Internal initiation of translation of the TrkB mRNA is mediated by multiple regions within the 5′ leader

Translational regulation of the dendritically localized mRNA encoding for the neurotrophin receptor TrkB has important ramifications for synaptic function. We examined whether the TrkB mRNA is translated through an internal initiation entry site (IRES). The human TrkB 5' leaders are derived from the use of alternative promoters and alternative splicing, but all 5' leaders share a common exon. Insertion of a full-length 5' leader, as well as the common exon into the intercistronic region of a dicistronic luciferase construct, yielded luciferase activity generated from the second cistron that was either equivalent or higher than that observed from the encephalomyocarditis virus IRES. Moreover, inhibiting cap-dependent translation ex vivo and in in vitro lysates had only a minimal effect on the translation of mRNA containing the TrkB 5' leader. Dissecting the 5' leader showed that the IRES is located in the exon common to all TrkB 5' leaders. Moreover, six regions ranging from 2 to 25 nt were identified that either promoted or inhibited IRES activity. Taken together, these results suggest that the 5' leader of the human TrkB mRNA contains multiple cis-elements that regulate internal initiation of translation and that this mechanism may contribute significantly to the translation of the TrkB mRNA in neuronal dendrites.