The MAP kinase pathway is upstream of the activation of GSK3β that enables it to phosphorylate MAP1B and contributes to the stimulation of axon growth

In pheochromocytoma 12 (PC12) cells and sympathetic neurons, nerve growth factor (NGF) engagement with the tropomyosin-related tyrosine kinase (TrkA) receptor activates the serine/threonine kinase glycogen synthase kinase 3β (GSK3β), enabling it to phosphorylate the microtubule-associated protein 1B (MAP1B). GSK3β phosphorylation of MAP1B acts as a molecular switch to regulate microtubule dynamics in growing axons, and hence the rate of axon growth. An important question relates to the identification of the upstream pathway linking the activation of GSK3β with TrkA engagement. TrkA can utilise a number of intracellular signalling pathways, including the mitogen-activated protein kinase (MAPK) pathway and the phosphatidylinositol-3 kinase (PI3K) pathway. We now show, using pharmacological inhibitor studies of PC12 cells and sympathetic neurons in culture and in vitro kinase and activation assays, that the MAPK pathway, and not the PI3K pathway, links NGF engagement with the TrkA receptor to GSK3β activation in PC12 cells and sympathetic neurons. We also show that activated GSK3β is a small fraction of the total GSK3β present in developing brain and that it is not part of a multiprotein complex. Thus, NGF drives increased neurite growth rates partly by coupling the MAPK pathway to the activation of GSK3β and thereby phosphorylation of MAP1B.