Multivalent Integrin-Specific Ligands Enhance Tissue Healing and Biomaterial Integration

Titanium implants coated with nanoclustered ligands for integrin adhesion receptors are tightly integrated into bone for orthopedic applications. Forming Bonds with Strangers Like a clique of teenagers, cells get nervous if they sense a stranger in their midst. This cellular stranger anxiety works against the surgeon who uses implants to repair broken bones or tissues. If human cells cannot bond with implanted foreign material, the implant will not be integrated into the existing tissue or, worse, will fall out. One solution is to find materials that can be disguised so as to fool human cells into acceptance. By coating titanium plugs with precisely configured bits of a common extracellular matrix protein, fibronectin, Petrie et al. have deceived surrounding cells into accepting their disguised device. To accomplish this con, they found the optimal configuration of fibronectin that binds a cell surface adhesion receptor, thereby enhancing tissue healing and integration of the titanium implant into bone. Cells can bind to fibronectin, part of the surrounding extracellular matrix, through adhesion receptors called integrins—dimeric transmembrane proteins that come in assorted varieties. This binding confers more than a physical link: It triggers signaling events in the cell that activate motility and metabolic changes, as well as locking the cell’s cytoplasm to the extracellular matrix through the membrane. The authors constructed an artificial extracellular matrix by securing a critical piece of fibronectin (FNIII7–10) to a customizable coiled-coil protein sequence via a flexible protein linker. By altering the coiled coils, these constructs could be assembled to present one, two, three, or five clustered integrin ligands from the fibronectin fragment. The flexible linker allowed the ligands 10 to 50 nm in which to move. The authors fixed the constructs to a titanium surface with a polymer coating and added cells with integrin on their surfaces. The trimeric and pentameric ligands bound and activated twice as much integrin as did the monomeric and dimeric ligands and were more effective at promoting osteoblastic differentiation from stem cells. To see whether the three- and five-ligand clusters improved integration into tissue, the authors implanted titanium plugs coated with the various constructs into holes in rat leg bones. Microscopy revealed that the three- and five-ligand coated implants had 50% more contact with the surrounding bone than did implants coated with monomers or dimers. Even more encouraging, these implants were 250% more securely fixed in place than the one- and two-ligand constructs and 400% more than polymer-coated titanium plugs. By using a life-like disguise to coat the surface, Petrie et al. have improved incorporation of titanium implants into bone. This result can be used by dental and orthopedic surgeons, who routinely use titanium implants in tooth and joint replacement. If coated with clustered fibronectin fragments, these implants may coax the surrounding cells into making firm contacts with their surfaces, securing their acceptance by their neighbors. Engineered biointerfaces covered with biomimetic motifs, including short bioadhesive ligands, are a promising material-based strategy for tissue repair in regenerative medicine. Potentially useful coating molecules are ligands for the integrins, major extracellular matrix receptors that require both ligand binding and nanoscale clustering for maximal signaling efficiency. We prepared coatings consisting of well-defined multimer constructs with a precise number of recombinant fragments of fibronectin (monomer, dimer, tetramer, and pentamer) to assess how nanoscale ligand clustering affects integrin binding, stem cell responses, tissue healing, and biomaterial integration. Clinical-grade titanium was grafted with polymer brushes that presented monomers, dimers, trimers, or pentamers of the α5β1 integrin–specific fibronectin III (7 to 10) domain (FNIII7–10). Coatings consisting of trimers and pentamers enhanced integrin-mediated adhesion in vitro, osteogenic signaling, and differentiation in human mesenchymal stem cells more than did surfaces presenting monomers and dimers. Furthermore, ligand clustering promoted bone formation and functional integration of the implant into bone in rat tibiae. This study establishes that a material-based strategy in which implants are coated with clustered bioadhesive ligands can promote robust implant-tissue integration.