Transient PKA activity is required for initiation but not maintenance of BDNF-mediated protection from nitric oxide-induced growth-cone collapse.

Growing axons during development are guided to their targets by the activity of their growth cones. Growth cones integrate positive and negative guidance cues in deciding the direction in which to extend. We demonstrated previously that treatment of embryonic retinal ganglion cells with brain-derived neurotrophic factor (BDNF) protects their growth cones from collapse induced by nitric oxide (NO). BDNF stabilizes growth-cone actin filaments against NO-induced depolymerization. In the present study, we examined the signaling mechanism involved in BDNF-mediated protection. We found that BDNF causes transient activation of protein kinase A (PKA) during the first 5 min of treatment. Treatment with PKA inhibitors before or in conjunction with BDNF treatment blocked the protective effects of BDNF. The effects of BDNF, however, were not blocked when addition of PKA inhibitors was delayed as little as 15 min after BDNF treatment. When cultures raised overnight in BDNF were treated with PKA inhibitors, BDNF-mediated protection did not end, demonstrating that the maintenance of the protective effects of BDNF is independent of PKA activity. The BDNF-induced activation of PKA was required for BDNF-mediated stabilization of growth-cone actin filaments against depolymerization by cytochalasin D. Finally, the initiation and maintenance of the protective effects of BDNF required protein synthesis. Collectively, these data demonstrate that PKA signaling is required only for an early phase of BDNF-mediated protection from NO-induced growth-cone collapse.