Local protein synthesis, actin dynamics, and LTP consolidation

Determinants of the induction of cortical plasticity by non-invasive brain stimulation in healthy subjects

The ability to induce cortical plasticity with non-invasive brain stimulation (NBS) techniques has provided novel and exciting opportunities for examining the role of the human cortex during a variety of behaviours. Additionally, and importantly, the induction of lasting changes in cortical excitability can, under some conditions, reversibly modify behaviour and interact with normal learning. Such findings have driven a large number of recent studies examining whether by using such approaches it might be possible to induce functionally significant changes in patients with a large variety of neurological and psychiatric conditions including stroke, Parkinson's disease and depression. However, even in neurologically normal subjects the variability in the neurophysiological and behavioural response to such brain stimulation techniques is high. This variability at present limits the therapeutic usefulness of these techniques. The cause of this variability is multifactorial and to some degree still unknown. However, a number of factors that can influence the induction of plasticity have been identified. This review will summarise what is known about the causes of variability in healthy subjects and propose additional factors that are likely to be important determinants. A greater understanding of these determinants is critical for optimising the therapeutic applications of non-invasive brain stimulation techniques.

Actin in dendritic spines: connecting dynamics to function

Dendritic spines are small actin-rich protrusions from neuronal dendrites that form the postsynaptic part of most excitatory synapses and are major sites of information processing and storage in the brain. Changes in the shape and size of dendritic spines are correlated with the strength of excitatory synaptic connections and heavily depend on remodeling of its underlying actin cytoskeleton. Emerging evidence suggests that most signaling pathways linking synaptic activity to spine morphology influence local actin dynamics. Therefore, specific mechanisms of actin regulation are integral to the formation, maturation, and plasticity of dendritic spines and to learning and memory.

Synaptic maturation of the Xenopus retinotectal system: effects of brain-derived neurotrophic factor on synapse ultrastructure

Synaptogenesis is a dynamic process that involves structural changes in developing axons and dendrites as synapses form and mature. The visual system of Xenopus laevis has been used as a model to study dynamic changes in axons and dendrites as synapses form in the living brain and the molecular mechanisms that control these processes. Brain-derived neurotrophic factor (BDNF) contributes to the establishment and refinement of visual connectivity by modulating retinal ganglion cell (RGC) axon arborization and presynaptic differentiation. Here, we have analyzed the ultrastructural organization of the Xenopus retinotectal system to understand better the maturation of this synaptic circuit and the relation between synapse ultrastructure and the structural changes in connectivity that take place in response to BDNF. Expression of yellow fluorescent protein (YFP) followed by preembedding immunoelectron microscopy was used to identify RGC axons specifically in living tadpoles. Injection of recombinant BDNF was used to alter endogenous BDNF levels acutely in the optic tectum. Our studies reveal a rapid transition from a relatively immature synaptic circuit in which retinotectal synapses are formed on developing filopodial-like processes to a circuit in which RGC axon terminals establish synapses with dendritic shafts and spines. Moreover, our studies reveal that BDNF treatment increases the number of spine synapses and docked vesicle number at YFP-identified synaptic sites within 24 hours of treatment. These fine structural changes at retinotectal synapses are consistent with the role that BDNF plays in the functional maturation of synaptic circuits and with dynamic, rapid changes in synaptic connectivity during development.

Brain-derived neurotrophic factor and the development of structural neuronal connectivity

During development, neural networks are established in a highly organized manner, which persists throughout life. Neurotrophins play crucial roles in the developing nervous system. Among the neurotrophins, brain-derived neurotrophic factor (BDNF) is highly conserved in gene structure and function during vertebrate evolution, and serves an important role during brain development and in synaptic plasticity. BDNF participates in the formation of appropriate synaptic connections in the brain, and disruptions in this process contribute to disorders of cognitive function. In this review, we first briefly highlight current knowledge on the expression, regulation, and secretion of BDNF. Further, we provide an overview of the possible actions of BDNF in the development of neural circuits, with an emphasis on presynaptic actions of BDNF during the structural development of central neurons.

Neuronal IP3 3-kinase is an F-actin-bundling protein: role in dendritic targeting and regulation of spine morphology

The actin microstructure in dendritic spines is involved in synaptic plasticity. Inositol trisphosphate 3-kinase A (ITPKA) terminates Ins(1,4,5)P(3) signals emanating from spines and also binds filamentous actin (F-actin) through its amino terminal region (amino acids 1-66, N66). Here we investigated how ITPKA, independent of its kinase activity, regulates dendritic spine F-actin microstructure. We show that the N66 region of the protein mediates F-actin bundling. An N66 fusion protein bundled F-actin in vitro, and the bundling involved N66 dimerization. By mutagenesis we identified a point mutation in a predicted helical region that eliminated both F-actin binding and bundling, rendering the enzyme cytosolic. A fusion protein containing a minimal helical region (amino acids 9-52, N9-52) bound F-actin in vitro and in cells, but had lower affinity. In hippocampal neurons, GFP-tagged N66 expression was highly polarized, with targeting of the enzyme predominantly to spines. By contrast, N9-52-GFP expression occurred in actin-rich structures in dendrites and growth cones. Expression of N66-GFP tripled the length of dendritic protrusions, induced longer dendritic spine necks, and induced polarized actin motility in time-lapse assays. These results suggest that, in addition to its ability to regulate intracellular Ca(2+) via Ins(1,4,5)P(3) metabolism, ITPKA regulates structural plasticity.

Emerging role for the cytoskeleton as an organizer and regulator of translation

The cytoskeleton is an intricate and dynamic fibrous network that has an essential role in the generation and regulation of cell architecture and cellular mechanical properties. The cytoskeleton also evolved as a scaffold that supports diverse biochemical pathways. Recent evidence favours the hypothesis that the cytoskeleton participates in the spatial organization and regulation of translation, at both the global and local level, in a manner that is crucial for cellular growth, proliferation and function.

Plasticity in the rat prefrontal cortex: linking gene expression and an operant learning with a computational theory

The plasticity in the medial Prefrontal Cortex (mPFC) of rodents or lateral prefrontal cortex in non human primates (lPFC), plays a key role neural circuits involved in learning and memory. Several genes, like brain-derived neurotrophic factor (BDNF), cAMP response element binding (CREB), Synapsin I, Calcium/calmodulin-dependent protein kinase II (CamKII), activity-regulated cytoskeleton-associated protein (Arc), c-jun and c-fos have been related to plasticity processes. We analysed differential expression of related plasticity genes and immediate early genes in the mPFC of rats during learning an operant conditioning task. Incompletely and completely trained animals were studied because of the distinct events predicted by our computational model at different learning stages. During learning an operant conditioning task, we measured changes in the mRNA levels by Real-Time RT-PCR during learning; expression of these markers associated to plasticity was incremented while learning and such increments began to decline when the task was learned. The plasticity changes in the lPFC during learning predicted by the model matched up with those of the representative gene BDNF. Herein, we showed for the first time that plasticity in the mPFC in rats during learning of an operant conditioning is higher while learning than when the task is learned, using an integrative approach of a computational model and gene expression.

Differential gene expression in the rat hippocampus during learning of an operant conditioning task

Changes in transcription levels of brain-derived neurotrophic factor (BDNF), cyclic adenosine monophosphate (cAMP) response element binding (CREB), Synapsin I, Ca(2+)/calmodulin-dependent protein kinase II (CamKII), activity-regulated cytoskeleton-associated protein (Arc), c-jun and c-fos have been associated to several learning paradigms in different brain areas. In this study, we measured mRNA expression in the hippocampus by real time (RT)-PCR mRNA levels of BDNF, CREB, Synapsin I, CamKII, Arc, c-jun and c-fos, during learning and operant conditioning task. Experimental groups were as follows: control (C, the animals never left the bioterium), when the animals reached 50-65% of the expected response (Incompletely Trained, IT), when animals reached 100% of the expected response with a latency time lower than 5 s (Trained, Tr), Box Control of Incompletely Trained (BCIT), animals spent the same time as the IT in the operant conditioning box and Box Control of Trained (BCTr) animals spent the same time as the Tr in the operant conditioning box. All rats were killed at the same time by cervical dislocation 15 min after training and hippocampi were removed and processed. We found increments of mRNA levels of most genes (BDNF, CREB, Synapsin I, Arc, c-jun and c-fos) in IT and Tr groups compared to their box controls, but increments in Tr were smaller compared with IT. These results describe a differential gene expression in the rat hippocampus when the animals are learning and when animals have already learned. Taking together the results presented herein with the known functions of these genes, we propose a link between changes in gene expression in the hippocampus and different degrees of cellular activation and plasticity during learning of an operant conditioning task.

Drebrin a knockout eliminates the rapid form of homeostatic synaptic plasticity at excitatory synapses of intact adult cerebral cortex

Homeostatic synaptic plasticity (HSP) is important for maintaining neurons' excitability within the dynamic range and for protecting neurons from unconstrained long-term potentiation that can cause breakdown of synapse specificity (Turrigiano [2008] Cell 135:422-435). Knowledge of the molecular mechanism underlying this phenomenon remains incomplete, especially for the rapid form of HSP. To test whether HSP in adulthood depends on an F-actin binding protein, drebrin A, mice deleted of the adult isoform of drebrin (DAKO) but retaining the embryonic isoform (drebrin E) were generated. HSP was assayed by determining whether the NR2A subunit of N-methyl-D-aspartate receptors (NMDARs) can rise rapidly within spines following the application of an NMDAR antagonist, D-APV, onto the cortical surface. Electron microscopic immunocytochemistry revealed that, as expected, the D-APV treatment of wild-type (WT) mouse cortex increased the proportion of NR2A-immunolabeled spines within 30 minutes relative to basal levels in hemispheres treated with an inactive enantiomer, L-APV. This difference was significant at the postsynaptic membrane and postsynaptic density (i.e., synaptic junction) as well as at nonsynaptic sites within spines and was not accompanied by spine size changes. In contrast, the D-APV treatment of DAKO brains did not augment NR2A labeling within the spine cytoplasm or at the synaptic junction, even though basal levels of NR2A were not significantly different from those of WT cortices. These findings indicate that drebrin A is required for the rapid (<30 minutes) form of HSP at excitatory synapses of adult cortices, whereas drebrin E is sufficient for maintaining basal NR2A levels within spines.

New insights in the biology of BDNF synthesis and release: implications in CNS function

BDNF has pleiotropic effects on neuronal development and synaptic plasticity that underlie circuit formation and cognitive function. Recent breakthroughs reveal that neuronal activity regulates BDNF cell biology, including Bdnf transcription, dendritic targeting and trafficking of BDNF mRNA and protein, and secretion and extracellular conversion of proBDNF to mature BDNF. Defects in these mechanisms contribute differentially to cognitive dysfunction and anxiety-like behaviors. Here we review recent studies, presented at a symposium at Neuroscience 2009, that describe regulatory mechanisms that permit rapid and dynamic refinement of BDNF actions in neurons.

Spine Modifications Associated with Long-Term Potentiation

Modification of neuronal connections is essential for the development of the nervous system and learning and memory functions of the mature brain. Structural modifications, such as modification of dendritic spines where the modified synapses reside, accompany and may even be required for these functional modifications. Recent advances in fluorescence microscopy, coupled with molecular approaches, prompted a rapid advance in the authors’ understanding of spine remodeling associated with synaptic plasticity, especially long-term potentiation. In this article, they review recent progress in this field, with focus on the potential functions of spine remodeling and key issues to be resolved.

Signals, synapses, and synthesis: how new proteins control plasticity

Localization of mRNAs to dendrites and local protein synthesis afford spatial and temporal regulation of gene expression and endow synapses with the capacity to autonomously alter their structure and function. Emerging evidence indicates that RNA binding proteins, ribosomes, translation factors and mRNAs encoding proteins critical to synaptic structure and function localize to neuronal processes. RNAs are transported into dendrites in a translationally quiescent state where they are activated by synaptic stimuli. Two RNA binding proteins that regulate dendritic RNA delivery and translational repression are cytoplasmic polyadenylation element binding protein and fragile X mental retardation protein (FMRP). The fragile X syndrome (FXS) is the most common known genetic cause of autism and is characterized by the loss of FMRP. Hallmark features of the FXS include dysregulation of spine morphogenesis and exaggerated metabotropic glutamate receptor-dependent long term depression, a cellular substrate of learning and memory. Current research focuses on mechanisms whereby mRNAs are transported in a translationally repressed state from soma to distal process and are activated at synaptic sites in response to synaptic signals.

BDNF signaling in the formation, maturation and plasticity of glutamatergic and GABAergic synapses

In the past 15 years numerous reports provided strong evidence that brain-derived neurotrophic factor (BDNF) is one of the most important modulators of glutamatergic and GABAergic synapses. Remarkable progress regarding localization, kinetics, and molecular mechanisms of BDNF secretion has been achieved, and a large number of studies provided evidence that continuous extracellular supply of BDNF is important for the proper formation and functional maturation of glutamatergic and GABAergic synapses. BDNF can play a permissive role in shaping synaptic networks, making them more susceptible for the occurrence of plastic changes. In addition, BDNF appears to be also an instructive factor for activity-dependent long-term synaptic plasticity. BDNF release just in response to synaptic stimulation might be a molecular trigger to convert high-frequency synaptic activity into long-term synaptic memories. This review attempts to summarize the current knowledge in synaptic secretion and synaptic action of BDNF, including both permissive and instructive effects of BDNF in synaptic plasticity.

The Arc of synaptic memory

The immediate early gene Arc is emerging as a versatile, finely tuned system capable of coupling changes in neuronal activity patterns to synaptic plasticity, thereby optimizing information storage in the nervous system. Here, we attempt to overview the Arc system spanning from transcriptional regulation of the Arc gene, to dendritic transport, metabolism, and translation of Arc mRNA, to post-translational modification, localization, and degradation of Arc protein. Within this framework we discuss the function of Arc in regulation of actin cytoskeletal dynamics underlying consolidation of long-term potentiation (LTP) and regulation of AMPA-type glutamate receptor endocytosis underlying long-term depression (LTD) and homeostatic plasticity. Behaviorally, Arc has a key role in consolidation of explicit and implicit forms of memory, with recent work implicating Arc in adaptation to stress as well as maladaptive plasticity connected to drug addiction. Arc holds considerable promise as a “master regulator” of protein synthesis-dependent forms of synaptic plasticity, but the mechanisms that modulate and switch Arc function are only beginning to be elucidated.

Dendritic spine formation and stabilization

Formation, elimination and remodeling of excitatory synapses on dendritic spines represent a continuous process that shapes the organization of synaptic networks during development. The molecular mechanisms controlling dendritic spine formation and stabilization therefore critically determine the rules of network selectivity. Recent studies have identified new molecules, such as Ephrins and Telencephalin that regulate filopodia motility and their transformation into dendritic spines. Trans-synaptic signaling involving nitric oxide, protease, adhesion molecules and Rho GTPases further controls contact formation or the structural remodeling of spines and their stability. Evidence also suggests that activity and induction of plasticity participate to the selection of persistent spines. Together these new data provide a better understanding of the mechanisms, speed and steps leading to the establishment of a stable excitatory synapse.

Regulation of glutamate receptor subunit availability by microRNAs

The efficacy of synaptic transmission depends, to a large extent, on postsynaptic receptor abundance. The molecular mechanisms controlling receptor abundance are poorly understood. We tested whether abundance of postsynaptic glutamate receptors (GluRs) in Drosophila neuromuscular junctions is controlled by microRNAs, and provide evidence that it is. We show here that postsynaptic knockdown of dicer-1, the endoribonuclease necessary for microRNA synthesis, leads to large increases in postsynaptic GluR subunit messenger RNA and protein. Specifically, we measured increases in GluRIIA and GluRIIB but not GluRIIC. Further, knockout of MiR-284, a microRNA predicted to bind to GluRIIA and GluRIIB but not GluRIIC, increases expression of GluRIIA and GluRIIB but not GluRIIC proportional to the number of predicted binding sites in each transcript. Most of the de-repressed GluR protein, however, does not appear to be incorporated into functional receptors, and only minor changes in synaptic strength are observed, which suggests that microRNAs primarily regulate Drosophila receptor subunit composition rather than overall receptor abundance or synaptic strength.

Molecular mechanisms of psychostimulant-induced structural plasticity

Drug addiction is characterized by persistent behavioral and cellular plasticity throughout the brain's reward regions. Among the many neuroadaptations that occur following repeated drug administration are alterations in cell morphology including changes in dendritic spines. While this phenomenon has been well documented, the underlying molecular mechanisms are poorly understood. Here, within the context of drug abuse, we review and integrate several of the established pathways known to regulate synaptic remodeling, and discuss the contributions of neurotrophic and dopamine signaling in mediating this structural plasticity. Finally, we discuss how such upstream mechanisms could regulate actin dynamics, the common endpoint involved in structural remodeling in neurons.

Synaptic output of individual layer 4 neurons in guinea pig visual cortex

More than 90% of geniculocortical axons from the dorsal lateral geniculate nucleus of the thalamus innervate layer 4 (L4) of V1 (primary visual cortex). Excitatory neurons, which comprise >80% of the neuronal population in L4, synapse mainly onto adjacent L4 neurons and layer 2/3 (L2/3) neurons. It has been suggested that intralaminar L4-L4 connections contribute to amplifying and refining thalamocortical signals before routing to L2/3. To unambiguously probe the properties of the synaptic outputs from these L4 excitatory neurons, we used multiple simultaneous whole-cell patch-clamp recording and stimulation from two to four neighboring L4 neurons. We recorded uEPSCs (evoked unitary synaptic currents) in response to pairs of action potentials elicited in single presynaptic L4 neurons for 102 L4 cell pairs and found that their properties are more diverse than previously described. Averaged unitary synaptic response peak amplitudes spanned almost three orders of magnitude, from 0.42 to 192.92 pA. Although connections were, on average, reliable (average failure rate, 25%), we recorded a previously unknown subset of unreliable (failure rates from 30 to 100%) and weak (averaged response amplitudes, <5 pA) connections. Reliable connections with high probability of neurotransmitter release responded to paired presynaptic pulses with depression, whereas unreliable connections underwent paired-pulse facilitation. Recordings from interconnected sets of L4 triplets revealed that synaptic response amplitudes and reliability were equally variable between independent cell pairs and those that shared a common presynaptic or postsynaptic cell, suggesting local perisynaptic influences on the development and/or state of synaptic function.

Selective survival and maturation of adult-born dentate granule cells expressing the immediate early gene Arc/Arg3.1

Progenitor cells in the adult dentate gyrus provide a constant supply of neuronal precursors, yet only a small fraction of these cells survive and develop into mature dentate granule cells (DGCs). A major challenge of current research is thus to understand the stringent selection process that governs the maturation and functional integration of adult-born DGCs. In mature DGCs, high-frequency stimulation (HFS) of the perforant path input elicits robust expression of the immediate early gene Arc/Arg3.1, trafficking of its mRNA to dendrites, and local synthesis of the protein necessary for consolidation of long-term potentiation (LTP). Given the synaptic commitment inherent in LTP consolidation, we considered that HFS-evoked expression of Arc could be used to timemap the functional integration of newborn DGCs. Dividing cells were birthmarked by BrdU-labeling at 1, 7, 14, 21, or 28 days prior to induction of LTP and expression of Arc was examined by confocal microscopy. Contrary to expectation, LTP did not induce Arc expression in newborn cells at any age, suggesting they might be refractory to synaptically-evoked Arc expression for at least one month. Importantly, however, spontaneous expression of Arc was detected in BrdU-labeled cells and strongly associated with the survival and maturation of NeuN-positive DGCs. Moreover, Arc expression at the earliest ages (1 and 7 days), clearly precedes the formation of glutamatergic synapses on new neurons. These results suggest an unexpected early role for Arc in adult-born DGCs, distinct from its functions in LTP, LTD, and homeostatic synaptic plasticity.

Object-place recognition learning triggers rapid induction of plasticity-related immediate early genes and synaptic proteins in the rat dentate gyrus

Long-term recognition memory requires protein synthesis, but little is known about the coordinate regulation of specific genes. Here, we examined expression of the plasticity-associated immediate early genes (Arc, Zif268, and Narp) in the dentate gyrus following long-term object-place recognition learning in rats. RT-PCR analysis from dentate gyrus tissue collected shortly after training did not reveal learning-specific changes in Arc mRNA expression. In situ hybridization and immunohistochemistry were therefore used to assess possible sparse effects on gene expression. Learning about objects increased the density of granule cells expressing Arc, and to a lesser extent Narp, specifically in the dorsal blade of the dentate gyrus, while Zif268 expression was elevated across both blades. Thus, object-place recognition triggers rapid, blade-specific upregulation of plasticity-associated immediate early genes. Furthermore, Western blot analysis of dentate gyrus homogenates demonstrated concomitant upregulation of three postsynaptic density proteins (Arc, PSD-95, and α-CaMKII) with key roles in long-term synaptic plasticity and long-term memory.

The immediate early gene arc/arg3.1: regulation, mechanisms, and function

In a manner unique among activity-regulated immediate early genes (IEGs), mRNA encoded by Arc (also known as Arg3.1) undergoes rapid transport to dendrites and local synaptic translation. Despite this intrinsic appeal, relatively little is known about the neuronal and behavioral functions of Arc or its molecular mechanisms of action. Here, we attempt to distill recent advances on Arc spanning its transcriptional and translational regulation, the functions of the Arc protein in multiple forms of neuronal plasticity [long-term potentiation (LTP), long-term depression (LTD), and homeostatic plasticity], and its broader role in neural networks of behaving animals. Worley and colleagues have shown that Arc interacts with endophilin and dynamin, creating a postsynaptic trafficking endosome that selectively modifies the expression of AMPA-type glutamate receptors at the excitatory synapses. Both LTD and homeostatic plasticity in the hippocampus are critically dependent on Arc-mediated endocytosis of AMPA receptors. LTD evoked by activation of metabotropic glutamate receptors depends on rapid Arc translation controlled by elongation factor 2. Bramham and colleagues have shown that sustained translation of newly induced Arc mRNA is necessary for cofilin phosphorylation and stable expansion of the F-actin cytoskeleton underlying LTP consolidation in the dentate gyrus of live rats. In addition to regulating F-actin, Arc synthesis maintains the activity of key translation factors during LTP consolidation. This process of Arc-dependent consolidation is activated by the secretory neurotrophin, BDNF. Moore and colleagues have shown that Arc mRNA is a natural target for nonsense-mediated mRNA decay (NMD) by virtue of its two conserved 3'-UTR introns. NMD and other related translation-dependent mRNA decay mechanisms may serve as critical brakes on protein expression that contribute to the fine spatial-temporal control of Arc synthesis. In studies in behaving rats, Guzowski and colleagues have shown that location-specific firing of CA3 and CA1 hippocampal neurons in the presence of theta rhythm provides the necessary stimuli for activation of Arc transcription. The impact of Arc transcription in memory processes may depend on the specific context of coexpressed IEGs, in addition to posttranscriptional regulation of Arc by neuromodulatory inputs from the amygdala and other brain regions. In sum, Arc is emerging as a versatile, finely tuned system capable of coupling changes in neuronal activity patterns to diverse forms of synaptic plasticity, thereby optimizing information storage in active networks.