The eIF2α kinase PERK limits the expression of hippocampal metabotropic glutamate receptor-dependent long-term depression.

The proper regulation of translation is required for the expression of long-lasting synaptic plasticity. A major site of translational control involves the phosphorylation of eukaryotic initiation factor 2 α (eIF2α) by PKR-like endoplasmic reticulum (ER) kinase (PERK). To determine the role of PERK in hippocampal synaptic plasticity, we used the Cre-lox expression system to selectively disrupt PERK expression in the adult mouse forebrain. Here, we demonstrate that in hippocampal area CA1, metabotropic glutamate receptor (mGluR)-dependent long-term depression (LTD) is associated with increased eIF2α phosphorylation, whereas stimulation of early- and late-phase long-term potentiation (E-LTP and L-LTP, respectively) is associated with decreased eIF2α phosphorylation. Interesting, although PERK-deficient mice exhibit exaggerated mGluR-LTD, both E-LTP and L-LTP remained intact. We also found that mGluR-LTD is associated with a PERK-dependent increase in eIF2α phosphorylation. Our findings are consistent with the notion that eIF2α phosphorylation is a key site for the bidirectional control of persistent forms of synaptic LTP and LTD and suggest a distinct role for PERK in mGluR-LTD.

Translational control by eIF2α kinases in long-lasting synaptic plasticity and long-term memory.

Although the requirement for new protein synthesis in synaptic plasticity and memory has been well established, recent genetic, molecular, electrophysiological, and pharmacological studies have broadened our understanding of the translational control mechanisms that are involved in these processes. One of the critical translational control points mediating general and gene-specific translation depends on the phosphorylation of eukaryotic initiation factor 2 alpha (eIF2α) by four regulatory kinases. Here, we review the literature highlighting the important role for proper translational control via regulation of eIF2α phosphorylation by its kinases in long-lasting synaptic plasticity and long-term memory.

Suppression of eIF2α kinases alleviates Alzheimer's disease-related plasticity and memory deficits.

Expression of long-lasting synaptic plasticity and long-term memory requires protein synthesis, which can be repressed by phosphorylation of eukaryotic initiation factor 2 α-subunit (eIF2α). Elevated phosphorylation of eIF2α has been observed in the brains of Alzheimer's disease patients and Alzheimer's disease model mice. Therefore, we tested whether suppressing eIF2α kinases could alleviate synaptic plasticity and memory deficits in Alzheimer's disease model mice. Genetic deletion of eIF2α kinase PERK prevented enhanced phosphorylation of eIF2α and deficits in protein synthesis, synaptic plasticity and spatial memory in mice that express familial Alzheimer's disease-related mutations in APP and PSEN1. Similarly, deletion of another eIF2α kinase, GCN2, prevented impairments of synaptic plasticity and defects in spatial memory exhibited by the Alzheimer's disease model mice. Our findings implicate aberrant eIF2α phosphorylation as a previously unidentified molecular mechanism underlying Alzheimer's disease-related synaptic pathophysioloy and memory dysfunction and suggest that PERK and GCN2 are potential therapeutic targets for treatment of individuals with Alzheimer's disease.

Brain-specific disruption of the eIF2α kinase PERK decreases ATF4 expression and impairs behavioral flexibility.

Translational control depends on phosphorylation of eIF2α by PKR-like ER kinase (PERK). To examine the role of PERK in cognitive function, we selectively disrupted PERK expression in the adult mouse forebrain. In the prefrontal cortex (PFC) of PERK-deficient mice, eIF2α phosphorylation and ATF4 expression were diminished and were associated with enhanced behavioral perseveration, decreased prepulse inhibition, reduced fear extinction, and impaired behavioral flexibility. Treatment with the glycine transporter inhibitor SSR504734 normalized eIF2α phosphorylation, ATF4 expression, and behavioral flexibility in PERK-deficient mice. Moreover, the expression levels of PERK and ATF4 were reduced in the frontal cortex of human patients with schizophrenia. Together, our findings reveal that PERK plays a critical role in information processing and cognitive function and that modulation of eIF2α phosphorylation and ATF4 expression may represent an effective strategy for treating behavioral inflexibility associated with several neurological disorders such as schizophrenia.

Transgenic brain-derived neurotrophic factor expression causes both anxiogenic and antidepressant effects.

Although neurotrophins have been postulated to have antidepressant properties, their effect on anxiety is not clear. We find that transgenic overexpression of the neurotrophin BDNF has an unexpected facilitatory effect on anxiety-like behavior, concomitant with increased spinogenesis in the basolateral amygdala. Moreover, anxiogenesis and amygdalar spinogenesis are also triggered by chronic stress in control mice but are occluded by BDNF overexpression, thereby suggesting a role for BDNF signaling in stress-induced plasticity in the amygdala. BDNF overexpression also causes antidepressant effects, because transgenic mice exhibit improved performance on the Porsolt forced-swim test and an absence of chronic stress-induced hippocampal atrophy. Thus, structural changes in the amygdala and hippocampus, caused by genetic manipulation of the same molecule BDNF, give rise to contrasting effects on anxiety and depressive symptoms, both of which are major behavioral correlates of stress disorders.

Serum response factor, an enriched cardiac mesoderm obligatory factor, is a downstream gene target for Tbx genes.

We tested the idea that T-box factors direct serum response factor (SRF) gene activity early in development. Analysis of SRF-LacZ "knock-in" mice showed highly restricted expression in early embryonic cardiac and skeletal muscle mesoderm and neuroectoderm. Examination of the SRF gene for regulatory regions by linking the promoter and 5'-flanking sequences, up to 5.5 kb, failed to target LacZ transgene activity to the heart and the tail pre-somitic mesenchyme. However, linkage of a minimal SRF promoter with the SRF 3'-untranslated region (UTR), inundated with multimeric T-box binding sites (TBEs), restored robust reporter gene activity to embryonic heart and tail. Finer dissection of the 3'-UTR to a small cluster of TBEs also stimulated transgene activity in the cardiac forming region and the tail, however, when the TBEs contained within these DNA sequences were mutated, preventing Tbx binding, transgene activity was lost. Tbx2, Tbx5, and the cardiac-enriched MYST family histone acetyltransferase TIP60, were observed to be mutual interactive cofactors through the TIP60 zinc finger and the T-box of the Tbx factors. In SRF-null ES cells, TIP60, Tbx2, and Tbx5 were sufficient to stimulate co-transfected SRF reporter activity, however this activity required the presence of the SRF 3'-UTR. SRF gene transactivation was blocked by two distinct TIP60 mutants, in which either the histone acetyltransferase domain was inactivated or the Zn finger-protein binding domain was excised. Our study supports the idea that SRF embryonic cardiac gene expression is dependent upon the SRF 3'-UTR enhancer, Tbx2, Tbx5, and TIP60 histone acetyltransferase activity.

Regulation of histone acetylation during memory formation in the hippocampus.

Formation of long term memory begins with the activation of many disparate signaling pathways that ultimately impinge on the cellular mechanisms regulating gene expression. We investigated whether mechanisms regulating chromatin structure were activated during the early stages of long term memory formation in the hippocampus. Specifically, we investigated hippocampal histone acetylation during the initial stages of consolidation of long term association memories in a contextual fear conditioning paradigm. Acetylation of histone H3 in area CA1 of the hippocampus was regulated in contextual fear conditioning, an effect dependent on activation of N-methyl-D-aspartic acid (NMDA) receptors and ERK, and blocked using a behavioral latent inhibition paradigm. Activation of NMDA receptors in area CA1 in vitro increased acetylation of histone H3, and this effect was blocked by inhibition of ERK signaling. Moreover, activation of ERK in area CA1 in vitro through either the protein kinase C or protein kinase A pathways, biochemical events known to be involved in long term memory formation, also increased histone H3 acetylation. Furthermore, we observed that elevating levels of histone acetylation through the use of the histone deacetylase inhibitors trichostatin A or sodium butyrate enhanced induction of long term potentiation at Schaffer-collateral synapses in area CA1 of the hippocampus, a candidate mechanism contributing to long term memory formation in vivo. In concert with our findings in vitro, injection of animals with sodium butyrate prior to contextual fear conditioning enhanced formation of long term memory. These results indicate that histone-associated heterochromatin undergoes changes in structure during the formation of long term memory. Mimicking memory-associated changes in heterochromatin enhances a cellular process thought to underlie long term memory formation, hippocampal long term potentiation, and memory formation itself.