Journal of Neuroscience, Vol 16, 2294-2306, Copyright © 1996 by Society for Neuroscience
Inhibition of protein tyrosine kinases impairs axon extension in the embryonic optic tract
T Worley and C Holt
Department of Biology, University of California at San Diego, La Jolla, 92093, USA.
The role of protein tyrosine kinase (PTK) activity in the development of
the retinal projection was examined in vivo by applying inhibitors of
cytoplasmic PTKs, herbimycin A and lavendustin A, to intact brain
preparations of Xenopus embryos. The inhibitors were present during the
period when retinal ganglion cell axons first navigate through the optic
tract to reach their target, the optic tectum. A majority of
inhibitor-treated retinal axons stalled at the beginning of the optic
tract, leading to an 80% reduction in projection length at the highest
doses. All inhibitor-treated axons that did extend into the optic tract
exhibited normal pathfinding behavior. Tyrosine kinase assays of
inhibitor-treated brains demonstrated that at doses at which retinal axon
extension was severely impaired, PTK activity, including that of src family
proteins, was reduced by 50-60%. Consistent with the in vivo findings, PTK
inhibitors reduced neurite outgrowth from cultured retinal neurons by
70-80%. This contrasts with the strong enhancement of outgrowth induced by
the same inhibitors in cultured chick ciliary ganglion neurons and suggests
that the mediation of outgrowth by PTK activity may vary in different
neuronal types. Inhibitor-treated growth cones cultured on laminin were
larger than normal, suggesting that tyrosine phosphorylation can modulate
growth cone-substrate adhesive interactions. Our results in vivo and in
vitro provide complementary evidence that retinal axon outgrowth is
inhibited by pharmacological blockers of PTK activity and indicate that
inhibitor-sensitive PTKs normally play a role in promoting retinal neurite
extension.