Genetic deletion of fragile X mental retardation protein (FMRP) has been shown to enhance mGluR-dependent long-term depression (LTD). Herein, we demonstrate that mGluR-LTD induces a transient, translation-dependent increase in FMRP that is rapidly degraded by the ubiquitin-proteasome pathway. Moreover, proteasome inhibitors abolished mGluR-LTD, and LTD was absent in mice that overexpress human FMRP. Neither translation nor proteasome inhibitors blocked the augmentation of mGluR-LTD in FMRP-deficient mice. In addition, mGluR-LTD is associated with rapid increases in the protein levels of FMRP target mRNAs in wild-type mice. Interestingly, the basal levels of these proteins were elevated and their synthesis was improperly regulated during mGluR-LTD in FMRP-deficient mice. Our findings indicate that hippocampal mGluR-LTD requires the rapid synthesis and degradation of FMRP and that mGluR-LTD triggers the synthesis of FMRP binding mRNAs. These findings indicate that the translation, ubiquitination, and proteolysis of FMRP functions as a dynamic regulatory system for controlling synaptic plasticity.

Protein synthesis is required for the expression of enduring memories and long-lasting synaptic plasticity. During cellular proliferation and growth, S6 kinases (S6Ks) are activated and coordinate the synthesis of de novo proteins. We hypothesized that protein synthesis mediated by S6Ks is critical for the manifestation of learning, memory, and synaptic plasticity. We have tested this hypothesis with genetically engineered mice deficient for either S6K1 or S6K2. We have found that S6K1-deficient mice express an early-onset contextual fear memory deficit within one hour of training, a deficit in conditioned taste aversion (CTA), impaired Morris water maze acquisition, and hypoactive exploratory behavior. In contrast, S6K2-deficient mice exhibit decreased contextual fear memory seven days after training, a reduction in latent inhibition of CTA, and normal spatial learning in the Morris water maze. Surprisingly, neither S6K1-nor S6K2-deficient mice exhibited alterations in protein synthesis-dependent late-phase long-term potentiation (L-LTP). However, removal of S6K1, but not S6K2, compromised early-phase LTP expression. Furthermore, we observed that S6K1-deficient mice have elevated basal levels of Akt phosphorylation, which is further elevated following induction of L-LTP. Taken together, our findings demonstrate that removal of S6K1 leads to a distinct array of behavioral and synaptic plasticity phenotypes that are not mirrored by the removal of S6K2. Our observations suggest that neither gene by itself is required for L-LTP but instead may be required for other types of synaptic plasticity required for cognitive processing.

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...

It is widely accepted that protein synthesis, including local protein synthesis at synapses, is required for several forms of synaptic plasticity. Local protein synthesis enables synapses to control synaptic strength independent of the cell body via rapid protein production from pre-existing mRNA. Therefore, regulation of translation initiation is likely to be intimately involved in modulating synaptic strength. Our understanding of the translation-initiation process has expanded greatly in recent years. In this review, we discuss various aspects of translation initiation, as well as signaling pathways that might be involved in coupling neurotransmitter and neurotrophin receptors to the translation machinery during various forms of synaptic plasticity.

Metaplasticity regulates the threshold for modification of synaptic strength and is an important regulator of learning rules; however, it is not known whether these cellular mechanisms for homeostatic regulation of synapses contribute to particular forms of learning. Conditional ablation of mGluR5 in CA1 pyramidal neurons resulted in the inability of low-frequency trains of afferent activation to prime synapses for subsequent theta burst potentiation. Priming-induced metaplasticity requires mGluR5-mediated mobilization of endocannabinoids during the priming train to induce long-term depression of inhibition (I-LTD). Mice lacking priming-induced plasticity had no deficit in spatial reference memory tasks, but were impaired in an associative task with a temporal component. Conversely, enhancing endocannabinoid signaling facilitated temporal associative memory acquisition and, after training animals in these tasks, ex vivo I-LTD was partially occluded and theta burst LTP was enhanced. Together, these results suggest a link between metaplasticity mechanisms in the hippocampus and the formation of temporal associative memories.

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 a (eIF2a) 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 eIF2a phosphorylation, whereas stimulation of early-and late-phase long-term potentiation (E-LTP and L-LTP, respectively) is associated with decreased eIF2a 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 eIF2a phosphorylation. Our findings are consistent with the notion that eIF2a 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.

Summary Age-related increase in L-type Ca2+ channel (LTCC) expression in hippocampal pyramidal neurons has been hypothesized to underlie the increased Ca2+ influx and subsequent reduced intrinsic neuronal excitability of these neurons that lead to age-related cognitive deficits. Here, using specific antibodies against Cav1.2 and Cav1.3 subunits of LTCCs, we systematically re-examined the expression of these proteins in the hippocampus from young (3–4 month old) and aged (30–32 month old) F344xBN rats. Western blot analysis of the total expression levels revealed significant reductions in both Cav1.2 and Cav1.3 subunits from all three major hippocampal regions of aged rats. Despite the decreases in total expression levels, surface biotinylation experiments revealed significantly higher proportion of expression on the plasma membrane of Cav1.2 in the CA1 and CA3 regions and of Cav1.3 in the CA3 region from aged rats. Furthermore, the surface biotinylation results were supported by immunohistochemical analysis that revealed significant increases of Cav1.2 immunoreactivity in the CA1 and CA3 regions of aged hippocampal pyramidal neurons. In addition, we found a significant increase in the level of phosphorylated Cav1.2 on the plasma membrane in the dentate gyrus of aged rats. Taken together, our present findings provide clear evidence that age-related cognitive deficits cannot be attributed to a global change in L-type channel expression nor to the level of phosphorylation of Cav1.2 on the plasma membrane of hippocampal neurons. Rather increased expression and density of functional LTCCs on the plasma membrane may underlie the age-related increase in L-type Ca2+ channel activity in CA1 pyramidal neurons.

B-ephrin - EphB receptor signaling modulates NMDA receptors by inducing tyrosine phosphorylation of NR2 subunits. Ephrins and EphB RTKs are localized to postsynaptic compartments in the CA1, and therefore potentially interact in a non-canonical cis-configuration. However, it is not known whether cis- configured receptor-ligand signaling is utilized by this class of RTKs, and whether this might influence excitatory synapses. We found that ablation of ephrin-B3 results in an enhancement of the NMDA receptor component of synaptic transmission relative to the AMPA receptor component in CA1 synapses. Synaptic AMPA receptor expression is reduced in ephrin-B3 knockout mice, and there is a marked enhancement of tyrosine phosphorylation of the NR2B receptor subunit. In a reduced system co-expression of ephrin-B3 attenuated EphB2-mediated NR2B tyrosine phosphorylation. Moreover, phosphorylation of EphB2 was elevated in the hippocampus of ephrin-B3 knockout mice, suggesting that regulation of EphB2 activity is lost in these mice. Direct activation of EphB RTKs resulted in phosphorylation of NR2B and a potential signaling partner, the non-receptor tyrosine kinase Pyk2. Our data suggests that ephrin-B3 limits EphB RTK-mediated phosphorylation of the NR2B subunit through an inhibitory cis- interaction which is required for the correct function of glutamatergic CA1 synapses.