FK506 Enhancement of Neuromuscular Junction Recovery After Nerve Injury Is Macrophage-Dependent

Introduction: Motor recovery following nerve injury is dependent on time required for muscle reinnervation. This process is imperfect, however, and recovery is often incomplete. At the neuromuscular junction (NMJ), macrophage signaling aids muscle reinnervation. Tacrolimus (FK506) treatment speeds functional recovery through unknown mechanisms. This study investigated whether macrophages were required for FK506 neuroenhancing effects. Methods: Wildtype (WT) mice and mice with impaired macrophage recruitment to injury sites (Ccr2^−/−) were injected subcutaneously with either saline or FK506 for 3 days prior to sciatic nerve transection and immediate repair and then daily for 4 weeks. Functional recovery was assessed by grid walk and muscle force. Morphometric NMJ and macrophage analyses were conducted in extensor digitorum longus muscles. Results: FK506-injected WT mice showed increased proportions of fully reinnervated NMJs and terminal Schwann cells/NMJ (p < 0.05), improved recovery of tetanic muscle force (p < 0.05), and improved grid walking (p < 0.05) relative to controls. Ccr2^−/− mice showed no enhancements in recovery; Ccr2^−/− mice treated with FK506 did not differ from controls on any tested metric. We also observed at the NMJ of WT mice increased macrophage numbers with FK506 treatment and increased macrophages expressing FK506 binding protein, FKBP52, after nerve injury. Discussion: These results show that macrophages are required for FK506-mediated improvements in NMJ reinnervation and muscle function. These data implicate macrophages in the mechanism underlying FK506-mediated enhancement of motor recovery after nerve injury. Enhanced knowledge of the neuroenhancing mechanism of FK506 may identify new clinically relevant therapeutic targets.