Rationale: Perlecan is an important heparan sulphate proteoglycan (HSPG) in the articular cartilage extracellular matrix (ECM). While perlecan has been postulated to be a growth factor reservoir and signalling modulator, its role in binding and modulating bone morphogenetic proteins (BMP), an important family of growth factors in the musculoskeletal system, is largely not understood. Recombinant BMPs have been available clinically for more than a decade, the use of rBMPs in articular cartilage is limited by its efficacy and adverse effects. Previous studies have focused on the interaction between heparan sulphate and BMPs which is important in understanding the growth factors’ biological activities. However, this interaction must be considered in the context of the presence of their antagonists.
Objective: To investigate the interaction of perlecan, BMPs and BMP antagonists.
Methods and Results: RT-PCR and immuno-localisation showed expression of perlecan, BMPs and BMP antagonists. Using an ELISA-based assay, binding of recombinant BMP-2 and BMP-4 to immunopurified perlecan protein core or the perlecan HS chains were characterized. The minimal binding of recombinant BMP-2 was confirmed using surface plasmon resonance (SPR) study. SPR also showed that BMP antagonists, follistatins and sclerostin, had higher affinity to perlecan when compared to recombinant BMP-2 and BMP-4.
Conclusions: BMPs, BMP antagonists and perlecan are all expressed by chondrocytes in culture. The low affinity of recombinant BMP-2 to immunopurified perlecan did not appear to be HS-dependent. BMP antagonists, follistatin and sclerostin, had higher affinity to perlecan than recombinant BMPs. This observation offers explanation to some features observed in perlecan-deficient mice which highlights the pivotal role that BMP antagonists may have in BMP biological activities in extracellular matrix.