Multiple Distinct Signal Pathways, Including an Autocrine Neurotrophic Mechanism, Contribute to the Survival-Promoting Effect of Depolarization on Spiral Ganglion NeuronsIn Vitro

Abstract

We have shown previously that BDNF, neurotrophin-3 (NT-3), chlorphenylthio-cAMP (cpt-cAMP) (a permeant cAMP analog), and membrane depolarization promote spiral ganglion neuron (SGN) survival in vitro in an additive manner, depolarization having the greatest efficacy. Expression of both BDNF and of NT-3 is detectable in cultured SGNs after plating in either depolarizing or nondepolarizing medium. These neurotrophins promote survival by an autocrine mechanism; TrkB-IgG or TrkC-IgG, which block neurotrophin binding to, respectively, TrkB and TrkC, partially inhibit the trophic effect of depolarization. The mitogen-activated protein kinase kinase inhibitor PD98059 and the phosphatidylinositol-3-OH kinase inhibitor LY294002 both abolish trophic support by neurotrophins but only partially inhibit support by depolarization. Inhibition by these compounds is not additive with inhibition by Trk-IgGs. The cAMP antagonist Rp-adenosine-3Ј,5Ј-cyclic-phosphorothioate (Rp-cAMPS) abolishes survival attributable to cptcAMP but has no effect on that attributable to neurotrophins, nor do inhibitors of neurotrophin-dependent survival affect survival attributable to cpt-cAMP. However, Rp-cAMPS does partially inhibit depolarization-dependent survival, an inhibition that is additive with that by Trk-IgGs, PD98059, or LY294002. Moreover, Rp-cAMPS prevents depolarization-dependent survival of PC12 cells maintained in subthreshold levels of NGF. Inhibition of Ca 2ϩ /calmodulin-dependent protein kinases (CaMKs) with KN-62 reduces SGN survival independently of Rp-cAMPS, Trk-IgGs, and LY294002 and additively with them. Combined inhibition of Trk, cAMP, and CaMK signaling prevents depolarization-dependent survival. Thus, survival of SGNs under depolarizing conditions involves additivity among a depolarization-independent autocrine pathway, a cAMPdependent pathway, and a CaMK-dependent pathway.

Complex Regulation of Spiral Ganglion Neuron Firing Patterns by Neurotrophin-3

“…Because NT-3 has been shown to promote survival of spiral ganglion neurons in vivo and in vitro (Avila et al, 1993;Mou et al, 1997;Hansen et al, 2001;Fritzsch et al, 2004), one mechanism could be that NT-3 selectively promotes the survival of a subpopulation of spiral ganglion neurons. Our laboratory has shown previously that putative type II spiral ganglion neurons differ in the base from their type I counterparts by demonstrating slow accommodation, longer latencies, and prolonged membrane time constants (Reid et al, 2004), similar to neurons exposed to the appropriate concentrations of NT-3 in the present study.…”

Enhanced bioavailability of nerve growth factor with phytantriol lipid-based crystalline nanoparticles in cochlea

“…Evidence suggests that electrical stimulation, such as the stimulation by cochlear implants, can lead to increased survival of SGCs after loss of hair cells; however, the effects of electrical stimulation alone were found to be less than the clinically acceptable level of survival for SGCs -even when stimulation was initiated immediately after deafening. 31,32 Some studies further indicate that electrical stimulation may attenuate the rapid degeneration of spiral ganglion neurons that follows withdrawal of neurotrophin therapy. For example, Maruyama et al demonstrated that SGCs survival was preserved after termination of intracochlear infusion with a glial cell line-derived neurotrophic factor in combination with the use of an electrical stimulation from a cochlear implant.…”

Stimulation of prostaglandin E2-EP3 receptors exacerbates stroke and excitotoxic injury

“…Cyclic AMP elicits a wide range of cellular functions; it is reported to be involved in neuronal survival, axonal regeneration, and enhancement of neurite outgrowth (Hansen et al, 2001;Kao et al, 2002;Rydel and Greene, 1988). Activation of the EP3 receptor has been shown to inhibit cAMP synthesis in murine platelets (Fabre et al, 2001).…”