Eight years ago we reviewed the role of bone morphogenetic proteins (BMPs) in articular cartilage repair in the last edition of this series on BMPs. Since that time our understanding of the function of BMPs and especially BMP-7, also called osteogenic protein-1 (OP-1), in cartilage homeostasis and repair has significantly increased. The primary focus of this chapter is the potential therapeutic opportunity for BMPs in the treatment of osteoarthritis. The intervening data confirm that among BMPs, BMP-7 exhibits the most robust evidence supporting its use for cartilage repair and regeneration. In the current review, we continue to unravel more of the underlying mechanisms of the anabolic and anti-catabolic activities of BMPs to provide a better understanding of the interactions between BMPs and signaling pathways and highlight the increased role BMP-7 and other BMPs play in human cartilage homeostasis. In regard to in vivo activities, exciting new data have been published demonstrating that BMP-7, in multiple models of osteoarthritis, can delay or inhibit degradation of the articular cartilage. Most interesting is that for the first time, a clinical trial has been reported, and Phase I data evaluation of the effect of a single injection of BMP-7 into osteoarthritic knees demonstrated enough of a positive response to warrant a Phase II study. Together, recent studies continue to indicate a significant opportunity for BMPs and particularly BMP-7 as therapeutics for osteoarthritis.

INTRODUCTION:The purpose of this study was to correlate the level of anabolic and catabolic biomarkers in synovial fluid (SF) from patients with rheumatoid arthritis (RA), patients with osteoarthritis (OA) and asymptomatic organ donors.METHODS:SF was collected from the knees of 45 OA, 22 RA patients and 20 asymptomatic organ donors. Eight biomarkers were selected and analyzed by using an enzyme-linked immunosorbent assay: interleukin (IL)-1, IL-6, IL-8 and IL-11; leukemia-inhibitory factor (LIF); cartilage oligomeric protein (COMP); osteocalcin; and osteogenic protein 1 (OP-1). Data are expressed as medians (interquartile ranges). The effects of sex and disease activity were assessed on the basis of the Western Ontario and McMaster Universities index score for patients with OA and on the basis of white blood cell count, erythrocyte sedimentation rate and C-reactive protein level for patients with RA.RESULTS:The mean ages (± SD) of the patients were as follows: 53 ± 9 years for patients with OA, 54 ± 11 years for patients with RA and 52 ± 7 years for asymptomatic organ donors. No effect of participants' sex was identified. In the SF of patients with RA, four of five cytokines were higher than those in the SF of patients with OA and those of asymptomatic organ donors. The most significant differences were found for IL-6 and IL-8, where IL-6 concentration in SF of patients with RA was almost threefold higher than that in patients with OA and fourfold higher than that in asymptomatic donor controls: 354.7 pg/ml (1,851.6) vs. 119.4 pg/ml (193.2) vs. 86.97 pg/ml (82.0) (P < 0.05 and P < 0.05, respectively). IL-8 concentrations were higher in SF of patients with RA than that in patients with OA as well as that in asymptomatic donor controls: 583.6 pg/ml (1,086.4) vs. 429 pg/ml (87.3) vs. 451 pg/ml (170.1) (P < 0.05 and P < 0.05, respectively). No differences were found for IL-11 in the SF of patients with RA and that of patients with OA, while a 1.4-fold difference was detected in the SF of patients with OA and that of asymptomatic donor controls: 296.2 pg/ml (257.2) vs. 211.6 pg/ml (40.8) (P < 0.05). IL-1 concentrations were the highest in the SF of RA patients (9.26 pg/ml (11.1)); in the SF of asymptomatic donors, it was significantly higher than that in patients with OA (9.083 pg/ml (1.6) vs. 7.76 pg/ml (2.6); P < 0.05). Conversely, asymptomatic donor control samples had the highest LIF concentrations: 228.5 pg/ml (131.6) vs. 128.4 pg/ml (222.7) in the SF of patients with RA vs. 107.5 pg/ml (136.9) in the SF of patients with OA (P < 0.05). OP-1 concentrations were twofold higher in the SF of patients with RA than those in patients with OA and threefold higher than those in asymptomatic donor control samples (167.1 ng/ml (194.8) vs. 81.79 ng/ml (116.0) vs. 54.49 ng/ml (29.3), respectively; P < 0.05). The differences in COMP and osteocalcin were indistinguishable between the groups, as were the differences between active and inactive OA and RA.CONCLUSIONS:Activation of selected biomarkers corresponds to the mechanisms that drive each disease. IL-11, LIF and OP-1 may be viewed as a cluster of biomarkers significant for OA; while profiling of IL-1, IL-6, IL-8, LIF and OP-1 may be more significant in RA. Larger, better-defined patient cohorts are necessary to develop a biomarker algorithm for prognostic use.

INTRODUCTION:The objective of this study was to investigate which genes are regulated by osteogenic protein-1 (OP-1) in human articular chondrocytes using Affimetrix gene array, in order to understand the role of OP-1 in cartilage homeostasis.METHODS:Chondrocytes enzymatically isolated from 12 normal ankle cartilage samples were cultured in high-density monolayers and either transfected with OP-1 antisense oligonucleotide in the presence of lipofectin or treated with recombinant OP-1 (100 ng/ml) for 48 hours followed by RNA isolation. Gene expression profiles were analyzed by HG-U133A gene chips from Affimetrix. A cut-off was chosen at 1.5-fold difference from controls. Selected gene array results were verified by real-time PCR and by in vitro measures of proteoglycan synthesis and signal transduction.RESULTS:OP-1 controls cartilage homeostasis on multiple levels including regulation of genes responsible for chondrocyte cytoskeleton (cyclin D, Talin1, and Cyclin M1), matrix production, and other anabolic pathways (transforming growth factor-beta (TGF-β)/ bone morphogenetic protein (BMP), insulin-like growth factor (IGF), vascular endothelial growth factor (VEGF), genes responsible for bone formation, and so on) as well as regulation of cytokines, neuromediators, and various catabolic pathways responsible for matrix degradation and cell death. In many of these cases, OP-1 modulated the expression of not only the ligands, but also their receptors, mediators of downstream signaling, kinases responsible for an activation of the pathways, binding proteins responsible for the inhibition of the pathways, and transcription factors that induce transcriptional responses.CONCLUSIONS:Gene array data strongly suggest a critical role of OP-1 in human cartilage homeostasis. OP-1 regulates numerous metabolic pathways that are not only limited to its well-documented anabolic function, but also to its anti-catabolic activity. An understanding of OP-1 function in cartilage will provide strong justification for the application of OP-1 protein as a therapeutic treatment for cartilage regeneration and repair.

Objective. Two major receptor-activated Smad (R-Smad) signaling pathways, bone morphogenetic protein (BMP) and MAPK, were examined in a model of interleukin-1 beta (IL-1 beta)-induced cartilage degeneration to investigate the effect of IL-1 beta on osteogenic protein 1 (OP-1) signaling in adult human articular chondrocytes.Methods. Chondrocytes from the ankles of 26 normal human donors were cultured in high-density monolayers in serum-free medium. The effect of IL-1 beta on BMP receptors was studied by reverse transcription-polymerase chain reaction and flow cytometry. Phosphorylation of R-Smads was tested in cells treated with IL-1 beta (10 ng/ml), OP-1 (100 ng/ml), or the combination of IL-1 beta and OP-1. Cell lysates were analyzed by Western blotting with polyclonal antibodies against 2 R-Smad phosphorylation sites (BMP- and MAPK-mediated) or with total, nonphosphorylated R-Smad as a control. To identify which MAPKs play a role in IL-1 beta activation of the linker region, chondrocytes were pre-incubated with specific MAPK inhibitors (PD98059 for MAP/ERK, SP600125 for JNK, and SB203580 for p38).Results. IL-1 beta reduced the number of activin receptor-like kinase 2 (ALK-2) and ALK-3 receptors, inhibited expression of Smad1 and Smad6, delayed and prematurely terminated the onset of OP-1-mediatedR-Smad phosphorylation, and affected nuclear translocation of R-Smad/Smad4 complexes. The alternative phosphorylation of R-Smad in the linker region via the MAPK pathway (primarily p38 and JNK) was observed to be a possible mechanism through which IL-1 beta offsets OP-1 signaling and the response to OP-1. Conversely, OP-1 was found to directly inhibit phosphorylation of p38.Conclusion. These findings describe new mechanisms of the crosstalk between OP-1 and IL-1 beta in chondrocytes. The study also identifies potential targets for therapeutic interventions in the treatment of cartilage-degenerative processes.

In vivo studies were used to characterize a model of cartilage injury leading to osteoarthritis progression in the medial femorotibial joint of sheep. In three subsequent studies, bilateral impact injuries were created and one joint received intraarticular injections of 340 µg of rhBMP‐7 protein in a collagen particle carrier while the contralateral knee received the vehicle alone. Sheep were allocated to three groups that received intraarticular injections on day 0 (group A), 21 (group B), or 90 (group C) after experimental knee injury. In each group the, joints were evaluated for signs of osteoarthritis progression 90 days after the last treatment using India ink stained area, OARSI histological scoring, cartilage sGAG content, immunostaining for apoptosis (TUNEL), caspase‐3, collagen degradation (Col 2 3/4C short collagen epitope), and the endogenous (pro‐) form of BMP‐7 protein. Knee joints that received rhBMP‐7 immediately after injury had small focal lesions at the injury site that did not progress into the surrounding cartilage. Joints that received BMP‐7 3 weeks after injury were improved and had limited progression compared to controls, but joints that received the protein 12 weeks after injury had no statistically significant improvement. These studies suggest that BMP‐7 may be chondroprotective after traumatic injury in patients if it is administered within 3 to 4 weeks of the index injury. The mechanism of protection after sublethal injury appeared to be an increased survival of chondrocytes that are able to participate in the repair process. © 2009 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 27: 602–611, 2009

This study investigated metabolism of autologous chondrocytes after initial expansion immediately before implantation. Chondrocytes cultured in either monolayers or alginate beads were treated with insulin-like growth factor-1 (IGF-1), osteogenic protein-1 (OP-1), or a combination. Proteoglycan synthesis and DNA content were tested in both cultures. Alginate beads also were analyzed with live/dead cell assay, safranin O/fast green stain for histology, and immunohistochemistry with antibodies against collagen type II and VI, aggrecan, decorin, and fibronectin. In monolayers, autologous chondrocytes changed their morphologic appearance. In alginate, they maintained chondrocytic phenotype. Growth factors, especially combined, promoted cell survival and induced chondrocyte proliferation. OP-1 stimulated the largest cartilage-specific matrix and the most accumulation of collagen type II and fibronectin, although the overall matrix synthesized by autologous chondrocyte implantation cells was smaller than that produced by normal chondrocytes. The clinical implications of this study suggest a significant promise for anabolic growth factors in cartilage repair as a potential modifying therapy for the enhancement of chondrocytic phenotype of autologous chondrocytes.

Cartilage repair and regeneration is a major obstacle in orthopedic medicine. The importance is enormous since osteoarthritis (OA) is a major cause of disability among the adult population in the United States and degenerative disc disease (DDD) is responsible for a significant amount of the chronic back pain. OA is considered a process of attempted, but gradually failing, repair of damaged cartilage extracellular matrix, as the balance between synthesis and breakdown of matrix components is disturbed and shifted towards catabolism. In recent times, members of the bone morphogenetic protein (BMP) family of proteins have demonstrated a great potential as anabolic factors for cartilage repair because of their ability to induce matrix synthesis and promote repair in cartilage.

We compared anabolic and anti-catabolic activities of selected bone morphogenetic proteins (BMP-2, -4, -6, and -7) and cartilage-derived morphogenetic proteins (CDMP-1 and -2) in human normal adult articular chondrocytes. Ankle chondrocytes were cultured in alginate beads in the presence of 10% serum and treated with either growth factors only (each at 100 ng/ml) or the combination of interleukin-1 (IL-1 beta) (0.1 ng/ml) and BMPs. Chondrocyte metabolism was assessed by proteoglycan (PG) synthesis, content, DNA content, and cell survival. The results showed that BMP-2, -4, and -7 were more potent in stimulating PGs than other growth factors tested. The highest levels of PG synthesis were detected at day 9 in the presence of BMP-7. With regard to anti-catabolic properties, the effect depended upon treatment scheme (simultaneous or sequential). Under simultaneous cultures, BMP-2, -4, and -6 failed to counteract IL-1 beta induced inhibition of PG synthesis, while the CDMPs restored this parameter to serum control levels. Only BMP-7 showed consistent and pronounced anti-catabolic activity in either culture treatment scheme. None of the factors induced cell death or chondrocyte proliferation. In conclusion, the growth factors tested showed different levels of effects on human chondrocytes in culture, but only BMP-7 displayed both strong anabolic and anti-catabolic properties.

Experimental animal models of disc degeneration have been used to assess the biomechanical behavior, biochemical composition, and biological changes in the intervertebral discs. The objective of our study was to evaluate the anabolic and anti‐catabolic effects of intradiscal injection of Osteogenic Protein‐1 (OP‐1) by histology and immunohistochemistry in disc degeneration model. Thirty‐four rats were divided into five groups: intact control; sham control; compressed nucleus pulposus (NP) injected with saline; and two OP‐1 groups: COP‐1 group (compression was continued after intradiscal OP‐1 injection) and ROP‐1 group (compression was released at the time of OP‐1 injection). Anabolic and anti‐catabolic effects of OP‐1 were evaluated by histology and immunohistochemistry with the following antibodies: anti‐pro‐ and anti‐mature OP‐1, anti‐MMP‐13, anti‐aggrecanase, anti‐substance P, anti‐tumor necrosis factor‐α (TNF‐α), and anti‐interleukin‐1β (IL‐1β). The OP‐1 injection to the degenerative disc stimulated an anabolic response characterized by the restoration of the normal morphology of the disc, increased Safranin O staining in the NP, extention of the extracellular matrix, and stimulation of endogenous OP‐1 synthesis in the NP, annulus fibrosis (AF), and end‐plate. The anti‐catabolic effect of OP‐1 was documented by reduced immunostaining for aggrecanase, MMP‐13, substance P, TNF‐α, and IL‐1β. This study confirmed the anti‐catabolic activity of OP‐1 as demonstrated previously in human articular cartilage and provided critical evidence for the potential of OP‐1 therapy in the treatment of disc degeneration. Because substance P is a neuropeptide linked with inflammation and pain, a reduction in the level of this protein may support our previously reported results on the effect of OP‐1 on pain‐related behavior. © 2007 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 25:517–530, 2007

Three years ago we published a book chapter on the role of bone morphogenetic proteins (BMPs) in cartilage repair. Since that time our understanding of the function of osteogenic protein-1 (OP-1) or BMP-7 in cartilage homeostasis and repair has substantially improved and therefore we decided to devote a current review solely to this BMP. Here we summarise the information accumulated on OP-1 from in vitro and ex vivo studies with cartilage cells and tissues as well as from in vivo studies of cartilage repair in various animal models. The primary focus is on articular chondrocytes and cartilage, but data will also be presented on nonarticular cartilage, particularly from the intervertebral disc. The data show that OP-1 is a unique growth factor which, unlike other members of the same BMP family, exhibits in addition to its strong pro-anabolic activity very prominent anti-catabolic properties. Animal studies have demonstrated that OP-1 has the ability to repair cartilage in vivo in various models of articular cartilage degradation, including focal osteochondral and chondral defects and osteoarthritis, as well as models of degeneration in intervertebral disc cartilage. Together our findings indicate a significant promise for OP-1 as therapeutic in cartilage repair.

The measurement of body fluid levels of biochemical markers in joint tissues has begun to provide clinically useful information. Synovial fluid (SF) plays an important role in articular joint lubrication, nutrition, and metabolism of cartilage and other connective tissues within the joint. The purpose of our study was to identify and characterize osteogenic protein 1 (OP-1) in SF from patients with rheumatoid arthritis (RA) or with osteoarthritis (OA) and to correlate levels of OP-1 with those of hyaluronan (HA) and antigenic keratan sulfate (AgKS). SF was aspirated from the knees of patients with either RA or OA and from the knees of asymptomatic organ donors with no documented history of joint disease. The presence of detectable OP-1 in SF was demonstrated by western blots with specific anti-pro-OP-1 and anti-mature OP-1 antibodies. Measurement of levels of OP-1, HA and AgKS was performed using ELISAs. OP-1 was identified in human SF in two forms, pro-OP-1 and active (mature) OP-1 – mature OP-1 being detected only in SF from OA patients and RA patients. Levels of OP-1 and HA were higher in RA patients than in OA patients and asymptomatic donors, while the level of AgKS was highest in SF from asymptomatic donors. Statistically significant differences were found between SF levels of OP-1 in RA and OA patients and between SF levels of AgKS among the three groups tested. The SF content of OP-1 tended to correlate positively with HA levels, but negatively with AgKS concentrations. In conclusion, the results of this study suggest that measurement of OP-1 in joint fluid may have value in the clinical evaluation of joint disease processes.