Nerve growth cone guidance mediated by G protein-coupled receptors

Growing axons navigate by responding to chemical guidance cues. Here we report that growth cones of rat cerebellar axons in culture turned away from a gradient of SDF-1, a chemokine that attracts migrating leukocytes and cerebellar granule cells via a G protein-coupled receptor (GPCR). Similarly, Xenopus spinal growth cones turned away from a gradient of baclofen, an agonist of the GABA_B receptor. This response was mediated by G_i and subsequent activation of phospholipase C (PLC), which triggered two pathways: protein kinase C (PKC) led to repulsion, and inositol 1, 4, 5-triphosphate (IP₃) receptor activation led to attractive turning. Under normal culture conditions, PKC-dependent repulsion dominated, but the repulsion could be converted to attraction by inhibiting PKC or by elevating cytosolic cGMP. Thus, GPCRs can mediate both repulsive and attractive axon guidance in vitro, and chemokines may serve as guidance cues for axon pathfinding.