Sulfated chitosan enhances BMP-2-mediated spinal fusion via skeletal stem cell rejuvenation in aging

Spinal degenerative diseases in elderly patients often require spinal fusion, but outcomes are limited by an aging microenvironment and stem cell dysfunction or depletion. In this study, we developed a bone morphogenetic protein-2 (BMP-2)/sulfated chitosan (SCS)/calcium phosphate cement (CPC) composite scaffold to enhance spinal fusion in aged mice. BMP-2/SCS significantly improved spinal fusion success rates (83.3 %) compared to BMP-2 alone (16.7 %) and high-dose BMP-2 (50 %). The BMP-2/SCS group promoted robust new bone formation and H-type vessel development, facilitating vascular-bone coupling in the fusion region. Mechanistically, SCS suppressed BMP-2-induced osteoclast overactivation and reduced MMP-9 secretion, leading to vertebral skeletal stem cell (vSSC) rejuvenation. Rejuvenated vSSCs primarily differentiated into the osteogenic bone/cartilage lineage while stromal lineage differentiation was suppressed. In contrast, co-administration of BMP-2 and alendronate sodium abolished BMP-2-mediated increases in vSSC numbers, vSSC rejuvenation, and bone integration, highlighting the indispensable role of osteoclast activity in BMP-2-induced bone regeneration. Collectively, this study demonstrates that BMP-2/SCS scaffolds effectively reverse age-related deficiencies by creating a rejuvenated bone microenvironment, promoting vascular-bone coupling, and enhancing osteogenesis, offering a promising strategy to improve spinal fusion outcomes in elderly patients.