Impairment of peripheral nerve regeneration by insufficient activation of the HGF/c-Met/c-Jun pathway in aged mice.

Aging in the peripheral nervous system (PNS) leads to its dysfunction and lowers regenerative capacity after injury. Based on the pro-regenerative effect of hepatocyte growth factor (HGF) in injured peripheral nerves, we investigated whether this growth factor is involved in the age-related degeneration of the PNS. We observed that the capacity for nerve regeneration was significantly reduced under aging conditions as indicated by the decreased level of SCG10-positive axons. Functional recovery was also impaired. We further tested whether the HGF/c-Met pathway was involved, and the activation of the c-Met receptor upon nerve injury was significantly reduced, whereas the production of HGF protein was still comparable to that in young mice. Moreover, the phosphorylation and expression of c-Jun, a key regeneration-associated gene, was also lowered in aged animals. In addition, exogenous administration of the HGF expressing plasmid DNA significantly ameliorated the pain-like behavior in young animals, however, such analgesic activity was impaired in aged mice. These data suggested that the HGF/c-Met pathway might be involved in the age-related impairment of regenerative capacity in the PNS.