Deletion of SIRPα (signal regulatory protein-α) promotes phagocytic clearance of myelin debris in Wallerian degeneration, axon regeneration, and recovery from nerve injury

Background: Recovery of function from traumatic nerve injury depends on the ability of severed axons to grow/regenerate back to their target tissues. This is achieved by successfully crossing the lesion site where physical impact severed axons, determined by the type of trauma, followed by successfully growing throughout the Wallerian degenerating nerve segment located distal to and beyond the lesion site, determined by the nature of Wallerian degeneration. The protracted removal of myelin debris in Wallerian degeneration, which leads residual myelin debris to slow down axon growth, impedes recovery of function. We focused in this study on mechanism(s) that delay the removal of myelin debris in Wallerian degeneration and so impede recovery. Previously, we showed that myelin debris inhibited its own phagocytosis in primary cultured macrophages and microglia as CD47 on myelin ligated SIRP alpha (signal regulatory protein-alpha) on phagocytes, and sequentially, SIRP alpha generated don't eat me signaling. We also demonstrated that serum inhibited phagocytosis in a SIRP alpha-dependent manner. Herein, we aimed to determine whether SIRP alpha-dependent inhibition of phagocytosis in macrophages impedes the in vivo removal of myelin debris in Wallerian degeneration, further leading to impaired healing. Methods: Using SIRP alpha null (SIRP alpha-/-) and littermate wild-type (SIRP alpha+/+) mice, we studied the recovery of sensory and motor functions from nerve injury and, further, axon regeneration, SIRP alpha expression, myelin debris removal, and the phagocytic capacity and presence of macrophages in Wallerian degeneration. Results: Myelin debris removal, axon regeneration, and the recovery of functions were all faster in SIRP alpha-/- mice than in wild-type mice. Between the two cell types that mostly scavenge myelin debris, macrophages but not Schwann cells expressed SIRP alpha in wild-type mice, and furthermore, SIRP alpha-/- macrophages phagocytosed significantly more than wild-type macrophages. Conclusions: Our findings suggest an intrinsic normally occurring SIRP alpha-dependent mechanism that impedes the in vivo removal of myelin debris in Wallerian degeneration by inhibiting the phagocytosis of myelin debris in macrophages, hence preventing fast growing axons from fully implementing their regenerative potential. Thus, accelerating the removal of myelin debris by eliminating SIRP alpha-dependent inhibition of phagocytosis will most likely advance recovery of functions from nerve injury.