It has been well known that when animals are treated with protein synthesis inhibitors, such as anisomycin, which stop the production of proteins in the animals' brains, these animals lose their long-term memory. This observation has led us to predict that the formation of long-term memory requires new protein synthesis. Furthermore, certain types of memories are dependent on the hippocampus for a short period of time following training, after which they are no longer susceptible to hippocampal manipulations. This process has been called ?systems consolidation?, a process by which memory is presumably transferred from hippocampus to cortex. However, recent studies have suggested that, after having completed the initial cellular consolidation process, a memory once again engages the hippocampus when recalled. This phenomenon is now called reconsolidation, which seems to be protein synthesis-dependent. Here a simple question arises as to whether memories reach a fixed stable state, or if they are subject to change every time they are activated. When the possibility is added that memories ?move? from one area of the brain to another after a certain period of time, we may call into question the conventional view of stable memory or its consolidation. The caveat of the research of this field is that most of the studies use chemicals like anisomycin to inhibit protein synthesis. Recent studies have demonstrated that a protein synthesis inhibitor induces mRNA expression, a process called super-induction. It can occur in one of the three ways, including (i) mRNA stabilization, (ii) activation of intracellular signaling pathways, or (iii) interference with transcriptional down-regulation. In addition, anisomycin has been shown to activate MAP kinase pathway in mammalian cells. In order to overcome these drawbacks, inducible genetic manipulation of protein synthesis knockdown in the live animals has been desired for the study of memory consolidation at both a cellular and systems level. Since last year, we have started a project to develop inducible genetic suppression of protein synthesis in the mouse brain. It is known that double-strand RNA-dependent protein kinase R (PKR) phosphorylates eIF2a, a key enzyme to initiate peptide elongation during protein translation process, and inhibit synthesis of most proteins in a cell. Taking the advantage of inducible dimerization of PKR domain upon a drug administration, that is required for kinase activation, Dr. Zhihong Jiang prepared a cDNA construct of loxP-LacZ-loxP-FKBP-PKR under the control of Cam Kinase II promoter. The transgenic core facility (Dr. Jim Pickel) injected the construct into mouse eggs to create its transgenic mice, and she has obtained several lines which shows high expression of LacZ in the mouse forebrain. Once these lines are crossed with a particular Cre-recombinase transgenic line, such as CA1-Cre line, we expect to create CA1-restricted genetic protein synthesis knockdown mice in an inducible manner. In a separate project, she is also trying to create hippocampal CA1-restricted Cre transgenic lines using BAC transgenic technology. The double conditional transgenic line inter-crossed between FKBP-PKR and CA1-Cre would be quite beneficial for the study of system consolidation of hippocampus-dependent memory. The key point in this project is whether protein synthesis inhibition is sufficient to take place in a brain cell where Cre recombinase is highly expressed upon the drug administration. The experiment to test this issue is now underway.
| Nakazawa, Kazu; McHugh, Thomas J; Wilson, Matthew A et al. (2004) NMDA receptors, place cells and hippocampal spatial memory. Nat Rev Neurosci 5:361-72 |
