Among several molecular processes during the translation of mRNA to protein, mRNA translation initiation is known to be a rate-limiting step of protein synthesis. Importantly, translation initiation is generally regulated by the protein phosphorylation of eukaryotic initiation factors (eIFs) or their regulators. We have focused on a particular phosphorylation event, reversible phosphorylation of the subunit of eIF2, which is the best characterized translational mechanism. Recently, it has been reported that reduced phosphorylation of eIF2 is associated with a decrease in translation of a transcription factor, ATF4. This effect has further been associated with increased synaptic plasticity and enhanced memory, suggesting that proper regulation of eIF2 phosphorylation is critical for establishing long-lasting synaptic changes and memory formation. To investigate the effect of eIF2 phosphorylation on synaptic plasticity and memory formation in vivo, we generated inducible transgenic mice, in which a protein kinase that phosphorylates eIF2, namely double-strand RNA dependent protein kinase (PKR), is overexpressed predominantly in the hippocampal CA1 pyramidal cells and activated upon drug administration. Thus, we expected that two mode of protein synthesis inhibition possible take place;one is the increased eIF2 phosphorylation which represses general or global translation, and another is, concomitantly, the increased eIF2 phosphorylation which results in gene-specific translation suppression by the increase in ATF4, because ATF4 is a potent inhibitors of CRE (cAMP-responsive element)-dependent gene transcription. The cellular and behavioral analysis of this transgenic line was designed to allow us to directly evaluate the de novo protein synthesis hypothesis for long-term memory formation and memory consolidation. We first confirmed using our DNA construct that activation of PKR kinase activity is drug-induced by applying a chemical inducer, AP20187, in hippocampal cell culture. Next, we tested the effect of PKR activation in the mouse brain on the plasticity and memory consolidation by using inducible transgenic mice, in which PKR is overexpressed predominantly in the hippocampal CA1 pyramidal cells and activated upon drug administration. We confirmed that eIF2 phosphorylation and subsequent ATF4 translation is induced upon drug administration predominantly in hippocampal CA1 pyramidal cells, a major output cell of hippocampal memory to cortical/subcortical areas. Consequently, we observed a decrease in CRE-dependent gene transcription. Remarkably, the transgenic mice exhibited a deficit in synaptic plasticity and hippocampal-dependent memory consolidation following drug administration. This result suggests that gene-specific translation, including ATF4 and its downstream molecular pathway, in CA1 pyramidal cells is critical for long-term memory formation and plasticity. Importantly, although we observed significant behavioral deficits, overall levels of protein synthesis in CA1 were not changed after PKR activation. Conversely, inhibition of general translation by low-dose anisomycin failed to block hippocampal-dependent memory consolidation. Taken together, these results indicated that CA1-restricted genetic manipulation of particular mRNA translations is sufficient to impair the consolidation and that consolidation of memories in CA1 pyramidal cells through eIF2 dephosphorylation depends more on transcription/translation of particular genes rather than overall levels of general translation. The present study sheds light on the critical importance of gene-specific translations for hippocampal memory consolidation.
| Jiang, Zhihong; Belforte, Juan E; Lu, Yuan et al. (2010) eIF2alpha Phosphorylation-dependent translation in CA1 pyramidal cells impairs hippocampal memory consolidation without affecting general translation. J Neurosci 30:2582-94 |
| Je, Hyun-Soo; Lu, Yuan; Yang, Feng et al. (2009) Chemically inducible inactivation of protein synthesis in genetically targeted neurons. J Neurosci 29:6761-6 |
