Degradation products of magnesium implant synergistically enhance bone regeneration: Unraveling the roles of hydrogen gas and alkaline environment

Biodegradable magnesium (Mg) implant generally provides temporary fracture fixation and facilitates bone regeneration. However, the exact effects of generated Mg ions (Mg²⁺), hydrogen gas (H₂), and hydroxide ions (OH^-) by Mg degradation on enhancing fracture healing are not fully understood. Here we investigate the <i>in vivo</i> degradation of Mg intramedullary nail (Mg-IMN), revealing the generation of these degradation products around the fracture site during early stages. Bulk-RNA seq indicates that H₂ and alkaline pH increase periosteal cell proliferation, while Mg²⁺ may mainly enhance extracellular matrix formation and cell adhesion in the femur <i>ex vivo</i>. <i>In vivo</i> studies further reveal that H₂, Mg²⁺ and alkaline pH individually generate comparable effects to the enhanced bone regeneration in the Mg-IMN group. Mechanistically, the degradation products elevate sensory calcitonin gene-related peptide (CGRP) and simultaneously suppress adrenergic factors in newly formed bone. H₂ and Mg²⁺, instead of alkaline pH, increase CGRP synthesis and inhibit adrenergic receptors. Our findings, for the first time, elucidate that Mg²⁺, H₂, and alkaline pH environment generated by Mg-IMN act distinctly and synergistically mediated by the skeletal interoceptive regulation to accelerate bone regeneration. These findings may advance the understanding on biological functions of Mg-IMN in fracture repair and even other bone disorders.