eIF2α Phosphorylation Bidirectionally Regulates the Switch from Short- to Long-Term Synaptic Plasticity and Memory

SummaryThe late phase of long-term potentiation (LTP) and memory (LTM) requires new gene expression, but the molecular mechanisms that underlie these processes are not fully understood. Phosphorylation of eIF2α inhibits general translation but selectively stimulates translation of ATF4, a repressor of CREB-mediated late-LTP (L-LTP) and LTM. We used a pharmacogenetic bidirectional approach to examine the role of eIF2α phosphorylation in synaptic plasticity and behavioral learning. We show that in eIF2α+/S51A mice, in which eIF2α phosphorylation is reduced, the threshold for eliciting L-LTP in hippocampal slices is lowered, and memory is enhanced. In contrast, only early-LTP is evoked by repeated tetanic stimulation and LTM is impaired, when eIF2α phosphorylation is increased by injecting into the hippocampus a small molecule, Sal003, which prevents the dephosphorylation of eIF2α. These findings highlight the importance of a single phosphorylation site in eIF2α as a key regulator of L-LTP and LTM formation

Toward reconsolidation blockade as a novel treatment for PTSD

Rationale/statement of the problem : Animal research has challenged the permanency of memory by suggesting that reactivation (retrieval) of a specific memory may return it to a labile state from which it must be “re-consolidated” if it is to persist. Pharmacologically blocking reconsolidation offers the therapeutic possibility of weakening traumatic memories in post-traumatic stress disorder (PTSD). Methods : We have been testing the above hypothesis using systemic drugs approved for human use. In rats we employ classical conditioning consisting of pairing a tone CS with a shock US on Day 1 (acquisition), presenting the tone without shock on Day 2 (reactivation) followed by drug, and then re-presenting the CS alone on Days 3 and 10 (tests). We have also used slice electrophysiology to measure the increase in cortico- and thalamo-lateral amygdala synaptic efficacy as a result of the tone-shock association, and then decrement in this efficacy following reconsolidation blockade. In PTSD subjects, we have administered oral drug along with verbal or written narration of the traumatic event (reactivation) and subsequently measured the strength of the traumatic memory via psychophysiological recording during script-driven imagery, and/or symptom report. Results : In animal work, we have found that the antiglucocorticoid receptor mifepristone, the protein synthesis inhibitor rapamycin, and the alpha-2-adrenergic autoreceptor agonist clonidine all block partially reconsolidation of conditioned fear. Clonidine does so in a dose-dependent manner. Additionally, rapamycin reverses the synaptic enhancement described above, providing an underlying physiological basis for reconsolidation blockade. In humans, we have found that traumatic memory reactivation plus double-blind, placebo-controlled propranolol reduce physiologic responding during script-driven imagery, and that six weekly open-label propranolol plus memory reactivation sessions reduce PTSD symptoms to a similar degree as current cognitive behavioral treatments. A double-blind clinical trial is underway. Conclusion : The above results show progress toward a clinical application of reconsolidation blockade, but much more needs to be done before efficacy is demonstrated

The X-linked inhibitor of apoptosis regulates long-term depression and learning rate

Hippocampal long-term depression (LTD) is an active form of synaptic plasticity that is necessary for consolidation of spatialmemory, contextual fearmemory, and novelty acquisition. Recent studies have shown that caspases (CASPs) play an important role in NMDA receptor-dependent LTD and are involved in postsynaptic remodeling and synaptic maturation. In the present study, we examined the role of X-linked inhibitor of apoptosis (XIAP), a putative endogenous CASP inhibitor, in synaptic plasticity in the hippocampus. Analysis in acute brain slices and in cultured hippocampal neurons revealed that XIAP deletion increases CASP-3 activity, enhances α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor internalization, sharply increases LTD, and significantly reduces synapse density. In vivo behaviors related to memorywere also altered in XIAP-/- mice, with faster acquisition of spatial object location and increased fear memory observed. Together, these results indicate that XIAP plays an important physiologic role in regulating sublethal CASP-3 activity within central neurons and thereby facilitates synaptic plasticity and memory acquisition.Fil: Gibon, Julien. McGill University. Montreal Neurological Institute and Hospital; CanadáFil: Unsain, Nicolas. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Córdoba. Instituto de Investigación Médica Mercedes y Martín Ferreyra. Universidad Nacional de Córdoba. Instituto de Investigación Médica Mercedes y Martín Ferreyra; ArgentinaFil: Gamache, Karine. McGill University; CanadáFil: Thomas, Rhalena A.. McGill University. Montreal Neurological Institute and Hospital; CanadáFil: De Leon, Andres. McGill University. Montreal Neurological Institute and Hospital; CanadáFil: Johnstone, Aaron. McGill University. Montreal Neurological Institute and Hospital; CanadáFil: Nader, Karim. McGill University; CanadáFil: Séguéla, Philippe. McGill University. Montreal Neurological Institute and Hospital; CanadáFil: Barker, Philip A.. McGill University. Montreal Neurological Institute and Hospital; Canad