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

Abstract

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.

Dendritic BC1 RNA in translational control mechanisms

“…The former by definition applies only to mRNAs bearing CPEs, whereas the latter controls eIF4-PABP-mediated translation, including cap-dependent translation and IRES-mediated translation using EMCV-like mechanisms. Several dendritic mRNAs have been reported to harbor IRES elements, indicating that they may be locally translated in a 5Ј end-independent manner (Pinkstaff et al, 2001;Dobson et al, 2005). Internal ribosome entry would provide a means for independent regulation of translation initiation in dendrites, differentiating it from the mostly capdependent mode in somatic translation.…”

Alternatively spliced isoforms of the human elk-1 mRNA within the 5′ UTR: implications for ELK-1 expression

“…during conditions of cellular stress or stages of the cell cycle). Alternative splicing coupled with the regulation of IRES activity has been reported for the mRNA encoding the neurotrophin receptor TrkB (50). However, although both the elk-1 mRNA 5′ UTR L and 5′ UTR S gave weak but positive read-outs in the bicistronic assay, this was readily attributable to cryptic promoter activity from the transfected DNA plasmids.…”

Human TrkB gene: novel alternative transcripts, protein isoforms and expression pattern in the prefrontal cerebral cortex during postnatal development

“…2002). It has been shown that these five exons form an internal ribosome entry site (IRES) guiding the ribosome to the translational start site which is located in exon 5 (Dobson et al. 2005).…”