The establishment and characterization of an immortal cell line with a stable chondrocytic phenotype

Possible Repositioning of an Oral Anti-Osteoporotic Drug, Ipriflavone, for Treatment of Inflammatory Arthritis via Inhibitory Activity of KIAA1199, a Novel Potent Hyaluronidase

KIAA1199 has a strong hyaluronidase activity in inflammatory arthritis. This study aimed to identify a drug that could reduce KIAA1199 activity and clarify its effects on inflammatory arthritis. Rat chondrosarcoma (RCS) cells were strongly stained with Alcian blue (AB). Its stainability was reduced in RCS cells, which were over-expressed with the KIAA1199 gene (RCS-KIAA). We screened the drugs that restore the AB stainability in RCS-KIAA. The effects of the drug were evaluated by particle exclusion assay, HA ELISA, RT-PCR, and Western blotting. We further evaluated the HA accumulation and the MMP1 and three expressions in fibroblast-like synoviocytes (FLS). In vivo, the effects of the drug on symptoms and serum concentration of HA in a collagen-induced arthritis mouse were evaluated. Ipriflavone was identified to restore AB stainability at 23%. Extracellular matrix formation was significantly increased in a dose-dependent manner (p = 0.006). Ipriflavone increased the HA accumulation and suppressed the MMP1 and MMP3 expression on TNF-α stimulated FLS. In vivo, Ipriflavone significantly improved the symptoms and reduced the serum concentrations of HA. Conclusions: We identified Ipriflavone, which has inhibitory effects on KIAA1199 activity. Ipriflavone may be a therapeutic candidate based on its reduction of KIAA1199 activity in inflammatory arthritis.

Gene expression of type II collagen is regulated by direct interaction with Kruppel-like factor 4 and AT-rich interactive domain 5B

We have previously found and characterized two pairs of enhancer elements, E1 and E2, in the type II collagen alpha 1 chain (COL2A1) gene. Subsequent studies have suggested that these enhancers function differently in the regulation of gene expression. For example, histone deacetylase 10 modifies only the E2 enhancer region to affect gene expression. Therefore, in this study, we aimed to clarify the transcriptional complex formed at each enhancer region by identifying transcription factors that specifically bind to each enhancer element. To this end, we used chondrocytic cell lines established using our unique silent reporter system and overexpressed candidate transcription factors in these cells. We found two transcription factors, other than the SOX trio, that directly bound to COL2A1 and regulated its expression. The first was Kruppel-like factor-4 (KLF4), which bound to the promoter proximal region, and the second was AT-rich interactive domain 5B (ARID5B) which bound to the E1 enhancer element. Further studies are needed to identify factors that specifically bind to the E2 enhancer element. In any case, our findings provide an important insight into the molecular mechanisms underlying the regulation of COL2A1. In this paper, we reevaluated the previous analysis of transcription factors involved in the regulation of COL2A1 expression.

CCN3 (NOV) Drives Degradative Changes in Aging Articular Cartilage

Aging is a major risk factor of osteoarthritis, which is characterized by the degeneration of articular cartilage. CCN3, a member of the CCN family, is expressed in cartilage and has various physiological functions during chondrocyte development, differentiation, and regeneration. Here, we examine the role of CCN3 in cartilage maintenance. During aging, the expression of Ccn3 mRNA in mouse primary chondrocytes from knee cartilage increased and showed a positive correlation with p21 and p53 mRNA. Increased accumulation of CCN3 protein was confirmed. To analyze the effects of CCN3 in vitro, either primary cultured human articular chondrocytes or rat chondrosarcoma cell line (RCS) were used. Artificial senescence induced by H₂O₂ caused a dose-dependent increase in Ccn3 gene and CCN3 protein expression, along with enhanced expression of p21 and p53 mRNA and proteins, as well as SA-β gal activity. Overexpression of CCN3 also enhanced p21 promoter activity via p53. Accordingly, the addition of recombinant CCN3 protein to the culture increased the expression of p21 and p53 mRNAs. We have produced cartilage-specific CCN3-overexpressing transgenic mice, and found degradative changes in knee joints within two months. Inflammatory gene expression was found even in the rib chondrocytes of three-month-old transgenic mice. Similar results were observed in human knee articular chondrocytes from patients at both mRNA and protein levels. These results indicate that CCN3 is a new senescence marker of chondrocytes, and the overexpression of CCN3 in cartilage may in part promote chondrocyte senescence, leading to the degeneration of articular cartilage through the induction of p53 and p21.

MiT/TFE factors control ER-phagy via transcriptional regulation of FAM134B

Lysosomal degradation of the endoplasmic reticulum (ER) via autophagy (ER-phagy) is emerging as a critical regulator of cell homeostasis and function. The recent identification of ER-phagy receptors has shed light on the molecular mechanisms underlining this process. However, the signaling pathways regulating ER-phagy in response to cellular needs are still largely unknown. We found that the nutrient responsive transcription factors TFEB and TFE3-master regulators of lysosomal biogenesis and autophagy-control ER-phagy by inducing the expression of the ER-phagy receptor FAM134B. The TFEB/TFE3-FAM134B axis promotes ER-phagy activation upon prolonged starvation. In addition, this pathway is activated in chondrocytes by FGF signaling, a critical regulator of skeletal growth. FGF signaling induces JNK-dependent proteasomal degradation of the insulin receptor substrate 1 (IRS1), which in turn inhibits the PI3K-PKB/Akt-mTORC1 pathway and promotes TFEB/TFE3 nuclear translocation and enhances FAM134B transcription. Notably, FAM134B is required for protein secretion in chondrocytes, and cartilage growth and bone mineralization in medaka fish. This study identifies a new signaling pathway that allows ER-phagy to respond to both metabolic and developmental cues.

Biochemical characteristics of the chondrocyte-enriched SNORC protein and its transcriptional regulation by SOX9

Snorc (Small NOvel Rich in Cartilage) has been identified as a chondrocyte-specific gene in the mouse. Yet little is known about the SNORC protein biochemical properties, and mechanistically how the gene is regulated transcriptionally in a tissue-specific manner. The goals of the present study were to shed light on those important aspects. The chondrocyte nature of Snorc expression was confirmed in mouse and rat tissues, in differentiated (day 7) ATDC5, and in RCS cells where it was constitutive. Topological mapping and biochemical analysis brought experimental evidences that SNORC is a type I protein carrying a chondroitin sulfate (CS) attached to serine 44. The anomalous migration of SNORC on SDS-PAGE was due to its primary polypeptide features, suggesting no additional post-translational modifications apart from the CS glycosaminoglycan. A highly conserved SOX9-binding enhancer located in intron 1 was necessary to drive transcription of Snorc in the mouse, rat, and human. The enhancer was active independently of orientation and whether located in a heterologous promoter or intron. Crispr-mediated inactivation of the enhancer in RCS cells caused reduction of Snorc. Transgenic mice carrying the intronic multimerized enhancer drove high expression of a βGeo reporter in chondrocytes, but not in the hypertrophic zone. Altogether these data confirmed the chondrocyte-specific nature of Snorc and revealed dependency on the intronic enhancer binding of SOX9 for transcription.

Novel mTORC1 Mechanism Suggests Therapeutic Targets for COMPopathies

Cartilage oligomeric matrix protein (COMP) is a large, multifunctional extracellular protein that, when mutated, is retained in the rough endoplasmic reticulum (ER). This retention elicits ER stress, inflammation, and oxidative stress, resulting in dysfunction and death of growth plate chondrocytes. While identifying the cellular pathologic mechanisms underlying the murine mutant (MT)-COMP model of pseudoachondroplasia, increased midline-1 (MID1) expression and mammalian target of rapamycin complex 1 (mTORC1) signaling was found. This novel role for MID1/mTORC1 signaling was investigated since treatments shown to repress the pathology also reduced Mid1/mTORC1. Although ER stress-inducing drugs or tumor necrosis factor α (TNFα) in rat chondrosarcoma cells increased Mid1, oxidative stress did not, establishing that ER stress- or TNFα-driven inflammation alone is sufficient to elevate MID1 expression. Since MID1 ubiquitinates protein phosphatase 2A (PP2A), a negative regulator of mTORC1, PP2A was evaluated in MT-COMP growth plate chondrocytes. PP2A was decreased, indicating de-repression of mTORC1 signaling. Rapamycin treatment in MT-COMP mice reduced mTORC1 signaling and intracellular retention of COMP, and increased proliferation, but did not change inflammatory markers IL-16 and eosinophil peroxidase. Lastly, mRNA from tuberous sclerosis-1/2-null mice brain tissue exhibiting ER stress had increased Mid1 expression, confirming the relationship between ER stress and MID1/mTORC1 signaling. These findings suggest a mechanistic link between ER stress and MID1/mTORC1 signaling that has implications extending to other conditions involving ER stress.

5'-methylschweinfurthin G reduces chondrosarcoma tumor growth

New treatment options are urgently required in the field of chondrosarcoma, particularly of chondrosarcomas with a well-differentiated hyaline cartilage-like extracellular matrix (e.g., collagen II and proteoglycan-rich) phenotype, notoriously resistant to drug penetration, and having potential of progression towards higher grade. We investigated the feasibility of using 5'-methylschweinfurthin G (MeSG) as a tumor suppressor agent in the Swarm rat chondrosarcoma, an intermediate- to high-grade chondrosarcoma model, having a hyaline cartilage-like phenotype. Tumor cell culture studies were performed to identify their proliferative and cytotoxicity sensitivity to MeSG. Tumor burden mice were treated with MeSG and analyzed for tumor growth, morphology and regression. The chondrosarcoma tumor cells had a half maximum cytotoxicity concentration (IC₅₀ ) of 35 nM MeSG; approximately 300-fold less than freshly isolated rat chondrocytes (IC₅₀ of 11 µM). Multiple injections of MeSG (20 mg/kg, body weight) resulted in reduced/eliminated tumor growth over a 17-day period in mice, and an 83% reduction (p = 0.023) in tumor mass. Three out of ten MeSG treated mice had complete elimination of tumor. Tumors of treated mice had a decrease in chondrosarcoma cell proliferation (p = 0.012) and an increase in cell death (p = 0.030) compared with tumors of control mice. These findings in an animal model demonstrate the effectiveness of MeSG for treatment of rat chondrosarcomas, and may have the potential use as a therapeutic option for the difficult-to-treat intermediate-to high-grade hyaline cartilage-like chondrosarcoma. © 2017 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 36:1283-1293, 2018.

mTORC1 hyperactivation arrests bone growth in lysosomal storage disorders by suppressing autophagy

The mammalian target of rapamycin complex 1 (mTORC1) kinase promotes cell growth by activating biosynthetic pathways and suppressing catabolic pathways, particularly that of macroautophagy. A prerequisite for mTORC1 activation is its translocation to the lysosomal surface. Deregulation of mTORC1 has been associated with the pathogenesis of several diseases, but its role in skeletal disorders is largely unknown. Here, we show that enhanced mTORC1 signaling arrests bone growth in lysosomal storage disorders (LSDs). We found that lysosomal dysfunction induces a constitutive lysosomal association and consequent activation of mTORC1 in chondrocytes, the cells devoted to bone elongation. mTORC1 hyperphosphorylates the protein UV radiation resistance-associated gene (UVRAG), reducing the activity of the associated Beclin 1-Vps34 complex and thereby inhibiting phosphoinositide production. Limiting phosphoinositide production leads to a blockage of the autophagy flux in LSD chondrocytes. As a consequence, LSD chondrocytes fail to properly secrete collagens, the main components of the cartilage extracellular matrix. In mouse models of LSD, normalization of mTORC1 signaling or stimulation of the Beclin 1-Vps34-UVRAG complex rescued the autophagy flux, restored collagen levels in cartilage, and ameliorated the bone phenotype. Taken together, these data unveil a role for mTORC1 and autophagy in the pathogenesis of skeletal disorders and suggest potential therapeutic approaches for the treatment of LSDs.

In vitro chondrocyte toxicity following long-term, high-dose exposure to Gd-DTPA and a novel cartilage-targeted MR contrast agent

OBJECTIVE

To determine the concentrations exhibiting toxicity of a cartilage-targeted magnetic resonance imaging contrast agent compared with gadopentetate dimeglumine (Gd-DT-PA) in chondrocyte cultures.

MATERIALS AND METHODS

A long-term Swarm rat chondrosarcoma chondrocyte-like cell line was exposed for 48 h to 1.0-20 mM concentrations of diaminobutyl-linked nitroxide (DAB4-DLN) citrate, 1.0-20 mM Gd-DTPA, 1.0 μM staurosporine (positive control), or left untreated. Cell appearance, 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assays of metabolic activity, quantitative PicoGreen assays of DNA content, and calcein-AM viability assays were compared.

RESULTS

At 1.0-7.5 mM, minimal decrease in cell proliferation was found for both agents. At all doses of both agents, cell culture appearances were similar after 24 h of treatment. At the higher doses, differences in cell culture appearance were found after 48 h of treatment, with dose-dependent declines in chondrocyte populations for both agents. Concentration-dependent declines in DNA content and calcein fluorescence were found after 48 h of treatment, but beginning at a lower dose of DAB4-DLN citrate than Gd-DTPA. Dose-dependent decreases in MTT staining (cell metabolism) were apparent for both agents, but larger effects were evident at a lower dose for DAB-DLN citrate. Poor MTT staining of cells exposed for 48 h to 20 mM DAB4-DLN citrate probably indicates dead or dying cells.

CONCLUSION

The minimal effect of the long-term exposure of model chondrocyte cell cultures to DAB4-DLN citrate and Gd-DTPA concentrations up to 7.5 mM (3x typical arthrographic administration) is supporting evidence that these doses are acceptable for MR arthrography. The findings are reassuring given that the experimental exposure to the contrast agents at sustained concentrations was much longer than when used clinically.

Development and application of a new Silent reporter system to quantitate the activity of enhancer elements in the type II Collagen Gene

Type II collagen is a major component of cartilage, which provide structural stiffness to the tissue. As a sufficient amount of type II collagen is critical for maintaining the biomechanical properties of cartilage, its expression is tightly regulated in chondrocytes. Therefore, it is essential to elucidate in detail the transcriptional mechanism that controls expression of type II collagen, in particular by two enhancer elements we recently discovered. To systematically analyze and compare enhancer activities, we developed a novel reporter assay system that exploits site-specific integration of promoter and enhancer elements to activate a transcriptionally silent reporter gene. Using this system, we found that the enhancer elements have distinct characteristics, with one exhibiting additive effects and the other exhibiting synergistic effects when repeated in tandem.

FGF signalling regulates bone growth through autophagy

Skeletal growth relies on both biosynthetic and catabolic processes. While the role of the former is clearly established, how the latter contributes to growth-promoting pathways is less understood. Macroautophagy, hereafter referred to as autophagy, is a catabolic process that plays a fundamental part in tissue homeostasis. We investigated the role of autophagy during bone growth, which is mediated by chondrocyte rate of proliferation, hypertrophic differentiation and extracellular matrix (ECM) deposition in growth plates. Here we show that autophagy is induced in growth-plate chondrocytes during post-natal development and regulates the secretion of type II collagen (Col2), the major component of cartilage ECM. Mice lacking the autophagy related gene 7 (Atg7) in chondrocytes experience endoplasmic reticulum storage of type II procollagen (PC2) and defective formation of the Col2 fibrillary network in the ECM. Surprisingly, post-natal induction of chondrocyte autophagy is mediated by the growth factor FGF18 through FGFR4 and JNK-dependent activation of the autophagy initiation complex VPS34-beclin-1. Autophagy is completely suppressed in growth plates from Fgf18(-/-) embryos, while Fgf18(+/-) heterozygous and Fgfr4(-/-) mice fail to induce autophagy during post-natal development and show decreased Col2 levels in the growth plate. Strikingly, the Fgf18(+/-) and Fgfr4(-/-) phenotypes can be rescued in vivo by pharmacological activation of autophagy, pointing to autophagy as a novel effector of FGF signalling in bone. These data demonstrate that autophagy is a developmentally regulated process necessary for bone growth, and identify FGF signalling as a crucial regulator of autophagy in chondrocytes.

The transcription factors SOX9 and SOX5/SOX6 cooperate genome-wide through super-enhancers to drive chondrogenesis

SOX9 is a transcriptional activator required for chondrogenesis, and SOX5 and SOX6 are closely related DNA-binding proteins that critically enhance its function. We use here genome-wide approaches to gain novel insights into the full spectrum of the target genes and modes of action of this chondrogenic trio. Using the RCS cell line as a faithful model for proliferating/early prehypertrophic growth plate chondrocytes, we uncover that SOX6 and SOX9 bind thousands of genomic sites, frequently and most efficiently near each other. SOX9 recognizes pairs of inverted SOX motifs, whereas SOX6 favors pairs of tandem SOX motifs. The SOX proteins primarily target enhancers. While binding to a small fraction of typical enhancers, they bind multiple sites on almost all super-enhancers (SEs) present in RCS cells. These SEs are predominantly linked to cartilage-specific genes. The SOX proteins effectively work together to activate these SEs and are required for in vivo expression of their associated genes. These genes encode key regulatory factors, including the SOX trio proteins, and all essential cartilage extracellular matrix components. Chst11, Fgfr3, Runx2 and Runx3 are among many other newly identified SOX trio targets. SOX9 and SOX5/SOX6 thus cooperate genome-wide, primarily through SEs, to implement the growth plate chondrocyte differentiation program.

The SOX9 upstream region prone to chromosomal aberrations causing campomelic dysplasia contains multiple cartilage enhancers

Two decades after the discovery that heterozygous mutations within and around SOX9 cause campomelic dysplasia, a generalized skeleton malformation syndrome, it is well established that SOX9 is a master transcription factor in chondrocytes. In contrast, the mechanisms whereby translocations in the –­350/–50-kb region 5′ of SOX9 cause severe disease and whereby SOX9 expression is specified in chondrocytes remain scarcely known. We here screen this upstream region and uncover multiple enhancers that activate Sox9-promoter transgenes in the SOX9 expression domain. Three of them are primarily active in chondrocytes. E250 (located at –250 kb) confines its activity to condensed prechondrocytes, E195 mainly targets proliferating chondrocytes, and E84 is potent in all differentiated chondrocytes. E84 and E195 synergize with E70, previously shown to be active in most Sox9-expressing somatic tissues, including cartilage. While SOX9 protein powerfully activates E70, it does not control E250. It requires its SOX5/SOX6 chondrogenic partners to robustly activate E195 and additional factors to activate E84. Altogether, these results indicate that SOX9 expression in chondrocytes relies on widely spread transcriptional modules whose synergistic and overlapping activities are driven by SOX9, SOX5/SOX6 and other factors. They help elucidate mechanisms underlying campomelic dysplasia and will likely help uncover other disease mechanisms.

In vitro toxicity in long-term cell culture of MR contrast agents targeted to cartilage evaluation

OBJECTIVE

Contrast-enhanced magnetic resonance (MR) imaging methods have been proposed for non-invasive evaluation of osteoarthritis (OA). We measured cell toxicities of cartilage-targeted low-generation dendrimer-linked nitroxide MR contrast agents and gadopentetate dimeglumine (Gd-DTPA) on cultured chondrocytes.

DESIGN

A long-term Swarm rat chondrosarcoma chondrocyte-like cell line was exposed for 48-h to different salts (citrate, maleate, tartrate) and concentrations of generation one or two diaminobutyl-linked nitroxides (DAB4-DLN or DAB8-DLN), Gd-DTPA, or staurosporine (positive control). Impact on microscopic cell appearance, MTT spectrophotometric assays of metabolic activity, and quantitative PicoGreen assays of DNA content (cell proliferation) were measured and compared to untreated cultures.

RESULTS

Chondrocyte cultures treated with up to 7.5 mM Gd-DTPA for 48-h had no statistical differences in DNA content or MTT reaction compared to untreated cultures. At all doses, DAB4-DLN citrate treated cultures had results similar to untreated and Gd-DTPA-treated cultures. At doses >1 mM, DAB4-DLN citrate treated cultures showed statistically greater DNA and MTT reaction than maleate and tartrate DAB4-DLN salts. Cultures exposed to 5 mM or 7.5 mM DAB8-DLN citrate exhibited rounded cells, poor cell proliferation, and barely detectable MTT reaction. Treatment with 0.1 μM staurosporine caused chondrocyte death.

CONCLUSION

Long-term exposure, greater than clinically expected, to either DAB4-DLN citrate or Gd-DTPA had no detectable toxicity with results equivalent to untreated cultures. DAB4-DLN citrate was more biocompatible than either the maleate or tartrate salts. Cells exposed for 48-h to 5 mM or 7.5 mM DAB8-DLN salts demonstrated significant cell toxicity. Further evaluation of DAB8-DLN with clinically appropriate exposure times is required to determine the maximum useful concentration.

A dynamic cell adhesion surface regulates tissue architecture in growth plate cartilage

The architecture and morphogenetic properties of tissues are founded in the tissue-specific regulation of cell behaviors. In endochondral bones, the growth plate cartilage promotes bone elongation via regulated chondrocyte maturation within an ordered, three-dimensional cell array. A key event in the process that generates this cell array is the transformation of disordered resting chondrocytes into clonal columns of discoid proliferative cells aligned with the primary growth vector. Previous analysis showed that column-forming chondrocytes display planar cell divisions, and the resulting daughter cells rearrange by ∼90° to align with the lengthening column. However, these previous studies provided limited information about the mechanisms underlying this dynamic process. Here we present new mechanistic insights generated by application of a novel time-lapse confocal microscopy method along with immunofluorescence and electron microscopy. We show that, during cell division, daughter chondrocytes establish a cell-cell adhesion surface enriched in cadherins and β-catenin. Rearrangement into columns occurs concomitant with expansion of this adhesion surface in a process more similar to cell spreading than to migration. Column formation requires cell-cell adhesion, as reducing cadherin binding via chelation of extracellular calcium inhibits chondrocyte rearrangement. Importantly, physical indicators of cell polarity, such as cell body alignment, are not prerequisites for oriented cell behavior. Our results support a model in which regulation of adhesive surface dynamics and cortical tension by extrinsic signaling modifies the thermodynamic landscape to promote organization of daughter cells in the context of the three-dimensional growth plate tissue.

Swarm chondrosarcoma: a continued resource for chondroblastic-like extracellular matrix and chondrosarcoma biology research

Since its first description over four decades ago, the Swarm chondrosarcoma (Swarm rat chondrosarcoma, SRC) remains a valuable tool for studies of chondroblastic-like extracellular matrix (ECM) biology and as an animal model of human chondrosarcoma of histological grades I-III. Moreover, articular joints and skeletal anomalies such as arthritis as well as cartilage regeneration, skeletal development, tissue engineering, hard tissue tumorigenesis and space flight physiology are advanced through studies in hyaline cartilage-like models. With more than 500 articles published since the first report on the characteristics of mucopolysaccharides (glycosaminoglycans) of the tumor in 1971, several transplantable tumor and cell lines have been developed by multiple laboratories worldwide. This review describes the characterization of SRC tumors and cell lines, including the use of SRC lines as a resource for isolation and characterization of several ECM elements that have become vital for the advancement of our understanding of cartilage biology. Also presented is the importance of pertubation of ECM components and the influence of the tumor microenvironment on disease progression. Therapeutic failure and currently pursued avenues of intervention utilizing the SRC lines in treatment of chondrosarcoma are also discussed.

Sedlin controls the ER export of procollagen by regulating the Sar1 cycle

Newly synthesized proteins exit the endoplasmic reticulum (ER) via coat protein complex II (COPII) vesicles. Procollagen (PC), however, forms prefibrils that are too large to fit into typical COPII vesicles; PC thus needs large transport carriers, which we term megacarriers. TANGO1 assists PC packing, but its role in promoting the growth of megacarriers is not known. We found that TANGO1 recruited Sedlin, a TRAPP component that is defective in spondyloepiphyseal dysplasia tarda (SEDT), and that Sedlin was required for the ER export of PC. Sedlin bound and promoted efficient cycling of Sar1, a guanosine triphosphatase that can constrict membranes, and thus allowed nascent carriers to grow and incorporate PC prefibrils. This joint action of TANGO1 and Sedlin sustained the ER export of PC, and its derangement may explain the defective chondrogenesis underlying SEDT.

A newly identified enhancer element responsible for type II collagen gene expression

Type II collagen is a major component of cartilage where it is present at a high concentration, which is essential for the functional maintenance of the tissue. Therefore, any fundamental understanding of the physiology of cartilage tissue must include an understanding of the mechanism that allows the high level of expression of type II collagen gene, Col2a1, by chondrocytes. To this end, we developed a new reporter assay system based on the co-transfection of candidate enhancer elements and reporter construct into Swarm rat chondrosarcoma chondrocytes that allowed their stable expression. Using this system, we screened more than 70 kb of the Col2a1 gene and found an enhancer domain that is responsible for the regulation of its expression level. The domain is localized in intron 7, and consists of an 800-bp region that contains within it a previously unidentified domain, ∼140 bp in size.

D469del-COMP retention in chondrocytes stimulates caspase-independent necroptosis

Mutations in the cartilage oligomeric matrix protein gene (COMP) cause pseudoachondroplasia (PSACH). This dysplasia results from the intracellular retention of mutant COMP protein and premature death of growth-plate chondrocytes. Toward better understanding of these underlying mechanisms, we examined D469del-COMP activation of the unfolded protein response and cell death pathways in rat chondrosarcoma cells. Using an inducible expression system, we examined the effects of D469del-COMP retention after 4 days of mRNA expression and then 5 days without inducing agent. Retention of D469del-COMP stimulated Chop (Ddit3) and Gadd34 (Ppp1r15a) and triggered reactivation of protein translation that exacerbated intracellular retention. High levels of Nox4 and endoplasmic reticulum receptor stress-inducible Ero1β generated reactive oxygen species, causing oxidative stress. Increased expression of Gadd genes and presence of γH2AX indicated that DNA damage was occurring. The presence of cleaved apoptosis inducing factor (tAIF) and the absence of activated caspases indicated that retention of D469del-COMP triggers cell death in chondrocytes by necroptosis, a caspase-independent programmed necrosis. Loss of growth-plate chondrocytes by necroptosis was also found in our pseudoachondroplasia mouse model. These results suggest a model in which D469del-COMP expression induces persistent endoplasmic reticulum stress, oxidative stress, and DNA damage, thus priming chondrocytes for necroptosis. We define for the first time the precise mechanisms underlying D469del-COMP pathology in pseudoachondroplasia and suggest that oxidative stress and AIF may be promising therapeutic targets.

Microenvironment alters epigenetic and gene expression profiles in Swarm rat chondrosarcoma tumors

BACKGROUND

Chondrosarcomas are malignant cartilage tumors that do not respond to traditional chemotherapy or radiation. The 5-year survival rate of histologic grade III chondrosarcoma is less than 30%. An animal model of chondrosarcoma has been established--namely, the Swarm Rat Chondrosarcoma (SRC)--and shown to resemble the human disease. Previous studies with this model revealed that tumor microenvironment could significantly influence chondrosarcoma malignancy.

METHODS

To examine the effect of the microenvironment, SRC tumors were initiated at different transplantation sites. Pyrosequencing assays were utilized to assess the DNA methylation of the tumors, and SAGE libraries were constructed and sequenced to determine the gene expression profiles of the tumors. Based on the gene expression analysis, subsequent functional assays were designed to determine the relevancy of the specific genes in the development and progression of the SRC.

RESULTS

The site of transplantation had a significant impact on the epigenetic and gene expression profiles of SRC tumors. Our analyses revealed that SRC tumors were hypomethylated compared to control tissue, and that tumors at each transplantation site had a unique expression profile. Subsequent functional analysis of differentially expressed genes, albeit preliminary, provided some insight into the role that thymosin-β4, c-fos, and CTGF may play in chondrosarcoma development and progression.

CONCLUSION

This report describes the first global molecular characterization of the SRC model, and it demonstrates that the tumor microenvironment can induce epigenetic alterations and changes in gene expression in the SRC tumors. We documented changes in gene expression that accompany changes in tumor phenotype, and these gene expression changes provide insight into the pathways that may play a role in the development and progression of chondrosarcoma. Furthermore, specific functional analysis indicates that thymosin-β4 may have a role in chondrosarcoma metastasis.

RNAi reduces expression and intracellular retention of mutant cartilage oligomeric matrix protein

Mutations in cartilage oligomeric matrix protein (COMP), a large extracellular glycoprotein expressed in musculoskeletal tissues, cause two skeletal dysplasias, pseudoachondroplasia and multiple epiphyseal dysplasia. These mutations lead to massive intracellular retention of COMP, chondrocyte death and loss of growth plate chondrocytes that are necessary for linear growth. In contrast, COMP null mice have only minor growth plate abnormalities, normal growth and longevity. This suggests that reducing mutant and wild-type COMP expression in chondrocytes may prevent the toxic cellular phenotype causing the skeletal dysplasias. We tested this hypothesis using RNA interference to reduce steady state levels of COMP mRNA. A panel of shRNAs directed against COMP was tested. One shRNA (3B) reduced endogenous and recombinant COMP mRNA dramatically, regardless of expression levels. The activity of the shRNA against COMP mRNA was maintained for up to 10 weeks. We also demonstrate that this treatment reduced ER stress. Moreover, we show that reducing steady state levels of COMP mRNA alleviates intracellular retention of other extracellular matrix proteins associated with the pseudoachondroplasia cellular pathology. These findings are a proof of principle and the foundation for the development of a therapeutic intervention based on reduction of COMP expression.

Global demethylation of rat chondrosarcoma cells after treatment with 5-aza-2'-deoxycytidine results in increased tumorigenicity

Abnormal patterns of DNA methylation are observed in several types of human cancer. While localized DNA methylation of CpG islands has been associated with gene silencing, the effect that genome-wide loss of methylation has on tumorigenesis is not completely known. To examine its effect on tumorigenesis, we induced DNA demethylation in a rat model of human chondrosarcoma using 5-aza-2-deoxycytidine. Rat specific pyrosequencing assays were utilized to assess the methylation levels in both LINEs and satellite DNA sequences following 5-aza-2-deoxycytidine treatment. Loss of DNA methylation was accompanied by an increase in invasiveness of the rat chondrosarcoma cells, in vitro, as well as by an increase in tumor growth in vivo. Subsequent microarray analysis provided insight into the gene expression changes that result from 5-aza-2-deoxycytidine induced DNA demethylation. In particular, two genes that may function in tumorigenesis, sox-2 and midkine, were expressed at low levels in control cells but upon 5-aza-2-deoxycytidine treatment these genes became overexpressed. Promoter region DNA analysis revealed that these genes were methylated in control cells but became demethylated following 5-aza-2-deoxycytidine treatment. Following withdrawal of 5-aza-2-deoxycytidine, the rat chondrosarcoma cells reestablished global DNA methylation levels that were comparable to that of control cells. Concurrently, invasiveness of the rat chondrosarcoma cells, in vitro, decreased to a level indistinguishable to that of control cells. Taken together these experiments demonstrate that global DNA hypomethylation induced by 5-aza-2-deoxycytidine may promote specific aspects of tumorigenesis in rat chondrosarcoma cells.

Adsorption of follicular dendritic cell-secreted protein (FDC-SP) onto mineral deposits. Application of a new stable gene expression system

Follicular dendritic cell-secreted protein (FDC-SP) is a small secretory protein having structural similarities to statherin, a protein in saliva thought to play a role in calcium retention in saliva. In contrast, FDC-SP is thought to play a role in the immune system associated with germinal centers. We report here the very specific expression of FDC-SP in junctional epithelium at the gingival crevice. This region is very important for the host defense against pathogens and for periodontal protection. To be able to better understand the function of FDC-SP, we developed a novel gene expression system that exploited gene trapping and site-specific gene integration to introduce the protein into a mammalian cell culture system. Using this system we were able to express FDC-SP as a fusion protein with green fluorescent protein in an osteogenic progenitor cell line with long term stability, which we then used to find that the fusion protein specifically adsorbs onto mineral deposits and the surface of hydroxyapatite particles exogenously added to the culture. This adsorption was highly dependent on the structural integrity of FDC-SP. These results suggest that FDC-SP may play an important role, adsorbing onto the surface of cementum and alveolar bone adjacent to periodontal ligament and onto tooth surface at the gingival crevice.

COMP mutations: domain-dependent relationship between abnormal chondrocyte trafficking and clinical PSACH and MED phenotypes

Mutations in cartilage oligomeric matrix protein (COMP) produce clinical phenotypes ranging from the severe end of the spectrum, pseudoachondroplasia (PSACH), which is a dwarfing condition, to a mild condition, multiple epiphyseal dysplasia (MED). Patient chondrocytes have a unique morphology characterized by distended rER cisternae containing lamellar deposits of COMP and other extracellular matrix proteins. It has been difficult to determine why different mutations give rise to variable clinical phenotypes. Using our in vitro cell system, we previously demonstrated that the most common PSACH mutation, D469del, severely impedes trafficking of COMP and type IX collagen in chondrocytic cells, consistent with observations from patient cells. Here, we hypothesize that PSACH and MED mutations variably affect the cellular trafficking behavior of COMP and that the extent of defective trafficking correlates with clinical phenotype. Twelve different recombinant COMP mutations were expressed in rat chondrosarcoma cells and the percent cells with ER-retained COMP was assessed. For mutations in type 3 (T3) repeats, trafficking defects correlated with clinical phenotype; PSACH mutations had more cells retaining mutant COMP, while MED mutations had fewer. In contrast, the cellular trafficking pattern observed for mutations in the C-terminal globular domain (CTD) was not predictive of clinical phenotype. The results demonstrate that different COMP mutations in the T3 repeat domain have variable effects on intracellular transport, which correlate with clinical severity, while CTD mutations do not show such a correlation. These findings suggest that other unidentified factors contribute to the effect of the CTD mutations. J. Cell. Biochem. 103: 778-787, 2008. (c) 2007 Wiley-Liss, Inc.

Computational identification and functional validation of regulatory motifs in cartilage-expressed genes

Chondrocyte gene regulation is important for the generation and maintenance of cartilage tissues. Several regulatory factors have been identified that play a role in chondrogenesis, including the positive transacting factors of the SOX family such as SOX9, SOX5, and SOX6, as well as negative transacting factors such as C/EBP and delta EF1. However, a complete understanding of the intricate regulatory network that governs the tissue-specific expression of cartilage genes is not yet available. We have taken a computational approach to identify cis-regulatory, transcription factor (TF) binding motifs in a set of cartilage characteristic genes to better define the transcriptional regulatory networks that regulate chondrogenesis. Our computational methods have identified several TFs, whose binding profiles are available in the TRANSFAC database, as important to chondrogenesis. In addition, a cartilage-specific SOX-binding profile was constructed and used to identify both known, and novel, functional paired SOX-binding motifs in chondrocyte genes. Using DNA pattern-recognition algorithms, we have also identified cis-regulatory elements for unknown TFs. We have validated our computational predictions through mutational analyses in cell transfection experiments. One novel regulatory motif, N1, found at high frequency in the COL2A1 promoter, was found to bind to chondrocyte nuclear proteins. Mutational analyses suggest that this motif binds a repressive factor that regulates basal levels of the COL2A1 promoter.

FGF upregulates osteopontin in epiphyseal growth plate chondrocytes: Implications for endochondral ossification

Fibroblast growth factor receptor 3 (FGFR3) signaling pathways are essential for normal longitudinal bone growth. Mutations in this receptor lead to various human growth disorders, including Achondroplasia, disproportionately short-limbed dwarfism, characterized by narrowing of the hypertrophic region of the epiphyseal growth plates. Here we find that FGF9, a preferred ligand for FGFR3 rapidly induces the upregulation and secretion of the matrix resident phosphoprotein, osteopontin (OPN) in cultured chicken chondrocytes. This effect was observed as early as two hours post stimulation and at FGF9 concentrations as low as 1.25 ng/ml at both mRNA and protein levels. OPN expression is known to be associated with chondrocyte and osteoblast differentiation and osteoclast activation. Unexpectedly, FGF9 induced OPN was accompanied by inhibition of differentiation and increased proliferation of the treated chondrocytes. Moreover, FGF9 stimulated OPN expression irrespective of the differentiation stage of the cells or culture conditions. In situ hybridization analysis of epiphyseal growth plates from chicken or mice homozygous for the Achondroplasia, G369C/mFGFR3 mutation demonstrated co-localization of OPN expression and osteoclast activity, as evidenced by tartarate resistant acid phosphatase positive cells in the osteochondral junction. We propose that FGF signaling directly activates OPN expression independent of chondrocytes differentiation. This may enhance the recruitment and activation of osteoclasts, and increase in cartilage resorption and remodeling in the chondro-osseus border.

An inhibitor of the stretch-activated cation receptor exerts a potent effect on chondrocyte phenotype

Rat chondrosarcoma (RCS) cells are unusual in that they display a stable chondrocyte phenotype in monolayer culture. This phenotype is reflected by a rounded cellular morphology with few actin-containing stress fibers and production of an extracellular matrix rich in sulfated proteoglycans, with high-level expression of aggrecan, COMP, Sox9, and collagens type II, IX, and XI. Additionally, these cells do not express collagen type I. Here it is shown that in the absence of any mechanical stimulation, treatment of RCS cells with gadolinium chloride (Gd3+), a stretch-activated cation channel blocker, caused the cells to undergo de-differentiation, adopting a flattened fibroblast phenotype with the marked appearance of actin stress fibers and vinculin-containing focal contacts. This change was accompanied by a dramatic reduction in the expression of aggrecan, Sox9, collagen types II, IX, and XI, with a corresponding increase in the expression of collagen type I and fibronectin. These effects were found to be reversible by simple removal of Gd3+ from the medium. Gd3+ also had a similar effect on expression of chondrocyte marker genes in freshly isolated human chondrocytes. These data suggest that mechanoreceptor signaling plays a key role in maintenance of the chondrocyte phenotype, even in the absence of mechanical stimulation. Further, treatment of RCS cells with Gd3+ provides a tractable system for assessing the molecular events underlying the reversible differentiation of chondrocytes.

Cell-type specific trafficking of expressed mutant COMP in a cell culture model for PSACH

Pseudoachondroplasia (PSACH) is an autosomal dominant disease that mainly affects cartilage, resulting in skeletal dysplasias and early onset osteoarthritis. PSACH is caused by mutations in the cartilage oligomeric matrix protein (COMP) gene. PSACH chondrocytes accumulate unique COMP-containing lamellar structures in an expanded rough endoplasmic reticulum (rER). Although COMP is also present in tendon extracellular matrix (ECM), it does not accumulate in PSACH tendon cells, suggesting the disease involves a chondrocyte-specific trafficking problem. To investigate putative cell-specific trafficking differences, we generated a cell culture model utilizing expression of the common DeltaD469 COMP mutation. In rat chondrosarcoma (RCS) cells, we find delayed secretion and ER accumulation of DeltaD469 COMP, paralleling the altered trafficking defect in PSACH chondrocytes. Non-chondrocytic COS-1 cells, in contrast, efficiently trafficked and secreted both mutant and wild-type COMP. In chondrocytic cells, expression of DeltaD469 COMP led to ER accumulation of type IX collagen, but did not affect aggrecan trafficking. Endogenous rat COMP accumulated in the ER along with expressed DeltaD469 COMP in a stably expressing RCS clone, consistent with the dominant negative effect of PSACH. When these stably expressing cells were cultured to promote ECM deposition, the small amount of secreted mutant COMP disrupted assembly of the normal fibrillar meshwork and caused irregular aggregates of COMP and type IX collagen to form. Thus, in a new model that reflects the cellular pathology of PSACH, we establish trafficking differences for mutant COMP in chondrocytic and non-chondrocytic cells and demonstrate that mutant COMP interferes with assembly of a normal ECM.