Molecular mechanisms underlying a unique intermediate phase of memory in aplysia

Short- and long-term synaptic facilitation induced by serotonin at Aplysia sensory-motor (SN-MN) synapses has been widely used as a cellular model of short- and long-term memory for sensitization. In recent years, a distinct intermediate phase of synaptic facilitation (ITF) has been described at SN-MN synapses. Here, we identify a novel intermediate phase of behavioral memory (ITM) for sensitization in Aplysia and demonstrate that it shares the temporal and mechanistic features of ITF in the intact CNS: (1) it declines completely prior to the onset of LTM, (2) its induction requires protein but not RNA synthesis, and (3) its expression requires the persistent activation of protein kinase A. Thus, in Aplysia, the same temporal and molecular characteristics that distinguish ITF from other phases of synaptic plasticity distinguish ITM from other phases of behavioral memory.     

The Role of Proteases in Hippocampal Synaptic Plasticity: Putting Together Small Pieces of a Complex Puzzle

Long-term synaptic plasticity in the hippocampus is thought to underlie the formation of certain forms of memory, including spatial memory. The early phase of long-term synaptic potentiation and synaptic depression depends on post-translational modifications of synaptic proteins, while protein synthesis is also required for the late-phase of both forms of synaptic plasticity (L-LTP and L-LTD). Numerous pieces of evidence show a role for different types of proteases in synaptic plasticity, further increasing the diversity of mechanisms involved in the regulation of the intracellular and extracellular protein content. The cleavage of extracellular proteins is coupled to changes in postsynaptic intracellular mechanisms, and additional alterations in this compartment result from the protease-mediated targeting of intracellular proteins. Both mechanisms contribute to initiate signaling cascades that drive downstream pathways coupled to synaptic plasticity. In this review we summarize the evidence pointing to a role for extracellular and intracellular proteases, with distinct specificities, in synaptic plasticity. Where in the cells the proteases are located, and how they are regulated is also discussed. The combined actions of proteases and translation mechanisms contribute to a tight control of the synaptic proteome relevant for long-term synaptic potentiation and synaptic depression in the hippocampus. Additional studies are required to elucidate the mechanisms whereby these changes in the synaptic proteome are related with plasticity phenomena.     

Dynamic visualization of local protein synthesis in hippocampal neurons

Using pharmacological approaches, several recent studies suggest that local protein synthesis is required for synaptic plasticity. Convincing demonstrations of bona fide dendritic protein synthesis in mammalian neurons are rare, however. We developed a protein synthesis reporter in which the coding sequence of green fluorescent protein is flanked by the 5' and 3' untranslated regions from CAMKII-alpha, conferring both dendritic mRNA localization and translational regulation. In cultured hippocampal neurons, we show that BDNF, a growth factor involved in synaptic plasticity, stimulates protein synthesis of the reporter in intact, mechanically, or "optically" isolated dendrites. The stimulation of protein synthesis is blocked by anisomycin and not observed in untreated neurons. In addition, dendrites appear to possess translational hot spots, regions near synapses where protein synthesis consistently occurs over time.     

Protein homeostasis and synaptic plasticity

It is clear that de novo protein synthesis has an important function in synaptic transmission and plasticity. A substantial amount of work has shown that mRNA translation in the hippocampus is spatially controlled and that dendritic protein synthesis is required for different forms of long‐term synaptic plasticity. More recently, several studies have highlighted a function for protein degradation by the ubiquitin proteasome system in synaptic plasticity. These observations suggest that changes in synaptic transmission involve extensive regulation of the synaptic proteome. Here, we review experimental data supporting the idea that protein homeostasis is a regulatory motif for synaptic plasticity.     

NMDAR‐dependent Argonaute 2 phosphorylation regulates miRNA activity and dendritic spine plasticity

MicroRNAs (miRNAs) repress translation of target mRNAs by associating with Argonaute (Ago) proteins to form the RNA‐induced silencing complex (RISC), underpinning a powerful mechanism for fine‐tuning protein expression. Specific miRNAs are required for NMDA receptor (NMDAR)‐dependent synaptic plasticity by modulating the translation of proteins involved in dendritic spine morphogenesis or synaptic transmission. However, it is unknown how NMDAR stimulation stimulates RISC activity to rapidly repress translation of synaptic proteins. We show that NMDAR stimulation transiently increases Akt‐dependent phosphorylation of Ago2 at S387, which causes an increase in binding to GW182 and a rapid increase in translational repression of LIMK1 via miR‐134. Furthermore, NMDAR‐dependent down‐regulation of endogenous LIMK1 translation in dendrites and dendritic spine shrinkage requires phospho‐regulation of Ago2 at S387. AMPAR trafficking and hippocampal LTD do not involve S387 phosphorylation, defining this mechanism as a specific pathway for structural plasticity. This work defines a novel mechanism for the rapid transduction of NMDAR stimulation into miRNA‐mediated translational repression to control dendritic spine morphology.     

Biochemical mechanisms for translational regulation in synaptic plasticity

Changes in gene expression are required for long-lasting synaptic plasticity and long-term memory in both invertebrates and vertebrates. Regulation of local protein synthesis allows synapses to control synaptic strength independently of messenger RNA synthesis in the cell body. Recent reports indicate that several biochemical signalling cascades couple neurotransmitter and neurotrophin receptors to translational regulatory factors in protein synthesis-dependent forms of synaptic plasticity and memory. In this review, we highlight these translational regulatory mechanisms and the signalling pathways that govern the expression of synaptic plasticity in response to specific types of neuronal stimulation.     

ERK and mTOR signaling couple beta-adrenergic receptors to translation initiation machinery to gate induction of protein synthesis-dependent long-term potentiation

beta-Adrenergic receptors critically modulate long-lasting synaptic plasticity and long-term memory in the mammalian hippocampus. Persistent long-term potentiation of synaptic strength requires protein synthesis and has been correlated with some forms of hippocampal long-term memory. However, the intracellular processes that initiate protein synthesis downstream of the beta-adrenergic receptor are unidentified. Here we report that activation of beta-adrenergic receptors recruits ERK and mammalian target of rapamycin signaling to facilitate long-term potentiation maintenance at the level of translation initiation. Treatment of mouse hippocampal slices with a beta-adrenergic receptor agonist results in activation of eukaryotic initiation factor 4E and the eukaryotic initiation factor 4E kinase Mnk1, along with inhibition of the translation repressor 4E-BP. This coordinated activation of translation machinery requires concomitant ERK and mammalian target of rapamycin signaling. Taken together, our data identify distinct signaling pathways that converge to regulate beta-adrenergic receptor-dependent protein synthesis during long-term synaptic potentiation in the hippocampus. We suggest that beta-adrenergic receptors play a crucial role in gating the induction of long-lasting synaptic plasticity at the level of translation initiation, a mechanism that may underlie the ability of these receptors to influence the formation of long-lasting memories.     

Dual regulation of translation initiation and peptide chain elongation during BDNF-induced LTP in vivo: evidence for compartment-specific translation control

Protein synthesis underlying activity-dependent synaptic plasticity is controlled at the level of mRNA translation. We examined the dynamics and spatial regulation of two key translation factors, eukaryotic initiation factor 4E (eIF4E) and elongation factor-2 (eEF2), during long-term potentiation (LTP) induced by local infusion of brain-derived neurotrophic factor (BDNF) into the dentate gyrus of anesthetized rats. BDNF-induced LTP led to rapid, transient phosphorylation of eIF4E and eEF2, and enhanced expression of eIF4E protein in dentate gyrus homogenates. Infusion of the extracellular signal-regulated kinase (ERK) inhibitor U0126 blocked BDNF-LTP and modulation of the translation factor activity and expression. Quantitative immunohistochemical analysis revealed enhanced staining of phospho-eIF4E and total eIF4E in dentate granule cells. The in vitro synaptodendrosome preparation was used to isolate the synaptic effects of BDNF in the dentate gyrus. BDNF treatment of synaptodendrosomes elicited rapid, transient phosphorylation of eIF4E paralleled by enhanced expression of alpha-calcium/calmodulin-dependent protein kinase II. In contrast, BDNF had no effect on eEF2 phosphorylation state in synaptodendrosomes. The results demonstrate rapid ERK-dependent regulation of the initiation and elongation steps of protein synthesis during BDNF-LTP in vivo. Furthermore, the results suggest a compartment-specific regulation in which initiation is selectively enhanced by BDNF at synapses, while both initiation and elongation are modulated at non-synaptic sites.     

MiR‐134‐dependent regulation of Pumilio‐2 is necessary for homeostatic synaptic depression

Neurons employ a set of homeostatic plasticity mechanisms to counterbalance altered levels of network activity. The molecular mechanisms underlying homeostatic plasticity in response to increased network excitability are still poorly understood. Here, we describe a sequential homeostatic synaptic depression mechanism in primary hippocampal neurons involving miRNA‐dependent translational regulation. This mechanism consists of an initial phase of synapse elimination followed by a reinforcing phase of synaptic downscaling. The activity‐regulated microRNA miR‐134 is necessary for both synapse elimination and the structural rearrangements leading to synaptic downscaling. Results from miR‐134 inhibition further uncover a differential requirement for GluA1/2 subunits for the functional expression of homeostatic synaptic depression. Downregulation of the miR‐134 target Pumilio‐2 in response to chronic activity, which selectively occurs in the synapto‐dendritic compartment, is required for miR‐134‐mediated homeostatic synaptic depression. We further identified polo‐like kinase 2 (Plk2) as a novel target of Pumilio‐2 involved in the control of GluA2 surface expression. In summary, we have described a novel pathway of homeostatic plasticity that stabilizes neuronal circuits in response to increased network activity.     

Local protein synthesis in neuronal axons: why and how we study

Adaptive brain function and synaptic plasticity rely on dynamic regulation of local proteome. One way for the neuron to introduce new proteins to the axon terminal is to transport those from the cell body, which had long been thought as the only source of axonal proteins. Another way, which is the topic of this review, is synthesizing proteins on site by local mRNA translation. Recent evidence indicates that the axon stores a reservoir of translationally silent mRNAs and regulates their expression solely by translational control. Different stimuli to axons, such as guidance cues, growth factors, and nerve injury, promote translation of selective mRNAs, a process required for the axon’s ability to respond to these cues. One of the critical questions in the field of axonal protein synthesis is how mRNA-specific local translation is regulated by extracellular cues. Here, we review current experimental techniques that can be used to answer this question. Furthermore, we discuss how new technologies can help us understand what biological processes are regulated by axonal protein synthesis in vivo. [BMB Reports 2015; 48(3): 139-146]