Depolarization-dependent pro-survival signaling in spiral ganglion neurons

Membrane depolarization is an effective neurotrophic stimulus, with its trophic effect on spiral ganglion neurons (SGNs) even surpassing that of neurotrophins. Thus, SGN cultures are a favorable system to investigate pro-survival signal transduction downstream of depolarization.

Depolarization promotes SGN survival by recruiting three distinct kinase pathways: cyclic AMP-dependent protein kinase (PKA), Ca2+/calmodulin-dependent protein kinase II (CaMKII) and CaMKIV. CaMKIV mediates the pro-survival effect of depolarization by activating CREB in nucleus. However, the mechanisms by which PKA and CaMKII promote survival are still not clear. By targeting constitutively active PKA or a PKA inhibitor (PKI) to the outer mitochondrial membrane (OMM), we showed that PKA activity at the OMM is sufficient to support SGN survival in the absence of other trophic factors and necessary for cAMP-dependent SGN survival. It has been suggested that PKA can promote survival by inactivating pro-apoptotic protein Bad. By cotransfection of SGNs with OMM-PKA and wild-type Bad, we showed that this was the case. We further showed that Ser112 and Ser136 in Bad, but not Ser155, a hypothetical PKA target, were necessary for functional inactivation of Bad by PKA.

CaMKII mediates the third depolarization-dependent pro-survival pathway. A specific pro-survival target for CaMKII was identified through a separate investigation of the pro-apoptotic JNK-Jun signaling pathway, which we had identified as active in apoptotic SGNs in vivo. By measuring anti-phosphoJun immunofluorescence, we could quantify JNK-Jun activation in SGNs under different conditions. We showed that JNK inhibition or genetic deletion of JNK3 reduces SGN death after neurotrophic factor withdrawal. Neurotrophins have been shown to suppress JNK activation via their receptor protein tyrosine kinases (PTKs). By expressing constitutively active and dominant negative forms of candidate protein kinases, we identified a novel signaling pathway linking depolarization to JNK: Ca2+ entry - CaMKII - FAK/Pyk2 - PI-3-OH Kinase - Protein Kinase B - inhibition of MLKs (upstream activators of JNK). Thus, depolarization also recruits PTKs - the nonreceptor PTKs FAK and Pyk2 - to suppress JNK activation, implying a conserved PTK-PI3K-PKB pathway for suppression of pro-apoptotic JNK activation by neurotrophic stimuli.