title: Protein kinase A regulates the sensitivity of spinal commissural axon turning to netrin-1 but does not switch between chemoattraction and chemorepulsion.
abstract: Bifunctional axon guidance cues have been grouped into two classes depending on whether changes in intracellular cAMP or cGMP switch the response of the growth cone between attraction and repulsion. According to this model, axons respond to netrin-1, a group I guidance cue, as a chemoattractant when cAMP levels are high in the growth cone but switch and are repelled when the intraneuronal concentration of cAMP is low. The model is complicated by the proposal that cAMP-dependent kinase, protein kinase A (PKA), functions as a downstream effector for several guidance cues, including netrin-1, suggesting a close inter-relationship between guidance cue signal transduction and mechanisms regulating the switch between attraction and repulsion. Here, we examine possible interactions between netrin-1-mediated axon guidance and cAMP signaling in embryonic rat spinal commissural neurons. We report that netrin-1 does not alter the concentration of cAMP or PKA activity in these neurons across a wide range of netrin-1 concentrations and time points after application, leading us to conclude that netrin-1 does not regulate PKA in these cells. In contrast to the cyclic nucleotide switch model, we report that, despite inhibiting PKA, embryonic spinal commissural axons were always attracted to netrin-1 and never repelled. Instead, manipulating PKA regulated the sensitivity of chemoattraction to netrin-1: PKA inhibition reduced, and PKA activation increased, the distance over which axons turn toward a source of netrin-1. These findings indicate that the mechanisms underlying cyclic nucleotide-regulated switching are separable from the signal transduction mechanisms required for chemoattraction to netrin-1.
authors.affiliation: Centre for Neuronal Survival, Montreal Neurological Institute, Department of Neurology and Neurosurgery, McGill University, Montreal, Quebec, H3A 2B4, Canada.
authors.forename: Simon W
authors.initials: SW
authors.lastname: Moore
authors.forename: Timothy E
authors.initials: TE
authors.lastname: Kennedy
mesh_terms: D000818:Animals; D001369:Axons / Q000187:drug effects / Q000502:physiology*; D015153:Blotting, Western / Q000379:methods; D002227:Carbazoles / Q000494:pharmacology; D002478:Cells, Cultured; D005576:Colforsin / Q000494:pharmacology; D000242:Cyclic AMP / Q000378:metabolism; D017868:Cyclic AMP-Dependent Protein Kinases / Q000502:physiology*; D004305:Dose-Response Relationship, Drug; D004622:Embryo, Mammalian; D004791:Enzyme Inhibitors / Q000494:pharmacology; D004797:Enzyme-Linked Immunosorbent Assay / Q000379:methods; D006801:Humans; D007211:Indoles / Q000494:pharmacology; D009414:Nerve Growth Factors / Q000494:pharmacology / Q000502:physiology*; D000075388:Netrin-1; D009474:Neurons / Q000166:cytology* / Q000187:drug effects; D011758:Pyrroles / Q000494:pharmacology; D051381:Rats; D017207:Rats, Sprague-Dawley; D013116:Spinal Cord / Q000166:cytology*; D013997:Time Factors; D025521:Tumor Suppressor Proteins / Q000494:pharmacology / Q000502:physiology*
chemical_list: D002227:Carbazoles; D004791:Enzyme Inhibitors; D007211:Indoles; C000617351:NTN1 protein, human; D009414:Nerve Growth Factors; C000617354:Ntn1 protein, rat; D011758:Pyrroles; D025521:Tumor Suppressor Proteins; D000075388:Netrin-1; D005576:Colforsin; C057416:KT 5720; D000242:Cyclic AMP; D017868:Cyclic AMP-Dependent Protein Kinases