5‐HT stimulates eEF2 dephosphorylation in a rapamycin‐sensitive manner in Aplysia neurites

Abstract

In Aplysia, serotonin mediates behavioral sensitization by increasing the strength of the synapse between sensory and motor neurons, a process known as facilitation. The retention of long-term facilitation is blocked by rapamycin, an inhibitor of a specific translational pathway. One possible rapamycinsensitive target is the increased translation of 5¢-terminal oligopyrimidine mRNAs. These transcripts encode components of the translational machinery and have been proposed to be important for retention of long-term facilitation. We have cloned the 5¢-terminal oligopyrimidine mRNA encoding eukaryotic elongation factor 2 and shown that serotonin increased its translation in synaptosomes. Another possible rapamycinsensitive target is the inactivation of eukaryotic elongation factor 2 kinase. Eukaryotic elongation factor 2 kinase phosphorylates and inactivates eukaryotic elongation factor 2, blocking translational elongation. Serotonin application decreased eukaryotic elongation factor 2 phosphorylation in synaptosomes and in isolated neurites, and this was blocked by rapamycin. We propose a role for the rapamycin-sensitive pathway in neurons. Stimulation blocks translation by inducing calcium entry and phosphorylation of eukaryotic elongation factor 2. This block is reversed through activation of the rapamycin-sensitive system and dephosphorylation of eukaryotic elongation factor 2. Keywords: Aplysia, eukaryotic elongation factor 2, rapamycin, S6 kinase, synaptic plasticity, translation. The marine mollusk, Aplysia californica, is a model system for the study of learning and memory. The memory for sensitization of the gill-and siphon-withdrawal reflexes in Aplysia is manifested on a cellular level by facilitation of synaptic strength at identified sensory-to-motor neuron synapses (Bailey and Chen 1983;Bailey et al. 1996). Experiments on cultured Aplysia sensory-motor neuron synapses have revealed the existence of a persistent phase of long-term facilitation (LTF) whose induction requires local translation. While LTF induced by somatic treatment with serotonin (5-hydroxytryptamine, 5-HT) lasts only 24 h, when 5-HT is also added to the synapse, facilitation persists for 72 h. A 5 min application of 5-HT to the synapse was sufficient to activate local translation and to allow for persistent LTF when gene expression was activated independently (Casadio et al. 1999). This persistent phase of LTF is blocked if the specific translational inhibitor, rapamycin, is applied locally to the synapse (Casadio et al. 1999). Addition of 5-HT to Aplysia sensory cell neurites or pleural ganglia has been shown to cause an increase in the rate of translation that is sensitive to rapamycin (Yanow et al. 1998;Casadio et al. 1999), suggesting that a local rapamycin-sensitive up-regulation of protein synthesis may be specifically required for the retention of plasticity. Recently, a rapamycin-sensitive increase in the levels of the cytoplasmic polyadenylation element binding protein (CPEB) was shown to be an important component of t...

A recollection of mTOR signaling in learning and memory

“…Specifically, Late-LTP and hippocampus-dependent long-term memory are blocked in mice by forcing eEF2 phosphorylation through overexpression of eEF2K (Im et al 2009). S6K has also been linked to decreases in eEF2 phosphorylation after persistent activation in Aplysia neurites (Carroll et al 2004;Weatherill et al 2010) and after BDNF application in hippocampal cultures (Inamura et al 2005). However, the situation is probably more complicated.…”

Consolidation and translation regulation: Figure 1.

“…In Aplysia-dissociated sensory neurons, application of 5-HT results in a rapid decrease in eEF2 phosphorylation (p-eEF2) in a rapamycin-sensitive manner in neurites but an increase in p-eEF2 at the soma (Carroll et al 2004;Weatherill et al 2011). In contrast, chronic administration of the selective serotonin reuptake inhibitor, fluoxetine, induces eEF2 and eIF4E phosphorylation in the prefrontal cortex (PFC), hippocampus, and dentate gyrus in rats .…”

Regulation of global and specific mRNA translation by the mTOR signaling pathway

“…78 Notably, a number of studies have documented an elevation in the levels of elongation factors eEF1A and eEF2, as well as ribosomal protein S6, in response to synaptic activity in both mammals and Aplysia. [83][84][85][86][87][88] All of these proteins are encoded by TOP mRNAs, 64 suggesting that mTORC1 may participate in synaptic plasticity by locally increasing the availability of translation factors and other components of the translational apparatus. 78 Evidence for this comes from the observation that TOP mRNAs are transported into dendrites and axons where they are locally translated in an mTORC1-dependent manner.…”