The inhibition of phosphatidylinositol-3-kinase induces neurite retractionand activates GSK3

It has been extensively described that neuronal differentiation involves th e signalling through neurotrophin receptors to a Ras-dependent mitogen-acti vated protein kinase (MAPK) cascade. However, signalling pathways from othe r neuritogenic factors have not been well established. It has been reported that cAMP may activate protein kinase (PKA), and it has been shown that PK A-mediated stimulation of MAPK pathway regulates not only neuritogenesis bu t also survival. However, extracellular regulated kinases (ERKs) mediated p athways are not sufficient to explain all the processes which occur in neur onal differentiation. Our present data show that: in cAMP-mediated neuritog enesis, using the SH-SY5Y human neuroblastoma cell line, there exists a lin k between the activation of PKA and stimulation of phosphatidylinositol 3-k inase (PI3K). Both kinase activities are essential to the initial elongatio n steps. Surprisingly, this neuritogenic process appears to be independent of ERKs. While the activity of PI3K is essential for elongation and mainten ance of neurites, its inhibition causes retraction. In this neurite retract ion process, GSK3 is activated. Using both a pharmacological approach and g ene transfer of a dominant negative form of GSK3, we conclude that this ind uced retraction is a GSK3-dependent process which in turn appears to be a c ommon target for transduction pathways involved in lysophosphatidic acid-me diated and PI3K-mediated neurite retraction.