title: Influence of cAMP and protein kinase A on neurite length from spiral ganglion neurons.
abstract: Regrowth of peripheral spiral ganglion neuron (SGN) fibers is a primary objective in efforts to improve cochlear implant outcomes and to potentially reinnervate regenerated hair cells. Cyclic adenosine monophosphate (cAMP) regulates neurite growth and guidance via activation of protein kinase A (PKA) and Exchange Protein directly Activated by Cylic AMP (Epac). Here we explored the effects of cAMP signaling on SGN neurite length in vitro. We find that the cAMP analog, cpt-cAMP, exerts a biphasic effect on neurite length; increasing length at lower concentrations and reducing length at higher concentrations. This biphasic response occurs in cultures plated on laminin, fibronectin, or tenascin C suggesting that it is not substrate dependent. cpt-cAMP also reduces SGN neurite branching. The Epac-specific agonist, 8-pCPT-2'-O-Me-cAMP, does not alter SGN neurite length. Constitutively active PKA isoforms strongly inhibit SGN neurite length similar to higher levels of cAMP. Chronic membrane depolarization activates PKA in SGNs and also inhibits SGN neurite length. However, inhibition of PKA fails to rescue neurite length in depolarized cultures implying that activation of PKA is not necessary for the inhibition of SGN neurite length by chronic depolarization. Expression of constitutively active phosphatidylinositol 3-kinase, but not c-Jun N-terminal kinase, isoforms partially rescues SGN neurite length in the presence of activated PKA. Taken together, these results suggest that activation of cAMP/PKA represents a potential strategy to enhance SGN fiber elongation following deafness; however such therapies will likely require careful titration so as to promote rather than inhibit nerve fiber regeneration.
authors.affiliation: Department of Otolaryngology-Head and Neck Surgery, University of Iowa, Iowa, IA 52242, USA.
authors.forename: Ningyong
authors.initials: N
authors.lastname: Xu
authors.forename: Jonathan
authors.initials: J
authors.lastname: Engbers
authors.forename: Sobia
authors.initials: S
authors.lastname: Khaja
authors.forename: Linjing
authors.initials: L
authors.lastname: Xu
authors.forename: J Jason
authors.initials: JJ
authors.lastname: Clark
authors.forename: Marlan R
authors.initials: MR
authors.lastname: Hansen
mesh_terms: D000818:Animals; D000831:Animals, Newborn; D005576:Colforsin / Q000494:pharmacology; D000242:Cyclic AMP / Q000031:analogs & derivatives / Q000378:metabolism* / Q000494:pharmacology; D017868:Cyclic AMP-Dependent Protein Kinases / Q000037:antagonists & inhibitors / Q000235:genetics / Q000378:metabolism*; D004305:Dose-Response Relationship, Drug; D004789:Enzyme Activation; D020536:Enzyme Activators / Q000494:pharmacology; D020662:Guanine Nucleotide Exchange Factors / Q000378:metabolism; D048031:JNK Mitogen-Activated Protein Kinases / Q000378:metabolism; D008564:Membrane Potentials; D009416:Nerve Regeneration / Q000187:drug effects; D016501:Neurites / Q000201:enzymology; D009474:Neurons / Q000187:drug effects / Q000201:enzymology*; D058539:Phosphatidylinositol 3-Kinase / Q000378:metabolism; D047428:Protein Kinase Inhibitors / Q000494:pharmacology; D051381:Rats; D011993:Recombinant Fusion Proteins / Q000378:metabolism; D015398:Signal Transduction; D013136:Spiral Ganglion / Q000187:drug effects / Q000201:enzymology*; D013873:Thionucleotides / Q000494:pharmacology; D046509:Tissue Culture Techniques; D014162:Transfection
chemical_list: C473217:8-(4-chloro-phenylthio)-2'-O-methyladenosine-3'-5'-cyclic monophosphate; D020536:Enzyme Activators; D020662:Guanine Nucleotide Exchange Factors; D047428:Protein Kinase Inhibitors; D011993:Recombinant Fusion Proteins; D013873:Thionucleotides; D005576:Colforsin; C016957:adenosine-3',5'-cyclic phosphorothioate; D000242:Cyclic AMP; D058539:Phosphatidylinositol 3-Kinase; D017868:Cyclic AMP-Dependent Protein Kinases; D048031:JNK Mitogen-Activated Protein Kinases