AXON OUTGROWTH IS REGULATED BY AN INTRACELLULAR PURINE-SENSITIVE MECHANISM IN RETINAL GANGLION-CELLS

Although purinergic compounds are widely involved in the intra- and in tercellular communication of the nervous system, little is known of th eir involvement in the growth and regeneration of neuronal connections . In dissociated cultures, the addition of adenosine or guanosine in t he low micromolar range induced goldfish retinal ganglion cells to ext end lengthy neurites and express the growth-associated protein GAP-43. These effects were highly specific and did not reflect conversion of the nucleosides to their nucleotide derivatives; pyrimidines, purine n ucleotides, and membrane-permeable, nonhydrolyzable cyclic nucleotide analogs were all inactive. The activity of adenosine required its conv ersion to inosine, because inhibitors of adenosine deaminase rendered adenosine inactive. Exogenously applied inosine and guanosine act dire ctly upon an intracellular target, which may coincide with a kinase de scribed in PC12 cells. In support of this, the effects of the purine n ucleosides were blocked with purine transport inhibitors and were inhi bited competitively with the purine analog 6-thioguanine (6-TG). In PC 12 cells, others have shown that 6-TG blocks nerve growth factor-induc ed neurite outgrowth and selectively inhibits the activity of protein kinase N, a partially characterized, nerve growth factor-inducible ser ine-threonine kinase. In both goldfish and rat retinal ganglion cells, 6-TG completely blocked outgrowth induced by other growth factors, an d this inhibition was reversed with inosine. These results suggest tha t axon outgrowth in central nervous system neurons critically involves an intracellular purine-sensitive mechanism.