BDNF signaling contributes to protein-synthesis-dependent synaptic plasticity, but the dynamics of TrkB signaling and mechanisms of translation have not been defined. Here, we show that long-term potentiation (LTP) consolidation in the dentate gyrus of live rodents requires sustained (hours) BDNF-TrkB signaling. Surprisingly, this sustained activation maintains an otherwise labile signaling pathway from TrkB to MAP-kinase-interacting kinase (MNK). MNK activity promotes eIF4F translation initiation complex formation and protein synthesis in mechanistically distinct early and late stages. In early-stage translation, MNK triggers release of the CYFIP1/FMRP repressor complex from the 5'-mRNA cap. In late-stage translation, MNK regulates the canonical translational repressor 4E-BP2 in a synapse-compartment-specific manner. This late stage is coupled to MNK-dependent enhanced dendritic mRNA translation. We conclude that LTP consolidation in the dentate gyrus is mediated by sustained BDNF signaling to MNK and MNK-dependent regulation of translation in two functionally and mechanistically distinct stages.

AnxA2 participates in malignant cell transformation, and its overexpression and/or phosphorylation is associated with cancer progression and metastasis. Thus, tight regulation of AnxA2 function is an integral aspect of cellular homeostasis. The presence of AnxA2 in cancer cell-derived exosomes, as well as the potential regulation of exosomal AnxA2 by phosphorylation or other PTMs, are topics of great interest.

Messenger RNA trafficking, which provides a mechanism for local protein synthesis, is dependent on cis‐acting sequences in the 3′ untranslated regions (3′UTRs) of the mRNAs concerned acting together with trans‐acting proteins. The C‐MYC transcription factor is a proto‐oncogene product involved in cell proliferation, differentiation and apoptosis. Localization of c‐myc mRNA to the perinuclear cytoplasm and its association with the cytoskeleton is determined by a signal in the 3′UTR. Here we show the specific binding of a trans‐acting factor to the perinuclear localization element in the 3′UTR of c‐myc mRNA and identify this protein as annexin A2. Gel retardation and UV cross‐linking experiments showed that proteins in fibroblast extracts formed complexes with the region of c‐myc 3′UTR implicated in localization; a protein of ≈ 36 kDa exhibited specific, Ca2+‐dependent binding. Binding was reduced by introduction of a mutation that abrogates localization. Using RNA‐affinity columns followed by gel electrophoresis and mass spectrometry this protein was identified as annexin A2. The RNA–protein complex formed by cell extracts was further retarded by anti‐(annexin A2). Purified annexin A2 bound to the same region of the c‐myc 3′UTR but binding was reduced by introduction of a mutation, as with cell extracts. It is proposed that binding of annexin A2 to the localization signal in the c‐myc mRNA leads to association with the cytoskeleton and perinuclear localization. The data indicate a novel functional role for the RNA‐binding properties of annexin A2 in perinuclear localization of mRNA and the association with the cytoskeleton.

Annexin A2 is a multifunctional Ca(2+)- and lipid-binding protein. We previously showed that a distinct pool of cellular Annexin A2 associates with mRNP complexes or polysomes associated with the cytoskeleton. Here we report in vitro and in vivo experiments showing that Annexin A2 present in this subset of mRNP complexes interacts with its cognate mRNA and c-myc mRNA, but not with beta(2)-microglobulin mRNA translated on membrane-bound polysomes. The protein recognises sequence elements within the untranslated regions, but not within the coding region, of its cognate mRNA. Alignment of the Annexin A2-binding 3'-untranslated regions of annexin A2 mRNA from several species reveals a five nucleotide consensus sequence 5'-AA(C/G)(A/U)G. The Annexin A2-interacting region of the 3'-untranslated region can be mapped to a sequence of about 100 nucleotides containing two repeats of the consensus sequence. The binding elements appear to involve both single and double stranded regions, indicating that a specific higher order mRNA structure is required for binding to Annexin A2. We suggest that this type of interaction is representative for a group of mRNAs translated on cytoskeleton-bound polysomes.

Polysomes from Krebs II ascites and 3T3 cells were separated into three populations by using a sequential extraction method. Free polysomes were released by using a combination of low salt (25 mM KCl) and NP-40 detergent in the lysis buffer. The cytoskeletal bound polysomes were subsequently released by raising the salt concentration to 130 mM and finally, polysomes bound to the membranes of the endoplasmic reticulum were extracted by the combined treatment with Triton X-100 and deoxycholate. The results presented here illustrate that the three polysome-containing fractions differ in many parameters such as polysome profiles, cytoskeletal components and phospholipid content. When polyA-containing mRNA was isolated from the three polysome fractions and translated in an in vitro system, some differences were observed in the patterns of proteins being synthesized.