Osteoprotegerin-Mediated Osteoclast Inhibition Is Augmented on Nanoparticulate Mineralized Collagen Glycosaminoglycan Materials

Purpose: Coordination of bone formation and resorption is necessary for the success of bone regenerative strategies. Characterization of the instructive capabilities of extracellular matrix (ECM)-inspired materials for osteoprogenitor differentiation has sparked questions on the interactions between such materials and the host microenvironment. Previously, we demonstrated that a nanoparticulate mineralized collagen glycosaminoglycan (MC-GAG) scaffold is both highly osteogenic without the need for exogenous growth factor stimulation and induces secretion of osteoprotegerin (OPG), an endogenous decoy receptor against the receptor activator of nuclear factor-κB ligand (RANKL), a necessary osteoclastogenic factor. In this work, we combine an adenoviral mediated expression of OPG (AdOPG) in primary human mesenchymal stem cells (hMSCs) with MC-GAG to understand the role for osteoclast inactivation in augmentation of bone regeneration.

Methods: Control and AdOPG transduced primary hMSCs were cultured on Col-GAG or MC-GAG materials in osteogenic differentiation medium. OPG and RANKL expression were evaluated using quantitative reverse transcriptase polymerase chain reaction (QPCR), western blot analysis, and enzyme linked immunosorbent assay (ELISA). Co-cultures of control and AdOPG transduced hMSCs on Col-GAG and MC-GAG with primary human osteoclasts were performed. Osteogenic differentiation was evaluated with western blot, ELISA, and micro-computed tomography for osteogenic differentiation.  Simultaneously osteoclast activity was assessed with tartrate resistant acid phosphatase staining and resorption pit assays.

Results: hMSCs differentiated on MC-GAG expressed a lower ratio of endogenous RANKL/OPG protein compared to a non-mineralized collagen glycosaminoglycan (Col-GAG) scaffold. In both materials, the RANKL/OPG ratio was further lowered significantly in the presence of AdOPG compared to control hMSCs without significant difference between the materials. We established a co-culture system to understand the interplay between differentiating hMSCs on Col-GAG or MC-GAG and differentiating human primary pre-osteoclasts. Control hMSCs on Col-GAG or MC-GAG in co-cultures did not differ in viability or proliferation compared to each other or to hMSC single cultures. However, AdOPG-transduced hMSCs on MC-GAG was modestly decreased in viability or proliferation compared to the Col-GAG counterpart in co-cultures. Co-cultures with differentiating osteoclasts increased hMSC mineralization with or without AdOPG, particularly in MC-GAG. In contrast, the viability and proliferation of osteoclasts in co-culture were significantly decreased in the presence of AdOPG-transduced hMSCs. While co-culture with control hMSCs on either Col-GAG or MC-GAG upregulated the resorptive activity of osteoclasts, AdOPG-transduced hMSCs reduced the resorption with a greater effect on MC-GAG compared to Col-GAG. 

Conclusions: The addition of osteoprotegerin to MC-GAG-mediated hMSC osteogenic differentiation simultaneously diminishes osteoclast resorptive capacity without affecting the positive regulatory effects on osteogenic differentiation.