Membrane depolarization inhibits spiral ganglion neurite growth via activation of multiple types of voltage sensitive calcium channels and calpain

Abstract

The effect of membrane electrical activity on spiral ganglion neuron (SGN) neurite growth remains unknown despite its relevance to cochlear implant technology. We demonstrate that membrane depolarization delays the initial formation and inhibits the subsequent extension of cultured SGN neurites. This inhibition depends directly on the level of depolarization with higher levels of depolarization causing retraction of existing neurites. Cultured SGNs express subunits for L-type, N-type, and P/Q type voltage-gated calcium channels (VGCCs) and removal of extracellular Ca 2+ or treatment with a combination of L-type, N-type, P/Q-type VGCC antagonists rescues SGN neurite growth under depolarizing conditions. By measuring the fluorescence intensity of SGNs loaded with the fluorogenic calpain substrate t-butoxy carbonyl-Leu-Met-chloromethylaminocoumarin (20 μM), we demonstrate that depolarization activates calpains. Calpeptin (15 μM), a calpain inhibitor, prevents calpain activation by depolarization and rescues neurite growth in depolarized SGNs suggesting that calpain activation contributes to the inhibition of neurite growth by depolarization. Keywords auditory neuron; axon growth; Ca 2+ /calmodulin dependent kinase II

Kv7-type Channel Currents in Spiral Ganglion Neurons

“…Indeed, the effect of linopirdine plummeted significantly by reduced extracellular Ca 2ϩ and by application of Ca 2ϩ channel blockers in basal SGNs. However, the results are in keeping with a recent report demonstrating that SGN membrane depolarization may inhibit neurite outgrowth by way of activation of multiple voltage-dependent Ca 2ϩ channels and calpain (25). Our results imply that although a moderate rise in intracellular Ca 2ϩ may mediate activation of factors that promote neurite outgrowth and survival, a sustained increase in Ca 2ϩ may result in Ca 2ϩ -dependent cell death (29 -32).…”

Mechanisms of Calmodulin Regulation of Different Isoforms of Kv7.4 K+ Channels

“…Reduction and/or removal of the capacity to generate excitability will not only alter information coding but also reduce growth and survival of SGNs as well as survival of neurons in the cochlear nucleus (50,51). This, conceivably, may result in progressive degeneration of upstream neurons.…”

CaBP1 regulates Cav1 L-type Ca2+ channels and their coupling to neurite growth and gene transcription in mouse spiral ganglion neurons

“…Of the multiple classes of Ca v channels, Ca v 1 L-type channels are key mediators of Ca 2+ signals controlling neurogenesis (Marschallinger et al, 2015; Temme et al, 2016; Volkening et al, 2017), neurite growth (Audesirk et al, 1990; Robson and Burgoyne, 1989; Roehm et al, 2008; Schindelholz and Reber, 2000), and gene transcription (Dolmetsch et al, 2001; Graef et al, 1999; Oliveria et al, 2007). Mutations affecting the major Ca v 1 channels in the brain, Ca v 1.2 and Ca v 1.3, are linked to a variety of neurological and psychiatric disorders (Kabir et al, 2017; Pinggera and Striessnig, 2016).…”