A purine-sensitive pathway regulates multiple genes involved in axon regeneration in goldfish retinal ganglion cells

In lower vertebrates, retinal ganglion cells (RGCs) can regenerate their ax ons and reestablish functional connections after optic nerve injury. We sho w here that in goldfish RGCs, the effects of several trophic factors conver ge on a purine-sensitive signaling mechanism that controls axonal outgrowth and the expression of multiple growth-associated proteins. In culture, gol dfish RGCs regenerate their axons in response to two molecules secreted by optic nerve glia, axogenesis factor-1 (AF-1) and AF-2, along with ciliary n eurotrophic factor. The purine analog 6-thioguanine (6-TG) blocked outgrowt h induced by each of these factors. Previous studies in PC12 cells have sho wn that the effects of 6-TG on neurite outgrowth may be mediated via inhibi tion of a 47 kDa protein kinase. Growth factor-induced axogenesis in RGCs w as accompanied by many of the molecular changes that characterize regenerat ive growth in vivo, e.g., increased expression of GAP-43 and certain cell s urface glycoproteins. 6-TG inhibited all of these changes but not those ass ociated with axotomy per se, e.g., induction of jun family transcription fa ctors, nor did it affect cell survival. Additional studies using RGCs from transgenic zebrafish showed that expression of T alpha -1 tubulin is likewi se stimulated by AF-1 and blocked by 6-TG. The purine nucleoside inosine ha d effects opposite to those of 6-TG. Inosine stimulated outgrowth and the c haracteristic pattern of molecular changes in RGCs and competitively revers ed the inhibitory effects of 6-TG. We conclude that axon regeneration and t he underlying program of gene expression in goldfish RGCs are mediated via a common, purine-sensitive pathway.