The inhibition of phosphatidylinositol-3-kinase induces neurite retraction and activates GSK3

It has been extensively described that neuronal differentiation involves the signalling through neurotrophin receptors to a Ras-dependent mitogen-activated protein kinase (MAPK) cascade. However, signalling pathways from other neuritogenic factors have not been well established. It has been reported that cAMP may activate protein kinase (PKA), and it has been shown that PKA-mediated stimulation of MAPK pathway regulates not only neuritogenesis but also survival. However, extracellular regulated kinases (ERKs) mediated pathways are not sufficient to explain all the processes which occur in neuronal differentiation. Our present data show that: in cAMP-mediated neuritogenesis, using the SH-SY5Y human neuroblastoma cell line, there exists a link between the activation of PKA and stimulation of phosphatidylinositol 3-kinase (PI3K). Both kinase activities are essential to the initial elongation steps. Surprisingly, this neuritogenic process appears to be independent of ERKs. While the activity of PI3K is essential for elongation and maintenance of neurites, its inhibition causes retraction. In this neurite retraction process, GSK3 is activated. Using both a pharmacological approach and gene transfer of a dominant negative form of GSK3, we conclude that this induced retraction is a GSK3-dependent process which in turn appears to be a common target for transduction pathways involved in lysophosphatidic acid-mediated and PI3K-mediated neurite retraction.