Internal initiation of translation of five dendritically localized neuronal mRNAs

In neurons, translation of dendritically localized mRNAs is thought to play a role in affecting synaptic efficacy. Inasmuch as components of the trans lation machinery may be limiting in dendrites, we investigated the mechanis ms by which translation of five dendritically localized mRNAs is initiated. The 5' leader sequences of mRNAs encoding the activity-regulated cytoskele tal protein, the alpha subunit of calcium-calmodulin-dependent kinase II, d endrin, the microtubule-associated protein 2, and neurogranin (RC3) were ev aluated for their ability to affect translation in the 5' untranslated regi on of a monocistronic reporter mRNA, In both neural and nonneural cell line s, the activity-regulated cytoskeletal protein, microtubule-associated prot ein 2, and alpha -CaM Kinase II leader sequences enhanced translation, wher eas the dendrin and RC3 5' untranslated regions slightly inhibited translat ion as compared with controls, When cap-dependent translation of these cons tructs was suppressed by overexpression of a protein that binds the cap-bin ding protein elF4E, it was revealed that translation of these mRNAs had bot h cap-dependent and cap-independent components. The cap-independent compone nt was further analyzed by inserting the 5' leader sequences into the inter cistronic region of dicistronic mRNAs, All five leader sequences mediated i nternal initiation via internal ribosome entry sites (IRESes). The RC3 IRES was most active and was further characterized after transfection in primar y neurons. Although translation mediated by this IRES occurred throughout t he cell, it was relatively more efficient in dendrites. These data suggest that IRESes may increase translation efficiency at postsynaptic sites after synaptic activation.