mGluR-Dependent Long-Term Depression Is Associated with Increased Phosphorylation of S6 and Synthesis of Elongation Factor 1A but Remains Expressed in S6K-Deficient Mice

Abstract

Metabotropic glutamate receptor-dependent long-term depression (mGluR-LTD) in the hippocampus requires rapid protein synthesis, which suggests that mGluR activation is coupled to signaling pathways that regulate translation. Herein, we have investigated the signaling pathways that couple group I mGluRs to ribosomal S6 protein phosphorylation and 5oligopyrimidine tract (5TOP)-encoded protein synthesis during mGluR-LTD. We found that mGluR-LTD was associated with increased phosphorylation of p70S6 kinase (S6K1) and S6, as well as the synthesis of the 5TOP-encoded protein elongation factor 1A (EF1A). Moreover, we found that LTD-associated increases in S6K1 phosphorylation, S6 phosphorylation, and levels of EF1A were sensitive to inhibitors of phosphoinositide 3-kinase (PI3K), mammalian target of rapamycin (mTOR), and extracellular signal-regulated kinase (ERK). However, mGluR-LTD was normal in S6K1 knockout mice and enhanced in both S6K2 knockout mice and S6K1/S6K2 double knockout mice. In addition, we observed that LTD-associated increases in S6 phosphorylation were still increased in S6K1-and S6K2-deficient mice, whereas basal levels of EF1A were abnormally elevated. Taken together, these findings indicate that mGluR-LTD is associated with PI3K-, mTOR-, and ERK-dependent alterations in the phosphorylation of S6 and S6K. Our data also suggest that S6Ks are not required for the expression of mGluR-LTD and that the synthesis of 5TOP-encoded proteins is independent of S6Ks during mGluR-LTD.The activation of group I metabotropic glutamate receptors (mGluRs) is intimately involved in the regulation of protein synthesis in neurons. Brief application of the mGluR I agonist [3,5-RS] dihydroxyphenylhydrazine (DHPG) has been reported to enhance de novo protein synthesis in hippocampal slices (37) and synaptoneurosomes (12), alter mRNA granule distribution (3), enhance polyribosome formation in synaptoneurosomes (12, 52), and promote rapid dendritic protein synthesis during mGluRdependent long-term depression (mGluR-LTD) (19). These findings strongly suggest that mGluR-mediated signaling is directly coupled to the regulation of translation initiation in neurons.Multiple signaling pathways critical for regulating protein synthesis have been reported to be required for mGluR-LTD. Both mammalian target of rapamycin (mTOR) and extracellular signal-regulated kinase (ERK) are activated during mGluR-LTD, and inhibitors of these kinases, as well as inhibitors of phosphoinositide-3-kinase (PI3K), have been shown to block mGluR-LTD (4, 9, 16). Substrates of mTOR and ERK that are known to be involved in translational control during mGluR-LTD include eukaryotic initiation factor 4E and its repressor 4E-binding protein (4). S6 kinases also are known to integrate mTOR, PI3K, and ERK signaling to influence protein synthesis; however, the requirement of S6 kinases for the expression of mGluR-LTD has not been examined.In mammals, S6 kinases are encoded by two separate genes, S6K1 and S6K2, which are approximately 80% homologous (8). One of t...

Synaptic activation of ribosomal protein S6 phosphorylation occurs locally in activated dendritic domains

“…Previous studies have reported increases in 35S-methionine incorporation (Kelleher et al 2004b), induced expression of 5TOP mRNAs (Tsokas 2005;Tsokas et al 2007;Gobert et al 2008), and increases in protein synthesis (Kelleher et al 2004b;Klann and Dever 2004;Antion et al 2008a) after LTP induction. One possible explanation is that mechanisms of perforant path LTP are different than forms of LTP studies in hippocampal slices in vitro.…”

Translational Control Mechanisms in Long-lasting Synaptic Plasticity and Memory

“…In contrast, in Aplysia neurons, S6K, but not 4E-BP, appears to be the major effector of mTORC1 for long-term facilitation (44). In mammals, S6K appears to be involved mainly in the regulation of long-lasting decreases in synaptic strength because metabotropic glutamate receptor-induced LTD is enhanced in slices from S6K2 Ϫ/Ϫ mice (but not in those from S6K1 Ϫ/Ϫ mice) (42). Lack of S6K1 leads to a variety of complex behavioral phenotypes, which include hypoactive behavior, impaired short-term fear memory, deficient conditioned taste aversion and spatial memory and decreased contextual fear memory 7 days (but not 1 day) after training, and reduced latent inhibition of conditioned taste aversion and normal spatial memory as determined by the Morris water maze (43).…”

“…Ϫ/Ϫ mice display impaired protein synthesis-independent E-LTP but normal protein synthesis-dependent L-LTP (42,43), demonstrating that S6K does not control the translation of mRNAs that underlie L-LTP. In contrast, in Aplysia neurons, S6K, but not 4E-BP, appears to be the major effector of mTORC1 for long-term facilitation (44).…”

Calcium/Calmodulin-Dependent Protein Kinase II Mediates Group I Metabotropic Glutamate Receptor-Dependent Protein Synthesis and Long-Term Depression in Rat Hippocampus

“…There are at least two signaling pathways that link mGluR activation to the initiation of translation and LTD: activation of extracellular signal-regulated kinase (ERK) by mitogen-activated protein kinase kinase (MEK), and activation of mammalian target of rapamycin (mTOR) through phosphoinositide-3-kinase (PI3K) and Akt. Thus, (RS)-3,5-dihydrophenylglycine (DHPG) increases levels of phosphorylated ERK and mTOR (Choe and Wang, 2001a,b;Gallagher et al, 2004;Banko et al, 2006;Antion et al, 2008) and initiates translation through the phosphorylation and activation of eIF4E (Banko et al, 2006). Furthermore, mGluR activation increases other regulators of protein synthesis such as mRNA polyadenylation (Shin et al, 2004) and p70S6 kinase activity (Antion et al, 2008).…”