Fibroblast growth factor 2 drives changes in gene expression following injury to murine cartilage in vitro and in vivo.

OBJECTIVE: The articular cartilage is known to be highly mechanosensitive, and a number of mechanosensing mechanisms have been proposed as mediators of the cellular responses to altered mechanical load. These pathways are likely to be important in tissue homeostasis as well as in the pathogenesis of osteoarthritis. One important injury-activated pathway involves the release of pericellular fibroblast growth factor 2 (FGF-2) from the articular cartilage. Using a novel model of murine cartilage injury and surgically destabilized joints in mice, we examined the extent to which FGF-2 contributes to the cellular gene response to injury. METHODS: Femoral epiphyses from 5-week-old wild-type mice were avulsed and cultured in serum-free medium. Explant lysates were Western blotted for phospho-JNK, phospho-p38, and phospho-ERK or were fixed for immunohistochemical analysis of the nuclear translocation of p65 (indicative of NF-κB activation). RNA was extracted from injured explants, rested explants that had been stimulated with recombinant FGF-2 or FGF-18, or whole joints from either wild-type mice or FGF-2(-/-) mice. Reverse transcription-polymerase chain reaction was performed to examine a number of inflammatory response genes that had previously been identified in a microarray analysis. RESULTS: Murine cartilage avulsion injury resulted in rapid activation of the 3 MAP kinase pathways as well as NF-κB. Almost all genes identified in murine joints following surgical destabilization were also regulated in cartilage explants upon injury. Many of these genes, including those for activin A (Inhba), tumor necrosis factor-stimulated gene 6 (Tnfaip6), matrix metalloproteinase 19 (Mmp19), tissue inhibitor of metalloproteinases 1 (Timp1), and podoplanin (Pdpn), were significantly FGF-2 dependent following injury to cartilage in vitro and to joint tissues in vivo. CONCLUSION: FGF-2-dependent gene expression occurs in vitro and in vivo in response to cartilage/joint injury in mice.

Joint immobilization prevents murine osteoarthritis and reveals the highly mechanosensitive nature of protease expression in vivo.

OBJECTIVE: Mechanical joint loading is critical for the development of osteoarthritis (OA). Although once regarded as a disease of cartilage attrition, OA is now known to be controlled by the expression and activity of key proteases, such as ADAMTS-5, that drive matrix degradation. This study was undertaken to investigate the link between protease expression and mechanical joint loading in vivo. METHODS: We performed a microarray analysis of genes expressed in the whole joint following surgical induction of murine OA (by cutting the medial meniscotibial ligament). Gene expression changes were validated by reverse transcriptase-polymerase chain reaction in whole joints and microdissected tissues of the joint, including the articular cartilage, meniscus, and epiphysis. Following surgery, mouse joints were immobilized, either by prolonged anesthesia or by sciatic neurectomy. RESULTS: Many genes were regulated in the whole joint within 6 hours of surgical induction of OA in the mouse. These included Arg1, Ccl2, Il6, Tsg6, Mmp3, Il1b, Adamts5, Adamts4, and Adamts1. All of these were significantly regulated in the articular cartilage. When joints were immobilized by prolonged anesthesia, regulation of the vast majority of genes was abrogated. When joints were immobilized by sciatic neurectomy, regulation of selected genes was abrogated, and OA was prevented up to 12 weeks postsurgery. CONCLUSION: These findings indicate that gene expression in the mouse joint following the induction of OA is rapid and highly mechanosensitive. Regulated genes include the known pathogenic protease ADAMTS-5. Targeting the mechanosensing mechanisms of joint tissue may offer new strategies for disease modification.

Does Pain at an Earlier Stage of Chondropathy Protect Female Mice Against Structural Progression After Surgically Induced Osteoarthritis?

OBJECTIVE: Female C57BL/6 mice exhibit less severe chondropathy than male mice. This study was undertaken to test the robustness of this observation and explore underlying mechanisms. METHODS: Osteoarthritis was induced in male and female C57BL/6 or DBA/1 mice (n = 6-15 per group) by destabilization of the medial meniscus (DMM) or partial meniscectomy (PMX). Some mice were ovariectomized (OVX) (n = 30). In vivo repair after focal cartilage defect or joint immobilization (sciatic neurectomy) following DMM was assessed. Histologic analysis, evaluation of gene expression in whole knees, and behavioral analysis using Laboratory Animal Behavior Observation Registration and Analysis System (LABORAS) and Linton incapacitance testing (n = 7-10 mice per group) were performed. RESULTS: Female mice displayed less severe chondropathy (20-75% reduction) across both strains and after both surgeries. Activity levels after PMX were similar for male and female mice. Some repair-associated genes were increased in female mouse joints after surgery, but no repair differences were evident in vivo. Despite reduced chondropathy, female mice developed pain-like behavior at the same time as male mice. At the time of established pain-like behavior (10 weeks after PMX), pain-associated genes were significantly up-regulated in female mice, including Gdnf (mean ± SEM fold change 2.54 ± 0.30), Nrtn (6.71 ± 1.24), Ntf3 (1.92 ± 0.27), and Ntf5 (2.89 ± 0.48) (P < 0.01, P < 0.01, P < 0.05, and P < 0.001, respectively, versus male mice). Inflammatory genes were not regulated in painful joints in mice of either sex. CONCLUSION: We confirm strong structural joint protection in female mice that is not due to activity or intrinsic repair differences. Female mice develop pain at the same time as males, but induce a distinct set of neurotrophins. We speculate that heightened pain sensitivity in female mice protects the joint by preventing overuse.

TSG-6 Is Weakly Chondroprotective in Murine OA but Does not Account for FGF2-Mediated Joint Protection.

OBJECTIVE: Tumor necrosis factor α-stimulated gene 6 (TSG-6) is an anti-inflammatory protein highly expressed in osteoarthritis (OA), but its influence on the course of OA is unknown. METHODS: Cartilage injury was assessed by murine hip avulsion or by recutting rested explants. Forty-two previously validated injury genes were quantified by real-time polymerase chain reaction in whole joints following destabilization of the medial meniscus (DMM) (6 hours and 7 days). Joint pathology was assessed at 8 and 12 weeks following DMM in 10-week-old male and female fibroblast growth factor 2 (FGF2)-/- , TSG-6-/- , TSG-6tg (overexpressing), FGF2-/- ;TSG-6tg (8 weeks only) mice, as well as strain-matched, wild-type controls. In vivo cartilage repair was assessed 8 weeks following focal cartilage injury in TSG-6tg and control mice. FGF2 release following cartilage injury was measured by enzyme-linked immunosorbent assay. RESULTS: TSG-6 messenger RNA upregulation was strongly FGF2-dependent upon injury in vitro and in vivo. Fifteeen inflammatory genes were significantly increased in TSG-6-/- joints, including IL1α, Ccl2, and Adamts5 compared with wild type. Six genes were significantly suppressed in TSG-6-/- joints including Timp1, Inhibin ßA, and podoplanin (known FGF2 target genes). FGF2 release upon cartilage injury was not influenced by levels of TSG-6. Cartilage degradation was significantly increased at 12 weeks post-DMM in male TSG-6-/- mice, with a nonsignificant 30% reduction in disease seen in TSG-6tg mice. No differences were observed in cartilage repair between genotypes. TSG-6 overexpression was unable to prevent accelerated OA in FGF2-/- mice. CONCLUSION: TSG-6 influences early gene regulation in the destabilized joint and exerts a modest late chondroprotective effect. Although strongly FGF2 dependent, TSG-6 does not explain the strong chondroprotective effect of FGF2.

Treatment of murine osteoarthritis with TrkAd5 reveals a pivotal role for nerve growth factor in non-inflammatory joint pain.

The origin of pain in osteoarthritis is poorly understood, but it is generally thought to arise from inflammation within the innervated structures of the joint, such as the synovium, capsule and bone. We investigated the role of nerve growth factor (NGF) in pain development in murine OA, and the analgesic efficacy of the soluble NGF receptor, TrkAD5. OA was induced in mice by destabilisation of the medial meniscus and pain was assessed by measuring hind-limb weight distribution. RNA was extracted from joints, and NGF and TNF expressions were quantified. The effect of tumour necrosis factor (TNF) and neutrophil blockade on NGF expression and pain were also assessed. NGF was induced in the joints during both post-operative (day 3) and OA (16weeks) pain, but not in the non-painful stage of disease (8weeks post-surgery). TrkAd5 was highly effective at suppressing pain in both phases. Induction of NGF in the post-operative phase of pain was TNF-dependent as anti-TNF reduced NGF expression in the joint and abrogated pain. However, TNF was not regulated in the late OA joints, and pain was not affected by anti-TNF therapy. Fucoidan, by suppressing cellular infiltration into the joint, was able to suppress post-operative, but not late OA pain. These results indicate that NGF is an important mediator of OA pain and that TrkAd5 represents a potent novel analgesic in this condition. They also suggest that, unlike post-operative pain, induction of pain in OA may not necessarily be driven by classical inflammatory processes.

Fibroblast growth factor 2 is an intrinsic chondroprotective agent that suppresses ADAMTS-5 and delays cartilage degradation in murine osteoarthritis.

OBJECTIVE: We have previously identified in articular cartilage an abundant pool of the heparin-binding growth factor, fibroblast growth factor 2 (FGF-2), which is bound to the pericellular matrix heparan sulfate proteoglycan, perlecan. This pool of FGF-2 activates chondrocytes upon tissue loading and is released following mechanical injury. In vitro, FGF-2 suppresses interleukin-1-driven aggrecanase activity in human cartilage explants, suggesting a chondroprotective role in vivo. We undertook this study to investigate the in vivo role of FGF-2 in murine cartilage. METHODS: Basal characteristics of the articular cartilage of Fgf2(-/-) and Fgf2(+/+) mice were determined by histomorphometry, nanoindentation, and quantitative reverse transcriptase-polymerase chain reaction. The articular cartilage was graded histologically in aged mice as well as in mice in which osteoarthritis (OA) had been induced by surgical destabilization of the medial meniscus. RNA was extracted from the joints of Fgf2(-/-) and Fgf2(+/+) mice following surgery and quantitatively assessed for key regulatory molecules. The effect of subcutaneous administration of recombinant FGF-2 on OA progression was assessed in Fgf2(-/-) mice. RESULTS: Fgf2(-/-) mice were morphologically indistinguishable from wild-type (WT) animals up to age 12 weeks; the cartilage thickness and proteoglycan staining were equivalent, as was the mechanical integrity of the matrix. However, Fgf2(-/-) mice exhibited accelerated spontaneous and surgically induced OA. Surgically induced OA in Fgf2(-/-) mice was suppressed to levels in WT mice by subcutaneous administration of recombinant FGF-2. Increased disease in Fgf2(-/-) mice was associated with increased expression of messenger RNA of Adamts5, the key murine aggrecanase. CONCLUSION: These data identify FGF-2 as a novel endogenous chondroprotective agent in articular cartilage.