Functional requirement of CCN2 for intramembranous bone formation in embryonic mice

Connective tissue growth factor enhances TGF-β1-induced osteogenic differentiation via activation of p38 MAPK in mesenchymal stem cells

OBJECTIVES

Cellular differentiation is based on the effects of various growth factors. Transforming growth factor (TGF)-β1 plays a pivotal role in inducing osteogenic differentiation of mesenchymal stem cells (MSCs). In this study, we investigated the influence of connective tissue growth factor (CTGF), known to function synergistically with TGF-β1, on osteogenic differentiation in MSCs.

METHODS

UE7T-13 cells were treated with TGF-β1 and/or CTGF. Subsequently, protein levels of intracellular signaling pathway molecules were determined through western blot analysis. The mRNA expression levels of osteogenic differentiation markers were investigated using reverse transcription-quantitative polymerase chain reaction. Bone matrix mineralization was evaluated through alizarin red staining.

RESULTS

Co-treatment with TGF-β1 and CTGF resulted in the suppression of TGF-β1-induced phosphorylation of extracellular signal-regulated kinase 1/2, an intracellular signaling pathway molecule in MSCs, while significantly enhancing the phosphorylation of p38 mitogen-activated protein kinase (MAPK). In MSCs, co-treatment with CTGF and TGF-β1 led to increased expression levels of alkaline phosphatase and type I collagen, markers of osteogenic differentiation induced by TGF-β1. Osteopontin expression was observed only after TGF-β1 and CTGF co-treatment. Notably, bone sialoprotein and osteocalcin were significantly upregulated by treatment with CTGF alone. Furthermore, CTGF enhanced the TGF-β1-induced mineralization in MSCs, with complete suppression observed after treatment with a p38 MAPK inhibitor.

CONCLUSIONS

CTGF enhances TGF-β1-induced osteogenic differentiation and subsequent mineralization in MSCs by predominantly activating the p38 MAPK-dependent pathway.

Controversial causal association between IGF family members and osteoporosis: a Mendelian randomization study between UK and FinnGen biobanks

Objectives

Osteoporosis, a prevalent skeletal disorder characterized by reduced bone strength, is closely linked to the IGF system, crucial for skeletal metabolism. However, the precise nature of this relationship remains elusive. In this study, we employed Mendelian randomization (MR) to unravel the associations between genetically predicted serum IGF system member levels and osteoporosis.

Methods

A two-sample MR approach was employed to investigate these causal associations based on two individual datasets. Predictions of 14 serum levels of IGF system members were made using 11,036,163 relevant Single Nucleotide Polymorphisms (SNPs) within a cohort of 4,301 individuals of European descent. Genetic association estimates for osteoporosis were derived from two publicly available GWAS consortia: the Finnish consortium from the FinnGen biobank, comprising 212,778 individuals of Finnish descent (3,203 cases and 209,575 controls), and the UK consortium from the UK Biobank, including 337,159 individuals of European descent (5,266 cases and 331,893 controls).

Results

According to the UK dataset, IGF-1 levels were associated with a reduced risk of osteoporosis, as indicated by the weighted median method (Odds Ratio [OR] = 0.998, 95% CI = 0.997-1.000, P = 0.032). Additionally, higher levels of IGFBP-3 were linked to a decreased risk of osteoporosis using the Inverse-Variance Weighted (IVW) method (OR = 0.999, 95% CI = 0.998-1.000, P = 0.019), and CTGF levels exhibited a negative association with osteoporosis, as determined by the weighted median method (OR = 0.998, 95% CI = 0.996-0.999, P = 0.004). In the FinnGen dataset, IGF-1 and IGFBP-3 were not identified to be associated with osteoporosis. While, IGF-LR1 levels displayed a negative association with osteoporosis, according to the MR-Egger method (OR = 0.886, 95% CI = 0.795-0.987, P = 0.036), while CYR61 was linked to an increased risk of osteoporosis based on both the weighted median and IVW methods (OR = 1.154, 95% CI = 1.009-1.319, P = 0.037, and OR = 1.115, 95% CI = 1.022-1.215, P = 0.014, respectively).

Conclusion

This study provides compelling evidence that certain IGF family members play a role in the pathogenesis of osteoporosis between different datasets, indicating population specific causal effects between IGF family and osteoporosis. Although the results from both datasets demonstrated that IGF family involved in the pathogenesis of osteoporosis, but the responding key molecules might be various among different population. Subsequent research is warranted to evaluate the potential of these biomarkers as targets for osteoporosis prevention and treatment in specific population.

The regulation and functions of the matricellular CCN proteins induced by shear stress

Shear stress is a frictional drag generated by the flow of fluid, such as blood or interstitial fluid, and plays a critical role in regulating cellular gene expression and functional phenotype. The matricellular CCN family proteins are dynamically regulated by shear stress of different flow patterns, and their expression significantly alters the microenvironment of cells. Secreted CCN proteins mainly bind to several cell surface integrin receptors to mediate their diverse functions in regulating cell survival, function, and behavior. Gene-knockout studies indicate major functions of CCN proteins in the cardiovascular and skeletal systems, the two primary systems in which CCN expressions are regulated by shear stress. In the cardiovascular system, the endothelium is directly exposed to vascular shear stress. Unidirectional laminar blood flow generates laminar shear stress, which promotes a mature endothelial phenotype and upregulates anti-inflammatory CCN3 expression. In contrast, disturbed flow generates oscillatory shear stress, which induces endothelial dysfunction through the induction of CCN1 and CCN2. Shear-induced CCN1 binds to integrin α6β1 and promotes superoxide production, NF-κB activation, and inflammatory gene expression in endothelial cells. Although the interaction between shear stress and CCN4-6 is not clear, CCN 4 exhibits a proinflammatory property and CCN5 inhibits vascular cell growth and migration. The crucial roles of CCN proteins in cardiovascular development, homeostasis, and disease are evident but not fully understood. In the skeletal system, mechanical loading on bone generates shear stress from interstitial fluid in the lacuna-canalicular system and promotes osteoblast differentiation and bone formation. CCN1 and CCN2 are induced and potentially mediate fluid shear stress mechanosensing in osteocytes. However, the exact roles of interstitial shear stress-induced CCN1 and CCN2 in bone are still not clear. In contrast to other CCN family proteins, CCN3 inhibits osteoblast differentiation, although its regulation by interstitial shear stress in osteocytes has not been reported. The induction of CCN proteins by shear stress in bone and their functions remain largely unknown and merit further investigation. This review discusses the expression and functions of CCN proteins regulated by shear stress in physiological conditions, diseases, and cell culture models. The roles between CCN family proteins can be compensatory or counteractive in tissue remodeling and homeostasis.

Fibroblast Growth Factors and Cellular Communication Network Factors: Intimate Interplay by the Founding Members in Cartilage

Fibroblast growth factors (FGFs) constitute a large family of signaling molecules that act in an autocrine/paracrine, endocrine, or intracrine manner, whereas the cellular communication network factors (CCN) family is composed of six members that manipulate extracellular signaling networks. FGFs and CCNs are structurally and functionally distinct, except for the common characteristics as matricellular proteins. Both play significant roles in the development of a variety of tissues and organs, including the skeletal system. In vertebrates, most of the skeletal parts are formed and grow through a process designated endochondral ossification, in which chondrocytes play the central role. The growth plate cartilage is the place where endochondral ossification occurs, and articular cartilage is left to support the locomotive function of joints. Several FGFs, including FGF-2, one of the founding members of this family, and all of the CCNs represented by CCN2, which is required for proper skeletal development, can be found therein. Research over a decade has revealed direct binding of CCN2 to FGFs and FGF receptors (FGFRs), which occasionally affect the biological outcome via FGF signaling. Moreover, a recent study uncovered an integrated regulation of FGF and CCN genes by FGF signaling. In this review, after a brief introduction of these two families, molecular and genetic interactions between CCN and FGF family members in cartilage, and their biological effects, are summarized. The molecular interplay represents the mutual involvement of the other in their molecular functions, leading to collaboration between CCN2 and FGFs during skeletal development.

Multifunctional regulatory protein connective tissue growth factor (CTGF): A potential therapeutic target for diverse diseases

Connective tissue growth factor (CTGF), a multifunctional protein of the CCN family, regulates cell proliferation, differentiation, adhesion, and a variety of other biological processes. It is involved in the disease-related pathways such as the Hippo pathway, p53 and nuclear factor kappa-B (NF-κB) pathways and thus contributes to the developments of inflammation, fibrosis, cancer and other diseases as a downstream effector. Therefore, CTGF might be a potential therapeutic target for treating various diseases. In recent years, the research on the potential of CTGF in the treatment of diseases has also been paid more attention. Several drugs targeting CTGF (monoclonal antibodies FG3149 and FG3019) are being assessed by clinical or preclinical trials and have shown promising outcomes. In this review, the cellular events regulated by CTGF, and the relationships between CTGF and pathogenesis of diseases are systematically summarized. In addition, we highlight the current researches, focusing on the preclinical and clinical trials concerned with CTGF as the therapeutic target.

Functions of Matricellular Proteins in Dental Tissues and Their Emerging Roles in Orofacial Tissue Development, Maintenance, and Disease

Matricellular proteins (MCPs) are defined as extracellular matrix (ECM) associated proteins that are important regulators and integrators of microenvironmental signals, contributing to the dynamic nature of ECM signalling. There is a growing understanding of the role of matricellular proteins in cellular processes governing tissue development as well as in disease pathogenesis. In this review, the expression and functions of different MP family members (periostin, CCNs, TSPs, SIBLINGs and others) are presented, specifically in relation to craniofacial development and the maintenance of orofacial tissues, including bone, gingiva, oral mucosa, palate and the dental pulp. As will be discussed, each MP family member has been shown to have non-redundant roles in development, tissue homeostasis, wound healing, pathology and tumorigenesis of orofacial and dental tissues.

Differential intolerance to loss of function and missense mutations in genes that encode human matricellular proteins

Targeted gene disruption in mice has provided valuable insights into the functions of matricellular proteins. Apart from missense and loss of function mutations that have been associated with inherited diseases, however, their functions in humans remain unclear. The availability of deep exome sequencing data from over 140,000 individuals in the Genome Aggregation Database provided an opportunity to examine intolerance to loss of function and missense mutations in human matricellular genes. The probability of loss-of-function intolerance (pLI) differed widely within members of the thrombospondin, CYR61/CTGF/NOV (CCN), tenascin, small integrin-binding ligand N-linked glycoproteins (SIBLING), and secreted protein, acidic and rich in cysteine (SPARC) gene families. Notably, pLI values in humans had limited correlation with viability of the corresponding homozygous null mice. Among the thrombospondins, only THBS1 was highly loss-intolerant (pLI = 1). In contrast, Thbs1 is not essential for viability in mice. Several known thrombospondin-1 receptors were similarly loss-intolerant, although thrombospondin-1 is not the exclusive ligand for some of these receptors. The frequencies of missense mutations in THBS1 and the gene encoding its signaling receptor CD47 indicated conservation of some residues implicated in specific receptor binding. Deficits in missense mutations were also observed for other thrombospondin genes and for SPARC, SPOCK1, SPOCK2, TNR, and DSPP. The intolerance of THBS1 to loss of function in humans and elevated pLI values for THBS2, SPARC, SPOCK1, TNR, and CCN1 support important functions for these matricellular protein genes in humans, some of which may relate to functions in reproduction or responding to environmental stresses.

Evaluation of the mechanical properties and biocompatibility of gypsum-containing calcium silicate cements

Mineral trioxide aggregate (MTA) cement is widely used in the field of endodontic treatment. We herein synthesized calcium silicates from calcium carbonate and silicon dioxide, with the aim of reducing the cost associated with the MTA. Additionally, we prepared gypsum-containing calcium silicate cement to reduce the setting time while enhancing the mechanical strength. We evaluated the physical properties of this cement and investigated the response of human dental pulp stem cells (hDPSCs) grown in culture media containing cement eluate. Our results revealed that calcium silicates could be easily synthesized in lab-scale. Furthermore, we demonstrate that gypsum addition helps shorten the setting time while increasing the compressive strength of dental cements. The synthesized gypsum-containing calcium silicate cement showed minimal cytotoxicity and did not inhibit the proliferation of hDPSCs. These results suggested that the newly developed calcium silicate material could be a promising pulp capping material.

Connective tissue growth factor promotes chemotaxis of preosteoblasts through integrin α5 and Ras during tensile force-induced intramembranous osteogenesis

In vertebrates, new bone formation via intramembranous osteogenesis is a critical biological event for development, remodeling, and fracture healing of bones. Chemotaxis of osteoblast lineage cells is an essential cellular process in new bone formation. Connective tissue growth factor (CTGF) is known to exert chemotactic properties on various cells; however, details of CTGF function in the chemotaxis of osteoblast lineage cells and underlying molecular biological mechanisms have not been clarified. The aim of the present study was to evaluate the chemotactic properties of CTGF and its underlying mechanisms during active bone formation through intramembranous osteogenesis. In our mouse tensile force-induced bone formation model, preosteoblasts were aggregated at the osteogenic front of calvarial bones. CTGF was expressed at the osteogenic front, and functional inhibition of CTGF using a neutralizing antibody suppressed the aggregation of preosteoblasts. In vitro experiments using μ-slide chemotaxis chambers showed that a gradient of CTGF induced chemotaxis of preosteoblastic MC3T3-E1 cells, while a neutralizing integrin α5 antibody and a Ras inhibitor inhibited the CTGF-induced chemotaxis of MC3T3-E1 cells. These findings suggest that the CTGF-integrin α5-Ras axis is an essential molecular mechanism to promote chemotaxis of preosteoblasts during new bone formation through intramembranous osteogenesis.

Controlled Co-delivery of Growth Factors through Layer-by-Layer Assembly of Core-Shell Nanofibers for Improving Bone Regeneration

The regeneration of bone tissue is regulated by both osteogenic and angiogenic growth factors which are expressed in a coordinated cascade of events. The aim of this study was to create a dual growth factor-release system that allows for time-controlled release to facilitate bone regeneration. We fabricated core-shell SF/PCL/PVA nanofibrous mats using coaxial electrospinning and layer-by-layer (LBL) techniques, where bone morphogenetic protein 2 (BMP2) was incorporated into the core of the nanofibers and connective tissue growth factor (CTGF) was attached onto the surface. Our study confirmed the sustained release of BMP2 and a rapid release of CTGF. Both in vitro and in vivo experiments demonstrated improvements in bone tissue recovery with the dual-drug release system. In vivo studies showed improvement in bone regeneration by 43% compared with single BMP2 release systems. Time-controlled release enabled by the core-shell nanofiber assembly provides a promising strategy to facilitate bone healing.

Flavonoid compound icariin enhances BMP-2 induced differentiation and signalling by targeting to connective tissue growth factor (CTGF) in SAMP6 osteoblasts

BACKGROUND

Icariin, a major active flavonoid glucoside, is widely used for the treatment of bone injury and rebuilding in the clinic because of its roles in suppressing osteoblastogenesis and promoting osteogenesis. The senescence-accelerated mouse SAMP6 was accepted as a useful murine model to reveal the mechanism of senile osteoporosis and the therapeutic mechanism of drug activity. However, little is known about the characteristics of SAMP6 osteoblasts and the associated regulatory roles of icariin.

METHODS

We isolated and cultured osteoblasts from SAMP6 or SAMR1 mice and compared their proliferation, migration, and differentiation by performing the CCK-8 assay, cell counting assay, EdU staining, cell cycle analysis, ALP staining and activity measurement, Alizarin red staining, and RT-qPCR analysis to measure the levels of osteoblast markers, including RUNX2, Colla1 and Oc. To assess the effects of icariin on BMP-2-induced osteoblast differentiation, after BMP-2 treatment, osteoblast markers were analyzed by RT-qPCR and semi-quantitative Western blotting. The effects of icariin on connective tissue growth factor (CTGF) were measured by RT-qPCR. shRNA targeting CTGF mRNA was employed to knockdown its expression level in osteoblasts.

RESULTS

The SAMP6 osteoblasts presented decreased the development and differentiation activity compared with SAMR1 osteoblasts, indicating that they are the potential mechanisms underlying age-associated disease. Moreover, SAMP6 osteoblasts presented upregulated CTGF compared with SAMR1 osteoblasts. Icariin enhanced BMP-2-induced osteoblast differentiation by downregulating CTGF expression, which tightly regulates osteoblast differentiation. By downregulating CTGF, icariin treatment upregulated phosphate-Smad1/5/8, indicating its activating effects on the BMP signaling pathway.

CONCLUSION

These results suggest that decreased osteoblast development and function potentially contributes to age-associated disease. Icariin exerts enhancing effects on BMP-2-mediated osteoblast development via downregulating CTGF.

Connective tissue growth factor-specific monoclonal antibody inhibits growth of malignant mesothelioma in an orthotopic mouse model

Malignant mesothelioma is an aggressive neoplasm with no particularly effective treatments. We previously reported that overexpression of connective tissue growth factor (CTGF/CCN2) promotes mesothelioma growth, thus suggesting it as a novel molecular target. A human monoclonal antibody that antagonizes CTGF (FG-3019, pamrevlumab) attenuates malignant properties of different kinds of human cancers and is currently under clinical trial for the treatment of pancreatic cancer. This study reports the effects of FG-3019 on human mesothelioma in vitro and in vivo. We analyzed the effects of FG-3019 on the proliferation, apoptosis, migration/invasion, adhesion and anchorage-independent growth in three human mesothelioma cell lines, among which ACC-MESO-4 was most efficiently blocked with FG-3019 and was chosen for in vivo experiments. We also evaluated the coexistent effects of fibroblasts on mesothelioma in vitro, which are also known to produce CTGF in various pathologic situations. Coexistent fibroblasts in transwell systems remarkably promoted the proliferation and migration/invasion of mesothelioma cells. In orthotopic nude mice model, FG-3019 significantly inhibited mesothelioma growth. Histological analyses revealed that FG-3019 not only inhibited the proliferation but also induced apoptosis in both mesothelioma cells and fibroblasts. Our data suggest that FG-3019 antibody therapy could be a novel additional choice for the treatment of mesothelioma.

Multiple enhancer regions govern the transcription of CCN2 during embryonic development

CCN2 is a critical matricellular protein that is expressed in several cells with major implications in physiology and different pathologies. However, the transcriptional regulation of this gene remains obscure. We used the Encyclopaedia of DNA Elements browser (ENCODE) to visualise the region spanning from 300 kb upstream to the CCN2 start site in silico in order to identify enhancer regions that regulate transcription of this gene. Selection was based on three criteria associated with enhancer regions: 1) H3K4me1 and H3K27ac histone modifications, 2) DNase I hypersensitivity of chromatin and 3) inter-species conservation. Reporter constructs were created with sequences spanning each of the regions of interest placed upstream of an Hsp68 silent proximal promoter sequence in order to drive the expression of β-galactosidase transgene. Each of these constructs was subsequently used to create transgenic mice in which reporter gene production was assessed at the E15.5 developmental stage. Four functional enhancers were identified, with each driving distinct, tissue-specific patterns of transgene expression. An enhancer located -100 kb from the CCN2 transcription start site facilitated expression within vascular tissue. An enhancer -135 kb upstream of CCN2 drove expression within the articular chondrocytes of synovial joints. The other two enhancers, located at -198 kb and -229 kb, mediated transgene expression within dermal fibroblasts, however the most prevalent activity was found within hypertrophic chondrocytes and periosteal tissue, respectively. These findings suggest that the global expression of CCN2 during development results from the activity of several tissue-specific enhancer regions in addition to proximal regulatory elements that have previously been demonstrated to drive transcription of the gene during development.

Immunohistochemical Analysis of CCN Proteins in Calcified Tissues

Immunohistochemistry is a major technique to determine the distribution and localization of differentially produced proteins in the context of an intact tissue. It exploits one of the properties of antibodies, specific binding to an antigen, i.e., to the epitope of its target protein, in combination with a color-developing enzymatic reaction or tagged fluorophore. We have clarified the spatial and temporal expression patterns of CCN family proteins in several different types of animal tissues by using this immunohistochemical technique to support our corresponding data obtained in vitro. In this chapter, we provide our protocol for immunohistochemistry optimized for paraffin-embedded sections after having determined the optimal conditions for the use of antibodies against each member of the CCN family.

Production of Recombinant CCN2 Protein by Mammalian Cells

Recombinant CCN2 protein (rCCN2) is available from many companies; however, most of them are produced in E. coli. To investigate true functions of rCCN2, glycosylated protein with proper folding needs to be used. Therefore, we use rCCN2 produced by mammalian cells. Conditioned medium (CM) of HeLa cells stably transfected with a CCN2 expression vector are collected, and the recombinant CCN2 protein produced and secreted into the CM is purified by two-step chromatography, first with a heparin affinity column and then with an anti-CCN2 affinity column prepared with a monoclonal antibody against CCN2. The purified rCCN2 shows the bands of 36-38 kDa with sliver staining after gel electrophoresis, which can be confirmed by Western blotting. This chapter describes these methods in detail.

Analysis of Expression of CCN Family Genes in Skeletal Tissue-Derived Cells

The quantitative reverse transcription polymerase chain reaction or real-time PCR has become a routine technique for the detection and comparison of amounts of specific mRNA transcripts, done by measuring amplified levels of specific cDNAs. In this chapter, we provide our real-time RT-PCR experimental procedure using SYBR Green I for the quantitative analysis of CCN family gene expression. Especially, we describe the extraction and purification steps for RNA derived from mesenchymal cells, such as chondrocytes and osteoblasts that produce a large amount of extracellular matrix in detail.

Analysis of Signaling Pathways Activated by CCN Proteins

CCN family proteins activate multiple intracellular phosphorylated kinase cascades to yield the multiple physiological functions of a variety of target cells. In this chapter, we describe our protocol examining the effects of these proteins on signal transduction pathways, especially mitogen-activated protein kinase cascades, activated by CCN member proteins, which examinations have been carried out mainly by using Western blotting methodologies.

Connective-Tissue Growth Factor (CTGF/CCN2) Induces Astrogenesis and Fibronectin Expression of Embryonic Neural Cells In Vitro

Connective-tissue growth factor (CTGF) is a modular secreted protein implicated in multiple cellular events such as chondrogenesis, skeletogenesis, angiogenesis and wound healing. CTGF contains four different structural modules. This modular organization is characteristic of members of the CCN family. The acronym was derived from the first three members discovered, cysteine-rich 61 (CYR61), CTGF and nephroblastoma overexpressed (NOV). CTGF is implicated as a mediator of important cell processes such as adhesion, migration, proliferation and differentiation. Extensive data have shown that CTGF interacts particularly with the TGFβ, WNT and MAPK signaling pathways. The capacity of CTGF to interact with different growth factors lends it an important role during early and late development, especially in the anterior region of the embryo. ctgf knockout mice have several cranio-facial defects, and the skeletal system is also greatly affected due to an impairment of the vascular-system development during chondrogenesis. This study, for the first time, indicated that CTGF is a potent inductor of gliogenesis during development. Our results showed that in vitro addition of recombinant CTGF protein to an embryonic mouse neural precursor cell culture increased the number of GFAP- and GFAP/Nestin-positive cells. Surprisingly, CTGF also increased the number of Sox2-positive cells. Moreover, this induction seemed not to involve cell proliferation. In addition, exogenous CTGF activated p44/42 but not p38 or JNK MAPK signaling, and increased the expression and deposition of the fibronectin extracellular matrix protein. Finally, CTGF was also able to induce GFAP as well as Nestin expression in a human malignant glioma stem cell line, suggesting a possible role in the differentiation process of gliomas. These results implicate ctgf as a key gene for astrogenesis during development, and suggest that its mechanism may involve activation of p44/42 MAPK signaling. Additionally, CTGF-induced differentiation of glioblastoma stem cells into a less-tumorigenic state could increase the chances of successful intervention, since differentiated cells are more vulnerable to cancer treatments.

Connective Tissue Growth Factor reporter mice label a subpopulation of mesenchymal progenitor cells that reside in the trabecular bone region

Few gene markers selectively identify mesenchymal progenitor cells inside the bone marrow. We have investigated a cell population located in the mouse bone marrow labeled by Connective Tissue Growth Factor reporter expression (CTGF-EGFP). Bone marrow flushed from CTGF reporter mice yielded an EGFP+ stromal cell population. Interestingly, the percentage of stromal cells retaining CTGF reporter expression decreased with age in vivo and was half the frequency in females compared to males. In culture, CTGF reporter expression and endogenous CTGF expression marked the same cell types as those labeled using Twist2-Cre and Osterix-Cre fate mapping approaches, which previously had been shown to identify mesenchymal progenitors in vitro. Consistent with this past work, sorted CTGF+ cells displayed an ability to differentiate into osteoblasts, chondrocytes, and adipocytes in vitro and into osteoblast, adipocyte, and stromal cell lineages after transplantation into a parietal bone defect. In vivo examination of CTGF reporter expression in bone tissue sections revealed that it marked cells highly localized to the trabecular bone region and was not expressed in the perichondrium or periosteum. Mesenchymal cells retaining high CTGF reporter expression were adjacent to, but distinct from mature osteoblasts lining bone surfaces and endothelial cells forming the vascular sinuses. Comparison of CTGF and Osterix reporter expression in bone tissue sections indicated an inverse correlation between the strength of CTGF expression and osteoblast maturation. Down-regulation of CTGF reporter expression also occurred during in vitro osteogenic differentiation. Collectively, our studies indicate that CTGF reporter mice selectively identify a subpopulation of bone marrow mesenchymal progenitor cells that reside in the trabecular bone region.

Cellular and molecular actions of CCN2/CTGF and its role under physiological and pathological conditions

CCN family protein 2 (CCN2), also widely known as connective tissue growth factor (CTGF), is one of the founding members of the CCN family of matricellular proteins. Extensive investigation on CCN2 over decades has revealed the novel molecular action and functional properties of this unique signalling modulator. By its interaction with multiple molecular counterparts, CCN2 yields highly diverse and context-dependent biological outcomes in a variety of microenvironments. Nowadays, CCN2 is recognized to conduct the harmonized development of relevant tissues, such as cartilage and bone, in the skeletal system, by manipulating extracellular signalling molecules involved therein by acting as a hub through a web. However, on the other hand, CCN2 occasionally plays profound roles in major human biological disorders, including fibrosis and malignancies in major organs and tissues, by modulating the actions of key molecules involved in these clinical entities. In this review, the physiological and pathological roles of this unique protein are comprehensively summarized from a molecular network-based viewpoint of CCN2 functionalities.

Connective tissue growth factor (CTGF/CCN2) negatively regulates BMP-2 induced osteoblast differentiation and signaling

Connective tissue growth factor (CTGF/CCN2) and bone morphogenetic protein (BMP)-2 are both produced and secreted by osteoblasts. Both proteins have been shown to have independent effects in regulating osteoblast proliferation, maturation and mineralization. However, how these two proteins interact during osteoblast differentiation remains unknown. In this study, we utilized two cell culture model systems, osteoblasts derived from CTGF knockout (KO) mice and osteoblasts infected with an adenovirus which over-expresses CTGF (Ad-CTGF), to investigate the effects of CTGF and BMP-2 on osteoblast development and function in vitro. Contrary to a previously published report, osteoblast maturation and mineralization were similar in osteogenic cultures derived from KO and WT calvaria in the absence of BMP-2 stimulation. Interestingly, in KO and WT osteoblast cultures stimulated with BMP-2, the KO osteoblasts exhibited enhanced osteoblast differentiation. This increase in osteoblast differentiation was accompanied by increased protein levels of phosphorylated Smad 1/5/8 and mRNA expression levels of bone morphogenetic protein receptor Ib. We also examined osteoblast differentiation in cultures that were infected with an adenoviral-CTGF vector (Ad-CTGF) and in controls. Continuous over-expression of CTGF resulted in decreased osteoblast maturation and mineralization in both unstimulated and BMP-2 stimulated cultures. Impaired osteoblast differentiation in cultures over-expressing CTGF was accompanied by decreased protein levels of phosphorylated Smad 1/5/8. Collectively, the data from these studies demonstrate that CTGF acts to negatively regulate BMP-2 induced signaling and osteoblast differentiation, and warrant additional studies to determine the precise mechanism(s) responsible for this effect. J. Cell. Physiol. 229: 672-681, 2014. © 2013 Wiley Periodicals, Inc.

Compressive force-produced CCN2 induces osteocyte apoptosis through ERK1/2 pathway

Osteocytes produce various factors that mediate the onset of bone formation and resorption and play roles in maintaining bone homeostasis and remodeling in response to mechanical stimuli. One such factor, CCN2, is thought to play a significant role in osteocyte responses to mechanical stimuli, but its function in osteocytes is not well understood. Here, we showed that CCN2 induces apoptosis in osteocytes under compressive force loading. Compressive force increased CCN2 gene expression and production, and induced apoptosis in osteocytes. Application of exogenous CCN2 protein induced apoptosis, and a neutralizing CCN2 antibody blocked loading-induced apoptosis. We further examined how CCN2 induces loaded osteocyte apoptosis. In loaded osteocytes, extracellular signal-regulated kinase 1/2 (ERK1/2) was activated, and an ERK1/2 inhibitor blocked loading-induced apoptosis. Furthermore, application of exogenous CCN2 protein caused ERK1/2 activation, and the neutralizing CCN2 antibody inhibited loading-induced ERK1/2 activation. Therefore, this study demonstrated for the first time to our knowledge that enhanced production of CCN2 in osteocytes under compressive force loading induces apoptosis through activation of ERK1/2 pathway.

CCN2: a master regulator of the genesis of bone and cartilage

CCN family member 2 (CCN2), also known as connective tissue growth factor (CTGF), has been suggested to be an endochondral ossification genetic factor that has been termed “ecogenin”, because in vitro studies revealed that CCN2 promotes the proliferation and differentiation of growth-plate chondrocytes, osteoblasts, and vascular endothelial cells, all of which play important roles in endochondral ossification. In addition to its action toward these three types of cells, CCN2 was recently found to promote the formation of osteoclasts in vitro, which cells play an important role in the replacement of cartilage by bone during endochondral ossification, thus strengthening the “ecogenin” hypothesis. For confirmation of this hypothesis, transgenic mice over-expressing CCN2 in cartilage were generated. The results proved the hypothesis; i.e., the over-expression of CCN2 in cartilage stimulated the proliferation and differentiation of growth-plate chondrocytes, resulting in the promotion of endochondral ossification. In addition to its “ecogenin” action, CCN2 had earlier been shown to promote the differentiation of various cartilage cells including articular cartilage cells. In accordance with these findings, cartilage-specific overexpression of CCN2 in the transgenic mice was shown to protect against the development of osteoarthritic changes in aging articular cartilage. Thus, CCN2 may also play a role as an anti-aging (chondroprotective) factor, stabilizing articular cartilage. CCN2 also had been shown to promote intramembranous ossification, regenerate cartilage and bone, and induce angiogenesis in vivo. For understanding of the molecular mechanism underlying such multifunctional actions, yeast two-hybrid analysis, protein array analysis, solid-phase binding assay, and surface plasmon resonance (SPR) analysis have been used to search for binding partners of CCN2. ECMs such as fibronectin and aggrecan, growth factors including BMPs and FGF2 and their receptors such as FGFR1 and 2 and RANK, as well as CCN family members themselves, were shown to bind to CCN2. Regarding the interaction of CCN2 with some of them, various binding modules in the CCN2 molecule have been identified. Therefore, the numerous biological actions of CCN2 would depend on what kinds of binding partners and what levels of them are present in the microenvironment of different types of cells, as well as on the state of differentiation of these cells. Through this mechanism, CCN2 would orchestrate various signaling pathways, acting as a signal conductor to promote harmonized skeletal growth and regeneration.

Molecular regulation of CCN2 in the intervertebral disc: lessons learned from other connective tissues

Connective tissue growth factor (CCN2/CTGF) plays an important role in extracellular matrix synthesis, especially in skeletal tissues such as cartilage, bone, and the intervertebral disc. As a result there is a growing interest in examining the function and regulation of this important molecule in the disc. This review discusses the regulation of CCN2 by TGF-β and hypoxia, two critical determinants that characterize the disc microenvironment, and discusses known functions of CCN2 in the disc. The almost ubiquitous regulation of CCN2 by TGF-β, including that seen in the disc, emphasizes the importance of the TGF-β-CCN2 relationship, especially in terms of extracellular matrix synthesis. Likewise, the unique cross-talk between CCN2 and HIF-1 in the disc highlights the tissue and niche specific mode of regulation. Taken together the current literature supports an anabolic role for CCN2 in the disc and its involvement in the maintenance of tissue homeostasis during both health and disease. Further studies of CCN2 in this tissue may reveal valuable targets for the biological therapy of disc degeneration.

The skeletal site-specific role of connective tissue growth factor in prenatal osteogenesis

BACKGROUND

Connective tissue growth factor (CTGF/CCN2) is a matricellular protein that is highly expressed during bone development. Mice with global CTGF ablation (knockout, KO) have multiple skeletal dysmorphisms and perinatal lethality. A quantitative analysis of the bone phenotype has not been conducted.

RESULTS

We demonstrated skeletal site-specific changes in growth plate organization, bone microarchitecture, and shape and gene expression levels in CTGF KO compared with wild-type mice. Growth plate malformations included reduced proliferation zone and increased hypertrophic zone lengths. Appendicular skeletal sites demonstrated decreased metaphyseal trabecular bone, while having increased mid-diaphyseal bone and osteogenic expression markers. Axial skeletal analysis showed decreased bone in caudal vertebral bodies, mandibles, and parietal bones in CTGF KO mice, with decreased expression of osteogenic markers. Analysis of skull phenotypes demonstrated global and regional differences in CTGF KO skull shape resulting from allometric (size-based) and nonallometric shape changes. Localized differences in skull morphology included increased skull width and decreased skull length. Dysregulation of the transforming growth factor-β-CTGF axis coupled with unique morphologic traits provides a potential mechanistic explanation for the skull phenotype.

CONCLUSIONS

We present novel data on a skeletal phenotype in CTGF KO mice, in which ablation of CTGF causes site-specific aberrations in bone formation.

The role of connective tissue growth factor (CTGF/CCN2) in skeletogenesis

Connective tissue growth factor (CTGF) is a 38 kDa, cysteine rich, extracellular matrix protein composed of 4 domains or modules. CTGF has been shown to regulate a diverse array of cellular functions and has been implicated in more complex biological processes such as angiogenesis, chondrogenesis, and osteogenesis. A role for CTGF in the development and maintenance of skeletal tissues first came to light in studies demonstrating its expression in cartilage and bone cells, which was dramatically increased during skeletal repair or regeneration. The physiological significance of CTGF in skeletogenesis was confirmed in CTGF-null mice, which exhibited multiple skeletal dysmorphisms as a result of impaired growth plate chondrogenesis, angiogenesis, and bone formation/mineralization. Given the emerging importance of CTGF in osteogenesis and chondrogenesis, this review will focus on its expression in skeletal tissues, its effects on osteoblast and chondrocyte differentiation and function, and the skeletal implications of ablation or over-expression of CTGF in knockout or transgenic mouse models, respectively. In addition, this review will examine the role of integrin-mediated signaling and the regulation of CTGF expression as it relates to skeletogenesis. We will emphasize CTGF studies in bone or bone cells, and will identify opportunities for future investigations concerning CTGF and chondrogenesis/osteogenesis.

Differential roles of CCN family proteins during osteoblast differentiation: Involvement of Smad and MAPK signaling pathways

CCN family proteins play diverse roles in many aspects of cellular processes such as proliferation, differentiation, adhesion, migration, angiogenesis and survival. In the bone tissue of vertebrate species, the expression of most CCN family members has been observed in osteoblasts. However, their spatial and temporal distributions, as well as their functions, are still only partially understood. In this study, we evaluated the localization of CCN family members in skeletal tissue in vivo and comparatively analyzed the gene expression patterns and functions of the members in murine osteoblasts in primary culture. Immunofluorescent analyses revealed that the CCN family members were differentially produced in osteoblasts and osteocytes. The presence of all Ccn transcripts was confirmed in those osteoblasts. Among the members, CCN1, CCN2, CCN4 and CCN5 were found in osteocytes. CCN4 and CCN5 were distributed in osteocytes located inside of bone matrix as well. Next, we investigated the expression pattern of Ccn family members during osteoblast differentiation. Along with differentiation, most of the members followed proper gene expression patterns; whereas, Ccn4 and Ccn5 showed quite similar patterns. Furthermore, we evaluated the effects of CCN family members on the osteoblastic activities by using recombinant CCN proteins and RNA interference method. Five members of this family displayed positive effects on osteoblast proliferation or differentiation. Of note, CCN3 drastically inhibited the osteoblast activities. Each Ccn specific siRNA could modulate osteoblast activities in a manner expected by the observed effect of respective recombinant CCN protein. In addition, we found that extracellular signal-regulated kinase1/2 and p38 mitogen-activated protein kinase pathways were critically involved in the CCN family member-mediated modification of osteoblast activities. Collectively, all Ccn family members were found to be differentially expressed along with differentiation and therefore could participate in progression of the osteoblast lineage.

Roles for CCN2 in normal physiological processes

CCN2, also known as connective tissue growth factor, is a member of the CCN (CCN1-6) family of modular matricellular proteins. Analysis of CCN2 function in vivo has focused primarily on its key role as a mediator of excess ECM synthesis in multiple fibrotic diseases. However, CCN2 and related family members are widely expressed during development. Recent studies using new genetic models are revealing that CCN2 has essential roles in the development of many tissues. This review focuses on current and emerging data on CCN2 and its functions in chondrogenesis and angiogenesis, and on new studies showing that CCN2 has essential functions during embryonic and postnatal development in a number of epithelial tissues.

CCN3-mediated promotion of sulfated proteoglycan synthesis in rat chondrocytes from developing joint heads

Chondrocytes forming articular cartilage are embedded in a vast amount of extracellular matrix having physical stiffness and elasticity, properties that support the mechanical load from bones and enable the flexible movement of synovial joints. Unlike chondrocytes that conduct the growth of long bones by forming the growth plate, articular chondrocytes show suppressed cell proliferation, unless these cells are exposed to pathological conditions such as mechanical overload. In the present study, we found that one of the members of the CCN family, CCN3, was significantly expressed in chondrocytes isolated from the epiphyseal head in developing rat synovial joints. Evaluation of the effect of recombinant CCN3 on those chondrocytes revealed that CCN3 promoted proteoglycan synthesis, whereas this factor repressed the proliferation of the same cells. These results suggest a critical role for CCN3 in the regulation of the biological properties of articular chondrocytes.

Association of the metastatic phenotype with CCN family members among breast and oral cancer cells

The CCN family of proteins consists of six members with conserved structural features. These proteins play several roles in the physiology and pathology of cells. Among the pathological roles of the CCN family, one of the most important and controversial ones is their role in the expansion and metastasis of cancer. Up to now a number of reports have described the possible role of each CCN family member independently. In this study, we comprehensively analyzed the roles of all six CCN family members in cell growth, migration and invasion of breast cancer cells in vitro and in vivo. As a result, we found the CCN2/CCN3 ratio to be a parameter that is associated with the metastatic phenotype of breast cancer cells that are highly metastatic to the bone. The same analysis with cell lines from oral squamous carcinomas that are not metastatic to the bone further supported our notion. These results suggest the functional significance of the interplay between CCN family members in regulating the phenotype of cancer cells.

The role of CCN2 in cartilage and bone development

CCN2, a classical member of the CCN family of matricellular proteins, is a key molecule that conducts cartilage development in a harmonized manner through novel molecular actions. During vertebrate development, all cartilage is primarily formed by a process of mesenchymal condensation, while CCN2 is induced to promote this process. Afterwards, cartilage develops into several subtypes with different fates and missions, in which CCN2 plays its proper roles according to the corresponding microenvironments. The history of CCN2 in cartilage and bone began with its re-discovery in the growth cartilage in long bones, which determines the skeletal size through the process of endochondral ossification. CCN2 promotes physiological developmental processes not only in the growth cartilage but also in the other types of cartilages, i.e., Meckel's cartilage representing temporary cartilage without autocalcification, articular cartilage representing hyaline cartilage with physical stiffness, and auricular cartilage representing elastic cartilage. Together with its significant role in intramembranous ossification, CCN2 is regarded as a conductor of skeletogenesis. During cartilage development, the CCN2 gene is dynamically regulated to yield stage-specific production of CCN2 proteins at both transcriptional and post-transcriptional levels. New functional aspects of known biomolecules have been uncovered during the course of investigating these regulatory systems in chondrocytes. Since CCN2 promotes integrated regeneration as well as generation (=development) of these tissues, its utility in regenerative therapy targeting chondrocytes and osteoblasts is indicated, as has already been supported by experimental evidence obtained in vivo.

CCN family 2/connective tissue growth factor (CCN2/CTGF) promotes osteoclastogenesis via induction of and interaction with dendritic cell-specific transmembrane protein (DC-STAMP)

CCN family 2/connective tissue growth factor (CCN2/CTGF) promotes endochondral ossification. However, the role of CCN2 in the replacement of hypertrophic cartilage with bone is still unclear. The phenotype of Ccn2 null mice, having an expanded hypertrophic zone, indicates that the resorption of the cartilage extracellular matrix is impaired therein. Therefore, we analyzed the role of CCN2 in osteoclastogenesis because cartilage extracellular matrix is resorbed mainly by osteoclasts during endochondral ossification. Expression of the Ccn2 gene was upregulated in mouse macrophage cell line RAW264.7 on day 6 after treatment of glutathione S transferase (GST) fusion mouse receptor activator of NF-κB ligand (GST-RANKL), and a combination of recombinant CCN2 (rCCN2) and GST-RANKL significantly enhanced tartrate-resistant acid phosphatase (TRACP)-positive multinucleated cell formation compared with GST-RANKL alone. Therefore, we suspected the involvement of CCN2 in cell-cell fusion during osteoclastogenesis. To clarify the mechanism, we performed real-time PCR analysis of gene expression, coimmunoprecipitation analysis, and solid-phase binding assay of CCN2 and dendritic cell-specific transmembrane protein (DC-STAMP), which is involved in cell-cell fusion. The results showed that CCN2 induced and interacted with DC-STAMP. Furthermore, GST-RANKL-induced osteoclastogenesis was impaired in fetal liver cells from Ccn2 null mice, and the impaired osteoclast formation was rescued by the addition of exogenous rCCN2 or the forced expression of DC-STAMP by a retroviral vector. These results suggest that CCN2 expressed during osteoclastogenesis promotes osteoclast formation via induction of and interaction with DC-STAMP.

WISP-1/CCN4 regulates osteogenesis by enhancing BMP-2 activity

Wnt-induced secreted protein 1 (WISP-1/CCN4) is a member of the CCN family that is highly expressed in skeletal tissue and in osteoprogenitor cells induced to differentiate in vitro. To determine the function of WISP-1 during osteogeneis, osteogenic bone marrow stromal cells (BMSCs) were transduced with WISP-1 adenovirus (adWISP-1) in the presence or absence of bone morphogenetic protein 2 (BMP-2) adenovirus (adBMP-2). WISP-1 overexpression enhanced the ability of BMP-2 to direct BMSCs toward osteogenic differentiation and appeared to work by stimulating Smad-1/5/8 phosphorylation and activation. The ability of WISP-1 to enhance BMP-2 activity also was shown in vivo using an ectopic osteogenesis assay with BMSCs transduced with WISP-1, BMP-2, or both. When BMSCs were infected with lentivirus containing human WISP1 shRNA, they formed less bone in vivo and were less responsive to BMP-2, confirming that WISP-1 and BMP-2 have a functional interaction. Immunoprecipitation (IP) and Western blot analysis showed that WISP-1 bound directly to BMP-2 and showed that WISP-1 increased BMP-2 binding to hBMSCs in a dose-dependent fashion. To understand how WISP-1 enhanced BMP-2 signaling, the influence of WISP-1 on integrin expression was analyzed. WISP-1 induced the mRNA and protein levels of α(5)-integrin and, further, was found to bind to it. Antibody-blocking experiments showed that the BMP-2 binding to BMSCs that was enhanced by WISP-1 was completely neutralized by treatment with anti-integrin α(5)β(1) antibody. Pilot studies and the use of transgenic mice that overexpressed human WISP-1 in preosteoblasts had increased bone mineral density (BMD), trabecular thickness, and bone volume (BV/TV) over wild-type controls, supporting observations using human osteoprogenitors that WISP-1 has a positive influence on osteogenesis in vivo. In conclusion, these studies show, for the first time, that WISP-1 has a positive influence on bone cell differentiation and function and may work by enhancing the effects of BMP-2 to increase osteogenesis through a mechanism potentially involving binding to integrin α(5)β(1).

Temporal and spatial expression of CCN genes in zebrafish

The six mammalian CCN genes (Cyr61, CTGF, Nov, WISP1, WISP2, WISP3) encode a family of secreted, cysteine-rich, multimodular proteins having roles in cell proliferation, adhesion, migration, and differentiation during embryogenesis, wound healing, and angiogenesis. We used bioinformatics to identify 9 CCN genes in zebrafish (zCCNs), 6 of which have not been previously described. When compared with mammalian CCN family members, 3 were paralogs of Cyr61, 2 of CTGF, 2 of WISP1, 1 of WISP2, and 1 of WISP3. No paralog of Nov was found. In situ hybridization was performed to characterize the sites of expression of the zCCNs during early zebrafish development. zCCNs demonstrated both unique and overlapping patterns of expression, suggesting potential division of labor between orthologous genes and providing an alternate approach to gene function studies that will complement studies in mammalian models. Developmental Dynamics 239:1755–1767, 2010. © 2010 Wiley-Liss, Inc.

Osteogenic differentiation of mesenchymal stem cells promoted by overexpression of connective tissue growth factor

OBJECTIVE

Large segmental bone defect repair remains a clinical and scientific challenge with increasing interest focusing on combining gene transfection with tissue engineering techniques. The aim of this study is to investigate the effect of connective tissue growth factor (CTGF) on the proliferation and osteogenic differentiation of the bone marrow mesenchymal stem cells (MSCs).

METHODS

A CTGF-expressing plasmid (pCTGF) was constructed and transfected into MSCs. Then expressions of bone morphogenesis-related genes, proliferation rate, alkaline phosphatase activity, and mineralization were examined to evaluate the osteogenic potential of the CTGF gene-modified MSCs.

RESULTS

Overexpression of CTGF was confirmed in pCTGF-MSCs. pCTGF transfection significantly enhanced the proliferation rates of pCTGF-MSCs (P<0.05). CTGF induced a 7.5-fold increase in cell migration over control (P<0.05). pCTGF transfection enhanced the expression of bone matrix proteins, such as bone sialoprotein, osteocalcin, and collagen type I in MSCs. The levels of alkaline phosphatase (ALP) activities of pCTGF-MSCs at the 1st and 2nd weeks were 4.0- and 3.0-fold higher than those of MSCs cultured in OS-medium, significantly higher than those of mock-MSCs and normal control MSCs (P<0.05). Overexpression of CTGF in MSCs enhanced the capability to form mineralized nodules.

CONCLUSION

Overexpression of CTGF could improve the osteogenic differentiation ability of MSCs, and the CTGF gene-modified MSCs are potential as novel cell resources of bone tissue engineering.

Strategies for blocking the fibrogenic actions of connective tissue growth factor (CCN2): From pharmacological inhibition in vitro to targeted siRNA therapy in vivo

Connective tissue growth factor (CCN2) is a major pro-fibrotic factor that frequently acts downstream of transforming growth factor beta (TGF-beta)-mediated fibrogenic pathways. Much of our knowledge of CCN2 in fibrosis has come from studies in which its production or activity have been experimentally attenuated. These studies, performed both in vitro and in animal models, have demonstrated the utility of pharmacological inhibitors (e.g. tumor necrosis factor alpha (TNF-alpha), prostaglandins, peroxisome proliferator-activated receptor-gamma (PPAR-gamma) agonists, statins, kinase inhibitors), neutralizing antibodies, antisense oligonucleotides, or small interfering RNA (siRNA) to probe the role of CCN2 in fibrogenic pathways. These investigations have allowed the mechanisms regulating CCN2 production to be more clearly defined, have shown that CCN2 is a rational anti-fibrotic target, and have established a framework for developing effective modalities of therapeutic intervention in vivo.

Cooperative regulation of chondrocyte differentiation by CCN2 and CCN3 shown by a comprehensive analysis of the CCN family proteins in cartilage

CCN2 is best known as a promoter of chondrocyte differentiation among the CCN family members, and its null mice display skeletal dysmorphisms. However, little is known concerning roles of the other CCN members in chondrocytes. Using both in vivo and in vitro approaches, we conducted a comparative analysis of CCN2-null and wildtype mice to study the roles of CCN2 and the other CCN proteins in cartilage development. Immunohistochemistry was used to evaluate the localization of CCN proteins and other chondrocyte-associated molecules in the two types of mice. Moreover, gene expression levels and the effects of exogenous CCN proteins on chondrocyte proliferation, differentiation, and the expression of chondrocyte-associated genes in their primary chondrocytes were evaluated. Ccn3 was dramatically upregulated in CCN2-null cartilage and chondrocytes. This upregulation was associated with diminished cell proliferation and delayed differentiation. Consistent with the in vivo findings, CCN2 deletion entirely retarded chondrocyte terminal differentiation and decreased the expression of several chondrocyte-associated genes in vitro, whereas Ccn3 expression drastically increased. In contrast, the addition of exogenous CCN2 promoted differentiation strongly and induced the expression of the associated genes, whereas decreasing the Ccn3 expression. These findings collectively indicate that CCN2 induces chondrocyte differentiation by regulating the expression of chondrocyte-associated genes but that these effects are counteracted by CCN3. The lack of CCN2 caused upregulation of CCN3 in CCN2-null mice, which resulted in the observed phenotypes, such as the resultant delay of terminal differentiation. The involvement of the PTHrP-Ihh loop in the regulation of CCN3 expression is also suggested.

Connective tissue growth factor enhances osteoblastogenesis in vitro

Connective tissue growth factor (CTGF), a member of the CCN family of proteins, is expressed by osteoblasts, but its function in cells of the osteoblastic lineage has not been established. We investigated the effects of CTGF overexpression by transducing murine ST-2 stromal cells with a retroviral vector, where CTGF is under the control of the cytomegalovirus promoter. Overexpression of CTGF in ST-2 cells increased alkaline phosphatase activity, osteocalcin and alkaline phosphatase mRNA levels, and mineralized nodule formation. CTGF overexpression decreased the effect of bone morphogenetic protein-2 on Smad 1/5/8 phosphorylation and of Wnt 3 on cytosolic beta-catenin, indicating that the stimulatory effect on osteoblastogenesis was unrelated to BMP and Wnt signaling. CTGF overexpression suppressed Notch signaling and induced the transcription of hairy and E (spl)-1 (HES)-1, by Notch-independent mechanisms. CTGF induced nuclear factor of activated T cells (NFAT) transactivation by a calcineurin-dependent mechanism. Down-regulation of CTGF enhanced Notch signaling and decreased HES-1 transcription and NFAT transactivation. Similar effects were observed following forced CTGF overexpression, the addition of CTGF protein, or the transduction of ST-2 cells with a retroviral vector expressing HES-1. In conclusion, CTGF enhances osteoblastogenesis, possibly by inhibiting Notch signaling and inducing HES-1 transcription and NFAT transactivation.

The CCN family of genes: a perspective on CCN biology and therapeutic potential

The CCN family of genes currently comprises six secreted proteins (designated CCN1-6 after Cyr61/CCN1; ctgf/CCN2; Nov/CCN3; WISP1/CCN4; WISP2/CCN5, WISP3/CCN6) with a similar mosaic primary structure. It is now well accepted that CCN proteins are not growth factors but matricellular proteins that modify signaling of other molecules, in particular those associated with the extracellular matrix. CCN proteins are involved in mitosis, adhesion, apoptosis, extracellular matrix production, growth arrest and migration of multiple cell types. Since their first identification as matricellular factors, the CCN proteins now figure prominently in a variety of major diseases and are now considered valid candidates for therapeutic targeting. Dissection of the molecular mechanisms governing the biological properties of these proteins is being actively pursued by an expanding network of scientists around the globe who will meet this year at the 5th International Workshop on the CCN family of Genes, organized by the International CCN Society ( http://ccnsociety.com ), home for an international cadre of collaborators working in the CCN field.