title: BDNF synthesis in spiral ganglion neurons is constitutive and CREB-dependent.
abstract: Brain-derived neurotrophic factor (BDNF), which supports spiral ganglion neuron (SGN) survival in vivo and in vitro, is synthesized by SGNs. The BDNF gene generates multiple different transcripts, each from its own promoter region. Using reverse transcriptase-polymerase chain reaction (RT-PCR), we find that SGNs express only the downstream transcripts III and IV in vivo and in vitro. Using RT-PCR assays of BDNF transcripts and transfection of BDNF promoter-reporter constructs, we tested the hypothesis, originally derived from studies of cortical neurons, that depolarization induces BDNF expression via a signaling pathway that includes Ca2+/calmodulin-dependent kinases (CaMKs) and the transcription factor, Ca2+/cyclic AMP response element binding protein (CREB). In contrast, we found that in SGNs in vivo BDNF expression is constitutive and is not increased by electrical activation. Similarly, BDNF expression in vitro is not increased by stimuli that activate CREB, including depolarization, cAMP, or transfection of activated CaMK mutants. However, transfection of dominant-negative CREB mutants did abrogate gene expression driven by BDNF promoters III and IV, indicating that CREB is necessary for constitutive BDNF expression. Thus, BDNF synthesis within SGNs makes possible an autocrine or paracrine mechanism that can contribute to support SGN survival but SGNs are distinctive in that this mechanism is constitutive and not activity-regulated.
authors.affiliation: Department of Biological Sciences, University of Iowa, Iowa City 52242-1324, USA.
authors.forename: X M
authors.initials: XM
authors.lastname: Zha
authors.forename: J F
authors.initials: JF
authors.lastname: Bishop
authors.forename: M R
authors.initials: MR
authors.lastname: Hansen
authors.forename: L
authors.initials: L
authors.lastname: Victoria
authors.forename: P J
authors.initials: PJ
authors.lastname: Abbas
authors.forename: M M
authors.initials: MM
authors.lastname: Mouradian
authors.forename: S H
authors.initials: SH
authors.lastname: Green
mesh_terms: D000818:Animals; D019208:Brain-Derived Neurotrophic Factor / Q000096:biosynthesis* / Q000235:genetics; D017871:Calcium-Calmodulin-Dependent Protein Kinases / Q000502:physiology; D002478:Cells, Cultured; D000242:Cyclic AMP / Q000502:physiology; D017362:Cyclic AMP Response Element-Binding Protein / Q000502:physiology*; D004594:Electrophysiology; D009474:Neurons / Q000378:metabolism* / Q000502:physiology; D011401:Promoter Regions, Genetic / Q000502:physiology; D051381:Rats; D013136:Spiral Ganglion / Q000166:cytology / Q000378:metabolism* / Q000502:physiology; D015854:Up-Regulation
chemical_list: D019208:Brain-Derived Neurotrophic Factor; D017362:Cyclic AMP Response Element-Binding Protein; D000242:Cyclic AMP; D017871:Calcium-Calmodulin-Dependent Protein Kinases