healing process of an articular cartilage defect by stimulating the synthesis of the cartilage matrix constituents: type II collagen and aggrecan. BMP-4 has also been shown to suppress chondrogenic hypertrophy and maintain regenerated cartilage. Use of an appropriate carrier for BMP-4 is crucial for successful reconstruction of cartilage defects. Due to the relatively short half-life in vivo of BMP-4, there is a need to localize and maintain the delivery of BMP-4 to the injury site. Additionally, the delivery of MSCs to the wound site could improve cartilage regeneration; therefore, the carrier should function both as a cell and a protein delivery vehicle.

CONCLUSION

The role of BMP-4 in chondrogenesis is significant, and successful methods to deliver BMP-4, with or without MSCs, to the cartilage defect site are a promising therapy to treat cartilage defects.

Systematic assessment of the human osteoblast transcriptome in resting and induced primary cells

Osteoblasts are key players in bone remodeling. The accessibility of human primary osteoblast-like cells (HObs) from bone explants makes them a lucrative model for studying molecular physiology of bone turnover, for discovering novel anabolic therapeutics, and for mesenchymal cell biology in general. Relatively little is known about resting and dynamic expression profiles of HObs, and to date no studies have been conducted to systematically assess the osteoblast transcriptome. The aim of this study was to characterize HObs and investigate signaling cascades and gene networks with genomewide expression profiling in resting and bone morphogenic protein (BMP)-2- and dexamethasone-induced cells. In addition, we compared HOb gene expression with publicly available samples from the Gene Expression Omnibus. Our data show a vast number of genes and networks expressed predominantly in HObs compared with closely related cells such as fibroblasts or chondrocytes. For instance, genes in the insulin-like growth factor (IGF) signaling pathway were enriched in HObs (P = 0.003) and included the binding proteins (IGFBP-1, -2, -5) and IGF-II and its receptor. Another HOb-specific expression pattern included leptin and its receptor (P < 10(-8)). Furthermore, after stimulation of HObs with BMP-2 or dexamethasone, the expression of several interesting genes and pathways was observed. For instance, our data support the role of peripheral leptin signaling in bone cell function. In conclusion, we provide the landscape of tissue-specific and dynamic gene expression in HObs. This resource will allow utilization of osteoblasts as a model to study specific gene networks and gene families related to human bone physiology and diseases.

JinSanE decoction, a chinese herbal medicine, inhibits expression of TGF-beta1/Smads in experimental hepatic fibrosis in rats

The study is to investigate the effects of a Chinese herbal medicine, JinSanE decoction, on the TGF-beta1/Smads signal transduction pathway in a carbon tetrachloride (CCl(4))-induced hepatic fibrosis model in rats. Rats were randomly divided into 4 study groups: namely, a normal control group, a hepatic fibrosis model group, and 2 treatment groups with different doses of JinSanE (6 and 12 g/kg). Ten rats in each group were sacrificed at 4 and 8 weeks after exposure to CCl(4) respectively. The levels of TGF-beta1 and TRII mRNA in liver tissue were analyzed by RT-PCR. The expressions of TGF-beta1, Smad3 and Smad7 in liver tissues were evaluated by immunohistochemistry. The liver histopathology was examined by hematoxylin and eosin (HE) staining and electron microscopy respectively. The liver hydroxyproline (HYP), liver function and hyaluronic acid (HA) were examined by biochemistry and radioimmunoassay (RIA) respectively. Compared with the hepatic fibrosis model group, the levels of TGF-beta1, TRII mRNA and Smad3 expression significantly decreased in the 2 treatment groups. The expression of Smad7 was significantly increased in the liver of the rats treated with JinSanE (p < 0.05 or p < 0.01). The histological changes of fibrotic liver were obviously improved in the treatment rats. The levels of liver HYP, serum liver function and HA were also remarkably improved in the treatment rats. Moreover, the effects of JinSanE occurred in a dose- and time-dependent manner in the process of the protection of liver injury and fibrosis. JinSanE decoction had a protective effect on liver injury and could ameliorate hepatic fibrosis in rats. The mechanisms might be associated with their effects of down-regulating TGF-beta1, TRII mRNA and Smad3, and up-regulating Smad7.

Up-regulation of bone morphogenetic protein receptor IB by growth factors enhances BMP-2-induced human bone cell functions

Bone morphogenetic proteins (BMP) stimulate osteoblast differentiation by signal transduction via three BMP receptors (BMPR-IA, -IB, and -II). Several growth factors, including transforming growth factor-beta1 (TGF-beta1), fibroblast growth factor-2 (FGF-2) and platelet-derived growth factor-AB (PDGF-AB), have also been shown to play an important part in osteogenesis. The mechanism underlying these activities is unclear, but these growth factors could modulate the BMP/BMPR pathway by up-regulating BMPR expression, thereby enhancing the osteogenic responses of bone cells to the BMP. In this study we have therefore examined the effects of TGF-beta1, FGF-2, and PDGF-AB on BMPR expression and BMP-2-mediated osteoblast functions in primary human bone cells. The results showed that although the ligand BMP-2 and growth factors had little effect on BMPR-IA and -II transcript expression, they significantly up-regulated BMPR-IB mRNA specifically. However, only the growth factors, but not the ligand BMP-2, increased the surface expression of the BMPR-IB antigen, which was found to be due to a differential effect of BMP-2 and the growth factors on the Smurf1/Smad6-induced breakdown process. Pre-incubation of the cells with the growth factors significantly augmented BMP-2-induced Smad1/5/8 phosphorylation, and Dlx5 expression ALP activity, compared with that of cells treated with BMP-2 alone. When cells were transfected with siRNA targeting BMPR-IB, the growth factors neither up-regulated BMPR-IB transcript expression nor enhanced BMP-2-induced Smad1/5/8 phosphorylation, Dlx5 expression and ALP activity. The results indicate that increased BMPR-IB by TGF-beta1, FGF-2, and PDGF-AB significantly enhances BMP-2-induced osteogenic functions in vitro, suggesting that they might positively modulate bone formation by up-regulating BMPR-IB in vivo.

Effects of perindopril and valsartan on expression of transforming growth factor-beta-Smads in experimental hepatic fibrosis in rats

BACKGROUND

Previous studies have shown that the renin-angiotensin system (RAS) plays an important role in the pathogenesis of hepatic fibrosis, and blockers of the RAS may be active as an antifibrogenic goal. However, the potential role of RAS inhibition on expression transforming growth factor (TGF)-beta-Smads in hepatic fibrosis remains unknown. The aim of this study was to investigate the effect and mechanism of an angiotensin-converting enzyme inhibitor (perindopril) and an angiotensin II receptor blocker (valsartan) on TGF-beta1 and TGF receptor II (TRII) mRNA, Smad3 and Smad7 in fibrotic hepatic livers in rats.

METHODS

Sixty Wistar rats were randomly divided into four study groups (n = 15 for each group), including normal controls, hepatic fibrosis models, and two treated groups with either perindopril or valsartan, starting from the fourth week after being exposed to carbon tetrachloride (CCl(4)) for 4 weeks. The levels of TGF-beta and TRII mRNA in liver tissue were analyzed by RT-PCR. The expressions of TGF-beta1, Smad3 and Smad7 in liver tissues were evaluated by immunohistochemistry. The liver histopathology was examined by hematoxylin and eosin (HE) staining and by electron microscopy, respectively. The liver function and serum hyaluronic acid were also assayed by biochemistry and radioimmunoassay.

RESULTS

Compared with the hepatic fibrosis models, the levels of TGF-beta1, TRII mRNA and the expression Smad3 significantly decreased in the two treated groups, and the expression of Smad7 was significantly increased in the liver of rats treated with perindopril or valsartan (P < 0.05 or P < 0.01). The histological changes and ultrastructure of fibrotic liver, liver function and hyaluronic acid also remarkably improved in the treated rats.

CONCLUSIONS

The angiotensin-converting enzyme inhibitors perindopril and valsartan have a protective effect on liver injury and can ameliorate hepatic fibrosis in rats induced by CCl(4). The mechanisms may be associated with their effects of down-regulating TGF-beta1, TRII mRNA and smad3, and up-regulating Smad7.

Upregulation of Id-1 via BMP-2 receptors induces reactive oxygen species in podocytes

Bone morphogenetic proteins (BMPs) are secreted signaling molecules, which play a major role in kidney development and disease. Here, we show the existence of mRNA for BMP-2 and for the BMP receptors BMPR1A, BMPR1B, BMPRII, ACVR1A, ACVR2, and ACVR2B in differentiated mouse podocytes and the protein expression of BMPR1A in human glomerular podocytes. BMP-2 dose dependently increases the free cytosolic Ca(2+) concentration in podocytes proving the existence of a functional receptor in these cells. Recent data indicate that in a myoblastic cell line and in a breast cancer cell line, BMP-2 increases the expression of Id-1, a negative regulator of basic helix-loop-helix transcription factors, but the role of BMP-2 stimulated Id-1 expression in the kidney has not been further characterized. Here, we show that BMP-2 increases the expression of Id-1 in differentiated podocytes. To investigate a role of Id-1 for podocyte function, overexpression of Id-1 was induced in differentiated mouse podocytes. Id-1-overexpressing podocytes show an increased NADPH-dependent production of reactive oxygen species (ROS). This effect can be evoked by BMP-2 and can be antagonized by anti-Id-1 antisense oligonucleotides. The data indicate that BMP-2 may, via an increased expression of Id-1 and an increased generation of ROS, contribute to important cellular functions in podocytes. ROS supposedly play a major role in cell adhesion, cell injury, ion transport, fibrogenesis, angiogenesis and are involved in the pathogenesis of membranous nephropathy.

Smad3-deficient chondrocytes have enhanced BMP signaling and accelerated differentiation

Smad3 deficiency accelerates chondrocyte maturation and leads to osteoarthritis. Primary chondrocytes without Smad3 lack compensatory increases of TGF-beta signaling factors, but BMP-related gene expression is increased. Smad2 or Smad3 overexpression and BMP blockade abrogate accelerated maturation in Smad3-/- chondrocytes. BMP signaling is increased in TGF-beta deficiency and is required for accelerated chondrocyte maturation.

INTRODUCTION

Disruption of TGF-beta signaling results in accelerated chondrocyte maturation and leads to postnatal dwarfism and premature osteoarthritis. The mechanisms involved in this process were studied using in vitro murine chondrocyte cultures.

MATERIALS AND METHODS

Primary chondrocytes were isolated from the sterna of neonatal wildtype and Smad3-/- mice. Expressions of maturational markers, as well as genes involved in TGF-beta and BMP signaling were examined. Chondrocytes were treated with TGF-beta and BMP-2, and effects on maturation-related genes and BMP/TGF-beta responsive reporters were examined. Recombinant noggin or retroviral vectors expressing Smad2 or Smad3 were added to the cultures.

RESULTS

Expression of colX and other maturational markers was markedly increased in Smad3-/- chondrocytes. Smad3-/- chondrocytes lacked compensatory increases in Smad2, Smad4, TGFRII, Sno, or Smurf2 and had reduced expression of TGF-beta1 and TGFRI. In contrast, Smad1, Smad5, BMP2, and BMP6 expression was increased, suggesting a shift from TGF-beta toward BMP signaling. In Smad3-/- chondrocytes, alternative TGF-beta signaling pathways remained responsive, as shown by luciferase assays. These non-Smad3-dependent TGF-beta pathways reduced colX expression and alkaline phosphatase activity in TGF-beta-treated Smad3-/- cultures, but only partially. In contrast, Smad3-/- chondrocytes were more responsive to BMP-2 treatment and had increased colX expression, phosphoSmads 1, 5, and 8 levels, and luciferase reporter activity. Overexpression of both Smad2 and Smad3 blocked spontaneous maturation in Smad3-deficient chondrocytes. Maturation was also abrogated by the addition of noggin, an extracellular BMP inhibitor.

CONCLUSIONS

These findings show a key role for BMP signaling during the chondrocyte maturation, occurring with loss of TGF-beta signaling with important implications for osteoarthritis and cartilage diseases.

Effects of Chinese traditional compound, JinSanE, on expression of TGF-β1 and TGF-β1 type II receptor mRNA, Smad3 and Smad7 on experimental hepatic fibrosis in vivo

AIM: The transforming growth factor-beta (TGF-β)/Smad signaling pathway system plays a prominent role in the control of cell growth and extracellular matrix formation in the progression of liver fibrogenesis. Smad proteins can either positively or negatively regulate TGF-β responses. In this study, the therapeutic effects of Chinese traditional compound decoction, JinSanE, and the changes of TGF-β/Smad signaling pathway system in carbon tetrachloride (CCl₄)-induced rat experimental liver fibrosis were examined.

METHODS: Seventy-two healthy Wistar rats were assigned to groups including normal control group, CCl₄ model group, JinSanE treatment group I and JinSanE treatment group II. Each group contained 18 rats. All groups, except the normal control group, received CCl₄ subcutaneous injection for 8 wk. Rats in JinSanE groups I and II were orally treated with JinSanE daily at the 1^st and 5^th wk, respectively, after exposure to CCl₄. The expression of TGF-β1 and TGF-β1 type II receptor (TRII) mRNA in the liver was determined by reverse transcription polymerase chain reaction, and the expression of TGF-β1, Smad3 and Smad7 by immunohistochemistry. The liver histopathology was also examined by HE staining and observed under electron microscope. The activities of several serum fibrosis-associated enzymes, alanine aminotransferase (ALT), aspartate aminotransferase (AST), the levels of serum hyaluronic acid (HA) were assayed.

RESULTS: Hepatic fibrosis caused by CCl₄ was significantly inhibited in the JinSanE-treated groups. The degrees of necrosis/degeneration and fibrosis scores were significantly lower in the JinSanE-treated groups than in the model control group. The expression of TGF-β1, TRII and Smad3 was significantly higher in the model group than that in the JinSanE-treated groups, and the active/total TGF-β1 ratio in the JinSanE groups was suppressed. Expression of TRII mRNA and Smad3 proteins showed a distribution pattern similar to that of TGF-β1 with a direct correlation in terms of the degree of hepatic fibrosis. The amount of positive staining Smad7 cells was significantly less in the model group than in the JinSanE-treated groups and the normal group. The contents of ALT, AST and HA were significantly lower in the JinSanE-treated groups than those in the model group.

CONCLUSION: Traditional Chinese medicine, JinSanE, prevents the progression of hepatic damage and fibrosis through the inhibition of TGF-β1, TRII and Smad3 signal proteins, and increases expression of Smad7 signal protein in vivo.

BMP signaling in skeletal development

Development of the vertebrate skeleton, a complex biological event that includes diverse processes such as formation of mesenchymal condensations at the sites of future skeletal elements, osteoblast and chondrocyte differentiation, and three dimensional patterning, is regulated by many growth factors. Bone morphogenetic proteins (BMPs), members of the TGF-beta superfamily, play a pivotal role in the signaling network and are involved in nearly all processes associated with skeletal morphogenesis. BMP signals are transduced from the plasma membrane receptors to the nucleus through both Smad pathway and non-Smad pathways, and regulated by many extracellular and intercellular proteins that interact with BMPs or components of the BMP signaling pathways. To gain a better understanding of the molecular mechanisms underlying the role of BMP in early skeletal development, it is necessary to elucidate the BMP signaling transduction pathways in chondrocytes and osteoblasts. The major objective of this review was to summarize BMP signaling pathways in the context of craniofacial, axial, and limb development. In particular, this discourse will focus on recent advances of the role of different ligands, receptors, Smads, and BMP regulators in osteoblast and chondrocyte differentiation during embryonic development.

Deficiencies of physiologic calcification inhibitors and low-grade inflammation in arterial calcification: lessons for cartilage calcification

Apart from clinical parallels, similarities in the pathogenesis of arterial and articular cartilage calcification have come to light in recent years. These include the roles of aging, of chronic low-grade inflammation and of genetic and acquired dysregulation of inorganic pyrophosphate (PP(i)) metabolism. This review focuses on recent developments in understanding the pathogenesis of artery calcification pertinent to interpretation of the mechanistic basis for articular cartilage calcification in aging and osteoarthritis.

Protein kinases in chondrocyte signaling and osteoarthritis

Protein kinases, particularly mitogen-activated protein kinases and receptor-tyrosine kinases play crucial roles in mammalian cellular metabolism by regulating intracellular signaling pathways that control proliferation, differentiation, cytokine gene induction and cytokine responsiveness, matrix metalloproteinase gene expression, mechanical transduction, as well as programmed cell death (apoptosis). Many of these pathways are also important components of cartilage homeostasis because alterations in intracellular signaling pathways appear to play a prominent role in chondrocyte dysfunction that is part of osteoarthritis pathogenesis and disease progression. Several mitogen-activated protein kinases and receptor-tyrosine kinases have been characterized as participating in chondrocyte signaling pathways. They are c-Jun-amino-terminal protein kinase, p38 kinase, extracellular signal-regulated protein kinase, and Ror2. Janus kinases and signal transducers and activators of transcription factors (Janus kinase/signal transducers and activators of transcription pathway) are also implicated in modulating the chondrogenic phenotype. Mitogen-activated protein kinase activation is required for their role as phosphorylating enzymes. Activation results from mitogen-activated protein kinase phosphorylation carried out by at least seven upstream kinases known as mitogen-activated protein kinase kinases. Additional upstream kinases (for example, MKKKKs and MKKKs) often require low molecular weight GTP-binding proteins to mediate the mitogen-activated protein-kinase kinases cascade. Identifying the functions of mitogen-activated protein kinases in normal and aging cartilage and the extent to which mitogen-activated protein kinases may be altered in osteoarthritis cartilage and synovium will be critical for developing novel therapies for osteoarthritis management.

Distinct functions of BMP4 and GDF5 in the regulation of chondrogenesis

Bone morphogenetic protein 4 (BMP4) and growth/differentiation factor 5 (GDF5) are closely related protein family members and regulate early cartilage patterning and differentiation. In this study, we compared the functional outcome of their actions systematically at various stages of chondrogenesis in mouse embryonic limb bud mesenchyme grown in micromass cultures. Overall, both growth factors enhanced cartilage growth and differentiation in these cultures. Uniquely, BMP4 not only accelerated the formation and maturation of cartilaginous nodules, but also induced internodular mesenchymal cells to express cartilage differentiation markers. On the other hand, GDF5 increased the number of prechondrogenic mesenchymal cell condensation and cartilaginous nodules, without altering the overall pattern of differentiation. In addition, GDF5 caused a more sustained elevated expression level of Sox9 relative to that associated with BMP4. BMP4 accelerated chondrocyte maturation throughout the cultures and sustained an elevated level of Col10 expression, whereas GDF5 caused a transient increase in Col10 expression. Taken together, we conclude that BMP4 is instructive to chondrogenesis and induces mesenchymal cells toward the chondrogenic lineage. Furthermore, BMP4 accelerates the progression of cartilage differentiation to maturation. GDF5 enhances cartilage formation by promoting chondroprogenitor cell aggregation, and amplifying the responses of cartilage differentiation markers. These differences may serve to fine-tune the normal cartilage differentiation program, and can be exploited for the molecular manipulation in biomimetics.

Chondrosarcoma cell differentiation

A mixed population of lymphocytes from a healthy donor co-existed with an established culture of allogeneic chondrosarcoma cells, during which time the tumor cells changed from malignantly transformed to benign fibroblast-like morphology; from multilayered to a monolayered growth pattern; lost their potency to grow in colonies in soft agar; and showed signs of senescence. A discussion of possible molecular mechanisms for this event is offered. If there are as yet undiscovered lymphokines that can induce reversal of the malignant geno/phenotype, the cognate gene(s) should be cloned for genetic engineering and for the mass production of the corresponding molecular mediators for clinical trials.