N -methyl- d -aspartate receptor activation and visual activity induce elongation factor-2 phosphorylation in amphibian tecta: A role for N -methyl- d -aspartate receptors in controlling protein synthesis

Abstract

N -methyl- d -aspartate receptor (NMDAR) activation has been implicated in forms of synaptic plasticity involving long-term changes in neuronal structure, function, or protein expression. Transcriptional alterations have been correlated with NMDAR-mediated synaptic plasticity, but the problem of rapidly targeting new proteins to particular synapses is unsolved. One potential solution is synapse-specific protein translation, which is suggested by dendritic localization of numerous transcripts and subsynaptic polyribosomes. We report here a mechanism by which NMDAR activation at synapses may control this protein synthetic machinery. In intact tadpole tecta, NMDAR activation leads to phosphorylation of a subset of proteins, one of which we now identify as the eukaryotic translation elongation factor 2 (eEF2). Phosphorylation of eEF2 halts protein synthesis and may prepare cells to translate a new set of mRNAs. We show that NMDAR activation-induced eEF2 phosphorylation is widespread in tadpole tecta. In contrast, in adult tecta, where synaptic plasticity is reduced, this phosphorylation is restricted to short dendritic regions that process binocular information. Biochemical and anatomical evidence shows that this NMDAR activation-induced eEF2 phosphorylation is localized to subsynaptic sites. Moreover, eEF2 phosphorylation is induced by visual stimulation, and NMDAR blockade before stimulation eliminates this effect. Thus, NMDAR activation, which is known to mediate synaptic changes in the developing frog, could produce local postsynaptic alterations in protein synthesis by inducing eEF2 phosphorylation.

The target of rapamycin (TOR) proteins

“…For example, glutamate or NMDA treatment of cortical neurons in culture leads to a rapid and pronounced increase in eEF2 phosphorylation, and a decrease in translation rates in cell bodies and proximal (but not distal) cell processes (121). Activation of the NMDA receptor also leads to eEF2 phosphorylation, in tadpole tecta (122). It is tempting to speculate that mTOR could inhibit eEF2 phosphorylation in active synapses to locally derepress translation.…”

“…It is tempting to speculate that mTOR could inhibit eEF2 phosphorylation in active synapses to locally derepress translation. It has also been suggested that eEF2 phosphorylation could actually enhance the translation of specific mRNAs localized to dendrites by driving these mRNAs from untranslated ribonucleotide particles or small polysomes into larger polysomes (122)(123)(124)(125).…”

Protein synthesis in the dendrite

“…Other studies have indicated a possible inhibitory role of synaptic activity on translation. In adult frog tecta, activation of NMDA receptors and visual activity led to activation of calcium/calmodulindependent EF2 kinase, resulting in the phosphorylation of eEF2 at subsynaptic sites (Scheetz et al 1998). It is known that phosphorylation of eEF2 arrests translation elongation.…”

“…Recent studies showed that the eukaryotic translation elongation factor 2 (eEF2) is phosphorylated in response to N-methyl-D-aspartate (NMDA) receptor activation and visual activity in the intact tadpole tecta; phosphorylation of eEF2 occurs at the subsynaptic regions (Scheetz et al 1998). In eukaryotic cells, phosphorylation of eEF2 is associated with reduced protein synthesis, probably by halting ribosomal translocation (Narin & Palfrey 1996).…”

Metabotropic glutamate receptor-initiated translocation of protein kinase p90rsk to polyribosomes: A possible factor regulating synaptic protein synthesis

“…However, in neuronal tissue no receptor-mediated mechanism that up-regulates localized postsynaptic translation has been described. A receptor-mediated inhibition of protein synthesis in neurons has been described that involves N-methyl-D-aspartate-induced phosphorylation of eukaryotic elongation factor 2 (eEF-2) by activation of calcium͞calmodulin-dependent protein kinase III (EF-2 kinase) (21,22).…”