SIGNAL-TRANSDUCTION IN VERTEBRATE GROWTH CONES NAVIGATING IN-VIVO

Navigating growth cones need signal transduction machinery to amplify and transmit the effects of extracellular signals throughout the growt h cone. In culture, many drugs that affect second messengers are known to modulate neurite extension (with different effects on differ ent n eurons), and gradients of calcium influx and cyclic nucleotide analogs can cause growth cones to turn. However, it is not clear which of the se responses are physiologically relevant, as axons grow through much more complex environments in vivo. The ''exposed brain'' preparation i n Xenopus embryos provides an experimentally tractable system in which it is possible to study growth, pathfinding, and target recognition o f retinal growth cones in vivo, while pharmacologically manipulating t heir signal transduction systems, These growth cones can also be easil y studied in explant culture. We describe preliminary results of paral lel in vivo and in vitro experiments using an array of drugs that pert urb transduction molecules. Surprisingly, calcium ionophores and cycli c nucleotide analogs have no significant effect on retinal axon growth or pathfinding. Several agents including herbimycin A, ML-7, mastopar an, and RHC80267 inhibit retinal axon growth, both in vivo and in vitr o, suggesting that tyrosine kinases, myosin, heterotrimeric G-proteins , and diacylglycerol lipase are important for retinal growth cones nav igating in the optic pathway.