Abstract
Protein conjugation to biomaterial scaffolds is a powerful approach for tissue engineering. However, typical chemical conjugation methods lack site-selectivity and can negatively impact protein bioactivity. To overcome this problem, a site-selective strategy is reported here for installing tetrazine groups on terminal poly-histidines (His-tags) of recombinant proteins. These tetrazine groups are then leveraged for bio-orthogonal conjugation to poly(ethylene glycol) (PEG) hydrogel microparticles, which are subsequently assembled into microporous annealed particle (MAP) hydrogels. Efficacy of the strategy is demonstrated using recombinant, green fluorescent protein with a His tag (His-GFP), which enhanced fluorescence of the MAP hydrogels compared to control protein lacking tetrazine groups. Subsequently, to demonstrate efficacy with a therapeutic protein, recombinant human bone morphogenetic protein-2 (His-BMP2) was conjugated. Human mesenchymal stem cells growing in the MAP hydrogels responded to the conjugated BMP2 and significantly increased mineralization after 21 days compared to controls. Thus, this site-selective protein modification strategy coupled with bio-orthogonal click chemistry is expected to be useful for bone defect repair and regeneration therapies. Broader application to the integration of protein therapeutics with biomaterials is also envisioned.