Critical role of p38 MAPK for regeneration of the sciatic nerve following crush injury in vivo

Background: The physiological function of p38 alpha, which is an isoform of p38 MAPK, has been investigated previously in several studies using pharmacological inhibitors. However, the results regarding whether p38 alpha promotes or inhibits nerve regeneration in vivo have been controversial. Methods: We generated novel p38 alpha mutant mice (sem mice) with a point mutation in the region encoding the p38 alpha substrate-docking-site, which serves as a limited loss-of-function model of p38 alpha. In the present study, we utilized sem mice and wild-type littermates (wt mice) to investigate the physiological role of p38 alpha in nerve regeneration following crush injuries. Results: At four weeks after crush injury, the average axon diameter and the average axon area in sem mice were significantly smaller than those in wt mice. The average myelin sheath thickness in sem mice was reduced compared to wt mice, but no significant difference was observed in the G-ratio between the two groups. The sciatic functional index value demonstrated that functional nerve recovery in sem mice following crush injury was delayed, which is consistent with the histological findings. To investigate the underlying mechanisms of these findings, we examined inflammatory responses of the sciatic nerve by immunohistochemistry and western blotting. At an early phase following crush injury, sem mice showed remarkably lower expression of inflammatory cytokines, such as TNF-alpha and IL-1 beta, than wt mice. The expression of Caspase-3 and Tenascin-C were also lower in sem mice. Conversely, at a late phase of the response, sem mice showed considerably higher expression of TNF-alpha and of IL-1 beta with lower expression of S-100 than wt mice. Conclusions: This is the first study of the physiological role of p38 MAPK in nerve regeneration that does not rely on the use of pharmacological inhibitors. Our results indicate that p38 alpha insufficiency may cause an inflammatory disorder, resulting in a delay of histological and functional nerve recovery following crush injury. We conclude that p38 MAPK has an important physiological role in nerve regeneration and may be important for controlling both initiation of inflammation and recovery from nerve injury.