Ciliary neurotrophic factor is an axogenesis factor for retinal ganglion cells

Although mature mammalian retinal ganglion cells normally fail to regrow injured axons, exposure to the molecular environment of the peripheral nervous system stimulates regenerative growth. The present study used dissociated rat retinal ganglion cells purified by immunopanning to identify peripheral nervous system-derived factors that promote axonal outgrowth. Of the multiple growth factors investigated, only ciliary neurotrophic factor and the related cytokine, leukemia inhibitory factor, had striking neuritogenic activity, with half-maximal effects at 1-2 ng/ml. Brain-derived neurotrophic factor stimulated retinal ganglion cell survival nearly as well as ciliary neurotrophic factor, but had only minor effects on outgrowth. Thus, the neuritogenic effects of ciliary neurotrophic factor are not a simple consequence of increased survival. Ciliary neurotrophic factor-stimulated outgrowth was correlated with increased expression of the growth-associated membrane phosphoprotein, GAP-43, a hallmark of optic nerve regeneration in vivo. A high molecular weight fraction from media conditioned by rat optic or sciatic nerve mimicked the effect of ciliary neurotrophic factor in inducing axonal outgrowth. Ciliary neurotrophic factor was detected in the conditioned media on western blots, and the biological activity of the conditioned media was neutralized with an anti-ciliary neurotrophic factor antibody. These results indicate that ciliary neurotrophic factor has specific effects on axon outgrowth in retinal ganglion cells that are dissociable from its effects on cell survival, and that ciliary neurotrophic factor accounts for most of the axon-promoting activity for retinal ganglion cells present in either the sciatic or optic nerve.