Immunomodulated Bone Regeneration Using a Conductive Micro-hydrogel System Through Mitochondrial Transfer and Mitochondrial Metabolic Regulation

Biomaterial-based bone tissue engineering offers a promising prospect for the treatment of bone defects. In particular, the ability of biomaterials to regulate the immune microenvironment of the defect site is essential for effective bone regeneration. Electro-biomaterials have been confirmed to induce macrophage M2 polarization through metabolic pathways, thereby enhancing bone regeneration. Considering the central role of mitochondria in cellular metabolism and their ability to influence the function of neighboring cells through intercellular transfer, and inspired by the fact that tumor cells can uptake mitochondria from immune cells to generate energy, we hypothesize that the metabolic activation of immune cells by electro-materials can be transmitted to preosteoblasts through mitochondria to promote bone repair. Therefore, this study proposed a conductive micro-hydrogel (CMH) system composed of conductive hydrogel microspheres made from GelMA and poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS), which served as scaffolds for defect filling, and a biomimetic periosteum made from poly-l-lactic acid (PLLA) and polydopamine (PDA) for microsphere immobilization and isolation of soft tissue. The microspheres exhibited excellent tissue support and degradation properties, their high specific surface area enhanced tissue remodeling, and their good conductivity eliminated free radicals and induced macrophage M2 polarization, which were confirmed by tests of mechanical property, swelling and degradation, conductivity and assays of cellular biocompatibility, ROS generation, and macrophage phenotype. In vivo experiments using a rat mandibular defect model confirmed the excellent bone repair capabilities of the CMH system, and transcriptomics, metabolomics, and metabolic testing revealed that the CMH system upregulated the oxidative phosphorylation pathway of macrophage, enhancing mitochondrial respiration and ATP production. Mitochondrial tracing experiments demonstrated the transfer of macrophage mitochondria to preosteoblasts, resulting in enhanced metabolic activity and osteogenic differentiation of preosteoblasts. This study may be the first suggest that conductive biomaterials facilitate osteogenic immunomodulation through mitochondrial transfer, which provides a promising method for regulating the immune microenvironment and reveals a novel pathway by which M2 macrophages enhance osteogenesis. Overall design: RAW 264.7 macrophages were cultured with CMH system group or control GelMA group under static and dynamic conditions for 3 d as aforementioned. Total RNA was extracted using TRIzol reagent (Invitrogen, Carlsbad, CA, USA). RNA sequencing was performed by Shanghai Genechem Co., Ltd (China). The thresholds for differentially expressed transcripts were P<0.05 and an absolute fold change=1. Differentially expressed transcripts were subjected to function and signaling pathway enrichment analysis using the GO (Gene Oncology) and KEGG (Kyoto Encyclopedia of Genes and Genomes) databases. GSEA (v4.1.0) was used to analyze the differences in a pre-defined gene set.