Control of neurite outgrowth and growth cone motility by phosphatidylinositol-3-kinase

Phosphatidylinositol-3-kinase (PI-3K) has been reported to affect neurite outgrowth both in vivo and in vitro. Here we investigated the signaling pathways by which PI-3K affects neurite outgrowth and growth cone motility in identified snail neurons in vitro. Inhibition of PI-3K with wortmannin (2 microM) or LY 294002 (25 microM) resulted in a significant elongation of filopodia and in a slow-down of neurite outgrowth. Experiments using cytochalasin and blebbistatin, drugs that interfere with actin polymerization and myosin II activity, respectively, demonstrated that filopodial elongation resulting from PI-3K inhibition was dependent on actin polymerization. Inhibition of strategic kinases located downstream of PI-3K, such as Akt, ROCK, and MEK, also caused significant filopodial elongation and a slow-down in neurite outgrowth. Another growth cone parameter, filopodial number, was not affected by inhibition of PI-3K, Akt, ROCK, or MEK. A detailed study of growth cone behavior showed that the filopodial elongation induced by inhibiting PI-3K, Akt, ROCK, and MEK was achieved by increasing two motility parameters: the rate with which filopodia extend (extension rate) and the time that filopodia spend elongating. Whereas the inhibition of ROCK or Akt (both activated by the lipid kinase activity of PI-3K) and MEK (activated by the protein kinase activity of PI-3K) had additive effects, simultaneous inhibition of Akt and ROCK showed no additive effect. We further demonstrate that the effects on filopodial dynamics investigated were calcium-independent. Taken together, our results suggest that inhibition of PI-3K signaling results in filopodial elongation and a slow-down of neurite advance, reminiscent of growth cone searching behavior.