Interaction of neurotrophin signaling with Bcl‐2 localized to the mitochondria and endoplasmic reticulum on spiral ganglion neuron survival and neurite growth

Abstract

Enhanced spiral ganglion neuron (SGN) survival and regeneration of peripheral axons following deafness will likely enhance the efficacy of cochlear implants. Overexpression of Bcl‐2 prevents SGN death but inhibits neurite growth. Here we assessed the consequences of Bcl‐2 targeted to either the mitochondria (GFP‐Bcl‐2‐Maob) or the endoplasmic reticulum (ER, GFP‐Bcl‐2‐Cb5) on cultured SGN survival and neurite growth. Transfection of wild‐type GFP‐Bcl‐2, GFP‐Bcl‐2‐Cb5, or GFP‐Bcl‐2‐Maob increased SGN survival, with GFP‐Bcl‐2‐Cb5 providing the most robust response. Paradoxically, expression of GFP‐Bcl‐2‐Maob results in SGN death in the presence of neurotrophin‐3 (NT‐3) and brain‐derived neurotrophic factor (BDNF), neurotrophins that independently promote SGN survival via Trk receptors. This loss of SGNs is associated with cleavage of caspase 3 and appears to be specific for neurotrophin signaling, insofar as coexpression of constitutively active mitogen‐activated kinase kinase (MEKΔEE) or phosphatidyl inositol‐3 kinase (P110), but not other prosurvival stimuli (e.g., membrane depolarization), also results in the loss of SGNs expressing GFP‐Bcl‐2‐Maob. MEKΔEE and P110 promote SGN survival, whereas P110 promotes neurite growth to a greater extent than NT‐3 or MEKΔEE. However, wild‐type GFP‐Bcl‐2, GFP‐Bcl‐2‐Cb5, and GFP‐Bcl‐2‐Maob inhibit neurite growth even in the presence of neurotrophins, MEKΔEE, or P110. Historically, Bcl‐2 has been thought to act primarily at the mitochondria to prevent neuronal apoptosis. Nevertheless, our data show that Bcl‐2 targeted to the ER is more effective at rescuing SGNs in the absence of trophic factors. Additionally, Bcl‐2 targeted to the mitochondria results in SGN death in the presence of neurotrophins.