Loss of EGFR activity in skeletal stem/progenitor cells is associated with impaired fracture healing in aged mice.

Abstract

The cellular and molecular events responsible for fracture healing becoming delayed with aging remain unclear. Epidermal growth factor receptor (EGFR) signaling has been reported to play a critical role in bone regeneration. However, knowledge of its specific function in skeletal stem cells during aging-induced fracture delayed union remains scant. In the present study, we first demonstrated that EGFR activity in skeletal stem/progenitor cells decreased as mice aged and thus resulted in delayed fracture healing. To further investigate whether the EGFR signaling can be targeted as a potential therapy for aging-induced fracture delayed union, we designed a two-pronged approach: one involved crossing <i>Prx1-Cre</i> with <i>Egfr</i> ^{<i>flox/flox</i>} mice to generate a model with stem/progenitor-specific EGFR inactivation (<i>Egfr iCKO</i>), while the other entailed overexpressing heparin-binding EGF-like growth factor (HBEGF), an EGFR ligand, to generate a stem/progenitor-specific EGFR overactivation model. Our findings revealed that <i>Egfr iCKO</i> mice developed obvious delayed fracture healing. Conversely, <i>Prx1-Cre HBEGF-overexpressing</i> aged mice exhibited accelerated fracture healing due to promotion of osteogenesis and angiogenic coupling, as well as inhibition of cellular senescence. Based on these results, we developed an injectable, self-healing, adhesive hydrogel, which sustainably released HBEGF in situ at the fracture site. This hydrogel effectively promoted cartilage-to-bone transition as well as the fracture healing process in aged mice. Together, our findings demonstrate that EGFR signaling is a molecular mechanism involved in healing fractures in the elderly and provide a promising therapy to target EGFR signaling for the treatment of fracture delayed union caused by aging.