Dual role of cyclic AMP-dependent protein kinase in neuritogenesis and synaptogenesis during neuronal differentiation

To create precise neural circuits in the nervous system, neuritogenesis and synaptogenesis are the critical cellular processes during neuronal differentiation. We examined the cyclic AMP (cAMP)-responsible signaling pathways for regulating neuritogenesis and synaptogenesis in NG108-15 cells. A rise in intracellular cAMP concentration by a membrane-permeable cAMP analog, dibutyryl cAMP (DBcAMP), led to an increase in the number of neurites and varicosities. Inhibition of cAMP-dependent protein kinase (PKA) activity by a PKA inhibitor (H89) accelerated this neuritogenesis and neurite outgrowth rate. Treatment with H89, however, decreased the number of varicosities and the frequency of postsynaptic miniature current recorded in the cultured cells, resulting in suppression of synaptogenesis. Immunoblot analyses revealed that PKA activity mediates phosphorylation of a gene transcription factor, cAMP-response element binding protein (CREB). On the other hand, inhibition of a mitogen-activated protein kinase (MAPK)/extracellular signal-regulated kinase (ERK) pathway by a MAPK/ERK kinase (MEK) inhibitor (PD98059) suppressed both neuritogenesis and neurite outgrowth without CREB phosphorylation. These results suggest strongly that PKA simultaneously plays two different roles in neuronal differentiation: inhibition of neuritogenesis and stimulation of synaptogenesis, via CREB-mediated gene expression.