The regenerative effects of CCN2 independent modules on chondrocytes in vitro and osteoarthritis models in vivo

Inverse genetics tracing the differentiation pathway of human chondrocytes

OBJECTIVE

Mammalian somatic cells can be reprogrammed into induced pluripotent stem cells (iPSCs) via the forced expression of Yamanaka reprogramming factors. However, only a limited population of the cells that pass through a particular pathway can metamorphose into iPSCs, while the others do not. This study aimed to clarify the pathways that chondrocytes follow during the reprogramming process.

DESIGN

The fate of human articular chondrocytes under reprogramming was investigated through a time-coursed single-cell transcriptomic analysis, which we termed an inverse genetic approach. The iPS interference technique was also employed to verify that chondrocytes inversely return to pluripotency following the proper differentiation pathway.

RESULTS

We confirmed that human chondrocytes could be converted into cells with an iPSC phenotype. Moreover, it was clarified that a limited population that underwent the silencing of SOX9, a master gene for chondrogenesis, at a specific point during the proper transcriptome transition pathway, could eventually become iPSCs. Interestingly, the other cells, which failed to be reprogrammed, followed a distinct pathway toward cells with a surface zone chondrocyte phenotype. The critical involvement of cellular communication network factors (CCNs) in this process was indicated. The idea that chondrocytes, when reprogrammed into iPSCs, follow the differentiation pathway backward was supported by the successful iPS interference using SOX9.

CONCLUSIONS

This inverse genetic strategy may be useful for seeking candidates for the master genes for the differentiation of various somatic cells. The utility of CCNs in articular cartilage regeneration is also supported.

Positive Regulation of S-Adenosylmethionine on Chondrocytic Differentiation via Stimulation of Polyamine Production and the Gene Expression of Chondrogenic Differentiation Factors

S-adenosylmethionine (SAM) is considered to be a useful therapeutic agent for degenerative cartilage diseases, although its mechanism is not clear. We previously found that polyamines stimulate the expression of differentiated phenotype of chondrocytes. We also found that the cellular communication network factor 2 (CCN2) played a huge role in the proliferation and differentiation of chondrocytes. Therefore, we hypothesized that polyamines and CCN2 could be involved in the chondroprotective action of SAM. In this study, we initially found that exogenous SAM enhanced proteoglycan production but not cell proliferation in human chondrocyte-like cell line-2/8 (HCS-2/8) cells. Moreover, SAM enhanced gene expression of cartilage-specific matrix (aggrecan and type II collagen), Sry-Box transcription factor 9 (SOX9), CCN2, and chondroitin sulfate biosynthetic enzymes. The blockade of the methionine adenosyltransferase 2A (MAT2A) enzyme catalyzing intracellular SAM biosynthesis restrained the effect of SAM on chondrocytes. The polyamine level in chondrocytes was higher in SAM-treated culture than control culture. Additionally, Alcian blue staining and RT-qPCR indicated that the effects of SAM on the production and gene expression of aggrecan were reduced by the inhibition of polyamine synthesis. These results suggest that the stimulation of polyamine synthesis and gene expression of chondrogenic differentiation factors, such as CCN2, account for the mechanism underlying the action of SAM on chondrocytes.

Proteomic profiling of human menisci from mild joint degeneration and end-stage osteoarthritis versus healthy controls

Objective

To gain new insight into the molecular changes of the meniscus by comparing the proteome profiles of healthy controls with mild degeneration and end-stage osteoarthritis (OA).

Method

We obtained tissue plugs from lateral and medial menisci of 37 individuals (central part of the posterior horn) classified as healthy (n ​= ​12), mild signs of joint damage (n ​= ​13) and end-stage OA (n ​= ​12). The protein profile was analysed by nano-liquid chromatography-mass spectrometry using data-independent acquisition and quantified by Spectronaut. Linear-mixed effects modelling was applied to extract the between-group comparisons.

Results

A similar protein profile was observed for the mild group as compared to healthy controls while the most different group was end-stage OA mainly for the medial compartment. When a pattern of gradual change in protein levels from healthy to end-stage OA was required, a 42-proteins panel was identified, suggesting a potential role in OA development. The levels of QSOX1 were lower and G6PD higher in the mild group following the proposed protein abundance pattern. Qualitative protein changes suggest lower levels of CYTL1 as a potential biomarker of early joint degradation.

Conclusion

For future targeted proteomic approaches, we propose a candidate panel of 42 proteins based on gradually altered meniscal posterior horn protein abundance patterns associated with joint degradation.

Do not overwork: cellular communication network factor 3 for life in cartilage

Cellular communication network factor (CCN) 3, which is one of the founding members of the CCN family, displays diverse functions. However, this protein generally represses the proliferation of a variety of cells. Along with skeletal development, CCN3 is produced in cartilaginous anlagen, growth plate cartilage and epiphysial cartilage. Interestingly, CCN3 is drastically induced in the growth plates of mice lacking CCN2, which promotes endochondral ossification. Notably, chondrocytes in these mutant mice with elevated CCN3 production also suffer from impaired glycolysis and energy metabolism, suggesting a critical role of CCN3 in cartilage metabolism. Recently, CCN3 was found to be strongly induced by impaired glycolysis, and in our study, we located an enhancer that mediated CCN3 regulation via starvation. Subsequent investigations specified regulatory factor binding to the X-box 1 (RFX1) as a transcription factor mediating this CCN3 regulation. Impaired glycolysis is a serious problem, resulting in an energy shortage in cartilage without vasculature. CCN3 produced under such starved conditions restricts energy consumption by repressing cell proliferation, leading chondrocytes to quiescence and survival. This CCN3 regulatory system is indicated to play an important role in articular cartilage maintenance, as well as in skeletal development. Furthermore, CCN3 continues to regulate cartilage metabolism even during the aging process, probably utilizing this regulatory system. Altogether, CCN3 seems to prevent "overwork" by chondrocytes to ensure their sustainable life in cartilage by sensing energy metabolism. Similar roles are suspected to exist in relation to systemic metabolism, since CCN3 is found in the bloodstream.

CCN Proteins (Cellular Communication Network Factors): Expanding Their Repertoire Toward a New Concept

I herein report the general structures and functions of CCN proteins and possible molecular mechanisms involved in the unique biological actions of this family of intercellular signaling regulators, which are considered matricellular proteins and were once referred to as "signal conductors" but have recently been renamed "Cellular Communication Network Factors." Their repertoire of functions beyond their role as matricellular proteins is also described to aid in future studies. Advanced research concerning their relevance to pathology is briefly introduced as well. The information provided in this chapter is expected to be useful for readers of subsequent chapters.

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.

Molecular and Genetic Interactions between CCN2 and CCN3 behind Their Yin-Yang Collaboration

Cellular communication network factor (CCN) 2 and 3 are the members of the CCN family that conduct the harmonized development of a variety of tissues and organs under interaction with multiple biomolecules in the microenvironment. Despite their striking structural similarities, these two members show contrastive molecular functions as well as temporospatial emergence in living tissues. Typically, CCN2 promotes cell growth, whereas CCN3 restrains it. Where CCN2 is produced, CCN3 disappears. Nevertheless, these two proteins collaborate together to execute their mission in a yin–yang fashion. The apparent functional counteractions of CCN2 and CCN3 can be ascribed to their direct molecular interaction and interference over the cofactors that are shared by the two. Recent studies have revealed the mutual negative regulation systems between CCN2 and CCN3. Moreover, the simultaneous and bidirectional regulatory system of CCN2 and CCN3 is also being clarified. It is of particular note that these regulations were found to be closely associated with glycolysis, a fundamental procedure of energy metabolism. Here, the molecular interplay and metabolic gene regulation that enable the yin–yang collaboration of CCN2 and CCN3 typically found in cartilage development/regeneration and fibrosis are described.

CTGF as a multifunctional molecule for cartilage and a potential drug for osteoarthritis

CTGF is a multifunctional protein and plays different roles in different cells and under different conditions. Pamrevlumab, a monoclonal antibody against CTGF, is an FDA approved drug for idiopathic pulmonary fibrosis (IPF) and Duchenne muscular dystrophy (DMD). Recent studies have shown that CTGF antibodies may potentially serve as a new drug for osteoarthritis (OA). Expression of CTGF is significantly higher in OA joints than in healthy counterparts. Increasing attention has been attracted due to its interesting roles in joint homeostasis. Joint homeostasis relies on normal cellular functions and cell-cell interactions. CTGF is essential for physiological activities of chondrocytes. Abnormal CTGF expression may cause cartilage degeneration. In this review, the physiological functions of CTGF in chondrocytes and related mechanisms are summarized. Changes in the related signaling pathways due to abnormal CTGF are discussed, which are contributing factors to inflammation, cartilage degeneration and synovial fibrosis in OA. The possibility of CTGF as a potential therapeutic target for OA treatment are reviewed.

RFX1-mediated CCN3 induction that may support chondrocyte survival under starved conditions

Cellular communication network factor (CCN) family members are multifunctional matricellular proteins that manipulate and integrate extracellular signals. In our previous studies investigating the role of CCN family members in cellular metabolism, we found three members that might be under the regulation of energy metabolism. In this study, we confirmed that CCN2 and CCN3 are the only members that are tightly regulated by glycolysis in human chondrocytic cells. Interestingly, CCN3 was induced under a variety of impaired glycolytic conditions. This CCN3 induction was also observed in two breast cancer cell lines with a distinct phenotype, suggesting a basic role of CCN3 in cellular metabolism. Reporter gene assays indicated a transcriptional regulation mediated by an enhancer in the proximal promoter region. As a result of analyses in silico, we specified regulatory factor binding to the X-box 1 (RFX1) as a candidate that mediated the transcriptional activation by impaired glycolysis. Indeed, the inhibition of glycolysis induced the expression of RFX1, and RFX1 silencing nullified the CCN3 induction by impaired glycolysis. Subsequent experiments with an anti-CCN3 antibody indicated that CCN3 supported the survival of chondrocytes under impaired glycolysis. Consistent with these findings in vitro, abundant CCN3 production by chondrocytes in the deep zones of developing epiphysial cartilage, which are located far away from the synovial fluid, was confirmed in vivo. Our present study uncovered that RFX1 is the mediator that enables CCN3 induction upon cellular starvation, which may eventually assist chondrocytes in retaining their viability, even when there is an energy supply shortage.

CCN2 (Cellular Communication Network factor 2) in the bone marrow microenvironment, normal and malignant hematopoiesis

CCN2, formerly termed Connective Tissue Growth Factor, is a protein belonging to the Cellular Communication Network (CCN)-family of secreted extracellular matrix-associated proteins. As a matricellular protein it is mainly considered to be active as a modifier of signaling activity of several different signaling pathways and as an orchestrator of their cross-talk. Furthermore, CCN2 and its fragments have been implicated in the regulation of a multitude of biological processes, including cell proliferation, differentiation, adhesion, migration, cell survival, apoptosis and the production of extracellular matrix products, as well as in more complex processes such as embryonic development, angiogenesis, chondrogenesis, osteogenesis, fibrosis, mechanotransduction and inflammation. Its function is complex and context dependent, depending on cell type, state of differentiation and microenvironmental context. CCN2 plays a role in many diseases, especially those associated with fibrosis, but has also been implicated in many different forms of cancer. In the bone marrow (BM), CCN2 is highly expressed in mesenchymal stem/stromal cells (MSCs). CCN2 is important for MSC function, supporting its proliferation, migration and differentiation. In addition, stromal CCN2 supports the maintenance and longtime survival of hematopoietic stem cells, and in the presence of interleukin 7, stimulates the differentiation of pro-B lymphocytes into pre-B lymphocytes. Overexpression of CCN2 is seen in the majority of B-acute lymphoblastic leukemias, especially in certain cytogenetic subgroups associated with poor outcome. In acute myeloid leukemia, CCN2 expression is increased in MSCs, which has been associated with leukemic engraftment in vivo. In this review, the complex function of CCN2 in the BM microenvironment and in normal as well as malignant hematopoiesis is discussed. In addition, an overview is given of data on the remaining CCN family members regarding normal and malignant hematopoiesis, having many similarities and some differences in their function.

Bipartite regulation of cellular communication network factor 2 and fibroblast growth factor 1 genes by fibroblast growth factor 1 through histone deacetylase 1 and fork head box protein A1

Fibroblast growth factor 1 (FGF-1) is the first FGF family member, and it induces proliferation of fibroblasts and other types of the cells. However, recent studies are uncovering unexpected functions of this molecule. Our previous study redefined this growth factor as a catabolic molecule produced in cartilage upon metabolic insult. Indeed, FGF-1 was found to repress the gene expression of cellular communication network factor 2 (CCN2), which protects and regenerates cartilage, amplifying its own production through positive feedback regulation. In the present study, we investigated the molecular mechanism of this bipartite CCN2 repression and FGF1 activation by FGF-1 in chondrocytes. Repression of CCN2 and induction of FGF1 in human chondrocytic cells were both partly abolished by valproic acid, an inhibitor of histone deacetylase 1 (HDAC1), indicating the involvement of chromatin remodeling by histone acetylation in this system. In contrast, RNA degradation analysis suggested no contribution of post-transcriptional regulation of the mRNA stability to the effects conferred by FGF-1. Suspecting a regulation by a specific transcription factor, we next sought a candidate in silico from a large dataset. As a result, we found fork head box protein A1 (FOXA1) as the transcription factor that bound to both CCN2 and FGF1 loci. Functional analysis demonstrated that FOXA1 silencing significantly attenuated the CCN2 repression and FGF1 induction caused by FGF1. These findings collectively indicate that the bipartite regulation by FGF-1 is enabled by the combination of chromatin remodeling by HDACs and transcriptional modulation by FOXA1 with unknown transcriptional coactivators of opposite functionalities.

The thrombospondin module 1 domain of the matricellular protein CCN3 shows an atypical disulfide pattern and incomplete CWR layers

The members of the CCN (Cyr61/CTGF/Nov) family are a group of matricellular regulatory proteins that are essential to a wide range of functional pathways in cell signalling. Through interacting with extracellular matrix components and growth factors via one of their four domains, the CCN proteins are involved in critical biological processes such as angiogenesis, cell proliferation, bone development, fibrogenesis and tumorigenesis. Here, the crystal structure of the thrombospondin module 1 (TSP1) domain of CCN3 (previously known as Nov) is presented, which shares a similar three-stranded fold with the thrombospondin type 1 repeats of thrombospondin-1 and spondin-1, but with variations in the disulfide connectivity. Moreover, the CCN3 TSP1 domain lacks the typical π-stacked ladder of charged and aromatic residues on one side of the domain that is seen in other TSP1 domains. Using conservation analysis among orthologous domains, it is shown that a charged cluster in the centre of the domain is the most conserved site and this cluster is predicted to be a potential functional epitope for heparan sulfate binding. This variant TSP1 domain has also been used to revise the sequence determinants of TSP1 domains and to derive improved Pfam sequence profiles for the identification of novel TSP1 domains in more than 10 000 proteins across diverse phyla.

The pathogenic role of connective tissue growth factor in osteoarthritis

Osteoarthritis (OA) is the most common form of arthritis, and connective tissue growth factor (CTGF) is found to be up-regulated in adjacent areas of cartilage surface damage. CTGF is present in osteophytes of late stage OA. In the present study, we have reviewed association of CTGF in the development and progression of OA and the potential effects of CTGF as a therapeutic agent for the treatment of OA. We have reviewed the recent articles on CTGF and OA in databases like PubMed, google scholar, and SCOPUS and collected the information for the articles. CTGF is usually up-regulated in synovial fluid of OA that stimulates the production of inflammatory cytokines. CTGF also activates nuclear factor-κB, increases the production of chemokines and cytokines, and up-regulates matrix metalloproteinases-3 (MMP-3) that in turn leads to the reduction in proteoglycan contents in joint cartilage. Consequently, cartilage homeostasis is imbalanced that might contribute to the pathogenesis of OA by developing synovial inflammation and cartilage degradation. CTGF might serve as a useful biomarker for the prognosis and treatment of OA, and recent studies have taken attempt to use CTGF as therapeutic target of OA. However, more investigations with clinical trials are necessary to validate the possibility of use of CTGF as a biomarker in OA diagnosis and therapeutic target for OA treatment.

Possible reparative effect of low-intensity pulsed ultrasound (LIPUS) on injured meniscus

Menisci are a pair of crescent-shaped fibrocartilages, particularly of which their inner region of meniscus is an avascular tissue. It has characteristics similar to those of articular cartilage, and hence is inferior in healing. We previously reported that low-intensity pulsed ultrasound (LIPUS) treatment stimulates the production of CCN2/CTGF, a protein involved in repairing articular cartilage, and the gene expression of major cartilage matrices such as type II collagen and aggrecan in cultured chondrocytes. Therefore, in this present study, we investigated whether LIPUS has also favorable effect on meniscus cells and tissues. LIPUS applied with a 60 mW/cm² intensity for 20 min stimulated the gene expression and protein production of CCN2 via ERK and p38 signaling pathways, as well as gene expression of SOX9, aggrecan, and collagen type II in human inner meniscus cells in culture, and slightly stimulated the gene expression of CCN2 and promoted the migration in human outer meniscus cells in culture. LIPUS also induced the expression of Ccn2, Sox9, Col2a1, and Vegf in rat intact meniscus. Furthermore, histological evaluations showed that LIPUS treatment for 1 to 4 weeks promoted healing of rat injured lateral meniscus, as evidenced by better and earlier angiogenesis and extracellular matrix synthesis. The data presented indicate that LIPUS treatment might prevent meniscus from degenerative change and exert a reparative effect on injured meniscus via up-regulation of repairing factors such as CCN2 and that it might thus be useful for treatment of an injured meniscus as a non-invasive therapy.

New Functions of Classical Compounds against Orofacial Inflammatory Lesions

Anti-inflammatory agents have been widely used to ameliorate severe inflammatory symptoms of a number of diseases, and such therapeutics are particularly useful for diseases with intolerable pain without significant mortality. A typical example of this is a disease known as stomatitis; although stomatitis itself is not a life-threatening disease, it severely impairs the individual’s quality of life, and thus a standard therapeutic strategy for it has already been established. The topical application of a bioactive agent is quite easy, and a strong anti-inflammatory agent can be used without significant adverse effects. In contrast, natural products with relatively mild bioactivity are used for systemic intervention. However, new aspects of classical drugs used in these established therapeutic methods have recently been discovered, which is expanding the utility of these compounds to other oral diseases such as osteoarthritis of temporomandibular joints (TMJ-OA). In this review article, after summarizing the general concept and pathobiology of stomatitis, its established therapeutics are explained. Thereafter, recent advances in the research into related compounds, which is uncovering new biological functions of the agents used therein, are introduced. Indeed, regenerative therapeutics for TMJ-OA may be developed with the classical compounds currently being used.

NELL-1, HMGB1, and CCN2 Enhance Migration and Vasculogenesis, But Not Osteogenic Differentiation Compared to BMP2

Currently, autografts still represent the gold standard treatment for the repair of large bone defects. However, these are associated with donor-site morbidity and increased pain, cost, and recovery time. The ideal therapy would use biomaterials combined with bone growth factors to induce and instruct bone defect repair without the need to harvest patient tissue. In this line, bone morphogenetic proteins (BMPs) have been the most extensively used agents for clinical bone repair, but at supraphysiological doses that are not without risk. Because of the need to eliminate the risks of BMP2 use in vivo, we assessed the ability of three putative osteogenic factors, nel-like molecule type 1 (NELL-1), high mobility group box 1 (HMGB1), and CCN2, to enhance the essential processes for bone defect repair in vitro and compared them to BMP2. Although it has been reported that NELL-1, HMGB1, and CCN2 play a role in bone formation, less is known about the contribution of these proteins to the different events involved, such as cell migration, osteogenesis, and vasculogenesis. In this study, we investigated the effects of different doses of NELL-1, HMGB, CCN2, and BMP2 on these three processes as a model for the recruitment and differentiation of resident cells in the in vivo bone defect repair situation, using cells of human origin. Our data demonstrated that NELL-1, HMGB1, and CCN2 significantly induced mesenchymal stem cell migration (from 1.58-fold increase compared to control), but BMP2 did not. Interestingly, only BMP2 increased osteogenesis in marrow stromal cells, whereas it inhibited osteogenesis in preosteoblasts. Moreover, the four proteins studied promoted significantly endothelial cell migration, reaching a maximum of 2.4-fold increase compared to control, and induced formation of tube-like structures. NELL-1, HMGB1, and CCN2 had these effects at relatively low doses compared to BMP2. This work indicates that NELL-1, HMGB1, and CCN2 might enhance bone defect healing via the recruitment of endogenous cells and induction of vascularization and act via different processes than BMP2.

Thrombospondin-1 Partly Mediates the Cartilage Protective Effect of Adipose-Derived Mesenchymal Stem Cells in Osteoarthritis

Objective

Assuming that mesenchymal stem cells (MSCs) respond to the osteoarthritic joint environment to exert a chondroprotective effect, we aimed at investigating the molecular response setup by MSCs after priming by osteoarthritic chondrocytes in cocultures.

Methods

We used primary human osteoarthritic chondrocytes and adipose stem cells (ASCs) in mono- and cocultures and performed a high-throughput secretome analysis. Among secreted proteins differentially induced in cocultures, we identified thrombospondin-1 (THBS1) as a potential candidate that could be involved in the chondroprotective effect of ASCs.

Results

Secretome analysis revealed significant induction of THBS1 in ASCs/chondrocytes cocultures at mRNA and protein levels. We showed that THBS1 was upregulated at late stages of MSC differentiation toward chondrocytes and that recombinant THBS1 (rTHBS1) exerted a prochondrogenic effect on MSC indicating a role of THBS1 during chondrogenesis. However, compared to control ASCs, siTHBS1-transfected ASCs did not decrease the expression of hypertrophic and inflammatory markers in osteoarthritic chondrocytes, suggesting that THBS1 was not involved in the reversion of osteoarthritic phenotype. Nevertheless, downregulation of THBS1 in ASCs reduced their immunosuppressive activity, which was consistent with the anti-inflammatory role of rTHBS1 on T lymphocytes. THBS1 function was then evaluated in the collagenase-induced OA model by comparing siTHBS1-transfected and control ASCs. The protective effect of ASCs evaluated by histological and histomorphological analysis of cartilage and bone was not seen with siTHBS1-transfected ASCs.

Conclusion

Our data suggest that THBS1 did not exert a direct protective effect on chondrocytes but might reduce inflammation, subsequently explaining the therapeutic effect of ASCs in OA.

Metabolic regulation of the CCN family genes by glycolysis in chondrocytes

The CCN family consists of 6 genes in the mammalian genome and produces multifunctional proteins involved in a variety of biological processes. Recent reports indicate the profound roles of CCN2 in energy metabolism in chondrocytes, and Ccn2 deficiency is known to alter the expression of 2 other family members including Ccn3. However, almost nothing is known concerning the regulation of the CCN family genes by energy metabolism. In order to gain insight into this critical issue, we initially and comprehensively evaluated the effect of inhibition of glycolysis on the expression of all of the CCN family genes in chondrocytic cells. Upon the inhibition of a glycolytic enzyme, repression of CCN2 expression was observed, whereas CCN3 expression was conversely induced. Similar repression of CCN2 was conferred by the inhibition of aerobic ATP production, which, however, did not induce CCN3 expression. In contrast, glucose starvation significantly enhanced the expression of CCN3 in those cells. The results of a reporter gene assay using a molecular construct containing a CCN3 proximal promoter revealed a dose-dependent induction of the CCN3 promoter activity by the glycolytic inhibitor in chondrocytic cells. These results unveiled a critical role of glycolytic activity in the regulation of CCN2 and CCN3, which activity mediated the mutual regulation of these 2 major CCN family members in chondrocytes.

Novel role of CCN3 that maintains the differentiated phenotype of articular cartilage

Knowledge of the microenvironment of articular cartilage in health and disease is the key to accomplishing fundamental disease-modifying treatments for osteoarthritis. The proteins comprising the CCN Family are matricellular proteins with a remarkable relevance within the context of cartilage metabolism. CCN2 displays a great capability for regenerating articular cartilage, and CCN3 has been shown to activate the expression of genes related to articular chondrocytes and to repress genes related to endochondral ossification in epiphyseal chondrocytes. Moreover, mice lacking CCN3 protein have been shown to display ostearthritic changes in their knee articular cartilage. In this study, we employed a monoiodoacetic acid (MIA)-induced osteoarthritic model to investigate whether osteoarthritic changes in the cartilage are reciprocally accompanied by CCN3 down-regulation and an inducible overexpression system to evaluate the effects of CCN3 on articular chondrocytes in vitro. Finally, we also investigated the effects of exogenous CCN3 in vivo during the early stages of MIA-induced osteoarthritis. We discovered that CCN3 is expressed by articular chondrocytes in normal rat knees, whereas it is rapidly down-regulated in osteoarthritic knees. In vitro, we also discovered that CCN3 increases the proteoglycan accumulation, the gene expression of type II collagen, tenascin-C and lubricin, as well as the protein production of tenascin-C and lubricin in articular chondrocytes. In vivo, it was discovered that exogenous CCN3 increased tidemark integrity and produced an increased production of lubricin protein. The potential utility of CCN3 as a future therapeutic agent and possible strategies to improve its therapeutic functions are also discussed.

Catabolic effects of FGF-1 on chondrocytes and its possible role in osteoarthritis

Fibroblast growth factor 1 (FGF-1) is a classical member of the FGF family and is produced by chondrocytes cultured from osteoarthritic patients. Also, this growth factor was shown to bind to CCN family protein 2 (CCN2), which regenerates damaged articular cartilage and counteracts osteoarthritis (OA) in an animal model. However, the pathophysiological role of FGF-1 in cartilage has not been well investigated. In this study, we evaluated the effects of FGF-1 in vitro and its production in vivo by use of an OA model. Treatment of human chondrocytic cells with FGF-1 resulted in marked repression of genes for cartilaginous extracellular matrix components, whereas it strongly induced matrix metalloproteinase 13 (MMP-13), representing its catabolic effects on cartilage. Interestingly, expression of the CCN2 gene was dramatically repressed by FGF-1, which repression eventually caused the reduced production of CCN2 protein from the chondrocytic cells. The results of a reporter gene assay revealed that this repression could be ascribed, at least in part, to transcriptional regulation. In contrast, the gene expression of FGF-1 was enhanced by exogenous FGF-1, indicating a positive feedback system in these cells. Of note, induction of FGF-1 was observed in the articular cartilage of a rat OA model. These results collectively indicate a pathological role of FGF-1 in OA development, which includes an insufficient cartilage regeneration response caused by CCN2 down regulation.

The CCN Proteins: An Overview

I introduce the general structures and functions of CCN proteins and possible molecular mechanisms regarding the unique biological actions of this new family of signaling regulators, which may be referred to as "signal conductors." Relevance to pathology is also briefly introduced. The information provided in this overview should be useful for readers of the following chapters.

In Vivo Evaluation of Cartilage Regenerative Effects of CCN2 Protein

CCN family protein 2/connective tissue growth factor (CCN2/CTGF) is a unique growth factor that promotes the proliferation and differentiation, but not the hypertrophy of articular chondrocytes in vitro. Based on these findings, we previously evaluated the cartilage-regenerative effects of recombinant CCN2 protein (rCCN2) by using both mono-iodoacetate (MIA) injection into the rat joint cavity and formation of full-thickness defects of rat articular cartilage in vivo, and our results suggested the utility of CCN2 in the regeneration of articular cartilage. This chapter entails helpful tips to apply these two animal models for the evaluation of cartilage-regenerative effects of CCN2 or its derivatives.

Involvement of multiple CCN family members in platelets that support regeneration of joint tissues

OBJECTIVES

Platelet-rich plasma (PRP) has been widely used to enhance the regeneration of damaged joint tissues, such as osteoarthritic and rheumatoid arthritic cartilage. The aim of this study is to clarify the involvement of all of the CCN family proteins that are crucially associated with joint tissue regeneration.

METHODS

Cyr61-CTGF-NOV (CCN) family proteins in human platelets and megakaryocytic cells were comprehensively analyzed by Western blotting analysis. Production of CCN family proteins in megakaryocytes in vivo was confirmed by immunofluorescence analysis of mouse bone marrow cells. Effects of CCN family proteins found in platelets on chondrocytes were evaluated by using human chondrocytic HCS-2/8 cells.

RESULTS

Inclusion of CCN2, a mesenchymal tissue regenerator, was confirmed. Of note, CCN3, which counteracts CCN2, was newly found to be encapsulated in platelets. Interestingly, these two family members were not detectable in megakaryocytic cells, but their external origins were suggested. Furthermore, we found for the first time CCN5 and CCN1 that inhibits ADAMTS4 in both platelets and megakaryocytes. Finally, application of a CCN family cocktail mimicking platelets onto HCS-2/8 cells enhanced their chondrocytic phenotype.

CONCLUSIONS

Multiple inclusion of CCN1, 2 and 3 in platelets was clarified, which supports the harmonized regenerative potential of PRP in joint therapeutics.

Role of CCN2 in Amino Acid Metabolism of Chondrocytes

CCN2/connective tissue growth factor (CTGF) is a multi-functional molecule that promotes harmonized development and regeneration of cartilage through its matricellular interaction with a variety of extracellular biomolecules. Thus, deficiency in CCN2 supply profoundly affects a variety of cellular activities including basic metabolism. A previous study showed that the expression of a number of ribosomal protein genes was markedly enhanced in Ccn2-null chondrocytes. Therefore, in this study, we analyzed the impact of CCN2 on amino acid and protein metabolism in chondrocytes. Comparative metabolome analysis of the amino acids in Ccn2-null and wild-type mouse chondrocytes revealed stable decreases in the cellular levels of all of the essential amino acids. Unexpectedly, uptake of such amino acids was rather enhanced in Ccn2-null chondrocytes, and the addition of exogenous CCN2 to human chondrocytic cells resulted in decreased amino acid uptake. However, as expected, amino acid consumption by protein synthesis was also accelerated in Ccn2-null chondrocytes. Furthermore, we newly found that expression of two genes encoding two glycolytic enzymes, as well as the previously reported Eno1 gene, was repressed in those cells. Considering the impaired glycolysis and retained mitochondrial membrane potential in Ccn2-null chondrocytes, these findings suggest that Ccn2 deficiency induces amino acid shortage in chondrocytes by accelerated amino acid consumption through protein synthesis and acquisition of aerobic energy. Interestingly, CCN2 was found to capture such free amino acids in vitro. Under physiological conditions, CCN2 may be regulating the levels of free amino acids in the extracellular matrix of cartilage.

CCN2 enhances RANKL-induced osteoclast differentiation via direct binding to RANK and OPG

CCN family protein 2/connective tissue growth factor (CCN2/CTGF) is a multi-potent factor for mesenchymal cells such as chondrocytes, osteoblasts, osteoclasts, and endothelial cells. CCN2 is also known as a modulator of other cytokines and receptors via direct molecular interactions with them. We screened additional factors binding to CCN2 and found receptor activator of NF-kappa B (RANK) as one of them. RANK is also known as TNF-related activation-induced cytokine (TRANCE) receptor, and its signaling plays a critical role in osteoclastogenesis. Notable affinity between CCN2 and RANK was confirmed by using surface plasmon resonance (SPR) analysis. In fact, CCN2 enhanced the RANK-mediated signaling, such as occurs in NF-kappa B, p38 and JNK pathways, in pre-osteoclastic RAW264.7 cells; whereas CCN2 had no influence on RANK-RANK ligand (RANKL) binding. Moreover, CCN2 also significantly bound to osteoprotegerin (OPG), which is a decoy receptor of RANKL. Of note, OPG markedly inhibited the binding between CCN2 and RANK; and CCN2 canceled the inhibitory effect of OPG on osteoclast differentiation. These findings suggest CCN2 as a candidate of the fourth factor in the RANK/RANKL/OPG system for osteoclastogenesis, which regulates OPG and RANK via direct interaction.

CCN2 as a novel molecule supporting energy metabolism of chondrocytes

CCN2/connective tissue growth factor (CTGF) is a unique molecule that promotes both chondrocytic differentiation and proliferation through its matricellular interaction with a number of extracellular biomolecules. This apparently contradictory functional property of CCN2 suggests its certain role in basic cellular activities such as energy metabolism, which is required for both proliferation and differentiation. Comparative metabolomic analysis of costal chondrocytes isolated from wild-type and Ccn2-null mice revealed overall impaired metabolism in the latter. Among the numerous metabolites analyzed, stable reduction in the intracellular level of ATP, GTP, CTP, or UTP was observed, indicating a profound role of CCN2 in energy metabolism. Particularly, the cellular level of ATP was decreased by more than 50% in the Ccn2-null chondrocytes. The addition of recombinant CCN2 (rCCN2) to cultured Ccn2-null chondrocytes partly redeemed the cellular ATP level attenuated by Ccn2 deletion. Next, in order to investigate the mechanistic background that mediates the reduction in ATP level in these Ccn2-null chondrocytes, we performed transcriptome analysis. As a result, several metabolism-associated genes were found to have been up-regulated or down-regulated in the mutant mice. Up-regulation of a number of ribosomal protein genes was observed upon Ccn2 deletion, whereas a few genes required for aerobic and anaerobic ATP production were down-regulated in the Ccn2-null chondrocytes. Among such genes, reduction in the expression of the enolase 1 gene was of particular note. These findings uncover a novel functional role of CCN2 as a metabolic supporter in the growth-plate chondrocytes, which is required for skeletogenesis in mammals.

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.

Direct interaction between CCN family protein 2 and fibroblast growth factor 1

In an attempt to find out a new molecular counterpart of CCN family protein 2 (CCN2), a matricellular protein with multiple functions, we performed an interactome analysis and found fibroblast growth factor (FGF) -1 as one of the candidates. Solid-phase binding assay indicated specific binding between CCN2 and FGF-1. This binding was also confirmed by surface plasmon resonance (SPR) analysis that revealed a dissociation constant (Kd) of 3.98 nM indicating strong molecular interaction between the two. RNA analysis suggested that both FGF-1 and CCN2 could be produced by chondrocytes and thus their interaction in the cartilage is possible. These findings for the first time indicate the direct interaction of CCN2 and FGF-1 and suggest the co-presence of these molecules in the cartilage microenvironment. CCN2 is a well-known promoter of cartilage development and regeneration, whereas the physiological and pathological role of FGF-1 in cartilage mostly remains unclear. Biological role of FGF-1 itself in cartilage is also suspected.