Erythropoietin M2 macrophage Signaling Promotes Schwann Cells Clearance and Functional Recovery Following Peripheral Nerve Injury

Abstract

BackgroundTraumatic peripheral nerve injury (TPNI) obliterates axons and Schwann cells (SCs) that must be cleared through phagocytosis to allow surviving SCs to de‐differentiate and divide and guide the regeneration process. In TPNI, macrophage (MØ) infiltration and early phenotypic transitions (M1 to M2) are critical for the phagocytosis of SCs debris. Delayed phagocytosis critically aggravates inflammation and impairs MØ function, which leads to failure to advance SC orchestration and neuronal regeneration. Erythropoietin (EPO), a pluripotent hormone, helps to guide MØs and SC transition post‐TPNI. The significance of the effects of EPO on debris clearance, apoptosis, myelination, and functional improvement is unknown, despite much effort motivating clinical translation of EPO for TPNI. We hypothesized that a role in supporting M2 phenotype macrophages after TPNI might explain EPO’s neuroprotective function through SCs debris clearance.Methods and ResultsWe evaluated EPO’s effect on sciatic nerve crush injury (SNCI) and found EPO treatment (5000 IU/kg, intraperitoneally, post‐surgery days 1, 2, and 3,) increased myelination and sciatic functional index (SFI) and augmented anti‐apoptosis and phagocytosis of myelin debris via CD206+ macrophages when compared to saline treatment in the mouse. EPO increased efferocytosis of apoptotic sciatic nerve derived Schwann cells (SNSCs) both in bone marrow derived macrophages (BMMØs) and peritoneal derived macrophages (PMØs) in‐vitro, as well as in PMØs in‐vivo by immunofluorescence (IF) and flow cytometry. EPO treatment significantly attenuated BMMØ pro‐inflammatory genes (IL1b, iNOS, and CD68) and augmented anti‐inflammatory genes (IL10, and CD163) in addition to the cell surface marker CD206 expression after lipopolysaccharide (LPS) induction. EPO also showed anti‐apoptotic (Annexin V/ 7AAD) effects in BMMØs. Our data demonstrate that EPO promotes the M2 phenotype macrophages to ameliorate SN apoptosis and efferocytosis of dying SCs and myelin debris and improves SN functional recovery following SNCI.ConclusionsTo the best of our knowledge, this is the first study to demonstrate EPO’s ameliorative effect on the progression of post‐TPNI via M2 phenotype macrophage phagocytosis of SCs debris with early anti‐apoptotic and anti‐inflammatory effects. These data may support the findings of other researchers and might shed light on the role of EPO in improving functional recovery in the traumatized limb.