Axotomized frog sciatic nerve releases diffusible neurite-promoting factors

Using the bullfrog (Rana catesbeiana) dorsal root ganglia (DRG) and its sciatic nerve (ScN) as a model system, we have previously described neuronal and non-neuronal molecular changes associated with the early regenerative response of DRG neurons to axotomy. Since diffusible molecular factors, released by axotomized ScN, might function to stimulate axon regrowth, we have assayed the ability of ScN-conditioned bath to promote in vitro neurite outgrowth from PC-12 cells. Diffusible ScN proteins were collected by incubating segments of normal or axotomized ScN in a small volume of RPMI media for 4 h (nerve bath). The nerve baths, supplemented with serum, were then added to PC-12 cell cultures to assay for the presence of neurite growth factors released by ScN. Results showed that nerve baths, collected from sham-operated or axotomized ScN, could not induce the differentiation of PC-12 into neurite-bearing cells. Therefore, in all subsequent neurite growth assay experiments, an exogenous source of nerve growth factor (NGF) (50 ng/ml) was added to the nerve baths or unconditioned media to generate and maintain PC-12 neuritic structure. We found that nerve baths, collected from previously axotomized (at least 3 days post-injury) nerve, contained diffusible factors which enhanced PC-12 neurite growth, relative to unconditioned media. No neurite growth factors were observed to be released by sham-operated ScN or 1-day post-axotomized ScN. Further experiments were conducted to identify the diffusible neurite growth factors released from axotomized ScN. We showed that the release (if any) of endogenous diffusible NGF or laminin from axotomized nerve could not have accounted for the facilitation of neurite growth. Analysis of radiolabelled ScN proteins by two-dimensional polyacrylamide gel could not have accounted for the facilitation of neurite growth. Analysis of radiolabelled ScN proteins by two-dimensional polyacrylamide gel electrophoresis (2D-PAGE) showed that the relative abundance of two diffusible proteins (M(r) approximately 35 and 70 kDa) in the nerve bath was directly correlated with the ability of the nerve bath to facilitate PC-12 neurite growth.