Distinct roles ofBdnf IandBdnf IVtranscript variant expression in hippocampal neurons

Abstract

SummaryBrain-derived neurotrophic factor (Bdnf) plays a critical role in brain development, dendritic growth, synaptic plasticity, as well as learning and memory. The rodentBdnfgene contains nine 5′ non-coding exons (I-IXa), which are spliced to a common 3′ coding exon (IX). Transcription of individualBdnfvariants, which all encode the same BDNF protein, is initiated at unique promoters upstream of each non-coding exon, enabling precise spatiotemporal and activity-dependent regulation ofBdnfexpression. Although prior evidence suggests thatBdnftranscripts containing exonI(Bdnf I) or exonIV(Bdnf IV) are uniquely regulated by neuronal activity, the functional significance of differentBdnftranscript variants remains unclear. To investigate functional roles of activity-dependentBdnf IandIVtranscripts, we used a CRISPR activation (CRISPRa) system in which catalytically-dead Cas9 (dCas9) fused to a transcriptional activator (VPR) is targeted to individualBdnfpromoters with single guide RNAs (sgRNAs), resulting in transcript-specificBdnfupregulation.Bdnf Iupregulation is associated with gene expression changes linked to dendritic growth, whileBdnf IVupregulation is associated with genes that regulate protein catabolism. Upregulation ofBdnf I, but notBdnf IV, increased mushroom spine density, volume, length, and head diameter, and also produced more complex dendritic arbors in cultured rat hippocampal neurons. In contrast, upregulation ofBdnf IV, but notBdnf I, in the rat hippocampus attenuated contextual fear expression. Our data suggest that whileBdnf IandIVare both activity-dependent, BDNF produced from these promoters may serve unique cellular, synaptic, and behavioral functions.