FGF9 alters the Wallerian degeneration process by inhibiting Schwann cell transformation and accelerating macrophage infiltration

In vitro experiments have proved that Fibroblast Growth Factor 9 (FGF9) was decreased in Schwann cells (SCs) in which Wallerian degeneration (WD) occurred after nerve injury. We hypothesize that FGF9 downregulation in WD has some biological influence on Schwann cells (SCs) and macrophages - the two most important cell components involved in WD. In this study, we employed strategies to regulate FGF9 in sciatic nerve crush by generating a mouse model, wherein Fgf9 was specifically knocked-out in SCs, and an intraneural injection of human FGF9 protein administered to overexpress FGF9 independently. Furthermore, an inhibitor of extracellular-regulated kinases 1/2 (ERK1/2), PD0325901, was used to clarify the underlying downstream mechanism of ERK1/2 activated by FGF9. Analysis of WD revealed the novel features of FGF9: (i) FGF9 was widely expressed in axons and SCs, and was decreased during WD process. (ii) Fgf9 knockout in SCs impaired the debris clearance and eventually impeded the regeneration of nerve fibers after damage. (iii) Fgf9 knockout in SCs promoted the dedifferentiation of SCs and delayed the infiltration of macrophages by decreasing Mcp1, Tnfα, Il1β levels and leaky blood-nerve-barrier (BNB) in WD. (iv) FGF9 injection preserved the nerve fibers, inhibited SCs dedifferentiation and accelerated macrophages infiltration. (v) ERK1/2 phosphorylation was increased by exogenous FGF9 injection. P75, Cyclin D1, Mcp1, Tnfα, Il1β, c-Jun changes by FGF9 intraneural injection were partially reversed by the ERK1/2 inhibitor. Conclusion was that FGF9 inhibited the dedifferentiation of SCs and accelerated the accumulation of macrophages in WD, and exogenous FGF9 took effects partially by ERK1/2.