Morphogen Delivery by Osteoconductive Nanoparticles Instructs Stromal Cell Spheroid Phenotype

Mesenchymal stem/stromal cells (MSCs) exhibit a rapid loss in osteogenic phenotype upon removal of osteoinductive cues, as commonly occurs during transplantation. Osteogenic differentiation can be more effectively but not fully maintained by aggregating MSCs into spheroids. Therefore, the development of effective strategies that prolong the efficacy of inductive growth factors would be advantageous for advancing cell-based therapies. To address this challenge, osteoinductive bone morphogenetic protein-2 (BMP-2) is adsorbed to osteoconductive hydroxyapatite (HA) nanoparticles for incorporation into MSC spheroids. MSC induction is evaluated in osteogenic conditions and retention of the osteogenic phenotype in the absence of other osteogenic cues. HA is more uniformly incorporated into spheroids at lower concentrations, while BMP-2 dosage is dependent upon initial morphogen concentration. MSC spheroids containing BMP-2-loaded HA nanoparticles exhibit greater alkaline phosphatase activity and more uniform spatial expression of osteocalcin compared to spheroids with uncoated HA nanoparticles. Spheroids cultured in media containing soluble BMP-2 demonstrate differentiation only at the spheroid periphery. Furthermore, the osteogenic phenotype of MSC spheroids is better retained with BMP-2-laden HA upon the removal of soluble osteogenic cues. These findings represent a promising strategy for simultaneous delivery of osteoconductive and osteoinductive signals for enhancing MSC participation in bone formation.