Salicylate alters the expression of calcium response transcription factor 1 in the cochlea:: Implications for brain-derived neurotrophic factor transcriptional regulation

Brain-derived neurotrophic factor (BDNF) is a key neurotrophin whose expression is altered in response to neurological activity, influencing both short- and long-term synaptic changes. The BDNF gene consists of eight upstream exons (I-VII), each of which has a distinct promoter and can be independently spliced to the ninth coding exon (IX). We showed recently that the expression of BDNF exon IV in the cochlea is altered after exposure to salicylate, an ototoxic drug that in high doses is able to induce hearing loss and tinnitus. These changes were a crucial trigger for plasticity changes in the central auditory system. BDNF exon IV expression is regulated via interaction between calcium-response elements CaRE1, CaRE2, and CaRE3/Cre (CaREs) that are bound by the transcription factors CaRF1, upstream stimulatory factors 1 and 2 (USF1/2), and cAMP/Ca2+ response element-binding protein (CREB), respectively. To determine whether the salicylate-induced changes in cochlear BDNF exon IV expression include a differential use of the CaRE binding proteins, we studied the level of the corresponding binding proteins in the spiral ganglion neurons before and after systemic application of concentrated salicylate using in situ hybridization and RT-PCR. BDNF exon IV and CaRF1 expression were up-regulated after application of salicylate, whereas USF1/2 and CREB mRNA expression remained unaffected. The changes in BDNF exon IV and CaRF1 expression were also dose-dependent. The data show Ca2+ and CaRF1 as messengers of trauma (salicylate)-induced altered BDNF levels in the cochlea. Furthermore, they also provide the first evidence that a differential regulation of BDNF transcription factors might participate in BDNF-mediated plasticity changes.