Rapid actin-based plasticity in dendritic spines

Molecular changes associated with hippocampal long-lasting depression induced by the serine protease subtilisin-A

The serine protease subtilisin-A (SubA) induces a form of long-term depression (LTD) of synaptic transmission in the rat hippocampus, and molecular changes associated with SubA-induced LTD (SubA-LTD) were explored by using recordings of evoked postsynaptic potentials and immunoblotting. SubA-LTD was prevented by a selective inhibitor of SubA proteolysis, but the same inhibitor did not affect LTD induced by electrical stimulation or activation of metabotropic glutamate receptors. SubA-LTD was reduced by the protein kinase inhibitors genistein and lavendustin A, although not by inhibitors of p38 mitogen-activated protein kinase, glycogen synthase kinase-3, or protein phosphatases. It was also reduced by (RS)-α-methyl-4-carboxyphenylglycine, a broad-spectrum antagonist at metabotropic glutamate receptors. Inhibition of the Rho kinase enzyme Rho-associated coiled-coil kinase reduced SubA-LTD, although inhibitors of the RhoGTPase-activating enzymes farnesyl transferase and geranylgeranyl transferase did not. In addition, a late phase of SubA-LTD was dependent on new protein synthesis. There was a small, non-significant difference in SubA-LTD between wild-type and RhoB(-/-) mice. Marked decreases were seen in the levels of Unc-5H3, a protein that is intimately involved in the development and plasticity of glutamatergic synapses. Smaller changes were noted, at higher concentrations of SubA, in Unc-5H1, vesicle-associated membrane protein-1 (synaptobrevin), and actin, with no changes in the levels of synaptophysin, synaptotagmin, RhoA, or RhoB. None of these changes was associated with LTD induced electrically or by the metabotropic glutamate receptor agonist (RS)-3,5-dihydroxyphenylglycine. These results indicate that SubA induces molecular changes that overlap with other forms of LTD, but that the overall molecular profile of SubA-LTD is quite different.

General considerations for live imaging of developing hippocampal neurons in culture

Dissociated cell cultures of the rodent hippocampus have become a standard model for studying many facets of neural development, including the development of polarity, axonal and dendritic growth, and synapse formation. The cultures are quite homogeneous--∼90% of the cells are pyramidal neurons--and it is relatively easy to express green fluorescent protein (GFP)-tagged proteins by transfection. This article describes the cultures and the key features of the system used to image them. It also includes suggestions on labeling cells with GFP-tagged proteins. It concludes with a discussion of the advantages and disadvantages of this culture system.

EphA signaling promotes actin-based dendritic spine remodeling through slingshot phosphatase

Actin cytoskeletal remodeling plays a critical role in transforming the morphology of subcellular structures across various cell types. In the brain, restructuring of dendritic spines through actin cytoskeleletal reorganization is implicated in the regulation of synaptic efficacy and the storage of information in neural circuits. However, the upstream pathways that provoke actin-based spine changes remain only partly understood. Here we show that EphA receptor signaling remodels spines by triggering a sequence of events involving actin filament rearrangement and synapse/spine reorganization. Rapid EphA signaling over minutes activates the actin filament depolymerizing/severing factor cofilin, alters F-actin distribution in spines, and causes transient spine elongation through the phosphatases slingshot 1 (SSH1) and calcineurin/protein phosphatase 2B (PP2B). This early phase of spine extension is followed by synaptic reorganization events that take place over minutes to hours and involve the relocation of pre/postsynaptic components and ultimately spine retraction. Thus, EphA receptors utilize discrete cellular and molecular pathways to promote actin-based structural plasticity of excitatory synapses.

β-Pix modulates actin-mediated recruitment of synaptic vesicles to synapses

Presynaptic compartments are formed through the recruitment of preassembled clusters of proteins to points of cell-cell contact, however, the molecular mechanism(s) underlying this process remains unclear. We demonstrate that clusters of polymerized actin can recruit and maintain synaptic vesicles to discrete sites along the axon, and that cadherin/β-catenin/scribble/β-pix complexes play an important role in this event. Previous work has demonstrated that β-catenin and scribble are important for the clustering of vesicles at synapses. We demonstrate that β-pix, a Rac/Cdc42 guanine nucleotide exchange factor (GEF), forms a complex with cadherin, β-catenin, and scribble at synapses and enhances localized actin polymerization in rat hippocampal neurons. In cells expressing β-pix siRNA or dominant-negative β-pix that lacks its GEF activity, actin polymerization at synapses is dramatically reduced, and synaptic vesicle localization is disrupted. This β-pix phenotype can be rescued by cortactin overexpression, suggesting that β-pix-mediated actin polymerization at synapses regulates vesicle localization.

Magnetic resonance microscopy of human and porcine neurons and cellular processes

With its unparalleled ability to safely generate high-contrast images of soft tissues, magnetic resonance imaging (MRI) has remained at the forefront of diagnostic clinical medicine. Unfortunately due to resolution limitations, clinical scans are most useful for detecting macroscopic structural changes associated with a small number of pathologies. Moreover, due to a longstanding inability to directly observe magnetic resonance (MR) signal behavior at the cellular level, such information is poorly characterized and generally must be inferred. With the advent of the MR microscope in 1986 came the ability to measure MR signal properties of theretofore unobservable tissue structures. Recently, further improvements in hardware technology have made possible the ability to visualize mammalian cellular structure. In the current study, we expand upon previous work by imaging the neuronal cell bodies and processes of human and porcine α-motor neurons. Complimentary imaging studies are conducted in pig tissue in order to demonstrate qualitative similarities to human samples. Also, apparent diffusion coefficient (ADC) maps were generated inside porcine α-motor neuron cell bodies and portions of their largest processes (mean=1.7 ± 0.5 μm²/ms based on 53 pixels) as well as in areas containing a mixture of extracellular space, microvasculature, and neuropil (0.59 ± 0.37 μm²/ms based on 33 pixels). Three-dimensional reconstruction of MR images containing α-motor neurons shows the spatial arrangement of neuronal projections between adjacent cells. Such advancements in imaging portend the ability to construct accurate models of MR signal behavior based on direct observation and measurement of the components which comprise functional tissues. These tools would not only be useful for improving our interpretation of macroscopic MRI performed in the clinic, but they could potentially be used to develop new methods of differential diagnosis to aid in the early detection of a multitude of neuropathologies.

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.

Methods for three-dimensional analysis of dendritic spine dynamics

Dendritic spines are small bulbous expansions that receive input from a single excitatory synapse. Although spines are often characterized by a mushroom-like morphology, they come in a wide range of sizes and shapes, even within the same dendrite. In a developing brain, spines exhibit a high degree of structural and functional plasticity, reflecting the formation and elimination of synapses during the maturation of neuronal circuits. The morphology of spines in developing neurons is affected by synaptic activity, hence contributing to the experience-dependent refinement of neuronal circuits, learning, and memory. Thus, understanding spine dynamics and its regulation is of central importance to studies of synaptic plasticity in the brain. The challenge has been to develop a computer-based assay that will quantitatively assess the three-dimensional change in spine movements caused by various stimuli and experimental conditions. Here, we provide detailed protocols for cell plating, transient transfections, and time-lapse imaging of dendritic spines. For the analysis of dendritic spine dynamics, we present two methods based on quantitative three-dimensional measurements.

Rapid experience-dependent plasticity of synapse function and structure in ferret visual cortex in vivo

The rules by which visual experience influences neuronal responses and structure in the developing brain are not well understood. To elucidate the relationship between rapid functional changes and dendritic spine remodeling in vivo, we carried out chronic imaging experiments that tracked visual responses and dendritic spines in the ferret visual cortex following brief periods of monocular deprivation. Functional changes, which were largely driven by loss of deprived eye responses, were tightly regulated with structural changes at the level of dendritic spines, and occurred very rapidly (on a timescale of hours). The magnitude of functional changes was correlated with the magnitude of structural changes across the cortex, and both these features reversed when the deprived eye was reopened. A global rule governed how the responses to the two eyes or changes in spines were altered by monocular deprivation: the changes occurred irrespective of regional ocular dominance preference and were independently mediated by each eye, and the loss or gain of responses/spines occurred as a constant proportion of predeprivation drive by the deprived or nondeprived eye, respectively.

Depolarization gates spine calcium transients and spike-timing-dependent potentiation

Timing-dependent long-term potentiation (t-LTP) is induced when synaptic activity is immediately followed by one or more back-propagating action potentials (bAPs) in the postsynaptic cell. As a mechanistic explanation, it has been proposed that the bAP removes the Mg2+ block of synaptic NMDA receptors, allowing for rapid Ca2+ entry at the active synapse. Recent experimental studies suggest that this model is incomplete: NMDA receptor-based coincidence detection requires strong postsynaptic depolarization, usually provided by AMPA receptor currents. Apparently, the brief AMPA-EPSP does not only enable t-LTP, it is also responsible for the very narrow time window for t-LTP induction. The emerging consensus puts the spine in the center of coincidence detection, as active conductances on the spine together with the electrical resistance of the spine neck regulate the depolarization of the spine head and thus Ca2+ influx during pairing. A focus on postsynaptic voltage during synaptic activation not only encompasses spike-timing-dependent plasticity (STDP), but explains also the cooperativity and frequency-dependence of plasticity.

Dendritic spines and distributed circuits

Dendritic spines receive most excitatory connections in pyramidal cells and many other principal neurons. But why do neurons use spines, when they could accommodate excitatory contacts directly on their dendritic shafts? One suggestion is that spines serve to connect with passing axons, thus increasing the connectivity of the dendrites. Another hypothesis is that spines are biochemical compartments that enable input-specific synaptic plasticity. A third possibility is that spines have an electrical role, filtering synaptic potentials and electrically isolating inputs from each other. In this review, I argue that, when viewed from the perspective of the circuit function, these three functions dovetail with one another to achieve a single overarching goal: to implement a distributed circuit with widespread connectivity. Spines would endow these circuits with nonsaturating, linear integration and input-specific learning rules, which would enable them to function as neural networks, with emergent encoding and processing of information.

Synapse adhesion: a dynamic equilibrium conferring stability and flexibility

Cell adhesion molecules (CAMs) linked to cytoskeleton generate stable cell-cell junctions. Cadherins provide a canonical example, but paradoxically, they participate in a multitude of transient and regulatable interactions. Their extracellular binding generates weak adhesion that is modified by clustering; interactions with F-actin are regulated, can be transient, and can alter F-actin dynamics. Additionally, cadherin recycling from the cell surface can modify the size and location of junctions and strength of adhesion. In epithelial cells, this ongoing dynamic behavior is important for maintaining stable junctions. Recent work supports that cadherins act similarly at synapses where their actions are likely to be shared by integrins and other actin-linked CAMs. Together the collaborative activities of such CAMs provide a stable, but flexible structure that can promote and support changes in synapse shape and size while maintaining stable junctions to permit information flow.

Moderate traumatic brain injury causes acute dendritic and synaptic degeneration in the hippocampal dentate gyrus

Hippocampal injury-associated learning and memory deficits are frequent hallmarks of brain trauma and are the most enduring and devastating consequences following traumatic brain injury (TBI). Several reports, including our recent paper, showed that TBI brought on by a moderate level of controlled cortical impact (CCI) induces immature newborn neuron death in the hippocampal dentate gyrus. In contrast, the majority of mature neurons are spared. Less research has been focused on these spared neurons, which may also be injured or compromised by TBI. Here we examined the dendrite morphologies, dendritic spines, and synaptic structures using a genetic approach in combination with immunohistochemistry and Golgi staining. We found that although most of the mature granular neurons were spared following TBI at a moderate level of impact, they exhibited dramatic dendritic beading and fragmentation, decreased number of dendritic branches, and a lower density of dendritic spines, particularly the mushroom-shaped mature spines. Further studies showed that the density of synapses in the molecular layer of the hippocampal dentate gyrus was significantly reduced. The electrophysiological activity of neurons was impaired as well. These results indicate that TBI not only induces cell death in immature granular neurons, it also causes significant dendritic and synaptic degeneration in pathohistology. TBI also impairs the function of the spared mature granular neurons in the hippocampal dentate gyrus. These observations point to a potential anatomic substrate to explain, in part, the development of posttraumatic memory deficits. They also indicate that dendritic damage in the hippocampal dentate gyrus may serve as a therapeutic target following TBI.

Spinal cord injury, dendritic spine remodeling, and spinal memory mechanisms

Spinal cord injury (SCI) often results in the development of neuropathic pain, which can persist for months and years after injury. Although many aberrant changes to sensory processing contribute to the development of chronic pain, emerging evidence demonstrates that mechanisms similar to those underlying classical learning and memory can contribute to central sensitization, a phenomenon of amplified responsiveness to stimuli in nociceptive dorsal horn neurons. Notably, dendritic spines have emerged as major players in learning and memory, providing a structural substrate for how the nervous system modifies connections to form and store information. Until now, most information regarding dendritic spines has been obtained from studies in the brain. Recent experimental data in the spinal cord, however, demonstrate that Rac1-regulated dendritic spine remodeling occurs on second-order wide dynamic range neurons and accompanies neuropathic pain after SCI. Thus, SCI-induced synaptic potentiation engages a putative spinal memory mechanism. A compelling, novel possibility for pain research is that a synaptic model of long-term memory storage could explain the persistent nature of neuropathic pain. Such a conceptual bridge between pain and memory could guide the development of more effective strategies for treatment of chronic pain after injury to the nervous system.

Beta- and gamma-cytoplasmic actins are required for meiosis in mouse oocytes

In mammals, female meiosis consists of two asymmetric cell divisions, which generate a large haploid oocyte and two small polar bodies. Asymmetric partitioning of the cytoplasm results from migration of the meiotic spindle toward the cortex and requires actin filaments. However, the subcellular localization and the role of the existing two cytoplasmic actin (CYA) isoforms, beta and gamma, have not been characterized. We show that beta- and gamma-CYA are differentially distributed in the maturing oocyte from late metaphase I as well as in preimplantation embryos. Gamma-CYA is preferentially enriched in oocyte cortices and is absent from all cell-cell contact areas from metaphase II until the blastocyst stage. Beta-CYA is enriched in contractile structures, at cytokinesis, at cell-cell contacts, and around the forming blastocoel. Alteration of beta- or gamma-CYA function by isoform-specific antibody microinjection suggests that gamma-CYA holds a major and specific role in the establishment and/or maintenance of asymmetry in meiosis I and in the maintenance of overall cortical integrity. In contrast, beta- and gamma-CYA, together, appear to participate in the formation and the cortical anchorage of the second meiotic spindle in waiting for fertilization. Finally, differences in gamma-CYA expression are amongst the earliest markers of cell fate determination in development.

Rapid and reversible formation of spine head filopodia in response to muscarinic receptor activation in CA1 pyramidal cells

A key feature at excitatory synapses is the remodelling of dendritic spines, which in conjunction with receptor trafficking modifies the efficacy of neurotransmission. Here we investigated whether activation of cholinergic receptors, which can modulate synaptic plasticity, also mediates changes in dendritic spine structure. Using confocal time-lapse microscopy in mouse slice cultures we found that brief activation of muscarinic receptors induced the emergence of fine filopodia from spine heads in all CA1 pyramidal cells examined. This response was widespread occurring in 48% of imaged spines, appeared within minutes, was reversible, and was blocked by atropine. Electron microscopic analyses showed that the spine head filopodia (SHFs) extend along the presynaptic bouton. In addition, the decay time of miniature EPSCs was longer after application of the muscarinic acetylcholine receptor agonist methacholine (MCh). Both morphological and electrophysiological changes were reduced by preventing microtubule polymerization with nocodazole. This extension of SHFs during cholinergic receptor activation represents a novel structural form of subspine plasticity that may regulate synaptic properties by fine-tuning interactions between presynaptic boutons and dendritic spines.

Modulation of dendritic spine development and plasticity by BDNF and vesicular trafficking: fundamental roles in neurodevelopmental disorders associated with mental retardation and autism

The process of axonal and dendritic development establishes the synaptic circuitry of the central nervous system (CNS) and is the result of interactions between intrinsic molecular factors and the external environment. One growth factor that has a compelling function in neuronal development is the neurotrophin brain-derived neurotrophic factor (BDNF). BDNF participates in axonal and dendritic differentiation during embryonic stages of neuronal development, as well as in the formation and maturation of dendritic spines during postnatal development. Recent studies have also implicated vesicular trafficking of BDNF via secretory vesicles, and both secretory and endosomal trafficking of vesicles containing synaptic proteins, such as neurotransmitter and neurotrophin receptors, in the regulation of axonal and dendritic differentiation, and in dendritic spine morphogenesis. Several genes that are either mutated or deregulated in neurodevelopmental disorders associated with mental retardation have now been identified, and several mouse models of these disorders have been generated and characterized. Interestingly, abnormalities in dendritic and synaptic structure are consistently observed in human neurodevelopmental disorders associated with mental retardation, and in mouse models of these disorders as well. Abnormalities in dendritic and synaptic differentiation are thought to underlie altered synaptic function and network connectivity, thus contributing to the clinical outcome. Here, we review the roles of BDNF and vesicular trafficking in axonal and dendritic differentiation in the context of dendritic and axonal morphological impairments commonly observed in neurodevelopmental disorders associated with mental retardation.

Inhibition of matrix metalloproteinases prevents the potentiation of nondeprived-eye responses after monocular deprivation in juvenile rats

The ocular dominance (OD) shift induced by monocular deprivation (MD) during the critical period is mediated by an initial depression of deprived-eye responses followed by an increased responsiveness to the nondeprived eye. It is not fully clear to what extent these 2 events are correlated and which are their physiological and molecular mediators. The extracellular synaptic environment plays an important role in regulating visual cortical plasticity. Matrix metalloproteinases (MMPs) are a family of activity-dependent zinc-dependent extracellular endopeptidases mediating extracellular matrix remodeling. We investigated the effects of MMP inhibition on OD plasticity in juvenile monocularly deprived rats. By using electrophysiological recordings, we found that MMP inhibition selectively prevented the potentiation of neuronal responses to nondeprived-eye stimulation occurring after 7 days of MD and potentiation of deprived-eye responses occurring after eye reopening. Three days of MD only resulted in a depression of deprived-eye responses insensitive to MMP inhibition. MMP inhibition did not influence homeostatic plasticity tested in the monocular cortex but significantly prevented an increase in dendritic spine density present after 7 days MD in layer II-III pyramids.

Structural modulation of dendritic spines during synaptic plasticity

The majority of excitatory synaptic input in the brain is received by small bulbous actin-rich protrusions residing on the dendrites of glutamatergic neurons. These dendritic spines are the major sites of information processing in the brain. This conclusion is reinforced by the observation that many higher cognitive disorders, such as mental retardation, Rett syndrome, and autism, are associated with aberrant spine morphology. Mechanisms that regulate the maturation and plasticity of dendritic spines are therefore fundamental to understanding higher brain functions including learning and memory. It is well known that activity-driven changes in synaptic efficacy modulate spine morphology due to alterations in the underlying actin cytoskeleton. Recent studies have elucidated numerous molecular regulators that directly alter actin dynamics within dendritic spines. This review will emphasize activity-dependent changes in spine morphology and highlight likely roles of these actin-binding proteins.

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.

Which Comes First in Fragile X Syndrome, Dendritic Spine Dysgenesis or Defects in Circuit Plasticity?

The salient neuropathological defect in fragile X syndrome is the overabundance of immature dendritic spines in cortical pyramidal neurons. This review examines this anatomical synaptic defect in the context of other alterations in synaptic and circuit plasticity in fragile X mice. In theory, abnormal spines could lead to dysfunctional circuits and vice versa, so it is still not clear which problem comes first. Because of the tight structure-function relationships at the synapse, and given the significant overlap between signaling pathways that regulate spine shape/dynamics and long-term synaptic plasticity (both of which involve proteins regulated by fragile X mental retardation protein [FMRP]), it is argued that the two defects cannot be separated. It will be critical to determine whether neurons that lack FMRP and demonstrate alterations in long-term potentiation/depression also fail to undergo the expected enlargement/shrinkage of dendritic spines associated with those forms of synaptic plasticity or to establish clear links from FMRP signaling to either spine instability or defective synaptic plasticity, especially during critical periods of brain development. The resulting data will be vital in guiding translational research that can identify novel molecular targets for therapy in this devastating disorder.

EphA4 expression promotes network activity and spine maturation in cortical neuronal cultures

Background

Neurons form specific connections with targets via synapses and patterns of synaptic connectivity dictate neural function. During development, intrinsic neuronal specification and environmental factors guide both initial formation of synapses and strength of resulting connections. Once synapses form, non-evoked, spontaneous activity serves to modulate connections, strengthening some and eliminating others. Molecules that mediate intercellular communication are particularly important in synaptic refinement. Here, we characterize the influences of EphA4, a transmembrane signaling molecule, on neural connectivity.

Results

Using multi-electrode array analysis on in vitro cultures, we confirmed that cortical neurons mature and generate spontaneous circuit activity as cells differentiate, with activity growing both stronger and more patterned over time. When EphA4 was over-expressed in a subset of neurons in these cultures, network activity was enhanced: bursts were longer and were composed of more spikes than in control-transfected cultures. To characterize the cellular basis of this effect, dendritic spines, the major excitatory input site on neurons, were examined on transfected neurons in vitro. Strikingly, while spine number and density were similar between conditions, cortical neurons with elevated levels of EphA4 had significantly more mature spines, fewer immature spines, and elevated colocalization with a mature synaptic marker.

Conclusions

These results demonstrate that experimental elevation of EphA4 promotes network activity in vitro, supporting spine maturation, producing more functional synaptic pairings, and promoting more active circuitry.

Mild traumatic brain injury results in extensive neuronal degeneration in the cerebral cortex

Mild traumatic brain injury (mTBI) leads to long-term cognitive and emotional difficulties and behavioral disturbances, but the diagnosis and treatment of mTBI have historically been hampered by a lack of evidence-based correlates of these clinical manifestations. Unlike moderate and severe TBI, mTBI does not show significant tissue lesions or cavities in the cortex. Moreover, neuroimaging by magnetic resonance imaging or computed tomography is usually negative, suggesting that the damage is beyond the resolution of current structure-based scanning technologies. Therefore, we investigated the morphologies of spared neurons in the mouse cortex after mTBI in a controlled cortical impact injury model. Our results indicate that, although mTBI caused only a mild extent of cell death, it led to extensive dendrite degeneration and synapse reduction in the cortex in this model. This study sheds light on the neuropathologic consequences of mTBI in humans and suggests that neurodegeneration may be a novel target for developing diagnostic methods and therapeutic approaches for mTBI.

Development and application of probes for labeling the actin cytoskeleton in living plant cells

The actin cytoskeleton is one of the most important components of eukaryotic cytoskeletons. It participates in numerous crucial procedures of cells and has been studied by using various methods. The development and application of appropriate probes for actin visualization is the first and foremost step for functional analysis of actin in vivo. Since the actin cytoskeleton is a highly dynamic and sensitive structure, methods previously used to visualize actin often harm cells and cannot reveal the native state of the actin cytoskeleton in living cells. The development of labeling technologies for living plant cells, especially the emergence and application of green fluorescent protein-tagged actin markers, has provided new insights into the structure and function of the actin cytoskeleton in vivo. There has been a number of probes for actin labeling in living plant cells though they each present different advantages and defects. In this review, we discuss and compare those widely used methods for actin visualization and analysis.

The role of amygdaloid brain-derived neurotrophic factor, activity-regulated cytoskeleton-associated protein and dendritic spines in anxiety and alcoholism

Innate anxiety appears to be a robust factor in the promotion of alcohol intake, possibly due to the anxiolytic effects of self-medication with alcohol. Brain-derived neurotrophic factor (BDNF) and its downstream target, activity-regulated cytoskeleton-associated (Arc) protein, play a role in the regulation of synaptic function and structure. In order to examine the role of the BDNF-Arc system and associated dendritic spines in the anxiolytic effects of ethanol, we investigated the effects of acute ethanol exposure on anxiety-like behaviors of alcohol-preferring (P) and -nonpreferring (NP) rats. We also examined changes in the expression of BDNF and Arc, and dendritic spine density (DSD), in amygdaloid brain regions of P and NP rats with or without ethanol exposure. It was found that in comparison with NP rats, P rats displayed innate anxiety-like behaviors, and had lower mRNA and protein levels of both BDNF and Arc, and also had lower DSD in the central amygdala (CeA) and medial amygdala (MeA), but not in the basolateral amygdala (BLA). Acute ethanol treatment had an anxiolytic effect in P, but not in NP rats, and was associated with an increase in mRNA and protein levels of BDNF and Arc, and in DSD in the CeA and MeA, but not BLA. These results suggest that innate deficits in BDNF-Arc levels, and DSD, in the CeA and MeA may be involved in the anxiety-like and excessive alcohol-drinking behaviors of P rats, as ethanol increased these amygdaloid synaptic markers and produced anxiolytic effects in P rats, but not NP rats.

A global spatial similarity optimization scheme to track large numbers of dendritic spines in time-lapse confocal microscopy

Dendritic spines form postsynaptic contact sites in the central nervous system. The rapid and spontaneous morphology changes of spines have been widely observed by neurobiologists. Determining the relationship between dendritic spine morphology change and its functional properties such as memory learning is a fundamental yet challenging problem in neurobiology research. In this paper, we propose a novel algorithm to track the morphology change of multiple spines simultaneously in time-lapse neuronal images based on nonrigid registration and integer programming. We also propose a robust scheme to link disappearing-and-reappearing spines. Performance comparisons with other state-of-the-art cell and spine tracking algorithms, and the ground truth show that our approach is more accurate and robust, and it is capable of tracking a large number of neuronal spines in time-lapse confocal microscopy images.

Phosphoinositide-3-kinase activation controls synaptogenesis and spinogenesis in hippocampal neurons

The possibility of changing the number of synapses may be an important asset in the treatment of neurological diseases. In this context, the synaptogenic role of the phosphoinositide-3-kinase (PI3K) signaling cascade has been previously demonstrated in Drosophila. This study shows that treatment with a PI3K-activating transduction peptide is able to promote synaptogenesis and spinogenesis in primary cultures of rat hippocampal neurons, as well as in CA1 hippocampal neurons in vivo. In culture, the peptide increases synapse density independently of cell density, culture age, dendritic complexity, or synapse type. The induced synapses also increase neurotransmitter release from cultured neurons. The synaptogenic signaling pathway includes PI3K-Akt. Furthermore, the treatment is effective on adult neurons, where it induces spinogenesis and enhances the cognitive behavior of treated animals in a fear-conditioning assay. These findings demonstrate that functional synaptogenesis can be induced in mature mammalian brains through PI3K activation.

The actin binding domain of βI-spectrin regulates the morphological and functional dynamics of dendritic spines

Actin microfilaments regulate the size, shape and mobility of dendritic spines and are in turn regulated by actin binding proteins and small GTPases. The βI isoform of spectrin, a protein that links the actin cytoskeleton to membrane proteins, is present in spines. To understand its function, we expressed its actin-binding domain (ABD) in CA1 pyramidal neurons in hippocampal slice cultures. The ABD of βI-spectrin bundled actin in principal dendrites and was concentrated in dendritic spines, where it significantly increased the size of the spine head. These effects were not observed after expression of homologous ABDs of utrophin, dystrophin, and α-actinin. Treatment of slice cultures with latrunculin-B significantly decreased spine head size and decreased actin-GFP fluorescence in cells expressing the ABD of α-actinin, but not the ABD of βI-spectrin, suggesting that its presence inhibits actin depolymerization. We also observed an increase in the area of GFP-tagged PSD-95 in the spine head and an increase in the amplitude of mEPSCs at spines expressing the ABD of βI-spectrin. The effects of the βI-spectrin ABD on spine size and mEPSC amplitude were mimicked by expressing wild-type Rac3, a small GTPase that co-immunoprecipitates specifically with βI-spectrin in extracts of cultured cortical neurons. Spine size was normal in cells co-expressing a dominant negative Rac3 construct with the βI-spectrin ABD. We suggest that βI-spectrin is a synaptic protein that can modulate both the morphological and functional dynamics of dendritic spines, perhaps via interaction with actin and Rac3.

In vivo time-lapse imaging and serial section electron microscopy reveal developmental synaptic rearrangements

Dendrites, axons, and synapses are dynamic during circuit development; however, changes in microcircuit connections as branches stabilize have not been directly demonstrated. By combining in vivo time-lapse imaging of Xenopus tectal neurons with electron microscope reconstructions of imaged neurons, we report the distribution and ultrastructure of synapses on individual vertebrate neurons and relate these synaptic properties to dynamics in dendritic and axonal arbor structure over hours or days of imaging. Dynamic dendrites have a high density of immature synapses, whereas stable dendrites have sparser, mature synapses. Axons initiate contacts from multisynapse boutons on stable branches. Connections are refined by decreasing convergence from multiple inputs to postsynaptic dendrites and by decreasing divergence from multisynapse boutons to postsynaptic sites. Visual deprivation or NMDAR antagonists decreased synapse maturation and elimination, suggesting that coactive input activity promotes microcircuit development by concurrently regulating synapse elimination and maturation of remaining contacts.

Regulation of synapse structure and function by distinct myosin II motors

Ongoing synaptic function and rapid, bidirectional plasticity are both controlled by regulatory mechanisms within dendritic spines. Spine actin dynamics maintain synapse structure and function, and cytoskeletal rearrangements in these structures trigger structural and functional plasticity. Therefore, proteins that interact with actin filaments are attractive candidates to regulate synaptic actin dynamics and, thus, synapse structure and function. Here, we have cloned the rat isoform of class II myosin heavy chain MyH7B in brain. Unexpectedly, this isoform resembles muscle-type myosin II rather than the ubiquitously expressed nonmuscle myosin II isoforms, suggesting that a rich functional diversity of myosin II motors may exist in neurons. Indeed, reducing the expression of MyH7B in mature neurons caused profound alterations to dendritic spine structure and excitatory synaptic strength. Structurally, dendritic spines had large, irregularly shaped heads that contained many filopodia-like protrusions. Neurons with reduced MyH7B expression also had impaired miniature EPSC amplitudes accompanied by a decrease in synaptic AMPA receptors, which was linked to alterations of the actin cytoskeleton. MyH7B-mediated control over spine morphology and synaptic strength was distinct from that of a nonmuscle myosin, myosin IIb. Interestingly, when myosin IIb expression and MyH7B expression were simultaneously knocked-down in neurons, a third, more pronounced phenotype emerged. Together, our data provide evidence that distinct myosin II isoforms work together to regulate synapse structure and function in cultured hippocampal neurons. Thus, myosin II motor activity is emerging as a broad regulatory mechanism for control over complex actin networks within dendritic spines.

Multifunctional roles of activins in the brain

Activins, which are members of the TGF-β superfamily, were initially isolated from gonads and served as modulators of follicle-stimulating hormone secretion. Activins regulate various biological functions, including induction of the dorsal mesoderm, craniofacial development, and differentiation of numerous cell types. Activin receptors are highly expressed in neuronal cells, and activin mRNA expression is upregulated by neuronal activity. Activins also exhibit neuroprotective action during excitotoxic brain injury. However, very little is known about the functional roles of activins in the brain. We recently generated various types of transgenic mice, demonstrating that activins regulate spine formation, behavioral activity, anxiety, adult neurogenesis, late-phase long-term potentiation, and maintenance of long-term memory. The present chapter describes recent progress in the study of the role of activin in the brain.

Vasodilator-stimulated phosphoprotein (VASP) induces actin assembly in dendritic spines to promote their development and potentiate synaptic strength

Dendritic spines are small actin-rich structures that receive the majority of excitatory synaptic input in the brain. The actin-based dynamics of spines are thought to mediate synaptic plasticity, which underlies cognitive processes, such as learning and memory. However, little is known about the molecular mechanisms that regulate actin dynamics in spines and synapses. In this study we show the multifunctional actin-binding protein vasodilator-stimulated phosphoprotein (VASP) regulates the density, size, and morphology of dendritic spines by inducing actin assembly in these structures. Knockdown of endogenous VASP by siRNA led to a significant decrease in the density of spines and synapses, whereas expression of siRNA-resistant VASP rescued this defect. The ability of VASP to modulate spine and synapse formation, maturation, and spine head enlargement is dependent on its actin binding Ena/VASP homology 2 (EVH2) domain and its EVH1 domain, which contributes to VASP localization to actin-rich structures. Moreover, VASP increases the amount of PSD-scaffolding proteins and the number of surface GluR1-containing α-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid receptors (AMPARs) in spines. VASP knockdown results in a reduction in surface AMPAR density, suggesting a role for this protein in regulating synaptic strength. Consistent with this, VASP significantly enhances the retention of GluR1 in spines as determined by fluorescence recovery after photobleaching and increases AMPAR-mediated synaptic transmission. Collectively, our results suggest that actin polymerization and bundling by VASP are critical for spine formation, expansion, and modulating synaptic strength.

Lateral diffusion of inositol 1,4,5-trisphosphate receptor type 1 in Purkinje cells is regulated by calcium and actin filaments

Inositol 1,4,5-trisphosphate receptor type 1 (IP(3) R1) is an intracellular Ca(2+) release channel that plays crucial roles in the functions of Purkinje cells. The dynamics of IP(3) R1 on the endoplasmic reticulum membrane and the distribution of IP(3) R1 in neurons are thought to be important for the spatial regulation of Ca(2+) release. In this study, we analyzed the lateral diffusion of IP(3) R1 in Purkinje cells in cerebellar slice cultures using fluorescence recovery after photobleaching. In the dendrites of Purkinje cells, IP(3) R1 showed lateral diffusion with an effective diffusion constant of approximately 0.30 μm(2) /s, and the diffusion of IP(3) R1 was negatively regulated by actin filaments. We found that actin filaments were also involved in the regulation of IP(3) R1 diffusion in the spine of Purkinje cells. Glutamate or quisqualic acid stimulation, which activates glutamate receptors and leads to a Ca(2+) transient in Purkinje cells, decreased the diffusion of IP(3) R1 and increased the density of actin in spines. These findings indicate that the neuronal activity-dependent augmentation of actin contributes to the stabilization of IP(3) R1 in spines.

Spike timing-dependent plasticity as the origin of the formation of clustered synaptic efficacy engrams

Synapse location, dendritic active properties and synaptic plasticity are all known to play some role in shaping the different input streams impinging onto a neuron. It remains unclear however, how the magnitude and spatial distribution of synaptic efficacies emerge from this interplay. Here, we investigate this interplay using a biophysically detailed neuron model of a reconstructed layer 2/3 pyramidal cell and spike timing-dependent plasticity (STDP). Specifically, we focus on the issue of how the efficacy of synapses contributed by different input streams are spatially represented in dendrites after STDP learning. We construct a simple feed forward network where a detailed model neuron receives synaptic inputs independently from multiple yet equally sized groups of afferent fibers with correlated activity, mimicking the spike activity from different neuronal populations encoding, for example, different sensory modalities. Interestingly, ensuing STDP learning, we observe that for all afferent groups, STDP leads to synaptic efficacies arranged into spatially segregated clusters effectively partitioning the dendritic tree. These segregated clusters possess a characteristic global organization in space, where they form a tessellation in which each group dominates mutually exclusive regions of the dendrite. Put simply, the dendritic imprint from different input streams left after STDP learning effectively forms what we term a “dendritic efficacy mosaic.” Furthermore, we show how variations of the inputs and STDP rule affect such an organization. Our model suggests that STDP may be an important mechanism for creating a clustered plasticity engram, which shapes how different input streams are spatially represented in dendrite.

Advanced microscopy techniques for quantitative analysis in neuromorphology and neuropathology research: current status and requirements for the future

Visualizing neuromorphology and in particular neuropathology has been the focus of many researchers in the quest to solve the numerous questions that are still remaining related to several neurological and neuropsychiatric diseases. Over the last years, intense research into microscopy techniques has resulted in the development of various new types of microscopes, software imaging systems, and analysis programs. This review briefly discusses some key techniques, such as confocal stereology and automated neuron tracing and reconstruction, and their applications in neuroscience research. Special emphasis is placed on needs for further developments, such as the demand for higher-throughput analyses in quantitative neuromorphology. These developments will advance basic neuroscience research as well as pharmaceutical and biotechnology research and development.

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.

Is bigger always better? A critical appraisal of the use of volumetric analysis in the study of the hippocampus

A well-developed spatial memory is important for many animals, but appears especially important for scatter-hoarding species. Consequently, the scatter-hoarding system provides an excellent paradigm in which to study the integrative aspects of memory use within an ecological and evolutionary framework. One of the main tenets of this paradigm is that selection for enhanced spatial memory for cache locations should specialize the brain areas involved in memory. One such brain area is the hippocampus (Hp). Many studies have examined this adaptive specialization hypothesis, typically relating spatial memory to Hp volume. However, it is unclear how the volume of the Hp is related to its function for spatial memory. Thus, the goal of this article is to evaluate volume as a main measurement of the degree of morphological and physiological adaptation of the Hp as it relates to memory. We will briefly review the evidence for the specialization of memory in food-hoarding animals and discuss the philosophy behind volume as the main currency. We will then examine the problems associated with this approach, attempting to understand the advantages and limitations of using volume and discuss alternatives that might yield more specific hypotheses. Overall, there is strong evidence that the Hp is involved in the specialization of spatial memory in scatter-hoarding animals. However, volume may be only a coarse proxy for more relevant and subtle changes in the structure of the brain underlying changes in behaviour. To better understand the nature of this brain/memory relationship, we suggest focusing on more specific and relevant features of the Hp, such as the number or size of neurons, variation in connectivity depending on dendritic and axonal arborization and the number of synapses. These should generate more specific hypotheses derived from a solid theoretical background and should provide a better understanding of both neural mechanisms of memory and their evolution.

Disruption of fear memory consolidation and reconsolidation by actin filament arrest in the basolateral amygdala

The dynamic re-arrangement of actin filaments is an essential process in the plasticity of synaptic connections during memory formation. In this study, we determined in mice effects of actin filament arrest in the basolateral complex of the amygdala (BLA) at different time points after memory acquisition and re-activation, using the fungal cytotoxin phalloidin. Our data show a selective disruption of auditory cued but not contextual fear memory, when phalloidin was injected 6h after conditioning. In contrast, no effect was observed when phalloidin was applied after 24h, ruling out an interference with the retrieval or expression of conditioned fear. A comparable result was obtained after memory re-activation, hence suggesting similar actin-dependent mechanisms to be active during consolidation and reconsolidation of auditory fear memory. Biochemical analysis showed that phalloidin-mediated filament arrest leads to a transient increase of highly cross-linked actin filaments in the BLA, evident 2h after injection. Together, these observations indicate that dynamic re-arrangements of actin filaments in the BLA during a late phase of fear memory consolidation and reconsolidation are critical for fear memory storage.

Disrupted-in-Schizophrenia 1 (DISC1) regulates spines of the glutamate synapse via Rac1

Synaptic spines are dynamic structures that regulate neuronal responsiveness and plasticity. We examined the role of the schizophrenia risk factor DISC1 in the maintenance of spine morphology and function. We found that DISC1 anchored Kalirin-7 (Kal-7), regulating access of Kal-7 to Rac1 and controlling the duration and intensity of Rac1 activation in response to NMDA receptor activation in both cortical cultures and rat brain in vivo. These results explain why Rac1 and its activator (Kal-7) serve as important mediators of spine enlargement and why constitutive Rac1 activation decreases spine size. This mechanism likely underlies disturbances in glutamatergic neurotransmission that have been frequently reported in schizophrenia that can lead to alteration of dendritic spines with consequential major pathological changes in brain function. Furthermore, the concept of a signalosome involving disease-associated factors, such as DISC1 and glutamate, may well contribute to the multifactorial and polygenetic characteristics of schizophrenia.

Synaptic adaptations of CA1 pyramidal neurons induced by a highly effective combinational antidepressant therapy

BACKGROUND

Antidepressants (AD) need to be chronically administered (weeks to months) to provide beneficial effects. Evidence suggests that combined administration of inhibitors of monoamine reuptake and phosphodiesterase type 4 allows a highly effective therapeutic action. Also, this coadministration more rapidly boosts the cyclic adenosine monophosphate (cAMP) pathway, which is normally activated during chronic treatment of single compounds. Little is known, however, about how this augmentation therapy affects the core mechanism of glutamatergic plasticity. We therefore investigated how in vivo combinational subchronic rolipram and imipramine (scRI) treatment affects depressive behavior, cAMP-dependent transcription, and glutamatergic transmission in the hippocampus, a region critically implicated in depression.

METHODS

Antidepressant properties of scRI were investigated through the forced swim test. Changes in cAMP-dependent transcription and synaptic transmission of CA1 pyramidal cells were explored with green fluorescent protein, enzyme-linked immunosorbent assay, electrophysiology recordings, and Golgi-Cox staining.

RESULTS

We demonstrate that scRI displays robust antidepressant properties compared with single-drug treatments and increases hippocampal c-Fos expression and brain-derived neurotrophic factor protein levels. These effects are accompanied by a specific increase in alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid and N-methyl-D-aspartate receptors in already existing synapses. Finally, both acute and subchronic treatments led to enhancement of long-term potentiation but differently affected spine density and morphology, with scRI administration specifically resulting in a large increase in stubby spines.

CONCLUSIONS

We conclude that scRI is highly effective in providing antidepressive effects, including the hippocampal transcriptional alterations normally observed with longer single-drug treatments. Furthermore, we identified scRI-induced modifications in glutamatergic transmission that probably underlie the beneficial action of this combinational therapy.

Neuropeptide Release is Impaired in a Mouse Model of Fragile X Mental Retardation Syndrome

Fragile X syndrome (FXS), an inherited disorder characterized by mental retardation and autismlike behaviors, is caused by the failure to transcribe the gene for fragile X mental retardation protein (FMRP), a translational regulator and transporter of select mRNAs. FXS model mice (Fmr1 KO mice) exhibit impaired neuropeptide release. Release of biogenic amines does not differ between wild-type (WT) and Fmr1 KO mice. Rab3A, an mRNA cargo of FMRP involved in the recruitment of vesicles, is decreased by ∼50% in synaptoneurosomes of Fmr1 KO mice; however, the number of dense-core vesicles (DCVs) does not differ between WT and Fmr1 KO mice. Therefore, deficits associated with FXS may reflect this aberrant vesicle release, specifically involving docking and fusion of peptidergic DCVs, and may lead to defective maturation/maintenance of synaptic connections.

Mechanisms of synapse and dendrite maintenance and their disruption in psychiatric and neurodegenerative disorders

Emerging evidence indicates that once established, synapses and dendrites can be maintained for long periods, if not for the organism's entire lifetime. In contrast to the wealth of knowledge regarding axon, dendrite, and synapse development, we understand comparatively little about the cellular and molecular mechanisms that enable long-term synapse and dendrite maintenance. Here, we review how the actin cytoskeleton and its regulators, adhesion receptors, and scaffolding proteins mediate synapse and dendrite maintenance. We examine how these mechanisms are reinforced by trophic signals passed between the pre- and postsynaptic compartments. We also discuss how synapse and dendrite maintenance mechanisms are compromised in psychiatric and neurodegenerative disorders.

Rho kinase inhibition protects CA1 cells in organotypic hippocampal slices during in vitro ischemia

The actin cytoskeleton is a dynamic superstructure that regulates multiple cellular functions and that has been implicated in cell death regulation. We investigated whether modulating the neuronal actin cytoskeleton polymerization by Rho-GTPase kinase (ROCK) inhibition influences cell death in hippocampal neuronal cultures and in murine organotypic hippocampal slice cultures subjected to in vitro ischemia (IVI). During IVI, spines on vehicle treated hippocampal neurons collapsed and large dendritic actin aggregates were formed. Following ROCK inhibition by Y27632, the actin aggregates were markedly smaller while large filopodia extended from the dendritic trunk. Y27632 also provided strong neuroprotection of hippocampal pyramidal CA1 neurons, which was of similar magnitude as protection by NMDA receptor blockade. Likewise, treatment with the F-actin depolymerizing agent latrunculin during IVI diminished actin aggregation and mitigated cell death following IVI. We propose that ROCK inhibition protects neurons against ischemic damage by disrupting actin polymerization thereby mitigating NMDA receptor induced toxicity and releasing ATP bound to actin for cellular energy use. We conclude that ROCK inhibitors abrogate multiple detrimental processes and could therefore be useful in stroke therapy.

Reciprocal interactions regulate targeting of calcium channel beta subunits and membrane expression of alpha1 subunits in cultured hippocampal neurons

Auxiliary beta subunits modulate current properties and mediate the functional membrane expression of voltage-gated Ca(2+) channels in heterologous cells. In brain, all four beta isoforms are widely expressed, yet little is known about their specific roles in neuronal functions. Here, we investigated the expression and targeting properties of beta subunits and their role in membrane expression of Ca(V)1.2 alpha(1) subunits in cultured hippocampal neurons. Quantitative reverse transcription-PCR showed equal expression, and immunofluorescence showed a similar distribution of all endogenous beta subunits throughout dendrites and axons. High resolution microscopy of hippocampal neurons transfected with six different V5 epitope-tagged beta subunits demonstrated that all beta subunits were able to accumulate in synaptic terminals and to colocalize with postsynaptic Ca(V)1.2, thus indicating a great promiscuity in alpha(1)-beta interactions. In contrast, restricted axonal targeting of beta(1) and weak colocalization of beta(4b) with Ca(V)1.2 indicated isoform-specific differences in local channel complex formation. Membrane expression of external hemagglutinin epitope-tagged Ca(V)1.2 was strongly enhanced by all beta subunits in an isoform-specific manner. Conversely, mutating the alpha-interaction domain of Ca(V)1.2 (W440A) abolished membrane expression and targeting into dendritic spines. This demonstrates that in neurons the interaction of a beta subunit with the alpha-interaction domain is absolutely essential for membrane expression of alpha(1) subunits, as well as for the subcellular localization of beta subunits, which by themselves possess little or no targeting properties.