BMP2 stimulates bone formation and signals preferably through BMP receptor (BMPR) 1A, whereas GDF5 is a cartilage inducer and signals preferably through BMPR1B. Consequently, BMPR1A and BMPR1B are believed to be involved in bone and cartilage formation, respectively. However, their function is not yet fully clarified. In this study, GDF5 mutants with a decreased affinity for BMPR1A were generated. These mutants, and wild-type GDF5 and BMP2, were tested for their ability to induce dimerization of BMPR1A or BMPR1B with BMPR2, and for their chondrogenic, hypertrophic and osteogenic properties in chondrocytes, in the multipotent mesenchymal precursor cell line C3H10T1/2 and the human osteosarcoma cell line Saos-2. Mutants with the lowest potency for inducing BMPR1A–BMPR2 dimerization exhibited minimal chondrogenic and osteogenic activities, indicating that BMPR1A is necessary for chondrogenic and osteogenic differentiation. BMP2, GDF5 and the GDF5 R399E mutant stimulated expression of chondrogenic and hypertrophy markers in C3H10T1/2 cells and chondrocytes. However, GDF5 R399E, which induces the dimerization of BMPR1B and BMPR2 more potently than GDF5 or BMP2, displayed reduced hypertrophic activity. Therefore, we postulate that stronger BMPR1B signaling, compared to BMPR1A signaling, prevents chondrocyte hypertrophy and acts as a cartilage stabilizer during joint morphogenesis.This article has an associated First Person interview with the first author of the paper.

The growth and differentiation factor 5 (GDF‐5) is known to play a key role in cartilage morphogenesis and homeostasis, and a single‐nucleotide polymorphism in its promoter sequence was found to be associated with osteoarthritis (OA). In addition, GDF‐5 was shown to promote extracellular matrix (ECM) production in healthy chondrocytes, to stimulate chondrogenesis of mesenchymal stem cells (MSCs) and to protect against OA progression in vivo. Therefore, GDF‐5 appears to be a promising treatment for osteoarthritis. However, GDF‐5 also promotes osteogenesis and hypertrophy, limiting its therapeutic utility. To circumvent this, a GDF‐5 mutant with lower hypertrophic and osteogenic properties was engineered: M1673. The present study aimed to evaluate and compare the effects of GDF‐5 and M1673 on primary porcine and human OA chondrocytes. We found that both GDF‐5 and M1673 can robustly stimulate ECM accumulation, type II collagen and aggrecan expression in porcine and human OA chondrocytes in 3D culture. In addition, both molecules also down‐regulated MMP13 and ADAMTS5 expression. These results suggest that M1673 retained the anabolic and anti‐catabolic effects of GDF‐5 on chondrocytes and is an alternative to GDF‐5 for osteoarthritis.

The environment surrounding chondrocytes changes drastically in osteoarthritis (OA). For instance, the osmolarity in cartilage (ranging from 350 to 460 mOsm in healthy tissue) decreases during the progression of OA, reaching 270 mOsm. The objective of this study was to evaluate how osmolarity influences human OA chondrocytes. For this purpose, the osmolarity of the culture medium (340 mOsm) was increased to 380, 420 or 460 mOsm and its effect on the phenotype, matrix production, protease expression, cytokine release and growth and differentiation factor-5 (GDF-5) receptor expression in human OA chondrocytes was evaluated in a monolayer. Afterwards, the same parameters, as well as the responsiveness to GDF-5, were evaluated in 3D culture at 340 and 380 mOsm. Our results revealed that increasing the medium osmolarity increased matrix production but also reduced cytokine release, type I collagen and protease expression. It was also demonstrated that at 380 mOsm, the response to GDF-5 in 3D culture was more robust than at 340 mOsm. For the first time, it was established that a decreased osmolarity plays a role in sustaining inflammation and catabolic activities in OA chondrocytes and decreases their responsiveness to GDF-5. This indicates that osmolarity is a critical aspect of OA pathobiology.