Tricarboxylic Acid Cycle Metabolite-Coordinated Biohydrogels Augment Cranial Bone Regeneration Through Neutrophil-Stimulated MSCs Recruitment and Histone Acetylation-Mediated Osteogenesis [scRNA-Seq]

Cranial bone defect is a major clinical challenge that increases the life burden of patients. Tricarboxylic acid (TCA) cycle metabolites have recently drawn considerable attention in the field of bone tissue regeneration due to their bio-safety, low cost, structural stability, and effectiveness. However, the development of TCA cycle metabolite-modified biomimetic grafts for skull bone regeneration via a facile and general approach remains lacking. Moreover, the mechanism underlying the release of TCA cycle metabolites from biomaterials in immune responses and mesenchymal stem cells (MSCs) fate (migration and differentiation) remain unknown. Herein, inspired by the Hofmeister effects, we developed a series of TCA metabolite (in the form of sodium salt)-coordinated biomimetic hydrogels (CGG) with strong mechanical and anti-swelling performances. Importantly, the sodium citrate (Na3Cit)-treated CGG hydrogels (CGG-Cit) with the highest mechanical modulus and strength significantly promoted skull bone regeneration in rats and mice. Mechanistically, using a transgenic mouse model, bulk RNA sequencing, and single-cell RNA sequencing, we demonstrated that CGG-Cit promotes Gli1+ MSCs migration via neutrophil-secreted OSM. Our results also indicate that CGG-Cit improved osteogenesis via enhanced H3K9ac modification on osteogenic master genes. Taken together, the immune microenvironments and MSCs fate-regulated biomimetic hydrogels developed herein represent a highly efficient and facile approach toward skull bone tissue regeneration with great potential for bench-to-bedside translation. Overall design: To test whether distinct cell populations exist and affect progenitor cell recruitment, we dissected calvaria defects on day 14 post-surgery for enzymatic digestion before conducting droplet-based scRNA-seq. Briefly, we created 2 mm calvarial bone defects on the parietal bones and implanted CGG hydrogels or CGG-Cit hydrogels. At 2 weeks post-surgery, the defect regions were carefully trimmed, and tissues were digested in 0.3% collagenase in a-MEM. Cell suspensions were filtered through 70 µm strainers, and single cells were subjected to RNA sequencing.