Synapse-specific, long-term facilitation of aplysia sensory to motor synapses: a function for local protein synthesis in memory storage

FMRP and Ataxin-2 function together in long-term olfactory habituation and neuronal translational control

Fragile X mental retardation protein (FMRP) and Ataxin-2 (Atx2) are triplet expansion disease- and stress granule-associated proteins implicated in neuronal translational control and microRNA function. We show that Drosophila FMRP (dFMR1) is required for long-term olfactory habituation (LTH), a phenomenon dependent on Atx2-dependent potentiation of inhibitory transmission from local interneurons (LNs) to projection neurons (PNs) in the antennal lobe. dFMR1 is also required for LTH-associated depression of odor-evoked calcium transients in PNs. Strong transdominant genetic interactions among dFMR1, atx2, the deadbox helicase me31B, and argonaute1 (ago1) mutants, as well as coimmunoprecitation of dFMR1 with Atx2, indicate that dFMR1 and Atx2 function together in a microRNA-dependent process necessary for LTH. Consistently, PN or LN knockdown of dFMR1, Atx2, Me31B, or the miRNA-pathway protein GW182 increases expression of a Ca2+/calmodulin-dependent protein kinase II (CaMKII) translational reporter. Moreover, brain immunoprecipitates of dFMR1 and Atx2 proteins include CaMKII mRNA, indicating respective physical interactions with this mRNA. Because CaMKII is necessary for LTH, these data indicate that fragile X mental retardation protein and Atx2 act via at least one common target RNA for memory-associated long-term synaptic plasticity. The observed requirement in LNs and PNs supports an emerging view that both presynaptic and postsynaptic translation are necessary for long-term synaptic plasticity. However, whereas Atx2 is necessary for the integrity of dendritic and somatic Me31B-containing particles, dFmr1 is not. Together, these data indicate that dFmr1 and Atx2 function in long-term but not short-term memory, regulating translation of at least some common presynaptic and postsynaptic target mRNAs in the same cells.

RNA transport and long-term memory storage

Several studies have shown that synthesis of new proteins at the synapse is a prerequisite for the storage of long-term memories. Relatively little is known about the availability of distinct mRNA populations for translation at specific synapses, the process that determines mRNA localization, and the temporal designations of localized mRNA translation during memory storage. Techniques such as synaptosome preparation and microdissection of distal neuronal processes of cultured neurons and dendritic layers in brain slices are general approaches used to identify localized RNAs. Exploration of the association of RNA-binding proteins to the axonal transport machinery has led to the development of a strategy to identify RNAs that are transported from the cell body to synapses by molecular motor kinesin. In this article, RNA localization at the synapse, as well as its mechanisms and significance in understanding long-term memory storage, are discussed.

α7 Nicotinic receptor agonist enhances cognition in aged 3xTg-AD mice with robust plaques and tangles

Alzheimer disease (AD) is a progressive neurodegenerative disorder with associated memory loss, spatial disorientation, and other psychiatric problems. Cholinergic system dysfunction is an early and salient feature of AD, and enhancing cholinergic signaling with acetylcholinesterase inhibitors is currently the primary strategy for improving cognition. The beneficial effects of acetylcholinesterase inhibitors, however, are typically short-lived and accompanied by adverse effects. Recent evidence suggests that activating α7 nicotinic acetylcholine receptors (α7 nAChR) may facilitate the specific modulation of brain cholinergic signaling, leading to cognitive enhancement and possibly to amelioration of AD pathologic findings. In the present study, we determined the effect of long-term treatment with the selective α7 nAChR agonist A-582941 in aged 3xTg-AD mice with robust AD-like pathology, which is particularly significant not only because this is the only mouse model that co-develops amyloid plaques and neurofibrillary tangles but also because it enabled us to explore whether A-582941 is able to restore brain function after the severe damage associated with AD. Analysis of β-amyloid deposits, tau phosphorylation, and inflammatory cells revealed that, overall, pathologic findings were unchanged. Rather, α7 nAChR activation induced expression of c-Fos and brain-derived neurotrophic factor and phosphorylation of cyclic adenosine monophosphate response element binding and neurotrophic tyrosine receptor kinase type 2. More important, A-582941 completely restored cognition in aged 3xTg-AD mice to the level of that in age-matched nontransgenic mice. These novel findings indicate that activating α7 nAChR is a promising treatment for cognitive impairment in AD.

Activation of mammalian target of rapamycin contributes to pain nociception induced in rats by BmK I, a sodium channel-specific modulator

The mammalian target of rapamycin (mTOR) pathway is essential for maintenance of the sensitivity of certain adult sensory neurons. Here, we investigated whether the mTOR cascade is involved in scorpion envenomation-induced pain hypersensitivity in rats. The results showed that intraplantar injection of a neurotoxin from Buthus martensii Karsch, BmK I (10 μg), induced the activation of mTOR, as well as its downstream molecules p70 ribosomal S6 protein kinase (p70 S6K) and eukaryotic initiation factor 4E-binding protein 1 (4E-BP1), in lumbar 5-6 dorsal root ganglia neurons on both sides in rats. The activation peaked at 2 h and recovered 1 day after injection. Compared with the control group, the ratios of p-mTOR/p-p70 S6K/p-4EBP1 in three types of neurons changed significantly. The cell typology of p-mTOR/p-p70 S6K/p-4E-BP1 immuno-reactive neurons also changed. Intrathecal administration of deforolimus, a specific inhibitor of mTOR, attenuated BmK I-induced pain responses (spontaneous flinching, paroxysmal pain-like behavior, and mechanical hypersensitivity). Together, these results imply that the mTOR signaling pathway is mobilized by and contributes to experimental scorpion sting-induced pain.

Activation of mammalian target of rapamycin mediates rat pain-related responses induced by BmK I, a sodium channel-specific modulator

The mammalian target of rapamycin (mTOR) is known to regulate cell proliferation and growth by controlling protein translation. Recently, it has been shown that mTOR signaling pathway is involved in long-term synaptic plasticity. However, the role of mTOR under different pain conditions is less clear. In this study, the spatiotemporal activation of mTOR that contributes to pain-related behaviors was investigated using a novel animal inflammatory pain model induced by BmK I, a sodium channel-specific modulator purified from scorpion venom. In this study, intraplantar injections of BmK I were found to induce the activation of mTOR, p70 ribosomal S6 protein kinase (p70 S6K) and eukaryotic initiation factor 4E-binding protein 1 (4E-BP1) in rat L5-L6 spinal neurons. In the spinal cord, mTOR, p70 S6K and 4E-BP1 were observed to be activated in the ipsilateral and contralateral regions, peaking at 1-2 h and recovery at 24 h post-intraplantar (i.pl.) BmK I administration. In addition, intrathecal (i.t.) injection of rapamycin - a specific inhibitor of mTOR - was observed to result in the reduction of spontaneous pain responses and the attenuation of unilateral thermal and bilateral mechanical hypersensitivity elicited by BmK I. Thus, these results indicate that the mTOR signaling pathway is mobilized in the induction and maintenance of pain-activated hypersensitivity.

The Drosophila transcription factor Adf-1 (nalyot) regulates dendrite growth by controlling FasII and Staufen expression downstream of CaMKII and neural activity

Memory deficits in Drosophila nalyot mutants suggest that the Myb family transcription factor Adf-1 is an important regulator of developmental plasticity in the brain. However, the cellular functions for this transcription factor in neurons or molecular mechanisms by which it regulates plasticity remain unknown. Here, we use in vivo 3D reconstruction of identifiable larval motor neuron dendrites to show that Adf-1 is required cell autonomously for dendritic development and activity-dependent plasticity of motor neurons downstream of CaMKII. Adf-1 inhibition reduces dendrite growth and neuronal excitability, and results in motor deficits and altered transcriptional profiles. Surprisingly, analysis by comparative chromatin immunoprecipitation followed by sequencing (ChIP-Seq) of Adf-1, RNA Polymerase II (Pol II), and histone modifications in Kc cells shows that Adf-1 binding correlates positively with high Pol II-pausing indices and negatively with active chromatin marks such as H3K4me3 and H3K27ac. Consistently, the expression of Adf-1 targets Staufen and Fasciclin II (FasII), identified through larval brain ChIP-Seq for Adf-1, is negatively regulated by Adf-1, and manipulations of these genes predictably modify dendrite growth. Our results imply mechanistic interactions between transcriptional and local translational machinery in neurons as well as conserved neuronal growth mechanisms mediated by cell adhesion molecules, and suggest that CaMKII, Adf-1, FasII, and Staufen influence crucial aspects of dendrite development and plasticity with potential implications for memory formation. Further, our experiments reveal molecular details underlying transcriptional regulation by Adf-1, and indicate active interaction between Adf-1 and epigenetic regulators of gene expression during activity-dependent neuronal plasticity.

Local gene expression in nerve endings

At the Nobel lecture for physiology in 1906, Ramón y Cajal famously stated that "the nerve elements possess reciprocal relationships in contiguity but not in continuity," summing up the neuron doctrine. Sixty years later, by the time the central dogma of molecular biology formulated the axis of genetic information flow from DNA to mRNA, and then to protein, it became obvious that neurons with extensive ramifications and long axons inevitably incur an innate problem: how can the effect of gene expression be extended from the nucleus to the remote and specific sites of the cell periphery? The most straightforward solution would be to deliver soma-produced proteins to the target sites. The influential discovery of axoplasmic flow has supported this scheme of protein supply. Alternatively, mRNAs can be dispatched instead of protein, and translated locally at the strategic target sites. Over the past decades, such a local system of protein synthesis has been demonstrated in dendrites, axons, and presynaptic terminals. Moreover, the local protein synthesis in neurons might even involve intercellular trafficking of molecules. The innovative concept of glia-neuron unit suggests that the local protein synthesis in the axonal and presynaptic domain of mature neurons is sustained by a local supply of RNAs synthesized in the surrounding glial cells and transferred to these domains. Here, we have reviewed some of the evidence indicating the presence of a local system of protein synthesis in axon terminals, and have examined its regulation in various model systems.

Roles of microRNAs in the Caenorhabditis elegans nervous system

The first microRNA was discovered in Caenorhabditis elegans in 1993, and since then, thousands of microRNAs have been identified from almost all eukaryotic organisms examined. MicroRNAs function in many biological events such as cell fate determination, metabolism, apoptosis, and carcinogenesis. So far, more than 250 microRNAs have been identified in C. elegans; however, functions for most of these microRNAs are still unknown. A small number of C. elegans microRNAs are associated with known physiological roles such as developmental timing, cell differentiation, stress response, and longevity. In this review, we summarize known roles of microRNAs in neuronal differentiation and function of C. elegans, and discuss interesting perspectives for future studies.

Increasing our understanding of human cognition through the study of Fragile X Syndrome

Fragile X Syndrome (FXS) is considered the most common form of inherited intellectual disability. It is caused by reductions in the expression level or function of a single protein, the Fragile X Mental Retardation Protein (FMRP), a translational regulator which binds to approximately 4% of brain messenger RNAs. Accumulating evidence suggests that FXS is a complex disorder of cognition, involving interactions between genetic and environmental influences, leading to difficulties in acquiring key life skills including motor skills, language, and proper social behaviors. Since many FXS patients also present with one or more features of autism spectrum disorders (ASDs), insights gained from studying the monogenic basis of FXS could pave the way to a greater understanding of underlying features of multigenic ASDs. Here we present an overview of the FXS and FMRP field with the goal of demonstrating how loss of a single protein involved in translational control affects multiple stages of brain development and leads to debilitating consequences on human cognition. We also focus on studies which have rescued or improved FXS symptoms in mice using genetic or therapeutic approaches to reduce protein expression. We end with a brief description of how deficits in translational control are implicated in FXS and certain cases of ASDs, with many recent studies demonstrating that ASDs are likely caused by increases or decreases in the levels of certain key synaptic proteins. The study of FXS and its underlying single genetic cause offers an invaluable opportunity to study how a single gene influences brain development and behavior.

A comparative quantitative assessment of axonal and dendritic mRNA transport in maturing hippocampal neurons

Translation of mRNA in axons and dendrites enables a rapid supply of proteins to specific sites of localization within the neuron. Distinct mRNA-containing cargoes, including granules and mitochondrial mRNA, are transported within neuronal projections. The distributions of these cargoes appear to change during neuronal development, but details on the dynamics of mRNA transport during these transitions remain to be elucidated. For this study, we have developed imaging and image processing methods to quantify several transport parameters that can define the dynamics of RNA transport and localization. Using these methods, we characterized the transport of mitochondrial and non-mitochondrial mRNA in differentiated axons and dendrites of cultured hippocampal neurons varying in developmental maturity. Our results suggest differences in the transport profiles of mitochondrial and non-mitochondrial mRNA, and differences in transport parameters at different time points, and between axons and dendrites. Furthermore, within the non-mitochondrial mRNA pool, we observed two distinct populations that differed in their fluorescence intensity and velocity. The net axonal velocity of the brighter pool was highest at day 7 (0.002±0.001 µm/s, mean ± SEM), raising the possibility of a presynaptic requirement for mRNA during early stages of synapse formation. In contrast, the net dendritic velocity of the brighter pool increased steadily as neurons matured, with a significant difference between day 12 (0.0013±0.0006 µm/s ) and day 4 (−0.003±0.001 µm/s) suggesting a postsynaptic role for mRNAs in more mature neurons. The dim population showed similar trends, though velocities were two orders of magnitude higher than of the bright particles. This study provides a baseline for further studies on mRNA transport, and has important implications for the regulation of neuronal plasticity during neuronal development and in response to neuronal injury.

Genomics and proteomics in solving brain complexity

The human brain is extraordinarily complex, composed of billions of neurons and trillions of synaptic connections. Neurons are organized into circuit assemblies that are modulated by specific interneurons and non-neuronal cells, such as glia and astrocytes. Data on human genome sequences predicts that each of these cells in the human brain has the potential of expressing ∼20 000 protein coding genes and tens of thousands of noncoding RNAs. A major challenge in neuroscience is to determine (1) how individual neurons and circuitry utilize this potential during development and maturation of the nervous system, and for higher brain functions such as cognition, and (2) how this potential is altered in neurological and psychiatric disorders. In this review, we will discuss how recent advances in next generation sequencing, proteomics and bioinformatics have transformed our understanding of gene expression and the functions of neural circuitry, memory storage, and disorders of cognition.

Intra-axonal synthesis of eukaryotic translation initiation factors regulates local protein synthesis and axon growth in rat sympathetic neurons

Axonal protein synthesis is a complex process involving selective mRNA localization and translational regulation. In this study, using in situ hybridization and metabolic labeling, we show that the mRNAs encoding eukaryotic translation initiation factors eIF2B2 and eIF4G2 are present in the axons of rat sympathetic neurons and are locally translated. We also report that a noncoding microRNA, miR16, modulates the axonal expression of eIF2B2 and eIF4G2. Transfection of axons with precursor miR16 and anti-miR16 showed that local miR16 levels modulated axonal eIF2B2 and eIF4G2 mRNA and protein levels, as well as axon outgrowth. siRNA-mediated knock-down of axonal eIF2B2 and eIF4G2 mRNA also resulted in a significant decrease in axonal eIF2B2 and eIF4G2 protein. Moreover, results of metabolic labeling studies showed that downregulation of axonal eIF2B2 and eIF4G2 expression also inhibited local protein synthesis and axon growth. Together, these data provide evidence that miR16 mediates axonal growth, at least in part, by regulating the local protein synthesis of eukaryotic translation initiation factors eIF2B2 and eIF4G2 in the axon.

Synaptic tagging and capture in the living rat

In isolated hippocampal slices, decaying long-term potentiation (LTP) can be stabilized, and converted to late-LTP lasting many hours, by prior or subsequent strong high-frequency tetanization of an independent input to a common population of neurons—a phenomenon known as ‘synaptic tagging and capture’. Here we show that the same phenomenon occurs in the intact rat. Late-LTP can be induced in CA1 during the inhibition of protein synthesis if an independent input is strongly tetanized beforehand. Conversely, declining early-LTP induced by weak tetanization can be converted into lasting late-LTP by subsequent strong tetanization of a separate input. These findings indicate that synaptic tagging and capture is not limited to in vitro preparations; the past and future activity of neurons plays a critical role in determining the persistence of synaptic changes in the living animal, thus providing a bridge between cellular studies of protein-synthesis-dependent synaptic potentiation and behavioural studies of memory persistence.

Endocannabinoid-dependent long-term depression in a nociceptive synapse requires coordinated presynaptic and postsynaptic transcription and translation

Endocannabinoids (eCBs) play an important role in long-term regulation of synaptic signaling in both vertebrates and invertebrates. In this study, the role of transcription- and translation-dependent processes in presynaptic versus postsynaptic neurons was examined during eCB-mediated synaptic plasticity in the CNS of the leech. Low-frequency stimulation (LFS) of non-nociceptive afferents elicits eCB-dependent long-term depression (eCB-LTD) heterosynaptically in nociceptive synapses that lasts at least 2 h. Bath application of emetine, a protein synthesis inhibitor, blocked eCB-LTD after afferent LFS or exogenous eCB application, indicating that this depression was translation dependent. Bath application of actinomycin D, an irreversible RNA synthesis inhibitor, or 5,6-dichlorobenzimidazole 1-β-d-ribofurandoside (DRB), a reversible RNA synthesis inhibitor, also prevented eCB-LTD. Selective injection of DRB or emetine into the presynaptic or postsynaptic neuron before LFS indicated that eCB-LTD required transcription and translation in the postsynaptic neuron but only translation in the presynaptic cell. Depression observed immediately after LFS was also blocked when these transcription- and translation-dependent processes were inhibited. It is proposed that induction of eCB-LTD in this nociceptive synapse requires the coordination of presynaptic protein synthesis and postsynaptic mRNA and protein synthesis. These findings provide significant insights into both eCB-based synaptic plasticity and understanding how activity in non-nociceptive afferents modulates nociceptive pathways.

A strategy to capture and characterize the synaptic transcriptome

Here we describe a strategy designed to identify RNAs that are actively transported to synapses during learning. Our approach is based on the characterization of RNA transport complexes carried by molecular motor kinesin. Using this strategy in Aplysia, we have identified 5,657 unique sequences consisting of both coding and noncoding RNAs from the CNS. Several of these RNAs have key roles in the maintenance of synaptic function and growth. One of these RNAs, myosin heavy chain, is critical in presynaptic sensory neurons for the establishment of long-term facilitation, but not for its persistence.

Differentiation in the protein synthesis-dependency of persistent synaptic plasticity in mossy fiber and associational/commissural CA3 synapses in vivo

Long-term potentiation (LTP) and long-term depression (LTD) are two mechanisms involved in the long-term storage of information in hippocampal synapses. In the hippocampal CA1 region, the late phases of LTP and LTD are protein-synthesis dependent. In the dentate gyrus, late-LTP but not LTD requires protein synthesis. The protein synthesis-dependency of persistent plasticity at CA3 synapses has not yet been characterized. Here, the roles of protein transcription and translation at mossy fiber (mf) and associational/commissural (AC)- synapses were studied in freely behaving rats. In control animals, low-frequency stimulation (LFS) evoked robust LTD (>24 h), whereas high-frequency stimulation (HFS) elicited robust LTP (>24 h) at both mf-CA3 and AC-CA3 synapses. Translation inhibitors prevented early and late phases of LTP and LTD at mf-CA3 synapses. In contrast, at AC–CA3 synapses, translation inhibitors prevented intermediate/late-LTP and late-LTD only. Transcription effects were also synapse-specific: whereas transcription inhibitors inhibited late-LTP and late-LTD (>3 h) at mf-CA3 synapses, at AC–CA3 synapses, protein transcription affected early-LTP and late-LTD. These results show that the AC-CA3 and mf-CA3 synapses display different properties in terms of their protein synthesis dependency, suggesting different roles in the processing of short- and long term synaptic plasticity.

Retrograde changes in presynaptic function driven by dendritic mTORC1

Mutations that alter signaling through the mammalian target of rapamycin complex 1 (mTORC1), a well established regulator of neuronal protein synthesis, have been linked to autism and cognitive dysfunction. Although previous studies have established a role for mTORC1 as necessary for enduring changes in postsynaptic function, here we demonstrate that dendritic mTORC1 activation in rat hippocampal neurons also drives a retrograde signaling mechanism promoting enhanced neurotransmitter release from apposed presynaptic terminals. This novel mode of synaptic regulation conferred by dendritic mTORC1 is locally implemented, requires downstream synthesis of brain-derived neurotrophic factor as a retrograde messenger, and is engaged in an activity-dependent fashion to support homeostatic trans-synaptic control of presynaptic function. Our findings thus reveal that mTORC1-dependent translation in dendrites subserves a unique mode of synaptic regulation, highlighting an alternative regulatory pathway that could contribute to the social and cognitive dysfunction that accompanies dysregulated mTORC1 signaling.

Training old rats selectively modulates synaptosomal protein synthesis

We have previously shown that the local synthesis of two synaptic proteins of 66.5-kDa and 87.6-kDa is selectively enhanced in male adult rats trained for a two-way active avoidance task. We report here that a comparable but not identical response occurs in 2-year-old male rats trained for the same task. In the latter age group, the local synthesis of the 66.5-kDa protein markedly increases in cerebral cortex, brainstem, and cerebellum, with a somewhat lower increment in synthesis of the 87.6-kDa protein. On the other hand, the newly synthesized 87.6-kDa protein correlates with avoidances and escapes and inversely correlates with freezings in cerebral cortex and brainstem, whereas the correlations of the newly synthesized 66.5-kDa protein remain below significance. These correlative patterns are sharply at variance with those present in trained adult rats. Our data confirm that the local system of synaptic protein synthesis is selectively modulated by training and show that the synaptic response of old rats differs from that of adult rats as reflected in behavioral responses.

Drosophila CPEB Orb2A mediates memory independent of Its RNA-binding domain

Long-term memory and synaptic plasticity are thought to require the synthesis of new proteins at activated synapses. The CPEB family of RNA binding proteins, including Drosophila Orb2, has been implicated in this process. The precise mechanism by which these molecules regulate memory formation is however poorly understood. We used gene targeting and site-specific transgenesis to specifically modify the endogenous orb2 gene in order to investigate its role in long-term memory formation. We show that the Orb2A and Orb2B isoforms, while both essential, have distinct functions in memory formation. These two isoforms have common glutamine-rich and RNA-binding domains, yet Orb2A uniquely requires the former and Orb2B the latter. We further show that Orb2A induces Orb2 complexes in a manner dependent upon both its glutamine-rich region and neuronal activity. We propose that Orb2B acts as a conventional CPEB to regulate transport and/or translation of specific mRNAs, whereas Orb2A acts in an unconventional manner to form stable Orb2 complexes that are essential for memory to persist.

Role of signal transduction crosstalk between adenylyl cyclase and MAP kinase in hippocampus-dependent memory

One of the intriguing questions in neurobiology is how long-term memory (LTM) traces are established and maintained in the brain. Memory can be divided into at least two temporally and mechanistically distinct forms. Short-term memory (STM) lasts no longer than several hours, while LTM persists for days or longer. A crucial step in the generation of LTM is consolidation, a process in which STM is converted to LTM. Hippocampus-dependent LTM depends on activation of Ca(2+), Erk/MAP kinase (MAPK), and cAMP signaling pathways, as well as de novo gene expression and translation. One of the transcriptional pathways strongly implicated in LTM is the CREB/CRE (calcium, cAMP response element) transcriptional pathway. Interestingly, this transcriptional pathway may also contribute to other forms of neuroplasticity including adaptive responses to drugs. Evidence discussed in this review indicates that activation of the Erk1/2 MAP Kinase (MAPK)/CRE transcriptional pathway during the formation of hippocampus-dependent memory depends on calmodulin (CaM)-stimulated adenylyl cyclases.

MicroRNAs shape the neuronal landscape

The nervous system equips us with capability to adapt to many conditions and circumstances. We rely on an armamentarium of intricately formed neural circuits for many of our adaptive strategies. However, this capability also depends on a well-conserved toolkit of different molecular mechanisms that offer not only compensatory responses to a changing world, but also provide plasticity to achieve changes in cellular state that underlie a broad range of processes from early developmental transitions to life-long memory. Among the molecular tools that mediate changes in cellular state, our understanding of posttranscriptional regulation of gene expression is expanding rapidly. Part of the "epigenetic landscape" that shapes the deployment and robust regulation of gene networks during the construction and the remodeling of the brain is the microRNA system controlling both levels and translation of messenger RNA. Here we consider recent advances in the study of microRNA-mediated regulation of synaptic form and function.

Mapping molecular memory: navigating the cellular pathways of learning

A consolidated map of the signalling pathways that function in the formation of short- and long-term cellular memory could be considered the ultimate means of defining the molecular basis of learning. Research has established that experience-dependent activation of these complex cellular cascades leads to many changes in the composition and functioning of a neuron's proteome, resulting in the modulation of its synaptic strength and structure. However, although generally accepted that synaptic plasticity is the mechanism whereby memories are stored in the brain, there is much controversy over whether the site of this neuronal memory expression is predominantly pre- or postsynaptic. Much of the early research into the neuromolecular mechanisms of memory performed using the model organism, the marine snail Aplysia, has focused on the associated presynaptic events. Recently however, postsynaptic mechanisms have been shown to contribute definitively to long term memory processes, and are in fact critical for persistent learning-induced synaptic changes. In this review, in which we aimed to integrate many of the early and recent advances concerning coordinated neuronal signaling in both the pre- and postsynaptic neurons, we have provided a detailed account of the diverse cellular events that lead to modifications in synaptic strength. Thus, a comprehensive synaptic model is presented that could explain a few of the shortcomings that arise when the presynaptic and postsynaptic changes are considered separately. Although it is clear that there is still much to be learnt and that the exact nature of many of the signalling cascades and their components are yet to be fully understood, this still incomplete but integrated illustrative map of the cellular pathways involved provides an overview which expands understanding of the neuromolecular mechanisms of learning and memory.

The local transcriptome in the synaptic neuropil revealed by deep sequencing and high-resolution imaging

In neurons, dendritic protein synthesis is required for many forms of long-term synaptic plasticity. The population of mRNAs that are localized to dendrites, however, remains sparsely identified. Here, we use deep sequencing to identify the mRNAs resident in the synaptic neuropil in the hippocampus. Analysis of a neuropil data set yielded a list of 8,379 transcripts of which 2,550 are localized in dendrites and/or axons. Using a fluorescent barcode strategy to label individual mRNAs, we show that their relative abundance in the neuropil varies over 3 orders of magnitude. High-resolution in situ hybridization validated the presence of mRNAs in both cultured neurons and hippocampal slices. Among the many mRNAs identified, we observed a large fraction of known synaptic proteins including signaling molecules, scaffolds and receptors. These results reveal a previously unappreciated enormous potential for the local protein synthesis machinery to supply, maintain and modify the dendritic and synaptic proteome.

Synapses and memory storage

The synapse is the functional unit of the brain. During the last several decades we have acquired a great deal of information on its structure, molecular components, and physiological function. It is clear that synapses are morphologically and molecularly diverse and that this diversity is recruited to different functions. One of the most intriguing findings is that the size of the synaptic response in not invariant, but can be altered by a variety of homo- and heterosynaptic factors such as past patterns of use or modulatory neurotransmitters. Perhaps the most difficult challenge in neuroscience is to design experiments that reveal how these basic building blocks of the brain are put together and how they are regulated to mediate the information flow through neural circuits that is necessary to produce complex behaviors and store memories. In this review we will focus on studies that attempt to uncover the role of synaptic plasticity in the regulation of whole-animal behavior by learning and memory.

The molecular biology of memory: cAMP, PKA, CRE, CREB-1, CREB-2, and CPEB

The analysis of the contributions to synaptic plasticity and memory of cAMP, PKA, CRE, CREB-1, CREB-2, and CPEB has recruited the efforts of many laboratories all over the world. These are six key steps in the molecular biological delineation of short-term memory and its conversion to long-term memory for both implicit (procedural) and explicit (declarative) memory. I here first trace the background for the clinical and behavioral studies of implicit memory that made a molecular biology of memory storage possible, and then detail the discovery and early history of these six molecular steps and their roles in explicit memory.

Self-injurious behaviour in intellectual disability syndromes: evidence for aberrant pain signalling as a contributing factor

BACKGROUND

In most individuals, injury results in activation of peripheral nociceptors (pain-sensing neurons of the peripheral nervous system) and amplification of central nervous system (CNS) pain pathways that serve as a disincentive to continue harmful behaviour; however, this may not be the case in some developmental disorders that cause intellectual disability (ID). Moreover, individuals affected by ID disorders may initiate self-injurious behaviour to address irritating or painful sensations. In normal individuals, a negative feedback loop decreases sensation of pain, which involves descending inhibitory neurons in the CNS that attenuate spinal nociceptive processing. If spinal nociceptive signalling is impaired in these developmental disorders, an exaggerated painful stimulus may be required in order to engage descending anti-nociceptive signals.

METHODS

Using electronic databases, we conducted a review of publications regarding the incidence of chronic pain or altered pain sensation in ID patients or corresponding preclinical models.

RESULTS

There is a body of evidence indicating that individuals with fragile X mental retardation and/or Rett syndrome have altered pain sensation. These findings in humans are supported by mechanistic studies using genetically modified mice harbouring mutations consistent with the human disease. Thus, once self-injurious behaviour is initiated, the signal to stop may be missing. Several developmental disorders that cause ID are associated with increased incidence of gastroesophageal reflux disease (GERD), which can cause severe visceral pain. Individuals affected by these disorders who also have GERD may self-injure as a mechanism to engage descending inhibitory circuits to quell visceral pain. In keeping with this hypothesis, pharmacological treatment of GERD has been shown to be effective for reducing self-injurious behaviour in some patients. Hence, multiple lines of evidence suggest aberrant nociceptive processing in developmental disorders that cause ID.

CONCLUSIONS

There is evidence that pain pathways and pain amplification mechanisms are altered in several preclinical models of developmental disorders that cause ID. We present hypotheses regarding how impaired pain pathways or chronic pain might contribute to self-injurious behaviour. Studies evaluating the relationship between pain and self-injurious behaviour will provide better understanding of the mechanisms underlying self-injurious behaviour in the ID population and may lead to more effective treatments.

Molluscan neurons in culture: shedding light on synapse formation and plasticity

From genes to behaviour, the simple model system approach has played many pivotal roles in deciphering nervous system function in both invertebrates and vertebrates. However, with the advent of sophisticated imaging and recording techniques enabling the direct investigation of single vertebrate neurons, the utility of simple invertebrate organisms as model systems has been put to question. To address this subject meaningfully and comprehensively, we first review the contributions made by invertebrates in the field of neuroscience over the years, paving the way for similar breakthroughs in higher animals. In particular, we focus on molluscan (Lymnaea, Aplysia, and Helisoma) and leech (Hirudo) models and the pivotal roles they have played in elucidating mechanisms of synapse formation and plasticity. While the ultimate goal in neuroscience is to understand the workings of the human brain in both its normal and diseased states, the sheer complexity of most vertebrate models still makes it difficult to define the underlying principles of nervous system function. Investigators have thus turned to invertebrate models, which are unique with respect to their simple nervous systems that are endowed with a finite number of large, individually identifiable neurons of known function. We start off by discussing in vivo and semi-intact preparations, regarding their amenability to simple circuit analysis. Despite the 'simplicity' of invertebrate nervous systems however, it is still difficult to study individual synaptic connections in detail. We therefore emphasize in the next section, the utility of studying identified invertebrate neurons in vitro, to directly examine the development, specificity, and plasticity of synaptic connections in a well-defined environment, at a resolution that it is still unapproachable in the intact brain. We conclude with a discussion of the future of invertebrates in neuroscience in elucidating mechanisms of neurological disease and developing neuron-silicon interfaces.

Generation of a pain memory in the primary afferent nociceptor triggered by PKCε activation of CPEB

Isolectin B(4)-positive [IB(4)(+)] primary afferent nociceptors challenged with an inflammatory or neuropathic insult develop a PKCε-dependent long-lasting hyperalgesic response to a subsequent challenge by the proinflammatory cytokine prostaglandin E(2) (PGE(2)), a phenomenon known as hyperalgesic priming. Here we demonstrate that the neuroplasticity underlying nociceptor priming requires 72 h to be established; rats that have been challenged with the inflammatory mediator TNFα 24 or 48 h ahead of PGE(2) do not show the enhanced and prolonged hyperalgesic response by which primed IB(4)(+)-nociceptors are being characterized. Moreover, as the underlying plasticity can be interrupted by the peripheral administration of the protein translation inhibitor anisomycin it is reflected by changes in the peripheral protein expression pattern. Finally, the induction of priming by the selective PKCε agonist, psi ε receptor for activated c kinase (ψεRACK) can be prevented, but not reversed by intrathecal injections of antisense oligodeoxynucleotides for the cytoplasmic polyadenylation element binding protein (CPEB) mRNA, a master regulator of protein translation that coimmunoprecipitated with PKCε and is almost exclusively expressed by IB(4)(+)-nociceptors. Our results suggest that CPEB is downstream of PKCε in the cellular signaling cascade responsible for the induction of priming, raising the intriguing possiblity that prion-like misfolding could be a responsible mechanism for the chronification of pain.

Dendritically targeted Bdnf mRNA is essential for energy balance and response to leptin

Mutations in the Bdnf gene, which produces transcripts with either short or long 3' untranslated regions (3' UTRs), cause human obesity; however, the precise role of brain-derived neurotrophic factor (BDNF) in the regulation of energy balance is unknown. Here we show the relationship between Bdnf mRNA with a long 3' UTR (long 3' UTR Bdnf mRNA), leptin, neuronal activation and body weight. We found that long 3' UTR Bdnf mRNA was enriched in the dendrites of hypothalamic neurons and that insulin and leptin could stimulate its translation in dendrites. Furthermore, mice harboring a truncated long Bdnf 3' UTR developed severe hyperphagic obesity, which was completely reversed by viral expression of long 3' UTR Bdnf mRNA in the hypothalamus. In these mice, the ability of leptin to activate hypothalamic neurons and inhibit food intake was compromised despite normal activation of leptin receptors. These results reveal a novel mechanism linking leptin action to BDNF expression during hypothalamic-mediated regulation of body weight, while also implicating dendritic protein synthesis in this process.

Local RNA translation at the synapse and in disease

Local regulation of protein synthesis in neurons has emerged as a leading research focus because of its importance in synaptic plasticity and neurological diseases. The complexity of neuronal subcellular domains and their distance from the soma demand local spatial and temporal control of protein synthesis. Synthesis of many synaptic proteins, such as GluR and PSD-95, is under local control. mRNA binding proteins (RBPs), such as FMRP, function as key regulators of local RNA translation, and the mTORC1 pathway acts as a primary signaling cascade for regulation of these proteins. Much of the regulation occurs through structures termed RNA granules, which are based on reversible aggregation of the RBPs, some of which have aggregation prone domains with sequence features similar to yeast prion proteins. Mutations in many of these RBPs are associated with neurological diseases, including FMRP in fragile X syndrome; TDP-43, FUS (fused in sarcoma), angiogenin, and ataxin-2 in amyotrophic lateral sclerosis; ataxin-2 in spinocerebellar ataxia; and SMN (survival of motor neuron protein) in spinal muscular atrophy.

PATHOLOGIES OF AXONAL TRANSPORT IN NEURODEGENERATIVE DISEASES

Gene products such as organelles, proteins and RNAs are actively transported to synaptic terminals for the remodeling of pre-existing neuronal connections and formation of new ones. Proteins described as molecular motors mediate this transport and utilize specialized cytoskeletal proteins that function as molecular tracks for the motor based transport of cargos. Molecular motors such as kinesins and dynein's move along microtubule tracks formed by tubulins whereas myosin motors utilize tracks formed by actin. Deficits in active transport of gene products have been implicated in a number of neurological disorders. We describe such disorders collectively as "transportopathies". Here we review current knowledge of critical components of active transport and their relevance to neurodegenerative diseases.

Roles of neuronal activity-induced gene products in Hebbian and homeostatic synaptic plasticity, tagging, and capture

The efficiency of synaptic transmission undergoes plastic modification in response to changes in input activity. This phenomenon is most commonly referred to as synaptic plasticity and can involve different cellular mechanisms over time. In the short term, typically in the order of minutes to 1 h, synaptic plasticity is mediated by the actions of locally existing proteins. In the longer term, the synthesis of new proteins from existing or newly synthesized mRNAs is required to maintain the changes in synaptic transmission. Many studies have attempted to identify genes induced by neuronal activity and to elucidate the functions of the encoded proteins. In this chapter, we describe our current understanding of how activity can regulate the synthesis of new proteins, how the distribution of the newly synthesized protein is regulated in relation to the synapses undergoing plasticity and the function of these proteins in both Hebbian and homeostatic synaptic plasticity.

A Hypothesis about the Biological Basis of Expert Intuition

It is well established that intuition plays an important role in experts’ decision making and thinking generally. However, the theories that have been developed at the cognitive level have limits in their explanatory power and lack detailed explanation of the underlying biological mechanisms. In this paper, we bridge this gap by proposing that Hebb's (1949) concept of cell assembly is the biological realization of Simon's (1974) concept of chunking. This view provides mechanisms at the biological level that are consistent with both biological and psychological findings. To further address the limits of previous theories, we introduce emotions as a component of intuition by showing how they modulate the perception-memory interaction. The idea that intuition lies at the crossroads between perception, knowledge, and emotional modulation sheds new light on the phenomena of expertise and intuition.

Reelin supplementation enhances cognitive ability, synaptic plasticity, and dendritic spine density

Apolipoprotein receptors belong to an evolutionarily conserved surface receptor family that has intimate roles in the modulation of synaptic plasticity and is necessary for proper hippocampal-dependent memory formation. The known lipoprotein receptor ligand Reelin is important for normal synaptic plasticity, dendritic morphology, and cognitive function; however, the in vivo effect of enhanced Reelin signaling on cognitive function and synaptic plasticity in wild-type mice is unknown. The present studies test the hypothesis that in vivo enhancement of Reelin signaling can alter synaptic plasticity and ultimately influence processes of learning and memory. Purified recombinant Reelin was injected bilaterally into the ventricles of wild-type mice. We demonstrate that a single in vivo injection of Reelin increased activation of adaptor protein Disabled-1 and cAMP-response element binding protein after 15 min. These changes correlated with increased dendritic spine density, increased hippocampal CA1 long-term potentiation (LTP), and enhanced performance in associative and spatial learning and memory. The present study suggests that an acute elevation of in vivo Reelin can have long-term effects on synaptic function and cognitive ability in wild-type mice.

MicroRNA networks direct neuronal development and plasticity

MicroRNAs (miRNAs) constitute a class of small, non-coding RNAs that act as post-transcriptional regulators of gene expression. In neurons, the functions of individual miRNAs are just beginning to emerge, and recent studies have elucidated roles for neural miRNAs at various stages of neuronal development and maturation, including neurite outgrowth, dendritogenesis, and spine formation. Notably, miRNAs regulate mRNA translation locally in the axosomal and synaptodendritic compartments, and thereby contribute to the dynamic spatial organization of axonal and dendritic structures and their function. Given the critical role for miRNAs in regulating early brain development and in mediating synaptic plasticity later in life, it is tempting to speculate that the pathology of neurological disorders is affected by altered expression or functioning of miRNAs. Here we provide an overview of recently identified mechanisms of neuronal development and plasticity involving miRNAs, and the consequences of miRNA dysregulation.

Rapid increase in clusters of synaptophysin at onset of homosynaptic potentiation in Aplysia

Imaging studies have shown that even the earliest phases of long-term plasticity are accompanied by the rapid recruitment of synaptic components, which generally requires actin polymerization and may be one of the first steps in a program that can lead to the formation of new stable synapses during late-phase plasticity. However, most of those results come from studies of long-term potentiation in rodent hippocampus and might not generalize to other forms of synaptic plasticity or plasticity in other brain areas and species. For example, recruitment of presynaptic proteins during long-term facilitation by 5HT in Aplysia is delayed for several hours, suggesting that whereas activity-dependent forms of plasticity, such as long-term potentiation, involve rapid recruitment of presynaptic proteins, neuromodulatory forms of plasticity, such as facilitation by 5HT, involve more delayed recruitment. To begin to explore this hypothesis, we examined an activity-dependent form of plasticity, homosynaptic potentiation produced by tetanic stimulation of the presynaptic neuron in Aplysia. We found that homosynaptic potentiation involves presynaptic but not postsynaptic actin and a rapid (under 10 min) increase in the number of clusters of the presynaptic vesicle-associated protein synaptophysin. These results indicate that rapid recruitment of synaptic components is not limited to hippocampal potentiation and support the hypothesis that activity-dependent types of plasticity involve rapid recruitment of presynaptic proteins, whereas neuromodulatory types of plasticity involve more delayed recruitment.

Persistent long-term synaptic plasticity requires activation of a new signaling pathway by additional stimuli

Most memories are strengthened by additional stimuli, but it is unclear how additional stimulation or training reinforces long-term memory. To address this we examined whether long-term facilitation (LTF) of Aplysia sensorimotor synapses in cell culture-a cellular correlate of long-term sensitization of defensive withdrawal reflexes in Aplysia californica-can be prolonged by additional stimulation. We found that 1 d treatment with serotonin (5-HT; five brief applications at 20 min intervals) produced LTF lasting ∼3 d, whereas 2 d of such 5-HT treatments induced a persistent LTF lasting >7 d. Incubation with the protein synthesis inhibitor rapamycin during the second set of 5-HT treatments abolished all facilitation, and synapse strength returned prematurely to baseline. Persistent LTF required more persistent elevation in the expression of the neurotrophin-like peptide sensorin and its secretion. Activation of protein kinase C (PKC) during the second day of 5-HT treatments, not required for LTF or changes in sensorin expression during the first set of 5-HT treatments, is critical for persistent LTF and replaces phosphoinositide 3 kinase (PI3K) activity in mediating the increase in sensorin expression. In contrast, activations of PKC during the first day of 5-HT treatments and PI3K during the second day of 5-HT treatments are unnecessary for persistent LTF or the increases in sensorin expression. Thus, additional stimuli make preexisting plasticity labile as they recruit a new signaling cascade to regulate the synthesis of a neurotrophin-like peptide required for persistent alterations in synaptic efficacy.

Endogenous ion channel complexes: the NMDA receptor

Ionotropic receptors, including the NMDAR (N-methyl-D-aspartate receptor) mediate fast neurotransmission, neurodevelopment, neuronal excitability and learning. In the present article, the structure and function of the NMDAR is reviewed with the aim to condense our current understanding and highlight frontiers where important questions regarding the biology of this receptor remain unanswered. In the second part of the present review, new biochemical and genetic approaches for the investigation of ion channel receptor complexes will be discussed.

Neurexin-neuroligin transsynaptic interaction mediates learning-related synaptic remodeling and long-term facilitation in aplysia

Neurexin and neuroligin, which undergo heterophilic interactions with each other at the synapse, are mutated in some patients with autism spectrum disorder, a set of disorders characterized by deficits in social and emotional learning. We have explored the role of neurexin and neuroligin at sensory-to-motor neuron synapses of the gill-withdrawal reflex in Aplysia, which undergoes sensitization, a simple form of learned fear. We find that depleting neurexin in the presynaptic sensory neuron or neuroligin in the postsynaptic motor neuron abolishes both long-term facilitation and the associated presynaptic growth induced by repeated pulses of serotonin. Moreover, introduction into the motor neuron of the R451C mutation of neuroligin-3 linked to autism spectrum disorder blocks both intermediate-term and long-term facilitation. Our results suggest that activity-dependent regulation of the neurexin-neuroligin interaction may govern transsynaptic signaling required for the storage of long-term memory, including emotional memory that may be impaired in autism spectrum disorder.

The tail-elicited tail withdrawal reflex of Aplysia is mediated centrally at tail sensory-motor synapses and exhibits sensitization across multiple temporal domains

The defensive withdrawal reflexes of Aplysia californica have provided powerful behavioral systems for studying the cellular and molecular basis of memory formation. Among these reflexes the tail-elicited tail withdrawal reflex (T-TWR) has been especially useful. In vitro studies examining the monosynaptic circuit for the T-TWR, the tail sensory-motor (SN-MN) synapses, have identified the induction requirements and molecular basis of different temporal phases of synaptic facilitation that underlie sensitization in this system. They have also permitted more recent studies elucidating the role of synaptic and nuclear signaling during synaptic facilitation. Here we report the development of a novel, compartmentalized semi-intact T-TWR preparation that allows examination of the unique contributions of processing in the SN somatic compartment (the pleural ganglion) and the SN-MN synaptic compartment (the pedal ganglion) during the induction of sensitization. Using this preparation we find that the T-TWR is mediated entirely by central connections in the synaptic compartment. Moreover, the reflex is stably expressed for at least 24 h, and can be modified by tail shocks that induce sensitization across multiple temporal domains, as well as direct application of the modulatory neurotransmitter serotonin. This preparation now provides an experimentally powerful system in which to directly examine the unique and combined roles of synaptic and nuclear signaling in different temporal domains of memory formation.

Serotonin- and training-induced dynamic regulation of CREB2 in Aplysia

Long-term memory and plasticity, including long-term synaptic facilitation (LTF) of the Aplysia sensorimotor synapse, depend on the activation of transcription factors that regulate genes necessary for synaptic plasticity. In the present study we found that treatment with 5-HT and behavioral training produce biphasic changes in the expression of CREB2, a transcriptional repressor. An immediate increase in CREB2 protein was followed by a subsequent decrease. The effects of these treatments persist for at least 24 h and are observed in isolated sensory neurons. This study suggests that the dynamics of CREB2 expression could contribute to the consolidation of memory.