Role of cdk5 and tau phosphorylation in heterotrimeric G protein-mediated retinal growth cone collapse

During axonal growth, repulsive guidance cues cause growth cone collapse an d retraction. In the chick embryo, membranes from the posterior part of the optic tectum containing ephrins are original collapsing factors for axons growing from the temporal retina. We investigated signal transduction pathw ays in retinal axons underlying this membrane-evoked collapse. Perturbation experiments using pertussis toxin (PTX) showed that membrane-induced colla pse is mediated via G(o/i) proteins, as is the case for semaphorin/collapsi n-1-induced collapse. Studies with Indo-1 revealed that growth cone collaps e by direct activation of G(o/i) proteins with mastoparan did not cause ele vation of the intracellular Ca2+ level, and thus this signal transduction p athway is Ca2+ independent. Application of the protein phosphatase Inhibito r okadaic acid alone induced growth cone collapse in retinal culture, sugge sting signals involving protein dephosphorylation, In addition, pretreatmen t of retinal axons with olomoucine, a specific inhibitor of cdk5 (tau kinas e II), prevented mastoparan-evoked collapse. Olomoucine also blocks caudal tectal membrane-mediated collapse. These results suggest that rearrangement of the cytoskeleton is mediated by tau phosphorylation, Immunostaining vis ualized complementary distributions of tau phospho- and dephosphoisoforms w ithin the growth cone, which also supports the involvement of tau, Taking t hese findings together, we conclude that cdk5 and tau phosphorylation proba bly lie downstream of growth cone collapse signaling mediated by PTX-sensit ive G proteins. (C) 1999 John Wiley & Sons. Inc.