After loss of hair cells, the deafferented spiral ganglion neurons (SGNs) lose their peripheral process and gradually die. SGN degeneration reduces the efficacy of cochlear implants, currently the only treatment for sensorineural deafness. Electrical stimulation promotes survival of deafferented SGNs in vivo, raising the possibility of using electrical stimulation to maintain survival of SGNs in deaf individuals - in effect allowing cochlear implants to replace the trophic as well as the sensory function of hair cells. We use in vitro and in vivo approaches to determine how electrical activity prevents SGN death and apply this knowledge to prevention of SGN degeneration in vivo. We showed that SGN death in deafened rats is correlated with increased proapoptotic signaling in the JNK-Jun pathway. Early in the post-deafening period there is also decreased prosurvival signaling in SGNs, evident as decreased CREB phosphorylation.
Aim 1 uses intracochlear infusion of a JNK inhibitor and JNK3-/- mice to determine whether JNK activity is necessary for SGN death in vivo and, if so, when is it necessary. We also ask the extent to which JNK inhibition or JNK3 deletion promotes degeneration of peripheral processes. Because Jun phosphorylation and SGN death occur long after hair cells have died, we ask, in Aim 2, whether other post-deafening degenerative changes in the cochlea can account for SGN death, focusing on the death of glial cells, peripheral process degeneration, and loss of NT-3 expression. We next turn to the question of how membrane electrical activity promotes SGN survival. We have developed molecular reagents to selectively activate or silence individual intracellular signaling pathways in specific subcellular compartments. Using these, we showed that CaMKII links depolarization to suppression of proapoptotic JNK signaling. We further show that CaMKII does so by recruiting nonreceptor protein-tyrosine kinases, FAK and Pyk2, and protein kinase B (PKB). This is reminiscent of the mechanism by which peptide neurotrophic factors suppress JNK signaling via their receptor protein-trosine kinases and PKB.
In Aim 3, we further develop this novel signaling pathway, and parallelism with neurotrophins, by testing the role of Rac/Cdc42 small GTPases in suppression of JNK signaling by the depolarization-CaMKII-Pyk2/FAK pathway. As in our previous studies, the experimental approach using transfection into cultured SGNs of inhibitory and gain-of-function constructs targeting specific steps in the proposed pathway. Physiologi,cal activity and stimulation by cochlear implants consists of impulses of various frequencies.
In Aim 4, we extend our studies of intracellular signaling to patterned electrical activity using a system for in vitro electrical stimulation (ES). We ask whether patterned ES recruits the novel signaling pathways we have identified in depolarized SGNs. We also ask in Aim 4 what is the optimal frequency for suppression of proapoptotic signaling in deafferented SGNs in vivo and whether in vivo ES also recruits FAK/Pyk2 in a CaMKII-dependent manner.Sensorineural hearing loss affects about 20,000,000 Americans and the only current means to replace the function of the lost sensory cells is the cochlear implant, which directly stimulates cochlear neurons. Our research focuses on improving the survival and function of surviving neurons in order to improve the long-term efficacy of cochlear implants, currently used by over 40,000 Americans.
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