Key result
The mouse TrkB mRNA contains two functionally independent internal ribosomal entry sites (IRESes) within its 5' leader that are differentially regulated by the polypyrimidine tract binding protein (PTB1).
The study identifies two functionally independent and differentially regulated internal ribosomal entry sites within the mouse TrkB 5' leader, highlighting complex translational control mechanisms.
No direct clinical implications; leaves open relevance of PTB1-regulated TrkB IRESes to human disease.
A single internal ribosomal entry site (IRES) in conjunction with IRES transactivating factors (ITAFs) is sufficient to recruit the translational machinery to a eukaryotic mRNA independent of the cap structure. However, we demonstrate that the mouse TrkB mRNA contains two independent IRESes. The mouse TrkB mRNA consists of one of two 5' leaders (1428 nt and 448 nt), both of which include the common 3' exon (Ex2, 344 nt). Dicistronic RNA transfections and in vitro translation of monocistronic RNA demonstrated that both full-length 5' leaders, as well as Ex2, exhibit IRES activity indicating the IRES is located within Ex2. Additional analysis of the upstream sequences demonstrated that the first 260 nt of exon 1 (Ex1a) also contains an IRES. Dicistronic RNA transfections into SH-SY5Y cells showed the Ex1a IRES is constitutively active. However, the Ex2 IRES is only active in response to retinoic acid induced neural differentiation, a state which correlates with the synthesis of the ITAF polypyrimidine tract binding protein (PTB1). Correspondingly, addition or knock-down of PTB1 altered Ex2, but not Ex1a IRES activity in vitro and ex vivo, respectively. These results demonstrate that the two functionally independent IRESes within the mouse TrkB 5' leader are differentially regulated, in part by PTB1.
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Timmerman et al. (2008) studied this question. Dicistronic RNA transfections and in vitro translation vs. Control constructs (e.g., beta-globin 5' leader) was evaluated on IRES activity (Photinus:Renilla luciferase ratio). The mouse TrkB mRNA contains two functionally independent internal ribosomal entry sites (IRESes) within its 5' leader that are differentially regulated by the polypyrimidine tract binding protein (PTB1).
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