Manganese supplementation of orthopedic implants: a new strategy for enhancing integrin-mediated cellular responses

Integrin-mediated osteoblast adhesion to adsorbed extracellular ligands on orthopedic implants is crucial for the subsequent osteoblast behaviors and ultimate osseointegration. Considerable research efforts have focused on the development of implant surfaces that promote the adsorption of extracellular ligands, but ignored the fact that integrin binding to ligands requires divalent cations (such as Mn2+). Here, three kinds of Mn-doped nanowire-structured TiO2 coatings with 1.9, 3.9, and 8.8 wt% dopant contents (Mn1-, Mn2-, and Mn3-TiO2) were synthesized on Ti implants to enhance integrin-mediated osteoblastic responses. The Mg-doped and undoped TiO2 nanocoatings served as the control. Mn element was not only successfully incorporated into the TiO2 matrix, but also formed an oxygen-deficient Mn oxide on the nanowire surface. Although the adsorbed fibronectin (Fn) amount on Mn-doped nanocoatings and its unfolded status were slightly attenuated with increasing Mn amount, the interaction between the coating extract and Fn demonstrated a Mn2+-induced unfolding of Fn with the exposure of the RGD motif. Compared to the Mn1-, Mn2- and Mg-doped TiO2 nanocoatings, the Mn3-TiO2 nanocoating significantly upregulated the expression of integrin α5β1 probably through increasing the ligand-binding affinity of the integrin rather than integrin binding sites in Fn. Consistent with the activation trend of integrin α5β1, the Mn3-TiO2 nanocoating enhanced cell adhesion with the long stretched structure of actin fibers and extensive formation of vinculin focal adhesion spots and upregulated the levels of alkaline phosphatase and osteocalcin activities. Therefore, Mn supplementation of orthopedic implants may be a promising way to improve osteogenesis at the implant surface.